Atomic-scale spin sensing of a 2D $d$-wave altermagnet via helical tunneling

TL;DR

This paper demonstrates atomic-scale spin-resolved measurements of a 2D $d$-wave altermagnet using helical edge modes of a topological insulator, revealing its unique dual-space spin structure and excitations.

Contribution

It introduces a novel spin-resolved scanning tunneling microscopy technique utilizing topological insulator edge modes to probe altermagnetic order at atomic scale.

Findings

Visualization of checkerboard antiferromagnetic order in real space

Direct imaging of d-wave spin splitting in momentum space

Observation of unidirectional, spin-polarized quasiparticle excitations

Abstract

Altermagnetism simultaneously possesses nonrelativistic spin responses and zero net magnetization, thus combining advantages of ferromagnetism and antiferromagnetism. This superiority originates from its unique dual feature, i.e., opposite-magnetic sublattices in real space and alternating spin polarization in momentum space enforced by the same crystal symmetry. Therefore, the determination of an altermagnetic order and its unique spin response inherently necessitates atomic-scale spin-resolved measurements in real and momentum spaces, an experimental milestone yet to be achieved. Here, via utilizing the helical edge (hinge) modes of a higher order topological insulator as the spin sensor, we realize spin-resolved scanning tunneling microscopy which enables us to pin down the dual-space feature of a layered $d$-wave altermagnet, KV$_2$Se$_2$O. In real space, atomic-registered mapping demonstrates the checkerboard antiferromagnetic order together with density-wave lattice modulation, and in momentum space, spin-resolved spectroscopic imaging provides a direct visualization of d-wave spin splitting of the band structure. Critically, using this new topology-guaranteed spin filter we directly reveal the unidirectional, spin-polarized quasiparticle excitations originating from the crystal symmetry-paired X and Y valleys around opposite magnetic sublattices simultaneously --the unique spin response for $d$-wave altermagnetism. Our experiments establish a solid basis for the exploration and utilization of altermagnetism in layered materials and further facilitate access to atomic-scale spin sensing and manipulating of 2D quantum materials.