Conductivity anisotropy and pseudogap in the microwave response of high-T_csuperconductors

Abstract

Results of investigating the temperature dependences of the surface impedance Z(T)=R(T)+iX(T) and complex conductivity r T r H T À ir HH T in the ab-planes and along the c-axis of high-T c superconductors (HTSCs) are reviewed and critically analyzed. An electrodynamic method is considered for extracting all components of r T and ZT tensors from quantities measured in microwave experiments. The main attention is focused on the evolution of ZT and r T dependences in a YBa 2 Cu 3 O 7±x crystal with the oxygen content variation. Possible mechanisms of conductivity are discussed in the framework of models for normal, superconducting, and pseudogap states of HTSCs.

Microwave response of a cylindrical cavity made of bulk MgB2 superconductor

“…In order to identify possible spurious effects in the measurement of the mw response of the cavity, we have measured the mw surface resistance of a small plate-like sample of MgB 2 extracted from the same specimen from which the cavity has been obtained. These measurements have been performed by the technique of hot-finger cavity perturbation [10], using a Nb cavity resonating at ≈ 9.4 GHz. Fig.…”

“…Firstly, grain boundaries in these materials are weakly coupled giving rise to reduction of the critical current and/or nonlinear effects, which worsen the device performance [9]; furthermore, the process necessary to obtain bulk HTS in a performing textured form is very elaborated. For these reasons, in several applications, most of the superconducting cavities are still manufactured with Nb, requiring liquid He as refrigerator [4,7,10].…”

Calculation of the excess current and the pseudogap in high-temperature superconductors YBa<inf>2</inf>Cu<inf>3</inf>O<inf>6,85</inf> and Bi<inf>1,6</inf>Pb<inf>0,4</inf>Sr<inf>1,8</inf>Ca<inf>2,2</inf>Cu<inf>3</inf>O<inf>10</inf> by the Monte Carlo method

“…One of the urgent problems of the condensed matter physics is the study of nature and properties of pseudogap states of cuprate high temperature superconductors (HTSC) created at the decrease in concentration of hole charge carriers lower than optimal level for superconductivity [1,2]. Despite of the efforts put by the experimenters and theorists the pseudogap nature is still not obvious.…”

Non-separable pairing interaction kernels applied to superconducting cuprates

“…Measurements of the superfluid density n s (T, x) [9] provide another connection to our model. The x dependence n s (T = 0, x) ∝ x − x 1 is the same as the that of anti-pseudo gap…”

“…[7,8] Current theories either promote the pseudogap state as a pairing precursor of the SC state, or as a competitor due to unrelated dynamical fluctuations. [9,10] Hence, the origin and the effect of the pseudogap on the SC state remains unresolved. [2,7,11,9] The SPM gives new insight here.…”