# Understanding low-temperature bulk transport in samarium hexaboride without relying on in-gap bulk states

## TL;DR

This paper introduces a model explaining the discrepancy between transport and spectroscopy gaps in SmB$_6$ by considering it as an intrinsic semiconductor with a diverging depletion length at low temperatures, accounting for observed transport phenomena.

## Contribution

The paper presents a self-consistent bulk band bending model for SmB$_6$, linking surface and bulk transport properties without relying on in-gap bulk states.

## Key findings

- Good agreement with transport data from 4-40 K
- Explains the 10-12 K crossover in transport behavior
- Accounts for the difference between spectroscopic and transport gaps

## Abstract

We present a new model to explain the difference between the transport and spectroscopy gaps in samarium hexaboride (SmB$_6$), which has been a mystery for some time. We propose that SmB$_6$ can be modeled as an intrinsic semiconductor with a depletion length that diverges at cryogenic temperatures. In this model, we find a self-consistent solution to Poisson's equation in the bulk, with boundary conditions based on Fermi energy pinning due to surface charges. The solution yields band bending in the bulk; this explains the difference between the two gaps because spectroscopic methods measure the gap near the surface, while transport measures the average over the bulk. We also connect the model to transport parameters, including the Hall coefficient and thermopower, using semiclassical transport theory. The divergence of the depletion length additionally explains the 10-12 K feature in data for these parameters, demonstrating a crossover from bulk dominated transport above this temperature to surface-dominated transport below this temperature. We find good agreement between our model and a collection of transport data from 4-40 K. This model can also be generalized to materials with similar band structure.