Electron–Phonon Coupling Constant of Uranium and Lutetium

Journal of Superconductivity and Novel Magnetism - According to the importance of electron–phonon interaction, we present an ab initio study of the electron–phonon coupling constant of...     

Electron–Phonon Coupling and Properties of Doped BaBiO3

Density functional calculations based on local density approximation (LDA) of the properties of doped barium bismuthates are reported. Using a linear-response approach within the linear-muffin-tin-orbital method the phonon spectrum of Ba_0.6K_0.4BiO₃ is calculated. The electron–phonon coupling constant is then evaluated for a grid of phonon wavevectors using the self-consistent change in the potential due to phonon distortion. Anharmonic contributions to from the tilting of oxygen octahedra are also evaluated on the basis of the frozen-phonon approach.     

Electron–Phonon Interaction and Optical Spectra of Metals

Observed optical reflectivity in the infrared spectral region is compared with theoretical predictions in a strongly coupled electron–phonon system. Starting from a Fröhlich Hamiltonian, the spectral functions and their temperature dependence are derived. A full analysis including vertex corrections leads to an expression for the optical conductivity () that can be formulated in terms of the well-known optical conductivity for a quasi-isotropic system without vertex corrections. A numerical comparison between the full result and the so-called extended Drude formula, its weak coupling expansion, shows little difference over a wide range of coupling constants. Normal-state optical spectra for the high-T _c superconductors YBa₂Cu₃O₇ and La_{2 – x} Sr _x CuO₄ at optimal doping are compared with the results of model calculations. Taking the plasma frequency and from band structure calculations, the model has only one free parameter, the electron–phonon coupling constant . In both materials the overall behavior of the reflectivity can be well accounted for over a wide frequency range. Systematic differences exist only in the mid-infrared region. They become more pronounced with increasing frequency, which indicates that a detailed model for the optical response should include temperature-dependent mid-infrared bands.     

Effect of the Electron–Phonon Coupling on the Effective Thermal Conductivity of Metallic Bilayers

Systems consisting of metallic layers are commonly used in many applications for microelectronics, data storage, protection coatings, and microelectro-mechanical systems. The physical properties of such systems are strongly determined by the flow of the electron and phonon gases and their interactions. In this study, the effective thermal conductivity of a metal–metal bilayer system is studied using the two-temperature model of heat conduction. By defining the total interfacial thermal resistance, it is shown that the thermal conductivity of the bilayer system depends on the ratio between the thicknesses of the metallic layers and their intrinsic coupling length and it has a simple interpretation as the sum of thermal resistances in series. It is demonstrated that the total interfacial thermal resistance can be minimized by choosing appropriately the thermal and geometrical properties of the component layers. The proposed approach could be useful for thermally characterizing and guiding the design of novel metal–metal-layered systems involved in diverse technological applications.     

The Influence of Electron–Phonon Coupling and Lattice Vibrations on the Rashba Coupling Induced Domain Wall Spin Current and Spin Torque

In the current work, a detailed calculation has been presented for spin-current and spin torque in a domain wall (DW). Specifically, we analyze both spin and momentum relaxation that are relevant to the spin-transport process. It was shown that the vibrational induced relaxations play important roles in the amount of the spin-current generated by the Rashba interaction.     

Electron–Phonon Coupling in Ti/TiN MKIDs Multilayer Microresonator

Over the last few years, there has been a growing interest toward the use of superconducting microwave microresonators operated in quasi-thermal equilibrium mode, especially applied to single particle detection. Indeed, previous devices designed and tested by our group with X-ray sources in the keV range evidenced that several issues arise from the attempt of detection through athermal quasiparticles produced within direct strikes of X-rays in the superconductor material of the resonator. In order to prevent issues related to quasiparticles self-recombination and to avoid exchange of athermal phonons with the substrate, our group focused on the development of thermal superconducting microresonators. In this configuration, resonators composed of multilayer films of Ti/TiN sense the temperature of an absorbing material. To maximize the thermal response, low-critical-temperature films are preferable. By lowering the critical temperature, though, the maximum probing power bearable by the resonators decreases abruptly because of the weakening of the electron–phonon coupling. A proper compromise between the value of critical temperature (and hence sensitivity to energy deposition) and readout power bearable by the device has to be found in order to avoid signal-to-noise ratio degradation. In this contribution, we report the latest measurement of the electron–phonon coupling.     

Quantum Noise of Electron–Phonon Heat Current

We analyze heat current fluctuations between electrons and phonons in a metal. In equilibrium we recover the standard result consistent with the fluctuation–dissipation theorem. Here we show that heat current noise at finite frequencies remains non-vanishing down to zero temperature. From the experimental point of view, it is a small effect and up to now elusive. We briefly discuss the impact of electron–phonon heat current fluctuations on calorimetry, particularly in the regime of single microwave-photon detection.     

Bipolaron Localization for Increasing Electron–Phonon Coupling in a Small Cluster

By exact diagonalization of a small cluster, we show that an interplay of electron–phonon and onsite electron–electron interactions results in intersite or onsite two-electron (bipolaronic) solutions of the Holstein–Hubbard model, depending on the strengths of the interactions. On this basis, we argue that the decrease in the superconducting transition temperature of Bi-2212 compounds, following the enhancement of the electron–phonon interaction recently reported by Devereaux et al. [10] might be a consequence of the above mechanism, which leads to a transition from itinerant (intersite) to bound immobile (onsite) bipolarons, thus effectively reducing the number of superconducting carriers.     

Pseudo-gap and Vertex Correction of Electron–Phonon Interaction in High Transition-Temperature Superconductors

The strong-coupling Eliashberg theory plus vertex correction is used to calculate the maps of transition temperature (T _c) in parameter-space characterizing superconductivity. Based on these T _c maps, complex crossover behaviors are found when electron?Cphonon interaction increases from weak-coupling region to strong-coupling region. The doping-dependent T _c of cuprate superconductors and most importantly the emergence of pseudo-gap region can be explained as the effects of vertex correction.     

Explanation of Temperature-Dependent Resistivity of Sm-Doped Cuprates by Electron–Phonon Scattering

The resistivity of electron-doped cuprate Sm_1.85Ce_0.15CuO_{4 –} is theoretically analyzed within the framework of electron–phonon i.e., Bloch–Gruneisen (BG) model of resistivity. Characteristic temperatures as the Debye temperature and the Einstein temperature were first derived from an overlap repulsive potential. The optical phonons of the oxygen-breathing mode yield a relatively larger contribution to the resistivity compared to the contribution of acoustic phonons above 220 K. While to that, below this temperature, acoustic phonon is a major cause of resistivity. Estimated contribution to resistivity by considering both phonons i.e., _ac (acoustic phonons) and _op (optical phonons), along with the zero limited resistivity, when subtracted from single crystal data, infers a quadratic temperature dependence over most of the temperature range (25 T 300). Power temperature dependence of _diff.{=[ _exp. – ( ₀ + _e-ph(= _ac + _op))]} points the contribution of electron–electron inelastic scattering. The present analysis allows us to infer that the single crystal experimental data is well approximated within the framework of BG electron–phonon model of resistivity. Further calculations of superconducting transition temperature and isotope effect exponent from Kresin's strong coupling theory indicates that the electron–phonon interaction plays an important role in the attractive pairing mechanism.     

The Electron–Phonon Interaction of Low-Dimensional and Multi-Dimensional Materials from He Atom Scattering

Atom scattering is becoming recognized as a sensitive probe of the electron–phonon interaction parameter λ at metal and metal-overlayer surfaces. Here, the theory is developed, linking λ to the thermal attenuation of atom scattering spectra (in particular, the Debye–Waller factor), to conducting materials of different dimensions, from quasi-1D systems such as W(110):H(1 × 1) and Bi(114), to quasi-2D layered chalcogenides, and high-dimensional surfaces such as quasicrystalline 2ML-Ba(0001)/Cu(001) and d-AlNiCo(00001). Values of λ obtained using He atoms compare favorably with known values for the bulk materials. The corresponding analysis indicates in addition, the number of layers contributing to the electron–phonon interaction, which is measured in an atom surface collision.     

Electrical Resistivity of the Mg(B1−x C x )2 Superconductors: Role of Electron–Electron, and Electron–Phonon Interactions

In this paper, we undertake a quantitative analysis of temperature-dependent resistivity Mg(B_1?x C _x )₂ superconductors. Due to inherent two energy gaps, the elastic scatterings of electrons from impurities have first been estimated and within a two-band picture, the impurity-limited resistivity due to π band carriers $\rho_{0}^{\pi} $ is larger as compared to the contribution from σ band carriers. An effective inter-ionic interaction potential (EIoIP) with the long-range Coulomb, van der Waals interaction and the short-range repulsive interaction within the Hafemeister and Flygare approach have allowed us to determine the Debye and Einstein temperature. An investigation exhibiting the mechanism of Mg(B_1?x C _x )₂ (0.0≤x≤0.125) was accomplished by comparing to the resistivity estimated by considering both phonons, with that of reported metallic resistivity, accordingly ρ _diff.=[ρ _exp.?{ρ ₀+ρ _e-ph (=ρ _ac+ρ _op)}] have been analysed through electron–electron scattering. The quadratic temperature dependence of $\rho_{\mathrm {diff}} = [\rho_{\mathrm{exp}.} - \{\rho_{0} + \rho_{\text{e-ph}}\ (= \rho ^{\sigma}_{\text{e-ph}} + \rho^{\pi}_{\text{e-ph}})\}]$ is understood in terms of inelastic electron–electron scattering. The comparison of transport parameter with single crystal data appears consistent within the two-band scheme for resistivity that we have presented.     

Electron–Phonon Interaction and Phonon Renormalization in the Lamellar Cobaltate Na x CoO2

We study theoretically the electron–phonon interaction in Na _x CoO₂. For the A _1g and E _1g phonon modes found in Raman experiments, we calculate the matrix elements of the electron–phonon interaction. Analyzing the feedback effect of the conduction electrons on the phonon frequency ω, we investigate the doping dependence of these two phonon modes. Due to the momentum dependence of the electron–phonon interaction, we find the strongest renormalization of the E _1g mode around the Brillouin zone boundary which should be observed in the neutron scattering. At the same time, the A _1g mode shows the strongest coupling to the conducting electrons around the Γ point and reveals its doping dependence in the Raman experiments. Our results shed light on the possible importance of the electron–phonon interaction in the lamellar sodium cobaltates.     

Nonuniform Electron–Hole Pairing in Graphene Bilayer

Electron–hole (e–h) pairing caused by Coulomb interaction in the system of independently gated graphene layers is considered. The influence of the mismatch of concentrations of e and h and trigonal warping of their spectrum on critical temperature is studied. We predict the appearance of the state with finite value of Cooper pair momentum–Larkin–Ovchinnikov–Fulde–Ferrell-like (LOFF) state at mismatch of the concentrations of e and above the critical value. Internal Josephson effect in LOFF-like state caused by interlayer tunneling of e and h is considered. We suggest a new phase-sensitive experiment based on the internal Josephson effect for probing spatial structure of order parameter in a LOFF-like state.     

The Decoherence of GaAs Quantum Dot Qubit Due to Electron–Phonon Interactions

We investigate electron charge decoherence in a GaAs single-electron semiconductor quantum dot through electron–phonon interaction. We analytically and numerically evaluate decoherence time within the Lee–Low–Pines–Huybrecht variational calculation for all coupling strengths. The dependence of decoherence time on the electron-LO-phonon coupling strength and the size of quantum dot is investigated. Our results suggest that electron–phonon interaction has very important effects on charge decoherence.     

Properties of high-T C Copper Oxides from Band Models of Spin–Phonon Coupling

The mechanism of spin–phonon coupling (SPC) and possible consequences for the properties of high-T _C copper oxides are presented. The results are based on ab initio LMTO band calculations and a nearly free-electron (NFE) model of the band near E _F . Many observed properties are compatible with SPC, as for the relation between doping and for spin excitations and their energy dependence. The main pseudogap is caused by SPC and waves along [1,0,0], but it is suggested that secondary waves, generated along [1,1,0], contribute to a ‘waterfall’ structure. Conditions for optimal T _C , and the possibilities for spin enhancement at the surface are discussed.      

Influence of Electron–Phonon Interaction on Transition Temperature and Isotope Effect of Ba0.6K0.4BiO3

We employ a three-square-well model for the three interactions namely, electron-acoustic phonon, electron-optical phonon, and Coulomb in the calculation of superconducting transition temperature (T _c) and isotope effect coefficient () for cubic perovskite Ba_0.6K_0.4BiO₃. The analytical solutions for the energy gap equation allow us to understand the relative interplay of these interactions. To correlate the T _c with various coupling strengths as electron-acoustic phonon (_ac), electron-optical phonon (_op) and Coulomb (^*), we present curves of T _c with them. The values of the coupling strength and of the Coulomb interaction parameter indicate that the superconductor is in the intermediate coupling regime. The superconducting transition temperature of optimally doped Ba—K—BiO is estimated as 29 K for _ac of 0.3, _op of 0.2, and ^* of 0.12. The present approach also explains the reported oxygen isotope effect in the test material. We suggest from these results that both the acoustic and optical phonons within the framework of a three-square-well scheme consistently explains the effective electron–electron interaction leading to superconductivity in doped cubic perovskites.     

Influence of Electron–Phonon Interaction on Superconducting State Parameters of MgB2

A three-square well model is employed for the three interactions namely, electron–acoustic phonon, electron–optical phonon, and Coulomb in the calculation of superconducting transition temperature (T _c) for layered structure MgB₂. The analytical solutions for the energy gap equation allow us to understand the relative interplay of these interactions. The values of the coupling strength and of the Coulomb interaction parameter indicate that the test material is in the intermediate coupling regime. The superconducting transition temperature of MgB₂ is estimated as 41 K for _ac 0.3, _op 0.1, and * 0.07. We suggest from these results that both the acoustic and optical phonons within the framework of a three-square well scheme consistently explains the effective electron–electron interaction leading to superconductivity in layered structure MgB₂.     

Study of the Electron–Phonon Relaxation in Thin Metal Films Using Transient Thermoreflectance Technique

Electron–phonon (e–ph) relaxation in thin metal films is an important consideration in many ultra-small and ultra-fast applications. In this work, e–ph relaxation in thin gold and aluminum films has been studied using the transient thermoreflectance technique which is demonstrated sensitive enough to study the relaxation process. The optical properties of the thin metal films are different from those of bulk metal and have been measured. Based on confirmation of the measurements, the effects of metal type, film thickness, and interface on e–ph relaxation have been experimentally studied. The thermoreflectance traces of gold and aluminum films have been compared. The results show that the e–ph relaxation and the effect of electron and lattice temperatures on the thermoreflectance of gold and aluminum are quite different. The e–ph relaxation is independent of film thickness and interface.