The superconducting diode effect in Josephson junctions fabricated from structurally chiral Mo$_3$Al$_2$C

Abstract

The superconducting diode effect occurs in superconducting materials in which both spin and inversion symmetry are broken. The recently observed chirality-induced spin selectivity effect demonstrates that chiral materials break both symmetries. Thus a Josephson junction interface with the left-handed structure on one side of the junction and the right-handed structure on the other should exhibit a diode effect. Here, we report the electrical transport properties of right-handed/left-handed and right-handed/right-handed devices fabricated from single crystals of the structurally chiral superconductor Mo$_3$Al$_2$C. Fraunhofer-like magnetic diffraction patterns confirm the presence of Josephson effect in all but one of our devices. A magnetic field-induced superconducting diode effect is demonstrated in the right-handed/left-handed devices by a statistically significant difference in $I_{c+}$ and $I_{c-}$, with a maximum asymmetry of 5\%. The intrinsic superconducting diode effect is not observed in the right-handed/right-handed devices. We provide an explanation for the presence of the superconducting diode effect in the right-handed/left-handed devices.

Figures (33)

• Figure 1: (a) Crystal structure of Mo$_3$Al$_2$C looking along the $a$ axis. (b) Schematic diagram of a vertical Josephson junction with a right-handed (P4$_1$32) and left-handed (P4$_3$32) crystal. (c) An SEM image of a single crystal of Mo$_3$Al$_2$C. (d) A single crystal of Mo$_3$Al$_2$C positioned on a "v" of silver paint on a mica substrate to make electrical contact. (e) Two Mo$_3$Al$_2$C crystals stacked together in preparation to be pressed together. The crystals are located where the silver paint for each crystal overlaps, shown by the black box. (f) The final device with the crossbar screwed down to push the two crystals together and with electrical connections made to a Quantum Design PPMS universal sample mount.
• Figure 1: Powder X-ray diffraction pattern of of ground metallic pieces of Mo$_3$Al$_2$C.
• Figure 2: (a) Resistance versus temperature and (b) voltage and $dV/dI$ versus excitation current for a single crystal device. The inset in (a) shows the electrical connections made to the device, where there is a current and voltage lead on each side of the junction.
• Figure 2: (a) Magnetic susceptibility versus temperature and (b) magnetization versus magnetic field for a polycrystalline sample of Mo$_3$Al$_2$C. A superconducting transition is observed at $T_{c} = 8.5$ K, and the critical field is estimated to be $H~=~120$ Oe at $T = 2$ K.
• Figure 3: Resistance versus temperature for the P4$_1$32/P4$_3$32 and P4$_1$32/P4$_1$32 devices.