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 H c tends to saturate as the temperature approaches 0 K, which is different from the upturn trend of H c 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.

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The superconductor-insulator transition (SIT), one of the most fascinating quantum phase transitions, is closely related to the competition between superconductivity and carrier localization in disordered thin films. Here, superconducting TiO x films with different oxygen contents were grown on Al 2 O 3 substrates by a pulsed laser deposition technique. The increasing oxygen content leads to an increase of disorder, a reduction of carrier density, an enhancement of carrier localization, and therefore a decrease of superconducting transition temperature. A fascinating SIT emerges in cubic TiO x films with increasing oxygen content and its critical sheet resistance is close to the quantum resistance h/(2e) 2 ~6.45 kΩ. The scaling analyses of magnetic field-tuned SIT exhibit that the critical exponent products zν increase from 1.02 to 1.31 with increasing disorder. Based on the results, the SIT can be described by the "dirty boson" model, and a schematic phase diagram for TiO x films was constructed.

Recently, 2D Mo2C, a new member of the MXene family, has attracted much attention due to the exotic superconducting properties discovered in 2D α-Mo2C. Here, not only 2D α-Mo2C but also 2D β-Mo2C crystal sheets with distinct disordered carbon distributions were successfully grown. 2D β-Mo2C shows a much stronger superconductivity than 2D α-Mo2C, and their superconductivities have different hydrostatic pressure responses. The superconducting transition temperature Tc of 2D α-Mo2C shows a dome-shaped profile under pressure, implying the existence of two competing effects arising from phononic and electronic properties, while for 2D β-Mo2C, Tc decreases monotonically with increasing pressure, possibly due to phonon stiffening. These results indicate that the electronic properties have a more important influence on the superconductivity in 2D α-Mo2C compared to 2D β-Mo2C. The ordered and disordered carbon distributions in 2D α-Mo2C and β-Mo2C, respectively, may be the underlying origin for their different electronic and superconducting properties.

There are many fascinating properties in titanium oxides such as the enhanced superconductivity in cubic TiO epitaxial thin films with the superconducting transition temperature of 7.65 K, which is much higher than that of the bulk polycrystalline TiO. To explore the superconductivity of the TiO thin films in more detail, we investigated the magnetic field and temperature dependences of the current–voltage (I–V) characteristics and the critical current density Jc in magnetic fields perpendicular to the film surface. The I–V curves show a quasi-two-dimensional vortex glass (VG) scaling collapse in different magnetic fields, and a vortex phase diagram is constructed from the VG and vortex liquid regions to the normal state. Through the critical current density investigation, we found that δl pinning dominates the pinning behavior, which is in accordance with the analyzed results of the vortex pinning force associated with defects in the film, such as grain boundaries. The findings of the magnetic phase diagram and critical current properties should be helpful for practical applications of the TiO family.