Energy gap of intermediate-valentSmB6studied by point-contact spectroscopy

Abstract

We have investigated the intermediate valence narrow-gap semiconductor ${\mathrm{SmB}}_{6}$ at low temperatures using both conventional spear-anvil type point contacts as well as mechanically controllable break junctions. The zero-bias conductance varied between less than $0.01 \ensuremath{\mu}\mathrm{S}$ and up to 1 mS. The position of the spectral anomalies, which are related to the different activation energies and band gaps of ${\mathrm{SmB}}_{6},$ did not depend on the the contact size. Two different regimes of charge transport could be distinguished: Contacts with large zero-bias conductance are in the diffusive Maxwell regime. They had spectra with only small nonlinearities. Contacts with small zero-bias conductance are in the tunneling regime. They had larger anomalies, but still indicating a finite $45%$ residual quasiparticle density of states at the Fermi level at low temperatures of $T=0.1 \mathrm{K}.$ The density of states derived from the tuneling spectra can be decomposed into two energy-dependent parts with ${E}_{g}=21 \mathrm{meV}$ and ${E}_{d}=4.5 \mathrm{meV}$ wide gaps, respectively.