Influence Of Electron-phonon Coupling Research Articles

Temperature-dependent electronic, optical, and solar cell device properties of AlAs and AlSb semiconductors and their p-n homojunctions

AlSb and AlAs have attracted renewed interest in recent years as promising photovoltaic (PV) materials due to significant light absorption, eco-friendliness, and low cost. Here, density functional theory (DFT) in combination with the non-equilibrium Green's function method (NEGF) has been used to compute the temperature-dependent PV parameters of nanoscale AlSb and AlAs p-n junction solar cell devices. In particular, the influence of electron-phonon coupling (EPC) and spin-orbit coupling (SOC) on the PV properties of these devices is investigated. Our results highlight the importance of inclusion of EPC in evaluating the ab initio PV properties of p-n junctions at finite temperatures. At non-zero temperatures, the PV parameters can differ significantly compared to those at zero temperature due to phonon-assisted tunnelling of charge carriers across the p-n junction. In particular, the mobility of low-effective-mass carriers at the p-n junction is found to be enhanced by phonon scattering, which in turn results in improved PV transport properties. The short-circuit current (Jsc) at 300 K proved to be 21.7% (16.2%) higher for AlSb (AlAs) than that at 0 K. On the other hand, the open-circuit voltage (Voc) at 300 K proved to be 19.1% (16.7%) lower for AlSb (AlAs) than that at 0 K. Jsc is higher in AlSb than in AlAs due to higher phonon-influenced mobility of low-effective-mass carriers. Overall, our study strongly suggests the importance of inclusion of EPC and hence phonon scattering in the first-principles quantitative determination of PV parameters of solar cells. The estimates obtained in this study may be utilized as input ab initio parameters in continuum-model-based multi-scale simulations of AlSb and AlAs p-n homo-junction solar cells.

Polaronic Effects on the Electron Raman Scattering in a Spherical Anisotropic Quantum Dot

We study the polaronic effect on the differential cross section (DCS) for an electron Raman scattering process in a spherical anisotropic semiconductor quantum dot in the presence of a homogeneous external electric field. The Fröhlich coupling of electrons with confined and surface polar optical phonon modes for resonance Raman scattering is considered. The influence of electron-phonon coupling on the energy spectrum is taken into account in the framework of perturbation theory. The emission spectra are discussed for different anisotropy cases. Resonant peaks in the spectra are found and interpreted.

Influence of electron-phonon coupling on the low-temperature phases of metallic single-wall carbon nanotubes

We investigate the effect of electron-phonon coupling on low temperature phases in metallic single-wall carbon nanotubes. We obtain low-temperature phase diagrams of armchair and zigzag type nanotubes with screened interactions with a weak-coupling renormalization group approach. In the absence of electron-phonon coupling, two types of nanotubes have similar phase diagrams. A $D$-Mott phase or $d$-wave superconductivity appears when the on-site interaction is dominant, while a charge-density wave or an excitonic insulator phase emerges when the nearest neighbor interaction becomes comparable to the on-site interaction. The electron-phonon coupling, treated by a two-cutoff scaling scheme, leads to different behavior in two types of nanotubes. For strong electron-phonon interactions, phonon softening is induced and a Peierls insulator phase appears in armchair nanotubes. We find that this softening of phonons may occur for any intraband scattering phonon mode. On the other hand, the effect of electron-phonon coupling is negligible for zigzag nanotubes. The distinct behavior of armchair and zigzag nanotubes against lattice distortion is explained by analysis of the renormalization group equations.

The role of phonon-phonon and electron-phonon scattering in thermal transport in PdCoO2.

The layered metal oxide PdCoO2 is of fundamental interest in solid-state physics due to its unique nature as a two-dimensional electron gas. It is well known that electron-phonon scattering plays the leading role in the electrical transport of lots of materials; however, the influence of electron-phonon coupling on the thermal transport in PdCoO2 is rarely studied. Herewith we employ ab initio calculations to study the phonon lifetimes of PdCoO2 due to electron-phonon scattering and anharmonic phonon-phonon interactions. It is found that electron-phonon scattering has a large correction to the lattice thermal conductivity of PdCoO2 for both the in-plane and cross-plane directions, which is reduced by 21% and 27%, respectively. Moreover, the correction becomes larger and is more significant for the in-plane direction with decreasing temperature. As a result, the cross-plane lattice thermal transport overwhelms the in-plane value at low temperatures. This study provides useful guidance for the development of relevant devices involving two-dimensional electron gas systems.

The influence of electron-phonon coupling and spin fluctuations on the superconductivity of the Ti-V alloys

We report a study of the normal and superconducting state properties of the Ti x V1−x alloys for x = 0.4, 0.6, 0.7 and 0.8 with the help of dc magnetization, electrical resistivity and heat capacity measurements along with the electronic structure calculation. The superconducting transition temperature T c of these alloys is higher than that of elemental Ti and is also higher than elemental V for x ≤ 0.7. The roles of electron density of states, electron-phonon coupling and spin fluctuations in the normal and superconducting state properties of these alloys have been investigated in detail. The experimentally observed value of T c is found to be considerably lower than that estimated on the basis of electron density of states and electron-phonon coupling in the x = 0.4, 0.6 and 0.7 alloys. There is some evidence as well for the preformed Cooper pair in all these Ti-V alloys in the temperature regime well above T c . Similar to x = 0.6 [Md. Matin, L.S. Sharath Chandra, R.K. Meena, M.K. Chattopadhyay, A.K. Sinha, M.N. Singh, S.B. Roy, Physica B 436, 20 (2014)], the normal state properties of the x = 0.4 alloy showed the signature of the presence of spin fluctuations. The difference between the experimentally observed T c and that estimated by considering electron density of states and electron-phonon coupling in the x = 0.4, 0.6 and 0.7 alloys is attributed to the possible influence of these spin fluctuations. We show that the non-monotonous variation of T c as a function of x in the Ti x V1−x alloys is due to the combined effects of the electron-phonon coupling and the spin fluctuations.

The Kondo effect in quantum dots coupled to ferromagnetic leads with noncollinearmagnetizations: effects due to electron–phonon coupling

Spin-polarized transport through a quantum dot strongly coupled to ferromagneticelectrodes with noncollinear magnetic moments is analysed theoretically in terms of thenonequilibrium Green function formalism based on the equation of motion method.Electrons in the dot are assumed to be coupled to a phonon bath. The influence ofelectron–phonon coupling on tunnelling current, linear and nonlinear conductance, andtunnel magnetoresistance is studied in detail. Variation of the main Kondo peaks andphonon satellites with the angle between the magnetic moments of the leads isanalysed.

Line shape of emission spectra of the luminescent polymer poly(p-phenylene vinylene)

The influence of electron-phonon coupling in the emission spectra of self-assembly (SA) films of poly(p-phenylene vinylene) (PPV) is discussed. PPV is an important polymeric system because it exhibits strong luminescence effect. A semi-empirical model was used to analyze the photoluminescence spectra of PPV. This model assumes that defects along the molecule give rise to a distribution of conjugated segments of different lengths. In addition, it was considered that electronic transition was essentially coupled with three effective phonon modes with correspondent energies at 62, -139 and 192 meV, which were observed experimentally.

Role of electron-phonon coupling on collision cascade development in Ni, Pd, and Pt

Electron-phonon coupling in energetic collision cascades is believed to greatly enhance the cooling rate of heat spikes in many metals. Previous studies have not been able to conclusively determine the magnitude of the coupling, however. By directly comparing ion-beam mixing experiments and molecular dynamics simulations of collision cascades in Ni, Pd, and Pt metals, in which the coupling is believed to be most important, we show that the influence of electron-phonon coupling on mixing can be no more than about 30%, roughly an order of magnitude less than the most widely used models predict.

Influence of electron-phonon interaction to the periodic Anderson lattice

In heavy fermion compounds, the hybridization effect between the conduction and localized f-orbital electrons gives rise to a breathing phonon mode. This breathing mode couples with the conduction electrons and renormalizes the conduction bandwidth. Using the perturbation method for the one-dimensional periodic Anderson lattice, we study the influence of electron-phonon coupling on the hybridization effect and the stability of paramagnetic and antiferromagnetic states. Our results show that both the hybridization effect and Kondo temperature are reduced. This effect is more significant at large hybridization interaction in that it will destroy the stability of the antiferromagnetic state. However, this is already beyond the parameter ranges for the heavy fermion compounds.

Influence of ion-electron interaction on the formation mechanism of depleted zones in displacement cascades in metals.

The mechanism of redistribution of vacancies in the depleted zone of a displacement cascade in copper has been investigated by molecular-dynamics simulations. A simplified model of the thermal spike was used, for which the calculation cell was cooled down along one axis after vacancies and kinetic energy had been introduced in the center of it. The influence of electron-phonon coupling on the sweeping of vacancies in this highly disordered hot core as it cools was then investigated. The characteristic cooling time, \ensuremath{\tau}, for transfer of heat to the electron system by ion-electron interaction was a variable parameter and was varied from 1 to 10 ps. According to Finnis et al. [Phys. Rev. B 44, 291 (1991)] the values of \ensuremath{\tau}\ensuremath{\le}1 and \ensuremath{\tau}\ensuremath{\ge}5 ps are typical of metals such as Ni and Cu, respectively. In the conditions of the nonuniform distribution of the thermal spike, the maximum size of the real melt becomes a function of \ensuremath{\tau}. The influence of the strong coupling on the vacancy redistribution is more strongly pronounced for thermal spikes in which the average atomic kinetic energy in the melt is close to the value (3${\mathit{k}}_{\mathit{B}}$${\mathit{T}}_{\mathit{m}}$+L), where ${\mathit{T}}_{\mathit{m}}$ is the melting temperature and L is the latent heat of fusion. In this case the time to form the liquid structure by melting is comparable with the time to cool the zone to below the temperature of crystallization. It has been demonstrated that the coupling also influences the number of vacancies trapped by the melt during the advance of the solid/liquid interface. It is shown that in metals with strong electron-phonon coupling, the highly disordered structure of the melted region can be frozen in when a depleted zone with a high concentration of vacancies (\ensuremath{\ge}2--5 at. %) solidifies.

Slave-boson phase diagram of the two-dimensional extended Hubbard model: Influence of electron-phonon coupling

We present a detailed study of the extended Hubbard-Peierls model on a square lattice using the slave-boson method proposed by Kotliar and Ruckenstein. The emphasis is on the investigation of the ground state phase diagram. To compare the relative stability of several homogenous phases, the effective bosonized action was evaluated by means of a two-sublattice saddlepoint approximation which allows for the symmetry broken states compatible with the underlying bipartite lattice structure. Paying particular attention to the interplay of electron-electron and electron-phonon interaction, we take into account various types of magnetic ordered phases, i.e. para-, ferro-, ferri-, and antiferromagnetic states, as well as charge ordered phases, e.g. a static (π, π) Peierls distorted state. Furthermore the approach has been applied to the following special cases: the Hubbard model, the extended Hubbard model, and the Hubbard-Peierls model. A careful numerical solution of the corresponding self-consistency equations enables us to map out the ground-state phase diagrams of the various models at arbitrary band filling over the whole range of interaction strength. In the phase diagram of the Hubbard model we found a large region with ferrimagnetic order away from half-filling. The phase diagram of the halffilled band extended Hubbard model shows a first-order transition from a spin-density-wave to a charge-density-wave state which is displaced from the mean-field lineU=4V towards largerV. At large negativeU andV we obtain a domain with charge separation. The phase compares favorably with earlier quantum Monte-Carlo results. Including the local electron-phonon coupling the charge-density-wave region is considerably enlarged. Away from half-filling the phase diagram becomes more complex: besides the pure magnetic phases we obtain ferri- and paramagnetic states which show additional charge-density order. Aspects of phase separation are discussed. Finally we investigate the variation of the different gap and order parameters along characteristic lines in the parameter space and determine the renormalized quasiparticle bands.

Influence of Electron-Phonon Coupling on Transport near a Mobility Edge

The authors treat electronic transport in disordered systems, including the electron-phonon interaction, on both sides of a mobility edge ${\ensuremath{\epsilon}}_{c}$ on the same footing. By application of a mode-coupling approximation self-consistent equations are derived from which one gets the matrices describing density relaxation due to both coherent tunneling processes and hopping processes. The resulting conductivity interpolates between a metallic behavior well above and a hopping behavior well below ${\ensuremath{\epsilon}}_{c}$.

Influence of electron-phonon coupling on superconducting contacts and the properties of superconductor-semimetal (or semiconductor) systems

Consideration of the proximity effect, electron-phonon coupling, and size-quantization results in peculiar behavior of the critical temperature ${T}_{c}$ of a contact containing superconducting and semimetal (or semiconductor) thin films. The method of thermodynamic Green's functions is applied. The dependence of ${T}_{c}$ on the thickness of the barrier is discussed. Size quantization leads to additional oscillations of ${T}_{c}$ and to the possibility of observing peculiar charge-density waves. The experimental data are discussed.

The influence of electron-phonon coupling on the transition temperature into the SDW and CDW states

In the framework of the two-band model including the intraband electron-phonon coupling, the equation for SDW and CDW states transition temperatures was obtained. The analytical solution is given. Commensurate antiferromagnetic chromium alloys are discussed.

Influence of electron-phonon coupling on the energy of an exciton bound to a donor-acceptor pair

The influence of electron-phonon coupling on the binding energy of excitons bound to donor-acceptor pairs is investigated. It is shown that the experimental data in ZnSe agree very well with a theory which takes into account Coulomb overlap and the configuration interaction. The contribution of electron-phonon coupling is small, but not negligible.