Chemical and valence reconstruction at the surface ofSmB6revealed by means of resonant soft x-ray reflectometry

Abstract

Samarium hexaboride (SmB6), 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 4,5 absorption edge of Sm and photoemission data, to establish laterally averaged chemical and valence depth profiles at the surface of SmB6. We show that upon cleaving, the highly polar (001) surface of SmB6 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.

Surface- and pressure-induced bulk Kondo breakdown in SmB6

“…The strong hybridization also leads to a mixture of the Sm 2+ (4f 6 ) and Sm 3+ (4f 5 ) configurations, with the electronic structure described by the Hund's rule ground states with total orbital momenta of J = 0 and 5/2, respectively. The bulk and surface electronic properties have been investigated using a wide range of experimental methods, highpressure x-ray-absorption spectroscopy experiments [17,18], scanning tunneling spectroscopy [19][20][21], angle-resolved photoelectron spectroscopy [22][23][24], hard x-ray photoelectron spectroscopy [25,26], and resonant soft x-ray reflectometry [27] providing additional confirmation of surface states in SmB 6 .…”

Magnetic dichroism in the Kondo insulator SmB6

“…The stoichiometric sample central to this study was cleaved more than 24 hours before measurement, sufficient time for complete surface reconstruction. [32] The isotropic and dichroic x-ray absorption spectra were calculated using Xclaim [30] in the atomic limit [33,34], which is appropriate for the largely localized rare earth 4f electrons. The Hamiltonian includes spin-orbit interaction in the 3d and 4f orbitals and Coulomb interactions in the 4f shell and between the 4f shell and the 3d core hole.…”

An STM Perspective on Hexaborides: Surface States of the Kondo Insulator SmB6

“…This reduced hybridization decreases the Kondo temperature at the surface, , relative to the bulk and implies a shift of the Sm valence at the surface toward Sm 3 + . Such a shift has been observed in X‐ray absorption spectroscopy (XAS) [ 112,113,135 ] and the reduced [ 140 ] conforms to the Kondo breakdown scenario. [ 141,142 ] In consequence, an assignment of the surface termination without relying on band‐bending effects appears preferable.…”

“…This is consistent with our observation that the latter can be found only extremely rarely while the former is encountered every now and then, and conforms to findings of an X‐ray absorption study. [ 135 ] An annealed Sm surface did not reveal the intermittent protrusions due to remaining disorder. [ 133 ] As mentioned, compared to the B‐ and Sm‐terminated surfaces, the 2 × 1 reconstruction is found somewhat more frequently which is energetically favorable.…”