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

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 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.

Superconductor-Metal Quantum Transition at the EuO/KTaO3 Interface*

“…Experimentally, the quantum Griffiths singularity has been observed in 2D superconducting Ga thin films [10], where the origin is theoretically discussed to be that the quenched disorder strongly influences the phase transition behavior and the resultant large rare superconducting regions are linked via long-range Josephson coupling. Subsequently, various low-dimensional superconducting systems have been shown to exhibit the quantum Griffiths singularity [11][12][13][14][15][16], albeit with detailed differences concerning the evolution of the dynamical critical exponent. Thus, testifying the quantum Griffiths singularity and the universality in a newly-discovered superconducting system is worth of more effort.…”

Single-crystalline epitaxial TiO film: A metal and superconductor, similar to Ti metal

“…Owing to the strong tendency of forming Ti 3+ and Ti 4+ instead of Ti 2+ , the oxygen pressure in the growth chamber is maintained at ~5.0 × 10 −9 torr during film growth and is precisely controlled within a small pressure window of ±1.0 × 10 −10 torr, to enable the persistent observation of a single rock salt phase and intensity oscillation in reflection high-energy electron diffraction (RHEED) spots, indicating layer-by-layer growth of atomically smooth, singledomain films. The precise control of oxygen pressure, the extremely low chamber base pressure (less than 1.0 × 10 −11 torr, much lower than any other growth method used for TiO), and the epitaxial stabilization provided by the nonpolar MgO substrate surface and the epitaxial growth along the nonpolar <001> direction are the main advantages of the MBE growth method, compared to the bulk samples (8)(9)(10)(14)(15)(16)(17)(18) and the films grown by pulsed laser deposition (PLD) (19)(20)(21)(22). As shown in Fig.…”

“…The main problem in experiment is the strong tendency of Ti 2+ to form Ti 3+ or Ti 4+ , and so achieving high-quality single-crystal TiO is difficult. The recently reported TiO(111) films with superconducting T c ~ 7 K and their -Al 2 O 3 (0001) substrates (19)(20)(21)(22) have the well-known "polar catastrophe" problem (23) resulting in the formation of domain structures (22) and probable change of stoichiometry from TiO, which leaves the origin of observed superconductivity an open question. The nature of the TiO ground state and whether the reported superconductivity is intrinsic remains a long standing but rather important question in condensed matter physics.…”

Epitaxial stabilization of an orthorhombic Mg-Ti-O superconductor

“…The family of titanium oxide superconductors (TOS) is an ideal candidate for contrastive study of high-temperature superconductivity. Many intriguing phenomena similar to that of high-temperature superconductors have been unveiled in TOS, such as pseudogap [16] and exotic superconductormetal [17] or -insulator [18][19][20] transition. Although TOS have various structures, e.g., cubic spinel LiTi 2 O 4 [21], cubic rock-salt TiO [22], triclinic Ti 4 O 7 [23], and monoclinic γ-Ti 3 O 5 [23], they have a notably common structural unit, Ti-O bond.…”

“…In the case of LiTi 2 O 4 , the superconductor with the highest superconducting transition temperature T c ∼ 13 K among the family, the substantial Ti-O hybridization [24] contributes remarkably to the superconductivity [25]. In contrast, stoichiometric TiO * qianli@iphy.ac.cn exhibits a much lower T c ∼ 0.5 K than oxygen-rich TiO 1+δ samples (up to ∼ 7 K) [17,18,22,26,27] due to the direct Ti-Ti bonding [28]. Besides, the coexistence of superconductivity with other collective excitations, e.g., orbital-related state in LiTi 2 O 4 [29] and ferromagnetism in Mg-doped TiO [30], also indicates that the family provides a promising window for superconductivity in the vicinity of a competing order, other than cuprates [8] and iron-based superconductors [31].…”