bluuebunny/crossref_metadata_2025_split huggingface dataset:
Dephasing processes in II–VI quantum dots
Authors: Woggon U. , Portuné M. , Klingshirn C. , Giessen H. , Fluegel B. , Mohs G. , Peyghambarian N. 
Published: July 2007

AbstractThe loss of phase coherence in II–VI quantum dots is investigated using experimental methods working both in the frequency and in the time domain. Femtosecond differential absorption experiments reveal narrow resonances at very early times after excitation and low excitation densities for CdSe quantum dots. A substructure is detected and explained by transitions from the series of confined hole states. By nondegenerate four‐wave mixing experiments (NDFWM) the dephasing time is determined for CdS quantum dots with radii larger than the Bohr radius. Temperature and excitation intensity dependent dephasing times between ≈︁ 20 fs and some 100 fs are measured and attributed to electron‐hole pair–phonon coupling and carrier–carrier scattering.
InAs/GaAs quantum dots radiative recombination from zero‐dimensional states
Authors: Grundmann M. , Ledentsov N. N. , Heitz R. , Eckey L. , Christen J. , Böhrer J. , Bimberg D. , Ruvimov S. S. , Werner P. , Richter U. , Heydenreich J. , Ustinov V. M. , Egorov A. Yu. , Zhukov A. E. , Kopev P. S. , Alferov Zh. I. 
Published: July 2007

AbstractNanometer‐scale quantum dots are fabricated using the Stranski‐Krastanov growth mode of InAs on GaAs (001). For an average coverage of four monolayers, 1011cm−2 pyramidal‐shaped dots (12 ± 1 nm base along 〈100〉, 5 ± 1 nm high) are formed as observed in plane‐view and cross‐section transmission electron microscopy. The quantum dots exhibit short‐range order, aligning along rows in 〉100〉 directions. The three‐dimensional confinement of the wave function results in ultrasharp luminescence lines (full width at half maximum < 0.15 meV) from individual dots as revealed with highly spatially resolved cathodoluminescence. Even at elevated temperatures extremely sharp lines are observed, proving the δ‐like zero‐dimensional electronic density of states.
Coulomb effects in femtosecond nonlinear transmission of semiconductor nanocrystals
Authors: Klimov V. , Hunsche S. , Kurz H. 
Published: July 2007

AbstractThe dynamics of carrier‐induced absorption changes in CdSe nanocrystals excited by 2 and 4 eV photons are investigated with femtosecond pump and probe spectroscopy. After excitation with 4 eV photons a transient red shift of the lowest optical transition is observed in the initial phase of carrier relaxation. This shift is attributed to the Coulomb two‐electron‐hole‐pair interaction (a biexciton effect).
Linear polarization of photoluminescence and Raman scattering in open InGaAs/InP quantum well wires
Authors: Gippius N. A. , Tikhodeev S. G. , Rubio J. , Calleja J. M. , Ils P. , Forchel A. , Kulakovskii V. D. 
Published: July 2007

AbstractThe photoluminescence and Raman scattering intensities of InGaAs/InP quantum well wires with wire widths Lx between 10 nm and 1 pm are strongly polarized parallel to the wire axis. This effect is mainly due to the spatial redistribution of the electric component of the electromagnetic field in the vicinity of the quantum wire.
Strong optical nonlinearities in quantum wires and dots of porous silicon
Authors: Dneprovskii V. , Eev A. , Gushina N. , Okorokov D. , Panov V. , Karavanskii V. , Maslov A. , Sokolov V. , Dovidenko E. 
Published: July 2007

AbstractThe increase of transmission at discrete frequencies (bleaching bands) is observed in porous silicon platelets pumped by picosecond laser pulses. It is attributed to a saturation of optical transitions between the energy levels of electrons and holes spatially confined within quasi‐zero‐dimensional (quantum dots) and quasi‐one‐dimensional (quantum wires) nanostructures. The results of independent measurements using transmission and reflection electron and scanning tunneling microscopy have confirmed the existence of quantum dots and wires of corresponding size. A considerable slowing of energy relaxation from upper to lower levels of size quantization compared with intraband relaxation in the bulk has been observed in the cooled (80 K) platelets of porous silicon.
The ultrafast response of a scanning tunneling microscope
Authors: Weiss S. , Botkin D. , Ogletree D. F. , Salmeron M. , Chemla D. S. 
Published: July 2007

AbstractThe development of the ultrafast scanning tunneling microscope is reviewed. It is shown that the response of the tunneling gap of a scanning tunneling microscope to excitation by a subpicosecond electrical pulse has a capacitive component, the origin of which is quantum mechanical.
Femtosecond dynamics of excitons in quantum wells and quantum well microcavities
Authors: Wang H. , Shah J. , Damen T. C. , Pfeiffer L. N. , Cunningham J. E. 
Published: July 2007

AbstractInvestigations of femtosecond dynamics of excitons in two different regimes are reported: 1. perturbative or weak coupling regime where the exciton decay is irreversible (i.e. excitons in quantum wells), and 2. non‐perturbative or strong coupling regime (i.e. quantum well excitons in microcavities) where the coherent exchange of energy between the coupled cavity and exciton modes dominates the femtosecond dynamics. In the perturbative regime, the first femtosecond luminescence measurements are reported, which show (i) a finite rise time for the emission from resonantly‐excited excitons and (ii) femtosecond quantum beats of heavy‐hole and light‐hole excitons. In microcavities, the first nonlinear measurements are reported, which show quantum beats from the coupled cavity‐exciton modes, and provide information about the system not available from linear measurements.
Influence of inhomogeneous broadening on spectrally resolved four‐wave mixing in semiconductors
Authors: Pantke K.‐H. , Erland J. , Hvam J. M. 
Published: July 2007

AbstractThe influence of inhomogeneous broadening on results obtained from spectrally resolved transient four‐wave mixing in semiconductors is studied and the homogeneous and inhomogeneous linewidths are simultaneously deduced. Experimental results obtained from the impuritybound exciton I1 in CdSe and the heavy‐hole and light‐hole excitons in GaAs multiple quantum well structures are presented. Furthermore, the case is investigated when more resonances are coherently excited leading to quantum beats.
Influence of disorder and exciton‐exciton interactions four‐wave mixing in quantum wells
Authors: Bongiovanni G. , Congiu F. , Mura A. , Staehli J. L. 
Published: July 2007

AbstractThe interplay of disorder effects and exciton‐exciton interactions in the four‐wave‐mixing process in quantum wells is investigated. Experiments are performed employing femtosecond and picosecond pulses to have either good temporal or good spectral resolution. It is shown that the localisation of excitons is of primary importance in the generation of the nonlinear signal. It is shown which states contribute and how to the FWM process.
Temperature dependence of Anderson parameter and thermal expansivity
Authors: Kumar M. , Bedi S. S. 
Published: July 2007

AbstractTwo different methods are proposed to investigate the thermal expansivity as a function of temperature by considering the temperature dependence of the Anderson parameter. The validity of the methods is examined by computing the thermal expansion coefficient ranging from room temperature up to the melting temperatures for sixteen alkali halide crystals. The results are discussed in the light of recent research in high temperature physics. The simplicity of one method over the other is discussed.
Semiconductor heterointerface characterisation via effective harmonic oscillator simulation
Authors: Anagnostakis E. A. , Theodorou D. E. 
Published: July 2007

AbstractThe persistent photoconductivity (PP) measurements concerning the two‐dimensional electron gas (2DEG) hosted by the quantum well (QW) of an illuminated typical semiconductor heterojunction (HJ) are processed to yield the respective heterointerface (HI) characterisation in terms of fundamental parameters identifying the HJ QW. The procedure involves the employment of an effective harmonic oscillator as simulating the QW electron‐confining potential, as well as the introduction of a HI descriptor incorporating the QW spatial width along with the generic oscillator eigenfrequency.
Distorted flux lines in a uniaxial anisotropic superconducting spherical shell
Authors: Saif A. G. , El‐Sabagh M. A. 
Published: July 2007

AbstractA model of a uniaxial anisotropic superconducting spherical shell is established to verify the properties of distorted flux lines. In the frame of the generalized spherical functions (GSF), the field distribution inside and outside the high‐temperature superconducting region are determined. Using these field solutions, the current density and the forces on the vortex segments are calculated. Moreover, the Gibbs free energy of the system is obtained in compact form which can be easily applied to real‐life problems. In the isotropic case as well as if the shell tends to a rigid sphere agreement with previous results is achieved.
Effect of interlayer coupling on Néel, temperature in copper oxide based antiferromagnets
Authors: Ajay  , Patra S. , Tripathi R. S. 
Published: July 2007

AbstractThe role of interlayer coupling (J⟂ on the Néel temperature of (1–2–3) compounds is studied by taking the three‐dimensional anisotropic Heisenberg antiferromagnetic model. The two‐sublattice model of the antiferromagnet is considered. The double‐time Green function technique is employed and the higher‐order Green functions are decoupled using the Callen decoupling approximation. The Néel temperature (TN) for YBa2Cu3O6+x (1–2–3) compounds as a function of doping concentrations (x) is estimated. The intraplanar coupling strength (J‖) is treated as a constant throughout the numerical calculations. The theoretical results are compared with that of the random phase approximation (RPA) and existing experimental results.
Magnetic properties of thin films containing itinerant electrons interacting with localized spins. The problem of Interlayer exchange coupling
Authors: Urbaniak‐Kucharczyk A. 
Published: July 2007

AbstractThe model of indirect interaction between localized spins via conduction electrons in thin magnetic films is applied to the calculation of the interlayer exchange coupling parameter. The case of the spacer layer of f. c. c. structure is analyzed in detail. The oscillatory character of the exchange coupling parameter as a function of the spacer thickness as well as of the thickness of the magnetic layer is found for various orientations of the spacer.
Impurity‐related dynamical optical switching in CdS
Authors: Schmolke R. , Schöll E. , Nägele M. , Gutowski J. 
Published: July 2007

AbstractNonlinear optical transmission in unintentionally or lightly doped CdS crystals is studied theoretically and experimentally near the band gap. A spatially homogeneous three‐level model is developed which explains the observed optical switching process and its dependence upon incident intensity, wavelength, lattice temperature, and surface reflectivity. It is shown that the switching is due to the dynamics of the generation‐recombination processes involving donors, acceptors, and the conduction band, and not due to transmissive optical bistability.
Pressure Broadening and Motional Narrowing in Excitonic States of J‐Aggregates
Authors: Pschierer H. , Friedrich J. 
Published: July 2007

AbstractThe broadening of spectral holes burnt into the J‐band of molecular aggregates of the pseudoisocyanine‐type under isotropic pressure conditions is measured and compared with the respective behavior of holes burnt into the monomer absorption. From the ratio of the pressure broadening the coherence length of the exciton on the aggregate chain can be determined. 120 and 101 monomer units for the two dyes investigated are found.
Light‐Induced Structural Changes of J‐Aggregates of Organic Dyes at Low Temperature
Authors: Lindrum M. , Daehne S. 
Published: July 2007

AbstractNarrow‐band laser irradiation of the J‐aggregate of a tetrachlorobenzimidocarbocyanine dye at low temperature leads to drastic changes of the absorption profile. ΔOD spectra consist of broad features with both decrease (“hole”) and increase (“antihole”) of absorption where the latter appears exclusively blue‐shifted with respect to the hole. Further irradiation at shorter wavelengths shifts the absorption successively to the blue. The mechanism of the observed spectral changes is assumed to be due to specific structural changes in the J‐aggregate: aggregate states which absorb at longer wavelengths are unstable and can be transformed to more stable ones.
Ultrasonic Attenuation in Semiconductor Quantum Heterostructures under a High Magnetic Field
Authors: Rodrigues C. , Fonseca A. L. A. , Nunes O. A. C. 
Published: July 2007

AbstractA theory of ultrasonic attenuation in a quasi‐two‐dimensional electron gas in a quantum well semiconductor heterostructure in the presence of a quantizing magnetic field is presented. The effect of the high magnetic field on the ultrasonic attenuation coefficient is studied in the extreme quantum limit, when all the electrons are accomodated in the lowest Landau level and electro‐subband. Numerical examples are shown and discussed.
Nonradiative Capture of Electrons in Quantum Wells and Wires by Shallow Donor Impurities
Authors: Sirenko Y. M. , Mitin V. 
Published: July 2007

AbstractThe capture of electrons in a quantum well or wire by shallow donor impurities due to the interaction with an acoustic phonon is considered. Wave functions of several lowest bound states are found by the variational method. The rates of transitions between the free and bound states as well as between different bound states are calculated. In the case of well or wire width L less than or of the order of the effective Bohr radius and greater than the phonon wavelength, the transition rates are proportional to ϵ−5 (nϵ + 1/2 ± 1/2) L−6, where ϵ is the transferred energy, and nϵ a phonon occupation number.
Oxygen Content, Crystal Structure, and Superconductivity in YSr2Cu2.75Mo0.25O7+δ
Authors: Cao Guanghan , Qian Yitai , Wei Qingqiao , Chen Zuyao , Lu Bin , Mao Zhiqiang , Shi Lei , Zhang Yuheng 
Published: July 2007

AbstractPolycrystalline samples of YSr2Cu2.75Mo0.25O7+δ with different oxygen contents are investigated by powder X‐ray diffraction and selected area electron diffraction. The temperature dependence of resistivity and the oxygen content are determined. The air‐sintered sample is tetragonal and semiconductive, while the O2‐sintered sample is orthorhombic and superconducting at 26 K. It is suggested that the structural variation originates from the (Cu(1), Mo)O1+δ layers and the superconductivity is induced by the excess oxygen rather than charge redistribution between Cu(1) and Cu(2).
Tranverse Scattering Resonances in Quantum Wires
Authors: Wilkinson M. , Austin E. J. 
Published: July 2007

AbstractA theory for the effect of scattering by impurities in a quantum wire or electron waveguide is described. Scattering is strongly influenced by resonances which occur when a new mode of propagation appears as the Fermi energy is increased. These resonances can either increase or decrease the probability of backscattering by many orders of magnitude.
High Field Positive Magnetoresistance in the Variable Range Hopping Regime in n‐Type CuInSe2
Authors: Essaleh L. , Wasim S. M. , Galibert J. , Leotin J. 
Published: July 2007

AbstractElectrical conduction by the variable range hopping mechanism of Mott type is observed in n‐type CuInSe2 over a wide temperature range up to about 50 K. A detailed study of the field dependence up to 15 T of the localization length ξ and T0 is made at different temperatures. The increase of ξ with B at lower values is consistent with the magnetic field induced delocalization effect whereas at higher values it decreases due to the shrinkage of the wave functions of the impurity states. The variation of experimental T0 with B shows a similar trend as predicted by the theory but its magnitude is much smaller. This is attributed to the increase in the dielectric constant due to correlation effects not taken into account in the theoretical expression for the density of states.
The Role of an Applied Transverse Field in a Diluted Mixed Spin Ising System
Authors: Kaneyoshi T. 
Published: July 2007

AbstractThe phase diagrams and magnetization curves of a diluted mixed spin‐1/2 and spin‐1 Ising system on honeycomb lattice in a transverse field Ω are investigated by the use of the effective‐field theory with correlations. The influence of Ω on the magnetic properties is discussed and some interesting phenomena are found in them, such as the transverse field induced magnetic ordering in the system with a concentration at or very near the critical concentration where the system with Ω = 0 becomes paramagnetic.
Phase Diagram and Magnetization Moments of the Spin‐5/2 Diluted Ising Ferromagnet in a Transverse Field
Authors: Saber M. , Tucker J. W. 
Published: July 2007

AbstractThe phase diagram and the magnetization moments of the spin‐5/2 diluted Ising ferromagnet in a transverse field is studied by an effective field method within a single‐site cluster theory. The effective field equations are derived using a probability distribution obtained with the aid of generalized van der Waerden identities that account for the single‐site spin kinematic relations. Correlations between the site disorder and the local configurational‐dependent thermal averages of the spin variable are included. The dependence of the transition temperature for longitudinal ordering is studied both as a function of the transverse field strength and the site dilution. Selected results for the thermal dependence of the longitudinal and transverse components of the magnetization and their higher moments are also presented.
The Coherent Potential Approximation in the Description of Spin Wave Resonance
Authors: Urbaniak‐Kucharczyk A. 
Published: July 2007

AbstractThe coherent potential approximation method is applied to spin wave resonance in thin ferromagnetic films. The approach takes into account a natural disorder connected with different states of spin components. The resonance spectra with a finite line‐width are calculated for the b.c.c. (100) film and S = 1/2. The results obtained show that the half‐widths of the surface as well as the volume peaks depend on the surface anisotropy parameter.
Excitonic Transitions and Band Offset Calculations in a Short Period ZnS‐ZnSe Superlattice
Authors: Abounadi A. , Rajira A. , Averous M. , Calas J. 
Published: July 2007

AbstractAn investigation of the photoreflectance and photoluminescence spectra of a 45 period ZnS‐ZnSe strained layer superlattice having well and barrier widths of 3.3 and 3.7 nm, respectively, is reported. Heavy hole (e1hh1) and light hole (e1lh1) excitons are well resolved in the 2 K photoreflectance spectrum. The energies of these transitions are calculated within the envelope function framework, generalized to strained layers. The valence and conduction band offsets are the principal parameters in this calculation. Thus, two different approaches are set in competition to determine these quantities. The first is based on theoretical models while the second uses a “semi‐experimental” method. The latter method gives a free‐standing type‐I superlattice with optimal agreement between photoreflectance measurements and theory. Also the importance of a precise knowledge of the exciton binding energy (enhanced because of the reduced dimensionality) for an accurate band offset determination is shown. This binding energy is estimated for the heavy hole exciton from a variational calculation.
Cd+(5s1) Centres in Rb2ZnCl4 Single Crystals
Authors: Nistor S. V. , Bouwen A. , Schoemaker D. 
Published: July 2007

AbstractElectron paramagnetic resonance and optical absorption spectra attributed to 5s1 trapped electron Cd+(I) and Cd+(II) centres are observed over a broad temperature range in Rb2ZnCl4 single crystals doped with cadmium, after X‐ray irradiation at 80 K and room temperature, respectively. The low temperature Cd+(I) centre, which seems to be a substitutional Cd+ ion in a Zn2+ site, transforms into a Cd+(II) centre at 190 K by trapping an anion vacancy, which has become mobile at this temperature, in its nearest‐neighbour positions. The suggested structural models of the Cd+ centres are supported by the analysis of the spectral data considering covalency effects.
Equations of State and Pressure Dependence of Bulk Modulus for NaCl Crystals
Authors: Thomas L. M. , Shanker J. 
Published: July 2007

AbstractVarious equations of state reported in the literature are used to investigate the variation of the bulk modulus for NaCl crystals under the effect of pressure up to 300 × 108 Pa. The results are compared with the corresponding values obtained in the present study using interionic potentials as well as with the available experimental data. Values of bulk modulus as a function of pressure are also calculated by taking into account the variation of the Anderson parameter with pressure. The results thus obtained are found to present best agreement with those estimated from the universal equation of state.
X‐Ray Study of the System BP1−xBPIx
Authors: Santos M. Lopes Dos , Kiat J. M. , Almeida A. , Chaves M. R. , Klöpperpieper A. , Albers J. 
Published: July 2007

AbstractAccurate X‐ray diffraction measurements are carried out as function of temperature in antiferroelectric betaine phosphate (BP), ferroelectric betaine phosphite (BPI), and mixed compounds of BP1−xBPIx. In agreement with previous studies, the phases which derive from a high‐temperature phase with P21/c symmetry are clearly detected. However, in contrast with previous assignments, for 0.90 < x ≦ 1.0 a transition occurs to the P21 ferroelectric phase. For 0 ≦ x < 0.20, the two successive transitions are to a P21, phase and subsequently to a Pc phase with a simultaneous doubling of the unit cell. For 0.9 ≦ x < 1.0 the results confirm previous assignments of a single transition to a P21 phase. In the intermediate region 0.20 < x < 0.90 a clear evidence of an isomorphous transition to a glass phase is obtained, where the relative dominance of the ferroelectric or antiferroelectric interaction depends on the concentration values. Furthermore an orientational glass phase near x = 0.80 is identified. The existence of a strong anisotropy in all the studied elements of the family and of a weak coupling between the ordering of the dipoles and the lattice is evidenced. The role of the absent oxygen in the behaviour of the lattice parameters as well as its subsequent influence on the competitive interactions between dipoles are discussed.
Energy Transfer from Gd3+ to Er3+ in Y0.34Gd0.65Er0.01F3
Authors: Collins J. M. , Dibartolo B. 
Published: July 2007

AbstractThe energy transfer from the 6P7/2 level of Gd3+ to the 2P3/2 level of Er3+ is studied in the phosphor Y0.34Gd0.65Er0.01F3. The lifetime of the 6P7/2 level of Gd3+ decreases substantially in the presence of Er, indicating that the transfer is nonradiative. The transfer is studied in the temperature range from 20 to 300 K, and is found to be more efficient at low temperatures. A model is proposed to explain this behavior, in which we consider the distribution of the ions in the Stark levels involved in the transfer, and the presence of one‐ and two‐phonon processes.