arXiv subjects

J. D. Watson

Publications and source records attributed to J. D. Watson.

At least 19 recordsLinked to original sources

arXiv2023

An rf Quantum Capacitance Parametric Amplifier

We demonstrate a radio-frequency parametric amplifier that exploits the gate-tunable quantum capacitance of an ultra high mobility two dimensional electron gas (2DEG) in a GaAs heterostructure at cryogenic temperatures. The prototype narrowband amplifier exhibits a gain greater than 20 dB up to an input power of - 66 dBm (1 dB compression), and a noise temperature TN of 1.3 K at 370 MHz. In contrast to superconducting amplifiers, the quantum capacitance parametric amplifier (QCPA) is operable at tesla-scale magnetic fields and temperatures ranging from milli kelvin to a few kelvin. These attributes, together with its low power (microwatt) operation when compared to conventional transistor amplifiers, suggest the QCPA may find utility in enabling on-chip integrated readout circuits for semiconductor qubits or in the context of space transceivers and radio astronomy instruments.

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arXiv2021

Anomalous nematic-to-stripe phase transition driven by in-plane magnetic fields

Anomalous nematic states, recently discovered in ultraclean two-dimensional electron gas, emerge from quantum Hall stripe phases upon further cooling. These states are hallmarked by a local minimum (maximum) in the hard (easy) longitudinal resistance and by an incipient plateau in the Hall resistance in nearly half-filled Landau levels. Here, we demonstrate that a modest in-plane magnetic field, applied either along $\left < 110 \right >$ or $\left < 1\bar10 \right >$ crystal axis of GaAs, destroys anomalous nematic states and restores quantum Hall stripe phases aligned along their native $\left < 110 \right >$ direction. These findings confirm that anomalous nematic states are distinct from other ground states and will assist future theories to identify their origin.

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arXiv2021

Stability of multielectron bubbles in high Landau levels

We study multielectron bubble phases in the $N=2$ and $N=3$ Landau levels in a high mobility GaAs/AlGaAs sample. We found that the longitudinal magnetoresistance versus temperature curves in the multielectron bubble region exhibit sharp peaks, irrespective of the Landau level index. We associate these peaks with an enhanced scattering caused by thermally fluctuating domains of a bubble phase and a uniform uncorrelated electron liquid at the onset of the bubble phases. Within the $N=3$ Landau level, onset temperatures of three-electron and two-electron bubbles exhibit linear trends with respect to the filling factor; the onset temperatures of three-electron bubbles are systematically higher than those of two-electron bubbles. Furthermore, onset temperatures of the two-electron bubble phases across $N=2$ and $N=3$ Landau levels are similar, but exhibit an offset. This offset and the dominant nature of the three-electron bubbles in the $N=3$ Landau level reveals the role of the short-range part of the electron-electron interaction in the formation of the bubbles.

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arXiv2020

Observation of Photoinduced Terahertz Gain in GaAs Quantum Wells: Evidence for Radiative Two-Exciton-to-Biexciton Scattering

We have observed photoinduced negative optical conductivity, or gain, in the terahertz frequency range in a GaAs multiple-quantum-well structure in a strong perpendicular magnetic field at low temperatures. The gain is narrow-band: it appears as a sharp peak (linewidth $<$0.45 meV) whose frequency shifts with applied magnetic field. The gain has a circular-polarization selection rule: a strong line is observed for hole-cyclotron-resonance-active polarization. Furthermore, the gain appears only when the exciton $1s$ state is populated, which rules out intraexcitonic transitions to be its origin. Based on these observations, we propose a possible process in which the stimulated emission of a terahertz photon occurs while two free excitons scatter into one biexciton in an energy and angular-momentum conserving manner.

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arXiv2020

Anomalous Nematic States in High Half-Filled Landau Levels

It is well established that the ground states of a two-dimensional electron gas with half-filled high ($N \ge 2$) Landau levels are compressible charge-ordered states, known as quantum Hall stripe (QHS) phases. The generic features of QHSs are a maximum (minimum) in a longitudinal resistance $R_{xx}$ ($R_{yy}$) and a non-quantized Hall resistance $R_H$. Here, we report on emergent minima (maxima) in $R_{xx}$ ($R_{yy}$) and plateau-like features in $R_H$ in half-filled $N \ge 3$ Landau levels. Remarkably, these unexpected features develop at temperatures considerably lower than the onset temperature of QHSs, suggesting a new ground state.

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arXiv2019

Electron Bubbles and the Structure of the Orbital Wavefunction

Stripe-like and bubble-like patterns spontaneously form in numerous physical, chemical, and biological systems when competing long-range and short-range interactions banish uniformity. Stripe-like and the related nematic morphology are also under intense scrutiny in various strongly correlated electron systems. In contrast, the electronic bubble morphology is rare. Some of the most intriguing electron bubbles develop in the two-dimensional electron gas subjected to a perpendicular magnetic field. However, in contrast to bubbles forming in classical systems such as the Turing activator-inhibitor reaction or Langmuir films, bubbles in electron gases owe their existence to elementary quantum mechanics: they are stabilized as wavefunctions of individual electrons overlap. Here we report a rich pattern of multi-electron bubble phases in a high Landau level and we conclude that this richness is due to the nodal structure of the orbital component of the electronic wavefunction.

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arXiv2019

Effect of density on microwave-induced resistance oscillations in back-gated GaAs quantum wells

We report on microwave-induced resistance oscillations (MIROs) in a tunable-density 30-nm-wide GaAs/AlGaAs quantum well. We find that the MIRO amplitude increases dramatically with carrier density. Our analysis shows that the anticipated increase in the effective microwave power and quantum lifetime with density is not sufficient to explain the observed growth of the amplitude. We further observe that the fundamental oscillation extrema move towards cyclotron resonance with increasing density, which also contradicts theoretical predictions. These findings reveal that the density dependence is not properly captured by existing theories, calling for further studies.

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arXiv2019

Effect of illumination on quantum lifetime in GaAs quantum wells

Low-temperature illumination of a two-dimensional electron gas in GaAs quantum wells is known to greatly improve the quality of high-field magnetotransport. The improvement is known to occur even when the carrier density and mobility remain unchanged, but what exactly causes it remains unclear. Here, we investigate the effect of illumination on microwave photoresistance in low magnetic fields. We find that the amplitude of microwave-induced resistance oscillations grows dramatically after illumination. Dingle analysis reveals that this growth reflects a substantial increase in the single-particle (quantum) lifetime, which likely originates from the light-induced redistribution of charge enhancing the screening capability of the doping layers.

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arXiv2019

Two- and three-electron bubbles in Al$_{x}$Ga$_{1-x}$As/Al$_{0.24}$Ga$_{0.76}$As quantum wells

We report on transport signatures of eight distinct bubble phases in the $N=3$ Landau level of a Al$_{x}$Ga$_{1-x}$As/Al$_{0.24}$Ga$_{0.76}$As quantum well with $x = 0.0015$. These phases occur near partial filling factors $ν^\star \approx 0.2\,(0.8)$ and $ν^\star \approx 0.3\,(0.7)$ and have $M = 2$ and $M = 3$ electrons (holes) per bubble, respectively. We speculate that a small amount of alloy disorder in our sample helps to distinguish these broken symmetry states in low-temperature transport measurements.

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arXiv2017

Device Architecture for Coupling Spin Qubits Via an Intermediate Quantum State

We demonstrate a scalable device architecture that facilitates indirect exchange between singlet-triplet spin qubits, mediated by an intermediate quantum state. The device comprises five quantum dots, which can be independently loaded and unloaded via tunneling to adjacent reservoirs, avoiding charge latch-up common in linear dot arrays. In a step towards realizing two-qubit entanglement based on indirect exchange, the architecture permits precise control over tunnel rates between the singlet-triplet qubits and the intermediate state. We show that by separating qubits by 1 um, the residual capacitive coupling between them is reduced to 7 ueV.

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arXiv2017

Gate-Sensing Charge Pockets in the Semiconductor Qubit Environment

We report the use of dispersive gate sensing (DGS) as a means of probing the charge environment of heterostructure-based qubit devices. The DGS technique, which detects small shifts in the quantum capacitance associated with single-electron tunnel events, is shown to be sensitive to pockets of charge in the potential-landscape likely under, and surrounding, the surface gates that define qubits and their readout sensors. Configuring a quantum point contact (QPC) as a localized emitter, we show how these charge pockets are activated by the relaxation of electrons tunneling through a barrier. The presence of charge pockets creates uncontrolled offsets in gate-bias and their thermal activation by on-chip tunnel currents suggests further sources of charge-noise that lead to decoherence in semiconductor qubits.

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arXiv2017

Theory of topological excitations and metal-insulator transition in reentrant integer quantum Hall effect

The reentrant integer quantum Hall effects (RIQHE) are due to formation of electronic crystals. We show analytically and numerically that topological textures in the charge density distribution in these crystals in the vicinity of charged defects strongly reduce energy required for current-carrying excitations. The theory quantitatively explains sharp insulator-metal transitions experimentally observed in RIQHE states. The insulator to metal transition in RIQHE emerges as a thermodynamic unbinding transition of topological charged defects.

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arXiv2017

Effect of density on quantum Hall stripe orientation in tilted magnetic fields

We investigate quantum Hall stripes under in-plane magnetic field $B_\parallel$ in a variable-density two-dimensional electron gas. At filling factor $ν= 9/2$, we observe one, two, and zero $B_\parallel$-induced reorientations at low, intermediate, and high densities, respectively. The appearance of these distinct regimes is due to a strong density dependence of the $B_\parallel$-induced orienting mechanism which triggers the second reorientation, rendering stripes \emph{parallel} to $B_\parallel$. In contrast, the mechanism which reorients stripes perpendicular to $B_\parallel$ showed no noticeable dependence on density. Measurements at $ν= 9/2$ and $11/2$ at the same, tilted magnetic field, allows us to rule out density dependence of the native symmetry-breaking field as a dominant factor. Our findings further suggest that screening might play an important role in determining stripe orientation, providing guidance in developing theories aimed at identifying and describing native and $B_\parallel$-induced symmetry-breaking fields.

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arXiv2017

Apparent temperature-induced reorientation of quantum Hall stripes

Our magnetotransport measurements of quantum Hall stripes in a high-quality GaAs quantum well in a slightly tilted magnetic field reveal that the orientation of stripes can be changed by temperature. Field-cooling and field-warming measurements, as well as observation of hysteresis at intermediate temperatures allow us to conclude that the observed temperature-induced reorientation of stripes is owing to the existence of two distinct minima in the symmetry-breaking potential. We also find that the native symmetry-breaking mechanism does not depend on temperature and that low-temperature magnetotransport data should be treated with caution as they do not necessarily reveal the true ground state, even in the absence of hysteresis.

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arXiv2016

Evidence for a new symmetry breaking mechanism reorienting quantum Hall nematics

We report on the effect of in-plane magnetic field $B_\parallel$ on stripe phases in higher ($N=2,3$) Landau levels of a high-mobility 2D electron gas. In accord with previous studies, we find that a modest $B_\parallel$ applied parallel to the native stripes aligns them perpendicular to it. However, upon further increase of $B_\parallel$, stripes are reoriented back to their native direction. Remarkably, applying $B_\parallel$ perpendicular to the native stripes also aligns stripes parallel to it. Thus, regardless of the initial orientation of stripes with respect to $B_\parallel$, stripes are ultimately aligned \emph{parallel} to $B_\parallel$. These findings provide evidence for a $B_\parallel$-induced symmetry breaking mechanism which challenge current understanding of the role of $B_\parallel$ and should be taken into account when determining the strength of the native symmetry breaking potential. Finally, our results might indicate nontrivial coupling between the native and external symmetry breaking fields, which has not yet been theoretically considered.

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arXiv2016

Reorientation of quantum Hall stripes within a partially filled Landau level

We investigate the effect of the filling factor on transport anisotropies, known as stripes, in high Landau levels of a two-dimensional electron gas. We find that at certain in-plane magnetic fields, the stripes orientation is sensitive to the filling factor within a given Landau level. This sensitivity gives rise to the emergence of stripes away from half-filling while an orthogonally-oriented, native stripes reside at half-filling. This switching of the anisotropy axes within a single Landau level can be attributed to a strong dependence of the native symmetry breaking potential on the filling factor.

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arXiv2016

Resistively detected high-order magnetoplasmons in a high-quality 2D electron gas

We report on high-order magnetoplasmon resonances detected in photoresistance in high-mobility GaAs quantum wells. These resonances manifest themselves as a series of photoresistance extrema in the regime of Shubnikov-de Haas oscillations. Extending to orders above 20, the extrema exhibit alternating strength, being less (more) pronounced at even (odd) order magnetoplasmon modes. This experimental technique provides sensitive and elegant means to detect and investigate multiple magnetoplasmon modes and could be applied to other systems.

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arXiv2016

Spatially resolved breakdown in reentrant quantum Hall states

Reentrant integer quantum Hall (RIQH) states are believed to be correlated electron solid phases, though their microscopic description remains unclear. As bias current increases, longitudinal and Hall resistivities measured for these states exhibit multiple sharp breakdown transitions, a signature unique to RIQH states. We present spatially-resolved measurements of RIQH breakdown that indicate these breakdown signatures can be ascribed to a phase boundary between broken-down and unbroken regions, spreading chirally from source and drain contacts as a function of bias current and passing voltage probes one by one. The chiral sense of the spreading is not set by the chirality of the edge state itself, instead depending on electron- or hole-like character of the RIQH state.

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