Chemical and valence reconstruction at the surface of SmB6 revealed with resonant soft x-ray reflectometry

TL;DR

This study uses resonant soft x-ray reflectometry and theoretical modeling to reveal how the surface of SmB6 undergoes chemical and valence reconstruction after cleaving, affecting its surface termination and properties.

Contribution

It provides the first detailed depth profiles of chemical and valence states at the surface of SmB6, clarifying surface reconstruction mechanisms and their temperature dependence.

Findings

Surface undergoes significant reconstruction after cleaving.

Boron termination is established regardless of initial surface.

Reconstruction occurs faster at room temperature than below 50 K.

Abstract

Samarium hexaboride (SmB$_6$), a Kondo insulator with mixed valence, has recently attracted much attention as a possible host for correlated topological surface states. Here, we use a combination of x-ray absorption and reflectometry techniques, backed up with a theoretical model for the resonant $M_{4,5}$ absorption edge of Sm and photoemission data, to establish laterally averaged chemical and valence depth profiles at the surface of SmB$_6$. We show that upon cleaving, the highly polar (001) surface of SmB$_6$ undergoes substantial chemical and valence reconstruction, resulting in boron termination and a Sm$^{3+}$ dominated sub-surface region. Whereas at room temperature, the reconstruction occurs on a time scale of less than two hours, it takes about 24 hours below 50 K. The boron termination is eventually established, irrespective of the initial termination. Our findings reconcile earlier depth resolved photoemission and scanning tunneling spectroscopy studies performed at different temperatures and are important for better control of polarity and, as a consequence, surface states in this system.