Quantum Griffiths singularities in TiO superconducting thin films with insulating normal states

Chao Zhang,Qi Li,Tianyi Wang,Xiang Liu,Xiaoguang Li,Yuewei Yin,Qiaoling Chen,Yunjie Fan

doi:10.1038/s41427-019-0181-3

Abstract

A superconductor–metal transition (SMT) with an unconventional diverging dynamic critical exponent was recently discovered, and it drew tremendous attention because this signature of a quantum Griffiths singularity (QGS) was thought to be a common characteristic of low-disorder crystalline superconductors. However, because the QGS was observed only in limited materials with metallic normal states, the question of whether the QGS exists in other superconducting systems is still unanswered. In this paper, a superconductor–insulator transition (SIT) is observed in TiO thin films with insulating normal states, which offers a more universal platform for investigating the QGS. A thickness-tuned SIT is obtained when the magnetic field is zero. Importantly, a magnetic field-tuned SIT with a diverging dynamic critical exponent, which is direct evidence of a QGS, is observed in TiO thin films with different thicknesses. By constructing a comprehensive phase diagram, it is demonstrated that the critical magnetic field Hc tends to saturate as the temperature approaches 0 K, which is different from the upturn trend of Hc observed in SMT systems and probably due to the weaker Josephson coupling of the locally ordered superconducting islands (rare regions) in a weakly insulating normal state background. The results extend the QGS scenario from only SMT systems to SIT systems, and they provide vital evidence that QGSs are common in crystalline superconducting thin films, which has possible applications in quantum-computing devices.

Highlights

The superconductor–insulator transition (SIT/superconductor–metal transition (SMT)), a remarkable paradigm of the quantum phase transition (QPT), has been an important issue because this exotic phenomenon extends our understanding of the novel physical nature of the QPT and contributes to the design of quantum devices, such as superconducting qubits[1,2]
This phenomenon occurs because some areas of the TiO present a different crystalline orientation, which can be converted to that shown in Fig. 1b by a rotation of 60° around the [111] axis of the TiO thin film
It is noteworthy that triangular facets were observed on the surface, which reflects the more stable (100) facets of the rock-salt type TiO deposited on the hexagonal α-Al2O3 substrate[22,26]

Summary

Introduction

SMT), a remarkable paradigm of the quantum phase transition (QPT), has been an important issue because this exotic phenomenon extends our understanding of the novel physical nature of the QPT and contributes to the design of quantum devices, such as superconducting qubits[1,2]. In 2015, a totally different type of SMT called a quantum Griffiths singularity (QGS) was strikingly reported in a molecular-beam-epitaxy (MBE)-grown Ga trilayer[14] that shows diverging dynamic exponents and a continuous line of QCPs. In 2015, a totally different type of SMT called a quantum Griffiths singularity (QGS) was strikingly reported in a molecular-beam-epitaxy (MBE)-grown Ga trilayer[14] that shows diverging dynamic exponents and a continuous line of QCPs Since this discovery, examples of SMTs with a QGS have been observed in a NbSe2 monolayer[15], a

Methods

Results

Conclusion