No abstract

We study the Zeeman spin-splitting in hole quantum wires oriented along the [011] and [011] crystallographic axes of a high mobility undoped (100)-oriented AlGaAs/GaAs heterostructure. Our data shows that the spin-splitting can be switched 'on' (finite g * ) or 'off' (zero g * ) by rotating the field from a parallel to a perpendicular orientation with respect to the wire, and the properties of the wire are identical for the two orientations with respect to the crystallographic axes. We also find that the g-factor in the parallel orientation decreases as the wire is narrowed. This is in contrast to electron quantum wires, where the g-factor is enhanced by exchange effects as the wire is narrowed. This is evidence for a k-dependent Zeeman splitting that arises from the spin-3 2 nature of holes.

Electrically defined semiconductor quantum dots are attractive systems for spin manipulation and quantum information processing. Heavy-holes in both Si and GaAs are promising candidates for all-electrical spin manipulation, owing to the weak hyperfine interaction and strong spin-orbit interaction. However, it has only recently become possible to make stable quantum dots in these systems, mainly due to difficulties in device fabrication and stability.Here we present electrical transport measurements on holes in a gate-defined double quantum dot in a GaAs/Al x Ga 1−x As heterostructure. We observe clear Pauli spin blockade and demonstrate that the lifting of this spin blockade by an external magnetic field is highly anisotropic.

We have fabricated and studied a ballistic one-dimensional p-type quantum wire using an undoped AlGaAs/GaAs heterostructure. The absence of modulation doping eliminates remote ionized impurity scattering and allows high mobilities to be achieved over a wide range of hole densities, and in particular, at very low densities where carrier-carrier interactions are strongest. The device exhibits clear quantized conductance plateaus with highly stable gate characteristics. These devices provide opportunities for studying spin-orbit coupling and interaction effects in mesoscopic hole systems in the strong interaction regime where r s > 10.

We have studied ballistic transport in a 1D channel formed using surface gate techniques on a back-gated, high-mobility, bilayer 2D hole system. At millikelvin temperatures, robust conductance quantization is observed in the quantum wire formed in the top layer of the bilayer system, without the gate instabilities that have hampered previous studies of 1D hole systems.Using source drain bias spectroscopy, we have measured the 1D subband spacings, which are 5-10 times smaller than in comparable GaAs electron systems, but 2-3 times larger than in previous studies of 1D holes. We also report the first observation of the anomalous conductance plateau at G = 0.7 × 2e 2 /h in a 1D hole system.

We report the observation of Kondo physics in a spin-3 2 hole quantum dot. The dot is formed close to pinch-off in a hole quantum wire defined in an undoped AlGaAs/GaAs heterostructure. We clearly observe two distinctive hallmarks of quantum dot Kondo physics. First, the Zeeman spin-splitting of the zero-bias peak in the differential conductance is independent of gate voltage. Second, this splitting is twice as large as the splitting for the lowest one-dimensional subband. We show that the Zeeman splitting of the zero-bias peak is highly-anisotropic, and attribute this to the strong spin-orbit interaction for holes in GaAs.PACS numbers: 72.15. Qm, 75.70.Tj The observation of an unexpected minimum in the low temperature resistance of metals by de Haas in 1933 was ultimately explained thirty years later by Kondo as being due to interactions between a single magnetic impurity and the sea of conduction electrons in a metal [1,2]. More recently there has been a resurgence of interest in the Kondo effect, following the discovery that the conductance of a few electron quantum dot in the Coulomb blockade regime is enhanced when the dot contains an odd number of electrons [3][4][5]. There is a direct analogy with the Kondo effect in metals, with the localized electron in the quantum dot acting as a magnetic impurity that interacts with the two-dimensional sea of electrons in the source and drain reservoirs.Studies of the Kondo effect in bulk systems have progressed since the 1960s, with the focus shifting towards manifestations of Kondo physics in the strongly correlated electron systems formed in cuprates and heavyfermion metals [6]. More precise control via improved electrostatic gate design has similarly allowed progress towards the study of more exotic manifestations of Kondo phenomena in quantum dots such as the integer-spin [7][8][9], two-impurity [10], and orbital Kondo effects [11]. Thus far all quantum dot Kondo studies have involved electrons, and GaAs hole quantum dots present an interesting next step. Holes in GaAs originate from p-like orbitals and behave as spin-3 2 particles due to strong spinorbit coupling [12]. In two-and one-dimensional systems, the spin-3 2 nature of holes leads to remarkable, highlyanisotropic phenomena [13-17] not observed in electron systems, and new physics is expected for hole quantum dots also [18]. Studies of Kondo physics in hole quantum dots may also provide useful connections to recent studies in bulk strongly correlated systems [19,20].Here we report the observation of the Kondo effect in a GaAs hole quantum dot. Due to the poor stability of conventional gate-defined modulation doped struc- * klochan@phys.unsw.edu. A key advantage to this approach is the ability to obtain an independent estimate of the effective Landé g-factor g * . Using this we have fabricated a small hole quantum dot and conclusively demonstrate the "smoking gun" for Kondo physics [23] -a splitting of the zero-bias peak in the differential conductance that opens as 2g * µ B B in response to an in-plane...

We have fabricated AlGaAs/GaAs heterostructure devices in which the conduction channel can be populated with either electrons or holes simply by changing the polarity of a gate bias. The heterostructures are entirely undoped, and carriers are instead induced electrostatically. We use these devices to perform a direct comparison of the scattering mechanisms of two-dimensional (2D) electrons (µ peak = 4×10 6 cm 2 /Vs) and holes (µ peak = 0.8 × 106 cm 2 /Vs) in the same conduction channel with nominally identical disorder potentials. We find significant discrepancies between electron and hole scattering, with the hole mobility being considerably lower than expected from simple theory.Modulation-doped Al x Ga 1−x As/GaAs heterostructures have formed the starting point for innumerable studies of low dimensional electron and hole systems. 1In modulation doped heterostructures the spatial separation of the dopants and the channels significantly reduces scattering, so that very high electron or hole mobilities can be achieved at low temperatures.2 However, the type of charge carrier is determined by the dopants used during the heterostructure growth, which makes it very difficult to fabricate ambipolar devices that can operate with both electron and hole conduction. The ability to switch seamlessly between electrons and holes in the same device would be of interest for studies of scattering, interaction effects and spin related phenomena, since the two types of charge carriers have very different effective masses, bandstructures and spin properties.To create ambipolar devices we eschew conventional modulation doping techniques, using instead a gate electrode to populate the channel electrostatically. The challenge with this approach is to make good electrical contact to the 2D electrons or holes in the channel without forming an unwanted contact to the gate electrode which must overlap the ohmic contact. Hirayama et al. 3,4used ion-implantation to overcome this problem and fabricate ambipolar devices on AlGaAs/GaAs heterostructures. Thick high Al content AlGaAs diffusion barriers were used to suppress leakage between the top gate and the ohmic contacts. However, a high temperature anneal (∼ 800• C) is required after the ion implantation to activate the dopants, which is higher than the wafer growth temperature and may have adverse effects on the heterostructure and the carrier mobility. Here we describe ambipolar devices fabricated without the need for ion implantation and subsequent dopant activation anneals. With this device design we are able to make high quality ambipolar 2D systems, and compare the transport lifetime of electrons and holes formed in the same channel, with the same scattering potential.We have fabricated devices from a number of different undoped heterostructures, extending the approach described in Refs. 5 and 6 for making unipolar devices. Ohmic contacts are fabricated by standard optical lithography techniques, so there is no need for ion implantation. Here we present data from wafer B13520 grown

We study the Zeeman splitting in induced ballistic 1D quantum wires aligned along the [233] and [011] axes of a high mobility (311)A undoped heterostructure. Our data shows that the g-factor anisotropy for magnetic fields applied along the high symmetry [011] direction can be explained by the 1D confinement only. However when the magnetic field is along [233] there is an interplay between the 1D confinement and 2D crystal anisotropy. This is highlighted for the [233] wire by an unusual non-monotonic behavior of the g-factor as the wire is made narrower.