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G. Gervais

Publications and source records attributed to G. Gervais.

At least 19 recordsLinked to original sources

A First Bound on the Moffat Energy and Thorium-229 Clock as a Probe of the Nonlocal Time-Energy Structure and a Proposed Experiment for the use of Nuclear Entanglement and Squeezed States to Test Nonlocal Quantum Field Theory

In this paper we derive the nonlocal Time-Energy uncertainty principle and then apply the published $^{229}$Th nuclear clock data as a first probe of the nonlocality scale \(E_M\). By using the direct clock-energy channel, we find conservative lower bounds on \(E_M\) at the tens of MeV scale, with optimistic present-data estimates reaching the hundred MeV scale. Including the known nuclear sensitivity enhancement of the $^{229}$Th transition gives us stronger model dependent bounds in the GeV range, while nuclear-scale reference-energy scenarios can reach the TeV range. The conclusion we draw from this is that nuclear clocks already provide an experimental route from nonlocal time--energy uncertainty to measurable laboratory bounds on non-Planckian nonlocality. We explore the idea of using a squeezed-state experiment with the $^{229}\mathrm{Th}$ nuclear clock to test the time--energy structure of nonlocal quantum field theory. The idea is reasonably obtainable within the near future of nuclear clock experiments. One would prepare an ensemble of thorium nuclei in a coherent superposition of the nuclear ground state and the low-lying isomeric clock state, entangle the participating nuclei through a collective interaction, and generate a family of spin-squeezed states with a tunable squeezing parameter $r$. Then a phase-controlled analysis pulse will rotate the selected collective nuclear quadrature into a measurable ground-isomer population difference. Near a strongly polarized collective state the normalized operators $J_y/\sqrt{S}$ and $J_z/\sqrt{S}$ obey the same approximate canonical algebra as the phase and amplitude quadratures of a squeezed optical mode. We use this experiment to either probe or bound the nonlocal energy scale $E_M$.

quant-ph

Demonstration of a Field-Effect Three-Terminal Electronic Device with an Electron Mobility Exceeding 40 Million cm^2/(Vs)

We report the fabrication and operation of a source-drain-gate three-terminal field-effect electronic device with an electron mobility exceeding $40\times 10^6$ cm$^2$ / (Vs). Several devices were fabricated, with the highest achieved electron mobility obtained using a symmetrically-doped GaAs/AlGaAs quantum well forming a two-dimensional electron gas (2DEG) with a density of $1.47(1) \times 10^{11}$ cm$^{-2}$ and a pristine, pre-fabrication electron mobility of $44(2) \times 10^6$ cm$^2$/(\text{Vs}). To circumvent the well-known degradation of electron mobility during fabrication, devices were fabricated using a flip-chip technique where all lithographic processing steps were performed on a separate sapphire substrate. This method demonstrates the successful operation of various gate assembly designs on distinct 2DEGs without observable mobility degradation. This advance doubles the previous record for field-effect electronic device mobility and enables access to new regimes of quantum transport and applications that were previously unfathomable due to mobility limitations.

cond-mat.mes-hall

Bulk Thermal Conductance of the 5/2 and 7/3 Fractional Quantum Hall States in the Corbino Geometry

In this work, making use of time-resolved in situ Joule heating of a two-dimensional electron gas (2DEG) in the Corbino geometry, we report bulk thermal conductance measurements for the {\nu} = 5/2 and {\nu} = 7/3 fractional quantum Hall (FQH) states for electron temperatures ranging from 20 to 150 mK. We compare our findings with a recent study by Melcer et al. [Nature 625, 489 (2024)] that observed a finite bulk thermal conductivity \k{appa}xx in FQH states. In spite of the large size difference and the vastly different experimental schemes used to extract \k{appa}xx, we find in large part that both experiments yield similar results and conclude that the bulk of FQH states thermally conducts and violate the Wiedemann-Franz law by a wide margin. Slight discrepancies between both studies are further discussed in terms of particle-hole symmetry in the vicinity of the 5/2 and 7/3 FQH states.

cond-mat.mes-hall

Observation of Temperature Independent Anomalous Hall Effect in Thin Bismuth from Near Absolute Zero to 300 K Temperature

We report our discovery of a temperature independent anomalous Hall effect (AHE) from 15 mK to 300 K temperature occurring in a 68 nm thick transport device made out of pure bismuth. This surprising behaviour is accompanied with an expected temperature dependent longitudinal resistance consistent with semi-metallic bismuth, however it surprisingly showed no hint of a magnetoresistance for magnetic fields between $\pm30$ T. Even though bismuth is a diamagnetic material which {\it a priori} does not break time-reversal symmetry (TRS), our analysis of the reconstructed conductivities points towards the AHE to be of the intrinsic type, which does not emanate from magnetic impurities. Finally, as pure bismuth has been shown numerically to host a Berry curvature at its surface which breaks inversion symmetry, we propose it as a possible explanation for the temperature independent AHE observed here.

cond-mat.mes-hall

Two-dimensional hydrodynamic viscous electron flow in annular Corbino rings

The concept of fluidic viscosity is ubiquitous in our everyday life and for it to arise the fluidic medium must necessarily form a continuum where macroscopic properties can emerge. While a powerful concept for tangible liquids, hydrodynamic manifestation of collective flow in electronic systems such as two-dimensional electron gases (2DEGs) has only been shown recently to occur in graphene and GaAs/AlGaAs. Here, we present nonlocal electronic transport measurements in concentric annular rings formed in high-mobility GaAs/AlGaAs 2DEGs and the resulting data strongly suggest that viscous hydrodynamic flow can occur far away from the source-drain current region. Our conclusion of viscous electronic transport is further corroborated by simulations of the Navier-Stokes equations that are found to be in agreement with our measurements below 1K temperature. Most importantly, our work emphasizes the key role played by viscosity via electron-electron (e-e) interaction when hydrodynamic transport is restricted radially, and for which a priori should not have played a major role.

cond-mat.mes-hall

Anomalous Hall Effect in Thin Bismuth

Bismuth, the heaviest of all group V elements with strong spin-orbit coupling, is famously known to exhibit many interesting transport properties, and effects such as Shubnikov-de Haas and de Haas-van Alphen were first revealed in its bulk form. However, the transport properties have not yet been fully explored experimentally in thin bismuth nor in its 2D limit. In this work, bismuth flakes with average thicknesses ranging from 29 to 69 nm were mechanically exfoliated by a micro-trench technique and were used to fabricate four-point devices. Due to mixing of components, Onsager's relations were used to extract the longitudinal ($R_{xx}$) and Hall ($R_{xy}$) resistances where the latter shows a Hall anomaly that is consistent with the Anomalous Hall Effect (AHE). Our work strongly suggests that that there could be a hidden mechanism for time-reversal symmetry breaking in pure bismuth thin films.

cond-mat.mes-hall

Large composite fermion effective mass at filling factor 5/2

The 5/2 fractional quantum Hall effect in the second Landau level of extremely clean two-dimensional electron gases has attracted much attention due to its topological order predicted to host quasiparticles that obey non-Abelian quantum statistics and could serve as a basis for fault-tolerant quantum computations. While previous works have establish the Fermi liquid (FL) nature of its putative composite fermion (CF) normal phase, little is known regarding its thermodynamics properties and as a result its effective mass is entirely unknown. Here, we report on time-resolved specific heat measurements at filling factor 5/2, and we examine the ratio of specific heat to temperature as a function of temperature. Combining these specific heat data with existing longitudinal thermopower data measuring the entropy in the clean limit we find that, unless a phase transition/crossover gives rise to large specific heat anomaly, both datasets point towards a large effective mass in the FL phase of CFs at 5/2. We estimate the effective-to-bare mass ratio m*/me to be ranging from ~2 to 4, which is two to three times larger than previously measured values in the first Landau level.

cond-mat.mes-hall

Anomalous Electronic Transport in High Mobility Corbino Rings

We report low-temperature electronic transport measurements performed in two multi-terminal Corbino samples formed in GaAs/Al-GaAs two-dimensional electron gases (2DEG) with both ultra-high electron mobility ($\gtrsim 20\times 10^6$ $cm^2/Vs)$ and with distinct electron density of $1.7$ and $3.6\times 10^{11}~cm^{-2}$. In both Corbino samples, a non-monotonic behavior is observed in the temperature dependence of the resistance below 1~$K$. Surprisingly, a sharp {\it decrease} in resistance is observed with {\it increasing} temperature in the sample with lower electron density, whereas an opposite behavior is observed in the sample with higher density. To investigate further, transport measurements were performed in large van der Pauw samples having identical heterostructures, and as expected they exhibit resistivity that is monotonic with temperature. Finally, we discuss the results in terms of various lengthscales leading to ballistic and hydrodynamic electronic transport, as well as a possible Gurzhi effect.

cond-mat.mes-hall

Noise and fluctuations in nanoscale gas flow

We theoretically calculate the fundamental noise that is present in gaseous (dilute fluid) flow in channels in the classical and degenerate quantum regime, where the Fermi-Dirac and Bose- Einstein distribution must be considered. Results for both regimes are analogous to their electrical counterparts. The quantum noise is calculated for a two terminal system and is a complicated function of the thermal and shot noise with the thermal noise dominating when $2k_BT >> m\Delta P$ and vice versa. The cumulant generating function for mass flow, which generates all the higher order statistics related to our mass flow distribution, is also derived and is used to find an expression for the third cumulant of flow across a fluidic channel.

cond-mat.mes-hall

Quasi-two-dimensional thermoelectricity in SnSe

Stannous selenide is a layered semiconductor that is a polar analogue of black phosphorus, and of great interest as a thermoelectric material. Unusually, hole doped SnSe supports a large Seebeck coefficient at high conductivity, which has not been explained to date. Angle resolved photo-emission spectroscopy, optical reflection spectroscopy and magnetotransport measurements reveal a multiple-valley valence band structure and a quasi two-dimensional dispersion, realizing a Hicks-Dresselhaus thermoelectric contributing to the high Seebeck coefficient at high carrier density. We further demonstrate that the hole accumulation layer in exfoliated SnSe transistors exhibits a field effect mobility of up to $250~\mathrm{cm^2/Vs}$ at $T=1.3~\mathrm{K}$. SnSe is thus found to be a high quality, quasi two-dimensional semiconductor ideal for thermoelectric applications.

cond-mat.mtrl-sci

Large magnetoresistance by Pauli blockade in hydrogenated graphene

We report the observation of a giant positive magnetoresistance in millimetre scale hydrogenated graphene with magnetic field oriented in the plane of the graphene sheet. A positive magnetoresistance in excess of 200\% at a temperature of 300 mK was observed in this configuration, reverting to negative magnetoresistance with the magnetic field oriented normal to the graphene plane. We attribute the observed positive, in-plane, magnetoresistance to Pauli-blockade of hopping conduction induced by spin polarization. Our work shows that spin polarization in concert with electron-electron interaction can play a dominant role in magnetotransport within an atomic monolayer.

cond-mat.mes-hall

Competing charge density waves probed by non-linear transport and noise in the second and third Landau levels

Charge density waves (CDW) in the second and third Landau levels (LL) are investigated by both non-linear electronic transport and noise. The use of a Corbino geometry ensures that only bulk properties are probed, with no contribution from edge states. Sliding transport of CDWs is revealed by narrow band noise in re-entrant quantum Hall states R2a and R2c of the second LL as well as in pinned CDWs of the third LL. Competition between various phases - stripe, pinned CDW or fractional quantum Hall liquid - in both LL are clearly revealed by combining noise data with maps of conductivity versus magnetic field and bias voltage.

cond-mat.mes-hall

Specific heat and entropy of fractional quantum Hall states in the second Landau level

Specific heat has had an important role in the study of superfluidity and superconductivity, and could provide important information about the fractional quantum Hall effect as well. However, traditional measurements of the specific heat of a two-dimensional electron gas are difficult due to the large background contribution of the phonon bath, even at very low temperatures. Here, we report measurements of the specific heat per electron in the second Landau level by measuring the thermalization time between the electrons and phonons. We observe activated behaviour of the specific heat of the 5/2 and 7/3 fractional quantum Hall states, and extract the entropy by integrating over temperature. Our results are in excellent agreement with previous measurements of the entropy via longitudinal thermopower. Extending the technique to lower temperatures could lead to detection of the non-Abelian entropy predicted for bulk quasiparticles at 5/2 filling

cond-mat.mes-hall

Magnetic refrigeration with paramagnetic semiconductors at cryogenic temperatures

We propose paramagnetic semiconductors as active media for refrigeration at cryogenic temperatures by adiabatic demagnetization. The paramagnetism of impurity dopants or structural defects can provide the entropy necessary for refrigeration at cryogenic temperatures. We present a simple model for the theoretical limitations to specific entropy and cooling power achievable by demagnetization of various semiconductor systems. Performance comparable to that of the hydrate (CMN) is predicted.

cond-mat.mes-hall

Dephasing in strongly anisotropic black phosphorus

Weak localization was observed and determined in a black phosphorus (bP) field-effect transistor 65 nm thick. The weak localization behaviour was found to be in excellent agreement with the Hikami-Larkin-Nagaoka model for fields up to 1~T, from which characteristic scattering lengths could be inferred. The dephasing length $L_ϕ$ was found to increase linearly with increasing hole density attaining a maximum value of 55 nm at a hole density of approximately $10^{13} cm^{-2}$ inferred from the Hall effect. The temperature dependence of $L_ϕ$ was also investigated and above 1~K, it was found to decrease weaker than the $L_ϕ\propto T^{-\frac{1}{2}}$ dependence characteristic of electron-electron scattering in the presence of elastic scattering in two dimensions. Rather, the observed power law was found to be close to that observed previously in other quasi-one-dimensional systems such as metallic nanowires and carbon nanotubes. We attribute our result to the crystal structure of bP which host a `puckered' honeycomb lattice forming a strongly anisotropic medium

cond-mat.mes-hall

Dual gate black phosphorus velocity modulated transistor

The layered semiconductor black phosphorus has attracted attention as a 2D atomic crystal that can be prepared in ultra-thin layers for operation as field effect transistors. Despite the susceptibility of black phosphorus to photo-oxidation, improvements to the electronic quality of black phosphorus devices has culminated in the observation of the quantum Hall effect. In this work, we demonstrate the room temperature operation of a dual gated black phosphorus transistor operating as a velocity modulated transistor, whereby modification of hole density distribution within a black phosphorus quantum well leads to a two-fold modulation of hole mobility. Simultaneous modulation of Schottky barrier resistance leads to a four-fold modulation of transcon- ductance at a fixed hole density. Our work explicitly demonstrates the critical role of charge density distribution upon charge carrier transport within 2D atomic crystals.

cond-mat.mes-hall

Measurement of Topological Berry Phase in Highly Disordered Graphene

We have observed the quantum Hall effect (QHE) and Shubnikov-de Haas (SdH) oscillations in highly disordered graphene at magnetic fields up to 65 T. Disorder was introduced by hydrogenation of graphene up to a ratio H/C $\approx 0.1\%$. The analysis of SdH oscillations and QHE indicates that the topological part of the Berry phase, proportional to the pseudo-spin winding number, is robust against introduction of disorder by hydrogenation in large scale graphene.

cond-mat.mes-hall

Second Landau Level Fractional Quantum Hall Effects in the Corbino Geometry

For certain measurements, the Corbino geometry has a distinct advantage over the Hall and van der Pauw geometries, in that it provides a direct probe of the bulk 2DEG without complications due to edge effects. This may be important in enabling detection of the non-Abelian entropy of the 5/2 fractional quantum Hall state via bulk thermodynamic measurements. We report the successful fabrication and measurement of a Corbino-geometry sample in an ultra-high mobility GaAs heterostructure, with a focus on transport in the second and higher Landau levels. In particular, we report activation energy gaps of fractional quantum Hall states, with all edge effects ruled out, and extrapolate the conductivity prefactor from the Arrhenius fits. Our results show that activated transport in the second Landau level remains poorly understood. The development of this Corbino device opens the possibility to study the bulk of the 5/2 state using techniques not possible in other geometries.

cond-mat.mes-hall