Surface-dominated conduction up to 240 K in the Kondo insulator SmB6 under strain

Abstract

SmB 6 is a strongly correlated mixed-valence Kondo insulator 1,2 with a newly discovered surface state 3,4 , proposed to be of non-trivial topological origin 5,6 . However, the surface state dominates electrical conduction only below T * ≈ 4 K (ref. 3), limiting its scientific investigation and device application. Here, we report the enhancement of T * in SmB 6 under the application of tensile strain. With 0.7% tensile strain we report surface-dominated conduction at up to a temperature of 240 K, persisting even after the strain has been removed. This can be explained in the framework of strain-tuned temporal and spatial fluctuations of f-electron configurations, which might be generally applied to other mixed-valence materials. We note that this amount of strain can be induced in epitaxial SmB 6 films via substrate in potential device applications.SmB 6 is a prototypical Kondo insulator, a material where the quantum mechanical hybridization between the conduction and f -electronic orbitals leads to an opening of a narrow Kondo gap near the Fermi energy 1,2 . Experimental observations of the robust surface conductivity on the background of the fully insulating bulk 3,4 and quantum oscillations from a two-dimensional surface 7 both contribute to the highly non-trivial nature of an insulating state in SmB 6 . SmB 6 has so far attracted much attention as a prominent candidate material for a strongly correlated topological insulator 5,6,8 . It is noteworthy to mention that the first application of SmB 6 as a radiofrequency micro-oscillator device 9 has been recently demonstrated, operating at cryogenic temperatures.Due to the small value of the bandgap ∆, scientific investigations and device applications of SmB 6 have thus far been limited to temperatures below the resistance saturation temperature T *

Dynamical Bonding Driving Mixed Valency in a Metal Boride

“…However, SmB 6 displays quantum oscillations. [4][5][6][7][8] While a 2D Fermi surface can be a result of the presence of topological surface states, 7 a 3D Fermi surface 9,10 is in contradiction with apparently insulating properties of the material. Many theories point to a topological type Kondo insulator allowing conduction only on the surface states, and some also point to composite particle excitons, 11 and even call SmB 6 a Majorana Fermion sea.…”

“…20 The energy of the localized states can rise in response to lattice compression, producing pressure effects. A seemingly disconnected theory for SmB 6 states that the ratio between Sm 2+ and Sm 3+ is held constant by the stiff, non-interacting boron network pressing against the hard sphere metal ions, while mixing Sm 2+ and Sm 3+ -cations of different radii-minimized the lattice energy. 1 The major assumption underlying this description is the lack of electronic Sm-B interactions.…”

“…Motivation for our choices of basis set and order of scalar relativistic correction can be found in a previous study of the SmB − 6 cluster (Ref 33, main text). The potential energy surface of SmB + 2 was modeled with Multi-reference Configuration Interactions (MRCI) starting from the 8-SA-CASSCF(14o,9e) wave-function, as implemented in the Molpro suite [5][6][7][8]. For each point in the energy surface, we considered the lowest doublet, quartet, sextet and octet A 1 , A 2 , B 1 and B 2 states.…”

Surface- and pressure-induced bulk Kondo breakdown in SmB6

“…A change in Sm valence at the * kaichen.hzg@gmail.com surface has been reported from x-ray-absorption spectroscopy (XAS) and x-ray magnetic circular dichroism (XMCD), which is claimed to be the definitive proof of changes in the electronic structure at the surface of SmB 6 [11]. A surface dominated conduction at 240 K has been achieved in epitaxial SmB 6 films with a small 0.7% tensile strain enhanced d-f hybridization [30], shedding light on the tuning of the ground state of the compound. For bulk SmB 6 , pressure-induced long-range magnetic ordering was reported in the metallic state above a critical pressure P c from nuclear forward scattering and specific-heat measurements [31], and the pressure-induced localization of 4f states was supported by 11 B-NMR results [32].…”

Study of gamma/X‐ray interaction in Kondo insulators

“…Kondo effect is an interesting phenomenon and is attracting scientific community. Previous researchers observed the Kondo effect in experiments …”