arXiv subjects

G. C. Gardner

Publications and source records attributed to G. C. Gardner.

41 records · Page 3Linked to original sources

arXiv2015

Shubnikov-de Haas oscillations in two-dimensional electron gas under subterahertz radiation

We report on magnetotransport measurements in a two-dimensional (2D) electron gas subject to subterahertz radiation in the regime where Shubnikov-de Haas oscillations (SdHO) and microwave-induced resistance oscillations (MIRO) coexist over a wide magnetic field range, spanning several harmonics of the cyclotron resonance. Surprisingly, we find that the SdHO amplitude is modified by the radiation in a non-trivial way owing to the oscillatory correction which has the same period and phase as MIRO. This finding challenges our current understanding of microwave photoresistance in 2D electron gas, calling for future investigations.

cond-mat.mes-hall↗

arXiv2015

Gapped Excitations of unconventional FQHE states in the Second Landau Level

We report the observation of low-lying collective charge and spin excitations in the second Landau level at ν = 2 + 1/3 and also for the very fragile states at ν = 2 + 2/5, 2 + 3/8 in inelastic light scattering experiments. These modes exhibit a clear dependence on filling factor and temperature substantiating the unique access to the characteristic neutral excitation spectra of the incompressible FQHE states. A detailed mode analysis reveals low energy modes at around 70 μeV and a sharp mode slightly below the Zeeman energy interpreted as gap and spin wave excitation, respectively. The lowest energy collective charge excitation spectrum at ν = 2 + 1/3 exhibits significant similarities and a universal scaling of the energies with its cousin state in the lowest Landau level at ν = 1/3 suggesting similar underlying physics. The observed excitation spectra facilitate to distinguish between theoretical descriptions of the nature of those FQHE states. A striking polarization dependence in light scattering is discussed in the framework of anisotropic electron phases that allow for the stabilization of nematic FQHE states.

cond-mat.mes-hall↗

arXiv2015

Observation of a transition from a topologically ordered to a spontaneously broken symmetry phase

Until the late 1980s, phases of matter were understood in terms of Landau's symmetry breaking theory. Following the discovery of the quantum Hall effect the introduction of a second class of phases, those with topological order, was necessary. Phase transitions within the first class of phases involve a change in symmetry, whereas those between topological phases require a change in topological order. However, in rare cases transitions may occur between the two classes in which the vanishing of the topological order is accompanied by the emergence of a broken symmetry. Here, we report the existence of such a transition in a two-dimensional electron gas hosted by a GaAs/AlGaAs crystal. When tuned by hydrostatic pressure, the $ν=5/2$ fractional quantum Hall state, believed to be a prototype non-Abelian topological phase, gives way to a quantum Hall nematic phase. Remarkably, this nematic phase develops spontaneously, in the absence of any externally applied symmetry breaking fields.

cond-mat.str-el↗

arXiv2014

Cryogenic Control Architecture for Large-Scale Quantum Computing

Solid-state qubits have recently advanced to the level that enables them, in-principle, to be scaled-up into fault-tolerant quantum computers. As these physical qubits continue to advance, meeting the challenge of realising a quantum machine will also require the engineering of new classical hardware and control architectures with complexity far beyond the systems used in today's few-qubit experiments. Here, we report a micro-architecture for controlling and reading out qubits during the execution of a quantum algorithm such as an error correcting code. We demonstrate the basic principles of this architecture in a configuration that distributes components of the control system across different temperature stages of a dilution refrigerator, as determined by the available cooling power. The combined setup includes a cryogenic field-programmable gate array (FPGA) controlling a switching matrix at 20 millikelvin which, in turn, manipulates a semiconductor qubit.

cond-mat.mes-hall↗

arXiv2014

The $ν=5/2$ Fractional Quantum Hall State in the Presence of Alloy Disorder

We report quantitative measurements of the impact of alloy disorder on the $ν=5/2$ fractional quantum Hall state. Alloy disorder is controlled by the aluminum content $x$ in the Al$_x$Ga$_{1-x}$As channel of a quantum well. We find that the $ν=5/2$ state is suppressed with alloy scattering. To our surprise, in samples with alloy disorder the $ν=5/2$ state appears at significantly reduced mobilities when compared to samples in which alloy disorder is not the dominant scattering mechanism. Our results highlight the distinct roles of the different types of disorder present in these samples, such as the short-range alloy and the long-range Coulomb disorder.

cond-mat.mes-hall↗