Direct measurement of the electron-phonon interaction in Pd,Mo and W by point-contact spectroscopy

The point-contact electron-phonon interaction function α²F has been obtained for the three transition metals Pd, Mo and W. The measured^Pα²F_p-spectra show clear structures which are in agreement with characteristic features in the phonon densities of states and calculated α²(ω)F(ω)-spectra.     

Singularities in V3Si point-contact spectra

The V₃Si point-contact has been studied. The low-frequency mode of electron-phonon interaction has been observed by eV ～ 16 meV unknown previously. The “dirty” point-contacts exhibited the effects of the negative contribution to the dynamic electric resistance which might be the manifestation of the electron-electron correlations.     

Electron-phonon relaxation time in mercury

Electron scattering times determined by Azbel'-Kaner cyclotron resonance in mercury are discussed in terms of the electron-phonon interaction. Anisotropy in the electron-phonon relaxation time is explained in terms of an orbital α _k ² (ω)F _k (ω) function which describes the electron-phonon interaction and the phonon spectrum. AT ⁵ temperature dependence of the electron-phonon relaxation frequency is also considered.     

Energy gaps and phonon structures in tunneling spectra of Bi2Sr2Ca1Cu2O8+x and Bi2Sr2Ca2Cu3O10+y superconductors

Polycrystalline Bi-2212 or Bi-2223 systems and single crystals of the Bi-2212 system were investigated at several temperatures by means of the break-junction and point-contact tunneling techniques, respectively. In the case of Bi-2223 we obtained an averaged gap value 2=65±4 meV. The Bi-2212 system yields 2= 45±5 meV in the case of polycrystalline samples and 2=47±5 meV in the case of single crystals. Hence there result BCS-ratios 2/k _B T _c of 7.3, 6.8 and 6.5, respectively. A dc-Josephson-type supercurrent could be observed in polycrystalline Bi-2212 samples in addition to the usual gap structure. The point-contact tunneling spectra of Bi-2212 single crystals exhibit structures in the second derivative d² I/d V ² which we interpret as inelastic tunneling processes due to electron-phonon scattering most likely in the barrier region. The phonon energies deduced from these structures are in accordance with data obtained from inelastic neutron scattering and Raman spectroscopy. Good agreement is obtained with a lattice dynamical calculation of the partial phonon densities of states of individual atoms in the unit cell.     

Optical properties of BiTeI semiconductor with a strong Rashba spin-orbit interaction

We have modeled the 4f ¹-5d ¹ absorption spectrum of a LiYF₄:Ce³⁺ crystal at zero temperature using a microscopic model of the electron-phonon interaction and the real spectrum of LiYF₄ lattice vibrations. Effects caused by mixing of the wave functions of different states of the 5d ¹ excited configuration of the Ce³⁺ ion, which is induced by the electron-phonon interaction, are considered based on the calculations of the second-, third-, and fourth-order exact moments of curvature of the spectrum envelope. We have shown that the large value of the splitting between the maxima of the bands in the absorption spectrum that correspond to transitions to the third and fourth 5d ¹ levels is a result of the nonadiabatic interaction of 5d electrons with lattice vibrations.     

Effect of electron-phonon interaction and temperature dependence of resistivity in some heavy fermion systems

Here we have considered the electron-phonon interaction in the Periodic Anderson Model (PAM) to describe the temperature dependence of resistivity in some heavy fermion (HF) systems. Since the resistivity is related to the imaginary part of the electron self energy, the expression of the same is evaluated from electron Green function by the double time temperature dependant Green function technique of the Zubarev type. In order to understand the effect of electron-phonon interaction in these systems, we couple the phonons to both the f-electrons as well as to the electrons of the hybridization band of both conduction and f-electrons. The influence of various model parameters, namely, the position of 4f level E ₀, the electron-phonon coupling strengths f ₁(q) and f ₂(q), the effecting coupling strength g = N(0)γ ₀²/ω ₀ have been studied on the temperature dependence of resistivity in HF systems. The numerical analysis is performed for q = 0 and for finite temperature in the static limit. The analysis of the results gives satisfactorily in comparison to the experimental observations.     

Point-contact spectroscopy of renium single crystal

An electron-phonon interaction in the single crystal renium has been studied by the method of point-contact spectroscopy. The transverse phonon peak (hω = 15 meV) and longitudinal one (hω = 25 meV) have been determined over the energy scale. Numerical estimations of the transport electron-phonon interaction function (λ_tr = 0.07) and of the average phonon frequency $\text{〈ω}{}^2\text{〉}{}^{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}\text{= 210 ± 20 K}$ are done which are supported by the data on temperature dependences of electro-resistivity and heat capacity.     

Temperature effects on the phonon spectrum in YBa2Cu3O7 single crystals and thin films

We have performed detailed investigations on the temperature dependence of the 335 cm^–1 phonon in single crystals and thin films of the YBa₂Cu₃O₇ superconductor. The frequency of this phonon exhibits a downshift of about 5 cm^–1 on passing the superconducting transition from above. The shift of the phonon in thin epitaxial films on MgO or SrTiO₃ substrates is only about 2.5 cm^–1. The width of the asymmetric phonon line displays a slight increase belowT _c due to the electron-phonon interaction in these systems.     

Theoretical analysis of the thermal conductivity of high-temperature superconductors

Summary The thermal conductivity of YBa₂Cu₃O_7−δ high-T _c superconductor is analysed self-consistently on both normal and superconducting states on the base of the Bardeen-Rickayzen-Tewordt extended theory to take into account the effects of magnetic field and superconducting fluctuations. It is shown that experimental data are in a quantitative agreement with theory even if the number and variation intervals of adjustable parameters are substantially reduced in comparison with previous works. Phonon relaxation rates due to different mechanisms of phonon scattering as well as the parameters of electron-phonon interaction are estimated. It is shown that thermal conductivity in YBa₂Cu₃O_7−δ is consistent with the BCS model with intermediate electron-phonon coupling λ=1–3 the phonon-electron and electron-phonon relaxation times near critical temperature are evaluated to be 10⁻¹⁰s and 10⁻¹²s, respectively.     

A new optical method for measuring the electron-phonon interaction parameter λ〈Ω2〉 using the spectral dependence of the relaxation rate

The spectral dependence of the electron-phonon relaxation rate γ_e−ph(ℏω) in metals is studied in pump-supercontinuum-probe (PSCP) experiments with femtosecond time resolution. Investigation of this spectral dependence, which exhibits a substantial slowing of the relaxation rate γ_e−ph(ℏω) near the Fermi level E _F , using the parametrization γ_e−ph(ℏω)∝λ〈Ω²〉 (ℏω−E _F )² makes it possible to determine directly the electron-phonon interaction parameter λ〈Ω²〉. The parameter λ〈Ω²〉 for YBa₂Cu₃O_7−δ is analyzed using this method. Pis’ma Zh. éksp. Teor. Fiz. 70, No. 5, 329–332 (10 September 1999)     

Anharmonic oxygen vibration and mode softening in Tl2Ba2Ca2Cu3O10 superconductor

The softening of the IR active 300 cm⁻¹ phonon mode in Tl-2223 superconductor, aroundT _c has been explained by considering a sixth order polarization potential at the off-center oxygen ion site in the Tl-O plane, and by using a nonlinear lattice dynamical theory. The present theory explains, more or less satisfactorily, the unusual temperature dependence of oxygen ion vibration and frequency shifts at higher temperature. The existence of strong nonlinear electron-phonon interaction atT _c has been emphasized.     

Electron—Phonon Interaction,Phonon and Electronic Structures of Layered Electride Ca2N

The phonon and electronic properties, the Eliashberg function and the temperature dependence of resistance of electride Ca₂N are investigated by the DFT-LDA (density functional theory in local density approximation) plane-wave method. The phonon dispersion, the partial phonon density of states and the atomic eigenvectors of zero-center phonons are studied. The electronic band dispersion and partial density of states conclude that Ca₂N is a metal and the Ca 3p, 4s and N 2p orbitals are hybridized. For the analysis of an electron-phonon interaction and its contribution of the Eliashberg function to resistance was calculated and a temperature dependence of resistance due to electron-phonon interaction was found.

     

Resonant tunneling of electrons interacting with phonons

The effect of the electron-phonon interaction on resonant tunneling of electrons through a two-barrier nanostructure is investigated in the framework of a consistent quantum-mechanical model. The wave function is determined by solving the Schrödinger equation with correct boundary conditions in the semiclassical approximation in the electron-phonon interaction. The current calculated with the help of the wave function is averaged over the phonon subsystem with the help of the Bloch theorem. The analytic expressions derived for static and varying currents in a resonance tunnel diode taking into account the electron-phonon interaction formally coincide with the Mössbauer effect probability. In the adiabatic limit and for a strong electron-phonon interaction, the static current decreases in proportion to η, while the varying low-frequency current is proportional to η². The shape of the resonance curve becomes Gaussian with a width of τ _ph ^?1 . The fundamental result is that the properties inherent in coherent tunneling are preserved even in the limit η?1 (which is often regarded as incoherent). The most striking effect (analogous to the Mössbauer effect) is the conservation of a narrow Lorentzian resonance curve in the limit η?1, ω_ph?Γ. This means that even for η?1, the resonance current is due to coherent electrons (experiencing interference), but their fraction decreases in view of the electron-phonon interaction. It is concluded that the application of the rate equations and other approximate methods disregarding interference may lead to incorrect results. The expressions for the high-frequency and nonlinear responses are also derived. The quantum-mechanical regime is found to be less sensitive to the effect of phonons than the classical regime.     

Magnetic and Transport Properties of Tm2Co7B3 Compound

Magnetic and transport properties of Tm₂Co₇B₃ compound have been studied. This compound crystallizes in the hexagonal Ce₂Co₇B₃ type structure. The coercivity (H _c) of the compound was determined from hysteresis measurements in fields up to 4 T. The temperature dependence of coercivity has been explained by a thermally activated process of domain wall motion. The resistivity at low temperatures shows a T ² dependence. At higher temperatures the resistivity is not a linear function of temperature, which indicates an electron-phonon interaction in the presence of a small s-d scattering.     

Superconducting state parameters of metallic glass Mg70Zn30 using linearized screened pseudopotential

A linearized form for the screened form factors of electron-phonon interaction in the metallic glass Mg₇₀Zn₃₀ is applied for the first time to predict the superconducting state parameters viz. electron-phonon coupling strength (λ), Coulomb pseudopotential (μ*), transition temperature (T _c), isotope effect exponent (α) and the interaction strength (N ₀ V). Computed results agree with the experimental data available in the literature.     

Femtosecond coherent spectroscopy of four-wave mixing and photon echoes in a GaAs/AlGaAs heterostructure at room temperature

The results from experiments employing coherent femtosecond spectroscopy in a layer of two-dimensional electron gas at the boundary of the GaAs/AlGaAs heterojunction at room temperature are presented. The decay curves of primary femtosecond photon echo are obtained. The decoherence time in two-dimensional electron gas depends strongly on the power of the exciting pulse and varies from 36 to 54 fs. The dephasing time is studied for the first time as a function of the power of exciting pulses at room temperature. It is established that this dependence obeys the law T ₂ ∼ N ^−0.22, which differs from the typical law T ₂ ∼ N ⁻¹ for unscreened electron-electron interaction in semiconductor crystals. Analysis shows that electron-phonon interaction plays an important part along with electron-electron interaction. The induced spin gratings in the GaAs/AlGaAs heterostructure are studied with an eye to their possible application in spintronics.     

Hydrostatic pressure effects on the superconducting properties of Ag1−xSn1+xSe2

The superconducting transition temperature, T_c, of NaCl structure compounds in the series Ag_1?xSn_1+xSe₂ is found to be exceptionally sensitive to hydrostatic pressures up to 21 kbar. For five samples of varying composition, T_c is suppressed smoothly at a rate of ?(6?8)×10^?5K·bar^?1. These results are discussed with respect to the volume sensitivity of the electron-phonon interaction responsible for superconductivity.     

Influence of isomorphous substitution and external hydrostatic pressure on the fundamental light-absorption edge in Sn(Pb)2P2S(Se)6 crystals

The influence of isomorphous substitution and high hydrostatic pressure on the behavior of the fundamental absorption edge (AE) inSn(Pb) ₂ P ₂ S(Se) ₆ ferroelectric crystals is studied. It is shown that the AE in these crystals is caused by an electron-phonon interaction, and oscillation frequencies of interacting phonons correspond to oscillations of the[P ₂ S(Se) ₆]⁴⁻ anion sublattice. Baric and concentration changes of the Uhrbach rule parameters are analyzed. Uzhgorod State University. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 9, pp. 23–28, September, 1999.     

Pressure dependence study on the electron-phonon coupling of thulium hexachloroelpasolite

Raman spectra of Cs₂NaTmCl₆ have been recorded using a diamond anvil cell at ambient temperature. The vibrational energy of each of the Raman-active TmCl₆⁻³ moiety modes increases linearly with pressure. The integrated band areas of the ν₁(a_1g) and ν₂(e_g) modes are independent of applied pressure. However, the band area of the ν₅(t_2g) mode shows an anomalous behaviour, which has been qualitatively interpreted as due to electron-phonon coupling of the aΓ₅ electronic state with the Γ₁+ν₅(t_2g) vibronic state. This interaction between the coupled states is strongest between ca. 10 and 13 GPa at ambient temperature. The results serve to emphasize the specificity of the occurrence of strong electron-phonon coupling for particular transitions of a given rare earth ion.     

Spectroscopy of phonon and vibronic states of YbAl3(BO3)4 single crystal

The polarized Raman and reflection spectra of a single crystal YbAl₃(BO₃)₄ at room temperature were studied. Raman active vibrational modes A ₁, E _TO, and E _LO are identified. In the Raman spectrum, we detected an intense line at a frequency of 1018 cm⁻¹, which refers to internal vibrations of the BO₃ group and is known to be promising for use in amplifiers based on stimulated Raman scattering. From the simulation of reflection spectra by the method of dispersion analysis the frequencies of A ₂ vibrational modes were determined. Intense bands observed in the low-temperature transmission spectra in the range of f-f transitions in the Yb³⁺ ion are attributed to electron-phonon transitions. The Raman lines are compared with electron-phonon lines in the transmission spectrum.