Valley Splittings in Si/SiGe Heterostructures from First Principles

This paper computes valley splittings in Si/SiGe superlattices using ab initio density functional theory (DFT), which provides an excellent description of interfaces, strains, and atomistic disorder. It also benchmarks atomistic tight-binding and "2k_0" theory against DFT, revealing limitations of semi-empirical methods in describing atomistic disorder.

The paper addresses a critical bottleneck in silicon spin qubit fidelity: the magnitude and controllability of valley splitting ($E_{VS}$) in Si/SiGe heterostructures. By employing first-principles calculations (Density Functional Theory - DFT), the authors move beyond the limitations of Effective Mass Theory (EMT), which often fails to capture atomistic interface effects accurately. The study provides a rigorous analysis of how interface sharpness, steps, and Ge concentration influence the valley phase and magnitude. However, the study faces limitations regarding computational scalability; modeling realistic, long-range disorder using full DFT is computationally prohibitive, requiring relatively small supercells that may not perfectly represent macroscopic device variations. Furthermore, while the theoretical predictions are robust, the direct correlation with experimental data often suffers from the inability to characterize buried interfaces with atomic precision in real devices.