Anomalous quantum Griffiths singularity in ultrathin crystalline lead films

Abstract

Superconductor-insulator/metal transition (SIT/SMT) represents a prototype of quantum phase transition, where quantum fluctuation plays a dominant role and dramatically changes the physical properties of low-dimensional superconducting systems. Recent observation of quantum Griffiths singularity (QGS) offers an essential perspective to understand the subtleties of quantum phase transition in two-dimensional superconductors. Here we study the magnetic field induced SMT in ultrathin crystalline Pb films down to ultralow temperatures. The divergent critical exponent is observed when approaching zero temperature quantum critical point, indicating QGS. Distinctively, the anomalous phase boundary of SMT that the onset critical field decreases with decreasing temperatures in low temperature regime distinguishes our observation from previous reports of QGS in various two-dimensional superconductors. We demonstrate that the anomalous phase boundary originates from the superconducting fluctuations in ultrathin Pb films with pronounced spin-orbit interaction. Our findings reveal a novel aspect of QGS of SMT in two-dimensional superconductors with anomalous phase boundary.

Recent Advances in 2D Superconductors

“…Similar phenomenon was also observed in the LaAlO 3 /SrTiO 3 (110) interface, [ 126 ] monolayer NbSe 2 , [ 127 ] ZnNCl, and artificial 2D superconducting graphene/Pb‐islands‐array hybrid, [ 128 ] but in 2019, as shown in Figure 4h, the anomalous quantum Griffiths singularity was observed in the 4 ML Pb thin films, which shows a reentrant phase boundary with decreased temperature. [ 129 ] Although the critical exponent zv of the anomalous QGS diverges with decreasing field, which is clearly different from that in the aforementioned systems, its characteristics can also be captured by activated scaling law, while the reentrant phase boundary can be explained quantitatively by the effect of superconducting fluctuation with strong spin–orbit interaction. To further reveal the underlying physics phenomena in the quantum phase transition, more artificial superconducting systems are needed.…”

“…h) Reproduced under the terms of the CC‐BY Creative Commons Attribution 4.0 International license ( https://creativecommons.org/licenses/by/4.0). [ 129 ] Copyright 2019, The Authors, published by Springer Nature.…”

Disorder effects on quantum transport and quantum phase transition in low-dimensional superconducting and topological systems

“…Besides, the weak Rashba-type SOI tends to polarize the spin to the in-plane direction, making the Cooper pairs more susceptible to the in-plane magnetic, and destructs the upturn at very low temperature. Lastly, the combination effect large intrinsic SOI and quantum fluctuation may also dramatically influence the perpendicular critical field B c;2 , including the quantum Griffiths singularity along the anomalous phase boundary [68]. The influence of large intrinsic SOI on the quantum fluctuation needs to be considered with the second order corrections [68,69], which is beyond the WHH framework.…”

“…al. have observed the QGS associated with the anomalous phase boundary of SMT in ultrathin crystalline Pb films at ultra-low temperatures [68]. Usually, the SMT shows monotonic phase boundary separating the regions of dR=dT < 0 and dR=dT > 0.…”

“…When approaching the real infiniterandomness quantum critical point B � c along this anomalous phase boundary, the system exhibits anomalous QGS behavior. Reproduced from Ref [124]…”