Normal-state Quasiparticle Research Articles

Spin singlet and quasiparticle excitations in cuprate superconductors

We followed step by step the transition from an antiferromagnetic (AF) Mott insulator to a superconducting (SC) metal in the Bi$_2$Sr$_2$CaCu$_{2}$O$_{8+\delta}$ (Bi-2212) cuprate using the electronic Raman scattering spectroscopy. This was achieved by tracking the doping dependence of the spin singlet excitation originate from the AF Mott insulator, the normal state quasiparticles excitation related to the mobile charge carriers and the Bogoliubov quasiparticles related to the SC gap. We show that the signature of the pseudogap phase which develops during this transition, can be interpreted as the blocking of charge carriers by the enhancement of the antiferromagnetic correlations as the temperature drops. We find that the energy scale of the pseudogap, $\Delta_{\textrm{pg}}(p)$, closely follows the one of the spin singlet excitation, $\Delta_{\textrm{sse}}(p)$, with doping $p$. The quasiparticles lifetime considerably increases with doping when the pseudogap collapses. We reveal that the maximum amplitude of the SC gap, $\Delta_{\textrm{sc}}^{\textrm{max}}$ and the SC transition temperature \Tc are linked in an extended range of doping such as $\Delta_{\textrm{sc}}^{\textrm{max}}(p) \propto \Delta_{\textrm{sse}}(p)\, T_c(p)$. This relation suggests that the AF correlations play a key role in the mechanism of superconductivity.

Anomalous acoustoelectric effect induced by clapping modes in chiral superconductors

Clapping modes, which are relative amplitude and phase modes between two chiral components of Cooper pairs, are bosonic collective modes inherent to chiral superconductors. These modes behave as long-lived bosons with masses smaller than the threshold energy, $2|\Delta|$, for decay into unbound fermion pairs. Here, we clarify that the real/imaginary clapping modes in chiral superconductors directly couple to acoustic wave propagation when the weak particle-hole asymmetry of the normal state quasiparticle dispersion is taken into account. The clapping modes driven by an acoustic wave generate an alternating electric current, that is, the acoustoelectric effect in superconductors. Significantly, the clapping modes give rise to a transverse electric current. When the sound velocity is comparable to the Fermi velocity, as in heavy fermion compounds, the transverse current is resonantly enhanced at energy below the threshold for continuum excitations. This resonance provides a smoking-gun evidence of chiral superconductivity.

Odd-frequency superconductivity induced by nonmagnetic impurities

A growing body of literature suggests that odd-frequency superconducting pair amplitudes can be generated in normal metal-superconductor junctions. The emergence of odd-frequency pairing in these systems is often attributed to the breaking of translation invariance. In this work, we study the pair symmetry of a one-dimensional $s$-wave superconductor in the presence of a single non-magnetic impurity and demonstrate that translation symmetry breaking is not sufficient for inducing odd-frequency pairing. We consider three kinds of impurities: a local perturbation of the chemical potential, an impurity possessing a quantum energy level, and a local perturbation of the superconducting gap. Surprisingly, we find local perturbations of the chemical potential do not induce any odd-frequency pairing, despite the fact that they break translation invariance. Moreover, although odd-frequency can be induced by both the quantum impurity and the perturbation of the gap, we find these odd-frequency amplitudes emerge from entirely different kinds of scattering processes. The quantum impurity generates odd-frequency pairs by allowing one of the quasiparticles belonging to an equal-time Cooper pair to tunnel onto the impurity state and then back to the superconductor, giving rise to odd-frequency amplitudes with a temporal broadening inversely proportional to the energy level of the impurity. In contrast to this, the perturbation of the gap leads to odd-frequency pairing by "gluing-together" normal state quasiparticles from different points in space and time, leading to odd-frequency amplitudes which are very localized in the time domain.

Weak-coupling analysis of quasiparticle excitations in Sr2RuO4 along the Γ−M cut

We examine normal-state quasiparticle excitations along the $\mathrm{\ensuremath{\Gamma}}\text{\ensuremath{-}}M$ cut in momentum space for the putative $p$-wave superconductor ${\mathrm{Sr}}_{2}{\mathrm{RuO}}_{4}$ on the basis of fluctuation exchange approximation calculations. We take as input first-principles derived parameters for the band structure and spin-orbit and electron-electron interactions. The numerical results are in excellent agreement with data from photoemission experiments and provide insight into the underlying quasiparticle properties. We find that, despite the correlation-induced effective mass increase near the Fermi surface, the full $\ensuremath{\beta}$ and $\ensuremath{\gamma}$ bandwidths are, if anything, increased by correlations. Furthermore, for the $\ensuremath{\gamma}$ band we find anomalous lifetime broadening and a significant temperature of variation of unoccupied state quasiparticle energies for temperatures between 25 and 100 K, both of which are accounted for by the momentum dependence of the electron self-energy. In addition to aiding our understanding of experimental data, these results point to the challenge of assigning appropriate Fermi-liquid parameters or momentum-independent self-energies for schemes that require such approximations in order to model ${\mathrm{Sr}}_{2}{\mathrm{RuO}}_{4}$.

Electron Bubbles in Superfluid $$^3$$He-A: Exploring the Quasiparticle–Ion Interaction

When an electron is forced into liquid $^3$He it forms an "electron bubble", a heavy ion with radius, $R\simeq 1.5$ nm, and mass, $M\simeq 100\,m_3$, where $m_3$ is the mass of a $^3$He atom. These negative ions have proven to be powerful local probes of the physical properties of the host quantum fluid, especially the excitation spectra of the superfluid phases. We recently developed a theory for Bogoliubov quasiparticles scattering off electron bubbles embedded in a chiral superfluid that provides a detailed understanding of the spectrum of Weyl Fermions bound to the negative ion, as well as a theory for the forces on moving electron bubbles in superfluid $^3$He-A (Shevtsov et al. in arXiv:1606.06240). This theory is shown to provide quantitative agreement with measurements reported by the RIKEN group [Ikegami et al., Science 341:59, 2013] for the drag force and anomalous Hall effect of moving electron bubbles in superfluid $^3$He-A. In this report, we discuss the sensitivity of the forces on the moving ion to the effective interaction between normal-state quasiparticles and the ion. We consider models for the quasiparticle-ion (QP-ion) interaction, including the hard-sphere potential, constrained random-phase-shifts, and interactions with short-range repulsion and intermediate range attraction. Our results show that the transverse force responsible for the anomalous Hall effect is particularly sensitive to the structure of the QP-ion potential, and that strong short-range repulsion, captured by the hard-sphere potential, provides an accurate model for computing the forces acting on the moving electron bubble in superfluid $^3$He-A.

Green's function approach to the non-equilibrium superconductivity near the critical line

In spite of the absent friction of super-currents, normal currents affect the superconducting condensate. The BCS approach with Bogoliubov-Valutin quasiparticles is not suited for description of the normal current near the critical line. We review an alternative theory of the non-equilibrium superconductivity dealing exclusively with normal-state quasiparticles. It is based on the Thouless T-matrix criterion, which is extended to non-equilibrium. The problem of selfconsistency of the T-matrix in the superconducting state is examined and solved with the help of the multiple-scattering theory.

Nernst effect in the electron-doped cuprate superconductorPr2−xCexCuO4: Superconducting fluctuations, upper critical fieldHc2, and the origin of theTcdome

The Nernst effect was measured in the electron-doped cuprate superconductor Pr2-xCexCuO4 (PCCO) at four concentrations, from underdoped (x=0.13) to overdoped (x=0.17), for a wide range of temperatures above the critical temperature Tc. A magnetic field H up to 15 T was used to reliably access the normal-state quasiparticle contribution to the Nernst signal, Nqp, which is subtracted from the total signal, N, to obtain the superconducting contribution, Nsc. As a function of H, Nsc peaks at a field H* whose temperature dependence obeys Hc2* ln(T/Tc), as it does in a conventional superconductor like Nb1-xSix. The doping dependence of the characteristic field scale Hc2* - shown to be closely related to the upper critical field Hc2 - tracks the dome-like dependence of Tc, showing that superconductivity is weakened below the quantum critical point where the Fermi surface is reconstructed, presumably by the onset of antiferromagnetic order. Our data at all dopings are quantitatively consistent with the theory of Gaussian superconducting fluctuations, eliminating the need to invoke unusual vortex-like excitations above Tc, and ruling out phase fluctuations as the mechanism for the fall of Tc with underdoping. We compare the properties of PCCO with those of hole-doped cuprates and conclude that the domes of Tc and Hc2 vs doping in the latter materials are also controlled predominantly by phase competition rather than phase fluctuations.

Low temperature specific heat of the hole-dopedBa0.6K0.4Fe2As2single crystals

Low temperature specific heat (SH) was measured on the FeAs-based superconducting single crystals ${\text{Ba}}_{0.6}{\text{K}}_{0.4}{\text{Fe}}_{2}{\text{As}}_{2}$. It is found that the sharp SH anomaly $\ensuremath{\Delta}C/T{\ensuremath{\mid}}_{{T}_{c}}$ in our samples reaches an unexpected high value of $98\text{ }\text{mJ}/\text{mol}\text{ }{\text{K}}^{2}$, about 1 order of magnitude larger than that of the F-doped system LnFeAsO ($\text{Ln}=\text{rare}$-earth elements), suggesting a very high normal-state quasiparticle density of states in the FeAs-122 system than in the FeAs-1111 system. Furthermore, we found that the electronic SH coefficient ${\ensuremath{\gamma}}_{e}(T)$ of ${\text{Ba}}_{0.6}{\text{K}}_{0.4}{\text{Fe}}_{2}{\text{As}}_{2}$ is weakly temperature dependent and increases almost linearly with the magnetic field in the low temperature region, which may indicate that the hole-doped FeAs-122 system contains a dominant component with a full superconducting gap, although we cannot rule out the possibility of a small component with an anisotropic or nodal gap. A detailed analysis reveals that the ${\ensuremath{\gamma}}_{e}(T)$ of ${\text{Ba}}_{0.6}{\text{K}}_{0.4}{\text{Fe}}_{2}{\text{As}}_{2}$ cannot be fitted with a single gap of $s$-wave symmetry, probably due to the multigap effect. These results indicate a clear difference between the properties of the superconducting state of the holed-doped ${\text{Ba}}_{0.6}{\text{K}}_{0.4}{\text{Fe}}_{2}{\text{As}}_{2}$ and the F-doped LnFeAsO ($\text{Ln}=\text{rare}$-earth elements) system, which we believe originated from the complex Fermi-surface structures in different systems.

Kink Structures of Spin-Polarized Electrons due to Electron–Magnon Scattering in Ferromagnetic State

We propose a model composed of spin-polarized itinerant electrons and bosonic spin-wave excitations, and study renormalization of the spin-polarized itinerant electron bands due to electron-magnon scattering. Spin-polarized kink structures are predicted in the normal state quasiparticle dynamics of ferromagnetic superconductor as UGe2. It is suggested that the angle-resolved photoemission experiment may be helpful in this respect.

Evidence for a metallic–like state in the T=0 K phase diagram of a high temperature superconductor

We examine the effects of a phenomenological pseudogap on the T=0 K phase diagram of a high temperature superconductor within a self-consistent model which exhibits a d-wave pairing symmetry. At the mean-field level the presence of a pseudogap in the normal phase of the high temperature superconductor is proved to be essential for the existence of a metallic--like state in the density versus interaction phase diagram. In the small density limit, at high attractive interaction, bosonic--like degrees of freedom are likely to emerge. Our result should be relevant for underdoped high temperature superconductors, where there is a strong evidence for the presence of a pseudogap in the excitation spectrum of the normal state quasiparticles.

Electronic structure of the trilayer cuprate superconductor Bi(2)Sr(2)Ca(2)Cu(3)O(10+delta).

The low-energy electronic structure of the nearly optimally doped trilayer cuprate superconductor Bi(2)Sr(2)Ca(2)Cu(3)O(10+delta) is investigated by angle-resolved photoemission spectroscopy. The normal state quasiparticle dispersion and Fermi surface and the superconducting d-wave gap and coherence peak are observed and compared with those of single- and bilayer systems. We find that both the superconducting gap magnitude and the relative coherence-peak intensity scale linearly with T(c) for various optimally doped materials.

Exactly solvable model of dissipative vortex tunneling

I consider the problem of vortex tunneling in a two-dimensional superconductor. The vortex dynamics is governed by the Magnus force and the Ohmic friction force. Under-barrier motion in the vicinity of the saddle point of the pinning potential leads to a model with quadratic Hamiltonian which can be analytically diagonalized. I find the dependence of the tunneling probability on the normal state quasiparticle relaxation time $\tau$ with a minimum at $\omega_0\tau\sim 1$, where $\omega_0$ is the level spacing of the quasiparticle bound states inside the vortex core. The results agree qualitatively with the available experimental data.

Role of the density-of-states superconducting fluctuations in the in-plane and out-of-plane transport properties of Bi 2Sr 2CaCu 2O 8+ x single crystals

We report transverse ( B⊥ j) and longitudinal ( B‖ j) in-plane ( j‖ ab-plane) and out-of-plane ( j‖ c-axis) magnetoresistance measurements on underdoped, optimally doped and overdoped Bi 2Sr 2CaCu 2O 8+ x single crystals. The measurements were performed in the temperature range from slightly above T c to 200 K and in magnetic fields up to 13 T. The in-plane magnetoresistance (MR) is positive for all temperatures, magnetic fields and different doping levels. Near T c, the in-plane MR is dominated by thermodynamic fluctuations, but at higher temperatures a smooth transition to the MR of normal-state quasiparticles takes place. In sharp contrast, the out-of-plane MR is negative for all temperatures and orientations of the magnetic field. We interpret this unconventional behaviour in the framework of the superconducting fluctuation theory, taking into account different weights of the Aslamazov–Larkin and single-carrier density-of-states contributions.

Normal state quasiparticle structure in the high- Tc superconductors

We present a study of the normal state electronic structure of the high- T c superconductors obtained by diagonalizing the one-body Green function of the interacting system. Different approximations to the self-energy are considered. These corrections split the input density of state that is calculated within the local density formalism. The energy-dependent self-energy transfers part of spectral weight to the lower and upper Hubbard peaks, changing also the hole distribution in the different p and d states.

Interpretation of bound states in inhomogeneous superconductors: the role of Andreev reflection

We study the quasiparticle bound states of superconductor - normal-metal - superconductor (S - N - S) junctions using the framework of Bogoliubov - de Gennes equations. We find that whilst the frequently employed semiclassical approximations give a good account of the overall spectra, a rich variety of interesting details get lost in making such simplifications. In particular, this is the case with the behaviour of states corresponding to large momenta parallel to the interfacial planes and with bound states above the gap. We compare the spectra of the S - N - S junction with an analogous non-superconducting semiconductor - metal - semiconductor system and show how the new features can be understood in simple physical terms. We also examine the limitations of using Andreev reflection alone to describe the trapping of normal-state quasiparticles inside normal regions embedded in a superconductor.

Anisotropic magnetoresistance in the normal state of oxygen-deficient YBa2Cu3O7?? thin films induced by superconducting fluctuations

We have investigated both the transverse and the longitudinal magnetoresistance of oxygen-deficient YBa2Cu3O7−δ thin films above their critical temperatureT c =55 K. The magnetoresistance is solely caused by the magnetic-field suppression of superconducting orderparameter fluctuations, existing in the films up to 143 K, i.e. 2.6T c . The fluctuation effect provides a reliable determination of the Ginzburg-Landau coherence lengths, ξ ab = 2.5 nm and ξ c = 0.09 nm, with the anisotropy enhanced by oxygen depletion. No signature of the Maki-Thompson fluctuation process or a magnetoresistance resulting from the cyclotron motion of the normalstate quasiparticles was found.

Temperature dependence of the Maki-Thompson fluctuation process in YBa 2Cu 3O 7 probed by magneto-transport measurements

The contributions of thermodynamic fluctuations of the superconducting order parameter to the normal-state magneto-conductivity tensor are investigated. An analysis of complementary measurements of the electrical resistivity, the Hall effect and the magnetoresistance at temperatures above T c in thin films of the high- T c superconductor YBa 2Cu 3O 7 provides a separation between the direct (Aslamazov-Larkin) and indirect (Maki-Thompson) fluctuation contributions. The direct contribution is not significantly affected by a possible unconventional pairing state and reveals consistent values for the coherence lengths in the ab plane, ξ ab (0) = 1.4 nm, and parallel to the c-axis, ξ c (0) = 0.15 nm, respectively. The excess Hall effect provides the most efficient way to detect the indirect fluctuation contribution, which is found to drop more rapidly towards higher temperatures than predicted for s wave pairing symmetry. An analysis of magnetoresistance measurements with proper correction for the magnetoresistance of the normal-state quasiparticles also indicates that the Maki-Thompson fluctuation process is smaller at higher temperatures than previously estimated.

Novel approach for the analysis of the fluctuation magnetoconductivity in YBa 2Cu 3O 7 thin films

We report measurements of the paraconductivity and of the fluctuation magnetoconductivity as a function of both the magnetic field up to 13 T and the temperature in thin epitaxial films of YBa 2Cu 3O 7. An analysis within the model for dirty-limit superconductors yields consistent values for the in-plane, the out-of-plane coherence lengths, and the quasiparticle phase coherence time. Although an estimation of the mean free carrier path indicates that YBa 2Cu 3O 7 is a clean superconductor, the appropriate clean-limit theory produces an unrealisticly short phase coherence time, around half the value of the transport scattering time, and fails to fit the paraconductivity data. We show that if the magnetoconductivity due to the orbital motion of the normal-state quasiparticles is taken into account, the Maki-Thompson fluctuation contribution at high temperatures is significantly smaller than predicted by both clean- and dirty-limit theories. This result suggests non-s-wave pairing in YBa 2Cu 3O 7.

Many-body local fields and quasiparticle renormalization effects in two-dimensional electronic systems

We have developed a new microscopic approach designed to study the effects of the electron-electron interaction on various normal state quasiparticle properties in two-dimensional electronic systems. In our treatment the effects of charge and spin fluctuation induced vertex corrections are explicitly accounted for through the introduction of appropriate Hubbard-type many-body local field corrections. Our analysis is based on a novel self-consistent determination of the many-body local fields in two-dimensional systems for which some useful and new exact results have been derived. Although quite general our theory finds its natural application in the calculation of the quasiparticle effective mass and the anomalous Landé g-factor in silicon inversion layers, which display a pronounced dependence on the carrier density, a clear signal of interaction effects. It will be shown that, within the Landau theory of the Fermi liquids, the concomitant inclusion of previously neglected many-body effects associated with charge and spin fluctuation induced vertex corrections is crucial in attaining a satisfactory description of the observed phenomena.

Polarization Potentials and Transport Properties of NormalHe3

Scattering amplitudes for quasiparticles in $^{3}\mathrm{He}$ are constructed from the previously determined polarization potentials of Aldrich and Pines. The calculated results for the thermal conductivity, viscosity, spin diffusion, and normal-state quasiparticle lifetime of normal $^{3}\mathrm{He}$ at all pressures up to 34 atm are in good agreement with experiment.