Collapse of the Cooper pair phase coherence length at a superconductor-to-insulator transition

Abstract

We present investigations of the superconductor to insulator transition (SIT) of uniform a-Bi films using a technique sensitive to Cooper pair phase coherence. The films are perforated with a nanohoneycomb array of holes to form a multiply connected geometry and subjected to a perpendicular magnetic field. Film magnetoresistances on the superconducting side of the SIT oscillate with a period dictated by the superconducting flux quantum and the areal hole density. The oscillations disappear close to the SIT critical point to leave a monotonically rising magnetoresistance that persists in the insulating phase. These observations indicate that the Cooper pair phase coherence length, which is infinite in the superconducting phase, collapses to a value less than the interhole spacing at this SIT. This behavior is inconsistent with the gradual reduction of the phase coherence length expected for a bosonic, phase fluctuation driven SIT. This result starkly contrasts with previous observations of oscillations persisting in the insulating phase of other films implying that there must be at least two distinct classes of disorder tuned SITs. 1 arXiv:1301.6155v2 [cond-mat.supr-con] 4 Feb 2013Superconductor to insulator quantum phase transitions (SIT) can be induced in a wide range of quasi two-dimensional superconducting systems, including elemental films, high T c superconductors, organic superconductors and superconductor graphene composites [1][2][3][4][5][6][7][8]. Remarkably, these transitions occur, nearly universally, at a critical normal state resistance, R Nc ≃ R Q = h (2e) 2 , and film resistances often show scaling behavior around this critical point. The most prominent theories that can account for these behaviors view the SIT as a Cooper pair or boson localization transition, rather than a Cooper pair breaking transition [9][10][11][12][13][14]. Recent experiments that probe films near the SIT in new ways have provided more details that support this "bosonic" picture of the SIT [15][16][17][18][19]. For example, scanning tunneling microscopy (STM) has revealed that spatial inhomogeneities develop in the order parameter on approaching the SIT suggesting that Cooper pairs localize into islands [19,20]. High frequency transport measurements indicate that a finite superfluid density persists in non-superconducting films [21]. Here, we describe experiments employing a technique that is uniquely sensitive to the length scale that diverges at a bosonic SIT: the phase coherence length ξ φ . This technique previously revealed that Cooper pairs maintain their phase coherence over 100's of nanometers through the SITs of two distinct film systems [15,22].For this work, we investigate a third film system [23][24][25] that is likely to provide the most stringent test of whether the bosonic SIT is generic to thin films. This phase sensitive technique requires patterning films with an array of nanometer-scale holes.The hole patterning creates a simply connected geometry that leads to Little-Parks-like (LP) oscillations...

Asymmetric avalanches in the condensate of a Zeeman-limited superconductor

“…Indeed, the upsweep branch (superheating branch) of the R n = 0.32 kΩ/sq film has many discontinuous jumps in resistance and is generally more ragged than the corresponding branch of the R n = 5.5 kΩ/sq film. These data suggest that the avalanche behavior is limited to films with modest disorder and that it is almost completely suppressed once the film resistance is of the order of the superconducting quantum resistance /(4e 2 ) ∼ 6.5 kΩ/sq 18,19 . For this reason we have focused our tunneling studies on moderately disordered films having normal state sheet resistances of a few hundred ohms.…”

Superconductor-insulator transition of Josephson-junction arrays on a honeycomb lattice in a magnetic field

“…We argued that the absence of secondary minima at f = 1/3 predicted by the model and the approximately linear behavior of the activation energy with film thickness are due to effects of Josephson-coupling disorder. The same system fabricated to be uniformly thick 20 is not described by the present model, which assumes superconducting "grains" and weak links on a length scale of nanohole size and should belong to a different universality. The f = 1/2 case is of particular interest since geometrical frustration combined with thermal fluctuations leads to an unusual phase transition as a function of temperature 17,25,26,29 .…”

“…In part, this is due to the lack of experimental data for artificial arrays in this geometry. Recently, however, there has been a growing interest in a related system, in the form of an ultra-thin superconducting film with a triangular lattice of nanoholes [18][19][20][21][22] . A simple model for this system consists of a Josephson-junction array on a honeycomb lattice, with the triangular lattice of nanoholes corresponding to the dual lattice, and it has already been used to investigated the thermal resistive transition in absence of charging effects 17 .…”

Fermionic scenario for the destruction of superconductivity in ultrathin MoC films evidenced by STM measurements

“…The recent measurements of the Little-Park oscillations in the magnetoresistance on uniform a-Bi films with nanohoneycomb array of holes also support the fermionic SIT [29]. Then, a question arises what physical parameter decides on the insulating ground state upon SIT, fermionic or Cooper-pair insulating one.…”