We measure transport through a Ga [Al]As heterostructure at temperatures between 32 mK and 30 K. Increasing the temperature enhances the electron-electron scattering rate and viscous effects in the two-dimensional electron gas arise. To probe this regime we measure so-called vicinity voltages and use a voltage-biased scanning tip to induce a movable local perturbation. We find that the scanning gate images differentiate reliably between the different regimes of electron transport. Our data are in good agreement with recent theories for interacting electron liquids in the ballistic and viscous regimes stimulated by measurements in graphene. However, the range of temperatures and densities where viscous effects are observable in Ga [Al]As are very distinct from the graphene material system. arXiv:1807.03177v3 [cond-mat.mes-hall]

We measure the magneto-conductance through a micron-sized quantum dot hosting about 500 electrons in the quantum Hall regime. In the Coulomb blockade, when the island is weakly coupled to source and drain contacts, edge reconstruction at filling factors between one and two in the dot leads to the formation of two compressible regions tunnel coupled via an incompressible region of filling factor ν = 1. We interpret the resulting conductance pattern in terms of a phase diagram of stable charge in the two compressible regions. Increasing the coupling of the dot to source and drain, we realize a Fabry-Pérot quantum Hall interferometer, which shows an interference pattern strikingly similar to the phase diagram in the Coulomb blockade regime. We interpret this experimental finding using an empirical model adapted from the Coulomb blockaded to the interferometer case. The model allows us to relate the observed abrupt jumps of the Fabry-Pérot interferometer phase to a change in the number of bulk quasiparticles. This opens up an avenue for the investigation of phase shifts due to (fractional) charge redistributions in future experiments on similar devices. :1910.12525v1 [cond-mat.mes-hall] arXiv

We present transport experiments performed in high quality quantum point contacts embedded in a GaAs two-dimensional hole gas. The strong spin-orbit interaction results in peculiar transport phenomena, including the previously observed anisotropic Zeeman splitting and level-dependent effective g-factors. Here we find additional effects, namely the crossing and the anti-crossing of spinsplit levels depending on subband index and magnetic field direction. Our experimental observations are reconciled in an heavy hole effective spin-orbit Hamiltonian where cubic-and quadratic-inmomentum terms appear. The spin-orbit components, being of great importance for quantum computing applications, are characterized in terms of magnitude and spin structure. In the light of our results, we explain the level dependent effective g-factor in an in-plane field. Through a tilted magnetic field analysis, we show that the QPC out-of-plane g-factor saturates around the predicted 7.2 bulk value.Spin-orbit interaction (SOI) is a relativistic effect that couples the motion of an electron to its spin [1]. For two-dimensional electron gases in the conduction band of III-V materials, SOI originates from bulk inversion asymmetry (Dresselhaus SOI [2]) and structure inversion asymmetry (Rashba SOI [3]) and takes the form, with σ the Pauli matrices and k the in-plane wavevector [4]. For two-dimensional hole gases (2DHGs) in the valence band of GaAs the situation is very different. Because of the non-zero orbital angular momentum, bulk SOI, and confinement in growth direction, SOI for holes is expected to be more pronounced than for their electronic counterparts, mainly of Rashba type and cubic in k [5,6]. The relevance of an additional term, quadratic in k and proportional to the in-plane components of the applied magnetic field B, was recently proposed [7][8][9][10]. Such a term is unique for heavy holes and very useful for exploiting SOI for quantum computing applications [7]. In this manuscript we show how the cubic and quadratic terms present in the bulk Hamiltonian can be separately addressed in the magnetoconductance of a quantum point contact (QPC) embedded in a 2DHG. Furthermore, our results offer a better understanding of the physics of ptype QPCs in terms of level dependent in-plane and outof-plane g-factors (g and g ⊥ respectively) and allow us to measure the bulk g ⊥ . The latter is particularly interesting, since the bulk g-factor anisotropy of p-type GaAs [11-13] makes it impossible to directly measure g ⊥ with conventional transport techniques [14]. Theoretical predictions for a [001]-growth 2DHG estimate g = 0 and g ⊥ = 7.2 [5,15]. It was argued [16] that in a QPC, in the limit of high subband index n, g ⊥ should approach the bulk value. So far, despite the tendency of g ⊥ to increase with n, this prediction was not experimentally confirmed.The experiment was performed using a carbon doped GaAs 2DHG grown along the [001] direction. A strong Rashba SOI is expected here due to the asymmetry of the confinement potential. A complete ...

The pattern of branched electron flow revealed by scanning gate microscopy shows the distribution of ballistic electron trajectories. The details of the pattern are determined by the correlated potential of remote dopants with an amplitude far below the Fermi energy. We find that the pattern persists even if the electron density is significantly reduced such that the change in Fermi energy exceeds the background potential amplitude. The branch pattern is robust against changes in charge carrier density, but not against changes in the background potential caused by additional illumination of the sample.

Storing, transmitting, and manipulating information using the electron spin resides at the heart of spintronics. Fundamental for future spintronics applications is the ability to control spin currents in solid state systems. Among the different platforms proposed so far, semiconductors with strong spin-orbit interaction are especially attractive as they promise fast and scalable spin control with all-electrical protocols. Here we demonstrate both the generation and measurement of pure spin currents in semiconductor nanostructures. Generation is purely electrical and mediated by the spin dynamics in materials with a strong spin-orbit field. Measurement is accomplished using a spin-to-charge conversion technique, based on the magnetic field symmetry of easily measurable electrical quantities. Calibrating the spin-to-charge conversion via the conductance of a quantum point contact, we quantitatively measure the mesoscopic spin Hall effect in a multiterminal GaAs dot. We report spin currents of 174 pA, corresponding to a spin Hall angle of 34%.The generation and detection of spin currents in nanostructures is the central challenge of semiconductor spintronics. On the one hand, spin injection cannot be easily achieved by coupling semiconductors to ferromagnets [1] because of the lack of control over material interfaces [2]. On the other hand, magnetoelectric alternatives exploiting the celebrated spin Hall effect (SHE) [3,4], have delivered only qualitative measurement protocols in transport experiments [5]. Alternatively to all-electrical setups, spin polarizing the current through a quantum point contact (QPC) with a magnetic field allows a quantitative control over spin current generation and detection at the nanoscale [6][7][8]. The latter approach typically requires such high magnetic fields (6 − 8 Tesla) that, as a drawback, the desired magnetoelectric effects are either suppressed or totally altered. This Letter reports two major advances of nanoscale semiconductor spintronics. Namely, we develop novel experimental methods to electrically generate and quantitatively measure spin currents in a two-dimensional semiconductor nanostructure.It is predicted that charge currents flowing through spin-orbit interaction (SOI)-coupled nanostructures are generically accompanied by spin currents, if the spinorbit time is shorter than the electron dwell time [9][10][11][12]. This spin current generation mechanism is purely electrical and based on the mesoscopic SHE (MSHE) [9,10], where the electronic orbital dynamics in chaotic nanostructures cooperates with the SOI to make transport spin dependent. We will consider an open three-terminal quantum dot as represented in Fig. 1(a), where each lead i is a QPC carrying N i spin degenerate modes. Running a charge current I between terminals 1 and 2, a spin current in all terminals, including 3, is expected due to the MSHE.For a weak SOI, the spin currents' amplitude fluctuates from sample to sample with zero average. For cavities with a strong SOI, geometric correlations between ...