Narrow Results

Starting from a microscopic t–J like model and a SU(2) spin-charge separation ansatz, a relativistic continuum gauge Lagrangian is obtained in the vicinity of a nodal point of the Fermi surface. The excitations in the pseudogap phase are described by topological excitations in the dual model which has a Z₂ global symmetry due to the effect of instantons. Confinement of spinon and holons emerge from this picture. The adjoint and fundamental strings are associated with stripes. As the spin gap decreases a local Z₂ symmetry emerges.

We present a self-consistent approach to deal with the pairing-fluctuation effects in quasi-two-dimensional superconducting systems. Besides the Cooper pairs in the Bose–Einstein condensate, there are pairs occupying the excited states, which results in the predominant fluctuations. The low-lying excited states are the collective modes. On the basis of ladder-diagram approximation, we treat the single particles and the pairs on an equal-footing manner. The Green's function of single particles is obtained as an analytic solution to a cubic equation. The bosonic degrees of freedom are relevant to the pseudogap physics in the high-T_c cuprates. The superconducting order parameter and the transition temperature are substantially reduced from the values of the mean-field theory. The calculated phase boundary of superconducting state can reasonably describe the experiment data for cuprates.

Experimental studies of the pairing state of cuprate superconductors reveal asymmetric behaviors of the hole-doped (p-type) and electron-doped (n-type) cuprates. The pairing symmetry, pseudogap phenomenon, low-energy spin excitations and the spatial homogeneity of the superconducting order parameter appear to be non-universal among the cuprates, which may be attributed to competing orders. We propose that the non-universal pseudogap and nano-scale variations in the quasiparticle spectra may be the result of a charge nematic (CN) phase stabilized by disorder in highly two-dimensional (2D) p-type cuprates. The CN phase is accompanied by gapped spin excitations and competes with superconductivity (SC). In contrast, gapless spin excitations may be responsible for the absence of pseudogap and the presence of excess sub-gap spectral weight in the momentum-independent quasiparticle spectra of n-type cuprates. The physical implications and further verifications for these conjectures are discussed.

We discuss the pseudogap phenomena in High-T_c superconductors. The superconducting fluctuation is taken into account as an origin of the pseudogap. Then, the T-matrix approach has been widely used. We report a development along this line. The higher order correction, which is important to capture the effects of the SC fluctuation more precisely, is investigated. It is shown that the vertex correction, which is not included in the self-consistent T-matrix approximation, enhances the pseudogap.

The orientational anisotropy of the pseudogap closing field H_pg is investigated in magnetic fields up to 60 T applied parallel (||ab) and normal (||c) to CuO₂ planes in Bi₂Sr₂CaCu₂O_8+y. We find that, in contrast to large field anisotropy related to the orbital motion of Cooper pairs below the upper critical field H_c2, the temperature-independent and small anisotropy of H_pg is due to anisotropy of the g-factor: , indicating correlations only in the spin-channel.

We investigate the effect of the opening of the pseudogap on the temperature dependence of the in-plane resistivity ρ_ab(T) for Bi 2212 epitaxial thin films having different oxygen contents. Using successive annealing treatments we were able to bring the films from a maximally overdoped state (characterized by a T_c of 63 K) to a strongly underdoped state (T_c<10 K). We also observed a significant change in the temperature dependence of the in-plane resistivity ρ_ab(T). This dependence presents two major characteristics: for the overdoped sample we observed an upward curvature from linearity while for underdoped samples we observed a downturn from linearity starting from a characteristic temperature T* which will be associated with the opening of the pseudogap in the excitation spectrum.

Model of a superconducting singular Fermi liquid with a divergent scattering amplitude for particles with the same momenta has been extended by the inclusion of an external magnetic field. Pseudogap-like behavior observed in the model is interpreted. The response of the system to a weak magnetic field acting on both: spin and charge degrees of freedom is studied within the Bogolubov method. The magnetization and the magnetic field penetration depth as well as the critical magnetic field and the critical current are found and discussed. Numerical results for a s-wave paired singular Fermi liquid are presented.

This paper extends the two-local-spin-mediated interaction (TLSMI) for the superconductivity in the CuO₂ plane of the high-T_c cuprates to the Cu-O chain superconductivity in YBa₂Cu₃O_7-δ (Y123), and sets up a theory for the chain superconductivity in Y123. We explain the observed 60 K chain superconductivity and predicts that the pseudogap should exist in the Cu-O chain of Y123.

Recent Nernst and interlayer transport experiments in Bi₂Sr₂CaCu₂O_8+y (BSCCO) high temperature superconductors report hugely different limiting magnetic fields. We demonstrate that both fields convert to the same pseudogap energy scale T* upon transformation as orbital and Zeeman critical fields, respectively. We suggest a consistent interpretation of this finding based on separation of spin and charge degrees of freedom residing in different regions of a truncated Fermi surface.

We present scanning tunneling spectroscopic and high-field thermodynamic studies of hole- and electron-doped (p- and n-type) cuprate superconductors. Our experimental results are consistent with the notion that the ground state of cuprates is in proximity to a quantum critical point (QCP) that separates a pure superconducting (SC) phase from a phase comprised of coexisting SC and a competing order, and the competing order is likely a spin-density wave (SDW). The effect of applied magnetic field, tunneling current, and disorder on the revelation of competing orders and on the low-energy excitations of the cuprates is discussed.

The in-plane resistivity ρ_ab(T) and out-of-plane resistivity ρ_c(T) are measured for Bi₂Sr₂CaCu₂O₈ superconducting thin films successfully deposited by in situ magnetron RF-sputtering on vicinal (100) SrTiO₃ substrates. Oriented film with c axis inclined with respect to the basal plane by a tilt angle α ≈ 4° was obtained. It was found that for the underdoped region ρ_ab(T) deviates from the high T-linear behavior, which evidences the opening of the pseudogap while from the out-of-plane resistivity measurements, we found that the temperature corresponding to the minimum between metallic and semiconducting behavior appears to reflect the opening of the pseudogap for transport along c axis. We also found that the opening of the pseudogap seen in ρ_ab(T) does not coincide with the pseudogap along c axis.

The out-of-plane resistivity ρ_c(T) is measured for Bi₂Sr₂CaCu₂O₈ superconducting thin films successfully deposited by in situ magnetron RF–sputtering on vicinal (100) SrTiO₃ substrates. Oriented film with c-axis inclined with respect to the basal plane by a tilt angle α≈4° was obtained. The oxygen content of the film was changed from maximally overdoped state to strongly underdoped nonsuperconducting states. It was found that the temperature corresponding to the minimum between metallic and semiconductive behavior appears to reflect the opening of the pseudogap for transport along c-axis. We found that the opening of the pseudogap along c-axis does not coincide with pseudogap seen in ab plane.

This paper demonstrates in theory that a bias-induced oscillatory current should occur in a tunneling junction made of the high-T_c cuprates in pseudogap state. This novel oscillatory effect can be utilized for the judging mechanism of pseudogap in the high-T_c cuprates and a series of non-superconducting materials. In application, this novel effect can be utilized for making electronic devices, using Cooper pairs and operating at room temperature.

We explore extremely overdoped regime in the phase diagram of cuprate superconductors. Interlayer tunneling transport measurements under high magnetic fields are employed to detect the field dependence of low-energy density of states (DOS) in hole-doped (BSCCO and TBCO), and electron-doped (SCCO) cuprates. We find a strong doping dependence of the pseudogap closing field whose Zeeman energy universally scales with the pseudogap onset temperature, indicating a preeminent role of spin-singlet correlations in forming the pseudogap regardless of the doping type. Excess dissipation due to the pseudogap is found to follow a 'universal' correlation with the pseudogap closing field. The pseudogap features appear absent in the extremely overdoped TBCO, pointing to a possible quantum critical point very near the edge of the superconducting 'dome'.

We have performed measurements of the out-of-plane resistivity ρ_c in magnetic fields up to 40 T and the scanning tunneling microscopy (STM) in the Pb-Substituted Bi2201. Two kinds of pseudogaps PG1 and PG2 are found at T*₁ and T*₂ below which the nonmetallic ρ_c starts to develop and to be suppressed in magnetic fields with decreasing temperature, respectively. The obtained phase diagram indicates that the pseudogap related to the superconductivity is not PG1 but PG2. STM images of the Bi(Pb)-O plane in the optimally doped sample show bright atom-strings at V ≤ 10 mV. The observed structure may indicate the one-dimensional correlation which is different from the 4a × 4a structure reported in other systems.

STM/STS is used to examine the charge order in the pseudogap state on underdoped Bi2212 crystals with different hole-doping levels (p). We report that although the data for the period of charge order are slightly scattered, it is nearly 4a × 4a (a: the lattice constant) and exhibits no obvious dependence on p in the p-range examined.

We report the tunneling conductance on a quadruple-layered cuprate, Ba₂Ca₃Cu₄O₈(O_1-xF_x)₂ (2x~1.4, T_c~100K) that includes inequivalent outer (OP) and inner (IP) CuO₂ planes in a unit cell. Typical tunneling conductance displays the spectral dip structures as well as the superconducting gap (SCG) with clear coherence peaks. However we sometimes observe a pseudogap (PG) like structure without coherence peak at 4.2K. The magnitude of PG at 4.2K is larger than that of SCG at 4.2K and its conductance curve displays a faint structure at low energy. Therefore its origin would be different from that of small-PG observed at above T_c. In addition, we find that there are two kinds of SCGs that originate from IP and OP, and this result suggests that the superconducting property at IP is different from that at OP because of the imbalance of doped carrier between IP and OP. The observation of large PG at low temperature suggests the existence of some form of hidden ordered state at T<T_c and it would coexist with superconductivity.

Recent development in the physics of high-temperature cuprate superconductivity is reviewed, with special emphasis on the phenomena of unconventional and non-universal low-energy excitations of hole- and electron-type cuprate superconductors and the possible physical origin. A phenomenology based on coexisting competing orders with cuprate superconductivity in the ground state appears to provide a consistent account for a wide range of experimental findings, including the presence (absence) of pseudogaps and Fermi arcs above the superconducting transition T_c in hole-type (electron-type) cuprate superconductors and the novel conductance modulations below T_c, particularly in the vortex state. Moreover, the competing order scenario is compatible with the possibility of pre-formed Cooper pairs and significant phase fluctuations in cuprate superconductors. The physical implications of the unified phenomenology and remaining open issues for the microscopic mechanism of cuprate superconductivity are discussed.

We study properties of charge fluids with random impurities or heavy polarons using a microscopic Hamiltonian with the full many-body Coulomb interaction. At zero temperature and high enough density the bosonic fluid is superconducting, but when density decreases the Coulomb interaction will be strongly over-screened and impurities or polarons begin to trap charge carriers forming bound quasiparticle like clusters, which we call Coulomb bubbles or clumps. These bubbles are embedded inside the superconductor and form nuclei of a new insulating state. The growth of a bubble is terminated by the Coulomb force. The fluid contains two groups of charge carriers associated with free and localized states. The insulating state arises via a percolation of the insulating islands of bubbles, which cluster and prevent the flow of the electrical supercurrent through the system.

Our results are applicable to HTSC. There the Coulomb fluids discussed in the paper correspond to mobile holes located on Cu sites and heavy polarons or charged impurities located on Oxygen sites. As a result of our calculations the following two-componet picture of two competing orders in cuprates arise. The mobile and localized states are competing with each other and their balance is controlled by doping. At high doping a large Fermi surface is open. There the density of real charge carriers is significantly larger than the density of the doped ones. When doping decreases more and more carriers are localized as Coulomb clumps which are creating around heavy polarons localized on Oxygen sites and forming a regular lattice. The picture is consistent with the Gorkov and Teitelbaum (GT) analysis ^1,2 of the transport, Hall effect data and the ARPES spectra as well as with nanoscale superstructures observed in Scanning Tunneling Microscope(STM) experiments [3-8]. The scenario of the clump formation may be also applicable to pnictides, where two types of clumps may arise even at very high temperatures.

The effect of random on-site disorder on s-wave (BCS) superconductors described by a two dimensional negative-U Hubbard model is studied using Bogoliubov-de Gennes (BdG) method. The mean field pairing amplitudes become spatially inhomogeneous at large values of disorder where the system breaks up into superconducting islands with large pairing amplitudes, separated by insulating strips. The amplitude fluctuations are correctly accounted for via BdG calculations, however it misses phase fluctuations which are inherent to low dimensions. The phase fluctuations affect superconducting properties strongly, and the effect is more pronounced in the limit of large disorder. We provide a close estimate of the actual transition temperature, T_c by incorporating phase fluctuations about the inhomogeneous BdG state. This is obtained by relating the jump in renormalized D_s (obtained from a self consistent Harmonic approximation on a phase-only Hamiltonian) at the Kosterlitz-Thouless (KT) transition and the KT transition temperature, T_KT obtained from D_s(T) = (2/π)T_KT as temperature tends to T_KT from below. This yields opening of a large region sandwiched between (obtained by the vanishing of ) and T_KT where there is no phase coherence between the pairs, however amplitude correlations continue to exist, reminiscent of a pseudogap phase in high-T_c superconductors which is marked by short ranged preformed pairs without any definite phase relation between them. Further, the appearance of the superconducting islands from a homogeneous phase indicates the evolution of a system consisting of large and overlapping pairs to one that contains short and tightly bound pairs - a scenario termed as BCS-BEC crossover. We have investigated in details the crossover phenomenon as a function of disorder and confirmed it's existence at small values of electron concentration, however, absent at larger densities.