Quasiparticles in a strongly correlated liquid with the fermion condensate: applications to high-temperature superconductors

A model of a strongly correlated electron liquid based on fermion condensation (FC) is extended to high-temperature superconductors. Within our model, the appearance of FC presents a boundary separating the region of a strongly interacting electron liquid from the region of a strongly correlated electron liquid. We study the superconductivity of a strongly correlated liquid and show that, under certain conditions, the superconductivity vanishes at temperatures T > T _c ≈ T _node, with the superconducting gap being smoothly transformed into a pseudogap. As a result, the pseudogap occupies only a part of the Fermi surface. The gapped area shrinks with increasing the temperature and vanishes at T = _T*. The single-particle excitation width is also studied. The quasiparticle dispersion in systems with FC can be represented by two straight lines, characterized by the effective masses and, intersecting near the binding energy that is on the order of the superconducting gap. It is argued that this strong change of the quasiparticle dispersion upon binding can be enhanced in underdoped samples because of strengthening the FC influence. The FC phase transition in the presence of the superconductivity is examined, and it is shown that this phase transition can be considered as driven by the kinetic energy.     

Critical temperature and condensate fraction of a fermion pair condensate

We report on measurements of the critical temperature and the temperature dependence of the condensate fraction for a fermion pair condensate of 6Li atoms. Bragg spectroscopy is employed to determine the critical temperature and the condensate fraction after a fast magnetic field ramp to the molecular side of the Feshbach resonance. Our measurements reveal evidence of level off of the critical temperature and limiting behavior of condensate fraction near the unitarity limit.     

Formation dynamics of a fermion pair condensate

The dynamics of pair condensate formation in a strongly interacting Fermi gas close to a Feshbach resonance was studied. We employed a phase-shift method in which the delayed response of the many-body system to a modulation of the interaction strength was recorded. The observable was the fraction of condensed molecules in the cloud after a rapid magnetic field ramp across the Feshbach resonance. The measured response time was slow compared to the rapid ramp, which provides final proof that the molecular condensates reflect the presence of fermion pair condensates before the ramp.     

On electron pairing in superconducting cuprates

The superconducting properties of materials of layered structure containing copper and other metal oxides are compared with the expectations ofa recently proposed electron pairing model 1. The role of the oxygen content of samples is emphasized. Evidence is found showing that superconduction is originated only in presence of coupled layers of metal oxides holding unpaired electrons. Received 8 November 2000     

Modeling study of peak-dip-hump structure in tunneling spectraof high-temperature superconducting cuprates

We propose new specific model for quasiparticle (QP) tunneling across thesuperconductor-insulator-normal metal (SIN) junction based on two mechanisms. Origin ofthe many features of the tunneling spectra, such as peak-dip-hump (PDH) structure, U- andV-shapes, temperature dependence of differential tunneling conductance, asymmetricconductance peaks, zero-bias conductance, subgap feature and gap inhomogeneity have beenexplained by the proposed model. We show that the energy scales of the binding energies oflarge polarons and polaronic Cooper pairs are identified by pseudogap (PG) crossovertemperature on the cuprate phase diagram.     

Theory of dual fermion superconductivity in hole-doped cuprates

Since the discovery of the cuprate high-temperature superconductivity in 1986, a universal phase diagram has been constructed experimentally and numerous theoretical models have been proposed. However, there remains no consensus on the underlying physics thus far. Here, we theoretically investigate the phase diagram of hole-doped cuprates based on an itinerant-localized dual fermion model, with the charge carriers doped on the oxygen sites and localized holes on the copper d _{x2 ? y2} orbitals. We analytically demonstrate that the puzzling anomalous normal state or the strange metal could simply stem from a free Fermi gas of carriers bathing in copper antiferromagnetic spin fluctuations. The short-range high-energy spin excitations also act as the “magnetic glue” of carrier Cooper pairs and induce d-wave superconductivity from the underdoped to overdoped regime, distinctly different from the conventional low-frequency magnetic fluctuation mechanism. We further sketch out the characteristic dome-shaped critical temperature T _c versus doping level. The emergence of the pseudogap is ascribed to the localization of partial carriers coupled to the local copper moments or a crossover from the strange metal to a nodal Kondo-like insulator. Our work provides a consistent theoretical framework to understand the typical phase diagram of hole-doped cuprates and paves a distinct way to the studies of both non-Fermi liquid and unconventional superconductivity in strongly correlated systems.     

Theory of high-temperature superconductivity in cuprates

A microscopic theory of electronic spectrum and superconducting pairing in the high-temperature cuprate superconductors is presented. The theory is based on consideration of strong electron correlations within the Bogolyubov polar model. The Dyson equation is derived by using the equation of motion method for the thermodynamic Green functions in terms of the Hubbard operators. The self-energy is evaluated in the noncrossing approximation for electron scattering on spin and charge fluctuations induced by kinematic interaction. The theory demonstrates that a strong Coulomb repulsion results in the anomalous electronic spectrum and unconventional (d-wave) superconducting pairing with high T _c mediated by the antiferromagnetic exchange and spin fluctuations.     

Non-fermi liquid and pairing in electron-doped cuprates

We study the normal state and pairing instability in electron-doped cuprates in a model with long-ranged antiferromagnetic spin fluctuations close to an antiferromagnetic quantum-critical point. We show that the fermionic self-energy has a non-Fermi-liquid form leading to peculiar frequency dependencies of the conductivity and the Raman response. We solve the pairing problem and demonstrate that T(c) is determined by the curvature of the Fermi surface, and the pairing gap delta (kappa, omega) is strongly nonmonotonic along the Fermi surface. The normal state frequency dependencies, the value of T(c) is approximately 10 K, and the kappa dependence of the gap agree with the experiment.     

Momentum distribution and condensate fraction of a fermion gas in the BCS-BEC crossover

By using the diffusion Monte Carlo method we calculate the one- and two-body density matrix of an interacting Fermi gas at T = 0 in the BCS to Bose-Einstein condensate (BEC) crossover. Results for the momentum distribution of the atoms, as obtained from the Fourier transform of the one-body density matrix, are reported as a function of the interaction strength. Off-diagonal long-range order in the system is investigated through the asymptotic behavior of the two-body density matrix. The condensate fraction of pairs is calculated in the unitary limit and on both sides of the BCS-BEC crossover.     

Low-temperature phase transitions in systems with fermion condensate

Phase transitions observed in electronic systems of solids in the vicinity of the quantum critical point where the effective mass diverges are analyzed within the framework of the theory of fermion condensation. It is shown that the disordered phase contains a fermion condensate. Its entropy is finite at T → 0 and initiates a chain of transitions occurring at extremely low temperatures. The results are in agreement with experiment.     

Zero sound in a mixture of a single-component fermion gas and a Bose-Einstein condensate

The resonant dynamics of mediated interactions supports zero sound in a cold atom degenerate mixture of a single-component fermion gas and a Bose-Einstein condensate. We characterize the onset of instability in the phase separation of an unstable mixture and we find a rich collective mode structure for stable mixtures with one undamped mode that exhibits an avoided crossing and a Landau-damped mode that terminates.     

The fermion condensate of the Nambu-Jona-Lasinio model in Light Cone quantisation

We investigate the incorporation of condensates in Light Cone quantisation in the frame-work of the Nambu-Jona-Lasinio model. Although it is shown that the physical and perturbative vacua are identical, a gap equation for the dynamical quark mass is obtained and chiral symmetry is dynamically broken. The complete fermion condensate is found in the perturbative vacuum. The corresponding Goldstone mode is a zero mass bound state of the Weinberg equation.     

A Cud-d excitation model for the pairing in the high-T c cuprates

A new model for the pairing mechanism in the ceramic superconductors is presented. Like the magnetic models, we assume the limit of large correlation energies for the Cud electrons. We postulate that the pairing of the Op conduction holes occurs viad–d orbital excitations within thee _g manifold of thed hole of Cu⁺⁺, which is split because of tetragonal or lower symmetry at the Cu sites. This valence conserving charge degree of freedom has been ignored in the magnetic pairing models. Thed–d excitation model may provide a simple qualitative understanding of many experimental results.     

Strong pairing fluctuations and lattice anomalies in the pseudogap phase of the cuprates

Abstract

The pseudogap state in underdoped cuprates shows many very anomalous features. Among them are an extended temperature region of pairing fluctuations above the superconducting transition temperature and an unusual giant phonon anomaly in the same temperature and hole density range. A recent theoretical proposal that ascribes these anomalies to the presence of strong phase fluctuations related to a Leggett mode, is summarised.     

Evolution of fermion pairing from three to two dimensions

We follow the evolution of fermion pairing in the dimensional crossover from three-dimensional to two-dimensional as a strongly interacting Fermi gas of ^{6}Li atoms becomes confined to a stack of two-dimensional layers formed by a one-dimensional optical lattice. Decreasing the dimensionality leads to the opening of a gap in radio-frequency spectra, even on the Bardeen-Cooper-Schrieffer side of a Feshbach resonance. The measured binding energy of fermion pairs closely follows the theoretical two-body binding energy and, in the two-dimensional limit, the zero-temperature mean-field Bose-Einstein-condensation to Bardeen-Cooper-Schrieffer crossover theory.     

Nuclear magnetic resonance study of the electron-doped high-temperature superconducting cuprates

Nuclear magnetic resonance (NMR) measurements have been made on two of the electron-doped high-temperature superconducting cuprates (HTSCs), Pr_2−xCe_xCuO₄ and Sr_0.9La_0.1CuO₂ that represent the two known electron-doped structures. The results are compared with the more-studied hole-doped HTSCs. We show that the electron and hole-doped HTSCs probe a similar antiferromagnetic spin fluctuation spectrum in the normal state, which provides support for theories of superconductivity where the pairing is mediated by antiferromagnetic spin fluctuations and the superconducting order parameter has a d_x²−y² symmetry. Contrary to results from underdoped and hole-doped HTSCs, there is no evidence for a normal-state pseudogap in the NMR data from measurements on the electron-doped HTSCs. Therefore, the electron-doped HTSCs can be better compared with overdoped and hole-doped HTSCs where the normal-state pseudogap is absent. The antiferromagnetic spin fluctuation spectrum as probed by the Cu spin–lattice relaxation rate, is independent of the doped electrons per Cu. A similar effect is observed in the overdoped and hole-doped HTSC, Y_1−xCa_xBa₂Cu₃O_7−δ for a hole concentration range of 0.063. The anomalous Cu NMR linewidth anisotropy observed in the electron-doped HTSCs suggests a small and static spin variation for temperatures up to room temperature.