Strong spin-orbit interaction and helical hole states in Ge/Si nanowires

Abstract

We study theoretically the low-energy hole states of Ge/Si core/shell nanowires. The low-energy valence band is quasi-degenerate, formed by two doublets of different orbital angular momentum, and can be controlled via the relative shell thickness and via external fields. We find that direct (dipolar) coupling to a moderate electric field leads to an unusually large spin-orbit interaction of Rashba-type on the order of meV which gives rise to pronounced helical states enabling electrical spin-control. The system allows for quantum dots and spin-qubits with energy levels that can vary from nearly zero to several meV, depending on the relative shell thickness.

Magnetic field evolution of spin blockade in Ge/Si nanowire double quantum dots

“…While this corresponds to a substantial spin-orbit interaction, it does not greatly exceed that measured in InAs or InSb nanowires, contrary to some expectations 13 . One possibility for this could be that α is not maximal for the field orientation at which data is obtained here as a consequence of spin-orbit anisotropy 30 , although magnetic field was not oriented in the direction expected for the spin-orbit field.…”

“…Heavy/light hole degeneracy may also influence the spin blockade regime 12 . On the other hand, spin-orbit interaction is predicted 13 and suggested by experiments [14][15][16][17] to be strong in Ge/Si core/shell nanowires. This offers a path to electrical spin manipulation 18,19 , as well as to realizing Majorana fermions [20][21][22][23] .…”

“…3 (see below), we include the likely scenario that g-factors in the two dots are significantly different. The effective g-factor for a localized hole depends on many microscopic characteristics, among which the details of the confining potential 13 , and is thus expected to differ from dot to dot. Based on the data shown in Fig.…”

Circuit quantum electrodynamics with a spin qubit

“…First, dynamical decoupling can be used to reduce the qubit decay rate to ∼ 1 MHz in the InAs system [13]. InAs could also be replaced by Ge/Si core/shell nanowires where hole spin-orbit coupling is predicted to be large [31]. Resonators can also be coupled to nuclear-spin-free Si/SiGe quantum dots by using micromagnets to create artificial spin-orbit fields [32].…”

Hole weak anti-localization in a strained-Ge surface quantum well

“…They are expected to display a relatively small in-plane effective mass [3,4], favoring lateral confinement, as well as long spin coherence times [5], stemming from a reduced hyperfine coupling (natural Ge is predominantly constituted of isotopes with zero nuclear spin and holes are less coupled to nuclear spins due to the p-wave symmetry of their Bloch states [6]). In addition, low-dimensional, SiGe-based structures benefit from a strong and electrically tunable spin orbit coupling [4,[7][8][9][10][11]. This property could be exploited for purely electrical spin control [5,[12][13][14].…”