M. Warden

Nonlinear microwave response to oscillating fields: Critical current and field penetration in high-Tc oxides

In the high- T c superconducting oxides, critical currents and magnetic field penetration can be measured using a novel technique: the nonlinear microwave response to scanned magnetic fields.

Unconventional angular dependence of the microscopic magnetic field distribution in single crystal Bi2Sr2CaCu2O8 from muon-spin rotation studies

Abstract The angular dependence of the microscopic magnetic field distribution in single-crystal Bi 2 Sr 2 CaCu 2 O 8 has been measured for a range of applied fields using the muon-spin rotation technique. The results show significant deviations from the predictions of the anisotropic 3D model, which may reflect the extremely anisotropic, quasi-2D nature of the flux structure. The temperature dependence of the field distribution at 400 mT has also been measured for a range of orientations, showing disagreement with the predictions of a simple two-fluid model.

Muon-spin rotation (μSR) study of the temperature dependence of the London penetration depth in copper oxide superconductors

Abstract A μSR study of the temperature dependence of the London penetration depth λ in sintered samples of YBa 2 Cu 3 O z (with various oxygen contents x ), YBa 2 Cu 4 O 8 and Bi 2 Sr 2 CaCu 2 O 8 is presented. It is found that the temperature behavior of λ of all these cuprate superconductors is consistent with conventional s -wave pairing. However, there are significant differences concerning the exact temperature dependence of λ in these materials. In YBa 2 Cu 3 O z with high x , the behavior of λ( T ) is well described by the two-fluid model (strong coupling), whereas λ( T ) in YBa 2 Cu 3 O z with low x , YBa 2 Cu 4 O 8 and Bi 2 Sr 2 CaCu 2 O 8 is in better agreement with weak-coupling BCS theory. Possible reasons for the different temperature behavior of λ in these materials are discussed.

Nonlinear microwave response of superconductors

Abstract Superconductors (SC) have been investigated with microwaves by registering the nonlinear response (RNR) as a function of the modulation field. Equidistant narrow lines in a single YBaCuO crystal found by electron spin resonance (ESR) could be registered by RNR as absorption bands. Some narrow lines are present or absent depending on sweep direction and excursion. In granular SC, thin YBaCuO films showed critical (junction) fields H * which are smaller (3μT) than in bulk samples (5μT). They are as small as in BiSrCaCuO bulk. Using RNR with intermittent modulation, the measured complex hysteresis response indicates a spin glass type of structure of the SC network, and it demonstrates that the usual narrow band ESR registers only prepared states and not initial states corresponding to the scanning field.

Nonlinear microwave absorption in Ba1-xKxBiO3

Abstract Nonlinear, field-modulated, direct microwave absorption (FMMA) was observed in copper-free Ba 1− x K x BiO 3 powder samples. The high-field, low-temperature results were compared to previous measurements of YBa CuO and BiSrCaCuO. The microwave wave determined critical field μ 0 H ∗ = 0.7 μ T and depinning current density J ∗ c ≈ 1 × 10 7 A/m 2 were obtained from the “Portis” model of flux pinning and depinning. These values were lower than the values obtained previously for YBaCuO and BiSrCaCuO as anticipated for a material with larger coherence length and comparable London penetration depth. A previously unobserved asymmetry of the FMMA in the field scan direction was also noted.

μSR investigation of the superconducting properties of YNi2B2C

Abstract Muon spin rotation (μSR) has been used to investigate the mixed state of the recently discovered superconductor YNi2B2C. The width of the μSR-lineshape at low temperature gives a measure of the superconducting penetration depth λ(T = 0), while the field dependence of the linewidth allows an estimate of the superconducting coherence length ξ(T = 0).

Microwave studies of the superconducting state in Rb3C60

Abstract Methods of electron-spin resonance (ESR) and direct, non-linear field-modulated microwave absorption (FMMA) were applied for the measurements in low- and high-purity samples of rubidium-doped fullerene, Rb3C60. The coexistence of the normal strong ESR signals and regular series of weak absorption lines similar to those seen in systems of Josephson junctions was observed in the high-purity sample. The possible influence of the vortex lattice on the ESR signals was also studied. We determined from FMMA investigations using the Portis model the critical field μ 0 H ∗ = 40 μ T , the depinning current density J c ∗ (μ 0 H 0 = 1 mT ) ∼ 4 × 10 8 A/m 2 in low magnetic field and J c ∗ (μ 0 H 0 > 100 mT ) ∼ 1.6 × 10 8 A/m 2 in higher fields. These values were generally one order of magnitude higher than the highest corresponding values previously observed in high-temperature superconductors (HTSs).

Magnetic and superconductivity properties of Rb3C60

Abstract The magnetic and superconductivity properties of rubidium-doped C60 were studied. The temperature dependence of the irreversibility line H ∗ (T ∗ ) as determined by SQUID magnetization measurements gave a power law (n ≈ 2) scaling of the reduced relative irreversibility temperature with irreversibility field. Strong ESR signals were observed, and the influence of vortex induced line-width broadening was studied. A coexistence of ESR signals and regular series of absorption lines, presumably due to Josephson Junctions, was observed.

Direct field modulated microwave absorption in Ba1−xKxBiO3

Abstract Non linear, field modulated, direct microwave absorption (MA) was studied in the high- T c copper-free isotropic Ba 1− x K x BiO 3 (BaKBiO) powder samples. The microwave determined critical magnetic field gradient and the corresponding depinning current density were compared with the values obtained previously for YBaCuO (YBCO) and BiSrCaCuO (BSCCO).