Single-Particle Density of States of a Superconductor with a Spatially Varying Gap and Phase Fluctuations

Recent experiments have shown that the superconducting energy gap in some cuprates is spatially inhomogeneous. Motivated by these experiments, and using exact diagonalization of a model d-wave Hamiltonian, combined with Monte Carlo simulations of a Ginzburg-Landau free energy functional, we have calculated the single-particle density of states LDOS$(\omega,r)$ of a model high-T$_c$ superconductor as a function of temperature. Our calculations include both quenched disorder in the pairing potential and thermal fluctuations in both phase and amplitude of the superconducting gap. Most of our calculations assume two types of superconducting regions: $\alpha$, with a small gap and large superfluid density, and $\beta$, with the opposite. If the $\beta$ regions are randomly embedded in an $\alpha$ host, the LDOS on the $\alpha$ sites still has a sharp coherence peak at $T = 0$, but the $\beta$ component does not, in agreement with experiment. An ordered arrangement of $\beta$ regions leads to oscillations in the LDOS as a function of energy. The model leads to a superconducting transition temperature $T_c$ well below the pseudogap temperature $T_{c0}$, and has a spatially varying gap at very low $T$, both consistent with experiments in underdoped Bi2212. Our calculated LDOS$(\omega,r)$ shows coherence peaks for $T T_c$, in agreement with previous work considering phase but not amplitude fluctuations in a homogeneous superconductor. Well above $T_c$, the gap in the LDOS disappears.Comment: 37 pages, 12 figures. Accepted by Phys. Rev. B. Scheduled Issue: 01 Nov 200

Dielectric resonator for measuring the magnetic penetration depth at low temperature in high-Tc superconducting thin films

Knowledge of magnetic penetration depth λ(T) at low temperatures allows one to determine the pairing state in the superconductors. A simple method for the evaluation of λ(T) of small (∼1 cm×∼1 cm), flat, high-Tc superconductive samples at low T is discussed. The resolution of the method is a few Å. In addition to high resolution, the method has several advantages including nondestructive analysis, flexibility in sample size, and minimal requirements on the dielectric resonator. The current distribution within the sample being tested can also be accurately calculated, the experimental setup is convenient, and the procedure is comparatively rapid and can be performed in the necktube of a liquid-helium storage Dewar. The measurements for YBCO thin films have been performed at 14.4 GHz. © 1995 American Institute of Physics.SCOPUS: ar.jinfo:eu-repo/semantics/publishe

Andreev reflections and tunneling spectroscopy on underdoped Nd1.85Ce0.15CuO4-δ

Andreev-reflection and tunneling studies have been carried out on a single crystal of underdoped Nd1.85Ce0.15CuO4-δ (NCCO) by using Ag, Pt-Ir and Nb tips to clarify the symmetry of its order parameter. Surprisingly, we observed in the NCCO, on the one hand, the Josephson current (with Nb tip) which indicates the presence of s-wave order parameter, on the other hand, a zero-bias conductance peak (with Ag tip) which is a manifestation of d-wave order parameter. In addition, we find the presence of two distinct energy scales in the underdoped NCCO: the smallest energy scale is associated with the Andreev gap and c-axis tunneling gap, and the second scale corresponds to the maximum in-plane tunneling gap. It seems that the phenomenon of superconductivity in electron- and hole-doped cuprates can be understood within a common scheme.SCOPUS: ar.jinfo:eu-repo/semantics/publishe

The distribution of the energy gap and Josephson IcRn product in Bi2Sr2CaCu2O8+x, by tunneling spectroscopy

We present direct measurements of the density of states by tunneling spectroscopy on slightly overdoped Bi2Sr2CaCu2O8+x (Bi2212) single crystals at low temperature using break-junction and point-contact techniques. We find that (i) the variation of the gap magnitude, Δ, between 20 and 36 meV is likely to be intrinsic to Bi2212, and (ii) there is a correlation between the maximum value of the Josephson IcRn product and the gap magnitude: IcRn decreases with the increase of Δ. The maximum IcRn value of 26 mV is observed at Δ = 20.5 meV. For Δ = 36.5 meV, the maximum measured value of IcRn is 7.3 mV. We conclude that (i) the distribution of the Josephson IcRn product as a function of gap magnitude cannot be explained by the presence of a single energy gap in Bi2212, and (ii) the coherence energy scale in Bi2212 has the maximum Josephson strength. © 2001 Plenum Publishing Corporation.SCOPUS: ar.jinfo:eu-repo/semantics/publishe

Evidence for a quasi-one-dimensional topological-excitation liquid in Bi2Sr2CaCu2O8+x from tunneling spectroscopy

Tunneling measurements have been carried out on heavily underdoped and slightly overdoped Bi2Sr2CaCu2O8+x (Bi2212) single crystals by using a break-junction technique. We find that in-plane tunneling spectra below Tc are the combination of incoherent part from the pseudogap (PG) and coherent quasi-particle (QP) peaks. There is a correlation between the magnitude of the PG and the magnitude of the superconducting gap in Bi2212. Analysis of the data suggests that the tunneling PG in Bi2212 is predominantly a charge-density-wave gap on dynamical charge stripes. We find that the QP peaks appear from the condensation of soliton-like excitations. Thus, we present evidence for a quasi-one-dimensional topological-excitation liquid in Bi2212. © 2001 Elsevier Science B.V. All rights reserved.SCOPUS: ar.jinfo:eu-repo/semantics/publishe

Temperature dependence of the principal properties of dielectric resonators

Dielectric resonators are widely used in microwave circuits due to their small size, low loss and high stability. Knowledge of the temperature dependence of the properties of dielectric resonators can be as important as precise knowledge of the properties themselves. A parallel-plate dielectric resonator method is used to determine the temperature dependence of the fr resonant frequency, the εr relative permittivity, the tan δ loss tangent, and the τf and τε temperature coefficients of dielectric resonators in a broad temperature range.SCOPUS: ar.jinfo:eu-repo/semantics/publishe