Low-frequency response in the surface superconducting state of single-crystal ZrB12

Physical Review B (Impact Factor: 3.66). 02/2005; 71(6). DOI: 10.1103/PhysRevB.71.064506
Source: arXiv

ABSTRACT A large nonlinear response of a single crystal of ZrB12 to an ac field (frequency 40–2500 Hz) for H0>Hc2 has been observed. Direct measurements of the ac wave form and the exact numerical solution of the Ginzburg-Landau equations, as well as the phenomenological relaxation equation, permit the study of the surface superconducting state dynamics. It is shown that the low-frequency response is defined by transitions between the metastable superconducting states under the action of an ac field. The relaxation rate that determines such transition dynamics is found.

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    ABSTRACT: We report results of experimental studies of the ac susceptibility of a Nb single crystal at low frequencies in swept magnetic fields applied parallel to the surface. Analysis of the experimental data shows that the swept magnetic field significantly changes the bulk conductivity for Hc1<H0<Hc2Hc1<H0<Hc2. It becomes dissipative with substantial frequency dispersion. At the surface, a layer with enhanced conductivity in comparison to the bulk was detected for Hc1<H0<Hc2Hc1<H0<Hc2. This layer provides a considerable contribution to the shielding and absorption of the ac field even in the mixed state. There is anomaly of an ac susceptibility in the field near the transition between Abrikosov’s phase and surface superconducting states. Conductivity of the surface layer in large dc fields Hc2<H0<Hc3Hc2<H0<Hc3, measured in point-by-point mode, and in a swept field is unexpectedly small in spite of the large value of the order parameter near the surface. We demonstrate that the swept magnetic field affects the ac response of the surface superconducting states.
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    ABSTRACT: We report results of measurements of the ac conductivity of a Nb superconducting thin film in a swept dc magnetic field. In the mixed state the swept dc field creates vortices at the film surface which pass through the film and form the observed ac conductivity. Vortex rate generation does not depend on the value of the dc field and there is a large plateau-like region of dc magnetic fields where the dissipation is approximately constant. A proposed phenomenological model describes quite well the main features of the ac response in these fields, including its dependency on the sweep rate, ac amplitude, frequency, and value of the second and third harmonics.
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    ABSTRACT: Magnetization measurements under hydrostatic pressure up to 10.5kbar in zirconium dodecaboride ZrB12 superconductor ( Tc~=6.0K at p=0 ) were carried out. A negative pressure effect on Tc with dTc/dp=-0.0225(3)K/kbar was observed. The electron-phonon coupling constant lambdael-ph decreases with increasing pressure with dlnlambdael-ph/dp~=-0.20%/kbar . The magnetic field penetration depth lambda was studied in the Meissner state and, therefore, probes mainly the surface of the sample. The absolute values of lambda and the superconducting energy gap at ambient pressure and zero temperature were found to be lambda(0)=140(30)nm and Delta0=1.251(9)meV , respectively. Delta0 scales linearly with Tc as 2Delta0/kBTc=4.79(1) . The studies of the pressure effect on lambda reveal that lambda-2 increases with pressure with dlnlambda-2(0)/dp=0.60(23)%/kbar . This effect cannot be explained within the framework of conventional adiabatic electron-phonon pairing, suggesting the possibility that close to the surface an unconventional nonadiabatic character of the electron-phonon coupling takes place.
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