Spin States of Holes in<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mi>Ge</mml:mi><mml:mo>/</mml:mo><mml:mi>Si</mml:mi></mml:math>Nanowire Quantum Dots

We investigate tunable hole quantum dots defined by surface gating $\mathrm{Ge}/\mathrm{Si}$ core-shell nanowire heterostructures. In single level Coulomb-blockade transport measurements at low temperatures spin doublets are found, which become sequentially filled by holes. Magnetotransport measurements allow us to extract a $g$ factor ${g}^{*}\ensuremath{\approx}2$ close to the value of a free spin-$1/2$ particle in the case of the smallest dot. In less confined quantum dots smaller $g$ factor values are observed. This indicates a lifting of the expected strong spin-orbit interaction effects in the valence band for holes confined in small enough quantum dots. By comparing the excitation spectrum with the addition spectrum we tentatively identify a hole exchange interaction strength $\ensuremath{\chi}\ensuremath{\approx}130\text{ }\text{ }\ensuremath{\mu}\mathrm{eV}$.