bluuebunny/crossref_metadata_2025 huggingface dataset:
Quantum Mechanical Properties of a Thin Cylinder. II. Magnetic Susceptibility and Electronic Specific Heat
Authors: Makar M. N. , Ahmed M. A.
Published: July 2007

The magnetic susceptibility, entropy, and electronic specific heat are calculated in the case of a hollow metallic cylinder placed in a magnetic field, both for weak and intermediate fields. Results are also presented for low and high temperatures. The obtained results show that the shell may acquire paramagnetic properties for some values of the field and diamagnetic properties for some other ones. However, this effect decays with increasing temperature and wall thickness. The electronic specific heat contributes strongly to the shell specific heat round a critical temperature while it becomes negligible near another temperature at which the specific heat due to lattice vibrations becomes dominant.
Static and Dynamic Behaviour of the Crystal Water Molecules in the Normal Phase of Deuterated Betaine Calciumchloride Dihydrate
Authors: Hacker U. , Holzer K.‐P. , Michel D. , Petersson J.
Published: July 2007

The static and dynamic behaviour of the deuterated crystal water molecules in the normal (N) phase of betaine calciumchloride dihydrate (BCCD) single crystals is investigated by means of quadrupolar perturbed 2H NMR. The temperature dependence both of the 2H NMR spectra and of the 2H spin‐lattice relaxation time 7i can be interpreted consistently by a thermally activated exchange process of the deuterons within the D2O molecule. At 170 K, under the condition of slow exchange, the electric field gradient (EFG) tensors of both deuterium sites are determined. From their principal axes and the quadrupole coupling constants information is derived about the structure of the C1D2O unit in BCCD in comparison with previous X‐ray studies.
Phase Sequence in Strongly Deuterated Betaine Arsenate
Authors: Almeida A. , Carvalho P. Simeão , Chaves M. R. , Klöpperpieper A. , Albers J.
Published: July 2007

Betaine arsenate (BA) exhibits a structural phase transition at ≈ 119 K and becomes ferroelectric below this temperature. The deuteration changes drastically its properties and an antiferroelectric phase emerges between the paraelectric and the low temperature ferroelectric phase, for deuterium content higher than 77%. A previous work explained the transition sequence of BA and deuterated BA (DBA) on the basis of competitive interactions. A detailed experimental study of the pyroelectric and dielectric properties of DBA at zero field and under dc electric fields is reported. This study, carried out in DBA with deuterium content of 85%, reveals a new polar phase. The phase diagram shows the existence of a field induced ferroelectric phase and the experimental data obtained provide evidence for the role played by competitive interactions in DBA.
Luminescence Study of Na6MnCl8 and Na6MnBr8 Microcrystals in NaCl and NaBr Lattices
Authors: de Lucas M. C. Marco , Rodriguez F. , Moreno M.
Published: July 2007

A detailed study of the luminescence and excitation spectra as well as of the lifetime (τ) on Na6MnX8 (X = Cl, Br) precipitates in the 300 to 2 K temperature range is made. In spite of the large distance between nearest Mn2+ ions (0.797 nm for Na6MnCl8), it is demonstrated that even at 2 K luminescence takes place at excitation traps after an exchange induced transfer between close Mn2+ ions. At variance to compounds like MnF2, only one trap (with depth of 350 and 480 cm−1 for X = Cl and Br, respectively) together with killers is mainly responsible for the observed temperature dependence of the luminescence. Aside from the inhomogeneous broadening of the emission band, the extrinsic character of the luminescence is underlined by the experimental Stokes shift observed at 10 K. The analysis of the bandwidth and the vibrational progressions on the 4T1g(G) band is perfectly explained in terms of coupling to the a1g mode. The increase of the Huang‐Rhys factor on passing from Na6MnCl8, S(a1g) = 5, to Na6MnBr8, S(a1g) = 8, is reasonably explained through a microscopic model. It is demonstrated that the lifetime‐temperature dependence is mainly governed by the overlap between the absorption and emission bands. This overlap is estimated subtracting the odd‐phonon contribution to the Stokes shift since phonon assistance is not required for excitation transfer through superexchange. The existence of Mn2+ −Mn2+ transfer down to 2 K is related to the perfect cubic symmetry of these compounds, and to a small coupling to the Jahn‐Teller eg mode. The different thermal shifts experienced by the luminescence and the excitation 4T1g(G) band are also discussed.
Lattice Dynamics of Chlorotrifluoromethane
Authors: Zeng W. Y. , Anderson A.
Published: July 2007

A simple force constant model is applied to the vibrations of crystalline CClF3 at low temperatures. The model uses twelve adjustable parameters (six intermolecular and six intramolecular) to calculate the frequencies of nine lattice modes and eighteen components of internal modes. Good agreement with recent Raman and infrared spectra is obtained. Principal intermolecular interactions, between various pairs of halogen atoms, are identified. Intramolecular force constants are only slightly changed from their gas phase values.
Core and Valence Thermal Vibrations in Diamond, Silicon, and Germanium
Authors: Saravanan R. , Balamurugan P. , Mohanlal S. K.
Published: July 2007

An analysis is made using published X‐ray data, for the thermal vibrations in diamond, silicon, and germanium. The overall thermal vibration is split into core and valence contributions, because valence electrons can oscillate with different phase and amplitude than core electrons due to bond polarizability. Using indigenously developed computer programs the published data are analyzed. In the first phase, the overall Debye‐Waller factor of diamond, Si, and Ge together with scaling and extinction factors are refined using the method of least squares. In the second phase, the core and valence contributions of the harmonic temperature factor are evaluated. Finally, in the third phase, the core and valence contributions of the anharmonic temperature factor are evaluated. The error in the fitting procedure is less than 1% for Si and Ge and about 3% for diamond.
Surface Polaritons at the Interface of Two Anisotropie Crystals
Authors: Beletskii N. N. , Brukva A. N. , Nikolaeva T. N. , Cherkashin V. P.
Published: July 2007

The paper presents the results of theoretical and experimental investigations of phonon polaritons at the free surface of the uniaxial single crystal Al2O3 and for the three‐layer structure “vacuum‐uniaxial ZnO crystal‐uniaxial Al2O3”. The optical axes of the crystals are assumed to be normal to the interfaces of the media. The conditions of the existence of surface polaritons of different types are determined. It is shown that with the help of the attenuated total reflection technique (ATR) these polaritons could be used for the contactless determination of the film thickness as well as for nondamaging control of the main parameters of uniaxial crystals.
Determination of the True Metal‐Ligand Bond Lengths from the Optical Spectrum for Transition‐Metal Ions in Crystals
Authors: Zhao M. G.
Published: July 2007

The dependence of the optical spectrum of the Mn2 + −6F− cluster in crystals upon the true metal‐ligand (M‐L) bond length is studied by using a new refinement technique for the determination of the structure. Calculations show that the present theory gives a unified explanation for the d‐d transition spectra, the paramagnetic properties, and thermal expansion of Rbmnf3 and Kmgf3:Mn2 +.
Pancake Vortex Pinning by Defects in Strongly Anisotropic High‐Temperature Superconductors
Authors: Boiko J. , Majewski P. , Aldinger F.
Published: July 2007

Defects of crystalline structures are potential pinning centres of the magnetic vortex dots (“pancake vortices”) within strongly anisotropic high‐Tc superconductors. The peculiarity of the interaction between the pancake vortex and various defects is analyzed and the values of the elementary pinning potential, Up, and that of the critical current density, Jc, are estimated.
Two‐Photon Absorption in an Indirect‐Gap Semiconductor Quantum Well System. I. Interband Transitions
Authors: Hassan A. R.
Published: July 2007

Phonon‐assisted two‐photon interband transitions in an indirect‐band‐gap semiconductor layered quantum well (QW) and quantum well wire (QWW) are theoretically studied. The spectral dependence of the two‐photon absorption (TPA) coefficients α(1), α(2), α(3) in one‐dimensional (ID), 2D, and also 3D (bulk) systems, respectively, are found to vary according to (2ħω ± ħΩ − EG)β, where ħω(ħΩ) is the photon (phonon) energy and EG is the effective indirect gap. β takes the values 0, 1, 2, and 3 depending on the dimension of the system and the form of momentum matrix elements. The analytical expressions of the electron‐photon and electron‐phonon matrix elements in QW systems are given. The numerical calculation of the TPA coefficients α(1), α(2), and α(3) in Si0.5Ge0.5 shows that: α(1)(α(2)), for polarization parallel to the confinement directions, is four (two) orders of magnitude bigger than α(3) for allowed‐allowed transitions. Furthermore, α(1) is two orders of magnitude larger than α(2), for both photon polarizations with respect to the confinement directions in QW systems. This enhancement of α(1) over α(2) and α(3) is interpreted as due to the additional lateral quantum confinement of the carriers in QWW and also due to the intra‐subband momentum matrix elements.
Channeling Radiation in a Crystal Undergoing an Action of Ultrasonic or Electromagnetic Waves
Authors: Dedkov G. V.
Published: July 2007

The radiation emitted by channeled electrons and positrons in a crystal undergoing an action of ultrasonic (electromagnetic) waves is considered in quantum approximation. The most general crystal potential is shown to consist of two terms: the stationary and nonstationary ones both depending on the atomic displacements caused by the wave. As a result the peak positions and intensities of channeling radiation depend on the wave intensity, too. At resonance between one of the perturbation harmonics of the crystal potential and the proper particle transition the calculated radiation rate exceeds that one without wave by several orders.
Lattice Dynamics of Muscovite Layered Silicates
Authors: Gupta H. C. , Sinha M. M. , Rawat C. C. S. , Tripathi B. B.
Published: July 2007

The lattice dynamics of muscovite layered silicates along the finite wave vector direction is investigated. The calculated results are compared with available experimental results. The present angular force model gives a satisfactory picture of the phonons in complex structures like muscovites and their intercalates.
Electronic Excitations Localized in NaBr and RbI Crystals Near Vacancy Defects of Different Sizes
Authors: Vasilchenko E. , Sarmukhanov E. , Elango A.
Published: July 2007

The luminescence and excitation spectra of electronic excitations localized near a single anion vacancy (eα), a divacancy (ed), and a vacancy quartet (eq) are investigated at 4.2 K in deformed NaBr and RbI crystals. It is shown that the dissociation of eq into a VK and an electron‐vacancy complex occurs together with their radiative decay in NaBr, KBr, and RbI. Based on the spectral proximity of optical characteristics of eq and Ex luminescence, on the one hand, and eα and π luminescence, on the other, it is concluded that by their nature eq excitations represent weak off‐center and eα excitations strong off‐center configurations.
Dependence of the Electrical Resistivity on the Degree of Short‐Range Order in Multicomponent Solid Solutions
Authors: Yu Ge
Published: July 2007

The effect of short‐range order on the electrical resistivity in multicomponent solid solutions is studied by applying the model of Rossiter and Wells in which a spherical Fermi surface and s‐s scattering mechanism are assumed. For the description of the state of short‐range order in multicomponent systems a set of generalized Warren‐Cowley parameters αiXY are defined, based on the same assumptions that were used in the earlier treatments of binary alloys, a linear combination between the short‐range order induced resistivity and αiXY is obtained. The possibility of the experimental verification of the derived relationship is further discussed.
Magnetic Properties of the Three‐Dimensional Ising Model with an Interface Amorphization
Authors: Benyoussef A. , Amraoui Y. El , Kenz A. El , Saber M.
Published: July 2007

A three‐dimensional ferromagnetic Ising model with an interface amorphization is investigated with the use of the effective field theory. Phase diagrams and reduced magnetization curves of interface and bulks are studied. We obtain a number of characteristic behaviours such as the possibility of reentrant phenomena and a large depression of interface magnetization.
Pyroelectric Effect, Optical Birefringence, and Non‐Linear Dielectric Constant in Betaine Arsenate
Authors: Lacèrda‐Aroso M. T. , Ribeiro J. L. , Chaves M. R. , Almeida A. , Vieira L. G. , Klöpperpieper A. , Albers J.
Published: July 2007

Detailed measurements of the pyroelectric coefficient and optical birefringence in betaine arsenate, in a temperature range centred at the ferroelectric phase transition (Tc2 ≈ 121 K) are reported. The critical behaviour of the spontaneous polarization is analyzed from these data and a cross‐over between two different critical regimes is evidenced. Results of linear and non‐linear dielectric constant measured along [001] are also reported.
Localization Properties of 1D Disordered Systems in the Presence of An Electric Field
Authors: Brezini A. , Sebbani M. , Depollier C.
Published: July 2007

A Kronig‐Penney model with a constant electric field is used to examine the transmission properties of electrons in one‐dimensional disordered systems. Disorder is introduced into the system by assuming that the strengths of the δ‐function potential are random variables uniformly distributed. In particular the cases where the strength varies from 0 to W (barriers) and from −W to 0 (wells) are considered. Extensive results are presented and compared to other theories.
Hydrogen Chemisorption on Supported Semiconductor Films
Authors: Sun Qiang , Xie Jianjun , Zhang Tag
Published: July 2007

Hydrogen chemisorption on metal supported semiconductor ZnO films is studied by using the Green function method and complex‐energy‐plane integration approach. The Anderson‐Newns model is used to calculate the chemisorption energy ΔE and the adatom charge transfer Δq. The variations of the adatom state level Ead and the corresponding localized adatom density of states (LDOS) with increasing ZnO film thickness are investigated. It is found that the hydrogen chemisorption energy and the adatom charge transfer depend on both the thickness of the ZnO film and the species of the metal support. The calculations show that ten layers of ZnO film can mirror the chemisorption properties of bulk ZnO. The adatom state level Ead which corresponds to a peak in Ldos, is found shifting upwards to a constant value as the ZnO layer number N increases from 1 to 10.
Pressure Dependent Properties of Boron Phosphide
Authors: Badi N. , Amrane N. , Abid H. , Driz M. , Soudini B. , Khelifa B. , Aourag H.
Published: July 2007

An adjusted local pseudopotential method is used to obtain the electronic band structure of boron phosphide under pressure. The electronic charge density distributions are computed at several points of the valence and conduction band edges. These charge densities are used to study the modification of the bonding and electronic properties of the compound with respect to different volumes ranging from 0.75 V0 to V0.
EPR Study of Low Symmetry Mn2+ Centers in LiNbO3. Superposition Model and Crystal Field Analysis of the Zero‐Field Splitting Parameters
Authors: Yeom T. H. , Choh S. H. , Chang Y. M. , Rudowicz C.
Published: July 2007

X‐band EPR spectra of Mn2+ : LiNbO3 single crystals at room temperature are analyzed with a spin‐Hamiltonian of C3 symmetry. Experimental data are interpreted using the superposition model in a twofold way. Firstly, the zero‐field splitting (ZFS) parameters b, b, b and b4−3 are predicted. While the results for b are not conclusive, those for the major fourth‐order ZFS term |v| = [(b)2 + (b4−3)2]1/2 indicate better agreement with experiment, assuming Mn2+ at the Nb site. Secondly, crystal field (CF) parameters are obtained from the superposition model using two sets of available structural data. Diagonalization of the CF Hamiltonian within the whole 3d5 configuration yields the theoretical ZF splittings. The results for Mn2+ at the Nb site agree better with the experimental data than those for Mn2+ at the Li site. Hence the superposition model analyses of the observed EPR spectra indicate that the spectra can be most likely attributed to Mn2+ ions at Nb sites. The ways to reconcile this conclusion with the opposite one from the low temperature ENDOR study are discussed.
Magnetoresistance and Hall Effect of Ferromagnetic Amorphous Alloys (Fe100‐xMnx)75P15C10
Authors: Bärner K. , Heinemann K. , Medvedeva I. V.
Published: July 2007

The Hall effect ϱH and the magnetoresistance Δϱ/ϱ of ferromagnetic amorphous ribbons of (Fe100−xMnx)75P15C10 are measured for 0 ≦ x ≦ 30, 77 K < T < 300 K, 0 < B0 < 0.4 T, and 0 < p < 2 × 108 Pa. Both transport properties are determined by the number and distribution of spin scattering centers, some of which can be reoriented in external magnetic fields.
Electron Transport in Vapor Deposited Molecular Glasses
Authors: Borsenberger P. M. , Detty M. R. , Magin E. H.
Published: July 2007

Electron mobilities are measured in a series of vapor deposited l,‐l‐dioxo‐4‐(dicyanomethylidene)thiopyran glasses. The results are described within the framework of a formalism based on disorder, due to Bässler and coworkers. The formalism is based on the assumption that charge transport occurs by hopping through a manifold of localized states with superimposed energetic and positional disorder. The key parameters of the formalism are σ, the energy width of the hopping site manifold, μ0, a prefactor mobility, and Σ, a parameter that describes the degree of positional disorder. Experimental values of σ are approximately 0.123 eV for all compounds. The width of the hopping site manifold can be described by arguments based on dipolar disorder, if it is assumed that the hopping site energies are determined by group dipole moments associated with the cyano and sulfone functionalities. The prefactor mobilities are between 1 and 4 × 10 −2 cm2/Vs. A comparison of these results with literature values for molecular glasses suggests that the prefactor mobilities may be dependent upon the dipole moment. Values of the positional disorder parameter Σ are approximately 2.0 and attributed to packing constraints.
Analysis of Hydrogen in Aerosol Particles of a‐Si: H Forming during the Pyrolysis of Silane
Authors: Onischuk A. A. , Strunin V. P. , Ushakova M. A. , Panfilov V. N.
Published: July 2007

The hydrogen content in aerosol particles of a‐Si: H, forming during pyrolysis of silane, is studied by means of the methods of hydrogen evolution and IR spectroscopy. The conclusion is drawn that the evolution peaks at 650,730, and 750 to 850 K correspond to hydrogen present in the dihydride groups of polysilane chains, the monohydride groups of the clustered phase, and the monohydride groups of the dilute phase, respectively. In the IR spectra the monohydride groups of the clustered phase cause absorption near 2000 cm‐−1 and the monohydride groups of the dilute phase near 2100 cm‐−1. Under low silane conversion the main fraction of hydrogen is contained in (SiH2)n, under conversion more than 50% in monohydride groups.
Laser‐Induced Coloration and Discoloration Processes in Alkali Halides
Authors: Bichevin V. , Käämbre H.
Published: July 2007

The coloration kinetics of KCI, KBr, KI, and NaCl crystals under XeCl excimer laser irradiation (hv = 4.02 eV) at room temperature as well as laser‐induced discoloration of preliminarily X‐ray coloured crystals, including samples with M and R centres are studied. It can be concluded from the results that the achieved colour centre concentrations are determined by a dynamical equilibrium between laser two‐quantum coloration and a concomitant one‐quantum bleaching of the created centres. Most probably the bleaching proceeds via the laser‐light‐induced destruction of the F centre complementaries, V centres and annihilation of some produced mobile products (H interstitials?) with F centres. The X‐ray‐created F centres seem to bleach more rapidly than the laser‐created ones.
Solid Solubility of Cu in Al under Pressure and Elastic Moduli
Authors: Soma T. , Ishizuka M. , Kagaya H.‐Matsuo
Published: July 2007

The solid solubility of Cu in Al under pressure is studied using the microscopic electronic theory. An increase of the solubility up to 10 at% Cu in Al is predicted under a pressure of 10 GPa. The obtained results for the lattice constant, the heat of solution, the pressure‐volume relation, and elastic moduli in the Al‐Cu system are presented theoretically.
Electronic Properties of Quasi‐One‐Dimensional Chains at Zero Temperature and T ≠ 0 K
Authors: Schulte J. , Böhm M. C.
Published: July 2007

The electronic properties of quasi‐one‐dimensional (1D) organic/organomctallic chains in the metallic phase are discussed in the theoretical framework of an analytic many‐body description. The dynamics of the charge and spin degrees of freedom are quantified by the respective mean‐square deviations (i.e. fluctuations). The conditions leading to a simultaneous and nonsimultaneous occurrence of charge and spin fluctuations are discussed. By means of the electronic charge fluctuations the probabilities Pi(n) to find n = 0, 1, 2 electrons at the i‐th lattice site are calculated. These elements are input parameters for the determination of the magnetic susceptibility, Knight shift, and electronic effective mass in the correlated ground state at T = 0 K and T ≠ 0 K. The material parameters are enhanced due to many‐particle interactions.
Semiconducting Properties of Hexagonal Chromium, Molybdenum, and Tungsten Disilicides
Authors: Filonov A. B. , Tralle I. E. , Dorozhkin N. N. , Migas D. B. , Shaposhnikov V. L. , Petrov G. V. , Anishchik V. M. , Borisenko V. E.
Published: July 2007

Self‐consistent electronic property simulation of hexagonal CrSi2, MoSi2, and WSi2 is performed within the local density approximation using the semirelativistic linear muffin‐tin orbital method. They are deduced to be narrow‐gap semiconductors with indirect band gaps of 0.29,0.07, and 0.07 eV, respectively. The effective masses of holes and electrons in the disilicides are of the same order. They are close to the free electron mass in CrSi2 and there is a certain reduction of the appropriate values from chromium to tungsten disilicides. Carrier mobility calculations show that the scattering on acoustic phonons is dominant for the disilicides. The hole mobilities at 300 K are 42, 190, and 209 cm2/V s for CrSi2, MoSi2, and WSi2, respectively.
Application of Lattice Inversion Method to Embedded‐Atom Method
Authors: Xie Qian , Huang Meichun
Published: July 2007

A model for the embedded‐atom method with the use of the lattice inversion method is presented. The lattice sums of electron density and pair potential are assumed to be exponential functions of the lattice parameter. The individual functions of potential and density are inverted from the corresponding lattice sums by using the lattice inversion method. The model parameters are explicitly written by five physical inputs, i.e. the equilibrium lattice constant, the bulk modulus, the Voigt average shear modulus, the sublimation energy, and the unrelaxed vacancy‐formation energy. As applications, the 〈100〉 uniaxial stress strain curves in the absence of lateral contraction and shear‐mode failure for Cu, Ag, Au, Ni, Pd, and Pt are calculated. The predictions of tensile strengths and failure strains by the present method are found smaller than those by the pair‐potential model. The results are in agreement with first‐principles calculation of Esposito et al. for Cu and with empirical calculations of Milstein for Ni.
Density of States of Disordered Coupled Peierls Chains
Authors: Mertsching J.
Published: July 2007

The electron density of states of coupled bond‐disordered Peierls chains is approximately calculated in two and three dimensions, regarding nearest‐neighbour chains to be independently disordered. Both the bond disorder due to lattice fluctuations and the chain coupling contribute to the tail states within the one‐dimensional gap. The results are discussed for trans‐polyacetylene.
MFRG‐DPIR Study of the Ising Model: Spin‐3/2 with Random and Spin‐5/2 with Fixed Transverse Field
Authors: Song W. J. , Yanc C. Z.
Published: July 2007

The phase diagram and other critical properties of the Ising model of spin‐3/2 in a random transverse field (RTIM) and spin‐5/2 in a transverse field (TIM) are investigated within the approximate scheme which is combined by mean‐field renormalization group theory (MFRG) with the discretized pathintegral representation (DPIR).
Thermoluminescence and Phosphorescence of VUV‐ and X‐Irradiated CaO Monocrystals at Low Temperatures
Authors: Dolgov S. A. , Kärner T. , Maaroos A. , Savikiiina T. I. , Vasilchenko E. A.
Published: July 2007

Thermoluminescence (TL) of CaO and CaO‐Bi monocrystals VUV‐irradiated at 80 K or X‐irradiated at 4.2 K is reported. Varying the energy of VUV‐irradiation, the TL creation spectra (TLCS) of various peaks are derived. From the analysis of these spectra it is concluded that in the excitation spectrum of CaO indirect transitions may play an important role already from Eexc = 6.5 eV. Long‐lived phosphorescence, as well as low‐temperature TL peaks at 20, 37, and ≈ 50 K of X‐irradiated CaO and CaO‐Bi monocrystals at LHeT are observed. The observed phosphorescence is interpreted as a tunneling recombination between holes and electrons, trapped, respectively, by cation vacancies (V centers) and unidentified donor centers, thermal destruction of which takes place at ≈50 K.
Ionization of Interstitial Iron Atoms in β‐Rhombohedral Boron
Authors: Kuhlmann U. , Werheit H. , Pelloth J. , Keune W. , Lundström T.
Published: July 2007

Mössbauer spectra of β‐rhombohedral boron with interstitially distributed iron atoms up to 3.33 at% are presented. Taking into account that the six D sites in the unit cell are saturated by two Fe atoms, the distribution of the Fe atoms on A and D sites proved to be simply statistical. At lower Fe contents single iron atoms on A sites are ionized to Fe3+, while towards higher Fe contents a rapidly increasing share of Fe2+ ions is found. On D sites the ionization of single Fe atoms is 2+, but it changes to 3+, when two Fe atoms are accommodated in the D sites of one unit cell. The isomer shift of the Mössbauer doublets yields an estimation of the electron transfer to the boron framework. Transition from p‐ to n‐type takes place, when six electrons per unit cell are transferred from the iron ions to the boron framework. This number corresponds to the sum of the unoccupied sites in the upper valence band and the possible electron traps generated by an electron–phonon interaction in the B12 icosahedra.
Infrared Vibrational Modes and Anisotropy of the Effective Ionic Charge in CuAlSe2, CuAlS2, and CuGaSe2 Crystals
Authors: Andriesh A. M. , Syrbu N. N. , Iovu M. S. , Tazlavan V. E.
Published: July 2007

Reflectivity spectra of CuAlS2, CuAlSe2, and CuGaSe2 crystals have been investigated in the wave number range 50 to 600 cm−1 for the polarizations E ‖ c and E ⊥ c. The fundamental phonon parameters, the limiting dielectric constants ε∞ and εS and the reflectivity spectra contours have been calculated by using classical dispersion relations for both E ‖ c and E ‖ c configurations. The Szigeti effective charges and the relative ion charges of Cu, Al, Ga, Se, S anions and cations have been calculated in dependence on the incident light polarization.
A New Approach to the Correlation of the Electrical Properties with Interband and Intraband Transitions of Thin Cr Films
Authors: Fouad S. S. , Ammar A. H. , El‐Fazary M. H.
Published: July 2007

Thin chromium films of thickness ranging from 25 to 80 nm are prepared by thermal evaporation under a vacuum of 1.33 × 10−3 Pa. The electrical resistivity was inversely proportional to the thickness of the film. The analysis of the electrical resistivity is treated in the frame of the effective mean free path theory of size effect developed by Pichard et al. Such analysis allows the determination of the mean free path l0, carrier concentration nc, relaxation time τ, and Fermi energy EF. The optical constants, n and k, of chromium thin films are determined in the spectral range of 200 to 25000 nm. The obtained results agree with the optical conductivities predicted theoretically by Moruzzi et al. In addition, the values of nc, σs, l0, and τ obtained electrically are found to match with those obtained optically.
Damping of Surface Plasmon–Phonon Polaritons
Authors: Barlas T. R. , Dmitruk N. L.
Published: July 2007

The influence of the bulk damping in polar crystals on the elementary properties of surface polaritons (SP) of a mixed plasmon–phonon type is studied theoretically. Two different methods for the calculation of SP damping are used. These methods are shown to give the same results. Definite combinations of the plasmon and phonon damping constants are found to lead to a minimum in the damping and a maximum in the planar propagation length versus wave vector dependences. An increase in the SP energy dissipation due to an increase in the damping constants of elementary excitations in a crystal and the SP scattering by surface roughness give rise to a reduction of the SP frequency, but the shape of the dispersion curve changes differently, what can be distinguished experimentally.
Electronic Transport through Silicon (a‐Si:H) Barriers in Hydrogenated Amorphous Germanium (a‐Ge:H)
Authors: Drüsedau Thilo P.
Published: July 2007

Films of a‐Ge:H containing single and double barriers of a‐Si:H with a total thickness of 10 nm are prepared by rf PCVD from germane and silane, respectively. The current–voltage characteristics of the sandwich structures are smooth and do not show any irregularities. The differential conductivity σ(E) of the structures is nearly step‐like with values of 3 × 10−8 and 10−5 ω−1 cm−1 for external fields below 200 and above 1000 V/cm, respectively. The ohmic conductivity (at lower fields) of the ultrathin a‐Si:H barriers is identical to bulk a‐Si:H and is 10−10 Ω−1 cm−1. This implies a discontinuity of the conduction band edge at the a‐Si:H/a‐Ge:H interface of 0.3 eV. For low fields and below 240 K, there is ohmic hopping with an activation energy of 0.3 eV. The internal field in the barrier reaches very large values of 4 × 105 V/cm. For the first time, the observation of a vanishing and a negative differential resistivity of the a‐Si:H barriers even at room temperature is reported. It is concluded that there is no real quantum confinement in heterostructures based on amorphous semiconductors.
Hydrostatic Deformation Potentials and Phase Transitions in CuGa(SxSe1−x)2 Alloys at High Pressure
Authors: González J. , Calderón E. , Capet F.
Published: July 2007

The optical absorption edge of CuGa(SxSe1−x)2 single crystals is measured as a function of hydrostatic pressure, covering the full stability range (0 to 30GPa) of the chalcopyrile phase. For 0 ⩽ x ⩽ 1, a linear increase of the direct energy gap with pressure is found. The corresponding gap deformation potential is calculated to be −4.5 eV for x = 0, −5.2 eV for x = 0.5, and −4.4 eV for x = 1. From changes in the light transmission characteristic of the samples under pressure, structural first‐order phase transitions are observed.
Electronic and Structural Properties of the Liquid (Tl1/2Pb1/2)‐Se System
Authors: Usuki T. , Itoh K. , Aita S. , Kameda Y. , Uemura O.
Published: July 2007

The electrical conductivity, magnetic susceptibility, and thermoelectric power in the liquid (Tl1/2Pb1/2) Se system are measured over wide temperature and composition ranges. It is observed that the electrical conductivity in the system sharply falls near the stoichiometric composition of Tl2Pb2Se3 and the magnetic susceptibility has a diamagnetic maximum at this composition, where the thermoelectric power changes its sign. In addition, the DTA investigation shows that the liquidus temperature of the system has a maximum around this composition. These results suggest that a short‐range order appears around the composition of Tl2Pb2Se3 in the liquid (Tl1/2Pb1/2)‐Se system. The delocalized electron analysis using both susceptibility and conductivity data suggests that the bonding nature of liquid Ti2Pb2Se3 is rather ionic. The radial distribution study by neutron diffraction also exhibits that the first peak position of g(r) for liquid Tl2Pb2Se3 is located near the ionic bond distance of metal and chalcogen atoms. On the other hand, covalent Se‐ Se bonds are formed in the Se‐rich composition range.
On the Phenomenological Theory of the Coherent Photogalvanic Effect in Glass
Authors: Sokolov V. O. , Sulimov V. B.
Published: July 2007

In connection with the charge‐separation model of photoinduced second‐harmonic generation in optical fibres based on the coherent photogalvanic effect the tensorial properties of the coherent photocurrent in glass are studied using a phenomenological approach with the randomness of the glass taken into account. Since the most general expression is used for the coherent photocurrent, this approach is valid for any microscopic model of the coherent photogalvanic effect. The dependence of the photocurrent on the polarization of fundamental and second‐harmonic waves is obtained. Directional photoionization of localized states is studied explicitly in connection with experiments with circularly polarized waves performed recently.
Device Structures and Characteristics of Laser Diodes with ZnMgSSe Cladding Layers
Authors: Ohata T. , Itoh S. , Nakayama N. , Matsumoto S. , Nakano K. , Ozawa M. , Okuyama H. , Tomiya S. , Ikeda M. , Ishibashi A.
Published: July 2007

ZnSe based laser diodes exhibit best initial performances such as low threshold current with ZnCdSe/ZnSSe/ZnMgSSe SCH‐SQW structure. The device lifetime is improved with the ZnCdSe/ZnSSe/ZnMgSSe SCH‐MQW structure. Laser emission of 471 nm wavelength is obtained under pulsed injection at room temperature with ZnSe/ZnMgSSe DH structure.
MBE of ZnSe Alloys to Achieve Room Temperature CW Laser Diodes
Authors: Han Jung , Gunshor R. L. , Nurmikko A. V.
Published: July 2007

Issues related to the growth and characterizations of II–VI blue‐green laser diodes are reviewed. The development of the Zn(Se, Te) graded contact scheme to p‐ZnSe is found to be effective in supporting the room‐temperature CW operation of green (508 nm) laser diodes with a threshold voltage of 5.8 V. Compositional control of the ZnMgSSe cladding layers and the reduction of nonradiative point defects in the ZnCdSe quantum wells will be discussed. The observation of persistent photoconductivity in nitrogen‐doped p‐ZnMgSSe at low temperature is reported. The increase of conductivity after illumination appears due to a metastable population of holes which are in thermodynamic equilibrium with hydrogenic acceptors having reduced activation energy. The experimental evidence suggests the presence of a DX‐like behavior for holes.
Study of ZnSe‐Based Layer Structures for Blue‐Green Laser Diodes
Authors: Pessa M. , Rakennus K. , Uusimaa P. , Savolainen P. , Salokatve A.
Published: July 2007

The possibilities of developing ZnSe blue‐green lasers with new materials and layer structures are studied. In particular, it is considered: (i) the use of an „inverted”︁ laser structure, n‐on‐p configuration, having p‐type barrier reduction layers of III–V compound semiconductors which are sandwiched between the substrate and the device and (ii) the use of ZnMnSSe as a cladding layer. Electronic band structures, current–voltage curves, and light–current curves are calculated for the lasers with inverted layer structures. Preliminary results of a study of ZnMnSSe layers grown by the molecular beam epitaxy method are also presented.
Semiconductor Modulators and Optical Switches
Authors: Cavenett B. C. , Wang S. Y. , Prior K. A.
Published: July 2007

Optoelectronic devices are important for laser modulators and Q‐switches as well as in optical communication and network systems where all optical switching is required. The development of II–VI semiconductor devices fabricated by molecular beam epitaxy and based on the quantum confined Stark effect in p–i–n structures where the i‐region is a set of quantum wells is reviewed. In particular, the characterisation of ZnSe/ZnCdSe/ZnSe devices is discussed.
Recent Results on ZnSe‐Based Vertical‐Cavity Surface Emitting Lasers Operating in the Blue
Authors: Floyd P. D. , Furdyna J. K. , Luo H. , Merz J. L. , Yamada Y. , Yokogawa T.
Published: July 2007

The first successful operation of vertical‐cavity surface emitting lasers (VCSELs) operating in the blue is achieved using ZnSe/ZnCdSe multilayers. The vertical‐cavity resonators are constructed by depositing SiO2/SiN distributed Bragg reflectors on both sides of the active ZnSe/ZnCdSe region. Pulsed optical pumping is used to demonstrate the feasibility of this configuration. Although our first attempts at VCSEL operation required significant threshold excitation power densities (75 kW/cm2 at 2 K for lasing at 477 nm), even modest modifications in design and fabrication of subsequent structures already show substantial improvement in these figures.
Deep Etched ZnSe‐Based Nanostructures for Future Optoelectronic Applications
Authors: Bacher G. , Illing M. , Forchel A. , Hommel D. , Jobst B. , Landwehr G.
Published: July 2007

By electron beam lithography and a subsequent wet/dry etch process, optically active nanostructures are developed based on ZnSe with lateral extensions down to 35 nm. Dry etching using Ar+ ions is found to generate very smooth etch surfaces, while the photoluminescence efficiency of narrow wires is much higher in wet chemically etched structures. As a first application of deep etched nanostructures, index coupled distributed feedback laser structures of second order are realized with periods down to 185 nm on the base of (Cd, Zn)Se/ZnSe/(Mg, Zn) (Se, S) vertical waveguide heterostructures. A clear correlation between the resonator period and the emission wavelength is observed, indicating a high coupling coefficient of the structures. For optical pumping using a pulsed N2‐laser, the laser threshold at room temperature is about 100 kW/cm2.
Review of Compensation Centres in ZnSe:N
Authors: Prior K. A.
Published: July 2007

Over the past few years nitrogen has been found to be by far the best dopant to use in the production of p‐type ZnSe, although it can be highly compensated. This review presents evidence for compensation within ZnSe:N due to the presence of selenium vacancy and dopant–vacancy clusters, followed by a comparison of the models which attempt to calculate the degree of compensation and then discuss evidence for the generation and motion of point defects within ZnSe:N at room temperature.
Doping Characteristics of N‐Doped p‐ZnSe and Cl‐Doped n‐ZnSe
Authors: Yao T. , Zhu Z.
Published: July 2007

Doping characteristics of N‐doped p‐ZnSe and Cl‐doped n‐ZnSe are extensively studied by Hall measurements, CV, PL, DLTS, ICTS, and ion‐beam analysis. N forms both deep donors and acceptors in heavily N‐doped ZnSe. The lattice location of the N atoms in heavily N‐doped ZnSe is studied by ion beam channeling to get insight into the compensation mechanism. Free excitonic emission shifts to the lower energy side with N concentration, indicating a shrinkage of band gap. It is suggested that the carrier compensation mechanism in heavily Cl‐doped ZnSe is due to degradation of crystallinity. The maximum carrier concentration is greatly enhanced by selective doping of Cl.
p‐Type Doping of ZnSe. On the Properties of Nitrogen in ZnSe:N
Authors: Kurtz E. , Einfeldt S. , Nürnberger J. , Zerlauth S. , Hommel D. , Landwehr G.
Published: July 2007

Growth and optical properties of MBE grown ZnSe:N are investigated. Doping levels between NA – ND = 1 × 1015 to 1 × 1018 cm−3 are achieved by using either a rf or ECR plasma source for nitrogen activation. The different behaviour of the photoluminescence for both doping methods is discussed. It is shown that during the operation of the rf plasma in the high brightness mode about 5 to 8% atomic nitrogen is present in the growth chamber. Results for different optical methods are presented. This includes direct observation of the Zeeman splitting of the acceptor bound exciton and the determination of the degree of compensation by poly‐and monochromatic transmission.
Exciton Dynamics and Gain Mechanisms in Optically Pumped ZnSe‐Based Laser Structures
Authors: Gutowski J. , Diessel A. , Neukirch U. , Weckendrup D. , Behr T. , Jobst B. , Hommel D.
Published: July 2007

Although lasing is meanwhile achieved in various ZnSe‐based heterostructures, the underlying mechanisms are still seriously under debate. The gain spectra of various MBE‐grown samples are measured using the variable‐stripe‐length method. A description of the gain spectra for low particle densities is possible in terms of the model of Ding et al. The responsible gain mechanism is assumed to be an induced recombination of strongly exchange‐interacting localized excitons, which, however, may be a questionable precondition. By means of time‐dependent measurements the localizing character of the active material is demonstrated. However, several indications are given that under certain circumstances more than one process contribute to lasing, in particular biexciton recombination seems to be involved in the gain mechanism.
Current Activity in CNRS‐Sophia Antipolis Regarding Wide‐Gap II–VI Materials
Authors: Faurie J. P. , Brunet P. , Morhain C. , Ongaretto C. , Tournié E.
Published: July 2007

Preliminary results about MBE growth and characterization of ZnSe‐based alloys and multi‐quantum wells (MQW) are presented. Undoped ZnSe and Zn1−xCdxSe compounds exhibiting structural and optical properties, comparable to the best results previously published, are grown. RHEED intensity oscillations are observed during ZnSe and Zn1−xCdxSe growth. Several Zn1−xCdxSe/ZnSe MQWs are grown. RHEED intensity analysis is used to accurately control the well thicknesses. The MQWs produce very intense and sharp photoluminescence peaks. The sharpness of interfaces is confirmed by X‐ray analysis. Cl‐doped ZnSe epilayers with doping levels at 77 K ranging from 6 × 1015 to 6 × 1018 cm−3 are obtained.
The InGaAlN System as a Competitor of ZnSe
Authors: Matsuoka T.
Published: July 2007

The InGaAlN system is introduced from the viewpoint of light emitting devices in comparison with ZnSe‐based materials. The properties and the current status of both materials are reviewed, and future prospects are discussed.
On the Structure of Micro‐ and Nano‐Crystalline Semiconductors and Semimetals
Authors: Gerlach E.
Published: July 2007

It is shown that the size distribution of the grains of micro‐ and nanocrystalline semiconductors and semimetals can be determined from a measurement of the dynamical conductivity Re σ(Ω). Here Ω has to be larger than the plasma resonance frequency, and the average grain size must be smaller than λF/2 (λF Fermi wavelength) which is about 100 nm for an electron concentration of 1015 cm−3.
On the Pressure and Volume Dependence of the Grüneisen Parameter and Debye Temperature for NaCl Crystals
Authors: Dixit S. , Shanker J.
Published: July 2007

An analysis of the pressure and volume dependence of the Grüneisen parameter γ and the Debye temperature θ is presented for NaCl crystals using the model developed by Kumari and Dass. It is pointed out that the parameter q = d l γ/d ln V should be taken nearly equal to unity for predicting γ and θ at different pressures. Various thermodynamic approximations implied in the analysis are discussed.
Effect of Interface Energy Anisotropy on Thermal Stability and Transformation of Lamellar Structures. I. Behaviour of Two‐Phase Systems under Annealing as Related to Particle Shape Equilibrium
Authors: Mazurski M. I. , Salishchev G. A.
Published: July 2007

Thermodynamical regularities determining the occurrence of this or that shape of a particle and its stability at high temperatures are analyzed on the basis of a comparison of the calculated values of interface energies of sphere and disk. The thermodynamical stability of lamellar and spherical shapes is found to be determined by the level of interface energy anisotropy. Peculiarities of phase morphology changes in two‐phase systems during annealing, predicted on this basis, agree with the experimental data.
On the Effective Electron Mass in Quantum Wires of Non‐Parabolic Semiconductors under a Parallel Magnetic Field
Authors: Bhattacharyya D. , Ghatak K. P.
Published: July 2007

The effective electron mass at the Fermi level in quantum wires of non‐parabolic semiconductors in the presence of a parallel magnetic field is studied on the basis of a newly derived 1D‐electron dispersion law. It is found, taking n‐Hg1−xCdxTe and In1−xGaxAsyP1‐y lattice matched to InP as examples, that the effective Fermi level mass depends on the size quantum numbers in addition to the Fermi energy due to the presence of a magnetic field. The effective masses corresponding to different subbands increase with increasing electron concentration and decreasing film thickness in various ways. In addition, the corresponding well‐known results of quantum wires of wide gap materials in absence of a magnetic field are also obtained as special cases of a generalized formulation under certain limiting conditions.
Evidence for Photoinduced Hole Coupling in BaTiO3:Co
Authors: Chanussot G.
Published: July 2007

A possible photoinduced correlation mechanism in BaTiO3:Co is described. The underlying microscopic process is a local hole photoinjection at a Co3+ site close to a preexisting metastable hole. The Jahn‐Teller field at the Co2+ site transfers this pair of holes in the next cell along the rhombic direction [111] before normal polaronic or band conduction. The spatial extension of this correlation depends on a characteristic „delocalization”︁ length. This unstable conductive state appears to be possibly frozen in a permanent way below Tc.
Apical Oxygen and Frustration Effects in CuO2 Lattice
Authors: Eid Kh. , Matlak M. , Zieliński J.
Published: July 2007

A microscopic model for the CuO2 lattice which takes into account two orbitals of copper (3d, 3d and three orbitals of oxygen (2px, y, z) is considered. Applying the canonical transformation to eliminate the d–p hybridizations, the effective spin‐dependent Hamiltonian is derived describing superexchange interaction (SX) and Ruderman‐Kittel‐Kasuya‐Yosida (RKKY)‐type interaction. The possible role of apical oxygen degrees of freedom (β (2pz − 2px(y) hopping integral), Δ (2pz − 3d hybridization), and ε (apical oxygen energy)) for antiferromagnetic frustration effects in the CuO2 plane due to SX is investigated. The presence of an apical orbital coupled to the in‐plane copper and oxygen orbitals can effectively enhance these effects. Therefore, both these interactions (SX and RKKY) can cooperate in destroying the antiferromagnetic order in the sublattice of copper spins upon doping. It is shown that the increase of the frustration due to doping favours the formation of the superconducting state. The RKKY interaction is of ferromagnetic character. For small hole concentrations SX and RKKY integrals are of the same order of magnitude but different in sign.