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

Abstract

High-field electrical transport and point-contact tunneling spectroscopy were used to investigate superconducting properties of the unique spinel oxide, LiTi 2 O 4−δ films with various oxygen content. We find that the upper critical field B c2 gradually increases as more oxygen impurities are brought into the samples by carefully tuning the deposition atmosphere. It is striking that although the superconducting transition temperature and energy gap are almost unchanged, an astonishing isotropic B c2 up to ∼ 26 Tesla is observed in oxygenrich sample, which is doubled compared to the anoxic sample and breaks the Pauli limit.Such anomalies of B c2 were rarely reported in other three dimensional superconductors.Combined with all the anomalies, three dimensional spin-orbit interaction induced by tiny oxygen impurities is naturally proposed to account for the remarkable enhancement of B c2 in oxygen-rich LiTi 2 O 4−δ films. Such mechanism could be general and therefore provides ideas for optimizing practical superconductors with higher B c2 .In this letter, we present systematic transport measurements and point-contact tunneling spectroscopy study of LiTi 2 O 4−δ films with various oxygen content. For oxygen-rich samples, a significantly enhanced B c2 up to 26 T at low temperatures is observed. Such doubled B c2 , compared with anoxic samples, is isotropic. Intriguingly, superconducting energy gap and T c are almost the same for all samples. 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

Fourfold Symmetric Superconductivity in Spinel Oxide LiTi2O4(001) Thin Films

“…The Werthamer–Helfand–Hohenberg (WHH) model is further used to fit the μ 0 H c 2 mid (also see section II in Supporting Information). The extracted μ 0 H c 2 mid at the zero temperature limit is 18.3 T, and the superconducting coherence length ξ GL ( T = 0 K) is thus estimated from the Ginzburg–Landau formula , μ 0 H c 2 mid = Φ 0 /(2 πξ GL 2 ) with the fluxoid quantization Φ 0 set as 4.2 nm, consistent with previous findings. , …”

Epitaxial stabilization of an orthorhombic Mg-Ti-O superconductor

“…The zero temperature upper critical field B c2 (T = 0) is calculated to be 13.3 T and 13.7 T in the case of B ⊥ film and B film, respectively, strikingly breaking the Pauli limit B p ∼ 12 T predicted by the weakcoupling BCS paramagnetic formula B p = 1.84T c (T c is taken as the temperature at which the resistance crosses 90% of the normal state resistance). Such a relatively high B c2 of isotropy mimics the behaviors of some other TOS [26,32,61], which possibly stems from the special electronic structures correlated to the strong coupling of 3d electrons [61]. Additionally, the Ginzburg-Landau coherence length ξ GL is estimated to be ∼ 4.01 nm following WHH formula ξ GL = φ 0 /(2πB orb c2 ), where B orb c2 = −0.69T c (dB c2 /dT )| T =T c is the orbital limited upper critical field [60].…”

“…Our work not only introduces a different member to the TOS family, but also demonstrates the considerable potenial of film deposition in exploring more superconductors or other functional materials via epitaxial stabi-lization. Furthermore, an isotropic B c2 breaking the Pauli limit is exhibited in the Mg: Ti 9 O 10 film, resembling those of some other TOS [26,32,61]. Considering their common structural unit, Ti-O bond, it should be believed that some ingredients not relying on specific structure may act as the superconducting gene [62] in the family.…”

Sc Modification Induced Short-range Ordering in ZnGa2O4&nbsp;Spinel Leading to High Microwave Dielectric Performance

“…Spinel-type compounds AB2O4 have been extremely investigated in science and technical fields such as phosphor [1][2], catalysts [3][4], semiconductors [5][6], superconductors [7][8] and microwave dielectric materials [9][10][11][12]. Among these applications, microwave dielectric materials in high frequency (5G) is one of the most important applications with wide investigations.…”