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 lighthole-, heavy-hole-and split-off-like branches with two-dimensional dispersion, effective masses enhanced by a factor of about 3.5 with respect to bulk Ge counterpart, 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 straightforward fabrication and CMOS compatibility, establishes Pt/Ge(111) as a simple and versatile platform for exploring correlated phenomena and implementing spintronic and quantum devices.