We report high-resolution specific heat data on a high-quality single crystal of the classical superconductor V3Si, which reveal tiny lambda-shape anomalies appearing superimposed onto the BCS specific heat jump at the superconducting transition in m
agnetic fields of 2 T and higher. The appearance of these anomalies is accompanied by a magnetic-field-induced broadening of the superconducting transition. We demonstrate, using scaling relations predicted by the fluctuation models of the 3d-XY and the 3d-Lowest-Landau-Level (3d-LLL) universality class, that the effect of critical fluctuations becomes experimentally observable due to of a magnetic field-induced enlargement of the regime of critical fluctuations. The scaling indicates that a reduction of the effective dimensionality due to the confinement of quasiparticles into low Landau levels is responsible for this effect.
The melting of the magnetic vortex lattice has been observed in high-Tc superconductors in many experiments by different groups and is regarded as confirmed. To date, only one group claims to have observed the vortex-lattice melting in the low-Tc sup
erconductor Nb3Sn in specific-heat measurements. We measured the same Nb3Sn single crystal with a differential-thermal analysis method. We report on the absence of any sign of vortex-lattice melting in our data and discuss the possible reasons for this discrepancy. In addition we confirm the observation of a small peak-like anomaly near the transition to superconductivity which is likely related to thermal fluctuations.
By the application of a small oscillating magnetic field parallel to the main magnetic field and perpendicular to the transport current, we were able to generate a voltage dip in the I-V curves of Nb$_3$Sn similar to the peak-effect pattern observed
in earlier resistivity measurements. The pattern was history dependent and exhibited a memory effect. In addition we observed in the I-V curves for a high shaking-field amplitude a step feature of unknown origin.
