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Recently, high-order sideband polarimetry has been established as an experimental method that links the polarization of sidebands to an interference of Bloch wavefunctions. However, the robustness of sideband polarizations to increasing dephasing remains to be explored. Here, we investigate the dependence of high-order sideband generation in bulk gallium arsenide on dephasing by tuning temperature. We find that the intensities of the sidebands, but not their polarizations, depend strongly on temperature. Using our polarimetry method, we are able to isolate the contributions of electron-heavy hole (HH) and electron-light hole (LH) pairs to sideband intensities, and separately extract the nonequilibrium dephasing coefficients associated with the longitudinal optical (LO) phonons and acoustic (A) phonons for each species of electron-hole pair. We find that $\Gamma_{\text{HH},\text{A}} = 6.1 \pm 1.6$ $\mu$eV/K, $\Gamma_{\text{LH},\text{A}} < 1.5$ $\mu$eV/K, $\Gamma_{\text{HH},\text{LO}} = 14 \pm 3$ meV, and $\Gamma_{\text{LH},\text{LO}} = 30 \pm 3$ meV., Comment: 10 pages, 4 figures

We report experimental observation and theoretical modeling of fluctuations in a non-equilibrium Bose-Einstein condensate of polaritons near the phase boundary for onset of condensation. We find a critical fluctuation effect in which the fluctuations have a maximum at the phase boundary. Surprisingly, we see these fluctuations on time scales very long compared to the intrinsic scattering time of the polaritons, indicating that the system has metastable telegraph-type switching between modes. The theoretical modeling reproduces the experimental effects, and indicates that classical noise is not sufficient to explain the experiments; quantum noise must be introduced in the model.

Equilibrium Bose-Einstein condensation of exciton-polaritons, demonstrated with a long-lifetime microcavity [Phys. Rev. Lett. 118, 016602 (2017)], has proven that driven-dissipative systems can undergo thermodynamic phase transitions in the limit where the quasiparticle lifetime exceeds the thermalization time. Here, we identify the role of dimensionality and polariton interactions in determining the degree of thermalization in optically generated traps. To distinguish the effect of trapping from interactions and lifetimes, we measured the polariton distribution under four nonresonant Gaussian pumps in a square geometry and compared it with polariton distributions measured with each pump individually. We found that significant redistribution of polaritons arises by trapping and modification of the density of states. Surprisingly efficient polariton-polariton scattering below the condensation threshold is evidenced by the depletion of the inflection-point polaritons. Our work provides a deeper understanding of polariton distributions and their interactions under various geometries of optically generated potentials.

A new reentrant insulating phase (RIP) in low magnetic fields has been reported in the literature in strongly interacting 2D carrier systems and was suggested to be related to the formation of a Wigner crystal [e.g. Qiu et al, PRL 108, 106404 (2012)]. We have studied the transformation between the metallic liquid phase and the low field RIP in a dilute 2D hole system with large interaction parameter $r_s$ (~20-30) in GaAs quantum wells. Instead of a sharp transition, increasing density (or lowering $r_s$) drives the RIP into a state where an incipient RIP coexists with the metallic 2D hole liquid. The non-trivial temperature dependent resistivity and the in-plane magnetic field induced enhancement of the RIP highlight the competition between two phases and the essential role of spin in this mixture phase, and are consistent with the Pomeranchuk effect in a mixture of Wigner crystal and Fermi liquid., arXiv admin note: text overlap with arXiv:1509.07463

The interaction between aligned dipoles is long-ranged and highly anisotropic: it changes from repulsive to attractive depending on the relative positions of the dipoles. We report on the observation of the attractive component of the dipolar coupling between excitonic dipoles in stacked semiconductor bilayers. We show that the presence of a dipolar exciton fluid in one bilayer modifies the spatial distribution and increases the binding energy of excitonic dipoles in a vertically remote layer. The binding energy changes are explained by a many-body polaron model describing the deformation of the exciton cloud due to its interaction with a remote dipolar exciton. The results open the way for the observation of theoretically predicted new and exotic collective phases, the realization of interacting dipolar lattices in semiconductor systems as well as for engineering and sensing their collective excitations., 11 Pages, 9 Figures

Negative longitudinal magnetoresistances (NLMRs) have been recently observed in a variety of topological materials and often considered to be associated with Weyl fermions that have a defined chirality. Here we report NLMRs in non-Weyl GaAs quantum wells. In the absence of a magnetic field the quantum wells show a transition from semiconducting-like to metallic behaviour with decreasing temperature. We observed pronounced NLMRs up to 9 Tesla at temperatures above the transition and weak NLMRs in low magnetic fields at temperatures close to the transition and below 5 K. The observed NLMRs show various types of magnetic field behaviour resembling those reported in topological materials. We attribute them to microscopic disorder and use a phenomenological three-resistor model to account for their various features. Our results showcase a new contribution of microscopic disorder in the occurrence of novel phenomena. They may stimulate further work on tuning electronic properties via disorder/defect nano-engineering.

One of the most intriguing and fundamental properties of topological materials is the correspondence between the conducting edge states and the gapped bulk spectrum. So far, it has been impossible to access the full evolution of edge states with critical parameters such as magnetic field due to poor resolution, remnant bulk conductivity, or disorder. Here, we use a GaAs cleaved edge quantum wire to perform momentum-resolved tunneling spectroscopy. This allows us to probe the evolution of the chiral quantum Hall edge states and their positions from the sample edge with unprecedented precision from very low magnetic fields all the way to high fields where depopulation occurs. We present consistent analytical and numerical models, inferring the edge states from the well known bulk spectrum, finding excellent agreement with the experiment -- thus providing direct evidence for the bulk to edge correspondence. In addition, we observe various features beyond the single-particle picture, such as Fermi level pinning, exchange-enhanced spin splitting and signatures of edge-state reconstruction., 8 pages, 4 color figures, including supplementary on arXiv

We measure the spin-resolved transport of dipolar excitons in a biased GaAs double quantum well structure. From these measurements we extract both spin lifetime and mobility of the excitons. We find that below a temperature of $4.8$K, there is a sharp increase in the spin lifetime of the excitons, together with a sharp reduction in their mobility. Below a critical power the spin lifetime increases with increasing mobility and density, while above the critical power the opposite trend is observed. We interpret this transition as an evidence of the interplay between two different spin dephasing mechanisms: at low mobility the dephasing is dominated by the hyperfine interaction with the lattice nuclei spins, while at higher mobility the spin-orbit interaction dominates, and a Dyakonov-Perel spin relaxation takes over. The excitation power and temperature regime where the hyperfine interaction induced spin dephasing is observed correlates with the regime where a dark dipolar quantum liquid was reported recently on a similar sample.

Some theories predict that the filling factor 5/2 fractional quantum Hall state can exhibit non-Abelian statistics, which makes it a candidate for fault-tolerant topological quantum computation. Although the non-Abelian Pfaffian state and its particle-hole conjugate, the anti-Pfaffian state, are the most plausible wave functions for the 5/2 state, there are a number of alternatives with either Abelian or non-Abelian statistics. Recent experiments suggest that the tunneling exponents are more consistent with an Abelian state rather than a non-Abelian state. Here, we present edge-current-tunneling experiments in geometrically confined quantum point contacts, which indicate that Abelian and non-Abelian states compete at filling factor 5/2. Our results are consistent with a transition from an Abelian state to a non-Abelian state in a single quantum point contact when the confinement is tuned. Our observation suggests that there is an intrinsic non-Abelian 5/2 ground state, but that the appropriate confinement is necessary to maintain it. This observation is important not only for understanding the physics of the 5/2 state, but also for the design of future topological quantum computation devices.

The possible phases and the nano-scale particle correlations of two-dimensional interacting dipolar particles is a long-sought problem in many-body physics. Here we observe a spontaneous condensation of trapped two-dimensional dipolar excitons with internal spin degrees of freedom from an interacting gas into a high density, closely packed liquid state made mostly of dark dipoles. Another phase transition, into a bright, highly repulsive plasma is observed at even higher excitation powers. The dark liquid state is formed below a critical temperature $T_c \approx 4.8K$, and it is manifested by a clear spontaneous spatial condensation to a smaller and denser cloud, suggesting an attractive part to the interaction which goes beyond the purely repulsive dipole-dipole forces. Contributions from quantum mechanical fluctuations are expected to be significant in this strongly correlated, long living dark liquid. This is a new example of a two-dimensional atomic-like interacting dipolar quantum liquid, but where the coupling of light to its internal spin degrees of freedom plays a crucial role in the dynamical formation and the nature of resulting ground state., Supplementary information follows the references section

Introduction of a Josephson field effect transistor (JoFET) concept sparked active research on proximity effects in semiconductors. Induced superconductivity and electrostatic control of critical current has been demonstrated in two-dimensional gases in InAs, graphene and topological insulators, and in one-dimensional systems including quantum spin Hall edges. Recently, interest in superconductor-semiconductor interfaces was renewed by the search for Majorana fermions, which were predicted to reside at the interface. More exotic non-Abelian excitations, such as parafermions (fractional Majorana fermions) or Fibonacci fermions may be formed when fractional quantum Hall edge states interface with superconductivity. In this paper we develop transparent superconducting contacts to high mobility two-dimensional electron gas (2DEG) in GaAs and demonstrate induced superconductivity across several microns. Supercurrent in a ballistic junction has been observed across 0.6 $��$m of 2DEG, a regime previously achieved only in point contacts but essential to the formation of well separated non-Abelian states. High critical fields ($>16$ Tesla) in NbN contacts enables investigation of a long-sought regime of an interplay between superconductivity and strongly correlated states in a 2DEG at high magnetic fields., 11 pages, 7 figures

Exciton-polaritons in a microcavity are composite two-dimensional bosonic quasiparticles, arising from the strong coupling between confined light modes in a resonant planar optical cavity and excitonic transitions, typically using excitons in semiconductor quantum wells (QWs) placed at the antinodes of the same cavity. Quantum phenomena such as Bose-Einstein condensation (BEC), quantized vortices, and macroscopic quantum states have been reported at temperatures from tens of Kelvin up to room temperatures, and polaritonic devices such as spin switches \cite{Amo2010} and optical transistors have also been reported. Many of these effects of exciton-polaritons depend crucially on the polariton-polariton interaction strength. Despite the importance of this parameter, it has been difficult to make an accurate experimental measurement, mostly because of the difficulty of determining the absolute densities of polaritons and bare excitons. Here we report the direct measurement of the polariton-polariton interaction strength in a very high-Q microcavity structure. By allowing polaritons to propagate over 40 $\mu$m to the center of a laser-generated annular trap, we are able to separate the polariton-polariton interactions from polariton-exciton interactions. The interaction strength is deduced from the energy renormalization of the polariton dispersion as the polariton density is increased, using the polariton condensation as a benchmark for the density. We find that the interaction strength is about two orders of magnitude larger than previous theoretical estimates, putting polaritons squarely into the strongly-interacting regime. When there is a condensate, we see a sharp transition to a different dependence of the renormalization on the density, which is evidence of many-body effects., Comment: 27 pages, 14 figures

A fabrication method for electronic quantum Hall Fabry-Perot interferometers (FPI) is presented. Our method uses a combination of e-beam lithography and low-damage dry-etching to produce the FPIs and minimize the excitation of charged traps or deposition of impurities near the device. Optimization of the quantum point contacts (QPC) is achieved via systematically varying the etch depth and monitoring the device resistance between segmented etching sessions. The etching is stopped when a desired value of resistance is obtained. Finer control of interference trajectories is obtained by the gate metallized inside the etched area by e-beam evaporation. Our approach allows for a control of the delicate potential bending near the quantum well by tuning the confining potential of the quantum point contacts., Comment: 3 pages, 3 figures

We report an on-chip solid-state Mach-Zehnder interferometer operating on two-dimensional (2D) plasmonic waves at microwave frequencies. Two plasmonic paths are defined with GaAs/AlGaAs 2D electron gas 80 nm below a metallic gate. The gated 2D plasmonic waves achieve a velocity of ~c/300 (c: free-space light speed). Due to this ultra-subwavelength confinement, the resolution of the 2D plasmonic interferometer is two orders of magnitude higher than that of its electromagnetic counterpart at a given frequency. This GHz proof-of-concept at cryogenic temperatures can be scaled to the THz IR range for room temperature operation, while maintaining the benefits of the ultra-subwavelength confinement., 18 pages, 11 figures. The article has been submitted to Applied Physics Letters. After it is published, it will be found at http://apl.aip.org/

We report new results of Bose-Einstein condensation of polaritons in specially designed microcavities with very high quality factor, on the order of $10^6$, giving the polariton lifetimes of the order of 100 ps. When the polaritons are created with an incoherent pump, a dissipationless, coherent flow of the polaritons occurs over hundreds of microns, which increases as density increases. At high density, this flow is suddenly stopped, and the gas becomes trapped in a local potential minimum, with strong coherence., Comment: 4 figures. Physical Review X (2013)

It has been puzzling that the resistivity of high mobility two-dimensional(2D) carrier systems in semiconductors with low carrier density often exhibits a large increase followed by a decrease when the temperature ($T$) is raised above a characteristic temperature comparable with the Fermi temperature ($T_F$). We find that the metallic 2D hole system (2DHS) in GaAs quantum well (QW) has a linear density ($p$) dependent conductivity, $\sigma\approx e\mu^*(p-p_0)$, in both the degenerate (T

We study the effects of a perpendicular magnetic field on the spin and charge distribution across a quantum wire. Using momentum resolved tunneling between two parallel wires we measure the dispersion relation for different perpendicular magnetic fields. We find that as the magnetic field increases, a strip of spin polarized electrons separates in the cross section of the wire. We provide a quantitative description of the emerging structure within a Hartree-Fock approximation, showing that it results from exchange interaction between electrons in the wire. We discuss the relation of these results to the structure of Quantum Hall edge states.

Inter-layer excitonic coherence in a quantum Hall bilayer with negligible tunneling is monitored by measurements of low-lying spin excitations. At $\nu_T =1$ new quasiparticle excitations are observed above a transition temperature revealing a competing metallic phase. For magnetic fields above an onset Zeeman energy this metallic phase has full spin polarization. A phase diagram in the parameter space of temperature and Zeeman energy reveals that the transition temperature increases at higher fields. This unexpected result suggests intriguing impacts of spin polarization in the highly correlated phases., Comment: 5 pages and 5 figures

Complexity in many-particle systems occurs through processes of qualitative differentiation. These are described by concepts such as emerging states with specific symmetries that are linked to order parameters. In quantum Hall phases of electrons in semiconductor double layers with large inter-layer electron correlation there is an emergent many body exciton phase with an order parameter that measures the condensate fraction of excitons across the tunneling gap. As the inter-layer coupling is reduced by application of an in-plane magnetic field, this excitonic insulating state is brought in competition with a Fermi-metal phase of composite fermions (loosely, electrons with two magnetic flux quanta attached) stabilized by intra-layer electron correlation. Here we show that the quantum phase transformation between metallic and excitonic insulating states in the coupled bilayers becomes discontinuous (first-order) by impacts of different terms of the electron-electron interactions that prevail on weak residual disorder. The evidence is based on precise determinations of the excitonic order parameter by inelastic light scattering measurements close to the phase boundary. While there is marked softening of low-lying excitations, our experiments underpin the roles of competing orders linked to quasi-particle correlations in removing the divergence of quantum fluctuations., Comment: 18 pages, 4 figures, supp. mat. contains 3 figures

Dilute arrays of GaAs/AlGaAs modulation-doped quantum dots (QDs) fabricated by electron-beam lithography and low impact reactive-ion etching exhibit highly homogeneous luminescence. Single quantum dots display spectral emission with peak energies and linewidths linked largely to the geometrical diameter of the dot and to the built-in electron population. Excitonic-like and biexcitonic-like emission intensities have activation energy of about 2 meV. These results highlight the potential of high quality nanofabricated QDs for applications in areas that require fine control of optical emission., Comment: 3 Figures

We show both experimentally and theoretically that the recently observed optically induced in-plane charge separation in quantum well (QW) structures and the exciton ring emission pattern at this charge separation boundary have an extremely long lifetime. The oppositely charged carriers remain separated and provide a reservoir of excitons at their boundary with a persistent emission which lasts hundreds of microseconds (orders of magnitude longer than their recombination time) after the external excitation is removed. This long lifetime is due to an interplay between the slow in-plane carrier diffusion and slow carrier tunnelling perpendicular to the QW plane., Comment: 13 pages, 4 figures

A recent single quantum wire is of sufficient quality to reveal new details of the photoluminescence (PL) evolution with increasing electron--hole (e--h) pair density. At a pair density of 3.6 $\times$ 10$^{3}$ cm$^{-1}$, the PL is characteristic of biexcitons shifted below the exciton peak by the 2.8-meV biexciton binding. At the pair density of 1.2 $\times$ 10$^{5}$ cm$^{-1}$, the biexciton peak broadens without energy shift to an e--h plasma. At all pair densities up to 30 K, neither the exciton peak nor the one-dimensional (1D) continuum edge shows any shift. In contrast to prevailing theories, the low-energy edge of the plasma PL line never crosses the exciton peak and never makes contact with the 1D e--h continuum., Comment: 4 pages, including 3 figures; revised with new data

About a year ago, two independent experiments [1,2], imaging indirect exciton luminescence from doped double quantum wells under applied bias and optical excitation, reported a very intriguing observation: under certain experimental conditions, the exciton luminescence exhibits a ring pattern with a dark region in between the center excitation spot and the luminescent ring that can extend more than a millimeter from the center spot. Initial speculations on the origin of this emission pattern included supersonic ballistic transport of excitons due to their dipole-dipole repulsion and Bose superfluidity of excitons. In this paper we show that the ring effect is also observed in single quantum well structures, where only direct excitons exist. More importantly, we find that these experimental results are quantitatively explained by a novel coupled 2D electron-hole plasma dynamics, namely, photoinduced in-plane charge separation. This charge separation explains extremely long luminescence times that may be more than a microsecond for the ring -- orders of magnitude longer than the emission lifetime of the excitons in the center spot. This method of continuously creating excitons may result in a highly dense exciton gas which is also well thermalized with the lattice (since the particles can cool over the very long luminescence time after their hot optical creation), thus opening up opportunities for a detailed study of quantum statistics. The in-plane separation of the charges into positive and negative regions, with a sharp interface between them is an interesting new example of nonequilibrium dynamics and pattern formation., 17 pages, 6 figures

The recent report on the observation of soft magnetorotons in the dispersion of charge-density excitations across the tunneling gap in coupled bilayers at total Landau level filling factor $\nu_T=1$ is reviewed. The inelastic light scattering experiments take advantage of the breakdown of wave-vector conservation that occurs under resonant excitation. The results offer evidence that in the quantum Hall state there is a roton that softens and sharpens markedly when the phase boundary for transitions to highly-correlated compressible states is approached. These findings are interpreted with Hartree-Fock evaluations of the dynamic structure factor. The model includes the effect of disorder in the breakdown of wave-vector conservation and resonance enhancement profiles within a phenomenological approach. These results link the softening of magnetorotons to enhanced excitonic Coulomb interactions in the ferromagnetic bilayers., Comment: 6 pages, 5 figures; conference: EP2DS-15

A multi-quantum wire laser operating in the 1-D ground state has been achieved in a very high uniformity structure that shows free exciton emission with unprecedented narrow width and low lasing threshold. Under optical pumping the spontaneous emission evolves from a sharp free exciton peak to a red-shifted broad band. The lasing photon energy occurs about 5 meV below the free exciton. The observed shift excludes free excitons in lasing and our results show that Coulomb interactions in the 1-D electron-hole system shift the spontaneous emission and play significant roles in laser gain., Comment: 4 pages, 4 figures, prepared by RevTeX

We present the first magneto-absorption studies of coupled electron double layers in the quantum Hall regime. Optical absorption spectra in the vicinity of total filling factor nu = 1 reveal intriguing behavior that have no analog in the single electron layer nu = 1 state and demonstrate the interplay between single-particle tunneling and inter-layer Coulomb effects. The spectra provide direct evidence of a ground-state that evolves from a region dominated by single-particle tunneling to a regime in which inter-layer Coulomb interactions determine the nature of the ground-state. Moreover the spectra provide the first direct evidence that the incompressible ground-state at nu = 1 in the many-body regime is not fully pseudospin polarized and is sensitive to the effects of quantum fluctuations in the pseudospin variable., Comment: 5 pages, 5 figures

We realize a nanoscale-area Mach-Zehnder interferometer with co-propagating quantum Hall spin-resolved edge states and demonstrate the persistence of gate-controlled quantum interference oscillations, as a function of an applied magnetic field, at relatively large temperatures. Arrays of top-gate magnetic nanofingers are used to induce a resonant charge transfer between the pair of spin-resolved edge states. To account for the pattern of oscillations measured as a function of magnetic field and gate voltage, we have developed a simple theoretical model which satisfactorily reproduces the data., Comment: 7 pages, 5 figures