Quantum phase transitions in two-dimensional superconductors: a review on recent experimental progress

Abstract

Superconductor–insulator/metal transition (SMT) as a paradigm of quantum phase transition has been a research highlight over the last three decades. Benefit from recent developments in the fabrication and measurements of two-dimensional (2D) superconducting films and nanodevices, unprecedented quantum phenomena have been revealed in the quantum phase transitions of 2D superconductors. In this review, we introduce the recent progress on quantum phase transitions in 2D superconductors, focusing on the quantum Griffiths singularity (QGS) and anomalous metal state. Characterized by a divergent critical exponent when approaching zero temperature, QGS of SMT is discovered in ultrathin crystalline Ga films and subsequently detected in various 2D superconductors. The universality of QGS indicates the profound influence of quenched disorder on quantum phase transitions. Besides, in a 2D superconducting system, whether a metallic ground state can exist is a long-sought mystery. Early experimental studies indicate an intermediate metallic state in the quantum phase transition of 2D superconductors. Recently, in high-temperature superconducting films with patterned nanopores, a robust anomalous metal state (i.e. quantum metal or Bose metal) has been detected, featured as the saturated resistance in the low temperature regime. Moreover, the charge-2e quantum oscillations are observed in nanopatterned films, indicating the bosonic nature of the anomalous metal state and ending the debate on whether bosons can exist as a metal. The evidences of the anomalous metal states have also been reported in crystalline epitaxial thin films and exfoliated nanoflakes, as well as granular composite films. High quality filters are used in these works to exclude the influence of external high frequency noises in ultralow temperature measurements. The observations of QGS and metallic ground states in 2D superconductors not only reveal the prominent role of quantum fluctuations and dissipations but also provide new perspective to explore quantum phase transitions in superconducting systems.

Two-Dimensional and Interface Superconductivity in Crystalline Systems

“…For most previous reports of SIT/SMT, there exists a single QCP with a single value of zν during the transition. 6 However, the previous consensus has been challenged by the discovery of the quantum Griffiths singularity (QGS), which was first observed in three-monolayer (3-ML) Ga films on GaN (0001) substrate. The ultrathin (2-ML) Ga films grown by MBE exhibit graphene or silicene-like structure, differing from the bulk counterparts with orthorhombic (α and γ phase) or monoclinic (β phase) structures.…”

“…The nature of the anomalous metallic state is beyond the fundamental understanding of superconductivity and, thus, has become one of the major open issues in condensed matter physics. 6,10 Nevertheless, the early experimental observations of the anomalous metallic state are questioned by the possible extrinsic effects like electromagnetic noise, 23−25 casting doubt on the existence of the anomalous metallic state.…”

“…For the magnetic field ( B ) driven SIT/SMT, the magnetoresistance isotherms at various temperatures ( T ) normally show a single crossing point corresponding to the quantum critical point (QCP) at zero temperature. , To further reveal the quantum criticality of SIT/SMT, finite size scaling analysis is used to uncover the scaling behavior of magnetoresistance isotherms and determine the value of zν (correlation length exponent z and dynamical critical exponent ν ). For most previous reports of SIT/SMT, there exists a single QCP with a single value of zν during the transition . However, the previous consensus has been challenged by the discovery of the quantum Griffiths singularity (QGS), which was first observed in three-monolayer (3-ML) Ga films on GaN (0001) substrate.…”

“…Notably, the formation of Cooper pairs and the global phase coherence can occur at different temperatures in 2D superconductors, potentially giving rise to extraordinary quantum phenomena. A well-established example would be the superconductor–insulator/metal transition (SIT/SMT), a paradigm of the quantum phase transition in 2D superconducting systems. − Furthermore, the anomalous metallic state, an intervening quantum metallic state during SIT/SMT, was observed with a saturating sheet resistance much smaller than R Q at ultralow temperatures. ,− The emergence of the anomalous metallic state goes beyond the scope of the established framework of SIT/SMT on 2D superconductivity, and the underlying mechanism of this metallic state remains elusive.…”

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

“…Quantum phase transition (QPT) in two-dimensional (2D) superconducting systems, such as ultrathin films and heterogeneous interfaces, is an important topic for the deep understanding of quantum states, where the ground state is changed at absolute zero temperature in response to a variation of external parameter of the Hamiltonian . Superconductor–insulator/metal transition (SIT/SMT) as a prototype of QPT has been extensively studied, and numerous experiments have been performed by tuning magnetic field, disorder, or charge density, which greatly enriches the physical characteristics of QPT around the quantum critical points. − One of the most striking phenomena is the quantum Griffiths singularity (QGS) − with the signature of divergent dynamic critical exponent approaching zero temperature, instead of a constant critical exponent or finite saturated values in conventional QPT. − A number of experiments have verified the existence of QGS near the quantum critical points of SIT/SMT in 2D superconducting systems, such as three-monolayer crystalline superconducting Ga film, LaAlO 3 /SrTiO 3 (110) interface, monolayer NbSe 2 , ion-gated crystalline ZrNCl and MoS 2 . Theoretically, the QGS behavior has been attributed to the emergence of rare superconducting puddles resulting from disorder-induced vortex glass-like phase. , However, it has been found that the in-plane QGS phase also exists in 2D crystalline PdTe 2 films and highly disordered β-W films without vortex formation, revealing a novel microscopic nature of the origination of QGS.…”