Superconductivity in 3D Nb–Cu nanocomposite granular films have been studied with varying thickness for two different compositions, Nb rich with 88 at% of Nb and Cu rich with 46 at% of Nb. For both compositions, the superconducting transition temperature (Tc) decreases with decreasing film thickness. For any thickness, doubling the Cu content in the films decreases the Tc by about 2 K. To explore if phase fluctuations play any role in superconductivity in these 3D films, the superfluid stiffness (JS) of the films was measured using low frequency two-coil mutual inductance (M) technique. Interestingly, the measurement of M in magnetic fields showed two peaks in the imaginary component of M for both Nb rich and Cu rich films. The two peaks were associated with the pair-breaking effect of the magnetic field on the intra and inter-granular coupling in these films consisting of random network of superconductor (S) and normal metal (N) nano-particles. Furthermore, JS was seen to decrease with decreasing film thickness and increasing Cu content. However, for all films studied JS remained higher than the superconducting energy gap (∆) indicating that phase fluctuations do not play any role in superconductivity in the film thickness and composition range investigated. Our results indicate that an interplay of quantum size effects (QSE) and superconducting proximity effect (SPE) controls the Tc with composition in these 3D nano-composite films.

Temporal variation of secondary cosmic rays (SCR) flux was measured during the total lunar eclipse on 10 December 2011 and the subsequent full moon on 8 January 2012 from Mumbai (Geomagnetic latitude: 10.6°N), India. The measurements were done by using NaI (Tl) scintillation detector with energy threshold of 200 keV. The SCR flux shows approximately 8.1% enhancement during the lunar eclipse as compared to the average of pre‐ and post‐eclipse periods. Weather parameters (temperature and relative humidity) were continuously monitored, and their correlations with temporal variation in SCR flux have been examined. The influences of geomagnetic field, interplanetary parameters, and tidal effect on SCR flux have been considered. Qualitative analysis of SCR flux variation indicates that local weather, interplanetary, and geomagnetic factors affecting SCR flux fail to explain the observed enhancement during the eclipse. Lunar tidal effect on magnetosphere and crust still remains a possible mechanism which needs to be investigated in detail. The enhancement during lunar eclipse and widely reported decrease during solar eclipses may unravel hitherto unnoticed factors modulating SCR flux.

We show that under an a.c. magnetic field excitation the vortex lattice in a superconductor with periodic array of holes can undergo a transition from a Mott-like state where each vortex is localized in a hole, to a metal-like state where the vortices get delocalized. The vortex dynamics is studied through the magnetic shielding response which is measured using a low frequency twocoil mutual inductance technique on a disordered superconducting NbN film having periodic array of holes. We observe that the shielding response of the vortex state is strongly dependent on the amplitude of the a.c. magnetic excitation. At low amplitude the shielding response varies smoothly with excitation amplitude, corresponding to elastic deformation of the vortex lattice. However, above a threshold value of excitation the response shows a series of sharp jumps, signaling the onset of the Mott to metal transition. Quantitative analysis reveals that this is a collective phenomenon which depends on the filling fraction of vortices in the antidot lattice.