Superconductivity-Related Insulating Behavior

Sambandamurthy, G.; Engel, L. W.; Johansson, Andreas; Shahar, D.

doi:10.1103/physrevlett.92.107005

Cited by 227 publications

(411 citation statements)

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“…Here we study the effect of intentional magnetic doping on transport properties of ultra-thin MoGe films that undergo a superconductor-insulator transition (SIT) [5][6][7][8][9][10]. The mechanism of the SIT remains an important unresolved problem in condensed matter physics.…”

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confidence: 99%

Effect of magnetic Gd impurities on the superconducting state of amorphous Mo-Ge thin films with different thickness and morphology

et al. 2012

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We studied the effect of magnetic doping with Gd atoms on the superconducting properties of amorphous Mo70Ge30 films. We observed that in uniform films deposited on amorphous Ge, the pair-breaking strength per impurity strongly decreases with film thickness initially and saturates at a finite value in films with thickness below the spin-orbit scattering length. The variation is likely caused by surface-induced magnetic anisotropy and is consistent with the fermionic mechanism of superconductivity suppression. In thin films deposited on SiN the pair-breaking strength becomes zero. Possible reasons for this anomalous response are discussed. The morphological distinctions between the films of the two types were identified using atomic force microscopy with a carbon nanotube tip.

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Section: Introductionmentioning

confidence: 99%

Effect of magnetic Gd impurities on the superconducting state of amorphous Mo-Ge thin films with different thickness and morphology

et al. 2012

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“…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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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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“…2b of Ref. 5. Within the percolation picture, the nonmonotonic MR arises from the film breaking down to SC and normal regions ͑described as localized electron glass͒.…”

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“…Since there is still controversy over its theoretical value, we chose to estimate it from the experimentally measured activation gap in the insulating phase. 5,6 When vortices condense, Cooper pair density fluctuations become gapped due to Higgs mechanism, and the gap which can be read off from the Lagrangian depends on m v . We conjecture that this gap is the activation gap.…”

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confidence: 99%

“…More recent experiments, however, showed the critical resistance to be nonuniversal. 3 Furthermore, in many field tuned experiments, a surprising metallic phase intervenes between the superconductor and insulator, [4][5][6] with a temperature-independent resistance below T ϳ 50 mK, and ͑at least in Ta films͒ a distinct nonlinear I-V characteristics. 7 Quite generically, 5,6,8 these films exhibit a peak in the magnetoresistance ͑MR͒ curve ͑particularly strong in InO and TiN͒ as in Fig.…”

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Investigating the superconductor-insulator transition in thin films using drag resistance: Theoretical analysis of a proposed experiment

2009

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The magnetically driven superconductor-insulator transition in amorphous thin films ͑e.g., InO and Ta͒ exhibits several mysterious phenomena, such as a putative metallic phase and a huge magnetoresistance peak. Unfortunately, several conflicting categories of theories, particularly quantum-vortex condensation, and normal region percolation, explain key observations equally well. We present a experimental setup, an amorphous thin-film bilayer, where a drag resistance measurement would clarify the role quantum vortices play in the transition, and hence decisively point to the correct picture. We provide a thorough analysis of the device, which shows that the vortex paradigm gives rise to a drag with an opposite sign and orders of magnitude larger than the drag measured if competing paradigms apply. DOI: 10.1103/PhysRevB.80.180503 PACS number͑s͒: 74.78.Db, 73.43.Nq, 74.25.Fy, 74.78.Fk The superconducting state and the metallic Fermi-liquid form the very basis of our understanding of correlated electron systems. Nevertheless, the transition between these two phases in disordered films is shrouded in mystery. Experiments probing this transition in amorphous thin films such as Ta, MoGe, InO, and TiN used a perpendicular magnetic field and disorder ͑tuned through film thickness͒ to destroy superconductivity. But instead of a superconductor-metal transition, they observed in many cases a superconductor-insulator transition ͑SIT͒. 1 The "dirty boson" model 2 propounded the notion that the insulator is the mark of vortex condensation, and that the SIT occurs at a universal critical resistance, R ᮀ = h / 4e 2 . More recent experiments, however, showed the critical resistance to be nonuniversal. 3 Furthermore, in many field tuned experiments, a surprising metallic phase intervenes between the superconductor and insulator, 4-6 with a temperature-independent resistance below T ϳ 50 mK, and ͑at least in Ta films͒ a distinct nonlinear I-V characteristics. 7 Quite generically, 5,6,8 these films exhibit a peak in the magnetoresistance ͑MR͒ curve ͑particularly strong in InO and TiN͒ as in Fig. 1͑a͒.Two competing categories of theories may account for these phenomena. On one hand, within the quantum vortex pictures, 2,9,10 the insulating phase implies vortex condensation, the intervening metallic phase is described as uncondensed vortex liquid ͑e.g., vortex Fermi liquid͒, and the high field nonmonotonic MR indicates the appearance of a finite electronic density of states ͑DOS͒ at the Fermi level. On the other hand, the percolation paradigm 11,12 describes the films as consisting of superconducting ͑SC͒ and normal puddles; at the MR peak SC puddles exhibit a Coulomb blockade, and the percolating normal regions consist of narrow conduction channels. Yet a third theory tries to account for the low field SC-metal transition using a phase glass model 13 ͑see, however, Ref. 14 which argues against these results͒ but does not address the full MR curve. Qualitatively, both paradigms above are consistent with MR observations, and recent...

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Superconductivity-Related Insulating Behavior

Cited by 227 publications

References 32 publications

Effect of magnetic Gd impurities on the superconducting state of amorphous Mo-Ge thin films with different thickness and morphology

Effect of magnetic Gd impurities on the superconducting state of amorphous Mo-Ge thin films with different thickness and morphology

Magnetic-flux periodic response of nanoperforated ultrathin superconducting films

Investigating the superconductor-insulator transition in thin films using drag resistance: Theoretical analysis of a proposed experiment

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