Density dependence of the excitation gaps in an undoped Si/SiGe double-quantum-well heterostructure

TL;DR

This study investigates how the excitation gaps in an undoped Si/SiGe double quantum well heterostructure vary with electron density, revealing unique behaviors at different filling factors and evidence of spontaneous inter-layer coherence.

Contribution

It provides the first detailed analysis of density-dependent excitation gaps in an undoped Si/SiGe double quantum well, highlighting deviations from single-layer valley splitting and identifying conditions for spontaneous inter-layer coherence.

Findings

Observation of integer quantum Hall states at ν_T=1 and 2 in both density regimes.

Identification of non-standard behavior of the ν_T=1 gap compared to single-layer valley splitting.

Evidence of spontaneous inter-layer coherence at certain density imbalances.

Abstract

We report low-temperature magneto-transport measurements of an undoped Si/SiGe asymmetric double quantum well heterostructure. The density in both layers is tuned independently utilizing a top and a bottom gate, allowing the investigation of quantum wells at both imbalanced and matched densities. Integer quantum Hall states at total filling factor $\nu_{\text{T}} = 1$ and $\nu_{\text{T}} = 2$ are observed in both density regimes, and the evolution of their excitation gaps is reported as a function of density. The $\nu_{\text{T}} = 1$ gap evolution departs from the behavior generally observed for valley splitting in the single layer regime. Furthermore, by comparing the $\nu_{\text{T}} = 2$ gap to the single particle tunneling energy, $\Delta_{\text{SAS}}$, obtained from Schr\"{o}dinger-Poisson (SP) simulations, evidence for the onset of spontaneous inter-layer coherence (SIC) is observed for a relative filling fraction imbalance smaller than ${\sim}50\%$