Effect of external electromagnetic radiation on the anomalous metallic behavior in superconducting Ta thin films

We investigated the transport characteristics of superconducting Ta thin films with three configurations of radio-frequency radiation filters: no filter, only room-temperature filter, and low- and room-temperature filters. We find that when the room-temperature filter is installed, the entire transition is shifted to higher temperatures, and once the zero-resistance state is achieved at zero magnetic field, no strong radiation effect is observed. When the magnetic field is turned on, the non-zero-resistance saturation at low temperatures is revealed without low-temperature filters, which has previously been considered to be a magnetic-field-induced quantum metallic phase. However, the insertion of the additional low-temperature filters weakens the saturation of the resistance, i.e., the signature of the metallic behavior. This observation suggests that the previously reported anomalous metallic state in Ta films is mainly induced by unfiltered radiation, and, thus, the intrinsic metallic ground state, if it exists, should be limited to a narrow range of magnetic fields near the critical point.

Figure 1

Temperature dependence of the sheet resistance at zero magnetic field for Ta1-1 (black), Ta1-2 (red), and Ta1-3 (blue). Solid and open symbols show $R$($T$) in the NF and RTF configurations, respectively. The solid and dashed curves are eye guides.

Figure 2

Temperature dependence of $R/R_N$ under various magnetic fields for Ta2-1. (a) Arrhenius plots under various magnetic fields between 0 and 0.6 T with a 0.1-T step. Solid and open symbols are measured data in the R + LTF and RTF configurations, respectively. Black solid lines are the results of a linear fitting to $R/R_N\propto \exp (-E_A/T)$ with the slopes of the lines representing $E_A$. Black dashed lines show the saturation of $R/R_N$ to finite values. (b) Magnetic-field dependence of $E_A$ shown in a semilogarithmic plot. The red solid line is a linear fit to $E_A\propto \ln \left(B_0/B\right)$ where $B_0=0.64\mathrm{T}$.

Figure 3

MR isotherms at various temperatures. (a) $R/R_N$ of Ta2-1 as a function of magnetic field measured in the R + LTF configuration. Black solid lines are measured at various temperatures between 150 and 500 mK with a 50-mK step. The data points with different colors are measured at $T$ = 100 ($▹$), 75 ($◃$), 50 ($◇$), 30 ($▿$), 20 ($▵$), and 15 ($○$) mK, respectively, and the dashed lines are eye guides. (b) Magnetic-field dependence of $R/R_N$ in the RTF configuration. The data points are measured at $T$ = 100, 75, 50, 30, and 20 mK, and the dashed lines are eye guides. The measurement temperatures are displayed in the same colors and symbols as those in (a). The green solid line exhibits the exponential dependence of $R$ ∼ exp($B$/B*).

Figure 4

Transport phase diagram of Ta2-1 on the B-T plane constructed from measurements in the R + LTF configuration except for the dashed lines. The red open circles, $B_{\mathrm{Cross}}$, are the crossing points of two adjacent MR isotherms [Fig. 3], and the black open squares, $T_{\mathrm{Cross}}$, are the crossover points between negative and positive $dR/dT$, obtained from the $R$($T$) curves under 0.65 and 0.7 T (see Fig. S3 in the Supplemental Material [31]). The blue- and red-shaded regions show negative and positive $dR/dT$, respectively. The gray-shaded area indicates the zero-resistance region in our measurement limit. Gray, red, blue, and black solid lines are isodissipative curves of $0.9R_N, 0.1R_N, 0.01R_N$, and $0.001R_N$ in the R + LTF configuration, respectively. The dashed lines (gray, red, blue, and black) are isodissipative curves ($0.9R_N, 0.1R_N, 0.01R_N$, and $0.001R_N$) obtained from measurements in the RTF configuration.