Emergent superconductivity in single-crystalline MgTi2O4 films via structural engineering

Epitaxial stabilization of an orthorhombic Mg-Ti-O superconductor

“…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.…”

“…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].…”

“…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]. Particularly, superconducting transition up to ∼ 5 K can be achieved by suppressing the orbital ordering in MgTi 2 O 4 [32], once known to be a band insulator with a helical dimerization pattern of alternating short and long Ti-Ti bonds [33][34][35][36]. Therefore, further study on TOS is desired to thoroughly understand the mechanism of superconductivity originated from 3d electrons.…”

“…Exotic behaviors comparable to high-temperature superconductors have been exhibited in the films of TOS [16-20, 29, 30]. Moreover, some new TOS, e.g., Ti 4 O 7 [23], γ-Ti 3 O 5 [23], orthorhombic Ti 2 O 3 [43], and MgTi 2 O 4 [32], which have never been reported to be superconducting in bulk, are disclosed by thin film deposition. Besides providing an elastic strain, the substrate can also tailor the properties of the film much by its crystallographic direction [44][45][46].…”

“…In the top panel, out-of-plane reflections corresponding to the MgTi 2 O 4 (00l) orientation only can be observed beside the peaks from the substrate, suggesting that (00l)-oriented spinel MgTi 2 O 4 film without impurity phase has been deposited on MAO (001) substrate. It should be noted that superconducting MgTi 2 O 4 film with T c0 can be achieved in a single-layered film, rather than in an engineered superlattice geometry [32], by further optimization of deposition process. For the film deposited on (111)-oriented MAO substrate [middle panel of Fig.…”