Size dependent nature of the magnetic-field driven superconductor-to-insulator quantum-phase transitions

Abstract

The nature of the magnetic-field driven superconductor-to-insulator quantum-phase transition in two-dimensional systems at zero temperature has been under debate since the 1980s, and became even more controversial after the observation of a quantum-Griffiths singularity. Whether it is induced by quantum fluctuations of the superconducting phase and the localization of Cooper pairs, or is directly driven by depairing of these pairs, remains an open question. We herein experimentally demonstrate that in weakly-pinning systems and in the limit of infinitely wide films, a sequential superconductor-to-Bose insulator-to-Fermi insulator quantum-phase transition takes place. By limiting their size to smaller than the effective penetration depth, however, the vortex interaction alters, and the superconducting state re-enters the Bose-insulating state. As a consequence, one observes a direct superconductor-to-Fermi insulator in the zero-temperature limit. In narrow films, the associated critical-exponent products diverge along the corresponding phase boundaries with increasing magnetic field, which is a hallmark of the quantum-Griffiths singularity.

Investigation of dark count rate in NbRe microstrips for single photon detection

“…For the NbN and WSi devices, we took the DCR values reported in [8,11], respectively. Then, we used the values of λ L (0) and ξ (0) reported in [4,8] for NbN and in [39] for WSi. In this way, the same analysis as A2 was performed, obtaining the energy barriers.…”

Quantum Griffiths Singularity in Ordered Artificial Superconducting-Islands-Array on Graphene

“…Namely, the hybrid system undergoes a superconductor to weakly localized metal transition, where the quantum critical field is around 115 mT in the zero-temperature limit. The sheet resistance at the quantum critical point (around 365 Ω) is far lower than the quantum resistance for Cooper pairs, R Q = h /4 e 2 ∼ 6450 Ω, with h the Planck constant and e the elementary charge, indicating that the unpaired normal electrons in graphene also contribute to total conductance. , …”

“…The sheet resistance at the quantum critical point (around 365 Ω) is far lower than the quantum resistance for Cooper pairs, R Q = h/4e 2 ∼ 6450 Ω, with h the Planck constant and e the elementary charge, indicating that the unpaired normal electrons in graphene also contribute to total conductance. 26,27 To further show the continuous SMT in our hybrid system, the out-of-plane field dependence of sheet resistance R−B curves at temperatures down to the quantum critical point is systematically measured, as displayed in Figure 2b, revealing a broad transition region ranging from 0.1 to 3.3 K. Different from conventional SMT with a single critical point, 14 the magnetoresistance isotherms cross each other at a series of points within a well-distinguished region, reminiscent of the quantum behavior of QGS. 7 The inset of Figure 2b shows the extracted with irrelevant correction in the low temperature limit (fitted by the red solid line).…”

Quantum phase transition in two-dimensional NbN superconducting thin films

“…Recent experimental results have indicated that an intermediate anomalous metallic state appears in some 2D superconductors, including * Corresponding author, e-mail: zhiqingli@tju.edu.cn amorphous Mo 43 Ge 57 [8] and Ta thin films [9], ZrNClbased electric-double layer transistors [3], mechanical exfoliated crystalline NbSe 2 [10] and WTe 2 films [11], and Josephson junction arrays [12,13]. More recently, it has been found that in some 2D metal superconductors (i.e., the normal states of these superconductors reveal metallic characteristics) the magnetoresistance isotherms do not cross at a fixed point (the critical field) but at multiple points, and the effective exponent zν determined at each crossing point diverges as the quantum phase transition is approached [14][15][16][17][18][19][20]. This phenomenon is the so-called quantum Griffiths singularity (QGS) of SMT.…”

“…The QGS of SMT was initially observed in 3-monolayer Ga film [14], and subsequently observed in some highly crystalline 2D superconductors with metallic normal states, such as LaAlO 3 /SrTiO 3 interface [15], macrosize monolayer NbSe 2 [16], and ion-gated ZrNCl and MoS 2 [17]. Since 2019, the QGS has also been observed in amorphous InO x [18], WSi [19], and β-W [20] films. It is believed that quenched disorder plays an important role in the occurrence of QGS in highly crystalline 2D superconductors [14,16,17].…”