Qualitative picture of a new mechanism for high-Tc superconductors

Xu et al. observed enhanced Nernst effect and Iguchi et al. observed patched diamagnetism, both well above $T_c$ in underdoped high-$T_c$ superconductors (HTSCs). A new mechanism is proposed here, which seems to naturally explain, at least qualitatively, these observations, as well as the d-wave nature and continuity of pseudogap and pairing gap, the tunneling conductance above $T_c$, as well as $T^*(x)$, $T_{\nu}(x)$, $T_c(x)$, etc. This mechanism combines features of dynamic charged stripes, preformed pairs, and spin-bags: At appropriete doping levels, the doped holes (and perhaps also electrons) will promote the formation of anti-phase islands in short-range anti-ferromagnetic order. On the boundary of each such island reside two doped carriers; the unscreened Coulomb repulsion between them stabilizes its size. Superconductivity results when such ``pre-formed pairs'' Bose-condense.Comment: 8 pages, 4 figures, New3SC-4 Conference Proceedings, to be published in ijmp

Origin of the zero-bias conductance peaks observed ubiquitously in high-T-c superconductors

Journals published by the American Physical Society can be found at http://journals.aps.org/The midgap surface states predicted previously to exist on non-{n0m} surfaces of d(xa2-xb2-) superconductors (SC's) can be extended to midgap interface states (MIS's), which exist at almost all interface between grains of such SC's of different principal axes orientations. They can Five rise to a zero-bias conductance peak (ZBCP) in quasiparticle tunneling along any axis as shown in our model calculation. When the counterelectrode is a low-T-c SC, its gap is shown to appear as a dip at the center of the (broadened) ZBCP. These and other results support the proposal that such MIS's are responsible for most if not all of the ZBCP's observed ubiquitously in tunneling experiments performed on high-T-c SC's

Theory of electron transport in normal metal/superconductor junctions

On the basis of the Keldysh method of non-equilibrium systems, we develop a theory of electron tunneling in normal-metal/superconductor junctions. By using the tunneling Hamiltonian model (being appropriate for the tight-binding systems), the tunneling current can be exactly obtained in terms of the equilibrium Green functions of the normal metal and the superconductor. We calculate the conductance of various junctions. The discrepancy between the present treatment and the well-known scheme by Blonder, Tinkham, and Klapwijk is found for some junctions of low interfacial potential barrier.Comment: 5 pages, 4 figure

Effect of unitary impurities in non-STM-types of tunneling in high-T_c superconductors

Based on an extended Hubbard model, we present calculations of both the local (i.e., single-site) and spatially-averaged differential tunneling conductance in d-wave superconductors containing nonmagnetic impurities in the unitary limit. Our results show that a random distribution of unitary impurities of any concentration can at most give rise to a finite zero-bias conductance (with no peak there) in spatially-averaged non-STM type of tunneling, in spite of the fact that local tunneling in the immediate vicinity of an isolated impurity does show a conductance peak at zero bias, whereas to give rise to even a small zero-bias conductance peak in the former type of tunneling the impurities must form dimers, trimers, etc. along the [110] directions. In addition, we find that the most-recently-observed novel pattern of the tunneling conductance around a single impurity by Pan et al. [Nature (London) 403,746 (2000)] can be explained in terms of a realistic model of the tunneling configuration which gives rise to the experimental results reported there. The key feature in this model is the blocking effect of the BiO and SrO layers which exist between the tunneling tip and the CuO_2 layer being probed.Comment: 9 pages, 7 ps-figures, to appear in Phys. Rev. B (Sep. 1, 2000); typos corrected, references added, figure 6 changed to expand the explanation on recent experimental measurements by S.H. Pan et al. [Nature (London) 403, 746 (2000)

Phase-Boundary of a Cubic Superconducting Circuit in a Magnetic-Field of Arbitrary Magnitude and Direction

Journals published by the American Physical Society can be found at http://journals.aps.org/An exact analytic expression for the mean-field phase boundary T(c)(H) of a cubic superconducting circuit in an arbitrary external-magnetic-field vector H is derived. The phase boundary of this circuit is shown to depend in a complex and sensitive way on both the magnitude and the direction of the magnetic field. Some practical applications of these properties are also suggested

Numerical Relaxation Approach for Solving the General Ginzburg-Landau Equations for Type-Ii Superconductors

Journals published by the American Physical Society can be found at http://journals.aps.org/A numerical relaxation approach for solving the general Ginzburg-Landau equations for type-II superconductors is developed. It is first applied to an isotropic type-II superconductor near H(c1) in order to establish the reliability and effectiveness of this approach. The strength of this approach should be in dealing with anisotropic and/or inhomogeneous systems. As an initial test of this strength, we have applied it to some anisotropic cases. Distributions of the superconducting order parameter and the local magnetic field, as well as the lower critical field for these cases, are presented

Predicted giant magnetic moment on non-{n0m} surfaces of d-wave superconductors

Journals published by the American Physical Society can be found at http://journals.aps.org/It has been noted that the sizable areal density of midgap states which must exist on any non-{n0m} surface of a d-wave superconductor can lead to a giant magnetic moment. Here we show that this effect is observable, and discuss two precise ways to observe it: (i) by directly measuring magnetic moment in a system with a large density of internal {110} surfaces, or (ii) by performing spin-polarized tunneling on a {110} surface. In both cases, a sufficiently large magnetic field should be applied in the [1 (1) over bar 0] direction. Observing these predictions in high-T(c) superconductors can provide a strong confirmation of the d-wave scenario for such materials. [S0163-1829(99)50942-9]

Exact Near-Onset Analysis of the Spin-Density-Wave Instability in Ferromagnetic Superconductors - the Linearly Polarized State

Journals published by the American Physical Society can be found at http://journals.aps.org