Evidence of local superconductivity in granular Bi nanowires fabricated by electrodeposition

An unusual enhancement of resistance (i.e., a “superresistivity”) below a certain characteristic temperature $T_{\text{sr}}$ was observed in granular Bi nanowires. This “superresistive” state was found to be dependent on the applied magnetic field as well as the excitation current. The suppression of $T_{\text{sr}}$ by magnetic field resembles that of a superconductor. The observed superresistivity appears to be related to the nucleation of local superconductivity inside the granular nanowire without long-range phase coherence. The phenomenon is reminiscent of the “Cooper-pair insulator” observed previously in ultrathin two-dimensional superconducting films and three-dimensional percolative superconducting films.

Figure 1

(Color online) $R$ vs $T$ curves of nine granular Bi nanowire samples ($d=70\text{nm}$, $L=6\mu \text{m}$) showing a super-resistive behavior at a well-defined $T_{\text{sr}}$. Sample B1 shows a $T_{\text{sr}}$ of 5.8 K, which will be discussed specifically in context.

Figure 2

(Color online) (a) $R$ vs $T$ curves of granular Bi nanowire sample B1 under different perpendicular magnetic fields $H$, measured with a small dc excitation current of 50 nA. The inset shows a TEM image of the wires. (b) $\text{ln}R$ vs $1/T^{1/2}$ curves at two specific magnetic fields of $H=0$ and 8.0 kOe.

Figure 3

(Color online) (a) $R$ vs $H$ curves of granular Bi nanowire sample B1, measured at different temperatures with a small dc excitation current of 50 nA. The inset is a blowup of the $R\text{−}H$ curves at 5.5 and 6.0 K, respectively. (b) Phase diagram of $H_{\text{sr}}$ vs $T$, obtained from the $R\text{−}H$ and $R\text{−}T$ curves shown in Figs. 3a, 4b.

Figure 4

(Color online) (a) $R$ vs $T$ of sample B1, measured with different dc excitation current. The inset is a blowup near the $T_c$. (b) $R$ vs $T$ curves under different $H$, measured with a higher dc current of $25\mu \text{A}$.

Figure 5

(Color online) Magnetoresistance $\Delta R/R\left(0\right)$ vs $H$ of sample B1 at different $T$, measured with a higher dc current, $25\mu \text{A}$. A positive MR effect was seen, which is opposite to that shown in Fig. 3a, measured with a small excitation current of 50 nA.