# A chiral fermionic valve driven by quantum geometry

## TL;DR

This paper demonstrates a chiral fermionic valve that separates fermions by chirality using quantum geometry in PdGa without needing a magnetic field.

## Contribution

The paper introduces a chiral fermionic valve based on quantum geometry in topological semimetals, enabling spatial separation of chiral currents without magnetic fields.

## Key findings

- Chiral fermions are spatially separated into distinct Chern-number-polarized states using quantum geometry.
- Quantum interference of chiral currents is observed in a Mach–Zehnder interferometer without magnetic fields.
- Devices made from PdGa exhibit a nonlinear Hall effect due to quantum-geometry-induced anomalous velocities.

## Abstract

Multifold topological semimetals host fermions with opposite chiralities at topological band crossings1–3. Chiral fermionic transport in topological systems often relies on high magnetic fields or magnetic dopants to suppress trivial transport and create an imbalance in occupancy of opposite Chern-number states4,5. Here we use the quantum geometry6,7 of topological bands to filter fermions by chirality into distinct Chern-number-polarized states. This allows for the real-space separation of currents with opposite fermionic chiralities, which we have demonstrated by observing their quantum interference in the absence of any magnetic field. Devices fabricated from single-crystal PdGa in a three-arm geometry exhibit quantum-geometry-induced anomalous velocities8,9 of chiral fermions, thereby exhibiting a nonlinear Hall effect. The resultant transverse chiral currents with opposite anomalous velocities are thereby spatially separated into the outer arms of the device. These chiral currents in opposing Chern number states also carry orbital magnetizations with opposite signs. The mesoscopic phase coherence of these chiral currents facilitated their quantum interference10 in a Mach–Zehnder interferometer. Our findings establish a chiral fermionic valve that exhibits three key properties: spatially separates chiral fermions into Chern-number polarized states by using their quantum geometry, enables tuneable current-induced magnetization and provides a platform for controllable quantum interference of chiral quasiparticles using an electric current and magnetic field.

Fermionic currents of opposing chirality can be spatially filtered without the need for a magnetic field using the quantum geometry of topological bands in single-crystal PdGa.