Phonon Subsystem Research Articles

Toward polaron kinetics in the bogoliubov reduced description method

In the framework of the Bogoliubov method of reduced description of nonequilibrium states, which is based on his functional hypothesis, we obtain a kinetic equation for arbitrary inhomogeneous electron states in a polar crystal in the presence of a strong electric field. Using the reduced description method, we study the equalization of velocities and temperatures of the polaron gas with a small density and the phonon subsystem in the presence of a weak electric field in the spatially homogeneous case. We establish that the nonequilibrium distribution function of a polaron differs from the Maxwell distribution even in the approximation that is linear in small differences between the subsystem velocities and temperatures. We calculate the corresponding relaxation times and polaron mobility.

Laser short-pulse heating of silicon-aluminum thin films

Energy transport in silicon-aluminum thin films is examined during the laser short-pulse irradiation subjected to the silicon film. The silicon film is considered to be at the top of the aluminum film. Thermal boundary resistance at the interface of the films is incorporated in the analysis. The absorption of laser radiation in the silicon and aluminum films is modeled using the transfer matrix method. Since the silicon film is dielectric, the phonon radiative transport basing the Boltzmann transport equation is incorporated to determine equivalent equilibrium temperature in the film while modified two-equation model is used to account for the non-equilibrium energy transport due to thermal separation of electron and phonon sub-systems in the aluminum film during the laser short-pulse heating process.

Search for a disordered phase in solidHe3deformedin situ

A disordered (glassy) state has been searched for in solid $^{3}\mathrm{He}$ deformed in the course of the experiment through precise measurements of pressure. The analysis of the temperature dependence of the crystal pressure measured at a constant volume shows that the main contribution to the pressure is made by the phonon subsystem, with the influence of the disordered phase being very weak. Annealing of the deformed crystal does not affect this state. The results obtained differ greatly from the corresponding data for solid $^{4}\mathrm{He}$ measured in the region of supersolid effects, where a pressure excessive in comparison with the phonon one was registered. The excess pressure had a quadratic dependence on temperature, which is typical of a disordered system. Absence of the distinct excess pressure in solid $^{3}\mathrm{He}$ is yet unclear; some speculative interpretations are suggested.

Polarization modes in the Ba2Mg2Fe12O22 multiferroic

The spectra of complex permittivity of a Ba2Mg2Fe12O22 single crystal belonging to the family of Y-type hexaferrites have been measured over a wide temperature range (10–300 K) with the aim of determining the dynamic parameters of the phonon and magnetic subsystems in the terahertz and infrared frequency ranges (3–4500 cm−1). A factor-group analysis of the vibrational modes has been performed, and the results obtained have been compared with the experimentally observed resonances. The oscillator parameters of all nineteen phonon modes of Eu symmetry, which are allowed by the symmetry of the Ba2Mg2Fe12O22 crystal lattice, have been calculated. It has been found that, at temperatures below 195 and 50 K, the spectral response exhibits new absorption lines due to magnetic excitations.

Photoinduced piezooptical changes caused by microsecond CO 2 Infrared lasers in lead-germanate rare earth tridoped glasses

We have observed substantial influence of the illumination by 3.5 μs CO 2 laser on the piezooptical coefficients of 59 PbO – 41 GeO 2 (in wt.%) glasses prepared with Tm 2O 3, Er 2O 3, Yb 2O 3 using the following concentrations (in wt.%): B–Tm 2O 3 (0.5)/Yb 2O 3 (1.0)/Er 2O 3(0.5); C–Tm 2O 3(0.5)/Yb 2O 3(1.0)/Er 2O 3 O(1.0); D–Tm 2O 3 (0.5)/Yb 2O 3 (1.0)/Er 2O 3 (2.0). We have found that the maximal changes were observed for B samples that contain the lowest concentration of Er 2O 3. The maximal changes are observed around 360 nm near the fundamental absorption edge. There exists some correlation between the maximal intensities of the photoluminescence and the maximal piezooptical changes. The CO 2 lasers cause preliminary photothermal changes. The existence of the maxima may be caused by a competition between the phonon and anharmonic phonon subsystems, which are very sensitive to the temperature.

Heating of nonequilibrium electrons by laser radiation in solid transparent dielectrics

A computer simulation of the heating of nonequilibrium electrons by an intense high-frequency electromagnetic field leading to the bulk damage of solid transparent dielectrics under single irradiation has been carried out. The dependences of the avalanche ionization rate on threshold field strength have been derived. Using the Fokker-Planck equation with a flux-doubling boundary condition is shown to lead to noticeable errors even at a ratio of the photon energy to the band gap ∼0.1. The series of dependences of the critical fields on pulse duration have been constructed for various initial lattice temperatures and laser wavelengths, which allow the electron avalanche to be identified as a limiting breakdown mechanism. The ratio of the energy stored in the electron subsystem to the excess (with respect to the equilibrium state) energy of the phonon subsystem by the end of laser pulse action has been calculated both with and without allowance for phonon heating. The influence of phonon heating on the impact avalanche ionization rate is analyzed.

Fluctuation properties of phonons generated by ultrafast optical excitation of a quantum dot

AbstractIn this paper, the fluctuation properties of phonons after the optical excitation of a quantum dot with an ultrashort laser pulse are studied theoretically. When a single pulse with pulse area π creates an exciton in the system, a coherent phonon state builds up. Fluctuations of coherent states are time independent and equal to the vacuum fluctuations. By an excitation with a π/2 pulse a superposition of ground and exciton state is created. In this case the phonon system becomes entangled with the electronic system. In the phonon subsystem this leads to the formation of a statistical mixture of the vacuum and the coherent state. The fluctuations of this mixture oscillate in time with both the single and the double phonon frequency, but never exhibit squeezing. The discrepancies with the predictions made in a Raman tensor model are briefly discussed. All scenarios are illustrated using the Wigner function.

A comprehensive model for the ultrashort visible light irradiation of semiconductors

We present a new model describing the irradiation of semiconductors with ultrashort laser pulses. Based on the earlier developed multiple rate equation [Rethfeld, Phys. Rev. Lett. 92, 187401 (2004)], the model additionally includes the interaction of electrons with the phononic subsystem of the lattice and allows for the direct determination of the conditions for crystal damage. In contrast to commonly used approaches based on the thermodynamic description of the material, our model is applicable to nonequilibrium electronic conditions. Accounting for the dynamic changes in the optical properties of the target (i.e., reflectivity, photoabsorption coefficients), the developed model allows for a comprehensive evaluation of the damage by tracing the changes in the optical parameters, lattice heating and subsequent melting. Our model effectively describes the dynamics of the electronic subsystem and lattice heating and the results are in very good agreement with experimental measurements on the transient reflectivity and the fluence damage threshold of silicon irradiated with a femtosecond laser pulse.

The stabilization of magnetic solitons in carbon nanotubes by a constant electric field at low temperatures

The propagation of a variable electromagnetic field in a bundle of zigzag carbon nanotubes in the presence of a constant electric field at low temperatures is considered. The electronic system of nanotubes is treated microscopically ignoring interactions with the phonon subsystem, because the electromagnetic field pulse is extremely short. An effective equation for the amplitude of the vector-potential of a variable electromagnetic field was obtained. Solutions similar to solitons that corresponded to isolated waves in the case of the cosine dispersion law for the electronic subsystem were revealed. The dependences of the obtained nonlinear solutions on problem parameters and an applied constant electric field were analyzed.

Behavior of quantum entropies in polaronic systems

Quantum entropies and state distances are analyzed in polaronic systems with short range (Holstein model) and long range (Fr$\ddot{o}$hlich model) electron-phonon coupling. These quantities are extracted by a variational wave function which describes very accurately polaron systems with arbitrary size in all the relevant parameter regimes. With the use of quantum information tools, the crossover region from weak to strong coupling regime can be characterized with high precision. Then, the linear entropy is found to be very sensitive to the range of the electron-phonon coupling and the adiabatic ratio. Finally, the entanglement entropy is studied as a function of the system size pointing out that it not bounded, but scales as the logarithm of the size either for weak electron-phonon coupling or for short range interaction. This behavior is ascribed to the peculiar coupling induced by the single electron itinerant dynamics on the phonon subsystem.

Relaxation dynamics of an exactly solvable electron-phonon model

We address the question whether observables of an exactly solvable model of electrons coupled to (optical) phonons relax into large time stationary state values and investigate if the asymptotic expectation values can be computed using a stationary density matrix. Two initial nonequilibrium situations are considered. A sudden quench of the electron-phonon coupling, starting from the noninteracting canonical equilibrium at temperature T in the electron as well as in the phonon subsystems, leads to a rather simple dynamics. A richer time evolution emerges if the initial state is taken as the product of the phonon vacuum and the filled Fermi sea supplemented by a highly excited additional electron. Our model has a natural set of constants of motion, with as many elements as degrees of freedom. In accordance with earlier studies of such type of models we find that expectation values which become stationary can be described by the density matrix of a generalized Gibbs ensemble which differs from that of a canonical ensemble. For the model at hand it appears to be evident that the eigenmode occupancy operators should be used in the construction of the stationary density matrix.

Energy exchange between electrons and phonons in quantum dot connected between pyramidal and abrupt transport nanojunctions

Electron–phonon scattering induced intensive heat generation is one of the major bottlenecks for high performance nanoelectronic circuits in miniaturizing their line widths beyond submicron scales. The existence of quantum confinement effects in nanoscaled conduction channels results in which the behaviors of electrons and phonons will become drastically different from those in bulk materials. This is especially true in the junction regions where the nanochannel is linked with the electronic and thermal reservoirs. The present study investigated the structural effect of pyramidal and abrupt junctions in heat exchange between electron and phonon subsystems in the transport through a quantum dot (QD). The numerical results indicated that by confining the electronic and phononic wave functions in the pyramidal junctions, a higher saturation heat exchange would be reached at a lower bias, compared to that of the abrupt junction. The pyramidal junction also becomes more subjected to size effects, where the saturation heat exchange decreases as the size of the junction increases. Such an effect is less significant in the abrupt junctions. Surface reconstruction induced bond stiffening in the pyramidal junctions plays a dominant role in modulating the junction heat exchange by blockading the phonon transportation between the reservoirs through the QD, which effectively reducing the amount of heat being generated. The results may provide new insights on the fundamental science and relationship between contact atomic structures and thermal dissipations in nanoelectronics.

Temperature dependence of electron-phonon thermalization and its correlation to ultrafast magnetism

We report a systematic measurement of the electron-phonon thermalization time scale as a function of the sample base temperature in Ni using femtosecond electron diffraction. A strong temperature dependence of this time scale has been observed in the vicinity of the Curie point. By assuming that ultrafast magnetism completes well before the electron-phonon thermalization, this correlation between thermalization time and sample base temperature fits a modified three-temperature model describing the energy transfer among charge, spin, and phonon subsystems. The results indicate that charges and spins can be characterized by a unified temperature during ultrafast lattice heating.

Cooperative Two-Quanta Phase Transitions in Quantum Optics and Superconductivity

The behavior of an electronic subsystem in strong interaction with phonon subsystem, or quantified electromagnetic field (QEF) is discussed. In this case the correlation effect between first and second order electron–phonon interaction (or atom–QEF interaction) takes place. It is shown that the temperature dependence of two-quanta exchange between Fermi sub-system and thermal reservoir gives a non-linear behavior of the order parameter in superconductivity and super-radiance, accompanied by an increase of the electron correlations with increasing temperature. The same effect is considered for two-quanta scattering processes, in which one quantum is absorbed and another is emitted. It is demonstrated, that the order parameter in such a system firstly increases with temperature achieving the maximal value. After that it decreases as in traditional phase transition effects.

Energy relaxation time in NbN and YBCO thin films under optical irradiation

For a systematic study of energy relaxation processes in thin NbN and YBCO films on sapphire substrates, a frequency domain technique has been set up and employed. The magnetron sputtered NbN films of 3 nm to 22 nm thickness and pulsed-laser deposited YBCO films with thicknesses between 20 nm and 45 nm were excited by amplitude-modulated optical radiation (λ = 850 nm). The response spectra were analyzed on basis of the two-temperature model of the energy dynamics in the interacting electron and phonon subsystems at quasi-equilibrium conditions. An increase of the energy relaxation time with increasing film thickness has been obtained for both NbN and YBCO thin film samples.

Nature of the low-energy excitations of a charge-ordered phase of La0.25Ca0.75MnO3 manganites

The nature of the low-energy excitations of polycrystalline and nanostructured La0.25Ca0.75MnO3 samples has been analyzed in order to investigate the mechanisms of charge ordering in manganites. It has been found that the electrodynamic response spectra of La0.25Ca0.75MnO3 in the energy range of 0.5 to 90 meV and the temperature range of 5 to 300 K have no resonance features that could be attributed to the collective excitations of the charge-ordered phase. It has been shown that the absorption lines observed at frequencies of 20–40 and 80–100 cm–1 are attributed to usual acoustic phonons becoming optically active owing to the structure phase transition and the appearance of a fourfold superstructure with a quadruple period along the crystallographic a axis. The suppression of the superstructure has been revealed in samples with nanocrystallites (≤40 nm). This suppression indicates a relatively weak coupling of the charge and magnetic order parameters with the phonon subsystem.

ELASTOOPTICAL SPECTRA OF NOVEL L-LYSINE MONOHYDROCHLORIDE DEHYDRATE SINGLE CRYSTALS

We have established highly anisotropic features of the piezooptical coefficient behavior in L-lysine monohydrochloride dihydrate (LLMHCl) single crystals (a semiorganic material with space group P2 1). These coefficients show substantial anisotropy for the diagonal and off-diagonal tensor coefficients. The XRD analysis, FTIR, optical and FT Raman spectra were performed. Following the obtained data, principal role of the phonon sub-system was established.

Electromagnetic solitons in carbon nanotubes at low temperatures

The propagation of a variable electromagnetic field in arrays of “zigzag” carbon nanotubes at low temperatures is considered. The electronic system of carbon nanotubes is analyzed using the Hamilton formalism with ignoring interactions with the phonon subsystem because the electromagnetic field pulse is extremely short. An effective equation for the amplitude of the electromagnetic field vector-potential was obtained. Solutions-analogues of solitons were revealed; these solutions corresponded to solitons for the cosine electronic subsystem dispersion law. The dependences of the nonlinear solutions obtained on problem parameters were analyzed.

Specific features of phonon subsystems in diborides of rare-earth elements

The temperature dependences of the lattice heat capacity of yttrium, terbium, erbium, and lutetium diborides have been studied using Montroll’s method of moments. The characteristic temperatures of the diborides are determined at absolute zero temperature and at the temperature tending to infinity. The geometric mean frequencies of phonon spectra have been calculated. The influence of the rare-earth ion mass and the lanthanide contraction phenomenon on the lattice dynamics has been analyzed for the rare-earth diborides.

Nonlinear conductivity of single-walled zigzag carbon nanotubes

The conductivity of the single-walled zigzag carbon nanotube system was studied in an alternating electric field with the intensity vector along the axis of nanotubes. The electronic carbon nanotube system was macroscopically considered in terms of the Boltzmann kinetic equation in the constant relaxation time approximation while omitting the interaction with the phonon subsystem. The nonlinear responses to the applied harmonic field were calculated and analyzed.