Lattice Subsystems Research Articles

Phase transitions and relaxor properties of doped quantum paraelectrics

Scaling properties of the paraelectric susceptibility with respect to temperature T and electric field E have been evidenced on solid solutions Sr 1− x Ca x TiO 3, x⪡1, and are explained within the framework of both the transverse Ising model (TIM) and an anharmonic oscillator model (AOM). The impurity-induced ferroelectric phase transitions occurring at x> x c≈0.002 are modeled using a heterogeneous TIM, which is solved numerically. Alternatively, we present a coupled mode model involving a TIM and an AOM for the impurity and the host lattice subsystems, respectively. Relaxor properties such as dielectric polydispersivity, quasi-first-order Raman scattering, bilinear coupling of optic and acoustic phonon modes and hyper-Rayleigh light scattering in the paraelectric regime are attributed to polar nanodomains due to fluctuations of defect-induced quenched random fields (RFs). Some of these features are modeled within the framework of the heterogeneous TIM including RF interactions.

Spin correlations and magnetonuclear cross-correlation in Sm(Sr)-Mn-O perovskites in the low-temperature phase

The results of measurements of spin correlations and magnetonuclear cross-correlations (MNCCs) in the system 154Sm1−xSrxMnO3 with the perovskite structure (x=0.25,0.4) in the low-temperature phase in magnetic fields 0<H<1 kOe are presented. These are the first measurements performed by the small-angle polarized-neutron scattering method (SAPNS). It is shown that ferromagnetic correlations with a scale of 180–250 A and MNCCs characterizing the intercoupling of the magnetic and lattice subsystems on this scale exist in the system. It is found that in the low-temperature phase the system exhibits spin-glass properties.

Polaron dynamics in a two-dimensional anharmonic Holstein model

A generalized two-dimensional semiclassical Holstein model with a realistic on-site potential that contains anharmonicity is studied. More precisely, the lattice subsystem of anharmonic on-site oscillators is supposed to have a restricting core. The core plays the role of an effective saturation nonlinearity for the polaron (self-trapped) solutions. We apply the ``logarithmic'' potential approximation which allows us to use effectively a variational approach, on one hand, and to study the realistic situation of the potential core and saturation nonlinearity, on the other hand. Analytical estimates suggest the existence of wide polarons, contrary to the case with harmonic on-site potential. Numerical simulations confirm these estimates and show stability of such polaron solutions. We develop a numerical technique which allows us to obtain the profile of extended moving polarons. Simulations show that these polarons can propagate for long distances on the plane retaining their shape and velocity. Collision effects of the two-dimensional polarons are also investigated.

Temperature Anomalies of Transport Properties in SnTe Epitaxial Thin Films

The temperature dependences of electrical conductivity σ, Hall coefficient R H and charge carrier mobility μ H in the temperature range of 77 to 300 K were obtained for SnTe epitaxial thin films of different thicknesses (0.2 to 0.6 μm) and with charge carrier concentration of pH77= 3.5 x 10 20 cm -3 . In the vicinity of 200 K anomalies of R H and μ H , were detected most pronounced for the sample with h =0.6 μm bearing evidence of qualitative changes in the electron and lattice subsystems of the crystal. The observed anomalies are attributed to the phase transition consisting of a redistribution of nonstoichiometric defects (cation vacancies) over the crystal lattice. It is suggested that in contrast to bulk crystals, in thin films more favorable conditions for a restruction of the defect subsystem are established which lead to configurational changes which reduce the total energy of the crystal. These processes are controlled to a great extent by kinetic factors.

Soliton-induced modification of the speed of sound in quasi-one-dimensional molecular crystals

The influence of the anharmonic vibron-phonon coupling, arising on account of a `dressing' effect, on the characteristics of the lattice subsystem in quasi-one-dimensional molecular crystals was examined within the `pseudo-harmonic' phonon approximation. It was found that solitons could induce specific modifications of the speed of sound which shows a temperature dependence quite different to that in the case of the linear excitations. The possibility of an indirect experimental verification of the existence of solitons in molecular chains is suggested on the basis of these predictions.

Electron relaxation phenomena on a copper surface via nonlinear ultrashort single-photon photoelectric emission

Time-resolved single-photon photoelectric emission measurements from polycrystalline copper, using an autocorrelation system of 450 fs duration at 248 nm laser pulses, are reported. This experimental technique makes possible the direct measurement of the electron energy relaxation time at the surface as well as the evaluation of the electron - phonon scattering time in metallic samples. A nonlinear increase of the electron photocurrent versus the laser intensity is observed. This nonlinearity is attributed to the non-thermal equilibrium between the electron gas and the lattice produced by the subpicosecond laser pulses and is related to the dynamics of energy relaxation between the electrons and the lattice. The influence of the non-thermal electrons on the photoemission and their thermalization time is discussed. Our experimental results may be explained by a model based on the Fowler - Dubridge equation for photocurrent and on thermal-balance equations between electron and lattice subsystems.

Nonlinear enhancement of the efficiency of the second harmonic radiation produced by ultrashort laser pulses on a gold surface

An unexpected nonlinear enhancement in the efficiency of the second harmonic generation as a function of the fundamental laser intensity is observed, when a gold surface is illuminated by few tens of GW/cm 2 subpicosecond pulses of an excimer-pumped dye laser at grazing incidence. The evidence of this effect is tested by time resolved autocorrelation measurements. This enhancement is attributed to the thermal nonequilibrium, between the electron gas and lattice subsystems, produced by the subpicosecond laser pulses on the metallic surface.

Dynamic Properties of Anharmonic Phonons at Low Temperatures

In this work we investigated the influence of low temperature lattice anharmonicity on the ground state properties of the lattice subsystem. The inspiration sprang from certain experiments revealing that strong low temperature anomalies of certain vibrational modes are common in most high temperature superconductors [1-5], and a recent study of the effect of phonon correlations on superconducting properties by one of the authors, Hakioglu et.al. [6]. We propose a model Hamiltonian comprising the harmonic and fourth order contributions and in conjunction put down an argument for the absence of the third order coupling. Relying on the fact that low temperature anharmonicity produces phase space correlations between otherwise independent phonon modes we assume lowest order pair correlations \phi^{ss'}(\vec{k}) = and proceed by writing down a squeezed variational ground state wavefunction depending on a single parameter, which is to be found via a self-consistent mean field theory within the LTDA model [7].

Soliton-phonon interaction in anharmonic quasi-one-dimensional ferromagnetic crystals: Soliton-induced modification of the speed of sound.

The influence of the spin-phonon coupling on soliton characteristics and the soliton response on the lattice subsystem in a quasi-one-dimensional magnetic chain is examined. It was found that, in the case of the ferromagnetic coupling, these correlations may induce reduction of the effective exchange integral which, in the temperature range where one-dimensional ordering prevails, may be maximally about 1%. Consequently the soliton energy and width suffer the negligible modification, too. On the other hand, the magnetic subsystem may have a certain impact on elastic subsystem characteristics, the speed of sound in particular. The character of these changes depends strongly on the type of the excitations of the magnetic subsystem---solitons or magnons.

The Role of Core Levels in Scintillation Processes

ABSTRACTThe Auger decay of a core hole results in appearance of several strongly correlated excitations. This excited region strongly polarizes the lattice and thus the defect creation is possible. In all cases the core hole causes the strong local perturbation of electronic and lattice subsystems. The creation of such excited region with mutual relaxation of correlated electrons and holes can result in the increase of the efficiency of energy transfer to activators, acceleration of the luminescence kinetics, and the appearance of radiation-induced luminescence centers.

Dynamics of the amide-I excitation in a molecular chain with thermalized acoustic and optical modes

An improved version of the Davydov soliton model to describe the physical mechanisms of the energy transfer in proteins has been introduced. The essential modifications have been done for the lattice subsystem: (1) the intrapeptide N-H bond is allowed to move together with its peptide group and therefore both the acoustic and optical modes appear to be coupled and (2) a harmonic on-site potential, which describes the influence of the environment of the chain, is introduced. The chain is considered to be in contact with a heat bath only via the acoustic mode, while the coupling of the amide-I excitation with the optical mode is shown to be crucial for the thermal stability of the Davydov soliton. The presence of a substrate on-site potential, modelling the influence of the environment, is shown to improve the soliton stability with respect to thermal oscillations since it essentially reduces the amplitude of acoustic oscillations.

Hydrodynamic description of femtosecond transient thermoreflectivity

Femtosecond transient reflectivity curves are computed using hydrodynamic approximation for describing the behaviour of the material in the collisionless free-electron domain. Two cases, the dominant electron-impurity scatterings and the dominant electron-phonon relaxations are considered to describe the heat diffusion in non-equilibrated lattice and electronic subsystems.

Generation of nonequilibrium electron and lattice temperatures in copper by picosecond laser pulses.

Picosecond laser pulses can be used to generate and probe nonequilibrium electron and lattice temperature differences in metals. This is demonstrated by transient thermoreflectance measurements on copper in the vicinity of the d-band absorption edge. Our measurements are in agreement with an energy-balance model based on a separation of the one-dimensional heat-flow equation into electron and lattice subsystems.

Launching of Darydov soliton: I. Soliton analysis

An improved version of the Davydov soliton model to describe the physical mechanisms of the energy transfer in proteins has been introduced. The essential modifications have been done for the lattice subsystem: (1) the intrapeptide N-H bond is allowed to move together with its peptide group and therefore both the acoustic and optical modes appear to be coupled and (2) a harmonic on-site potential, which describes the influence of the environment of the chain, is introduced. The chain is considered to be in contact with a heat bath only via the acoustic mode, while the coupling of the amide-I excitation with the optical mode is shown to be crucial for the thermal stability of the Davydov soliton. The presence of a substrate on-site potential, modelling the influence of the environment, is shown to improve the soliton stability with respect to thermal oscillations since it essentially reduces the amplitude of acoustic oscillations.

The influence of electron-electron and electron-phonon interactions on electron charge orderings in quasi-one-dimensional systems

We consider a model for the description of charge orderings and lattice distortions in quasi-one-dimensional narrow-band systems. The Coulomb interactions and the electron-lattice couplings are included in the model and our attention is focused on the effects of electron-phonon couplings via Coulomb forces. It has been found that these interactions can influence the electronic and lattice subsystems in a different way than the electron-phonon couplings usually considered; e.g. (i) they can contribute to the stabilization of the island-type charge ordering and induce tetramerization of the lattice in this state; (ii) they break the particle-hole symmetry and thus lead to temperature dependence of the chemical potential even in the half-filled band case; (iii) they can lead to the appearance of a disorder point at which the charge correlation functions change in character, and to anomalous dilatation of the lattice.

Thermal conductivity of P-type germanium between 0·2° and 4°K

The influence of the transient thermal effects on the partition of the energy of selfrecoils in germanium and silicon into energy eventually given to electrons and to atomic recoils respectively is studied. The transient effects are treated in the frame of the thermal spike model, which considers the electronic and atomic subsystems coupled through the electron–phonon interaction. For low energies of selfrecoils, we show that the corrections to the energy partition curves due to the energy exchange during the transient processes modify the Lindhard predictions. These effects depend on the initial temperature of the target material, as the energies exchanged between electronic and lattice subsystems have different signs for temperatures lower and higher than about 15 K. Many of the experimental data reported in the literature support the model.