bluuebunny/crossref_metadata_2025 huggingface dataset:
Effective‐field renormalization‐group approach for quenched site‐dilute ising models
Authors: Bobák A. , Jaščur M. , Mockovčiak S.
Published: July 2007

The quenched site‐dilute Ising model in two and three dimensions is studied by employing the effective‐field renormalization‐group method. The concentration dependence of the critical temperature and values of the critical concentration are obtained and compared to other different methods. Quite good results for a honeycomb lattice are achieved even by considering the smallest possible clusters. In particular, it is shown that the present formalism is able to distinguish between lattices of the same coordination number but of different dimensions.
Mercury Diffusion in Cadmium Telluride
Authors: Sadaiyandi K. , Karthikeyan P. , Ramachandran K.
Published: July 2007

Theoretical calculations are carried out for the slow diffusion of mercury in cadmium telluride. Single vacancy, single interstitial, and divacancy mechanisms are tried. The results suggest that the most possible mechanism for Hg diffusion in CdTe is the single vacancy mechanism.
Porous Silicon Photoluminescence Enhancement and Saturation Resulting from Thermal Annealing in Nitrogen Gas
Authors: Chen Zhiliang , Bosman Gijs
Published: July 2007

The results of thermal annealing at 800 and 900 °C of porous silicon samples in N2 ambient conditions are discussed as a function of annealing time, annealing procedure, and sample preparation. The results show that N2 annealing plays an important role in surface passivation and nanostructure feature size shrinkage and may result in photoluminescence (PL) intensity enhancement and saturation with annealing time under appropriate conditions. The experimental results strongly support the quantum confinement model for PL in porous silicon.
Bulk and Interface Polarons in Quantum Wires and Dots
Authors: Klimin S. N. , Pokatilov E. P. , Fomin V. M.
Published: July 2007

The vibrational modes of inertial polarization in the multilayer wire‐like and dot‐like structures are determined. The bulk confinement modes and the interface ones are separated exactly by a unitary transformation, various additional boundary conditions for the inertial polarization vector being taken into account. The Hamiltonian of the electron‐phonon interaction is deduced. The polaron ground state energy as well as the effective mass in the cylindrical quantum wire and polaronic shift of electron energy levels in the spherical quantum dot are calculated.
Electrodynamic Properties of an Edged High‐Temperature Superconducting Film
Authors: Saif A. G. , El‐Sabagh M. A.
Published: July 2007

The structure of the surface barrier and the effect of a transport current on the annihilation of this barrier are investigated for a high‐temperature type‐II superconducting film of arbitrary thickness. It is found that the surface barrier depends on the external field strength, the value of the transport current flowing through the sample, and the orientation of the external field. The results obtained are found in agreement with some previous ones.
Photoluminescence of Anodically Oxidized Porous Silicon
Authors: Filippov V. V. , Pershukevich P. P. , Bondarenko V. P. , Dorofeev A. M.
Published: July 2007

A photoluminescence (PL) study is carried out on porous silicon (PS) layers of 50 to 60% porosity which are anodically oxidized in ethylenglycol NH4NO3 electrolyte. Such oxidation is found to make the PL intensity stronger and the peak wavelength shorter. The emission of partially linearly polarized light is observed for PS samples under linearly polarized light excitation. Transmission IR and reflectance spectra are recorded to characterize porous silicon. As a result, a strong shift of the absorption band edge and high silicon oxide content are found in PS sampl. This shift is supposed to be due to the quantum confinement mechanism, but the features of PL speak in favour of the important role of oxide in the light emission from PS. In particular, the polarization of the emitted light can be due to the oxide‐induced elastic strains.
Aggregation of Lead Ions Doped in CsBr Crystal
Authors: Asada H.
Published: July 2007

The absorption spectrum of a CsBr : Pb2+ crystal shows remarkable isothermal changes. In the temperature range near 80 °C, the absorption constants of most of the component bands decay with time, while the 316 nm band grows. The change is caused by second‐order kinetics. Reaction velocities classify the bands into two groups, the defects responsible for the spectrum consist of two types of defects including I–V dipoles.
Electron–Phonon Interaction Influence on Electron and Phonon Excitations in Amorphous Metals
Authors: Vakarchuk I. O. , Myhal V. M. , Tkachuk V. M.
Published: July 2007

The influence of electron‐phonon interaction on electron and phonon excitations in amorphous metals is examined. The equation for the gap in the electron spectrum for the transition to the superconducting state is obtained. The renormalization of the energy spectrum and damping of electron and phonon excitations due to electron‐phonon interaction is considered.
Phonons in Ni31Dy69 Glass
Authors: Pratap A. , Bhandari D. , Saxena N. S. , Saksena M. P.
Published: July 2007

The effective interatomic potential in Ni31Dy69 glass is obtained with the experimental structure factor S(Q). This potential is compared with those computed using Percus‐Yevick (PY) and hypernetted chain (HNC) approximations. A theory of phonons in amorphous solids is employed to compute the eigenfrequencies of the longitudinal and transverse phonon modes with these diffcrcnt potentials and the results are compared with each other. The elastic and thermodynamic properties of this glass are also studied using the longitudinal and transverse sound velocities estimated from the slope of the long‐wavelength limit of the dispersion relations.
Correlation between the Ionicity Character and the Charge Density in Semiconductors
Authors: Zaoui A. , Ferhat M. , Khelifa B. , Dufour J. P. , Aourag H.
Published: July 2007

Using the electronic charge densities obtained by the empirical pseudopotential method for tetrahedrally bonded semiconductors, an ionicity scale is established that is in good agreement with the Phillips ionicity scale.
Theoretical X‐ray Emission Study of High‐ Tc Superconductor YBa2Cu3O7 Thin Films
Authors: Kurganskii S. I. , Kharchenko M. A. , Dubrovskii O. I. , Bugakov A. M. , Domashevskaya E. P.
Published: July 2007

The energy spectrum of YBa2Cu3O7 thin films is investigated by the film linearized augmented plane wave method (LAPW). On the basis of the results of the energy band structure calculation, theoretical calculations of X‐ray emission spectra are performed. The calculated spectra are compared with the results of other calculations and experimental researches. The reason of the observed systematic discrepancies in the fine structure is analysed. The obtained spectra confirm the resonance character of the interaction of copper d‐states with the nearest oxygen p‐states.
Study of the Critical Behaviour for the S = 5/2 Transverse Ising Model in a Random Field
Authors: Song W. J. , Yang C. Z.
Published: July 2007

With the mean‐field renormalization group theory (MFRG) and the discretized path‐integral representation (DPIR), a spin S = 5/2 transverse Ising model in a random field (RFTIM) is studied. The phase diagram and the other critical properties are investigated.
Experimental and Theoretical Investigation of Spin‐Hamiltonian Parameters for the Low Symmetry Fe3+ Centre in LiNbO3
Authors: Yeom T. H. , Chang Y. M. , Choh S. H. , Rudowicz C.
Published: July 2007

X‐band EPR spectra of Fe3+ in LiNbO3 single crystals at room temperature are analyzed with a spin‐Hamiltonian of C3 symmetry. First the zero‐field splitting (ZFS) parameters b, b, b, and b−34 are calculated using the superposition model (SPM). The SPM‐ZFS results for Fe3+ at the Nb site agree better with the experimental ones than those for Fe3+ at the Li site. Next the crystal field (CF) parameters are predicted using SPM and two sets of available structural data for LiNbO3. Diagonalization of the full Hamiltonian, including the CF Hamiltonian, within the whole 3d5 configuration yields the ZF splittings for Fe3+ centres. The ZF splittings obtained from the theoretical SPM‐CF calculations for Fe3+ at the Nb site agree better with the experimental EPR data than those for Fe3+ at the Li site. The present results indicate that the observed EPR spectra of Fe3+ : LiNbO3 can be attributed to Fe3+ ions at the Nb sites.
Quantum Boltzmann Transport Equation in the Presence of Interactions; Application to the 2D Hall Effect
Authors: van Vliet C. M. , Samson Th.
Published: July 2007

The quantum Boltzmann equation (QBE), derived in previous papers, is extended to systems with strong interparticle interactions. In particular, the non‐diagonal hierarchy of kinetic equations for [ℒ︁hin]N ccζ, N = 0, 1,…, ∞, where ℒ︁in is the interaction Liouvillian, is solved by iteration in order to obtain a generalized non‐diagonal QBE. The application to the 2D quantum Hall effect, integral and fractional, is discussed.
Strong Ultraviolet and Visible Radiative Channels of Nd3+ ‐Doped Insulating Fluoride Crystals. A New Laser Potentiality
Authors: Kaminskii A. A. , Mironov V. S. , Bagaev S. N. , Boulon G. , Djeu N.
Published: July 2007

For three Nd3+ ‐doped anisotropic LiKYF5, LiYF4, and KYF4 laser crystals, the main intensity characteristics of their intermanifold radiative transitions from the 2F(2)5/2, 2H(l)9/2, 4D3/2. 2P3/2, and 4F3/2 states are calculated and analyzed. Several strong ultraviolet and visible channels promising stimulated emission excitation that can considerably enhance the generating potentiality of those fluorides, are revealed. The analysis performed permits also to establish general regularities of the luminescence density distribution among intermanifold channels between high‐lying emitting states of Nd3+ ions both in the investigated fluorides and other crystals.
Analysis of Exciton Absorption Peak Broadening Contributions in InGaAs/InP Multi Quantum Wells
Authors: Arena C. , Rotelli B. , Tarricone L.
Published: July 2007

The excitonic absorption of InGaAs/InP multi quantum wells prepared by chemical beam epitaxy with different growth conditions and structure parameters are studied in the temperature range 8 to 300 K and in the spectral region where the heavy hole to electron (hh–e) transition corresponding to n = 1 arises. In particular, a lineshape modelling is attempted using a Gaussian function. The temperature variation of the full width at half maximum (FWHM) is discussed in terms of an intrinsic contribution Γ0 plus a thermal broadening due to the interaction with longitudinal optical (LO) phonons. The Γ0 value may be considered as a helpful parameter to investigate the structural quality of the samples studied, leading to the observation that absorption spectroscopy is a simple and powerful characterization technique of MQW.
The Effect of an Elliptic Hole on the Elastic Interaction between Screw Dislocations and a Macrocrack
Authors: Shiue Sham‐Tsong
Published: July 2007

The effect of an elliptic hole on the elastic interaction between a screw dislocation and a macrocrack is investigated by the conformal mapping technique. The stress field, the stress intensity factor at the crack tip, and the force on the dislocation are derived. Both the stress intensity factor at the crack tip and the force on the dislocation are strongly influenced by the elliptic hole, and are dependent on the positions of the lattice screw dislocation, the dislocations in the elliptic hole, and the geometry of the elliptic hole. If the lattice dislocation arises from the macrocrack or infinity, the lattice dislocation always has a shielding effect on the propagation of the crack. It is also found that an elliptic hole with a smaller minor semi‐axis b will induce a larger magnitude of the negative stress intensity factor at the crack tip, so it has a larger shielding effect on the crack propagation. Finally, several special cases of the results are also discussed.
Infrared Absorption in Anharmonic Crystals with Impurities: An Application to a Diatomic Linear KI‐Chain
Authors: Held Th. , Kuhn W. , Pfeiffer I.
Published: July 2007

In expansion of well‐known results of dynamic and optical properties of anharmonic ideal crystals, a perturbation theory, using Matsubara functions and diagrammatic treatment is carried out to describe anharmonic crystals with impurities. Taking explicity into account anharmonic contributions of the potential expansion up to fourth order, mass changes as well as force constant changes between impurity and host‐lattice particles, it is shown that the anharmonic phonon propagator can be written as a sum of terms, which are added up within a Dyson equation. The corresponding proper self‐energy includes impurity‐activated harmonic as well as impurity‐influenced anharmonic contributions. As an application the infrared absorption of a diatomic linear KI‐chain with next‐neighbour interaction, a single mass defect, and consideration of anharmonicities up to second order of the van Hove parameter (O(α2)) is chosen. Numerical calculations of temperature‐dependent lineshapes and shifts of defect and main absorption lines are presented and discussed together with results of defect‐induced in‐band absorption.
Analysis of Free Exciton Properties in GaAs Epitaxial Layers with the Improved Model of Photoconductivity Spectra
Authors: Wu Fencmei , Shi Yi , Parenteau M. , Jorio A. , Carlone C.
Published: July 2007

The transition energy, binding energy, intensity, broadening, and lifetime of the free‐exciton transitions in GaAs epitaxial layers are well investigated experimentally and theoretically. The improved model of photoconductivity spectra which accounts for the contribution of many excitonic levels and two broadening mechanisms is presented. Moreover, the variation of the free‐exciton properties with increasing epitaxial layer thickness is discussed.
Temperature Dependence of Exchange Narrowed ESR Linewidth of (MT)2(TCNQ)3·2 H2O Salt
Authors: Koshta A. A. , Graja A.
Published: July 2007

The electron spin resonance g‐factor, linewidth, and spin susceptibility of single crystals of the (MT)2(TCNQ)3·2 H2O salt are presented. A discussion of the ESR parameters is given. It is shown that the exchange narrowing mechanism determines the lineshape as well as its temperature and orientation dependencies.
Deflection of Laser Light by an Acoustic Wave in a Magnetised Semiconductor with High Dielectric Constant
Authors: Pragya D. , Ghosh S.
Published: July 2007

An analytical investigation is made for the scattering of laser light by an acoustic wave propagating obliquely to the externally applied magnetic field in a doped solid state plasma with high dielectric constant, consequent amplification of scattered modes is also carried out. The origin of this nonlinear interaction lies in the second‐order optical susceptibility arising due to the nonlinear current density in the medium. The influence of carrier density is also considered. The threshold value of the pump electric field is obtained. It is found that the threshold pump amplitude required for the onset of instability may be reduced by increasing the doping level and decreasing the obliquity of the magnetic field to the propagation direction. The steady state growth rate is also obtained. The gain constant increases explosively with a little increment in cyclotron frequency after it touches its minimum at ωc≈1013 s−1. The transmitted intensity of the scattered mode and the cell efficiency have also been decuded when the cell length is quite large compared to the pump wavelength. The transmitted intensity is found to have a maximum value when the magnetic field is applied at an angle of 60° to the photon propagation direction.
Plasmons in CaSO4
Authors: Yaskolko V. Ya.
Published: July 2007

With the help of optical loss spectra (OLS), energy loss spectra (ELS), and the dependence of stimulated electron emission on the energy of slow exciting electrons (ESEL), the true plasmon energies in CaSO4 (13.5, 18.1,20.9 eV) are obtained and the energy gap is confirmed (12 eV). The subzones in the valence band where plasma oscillations occur are determined, as well as the number of electrons in one plasmon. The band structure for CaSO4 is constructed and interband transitions are interpreted. From the data of CaSO4 the plasmon energies, the energy gaps, etc. for the Sr, Ba, Pb sulphates are calculated.
Reference System Approach in the Electron Liquid Theory: V. Application to the Two‐Component Fermi Systems
Authors: Vavrukh M. , Blazhievskii O.
Published: July 2007

The reference system approach developed in the electron liquid theory is generalized for the case of the two‐component Fermi system model with a long‐range interaction between particles. The degenerate subsystems of non‐interacting particles are considered as a reference system. The general relations are shown for a simple model, i.e. degenerate electroneutral two‐component Fermi system model of charged particles. Some energetic and structural characteristics of this strongly non‐ideal model in the metal phase are obtained.
Novel Slab Model Analysis for Overlayer Deposited Bent Step‐Index Single‐Mode Fiber Half‐Couplers
Authors: Haubenreisser W. , Ecke W. , Schröter S. , Schwotzer G. , Wjllsch R.
Published: July 2007

A Novel Slab Model (NSM) for the modal analysis of Overlayer (OL) deposited Bent Step‐Index Single‐Mode Fiber (BSSF) half‐couplers is presented. In this NSM the BSSF is replaced by a modal equivalent slab waveguide (ESW) structure. It is shown that within the framework of the NSM the modal features of actual OL‐BSSF structures can be described more correctly and consistently on the basis ofequivalent OL‐ESW structures. A perturbation method is developed to analyze modal properties of such OL‐ESW structures analytically. Our modal analysis comprises all hitherto investigated evanescent fiber‐optic devices based on OL‐BSSF structures.
On the Theory of Reflectivity by an X‐Ray Multilayer Mirror
Authors: Kohn V. G.
Published: July 2007

A general theory of reflectivity by a perfect X‐ray multilayer mirror based on the usage of a recurrent relation is developed. The reflectivity is described by the well‐known Fresnel formulae which take into account jumps of susceptibility at the boundary between the layers. The analytical solution in the frame of the most general dynamical approach is found and the condition of application of the kinematical approximation is discussed. Special attention is paid to the periodic multilayer structure containing two layers of different materials with more than one pair of these layers in the period.
Angle‐Resolved Photoemission Investigation of the Valence Bands of RuCl3
Authors: Carricaburu B. , Mamy R. , Pollini I.
Published: July 2007

The electronic structure of the layered compound RuCl3 is studied by means of X‐ray photoelectron spectroscopy and angle‐resolved ultraviolet photoelectron spectroscopy. The energy distribution curves are measured along different azimuthal directions ΓM, ΓM′, and ΓK in the Brillouin zone. The experimental valence bands of RuCl3 consisting of an uppermost non‐dispersive band of d symmetry at about 2 eV above the top of the valence p band, and two dispersive bands of p symmetry are studied by application of the E(k‖) method. The nature of the valence bands of RuCl3, are discussed in relation to the electronic structure of the metal RuO2 owing to their similar X‐ray photoelectron spectra.
On the Analytical FMR Theory in the Normal Configuration
Authors: Fraitová D.
Published: July 2007

The analytical expressions for dispersion relations, cross‐over effect conditions, effective penetration depth, surface impedance, and inflection linewidth for FMR in bulk metals in normal configuration are derived including electrical conductivity and the Landau‐Lifshits damping parameter (for the case of zero surface pinning). The possibility of excitation of Larmor and anti‐Larmor Kittel and surface modes in materials with negative surface anisotropy is investigated.
57Fe Mössbauer Spectra Study on Eu0.5RE0.5Fe0.5Mn0.5O3 Oxides
Authors: Li Liping , Wei Quan , Ren Xiaolin , Li Wancheng , Su Wenhui
Published: July 2007

The method of solid state reactions is used to synthesize the samples of Eu0,5RE0,5Fe0,5Mn0,5O3 (RE = La, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Y). XRD and 57Fe Mössbauer spectra are measured. It is found that the volume of the unit cell of the samples decreases with increasing atomic number of RE. The Fe ions are in the Fe3+ high spin state, and the relationship between the quadrupole splittings and the lattice distortion parameter D of the samples is linear.
Optical Oscillations and Their Peculiarities Caused by Internal Switching Delay in Optically Bistable CdS Crystals
Authors: Kažukauskas V. , Issler H. , Grohs J. , Klingshirn C.
Published: July 2007

Light oscillations due to internal switching delay in optically bistable CdS platelets are reported using a geometry with two exciting beams. It is shown that in the case when one of the exciting beams has the form of a long thin light strip, an internal delay occurs due to the final propagation speed of the bistable switching front along it (switching wave) which enables one to obtain oscillations using electrooptical feedback without external delay. If an external delay is applied to one of the beams, while the second one appears as a single light pulse peculiarities of the oscillations caused by the interaction of both delay signals are obtained, which are not observed if only one light beam is used.
Optical Properties of Undoped and V‐Doped VA−VIA−VIIA Single Crystals
Authors: Park Sang‐An , Kim Mi‐Yang , Lim Ji‐Young , Park Byong‐Seo , Koh Jeong‐Dae , Kim Wha‐Tek
Published: July 2007

SbSI:V, SbSeI:V, BiSI:V, and BiSeI:V single crystals are grown by the vertical Bridgman technique using ingots synthesized from mixtures of the elements (purity 99.9999%). The grown single crystals have an orthorhombic structure and an indirect energy band structure. The temperature dependence of the optical energy gap is well fitted by the Varshni equation. Impurity optical absorption peaks due to the doped vanadium appear and are assigned to the electron transitions between the energy levels of the V2+ ion located at Td sites of the host lattice.
Blue‐Green ZnSe Laser Diodes for Optoelectronics. Present State at Würzburg University
Authors: Landwehr G. , Hommel D.
Published: July 2007

A review is given on the research activities at Würzburg University in the field of blue‐green emitting ZnSe laser diodes. Of great importance for both the structural and electrical properties is the possibility to grow a GaAs buffer layer on the GaAs substrate prior to the II–VI growth. Pulsed room temperature lasing with a duty cycle up to 17% is obtained using quaternary ZnMgSSe cladding layers, ternary ZnSSe waveguides, and a single CdZnSe quantum well. Important issues for long‐term stability of such laser diodes like growth start, p‐type doping, and ohmic contacts are discussed.
Growth Kinetics in MOMBE‐ALE of ZnSe and CdSe as Determined by a New In‐Situ Optical Probing Method
Authors: Yoshikawa A. , Kobayashi M. , Tokita S.
Published: July 2007

The surface reaction mechanism in MOMBE‐ALE of ZnSe and CdSe using DMZn, DMCd, and H2Se as reactants is studied with a newly developed optical in‐situ monitoring method. The new method is named surface photo‐interference (SPI), because the concept of the SPI is essentially concerned with a photo‐interference in the heteroepilayer. Fairly low‐energy photons for which the epilayer is transparent can be used as probing light in SPI. Reflecting this feature, SPI is especially useful for studying heteroepitaxial growth kinetics of wide‐gap II‐VI compounds, because the effect of photocatalytic growth rate enhancement can be avoided when using such low energy photons as probing light. It is shown that there is a quite significant difference in the surface reaction mechanism between the MOMBE‐ALE of ZnSe and CdSe. That is, in the ALE of ZnSe when none of the source gases are per‐cracked, the reaction to form ZnSe can take place when the group‐II source (i.e., DMZn) is supplied, resulting in an alternate appearance of Zn terminated and H2Se terminated surfaces during growth. On the other hand, the reaction to form CdSe can do when the group‐VI source (i.e., H2Se) is supplied, resulting in an alternate appearance of Se terminated and DMCd terminated surfaces during growth.
Raman Scattering by Acceptors and Donors in p‐Type ZnSe
Authors: Davies J. J. , Wolverson D. , Boyce P. J.
Published: July 2007

Raman spectroscopy is used to study dopants introduced into ZnSe to produce p‐type material. Special attention is paid to nitrogen‐doped epitaxial layers and Raman scattering involving the electronic levels of nitrogen acceptors is reported. In these layers spin‐flip scattering by electrons at a new type of donor centre at a depth of about 50 meV is also observed. This new donor is believed to play an important role in the compensation process that limits the net acceptor concentrations attainable in this material.
Optimization of Ohmic Contacts to ZnSe by Parabolic Grading of ZnSe1−xTex Composition
Authors: Yang G. L. , Luo H. , Lewandowski L. , Furdyna J. K.
Published: July 2007

A new type of grading, namely, parabolic grading, for the purpose of improving ohmic contacts to p‐type ZnSe is presented. The valence band profiles as well as the critical grading length for a 1kT barrier are calculated for a parabolically graded Zn(Se, Te) structure. Comparison of our results with those of linearly graded structures shows the clear advantage of parabolic grading.
Effective Mass Theory for Strained Superlattices
Authors: Brezini A. , Hamdache F. , Behilil F. , Depollier C.
Published: July 2007

An exact model calculation for the band structure based on the one‐dimensional Kronig‐Penney model including the local potential induced by the monolayers, is developed and compared to the conventional one. Although it is formulated for strained superlattices, it may be generalized to any abrupt strained heterostructures.
A New Approximation for Block Diagonalizing the Eight‐Band k · p Hamiltonian
Authors: Enders P.
Published: July 2007

Kane's (1957) coordinate transformation for diagonalizing the eight‐bank k · p Hamilton matrix for GaAs‐like semiconductors into 4 × 4 blocks is reexamined, where the k2−terms (free electron and interaction with the remote bands) are taken into account fully, not only perturbatively. Then, the warping precludes an exact block diagonalization. A new approximation is proposed which allows for a 4 × 4 block diagonalization, again. Since this approximation preserves the cubic symmetry and its error vanishes along the symmetry directions (k ‖ [100], [110], [111], etc.), it remains small for general k‐directions. This is illustrated by numerical examples. The 4 × 4 block diagonalization allows for significant simplifications in calculating the band structure and related properties. In contrast to other approaches, a consistent calculation of the momentum matrix remains possible.
Hot Electron High‐Frequency Mobility in Wide‐ and Narrow‐Gap Semiconductors
Authors: Weng X. M. , Lei X. L.
Published: July 2007

Hot electron High‐frequency mobility in wide‐ and narrow‐gap semiconductors under the influence of a small ac electric field superimposed on a dc bias electric field using the balance equation theory recently developed for an arbitrary energy band is investigated. In the wide‐gap semiconductor SiC the frequency dependent mobility predicted by the balance equation approach is not in quantitative agreement with a previous calculation based on the Boltzmann equation with a displaced Maxwellian distribution function. In narrow‐gap semiconductors it is found that nonparabolicity strongly affects the frequency dependence of the hot‐electron small signal mobility, such that the curves of the real and imaginary parts of the ac frequency in a strongly nonparabolic system (InAs) appear completely different from those in a weakly or moderately nonparabolic system (GaAs).
Energy Loss Rate of Hot Electrons in n‐HgxCd1−xTe at High Temperatures in the Presence of a Quantizing Magnetic Field
Authors: Bhaumik S. , Sarkar C. K.
Published: July 2007

The energy loss rate of hot electrons due to longitudinal polar optical phonons in narrow‐gap semiconductors is calculated for the longitudinal configuration with classical and quantum screening for various lattice temperatures. The results are used to examine the influence of quantum screening on the energy loss rate. A relative comparison of the effects of quantum and classical screening on the energy loss rate due to optical phonons is made. The energy loss rate without screening is compared with the energy loss rate with classical and quantum screening.
Effective Magnetic Moments of Co and Fe Clusters
Authors: Mahendran M. , Iyakutti K.
Published: July 2007

The effective magnetic moment per atom, μeff, is calculated for ferromagnetic clusters of cobalt and iron in the mean‐field approximation. μeff is also computed by assuming that the clusters undergo superparamagnetic relaxation. The surface effects are included properly. The calculated μeff are in agreement with recent experimental results. The characteristic features of this study are: 1. μeff of these clusters are far below the bulk values of Co and Fe and 2. the exchange interaction and the internal magnetic field attain saturation as the cluster size increases.
Experimental verification of the new model for the two‐photon absorption of biexcitons in CuCl
Authors: Hasuo M. , Kawano H. , Nagasawa N.
Published: July 2007

Two different theoretical models of the giant two‐photon absorption due to excitonic molecules (xx) are examined experimentally. The conventional model (Hanamura, 1973) follows the sequence: photon + photon → exciton + photon → xx, while the self‐consistent model (Ivanov and Haug, 1993) gives the other scenario of the two‐photon absorption: photon + photon → exciton + exciton → xx. Our high‐precision measurement of the frequency dependence of the two‐photon xx absorption coefficient in CuCl allows us to discriminate these two models and supports the new self‐consistent model.
Photoinduced Fermi‐edge singularity in one‐dimensional semiconductors
Authors: Ogawa T.
Published: July 2007

A high‐density electron–hole system in one dimension (1D) is studied theoretically in terms of the photoinduced Fermi‐edge singularity (FES) in optical spectra, the anomaly of which results from the fluctuating exciton Bose condensation in 1D. Describing this system as the two‐band Tomonaga Luttinger (TL) liquid with the forward‐scattering correlations, the critical exponent of the FES is analytically obtained with the aid of the bosonization technique. Competition between the exciton condensation and other Fermi‐surface instabilities is also clarified within the framework of the two‐band g‐ology. Comparison with the FES in ordinary 1D conductors (heavily‐doped 1D semiconductors), which are described as the single‐band TL liquids, is made to clarify recoil effects of an optical hole on the FES. Effects of backward scatterings, random potentials, spin‐dependent electronic correlations, and spin splitting of the conduction band are also discussed.
Theory of plasmon‐assisted electron pairing in semiconductors
Authors: Blank R. , Haug H.
Published: July 2007

It is shown that plasmon‐assisted superconductivity can occur in an electron plasma of a modulationdoped quantum well structure or in an optically excited, cool quasi‐equilibrium electron–hole plasma in stressed quantum wells. For our analysis the quasi‐classical approximation (QCA) for non‐equilibrium Keldysh‐Green functions for carriers of a degenerate plasma is used. The screening is described in RPA, simplified further for some of the calculations by a damped plasmon‐pole approximation (PPA). Because the plasmon coupling is strong the appropriately modified Eliashberg equations are derived and analyzed for the complex frequency‐dependent normal and anomalous self‐energies. The plasmon frequency in a semiconductor plasma with a typical density of 1018 cm−3 is of the order of some 10 meV. If the spectral density of the plasmon is not too broad, a pairing instability is found also in the range of interparticle distances with rs < 1, where QCA and RPA are valid. Depending on the material, transition plasma temperatures of up to 30 K are obtained which can be enhanced further by quantum confinement and special properties of the band structure. The underlying virtual exchange of plasmons can only be obtained in a fully dynamical description, and is absent in the commonly used quasi‐static quasi‐particle approximations.
Excitonic nonlinearities and gain processes in binary and ternary ZnSe‐based quantum well structures
Authors: Lowisch M. , Kreller F. , Puls J. , Henneberger F.
Published: July 2007

The nonlinear optical properties of MQW structures with binary (ZnSe/ZnMnSe) and ternary (ZnCdSe/ZnSe) wells grown by MBE are compared. Pump‐probe experiments and high‐excitation PL measurements are carried out to study the excitonic nonlinearity as well as gain processes. It is found that the nonlinear optical response is qualitatively different in these two structures as a result of the different kinds of disorder present in the QWs.
Controlled spontaneous emission from semiconductor microcavities
Authors: Yamanishi M.
Published: July 2007

The advantages of the simultaneous quantum manipulation of electron and photon systems due to quantum‐confined Stark effect and cavity quantum electrodynamics, respectively, in semiconductor microstructures are clarified, manifesting systematic experimental results on high transfer efficiencies of spontaneous emissions and cavity‐induced fractional changes of exciton decay times around on‐axis resonance. The scheme for the simultaneous quantum manipulation is also expected to lead to attractive device concepts for highly efficient, high‐speed spontaneous light emitters, and sub‐Poissonian photon generators in a short time interval.
Saturation and losses in an optical semiconductor quantum well amplifier
Authors: Butty J. , Bongiovanni G. , Staehli J. L.
Published: July 2007

The results obtained from luminescence measurements made on one‐dimensional optical amplifiers
are reviewed. The investigated samples are GaAs/(Ga, AI)As waveguiding structures containing a
multiple quantum well structure as active element. The experimental data are compared with the
results of a numerical self‐consistent model of the optical amplifier. The observed spatial dependencies
of carrier and luminous densities are found to agree in a semi‐quantitative way with the theoretical
predictions. Saturation of optical amplification is caused by carrier depopulation through stimulated
recombination, and by the loss of light caused by scattering at sample defects and/or imperfect
waveguiding. It is argued that the discrepancies found on the high‐energy side of the grain spectra are
caused by reabsorption of amplified light at the edges of the amplifier.
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

The 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

Nanometer‐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

The 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

The 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

The 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

The 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

Investigations 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

The 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

The 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

Two 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

The 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.
Carrier drag by surfing in a coherent sound wave in solids. GME approach
Authors: Čápek V.
Published: July 2007

From the time‐dependent projection formalism leading to generalized master equations (GME) it is shown that a sufficiently intense quantum coherent phonon (sound) wave can drag carriers in a way reminding sea‐foam‐drag or surfing on sea waves.
Distorted flux lines in a uniaxial anisotropic superconducting spherical shell
Authors: Saif A. G. , El‐Sabagh M. A.
Published: July 2007

A 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

The 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

The 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

Nonlinear 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

The 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

Narrow‐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.