Nonreciprocal Transport in chiral Mo3Al2C Near the Superconducting to Normal Transition

TL;DR

The study addresses nonreciprocal transport in a bulk chiral superconductor Mo$_3$Al$_2$C, leveraging the coexistence of lattice chirality and a polar CDW phase. By employing AC transport to measure the second-harmonic resistance $R^{2\omega}$, the authors reveal a pronounced enhancement of nonreciprocity near the superconducting–normal boundary, with both magnetochiral and toroidal contributions tied to the current, magnetic field orientation, and CDW polarization. The results show a dome-shaped dependence of $R^{2\omega}$ on field and current, track the $H$–$T$ phase boundary from $R^{1\omega}$, and are supported by a heat-capacity signature indicating strong electron–phonon coupling ($T_c \approx 7.8$ K, $\Delta C_p/(\gamma T_c) \approx 2.15$). This work establishes Mo$_3$Al$_2$C as an intrinsic bulk platform for tunable nonreciprocal transport with potential applications in superconducting diode-like devices and motivates exploration of related materials with enhanced spin–orbit coupling.

Abstract

We investigate nonreciprocal electrical transport in bulk single-crystalline Mo3Al2C, a material known to host crystallographic chirality, a polar charge-density-wave instability, and a superconducting transition near 8 K. Using AC transport measurements to analyze the first-harmonic and second-harmonic resistance responses, we observe a distinct nonreciprocal second-harmonic signal that is significantly enhanced near the boundary of the normal and superconducting phases. Phenomenologically, this response arises from direction-dependent coupling between the external magnetic field and the current-induced intrinsic magnetization within the chiral lattice. Furthermore, a persistent nonreciprocal response observed under perpendicular magnetic fields suggests a toroidal-induced effect linked to the electric polarization emerging from the charge-density-wave phase. These results demonstrate that bulk Mo3Al2C serves as an intrinsic platform for tunable nonreciprocal transport rooted in the interplay of chirality, polarity, and superconductivity.

Figures (6)

• Figure 1: (a) Cubic structure (P4$_1$32) of Mo$_3$Al$_2$C above $T^*=155$ K where CDW happens (b) A diagram of the chiral structure of Mo$_3$Al$_2$C. The chiral current induces magnetic field which enhances or diminishes the external magnetic field, switching from the normal to superconducting state, vice versa. (c) A circuit image of Mo$_3$Al$_2$C. The number on the wire denotes the probe number. (d) A M-T phase diagram of Mo$_3$Al$_2$C. $T^*=155$ K denotes the temperature where CDW happens.
• Figure 2: Nonreciprocal magnetoresistance of (sample dimension). (a), (b) Plots of first (top) and second (bottom) harmonic resistances when the magnetic field is parallel and perpendicular to the current, respectively. The dashed line is drawn at the temperature where $R^{1\omega}$ drops by 50% for 3 T, comparing the transition temperature in $R^{1\omega}$ and $R^{2\omega}$. The $R^{2\omega}$ signals in both (a) and (b) are offset by $0.015$ m$\Omega$.
• Figure 3: Second harmonic resistance ($R^{2\omega}$) as a function of magnetic field for different currents (a, b) and temperatures (c, d), with the field parallel and perpendicular to the current. The $R^{2\omega}$ signals in all panels are offset by $0.015$ m$\Omega$.
• Figure 4: (a, b, c) Maximum value of $R^{2\omega}$ vs. field, current and temperature from Fig. 2a, 2b, 3a, 3b, 3c, and 3d, respectively. The uncertainty of each data point was determined to be the standard deviation of signal's noise level.
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