Hydrostatic pressure effect on the transport properties in TiO superconducting thin films

The superconducting properties of the TiO epitaxial thin films were systematically investigated under hydrostatic pressures $\left(P\right)$ up to 2.13 GPa. At ambient pressure, the normal state resistivity increases with decreasing temperature, and steeply increases below $T_{\mathrm{kink}}\sim 115\mathrm{K}$. With further reducing temperature to $T_c\sim 5.99\mathrm{K}$, the thin film enters into a superconducting state. Interestingly, the superconducting temperature $T_c$ gradually decreases upon increasing $P$, and the decreasing rate of $T_c$ with $P$ is much larger than the McMillan theoretical expectation. In contrast, $T_{\mathrm{kink}}$ increases with $P$ and a remarkable resistivity enhancement was observed in the temperature range between $T_{\mathrm{kink}}$ and $T_c$. The variations of $T_c,T_{\mathrm{kink}}$, and normal state resistivity under high pressure may be induced by the charge localization related to the atomic vacancies rearrangement in TiO thin film. Furthermore, the temperature dependencies of the upper critical field $H_{c2}\left(T\right)$ indicate that both the orbital and Pauli-paramagnetic pair-breaking effects should be taken into account. Finally, the thermally activated flux flow investigations under different pressures suggest that the pressure will suppress the thermal activate energy.

Figure 1

Temperature dependencies of resistivity $\left(\rho \text{−}T\right)$ of TiO thin film under various pressures up to 2.13 GPa (0, 0.16, 0.53, 0.98, 1.35, 2.13 GPa). The inset is an enlarged view of the resistivity near $T_c$.

Figure 2

Pressure dependencies of zero resistive transition temperature $T_c$ (cyan solid circle), kink temperature $T_{\mathrm{kink}}$ (purple solid square), and the residual resistivity ratio (RRR) (blue star) of the specimen. The dash lines present the linear fits of $T_c$ and $T_{\mathrm{kink}}$ with slopes of $-0.39$ and $5.1\mathrm{K}\mathrm{GP}{\mathrm{a}}^{-1}$, respectively. The solid blue line is the eye guide line for RRR.

Figure 3

(a) Fits of resistivity data with variable range hopping model at various pressures (0, 0.16, 0.53, 0.98, 1.35, and 2.13 GPa). (b) Characteristic temperatures at temperatures above $T_{\mathrm{kink}}$ (black) and below $T_{\mathrm{kink}}$ (red) plotted as a function of pressure.

Figure 4

(a) and (b) $\rho \text{−}T$ curves for TiO thin film in different magnetic fields under the selected pressures of 0 and 2.13 GPa. (c) $H_{c2}\left(T\right)$ and $H_{\mathrm{irr}}\left(T\right)$ as a function of temperature at the representative pressures, the red lines are the fits to the WHH model with consideration of the spin-paramagnetic and the spin-orbit effects (0, 0.98, and 2.13 GPa), and the black lines are the fits to the empirical equation $H_{\mathrm{irr}}\left(T\right)=H_{\mathrm{irr}}\left(0\right)\left[1-{\left(T/T_c\right)}^2\right]$ (0, 0.98, and 2.13 GPa). Inset: Analysis of $H_{c2}\left(T\right)$ (0 GPa) using the WHH theory without (black line) and with (red line) spin-paramagnetic effect and spin orbital scattering. (d) Pressure dependencies of $H_{c2}\left(0\right),H_{\mathrm{irr}}\left(0\right),{\xi }^{\mathrm{orb}}\left(0\right)$, and ξ(0).

Figure 5

$\ln \rho$ vs $1/T$ curves at (a) 0 GPa and (b) 2.13 GPa. The black solid lines are the regressive curves obtained using a modified TAFF method, and the purple dashed lines show a linear Arrhenius relation fitting for the resistivity broadenings.

Figure 6

(a) $U_0$ as a function of magnetic fields obtained by fitting the resistivity in TAFF region using the modified TAFF model. The black dash lines are the fittings using Eq. (7). (b) Pressure dependencies of $U_0$ in various magnetic fields.