Air-Stable Subsurface Two-Dimensional Hole Gas with Strong Spin-Orbit Interaction in Single Layer Pt on Ge.

Two-dimensional hole gases (2DHGs) offer enhanced spin-orbit coupling and correlations, making them attractive for spintronic and quantum devices. Yet, most realizations require complex epitaxial structures and protective encapsulation, limiting their applicability. Here, we show that a stable 2DHG emerges in Ge(111) upon deposition of a single monolayer of Pt. Angle-resolved photoemission spectroscopy reveals multiple Ge-derived hole subbands, including light-hole-, heavy-hole-, and split-off-like branches with two-dimensional dispersion, effective masses enhanced by a factor of about 3.5 compared to bulk Ge, and a spin-orbit splitting of about 0.3 eV. Remarkably, the subsurface hole bands persist after air exposure. Transport measurements confirm conductive behavior and hole-type carriers down to cryogenic temperatures. The coexistence of strong spin-orbit interaction and exceptional ambient robustness, combined with CMOS compatibility and without requiring complex heterostructures, gating, or encapsulation, establish Pt/Ge(111) as a versatile platform for correlated phenomena and spintronic and quantum devices.