Fluctuations, dissipation, and nonuniversal superfluid jumps in two-dimensional superconductors

Abstract

We report a comprehensive study of the complex ac conductivity of thin effectively two-dimensional amorphous superconducting InO x films at zero applied field. Below a temperature scale T c0 where the superconducting order parameter amplitude becomes well defined, there is a temperature where both the generalized superfluid stiffness acquires a frequency dependence and the dc magnetoresistance becomes linear in field. We associate this with a transition of the Kosterlitz-Thouless-Berezinskii ͑KTB͒ type. At our measurement frequencies the superfluid stiffness at T KTB is found to be larger than the universal value. Although this may be understood with a vortex dielectric constant of ⑀ v Ϸ 1.9 within the usual KTB theory, this is a relatively large value and indicates that such a system may be out of the domain of applicability of the low-fugacity ͑lowvortex-density͒ KTB treatment. This opens up the possibility that at least some of the discrepancy from a nonuniversal magnitude is intrinsic. Our finite-frequency measurements allow us access to a number of other phenomena concerning the charge dynamics in superconducting thin films, including an enhanced conductivity near the amplitude fluctuation temperature T c0 and a finite dissipation at low temperature which appears to be a universal aspect of highly disordered superconducting films.

Superconductivity in compensated and uncompensated semiconductors

“…The pseudogap state would be an insulating state in the highly disordered regime because both electrons and Cooper pairs are localized. This state is qualitatively the same as the 'hard gap insulator' proposed by Feigel'man et al [13] which seems to be realized in disordered thin films [54][55][56][57][58][59][60][61][62][63].…”

“…Presence of incoherent Cooper pairs has been indicated in the thin films [13,16,[56][57][58][59][60][61][62][63] as well as in high-T c cuprates [69][70][71]. The pseudogap of the excitation spectrum has been observed in both systems.…”

Electron and thermal transport via variable range hopping in MoSe2 single crystals

“…However, we observe a down turn in resistivity, which varies 5 linearly with temperature. This semiconductor to metal phase transition at T ∼ 31 K observed in our experiment is highly intriguing which maybe attributed to disorder controlled divergence in resistivity as discussed by earlier works on phase transitions in disordered systems [34][35][36][37][38] . In two dimensional quasi-1D systems of the MoSe 2 family, superconducting phase transition has been observed at lower temperatures [39][40][41][42][43] .…”

Universal Conductivity in a Two-Dimensional Superfluid-to-Insulator Quantum Critical System

“…On the one hand, optical conductivity measurements at frequencies ω/T ∼ 1 in 2D have so far been rather limited and ambiguous [10,11]. On the other hand, previous [12] as well as the most recent [13] numerical studies were severely affected by the fact that simulations were performed away from the critical point on the insulating side of the transition, see the supplementary material [14].…”