NMR observation of rattling phonons in the pyrochlore superconductor KOs2O6

We report nuclear magnetic resonance studies on the beta-pyrochlore oxide superconductor KOs2O6. The nuclear relaxation at the K sites is entirely caused by fluctuations of the electric field gradient, which we ascribe to highly anharmonic low frequency oscillation (rattling) of K ions. A phenomenological analysis shows a crossover from overdamped to underdamped behavior of the rattling phonons with decreasing temperature and its sudden sharpening below the superconducting transition temperature T(c). Suppression of the Hebel-Slichter peak in the relaxation rate at the O sites below T(c) also indicates strong electron-phonon coupling.     

In-plane conductivity of a layered large-bipolaron liquid

Distinctive normal-state properties of cuprate superconductors follow from their charge carriers forming a large-bipolaron liquid. The very weak scattering of the liquid’s slow-moving heavy-massed excitations by acoustic phonons yields a scattering rate that is less than the Debye frequency. The liquid’s moderate mobility, >1 cm²/V-sec at 300 K, results from its weak scattering compensating for its large mass. In resolution of a long-standing dilemma, the dc resistivity resulting from scattering by long-wavelength phonons remains nearly proportional to temperature to well below the Debye temperature. Above the Debye frequency, the frequency-dependent conductivity is dominated by excitation and photo-ionization of the liquid’s self-trapped electronic carriers. Below the Debye frequency, the frequency-dependent conductivity is dominated by the liquid’s Drude-like collective motion. The ‘gap’ between these two domains sharpens with decreasing temperature as phonon scattering of the liquid’s collective excitations diminishes. The high-frequency electronic excitations survive in the superconducting state.     

The rôle of one- and few-phonon processes in the quantum diffusion of light particles in superconductors

The hopping motion of charged light particles in superconducting metals is discussed within the framework of a two-state model. In addition to the coupling to the superconducting electrons, the influence of phonons is investigated. It is shown that sufficiently far below the critical temperature the hopping rate is dominated by one-phonon processes if the static energy shifts between the particle ground states (so-called energy asymmetries) are finite but smaller than twice the BCS energy gap. The rôle of special two-phonon processes (named diphonon processes) resulting from non-linear coupling to the lattice is investigated. In the limit of energy asymmetries larger than twice the BCS energy gap phonon coupling does not crucially influence the jump rates.This article was processed using Springer-Verlag T_EX Z. Physik B macro package 1.0 and the AMS fonts, developed by the American Mathematical Society.     

Experimental and Theoretical In Situ Spectral Magneto-Ellipsometry Study of Layered Ferromagnetic Structures

JETP Letters - The influence of antiadiabatic phonons on the superconducting transition temperature is considered within Eliashberg-McMillan approach in the model of discrete set of (optical)...     

Mode-selected electron-phonon coupling in superconducting Pb nanofilms determined from He atom scattering

The electron-phonon coupling (EPC) strength for each phonon mode in superconducting Pb films is measured by inelastic helium atom scattering (IHAS). This surprising ability of IHAS relies on two facts: (a) In ultrathin metal films, the EPC range exceeds the film thickness, thus enabling IHAS to detect most film phonons, even 1 nm below the surface; (b) IHAS scattering amplitudes from single phonons are shown, by first-principle arguments, to be proportional to the respective EPC strengths. Thus IHAS is the first experiment providing mode-selected EPC strengths (mode-lambda spectroscopy).     

Far-infrared spectra of imperfect YBaCuO films on Si substrates

We have measured the far-infrared (FIR) transmittance of low conductivity, imperfect YBCO films deposited on Si, at both 300 K and 20 K. Test reflectance measurements were also performed at room temperature. Our aim was both to study YBCO phonons by transmission measurements and to check if FIR spectra of imperfect YBCO films can be described in the framework of the BCS theory. In order to describe our measurements, we provide exact expressions for both transmittance and reflectance, averaged with respect to the interference fringes of the substrate. By modelling the complex dielectric constant of the film, we then study the YBCO phonons which are evident in the measured spectra. The comparison between spectra of normal and superconducting phases shows that the superconducting transition affects our spectra only at very low frequencies. By considering a two fluid model in which only a small fraction of electrons becomes superconducting and by assuming an energy gap of the order of 130 cm⁻¹, we find that our data can be explained in terms of the Mattis-Bardeen theory.     

Theoretical studies on magnetic superconductors

The discovery of magnetic superconductors has posed the problem of the coexistence of two kinds of orders (magnetic and superconducting) in some temperature intervals in these systems. New microscopic mechanisms developed by us to explain the coexistence and reentrant behaviour are reported. The mechanism for antiferromagnetic superconductors which shows enhancement of superconductivity below the magnetic transition is found relevant for rare-earth systems having less than half-filled f-atomic shells. The theory will be compared with the experimental results of SmRh₄B₄ system. A phenomenological treatment based on a generalized Ginzburg-Landau approach will also be presented to explain the anomalous behaviour of the second critical field in some antiferromagnetic superconductors. These magnetic superconductors provide two kinds of Bose fields, namely, phonons and magnons which interact with each other and also with the conduction electrons. Theoretical studies of the effects of the excitations of these modes on superconducting pairing and magnetic ordering in these systems will be discussed.     

电-声超导电性的对称理论

利用新的库珀对平均汤近似，导出了电－声系统中声子－库珀对有效作用，证明了声子通过交换虚的库珀对能够产生一个有效的吸引力，导致两动量相反的声子配对并构成稳定的声超导态。发现电子超导态与声子超导态的直积能构成稳定的电－声对称的高温超导基态。在声子－库珀对弱耦合极限下，基态对称破缺成为传统的ＢＣＳ基态。     

电—声超导电性的对称理论

利用新的库珀对平均汤近似，导出了电－声系统中声子－库珀对有效作用，证明了声子通过交换虚的库珀对能够产生一个有效的吸引力，导致两动量相反的声子配对并构成稳定的声超导态。发现电子超导态与声子超导态的直积能构成稳定的电－声对称的高温超导基态。在声子－库珀对弱耦合极限下，基态对称破缺成为传统的ＢＣＳ基态。     

Ultrasonic properties of superconducting amorphous PdZr

Propagation of longitudinal waves in superconducting amorphous Pd₃₀Zr₇₀ has been studied down to temperatures as low as 0.4 K. A strong decrease of the absorption is observed for temperatures lower than 1.5 K whereas no change of the absorption occurs when passing through the superconducting transition temperature. Below 1.5 K the absorption becomes strongly magnetic-field dependent. We compare its overall behaviour to a recently developed theory. At the lowest temperatures, the temperature variation of the sound velocity exhibits a contribution arising from resonant interaction of phonons with tunnelling centers, also present in this material.     

Interplay of superconductivity and rattling phenomena in beta-pyrochlore KOs2O6 studied by photoemission spectroscopy

The electronic structure near the Fermi level (EF) of the beta-pyrochlore superconductor KOs2O6 is studied using laser-excited ultrahigh-resolution photoemission spectroscopy. The superconducting gap clearly opens across the superconducting transition (Tc=9.6 K), with the strong electron-phonon coupling value of 2Delta(0)/k B Tc>or=4.56. A fitting analysis identifies clear anomalies at Tp=7.5 K in the temperature dependencies of the superconducting gap size and the quasiparticle relaxation lifetime. These anomalies and the fine spectral structures arising from phonons suggest that the existence of the rattling behavior of K ions significantly affects the superconductivity in KOs2O6.     

Perturbat ion Approach for the Localization Transition in the Dissipative Two-State System

Analysis of a dissipative two-state system at zero temperature shows that the model Hamiltonian may be exactly reduced to a modified quantum sine-Gordon model, which describes the effective interactions between the low-frequency phonons under coupling with a tunneling system. Directly considering the infrared divergence encountered in the conventional perturbation treatment, we have developed a new perturbation approach for the effective Hamiltonian, and derived the exact critical conditions for the localization transition. In the critical regime, a gap will be opened near zero momentum in the elementary excitation spectrum of the low-frequency phonons, and the corresponding ground state wave function is found to be a pairing quasi-particle state, analogous to the BCS superconducting state.     

Adiabatic electron-phonon interaction and high-temperature thermodynamics of A15 compounds

Inelastic neutron scattering was used to measure the phonon densities of states of the A15 compounds V3Si, V3Ge, and V3Co at temperatures from 10 to 1,273 K. It was found that phonons in V3Si and V3Ge, which are superconducting at low temperatures, exhibit an anomalous stiffening with increasing temperature, whereas phonons in V3Co have a normal softening behavior. First-principles calculations show that this anomalous increase in phonon frequencies at high temperatures originates with an adiabatic electron-phonon coupling mechanism. The anomaly is caused by the thermally induced broadening of sharp peaks in the electronic density of states of V3Si and V3Ge, which tends to decrease the electronic density at the Fermi level. These results show that the adiabatic electron-phonon coupling can influence the phonon thermodynamics at temperatures exceeding 1,000 K.     

Strong interaction of correlated electrons with phonons: Exchange of phonon clouds by polarons

The interaction of strongly correlated electrons with phonons in the framework of the Hubbard-Holstein model is investigated. The electron-phonon interaction is considered to be strong and is an important parameter of the model, in addition to the Coulomb repulsion of electrons and the band filling. This interaction with nondispersive optical phonons is transformed to the problem of mobile polarons using the canonical transformation of Lang and Firsov. We discuss the case where the on-site Coulomb repulsion is exactly canceled by the phonon-mediated attractive interaction. It is suggested that polarons exchanging phonon clouds can lead to polaron pairing and superconductivity. The fact that the frequency of the collective mode of phonon clouds is larger than the bare frequency then determines the superconducting transition temperature.     

A model of the T-dependent pseudogap and its competition with superconductivity in copper oxides

Results for pseudogaps are obtained from a band model, where the stability of the gap depends on the amplitudes of vibrational displacements, or magnetic moments, and their coupling to electrons. A one-particle gap is favored by normal thermal excitations of phonons or spin waves. Another gap can be generated by spontaneous waves at lower temperature, if the electronic energy gain overcomes the elastic/magnetic energy needed for increased amplitudes of the oscillations. This state is characterized by charge or spin density waves. The pseudogap has many features in common with the superconducting gap, and the model lends support to the interpretation that the pseudogap is a precursor of, and competes with, superconducting pairing.     

Temperature-dependent localized excitations of doped carriers in superconducting diamond

Laser-excited photoemission spectroscopy is used to show that the doped carriers in metallic or superconducting diamond couple strongly to the lattice via high-energy (approximately 150 meV) optical phonons, with direct observations of localized Franck-Condon multiphonon sidebands appearing as Fermi-edge replicas. It exhibits a temperature-dependent spectral weight transfer from higher to lower energy sidebands and zero-phonon Fermi-edge states. The quantified coupling strength shows a systematic increase on lowering temperature, implicating its relation to the normal state transport and superconductivity.     

电—声系统中的费密超导态

基于推广的电－声相互作用模型，考虑到电子的强关联作用，提出了电子算符的玻戈留玻夫近似，导出了声子在电子场中受到的有效作用；证明了波矢相反的纵声子通过交换虚电子能产生一个有效的吸引力，从而构成带有一个电子的声子对。在适当的声子数密度下，形成稳定的费密高温超导态。     

电-声系统中的费密超导态

基于推广的电－声相互作用模型，考虑到电子的强关联作用，提出了电子算符的玻戈留玻夫近似，导出了声子在电子场中受到的有效作用；证明了波矢相反的纵声子通过交换虚电子能产生一个有效的吸引力，从而构成带有一个电子的声子对。在适当的声子数密度下，形成稳定的费密高温超导态。     

The two-dimensional superconducting photonic crystal

The band structure of a two-dimensional superconducting photonic crystal is studied. The temperature dependence of the photonic band structure in a wide temperature region below the superconducting transition is analyzed. It is found that the photonic crystal has two full band gaps and two incomplete band gaps, which are shifted to the high frequency region with decreasing temperature.     

Interplay of structure and magnetism in ruthenocuprates: a Raman scattering and dilatometry study

We present a Raman scattering and dilatometry study of polycrystalline samples of the magnetic superconducting ruthenocuprates RuSr₂Gd_2−xCe_xCu₂O_10+δ (RuGd₁₂₂₂) and RuSr₂GdCu₂O₈ (RuGd₁₂₁₂). In the Raman spectra a high-temperature diffusive-like laser-tail develops below the magnetic ordering temperature (T_M) into an underdamped peak which shifts up to 130 cm⁻¹. A line assigned to O(Ru) phonons hardens, narrows and strengthens strongly below T_M. Finally, a phonon peak appears below T_M at 590 cm⁻¹. These three magnetic-order-dependent features are observed for RuGd₁₂₁₂ and for RuGd₁₂₂₂ with x=1.0, but do not appear for x=0.5. Dilatometry measurements, on the other hand, evidence a change of the expansion coefficient at T_M. These results point to a structural effect accompanying the magnetic order, and suggest a complex interplay of spin and lattice degrees of freedom in these ruthenocuprates.