Enhanced Superconductivity in TiO Epitaxial Thin Films

Titanium oxides have many fascinating optical and electrical properties, such as the superconductivity at 2.0 K in cubic titanium monoxide TiO polycrystalline bulk. However, the lack of TiO single crystals or epitaxial films has prevented systematic investigations on its superconductivity. Here, we report the basic superconductivity characterizations of cubic TiO films epitaxially grown on (0001)-oriented Al2O3 substrates. The magnetic and electronic transport measurements confirmed that TiO is a type-II superconductor and the record high Tc is about 7.4 K. The lower critical field (Hc1) at 1.9 K, the extrapolated upper critical field Hc2(0) and coherence length are about 18 Oe, 13.7 T and 4.9 nm, respectively. With increasing pressure, the value of Tc shifts to lower temperature while the normal state resistivity increases. Our results on the superconducting TiO films confirm the strategy to achieve higher Tc in epitaxial films, which may be helpful for finding more superconducting materials in various related systems.


Introduction
Transition metal oxides, such as titanium oxides, are a large family of materials with many fascinating electrical properties and applications [1][2][3] . Among various stable titanium oxides, the cubic metallic monoxide TiO is one of very interesting materials because of the extremely wide homogeneity range with oxygen content varying from about 0.80 to 1.30 [4][5][6][7] .
Electrical, optical, magnetic, and structural properties of bulk TiO have been widely investigated [8][9][10][11][12][13][14][15][16][17] . Especially, the superconductivity in TiO bulk materials was discovered in early 1965's by Hulm et al 18 . They reported that, for the bulk TiO with NaCl structure, its superconducting transition temperature T c increases from 0.2 K to 1.0 K with oxygen content increases from 0.9 to 1.1, but is below 0.08 K outside this range 19 . Further investigations showed that, high pressure annealing increases the oxygen content and lattice constant of cubic TiO bulk, and its superconducting transition temperature increases linearly with oxygen content to a maximum of 2.0 K 4,7 . However, the intrinsic superconducting properties of TiO, such as the lower and upper critical fields, superconducting coherence length and so on, are not clear yet, due to the difficulty in obtaining single crystals or epitaxial films. Even the zero resistance superconducting state has not been clearly reported. Therefore, high-quality TiO epitaxial thin films or single crystals are essential and highly desired for investigating the fundamental superconductivity of the system. There was only one ~10 nm TiO single crystalline film reported very recently, which was formed on TiO 2 substrate through surface chemical reduction method by a low-energy ion bombardment technique 20 .
Although, this method is good at creating a thin TiO film on TiO 2 substrate, no magnetic and electrical properties concerning the superconductivity were reported. In fact, it is still a challenge to prepare high quality TiO epitaxial films on different substrates through a more controllable method like magnetron sputtering or pulsed laser deposition techniques.
On the other hand, it is well known that, for epitaxial superconducting thin films, the superconductivity could be enhanced or even created by proper heterostructure interfaces, such as the enhanced T c above 100 K in the epitaxial FeSe films grown on SrTiO 3 substrates, which was explained in terms of the coupling between conduction electrons and the substrate phonons 21,22,23 , as well as the superconductivity created at the interface between two insulators like LaAlO 3 /SrTiO 3 24,25 . In addition, a suitable lattice mismatch induced strain can also enhance the T c as reported in La 1.9 Sr 0.1 CuO 4 26 . Therefore, it will be interesting to see whether the superconductivity in TiO films can be improved as well.
In the present work, we successfully prepared cubic TiO thin films on a (0001)-oriented α-Al 2 O 3 single crystal substrate by a pulsed laser deposition technique. The superconducting transition temperature of 7.4 K was observed for the TiO films, confirmed by magnetization and electrical transport measurements. The T c is almost 4 times higher than its bulk value and is suppressed with increasing pressure (Maximum pressure 1.8 GPa).

Results and Discussions
The TiO thin films with the thickness of ~80 nm were epitaxially grown on (0001)-oriented α-Al 2 O 3 single crystalline substrates (see Methods). The structural characterization of the TiO thin films was performed using high angle annular dark-field scanning transmission electron microscope (HAADF-STEM) and X-ray diffraction (XRD).
It was determined that the cubic TiO thin film grown on α-Al 2 O 3 (0001) substrate is of the [111] direction perpendicular to its surface. Figure 1  The superconducting property of the TiO film was also investigated through electrical transport measurements using the Hall bar pattern. The schematic diagram of the experimental setup is shown in the supplementary information Fig. S3. Fig. 3 shows the temperature dependent resistance (R xx ) in zero magnetic field and Hall resistance (R xy ) in 2 T magnetic field. In the R xx -T curve, the resistance rises with decreasing temperature first, and a kink (T kink ) appears around 130 K below which the resistance increases even steeper. This kink may be related to the charge localization 29,30 . When the temperature goes down further, the resistance decreases suddenly and the superconducting behavior occurs at ~7.4 K, as shown in the inset of Fig. 3(a). The superconducting resistive transition at zero field is relatively broad, which may be attributed to the following two aspects: i) inhomogeneous oxygen or titanium stoichiometry of the films, as shown in Fig. S2, and ii) a consequence of a Berezinskii-Kosterlitz-Thouless (BKT)-like transition, as described in the following equation 31 : where R N is normal-state sheet resistance, and b is a non-universal dimensionless numerical constant. The good fitting result (blue solid line in inset of Fig. 3(a) From the Hall result, it is confirmed that the n-type electronic charge carriers dominate the conduction mechanism, as shown in Fig. 3 Fig.   3(b), which may be related to the unusual vortex motion 32,33 .
with n=0.85. To investigate the anisotropic superconducting properties in more detail, we studied the magnetic field orientation dependence of the superconducting transition. Fig. 4(d) 8 shows the field dependences of the resistances at different angles β at 4.0 K, where β denotes the tilt angle between the normal of film plane and the field direction, as depicted in the inset of Fig. 4(d). One can see that H c2 changes a little, while H irr increases gradually with increasing β from H⊥(111) to H//(111). In conclusion, the superconducting cubic TiO thin films were epitaxially grown on the (0001)-oriented α-Al 2 O 3 single crystalline substrates, and the superconducting properties of 9 the films were systematically characterized. It was found that the superconducting transition temperature is enhanced to about 7.4 K, and the upper critical field at zero field H c2 (0) is about 13.7 T (H//(111)) in the TiO thin films. The high pressure experimental results indicated that increasing pressure weakens the superconductivity. Our work will facilitate prospects for understanding the superconducting mechanism in the titanium-based oxide superconductors as well as achieving higher temperature superconductivity.         titanium monoxide structure, which shows good consistency with experimental data shown in Fig. 1(b).  TiO film are determined using the criterions of 90% and 0.1% normal-state resistivity normalized to its normal state value R N (T) 3,4 . Here, the magnetic field independent R N (T) is extrapolated from R(T) curve at temperatures well above the transition temperature by a polynomial function.