Axionic tunneling from a topological Kondo insulator

TL;DR

This paper provides direct experimental evidence of axionic electrodynamics in a topological Kondo insulator using STM, revealing voltage-controlled magnetization effects consistent with axionic $E ext{ extperiodcentered}B$ coupling.

Contribution

It demonstrates a novel STM-based method to detect axionic physics through voltage-induced magnetization in a topological Kondo insulator.

Findings

Voltage-induced magnetization of about 0.1 μ_B at ~30 meV bias

Reversal of spin polarization sign with bias voltage

Evidence supporting axionic $E ext{ extperiodcentered}B$ coupling

Abstract

Discoveries over the past two decades have revealed the remarkable ability of quantum materials to emulate relativistic properties of the vacuum, from Dirac cones in graphene to the Weyl surface states of topological insulators. Yet the most elusive consequence of topology in quantum matter is the axionic $E\cdot B$ term in the electromagnetic response. Here we report a direct signature of axionic physics obtained through scanning tunneling microscopy (STM). Although recent STM experiments using SmB$_6$ nanowires have been interpreted as evidence for spin-polarized currents arising from topological surface states, we show that the observed spin polarization instead originates from axionic electrodynamics. Our analysis reveals a striking voltage-induced magnetization: extremely small voltages ($\sim$ 30 meV) generate tip moments of order 0.1 $\mu_B$ that reverse sign with the applied bias. The magnitude, tunability, and reversibility of this signal are consistent with an axionic $E \cdot B$ coupling, and fully account for the magnetic component of the tip density of states, ruling out static magnetism. Millivolt-scale control of spin polarization in a tunnel junction provides a new route for probing axionic electrodynamics and opens avenues for future STM and spintronics applications.