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A quantum point contact (QPC) is a very basic nano-electronic device: a short and narrow transport channel between two electron reservoirs. In clean channels electron transport is ballistic and the conductance $G$ is then quantised as a function of channel width with plateaus at integer multiples of $2e^2/h$ ($e$ is the electron charge and $h$ Planck's constant). This can be understood in a picture where the electron states are propagating waves, without need to account for electron-electron interactions. Quantised conductance could thus be the signature of ultimate control over nanoscale electron transport. However, even studies with the cleanest QPCs generically show significant anomalies on the quantised conductance traces and there is consensus that these result from electron many-body effects. Despite extensive experimental and theoretical studies understanding of these anomalies is an open problem. We report evidence that the many-body effects have their origin in one or more spontaneously localised states that emerge from Friedel oscillations in the QPC channel. Kondo physics will then also contribute to the formation of the many-body state with Kondo signatures that reflect the parity of the number of localised states. Evidence comes from experiments with length-tunable QPCs that show a periodic modulation of the many-body physics with Kondo signatures of alternating parity. Our results are of importance for assessing the role of QPCs in more complex hybrid devices and proposals for spintronic and quantum information applications. In addition, our results show that tunable QPCs offer a rich platform for investigating many-body effects in nanoscale systems, with the ability to probe such physics at the level of a single site., Comment: NOTE: 59 page document. Pages 1-17 main text (with 4 figures); pages 18-45 supplementary information; pages 46-59 example of full data set

We present a numerical study of dephasing of electron spin ensembles in a diffusive quasi-one-dimensional GaAs wire due to the D'yakonov-Perel' spin-dephasing mechanism. For widths of the wire below the spin precession length and for equal strength of Rashba and linear Dresselhaus spin-orbit fields a strong suppression of spin-dephasing is found. This suppression of spin-dephasing shows a strong dependence on the wire orientation with respect to the crystal lattice. The relevance for realistic cases is evaluated by studying how this effect degrades for deviating strength of Rashba and linear Dresselhaus fields, and with the inclusion of the cubic Dresselhaus term.

Ohmic contacts to a two-dimensional electron gas (2DEG) in GaAs/AlGaAs heterostructures are often realized by annealing of AuGe/Ni/Au that is deposited on its surface. We studied how the quality of this type of ohmic contact depends on the annealing time and temperature, and how optimal parameters depend on the depth of the 2DEG below the surface. Combined with transmission electron microscopy and energy-dispersive X-ray spectrometry studies of the annealed contacts, our results allow for identifying the annealing mechanism and proposing a model that can predict optimal annealing parameters for a certain heterostructure., Comment: 9 pages, 4 figures

We present a numerical study of spin relaxation in a semiclassical electron ensemble in a large ballistic quantum dot. The dot is defined in a GaAs/AlGaAs heterojunction system with a two-dimensional electron gas, and relaxation occurs due to Dresselhaus and Rashba spin orbit interaction. We find that confinement in a micronscale dot can result in strongly enhanced relaxation with respect to a free two-dimensional electron ensemble, contrary to the established result that strong confinement or frequent momentum scattering reduces relaxation. This effect occurs when the size of the system is on the order of the spin precession length, but smaller than the mean free path., Comment: 5 pages, 3 figures

We report electronic control and measurement of an imbalance between spin-up and spin-down electrons in micron-scale open quantum dots. Spin injection and detection was achieved with quantum point contacts tuned to have spin-selective transport, with four contacts per dot for realizing a non-local spin-valve circuit. This provides an interesting system for studies of spintronic effects since the contacts to reservoirs can be controlled and characterized with high accuracy. We show how this can be used to extract in a single measurement the relaxation time for electron spins inside the dot ~ 300 ps and the degree of spin polarization of the contacts P ~ 0.8., Comment: 4 pages, 4 figures

The conductance of a quantum point contact (QPC) shows several features that result from many-body electron interactions. The spin degeneracy in zero magnetic field appears to be spontaneously lifted due to the so-called 0.7 anomaly. Further, the g-factor for electrons in the QPC is enhanced, and a zero-bias peak in the conductance points to similarities with transport through a Kondo impurity. We report here how these many-body effects depend on QPC geometry. We find a clear relation between the enhanced g-factor and the subband spacing in our QPCs, and can relate this to the device geometry with electrostatic modeling of the QPC potential. We also measured the zero-field energy splitting related to the 0.7 anomaly, and studied how it evolves into a splitting that is the sum of the Zeeman effect and a field-independent exchange contribution when applying a magnetic field. While this exchange contribution shows sample-to-sample fluctuations and no clear dependence on QPC geometry, it is for all QPCs correlated with the zero-field splitting of the 0.7 anomaly. This provides evidence that the splitting of the 0.7 anomaly is dominated by this field-independent exchange splitting. Signatures of the Kondo effect also show no regular dependence on QPC geometry, but are possibly correlated with splitting of the 0.7 anomaly., Comment: SpinTech IV contribution; new version contains additional wafer info and minor modifications to content

The spin degeneracy of the lowest subband that carries one-dimensional electron transport in quantum point contacts appears to be spontaneously lifted in zero magnetic field due to a phenomenon that is known as the 0.7 anomaly. We measured this energy splitting, and studied how it evolves into a splitting that is the sum of the Zeeman effect and a field-independent exchange contribution when applying a magnetic field. While this exchange contribution shows sample-to-sample fluctuations, it is for all QPCs correlated with the zero-field splitting of the 0.7 anomaly. This provides evidence that the splitting of the 0.7 anomaly is dominated by this field-independent exchange splitting., Comment: 5 pages, 4 figures

We investigate quantum fluctuations in the non-local resistance of an open quantum dot which is connected to four reservoirs via quantum point contacts. In this four-terminal quantum dot the voltage path can be separated from the current path. We measured non-local resistance fluctuations of several hundreds of Ohms, which have been characterized as a function of bias voltage, gate voltage and perpendicular magnetic field. The amplitude of the resistance fluctuations is strongly reduced when the coupling between the voltage probes and the dot is enhanced. Along with experimental results, we present a theoretical analysis based on the Landauer-B\"{u}ttiker formalism. While the theory predicts non-local resistance fluctuations of considerably larger amplitude than what has been observed, agreement with theory is very good if an additional dephasing mechanism is assumed.