Source: https://www.physik.uni-wuerzburg.de/ep3/research/quantum-transport/publications/
Timestamp: 2019-04-26 16:33:05+00:00

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Evidence for the ballistic intrinsic spin Hall effect in HgTe nanostructures.Brüne, C.; Roth, A.; Novik, E. G.; König, M.; Buhmann, H.; Hankiewicz, E. M.; Hanke, W.; Sinova, J.; Molenkamp, L. W. in Nature Physics (2010). 6 448--454.
In the spin Hall effect, a current passed through a spin–orbit coupled electron gas induces a spin accumulation of inverse sign on either side of the sample. A number of possible mechanisms have been described, extrinsic as well as intrinsic ones, and they may occur in the ballistic as well as the diffusive transport regime. A central problem for experimentalists in studying the effect is the very small signals that result from the spin accumulation. Electrical measurements on metals have yielded reliable signatures of the spin Hall effect, but in semiconductors the spin accumulation could only be detected by optical techniques. Here we report experimental evidence for electrical manipulation and detection of the ballistic intrinsic spin Hall effect (ISHE) in semiconductors. We perform a non-local electrical measurement in nanoscale H-shaped structures built on high-mobility HgTe/(Hg, Cd)Te quantum wells. When the samples are tuned into the p-regime, we observe a large non-local resistance signal due to the ISHE, several orders of magnitude larger than in metals. In the n-regime, where the spin–orbit splitting is reduced, the signal is at least one order of magnitude smaller and vanishes for narrower quantum wells. We verify our experimental observations by quantum transport calculations.
%0 Journal Article %1 brune2010evidence %A Brüne, C. %A Roth, A. %A Novik, E. G. %A König, M. %A Buhmann, H. %A Hankiewicz, E. M. %A Hanke, W. %A Sinova, J. %A Molenkamp, L. W. %D 2010 %I Nature Publishing Group %J Nature Physics %P 448--454 %R 10.1038/nphys1655 %T Evidence for the ballistic intrinsic spin Hall effect in HgTe nanostructures %V 6 %X In the spin Hall effect, a current passed through a spin–orbit coupled electron gas induces a spin accumulation of inverse sign on either side of the sample. A number of possible mechanisms have been described, extrinsic as well as intrinsic ones, and they may occur in the ballistic as well as the diffusive transport regime. A central problem for experimentalists in studying the effect is the very small signals that result from the spin accumulation. Electrical measurements on metals have yielded reliable signatures of the spin Hall effect, but in semiconductors the spin accumulation could only be detected by optical techniques. Here we report experimental evidence for electrical manipulation and detection of the ballistic intrinsic spin Hall effect (ISHE) in semiconductors. We perform a non-local electrical measurement in nanoscale H-shaped structures built on high-mobility HgTe/(Hg, Cd)Te quantum wells. When the samples are tuned into the p-regime, we observe a large non-local resistance signal due to the ISHE, several orders of magnitude larger than in metals. In the n-regime, where the spin–orbit splitting is reduced, the signal is at least one order of magnitude smaller and vanishes for narrower quantum wells. We verify our experimental observations by quantum transport calculations.
Spin Hall Effects in HgTe Quantum Well Structures.Buhmann, Hartmut in International Journal of Modern Physics B (2009). 23(12n13) 2551--2555.
Due to a strong spin orbit interaction quantum well structures exhibit an unusual subband structure ordering which leads to some remarkable transport properties depending on the actual carrier density. Especially for quantum wells with an inverted band structure ordering, a strong Rashba-type spin orbit splitting gives rise to a strong spin Hall effect in the metallic regime and in the bulk insulating regime spin polarized edge channel transport leads to the formation of the quantum spin Hall effect. Gated quantum well structures have been used to explore these, the metallic and insulating, transport regimes experimentally.
%0 Journal Article %1 buhmann2009effects %A Buhmann, Hartmut %D 2009 %J International Journal of Modern Physics B %N 12n13 %P 2551--2555 %R 10.1142/S0217979209061974 %T Spin Hall Effects in HgTe Quantum Well Structures %V 23 %X Due to a strong spin orbit interaction quantum well structures exhibit an unusual subband structure ordering which leads to some remarkable transport properties depending on the actual carrier density. Especially for quantum wells with an inverted band structure ordering, a strong Rashba-type spin orbit splitting gives rise to a strong spin Hall effect in the metallic regime and in the bulk insulating regime spin polarized edge channel transport leads to the formation of the quantum spin Hall effect. Gated quantum well structures have been used to explore these, the metallic and insulating, transport regimes experimentally.
Nonlocal Transport in the Quantum Spin Hall State.Roth, Andreas; Brüne, Christoph; Buhmann, Hartmut; Molenkamp, Laurens W.; Maciejko, Joseph; Qi, Xiao-Liang; Zhang, Shou-Cheng in Science (2009). 325(5938) 294--297.
Nonlocal transport through edge channels holds great promise for low-power information processing. However, edge channels have so far only been demonstrated to occur in the quantum Hall regime, at high magnetic fields. We found that mercury telluride quantum wells in the quantum spin Hall regime exhibit nonlocal edge channel transport at zero external magnetic field. The data confirm that the quantum transport through the (helical) edge channels is dissipationless and that the contacts lead to equilibration between the counterpropagating spin states at the edge. The experimental data agree quantitatively with the theory of the quantum spin Hall effect. The edge channel transport paves the way for a new generation of spintronic devices for low-power information processing.
%0 Journal Article %1 roth2009nonlocal %A Roth, Andreas %A Brüne, Christoph %A Buhmann, Hartmut %A Molenkamp, Laurens W. %A Maciejko, Joseph %A Qi, Xiao-Liang %A Zhang, Shou-Cheng %D 2009 %I American Association for the Advancement of Science %J Science %N 5938 %P 294--297 %R 10.1126/science.1174736 %T Nonlocal Transport in the Quantum Spin Hall State %V 325 %X Nonlocal transport through edge channels holds great promise for low-power information processing. However, edge channels have so far only been demonstrated to occur in the quantum Hall regime, at high magnetic fields. We found that mercury telluride quantum wells in the quantum spin Hall regime exhibit nonlocal edge channel transport at zero external magnetic field. The data confirm that the quantum transport through the (helical) edge channels is dissipationless and that the contacts lead to equilibration between the counterpropagating spin states at the edge. The experimental data agree quantitatively with the theory of the quantum spin Hall effect. The edge channel transport paves the way for a new generation of spintronic devices for low-power information processing.
Nonlinear magnetogyrotropic photogalvanic effect.Diehl, H.; Shalygin, V. A.; Golub, L. E.; Tarasenko, S. A.; Danilov, S. N.; Bel'kov, V. V.; Novik, E. G.; Buhmann, H.; Brüne, C.; Molenkamp, L. W.; Ivchenko, E. L.; Ganichev, S. D. in Phys. Rev. B (2009). 80(7) 075311.
%0 Journal Article %1 diehl2009nonlinear %A Diehl, H. %A Shalygin, V. A. %A Golub, L. E. %A Tarasenko, S. A. %A Danilov, S. N. %A Bel'kov, V. V. %A Novik, E. G. %A Buhmann, H. %A Brüne, C. %A Molenkamp, L. W. %A Ivchenko, E. L. %A Ganichev, S. D. %D 2009 %I American Physical Society %J Phys. Rev. B %N 7 %P 075311 %R 10.1103/PhysRevB.80.075311 %T Nonlinear magnetogyrotropic photogalvanic effect %V 80 %X We report on the observation of magnetic-field-induced photocurrent in HgTe/HgCdTe quantum wells of different widths. Both the intrasubband and interband absorption of infrared/terahertz radiation in the heterostructures is shown to cause a dc electric current in the presence of an in-plane magnetic field. The photocurrent behavior upon variation in the radiation polarization, magnetic-field strength, and temperature is studied. At a moderate magnetic field the current exhibits a linear field dependence. At high magnetic fields, however, it becomes nonlinear and is dominated by a cubic in magnetic-field contribution. The latter effect is observed in quantum wells with the inverted band structure only. The experimental results are analyzed in terms of the phenomenological theory and microscopic models of magnetogyrotropic photogalvanic effect based on asymmetry of optical transitions and/or asymmetric relaxation of carriers in the momentum space. The effect is shown to be related to the gyrotropic properties of the structures. The developed theory of magnetogyrotropic photocurrent describes well all experimental results. It is shown that both intrasubband and interband optical transitions may lead to spin-related as well as to spin-independent magnetic-field-induced photocurrents.
The Quantum Spin Hall Effect: Theory and Experiment.König, Markus; Buhmann, Hartmut; Molenkamp, Laurens W.; Hughes, Taylor; Liu, Chao-Xing; Qi, Xiao-Liang; Zhang, Shou-Cheng in Journal of the Physical Society of Japan (2008). 77(3) 031007.
The search for topologically non-trivial states of matter has become an important goal for condensed matter physics. Recently, a new class of topological insulators has been proposed. These topological insulators have an insulating gap in the bulk, but have topologically protected edge states due to the time reversal symmetry. In two dimensions the helical edge states give rise to the quantum spin Hall (QSH) effect, in the absence of any external magnetic field. Here we review a recent theory which predicts that the QSH state can be realized in HgTe/CdTe semiconductor quantum wells (QWs). By varying the thickness of the QW, the band structure changes from a normal to an “inverted” type at a critical thickness d c . We present an analytical solution of the helical edge states and explicitly demonstrate their topological stability. We also review the recent experimental observation of the QSH state in HgTe/(Hg,Cd)Te QWs. We review both the fabrication of the sample and the experimental setup. For thin QWs with well width d QW <6.3 nm, the insulating regime shows the conventional behavior of vanishingly small conductance at low temperature. However, for thicker QWs ( d QW >6.3 nm), the nominally insulating regime shows a plateau of residual conductance close to 2 e^2 / h . The residual conductance is independent of the sample width, indicating that it is caused by edge states. Furthermore, the residual conductance is destroyed by a small external magnetic field. The quantum phase transition at the critical thickness, d c =6.3 nm, is also independently determined from the occurrence of a magnetic field induced insulator to metal transition.
%0 Journal Article %1 knig2008quantum %A König, Markus %A Buhmann, Hartmut %A Molenkamp, Laurens W. %A Hughes, Taylor %A Liu, Chao-Xing %A Qi, Xiao-Liang %A Zhang, Shou-Cheng %D 2008 %I Physical Society of Japan %J Journal of the Physical Society of Japan %N 3 %P 031007 %R 10.1143/jpsj.77.031007 %T The Quantum Spin Hall Effect: Theory and Experiment %V 77 %X The search for topologically non-trivial states of matter has become an important goal for condensed matter physics. Recently, a new class of topological insulators has been proposed. These topological insulators have an insulating gap in the bulk, but have topologically protected edge states due to the time reversal symmetry. In two dimensions the helical edge states give rise to the quantum spin Hall (QSH) effect, in the absence of any external magnetic field. Here we review a recent theory which predicts that the QSH state can be realized in HgTe/CdTe semiconductor quantum wells (QWs). By varying the thickness of the QW, the band structure changes from a normal to an “inverted” type at a critical thickness d c . We present an analytical solution of the helical edge states and explicitly demonstrate their topological stability. We also review the recent experimental observation of the QSH state in HgTe/(Hg,Cd)Te QWs. We review both the fabrication of the sample and the experimental setup. For thin QWs with well width d QW <6.3 nm, the insulating regime shows the conventional behavior of vanishingly small conductance at low temperature. However, for thicker QWs ( d QW >6.3 nm), the nominally insulating regime shows a plateau of residual conductance close to 2 e^2 / h . The residual conductance is independent of the sample width, indicating that it is caused by edge states. Furthermore, the residual conductance is destroyed by a small external magnetic field. The quantum phase transition at the critical thickness, d c =6.3 nm, is also independently determined from the occurrence of a magnetic field induced insulator to metal transition.
Quantum dot as thermal rectifier.Scheibner, R; König, M; Reuter, D; Wieck, A D; Gould, C; Buhmann, H; Molenkamp, L W in New Journal of Physics (2008). 10(8) 083016.
We report the observation of thermal rectification in a semiconductor quantum dot, as inferred from the asymmetric line shape of the thermopower oscillations. The asymmetry is observed at high in-plane magnetic fields and caused by the presence of a high orbital momentum state in the dot.
%0 Journal Article %1 scheibner2008quantum %A Scheibner, R %A König, M %A Reuter, D %A Wieck, A D %A Gould, C %A Buhmann, H %A Molenkamp, L W %D 2008 %J New Journal of Physics %N 8 %P 083016 %R 10.1088/1367-2630/10/8/083016 %T Quantum dot as thermal rectifier %V 10 %X We report the observation of thermal rectification in a semiconductor quantum dot, as inferred from the asymmetric line shape of the thermopower oscillations. The asymmetry is observed at high in-plane magnetic fields and caused by the presence of a high orbital momentum state in the dot.
The influence of interfaces and the modulation doping technique on the magneto-transport properties of HgTe based quantum wells.Becker, C.R.; Brüne, C.; Schäfer, M.; Roth, A.H.; Buhmann, H.; Molenkamp, L.W. in Physica Status Solidi C (2007). 4 3382.
The influence of interfaces in the structure of HgTe based quantum wells (QW's), their structure as well as the necessary technological processes on their transport properties have been investigated and either reduced or optimized. The mobility (μ) of the 2-dimensional electron gas (2DEG) has been shown to increase when the separation of the 2DEG from the ionized donors is increased, and when the separation of the QW structure from both the insulator on top and the CdTe buffer interface is increased. Furthermore, replacing wet chemical etching in the Hall bar photolithography procedure with a dry plasma etch process resulted in a 2.5 fold increase in the μ. Values for μ up to 0.7 × 10^6 cm^2/(Vs) at 4.2 K have been reproducibly achieved.
%0 Journal Article %1 becker2007influence %A Becker, C.R. %A Brüne, C. %A Schäfer, M. %A Roth, A.H. %A Buhmann, H. %A Molenkamp, L.W. %D 2007 %J Physica Status Solidi C %P 3382 %R 10.1002/pssc.200775402 %T The influence of interfaces and the modulation doping technique on the magneto-transport properties of HgTe based quantum wells %V 4 %X The influence of interfaces in the structure of HgTe based quantum wells (QW's), their structure as well as the necessary technological processes on their transport properties have been investigated and either reduced or optimized. The mobility (μ) of the 2-dimensional electron gas (2DEG) has been shown to increase when the separation of the 2DEG from the ionized donors is increased, and when the separation of the QW structure from both the insulator on top and the CdTe buffer interface is increased. Furthermore, replacing wet chemical etching in the Hall bar photolithography procedure with a dry plasma etch process resulted in a 2.5 fold increase in the μ. Values for μ up to 0.7 × 10^6 cm^2/(Vs) at 4.2 K have been reproducibly achieved.
Sequential and cotunneling behavior in the temperature-dependent thermopower of few-electron quantum dots.Scheibner, R.; Novik, E. G.; Borzenko, T.; König, M.; Reuter, D.; Wieck, A. D.; Buhmann, H.; Molenkamp, L. W. in Phys. Rev. B (2007). 75(4) 041301.
We have studied the temperature-dependent thermopower of gate-defined, lateral quantum dots in the Coulomb blockade regime using an electron heating technique. The line shape of the thermopower oscillations depends strongly on the contributing tunneling processes. Between 1.5K and 40mK a crossover from a pure sawtooth to an intermittent sawtoothlike line shape is observed. The latter is attributed to the increasing dominance of cotunneling processes in the Coulomb blockade regime at low temperatures.
%0 Journal Article %1 scheibner2007sequential %A Scheibner, R. %A Novik, E. G. %A Borzenko, T. %A König, M. %A Reuter, D. %A Wieck, A. D. %A Buhmann, H. %A Molenkamp, L. W. %D 2007 %I American Physical Society %J Phys. Rev. B %N 4 %P 041301 %R 10.1103/PhysRevB.75.041301 %T Sequential and cotunneling behavior in the temperature-dependent thermopower of few-electron quantum dots %V 75 %X We have studied the temperature-dependent thermopower of gate-defined, lateral quantum dots in the Coulomb blockade regime using an electron heating technique. The line shape of the thermopower oscillations depends strongly on the contributing tunneling processes. Between 1.5K and 40mK a crossover from a pure sawtooth to an intermittent sawtoothlike line shape is observed. The latter is attributed to the increasing dominance of cotunneling processes in the Coulomb blockade regime at low temperatures.
Quantum Spin Hall Insulator State in HgTe Quantum Wells.König, Markus; Wiedmann, Steffen; Brüne, Christoph; Roth, Andreas; Buhmann, Hartmut; Molenkamp, Laurens W.; Qi, Xiao-Liang; Zhang, Shou-Cheng in Science (2007). 318(5851) 766--770.
Recent theory predicted that the quantum spin Hall effect, a fundamentally new quantum state of matter that exists at zero external magnetic field, may be realized in HgTe/(Hg,Cd)Te quantum wells. We fabricated such sample structures with low density and high mobility in which we could tune, through an external gate voltage, the carrier conduction from n-type to p-type, passing through an insulating regime. For thin quantum wells with well width d < 6.3 nanometers, the insulating regime showed the conventional behavior of vanishingly small conductance at low temperature. However, for thicker quantum wells (d > 6.3 nanometers), the nominally insulating regime showed a plateau of residual conductance close to 2e^2/h, where e is the electron charge and h is Planck's constant. The residual conductance was independent of the sample width, indicating that it is caused by edge states. Furthermore, the residual conductance was destroyed by a small external magnetic field. The quantum phase transition at the critical thickness, d = 6.3 nanometers, was also independently determined from the magnetic field-induced insulator-to-metal transition. These observations provide experimental evidence of the quantum spin Hall effect.
%0 Journal Article %1 konig2007quantum %A König, Markus %A Wiedmann, Steffen %A Brüne, Christoph %A Roth, Andreas %A Buhmann, Hartmut %A Molenkamp, Laurens W. %A Qi, Xiao-Liang %A Zhang, Shou-Cheng %D 2007 %I American Association for the Advancement of Science %J Science %N 5851 %P 766--770 %R 10.1126/science.1148047 %T Quantum Spin Hall Insulator State in HgTe Quantum Wells %V 318 %X Recent theory predicted that the quantum spin Hall effect, a fundamentally new quantum state of matter that exists at zero external magnetic field, may be realized in HgTe/(Hg,Cd)Te quantum wells. We fabricated such sample structures with low density and high mobility in which we could tune, through an external gate voltage, the carrier conduction from n-type to p-type, passing through an insulating regime. For thin quantum wells with well width d < 6.3 nanometers, the insulating regime showed the conventional behavior of vanishingly small conductance at low temperature. However, for thicker quantum wells (d > 6.3 nanometers), the nominally insulating regime showed a plateau of residual conductance close to 2e^2/h, where e is the electron charge and h is Planck's constant. The residual conductance was independent of the sample width, indicating that it is caused by edge states. Furthermore, the residual conductance was destroyed by a small external magnetic field. The quantum phase transition at the critical thickness, d = 6.3 nanometers, was also independently determined from the magnetic field-induced insulator-to-metal transition. These observations provide experimental evidence of the quantum spin Hall effect.
Phase Effects in HgTe Quantum Structures.König, M.; Buhmann, H.; Becker, C.R.; Molenkamp, L.W. in Physica Status Solidi C (2007). 4 3374.
HgTe quantum well structures with high electron mobilities have been used to fabricate quantum interference devices. Aharonov-Bohm oscillations have been studied in the low and high magnetic field regime. In the latter case a decrease of the effective ring radius is observed. Additionally, as a consequence of the strong Rashba spin-orbit coupling within this material, it was possible to observe conductance oscillations which are due to the so-called Aharonov-Casher effect. These quantum interference effects are effectively controlled by the applied magnetic and electric field.
%0 Journal Article %1 knig2007phase %A König, M. %A Buhmann, H. %A Becker, C.R. %A Molenkamp, L.W. %D 2007 %J Physica Status Solidi C %P 3374 %R 10.1002/pssc.200775420 %T Phase Effects in HgTe Quantum Structures %V 4 %X HgTe quantum well structures with high electron mobilities have been used to fabricate quantum interference devices. Aharonov-Bohm oscillations have been studied in the low and high magnetic field regime. In the latter case a decrease of the effective ring radius is observed. Additionally, as a consequence of the strong Rashba spin-orbit coupling within this material, it was possible to observe conductance oscillations which are due to the so-called Aharonov-Casher effect. These quantum interference effects are effectively controlled by the applied magnetic and electric field.
Gate control of the giant Rashba effect in HgTe quantum wells.Hinz, J.; Buhmann, H.; Schäfer, M.; Hock, V.; Becker, C.R.; Molenkamp, L.W. in Semiconductor Science and Technology (2006). 21(4) 501.
HgTe/Hg 0.3 Cd 0.7 Te(0 0 1) quantum well structures fabricated with a Si–O–N insulator layer and an Au top gate electrode exhibit hysteresis effects in their gate-voltage dependent carrier density and thus a nonlinear variation of the Rashba spin–orbit splitting energy (Δ R ). Charging and discharging of states at the semiconductor insulator interface has been found to be responsible for this effect. The quantitative agreement with a simple capacitor model has been used to identify the maximum hysteresis-free gate-voltage range. A nearly linear variation of Δ R with applied gate voltage has been observed in this range.
%0 Journal Article %1 hinz2006control %A Hinz, J. %A Buhmann, H. %A Schäfer, M. %A Hock, V. %A Becker, C.R. %A Molenkamp, L.W. %D 2006 %J Semiconductor Science and Technology %N 4 %P 501 %R 10.1088/0268-1242/21/4/015 %T Gate control of the giant Rashba effect in HgTe quantum wells %V 21 %X HgTe/Hg 0.3 Cd 0.7 Te(0 0 1) quantum well structures fabricated with a Si–O–N insulator layer and an Au top gate electrode exhibit hysteresis effects in their gate-voltage dependent carrier density and thus a nonlinear variation of the Rashba spin–orbit splitting energy (Δ R ). Charging and discharging of states at the semiconductor insulator interface has been found to be responsible for this effect. The quantitative agreement with a simple capacitor model has been used to identify the maximum hysteresis-free gate-voltage range. A nearly linear variation of Δ R with applied gate voltage has been observed in this range.
Exchange Interaction in Magnetic HgMnTe-Quantum Well Structures.Liu, J.; Buhmann, H.; Novik, E.G.; Gui, Y.S.; Hock, V.; Becker, C.R.; Molenkamp, L.W. in Physica Status Solidi B (2006). 243 835.
Low temperature magnetotransport measurements have been used to study the exchange interactions in Hg(1–x)Mn(x)Te magnetic quantum wells (QWs). An analysis of Shubnikov–de Haas oscillations in conjunction with 8 × 8 k · p bandstructure calculations allow the exchange constants N0α and N0β , the antiferromagnetic temperature T0, and the effective spin S0 to be determined. The latter displays a distinct temperature dependence and appears to be enhanced compared with bulk materials of equivalent Mn concentrations.
%0 Journal Article %1 liu2006exchange %A Liu, J. %A Buhmann, H. %A Novik, E.G. %A Gui, Y.S. %A Hock, V. %A Becker, C.R. %A Molenkamp, L.W. %D 2006 %J Physica Status Solidi B %P 835 %R 10.1002/pssb.200564610 %T Exchange Interaction in Magnetic HgMnTe-Quantum Well Structures %V 243 %X Low temperature magnetotransport measurements have been used to study the exchange interactions in Hg(1–x)Mn(x)Te magnetic quantum wells (QWs). An analysis of Shubnikov–de Haas oscillations in conjunction with 8 × 8 k · p bandstructure calculations allow the exchange constants N0α and N0β , the antiferromagnetic temperature T0, and the effective spin S0 to be determined. The latter displays a distinct temperature dependence and appears to be enhanced compared with bulk materials of equivalent Mn concentrations.
Direct Observation of the Aharonov-Casher Phase.König, M.; Tschetschetkin, A.; Hankiewicz, E. M.; Sinova, Jairo; Hock, V.; Daumer, V.; Schäfer, M.; Becker, C. R.; Buhmann, H.; Molenkamp, L. W. in Phys. Rev. Lett. (2006). 96(7) 076804.
Ring structures fabricated from HgTe/HgCdTe quantum wells have been used to study Aharonov-Bohm type conductance oscillations as a function of Rashba spin-orbit splitting strength. We observe nonmonotonic phase changes indicating that an additional phase factor modifies the electron wave function. We associate these observations with the Aharonov-Casher effect. This is confirmed by comparison with numerical calculations of the magnetoconductance for a multichannel ring structure within the Landauer-Büttiker formalism.
%0 Journal Article %1 konig2006direct %A König, M. %A Tschetschetkin, A. %A Hankiewicz, E. M. %A Sinova, Jairo %A Hock, V. %A Daumer, V. %A Schäfer, M. %A Becker, C. R. %A Buhmann, H. %A Molenkamp, L. W. %D 2006 %I American Physical Society %J Phys. Rev. Lett. %N 7 %P 076804 %R 10.1103/PhysRevLett.96.076804 %T Direct Observation of the Aharonov-Casher Phase %V 96 %X Ring structures fabricated from HgTe/HgCdTe quantum wells have been used to study Aharonov-Bohm type conductance oscillations as a function of Rashba spin-orbit splitting strength. We observe nonmonotonic phase changes indicating that an additional phase factor modifies the electron wave function. We associate these observations with the Aharonov-Casher effect. This is confirmed by comparison with numerical calculations of the magnetoconductance for a multichannel ring structure within the Landauer-Büttiker formalism.
Trion formation in narrow GaAs quantum well structures.Teran, F. J.; Eaves, L.; Mansouri, L.; Buhmann, H.; Maude, D. K.; Potemski, M.; Henini, M.; Hill, G. in Phys. Rev. B (2005). 71(16) 161309.
We study the optical and electrical properties of n−i−n GaAs∕AlAs double-barrier resonant tunneling diodes. Under illumination, new resonances appear in the current-voltage curves due to tunneling of photoexcited holes. By tuning the bias and the intensity of illumination, we can control independently the number of electrons and holes tunneling into the quantum well. This allows us to create charge conditions in the well that favor the formation of either positively charged, neutral, or negatively charged excitons. Our measured value of the binding energy of the second electron in the negative trion is significantly larger than that of the second hole in the positive trion.
%0 Journal Article %1 teran2005trion %A Teran, F. J. %A Eaves, L. %A Mansouri, L. %A Buhmann, H. %A Maude, D. K. %A Potemski, M. %A Henini, M. %A Hill, G. %D 2005 %I American Physical Society %J Phys. Rev. B %N 16 %P 161309 %R 10.1103/PhysRevB.71.161309 %T Trion formation in narrow GaAs quantum well structures %V 71 %X We study the optical and electrical properties of n−i−n GaAs∕AlAs double-barrier resonant tunneling diodes. Under illumination, new resonances appear in the current-voltage curves due to tunneling of photoexcited holes. By tuning the bias and the intensity of illumination, we can control independently the number of electrons and holes tunneling into the quantum well. This allows us to create charge conditions in the well that favor the formation of either positively charged, neutral, or negatively charged excitons. Our measured value of the binding energy of the second electron in the negative trion is significantly larger than that of the second hole in the positive trion.
Thermopower of a Kondo Spin-Correlated Quantum Dot.Scheibner, R.; Buhmann, H.; Reuter, D.; Kiselev, M. N.; Molenkamp, L. W. in Phys. Rev. Lett. (2005). 95(17) 176602.
The thermopower of a Kondo-correlated gate-defined quantum dot is studied using a current heating technique. In the presence of spin correlations, the thermopower shows a clear deviation from the semiclassical Mott relation between thermopower and conductivity. The strong thermopower signal indicates a significant asymmetry in the spectral density of states of the Kondo resonance with respect to the Fermi energies of the reservoirs. The observed behavior can be explained within the framework of an Anderson-impurity model.
%0 Journal Article %1 scheibner2005thermopower %A Scheibner, R. %A Buhmann, H. %A Reuter, D. %A Kiselev, M. N. %A Molenkamp, L. W. %D 2005 %I American Physical Society %J Phys. Rev. Lett. %N 17 %P 176602 %R 10.1103/PhysRevLett.95.176602 %T Thermopower of a Kondo Spin-Correlated Quantum Dot %V 95 %X The thermopower of a Kondo-correlated gate-defined quantum dot is studied using a current heating technique. In the presence of spin correlations, the thermopower shows a clear deviation from the semiclassical Mott relation between thermopower and conductivity. The strong thermopower signal indicates a significant asymmetry in the spectral density of states of the Kondo resonance with respect to the Fermi energies of the reservoirs. The observed behavior can be explained within the framework of an Anderson-impurity model.
Odd filling factor quantum Hall sequence in magnetic type-III quantum wells.Buhmann, H.; Novik, E. G.; Daumer, V.; Liu, J.; Gui, Y. S.; Becker, C. R.; Molenkamp, L. W. in Appl. Phys. Lett. (2005). 86(21) 212104.
The magnetotransport properties of a series of Hg(1−y)Mn(y)Te quantum wells (y=2%) reveal anomalous sequences of quantum Hall plateaus and corresponding minima in the longitudinal magnetoresistance which correspond solely to odd filling factors. This anomaly originates from the very large spin splitting in this material, which exceeds the cyclotron energy even at moderate magnetic fields (B>2T). An 8×8 band k∙p approach has been used to calculate the magnetic field dependence of the Landau levels. Finite temperature and Landau level broadening considerations confirm that subband crossings in the vicinity of the Fermi energy are responsible for the suppression of even filling factor quantum Hall states.
%0 Journal Article %1 buhmann2005filling %A Buhmann, H. %A Novik, E. G. %A Daumer, V. %A Liu, J. %A Gui, Y. S. %A Becker, C. R. %A Molenkamp, L. W. %B Applied Physics Letters %D 2005 %I American Institute of Physics %J Appl. Phys. Lett. %N 21 %P 212104 %R 10.1063/1.1935753 %T Odd filling factor quantum Hall sequence in magnetic type-III quantum wells %V 86 %X The magnetotransport properties of a series of Hg(1−y)Mn(y)Te quantum wells (y=2%) reveal anomalous sequences of quantum Hall plateaus and corresponding minima in the longitudinal magnetoresistance which correspond solely to odd filling factors. This anomaly originates from the very large spin splitting in this material, which exceeds the cyclotron energy even at moderate magnetic fields (B>2T). An 8×8 band k∙p approach has been used to calculate the magnetic field dependence of the Landau levels. Finite temperature and Landau level broadening considerations confirm that subband crossings in the vicinity of the Fermi energy are responsible for the suppression of even filling factor quantum Hall states.
Band structure of semimagnetic Hg(1-y)Mn(y)Te quantum wells.Novik, E. G.; Pfeuffer-Jeschke, A.; Jungwirth, T.; Latussek, V.; Becker, C. R.; Landwehr, G.; Buhmann, H.; Molenkamp, L. W. in Phys. Rev. B (2005). 72(3) 035321.
The band structure of semimagnetic Hg(1−y)Mn(y)Te/Hg(1−x)Cd(x)Te type-III quantum wells (QW’s) has been calculated using an eight-band k∙p model in an envelope function approach. Details of the band structure calculations are given for the Mn-free case (y=0). A mean-field approach is used to take the influence of the sp−d exchange interaction on the band structure of QW’s with low Mn concentrations into account. The calculated Landau level fan diagram and the density of states of a Hg(0.98)Mn(0.02)Te/Hg(0.3)Cd(0.7)Te QW are in good agreement with recent experimental transport observations. The model can be used to interpret the mutual influence of the two-dimensional confinement and the sp−d exchange interaction on the transport properties of Hg(1−y)Mn(y)Te/Hg(1−x)Cd(x)Te QW’s.
%0 Journal Article %1 novik2005structure %A Novik, E. G. %A Pfeuffer-Jeschke, A. %A Jungwirth, T. %A Latussek, V. %A Becker, C. R. %A Landwehr, G. %A Buhmann, H. %A Molenkamp, L. W. %D 2005 %I American Physical Society %J Phys. Rev. B %N 3 %P 035321 %R 10.1103/PhysRevB.72.035321 %T Band structure of semimagnetic Hg(1-y)Mn(y)Te quantum wells %V 72 %X The band structure of semimagnetic Hg(1−y)Mn(y)Te/Hg(1−x)Cd(x)Te type-III quantum wells (QW’s) has been calculated using an eight-band k∙p model in an envelope function approach. Details of the band structure calculations are given for the Mn-free case (y=0). A mean-field approach is used to take the influence of the sp−d exchange interaction on the band structure of QW’s with low Mn concentrations into account. The calculated Landau level fan diagram and the density of states of a Hg(0.98)Mn(0.02)Te/Hg(0.3)Cd(0.7)Te QW are in good agreement with recent experimental transport observations. The model can be used to interpret the mutual influence of the two-dimensional confinement and the sp−d exchange interaction on the transport properties of Hg(1−y)Mn(y)Te/Hg(1−x)Cd(x)Te QW’s.
Low-field diffusion magnetothermopower of a high-mobility two-dimensional electron gas.Maximov, S.; Gbordzoe, M.; Buhmann, H.; Molenkamp, L. W.; Reuter, D. in Phys. Rev. B (2004). 70(12) 121308.
The low-magnetic field-diffusion thermopower of a high-mobility GaAs heterostructure has been measured directly on an electrostatically defined micron-scale Hall-bar structure (4μm×8μm) at low temperature (T=1.6K) in the field regime (B⩽1.2T) where the formation of edge states does not influence the measurements. The sample design allowed the determination of the field dependence of the thermopower both parallel and perpendicular to the temperature gradient, denoted respectively by Sxx (longitudinal thermopower) and Syx (the Nernst-Ettinghausen coefficient). The experimental data show clear oscillations in Sxx and Syx due to the formation of Landau levels and reveal that Syx≈120Sxx at a magnetic field of 1T, which agrees well with the theoretical prediction that the ratio of these tensor components is dependent on the carrier mobility: Syx/Sxx=2ωcτ.
%0 Journal Article %1 maximov2004lowfield %A Maximov, S. %A Gbordzoe, M. %A Buhmann, H. %A Molenkamp, L. W. %A Reuter, D. %D 2004 %I American Physical Society %J Phys. Rev. B %N 12 %P 121308 %R 10.1103/PhysRevB.70.121308 %T Low-field diffusion magnetothermopower of a high-mobility two-dimensional electron gas %V 70 %X The low-magnetic field-diffusion thermopower of a high-mobility GaAs heterostructure has been measured directly on an electrostatically defined micron-scale Hall-bar structure (4μm×8μm) at low temperature (T=1.6K) in the field regime (B⩽1.2T) where the formation of edge states does not influence the measurements. The sample design allowed the determination of the field dependence of the thermopower both parallel and perpendicular to the temperature gradient, denoted respectively by Sxx (longitudinal thermopower) and Syx (the Nernst-Ettinghausen coefficient). The experimental data show clear oscillations in Sxx and Syx due to the formation of Landau levels and reveal that Syx≈120Sxx at a magnetic field of 1T, which agrees well with the theoretical prediction that the ratio of these tensor components is dependent on the carrier mobility: Syx/Sxx=2ωcτ.
Interplay of Rashba, Zeeman and Landau splitting in a magnetic two dimensional electron gas.Gui, Y.S.; Becker, C.R.; Liu, J.; Daumer, V.; Hock, V.; Buhmann, H.; Molenkamp, L.W. in Europhys. Lett. (2004). 65(3) 393.
The transport properties of a magnetic two-dimensional electron gas consisting of a modulation doped n-type Hg(0.98)Mn(0.02)Te/Hg(0.3)Cd(0.7)Te quantum well, QW, have been investigated. By analyzing the Shubnikov-de Haas oscillations and the node positions of their beating patterns, we have been able to separate the gate-voltage-dependent Rashba spin-orbit splitting from the temperature-dependent giant Zeeman splitting. It has been experimentally demonstrated that the Rashba spin-orbit splitting is larger than or comparable to the sp-d exchange-interaction–induced giant Zeeman splitting in this magnetic 2DEG even at moderately high magnetic fields.
%0 Journal Article %1 gui2004interplay %A Gui, Y.S. %A Becker, C.R. %A Liu, J. %A Daumer, V. %A Hock, V. %A Buhmann, H. %A Molenkamp, L.W. %D 2004 %J Europhys. Lett. %N 3 %P 393 %R 10.1209/epl/i2003-10094-2 %T Interplay of Rashba, Zeeman and Landau splitting in a magnetic two dimensional electron gas %V 65 %X The transport properties of a magnetic two-dimensional electron gas consisting of a modulation doped n-type Hg(0.98)Mn(0.02)Te/Hg(0.3)Cd(0.7)Te quantum well, QW, have been investigated. By analyzing the Shubnikov-de Haas oscillations and the node positions of their beating patterns, we have been able to separate the gate-voltage-dependent Rashba spin-orbit splitting from the temperature-dependent giant Zeeman splitting. It has been experimentally demonstrated that the Rashba spin-orbit splitting is larger than or comparable to the sp-d exchange-interaction–induced giant Zeeman splitting in this magnetic 2DEG even at moderately high magnetic fields.
Growth and studies of Hg(1-x)Cd(x)Te based low dimensional structures.Becker, C.R.; Ortner, K.; Zhang, X.C.; Pfeuffer-Jeschke, A.; Latussek, V.; Gui, Y.S.; Daumer, V.; Buhmann, H.; Landwehr, G.; Molenkamp, L.W. in Physica E: Low-dimensional Systems and Nanostructures (2004). 20(3-4) 436--443.
The band structure of HgTe quantum wells (QWs) has been determined from absorption experiments on superlattices in conjunction with calculations based on an 8x8 k p model. The band structure combined with self-consistent Hartree calculations has enabled transport results to be quantitatively explained. Rashba spin–orbit, (SO) splitting has been investigated in n-type modulation doped HgTe QWs by means of Shubnikov–de Haas oscillations (SdH) in gated Hall bars. The heavy hole nature of the H1 conduction subband in QWs with an inverted band structure greatly enhances the Rashba SO splitting, with values up to 17 meV. By analyzing the SdH oscillations of a magnetic two-dimensional electron gas (2DEG) in modulation-doped n-type Hg(1−x)Mn(x)Te QWs, we have been able to separate the gate voltage-dependent Rashba SO splitting from the temperature-dependent giant Zeeman splitting, which are of comparable magnitudes. In addition, hot electrons and Mn ions in a magnetic 2DEG have been investigated as a function of current. Nano-scale structures of lower dimensions are planned and experiments on sub-micrometer magneto-transport structures have resulted in the first evidence for ballistic transport in quasi-1D HgTe QW structures.
%0 Journal Article %1 becker2004growth %A Becker, C.R. %A Ortner, K. %A Zhang, X.C. %A Pfeuffer-Jeschke, A. %A Latussek, V. %A Gui, Y.S. %A Daumer, V. %A Buhmann, H. %A Landwehr, G. %A Molenkamp, L.W. %D 2004 %I Elsevier BV %J Physica E: Low-dimensional Systems and Nanostructures %N 3-4 %P 436--443 %R 10.1016/j.physe.2003.08.053 %T Growth and studies of Hg(1-x)Cd(x)Te based low dimensional structures %V 20 %X The band structure of HgTe quantum wells (QWs) has been determined from absorption experiments on superlattices in conjunction with calculations based on an 8x8 k p model. The band structure combined with self-consistent Hartree calculations has enabled transport results to be quantitatively explained. Rashba spin–orbit, (SO) splitting has been investigated in n-type modulation doped HgTe QWs by means of Shubnikov–de Haas oscillations (SdH) in gated Hall bars. The heavy hole nature of the H1 conduction subband in QWs with an inverted band structure greatly enhances the Rashba SO splitting, with values up to 17 meV. By analyzing the SdH oscillations of a magnetic two-dimensional electron gas (2DEG) in modulation-doped n-type Hg(1−x)Mn(x)Te QWs, we have been able to separate the gate voltage-dependent Rashba SO splitting from the temperature-dependent giant Zeeman splitting, which are of comparable magnitudes. In addition, hot electrons and Mn ions in a magnetic 2DEG have been investigated as a function of current. Nano-scale structures of lower dimensions are planned and experiments on sub-micrometer magneto-transport structures have resulted in the first evidence for ballistic transport in quasi-1D HgTe QW structures.
Giant spin-orbit splitting in a HgTe quantum well.Gui, Y. S.; Becker, C. R.; Dai, N.; Liu, J.; Qiu, Z. J.; Novik, E. G.; Schäfer, M.; Shu, X. Z.; Chu, J. H.; Buhmann, H.; Molenkamp, L. W. in Phys. Rev. B (2004). 70(11) 115328.
We have investigated beating patterns in Shubnikov–de Haas oscillations for HgTe/Hg(0.3)Cd(0.7)Te (001) quantum wells with electron densities of 2–3×10^12cm^−2. Up to 12 beating nodes have been observed at magnetic fields between 0.9 and 6T. Zero-magnetic-field spin-orbit splitting energies up to 30meV have been directly determined from the node positions as well as from the intersection of self-consistently calculated Landau levels. These values, which exceed the thermal broadening of Landau levels, kBT, at room temperature, are in good agreement with Rashba spin-orbit splitting energies calculated by means of an 8×8k∙p Kane model. The experimental Shubnikov–de Haas oscillations are also in good agreement with numerical simulations based on this model.
%0 Journal Article %1 gui2004giant %A Gui, Y. S. %A Becker, C. R. %A Dai, N. %A Liu, J. %A Qiu, Z. J. %A Novik, E. G. %A Schäfer, M. %A Shu, X. Z. %A Chu, J. H. %A Buhmann, H. %A Molenkamp, L. W. %D 2004 %I American Physical Society %J Phys. Rev. B %N 11 %P 115328 %R 10.1103/PhysRevB.70.115328 %T Giant spin-orbit splitting in a HgTe quantum well %V 70 %X We have investigated beating patterns in Shubnikov–de Haas oscillations for HgTe/Hg(0.3)Cd(0.7)Te (001) quantum wells with electron densities of 2–3×10^12cm^−2. Up to 12 beating nodes have been observed at magnetic fields between 0.9 and 6T. Zero-magnetic-field spin-orbit splitting energies up to 30meV have been directly determined from the node positions as well as from the intersection of self-consistently calculated Landau levels. These values, which exceed the thermal broadening of Landau levels, kBT, at room temperature, are in good agreement with Rashba spin-orbit splitting energies calculated by means of an 8×8k∙p Kane model. The experimental Shubnikov–de Haas oscillations are also in good agreement with numerical simulations based on this model.
Current heating of a magnetic two-dimensional electron gas in Hg(1-x)Mn(x)Te∕Hg(0.3)Cd(0.7)Te quantum wells.Gui, Y. S.; Becker, C. R.; Liu, J.; König, M.; Daumer, V.; Kiselev, M. N.; Buhmann, H.; Molenkamp, L. W. in Phys. Rev. B (2004). 70(19) 195328.
Heating caused by electrons with excess kinetic energy has been investigated in a magnetic two-dimensional electron gas in Hg(1−x)Mn(x)Te/Hg(0.3)Cd(0.7)Te (001) quantum wells. The temperature of the Mn ions T_Mn has been determined by the node positions in the beating pattern in Shubnikov–de Haas oscillations. The experimental dependence of T_Mn on current and therefore on electron temperature, is in excellent agreement with a rate equation model.
%0 Journal Article %1 gui2004current %A Gui, Y. S. %A Becker, C. R. %A Liu, J. %A König, M. %A Daumer, V. %A Kiselev, M. N. %A Buhmann, H. %A Molenkamp, L. W. %D 2004 %I American Physical Society %J Phys. Rev. B %N 19 %P 195328 %R 10.1103/PhysRevB.70.195328 %T Current heating of a magnetic two-dimensional electron gas in Hg(1-x)Mn(x)Te∕Hg(0.3)Cd(0.7)Te quantum wells %V 70 %X Heating caused by electrons with excess kinetic energy has been investigated in a magnetic two-dimensional electron gas in Hg(1−x)Mn(x)Te/Hg(0.3)Cd(0.7)Te (001) quantum wells. The temperature of the Mn ions T_Mn has been determined by the node positions in the beating pattern in Shubnikov–de Haas oscillations. The experimental dependence of T_Mn on current and therefore on electron temperature, is in excellent agreement with a rate equation model.
Relaxation of high-energy quasiparticle distributions: Electron-electron scattering in a two-dimensional electron gas.Gurzhi, R. N.; Kopeliovich, A. I.; Kalinenko, A. N.; Yanovsky, A. V.; Bogachek, E. N.; Landman, Uzi; Buhmann, H.; Molenkamp, L. W. in Phys. Rev. B (2003). 68(16) 165318.
We have investigated beating patterns in Shubnikov–de Haas oscillations for HgTe/Hg(0.3)Cd(0.7)Te(001) quantum wells with electron densities of 2–3×10^12cm^−2. Up to 12 beating nodes have been observed at magnetic fields between 0.9 and 6T. Zero-magnetic-field spin-orbit splitting energies up to 30meV have been directly determined from the node positions as well as from the intersection of self-consistently calculated Landau levels. These values, which exceed the thermal broadening of Landau levels, kBT, at room temperature, are in good agreement with Rashba spin-orbit splitting energies calculated by means of an 8×8k∙p Kane model. The experimental Shubnikov–de Haas oscillations are also in good agreement with numerical simulations based on this model.
%0 Journal Article %1 gurzhi2003relaxation %A Gurzhi, R. N. %A Kopeliovich, A. I. %A Kalinenko, A. N. %A Yanovsky, A. V. %A Bogachek, E. N. %A Landman, Uzi %A Buhmann, H. %A Molenkamp, L. W. %D 2003 %I American Physical Society %J Phys. Rev. B %N 16 %P 165318 %R 10.1103/PhysRevB.68.165318 %T Relaxation of high-energy quasiparticle distributions: Electron-electron scattering in a two-dimensional electron gas %V 68 %X We have investigated beating patterns in Shubnikov–de Haas oscillations for HgTe/Hg(0.3)Cd(0.7)Te(001) quantum wells with electron densities of 2–3×10^12cm^−2. Up to 12 beating nodes have been observed at magnetic fields between 0.9 and 6T. Zero-magnetic-field spin-orbit splitting energies up to 30meV have been directly determined from the node positions as well as from the intersection of self-consistently calculated Landau levels. These values, which exceed the thermal broadening of Landau levels, kBT, at room temperature, are in good agreement with Rashba spin-orbit splitting energies calculated by means of an 8×8k∙p Kane model. The experimental Shubnikov–de Haas oscillations are also in good agreement with numerical simulations based on this model.
Quasiballistic transport in HgTe quantum-well nanostructures.Daumer, V; Golombek, I; Gbordzoe, M; Novik, E. G.; Hock, V; Becker, C. R.; Buhmann, H; Molenkamp, L. W. in Appl. Phys. Lett. (2003). 83(7) 1376--1378.
The transport properties of micrometer scale structures fabricated from high-mobility HgTe quantum wells have been investigated. A special photoresist and Ti masks were used, which allow for the fabrication of devices with characteristic dimensions down to 0.45 μm. Evidence that the transport properties are dominated by ballistic effects in these structures is presented. Monte Carlo simulations of semiclassical electron trajectories show good agreement with the experiment.
%0 Journal Article %1 daumer2003quasiballistic %A Daumer, V %A Golombek, I %A Gbordzoe, M %A Novik, E. G. %A Hock, V %A Becker, C. R. %A Buhmann, H %A Molenkamp, L. W. %B Applied Physics Letters %D 2003 %I American Institute of Physics %J Appl. Phys. Lett. %N 7 %P 1376--1378 %R 10.1063/1.1602170 %T Quasiballistic transport in HgTe quantum-well nanostructures %V 83 %X The transport properties of micrometer scale structures fabricated from high-mobility HgTe quantum wells have been investigated. A special photoresist and Ti masks were used, which allow for the fabrication of devices with characteristic dimensions down to 0.45 μm. Evidence that the transport properties are dominated by ballistic effects in these structures is presented. Monte Carlo simulations of semiclassical electron trajectories show good agreement with the experiment.
Electron-beam propagation in a two-dimensional electron gas.Novik, E. G.; Buhmann, H.; Molenkamp, L. W. in Phys. Rev. B (2003). 67(24) 245302.
A quantum-mechanical model based on a Green’s function approach has been used to calculate the transmission probability of electrons traversing a two-dimensional electron gas injected and detected via mode-selective quantum point contacts. Two-dimensional scattering potentials, backscattering, and temperature effects were included in order to compare the calculated results with experimentally observed interference patterns. The results yield information about the distribution, size, and the energetic height of the scattering potentials.
%0 Journal Article %1 novik2003electronbeam %A Novik, E. G. %A Buhmann, H. %A Molenkamp, L. W. %D 2003 %I American Physical Society %J Phys. Rev. B %N 24 %P 245302 %R 10.1103/PhysRevB.67.245302 %T Electron-beam propagation in a two-dimensional electron gas %V 67 %X A quantum-mechanical model based on a Green’s function approach has been used to calculate the transmission probability of electrons traversing a two-dimensional electron gas injected and detected via mode-selective quantum point contacts. Two-dimensional scattering potentials, backscattering, and temperature effects were included in order to compare the calculated results with experimentally observed interference patterns. The results yield information about the distribution, size, and the energetic height of the scattering potentials.
Competition Between Rashba Effect and Exchange Interaction in a Hg(0.98)Mn(0.02)Te Magnetic 2DEG.Liu, J.; Daumer, V.; Gui, Y. S.; Hock, V.; Becker, C. R.; Buhmann, H.; Molenkamp, L. W. in Journal of Superconductivity (2003). 16(2) 365--368.
Magnetotransport measurements are used to study the peculiar properties of a modulation doped n-type Hg(0.98)Mn(0.02)Te magnetic 2DEG. The Rashba effect and giant Zeeman spin splitting are observed simultaneously and can be separated by temperature and gate voltage dependent Shubnikov-de Haas oscillations. The Rashba effect is found to be the dominant term up to 5 T.
%0 Journal Article %1 liu2003competition %A Liu, J. %A Daumer, V. %A Gui, Y. S. %A Hock, V. %A Becker, C. R. %A Buhmann, H. %A Molenkamp, L. W. %D 2003 %J Journal of Superconductivity %N 2 %P 365--368 %R 10.1023/A:1023638125071 %T Competition Between Rashba Effect and Exchange Interaction in a Hg(0.98)Mn(0.02)Te Magnetic 2DEG %V 16 %X Magnetotransport measurements are used to study the peculiar properties of a modulation doped n-type Hg(0.98)Mn(0.02)Te magnetic 2DEG. The Rashba effect and giant Zeeman spin splitting are observed simultaneously and can be separated by temperature and gate voltage dependent Shubnikov-de Haas oscillations. The Rashba effect is found to be the dominant term up to 5 T.
Spectroscopical imaging of electron scattering in two-dimensional conductors.Yanovsky, A.V.; Predel, H.; Buhmann, H.; Gurzhi, R.N.; Kalinenko, A.N.; Kopeliovich, A.I.; Molenkamp, L.W. in Physica E: Low-dimensional Systems and Nanostructures (2002). 12(1-4) 241--243.
A new method to study electron scattering processes in conductors is proposed and experimentally realized. This method is based on the separation of the particles which are scattered at the different angles in a magnetic field. Experimental implementation of this method on GaAs(AlGaAs) heterojunctions has confirmed the ideas about essential differences of momentum transfer mechanisms in two- and three-dimensional systems. Most conspicuous is the increased importance of the small angular scattering for small excess energies.
%0 Journal Article %1 yanovsky2002spectroscopical %A Yanovsky, A.V. %A Predel, H. %A Buhmann, H. %A Gurzhi, R.N. %A Kalinenko, A.N. %A Kopeliovich, A.I. %A Molenkamp, L.W. %D 2002 %I Elsevier BV %J Physica E: Low-dimensional Systems and Nanostructures %N 1-4 %P 241--243 %R 10.1016/s1386-9477(01)00363-0 %T Spectroscopical imaging of electron scattering in two-dimensional conductors %V 12 %X A new method to study electron scattering processes in conductors is proposed and experimentally realized. This method is based on the separation of the particles which are scattered at the different angles in a magnetic field. Experimental implementation of this method on GaAs(AlGaAs) heterojunctions has confirmed the ideas about essential differences of momentum transfer mechanisms in two- and three-dimensional systems. Most conspicuous is the increased importance of the small angular scattering for small excess energies.
MBE growth and characterization of Hg based compounds and heterostructures.Becker, C.R; Zhang, X.C; Ortner, K; Schmidt, J; Pfeuffer-Jeschke, A; Latussek, V; Gui, Y.S; Daumer, V; Liu, J; Buhmann, H; Landwehr, G; Molenkamp, L.W in Thin Solid Films (2002). 412(1-2) 129--138.
The MBE growth of Hg(1−x)Cd(x)Te alloys and type III HgTe/Hg(1−x)Cd(x)Te heterostructures has been discussed, including similarities and differences between the (0 0 1) and (1 1 2)B orientations. Furthermore the MBE growth of HgTe based quantum wells (QWs) with the incorporation of Mn are additional topics. An investigation of the optical properties of type III superlattices with a normal band structure has lead to information about band structure of these heterostructures as well as information about the interface and the semimetallic QW. For example, by means of the full 8×8 Kane Hamiltonian in the envelope function approximation, it has been demonstrated that the energy separation between the H1–E1 and L1–E1 intersubband transition energies is primarily determined by the valence band offset, Λ, between HgTe and CdTe. This has led to unambiguous values for the offset and its temperature dependence, i.e. Λ(T)=570±60 meV and dΛ/dT=−0.40±0.04T meV/K. Furthermore the energy gap of HgTe at room temperature has also been determined. Magneto-transport measurements of n-type QWs show very pronounced Shubnikov-de Haas (SdH) oscillations and well developed quantum Hall plateaus for temperatures up to approximately 60 K. A large Rashba spin–orbit splitting of the first conduction subband, H1, has been observed in HgTe/Hg(1−x)Cd(x)Te QWs with an inverted band structure. Self-consistent Hartree calculations of the band structure based on the above model allows us to quantitatively describe the experimental results and demonstrates that the heavy hole nature of the H1 subband greatly influences the spatial distribution of electrons in the QW and thus enhances the Rashba spin splitting, i.e. ΔE_H1=βk∥^3. Furthermore, the presence of two periodic SdH oscillations in p-type QWs with an inverted band structure has been observed and is the first direct evidence that these heterostructures are indirect semiconductors. The influence of Mn in the upper barrier on the 2D electrons in the well has been investigated as a function of their separation. With spacer thicknesses of 10 and 15 nm, no appreciable change is observed, however, a reduction of the spacer thickness to 5 nm results in an increase in the maximum population difference between the two Rashba spin split H1 subbands by a factor of approximately two.
%0 Journal Article %1 becker2002growth %A Becker, C.R %A Zhang, X.C %A Ortner, K %A Schmidt, J %A Pfeuffer-Jeschke, A %A Latussek, V %A Gui, Y.S %A Daumer, V %A Liu, J %A Buhmann, H %A Landwehr, G %A Molenkamp, L.W %D 2002 %I Elsevier BV %J Thin Solid Films %N 1-2 %P 129--138 %R 10.1016/s0040-6090(02)00324-3 %T MBE growth and characterization of Hg based compounds and heterostructures %V 412 %X The MBE growth of Hg(1−x)Cd(x)Te alloys and type III HgTe/Hg(1−x)Cd(x)Te heterostructures has been discussed, including similarities and differences between the (0 0 1) and (1 1 2)B orientations. Furthermore the MBE growth of HgTe based quantum wells (QWs) with the incorporation of Mn are additional topics. An investigation of the optical properties of type III superlattices with a normal band structure has lead to information about band structure of these heterostructures as well as information about the interface and the semimetallic QW. For example, by means of the full 8×8 Kane Hamiltonian in the envelope function approximation, it has been demonstrated that the energy separation between the H1–E1 and L1–E1 intersubband transition energies is primarily determined by the valence band offset, Λ, between HgTe and CdTe. This has led to unambiguous values for the offset and its temperature dependence, i.e. Λ(T)=570±60 meV and dΛ/dT=−0.40±0.04T meV/K. Furthermore the energy gap of HgTe at room temperature has also been determined. Magneto-transport measurements of n-type QWs show very pronounced Shubnikov-de Haas (SdH) oscillations and well developed quantum Hall plateaus for temperatures up to approximately 60 K. A large Rashba spin–orbit splitting of the first conduction subband, H1, has been observed in HgTe/Hg(1−x)Cd(x)Te QWs with an inverted band structure. Self-consistent Hartree calculations of the band structure based on the above model allows us to quantitatively describe the experimental results and demonstrates that the heavy hole nature of the H1 subband greatly influences the spatial distribution of electrons in the QW and thus enhances the Rashba spin splitting, i.e. ΔE_H1=βk∥^3. Furthermore, the presence of two periodic SdH oscillations in p-type QWs with an inverted band structure has been observed and is the first direct evidence that these heterostructures are indirect semiconductors. The influence of Mn in the upper barrier on the 2D electrons in the well has been investigated as a function of their separation. With spacer thicknesses of 10 and 15 nm, no appreciable change is observed, however, a reduction of the spacer thickness to 5 nm results in an increase in the maximum population difference between the two Rashba spin split H1 subbands by a factor of approximately two.
Large Rashba spin-orbit splitting in gate controlled n-type modulation doped HgTe/Hg(0.3)Cd(0.7-x)Mn(x)Te quantum wells.Gui, Y.S.; Liu, J.; Daumer, V.; Becker, C.R.; Buhmann, H.; Molenkamp, L.W. in Physica E: Low-dimensional Systems and Nanostructures (2002). 12(1-4) 416--419.
%0 Journal Article %1 gui2002large %A Gui, Y.S. %A Liu, J. %A Daumer, V. %A Becker, C.R. %A Buhmann, H. %A Molenkamp, L.W. %D 2002 %I Elsevier BV %J Physica E: Low-dimensional Systems and Nanostructures %N 1-4 %P 416--419 %R 10.1016/s1386-9477(01)00320-4 %T Large Rashba spin-orbit splitting in gate controlled n-type modulation doped HgTe/Hg(0.3)Cd(0.7-x)Mn(x)Te quantum wells %V 12 %X A quantum-mechanical model based on a Green’s function approach has been used to calculate the transmission probability of electrons traversing a two-dimensional electron gas injected and detected via mode-selective quantum point contacts. Two-dimensional scattering potentials, backscattering, and temperature effects were included in order to compare the calculated results with experimentally observed interference patterns. The results yield information about the distribution, size, and the energetic height of the scattering potentials.
Growth and Magneto-Transport of Gate Controlled n Type HgTe/Hg(0.3)Cd(0.7)Te Quantum Wells with Inclusion of Mn.Becker, C.R.; Gui, Y.S.; Liu, J.; Daumer, V.; Ortner, K.; Hock, V.; Pfeuffer-Jeschke, A.; Buhmann, H.; Molenkamp, L.W. in Physica Status Solidi B (2002). 229(2) 775--779.
A series of gate controlled n-type asymmetrically modulation doped HgTe quantum wells (QWs) with Mn in one of the barriers have been investigated. From a Fourier analysis of the Shubnikov-de Haas (SdH) oscillations, the population difference of the two spin split H1 subbands has been determined for three QWs with different spacer thicknesses between the quantum well and the Mn ions. This population difference shows a significant enhancement in the QW with a 5 nm spacer compared to the QWs with 10 and 15 nm thick spacers. The latter results agree with the predictions for Rashba spin–orbit splitting, however, we must assume that Mn in the QW with the thin spacer increases the asymmetry of the QW.
%0 Journal Article %1 becker2001growth %A Becker, C.R. %A Gui, Y.S. %A Liu, J. %A Daumer, V. %A Ortner, K. %A Hock, V. %A Pfeuffer-Jeschke, A. %A Buhmann, H. %A Molenkamp, L.W. %D 2002 %J Physica Status Solidi B %N 2 %P 775--779 %R 10.1002/1521-3951(200201)229:2<775::AID-PSSB775>3.0.CO;2-W %T Growth and Magneto-Transport of Gate Controlled n Type HgTe/Hg(0.3)Cd(0.7)Te Quantum Wells with Inclusion of Mn %V 229 %X A series of gate controlled n-type asymmetrically modulation doped HgTe quantum wells (QWs) with Mn in one of the barriers have been investigated. From a Fourier analysis of the Shubnikov-de Haas (SdH) oscillations, the population difference of the two spin split H1 subbands has been determined for three QWs with different spacer thicknesses between the quantum well and the Mn ions. This population difference shows a significant enhancement in the QW with a 5 nm spacer compared to the QWs with 10 and 15 nm thick spacers. The latter results agree with the predictions for Rashba spin–orbit splitting, however, we must assume that Mn in the QW with the thin spacer increases the asymmetry of the QW.
Electron-wave diffraction by density inhomogeneities in two-dimensional electron gas.Novik, E.G.; Buhmann, H.; Maximov, S.; Molenkamp, L.W. in Physica E: Low-dimensional Systems and Nanostructures (2002). 12(1-4) 237--240.
A model for the simulation of the interference patterns of an electron beam in a 2DEG in the presence of a perpendicular magnetic field is presented. The model is based on the Green function method and includes a realistic description of the scattering potential leading to electron interference effects in the 2DEG. Detailed information about distribution, strength and the size of the scattering potential are obtained from the simulation of the experimentally observed interference patterns. The results confirm theoretical assumptions about density-fluctuation correlations in a 2DEG.
%0 Journal Article %1 novik2002electronwave %A Novik, E.G. %A Buhmann, H. %A Maximov, S. %A Molenkamp, L.W. %D 2002 %I Elsevier BV %J Physica E: Low-dimensional Systems and Nanostructures %N 1-4 %P 237--240 %R 10.1016/s1386-9477(01)00362-9 %T Electron-wave diffraction by density inhomogeneities in two-dimensional electron gas %V 12 %X A model for the simulation of the interference patterns of an electron beam in a 2DEG in the presence of a perpendicular magnetic field is presented. The model is based on the Green function method and includes a realistic description of the scattering potential leading to electron interference effects in the 2DEG. Detailed information about distribution, strength and the size of the scattering potential are obtained from the simulation of the experimentally observed interference patterns. The results confirm theoretical assumptions about density-fluctuation correlations in a 2DEG.
1D diffusion: a novel transport regime in narrow 2DEG channels.Buhmann, H.; Molenkamp, L.W. in Physica E: Low-dimensional Systems and Nanostructures (2002). 12(1-4) 715--718.
The transport properties of a two-dimensional electron gas (2DEG) in a narrow electron channel were studied as a function of magnetic field and electron current. When the channel width is smaller than the electron mean free path the non-linear resistance as a function of electron current has been interpreted in terms of Knudsen and Poiseuille transport regimes, in analogy to the molecular flow of gas atoms in a tube. However, a positive magneto-resistance is observed in an electron temperature range beyond the Knudsen regime which contradicts expected Poiseuille transport characteristics. We can trace this unexpected behaviour to the nature of electron–electron scattering processes in a system of reduced dimensionality. The character of the transport in this new regime can be viewed as a one-dimensional (1d) diffusion caused by the interplay of the two most dominant electron scattering processes in a 2DEG.
%0 Journal Article %1 buhmann2002diffusion %A Buhmann, H. %A Molenkamp, L.W. %D 2002 %I Elsevier BV %J Physica E: Low-dimensional Systems and Nanostructures %N 1-4 %P 715--718 %R 10.1016/s1386-9477(01)00387-3 %T 1D diffusion: a novel transport regime in narrow 2DEG channels %V 12 %X The transport properties of a two-dimensional electron gas (2DEG) in a narrow electron channel were studied as a function of magnetic field and electron current. When the channel width is smaller than the electron mean free path the non-linear resistance as a function of electron current has been interpreted in terms of Knudsen and Poiseuille transport regimes, in analogy to the molecular flow of gas atoms in a tube. However, a positive magneto-resistance is observed in an electron temperature range beyond the Knudsen regime which contradicts expected Poiseuille transport characteristics. We can trace this unexpected behaviour to the nature of electron–electron scattering processes in a system of reduced dimensionality. The character of the transport in this new regime can be viewed as a one-dimensional (1d) diffusion caused by the interplay of the two most dominant electron scattering processes in a 2DEG.
Rashba splitting in n-type modulation-doped HgTe quantum wells with an inverted band structure.Zhang, X. C.; Pfeuffer-Jeschke, A.; Ortner, K.; Hock, V.; Buhmann, H.; Becker, C. R.; Landwehr, G. in Phys. Rev. B (2001). 63(24) 245305.
Rashba spin splitting has been observed in the first conduction subband of n-type modulation-doped HgTe quantum wells (QW’s) with an inverted band structure via an investigation of Shubnikov–de Haas oscillations in gated Hall bars. In accordance with calculations, no spin splitting was observed in the second conduction subband (E2), but an obvious Rashba splitting is present in the first heavy-hole-like conduction subband (H1) that displays a large dependence on gate voltage. Self-consistent Hartree calculations of the band structure based on an 8×8k⋅p model are compared with experiment, which enables us to understand and quantitatively describe the experimental results. It has been shown that the heavy-hole nature of the H1 conduction subband greatly influences the spatial distribution of electrons in the QW and also enhances the Rashba spin splitting at large electron densities. These are unique features of type III heterostructures in the inverted band regime. The βk^3∥ dispersion predicted by an analytical model is a good approximation of the self-consistent Hartree calculations for small values of the in-plane wave-vector k∥ and has consequently been employed to describe the spin splitting of the H1 conduction subband rather than the commonly used αk∥ dispersion for the conduction subband in type I heterojunctions. The relative magnitude of Rashba splitting in the H1 and E2 subbands as well as the splitting of the H1 subband for different well widths are also presented.
%0 Journal Article %1 zhang2001rashba %A Zhang, X. C. %A Pfeuffer-Jeschke, A. %A Ortner, K. %A Hock, V. %A Buhmann, H. %A Becker, C. R. %A Landwehr, G. %D 2001 %I American Physical Society %J Phys. Rev. B %N 24 %P 245305 %R 10.1103/PhysRevB.63.245305 %T Rashba splitting in n-type modulation-doped HgTe quantum wells with an inverted band structure %V 63 %X Rashba spin splitting has been observed in the first conduction subband of n-type modulation-doped HgTe quantum wells (QW’s) with an inverted band structure via an investigation of Shubnikov–de Haas oscillations in gated Hall bars. In accordance with calculations, no spin splitting was observed in the second conduction subband (E2), but an obvious Rashba splitting is present in the first heavy-hole-like conduction subband (H1) that displays a large dependence on gate voltage. Self-consistent Hartree calculations of the band structure based on an 8×8k⋅p model are compared with experiment, which enables us to understand and quantitatively describe the experimental results. It has been shown that the heavy-hole nature of the H1 conduction subband greatly influences the spatial distribution of electrons in the QW and also enhances the Rashba spin splitting at large electron densities. These are unique features of type III heterostructures in the inverted band regime. The βk^3∥ dispersion predicted by an analytical model is a good approximation of the self-consistent Hartree calculations for small values of the in-plane wave-vector k∥ and has consequently been employed to describe the spin splitting of the H1 conduction subband rather than the commonly used αk∥ dispersion for the conduction subband in type I heterojunctions. The relative magnitude of Rashba splitting in the H1 and E2 subbands as well as the splitting of the H1 subband for different well widths are also presented.
Antiferromagnetic temperature and effective spin in n-type Hg1−xMnxTe.Gui, Y. S.; Liu, J; Ortner, K; Daumer, V; Becker, C. R.; Buhmann, H; Molenkamp, L. W. in Appl. Phys. Lett. (2001). 79(9) 1321--1323.
We have observed a characteristic modulation in the amplitude of the Shubnikov–de Haas (SdH) oscillations and a large shift of node position with increasing temperature in the semimagnetic semiconductor Hg(1−x)Mn(x)Te. The positions of the oscillation extrema are almost temperature independent, with the exception that their phase changes by π after passing the nodal point. These features can be explained in terms of the exchange interaction between conduction electrons and the localized spin moments of Mn ions. The antiferromagnetic temperature and effective spin have been deduced from the temperature-dependent node position in the SdH oscillations.
%0 Journal Article %1 gui2001antiferromagnetic %A Gui, Y. S. %A Liu, J %A Ortner, K %A Daumer, V %A Becker, C. R. %A Buhmann, H %A Molenkamp, L. W. %B Applied Physics Letters %D 2001 %I American Institute of Physics %J Appl. Phys. Lett. %N 9 %P 1321--1323 %R 10.1063/1.1397275 %T Antiferromagnetic temperature and effective spin in n-type Hg1−xMnxTe %V 79 %X We have observed a characteristic modulation in the amplitude of the Shubnikov–de Haas (SdH) oscillations and a large shift of node position with increasing temperature in the semimagnetic semiconductor Hg(1−x)Mn(x)Te. The positions of the oscillation extrema are almost temperature independent, with the exception that their phase changes by π after passing the nodal point. These features can be explained in terms of the exchange interaction between conduction electrons and the localized spin moments of Mn ions. The antiferromagnetic temperature and effective spin have been deduced from the temperature-dependent node position in the SdH oscillations.
Angle-Resolved Spectroscopy of Electron-Electron Scattering in a 2D System.Yanovski, A.V.; Predel, H.; Buhmann, H.; Gurzhi, R.N.; Kopeliovich, A.I.; Kalinenko, A.N.; Molenkamp, L.W. in Europhysics Lett. (2001). 56(5) 709.
Electron-beam propagation experiments have been used to determine the energy and angle dependence of electron-electron (ee) scattering in a two-dimensional electron gas (2DEG) in a very direct manner. The experimental results provide direct evidence for novel ee-scattering effects in 2D degenerate conductors. Most striking is the increased importance of small-angle scattering in a 2D system with decreasing excitation energy. In particular, in a 2DEG ee-scattering can, at sufficiently low energies, be purely dephasing in character, i.e. changing the phase but not the direction of electron motion.
%0 Journal Article %1 yanovski2001angleresolved %A Yanovski, A.V. %A Predel, H. %A Buhmann, H. %A Gurzhi, R.N. %A Kopeliovich, A.I. %A Kalinenko, A.N. %A Molenkamp, L.W. %D 2001 %J Europhysics Lett. %N 5 %P 709 %R 10.1209/epl/i2001-00578-y %T Angle-Resolved Spectroscopy of Electron-Electron Scattering in a 2D System %V 56 %X Electron-beam propagation experiments have been used to determine the energy and angle dependence of electron-electron (ee) scattering in a two-dimensional electron gas (2DEG) in a very direct manner. The experimental results provide direct evidence for novel ee-scattering effects in 2D degenerate conductors. Most striking is the increased importance of small-angle scattering in a 2D system with decreasing excitation energy. In particular, in a 2DEG ee-scattering can, at sufficiently low energies, be purely dephasing in character, i.e. changing the phase but not the direction of electron motion.
Thermopower of the molecular state in a double quantum dot.Chen, Xiaoshuang; Buhmann, H.; Molenkamp, L. W. in Phys. Rev. B (2000). 61(24) 16801--16806.
The thermopower of the molecular state in a double quantum dot is studied using a mixed model, which combines the classical capacitance model and a valence-electron approach. Our calculations show that close to the triple points where the resonances of the two dots intersect, there are additional steplike structures in the thermopower that result from coupling of the “topmost” electronic states. Far away from the triple points, we find that, with increasing detuning, the steplike structures shift toward the center of two neighboring Coulomb blockade oscillations, while very far away from the triple points, the transport resembles that of a single dot of lower electrostatic energy. Our results are in qualitative agreement with experimental findings on the conductance of Blick et al. [Phys. Rev. Lett. 80, 4032 (1998)].
%0 Journal Article %1 chen2000thermopower %A Chen, Xiaoshuang %A Buhmann, H. %A Molenkamp, L. W. %D 2000 %I American Physical Society %J Phys. Rev. B %N 24 %P 16801--16806 %R 10.1103/PhysRevB.61.16801 %T Thermopower of the molecular state in a double quantum dot %V 61 %X The thermopower of the molecular state in a double quantum dot is studied using a mixed model, which combines the classical capacitance model and a valence-electron approach. Our calculations show that close to the triple points where the resonances of the two dots intersect, there are additional steplike structures in the thermopower that result from coupling of the “topmost” electronic states. Far away from the triple points, we find that, with increasing detuning, the steplike structures shift toward the center of two neighboring Coulomb blockade oscillations, while very far away from the triple points, the transport resembles that of a single dot of lower electrostatic energy. Our results are in qualitative agreement with experimental findings on the conductance of Blick et al. [Phys. Rev. Lett. 80, 4032 (1998)].
Thermopower of quantum chaos.Buhmann, H; Molenkamp, L.W in Physica E: Low-dimensional Systems and Nanostructures (2000). 6(1-4) 400--403.
The thermopower of a chaotic quantum dot has been studied in the Coulomb and the ballistic transport regime. Theoretical examinations predict distinct features for quantum dots where the electron trajectories are chaotic. Accordingly, we give experimental evidence for a residual effective charging energy in the case of asymmetric ajusted leads (G⩽2e^2/h) and for non-Gaussian thermopower fluctuation distributions in the ballistic regime (G=4e^2/h).
%0 Journal Article %1 buhmann2000thermopower %A Buhmann, H %A Molenkamp, L.W %D 2000 %I Elsevier BV %J Physica E: Low-dimensional Systems and Nanostructures %N 1-4 %P 400--403 %R 10.1016/s1386-9477(99)00207-6 %T Thermopower of quantum chaos %V 6 %X The thermopower of a chaotic quantum dot has been studied in the Coulomb and the ballistic transport regime. Theoretical examinations predict distinct features for quantum dots where the electron trajectories are chaotic. Accordingly, we give experimental evidence for a residual effective charging energy in the case of asymmetric ajusted leads (G⩽2e^2/h) and for non-Gaussian thermopower fluctuation distributions in the ballistic regime (G=4e^2/h).
Probing the Potential Landscape Inside a Two-Dimensional Electron Gas.Koonen, J. J.; Buhmann, H.; Molenkamp, L. W. in Phys. Rev. Lett. (2000). 84(11) 2473--2476.
We report direct observations of the scattering potentials in a two-dimensional electron gas using electron-beam diffraction experiments. The diffracting objects are local density fluctuations caused by spatial and charge-state distribution of donors in the GaAs-(Al,Ga)As heterostructures. The scatterers can be manipulated externally by sample illumination or by cooling the sample down under depleted conditions.
%0 Journal Article %1 koonen2000probing %A Koonen, J. J. %A Buhmann, H. %A Molenkamp, L. W. %D 2000 %I American Physical Society %J Phys. Rev. Lett. %N 11 %P 2473--2476 %R 10.1103/PhysRevLett.84.2473 %T Probing the Potential Landscape Inside a Two-Dimensional Electron Gas %V 84 %X We report direct observations of the scattering potentials in a two-dimensional electron gas using electron-beam diffraction experiments. The diffracting objects are local density fluctuations caused by spatial and charge-state distribution of donors in the GaAs-(Al,Ga)As heterostructures. The scatterers can be manipulated externally by sample illumination or by cooling the sample down under depleted conditions.
New transport effects in a degenerate two-dimensional electron gas.Buhmann, H.; Gurzhi, R.N.; Kalinenko, A.N.; Kopeliovich, A.I.; Molenkamp, L.W.; Predel, H.; Yanovsky, A.V. in Physica B: Condensed Matter (2000). 284-288 1904--1905.
The influence of electron–electron (e–e) scattering on the transport properties of a two-dimensional electron-gas in (Al ,Ga)As-GaAs heterostructures is studied experimentally and theoretically. The important role of e–e collisions has been demonstrated. Due to phase space restrains small-angle scattering events are expected to dominate the propagation of non-equilibrium electrons. The experimental evidence is given via injection-energy-dependent electron beam experiments.
%0 Journal Article %1 buhmann2000transport %A Buhmann, H. %A Gurzhi, R.N. %A Kalinenko, A.N. %A Kopeliovich, A.I. %A Molenkamp, L.W. %A Predel, H. %A Yanovsky, A.V. %D 2000 %I Elsevier BV %J Physica B: Condensed Matter %P 1904--1905 %R 10.1016/s0921-4526(99)02996-8 %T New transport effects in a degenerate two-dimensional electron gas %V 284-288 %X The influence of electron–electron (e–e) scattering on the transport properties of a two-dimensional electron-gas in (Al ,Ga)As-GaAs heterostructures is studied experimentally and theoretically. The important role of e–e collisions has been demonstrated. Due to phase space restrains small-angle scattering events are expected to dominate the propagation of non-equilibrium electrons. The experimental evidence is given via injection-energy-dependent electron beam experiments.
Electrontextendashelectron scattering and the propagation of electron beams in a two-dimensional electron gas.Buhmann, H; Predel, H; Molenkamp, L.W; Gurzhi, R.N; Kopeliovich, A.I; Kalinenko, A.N; Yanovsky, A.V in Physica E: Low-dimensional Systems and Nanostructures (2000). 6(1-4) 310--313.
Experimental studies of electron-beam propagation in a degenerate 2DEG in a GaAs-(Al,Ga)As heterostructure are presented for a wide range of injection energies, 0<E⩽E_F. The electron beam is injected and detected in the 2DEG via electrostatically defined quantum point-contacts with typical distances of . Energy-dependent beam injection is used to reveal the characteristics of the electron-beam propagation. Considering the specific character for electron–electron scattering events in 2D systems linearized Boltzmann equations could be used to model the experimental observations.
%0 Journal Article %1 buhmann2000electrontextendashelectron %A Buhmann, H %A Predel, H %A Molenkamp, L.W %A Gurzhi, R.N %A Kopeliovich, A.I %A Kalinenko, A.N %A Yanovsky, A.V %D 2000 %I Elsevier BV %J Physica E: Low-dimensional Systems and Nanostructures %N 1-4 %P 310--313 %R 10.1016/s1386-9477(99)00163-0 %T Electrontextendashelectron scattering and the propagation of electron beams in a two-dimensional electron gas %V 6 %X Experimental studies of electron-beam propagation in a degenerate 2DEG in a GaAs-(Al,Ga)As heterostructure are presented for a wide range of injection energies, 0<E⩽E_F. The electron beam is injected and detected in the 2DEG via electrostatically defined quantum point-contacts with typical distances of . Energy-dependent beam injection is used to reveal the characteristics of the electron-beam propagation. Considering the specific character for electron–electron scattering events in 2D systems linearized Boltzmann equations could be used to model the experimental observations.
Effects of electron-electron scattering on electron-beam propagation in a two-dimensional electron gas.Predel, H.; Buhmann, H.; Molenkamp, L. W.; Gurzhi, R. N.; Kalinenko, A. N.; Kopeliovich, A. I.; Yanovsky, A. V. in Phys. Rev. B (2000). 62(3) 2057--2064.
We have studied experimentally and theoretically the influence of electron-electron collisions on the propagation of electron beams in a two-dimensional electron gas for excess injection energies ranging from zero up to the Fermi energy. We find that the detector signal consists of quasiballistic electrons, which either have not undergone any electron-electron collisions or have only been scattered at small angles. Theoretically, the small-angle scattering exhibits distinct features that can be traced back to the reduced dimensionality of the electron system. A number of nonlinear effects, also related to the two-dimensional character of the system, are discussed. In the simplest situation, the heating of the electron gas by the high-energy part of the beam leads to a weakening of the signal of quasiballistic electrons and to the appearance of thermovoltage. This results in a nonmonotonic dependence of the detector signal on the intensity of the injected beam, as observed experimentally.
%0 Journal Article %1 predel2000effects %A Predel, H. %A Buhmann, H. %A Molenkamp, L. W. %A Gurzhi, R. N. %A Kalinenko, A. N. %A Kopeliovich, A. I. %A Yanovsky, A. V. %D 2000 %I American Physical Society %J Phys. Rev. B %N 3 %P 2057--2064 %R 10.1103/PhysRevB.62.2057 %T Effects of electron-electron scattering on electron-beam propagation in a two-dimensional electron gas %V 62 %X We have studied experimentally and theoretically the influence of electron-electron collisions on the propagation of electron beams in a two-dimensional electron gas for excess injection energies ranging from zero up to the Fermi energy. We find that the detector signal consists of quasiballistic electrons, which either have not undergone any electron-electron collisions or have only been scattered at small angles. Theoretically, the small-angle scattering exhibits distinct features that can be traced back to the reduced dimensionality of the electron system. A number of nonlinear effects, also related to the two-dimensional character of the system, are discussed. In the simplest situation, the heating of the electron gas by the high-energy part of the beam leads to a weakening of the signal of quasiballistic electrons and to the appearance of thermovoltage. This results in a nonmonotonic dependence of the detector signal on the intensity of the injected beam, as observed experimentally.
Thermopower of a Chaotic Quantum Dot.Godijn, S. F.; Möller, S.; Buhmann, H.; Molenkamp, L. W.; van Langen, S. A. in Phys. Rev. Lett. (1999). 82(14) 2927--2930.
The thermovoltage of a chaotic quantum dot is measured using a current heating technique. The fluctuations in the thermopower as a function of magnetic field and dot shape display a non-Gaussian distribution, in agreement with simulations using random matrix theory. We observe no contributions from weak localization or short trajectories in the thermopower.
%0 Journal Article %1 godijn1999thermopower %A Godijn, S. F. %A Möller, S. %A Buhmann, H. %A Molenkamp, L. W. %A van Langen, S. A. %D 1999 %I American Physical Society %J Phys. Rev. Lett. %N 14 %P 2927--2930 %R 10.1103/PhysRevLett.82.2927 %T Thermopower of a Chaotic Quantum Dot %V 82 %X The thermovoltage of a chaotic quantum dot is measured using a current heating technique. The fluctuations in the thermopower as a function of magnetic field and dot shape display a non-Gaussian distribution, in agreement with simulations using random matrix theory. We observe no contributions from weak localization or short trajectories in the thermopower.
On electron-electron scattering mechanisms in 2D degenerated systems.Buhmann, H.; Gurzhi, R.N.; Kalinenko, A.N.; Kopeliovich, A.I.; Molenkamp, L.W.; Yanovsky, A.V. in Nanostructured Materials (1999). 12(5-8) 835--838.
The detailed quantitative theory of electron-electron scattering processes in 2D electron degenerated systems in GaAs(AlGaAs) heterostructures has been developed on the basis of analytical treatment and numerical calculations. We have found the conditions and intervals of values of the characteristic parameters in which specific properties of 2D relaxation predicted previously on the theoretical level manifest themselves. New effects, i.e. a secondary beam of electrons scattered back and a very narrow beam of holes moving in the direction of injection, have been found.
%0 Journal Article %1 buhmann1999electronelectron %A Buhmann, H. %A Gurzhi, R.N. %A Kalinenko, A.N. %A Kopeliovich, A.I. %A Molenkamp, L.W. %A Yanovsky, A.V. %D 1999 %I Elsevier BV %J Nanostructured Materials %N 5-8 %P 835--838 %R 10.1016/s0965-9773(99)00246-9 %T On electron-electron scattering mechanisms in 2D degenerated systems %V 12 %X The detailed quantitative theory of electron-electron scattering processes in 2D electron degenerated systems in GaAs(AlGaAs) heterostructures has been developed on the basis of analytical treatment and numerical calculations. We have found the conditions and intervals of values of the characteristic parameters in which specific properties of 2D relaxation predicted previously on the theoretical level manifest themselves. New effects, i.e. a secondary beam of electrons scattered back and a very narrow beam of holes moving in the direction of injection, have been found.
Charge fluctuations, chaotic trajectories, and the coulomb blockade.Buhmann, H.; Möller, S.; Molenkamp, L.W. in Festkörperprobleme / Advances in Solid State Physics, B. Kramer (ed.) (1999). (Vol. 38) 193--202.
Two different experimental methods have been used to determine the charging energy U* of a quantum dot as a function of the coupling to the external leads. A powerlaw scaling behaviour depending on the number of opening leads was observed for the weak coupling regime in both types of experiments, in agreement with theories based on the Tomonaga-Luttinger formalism. For stronger coupling (t > 0.5), in asymetric quantum dots with only one opening lead, U* appears to reach a constant value. This latter behaviour can be explained by taking into account chaotic motion of the electrons in quantum dots.
%0 Book Section %1 buhmann1999charge %A Buhmann, H. %A Möller, S. %A Molenkamp, L.W. %B Festkörperprobleme / Advances in Solid State Physics %D 1999 %E Kramer, B. %P 193--202 %T Charge fluctuations, chaotic trajectories, and the coulomb blockade %U http://www.springer.com/la/book/9783662161272 %V 38 %X Two different experimental methods have been used to determine the charging energy U* of a quantum dot as a function of the coupling to the external leads. A powerlaw scaling behaviour depending on the number of opening leads was observed for the weak coupling regime in both types of experiments, in agreement with theories based on the Tomonaga-Luttinger formalism. For stronger coupling (t > 0.5), in asymetric quantum dots with only one opening lead, U* appears to reach a constant value. This latter behaviour can be explained by taking into account chaotic motion of the electrons in quantum dots.
On dynamic properties of a two-dimensional degenerate electron gas.Buhmann, H; Molenkamp, L. W.; Gurzhi, R. N.; Kalinenko, A. N.; Kopeliovich, A. I.; Yanovsky, A. V. in Low Temperature Physics (1998). 24(10) 737--741.
A detailed theory of electron–electron scattering in two-dimensional degenerate systems in heterostructures is constructed as a result of analysis and numerical computations. The conditions are formulated and the values of characteristic parameters are obtained for which specific two-dimensional effects predicted earlier from theoretical considerations can be observed. New properties of scattering indicatrix, viz., a beam of electrons flying almost antiparallel to the primary beam and a very narrow beam of holes flying in the forward direction, are found.
%0 Journal Article %1 buhmann1998dynamic %A Buhmann, H %A Molenkamp, L. W. %A Gurzhi, R. N. %A Kalinenko, A. N. %A Kopeliovich, A. I. %A Yanovsky, A. V. %B Low Temperature Physics %D 1998 %I American Institute of Physics %J Low Temperature Physics %N 10 %P 737--741 %R 10.1063/1.593663 %T On dynamic properties of a two-dimensional degenerate electron gas %V 24 %X A detailed theory of electron–electron scattering in two-dimensional degenerate systems in heterostructures is constructed as a result of analysis and numerical computations. The conditions are formulated and the values of characteristic parameters are obtained for which specific two-dimensional effects predicted earlier from theoretical considerations can be observed. New properties of scattering indicatrix, viz., a beam of electrons flying almost antiparallel to the primary beam and a very narrow beam of holes flying in the forward direction, are found.
Magneto-thermopower of a chaotic, ballistic quantum dot.Buhmann, H.; Möller, S.; Godijn, S.F.; Molenkamp, L.W. in Physica B: Condensed Matter (1998). 256-258 198--202.
We present measurements of the magneto-thermopower fluctuations of a gate-defined, chaotic quantum dot in a (Al,Ga)As-semiconductor heterostructure. Current-heating techniques were employed to build up the necessary temperature difference across the quantum dot for the thermopower measurement. A statistical ensemble is generated by registering the magnetic-field and gate-voltage dependence for a number of slightly different dot sizes. The experimental results are in good agreement with the predicted non-gaussian distribution of the Random Matrix Theory. To our knowledge these are the first transport experiments that directly yield the non-gaussian distribution which are an unambiguous sign for the occurence of chaos.
%0 Journal Article %1 buhmann1998magnetothermopower %A Buhmann, H. %A Möller, S. %A Godijn, S.F. %A Molenkamp, L.W. %D 1998 %I Elsevier BV %J Physica B: Condensed Matter %P 198--202 %R 10.1016/s0921-4526(98)00515-8 %T Magneto-thermopower of a chaotic, ballistic quantum dot %V 256-258 %X We present measurements of the magneto-thermopower fluctuations of a gate-defined, chaotic quantum dot in a (Al,Ga)As-semiconductor heterostructure. Current-heating techniques were employed to build up the necessary temperature difference across the quantum dot for the thermopower measurement. A statistical ensemble is generated by registering the magnetic-field and gate-voltage dependence for a number of slightly different dot sizes. The experimental results are in good agreement with the predicted non-gaussian distribution of the Random Matrix Theory. To our knowledge these are the first transport experiments that directly yield the non-gaussian distribution which are an unambiguous sign for the occurence of chaos.
Classical rebound trajectories in nonlocal ballistic electron transport.Heindrichs, A. S. D.; Buhmann, H.; Godijn, S. F.; Molenkamp, L. W. in Phys. Rev. B (1998). 57(7) 3961--3965.
We demonstrate experimentally and by Monte Carlo simulation that the negative dips which occur at low magnetic fields on both sides of the main signal in nonlocal electron-beam measurements in semiconductor nanostructures result from electrons following classical rebound trajectories. We propose an alternative measurement geometry that eliminates these effects.
%0 Journal Article %1 PhysRevB.57.3961 %A Heindrichs, A. S. D. %A Buhmann, H. %A Godijn, S. F. %A Molenkamp, L. W. %D 1998 %I American Physical Society %J Phys. Rev. B %N 7 %P 3961--3965 %R 10.1103/PhysRevB.57.3961 %T Classical rebound trajectories in nonlocal ballistic electron transport %V 57 %X We demonstrate experimentally and by Monte Carlo simulation that the negative dips which occur at low magnetic fields on both sides of the main signal in nonlocal electron-beam measurements in semiconductor nanostructures result from electrons following classical rebound trajectories. We propose an alternative measurement geometry that eliminates these effects.
Charging Energy of a Chaotic Quantum Dot.Möller, S.; Buhmann, H.; Godijn, S. F.; Molenkamp, L. W. in Phys. Rev. Lett. (1998). 81(23) 5197--5200.
The scaling behavior of the charging energy of a quantum dot with asymmetrically adjusted tunnel barriers is measured through the amplitude of the Coulomb oscillations in the thermovoltage. For weak coupling between the dot and the reservoirs, we observe a linear scaling of the effective charging energy when the transmission probability of one tunnel barrier is increased. At higher transmission probabilities, we find a deviation from the linear scaling and a crossover to a constant value. This behavior is caused by the chaotic nature of the electron trajectories within the dot.
%0 Journal Article %1 moller1998charging %A Möller, S. %A Buhmann, H. %A Godijn, S. F. %A Molenkamp, L. W. %D 1998 %I American Physical Society %J Phys. Rev. Lett. %N 23 %P 5197--5200 %R 10.1103/PhysRevLett.81.5197 %T Charging Energy of a Chaotic Quantum Dot %V 81 %X The scaling behavior of the charging energy of a quantum dot with asymmetrically adjusted tunnel barriers is measured through the amplitude of the Coulomb oscillations in the thermovoltage. For weak coupling between the dot and the reservoirs, we observe a linear scaling of the effective charging energy when the transmission probability of one tunnel barrier is increased. At higher transmission probabilities, we find a deviation from the linear scaling and a crossover to a constant value. This behavior is caused by the chaotic nature of the electron trajectories within the dot.

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