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Register a new userAbsence of a Bulk Meissner State in RuSr2GdCu2O8
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C. W. Chu
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We have systematically investigated the magnetic, electrical, and structural properties of RuSr2GdCu2O8, in which a long-range ferromagnetic order and superconductivity have been previously reported to coexist. Based on the reversible magnetization results, we conclude that the bulk Meissner state does not exist in this compound and that the condensation energy associated with superconductivity is negligible. The absence of a bulk Meissner state and the superconductivity detected are thus attributed to the possible appearance of a sponge-like crypto-superconducting fine structure in RuSr2GdCu2O8 samples that are found to be chemically homogeneous to 1-2 mu m and electrically uniform to ~ 10 mu m across the sample.
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We report ac magnetic susceptibility and dc magnetization measurements on the superconducting ferromagnet UCoGe (with superconducting and Curie temperatures of $T_{{rm SC}} sim 0.5$~K and $T_{{rm Curie}} sim 2.5$~K, respectively). In the normal, ferromagnetic state ($T_{{rm SC}} < T < T_{{rm Curie}}$), the magnetization curve exhibits a hysteresis loop similar to that of a regular itinerant ferromagnet. Upon lowering the temperature below $T_{{rm SC}}$, the spontaneous magnetization is unchanged, but the hysteresis is markedly enhanced. Even deeply inside the superconducting state, ferromagnetism is not completely shielded, and there is no Meissner region, a magnetic field region of $H < H_{rm c1}$ (a lower critical field). From these results, we suggest that UCoGe is the first material in which ferromagnetism robustly survives in the superconducting state and a spontaneous vortex state without the Meissner state is realized.
A simple procedure to extract anisotropic London penetration depth components from the magnetic susceptibility measurements in realistic samples of cuboidal shape is described.
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In a magnetic field, superconductivity is manifested by total magnetic field expulsion (Meissner effect) or by the penetration of integer multiples of the flux quantum {Phi}_0. Here we present experimental results revealing magnetic dipoles formed by Meissner current flowing around artificially introduced topological defects (lattice of antidots). By using scanning Hall probe microscopy, we have detected ordered magnetic dipole lattice generated at spatially periodic antidots in a Pb superconducting film. While the conventional homogeneous Meissner state breaks down, the total magnetic flux of the magnetic dipoles remains quantized and is equal to zero. The observed magnetic dipoles strongly depend on the intensity and direction of the locally flowing Meissner current, making the magnetic dipoles an effective way to monitor the local supercurrent. We have also investigated the first step of the vortex depinning process, where, due to the generation of magnetic dipoles, the pinned Abrikosov vortices are deformed and shifted from their original pinning sites.
A detailed study of the effect caused by the partial substitution of Ru by Ir on the magnetic and superconducting properties of the ruthenocuprate Ru(1-x)Ir(x)Sr2GdCu2o8; 0 <= x <= 0.10; is presented. The combined experimental results of structural, electrical, and magnetic measurements indicate that Ir substitutes Ru for x <= 0.10 with no significant structural distortions. Ir-doping gradually suppresses both the magnetic and the superconducting states. However, all samples were observed to attain the zero-resistance state at temperatures >= 2 K up to the highest applied magnetic field of 18 T. The resistive upper-critical field Hc2 as a function of temperature has been determined for these polycrystalline samples. Values of Hc2(0) were found to be ~ 52 T, and weakly dependent on the Ir concentration. We have also observed that the superconducting transition width decreases and the slope of the resistive transition increases with increasing Ir doping, a feature which is much more pronounced at high applied magnetic fields. The double-peak structure observed in the derivative of the resistive curves has been related to an inhomogeneous nature of the physical grains which is enhanced due to the Ru substitution by Ir. This indicates that the Josephson-junction-array (JJA) model seems to be appropriated to describe the superconducting state in these ruthenocuprates. The low temperature rho(T) data along with the determined vortex thermal activation energy are consistent with a 2D vortex dynamics in these materials.
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