arXiv subjects

K. W. West

Publications and source records attributed to K. W. West.

At least 91 records · Page 5Linked to original sources

arXiv2018

Wigner solid pinning modes tuned by fractional quantum Hall states of a nearby layer

We study a bilayer system hosting exotic many-body states of two-dimensional electron systems (2DESs) in close proximity but isolated from one another by a thin barrier. One 2DES has low electron density and forms a Wigner solid (WS) at high magnetic fields. The other has much higher density and, in the same field exhibits fractional quantum Hall states (FQHSs). The WS manifests microwave resonances which are understood as pinning modes, collective oscillations of the WS within the small but finite ubiquitous disorder. Our measurements reveal a striking evolution of the pinning mode frequencies of the WS layer with the formation of the FQHSs in the nearby layer, evincing a strong coupling between the WS pinning modes and the state of the 2DES in the adjacent layer, mediated by screening.

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arXiv2018

High-mobility indirect excitons in wide single quantum well

Indirect excitons (IXs) are bound pairs of electrons and holes confined in spatially separated layers. We present wide single quantum well (WSQW) heterostructures with high IX mobility, spectrally narrow IX emission, voltage-controllable IX energy, and long and voltage-controllable IX lifetime. This set of properties shows that WSQW heterostructures provide an advanced platform both for studying basic properties of IXs in low-disorder environments and for the development of high mobility excitonic devices.

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arXiv2018

Critical Filling Factor for the Formation of a Quantum Wigner Crystal Screened by a Nearby Layer

One of the most fascinating ground states of an interacting electron system is the so-called Wigner crystal where the electrons, in order to minimize their repulsive Coulomb energy, form an ordered array. Here we report measurements of the critical filling factor ($ν_{C}$) below which a magnetic-field-induced, quantum Wigner crystal forms in a dilute, two-dimensional electron layer when a second, high-density electron layer is present in close proximity. The data reveal that the Wigner crystal forms at a significantly smaller $ν_{C}$ compared to the $ν_{C}$ ($\simeq 0.20$) in single-layer two-dimensional electron systems. The measured $ν_{C}$ exhibits a strong dependence on the interlayer distance, reflecting the interaction and screening from the adjacent, high-density layer.

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arXiv2018

Anomalous coupling between magnetic and nematic orders in quantum Hall systems

The interplay between different orders is of fundamental importance in physics. The spontaneous, symmetry-breaking charge order, responsible for the stripe or the nematic phase, has been of great interest in many contexts where strong correlations are present, such as high-temperature superconductivity and quantum Hall effect. In this article we show the unexpected result that in an interacting two-dimensional electron system, the robustness of the nematic phase, which represents an order in the charge degree of freedom, not only depends on the orbital index of the topmost, half-filled Landau level, but it is also strongly correlated with the magnetic order of the system. Intriguingly, when the system is fully magnetized, the nematic phase is particularly robust and persists to much higher temperatures compared to the nematic phases observed previously in quantum Hall systems. Our results give fundamental new insight into the role of magnetization in stabilizing the nematic phase, while also providing a new knob with which it can be effectively tuned.

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arXiv2018

Multivalley two-dimensional electron system in an AlAs quantum well with mobility exceeding $2\times10^6$ cm$^{2}$V$^{-1}$s$^{-1}$

Degenerate conduction-band minima, or `valleys', in materials such as Si, AlAs, graphene, and MoS$_2$ allow them to host two-dimensional electron systems (2DESs) that can access a valley degree of freedom. These multivalley 2DESs present exciting opportunities for both pragmatic and fundamental research alike because not only are they a platform for valleytronic devices, but they also provide a tool to tune and investigate the properties of complex many-body ground states. Here, we report ultra-high quality, modulation doped AlAs quantum wells containing 2DESs that occupy two anisotropic valleys and have electron mobilities peaking at $2.4\times10^6$ cm$^{2}$V$^{-1}$s$^{-1}$ at a density of $2.2\times10^{11}$ cm$^{-2}$. This is more than an order of magnitude improvement in mobility over previous results. The unprecedented quality of our samples is demonstrated by magneto-transport data that show high-order fractional quantum Hall minima up to the Landau level filling $ν=8/17$, and even the elusive $ν=1/5$ quantum Hall state.

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arXiv2018

Realization of a Valley Superlattice

In a number of widely-studied materials, such as Si, AlAs, Bi, graphene, MoS2, and many transition metal dichalcogenide monolayers, electrons acquire an additional, spin-like degree of freedom at the degenerate conduction band minima, also known as "valleys". External symmetry breaking fields such as mechanical strain, or electric or magnetic fields, can tune the valley-polarization of these materials making them suitable candidates for "valleytronics". Here we study a quantum well of AlAs, where the two-dimensional electrons reside in two energetically degenerate valleys. By fabricating a strain-inducing grating on the sample surface, we engineer a spatial modulation of the electron population in different valleys, i.e., a "valley superlattice" in the quantum well plane. Our results establish a novel manipulation technique of the valley degree of freedom, paving the way to realizing a valley-selective layered structure in multi-valley materials, with potential application in valleytronics.

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arXiv2018

Electron-Electron Interactions and the Paired-to-Nematic Quantum Phase Transition in the Second Landau Level

In spite of its ubiquity in strongly correlated systems, the competition of paired and nematic ground states remains poorly understood. Recently such a competition was reported in the two-dimensional electron gas at filling factor $ν=5/2$. At this filling factor a pressure-induced quantum phase transition was observed from the paired fractional quantum Hall state to the quantum Hall nematic. Here we show that the pressure induced paired-to-nematic transition also develops at $ν=7/2$, demonstrating therefore this transition in both spin branches of the second orbital Landau level. However, we find that pressure is not the only parameter controlling this transition. Indeed, ground states consistent with those observed under pressure also develop in a sample measured at ambient pressure, but in which the electron-electron interaction was tuned close to its value at the quantum critical point. Our experiments suggest that electron-electron interactions play a critical role in driving the paired-to-nematic transition.

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arXiv2018

Finite Temperature Behavior in the Second Landau Level of the Two-dimensional Electron Gas

Reports of weak local minima in the magnetoresistance at $ν=2+3/5$, $2+3/7$, $2+4/9$, $2+5/9$, $2+5/7$, and $2+5/8$ in the second Landau level of the electron gas in GaAs/AlGaAs left open the possibility of fractional quantum Hall states at these filling factors. In a high quality sample we found that the magnetoresistance exhibits peculiar features near these filling factors of interest. These features, however, cannot be associated with fractional quantum Hall states; instead they originate from magnetoresistive fingerprints of the electronic bubble phases. We found only two exceptions: at $ν=2+2/7$ and $2+5/7$ there is evidence for incipient fractional quantum Hall states at intermediate temperatures. As the temperature is lowered, these fractional quantum Hall states collapse due to a phase competition with bubble phases.

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arXiv2018

Direct observation of composite fermions and their fully spin-polarized Fermi sea near $ν=5/2$

The enigmatic even-denominator fractional quantum Hall state at Landau level filling factor $ν=5/2$ is arguably the most promising candidate for harboring Majorana quasi-particles with non-Abelian statistics and thus of potential use for topological quantum computing. The theoretical description of the $ν=5/2$ state is generally believed to involve a topological p-wave pairing of fully spin-polarized composite fermions through their condensation into a non-Abelian Moore-Read Pfaffian state. There is, however, no direct and conclusive experimental evidence for the existence of composite fermions near $ν=5/2$ or for an underlying fully spin-polarized Fermi sea. Here, we report the observation of composite fermions very near $ν=5/2$ through geometric resonance measurements, and find that the measured Fermi wavevector provides direct demonstration of a Fermi sea with full spin polarization. This lends crucial credence to the model of $5/2$ fractional quantum Hall effect as a topological p-wave paired state of composite fermions.

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arXiv2018

Thermopower and Nernst measurements in a half-filled lowest Landau level

Motivated by recent proposal by Potter et al. [Phys. Rev. X 6, 031026 (2016)] concerning possible thermoelectric signatures of Dirac composite fermions, we perform a systematic experimental study of thermoelectric transport of an ultrahigh-mobility GaAs/AlxGa1-xAs two dimensional electron system at filling factor v = 1/2. We demonstrate that the thermopower Sxx and Nernst Sxy are symmetric and anti-symmetric with respect to B = 0 T, respectively. The measured properties of thermopower Sxx at v = 1/2 are consistent with previous experimental results. The Nernst signals Sxy of v = 1/2, which have not been reported previously, are non-zero and show a power law relation with temperature in the phonon-drag dominant region. In the electron-diffusion dominant region, the Nernst signals Sxy of v = 1/2 are found to be significantly smaller than the linear temperature dependent values predicted by Potter et al., and decreasing with temperature faster than linear dependence.

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arXiv2018

Surface segregation and the Al problem in GaAs quantum wells

Low-defect two-dimensional electron systems (2DESs) are essential for studies of fragile many-body interactions that only emerge in nearly-ideal systems. As a result, numerous efforts have been made to improve the quality of modulation-doped Al$_x$Ga$_{1-x}$As/GaAs quantum wells (QWs), with an emphasis on purifying the source material of the QW itself or achieving better vacuum in the deposition chamber. However, this approach overlooks another crucial component that comprises such QWs, the Al$_x$Ga$_{1-x}$As barrier. Here we show that having a clean Al source and hence a clean barrier is instrumental to obtain a high-quality GaAs 2DES in a QW. We observe that the mobility of the 2DES in GaAs QWs declines as the thickness or Al content of the Al$_x$Ga$_{1-x}$As barrier beneath the QW is increased, which we attribute to the surface segregation of Oxygen atoms that originate from the Al source. This conjecture is supported by the improved mobility in the GaAs QWs as the Al cell is cleaned out by baking.

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arXiv2018

Stability of the 5/2 Fractional Quantum Hall State in a Corbino Disc Sample with In-plane Electric Fields

Among the various geometries to study the fractional quantum Hall effect in two dimensional electron gas, the Corbino disc owns the advantage to probe the bulk properties directly. In this work we explore the influence of in-plane electric fields on the stability of the 5/2 fractional quantum Hall state realized in Corbino geometry. The effect of weak electric fields is investigated at ultra-low temperatures in order to compare with a theoretical proposal of enhanced Pfaffian state under weak electric fields.

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arXiv2018

Intersubband scattering in n-GaAs/AlGaAs wide quantum wells

Slow magnetooscilations of the conductivity are observed in a 75 nm wide quantum well at heating of the two-dimensional electrons by a high-intensity surface acoustic wave. These magnetooscillations are caused by intersubband elastic scattering between the symmetric and asymmetric subbands formed due to an electrostatic barrier in the center of the quantum well. The tunneling splitting between these subbands as well as the intersubband scattering rate are determined.

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arXiv2018

Cyclotron Orbits of Composite Fermions in the Fractional Quantum Hall Regime

We study a bilayer GaAs hole system that hosts two distinct many-body phases at low temperatures and high perpendicular magnetic fields. The higher-density (top) layer develops a Fermi sea of composite fermions (CFs) in its half-filled lowest Landau level, while the lower-density (bottom) layer forms a Wigner crystal (WC) as its filling becomes very small. Owing to the inter-layer interaction, the CFs in the top-layer feel the periodic Coulomb potential of the WC in the bottom-layer. We measure the magnetoresistance of the top-layer while changing the bottom-layer density. As the WC layer density increases, the resistance peaks separating the adjacent fractional quantum Hall states in the top-layer change nonmonotonically and attain maximum values when the cyclotron orbit of the CFs encloses one WC lattice point. These features disappear at T = 275 mK when the WC melts. The observation of such geometric resonance features is unprecedented and surprising as it implies that the CFs retain a well-defined cyclotron orbit and Fermi wave vector even deep in the fractional quantum Hall regime, far from half-filling.

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arXiv2018

Split-gate device for indirect excitons

We present a concept and experimental proof of principle for split-gate devices for indirect excitons (IXs). The split-gate forms a narrow channel, a point contact, for IX current. Control of IX flow through the split-gate with both gate voltage and excitation power is demonstrated.

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arXiv2017

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.

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arXiv2017

Berry Phase and Anomalous Transport of the Composite Fermions at the Half-Filled Landau Level

The fractional quantum Hall effect (FQHE) in two-dimensional electron system (2DES) is an exotic, superfluid-like matter with an emergent topological order. From the consideration of Aharonov-Bohm interaction of electrons and magnetic field, the ground state of a half-filled lowest Landau level is mathematically transformed to a Fermi sea of composite objects of electrons bound to two flux quanta, termed composite fermions (CFs). A strong support for the CF theories comes from experimental confirmation of the predicted Fermi surface at $ν$ = 1/2 (where $ν$ is the Landau level filling factor) from the detection of the Fermi wave vector in the semi-classical geometrical resonance experiments. Recent developments in the theory of CFs have led to a prediction of a $π$ Berry phase for the CF circling around the Fermi surface at half-filling. In this paper we provide the first experimental evidence for the detection of the Berry phase of CFs in the fractional quantum Hall effect. Our measurements of the Shubnikov-de Haas oscillations of CFs as a function carrier density at a fixed magnetic field provide a strong support for an existence of a $π$ Berry phase at $ν$ = 1/2. We also discover that the conductivity of composite fermions at $ν$ = 1/2 displays an anomalous linear density dependence, whose origin remains mysterious yet tantalizing.

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arXiv2017

Simultaneous Measurements of Microwave Photoresistance and Cyclotron Reflection in the Multi-Photon Regime

We simultaneously measure photoresistance with electrical transport and plasmon-cyclotron resonance (PCR) using microwave reflection spectroscopy in high mobility GaAs/AlGaAs quantum wells under a perpendicular magnetic field. Multi-photon transitions are revealed as sharp peaks in the resistance and the cyclotron reflection on samples with various carrier densities. Our main finding is that plasmon coupling is relevant in the cyclotron reflection spectrum but has not been observed in the electrical conductivity signal. We discuss possible mechanisms relevant to reflection or dc conductivity signal to explain this discrepancy. We further confirm a trend that higher order multi-photon features can be observed using higher carrier density samples.

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