Evidence of Vortices on the Insulating Side of the Superconductor-Insulator Transition

Marković, Nina; Mack, A. M.; Martinez-Arizala, G.; Christiansen, C.; Goldman, A. M.

doi:10.1103/physrevlett.81.701

Cited by 58 publications

(40 citation statements)

References 34 publications

(36 reference statements)

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“…2 [6][7][8]. However, recent experiments [9][10][11] have challenged the general existence of such a transition, demonstrating that the apparent transition is only a crossover to a new metallic state at low temperatures.…”

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confidence: 99%

True superconductivity in a two-dimensional superconducting-insulating system

Mason¹,

Kapitulnik²

2001

Phys. Rev. B

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We present results on disordered amorphous films which are expected to undergo a field-tuned Superconductor-Insulator Transition. Based on low-field data and I-V characteristics, we find evidence of a low temperature Metal-to-Superconductor transition. This transition is characterized by hysteretic magnetoresistance and discontinuities in the I-V curves. The metallic phase just above the transition is different from the "Fermi Metal" before superconductivity sets in.PACS numbers: 74.20.M, 73.40.Hm The two-dimensional (2D) Superconductor-Insulator (SI) Transition has attracted widespread attention in recent years [1]. Initially, theory argued for the existence of a magnetic field tuned, continuous phase transition from a superconducting to an insulating state, with a quantum critical point and corresponding quantum critical scaling about a critical field H SI [2][3][4]. Experiments seemed to confirm this scenario, although there was some concern that the apparent critical exponents resembled those expected from classical percolation [5] and that the critical resistance at the transition was not the quantum of resistance for Cooper pairs, h/4e2 [6][7][8]. However, recent experiments [9][10][11] have challenged the general existence of such a transition, demonstrating that the apparent transition is only a crossover to a new metallic state at low temperatures. Similar results have been obtained on quantum Hall liquid-to-insulator transitions and on arrays of Josephson junctions; both are presumed to be in the same universality class as the 2D SI transition [12][13][14]. The question of whether a true superconducting phase is possible within this new experimental situation was an unresolved issue.In this paper, we present results on magnetoresistance and current-voltage measurements carried out on 2D films of MoGe below H SI . Observations of a dramatic drop in resistance at a critical magnetic field H SM << H SI , along with corresponding hysteresis near that field and instabilities in I-V curves, point to the existence of a low-field quantum phase transition, possibly a first order one, to a true superconducting state. The metallic phase above the transition is not a normal Fermi Liquid, at least in any simple sense, because its resistance is orders of magnitudes smaller than the normal state ("Fermi") resistance, it has very large magnetoresistance, and it exhibits non-linear transport unlike a conventional metal. The superconducting state can be characterized by vortices (far separated due to the low field) that are pinned on impurities to provide the true zero resistance.Samples for which we present data in this paper are 30Ȧ and 40Ȧ M o 43 Ge 57 thin films, sandwiched between insulating layers of amorphous Ge on SiN substrates. The 30Ȧ and 40Ȧ samples have sheet resistances at 4.2K of R N ∼1300 Ω/✷ and R N ∼800 Ω/✷, respectively; T C 's of 0.5K and 1K; H C2 's of 1.4T and 1.9T. Previous studies have determined the films to be amorphous and homogeneous on all relevant length scales. We patterned the films in...

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confidence: 99%

True superconductivity in a two-dimensional superconducting-insulating system

Mason¹,

Kapitulnik²

2001

Phys. Rev. B

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“…holes or magnetic impurities) should exhibit flux periodic behavior according to our results. Similar arrangements [8] could test the proposal that there are vortices on the insulating side of the superconductor to insulator transition.…”

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confidence: 99%

“…Presently, a great deal of attention focuses on low superfluid density superconductors for which fluctuations strongly influence and substantially broaden their phase transitions. These include the high temperature superconductors [2], and in particular, their underdoped versions [3,4,5], and ultrathin superconducting films near the superconductor to insulator transition (SIT) [6,7,8,9,10]. For the latter, resistive transitions, R(T ), can develop widths comparable to or greater than the apparent mean field transition temperature, T c0 [11].…”

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confidence: 99%

“…We use the quality of the flux response at different points along their transitions to determine the presence of well-defined vortices [17] and thus provide insight into where the amplitude fluctuation dominated transport gives over to phase fluctuation dominated transport on an R(T ). In addition, we propose that this method can be used to directly detect the existence of vortices in settings where their presence has been contentious [8].As a consequence of phase coherence, magnetic field periodic behavior is expected in films with multiply connected periodic geometries (see inset of Fig. 1) [18,19].…”

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confidence: 99%

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Magnetic-flux periodic response of nanoperforated ultrathin superconducting films

et al. 2006

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We have patterned a hexagonal array of nano-scale holes into a series of ultrathin, superconducting Bi/Sb films with transition temperatures 2.65 K < Tco <5 K. These regular perforations give the films a phase-sensitive periodic response to an applied magnetic field. By measuring this response in their resistive transitions, R(T ), we are able to distinguish regimes in which fluctuations of the amplitude, both the amplitude and phase, and the phase of the superconducting order parameter dominate the transport. The portion of R(T ) dominated by amplitude fluctuations is larger in lower Tco films and thus, grows with proximity to the superconductor to insulator transition.The superconductors originally considered by BCS exhibited spectacularly sharp transitions from a finite resistance to zero resistance as a function of temperature [1]. Fluctuation effects were negligible. Presently, a great deal of attention focuses on low superfluid density superconductors for which fluctuations strongly influence and substantially broaden their phase transitions. These include the high temperature superconductors [2], and in particular, their underdoped versions [3,4,5], and ultrathin superconducting films near the superconductor to insulator transition (SIT) [6,7,8,9,10]. For the latter, resistive transitions, R(T ), can develop widths comparable to or greater than the apparent mean field transition temperature, T c0 [11].To discuss the effects of fluctuations on the R(T ) it is helpful to consider the two component superconductor order parameter ψ = |ψ 0 |e iφ . In bulk elemental superconductors, the sharp R(T ) reflect the near simultaneous appearance of a finite amplitude, |ψ 0 |, and long range coherence of the phase, φ. In low superfluid density superconductors, however, the amplitude first forms at high temperatures and phase coherence develops at lower temperatures [2,12,13,14]. High on a transition quasiparticles transiently form Cooper pairs, which enhances the quasiparticle or fermionic contribution to the conductivity, σ f . These pair or amplitude fluctuations give rise to the initial drop in R(T ) [15]. At very low values of R, a substantial Cooper pair density exists and the conductivity of these bosons, σ b , controls R(T ). σ b is limited by the motion of vortices which causes fluctuations in the phase of the cooper pair condensate. In between, a two fluid model may best describe the transport [7].Physical interpretations of the R(T ) in high sheet resistance films especially those near the SIT relies heavily on distinguishing the amplitude and phase fluctuation dominated regimes [3,4,5,6]. Most often, explicit models for σ f (T ) and σ b (T ) do not exist as a guide and qualitative arguments must prevail. In this paper we present the magnetic flux response of the R(T ) of very low superfluid density (high sheet resistance) [11,16] films patterned with a nanoscale array of holes. We use the quality of the flux response at different points along their transitions to determine the presence of well-defined vortic...

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“…By means of STM, scanning tunneling spectroscopy (STS), and transport measurements various intriguing properties of the 2D Pb systems have been discovered. These include "magic film thickness", "preferential island heights", "oscillating superconductivity transition temperatures" and other novel behaviors because of the strong quantum confinement effect (Bao et al, 2005;Eom et al, 2006;Guo, et al, 2004;Herzog et al, 1996;Markovic et al, 1998;Parendo et al, 2004;Xiong et al, 1997).…”

Section: Pb Nanobeltsmentioning

confidence: 99%