Superconductor–Insulator Transitions in 2D: The Experimental Situation

Abstract

Superconductor±insulator (SI) transitions in ultrathin films have attracted significant attention over the last decade because of the possibility that they are quantum phase transitions. Magnetic field, film thickness, or carrier concentration can be used as control parameters. The bosonic pictures of these transitions proposed some years ago are only in qualitative agreement with experiment. In particular, the critical resistance appears not to be universal, and there are variations in the values of critical exponents. It has been concluded that in real films fermionic degrees of freedom must be taken into account. There are also indications that the phase diagram may include a significant metallic phase separating the superconducting and insulating phases, and that the transition may have a significant percolative aspect. The experimental situation will be broadly reviewed with attention paid to issues relating to materials and measurements.Introduction The investigation of superconductivity in the presence of disorder began 60 years ago with the work of Shalnikov [1]. The subject has played an on-going role in contemporary condensed matter physics. Anderson [2] showed that nonmagnetic impurities have no significant effect on the superconducting transition by pointing out that Cooper pairs form from time-reversed eigenstates, which are not strongly affected by disorder. This idea is only relevant to weakly disordered systems where electronic states are extended. One anticipates that as states become localized, superconductivity will vanish [3].The case of very thin films is of particular interest as two dimensions (2D) is quite special. The superconducting transition in 2D, occurs as a topological, or KosterlitzThouless-Berezinskii transition [4]. Weakly interacting 2D electron systems are always localized, even for arbitrarily weak disorder [5]. Bosonic models, which involve a quantum phase tansition have ben proposed as an explanation of 2D superconductor±insula-tor transitions [6].

Gate Voltage Tuned Quantum Superconductor to Insulator Transition in an Ultrathin Bismuth Film Revisited

“…Continuous quantum-phase transitions (QPT) are transitions at absolute zero in which the ground state of a system is changed by varying a parameter of the Hamiltonian [1,2,3]. The transitions between superconducting and insulating behavior in two-dimensional systems tuned by disorder, film thickness, magnetic field or with the electrostatic field effect are believed to be such transitions [2,4,5].…”

“…With D = 2 and ν = ∞, appropriate for the BKT-transition [8,9], this disorder should be irrelevant. In this context it is important to recognize that the existence of the BKT-transition (vortex-antivortex dissociation instability) in 4 He films is intimately connected with the fact that the interaction energy between vortex pairs depends logarithmic on the separation between them. As shown by Pearl [12] vortex pairs in thin superconducting films (charged superfluid) have a logarithmic interaction energy out to the characteristic length λ 2D = λ 2 /d, beyond which the interaction energy falls off as 1/r.…”

“…Here λ is the magnetic penetration depth of the bulk. As λ 2D increases the diamagnetism of the superconductor becomes less important and the vortices in a thin superconducting film become progressively like those in 4 He films [13]. According to this λ 2D >> min [W, L] is required, where W and L denote the width and the length of the perfect sample.…”

Doping Controlled Superconductor-Insulator Transition inBi2Sr2−xLaxCaCu2O8+Oh1, Crane2, Harlingen3 et al. 2006Phys. Rev. Lett.416290View full textAdd to dashboardCite

“…As a function of decreasing temperature, the resistance first decreases at high temperatures, reaches a minimum at an intermediate temperature, starts to increase at lower temperatures until it reaches a local maximum, and then it drops to zero resistance below the superconducting transition temperature. Although such a re-entrant behavior is both experimentally [1][2][3][4] and theoretically [5][6][7][8][9] in homogeneously disor dimensional (2D) thin films. These studies suggest that there exists a defect-sc dominated metallic (that is, resistance being temperature-independen zero-temperature critical sheet resistance at the transition from super perconducting as T 0, otherwise they are insulating.…”

“…First of all, the sheet resistance tends to diverge as T 0 at the critical doping, contrary to the common notion of finite critical sheet resistance not only in conventional disordered metals [1][2][3][4] but also in cuprates [12,13]. Above the critical ling function ng phase.…”