Magnetically induced metallic phase in superconducting tantalum films

We have studied the electronic transport properties of homogeneously disordered superconducting tantalum thin films in magnetic fields. The films exhibit three distinct transport regimes in the zero temperature limit, which we identify as superconducting, metallic, and insulating phases. The metallic phase is unexpected. The transport characteristics of this metallic phase are found to be similar to those of $\mathrm{MoGe}$ films, and high mobility dilute two-dimensional electrons or holes confined in semiconductor interface or transistor geometry.

Figure 1

The resistance is measured at $B=0$ with a dc current in the range $1–10\mathrm{nA}$ that is within the linear response regime. The thicknesses of the films are, from the top, 1.9, 2.0, 2.1, 2.3, 2.5, 2.8, 3.1, 3.4, 3.7, 4.0, 4.5, and $5.5\mathrm{nm}$. The dashed lines are for the insulating phase and the solid lines are for the superconducting phase. Inset: x-ray diffraction patterns of 15, 10, $5\mathrm{nm}$ thick Ta films, and a bare Si substrate.

Figure 2

(a) Sheet resistances are measured with a dc current of $1\mathrm{nA}$ at the indicated magnetic fields (from top to bottom). (b) Magnetoresistances are measured with a dc current of $1\mathrm{nA}$ at the indicated temperatures. At fields above (below) the crossing point, the top (bottom) trace corresponds to the lowest temperature. The crossing point at $0.26\mathrm{T}$ marked by an arrow defines $B_c$.

Figure 3

(a) Hysteretic voltage jumps in the superconducting phase at $B=0$ and $T=20\mathrm{mK}$. Arrows are to indicate the current increasing or decreasing direction. Inset: Smooth and reversible $I\text{−}V$ curves in the metallic phase at $B=0.06\mathrm{T}$ and $T=20\mathrm{mK}$. (b) Differential resistance across the metal-insulator boundary is shown. The magnetic field for each trace is indicated from top to bottom.

Figure 4

(a) Arrhenius plot of the data shown in Fig. 2a. The magnetic field for each trace is indicated. The high-temperature data are fitted to the activated behavior, $\rho \sim e^{-U\left(B\right)∕T}$ (dashed lines). The density of the data is reduced to make the dashed lines visible. $T_{\mathrm{eff}}$ is determined as illustrated for a data point $A$. (b) Activation energy $U\left(B\right)$. The solid line is a fit to the form, $U\left(B\right)\sim \ln (B_o∕B)$. (c) $T_{\mathrm{eff}}$ is plotted against the sample stage temperature. The magnetic fields indicated are for traces from top to bottom at the low temperature end. The dashed line is to indicate the activated behavior.