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Register a new userAngular Dependence of the Nonlinear Transverse Magnetic Moment of YBCO in the Meissner state
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The angular dependence of the nonlinear transverse magnetic moment of untwinned high-quality single crystals of optimally doped YBCO have been studied at a temperature of 2.5K using a low frequency AC technique. The absence of any signature at angular period 2pi/4is analyzed in light of the numerical predictions of such a signal for a pure d-wave order parameter with line nodes. Implications of this null result for the existence of a non-zero gap at all angles on the Fermi surface are discussed.
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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.
SQUID magnetization measurements in oriented powders of Y$_{1-x}$Ca$_{x}$Ba$% _{2}$Cu$_{3}$O$_{y}$, with $x$ ranging from 0 to 0.2, for $yapprox 6.1$ and $yapprox 6.97$, have been performed in order to study the doping dependence of the fluctuating diamagnetism above the superconducting transition temperature $T_{c}$. While for optimally doped compounds the diamagnetic susceptibility and the magnetization curves $-M_{fl}(T=const$) vs. $H$ are rather well justified on the basis of an anisotropic Ginzburg-Landau (GL) functional, in underdoped and overdoped regimes an anomalous diamagnetism is observed, with a large enhancement with respect to the GL scenario. Furthermore the shape of magnetization curves differs strongly from the one derived in that scheme. The anomalies are discussed in terms of phase fluctuations of the order parameter in a layered system of vortices and in the assumption of charge inhomogeneities inducing local, non percolating, superconducting regions with $T_{c}^{(loc)}$ higher than the resistive transition temperature $T_{c}$. The susceptibility displays activated temperature behavior, a mark characteristic of the vortex-antivortex description, while history dependent magnetization, with relaxation after zero-field cooling, is consistent with the hypothesis of superconducting droplets in the normal state. Thus the theoretical picture consistently accounts for most experimental findings.
We report the results of angle dependent resistivity of NdFeAsO$_{0.82}$F$_{0.18}$ single crystals in the superconducting state. By doing the scaling of resistivity within the frame of the anisotropic Ginzburg-Landau theory, it is found that the angle dependent resistivity measured under different magnetic fields at a certain temperature can be collapsed onto one curve. As a scaling parameter, the anisotropy $Gamma$ can be determined for different temperatures. It is found that $Gamma(T)$ increases slowly with decreasing temperature, varying from $Gamma simeq$ 5.48 at T=50 K to $Gamma simeq$ 6.24 at T=44 K. This temperature dependence can be understood within the picture of multi-band superconductivity.
In the spin-excitation-mediated pairing mechanism for superconductivity, the geometric frustration effects not only the spin configuration but also the superconducting-state properties. Within the framework of the kinetic-energy-driven superconducting mechanism, the doping and temperature dependence of the Meissner effect in triangular-lattice superconductors is investigated. It is shown that the magnetic-field-penetration depth exhibits an exponential temperature dependence due to the absence of the d-wave gap nodes at the Fermi surface. However, in analogy to the dome-like shape of the doping dependence of the superconducting transition temperature, the superfluid density increases with increasing doping in the lower doped regime, and reaches a maximum around the critical doping, then decreases in the higher doped regime.
In a recent Letter (Phys. Rev. Lett. 81, p.5640 (1998), cond-mat/9808249 v3), it was suggested that nonlocal effects may prevent observation of the nonlinear Meissner effect in YBCO. We argue that this claim is incorrect with regards to measurements of the nonlinear transverse magnetic moment, and that the most likely reason for a null result lies elsewhere.
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