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.

EuTiO 3 , a band insulator, and LaTiO 3 , a Mott insulator, are both antiferromagnetic with transition temperatures ~ 5.5 K and ~ 160 K, respectively. Here, we report the synthesis of Eu 1x La x TiO 3 thin films with x = 0 to 1 by oxide molecular beam epitaxy. The films in the full range have high crystalline quality and show no phase segregation, allowing us carry out transport measurements to study their electrical and magnetic properties. From x = 0.03 to 0.95, Eu 1-x La x TiO 3 films show conduction by electrons as charge carriers, with differences in carrier densities and mobilities, contrary to the insulating nature of pure EuTiO 3 and LaTiO 3 . Following a rich phase diagram, the magnetic ground states of the films vary with increasing La-doping level, changing Eu 1-x La x TiO 3 from an antiferromagnetic insulator to an antiferromagnetic metal, a ferromagnetic metal, a paramagnetic metal, and back to an antiferromagnetic insulator. These emergent properties reflect the evolutions of the band structure, mainly at the Ti t 2g bands near the Fermi level, when Eu 2+ are gradually replaced by La 3+ . This work sheds light on this method for designing the electrical and magnetic properties in strongly-correlated oxides and completes the phase diagram of the titanate Eu 1-x La x TiO 3 .

Spin degree of freedom generally plays an important role in unconventional superconductivity. In many of the iron-based compounds, superconductivity is found in close proximity to long-range antiferromagnetic order, whereas monolayer FeSe grown on SrTiO3, with enhanced superconductivity, exhibits no magnetic or nematic ordering. Here we grow monolayer and multilayer FeSe on antiferromagnetic EuTiO3(001) layers, in an effort to introduce a spin polarization in proximity to the superconductivity of FeSe. By X-ray magnetic dichroism, we observe an antiferromagnet–ferromagnet switching on Eu and Ti sites in EuTiO3 driven by the applied magnetic field, with no concomitant spin polarization on the Fe site of FeSe. Transport measurements show enhanced superconductivity of monolayer FeSe on EuTiO3 with a transition temperature of ~30 K. The band structure revealed by photoemission spectroscopy is analogous to that of FeSe/SrTiO3. Our work creates a platform for the interplay of spin and unconventional superconductivity in the two-dimensional limit.

Hole-doped perovskite bismuthates such as Ba1−xKxBiO3 and Sr1−xKxBiO3 are well-known bismuth-based oxide high-transition-temperature superconductors. Reported thin bismuthate films show relatively low quality, likely due to their large lattice mismatch with the substrate and a low sticking coefficient of Bi at high temperatures. Here, we report the successful epitaxial thin film growth of the parent compound strontium bismuthate SrBiO3 on SrO-terminated SrTiO3 (001) substrates by molecular beam epitaxy. Two different growth methods, high-temperature co-deposition or recrystallization cycles of low-temperature deposition plus high-temperature annealing, are developed to improve the epitaxial growth. SrBiO3 has a pseudocubic lattice constant ∼4.25Å, an ∼8.8% lattice mismatch on SrTiO3 substrate, leading to a large strain in the first few unit cells. Films thicker than 6 unit cells prepared by both methods are fully relaxed to bulk lattice constant and have similar quality. Compared to high-temperature co-deposition, the recrystallization method can produce higher quality 1-6 unit cell films that are coherently or partially strained. Photoemission experiments reveal the bonding and antibonding states close to the Fermi level due to Bi and O hybridization, in good agreement with density functional theory calculations. This work provides general guidance to the synthesis of high-quality perovskite bismuthate films.Hole-doped perovskite bismuthates Ba 1−x K x BiO 3 (x = 0.4) and Sr 1−x K x BiO 3 (x = 0.6) become superconducting below ∼30 K and ∼12 K, respectively [1,2]. Their parent compounds BaBiO 3 (BBO) and SrBiO 3 (SBO) have drawn intense interest [3][4][5][6][7][8][9][10][11][12][13][14][15][16] owing to the hightemperature superconductivity and also because of their indirect bandgap semiconductor nature rather than metals as would be expected from the half-filled Bi 6s band assuming the formal 4+ valence of Bi. The most common explanation of the nonmetallic behavior invokes the concept of charge disproportionation of Bi 4+ into the more stable and closed-shell atomic configurations of Bi 3+ (6s 2 ) and Bi 5+ (6s 0 ) [3,4,17]. This would inevitably result in a long Bi 3+ -O and a short Bi 5+ -O bond lengths ordered in the simplest conceivable structure with bond length alternation along the three crystallographic axes. Such bond disproportionation is observed in neutron and x-ray diffraction (XRD) [4,11] and in the resulting band structure [6,8]. However, x-ray photoemission spectroscopy (XPS) and density functional theory (DFT) calculations show that the charge difference between the two inequivalent Bi sites is trivial, at most a few tenths of one electron [8,10,[18][19][20][21]. By DFT calculation, Foyevtsova et al. recently demonstrates that the bands straddling the Fermi level are in fact strongly hybridized Bi 6s with O 2p molecular orbitals with A 1g symmetry and contain more O 2p character than Bi 6s character, resulting in the bonding states at ∼10 eV below the Fermi energy and the unoccupied antibonding ...

the fundamental interest of exploring the spin degree of freedom in unusual electronic environments, but also for exploiting magnetism and spin currents in multifunctional devices. [7,8] Ongoing studies of magnetic 2DEGs focus on novel applications, as well as the search for new magnetic phenomena via, e.g., doping and charge transfer, or imposing epitaxial strain via growth on substrates with different crystal symmetries and lattice parameters. [9][10][11][12][13][14] LaTiO 3 (LTO) and EuTiO 3 (ETO) are two antiferromagnetic (AFM) insulating oxides possessing closely lattice-matched perovskite structures. Along the (001) direction, LTO has formally alternating charged layers (La 3+ O 2-/Ti 3+ O 2-2 ) and ETO has charge-neutral layers (Eu 2+ O 2-/ Ti 4+ O 2-2 ), thus the LTO/ETO interface has