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Andrea Hofmann

Publications and source records attributed to Andrea Hofmann.

27 records · Page 2Linked to original sources
arXiv2019

Quantum dot thermometry at ultra-low temperature in a dilution refrigerator with a $^4$He immersion cell

Experiments performed at a temperature of a few millikelvin require effective thermalization schemes, low-pass filtering of the measurement lines and low-noise electronics. Here, we report on the modifications to a commercial dilution refrigerator with a base temperature of 3.5 mK that enable us to lower the electron temperature to 6.7 mK measured from the Coulomb peak width of a quantum dot gate-defined in an [Al]GaAs heteostructure. We present the design and implementation of a liquid $^4$He immersion cell tight against superleaks, implement an innovative wiring technology and develop optimized transport measurement procedures.

cond-mat.mes-hall↗
arXiv2017

Anisotropy and suppression of spin--orbit interaction in a GaAs double quantum dot

The spin-flip tunneling rates are measured in GaAs-based double quantum dots by time-resolved charge detection. Such processes occur in the Pauli spin blockade regime with two electrons occupying the double quantum dot. Ways are presented for tuning the spin-flip tunneling rate, which on the one hand gives access to measuring the Rashba and Dresselhaus spin--orbit coefficents. On the other hand they make it possible to turn on and off the effect of SOI with a high on/off-ratio. The tuning is accomplished by choosing the alignment of the tunneling direction with respect to the crystallographic axes, as well as by choosing the orientation of the external magnetic field with respect to the spin--orbit magnetic field. Spin-lifetimes of 10 s are achieved at a tunnel rate close to 1 kHz.

cond-mat.mes-hall↗
arXiv2016

Heat dissipation and fluctuations in a driven quantum dot

While thermodynamics is a useful tool to describe the driving of large systems close to equilibrium, fluctuations dominate the distribution of heat and work in small systems and far from equilibrium. We study the heat generated by driving a small system and change the drive parameters to analyse the transition from a drive leaving the system close to equilibrium to driving it far from equilibrium. Our system is a quantum dot in a GaAs/AlGaAs heterostructure hosting a two-dimensional electron gas. The dot is tunnel-coupled to one part of the two-dimensional electron gas acting as a heat and particle reservoir. We use standard rate equations to model the driven dot-reservoir system and find excellent agreement with the experiment. Additionally, we quantify the fluctuations by experimentally test the theoretical concept of the arrow of time, predicting our ability to distinguish whether a process goes in the forward or backward drive direction.

cond-mat.mes-hall↗
arXiv2016

Measuring the degeneracy of discrete energy levels using a GaAs/AlGaAs quantum dot

We demonstrate an experimental method for measuring quantum state degeneracies in bound state energy spectra. The technique is based on the general principle of detailed balance, and the ability to perform precise and efficient measurements of energy-dependent tunnelling-in and -out rates from a reservoir. The method is realized using a GaAs/AlGaAs quantum dot allowing for the detection of time-resolved single-electron tunnelling with a precision enhanced by a feedback-control. It is thoroughly tested by tuning orbital and spin-degeneracies with electric and magnetic fields. The technique also lends itself for studying the connection between the ground state degeneracy and the lifetime of the excited states.

cond-mat.mes-hall↗
arXiv2016

Non-local polarization feedback in a fractional quantum Hall ferromagnet

In a quantum Hall ferromagnet, the spin polarization of the two-dimensional electron system can be dynamically transferred to nuclear spins in its vicinity through the hyperfine interaction. The resulting nuclear field typically acts back locally, modifying the local electronic Zeeman energy. Here we report a non-local effect arising from the interplay between nuclear polarization and the spatial structure of electronic domains in a $ν=2/3$ fractional quantum Hall state. In our experiments, we use a quantum point contact to locally control and probe the domain structure of different spin configurations emerging at the spin phase transition. Feedback between nuclear and electronic degrees of freedom gives rise to memristive behavior, where electronic transport through the quantum point contact depends on the history of current flow. We propose a model for this effect which suggests a novel route to studying edge states in fractional quantum Hall systems and may account for so-far unexplained oscillatory electronic-transport features observed in previous studies.

cond-mat.mes-hall↗
arXiv2015

Transport Spectroscopy of a Spin-Coherent Dot-Cavity System

Quantum engineering requires controllable artificial systems with quantum coherence exceeding the device size and operation time. This can be achieved with geometrically confined low-dimensional electronic structures embedded within ultraclean materials, with prominent examples being artificial atoms (quantum dots) and quantum corrals (electronic cavities). Combining the two structures, we implement a mesoscopic coupled dot-cavity system in a high-mobility two-dimensional electron gas, and obtain an extended spin-singlet state in the regime of strong dot-cavity coupling. Engineering such extended quantum states presents a viable route for nonlocal spin coupling that is applicable for quantum information processing.

cond-mat.mes-hall↗
arXiv2015

Equilibrium free energy measurement of a confined electron driven out of equilibrium

We study out-of equilibrium properties of a quantum dot in a GaAs/AlGaAs two-dimensional electron gas. By means of single electron counting experiments, we measure the distribution of work and dissipated heat of the driven quantum dot and relate these quantities to the equilibrium free energy change, as it has been proposed by C. Jarzynski [Phys. Rev. Lett. {\bf78}, 2690 (1997)]. We discuss the influence of the degeneracy of the quantized energy state on the free energy change as well as its relation to the tunnel rates between the dot and the reservoir.

cond-mat.mes-hall↗
arXiv2011

The ideal of curves of genus 2 on rational normal scrolls

Given a smooth curve of genus 2 embedded in P^(d-2) with a complete linear system of degree d>=6, we list all types of rational normal scrolls arising from linear systems g^1_2 and g^1_3 on C. Furthermore, we give a description of the ideal of such a curve of genus 2 embedded in P^(d-2) as a sum of the ideal of the two-dimensional scroll defined by the unique g^1_2 on C and the ideal of a three-dimensional scroll arising from a g^1_3 on C and not containing the scroll defined by the g^1_2 on C.

math.AG↗