The effects of oxygen in spinel oxide Li1+xTi2−xO4−δ thin films

Abstract

The evolution from superconducting LiTi2O4-δ to insulating Li4Ti5O12 thin films has been studied by precisely tuning the oxygen pressure in the sample fabrication process. In superconducting LiTi2O4-δ films, with the increase of oxygen pressure, the oxygen vacancies are filled gradually and the c-axis lattice constant decreases. When the oxygen pressure increases to a certain critical value, the c-axis lattice constant becomes stable, which implies that the sample has been completely converted to Li4Ti5O12 phase. The two processes can be manifested by the angular bright-field images of the scanning transmission electron microscopy techniques. The transition temperature (Tch) of magnetoresistance from the positive to the negative shows a nonmonotonic behavior, i.e. first decrease and then increase, with the increase of oxygen pressure. We suggest that the decrease Tch can be attributed to the suppressing of orbital-related state, and the inhomogeneous phase separated regions contribute positive MR and thereby lead to the reverse relation between Tch and oxygen pressure.

Anomalies of upper critical field in the spinel superconductor LiTi2O4−δ

“…In combination with our previous work on this system [16,19], we conclude that the three dimensional spin-orbit interaction induced by oxygen impurities is essential to the significant isotropic enhancement of B c2 . The effects of oxygen impurities could be general in oxides, thus providing us new clues to generate other exotic phenomena such as Ising superconductivity [14,35], topological nontrivial state due to band inversion [36][37][38], vortex phase transition from liquid to solid [39].…”

“…τ = l/v F = 1.3 × 10 −14 s, where l is mean free path and v F is Fermi velocity [16]. However, the excellent fitting of S oxy provides an underestimation of τ (= 5 × 10 −16 s) if we assume the Fermi energy is constant, since such τ is even smaller than the lower limit τ min (= c/v F = 6.1 × 10 −15 s, where c is the out-of-plane lattice constant), restricted by the Mott-Ioffe-Regel limit [19,22]. Such confliction can be reconciled in case of a reduced E oxy stand for S oxy and S vac , respectively.…”

“…Temperature-dependent B c2 can be well fitted by the Werthamer-Helfand-Hohenberg (WHH) theory considering spin paramagnetism and spin-orbit scattering[20], yet it leads to an underestimation of relaxation time for oxygen-rich samples. Here we consider the three dimensional spin-orbit interaction induced by oxygen impurities, which is beyond the WHH theory, thereby giving an universal understanding of the enhancement of B c2 and other anomalies.High-quality LiTi 2 O 4−δ thin films were epitaxially grown on MgAl 2 O 4 (001) substrates by pulsed laser deposition under various oxygen pressures from basic vacuum (< 1 × 10 −6 Torr) to ∼ 5 × 10 −6 Torr [19]. The temperature-and field-dependence of resistance were measured by a standard four-probe method in PPMS-16T, and steady high magnetic field facility with field up to 33 T. Point-contact measurements were performed by employing a homemade probe using Pt/Ir…”

“…High-quality LiTi 2 O 4−δ thin films were epitaxially grown on MgAl 2 O 4 (001) substrates by pulsed laser deposition under various oxygen pressures from basic vacuum (< 1 × 10 −6 Torr) to ∼ 5 × 10 −6 Torr [19]. The temperature-and field-dependence of resistance were measured by a standard four-probe method in PPMS-16T, and steady high magnetic field facility with field up to 33 T. Point-contact measurements were performed by employing a homemade probe using Pt/Ir…”

“…To get further insight into the isotropic enhancement of B c2 , we choose RRR as a good quantity to distinguish different samples since RRR monotonically decreases with increasing the oxygen pressure [19]. parameter λ so [20,21].…”