Bose Condensation Of Excitons Research Articles

Radiation intensity fluctuations of an exciton Bose condensate in GaAs/Al0.33Ga0.67As double quantum wells

Fluctuations in time of the intensity of indirect exciton photoluminescence in GaAs/Al0.33Ga0.67As double quantum wells have been studied. An analysis of the behavior of the intensity fluctuations under variation in external control parameters, such as the temperature and the voltage applied perpendicular to the structure, has revealed that they directly reflect the manifestation of Bose condensation. It has been demonstrated that the current flowing perpendicular to the structure also undergoes pronounced fluctuations simultaneously with those of the intensity. A study of possible reasons that could account for the onset of such fluctuations suggests the need for taking into account tunneling transitions in the exciton Bose condensate system.

Influence of temperature on the Bose condensation of photons and excitons in optic microcavity

In this paper, an exciton-photon model is created in an optic microcavity, and then in Bose condensation (BC), the variations of chemical potential range and number density of particles with temperature and position are studied in cases: constant width and varying width. Taking a semiconductor optic microcavity GaAs as example, the influence of temperature on BC is analyzed. The result shows that the range of chemical potential is related to dielectric function and microcavity width, while the number densities of photons and excitons and the sum of both particle numbers are related not only to them but also to temperature. The theoretical temperature of BC of GaAs is close to the experimental value. The densities of photons and excitons are almost equal, and their distributions are restricted to r=0 when BC occurs. With the reduction of temperature the number densities of both particles increase and their distributions expand, and the number of photons is more than that of excitons no matter how the width of optic microcavity changes.

High-Q polariton modes in heterostructures with traps for dipolar excitons

Polariton modes are studied in two-dimensional traps based on quantum-well heterostructures allowing the production of a Bose — Einstein condensate of indirect excitons. The characteristic equation for the modes is derived using the boundary conditions on the exciton layer located inside a resonator formed by such a trap. The spectrum and the structure of high-Q modes are found analytically and numerically. It is shown that some of these modes become unstable at the high exciton density and long polarisation relaxation time. According to the estimates, this instability can take place in experiments on the Bose condensation of dipolar excitons, thus explaining the origin of their coherent emission.

Light emission from different ZnO junctions and nanostructures

AbstractWe will discuss our experimental results for optical spectra produced by hole‐injection from different p‐type organic and inorganic materials into n‐type ZnO nanowires. The influence of different growth techniques and conditions on the nanowires and their emission spectral characteristics will then be analyzed and discussed. The latest findings on the mixture of the green emission band responsible for visible light emission from ZnO and the blue light emission from the organic polymer will be presented. Different high brightness light emitting diodes (HB‐LEDs) from our grown ZnO nanowires are demonstrated. The p‐type multi layer organic structures contain PEDOT:PSS as hole injectors combined with a hole transporting layer, and in some structures, a final top electron blocking/hole barrier stepping layer is placed. The purpose of this layer is to adjust the hole and electron emission from the corresponding junction side to optimize the LED performance. Structural scanning electron microscopy (SEM), electrical (I –V characteristics), photoluminescence (PL) and electroluminescence (EL) characteristics of these devices are displayed. Theoretically, we study the superfluidity of a two‐dimensional system of excitonic polaritons in an optical microcavity with an embedded quantum well. Using the effective low‐energy action for thermodynamic phase fluctuations, we obtain an expression for the analogue of the superfluid density in the system in terms of the “current–current” correlation function. The Kosterlits–Thouless transition temperature to the superfluid state as a function of the controlling parameters is calculated. Two methods are considered for producing traps for a polariton system in an optical microcavity. The behaviour of a two‐component Bose condensate of photons and excitons is analyzed theoretically for both types of the trap. The Bose condensate is described by the coupled system of equations of the Gross–Pitaevskii type. The approximate wave functions and the spatial profiles of coupled photon and exciton condensates are obtained. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Gauge-Field Rotation of an Electrically Polarized Bose Condensate by a Radial Magnetic Field

Here it is shown that a condensate of electrically polarized bosons subject to a radial magnetic field must rotate due to the Aharonov-Bohm effect. As in mechanically rotated superfluids, rotation is accompanied by penetration of vortices into the condensate at some critical magnetic field. In the case of a Bose-condensed exciton cloud in a double-quantum well, a necessary electric polarization is provided with spatial separation of electrons and holes forming excitons. Penetration of vortices strongly affects the intensity and the angular distribution of photoluminescence from the exciton cloud. This effect can be used for an experimental manifestation of exciton Bose condensation.

Bose-Einstein condensation of dipolar excitons in quantum wells

The experiments on Bose-Einstein condensation (BEC) of dipolar (spatially-indirect) excitons in the lateral traps in GaAs/AlGaAs Schottky-diode heterostructures with double and single quantum wells are presented. The condensed part of dipolar excitons under detection in the far zone is placed in k-space in the range which is almost two orders of magnitude less than thermal exciton wave vector. BEC occurs spontaneously in a reservoir of thermalized excitons. Luminescence images of Bose-condensate of dipolar excitons exhibit along perimeter of circular trap axially symmetrical spatial structures of equidistant bright spots which strongly depend on excitation power and temperature. By means of two-beam interference experiments with the use of cw and pulsed photoexcitation it was found that the state of dipolar exciton Bose-condensate is spatially coherent and the whole patterned luminescence configuration in real space is described by a common wave function.

10.1007/s11447-008-2009-9

The Bose condensation of two-dimensional dipolar excitons in quantum wells is numerically studied by the diffusion Monte Carlo simulation method. The correlation, microscopic, thermodynamic, and spectral characteristics are calculated. It is shown that, in structures of coupled quantum wells, in which low-temperature features of exciton luminescence have presently been observed, dipolar excitons form a strongly correlated system.

Bose condensation of exciton polaritons in an optical microcavity

Two methods are considered for producing traps for exciton polaritons in an optical microcavity with an embedded quantum well. The first method for controlling polaritons consists in producing a polariton trap governed by the longitudinal confinement of photons. Traps of this type can be created using an optical microcavity with a variable width. In traps of the second type, the exciton confinement is ensured by a weak potential that is applied to a quantum well with excitons or when this well is subjected to an inhomogeneous deformation. The behavior of a two-component Bose condensate of photons and excitons is analyzed theoretically. The Bose condensate is described by the coupled system of equations of the Gross-Pitaevskii type. The approximate wave functions and the spatial profiles of coupled photon and exciton condensates are obtained.

Critical tunneling currents in the regime of bilayer excitons

We have investigated the tunneling properties of an electron double quantum well system where the lowest Landau level of each quantum well is half filled. This system is expected to be a Bose condensate of excitons. Our four-terminal dc measurements reveal a nearly vanishing interlayer voltage and the existence of critical tunneling currents Icritical which depend on the strength of the condensate state.

Bose condensation of two-dimensional dipolar excitons: Simulation by the quantum Monte Carlo method

The Bose condensation of two-dimensional dipolar excitons in quantum wells is numerically studied by the diffusion Monte Carlo simulation method. The correlation, microscopic, thermodynamic, and spectral characteristics are calculated. It is shown that, in structures of coupled quantum wells, in which low-temperature features of exciton luminescence have presently been observed, dipolar excitons form a strongly correlated system.

Bose condensation of excitons in two-layer electronic systems in the absence of magnetic field

A high narrow peak of interlayer differential tunnel conductivity is observed at low temperatures in heterostructures with two closely located electronic layers in the absence of magnetic field. The analysis of experimental results suggests that this peak is due to the interlayer phase coherence that arises in the system as a result of the Bose condensation of interlayer excitons (electron-hole pairs) belonging to different layers, in accordance with the recent theoretical predictions.

Observation of the low-temperature peak in the interlayer tunneling conductance in bilayer electron systems in the absence of the magnetic field

A high narrow peak in the interlayer differential tunnel conductance has been observed in heterostructures with two closely located electron layers at low temperatures. Analysis of the experimental results suggests that this peak is due to the interlayer phase coherence, which appears in the system under investigation owing to the Bose condensation of indirect excitons, i.e., pairs of electrons and holes from different layers in the absence of the magnetic field.

Secondary peak in the optical absorption spectra: A possible criterion for Bose condensation of excitons

Abstract By solving the BCS and Bethe–Salpeter equations, we confirm the result by Chu and Chang that a secondary peak appears in the optical absorption spectrum immediately after the 1S-exciton peak in the presence of a condensed phase. The observation of the secondary peak indicates the presence of exciton condensate.

Long-range coherence of interacting Bose gas of dipolar excitons

Experiments connected with dipolar exciton Bose condensation in lateral traps arereviewed. Observations of long-range coherence of condensate in ring electrostatic traps inSchottky-diode heterostructures with double and single quantum wells are presented anddiscussed.

Collective state of the Bose gas of interacting dipolar excitons

Experiments connected with dipolar exciton Bose condensation in ring lateral traps are presented. The long-range spatial coherence of condensate along the sample surface is observed and discussed for the first time.

Large-scale coherence of the bose condensate of spatially indirect excitons

The Bose condensation of spatially indirect (dipolar) excitons in a wide single quantum well in an electric field transverse to the heterolayers is analyzed. Voltage is applied between a metallic film on the surface (Schottky gate) and a conducting electron layer inside a heterostructure (integrated electrode). The excitation of dipolar excitons and observation of their luminescence are performed through circle windows in a metallic mask 5 μm in diameter. Excitons are collected in a ring lateral trap, which is formed along the window perimeter owing to the strongly inhomogeneous electric field. When the critical condensation conditions in pump and temperature are reached, a narrow line of dipolar excitons corresponding to the exciton condensate appears stepwise in the luminescence spectrum. Under these conditions, a spatially periodic structure of equidistant luminescence spots appears in the luminescence pattern that is observed through a window with a resolution of about 1 μm and is selected by means of an interference filter. An in situ optical Fourier transform of spatially periodic structures from the real space to the k space is derived. The resulting Fourier transforms reproducing the pattern of the luminescence intensity distribution in the far field exhibit the result of the destructive and constructive interference, as well as the fact that the luminescence is directed along the normal to the heterolayers. These results are consequences of the large-scale coherence of the condensed exciton state in the ring lateral trap. Direct measurements of double-beam interference from pairs of luminescence spots in the ring show that the spatial coherence length is no less than 4 μm. Such a large scale means that the experimentally observed periodic luminescence structures are described by a common wavefunction under the condition of the Bose condensation of dipolar excitons.

Experimental observation of phase coherence in bilayer systems in the absence of a magnetic field

A high narrow peak of differential interlayer tunneling conductance is observed in the absence of a magnetic field at low temperatures in heterostructures with two closely spaced electron layers. The analysis of experimental results suggests that this peak is a consequence of the interlayer phase coherence, which arises in the system owing to the Bose condensation of interlayer excitons, i.e., pairs formed by electrons and holes belonging to different layers, in accordance with the recent theoretical predictions.

Vortex description of fractionalized phases in an exciton Bose condensate

As a sequel to the previous work [Phys. Rev. B 72, 235104 (2005)] we present a vortex description of the fractionalized phase in exciton bose condensate. Magnetic flux line and monopole of the 3+1D emergent U(1) gauge theory are identified in the exciton picture. A bundle of vortex/anti-vortex pairs of all flavors of excitons corresponds to the magnetic flux line and a point at which the vortices and anti-vortices recombine is identified as magnetic monopole. This completes the magnetic sector of the low energy excitation in the fractionalized phase.

Temperature dependence of exciton Auger decay process in cuprous oxide

We report a theoretical and experimental study on Auger decay of excitons in Cu 2O at a wide range of temperatures, from 2 to 325 K, in single-photon surface excitation. We find that the Auger constant linearly increases with temperature above 100 K. This constant is important for experiments on Bose condensation of excitons.

Observation of the emergent photon in a metastable fractionalized phase of Bose-condensed excitons: Monte Carlo simulation

Using the Monte Carlo simulation, we studied fractionalized phases in a model of exciton Bose condensate and found evidence for an emergent photon in a finite size system. The fractionalized phase is a metastable Coulomb phase where an emergent photon arises as a gapless collective excitation of the excitons. We also studied a possibility of the fractionalized spiral (${\mathrm{spiral}}^{*}$) phase where fractionalization and long range spiral order of the exciton condensate coexist.