Quantum superconductor-insulator transition in titanium monoxide thin films with a wide range of oxygen contents

Abstract

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.

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

“…2a, the critical transition sheet resistance is~R Q = 6.45 kΩ/□, which is in agreement with the quantum resistance for pairs 10 . Similar SIT behaviors has been achieved by tuning the oxygen content in TiO x thin films 25 ; the ability to tune SITs by thickness and/or composition has been observed in other materials 4,29 as well. Figure 2b shows that the value of T c decreases as the thickness of the TiO thin films decreases, where T c is defined as the temperature corresponding to 90% of the sheet resistance in a normal state.…”

“…Crystalline superconducting titanium oxide thin films are good systems for studying the QPT. In previous studies, we succeeded in growing crystalline TiO thin films on α-Al 2 O 3 (0001) substrates 23,24 , and we found an oxygen-content-tuned SIT in~80 nm TiO x films, but no QGS was observed because of the narrow range of measured temperatures 25 . In the present paper, we grow crystalline TiO epitaxial thin films with different thicknesses.…”

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

“…We need experimental information about the properties and electronic structure of single-crystalline stoichiometric TiO. However, inconsistent results have been reported on the nature of TiO, showing different temperature-dependent resistivities (8)(9)(10)(14)(15)(16) and superconductivities with a wide-range of transition temperature T c from below 0.5 K up to ~7 K (14,15,(17)(18)(19)(20)(21)(22). 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 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.…”

“…In this work, benefited from the well-controlled oxygen pressure and the epitaxy stabilization provided by the lattice-matched nonpolar substrate surface and the epitaxial growth along a nonpolar direction, we have prepared stoichiometric and highly crystalline TiO(001) thin films using oxide molecular beam epitaxy (MBE) on MgO(001) substrates. Unlike the previous bulk powder samples (8)(9)(10)(14)(15)(16)(17)(18) and thin films grown along a polar direction (19)(20)(21)(22), our epitaxial thin films are single phase and single domain, allowing transport measurements to reveal the intrinsic properties of TiO.…”