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

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. Nanoscale films that transform into superconductors through an unusual quantum phase transition may aid development of high-speed electronics. Recent research has shown that at temperatures close to absolute zero, rare superconducting regions within larger metallic films, closely related to the quenched disorders, can produce energetic disruptions, called singularities, which help induce superconducting phase changes. Now Xiaoguang Li and Yuewei Yin from the University of Science and Technology of China in Hefei report that these singularities also exist in materials that are normally insulating. By fabricating titanium oxide thin films using highly precise laser deposition, the team observed superconductor-to-insulator transitions that could be manipulated by changing the film thickness or by applying magnetic fields. Analysis of the phase transition data suggests that similar singularities may be discovered in other thin crystalline superconductors. In this work, the quantum Griffiths singularity (QGS) of superconductor-insulator transition (SIT) is discovered in TiO thin films with insulating normal states. A magnetic field-tuned SIT with a diverging dynamic critical exponent, the direct evidence of QGS, is observed in TiO thin films with different thicknesses. The critical magnetic field Hc tends to saturate at low temperature, different from the upturn trend of Hc in superconductor-metal transition (SMT) systems. The results extend the QGS scenario from SMT systems to SIT systems, and provide decisive clues to conclude that the QGS is a common feature of crystalline superconducting thin films.