Exact Multi-Valley Envelope Function Theory of Valley Splitting in Si/SiGe Nanostructures

TL;DR

This paper develops an exact multi-valley envelope-function model for Si/SiGe nanostructures that overcomes limitations of traditional theories, accurately capturing valley splitting with non-local potentials and invariance properties.

Contribution

It introduces a non-local envelope-function theory combining Burt-Foreman approach with valley decomposition, ensuring invariance and addressing sharp interface effects.

Findings

The non-local model enforces band-limited envelopes and satisfies invariance.

Conventional local models violate energy-reference invariance due to spectral leakage.

A spectrally filtered local approximation closely matches the exact non-local theory.

Abstract

Valley splitting in strained Si/SiGe quantum wells is a central parameter for silicon spin qubits and is commonly described with envelope-function and effective-mass theories. These models provide a computationally efficient continuum description and have been shown to agree well with atomistic approaches when the confinement potential is slowly varying on the lattice scale. In modern Si/SiGe heterostructures with atomically sharp interfaces and engineered Ge concentration profiles, however, the slowly varying potential approximation underlying conventional (local) envelope-function theory is challenged. We formulate an exact multi-valley envelope-function model by combining Burt-Foreman-type envelope-function theory, which does not rely on the assumption of a slowly varying potential, with a valley-sector decomposition of the Brillouin zone. This construction enforces band-limited envelopes, which satisfy a set of coupled integro-differential equations with a non-local potential energy operator. Using degenerate perturbation theory, we derive the intervalley coupling matrix element within this non-local model and prove that it is strictly invariant under global shifts of the confinement potential (choice of reference energy). We then show that the conventional local envelope model generically violates this invariance due to spectral leakage between valley sectors, leading to an unphysical energy-reference dependence of the intervalley coupling. The resulting ambiguity is quantified by numerical simulations of various engineered Si/SiGe heterostructures. Finally, we propose a simple spectrally filtered local approximation that restores the energy-reference invariance exactly and provides a good approximation to the exact non-local theory.