One of the hallmarks of the Berezinskii-Kosterlitz-Thouless (BKT) transition in two-dimensional (2D) superconductors is the universal jump of the superfluid density, that can be indirectly probed via the non-linear exponent of the current-voltage IV characteristics. Here, we compare the experimental measurements of IV characteristics in two cases, namely NbN thin films and SrTiO3-based interfaces. While the former display a paradigmatic example of BKT-like non-linear effects, the latter do not seem to justify a BKT analysis. Rather, the observed IV characteristics can be well reproduced theoretically by modelling the effect of mesoscopic inhomogeneity of the superconducting state. Our results offer an alternative perspective on the spontaneous fragmentation of the superconducting background in confined 2D systems. arXiv:1905.01221v1 [cond-mat.supr-con]

We investigate the magnetic field variation of the thermally activated flux flow resistivity, ρ TAFF and flux flow critical current density, J c , in a weakly pinned thin film of the amorphous superconductor aMoGe, where vortices are in a fluid state over a large range of magnetic fields. We show that both quantities can be understood within the framework of collective pinning theory. In particular, our results demonstrate that a 'peak effect' can arise at the order-disorder transition of the vortex lattice even when both the ordered and disordered states are vortex fluids, such as the boundary between a hexatic vortex fluid and an isotropic vortex liquid.

Recently, detailed real space imaging using scanning tunneling spectroscopy of the vortex lattice in a weakly pinned a-MoGe thin film revealed that the vortex lattice melts in two steps with temperature or magnetic field, going first from a vortex solid to a hexatic vortex fluid and then from a hexatic vortex fluid to an isotropic vortex liquid. In this paper, we show that the resistance in the hexatic vortex fluid state is extremely sensitive to radio-frequency electromagnetic perturbation. In the presence of very low-amplitude excitation above few kilohertz, the resistance increases by several orders of magnitude. On the other hand when the superconductor is well shielded from external electromagnetic radiation, the dissipation in the sample is very small and the resistance is below our detection limit. 1 pratap@tifr.res.in

We investigate the evolution of superconductivity with decreasing film thickness in ultrathin amorphous MoGe (a-MoGe) films using a combination of sub-Kelvin scanning tunneling spectroscopy, magnetic penetration depth measurements and magneto-transport measurements. We observe that superconductivity is strongly affected by quantum and classical phase fluctuations for thickness below 5 nm. The superfluid density is strongly suppressed by quantum phase fluctuations at low temperatures and evolves towards a linear-T dependence at higher temperatures. This is associated with a rapid decrease in the superconducting transition temperature, Tc, and the emergence of a pronounced pseudogap above Tc. These observations suggest that at strong disorder the destruction of superconductivity follows a Bosonic route where the global superconducting state is destroyed by phase fluctuations even though the pairing amplitude remains finite.In the late fifties, Anderson 1 predicted that in an s-wave superconductor the attractive pairing interaction forming Cooper pairs would remain largely unaffected by the presence of non-magnetic impurities. This has been loosely interpreted to imply that the superconducting transition temperature, Tc will also not be strongly sensitive to disorder scattering. However, later experiments showed that this is valid only in the limit of weak disorder: In the presence of strong disorder, Tc gets gradually suppressed 2,3 and eventually the material is driven into a non-superconducting state. The mechanism driving the transition from a superconductor to an insulator or a metal has been a subject of considerable debate. In principle, the suppression of Tc with increase in disorder can happen from two origins. The first mechanism is through the loss of effective screening with increase in disorder that weakens the attractive pairing interaction, and thus suppresses the mean field transition temperature 4,5 . The second mechanism results from the decrease in superfluid density, ns, induced by disorder scattering, which renders the superconductor susceptible to phase fluctuations 6,7 . When ns is small, the phase coherent superconducting state can get destroyed due to strong phase fluctuations even when the pairing amplitude remains finite 8,9,10 .The superconductor to non-superconductor transition driven by these two mechanisms are often classified as the Fermionic and Bosonic routes respectively 11 . In the Fermionic route, the pairing attraction drops to zero at a critical disorder where superconductivity is destroyed. This non-superconducting state is either a bad metal or an Anderson insulator. In the Bosonic mechanism the pairing interaction remains finite and therefore signatures of Cooper pairing continue to survive even in the non-superconducting state.Experimentally, this manifests as a persistence of the superconducting gap in the electronic excitation spectrum, known as the pseudogap, even after the global superconducting state is destroyed 12,13,14,15 .However, recent studies indicate that th...

Within the Bardeen–Cooper–Schrieffer (BCS) theory, superconductivity is entirely governed by the pairing energy scale, which gives rise to the superconducting energy gap, Δ. However, another important energy scale, the superfluid phase stiffness, J, which determines the resilience of the superconductor to phase-fluctuations is normally ignored. The spectacular success of BCS theory owes to the fact that in conventional superconductors J is normally several orders of magnitude larger than Δ and thus an irrelevant energy scale. However, in certain situations such as in the presence of low carrier density, strong disorder, at low-dimensions or in granular superconductors, J can drastically come down and even become smaller than Δ. In such situations, the temperature and magnetic field evolution of superconducting properties is governed by phase fluctuations, which gives rise to novel electronic states where signatures of electronic pairing continue to exist even when the zero resistance state is destroyed. In this article, we will review the recent experimental developments on the study of phase fluctuations in conventional superconductors.

The vortex lattice in a Type II superconductor provides a versatile model system to investigate the order-disorder transition in a periodic medium in the presence of random pinning. Here, using scanning tunnelling spectroscopy in a weakly pinned Co0.0075NbSe2 single crystal, we show that the vortex lattice in a 3-dimensional superconductor disorders through successive destruction of positional and orientational order, as the magnetic field is increased across the peak effect. At the onset of the peak effect, the equilibrium quasi-long range ordered state transforms into an orientational glass through the proliferation of dislocations. At a higher field, the dislocations dissociate into isolated disclination giving rise to an amorphous vortex glass. We also show the existence of a variety of additional non-equilibrium metastable states, which can be accessed through different thermomagnetic cycling.

We investigate the performance of niobium nitride superconducting coplanar waveguide resonators toward realizing hybrid quantum devices with magnon-photon coupling. We find internal quality factors ∼ 20 000 at 20 mK base temperature, in zero magnetic field. We find that by reducing film thickness below 100 nm, an internal quality factor greater than 1000 can be maintained up to a parallel magnetic field of ∼ 1 T and a perpendicular magnetic field of ∼ 100 mT. We further demonstrate strong coupling of microwave photons in these resonators with magnons in chromium trichloride, a van der Waals antiferromagnet, which shows that these cavities serve as a good platform for studying magnon-photon coupling in 2D magnonics based hybrid quantum systems. We demonstrate strong magnon-photon coupling for both optical and acoustic magnon modes of an antiferromagnet.

Pinning and creep determine the current-voltage characteristic of a type II superconductor and thereby its potential for technological applications. The recent development of strong pinning theory provides us with a tool to assess a superconductor's electric properties in a quantitative way. Motivated by the observation of typical excess-current characteristics and field-scaling of critical currents, here, we analyze current-voltage characteristics measured on 2H-NbSe2 and a-MoGe type II superconductors within the setting provided by strong pinning theory. The experimentally observed shift and rounding of the voltage-onset is consistent with the predictions of strong pinning in the presence of thermal fluctuations. We find the underlying parameters determining pinning and creep and discuss their consistency.