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

Abstract

Titanium monoxide (TiO), an important member of the rock salt 3d transition-metal monoxides, has not been studied in the stoichiometric single-crystal form. It has been challenging to prepare stoichiometric TiO due to the highly reactive Ti2+. We adapt a closely lattice-matched MgO(001) substrate and report the successful growth of single-crystalline TiO(001) film using molecular beam epitaxy. This enables a first-time study of stoichiometric TiO thin films, showing that TiO is metal but in proximity to Mott insulating state. We observe a transition to the superconducting phase below 0.5 K close to that of Ti metal. Density functional theory (DFT) and a DFT-based tight-binding model demonstrate the extreme importance of direct Ti–Ti bonding in TiO, suggesting that similar superconductivity exists in TiO and Ti metal. Our work introduces the new concept that TiO behaves more similar to its metal counterpart, distinguishing it from other 3d transition-metal monoxides.

Superconducting Phase of $\mathrm{Ti_xO_y}$ Thin Films Grown by Molecular Beam Epitaxy

“…[1][2][3][4][5] The ability to utilize the epitaxial stabilization of a given phase in thin films synthesised with atomic-layer control provides a route to tune the electronic properties of TiO x . [6][7][8][9][10][11][12] Recent reports of superconductivity in TiO x thin films as high as 11 K has sparked renewed interest in superconducting binary oxides. [13] The transition temperature, T C has been reported to depend on the film thickness, [14,15] oxygen stoichiometry [16,17] and growth temperature [15].…”

Half‐Metallic CoO2 and Semiconducting NiO2 at High Pressures

“…Extreme pressures, such as those found in the interior of the Earth and of other planets, may change the oxidation states, spin states, and phase stabilities of metal oxides, producing new stoichiometries. The entanglement of charge, orbital, and spin degrees of freedom in d electrons are known in strongly correlated 3d transition-metal oxides (TMOs), [1][2][3][4][5][6] which appear in diverse stoichiometries with various oxidation states, structures, and properties. Transition-metal dioxides (TMO 2 ) with TM 4þ ions have attracted wide attention in the fields of materials science, condensed-matter physics, and geoscience due to their special high-pressure behavior.…”

Epitaxial stabilization of an orthorhombic Mg-Ti-O superconductor

“…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].…”

“…Nevertheless, extensive study on TOS is hampered by the lack of high-qualified single crystal sample owning to the thermodynamic or chemical instability of their crystal lattice [28,37]. Fortunately, many metastable or unstable phases can be stabilized in the form of single-crystalline films by the means of epitaxial stabilization [38][39][40][41][42].…”