Evidence for Spin-Triplet Electron Pairing in the Proximity-Induced Superconducting State of an Fe-Doped InAs Semiconductor

We provide evidence for spin-triplet electron pairing in proximity-induced superconductivity in a ferromagnetic semiconductor (In,Fe)As. As discovered in half-metallic materials, an extraordinarily long proximity range is observed. More surprising is a very strong concentration of supercurrent to the edges of the superconducting region, which is deduced from the extremely persistent oscillation of the critical current vs magnetic field. The maxima of the critical current appear not at the zero magnetic flux but at around the maximum magnetic disorder, reflecting the connectivity between the spin-triplet and singlet pairings. These spin-triplet natures in proximity superconductivity also reveal ferromagnetic properties of (In,Fe)As.

Figure 1

(a) Schematic of the layered structure. (b) $M$ (magnetization) -$H$ (magnetic field) curve of the IFA film at 10 K measured with a dc SQUID. The field direction is perpendicular to the film plane and the sweep range is $\pm 10\mathrm{kOe}$. The inset shows the temperature dependence of the magnetization in IFA at $H=40\mathrm{Oe}$ for field cooling (FC, $H=10\mathrm{kOe}$) and zero-field cooling (ZFC). The green open circles represent the inverse magnetic susceptibility indicating a Curie temperature of 128 K.

Figure 2

(a) Optical micrograph image of the junction J10. (b) Temperature dependence of the zero-bias resistance of J06 at zero magnetic field. The inset shows a close-up between 0 and 0.4 K. The resistance peaks are attributable to domain wall motion discussed in the text. (c) Differential resistance as a function of the bias current for each junction in zero field at 0.1 K. (d) Dependence of critical current and ratio of the zero-bias resistance $R_0$ to the normal resistance $R_{\mathrm{n}}$ on the gap length.

Figure 3

Color plot of the differential resistance of J06 as a function of magnetic field and bias current. The field direction is perpendicular to the film plane. The external field was swept from $+109$ to $-109\mathrm{Oe}$ for the upper panel, and the reverse for the lower panel, in which sweep directions are indicated by green arrows. The insets show close-ups around $+20$ and $-20\mathrm{Oe}$ for down sweep and up sweep, respectively.

Figure 4

Field dependence of the zero-bias differential resistance of J06. The field was swept sequentially via the following points: $+14\mathrm{kOe}$, $-290$, 290, $-290$, 290, $-90$, 90, $-90$, 90, $-30$, 30, $-30$, and 30 Oe. The panels are the data for the sweeps from (a) $-290$ to $-290\mathrm{Oe}$ via $+290\mathrm{Oe}$, (b) $-90$ to $-90\mathrm{Oe}$ via $+90\mathrm{Oe}$, and (c) $-30$ to $-30\mathrm{Oe}$ via $+30\mathrm{Oe}$.

Figure 5

(a) Color plot of the differential resistance of J06 as a function of the magnetic field and bias current at 0.35 K. After the field was swept to $-109\mathrm{Oe}$, the resistance was measured from $-36$ to $-8\mathrm{Oe}$. The arrows above the graph indicate the phase slip attributable to the domain wall motion. (b) Temperature dependence of the maximum critical current of J06 in the range between $-36$ and $-8\mathrm{Oe}$. The broken line is a fitting line of the data.