Spatially inhomogeneous electron state deep in the extreme quantum limit of strontium titanate

TL;DR

This study investigates the behavior of lightly-doped strontium titanate in the extreme quantum limit, revealing inhomogeneous electron states and nonlinear transport phenomena under high magnetic fields.

Contribution

It provides experimental evidence of electron inhomogeneity and nonlinear transport in the EQL regime of strontium titanate, supporting theories of correlated electron states and puddling effects.

Findings

Observation of re-entrant nonlinear I-V characteristics

Saturation of quantum-limiting field at low carrier density

Evidence of inhomogeneous electron states in high magnetic fields

Abstract

When an electronic system is subjected to a sufficiently strong magnetic field that the cyclotron energy is much larger than the Fermi energy, the system enters the "extreme quantum limit" (EQL) and becomes susceptible to a number of instabilities. Bringing a three-dimensional electronic system deeply into the EQL can be difficult, however, since it requires a small Fermi energy, large magnetic field, and low disorder. Here we present an experimental study of the EQL in lightly-doped single crystals of strontium titanate, which remain good bulk conductors down to very low temperatures and high magnetic fields. Our experiments probe deeply into the regime where theory has long predicted electron-electron interactions to drive the system into a charge density wave or Wigner crystal state. A number of interesting features arise in the transport in this regime, including a striking re-entrant nonlinearity in the current-voltage characteristics and a saturation of the quantum-limiting field at low carrier density. We discuss these features in the context of possible correlated electron states, and present an alternative picture based on magnetic-field induced puddling of electrons.