Little-Parks Oscillations in an Insulator

We present the results of a magnetoresistance study of the disorder-induced superconductor-insulator transition in an amorphous indium-oxide thin film patterned by a nanoscale periodic array of holes. We observed Little-Parks-like oscillations over our entire range of disorder spanning the transition. The period of oscillations was unchanged and corresponded to the superconducting flux quantum in the superconducting as well as in the insulating phases. Our results provide direct evidence for electron pairing in the insulator bordering with superconductivity.

Figure 1

(a) $R$ vs $T$ at $B=0\mathrm{T}$ in annealing states $A$ through $K$, spanning the range of disorder in nanopatterned $a\mathrm{\text{:}}\mathrm{InO}$ thin-film sample. (b) $R$ vs $B$ in superconducting state $J$ at $T=0.03$, 0.04, 0.06, 0.08, 0.1, 0.15, 0.2, 0.3, 0.35, 0.45, 0.55, 0.6, 0.7, 0.8, 0.9 K . Inset: Critical $B$ vs $R$ at room $T$ for some of the superconducting states. The dashed line is a linear fit.

Figure 2

$R$ vs $B$ at $T=0.08\mathrm{K}$ for superconducting state $K$. The period of LP oscillations was $0.35\pm 0.005\mathrm{T}$. Inset: $R$ isotherms taken at various $T$’s in state $K$.

Figure 3

(a) $R$ vs $B$ from different annealing states at $T=0.55\mathrm{K}$ spanning our entire range of disorder. The states $A$ through $G$ were insulating, while states $H$ through $K$ were superconducting. (b) $R$, plotted on a logarithmic scale, vs $B$ for insulating state $F$ and superconducting state $I$. The dashed line corresponds to ${\Phi }_0$ penetrating an area of one unit cell.

Figure 4

Nonlinear $I-V$ in insulating state $A$ at $T=0.025\mathrm{K}$ at three different $B$’s. Arrows indicate direction of voltage sweep. Inset: threshold voltages $V_{\mathrm{LH}}$ and $V_{\mathrm{HL}}$ vs $B$.

Figure 5

Superconducting state $H$ exhibits multiple SIT’s driven by $B$. Transition points indicated by the dashed line at $R=4.2\mathrm{k}\Omega /\square$.