Showing papers in "Physica Status Solidi B-basic Solid State Physics in 1983"

Solitons and proton motion in ice‐like structures

Abstract: The proton motion in hydrogen bonded quasi-one-dimensional molecular chains is investigated It is shown that elementary processes in one-dimensional ice lattice are nonlinear autolocalized collective exitations which can spread over macroscopic distances retaining the energy, momentum, and charge. The results obtained can be used to explain some phenomena in biological systems. [Russian Text Ignored].

Optical Phonons in Amorphous Silicon Oxides. I. Calculation of the Density of States and Interpretation of LoTo Splittings of Amorphous Sio2

Abstract: Until now there exists no calculation of the phonon density of states of amorphous SiO2 and related materials, which could interpret the experimentally detected splittings between longitudinal and transverse modes in their optical phonon spectra. This problem is solved by taking into account the influence of the vibrationally induced polarization on the motion of atoms. A simple model for the description of this polarization in non-crystalline SiO2 is developed and used for the calculation of the density of states of LO and TO phonons in the framework of a coherent potential approximation for structurally disordered systems. This new method leads also to the determination of the IR response function and to simple analytical expressions for the microscopic parameters of the model, i.e. the bond angle at the oxygen atom and transverse dynamic effective charges. The values of these parameters are determined from IR transmission spectra of thin SiO2 layers prepared with the help of different methods. The calculated density of states and IR response function are in good agreement with corresponding experimental results.

Determination of the charge carrier concentration and mobility in n-gap by Raman spectroscopy

Abstract: A method is presented which allows the determination of the charge carrier concentration and mobility in n-GaP by Raman spectroscopy of coupled plasmon-phonon modes. The results are compared with those of electrical measurements.

Special points of the brillouin zone and their use in the solid state theory

Abstract: Article de synthese en trois parties: 1) Considerations generales sur l'integration en des points particuliers de la zone de Brillouin; 2) Points de valeur moyenne et points particuliers de la zone de Brillouin pour des cristaux de differentes symetries; 3) Utilisation des points particuliers dans des calculs de structure de bande et de proprietes physiques de solides

Raman study of the phonon Halfwidths and the phonon-plasmon coupling in ZnO

Abstract: The LO-phonon—plasmon coupling in n-ZnO single crystals is investigated by Raman spectroscopy at room and liquid nitrogen temperatures. The energy shift and halfwidth broadening of the LO-phonons relative to the uncoupled modes are analyzed to get the concentration and mobility of the charge carriers. These results are compared with those from electrical measurements. The uncoupled phonon modes are measured in high-ohmic crystals in which the interaction with the charge carriers can be neglected. For the low energy E2 mode an extreme small halfwidth of 0.3 cm−1 is found. At 77 K with increasing intensity of the exciting continuous laser light (hω = = 2.54 eV < Eg = 3.3 eV, P = 800 mW) nonequilibrium carriers are generated not connected with an increase of the lattice temperature. [Russion Text Ignored]

The Relativistic Linear Muffin-Tin Orbital Method Application to Au

Abstract: Starting with the Dirac equation the relativistic version of the linear muffin-tin orbital method (RLMTO) is developed. It is considerably faster than the non-linear RAPW method. Application to the band structure of Au shows, that the results (valence band energies) of both methods deviate in the average by only 0.004 Ryd from each other. Ausgehend von der Dirac-Gleichung wird die relativistische Version der linearen Muffin-Tin-Orbital-Methode (RLMTO) entwickelt. Sie ist wesentlich schneller als die (nichtlineare) RAPW-Methode. Ihre Anwendung auf die Bandstruktur von Au zeigt, das die Ergebnisse (Valenzbandenergien) beider Methoden im Mittel nur 0,004 Ryd voneinander abweichen.

Exciton–phonon interaction in crystalline and vitreous SiO2

Abstract: An investigation of the Urbach rule in crystalline SiO2 (α-quartz), crystalline SiO2Ge, and vitreous SiO2 in the framework of the models of Dow and Redfield and of Toyozawa shows that the fundamental optical absorption edge of SiO2 is determined by strong exciton–phonon interactions. In the crystalline and vitreous SiO2 low-energy excitons with E = 9.1 eV (α-quartz) and E = 8.7 eV (glass) interact mainly with longitudinal optical phonons. In the crystalline SiO2Ge alloy the interaction with phonons is realized by excitons localized on germanium impurities. The exciton–phonon interaction in SiO2Ge and vitreous SiO2 is stronger than in crystalline SiO2 due to the higher degree of localization of low-energy excitons in these disturbed SiO2 forms. The strong exciton–phonon interaction leads to a momentary self-trapping of the low-energy excitons in crystalline and non-crystalline SiO2 and to a corresponding relaxation of the SiO2Ge excitons localized on Ge impurities. Numerical values of the fundamental exciton parameters are deduced and discussed. Eine Untersuchung der Urbach-Regel in kristallinem SiO2 (α-Quarz), kristallinem SiO2Ge und in SiO2-Glas im Rahmen der Modelle von Dow and Redfield sowie Toyozawa zeigt, das die fundamentale optische Absorptionskante von SiO2 durch starke Exziton–Phonon-Wechselwirkungen bestimmt wird. In kristallinem und glasernem SiO2 wechselwirken niederenergetische Exzitonen mit E = 9,1 eV (α-Quarz) und E = 8,7 eV (Glas) hauptsachlich mit longitudinalen optischen Phononen. In der kristallinen SiO2Ge-Verbindung wird die Wechselwirkung mit Phononen durch Exzitonen realisiert, die an Germaniumverunreinigungen lokalisiert sind. Die Exziton–Phonon-Wechselwirkung in SiO2Ge und glasernem SiO2 ist infolge der hoheren Lokalisierung der niederenergetischen Exzitonen in diesen gestorten SiO2-Formen starker als im kristallinen SiO2. Die starke Exziton–Phonon-Wechselwirkung fuhrt zu einem augenblicklichen Selbsteinfang der niederenergetischen Exzitonen im kristallinen und nichtkristallinen SiO2 und zu einer entsprechenden Relaxation der SiO2Ge-Exzitonen, die an Ge-Verunreinigungen lokalisiert sind. Numerische Werte der fundamentalen Exzitonparameter werden ermittelt und diskutiert.

On the Elastic Constants of a Nematic Liquid Crystal

Abstract: Comparing an extended form of the Landau-de Gennes model for the order parameter with the free elastic energy for the director field of a nematic liquid crystal the temperature dependence of the Frank elastic constants is found. The theoretical results are in agreement with both the recent measurements and with molecular statistical theories. Durch Vergleich eines erweiterten Landau-de Gennes Modells fur den Ordnungsparameter mit der freien elastischen Energie fur das Direktorfeld eines nematischen Flussigkristalls findet man die Temperaturabhangigkeit der Frankschen elastischen Konstanten. Die theoretischen Ergebnisse sind sowohl in Ubereinstimmung mit jungsten Messungen als auch mit molekular-statistischen Theorien.

The Photoacoustic Effect in Semiconductors

Abstract: A theory of the photoacoustic effect (PAE) in semiconductors is developed for the region of fundamental absorption, taking into account the generation of free electrons and holes, their diffusion, and recombination. The temperature on the illuminated surface of a semiconductor is calculated in a general form. The PA measurements allow to determine the diffusion length of carriers, the surface recombination velocity, and the bulk lifetime along with the light absorption coefficient. The qualitative difference of the results from the conventional Rosencwaig and Gersho (RG) theory is demonstrated. The problem of interpretating the experiments is discussed. [Russian Text Ignored]

Electron spin resonance of doped glow-discharge amorphous germanium

Abstract: ESR-spectra and transport properties of a-Ge:H, prepared by decomposition of GeH4 in a glow-discharge, are investigated for different phosphorus and boron doping. Undoped and moderately doped samples show the usual resonance at g = 2.0225 due to dangling bonds. For high doping levels, however, two new lines are found: high boron doping leads to a resonance at g = 2.0535 with a linewidth of 112 G, high phosphorus doping to a line at g = 2.0120 with ΔHpp = 33 G. These lines are attributed to localized holes and electrons in the tails of valence and conduction band, respectively. The linewidth of all resonances is the combination of a g-spectrum ΔHpp(g) and a considerably smaller residual linewidth ΔHpp(r) caused by unresolved hyperfine interaction. A simple relation between the g-shift g–g0 and ΔHpp(g) is found which explains the similar doping dependence of g-value and linewidth. From the dependence of the spin density Ns on the conductivity activation energy Eσ a model for the density of states in the mobility gap is deduced. A comparison between the ESR-data of a-Ge: H and a-Si: H shows that g-shifts and linewidths of the three resonances are for a-Ge: H by a factor of about six larger than for the corresponding lines of a-Si: H. ESR-Spektren und Transporteigenschaften von a-Ge:H, hergestellt durch Zersetzung von GeH4 in einer Glimmentladung, werden fur verschiedene Phosphor- und Bor-Dotierungen untersucht. Undotierte und masig dotierte Proben zeigen die ubliche Resonanz bei g = 2,0225, die von freien Valenzen herruhrt. Bei starker Dotierung werden zwei neue Linien gefunden: hohe Bor-Dotierung fuhrt zu einer Resonanz bei g = 2,0535 mit einer Linienbreite von 112 G, hohe Phosphor-Dotierung zu einer Linie bei g = 2,0120 mit ΔHpp = 33 G. Diese Linien werden lokalisierten Lochern und Elektronen in den Auslaufern von Valenz- und Leitungsband zugeordnet. Die Linienbreite aller Resonanzen ist die Uberlagerung eines g-Spektrums ΔHpp(g) und eines betrachlich schmaleren Anteils ΔHpp(r), der durch nicht aufgeloste Hyperfein-Wechselwirkung bedingt ist. Es besteht eine einfache Beziehung zwischen g-Verschiebung g–g0 und ΔHpp(g); sie erklart die ahnliche Dotierungsabhangigkeit von g-Wert und Linienbreite. Aus der Abhangigkeit der Spindichte Ns von der Leitfahigkeits-Aktivierungsenergie Eσ wird ein Modell fur die Zustandsdichte in der verbotenen Zone abgeleitet. Der Vergleich zwischen den ESR-Daten von a-Ge:H und a-Si:H zeigt, das die g-Verschiebungen und die Linienbreiten der drei Resonanzen fur a-Ge:H um etwa den Faktor sechs groser sind als fur die entsprechenden Linien von a-Si:H.

The Biexciton Levels and Nonlinear Optical Transitions in ZnO

Abstract: The controversy about the level scheme of the biexcitons in ZnO is solved by two-photon reabsorption spectroscopy. Three levels are detected, corresponding to the possible combinations of A and B valence band holes. The diamagnetic shift of these states is below 2 × 10−6 eV T−2 in agreement with the small exciton and biexciton radii. In forward two-photon Raman scattering, two excitation induced renormalizations of the polariton dispersion are observed, partly involving transitions to the biexciton level. The M-band luminescence and the resonance structure observed in backward two-photon Raman scattering are reconsidered in terms of the new level scheme. Das Problem des Termschemas der Biexzitonen in ZnO wird durch Zweiphotonen-Reabsorptionsspektroskopie gelost. Drei Biexzitonenniveaus werden beobachtet entsprechend den moglichen Kombinationen von Lochern aus den A- und B-Valenzbandern. Die diamagnetische Verschiebung dieser Zustande liegt unter 2 × 10−6 eV T−2 in Ubereinstimmung mit den kleinen Exzitonen- und Biexzitonenradien. Die Zweiphotonen-Ramanstreuung in Vorwartsrichtung zeigt zwei anregungsinduzierte Renormierungen der Polaritonendispersion, die sich teilweise auf Ubergange zum Biexziton zuruckfuhren lassen. Die Lumineszenz der M-Bande und die Resonanz in der Zweiphotonen-Ramanstreuung in Ruckwartsrichtung werden im Rahmen des neuen Biexzitonen-Termschemas diskutiert.

Influence of a Strong Electric Field on the Carrier Capture by Nonradiative Deep-Level Centers in GaAs

Abstract: The techniques of capacitance spectroscopy are used to measure directly electron and hole capture cross-sections of deep levels in a neutral material at zero electric field and in the depletion layer with a strong electric field. The data show that if at zero electric field the electron capture cross-sections onto the centers are thermally activated and described by the theory of nonradiative multiphonon capture, then in a strong electric field (F ≈ 104 V/cm) the capture cross-sections increase to the limiting large value (σ ≈ 10−14 to 10−12 cm2) and are only little dependent on temperature. It is shown that an electric field causes a simultaneous increase of the electron and hole capture cross-sections of the level EL2 by five orders of magnitude at low temperatures (T ≈ 90 K). [Russian Text Ignored]

The Rôle of Two‐Level Systems in Amorphous Metallic Alloys. I. Induced Anisotropy in Amorphous Ferromagnets

Abstract: The induced anisotropy in amorphous ferromagnets is attributed to the directional ordering of atom pair axes. The thermodynamics and micromagnetic background of this process is treated on the basis of two-level systems which are identified with mobile atom pairs in the neighbourhood of free volumes. The rǒle of ‘reversible’ and of ‘irreversible’ relaxation processes is discussed in detail. Die induzierte Anisotropie amorpher Ferromagnete wird der Richtungsordnung von Atompaarachsen zugeschrieben. Die thermodynamischen und mikromagnetischen Grundlagen dieses Prozesses werden mit Hilfe von Zwei-Niveau-Systemen behandelt, die als bewegliche Atompaare in der Umgebung freien Volumens identifiziert werden. Die Rolle der „reversiblen” und „irrever-siblen” Relaxationsprozesse wird ausfuhrlich diskutiert.

Elastic constants of the layered compounds Gas, GaSe, InSe, and their pressure dependence I. Experimental part

Abstract: The block diagonalization of the dynamical matrix of β-GaS is reported in the case of propagation perpendicular to the layers. A linear chain model that includes intralayer interactions between any atoms is introduced, the principal frequencies and the elastic constants C33 and C44 are calculated. Within the model, the existence of real coupling parameters is discussed, that leads to an evaluation of the intralayer forces contribution to the elastic constants. The evolution of the interlayer interactions with pressure is studied, using previously published experimental results. Finally, as far as possible, the theoretical formulas are applied to the similar structures of ϵ-GaSe and γ-InSe. La bloc-diagonalisation de la matrice dynamique de β-GaS est effectuee dans le cas d'une propagation perpendiculaire au plan des feuillets. Un modele de ehaine lineaire incluant des interactions intrafeuillets entre atomes quelconques est ensuite introduit, les frequences principales et les constantes elastiques C33 et C44 calculees. Dans le cadre du modele propose, l'existence de parametres de couplage reels est discutee, conduisant a l'evaluation de la contribution des interactions intrafeuillets aux constantes elastiques. L'evolution des liaisons interfeuillets avec la pression est etudiee, utilisant les resultats experimentaux publies anterieurement. Enfin, dans la mesure du possible, les formules etablies sont appliquees aux structures voisines de ϵ-GaSe et γ-InSe.

Electron–Phonon Interaction in Periodic Two‐Layer Structures

Abstract: The Hamiltonian of the interaction of electrons and spatially extended optical interface modes in two-layer periodic structures is developed. Der Hamiltonoperator der Wechselwirkung von Elektronen und raumlich ausgedehnten optischen Grenzflachenmoden in einer periodischen Zwei-Schichtstruktur wird abgeleitet.

Optical phonons and structure of TlGaS2, TlGaSe2, and TlInS2 layer single crystals

Abstract: The polarization properties of bands in Raman scattering and IR reflection spectra are studied experimentally in TlGaS2, TlGaSe2, and TlInS2 layer single crystals and in solid solutions based on them. Additionally the angular dependence of IR-active modes on the angle between the phonon wave vector and the direction of the optical axis c of single crystals is investigated. It is shown that the observed phonon spectra can be interpreted on the basis of a primitive cell which contains two tetragonal-symmetry layers bonded to each other by an inversion operation. Bands corresponding to cells with a mixed composition are observed in the Raman spectra of the solid solutions. [Russian Text Ignored].

Exoemission of Pyroelectrics

Abstract: The investigation of the current density, the angular and energy distributions, as well as the field dependence of the exoemission arising under the pyro- and piezoeffect conditions, shows that the electron emission occurs from a dielectric layer on the ferroelectric surface. The effect of exoemission depletion is found. The mechanism of the exoemission phenomenon is examined. The calculations of the barrier permeability show that the exoemission represents the tunnel electron emission from the polaron band or from the bottom of the conduction band. Eine Untersuchung der Stromdichte, der Winkel- und Energieverteilung und der Feldabhangigkeit der Exoemission unter pyro- und piezoelektrischen Bedingungen zeigt, das die Exoelektronen aus einer dielektrischen Schicht auf der ferroelektrischen Oberflache erfolgt. Der Effekt der Erschopfung der Exoemission wird gefunden. Der Mechanismus der Exoemission wird untersucht. Berechnungen der Barrierendurchlassigkeit zeigen, das die Exoemission Tunnelemission aus dem Polaronenband oder von der Unterkante des Leitungsbandes darstellt.

The Interpretation of Broadened Gd3+ EPR Spectra of Polycrystalline Ceramics

Abstract: An interpretation is given of the broadened EPR spectra of Gd3+ ions, which are frequently observed in polycrystalline ceramics and glasses. A reasonable simulation of the spectra is obtained, when these are regarded as a superposition of a large number of normal EPR powder spectra, each of which can be described with the usual spin-Hamitonian. These originate from the many different surroundings of the Gd3+ ions in these compounds due to the structural disorder. The result is, that only those transitions, which are independent of the zero field splitting parameters in the spin Hamiltonian, remain visible in the derivative spectrum. The interpretation is supported by the EPR powder spectrum of the Gd3+ doped compound CaHf4O9, which changes from broadened into clearly resolved, when the compound is ordered by an annealing process.

Soliton excitations in one-dimensional molecular systems

Abstract: It is shown that the presence or the absence of a soliton excitation threshold and the threshold values depend on the character of the excitation distribution at the initial time moment. A study is made of the time evolution of an excitation given at the initial moment as a hyperbolic secant, in form of a rectangular step, and as a decreasing exponential. Es wird gezeigt, das das Auftreten oder Nichtauftreten der Solitonenanregungsschwelle und die Schwellenwerte vom Charakter der Anregungsverteilung zum Aufangszeitpunkt abhangen. Die zeitliche Entwicklung einer am Anfangszeitpunkt als hyperbolische Sekante, in Form einer Rechteckstufe und als abfallende Exponentialfunktion gegebenen Anregung wird untersucht.

Far ultraviolet photoelectric study of thin SnSe evaporated films

Abstract: Energy distribution curves of photoemitted electrons are obtained for SnSe thin polycrystalline films in the 10 to 40 eV energy range. The method of preparation of the sample under ultrahigh vacuum conditions is described and the synthesis of the initial material. The quality of the sample is checked by Auger analysis which reveals a weak reactivity of the material. The EDC's show structures appearing at 0.9, 2.1 to 2.7, 3.9, 7.4 eV below the top of the valence band. Also the Sn 4d levels are investigated. The 4d(5/2) is found at 24.3 eV and the 4d(3/2) at 25.3 eV below the top of the valence band. Structures in the EDC's are compared with those of existing similar works and are interpreted by means of theoretical bands scheme. Les courbes de distribution energetique des electrons photoemis ont ete obtenues pour des couches minces polycristallines de SnSe dans la region 10 a 40 eV. Nous decrivons la methode de preparation de l'echantillon sous ultra-vide et la synthese du materiau de depart. La qualite de l'echantillon est contrǒlee par analyse Auger qui revele une faible reactivite du materiau. Les EDC montrent des structures apparaissant a 0,9; 2,1 a 2,7; 3,9; 7,4 eV en dessous du haut de la bande de valence. Nous avons aussi etudie les niveaux 4d de l'etain. Nous avons trouve les 4d(5/2) a 24,3 eV et les 4d(3/2) a 25,3 eV en dessous du sommet de la bande de valence. Les structures dans les EDC sont comparees avec celles de travaux similaires existant et sont interpretees au moyen de schemas des bandes theoriques.

Peculiarities of phase transitions and physical properties in (NH4)2SbF5

Abstract: The dynamics of the (NH4)2SbF5 crystal lattice showing the superionic phase transition is studied in a broad temperature interval by NMR and NQR methods, together with electrophysical and calorimetrical properties. The investigations establish the ionic state of the axial fluorine atom in relation to equatorial atoms in the [SbF5]2− complex. The successive phase transitions are revealed and the mechanism of phase transitions is elucidated. The phase transitions are caused by the reorientation movements of NH4 and [SbF5]2− groups. The paper also describes the influence of water solution inclusions in the crystal on the superionic phase transition.

The Internal Soft‐Mode Phase Transition in Solid Biphenyl

Abstract: The phonon dispersion curves in solid biphenyl are calculated. It is shown that the commensurate-incommensurate phase transition is induced by mixing of anharmonic molecular torsion and acoustic phonons. The parameters of the anharmonic potential for torsion vibrations and temperature dependence of the soft mode are obtained. Phononendispersionskurven in festem Biphenyl werden berechnet. Es wird gezeigt, das der Ubergang: kommensurable-inkommensurable Phase durch Mischung von anharmonischer Molekultorsion und akustischen Phononen induziert wird. Die Parameter des anharmonischen Potentials fur die Torsionsschwingungen und die Temperaturabhangigkeit der soft-Mode werden erhalten.

Inverse dielectric function for a semi‐infinite solid

Abstract: An old problem is addressed being the description of the dielectric response of electrons near the surface of an infinite half space to perturbing potentials and external charges. Taking into account spatial dispersion, expressions are derived for the screened potential and the inverse dielectric function within the approximation of specular electron reflection. The expressions generalize former results in a way that spatial symmetries of the problem are properly reflected. The boundary conditions are shown to follow from the definition of the screened potential. The inverse dielectric function obtained in the treatment fulfils the orthogonality relation to the dielectric function and the f-sum rule. Der dielektrische Response von Elektronen an der Oberflache eines unendlichen Halbraumes auf ausere Felder und Ladungen wird unter Berucksichtigung raumlicher Dispersion beschrieben. In der Naherung der spiegelartigen Elektronenreflexion werden Ausdrucke fur das abgeschirmte Potential und die inverse dielektrische Funktion abgeleitet, die bereits bekannte Resultate in der Hinsicht verallgemeinern, das sie die raumlichen Symmetrien des Problems richtig widerspiegeln. Es wird gezeigt, das die verwendeten Grenzbedingungen aus der Definition des abgeschirmten Potentials folgen. Die berechnete inverse dielektrische Funktion erfullt die Orthogonalitatsrelation und den f-Summensatz.

Autolocalized collective excitations in molecular chains with cubic anharmonicity

Abstract: The cubic anharmonicity effect on the soliton motion in molecular chains with electron(exciton)—phonon interaction is investigated. This anharmonicity is shown to remove the infinite increase of the soliton energy as the soliton velocity is reaching the longitudinal sound velocity. [Russian Text Ignored].

Optical properties of K2SeO4 in the vicinity of phase transitions

Abstract: The temperature dependence of the absorption edge of K2SeO4 is measured in the range 300 to 4.2 K. In this temperature range the dependence of the absorption coefficient on the energy of photons and temperature is described by Urbach's rule. The magnitudes of the band gap obtained are E∥ = 6.06 and E⟂ = 6.14 eV for light polarized parallel and perpendicular to the a-axis. The temperature gradient of the band gap (∂E/∂T)∥, ⟂ reveals a jump at the paraelectric-incommensurate phase transition T2 = 129.5 K for both polarizations of incident light in agreement with the second-order nature of the phase transition. In the paraelectric phase, phonons with eigenfrequencies ω∥ = 218 cm−1 and ω⟂ = 181 cm−1 are interacting with the optical transitions. A switch-over to the optical phonon of frequency ω∥,⟂ = 92 cm−1 is observed at T2 together with the change of the electron-optical-phonon coupling constants indicating the increase of the ionic character of the bonds in the incommensurate phase between the K+ ions and the (SeO4)2− tetrahedral groups. [Russian Text Ignored].