arXiv subjects

Thomas Szkopek

Publications and source records attributed to Thomas Szkopek.

11 recordsLinked to original sources

arXiv2025

Total Faraday rotation by the Hall effect in a 2D electron gas

We report the realization of near total Faraday rotation of $θ_F$=1.43 rad (82 degrees) on a single pass through a 2D electron gas (2DEG), approaching the ideal limit of $π/2$ rad (90 degrees). The corresponding Verdet constant V = $9.5\times10^{8}$ rad T$^{-1}$m$^{-1}$, exceeds by approximately one order of magnitude that reported in other material systems. Our measurements were conducted at microwave frequencies (f=9.2-11.2 GHz) in a 2DEG with a high dc mobility $μ$ = $7\times10^6$ cm$^2$V$^{-1}$s$^{-1}$, in a hollow waveguide at low-magnetic field (B < 200 mT). Near-total Faraday rotation is attributed to the Hall effect with weak radiative coupling to the 2DEG in the inertial, collisionless regime, $ωτ\gg 1$, where $τ$ is the charge transport scattering time. A conducting iris was used to realize weak radiative coupling. Under these conditions, Faraday rotation is strongly enhanced away from the dissipation peak at cyclotron resonance. Our work demonstrates that the classical Hall effect could be ideally suited for the implementation of ideal non-reciprocal devices.

cond-mat.mes-hall↗

arXiv2024

Method of Mechanical Exfoliation of Bismuth with Micro-Trench Structures

The discovery of graphene led to a burst in search for 2D materials originating from layered atomic crystals coupled by van der Waals force. While bulk bismuth crystals share this layered crystal structure, unlike other group V members of the periodic table, its interlayer bonds are stronger such that traditional mechanical cleavage and exfoliation techniques have shown to be inefficient. In this work, we present a novel mechanical cleavage method for exfoliating bismuth by utilizing the stress concentration effect induced by micro-trench SiO2 structures. As a result, the exfoliated bismuth flakes can achieve thicknesses down to the sub-10 nm range which are analyzed by AFM and Raman spectroscopy.

cond-mat.mes-hall↗

arXiv2024

Mass inversion at the Lifshitz transition in monolayer graphene by diffusive, high-density, on-chip, doping

Experimental setups for charge transport measurements are typically not compatible with the ultra-high vacuum conditions for chemical doping, limiting the charge carrier density that can be investigated by transport methods. Field-effect methods, including dielectric gating and ionic liquid gating, achieve too low a carrier density to induce electronic phase transitions. To bridge this gap, we developed an integrated flip-chip method to dope graphene by alkali vapour in the diffusive regime, suitable for charge transport measurements at ultra-high charge carrier density. We introduce a cesium droplet into a sealed cavity filled with inert gas to dope a monolayer graphene sample by the process of cesium atom diffusion, adsorption and ionization at the graphene surface, with doping beyond an electron density of $4.7\times10^{14}~\mathrm{cm}^{-2}$ monitored by operando Hall measurement. The sealed assembly is stable against oxidation, enabling measurement of charge transport versus temperature and magnetic field. Cyclotron mass inversion is observed via the Hall effect, indicative of the change of Fermi surface geometry associated with the Liftshitz transition at the hyperbolic $M$ point of monolayer graphene. The transparent quartz substrate also functions as an optical window, enabling non-resonant Raman scattering. Our findings show that chemical doping, hitherto restricted to ultra-high vacuum, can be applied in a diffusive regime at ambient pressure in an inert gas environment and thus enable charge transport studies in standard cryogenic environments

cond-mat.mes-hall↗

arXiv2022

Fluctuation-Dissipation in Thermoelectrics

Thermoelectric materials exhibit correlated transport of charge and heat. The Johnson-Nyquist noise formula $ 4 k_B T R $ for spectral density of voltage fluctuations accounts for fluctuations associated solely with Ohmic dissipation. Applying the fluctuation-dissipation theorem, we generalize the Johnson-Nyquist formula for thermoelectrics, finding an enhanced voltage fluctuation spectral density $4 k_B T R (1 + ZT)$ at frequencies below a thermal cut-off frequency $f_T$, where $ZT$ is the dimensionless thermoelectric material figure of merit. The origin of the enhancement in voltage noise is thermoelectric coupling of temperature fluctuations. We use a wideband ($f_T\sim1$ kHz), integrated thermoelectric micro-device to experimentally confirm our findings. Measuring the $ZT$ enhanced voltage noise, we experimentally resolve temperature fluctuations with an amplitude of $0.8~μ\mathrm{K} \mathrm{Hz}^{-1/2}$ at a mean temperature of 295 K. We find that thermoelectric devices can be used for thermometry with sufficient resolution to measure the fundamental temperature fluctuations described by the fluctuation-dissipation theorem.

cond-mat.mtrl-sci↗

arXiv2020

Quantitative Measurements of Giant and Quantized Microwave Faraday Rotation

We report {\it quantitative} microwave Faraday rotation measurements conducted with a high-mobility two-dimensional electron gas (2DEG) in a GaAs/AlGaAs semiconductor heterostructure. In a magnetic field, the Hall effect and the Faraday effect arise from the action of Lorentz force on electrons in the 2DEG. As with the Hall effect, a classical Faraday effect is observed at low magnetic field as well as a quantized Faraday effect at high magnetic field. The high electron mobility of the 2DEG enables a giant single-pass Faraday rotation of $θ_F^{max} \simeq 45^\circ$ $(\simeq0.8$~rad) to be achieved at a modest magnetic field of $B \simeq 100$~mT. In the quantum regime, we find that the Faraday rotation $θ_F$ is quantized in units of $α^*= 2.80(4)α$, where $α\simeq 1/137$ is the fine structure constant. The enhancement in rotation quantum $α^* > α$ is attributed to electromagnetic confinement within a waveguide structure.

cond-mat.mes-hall↗

arXiv2018

Non-Classical Longitudinal Magneto-Resistance in Anisotropic Black Phosphorus

Resistivity measurements of a few-layer black phosphorus (bP) crystal in parallel magnetic fields up to 45 T are reported as a function of the angle between the in-plane field and the source-drain (S-D) axis of the device. The crystallographic directions of the bP crystal were determined by Raman spectroscopy, with the zigzag axis found within 5° of the S-D axis, and the armchair axis in the orthogonal planar direction. A transverse magneto-resistance (TMR) as well as a classically-forbidden longitudinal magneto-resistance (LMR) are observed. Both are found to be strongly anisotropic and non-monotonic with increasing in-plane field. Surprisingly, the relative magnitude (in %) of the positive LMR is larger than the TMR above $\sim$32 T. Considering the known anisotropy of bP whose zigzag and armchair effective masses differ by a factor of approximately seven, our experiment strongly suggests this LMR to be a consequence of the anisotropic Fermi surface of bP.

cond-mat.mes-hall↗

arXiv2013

Terahertz Magnetoplasmon Energy Concentration and Splitting in Graphene PN Junctions

Terahertz plasmons and magnetoplasmons propagating along electrically and chemically doped graphene p-n junctions are investigated. It is shown that such junctions support non-reciprocal magnetoplasmonic modes which get concentrated at the middle of the junction in one direction and split away from the middle of the junction in the other direction under the application of an external static magnetic field. This phenomenon follows from the combined effects of circular birefringence and carrier density non-uniformity. It can be exploited for the realization of plasmonic isolators.

physics.optics↗

arXiv2005

Eigenvalue Estimation of Differential Operators

We demonstrate how linear differential operators could be emulated by a quantum processor, should one ever be built, using the Abrams-Lloyd algorithm. Given a linear differential operator of order 2S, acting on functions psi(x_1,x_2,...,x_D) with D arguments, the computational cost required to estimate a low order eigenvalue to accuracy Theta(1/N^2) is Theta((2(S+1)(1+1/nu)+D)log N) qubits and O(N^{2(S+1)(1+1/nu)} (D log N)^c) gate operations, where N is the number of points to which each argument is discretized, nu and c are implementation dependent constants of O(1). Optimal classical methods require Theta(N^D) bits and Omega(N^D) gate operations to perform the same eigenvalue estimation. The Abrams-Lloyd algorithm thereby leads to exponential reduction in memory and polynomial reduction in gate operations, provided the domain has sufficiently large dimension D > 2(S+1)(1+1/nu). In the case of Schrodinger's equation, ground state energy estimation of two or more particles can in principle be performed with fewer quantum mechanical gates than classical gates.

quant-ph↗

arXiv2005

What is the Fastest Speed at which a Single Electron Can Be Detected?

Electrometers measure electric charge, but there must be a fundamental speed limit to measuring one electric charge. Since there are no dimensional inputs to this question, the answer must be expressible in terms of the fundamental physical constants of Nature, e,h,m,c. In general the question should be posed without reference to any specific technology, but for definiteness, we analyze the field effect transistor, which is essentially an electrometer. In spite of selecting a specific technology, we find that the speed limit is related to a fundamental constant, the Rydberg frequency, or as appropriate, the semiconductor Rydberg frequency including the electron effective mass, and the relative dielectric constant. We do not know whether the Rydberg frequency represents the upper speed limit, but on dimensional grounds we claim that the final limit can only differ by some power of the fine-structure-constant.

quant-ph↗

arXiv2005

Optimal two-qubit quantum circuits using exchange interactions

The Heisenberg exchange interaction is a natural method to implement non-local (i.e., multi-qubit) quantum gates in quantum information processing. We consider quantum circuits comprising of $(SWAP)^α$ gates, which are realized through the exchange interaction, and single-qubit gates. A universal two-qubit quantum circuit is constructed from only three $(SWAP)^α$ gates and six single-qubit gates. We further show that three $(SWAP)^α$ gates are not only sufficient, but necessary. Since six single-qubit gates are known to be necessary, our universal two-qubit circuit is optimal in terms of the number of {\em both} $(SWAP)^α$ and single-qubit gates.

quant-ph↗

arXiv2004

Single photo-electron trapping, storage, and detection in a one-electron quantum dot

There has been considerable progress in electro-statically emptying, and re-filling, quantum dots with individual electrons. Typically the quantum dot is defined by electrostatic gates on a GaAs/AlGaAs modulation doped heterostructure. We report the filling of such a quantum dot by a single photo-electron, originating from an individual photon. The electrostatic dot can be emptied and reset in a controlled fashion before the arrival of each photon. The trapped photo-electron is detected by a point contact transistor integrated adjacent to the electrostatic potential trap. Each stored photo-electron causes a persistent negative step in the transistor channel current. Such a controllable, benign, single photo-electron detector could allow for information transfer between flying photon qubits and stored electron qubits.

quant-ph↗