Bose-Einstein Condensation Of Excitons Research Articles

Possibility of Exciton Bose-Einstein Condensation in CdSe Nanoplatelets.

The quasi-two-dimensional exciton subsystem in CdSe nanoplatelets is considered. It is theoretically shown that Bose-Einstein condensation (BEC) of excitons is possible at a nonzero temperature in the approximation of an ideal Bose gas and in the presence of an "energy gap" between the ground and the first excited states of the two-dimensional exciton center of inertia of the translational motion. The condensation temperature (Tc) increases with the width of the "gap" between the ground and the first excited levels of size quantization. It is shown that when the screening effect of free electrons and holes on bound excitons is considered, the BEC temperature of the exciton subsystem increases as compared to the case where this effect is absent. The energy spectrum of the exciton condensate in a CdSe nanoplate is calculated within the framework of the weakly nonideal Bose gas approximation, considering the specifics of two-dimensional Born scattering.

0D-2D heterostructure for making very large quantum registers using ‘itinerant’ Bose-Einstein condensate of excitons

Presence of coherent resonant tunneling in quantum dot (zero-dimensional) - quantum well (two-dimensional) heterostructure is necessary to explain the collective oscillations of average electrical polarization of excitonic dipoles over a macroscopically large area. This was measured using photo excited capacitance as a function of applied voltage bias. Resonant tunneling in this heterostructure definitely requires momentum space narrowing of charge carriers inside the quantum well and that of associated indirect excitons, which indicates bias dependent itinerant Bose-Einstein condensation of excitons. Observation of periodic variations in negative quantum capacitance points to in-plane coulomb correlations mediated by long range spatial ordering of indirect, dipolar excitons. Enhanced contrast of quantum interference beats of excitonic polarization waves even under white light and observed Rabi oscillations over a macroscopically large area also support the presence of density driven excitonic condensation having long range order. Periodic presence (absence) of splitting of excitonic peaks in photocapacitance spectra even demonstrate collective coupling (decoupling) between energy levels of the quantum well and quantum dots with applied biases, which can potentially be used for quantum gate operations. All these observations point to experimental control of macroscopically large, quantum state of a two-component Bose-Einstein condensate of excitons in this quantum dot - quantum well heterostructure. Therefore, in principle, millions of two-level excitonic qubits can be intertwined to fabricate large quantum registers using such hybrid heterostructure by controlling the local electric fields and also by varying photoexcitation intensities of overlapping light spots.

Excitonic Order in Strongly Correlated Systems with the Spin Crossover

Features of the formation of the magnetic structure and the exciton Bose–Einstein condensate phase of magnetic excitons in strongly correlated systems near the spin crossover have been considered with the effective Hamiltonian obtained from the two-band Hubbard–Kanamori model. The coexistence of antiferromagnetism and exciton condensate, as well as the appearance of the long-range excitonic antiferromagnetic order even in the absence of the interatomic exchange interaction, has been revealed. The role of the electron–phonon coupling has been considered.

Large cumulant eigenvalue as a signature of exciton condensation

The Bose-Einstein condensation of excitons into a single quantum state is known as exciton condensation. Exciton condensation, which potentially supports the frictionless flow of energy, has recently been realized in graphene bilayers and van der Waals heterostructures. Here we show that exciton condensates can be predicted from a combination of reduced density matrix theory and cumulant theory. We show that exciton condensation occurs if and only if there exists a large eigenvalue in the cumulant part of the particle-hole reduced density matrix. In the thermodynamic limit we show that the large eigenvalue is bounded from above by the number of excitons. In contrast to the eigenvalues of the particle-hole matrix, the large eigenvalue of the cumulant matrix has the advantage of providing a size-extensive measure of the extent of condensation. Here we apply this signature to predict exciton condensation in both the Lipkin model and molecular stacks of benzene. The computational signature has applications to the prediction of exciton condensation in both molecules and materials.

Twisting Enabled Charge Transfer Excitons in Epitaxially Fused Quantum Dot Molecules.

A heterojunction with type-II band alignment has long been considered as a prerequisite to realize charge transfer (CT) excitons which are highly appealing for exploration of quantum many-body phenomena, such as excitonic Bose-Einstein condensation and superfluidity. Herein, we have shown CT excitons can be activated via twisting in epitaxially fused heterodimer quantum dot (QD) molecules with quasi type-II band alignment, and even in QD homodimer molecules, therefore breaking the constraint of band alignment. The enabling power of twisting has been revealed. It modulates the orbital spatial localization toward charge separation that is mandatory for CT excitons. Meanwhile, it manifests an effective band offset that counterbalances the distinct many-body effects felt by excitons of different nature, thus ensuring the successful generation of CT excitons. The present work extends the realm of twistroincs into zero-dimensional materials and opens a novel pathway of manipulating the properties of QD materials and closely related molecular systems.

Radiative pattern of intralayer and interlayer excitons in two-dimensional WS2/WSe2 heterostructure

Two-dimensional (2D) heterostructures (HS) formed by transition-metal dichalcogenide (TMDC) monolayers offer a unique platform for the study of intralayer and interlayer excitons as well as moiré-pattern-induced features. Particularly, the dipolar charge-transfer exciton comprising an electron and a hole, which are confined to separate layers of 2D semiconductors and Coulomb-bound across the heterojunction interface, has drawn considerable attention in the research community. On the one hand, it bears significance for optoelectronic devices, e.g. in terms of charge carrier extraction from photovoltaic devices. On the other hand, its spatially indirect nature and correspondingly high longevity among excitons as well as its out-of-plane dipole orientation render it attractive for excitonic Bose–Einstein condensation studies, which address collective coherence effects, and for photonic integration schemes with TMDCs. Here, we demonstrate the interlayer excitons’ out-of-plane dipole orientation through angle-resolved spectroscopy of the HS photoluminescence at cryogenic temperatures, employing a tungsten-based TMDC HS. Within the measurable light cone, the directly-obtained radiation profile of this species clearly resembles that of an in-plane emitter which deviates from that of the intralayer bright excitons as well as the other excitonic HS features recently attributed to artificial superlattices formed by moiré patterns.

Towards high-temperature electron-hole condensate phases in monolayer tetrels metal halides: Ultra-long excitonic lifetimes, phase diagram and exciton dynamics

Excitons as composite bosons have been predicted to condense into Bose Einstein Condensation (BEC), Berezinskii-Kosterlitz-Thouless (BKT) superfluid or electron-hole liquid (EHL) states. However, short lifetime and small binding energy are the main obstacles that hinder the formation and observation of excitonic condensate at high temperature. Here we demonstrate excellent two-dimensional Tetrel Metal Halides (TMHs) platforms for excitonic quantum phase transition researches. We find that excitons in those monolayers possess effective masses similar to the free electron, promising large thermal de Broglie wavelengths. They can maintain stable bosonic characteristic at high temperature and concentration up to 1012 cm−2. The calculation on PbI2, PbBr2, PbCl2, SnI2, SnBr2, SnCl2 indicates BEC transition temperatures at 49 K, 103 K, 252 K, 36 K, 78 K, 152 K, comparable to the experimentally observed exciton BEC in MoSe2–WSe2 bilayer and 1 T-TiSe2 at 100 K and 190 K respectively. Besides, we confirm that the thermal equilibrium of excitonic subsystem can be established really fast, up to ∼10–100 fs. The excitons can live for ∼1–100 μs without direct radiation, beneficial for the formation and observation of condensate state. Our result paves the way not only for studies of quantum phase transition researches, but also for high-temperature excitonic devices.

High-Temperature Excitonic Bose–Einstein Condensate in Centrosymmetric Two-Dimensional Semiconductors

The realization of high-temperature excitonic Bose-Einstein condensation (BEC) in practical materials poses great challenges, because of strict constraints in symmetry, exciton binding, lifetime, and interaction. Here, using first-principles methods and symmetry analysis, we propose a new route to realize high-temperature excitonic BEC in centrosymmetric 2D materials, exploiting the parity symmetry of band edges and reduced Coulomb screening. We demonstrate it by taking monolayer TiS3 as an example, whose lowest-energy exciton shows small exciton mass, small Bohr radius, large binding, and long lifetime simultaneously. The phase diagram of electron-hole systems is further constructed, showing that both BEC and superfluidity can be realized at high temperature and in a broad range of exciton density. Importantly, we reveal that the high-temperature character of excitonic BEC is robust against thickness, beneficial for its experimental observation. By application of this general strategy to 2D materials in the database, monolayer AuBr and BiS2 are identified as promising candidates for high-temperature excitonic BEC.

Exciton Vortices in Two-Dimensional Hybrid Perovskite Monolayers* *Supported by the National Key R&D Programme of China (Grant Nos. 2017YFA0303400 and 2016YFE0110000), the National Natural Science Foundation of China (Grant Nos. 11574303 and 11504366), the Youth Innovation Promotion Association of Chinese Academy of Sciences (Grant No. 2018148), and the Strategic Priority Research Program of Chinese Academy of Sciences (Grant No. XDB28000000).

We study theoretically the exciton Bose–Einstein condensation and exciton vortices in a two-dimensional (2D) perovskite (PEA)2PbI4 monolayer. Combining the first-principles calculations and the Keldysh model, the exciton binding energy of in a (PEA)2PbI4 monolayer can approach hundreds of meV, which make it possible to observe the excitonic effect at room temperature. Due to the large exciton binding energy, and hence the high density of excitons, we find that the critical temperature of the exciton condensation could approach the liquid nitrogen regime. In the presence of perpendicular electric fields, the dipole-dipole interaction between excitons is found to drive the condensed excitons confined in (PEA)2PbI4 monolayer flakes into patterned vortices, as the evolution time of vortex patterns is comparable to the exciton lifetime.

On the Bose–Einstein Condensate of Excitons in Crystals with Defects

The possibility of formation of a Bose–Einstein condensate (BEC) of excitons in a model nonideal lattice of a molecular crystal is considered. The spectrum of corresponding exciton excitations is analyzed. The dependence of the chemical potential on the concentration of structural defects is numerically simulated within the virtual crystal approximation, and particular features of the appearance of an exciton condensate in a nonideal system are studied based on this simulation. Conditions under which an BEC of light and dark excitons can appear are revealed.

Polaron Shift of the Levels of a Quantum Wire in a Hybrid Structure with a Bose—Einstein Condensate

The wavefunction of the transverse motion of electrons in a quantum wire located near a two-dimensional layer of dipole excitons is perturbed by fluctuations of the exciton density through the electron-exciton interaction. This polaron effect is responsible for the shift of levels of the wire and can be observed through a change in the intersubband transition frequency. In this work, the polaron shift of the main intersubband transition frequency under the conditions of the Bose-Einstein condensation of excitons, as well as for the normal state of the degenerate Bose gas, is calculated.

Biexciton as a Feshbach resonance and Bose–Einstein condensation of paraexcitons in Cu2O

Paraexcitons, the lowest energy exciton states in Cu2O, have been considered a good system for realizing exciton Bose–Einstein condensation (BEC). The fact that their BEC has not been attained so far is attributed to a collision-induced loss, whose nature remains unclear. To understand collisional properties of cold paraexcitons governing their BEC, we perform a theoretical analysis of the s-wave paraexciton–paraexciton scattering at low temperatures. We show the two-channel character of the scattering, where incoming paraexcitons are coupled to a biexciton in a closed channel. Being embedded in the paraexciton scattering continuum, the biexciton is a Feshbach resonance giving rise to a paraexciton loss and a diminution of their background scattering length. In strain-induced traps, the biexciton effects generally increase with stress. Thus the scattering length a of trapped paraexcitons decreases monotonically with stress turning its sign as stress goes beyond a critical value. In the stress range with a < 0, the paraexciton loss increases with stress, whereas in that with a > 0 the loss is almost stress-independent. Importantly, that in the latter case the loss rate can be reduced to such small values that it has no effects on BEC by lowering temperatures to near one Kelvin and below. Our approximate calculations give the critical value of stress in the range just above one kilobar; thus BEC of strain-confined paraexcitons might be attained under low stress at a subkelvin temperature.

Hanle model of a spin-orbit coupled Bose-Einstein condensate of excitons in semiconductor quantum wells

We present a theoretical model of a driven-dissipative spin-orbit coupled Bose-Einstein condensate of indirect excitons in semiconductor quantum wells (QW's). Our steady-state solution of the problem shares analogies with the Hanle effect in an optical orientation experiment. The role of the spin pump in our case is played by Bose-stimulated scattering into a linearly-polarized ground state and the depolarization occurs as a result of exchange interaction between electrons and holes. Our theory agrees with the recent experiment [A. A. High et al., Phys. Rev. Lett. 110, 246403 (2013)], where spontaneous emergence of spatial coherence and polarization textures have been observed. As a complementary test, we discuss a configuration where an external magnetic field is applied in the structure plane.

Complexes of dipolar excitons in layered quasi-two-dimensional nanostructures

We discuss neutral and charged complexes (biexciton and trion) formed by indirect excitons in layered quasi-two-dimensional semiconductor heterostructures. Indirect excitons -- long-lived neutral Coulomb-bound pairs of electrons and holes of different layers -- have been known for semiconductor coupled quantum wells and are recently reported for van der Waals heterostructures such as bilayer graphene and transition metal dichalcogenides. Using the configuration space approach, we derive the analytical expressions for the trion and biexciton binding energies as functions of the interlayer distance. The method captures essential kinematics of complex formation to reveal significant binding energies, up to a few tens of meV for typical interlayer distances ~3-5 A, with the trion binding energy always being greater than that of the biexciton. Our results can contribute to the understanding of more complex many-body phenomena such as exciton Bose-Einstein condensation and Wigner-like electron-hole crystallization in layered semiconductor heterostructures.

Four-Body Scattering of Light as a Method of Detection of Bose-Einstein Condensate of Excitons

Process of four-particle light scattering in molecular crystals with participation of the exciton Bose-condensate is considered. The intensities and tensor of scattering are found for this effect. The frequency and polarization characteristics of this process are studies. It is shown that the investigation of the spectrum shapes of scattered radiation allows one to find and prove the existence of the Bose-Einstein condensate of excitons.

Halogenation of SiC for band-gap engineering and excitonic functionalization

The optical excitation spectra and excitonic resonances are investigated in systematically functionalized SiC with Fluorine and/or Chlorine utilizing density functional theory in combination with many-body perturbation theory. The latter is required for a realistic description of the energy band-gaps as well as for the theoretical realization of excitons. Structural, electronic and optical properties are scrutinized and show the high stability of the predicted two-dimensional materials. Their realization in laboratory is thus possible. Large band-gaps of the order of 4 eV are found in the so-called GW approximation, with the occurrence of bright excitons, optically active in the four investigated materials. Their binding energies vary from 0.9 eV to 1.75 eV depending on the decoration choice and in one case, a dark exciton is foreseen to exist in the fully chlorinated SiC. The wide variety of opto-electronic properties suggest halogenated SiC as interesting materials with potential not only for solar cell applications, anti-reflection coatings or high-reflective systems but also for a possible realization of excitonic Bose–Einstein condensation.

Spectroscopic signatures for the dark Bose-Einstein condensation of spatially indirect excitons

We study semiconductor excitons confined in an electrostatic trap of a GaAs bilayer heterostructure. We evidence that optically bright excitonic states are strongly depleted while cooling to sub-kelvin temperatures. In return, the other accessible and optically dark states become macroscopically occupied so that the overall exciton population in the trap is conserved. These combined behaviours constitute the spectroscopic signature for the mostly dark Bose-Einstein condensation of excitons, which in our experiments is restricted to a dilute regime within a narrow range of densities, below a critical temperature of about 1 K.

Exciton Bose-Einstein Condensation in Double Walled Carbon Nanotubes

ABSTRACTWe demonstrate theoretically the possibility for the Bose-Einstein condensation of excitons in properly selected double walled carbon nanotube structures. The condensation mechanism is enabled by the interaction of excitons residing on one tubule with the near-field generated by the plasmon mode of the other coaxial tubule, resulting in new hybridized bosonic quasiparticles called exciton-plasmons. We derive the dispersion relation for the exciton-plasmons, and calculate the exciton participation rate in the exciton-plasmon condensate. The requirements for forming the appropriate double walled carbon nanotube combinations capable of the optimum exciton-plasmon coupling regime needed to realize the condensation effect, as well as the possibility of experimental observation of the phenomenon, are discussed.

Quantum Hall drag of exciton condensate in graphene

Excitons are pairs of electrons and holes bound together by the Coulomb interaction. At low temperatures, excitons can form a Bose-Einstein condensate (BEC), enabling macroscopic phase coherence and superfluidity. An electronic double layer (EDL), in which two parallel conducting layers are separated by an insulator, is an ideal platform to realize a stable exciton BEC. In an EDL under strong magnetic fields, electron-like and hole-like quasi-particles from partially filled Landau levels (LLs) bind into excitons and condense. However, in semiconducting double quantum wells, this magnetic-field-induced exciton BEC has been observed only in sub-Kelvin temperatures due to the relatively strong dielectric screening and large separation of the EDL. Here we report exciton condensation in bilayer graphene EDL separated by a few atomic layers of hexagonal boron nitride (hBN). Driving current in one graphene layer generates a quantized Hall voltage in the other layer, signifying coherent superfluid exciton transport. Owing to the strong Coulomb coupling across the atomically thin dielectric, we find that quantum Hall drag in graphene appears at a temperature an order of magnitude higher than previously observed in GaAs EDL. The wide-range tunability of densities and displacement fields enables exploration of a rich phase diagram of BEC across Landau levels with different filling factors and internal quantum degrees of freedom. The observed robust exciton superfluidity opens up opportunities to investigate various quantum phases of the exciton BEC and design novel electronic devices based on dissipationless transport.

Exciting double bilayers

An excitonic Bose–Einstein condensate has so far been realized only in particular semiconductor heterostructure setups. Now, experiments show that such condensates can form in double graphene bilayers separated by hexagonal boron nitride.