Josephson diode and spin-valve effects on the surface of altermagnet CrSb

TL;DR

CrSb, an altermagnetic metal with spin-polarized topological surface states, enables proximity-induced superconductivity that is strongly modulated by magnetic orientation rather than solely by flux. The authors fabricate In-CrSb and In-CrSb-In junctions to reveal Josephson spin-valve and Josephson diode effects, with clear evidence of spin-valve behavior in double junctions and FFLO-like, nonmonotonic gap oscillations in a single interface. The findings highlight a dual role for surface and bulk spin textures in governing the supercurrent, suggesting CrSb as a robust platform for superconducting spintronics and for exploring finite-momentum pairing in altermagnetic systems.

Abstract

We experimentally investigate charge transport in In-CrSb and In-CrSb-In proximity devices, which are formed as junctions between superconducting indium leads and thick single crystal flakes of altermagnet CrSb. For double In-CrSb-In junctions, the obtained $dV/dI(B)$ curves are mirrored in respect to zero field for two magnetic field sweep directions, which is characteristic behavior of a Josephson spin valve. Also, we demonstrate Josephson diode effect by direct measurement of the critical current for two opposite directions in external magnetic field. We interpret these observations as a joint effect of the spin-polarized topological surface states and the altermagnetic spin splitting of the bulk bands in CrSb. For a single In-CrSb interface, the superconducting gap oscillates in magnetic field for both field orientations, which strongly resembles the transition into the Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) state. The latter is based on finite-momentum Cooper pairing against a background of the Zeeman splitting, so it is fully compatible with the requirements for the Josephson diode effect.

Figures (5)

• Figure 1: (Color online) (a) Optical image of the CrSb single-crystal flake (top image), which is placed on the pre-defined In leads pattern (the bottom one) to form 2 $\mu$m separated In-CrSb junctions. To investigate Josephson current through the CrSb surface, In-CrSb-In resistance is measured in a standard four-point technique. (b) Josephson $dV/dI(I)$ curves at 30 mK temperature, the blue and the red curves are for two opposite current sweep directions, respectively. Zero-resistance state can be clearly seen, which is accompanied by usual hysteresis for the critical and the return currents. The black curve demonstrates suppression of the Josephson effect at 1.2 K. The curves are obtained after sample cooling in zero magnetic field, before any sample magnetization.
• Figure 2: (Color online) (a) Josephson spin-valve effect as the $dV/dI(B)$ curves reversal for two opposite magnetic field sweep directions. After magnetization of the sample, the zero-resistance region is not only shifted to finite magnetic fields, but the $dV/dI(B)$ curves are mirrored in respect to zero field, including regions with finite resistance. (b) $dV/dI(I)$ curves after magnetization of the sample. Differential resistance is always finite at zero field now, but there is wide zero-resistance region at 13 mT magnetic field. Magnetic field is directed normally to the In-CrSb interfaces (i.e. to the CrSb flake), the data are obtained at 30 mk temperature.
• Figure 3: (Color online) Josephson spin-valve effect for the parallel to the In-CrSb interfaces magnetic field orientation at 30 mK temperature. (a) Before magnetization of the sample, $dV/dI(I)$ curves well reproduce ones from Fig. \ref{['fig1']} (b). The zero-resistance region is slightly asymmetric for every current sweep direction (red and blue curves), it is suppressed at 1.2 K temperature (the black one). (b) After magnetization of the sample, the $dV/dI(B)$ curves are mirrored in respect to zero field (see also the regions of finite resistance), confirming the Josephson spin valve behavior. Inset shows the Josephson diode effect as another demonstration of the $dV/dI(B)$ curves reversal: the measured critical currents $I_c^\pm(B)$ are asymmetric in respect to zero field, but they coincide well when drawn as $I_c^+(B)$ (positive $I_c$) and $-I_c^-(-B)$ (the inverted negative $I_c$ for the inverted field value) infgtJDEaunite.
• Figure 4: (Color online) Andreev behavior of $dV/dI$ differential resistance for a single In-CrSb junction. (a) $dV/dI(V)$ curves in zero magnetic field at two different temperatures, 30 mK and 1.2 K, respectively. Differential resistance is diminished within $\approx\pm 0.5$ mV bias interval (depicted by vertical dashed lines). Temperature has low effect on $dV/dI(V)$ curve, however, the zero-bias anomaly disappears at 1.2 K, as well as the fine subgap structures. (b) Magnetic field suppression of Andreev reflection as a waterfall plot (i.e., the $dV/dI(V)$ curves are shifted vertically). As expected, the superconducting gap is suppressed by magnetic field, but the suppression is non-monotonic: the gap oscillates with $\approx$13 mT period. The curves are obtained at 30 mK temperature for the parallel to the In-CrSb interface magnetic field.
• Figure 5: (Color online) Non-monotonic behavior of the proximized superconductivity on the surface of altermagnet CrSb as the detailed colormaps of $dV/dI(B,V)$ differential resistance at 30 mK temperature for two, normal (a) and in-plane (b), magnetic field orientations, respectively, and at 1.2 K for the in-plane field in (c). The superconducting gap oscillates for both field orientations with similar $\approx$13 mT period in (a) and (b), while it is suppressed earlier in normal to the In-CrSb interface magnetic field. The oscillations with the same period survive even at 1.2 K in (c).