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Register a new userLimited role of vortices in transport in highly disordered superconductors near $B_{c2}$
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At finite temperatures and magnetic fields, type-II superconductors in the mixed state have a non-zero resistance that is overwhelmingly associated with vortex motion. In this work we study amorphous indium oxide films, which are thicker than the superconducting coherence length, and show that near $B_{c2}$ their resistance in the presence of perpendicular and in-plane magnetic fields becomes almost isotropic. Up to a linear rescaling of the magnetic fields both the equilibrium resistance as well as the non-equilibrium current-voltage characteristics are insensitive to magnetic field orientation suggesting that, for our superconductors, there is no fundamental difference in transport between perpendicular and in-plane magnetic fields. Additionally we show that this near-isotropic behavior extends to the insulating phase of amorphous indium oxide films of larger disorder strength that undergo a magnetic field driven superconductor-insulator transition. This near-isotropic behavior raises questions regarding the role of vortices in transport and the origin of resistance in thin-film superconductors.
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We develop an analytical theory for quantum phase transitions driven by disorder in magnets and superconductors. We study these transitions with a cavity approximation which becomes exact on a Bethe lattice with large branching number. We find two different disordered phases, characterized by very different relaxation rates, which both exhibit strong inhomogeneities typical of glassy physics.
Highly disordered superconductors have a rich phase diagram. At a moderate magnetic field (B) the samples go through the superconductor-insulator quantum phase transition. In the insulating phase, the resistance increases sharply with B up to a magneto-resistance peak beyond which the resistance drops with B. In this manuscript we follow the temperature (T) evolution of this magneto-resistance peak. We show that as T is reduced, the peak appears at lower Bs approaching the critical field of the superconductor-insulator transition. Due to experimental limitations we are unable to determine whether the T=0 limiting position of the peak matches that of the critical field or is at comparable but slightly higher B. We show that, although the peak appears at different B values, its resistance follows an activated T dependence over a large T range with a prefactor that is very similar to the quantum of resistance for cooper-pairs.
An increasing current through a superconductor can result in a discontinuous increase in the differential resistance at the critical current. This critical current is typically associated either with breaking of Cooper-pairs (de-pairing) or with a collective motion of vortices (de-pinning). In this work we measure superconducting amorphous indium oxide films at low temperatures and high magnetic fields. Using heat-balance considerations we demonstrate that the current-voltage characteristics are well explained by electron overheating that occurs due to the thermal decoupling of the electrons from the host phonons. As a result the electrons overheat to a significantly higher temperature than that of the lattice. By solving the heat-balance equation we are able to accurately predict the critical currents in a variety of experimental conditions. The heat-balance approach stems directly from energy conservation. As such it is universal and applies to diverse situations from critical currents in superconductors to climate bi-stabilities that can initiate another ice-age. One disadvantage of the universal nature of this approach is that it is insensitive to the microscopic details of the system, which limits our ability to draw conclusions regarding the initial departure from equilibrium.
Recently we reported an enhanced superconductivity in restacked monolayer TaS_2 nanosheets compared with the bulk TaS_2, pointing to the exotic physical properties of low dimensional systems. Here we tune the superconducting properties of this system with magnetic field along different directions, where a strong Pauli paramagnetic spin-splitting effect is found in this system. Importantly, an unusual enhancement as high as 3.8 times of the upper critical field B_{c2}, as compered with the Ginzburg-Landau (GL) model and Tinkham model, is observed under the inclined external magnetic field. Moreover, with the out-of-plane field fixed, we find that the superconducting transition temperature T_c can be enhanced by increasing the in-plane field and forms a dome-shaped phase diagram. An extended GL model considering the special microstructure with wrinkles was proposed to describe the results. The restacked crystal structure without inversion center along with the strong spin-orbit coupling may also play an important role for our observations.
We analyze the structure of an $s-$wave superconducting gap in systems with electron-phonon attraction and electron-electron repulsion. Earlier works have found that superconductivity develops despite strong repulsion, but the gap, $Delta (omega_m)$, necessarily changes sign along the Matsubara axis. We analyze the sign-changing gap function from a topological perspective using the knowledge that a nodal point of $Delta (omega_m)$ is the center of dynamical vortex. We consider two models with different cutoffs for the repulsive interaction and trace the vortex positions along the Matsubara axis and in the upper frequency half plane upon changing the relative strength of the attractive and repulsive parts of the interaction. We discuss how the presence of dynamical vortices affects the gap structure along the real axis, detectable in ARPES experiments.
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