arXiv subjects

D. M. Zumbühl

Publications and source records attributed to D. M. Zumbühl.

25 records · Page 2Linked to original sources

arXiv2014

Electrical spin protection and manipulation via gate-locked spin-orbit fields

The spin-orbit (SO) interaction couples electron spin and momentum via a relativistic, effective magnetic field. While conveniently facilitating coherent spin manipulation in semiconductors, the SO interaction also inherently causes spin relaxation. A unique situation arises when the Rashba and Dresselhaus SO fields are matched, strongly protecting spins from relaxation, as recently demonstrated. Quantum computation and spintronics devices such as the paradigmatic spin transistor could vastly benefit if such spin protection could be expanded from a single point into a broad range accessible with in-situ gate-control, making possible tunable SO rotations under protection from relaxation. Here, we demonstrate broad, independent control of all relevant SO fields in GaAs quantum wells, allowing us to tune the Rashba and Dresselhaus SO fields while keeping both locked to each other using gate voltages. Thus, we can electrically control and simultaneously protect the spin. Our experiments employ quantum interference corrections to electrical conductivity as a sensitive probe of SO coupling. Finally, we combine transport data with numerical SO simulations to precisely quantify all SO terms.

cond-mat.mes-hall↗

arXiv2014

GaAs Quantum Dot Thermometry Using Direct Transport and Charge Sensing

We present measurements of the electron temperature using gate defined quantum dots formed in a GaAs 2D electron gas in both direct transport and charge sensing mode. Decent agreement with the refrigerator temperature was observed over a broad range of temperatures down to 10 mK. Upon cooling nuclear demagnetization stages integrated into the sample wires below 1 mK, the device electron temperature saturates, remaining close to 10 mK. The extreme sensitivity of the thermometer to its environment as well as electronic noise complicates temperature measurements but could potentially provide further insight into the device characteristics. We discuss thermal coupling mechanisms, address possible reasons for the temperature saturation and delineate the prospects of further reducing the device electron temperature.

cond-mat.mes-hall↗

arXiv2013

Transport spectroscopy of disordered graphene quantum dots etched into a single graphene flake

We present transport measurements on quantum dots of sizes 45, 60 and 80 nm etched with an Ar/O2-plasma into a single graphene sheet, allowing a size comparison avoiding effects from different graphene flakes. The transport gaps and addition energies increase with decreasing dot size, as expected, and display a strong correlation, suggesting the same physical origin for both, i.e. disorder-induced localization in presence of a small confinement gap. Gate capacitance measurements indicate that the dot charges are located in the narrow device region as intended. A dominant role of disorder is further substantiated by the gate dependence and the magnetic field behavior, allowing only approximate identification of the electron-hole crossover and spin filling sequences. Finally, we extract a g-factor consistent with g=2 within the error bars.

cond-mat.mes-hall↗

arXiv2013

Evidence for Helical Nuclear Spin Order in GaAs Quantum Wires

We present transport measurements of cleaved edge overgrowth GaAs quantum wires. The conductance of the first mode reaches 2 e^2/h at high temperatures T > 10 K, as expected. As T is lowered, the conductance is gradually reduced to 1 e^2/h, becoming T-independent at T < 0.1 K, while the device cools far below 0.1 K. This behavior is seen in several wires, is independent of density, and not altered by moderate magnetic fields B. The conductance reduction by a factor of two suggests lifting of the electron spin degeneracy in absence of B. Our results are consistent with theoretical predictions for helical nuclear magnetism in the Luttinger liquid regime.

cond-mat.mes-hall↗

arXiv2012

Breakdown of the Korringa Law of Nuclear Spin Relaxation in Metallic GaAs

We present nuclear spin relaxation measurements in GaAs epilayers using a new pump-probe technique in all-electrical, lateral spin-valve devices. The measured T1 times agree very well with NMR data available for T > 1 K. However, the nuclear spin relaxation rate clearly deviates from the well-established Korringa law expected in metallic samples and follows a sub-linear temperature dependence 1/T1 ~ T^0.6 for 0.1 K < T < 10 K. Further, we investigate nuclear spin inhomogeneities.

cond-mat.mes-hall↗

arXiv2011

The Nuclear Spin Environment in Lateral GaAs Spin Valves

The spin degree of freedom in solids offers opportunities beyond charge-based electronics and is actively investigated for both spintronics and quantum computation. However, the interplay of these spins with their native environment can give rise to detrimental effects such as spin relaxation and decoherence. Here, we use an all-electrical, lateral GaAs spin valve to manipulate and investigate the inherent nuclear spin system. Hanle satellites are used to determine the nuclear spin relaxation rates for the previously unexplored temperature range down to 100 mK, giving T1 times as long as 3 hours. Despite metallic temperature dependence of resistivity, the observed relaxation rates show a sub-linear temperature dependence. This contrasts the Korringa relaxation mechanism observed in metals but is not inconsistent with hyperfine-mediated relaxation in a disordered, interacting conductor not far from the metal-insulator transition. We discuss possible relaxation mechanisms and further investigate inhomogeneities in the nuclear spin polarization.

cond-mat.mes-hall↗

arXiv2010

Method for Cooling Nanostructures to Microkelvin Temperatures

We propose a new scheme aimed at cooling nanostructures to microkelvin temperatures, based on the well established technique of adiabatic nuclear demagnetization: we attach each device measurement lead to an individual nuclear refrigerator, allowing efficient thermal contact to a microkelvin bath. On a prototype consisting of a parallel network of nuclear refrigerators, temperatures of $\sim 1\,$mK simultaneously on ten measurement leads have been reached upon demagnetization, thus completing the first steps toward ultracold nanostructures.

cond-mat.mes-hall↗