Recent advances in nanoscience have demonstrated that fundamentally new physical phenomena may be found when the size of samples shrinks. In the area of superconductivity, the reduction of sample size has led to the observation of the paramagnetic Meissner effect in micronsize superconductors (1), the quantization of the Bose condensate in submicron samples (2), and ultimately the suppression of superconductivity in nanometer-scale superconductors (3,4). In this regime, it has also been recognized that the sample topology has particularly strong effects on superconductivity, as reflected in the characteristic features of the phase diagrams for singly-and doubly-connected samples (5,6).A unique feature of doubly-connected superconductors (independent of the sample size) is
We have measured upper-critical-field H c2 , specific heat C, and tunneling spectra of the intermetallic perovskite superconductor MgCNi 3 with a superconducting transition temperature T c Ϸ7.6 K. Based on these measurements and relevant theoretical relations, we have evaluated various superconducting parameters for this material, including the thermodynamic critical field H c (0), coherence length (0), penetration depth (0), lower-critical-field H c1 (0), and Ginzburg-Landau parameter (0). From the specific heat, we obtain the Debye temperature ⌰ D Ϸ284 K. We find a jump of ⌬C/␥T c ϭ2.1 at T c ͑where ␥ is the normal-state electronic specific coefficient͒, which is larger than the weak-coupling BCS value of 1.43, suggesting that MgCNi 3 may be a strong-coupling superconductor. In addition, we observed a pronounced zero-bias conductance peak ͑ZBCP͒ in the tunneling spectra. We discuss the possible physical origins of the observed ZBCP.
Epitaxial thin films of the n=1–5 members of the layered Srn+1RunO3n+1 oxide series were produced by reactive molecular-beam epitaxy. X-ray diffraction and high-resolution transmission electron microscopy confirm that these films are epitaxially oriented and nearly phase pure (>98%). The Sr2RuO4 (n=1) and Sr3Ru2O7 (n=2) samples show no ferromagnetic transition in the range from 5to300K, while the Sr4Ru3O10 (n=3), Sr5Ru4O13 (n=4), and Sr6Ru5O16 (n=5) samples show ferromagnetic transitions at 85, 95, and 130K, respectively.
Trilayered ruthenate Sr 4 Ru 3 O 10 exhibits an interesting itinerant metamagnetic transition for magnetic fields applied along in-plane directions. Our earlier work has revealed that this metamagnetic transition occurs via a phase separation process with magnetic domain formation ͓Z. Q. Mao et al., Phys. Rev. Lett. 96, 077205 ͑2006͔͒. We recently performed systematic investigations on its magnetotransport properties and constructed a magnetic field-temperature ͑H-T͒ phase diagram. In the phase separated regime of the phase diagram, due to domain boundary scattering, the resistivity is increased and shows nonmetallic temperature dependence, while outside the phase separation regime, the system shows Fermi-liquid ground-state properties, i.e., ϰ T 2 . The Fermi-liquid temperature is strongly suppressed near the metamagnetic transition. The transport properties in the mixed phase region are sensitive to the disorders, and the magnetoresistivity steps in this region are found to be suppressed by increasing the level of disorders. In addition, we discussed the possible mechanism of the metamagnetic transition of this material.
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