Dissipation Effects on the Superconductor-Insulator Transition in 2D Superconductors

Abstract

Results on the superconductor to insulator transition in two-dimensional films are analyzed in terms of coupling of the system to a dissipative bath. Upon lowering the temperature the parameter that controls this coupling becomes relevant and a wide range of metallic phase is recovered.PACS numbers: 73.40.Hm Quantum phase transitions (QPT) continue to attract intense theoretical and experimental interest. These transitions -where changing an external parameter in the Hamiltonian of the system induces a transition from one quantum ground state to another, fundamentally different one -have been invoked to explain data from various experiments, including quantum-Hall liquid to insulator, metal to insulator and superconductor to insulator experiments. In this paper we use experiments done on thin superconducting films which undergo a socalled Superconductor-Insulator (SI) transition to propose a new phase diagram for a QPT that includes a dissipation axis. The implications of our analysis go much beyond this problem and in fact bear on all QPT in two dimensions.The most common treatment of the SI transition is to map it onto the so-called "dirty-bosons" model. In this model Cooper pairs are the Bose particles with well formed (non-fluctuating) pair amplitudes. At zero temperature an SI transition is expected as a function of disorder, with vortex-antivortex pairs activated by both quantum fluctuations and disorder. Vortices of one vorticity are also induced by a magnetic field; with no disorder an Abrikosov lattice of these vortices realizes the superconducting phase. However, an arbitrary amount of disorder disrupts the lattice and a true superconducting phase is assumed to be recovered only at T=0. In the superconducting phase vortices are localized into a so-called vortex-glass phase [3] and the Cooper pairs are delocalized. Upon increasing the magnetic field above some critical field H c , vortices will delocalize with Cooper pairs localizing into a Bose-Glass phase. Further increase of the magnetic field dissociates the Cooper pairs, and fermion degrees of freedom then determine the properties of the system. At the SI transition, both vortices and Cooper pairs are delocalized in a "Bose-Metal" phase; thus, a finite conductivity is expected at the transition. The above scenario was the basis for a scaling theory proposed by Fisher [3] in which a field-tuned transition was considered as a continuous transition with an associated diverging length ξ ∼ (H c − H) −ν , as well as a finite, universal conductivity at the transition [4].The above "dirty-bosons" model was used to describe numerous experimental data in both disorder and field tuned transitions [1,5,6]. However, a difficulty in applying this model arose when the vortex glass phase displayed a temperature independent resistance that could be interpreted as quantum tunneling of vortices [2,7,8]. In particular, in previous experiments on amorphous MoGe [1] we showed scaling for a range of disorder; yet the superconducting phase at lower fields showed finit...

Magnetic-field-induced superconductor–metal-insulator transitions in bismuth metal graphite

“…Similar crossover behavior is observed in Si MOSFET, 3 and amorphous metal films of InO x , 5 MoGe, 7 and Bi. 8 The value of H c (≈ 25 kOe) for Bi-MG is almost the same as that for Si MOSFET 3 and amorphous MoGe film.…”

“…8 The value of H c (≈ 25 kOe) for Bi-MG is almost the same as that for Si MOSFET 3 and amorphous MoGe film. 7 Note that the Zeemann energy gSµ B H at 25 kOe corresponds to a thermal energy k B T H with T H = 1.7 K, where g = 2 and S = 1/2. The temperature T H is slightly lower than T c (= 2.48 K) for the superconductivity.…”

Absence of metal–insulator transition and coherent interlayer transport in oriented graphite in parallel magnetic fields

“…Instead we observe always a saturation or, upon applied field, a weak logarithmic increase of the resistivity decreasing temperature [16]. The saturation for T → 0 in the insulating side of the transition has been reported for various 2D systems as, for example, in Mo-Ge films [12], GaAs/AlGaAs heterostructures [17] and Josephson junction arrays [18]. There is no consent on the origin of this saturation.…”

“…B||c-axis. Based on magnetization data [2] and the found 2D scaling similar to that in the MIT of Si-MOSFETs (metal-oxide-semiconductor field-effecttransitor) [11], Mo-Ge [12] and Bi-films [13], the MIT in graphite has been interpreted in terms of superconducting fluctuations [1,3]. It has been shown that the density of states at the Fermi level is enhanced through topological disorder, thus leading to the possible occurrence of localized ferromagnetism and superconductivity [7].…”

Singular or non-Fermi liquids

“…Interesting new developments in this problem with references to substantial earlier work may be found in [176,177,247]. The problem of transitions in Josephson arrays [247] is a variant of such problems.…”