Bose Condensation Of Excitons Research Articles

Magnetic transitions and excitonic order enhancement in spin crossover strongly correlated electron systems

The effects of exciton Bose condensation in strongly correlated spin crossover systems are considered within the effective Hamiltonian obtained from the two-orbital Hubbard–Kanamori model. The spectrum of collective excitations at various points of the temperature-crystal field phase diagram is calculated. The role of the electron–phonon interaction is discussed. A novel mechanism of excitonic and related magnetic order photoenhancement in strongly correlated spin crossover systems based on the massive collective phase mode appearance is demonstrated. In addition, we have shown that exciton and associated with it magnetic ordering can be photoinduced outside the exciton condensate phase due to the gap presence in the spectrum of single excitations.

The spontaneous symmetry breaking in Ta2NiSe5 is structural in nature

The excitonic insulator is an electronically driven phase of matter that emerges upon the spontaneous formation and Bose condensation of excitons. Detecting this exotic order in candidate materials is a subject of paramount importance, as the size of the excitonic gap in the band structure establishes the potential of this collective state for superfluid energy transport. However, the identification of this phase in real solids is hindered by the coexistence of a structural order parameter with the same symmetry as the excitonic order. Only a few materials are currently believed to host a dominant excitonic phase, Ta2NiSe5 being the most promising. Here, we test this scenario by using an ultrashort laser pulse to quench the broken-symmetry phase of this transition metal chalcogenide. Tracking the dynamics of the material's electronic and crystal structure after light excitation reveals spectroscopic fingerprints that are compatible only with a primary order parameter of phononic nature. We rationalize our findings through state-of-the-art calculations, confirming that the structural order accounts for most of the gap opening. Our results suggest that the spontaneous symmetry breaking in Ta2NiSe5 is mostly of structural character, hampering the possibility to realize quasi-dissipationless energy transport.

Bose Condensation of Long-Living Direct Excitons in an Off-Resonant Cavity.

We propose a way to increase the lifetime of two-dimensional direct excitons and show the possibility to observe their macroscopically coherent state at temperatures much higher than that of indirect exciton condensation. For a single GaAs quantum well embedded in photonic layered heterostructures with subwavelength period, we predict the exciton radiative decay to be strongly suppressed. Quantum hydrodynamics joined with the Bogoliubov approach are used to study the Berezinskii-Kosterlitz-Thouless crossover in a finite exciton system with intermediate densities. Below the estimated critical temperatures, drastic growth of the correlation length is shown to be accompanied by a manyfold increase of the photoluminescence intensity.

Resonance Tunneling Phenomena in Two-Dimensional Multilayer van der Waals Crystalline Systems

Works, mostly experimental, concerning the most interesting features of application of the resonant tunneling spectroscopy to a new type of heterosystems, van der Waals heterostructures, have been briefly reviewed. These heterostructures appeared after the recent discovery of two-dimensional crystals, which are a new class of materials beginning with graphene. The role of the angular matching of crystal lattices of conducting graphene electrodes of van der Waals systems in carrier tunneling between them has been analyzed together with the closely related problems of satisfaction of conservation laws in tunneling transitions. Manifestations of multiparticle correlation interactions between carriers in van der Waals systems such as Wigner crystallization of electrons in a two-dimensional electron gas in a magnetic field and Bose condensation of excitons in parallel two-dimensional electron gases have been briefly discussed.

Higher-rank deconfined quantum criticality at the Lifshitz transition and the exciton Bose condensate

Deconfined quantum critical points are characterized by the presence of an emergent gauge field and exotic fractionalized particles, which exist as well-defined excitations only at the critical point. We here demonstrate the existence of quantum critical points described by an emergent tensor gauge theory featuring subdimensional excitations, in close relation to fracton theories. We begin by reexamining a previously studied deconfined quantum critical point between two valence bond solid (VBS) phases on a bilayer honeycomb lattice. We show that the critical theory maps onto a rank-two tensor gauge theory featuring one-dimensional particles. In a slightly different context, the same tensor gauge theory also describes a deconfined quantum critical point between a two-dimensional superfluid and a finite-momentum Bose condensate, both of which are dual to rank-one gauge theories. This represents an entirely new class of deconfined quantum criticality, in which a critical tensor gauge theory arises on top of a stable conventional gauge theory. Furthermore, we propose that this quantum critical point gives rise to a new finite-temperature phase of bosons, behaving as an exciton Bose condensate, in which excitons (boson-hole pairs) are condensed but individual bosons are not. We discuss how small modifications of this theory give rise to the stable quantum "exciton Bose liquid" phase studied by Paramekanti, Balents, and Fisher.

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.

Dynamical phase transitions and pattern formation induced by a pulse pumping of excitons to a system near a thermodynamic instability

We suggest a phenomenological theory of dynamical phase transitions and the subsequent spaciotemporal evolution induced by a short optical pulse in a system which is already prone to a thermodynamic instability. We address the case of pumping to excitons whose density contributes additively to the thermodynamic order parameter like for charge-transfer excitons in electronic charge-ordering transitions. To describe both thermodynamic and dynamical effects on equal footing, we adopt for the phase transition a view of the ``excitonic insulator'' (EI) and suggest a formation of the macroscopic quantum state for the pumped excitons. The double nature of the ensemble of excitons leads to an intricate time evolution: the dynamical transition between number-preserved and phase-locked regimes, macroscopic quantum oscillations from interference between the Bose condensate of excitons, and the ground state of the EI. Modeling for an extended sample shows also stratification in domains of low and high densities which evolve through local dynamical phase transitions and a sequence of domain merges.

Coherence of Bose-Einstein condensates of dipolar excitons in GaAs/AlGaAs heterostructures

Experiments relating to studies of the coherence of Bose condensates of dipolar excitons in GaAs/AlGaAs heterostructures with a wide, single quantum well and a Schottky gate are analyzed. Dipolar excitons were excited by light in an annular trap formed along the perimeter of a window in a metal gate with an applied electric voltage. A dual-beam interference technique involving interference combination of the amplitudes of the luminescence light field, together with subsequent analysis of first order correlators, is used to study the temporal (longitudinal) and spatial (transverse) coherence of the exciton condensates. It is found that the transverse coherence length of an exciton condensate is considerably longer than its thermal De Broglie wavelength. Experimental studies of the luminescence intensity correlator also confirm the coherence of the exciton Bose condensate.

The excitonic insulator route through a dynamical phase transition induced by an optical pulse

We consider a dynamical phase transition induced by a short optical pulse in a system prone to thermodynamical instability. We address the case of pumping to excitons whose density contributes directly to the order parameter. To describe both thermodynamic and dynamic effects on equal footing, we adopt a view of the excitonic insulator for the phase transition and suggest a formation of the Bose condensate for the pumped excitons. The work is motivated by experiments in donor-acceptor organic compounds with a neutral-ionic phase transition coupled to the spontaneous lattice dimerization and to charge transfer excitons. The double nature of the ensemble of excitons leads to an intricate time evolution, in particular to macroscopic quantum oscillations from the interference between the Bose condensate of excitons and the ground state of the excitonic insulator. The coupling of excitons and the order parameter also leads to self-trapping of their wave function, akin to self-focusing in optics. The locally enhanced density of excitons can surpass a critical value to trigger the phase transformation, even if the mean density is below the required threshold. The system is stratified in domains that evolve through dynamical phase transitions and sequences of merging.

Excitonic Magnetism in Van Vleck–typed4Mott Insulators

In Mott insulators with the t(2g)4 electronic configuration such as of Re3+, Ru4+, Os4+, and Ir5+ ions, spin-orbit coupling dictates a Van Vleck-type nonmagnetic ground state with an angular momentum J=0, and the magnetic response is governed by gapped singlet-triplet excitations. We derive the exchange interactions between these excitons and study their collective behavior on different lattices. In perovskites, a conventional Bose condensation of excitons into a magnetic state is found, while an unexpected one-dimensional behavior supporting spin-liquid states emerges in honeycomb lattices, due to the bond directional nature of exciton interactions in the case of 90° d-p-d bonding geometry.

On the problem of lasing in traps for the bose condensation of dipolar excitons

Polariton-mode lasing in traps for the Bose condensation of dipolar excitons is considered and conditions under which this type of lasing takes place are examined. The width of the spectrum of the polariton modes involved in lasing and the role of the spatial and spectral inhomogeneity of the exciton distribution are discussed. The possibility of lasing in a system close to the exciton Bose condensation threshold is investigated in detail. The effect of inhomogeneous broadening of the exciton spectral line on the stability of steady-state lasing is analyzed. Experiments that help to reveal contributions from different physical processes to polariton-mode lasing in semiconductor structures designed for the Bose condensation of excitons are proposed.

Coupled condensates of excitons and photons in the trap

The properties of traps for exciton polaritons in a semiconductor optical microcavity with an embedded quantum well were considered. The two-component Bose condensate of photons and excitons was described by a coupled system of equations similar to the Gross-Pitaevskii equations, and analytical solutions in the Thomas-Fermi approximation were obtained for traps with weak exciton confinement. For strong confinement, condensate behavior was investigated numerically, and spatial profiles of coupled condensates of excitons and photons (which appear to be different in general case) were determined.

Magnetic Field Splitting of the Spin Resonance inCeCoIn5

Neutron scattering in strong magnetic fields is used to show the spin resonance in superconducting CeCoIn(5) (T(c)=2.3 K) is a doublet. The underdamped resonance (ħΓ=0.069±0.019 meV) Zeeman splits into two modes at E(±)=ħΩ(0)±αμ(B)μ(0)H with α=0.96±0.05. A linear extrapolation of the lower peak reaches zero energy at 11.2±0.5 T, near the critical field for the incommensurate "Q phase." Kenzelmann et al. [Science 321, 1652 (2008)] This, taken with the integrated weight and polarization of the low-energy mode (E(-)), indicates that the Q phase can be interpreted as a Bose condensate of spin excitons.

Phase diagram of the bose condensation of dipolar excitons in GaAs/AlGaAs quantum-well heterostructures

Experimental studies of the phase diagram of Bose condensation in a system of spatially indirect dipolar excitons in GaAs/AlGaAs quantum wells are reviewed. The properties of spatially periodic patterns arising in the luminescence of the exciton Bose condensate in a ring-shaped potential trap and the coherence of the condensate luminescence are discussed.

On Bose condensation of excitons in quasi-two-dimensional semiconductor heterostructures

Two semiconductor systems, quantum wells with spatially indirect dipolar excitons and exciton polaritons in semiconductor microcavities, exhibiting the Bose condensation of excitons, are discussed.

Lasing threshold in traps for Bose-condensation of dipolar excitons

We consider exciton recombination lasing in heterostructure traps for Bose–Einstein condensation of dipolar excitons. We show that such structures suit well for class D lasers where cavity decay strongly exceeds polarization decay. We evaluate lasing threshold taking into account specific inhomogeneous broadening of the exciton spectral line owing to Bose–Einstein condensation phenomenon under quasi-equilibrium conditions.It is found that narrowing of the exciton momentum distribution just before the condensation onset considerably lowers lasing threshold. At the same time, it is pointed out that a subsequent formation of condensate itself does not help lasing much. We conclude that it is possible to achieve lasing on polariton modes in nowadays experiments aimed on Bose–Einstein condensation of excitons.

Conditions and features of the lasing in traps for the bose condensation of dipolar excitons

We study the conditions and features of the polariton mode lasing in traps for the Bose condensation of dipolar excitons. We discuss the spectral linewidth of lasing modes and the effects of spatial and spectral inhomogeneity of the exciton distribution. We study in detail the possibility of the polariton mode lasing in the vicinity of the Bose condensation threshold. We analyze the impact of the inhomogeneous broadening of the exciton line on the stability of stationary lasing. We also propose additional experiments aimed at obtaining more information on the polariton mode lasing in semiconductor structures for the Bose condensation of excitons.

Preformed Excitonic Liquid Route to a Charge Density Wave in2H−TaSe2

Recent experiments on 2H-TaSe(2) contradict the long-held view of the charge density wave arising from a nested band structure. An intrinsically strong coupling view, involving a charge density wave state arising as a Bose condensation of preformed excitons emerges as an attractive, albeit scantily investigated alternative. Using the local density approximation plus multiorbital dynamic mean field theory, we show that this scenario agrees with a variety of normal state data for 2H-TaSe(2). Based thereupon, the ordered states in a subset of dichalcogenides should be viewed as instabilities of a correlated, preformed excitonic liquid.

Bose-Einstein condensation of dipolar excitons in lateral traps

The phase diagram of Bose-Einstein condensates of dipolar excitons accumulated in lateral electrostatic traps is determined. Spatially periodic luminescence structures of the exciton Bose condensate and their critical dependence on pump level and temperature are observed. Coherence in the dipolar exciton Bose-Einstein condensates is studied by direct measurement of the 1-st and 2-nd order correlators with variations in pump level and temperature. These results are indicative of large-scale coherence of the exciton Bose condensate.

The Aharonov–Bohm effect in neutral liquids

The Aharonov–Bohm effect was discovered as a quantum-mechanical effect for charged particles, but it has its counterpart in classical wave mechanics. The Aharonov–Bohm interference arises at the scattering of a sound wave by a vortex in classical and quantum hydrodynamics. This interference leads to a transverse force between quasiparticles and vortices in superfluids and superconductors. The Aharonov–Bohm effect was also generalized to neutral particles with magnetic or electric dipole momenta. The Aharonov–Bohm effect for charge particles and its modification for magnetic momenta (the Aharonov–Casher effect) have already been experimentally observed, and the efforts to detect the Aharonov–Bohm effect for electrically polarized neutral particles are on the way. A possible system for this detection is a Bose-condensate of excitons in a double quantum well. Observation of the Aharonov–Bohm effect in this system would provide direct evidence of Bose–Einstein condensation.