Nonreciprocal charge transport in an iron-based superconductor with broken inversion symmetry engineered by a hydrogen-concentration gradient

TL;DR

This study demonstrates that a hydrogen concentration gradient in an iron-based superconductor induces nonreciprocal charge transport, revealing a new method to break inversion symmetry and achieve high-temperature nonreciprocal effects.

Contribution

The paper introduces concentration-gradient engineering as a novel approach to break inversion symmetry in superconductors, leading to high-temperature nonreciprocal charge transport.

Findings

Nonreciprocal charge transport observed near superconducting transition.

High-temperature vortex nonreciprocity above 40 K.

Hydrogen gradient creates asymmetric vortex pinning landscape.

Abstract

The breaking of spatial inversion symmetry in condensed matter gives rise to intriguing physical properties, such as ferroelectricity, piezoelectricity, spin-momentum locking, and nonreciprocal responses. Here we propose that a concentration gradient, which often persists as a quasi-stable nonequilibrium state with long relaxation times in solids, can serve as a general platform for inversion symmetry breaking. We demonstrate this concept in an epitaxial thin film of the hydrogen-doped SmFeAsO (Sm1111:H) superconductor with a depthwise hydrogen-concentration gradient introduced via an optimized topotactic reaction. This film exhibits nonreciprocal charge transport, meaning that the electrical resistance depends on the direction of the applied current, which serves as a key signature of broken inversion symmetry. A pronounced nonreciprocal signal emerges in the vicinity of the superconducting transition, which we attribute to vortex-motion nonreciprocity arising from an asymmetric pinning landscape created by the hydrogen-concentration gradient. Owing to the high critical temperature of Sm1111:H, vortex-origin nonreciprocity is observed above 40 K, representing the highest temperature reported to date among single bulk materials without an artificially hetero-layered structure. Our findings establish concentration-gradient engineering as a versatile and broadly applicable route for realizing inversion-broken states in otherwise centrosymmetric hosts, opening pathways toward a broader landscape of odd-parity-driven functionalities.