Effect of electron-phonon interaction on the thermoelectric properties of superlattices

The electron relaxation time on acoustical phonons, the electrical conductivity, and the phonon-drag thermopower of a semiconductor superlattice with quasi-two-dimensional quantum wells are calculated. The inelasticity of the scattering of charge carriers is taken into account. It is shown that the phonon-drag thermopower of a superlattice can be an order of magnitude greater than the corresponding thermopower of a bulk semiconductor. Pis’ma Zh. éksp. Teor. Fiz. 69, No. 4, 290–295 (25 February 1999)     

Phonon frequency shift and effect of correlation on the electron-phonon interaction in heavy fermion systems

The electron-phonon interaction in the periodic Anderson model (PAM) is considered. The PAM incorporates the effect of onsite Coulomb interaction (U) between f-electrons. The influence of Coulomb correlation U on the phonon response of the system is studied by evaluating the phonon spectral function for various parameters of the model. The numerical evaluation of the spectral function is carried out in the long wavelength limit at finite temperatures keeping only linear terms in U. The observed behaviour is found to agree well with the general features obtained experimentally for some heavy fermion (HF) systems.     

Pressure dependence of knight shift in lithium

A simple explanation of the recent experimental observation of Kushida and Hanabusa on the volume dependence of the average spin density P_F at the nucleus for lithium metal is given. It is argued that the increase in the direct contribution to P_F associated with the decrease in volume (increase in pressure) is substantially reduced by an increase in the negative core polarization contribution to P_F.     

Effect of electron-phonon interaction on optical response in one-dimensional cuprates

We examine the single-particle excitation and linear optical absorption spectra in the one-dimensional (1D) extended Hubbard-Holstein model. We perform dynamical density matrix renormalization group calculations with use of pseudo-site representation of phonons. We focus on the interplay among phonons and elementary excitations in 1D Mott insulators. The excitations in the Mott insulators are easily modified by the phonons. We discuss implications of the present results in light of spectroscopic measurements in 1D cuprates.     

Temperature dependence of the positive-muon knight shift in MnSi

The temperature dependence of the μ⁺ Knight shift in weakly-helimagnetic MnSi has been measured in the temperature range between 28 and 300 K. The observed shift is found to be directly proportional to the host susceptibility in the paramagnetic state with a hyperfine coupling constant of —4.8 kOe/_μB.     

Relativistic electron-phonon interaction

On the basis of a new relativistic one-particle equation it is possible to reformulate the problem of the relativistic electron-phonon interaction in terms of “oscillating rigid muffin-tins” such that it can be expressed as in the non-relativistic case. The improvement over the non-relativistic approximation is shown for lanthanum and tungsten.     

Pressure dependence of the muon knight shift in antimony

The pressure dependence of the muon Knight shift in antimony has been measured at 30K using polycrystalline samples and at 10K using single crystals. A considerably stronger pressure dependence is observed with the field parallel to the c-axis than perpendicular. The deduced linear parts of the isotropic and axial pressure dependence are dK_iso/dP=−0.19(3)%/kbar and dK_ax/dP−0.24 (5)%/kbar. First principle molecular-cluster calculations show the origin, of the huge Knight shift and its pressure dependence.     

Theory of temperature-dependent knight shift in narrow-gap semiconductors

We derive a theory of the temperature-dependent Knight shift in narrow-gap semiconductors. As an example, we calculate the temperature variation of the Knight shift of ²⁰⁷Pb in p-PbTe, which agrees fairly well with experiment.     

Interpretation of the knight shift in liquid and solid metals

It is suggested that the density of states in many solid metals is close to the free-electron value. This would explain the small change of the Knight shift on melting, and is supported by general theory and by some empirical evidence.     

Effect of inelastic electron-phonon interaction on tunneling magnetoresistance through ferromagnetic-organic molecule-ferromagnetic junction

We have studied the effect of inelastic electron-phonon interaction on the spin-dependent transport properties of a molecule, trans-polyacetylene (trans-PA), as a molecular bridge sandwiched between two ferromagnetic (FM) electrodes. The work is based on a tight-binding Hamiltonian model within the framework of a generalized Green’s function technique and relies on the Landauer-Büttiker formalism as the basis for studying the current-voltage characteristic of this system. We use the wide-band approximation for FM electrodes. It is shown that due to inelastic interactions, the spin currents increase in a finite value of voltage and tunnel magnetoresistance (TMR) decreases compared with TMR obtained in the absence of phonons.     

Resonance enhancement of electron-phonon interaction

A superconducting state near the phase transition curve from paramagnetic phase to superconducting phase for perovskites La_{2 ? x} Sr _x CuO₄ and YBa₂Cu₃O₆ is considered. Expressions for effective parameters of electron-spin-phonon interaction are obtained. It is shown that the critical temperature of the phase transition from paramagnetic phase to superconducting phase T _c is determined by the enhancement of electron-phonon interaction by spin fluctuations of exchange type.     

Effect of electron-phonon interaction on surface states in a ternary mixed crystal

A variational theory is proposed to study the surface states of electrons in a semi-infinite ternary mixed crystal, by taking the effect of electron-surface optical (SO) phonon interaction into account. The energy and the wave function of the electronic surface-states are calculated. The numerical results of the energies of the surface states of the polarons and the self-trapping energies are obtained as functions of the composition x and surface potential V₀ for several ternary mixed crystal materials. The results show that the electron-phonon interaction lowers the surface-state levels with the energies from several to scores of meV. It is also found that the self-trapping energy of the surface polaron has a minimum at some middle value of the composition x. It is indicated that the electron-phonon coupling effect can not be neglected. Received 4 January 1999 and Received in final form 7 January 2000     

Anisotropic electron-phonon interaction in the cuprates

We explore manifestations of electron-phonon coupling on the electron spectral function for two phonon modes in the cuprates exhibiting strong renormalizations with temperature and doping. Applying simple symmetry considerations and kinematic constraints, we find that the out-of-plane, out-of-phase O buckling mode (B(1g)) involves small momentum transfers and couples strongly to electronic states near the antinode while the in-plane Cu-O breathing modes involve large momentum transfers and couples strongly to nodal electronic states. Band renormalization effects are found to be strongest in the superconducting state near the antinode, in full agreement with angle-resolved photoemission spectroscopy data.     

Effect of high pressure on the electron-phonon interaction and superconductivity in ZrN and HfN

Ab initio calculations of the superconducting properties have been performed for zirconium and hafnium nitrides at normal and high pressures. The results for ZrN are in good agreement with the existing data of the tunnel experiments and measurements of the pressure derivative of the critical temperature T _c. It has been shown that the decrease in T _c under compression occurs due primarily to an increase in the phonon frequencies.     

Theory of knight shift due to indirect nuclear hyperfine interactions

We derive a theory of Knight shift (K) in solids including the effects of periodic potential, spin-orbit interaction, magnetic hyperfine interactions and indirect nuclear hyperfine interaction. We use a temperature dependent Green's function technique to evaluate the thermodynamic potential which is then used to obtain a general expresion for the Knight shift. Our formula for K is expressed as a sum of contributions due to conduction electrons and localized electrons of either d- or f-type: K_cond and K_loc. While K_cond is the same as our previous expression for K derived in the absence of localized magnetic moments, K_loc is a new contribution and is due to the hybridization of conduction and localized electron magnetic moments. We also briefly discuss the many-body effects on the different contributions to K. Finally, the importance of the present theory in possible applications to metals, alloys and compounds containing transition and rare-earth elements, and magnetic semiconductors is discussed.     

Electronic-structure dependence of the electron-phonon interaction in Ag

The linewidths of sp- and d-band derived electronic quantum-well states in thin films of Ag on Fe(100) are measured as a function of temperature to yield the electron-phonon coupling parameters. The results vary by a factor of up to 35 among the different states. The origin of these huge differences is traced to the decay path selection for the various initial states of the holes created by the photoemission process. The electron-phonon coupling parameter for the top d-band quantum-well state, 0.015+/-0.006, is the smallest ever reported.     

Effect of electron-phonon interaction on the binding energy of a wannier exciton in a polarizable medium

A new phenomenological effective interaction potential between an electron and a hole of a Wannier exciton in a polarizable medium is proposed. Binding energies of Wannier excitons are calculated variationally using this potential in several polar crystals. A close agreement between the theoretical values thus calculated and the experimental values is found in all polar crystals including thallium halides.