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

Abstract

We perform transport measurements on double quantum dots defined in Ge/Si core/shell nanowires and focus on Pauli spin blockade in the regime where tens of holes occupy each dot. We identify spin blockade through the magnetic field dependence of leakage current. We find both a dip and a peak in the leakage current at zero field. We analyze this behavior in terms of the quantum dot parameters such as coupling to the leads, interdot tunnel coupling as well as spin-orbit interaction. We find a lower bound for spin-orbit interaction with lso = 500 nm. We also extract large and anisotropic effective Landé g-factors, with larger g-factors in the direction perpendicular to the nanowire axis in agreement with previous studies and experiments but with larger values reported here.Studies of spin blockade in quantum dots are largely motivated by the proposals to build a spin-based quantum computer 1 , as spin blockade can be used for qubit initialization and readout 2,3 . At the same time, spin blockade and its lifting mechanisms offer a direct insight into spin relaxation and dephasing processes in semiconductors and provide deeper understanding of interactions between spin localized in a quantum dot and its environment, be it the lattice and its vibrations or nuclear spins, spin-orbit interaction, or coupling to spins in nearby dots or in the lead reservoirs 4-8 .Holes in Ge/Si nanowires offer a relatively unexplored platform for such studies 9 . On the one hand, hyperfine interaction is expected to be greatly reduced owing to the low abundance of nonzero nuclear spin isotopes in the group IV materials 10 . Moreover, holes weakly couple to nuclear spins due to their p-wave Bloch wave symmetry, thus they are expected to come with longer spin relaxation times 11 . 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 14-17 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] .In this work we perform transport measurements on electrostatically defined double quantum dots 2 made in Ge/Si core/shell nanowires, and detect Pauli spin blockade at several charge degeneracy points. We expand and adapt a previously developed rate equation model to analyze the magnetic-field evolution of the leakage current 24 . We also observe large and anisotropic g-factors in these dots, which supports recent theoretical predictions 25 and experimental observations 26,27 .The devices are fabricated on n-doped Si substrates covered with 500 nm of thermal SiO 2 and patterned with local gate arrays of Ti/Au stripes with center to center distance of 60 nm. The gates are covered by a 10 nm layer of HfO 2 dielectric. Using a micromanipulator 28 the nanowires with a typical length of 4 µm and diameter of 30 nm are placed on top of these gates as shown in the inset of Fig. 1. After wet etching with buffered hydrofluoric acid, we sputter 15...

A germanium hole spin qubit

“…In order to realize a spin 3/2 qubit in the DQD devices we rely on PSB as a spin-selective read-out mechanism 25 , 26 . PSB occurs in a (1, 1) → (2, 0) or an equivalent (2N−1, 2N−1) → (2 N, 2N−2) charge configuration (Fig.…”

The germanium quantum information route

“…This large anisotropy follows from an almost vanishing g-factor (g * min = 0.2) along the NW axis. Pauli spin blockade can elucidate the spin-flip mechanisms through the behaviour of the leakage current as a function of magnetic field 193,199,200 .…”

Transient effects in double quantum dot sandwiched laterally between superconducting and metallic leads

“…Since double quantum dot (DQD) configurations provide a versatile platform for the implementation of quantum information processing [10,11], such systems have also been considered in hybrid setups involving superconducting elements. Experimentally, their bound states have been probed by the tunneling spectroscopy, using InAs [12][13][14][15][16][17], InSb [18], Ge/Si [19] quantum dots or carbon nanotubes [20,21] contacted with superconducting lead(s), as well as by the scanning tunneling microscopy (STM) applied to the magnetic dimers deposited on su-perconducting substrates [6,[22][23][24]. The single V, Cr, Mn, Fe, and Co atoms deposited on aluminum have revealed that Cr and Mn atoms have contributions from different orbitals to subgap quasiparticles, whereas the other elements merely consist of one pair of the in-gap bound states [25].…”