Higgs modes in condensed matter physics have drawn attention because of analogies to the Higgs bosons of particle physics. Here we use a microscopic time-dependent mean-field theory to study the collective mode spectra of two-dimensional spatially indirect exciton (electron-hole pair) condensates, focusing on the Higgs modes, i.e., those that have a large weight in electron-hole pair amplitude response functions. We find that in the low exciton density (BEC) limit, the dominant Higgs modes of spatially indirect exciton condensates correspond to adding electron-hole pairs that are orthogonal to the condensed pair state. We comment on the previously studied Higgs-like collective excitations of superconductors in light of this finding.
The photoluminescence dynamics of a microscopic gas of indirect excitons trapped in coupled quantum wells is probed at very low bath temperature (approximately 350 mK). Our experiments reveal the non linear energy relaxation characteristics of indirect excitons. Particularly, we observe that the excitons dynamics is strongly correlated with the screening of structural disorder by repulsive exciton-exciton interactions. For our experiments where two-dimensional excitonic states are gradually defined, the distinctive enhancement of the exciton scattering rate towards lowest energy states with in…
We have succeeded in trapping indirect excitons in coupled quantum wells in a harmonic potential minimum via inhomogeneous applied stress and electric field. These excitons exhibit a strong Stark shift (over 60 meV), long lifetime (100 ns), and high diffusivity (1000 cm$^2$/s). This approach is very promising for obtaining the high exciton density needed for Bose condensation of excitons in two dimensions.
We propose the application of nonlinear optics for studies of spatially indirect excitons in coupled quantum wells. We demonstrate, that despite their vanishing oscillator strength, indirect excitons can strongly contribute to the photoinduced reflectivity and Kerr rotation. This phenomenon is governed by the interaction between direct and indirect excitons. Both dark and bright states of indirect excitons can be probed by these nonlinear optical techniques.
The exciton system in double quantum well is considered under condition when the ground state is the spatially indirect exciton. At high pumping growth of the exciton concentration can lead to so significant increase of the indirect exciton energy that becomes equal to the direct exciton energy. Then further increase of pumping leads to formation of mixed direct - indirect exciton phase. A rough estimate of the exciton energy in the mixed phase explains puzzling features of some recent exciton measurements. An experiment that would reveal main characteristic features of the mixed phase is sugg…
Spin transport of indirect excitons in GaAs/AlGaAs coupled quantum wells was observed by measuring the spatially resolved circular polarization of exciton emission. Exciton spin transport over several microns originates from a long spin relaxation time and long lifetime of indirect excitons.
Spin transport of indirect excitons in GaAs/AlGaAs coupled quantum wells was observed by measuring the spatially resolved circular polarization of the exciton emission. The exciton spin transport originates from the long spin relaxation time and long lifetime of the indirect excitons.
We propose an optoelectronic scheme to define and manipulate an indirect neutral exciton qubit within a quantum dot molecule. We demonstrate coherent dynamics of indirect excitons resilient against decoherence effects, including direct exciton spontaneous recombination. For molecules with large interdot separation, the exciton dressed spectrum yields an often overlooked avoided crossing between spatially indirect exciton states. Effective two level system Hamiltonians are extracted by Feshbach projection over the multilevel exciton configurations. An adiabatic manipulation of the qubit states …
A simple method to create and control magnetic potentials onto coupled quantum wells is demonstrated for indirect-exciton magnetic confinement. Localized inhomogeneous magnetic potentials with periodically distributed local minima and maxima, known as magnetic lattices, are projected into the plane of the coupled quantum wells and used for the spatially distributed two-dimensional indirect-exciton trapping, in which case localized indirect-excitons are observed. The indirect-exciton trapping mechanism is examined by controlling external magnetic field bias resulting in a shift of the localized…
We realized a potential energy gradient - a ramp - for indirect excitons using a shaped electrode at constant voltage. We studied transport of indirect excitons along the ramp and observed that the exciton transport distance increases with increasing density and temperature.
We show that the "dark condensates" that arise when excitons form a Bose-Einstein condensate in a material with an indirect bandgap are not completely dark to optical emission. Rather, such states are "leaky condensates" in which optical emission is facilitated by many-body interactions. We analyze the properties of these leaky condensates in the context of twisted bilayers of transition metal dichalcogenides, which host strongly interacting excitons and an indirect bandgap. We show that this interaction-driven "leaky" emission dominates photoluminescence at low temperatures, with distinctive …
In-plane magnetic field is found to change drastically the photoluminescence spectra and kinetics of interwell excitons in GaAs/AlGaAs coupled quantum wells. The effect is due to the in-plane magnetic field induced displacement of the interwell exciton dispersion in a momentum space, which results in the transition from the momentum-space direct exciton ground state to the momentum-space indirect exciton ground state. In-plane magnetic field is, therefore, an effective tool for the exciton dispersion engineering.
Bilayer electron-hole systems undergo excitonic condensation when the distance d between the layers is smaller than the typical distance between particles within a layer. All excitons in this condensate have a fixed dipole moment which points perpendicular to the layers, and therefore this condensate of dipoles couples to external electromagnetic fields. We study the transport properties of this dipolar condensate system based on a phenomenological model which takes into account contributions from the condensate and quasiparticles. We discuss, in particular, the drag and counterflow transport,…
We demonstrate that perfect conversion between charged supercurrents in superconductors and neutral supercurrents in electron-hole pair condensates is possible via a new Andreev-like scattering mechanism. As a result, when two superconducting circuits are coupled through a bilayer exciton condensate, the superflow in both layers is drastically modified. Depending on the phase biases the supercurrents can be completely blocked or exhibit perfect drag.
We present spatially- and spectrally-resolved photoluminescence measurements of indirect excitons in high magnetic fields. Long indirect exciton lifetimes give the opportunity to measure magnetoexciton transport by optical imaging. Indirect excitons formed from electrons and holes at zeroth Landau levels (0e - 0h indirect magnetoexcitons) travel over large distances and form a ring emission pattern around the excitation spot. In contrast, the spatial profiles of 1e - 1h and 2e - 2h indirect magnetoexciton emission closely follow the laser excitation profile. The 0e - 0h indirect magnetoexciton…
Bose condensation is responsible for many of the most spectacular effects in physics because it can promote quantum behavior from the microscopic to the macroscopic world. Bose condensates can be distinguished by the condensing object; electron-electron Cooper-pairs are responsible for superconductivity, Helium atoms for superfluidity, and ultracold alkali atoms in vapors for coherent matter waves. Electron-hole pair (exciton) condensation has maintained special interest because it has been difficult to realize experimentally, and because exciton phase coherence is never perfectly spontaneous.…
Long-lived indirect excitons (IXs) exhibit a rich phase diagram, including a Bose-Einstein condensate (BEC), a Wigner crystal, and other exotic phases. Recent experiments have hinted at a new, "classical" liquid of IXs above the BEC transition. To uncover the nature of this phase, we use a broad range of theoretical tools and find no evidence of a driving force towards classical condensation. Instead, we attribute the condensed phase to a quantum electron-hole liquid (EHL), first proposed by Keldysh for direct excitons. Taking into account the association of free carriers into bound excitons, …
We report on spatially- and time-resolved emission measurements and observation of transport of indirect excitons in ZnO/MgZnO wide single quantum wells.
Spatially indirect excitons with displaced wavefunctions of electrons and holes play a pivotal role in a large portfolio of fascinating physical phenomena and emerging optoelectronic applications, such as valleytronics, exciton spin Hall effect, excitonic integrated circuit and high-temperature superfluidity. Here, we uncover three types of spatially indirect excitons (including their phonon replicas) and their quantum-confined Stark effects in hexagonal boron nitride encapsulated bilayer WSe2, by performing electric field-tunable photoluminescence measurements. Because of different out-of-pla…
We report on the kinetics of a low-temperature gas of indirect excitons in the optically-induced exciton trap. The excitons in the region of laser excitation are found to rapidly -- within 4 ns -- cool to the lattice temperature T = 1.4 K, while the excitons at the trap center are found to be cold -- essentially at the lattice temperature -- even during the excitation pulse. The loading time of excitons to the trap center is found to be about 40 ns, longer than the cooling time yet shorter than the lifetime of the indirect excitons. The observed time hierarchy is favorable for creating a dense…
We report on the principle and realization of an excitonic device: a ramp that directs the transport of indirect excitons down a potential energy gradient created by a perforated electrode at constant voltage. The device provides an experimental proof of principle for controlling exciton transport with electrode density gradients. We observed that the exciton transport distance along the ramp increases with increasing exciton density. This effect is explained in terms of disorder screening by repulsive exciton-exciton interactions.
We demonstrate an electrostatic trap for indirect excitons in a field-effect structure based on coupled GaAs quantum wells. Within the plane of a double quantum well indirect excitons are trapped at the perimeter of a SiO2 area sandwiched between the surface of the GaAs heterostructure and a semitransparent metallic top gate. The trapping mechanism is well explained by a combination of the quantum confined Stark effect and local field enhancement. We find the one-dimensional trapping potentials in the quantum well plane to be nearly harmonic with high spring constants exceeding 10 keV/cm^2.
We introduced an elevated trap technique and exploited it for lowering the effective temperature of indirect excitons. We observed narrow photoluminescence lines which correspond to the emission of individual states of indirect excitons in a disorder potential. We studied the effect of exciton-exciton interaction on the localized and delocalized exciton states and found that the homogeneous line broadening increases with density and dominates the linewidth at high densities.
Exciton-polaritons are mixed light-matter quasiparticles. We have developed a statistical model describing stochastic exciton-photon transitions within a condensate of exciton polaritons. We show that the exciton-photon correlator depends on the "hidden variable" which characterizes the rate of exciton-photon transformations in the condensate. We discuss implications of this effect for the quantum statistics of photons emitted by polariton lasers.
Double-layer electron systems in the quantum Hall regime have excitonic condensate ground states when the layers are close together and the total Landau level filling factor is close to an odd integer. In this paper we discuss the microscopic physics of these states and recent progress toward building a theory of the anomalous collective transport effects that occur in separately contacted bilayers when the condensate is present.
We predict angle-dependent superfluidity for a new class of 2D materials -- transition metals trichalcogenides (TMTC). Within a mean-field approach superfluidity of indirect excitons in TMTC van der Waals heterostructures (vdWHs) is studied. We use different potentials for charged carriers interaction to analyze the influence of the screening on the studied phenomena. Our study demonstrates the angle-dependent superfluidity temperature in TMTC vdWHs: for a given density a maximum and minimum $T_c$ of superfluidity occurs along chain and $a$-directions, respectively. This work can guide experim…
The superfluid phase transition of bosons in a two-dimensional (2D) system with disorder and an external parabolic potential is studied. The theory is applied to experiments on indirect excitons in coupled quantum wells. The random field is allowed to be large compared to the dipole-dipole repulsion between excitons. The slope of the external parabolic trap is assumed to change slowly enough to apply the local density approximation (LDA) for the superfluid density, which allows us to calculate the Kosterlitz-Thouless temperature $T_{c}(n(r))$ at each local point $r$ of the trap. The superfluid…
The variety of excitonic states in tungsten-based dichalcogenide monolayers stems from unique interplay between the spin and valley degrees of freedom. One of the exciton species is the indirect exciton (momentum or valley dark), which is responsible to a series of resonances when the monolayer is charge neutral. We investigate the short-range electron-hole exchange interaction of the indirect exciton, as well as its recombination mechanism mediated by impurities. The analysis provides thorough understanding of the energy and polarization of the zero-phonon indirect exciton resonance in the em…
We investigate the transport properties of a bilayer exciton condensate that is contacted by four superconducting leads. We focus on the equilibrium regime and investigate how the Josephson currents induced in the bilayer by phase biases applied to the superconducting electrodes are affected by the presence of an exciton condensate in the bulk of the system. As long as the distance between the superconducting electrodes is much larger than the exciton coherence length, the Josephson current depends only on the difference between the phase biases in the two layers. This result holds true in bot…
We demonstrate experimental proof of principle for a stirring potential for indirect excitons. The azimuthal wavelength of this stirring potential is set by the electrode periodicity, the amplitude is controlled by the applied AC voltage, and the angular velocity is controlled by the AC frequency.
We study the photoluminescence dynamics of ultra-cold indirect excitons optically created in a double quantum well heterostructure. Above a threshold laser excitation, our experiments reveal the apparition of the so-called inner photoluminescence ring. It is characterized by a ring shaped photoluminescence which suddenly collapses once the laser excitation is terminated. We show that the spectrally resolved dynamics is in agreement with an excitonic origin for the inner-ring which is formed due to a local heating of indirect excitons by the laser excitation. To confirm this interpretation and …
We propose a coherent control scheme based on the optical Stark effect in optically generated excitons in quantum dot molecules (QDMs). We show that, by the combined action of voltage bias detuning sweeps and Rosen-Zener pulsed interactions, it is possible to dynamically generate and modify an anticrossing gap that emerges between the dressed energy levels of long-lived, spatially indirect excitons. We perform numerical and analytic non-perturbative calculations based on the Bloch-Feshbach formalism, which demonstrate that this effect induces a mechanism of coherent population trapping of indi…
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.
Indirect excitons in coupled quantum wells are long-living quasi-particles, explored in the studies of collective quantum states. We demonstrate, that despite the extremely low oscillator strength, their spin and population dynamics can by addressed by time-resolved pump-probe spectroscopy. Our experiments make it possible to unravel and compare spin dynamics of direct excitons, indirect excitons and residual free electrons in coupled quantum wells. Measured spin relaxation time of indirect excitons exceeds not only one of direct excitons, but also one of free electrons by two orders of magnit…
Monolayer transition metal dichalcogenides are a promising platform to investigate many-body interactions of excitonic complexes. In monolayer tungsten diselenide, the ground-state exciton is dark (spin-indirect), and the valley degeneracy allows low-energy dark momentum-indirect excitons to form. Interactions between the dark exciton species and the optically accessible bright exciton (X) are likely to play significant roles in determining the optical properties of X at high power, as well as limiting the ultimate exciton densities that can be achieved, yet so far little is known about these …
We study the effect of non-homogeneous out-of-plane magnetic field on the behaviour of 2D spatially indirect excitons. Due to the difference of magnetic field acting on electrons and holes the total Lorentz force affecting the center of mass motion of an indirect exciton appears. Consequently, an indirect exciton acquires an effective charge proportional to the gradient of the magnetic field. The appearance of the Lorentz force causes the Hall effect for neutral bosons which can be detected by measurement of the spatially inhomogeneous blueshift of the photoluminescence using counter-flow expe…
We analyse the interaction of exciton and exciton-polariton condensates in semiconductor microcavity with a coherent acoustic wave. An analytical solution for the dispersion of excitations of coupled condensate-phonon system is found in the approximation of k-independent interactions. Accounting for k-dependence results in a stronger modification of the dispersion, and even in the appearance of the "roton instability" region.
It is shown that exciton condensates exhibit an incredible richness in the role that fundamental symmetries play whether manifest or broken. We investigate the appearance of the singlet and the triplet excitonic condensates under spin-orbit coupling (SOC) and consider realistic couplings to the radiation field as manipulated by the fundamental symmetries related to spin, time, orbital and particle degrees of freedom. The role of the strong SOC in inducing a ground state with coexisting singlet and triplet components is shown. It is also shown that the ground state of an excitonic condensate, i…
We study transport of indirect excitons in GaAs/AlGaAs coupled quantum wells in linear lattices created by laterally modulated gate voltage. The localization-delocalization transition (LDT) for transport across the lattice was observed with reducing lattice amplitude or increasing exciton density. The exciton interaction energy at the transition is close to the lattice amplitude. These results are consistent with the model, which attributes the LDT to the interaction-induced percolation of the exciton gas through the external potential. We also discuss applications of the lattice potentials fo…
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.
Indirect excitons (IXs) in van der Waals transition-metal dichalcogenide (TMD) heterostructures are characterized by a high binding energy making them stable at room temperature and giving the opportunity for exploring fundamental phenomena in excitonic systems and developing excitonic devices operational at high temperatures. We present the observation of IXs at room temperature in van der Waals TMD heterostructures based on monolayers of MoS$_2$ separated by atomically thin hexagonal boron nitride. The IXs realized in the TMD heterostructure have lifetimes orders of magnitude longer than lif…
We investigated excitonic absorptions in GaAs/AlAs/GaAs single barrier heterostructure using both photocapacitance and photocurrent spectroscopies at room temperature. Photocapacitance spectra show well defined resonance peak of indirect excitons formed around the Gamma-AlAs barrier. Unlike DC-photocurrent spectra, frequency dependent photocapacitance spectra interestingly red shift, sharpen up and then decrease with increasing tunneling at higher biases. Such dissimilarities clearly point out that different exciton dynamics govern these two spectral measurements. We also argue why such quantu…
We studied, theoretically, the optical absorption spectra for a slightly asymmetric double quantum well (DQW), in the presence of electric and magnetic fields. Recent experimental results for a 10.18/3.82/9.61 nm GaAs Al(_{.33} )Ga(_{.67})As DQW show clearly the different behavior in the luminescence peaks for the indirect exciton (IX) and left direct exciton (DX) as a function of the external electric field. We show that the presence of a peak near the (DX) peak, attributed to an impurity bound left (DX) in the experimental results, could be a consequence of the non-trivial mixing between exc…
We propose a scheme for the controllable preparation of a cold indirect exciton condensate using dipolaritonic setup with an optical pumping. Dipolaritons are bosonic quasiparticles which arise from the coupling between cavity photon (C), direct exciton (DX), and indirect exciton (IX) modes, and appear in a double quantum well embedded in a semiconductor microcavity. Controlling the detuning between modes of the system, the limiting cases of exciton-polaritons and indirect excitons can be realized. Our protocol relies on the initial preparation of an exciton polariton condensate for the far bl…
We analyze the nonlinear transport properties of a bilayer exciton condensate that is contacted by four metallic leads by calculating the full counting statistics of electron transport for arbitrary system parameters. Despite its formal similarity to a superconductor the transport properties of the exciton condensate turn out to be completely different. We recover the generic features of exciton condensates such as counterpropagating currents driven by excitonic Andreev reflections and make predictions for nonlinear transconductance between the layers as well as for the current (cross)correlat…
The superfluidity and Bose-Einstein condensation of indirect excitons in two-dimensional quantum dot are studied by path-integral Monte Carlo simulations. The temperature dependence of superfluid and bose-condensed fraction are calculated at different strengths of interaction. Using the Kosterlitz-Thouless recursion relations, we also predict behavior of superfluid fraction in macroscopic large systems.
The objective of this work is to numerically integrate in space and time the effective-mass Schrodinger equation for an excitonic wave packet in a coupled quantum-well system under a time-dependent electric field. Taking as a starting point a time-dependent Hartree potential, we derive the nonlinear dynamical evolution of the excitonic wave function. We found that the system resonant condition can be modified owing to the reaction field generated by a charge dynamically trapped in the quantum-well pair. As a result, this study raises a broader question: is it possible to create an electrostati…
We present a generic model for the description of non-equilibrium Bose-Einstein condensates, suited for the modelling of non-resonantly pumped polariton condensates in a semiconductor microcavity. The excitation spectrum and scattering of the non-equilibrium condensate with a defect are discussed.
Using a nonlinear Schrödinger equation including short-range two-body attraction and three-body repulsion, we investigate the spatial distribution of indirect excitons in semiconductor coupled quantum wells. The results obtained can interpret the experimental phenomenon that annular exciton cloud first contracts then expands when the number of confined excitons is increased in impurity potential well, as observed by Lai \emph{et al.} [Lai $et al.$, Science \textbf{303}, 503 (2004)]. In particular, the model reconciles the patterns of exciton rings reported by Butov \emph{et al.} [Butov $et al.…
The spin structure and spin dynamics of excitons in an ensemble of (In,Al)As/AlAs quantum dots (QDs) with type-I band alignment, containing both direct and indirect band gap dots, are studied. Time-resolved and spectral selective techniques are used to distinguish between the direct and indirect QDs. The exciton fine structure is studied by means of optical alignment and optical orientation techniques in magnetic fields applied in the Faraday or Voigt geometries. A drastic difference in emission polarization is found for the excitons in the direct QDs involving a $Γ$-valley electron and the ex…
We report on the principle and realization of a new trap for excitons -- the diamond electrostatic trap -- which uses a single electrode to create a confining potential for excitons. We also create elevated diamond traps which permit evaporative cooling of the exciton gas. We observe collection of excitons towards the trap center with increasing exciton density. This effect is due to screening of disorder in the trap by the excitons. As a result, the diamond trap behaves as a smooth parabolic potential which realizes a cold and dense exciton gas at the trap center.
We derive an effective Bose-Hubbard model that predicts a phase transition from Bose-Einstein condensate to Mott insulator in two different systems subject to applied periodic potentials: microcavity exciton-polaritons and indirect excitons. Starting from a microscopic Hamiltonian of electrons and holes, we derive an effective Bose-Hubbard model for both systems and evaluate the on-site Coulomb interaction U and hopping transition amplitudes t. Experimental parameters required for observing a phase transition between a Bose-Einstein condensate and a Mott insulator are discussed. Our results su…
We report on a method for the realization of two-dimensional electrostatic lattices for excitons using patterned interdigitated electrodes. Lattice structure is set by the electrode pattern and depth of the lattice potential is controlled by applied voltages. We demonstrate square, hexagonal, and honeycomb lattices created by this method.
Under optical excitation, coupled quantum wells are known to reveal fascinating features in the photoluminescence pattern originating from dipole orientated indirect excitons. The appearance of an external ring has been attributed to macroscopic charge separation in the quantum well plane. We present a classical model of non-linear diffusion to account for the observed fragmentation of the external ring into a periodic array of islands. The model incorporates the Coulomb interactions between electrons, holes and indirect excitons. At low temperatures, these interactions lead to pattern formati…
We report experiments showing that spatially indirect excitons confined in a wide single quantum well can exhibit macroscopic spatial coherence. Extended coherence is spontaneously established in the regime where indirect excitons form a distinctive ring shaped pattern fragmented into microscopic beads. These contain a large concentration of indirect excitons at sub-Kelvin temperatures, the excitons spatial coherence being the greatest in the vicinity of the fragments.
With exciton lifetime much extended in semiconductor quantum-well structures, their transport and Bose-Einstein condensation become a focus of research in recent years. We reveal a momentum-space gauge field in the exciton center-of-mass dynamics due to Berry phase effects. We predict spin-dependent topological transport of the excitons analogous to the anomalous Hall and Nernst effects for electrons. We also predict spin-dependent circulation of a trapped exciton gas and instability in an exciton condensate in favor of vortex formation.
We show that a Bose-Einstein condensate consisting of dark excitons forms in GaAs coupled quantum wells at low temperatures. We find that the condensate extends over hundreds of micrometers, well beyond the optical excitation region, and is limited only by the boundaries of the mesa. We show that the condensate density is determined by spin flipping collisions among the excitons, which convert dark excitons into bright ones. The suppression of this process at low temperature yields a density buildup, manifested as a temperature-dependent blueshift of the exciton emission line. Measurements und…
