We study spin transport in the one-and two-electron regimes of parallel-coupled double quantum dots (DQDs). The DQDs are formed in InAs nanowires by a combination of crystal-phase engineering and electrostatic gating, with an interdot tunnel coupling (t) tunable by one order of magnitude. Large single-particle energy separations (up to 10 meV) and |g * | factors (∼10) enable detailed studies of the B-field-induced transition from a singlet-to-triplet ground state as a function of t. In particular, we investigate how the magnitude of the spin-orbit-induced singlet-triplet anticrossing depends on t. For cases of strong coupling, we find values of 230 µeV for the anticrossing using excited-state spectroscopy. Experimental results are reproduced by calculations based on rate equations and a DQD model including a single orbital in each dot.

PACS number(s): 73.21. La, 73.63.Kv, 71.70.Ej, 73.23.Hk We use tunneling spectroscopy to study the evolution of few-electron spin states in parallel InAs nanowire double quantum dots (QDs) as a function of level detuning and applied magnetic field. Compared to the much more studied serial configuration, parallel coupling of the QDs to source and drain greatly expands the probing range of excited state transport.Owing to a strong confinement, we can here isolate transport involving only the very first interacting single QD orbital pair. For the (2,0) -(1,1) charge transition, with relevance for spin-based qubits, we investigate the excited (1,1) triplet, and hybridization of the (2,0) and(1,1) singlets. An applied magnetic field splits the (1,1) triplet, and due to spin-orbit induced mixing with the (2,0) singlet, we clearly resolve transport through all triplet states near the avoided singlet-triplet crossings. Transport calculations, based on a simple model with one orbital on each QD, fully replicate the experimental data. Finally, we observe an expected mirrored symmetry between the 1-2 and 2-3 electron transitions resulting from the two-fold spin degeneracy of the orbitals. I.interaction provides large, orbital-dependent |g|-factors in QDs [3][4][5][6][7], important to various qubit concepts building on manipulation of individual spins [8] or on Majorana states [9]. In most studies involving DQDs, the QDs are oriented serially, one after the other, relative to a source and drain contact. One reason for this focus was that many new materials were first synthesized into narrow, elongated objects, such as nanowires or nanotubes [4][5][6]10]. Another motivation, important for devices, is that the serial DQD configuration enables probing of spin-states through Pauli spin blockade [11][12].At zero bias, transport in serial DQDs only occurs when states in both QDs, tuned by local gates, align with the contact chemical potential at so-called triple degeneracy points. With increasing source-drain bias, these points evolve into triangular windows, where sequential tunneling through excited states is also possible. However, from the point-of-view of tunneling spectroscopy, the triple-points only provide small keyholes through which one can glimpse the full spectrum. By instead parallel-coupling the DQD to source and drain, it becomes possible to track states also far away from these points, and to decouple level detuning from the source-drain bias. Such a modification considerably expands the spectroscopic information that can be gained, and is the basis for the work presented here.The general approach we adopt in this study follows the pioneering works by Hatano et al., who used hybrid vertical-lateral GaAs DQDs, parallel-coupled with hard-wall barriers to source and drain, and with an inter-dot tunnel coupling tunable with side-gates [13][14][15]. There, the authors studied the evolution of various states as function of electron numbers, level detuning and inter-dot tunnel coupling. However, the spectroscopic resol...

No abstract

Quantum wells in InAs/GaSb heterostructures can be tuned to a topological regime associated with the quantum spin Hall effect, which arises due to an inverted band gap and hybridized electron and hole states. Here, we investigate electron-hole hybridization and the fate of the quantum spin Hall effect in a quasi one-dimensional geometry, realized in a core-shell-shell nanowire with an insulator core and InAs and GaSb shells. We calculate the band structure for an infinitely long nanowire using k • p theory within the Kane model and the envelope function approximation, then map the result onto a BHZ model which is used to investigate finite-length wires. Clearly, quantum spin Hall edge states cannot appear in the core-shell-shell nanowires which lack onedimensional edges, but in the inverted band-gap regime we find that the finite-length wires instead host localized states at the wire ends. These end states are not topologically protected, they are four-fold degenerate and split into two Kramers pairs in the presence of potential disorder along the axial direction. However, there is some remnant of the topological protection of the quantum spin Hall edge states in the sense that the end states are fully robust to (time-reversal preserving) angular disorder, as long as the bulk band gap is not closed.