— We report the first systematic study of the effect of oxidizing annealing on the superconducting transition temperature T c of Bi 2 Sr 2 – x La x CuO 6 + δ crystals. Using a standard heat treatment procedure, we have obtained detailed T c ( x ) data for Bi 2 Sr 2 – x La x CuO 6 + δ crystals in the range x = 0.35–0.75. Comparison of the shape of the T c ( x ) curve for Bi 2 Sr 2 – x La x CuO 6 + δ with that for La 2 – x Sr x CuO 4 demonstrates a factor of 4 reduction in the dependence of hole concentration in the CuO 2 planes from a La 3+ substitution on the Sr 2+ sites becouse oxygen content (or index) varies. This finding can be accounted for in terms of changes in the amount of oxygen vacancies in the SrO planes. In particular, the oxygen index in Bi 2 Sr 2 – x La x CuO 6 + δ varies by 0.3 per formula unit.

We develop a self-consistent approach for calculating the local impedance at a rough surface of a chiral p-wave superconductor. Using the quasiclassical Eilenberger-Larkin-Ovchinnikov formalism, we numerically find the pair potential, pairing functions, and the surface density of states taking into account diffusive electronic scattering at the surface. The obtained solutions are then employed for studying the local complex conductivity and surface impedance in the broad range of microwave frequencies (ranging from subgap to above-gap values). We identify anomalous features of the surface impedance caused by generation of odd-frequency superconductivity at the surface. The results are compared with experimental data for Sr2RuO4 and provide a microscopic explanation of the phenomenological two-fluid model suggested earlier to explain anomalous features of the microwave response in this material.

We present the results of investigations of the temperature dependences of complex conductivity sigma(T) = sigma (')(T)-i sigma (aEuro3)(T) at frequency 9.4 GHz in series of single crystals V3+x Si1-x with different Si content. The data exhibit peculiarities typical for multiband superconductors, namely a nonlinear temperature dependence of resistivity above superconducting transition temperature T (c) , suppression of superconducting transition temperature T (c) by nonmagnetic impurities, a positive curvature of sigma (aEuro3)(T) curves close to T (c) , and a coherence peak in sigma (')(T) at T similar to T (c) /2. Using a two-band model in the weak-coupling regime, we demonstrate that the behavior of T (c) and the evolution of sigma(T) with Si-content variation are consistently described by changing of the interband scattering rate.

We present the results of investigations of the temperature dependences of complex conductivity σ(T) = σ ′(T)−i σ ″(T) at frequency 9.4 GHz in series of single crystals V3+x Si1−x with different Si content. The data exhibit peculiarities typical for multiband superconductors, namely a nonlinear temperature dependence of resistivity above superconducting transition temperature T c , suppression of superconducting transition temperature T c by nonmagnetic impurities, a positive curvature of σ ″(T) curves close to T c , and a coherence peak in σ ′(T) at T∼T c /2. Using a two-band model in the weak-coupling regime, we demonstrate that the behavior of T c and the evolution of σ(T) with Si-content variation are consistently described by changing of the interband scattering rate.

We present the results of investigations of the temperature dependences of complex conductivity σ ( T ) = σ ′ ( T )− i σ ″ ( T ) at frequency 9.4 GHz in series of single crystals V 3+ x Si 1− x with different Si content. The data exhibit peculiarities typical for multiband superconductors, namely a nonlinear temperature dependence of resistivity above superconducting transition temperature T c , suppression of superconducting transition temperature T c by nonmagnetic impurities, a positive curvature of σ ″ ( T ) curves close to T c , and a coherence peak in σ ′ ( T ) at T ∼ T c /2. Using a two-band model in the weak-coupling regime, we demonstrate that the behavior of T c and the evolution of σ ( T ) with Si-content variation are consistently described by changing of the interband scattering rate.

Precision measurements of the real and imaginary parts of the microwave surface impedance Z ac(T) = R ac(T) + iX ac(T) of the conducting ac layers of the k-(BEDT-TTF)2Cu[N(CN)2]Br crystals in the temperature interval of 0.5 < T < 100 K have demonstrated a series of features: (i) the temperature course of the field penetration depth is close to linear Δλac(T)∞ΔX ac(T) in the superconducting state at T Open image in new window T c ∼ 11.5 K; (ii) the curves R ac(T) = X ac(T) coincide at T c < T < 40 K; (iii) the X ac(T) value at T > 40 K increases in comparison with R ac(T); (iv) the dependence R ac(T) at T > 40 K is nonmonotonic in thin crystals. These features of the impedance Z ac(T) with increasing T are interpreted in terms of (i) the d-type symmetry of the superconducting order parameter, (ii) normal skin effect, (iii) manifestations of the antiferromagnetic fluctuations, and (iv) the size effect. The electrodynamic parameters of k-(BEDT-TTF)2Cu[N(CN)2]Br have been determined.

The normal state resistivity in multiband superconductors has been analyzed in the framework of Eliashberg theory. The results are compared with measurements of the temperature dependence of normal state resistivity of high-purity Ba0.68K0.32Fe2As2 single crystals with the highest reported transition temperature T c = 38.5 K. The experimental data demonstrate strong deviations from the Bloch-Grüneisen behavior, namely the tendency to saturation of the resistivity at high temperatures. The observed behavior of the resistivity is explained within the two band scenario when the first band is strongly coupled and relatively clean, while the second band is weakly coupled and is characterized by much stronger impurity scattering.

We use c-axis resistivity and magnetoresistance measurements to study theinterplay between antiferromagnetic (AF) and superconducting (SC) ordering inunderdoped RBa_2Cu_3O_6+x (R = Lu, Y) single crystals. Both orders are foundto emerge from an anisotropic 3D metallic state, upon which antiferromagnetismopposes superconductivity by driving the doped holes towards localization.Despite the competition, the superconductivity sets in before the AF order iscompletely destroyed and coexists with latter in a certain range of holedoping. We find also that strong magnetic fields affect the AF-SC interplay byboth suppressing the superconductivity and stabilizing the Neel order.

The remarkable sensitivity of the c-axis resistivity and magnetoresistance in cuprates to the spin ordering is used to clarify the doping-induced transformation from an antiferromagnetic (AF) insulator to a superconducting (SC) metal in RBa2Cu3O6+x (R = Lu, Y) single crystals. The established phase diagram demonstrates that the AF and SC regions apparently overlap: The superconductivity in RBa2Cu3O6+x, in contrast to La2-xSrxCuO4, sets in before the long-range AF order is completely destroyed by hole doping. Magnetoresistance measurements of superconducting crystals with low T-c <= 15-20K give a clear view of the magnetic-field induced superconductivity suppression and recovery of the long-range AF state. What still remains to be understood is whether the AF order actually persists in the SC state or just revives when the superconductivity is suppressed, and in the former case, whether the antiferromagnetism and superconductivity reside in nanoscopically separated phases or coexist on an atomic scale.

Common and distinctive features of the temperature dependences of microwave surface impedance Z(T)=R(T)+iX(T) and conductivity σ(T) in the ab-plane and along the c-axis of high-T c single crystals (HTSC) are discussed. The main attention is focused on an evolution of these dependences in YBaCuO crystal with the oxygen deficiency. Comparison of YBaCuO with other HTSC crystals reveals a number of peculiarities, namely, the linear dependence Rab(T) ∞ T up to T c/2 in HTSC single crystals with tetragonal lattice and up to T c/3 in YBaCuO where R ab (T) shows also a broad peak at T∼ Tc/2; breakdown of normal skin effect in some HTSC crystals; dramatic effect of pseudogap on the superfluid density in the heavily underdoped YBCO; several orders of magnitude higher residual surface resistance in HTSC when compared to conventional superconductors; etc. Possible explanations are discussed in the context of the specific features of HTSC structure and in the framework of recent models of quasiparticles’ c-transport and pseudogap state in HTSC.

The temperature dependences of the upper critical field B c2(T) and surface impedance Z(T) = R(T) + iX(T) have been measured in Ba1 − x KxBiO3 single crystals with transition temperatures 6 ≤ T c ≤ 32 K (0.6 > x > 0.4). A transition from the BCS to an unusual type of superconductivity has been revealed: B c2(T) curves of the crystals with T c > 20 K have positive curvature (as in some HTSCs), and those of the crystals with T c < 15 K described by the usual Werthamer-Helfand-Hohenberg (WHH) formula. The R(T) and X(T) dependences of the crystals with T c ≈ 32 K and T c ≈ 11 K in the temperature range T ≪ T c are linear (as in HTSCs) and exponential (BCS), respectively. The experimental results are discussed using the extended saddle point model by Abrikosov.

A method and a setup for making precision measurements of the temperature dependences of components of surface impedance Z ( T ) = R ( T ) + iX ( T ) of small superconductor crystals in the temperature range 0.4–120 K have been developed. The setup combines a high-quality-factor resonance system for measuring the microwave response of a sample at a frequency of 28 GHz and a refrigerating unit with the use of evaporation of 3 He vapors. Measurements of Z ( T ) of optimally doped YBa 2 Cu 3 O 6.93 single crystals in the superconducting and normal states were performed to illustrate the operation of the setup.

This paper develops the theoretical approach and describes the design of a practical test rig for measuring the microwave parameters of unclad and laminated dielectric substrates. The test rig is based on a sapphire whispering-gallery resonator and allows the measurement of the following parameters: dielectric constant (epsilon) of the dielectric substrate in the range from 2 to 10, loss tangent (tan delta) of the dielectric substrate in the range from 10(-4) to 10(-2), and microwave losses of copper coating of the substrate in the range from 0.03 to 0.3 Omega. Measurements of numerous commonly used microwave printed-circuit-board materials were performed at frequencies between 30-40 GHz and over a temperature range of -50 degreesC to +70 degreesC.

Results of investigating the temperature dependences of the surface impedance Z(T) = R(T) + iX(T) and complex conductivity σ(T) = σ'(T) – iσ''(T) in the ab-planes and along the c-axis of high-Tc superconductors (HTSCs) are reviewed and critically analyzed. An electrodynamic method is considered for extracting all components of (T) and (T) tensors from quantities measured in microwave experiments. The main attention is focused on the evolution of (T) and (T) dependences in a YBa2Cu3O7–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.

The investigation of the temperature dependences of microwave surface impedance and complex conductivity of V3Si single crystals with different stoichiometry allowed to observe a number of peculiarities which are in remarkable contradiction with single-gap Bardeen-Cooper-Schrieffer theory. At the same time, they can be well described by two-band model of superconductivity, thus strongly evidencing the existence of two distinct energy gaps with zero-temperature values Delta1 1.8Tc and Delta2 0.95Tc in V3Si.

The in-plane and out-of-plane surface impedance and microwave conductivity components of YBa2Cu3O7-x (0.07 less than or equal to x less than or equal to 0.47) single crystal are determined in the wide ranges of temperature T and carrier concentration p in CuO2 planes. The following features of the superfluid density n(s)(T,p) proportional tolambda(ab)(-2) (T, p) are observed at T < T-c/2 and 0.078 less than or equal to p less than or equal to 0.16: ab (i) n(s)(O, p) depends linearly on p, (ii) the derivative \dn(s)(T, p)/dT\(T-->O) depends on p slightly in the optimally and moderately doped regions (0.10 < p less than or equal to 0.16); however, it rapidly increases with p decreasing, and (iii) the latter finding is accompanied by the linear low-temperature dependence Deltan(s)(T) proportional to (-T) changing to Deltan(s)(T) proportional to (-rootT). For optimum oxygen content the temperature dependence of the normalized imaginary part of the c-axis conductivity lambda(c)(2)(O)/lambda(c)(2) (T) is found to be strikingly similar to that of lambda(ab)(2)(O)/lambda(ab)(2) and becomes more convex with p decreasing. C 2(0' P) values are roughly proportional to the normal state conductivities sigma(c)(T-c, p) along the c-axis. All these properties can be treated in the framework of d-density wave order of a pseudogap.