bluuebunny/crossref_metadata_2025 huggingface dataset:
SPIN-RELATED TRANSPORT AFFECTED BY COMPETITION BETWEEN SPIN-ORBIT INTERACTION AND ZEEMAN EFFECT
Authors: NITTA JUNSAKU
Published: June 2009

The spin dynamics in solid state systems is governed by the competition between spin-orbit interaction (SOI) and the Zeeman effect. The SOI couples orbital motion of electron spins with an electric field. The Zeeman effect lifts the spin degeneracy in a magnetic field. In InGaAs -based 2DEGs, it is known that the Rashba SOI energy E SOI can be controlled by an electric field applied on the gate electrode.1 In the presence of SOI, weak localization (WL) due to time reversal symmetric interference changes to weak anti-localization (WAL). We have found crossover from WL to WAL by applying the gate voltage in InGaAs 2DEGs. Applying an in-plane magnetic field to the 2DEG does not affect the orbital motion of the electrons, but only modifies the Zeeman spin splitting energy E Z . This allows tuning the ratio between E SOI and E Z very accurately.

We have studied how the interplay between SOI and Zeeman coupling affects the electron transport and the spin dynamics in InGaAs -based 2DEGs. From the quantitative analysis of the magnetoconductance, measured in the presence of an in-plane magnetic field, we conclude that this interplay results in a spin-induced breaking of time reversal symmetry (TRS) and in an enhancement of the spin relaxation time. Both effects are due to a partial alignment of the electron spin along the applied magnetic field, and are found to be in excellent agreement with recent theoretical predictions.2 We find that the electron dephasing time saturates when E Z becomes comparable to E SOI . Moreover, we show that the spin-induced electron dephasing time is a universal function of the ratio E Z /E SOI within the experimental accuracy, i.e. it is independent of any details of the quantum well.3 This universal behavior is explained by the recent theory.4

The suppression of WAL is observed by applying in-plane magnetic field because of the enhancement of the spin relaxation time, and this suppression also appears in narrow InGaAs wires since the effective magnetic field direction is confined by wires. In gate fitted narrow wires, the large enhancement of spin relaxation time is obtained when the Rashba SOI is decreased. The spin relaxation time is more than one order longer than that of 2DEG case. This enhancement suggests that the Rashba SOI strength approaches the Dresselhaus SOI strength. We have numerically investigated the angular dependence of in-plane magnetoconductance in disordered wires with both Rashba and Dresselhaus SOIs. A new method is proposed to determine the relative strength of Rashba and Dresselhaus SOI from transport measurements without the need of fitting parameters.5 This in-plane magnetic field measurement provides fruitful information on spin related transport.

Note from Publisher: This article contains the abstract only.
SPIN EFFECTS IN THE PHASE TRANSITION OF THE νT = 1 BILAYER ELECTRON SYSTEM
Authors: MURAKI KOJI , GIUDICI PAULA , KUMADA NORIO
Published: June 2009

We present tilted-field experiments on a bilayer electron system at νT = 1 with negligible tunneling and demonstrate that the spin degree of freedom plays a decisive role in the ground-state phase diagram of the system. We observe that the phase boundary separating the incompressible quantum Hall state and a compressible state at d/ℓB = 1.90 (d: interlayer distance, ℓB: magnetic length) in a perpendicular field shifts to higher densities with tilt until it saturates at d/ℓB = 2.33. We develop a model describing the energies of the competing phases and show that the observed shift of the phase boundary reflects the spin-polarization dependence of the Coulomb and Zeeman energies of the compressible state. A new phase diagram as a function of d/ℓB and the Zeeman energy is established and its implications as to the nature of the phase transition are discussed.
TRANSPORT IN A BILAYER SYSTEM AT HIGH LANDAU FILLING FACTOR
Authors: GUSEV G. M.
Published: June 2009

We have studied Shubnikov de Haas oscillations and the quantum Hall effect in GaAs -double well structures in tilted magnetic fields. We found strong magnetoresistance oscillations as a function of an in-plane magnetic field B∥ at ν = 4N + 3 and ν = 4N + 1 filling factors. At low perpendicular magnetic field B⊥, the amplitude of the conventional Shubnikov-de Haas (SdH) oscillations also exhibits B∥-periodic dependence at fixed values of B⊥. We interpret the observed oscillations as a manifestation of the interference between cyclotron orbits in different quantum wells.
PINNING MODES OF THE STRIPE PHASES OF 2D ELECTRON SYSTEMS IN HIGHER LANDAU LEVELS
Authors: SAMBANDAMURTHY G. , ZHU HAN , CHEN YONG P. , ENGEL L. W. , TSUI D. C. , PFEIFFER L. N. , WEST K. W.
Published: June 2009

Striking resonances, identified as pinning modes, have recently been observed in the rf spectra of the stripe phases of extremely low disorder two-dimensional electron systems with two or more filled Landau levels. We present spectra which detail the dependence of this resonance on the Landau filling factor ν, in broad ranges which include the stripe phases in the Landau levels with N = 2, 3 and 4.
TRANSPARENCY OF NARROW CONSTRICTIONS IN A GRAPHENE SINGLE ELECTRON TRANSISTOR
Authors: STAMPFER C. , SCHURTENBERGER E. , MOLITOR F. , GÜTTINGER J. , IHN T. , ENSSLIN K.
Published: June 2009

We report on electronic transport experiments on a graphene single electron transistor as function of a perpendicular magnetic field. The device, which consists of a graphene island connected to source and drain electrodes via two narrow graphene constrictions is electronically characterized and the device exhibits a characteristic charging energy of approx. 3.5 meV. We investigate the homogeneity of the two graphene "tunnel" barriers connecting the single electron transistor to source and drain contacts as function of laterally applied electric fields, which are also used to electrostatically tune the overall device. Further, we focus on the barrier transparency as function of an applied perpendicular magnetic field and we find an increase of transparency for increasing magnetic field and a source-drain current saturation for magnetic fields exceeding 5 T.
BAND STRUCTURE AND ELECTRON VELOCITY MEASUREMENT IN CARBON NANOTUBES AND GRAPHENE
Authors: NICHOLAS ROBIN J. , CHUANG KAI-CHIEH
Published: June 2009

We discuss recent measurements of the interband spectrocopy of carbon nanotubes and studies of cyclotron resonance in graphene, using these to examine the possible dependence of the band structure of graphene on the number of layers present and the role of Coulomb interactions. Cyclotron resonances gives a value for the electron velocity at the Dirac point of 1.093×106 ms-1, which is ~ 20% larger than would be expected from deductions of the band structure of carbon nanotubes. In addition, a significant asymmetry exists between band structure for electrons and holes, which gives rise to a 5% difference between the velocities at energies of 125 meV away from the Dirac point.
HIGH FIELD MAGNETO-OPTICAL SPECTROSCOPY OF HIGHLY ALIGNED INDIVIDUAL AND ENSEMBLE SINGLE-WALLED CARBON NANOTUBES
Authors: SHAVER J. , SRIVASTAVA A. , KONO J. , CROOKER S. A. , HTOON H. , KLIMOV V. I. , FAGAN J. A. , HOBBIE E. K. , UBRIG N. , PORTUGALL O. , PEREBEINOS V. , AVOURIS PH.
Published: June 2009

The tubular nature of single-walled carbon nanotube (SWCNT) crystals allows them to exhibit non-intuitive quantum phenomena when threaded by a magnetic flux, which breaks the time reversal symmetry and adds an Aharonov-Bohm phase to the circumferential boundary conditions on the electronic wave function. We demonstrate that such a symmetry-breaking magnetic field can dramatically "brighten" an optically-inactive, or dark, exciton state at low temperature. This phenomenon, magnetic brightening, can be understood as a consequence of interplay between the strong intervalley Coulomb mixing and field-induced lifting of valley degeneracy. Most recently, we made the direct observation of the dark excitonic state in individual SWCNTs using low-temperature micro-photoluminescence (PL) and and verified the importance of a parallel, tube-threading magentic field with ensemble spectroscopy. For micro-PL, a magnetic field up to 5 T, applied along the nanotube axis, brightened the dark state, leading to the emergence of a new emission peak. The peak rapidly grew in intensity with increasing field at the expense of the originally-dominant bright exciton peak and finally became dominant at fields > 3 T. The directly measured dark-bright splitting values were 1-4 meV for tube diameters 1.0-1.3 nm. For ensemble PL, we used fields up to 55 T in two collection geometries to demonstrate the importance of the tube-threading component. These experiments have provided one of the most critical tests for recently-proposed theories of 1-D excitons taking into account the strong 1-D Coulomb interactions and unique band structure on an equal footing.
PHOTOLUMINESCENCE AND PHOTOLUMINESCENCE EXCITATION SPECTROSCOPY OF SEMICONDUCTING SINGLE WALL CARBON NANOTUBES
Authors: PLENTZ FLÁVIO , RIBEIRO HENRIQUE B. , JORIO ADO , PIMENTA MARCOS A. , FANTINI C. , PERESSINOTTO V. S. T. , FURTADO C. A. , SANTOS A. P.
Published: June 2009

Since the discovery of single wall carbon nanotubes (SWNT) in 1991, this nanomaterial has received an enormous attention from the nanoscience and nanotechnology community, not only due to the clear prospects for applications such as novel nanoelectronic and nano-optoelectronic devices, but also because this unique one dimensional (1D) system offered a new possibility for the investigation of novel physical phenomena in low dimensions. In 2002 it was demonstrated that photoluminescence (PL) could be observed in suspensions of isolated SWNTs and, later on, that PL could also be observed from individual, suspended SWNTs. Since then, there has been an increasing amount of work directed towards the investigation of the optical properties of semiconducting SWNTs by PL and photoluminescence excitation spectroscopy (PLE), and the use of PL and PLE for the qualitative and quantitative identification of SWNTs species within and ensemble of carbon nanotubes. In 2005 it was shown that the observed optical transitions are associated to 1D excitons and, from the point of view of optical properties, the rich physics of excitons in SWNTs has received much attention. For instance, it is now clear that excitons and exciton-phonon interactions play a major role in the mechanisms responsible for the emission and the absorption of light in SWNTs. Also, the interaction of SWNTs with their vicinity, which includes the interaction with organic and inorganic molecules, and the modifications in the excitonic system caused by changes in the dielectric constant, can be readily investigated by PL and PLE.

In this talk we presented an overview of our recent work in the optical spectroscopy of SWNTs.1–3 In particular, we showed some of our results on the investigation of the exciton-phonon interaction in semiconducting SWNTs and on the modifications in the PL and PLE spectra associated to the interactions with its surrounding environment.

Note from Publisher: This article contains the abstract only.
NONLINEAR TRANSPORT OF 2D ELECTRONS IN MAGNETIC FIELD
Authors: VITKALOV SERGEY , ZHANG JING QIAO , BYKOV A. A. , TOROPOV A. I.
Published: June 2009

Electric field induced, spectacular reduction of longitudinal resistivity of two dimensional electrons placed in strong magnetic field is studied in broad range of temperatures. The data are in good agreement with theory, considering the strong nonlinearity of the resistivity as result of non-uniform spectral diffusion of 2D electrons induced by the electric field. Comparison with the theory gives inelastic scattering time τin of the 2D electrons. In temperature range T = 2 - 20 K for overlapping Landau levels, the inelastic scattering rate 1/τin is found to be proportional to T2, indicating dominant contribution of the electron-electron interaction to the inelastic electron relaxation. At strong magnetic field, at which Landau levels are well separated, the inelastic scattering rate is proportional to T3 at high temperatures. We suggest the electron-phonon scattering as the dominant mechanism of the inelastic electron relaxation in this regime. At low temperature and separated Landau levels an additional regime of the inelastic electron relaxation is observed: τin ~ T-1.26.
DIFFERENTIAL RESISTANCE OF TWO DIMENSIONAL ELECTRON SYSTEMS SUBJECT TO MICROWAVE RADIATION
Authors: KHODAS MAXIM , VAVILOV MAXIM G.
Published: June 2009

We present the expression for differential resistance of a disordered two-dimensional electron gas placed in a perpendicular magnetic field and subject to microwave irradiation. We demonstrate that in strong dc electric fields the current oscillates as a function of the strength of the applied constant electric field. We demonstrate that the amplitude of oscillations of the differential resistivity is characterized by the back-scattering rate off disorder. We argue that the dominant contribution to the non-linearity in strong electric fields originates from the modification of electron scattering off disorder by electric fields, or so-called "displacement" mechanism. The non-equilibrium mechanism, which is related to modification of electron distribution function by electric fields turns out to be inefficient in strong electric fields, although it describes current in weak electric fields. We further analyze the positions of maxima and minima of the differential resistance as a function of the applied electric field and frequency of microwave radiation.
MICROWAVE MAGNETOPLASMON ABSORPTION BY A 2DEG STRIPE
Authors: FEDORYCH O. M. , STUDENIKIN S. A. , MOREAU S. , POTEMSKI M. , SAKU T. , HIRAYAMA Y.
Published: June 2009

Microwave (MW) absorption by a high mobility 2DEG has been investigated experimentally using sensitive Electron Paramagnetic Resonance (EPR) cavity technique. It is found that MW absorption spectra are chiefly governed by confined magnetoplasmon excitations in a 2DEG stripe. Spectra of the 2D magnetoplasmons are studied as a function of magnetic field, MW frequency and carrier density. The electron concentration is tuned by illumination and monitored using optical photoluminescence technique.
PHOTOLUMINESCENCE STUDIES OF POSITIVELY CHARGED EXCITONS IN ASYMMETRIC GaAs/Ga1-xAlxAs QUANTUM WELLS WITH A TWO-DIMENSIONAL HOLE GAS
Authors: BRYJA LESZEK , WÓJS ARKADIUSZ , MISIEWICZ JAN , PLOCHOCKA PAULINA , POTEMSKI MAREK , REUTER D. , WIECK A.
Published: June 2009

Positive trions in a quasi-two-dimensional hole gas confined in symmetric and asymmetric GaAs quantum wells are studied by a combination of polarization-resolved photoluminescence and transport measurements in high magnetic fields B (up to 23 T) and low temperatures (down to 30 mK). The experiments are accompanied by realistic numerical calculations. The whole family of trions (the singlet and a pair of triplets) are observed. The singlet-triplet crossing of Coulomb energies is found: hidden in symmetric and visible in asymmetric structures.
MAGNETO-OPTICAL ABSORPTION AND PHOTOMAGNETISM IN EUROPIUM CHALCOGENIDES
Authors: HENRIQUES A. B. , RAPPL P. H. O. , ABRAMOF E.
Published: June 2009

The absorption threshold in EuTe and EuSe was investigated as a function of applied magnetic field in the Faraday geometry. A well-resolved doublet of sharp dichroic lines was observed when the magnetic field induced ferromagnetic alignment of the spins in the crystal lattice. In contrast, at zero magnetic field only a broad and featureless absorption onset is seen. These results are fully explained in terms of a model of electronic transitions between localized states at the Eu lattice site and a tight-binding conduction band, which incorporates the formation of spin domains. Based on this model, predictions are made concerning the possibility of inducing magnetization of the spin lattices by illuminating the material with circularly polarized light.
QUANTUM HALL EFFECT AND OHM METROLOGY
Authors: POIRIER W. , SCHOPFER F.
Published: June 2009

The quantum Hall effect (QHE) discovery has revolutionized the ohm metrology: the representation of the unit of resistance is now universal and the ohm can be maintained in each national metrology institute with a relative uncertainty of one part in 109. This breakthrough also results from the development of resistance comparison bridges using cryogenic current comparator (CCC). The fundamental properties of the QHE allow the realization of Quantum Hall Array Resistance Standards (QHARS) by combining a large number of single Hall bars connected in series and/or in parallel. These standards can be as accurate as a single Hall bar. More generally, the multiple connection technique allows metrologists to design useful circuits based on quantum Hall resistors like voltage dividers or Wheatstone bridges. The QHE Wheatstone bridge is particularly suitable for comparing quantum standards. By detecting the unbalance current of this bridge with a CCC, new universality tests of the QHE with a target uncertainty less than 1011 can be realized.
3D SiGe QUANTUM DOT CRYSTALS: STRUCTURAL CHARACTERIZATION AND ELECTRONIC COUPLING
Authors: FROMHERZ T. , STANGL J. , LECHNER R. T. , WINTERSBERGER E. , BAUER G. , HOLY V. , DAIS C. , MÜLLER E. , SIGG H. , Solak H. H. , GRÜTZMACHER D.
Published: June 2009

We report on the growth of SiGe quantum dot crystals which are realized by depositing Ge on a two-dimensionally pit-patterned Si substrate and subsequent growth of Si spacer and Ge island layers. Lateral periods of 100 nm are obtained by employing deep UV lithography using synchrotron radiation. The vertical period of the typically 10 period dot superlattices was of the order of 10 nm. Ordering of the islands was investigated by atomic force microscopy as well as by high resolution x-ray diffraction studies. From the quantitative evaluation of the x-ray diffraction data a mean Ge content of about 60% in the quantum dots was obtained and an rms. deviation from ideal lattice sites of about 3 nm was found. A simulation of the eigenenergies based on the nextnano3 simulation package was used to interpret the measured photoluminescence data.
MAGNETICALLY CONTROLLED TERAHERTZ ABSORPTION AND EMISSION IN CARBON NANOTUBES
Authors: PORTNOI M. E. , ROSENAU DA COSTA M. , KIBIS O. V. , SHELYKH I. A.
Published: June 2009

We show that an experimentally attainable magnetic field applied along the axis of a metallic carbon nanotube not only opens the gap in the nanotube energy spectrum but also allows optical transitions, which are forbidden in the absence of the field. Possible terahertz applications of this effect are discussed.
COLLECTIVE EXCITATIONS IN SEMICONDUCTOR AND GRAPHENE BILAYERS IN HIGH MAGNETIC FIELD
Authors: BISTI VERONIKA
Published: June 2009

The effect of space symmetry on magnetoplasmon charge density and spin-density excitations in a weakly tunnel coupled bilayer electron system at the filling factor ν = 4 is considered. The optic magnetoplasmon energy as well as the energy of intralayer magnetoexciton is not affected by space symmetry. Three other collective magnetoplasmon modes as well as three magnetoexciton modes are determined by relations between the asymmetry parameter, tunnel parameter, and the difference in energies of intralayer and interlayer magnetoexcitons. The charge-density excitations in bilayer graphene at the filling-factor ν ≪ 1 are considered. The presence of small asymmetry of graphene layers is taken into account. It is demonstarted that the magnetoplasmon energy depends on the bilayer ground state.
QUANTUM CASCADE NANOSTRUCTURES UNDER HIGH MAGNETIC FIELD
Authors: PÉRÉ-LAPERNE N. , DE VAULCHIER L. A. , GULDNER Y. , SIRTORI C. , BERGER V.
Published: June 2009

Magneto-transport measurements have been performed on two quantum cascade structures, a laser and a detector. These experiments lead to determine the different scattering processes involved in these devices. In the lasers we find both an interface roughness mechanism and LO-phonon scattering of hot carriers in the upper state's Landau levels. In detectors we discover that an inelastic mechanism increase the dark current whereas an elastic one limits the detectivity.
THE CIRCULAR PHOTOGALVANIC EFFECT IN TWO-DIMENSIONAL HOLE GASES IN PERPENDICULAR MAGNETIC FIELD
Authors: VASYUKOV D. A. , PLAUT A. S. , MACDONALD A. H. , HENINI M.
Published: June 2009

We have measured the circular photogalvanic effect in a GaAs two-dimensional hole gas in zero and perpendicular magnetic field. The increase of the photoinduced voltage in very small magnetic field, when excited at fixed photon energy, demonstrates behavior reminiscent of that seen in anomalous Hall measurements on ferromagnets. The spectral dependence of the photoinduced voltage in magnetic field exhibits a series of maxima and minima that oscillate around zero volts. When plotted, the energy positions of these voltage peaks form a clear Landau-level-like fan chart.
A NEW TWO-DIMENSIONAL ELECTRON-HOLE SYSTEM
Authors: KVON Z. D. , OLSHANETSKY E. B. , KOZLOV D. A. , MIKHAILOV N. N. , DVORETSKII S. A.
Published: June 2009

A two-dimensional electron-hole system consisting of light high-mobility electrons with a density of Ns = (4 - 7) × 1010 cm -2 and heavier lower-mobility holes with a density Ps = (0.7 - 1.6) × 1011 cm -2 has been discovered in a quantum well based on mercury telluride with the (013) surface orientation. The system exhibits a number of specific magnetotransport properties in both the classical magnetotransport (positive magnetoresistance and sign-variable Hall effect) and the quantum Hall effect regime. These properties are associated with the coexistence of two-dimensional electrons and holes and actually manifest the first realization of a two-dimensional semimetal.
TRANSPORT PROPERTIES OF TUNNEL-COUPLED ONE-DIMENSIONAL CHANNELS FROM AN ELECTRON BILAYER SYSTEM IN PERPENDICULAR MAGNETIC FIELD
Authors: BUCHHOLZ S. S. , FISCHER S. F. , KUNZE U. , SCHUH D. , ABSTREITER G.
Published: June 2009

In a tunnel coupled GaAs / AlGaAs electron bilayer system (BLS), we compare transport measurements of 1D quantum Hall (QH) edge channels with lithographically defined quantum point contacts (QPCs). The electron densities in both systems can be varied by a top-gate. In a perpendicular magnetic field, Landau level mixing is observed in the QH regime, and indications of crossings of spin split QPC subbands are detected.
NEGATIVE MAGNETORESISTANCE OF A SILICON 2DEG UNDER IN-PLANE MAGNETIC FIELD DUE TO SPIN-SPLITTING OF UPPER SUBBANDS
Authors: TAKASHINA K. , NIIDA Y. , RENARD V. T. , FUJIWARA A. , FUJISAWA T. , HIRAYAMA Y.
Published: June 2009

We examine the effect of an in-plane magnetic field on the resistance of a 2-dimensional electron system confined in a silicon quantum well when the Fermi energy is tuned through the upper valley-subband edge while the electrons are otherwise valley-polarized. In contrast to previous experiments on valley-degenerate systems which only showed positive magnetoresistance, when the Fermi energy is at or near the upper valley-subband edge, the magnetoresistance is found to show a distinct negative contribution which is interpreted as being due to spin polarization of the upper valley-subband.
THE MAGNETORESISTANCE OF CARBON-BLACK SAMPLES AT DIFFERENT TEMPERATURES
Authors: APARECIDO-FERREIRA A. , ALVES E. S. , RIBEIRO G. M. , SAMPAIO J. F.
Published: June 2009

We present experimental results for the magnetoresistance of carbon-black, in magnetic fields (B) up to 15 T, and its temperature dependence at temperatures (T) from 10 K to 1.2 K. At these temperatures the electrical conduction is given by variable range hopping with Coulomb gap. The results show a B2 dependence at low fields and a B1/2 dependence in the limits of high fields. We show that the model of hydrogenlike localized states cannot explain the experimental results.
MODELING OF TUNNELING MAGNETORESISTANCE IN (Ga,Mn)As–TRILAYERS
Authors: SANKOWSKI PIOTR , KACMAN PERLA
Published: June 2009

A model which combines the Landauer-Büttiker formalism with empirical multi-orbital tight-binding approach is used to predict conditions for improving the performance of tunneling magnetoresistance (TMR) in ( Ga , Mn ) As -based trilayer structures, in particular, for reducing the bias anomaly effect. The calculations show that two parameters, i.e., the spin splitting in the magnetic layers and the height of the tunneling barrier, define a two range decay of the TMR with bias. It is shown that the higher is the barrier in the spacer layer and the bigger is the spin-splitting in the magnetic layers, the slower is the decay of the TMR ratio with the applied voltage. The model predicts also that the TMR decay can be additionally reduced when the hole concentrations in the two magnetic layers are different. Finally, to account, at least partially, for the interface roughness, we allow for a Gaussian distribution of the ions in the interface region.
MAGNETIC RESONANT X-RAY DIFFRACTION APPLIED TO THE STUDY OFEuTeFILMS AND MULTILAYERS
Authors: DÍAZ B. , ABRAMOF E. , RAPPL P. H. O. , GRANADO E. , CHITTA V. A. , HENRIQUES A. B. , OLIVEIRA N. F.
Published: June 2009

In this work we use magnetic resonant x-ray diffraction to study the magnetic properties of a 1.5 µm EuTe film and an EuTe / PbTe superlattice (SL). The samples were grown by molecular beam epitaxy on (111) oriented BaF2substrates. The measurements were made at the Eu L2absorption edge, taking profit of the resonant enhancement of more than two orders in the magnetically diffracted intensity. At resonance, high counting rates above 11000 cps were obtained for the 1.5 µm EuTe film, allowing to check for the type II antiferromagnetic order of EuTe . An equal population of the three possible in-plane magnetic domains was found. The EuTe / PbTe SL magnetic peak showed a satellite structure, indicating the presence of magnetic correlations among the 5 ML (monolayers) EuTe layers across the 15 ML PbTe non-magnetic spacers. The temperature dependence of the integrated intensities of the film and the SL yielded different Nèel temperatures TN. The lower TNfor the SL is explained considering the higher influence of the surface atoms, with partial bonds lost.
MAGNETO-LUMINESCENCE OF GADOLINIUM DOPED GALLIUM NITRIDE
Authors: WYSMOLEK A. , KAMIŃSKA M. , TWARDOWSKI A. , POTEMSKI M. , BOĆKOWSKI M. , GRZEGORY I.
Published: June 2009

The results of luminescence measurements in magnetic fields up to 23T of strain free GaN bulk crystals doped with gadolinium are presented. The magnetic field splitting pattern observed for the Gd -related transitions in the "red" spectral range is consistent with its identification as due to intracenter transitions between Gd 3+ ion levels. The Gd -related emission structures observed close to the GaN bandgap show magnetic field behavior which could be understood in terms bound exciton or donor-acceptor pair recombination involving Gd ions. Disappointingly, no giant Zeeman splitting, which otherwise could confirm ferromagnetism in this material, was observed.
MAGNETIC FIELD INDUCED NEAR-BAND-GAP OPTICAL PROPERTIES IN EuTe LAYERS
Authors: HERNANDEZ F. G. G. , HENRIQUES A. B. , RAPPL P. H. O. , ABRAMOF E.
Published: June 2009

The magnetic response of the near-band-edge optical properties is studied in EuTe layers. In several magneto-optical experiments, the absorption and emission are described as well as the related Stokes shift. Specifically, we present the first experimental report of the photoluminescence excitation (PLE) spectrum in Faraday configuration. The PLE spectra shows to be related with the absorption spectra through the observation of resonance between the excitation light and the zero-field band-gap. A new emission line appears at 1.6 eV at a moderate magnetic field in the photoluminescence (PL) spectra. Furthermore, we examine the absorption and PL red-shift induced by the magnetic field in the light of the d - f exchange interaction energy involved in these processes. Whereas the absorption red-shift shows a quadratic dependence on the field, the PL red-shift shows a linear dependence which is explained by spin relaxation of the excited state.
EXACT AND APPROXIMATE RELATIONS FOR THE SPIN-DEPENDENCE OF THE EXCHANGE ENERGY IN HIGH MAGNETIC FIELDS
Authors: MORBEC J. M. , CAPELLE K.
Published: June 2009

The exchange energy of an arbitrary collinear-spin many-body system in an external magnetic field is a functional of the spin-resolved charge and current densities, Ex[n↑,n↓,j↑,j↓]. Within the framework of density-functional theory (DFT), we show that the dependence of this functional on the four densities can be fully reconstructed from either of two extreme limits: a fully polarized system or a completely unpolarized system. Reconstruction from the limit of an unpolarized system yields a generalization of the Oliver-Perdew spin scaling relations from spin-DFT to current-DFT. Reconstruction from the limit of a fully polarized system is used to derive the high-field form of the local-spin-density approximation to current-DFT and to magnetic-field DFT.
BOUND HYDROGENIC STATES IN A MAGNETIC FIELD: A FINITE-DIFFERENCE APPROACH
Authors: DE SOUZA GUSTAVO VANIN BERNARDINO , BRUNO-ALFONSO ALEXYS
Published: June 2009

A finite-difference scheme is used to calculate bound electronic states of an electron in a hydrogen atom subject to a magnetic field. The numerical results are in good agreement with exact results, in the absence of the magnetic field, and with a two-parameters variational calculation, when the magnetic field is applied.
CALCULATION OF MAGNETODONOR STATES IN InP AND POLARON EFFECTS
Authors: BRUNO-ALFONSO ALEXYS , HAI GUO-QIANG
Published: June 2009

Far-infrared transitions in polar semiconductors are known to be affected by the presence of shallow donor impurities, external magnetic fields and the electron-LO-phonon interaction. We calculate the magnetodonor states in indium phosphide by a diagonalization procedure, and introduce the electron-phonon interaction by the Fröhlich term. The main effects of this perturbation are calculated by a multi-level version of the Wigner-Brillouin theory. We determine the transition energies, from the ground state to excited states, and find good qualitative agreement with recently reported absorption-spectroscopy measurements in the 100-800 cm-1 range, with applied magnetic fields up to 30 T. Our calculations suggest that experimental peak splittings in the 400-450 cm-1 range are due to the electron-phonon interaction.
A CLOSED-CYCLE REFRIGERATOR BASED PULSED MAGNET SYSTEM
Authors: MURTHY O. V. S. N. , VENKATARAMAN V.
Published: June 2009

A low cost 12 T pulsed magnet system has been integrated with a closed-cycle helium refrigerator. The copper solenoid is directly immersed in liquid nitrogen for reduced electrical resistance and more efficient heat transfer. This ensures a minimal delay of few minutes between pulses. The sample is mounted on the cold finger of the refrigerator and, along with the surrounding vacuum shroud, is inserted into the bore of the solenoid. When combined with software lock-in signal processing to reduce noise, quick but accurate measurements can be performed at temperatures 4 K-300 K up to 12 T. Quantum Hall effect data in a p -channel SiGe / Si heterostructure has been used to calibrate the instrument against a commercial superconducting magnet. Its versatility as a routine characterization tool is demonstrated by measuring parallel conduction in Si / SiGe modulation doped heterostructures.
MODELING AND ANALYSIS OF KNOWLEDGE FLOWS IN SOFTWARE PROCESSES THROUGH THE EXTENSION OF THE SOFTWARE PROCESS ENGINEERING METAMODEL
Authors: RODRÍGUEZ-ELIAS OSCAR M. , MARTÍNEZ-GARCÍA ANA I. , VIZCAÍNO AURORA , FAVELA JESÚS , PIATTINI MARIO
Published: June 2009

Knowledge is a key asset in software engineering. Facilitating access to the knowledge that software engineers require for the task at hand can therefore bring many benefits. To accomplish this, it is important to understand how knowledge flows through the organization, to identify problems that may hinder a suitable flow, and to define strategies with which to address them. Process modeling has proved to be a useful technique for analyzing knowledge flows. Traditional process modeling languages do not, however, provide primitives to explicitly represent the knowledge involved in the processes within the models. In this paper, we illustrate how the Software Process Engineering Metamodel (SPEM) can be adapted, to be used as a process modeling language for analyzing knowledge flows in software processes. We have extended SPEM to represent knowledge and its sources in process models in an explicit way. We also discuss the experiences obtained from using this extension in a software organization and the lesson we have learned from it.
A PROPOSED ARCHITECTURE FOR SELF-ADAPTIVE EXPERT SYSTEMS
Authors: CHEN TSUNG-TENG , HO CHENG-SEEN
Published: June 2009

The pre-built knowledge of traditional expert systems is only capable of limited responses to changes in the operating environment. If the data input is imperfect, a traditional system may fail to reach any rational conclusions. In this paper, we introduce the concept of self-adaptability to the inference process of an expert system, and propose a model that is capable of handling unexpected user input effectively and efficiently. Such a system can formulate operational knowledge on the move for inference. With this self-adaptive capability, an expert system can reach useful conclusions, even when the input data is insufficient. The architecture of the proposed system encodes domain knowledge with semantic networks. It also defines four types of adaptation, namely, condition knowledge adaptation, operational knowledge adaptation, conclusion knowledge adaptation, and presentation adaptation, and focuses on how the first three contribute to the adaptive capability of the system. In addition, to enable a self-adaptive expert system to effectively produce better conclusions, two entropy-based measuring mechanisms are proposed: one minimizes the information loss during knowledge adaptation, while the other selects the best attribute relation during the generation of operational knowledge. We have proved that a self-adaptive expert system based on this architecture can always reach a regular conclusion or an abstract conclusion, which is a more meaningful conclusion by automatically modifying its operational knowledge in response to user feedback during the inference process, even in unexpected situations.
ALTERNATIVE TIME DOMAIN BOUNDARY INTEGRAL EQUATIONS FOR THE SCALAR WAVE EQUATION USING DIVERGENCE-FREE REGULARIZATION TERMS
Authors: NATSIOPOULOS GEORGIOS
Published: June 2009

In this short note alternative time domain boundary integral equations (TDBIE) for the scalar wave equation are formulated on a surface enclosing a volume. The technique used follows the traditional approach of subtracting and adding back relevant Taylor expansion terms of the field variable, but does not restrict this to the surface patches that contain the singularity only. From the divergence-free property of the added-back integrands, together with an application of Stokes' theorem, it follows that the added-back terms can be evaluated using line integrals defined on a cut between the surface and a sphere whose radius increases with time. Moreover, after a certain time, the line integrals may be evaluated directly. The results provide additional insight into the theoretical formulations, and might be used to improve numerical implementations in terms of stability and accuracy.
AN ANALYTICAL FRAMEWORK FOR EXPLAINING RELATIVE PERFORMANCE OF CAPM BETA AND DOWNSIDE BETA
Authors: GALAGEDERA DON U. A.
Published: June 2009

Even though investors' view of risk is generally regarded as related to the downside of the return distribution the CAPM beta is still a widely used measure of systematic risk. A number of studies compare the empirical performance of CAPM beta and downside beta in explaining the variation in portfolio returns and report mixed results. This paper provides a basis for explaining such mixed results. Using data generating processes in the mean-variance and mean-lower partial moment frameworks, analytical relationships between the CAPM beta and downside beta are derived. The derived relationships reveal that the association between the two systematic risk measures is to a great extent dependent on the volatility of the market portfolio returns and the deviation of the target rate from the risk-free rate. How the relationships derived here may be used in practice is demonstrated using empirical data.
Efficient and provably secure aggregation of encrypted data in wireless sensor networks
Authors: Castelluccia Claude , Chan Aldar C-F. , Mykletun Einar , Tsudik Gene
Published: June 2009

Wireless sensor networks (WSNs) are composed of tiny devices with limited computation and battery capacities. For such resource-constrained devices, data transmission is a very energy-consuming operation. To maximize WSN lifetime, it is essential to minimize the number of bits sent and received by each device. One natural approach is to aggregate sensor data along the path from sensors to the sink. Aggregation is especially challenging if end-to-end privacy between sensors and the sink (or aggregate integrity) is required. In this article, we propose a simple and provably secure encryption scheme that allows efficient additive aggregation of encrypted data. Only one modular addition is necessary for ciphertext aggregation. The security of the scheme is based on the indistinguishability property of a pseudorandom function (PRF), a standard cryptographic primitive. We show that aggregation based on this scheme can be used to efficiently compute statistical values, such as mean, variance, and standard deviation of sensed data, while achieving significant bandwidth savings. To protect the integrity of the aggregated data, we construct an end-to-end aggregate authentication scheme that is secure against outsider-only attacks, also based on the indistinguishability property of PRFs.
Deployment-aware modeling of node compromise spread in wireless sensor networks using epidemic theory
Authors: De Pradip , Liu Yonghe , Das Sajal K.
Published: June 2009

Motivated by recently surfacing viruses that can spread over the air interfaces, in this article, we investigate the potentially disastrous threat of node compromise spreading in wireless sensor networks. We assume such a compromise originating from a single infected node, can propagate to other sensor nodes via communication and pre-established mutual trust. We focus on the possible epidemic breakout of such propagations where the whole network may fall victim to the attack. Using epidemic theory, we model and analyze this spreading process and identify key factors determining potential outbreaks. In particular, we perform our study on random graphs precisely constructed according to the parameters of the network, such as distance, key sharing constrained communication and node recovery, thereby reflecting the true characteristics therein. Moreover, a comparative study of the epidemic propagation is performed based on the effects of two types of sensor deployment strategies, viz., uniform random and group-based deployment. The analytical results provide deep insights in designing potential defense strategies against this threat. Furthermore, through extensive simulations, we validate the model and perform investigations on the system dynamics. Our analysis and simulation results indicate that the uniform random deployment is more vulnerable to an epidemic outbreak than the group based deployment strategy.
Low-complexity coding and source-optimized clustering for large-scale sensor networks
Authors: Maierbacher Gerhard , Barros João
Published: June 2009

We consider the distributed source coding problem in which correlated data picked up by scattered sensors has to be encoded separately and transmitted to a common receiver, subject to a rate-distortion constraint. Although near-to-optimal solutions based on Turbo and LDPC codes exist for this problem, in most cases the proposed techniques do not scale to networks of hundreds of sensors. We present a scalable solution based on the following key elements: (a) distortion-optimized index assignments for low-complexity distributed quantization, (b) source-optimized hierarchical clustering based on the Kullback-Leibler distance and (c) sum-product decoding on specific factor graphs exploiting the correlation of the data.
Sensor network data fault types
Authors: Ni Kevin , Ramanathan Nithya , Chehade Mohamed Nabil Hajj , Balzano Laura , Nair Sheela , Zahedi Sadaf , Kohler Eddie , Pottie Greg , Hansen Mark , Srivastava Mani
Published: June 2009

This tutorial presents a detailed study of sensor faults that occur in deployed sensor networks and a systematic approach to model these faults. We begin by reviewing the fault detection literature for sensor networks. We draw from current literature, our own experience, and data collected from scientific deployments to develop a set of commonly used features useful in detecting and diagnosing sensor faults. We use this feature set to systematically define commonly observed faults, and provide examples of each of these faults from sensor data collected at recent deployments.
An interrelational grouping abstraction for heterogeneous sensors
Authors: Rajamani Vasanth , Kabadayi Sanem , Julien Christine
Published: June 2009

In wireless sensor network applications, the potential to use cooperation to resolve user queries remains largely untapped. Efficiently answering a user's questions requires identifying the correct set of nodes that can answer the question and enabling coordination between them. In this article, we propose a
query domain
abstraction that allows an application to dynamically specify the nodes best suited to answering a particular query. Selecting the ideal set of heterogeneous sensors entails answering two fundamental questions—
how
are the selected sensors related to one another, and
where
should the resulting sensor coalition be located. We introduce two abstractions, the
proximity function
and the
reference function
, to precisely specify each of these concerns within a query. All nodes in the query domain must satisfy any provided proximity function, a user-defined function that constrains the relative relationship among the group of nodes (e.g., based on a property of the network or physical environment or on logical properties of the nodes). The selected set of nodes must also satisfy any provided reference function, a mechanism to scope the location of the query domain to a specified area of interest (e.g., within a certain distance from a specified reference point). In this article, we model these abstractions and present a set of protocols that accomplish this task with varying degrees of correctness. We evaluate their performance through simulation and highlight the tradeoffs between protocol overhead and correctness.
A3
Authors: Hailpern Joshua , Karahalios Karrie , Halle James , Dethorne Laura , Coletto Mary-Kelsey
Published: June 2009

HCI studies assessing nonverbal individuals (especially those who do not communicate through traditional linguistic means: spoken, written, or sign) are a daunting undertaking. Without the use of directed tasks, interviews, questionnaires, or question-answer sessions, researchers must rely fully upon observation of behavior, and the categorization and quantification of the participant’s actions. This problem is compounded further by the lack of metrics to quantify the behavior of nonverbal subjects in computer-based intervention contexts. We present a set of dependent variables called A3 (pronounced A-Cubed) or Annotation for ASD Analysis, to assess the behavior of this demographic of users, specifically focusing on engagement and vocalization. This paper demonstrates how theory from multiple disciplines can be brought together to create a set of dependent variables, as well as demonstration of these variables, in an experimental context. Through an examination of the existing literature, and a detailed analysis of the current state of computer vision and speech detection, we present how computer automation may be integrated with the A3 guidelines to reduce coding time and potentially increase accuracy. We conclude by presenting how and where these variables can be used in multiple research areas and with varied target populations.
Efficient management of idleness in storage systems
Authors: Mi Ningfang , Riska Alma , Zhang Qi , Smirni Evgenia , Riedel Erik
Published: June 2009

Various activities that intend to enhance performance, reliability, and availability of storage systems are scheduled with low priority and served during idle times. Under such conditions, idleness becomes a valuable “resource” that needs to be efficiently managed. A common approach in system design is to be nonwork conserving by “idle waiting”, that is, delay the scheduling of background jobs to avoid slowing down upcoming foreground tasks.

In this article, we complement “idle waiting” with the “estimation” of background work to be served in every idle interval to effectively manage the trade-off between the performance of foreground and background tasks. As a result, the storage system is better utilized without compromising foreground performance. Our analysis shows that if idle times have low variability, then idle waiting is not necessary. Only if idle times are highly variable does idle waiting become necessary to minimize the impact of background activity on foreground performance. We further show that if there is burstiness in idle intervals, then it is possible to predict accurately the length of incoming idle intervals and use this information to serve more background jobs without affecting foreground performance.
POTSHARDS—a secure, recoverable, long-term archival storage system
Authors: Storer Mark W. , Greenan Kevin M. , Miller Ethan L. , Voruganti Kaladhar
Published: June 2009

Users are storing ever-increasing amounts of information digitally, driven by many factors including government regulations and the public's desire to digitally record their personal histories. Unfortunately, many of the security mechanisms that modern systems rely upon, such as encryption, are poorly suited for storing data for indefinitely long periods of time; it is very difficult to manage keys and update cryptosystems to provide secrecy through encryption over periods of decades. Worse, an adversary who can compromise an archive need only wait for cryptanalysis techniques to catch up to the encryption algorithm used at the time of the compromise in order to obtain “secure” data. To address these concerns, we have developed POTSHARDS, an archival storage system that provides long-term security for data with very long lifetimes without using encryption. Secrecy is achieved by using unconditionally secure secret splitting and spreading the resulting shares across separately managed archives. Providing availability and data recovery in such a system can be difficult; thus, we use a new technique, approximate pointers, in conjunction with secure distributed RAID techniques to provide availability and reliability across independent archives. To validate our design, we developed a prototype POTSHARDS implementation. In addition to providing us with an experimental testbed, this prototype helped us to understand the design issues that must be addressed in order to maximize security.