正向电压激发下CdS MIS发光二极管中的自由激子发光

在44~77K温度范围内.在正向电压激发下的Cds MIS二极管中,观测到了发射0、1或2个纵光学(O、1LO或2LO)声子的自由激子的辐射衰减.根据激子的动能分布,讨论了1LO和2LO声子协助的伴线的形状和温度依赖.这里激子的有效温度等于晶格温度.     

Free exciton emission in forward-biased CdS MIS diodes

The radiative decay of free excitons with the emission of 0, 1, or 2LO phonons has been observed in electroluminescence in Cds MIS diodes operated in forward bias at temperatures in the range 40 – 77K. The line shapes and temperature dependence of the ILO and 2LO phonon assisted replicas are discussed in terms of the distribution of kinetic energy of the excitons with an effective temperature equal to that of the lattice.     

Anomalous dephasing of bosonic excitons interacting with phonons in the vicinity of the Bose-Einstein condensation

The dephasing and relaxation kinetics of bosonic excitons interacting with a thermal bath of acoustic phonons is studied after coherent pulse excitation. The kinetics of the induced excitonic polarization is calculated within Markovian equations both for subcritical and supercritical excitation with respect to a Bose-Einstein condensation (BEC). For excited densities n below the critical density , an exponential polarization decay is obtained, which is characterized by a dephasing rate . This dephasing rate due to phonon scattering shows a pronounced exciton-density dependence in the vicinity of the phase transition. It is well described by the power law that can be understood by linearization of the equations around the equilibrium solution. Above the critical density we get a non-exponential relaxation to the final condensate value p⁰ with that holds for all densities. Furthermore we include the full self-consistent Hartree-Fock-Bogoliubov (HFB) terms due to the exciton-exciton interaction and the kinetics of the anomalous functions . The collision terms are analyzed and an approximation is used which is consistent with the existence of BEC. The inclusion of the coherent exciton-exciton interaction does not change the dephasing laws. The anomalous function F_k exhibits a clear threshold behaviour at the critical density. Received 13 December 1999     

Excitonic transitions in GaAs---GaAlAs superlattices under a weak electric field

We report a study of the photoluminescence spectra of GaAs---Ga_0.65Al_0.35As superlattice under an electric field of about 10 kVcm⁻¹, from 9 K up to 80 K; the ratio of the intensities of the −1 and 0 peaks of the Wannier-Stark ladder varies with the temperature T. To fit these variations, we assume for both transitions, a trapped exciton density of states with a gaussian distribution of eigenenergies, and a two dimensional density of states for free excitons. We show that above about 50 K there is a thermal equilibrium between direct and crossed free excitons whereas at low temperatures, the equilibrium statistics are not respected: the excitons trap into the direct state rather than to the crossed one.     

The kinetics of exciton photoluminescence in mercuric iodide

Time-resolved photoluminescence (TRPL) of red mercuric iodide single crystal is measured at low temperatures and its two-photon luminescence is measured at room temperature. Sharp near band-gap luminescence is observed around 530 nm and was ascribed to radiative annihilation of free and bound excitons; the phonon replica of exciton luminescence are found between 533 and 540 nm at low temperatures. TRPL experiment reveals that near band-gap luminescence comprises fast and slow decay components and shows the different relaxation processes between free and bound exciton annihilation. Luminescence of bound excitons steeply lowers with increasing temperature and disappears about 40 K. A luminescence tail band is observed around 540 nm that is ascribed to defects in the anion sublattice. The temporal behavior of the tail band is described by rate equations very well. A broad luminescent band appears at 630 nm. The decay curves suggest that the luminescence is ascribed to the radiative recombination of donor-acceptor pairs and there are two kinds of mechanisms to control the decay. At room temperature, a luminescent band appears at the band-gap region, which shows the band-gap at room temperature is about 2.125 eV.     

Ultrafast hopping dynamics of 5f electrons in the Mott insulator UO₂ studied by femtosecond pump-probe spectroscopy

We describe a femtosecond pump-probe study of ultrafast hopping dynamics of 5f electrons in the Mott insulator UO? following Mott-gap excitation at temperatures of 5-300 K. Hopping-induced response of the lattice and electrons is probed by transient reflectivity at mid- and above-gap photon energies, respectively. These measurements show an instantaneous hop, subsequent picosecond lattice deformation, followed by acoustic phonon emission and microsecond relaxation. Temperature-dependent studies indicate that the slow relaxation results from Hubbard excitons formed by U3?-U?? pairs.     

The theory of phonon relaxation in He(II) at low temperatures

We have investigated analytically relaxation processes in the phonon system in helium(II) at sufficiently low temperatures where rotons are not exited. In accordance with the recent experimental data, the phonon velocity dispersion is supposed to be positive, though small. Two different relaxation processes exist in the phonon system in this case: (i) the fast longitudinal relaxation establishing equilibrium phonon distribution along each direction in the momentum space with the temperature and the drift velocity depending on the direction; (ii) the slow transverse relaxation setting up equilibrium between different directions.Using the energy and momentum conservation and general principles of the irreversible thermodynamics we have derived the expression for the transverse relaxation operator. It appears to be a differential operator of the fourth order and depends on a function of the “phonon temperature” Θ that cannot be determined from the general consideration. We have calculated this function for the case of three-phonon collisions.Physical properties of the transverse relaxation operator are discussed and the corresponding boundary conditions are formulated. Several typical physical problems, both linear and nonlinear, which can be formulated in terms of the transverse relaxation operator, are enumerated. With the help of the diagram method the contribution of multiphonon collisions both in the longitudional and in the transverse relaxation is evaluated.     

Intrawell and interwell transfer of excitons in growth-interrupted quantum wells

Intrawell and interwell transfers of excitons are observed by a temperature-dependent continuous-wave photoluminescence study of growth-interrupted single quantum wells. The intrawell transfer among the interface localization areas suggests a thermodynamic equilibrium between energy relaxation via LO-phonon emission and thermal population via phonon absorption. Thermal population is dominant in wider wells while relaxation is clearly observable in a four-monolayer narrow well at low temperatures. Interwell transfer of excitons also occurs between two narrow wells.     

Emission of Cu<Subscript>2</Subscript>O Paraexcitons Confined by a Strain Trap: Hints of a Bose–Einstein Condensate?

We monitor the phonon sideband emission from paraexcitons confined in a strain trap in cuprous oxide at T = 1.25 K. On the low energy ank of the optical phonon replicas, both of Γ ₅ ^? and Γ ₃ ^? symmetry (the latter activated by application of a magnetic field), we detect sharp peaks that might represent indications for a paraexciton Bose–Einstein condensate. In contrast, such peaks are absent in the phonon-mediated emission of the orthoexcitons, and they also disappear at elevated temperatures. The results challenge our understanding of the involved physics, e.g., of the Auger recombination of excitons, which has so far been believed to prevent crossing the border to a condensate.     

Temperature effects on exciton–phonon coupling and Auger recombination in CdTe/ZnTe quantum dots

We report the results of experimental studies on temperature-dependent thermal escape and Auger recombination coefficients in CdTe/ZnTe quantum dots. We show that at low temperatures, there is a thermally activated transition between two different states separated by a localization energy of about 15.8 meV, while the primary non-radiative process at high temperatures is thermal escape assisted multi-longitudinal optical (LO) phonons absorption with three phonons. The most striking result is a rapid increase in the Auger coefficient and a reduction in the decay time with increasing temperature above 35 K. These results show that the Auger process is assisted by the participation of phonons with an energy threshold of 34.4 meV and an LO phonon energy of around 19 meV.     

Exciton decay in Cu2O at high density and low temperature: Auger recombination,spin–flip scattering,and molecule formation

In this paper, we examine the relative importance of three density-dependent decay mechanisms for excitons in cuprous oxide (Cu₂O). Rate-equation models including Auger recombination (A), spin–flip scattering (B) and capture into short-lived biexcitons (C) are compared to photoluminescence data for a crystal temperature of 2 K. Process B—converting two orthoexcitons into two paraexcitons by exchanging electrons and holes—leaves the total exciton number unchanged and is inconsistent with the late-time data. In processes A and C, paraexcitons re-generate high energy excitons in agreement with the late-time data; however, the existing theory of Auger recombination seems to eliminate process A, leaving molecule formation as the dominant process.     

Theory of indirect exciton photoluminescence in elevated quantum trap

Inspired by an experiment of indirect excitons photoluminescence (PL) in elevated quantum trap (High et al., 2009), we theoretically investigate the energy relaxation and nonlinear interactions of indirect excitons in coupled quantum wells. It is shown that, when increasing the laser power, the intensity reversion of two PL peaks is due to the phonon necklace effect. In addition, we use a nonlinear Schrödinger equation including attractive two-body, repulsive three-body interactions and the excitation power dependence of energy distribution to understand the exciton states. This model gives a natural account for the PL blue shift with the increase of the excitation power. This study thus provides an alternative way to understand the underlying physics of the exciton dynamics in coupled potential wells.     

Contribution to the theory of spin relaxation at finite temperatures in the odd-filling quantum Hall effect regime

Spin relaxation in a two-dimensional electron gas (2D EG) is treated as the establishment of equilibrium in a gas of spin excitons as a result of processes that change the number of spin excitons. Coalescence is the dominant channel above a temperature of the order of 1 K. The coalescence of excitons can occurr as a result of spin-orbit and Coulomb interactions in the 2D EG. The rate of coalescence falls exponentially at low temperatures. The relaxation time is calculated, and the critical temperature below which the main annihilation process becomes that due to the exciton-phonon interaction is determined. Pis’ma Zh. éksp. Teor. Fiz. 70, No. 8, 531–536 (25 October 1999)     

Exponential decay lifetimes of excitons in individual single-walled carbon nanotubes

The dynamics of excitons in individual semiconducting single-walled carbon nanotubes was studied using time-resolved photoluminescence (PL) spectroscopy. The PL decay from tubes of the same (n,m) type was found to be monoexponential, however, with lifetimes varying between less than 20 and 200 ps from tube to tube. Competition of nonradiative decay of excitons is facilitated by a thermally activated process, most likely a transition to a low-lying optically inactive trap state that is promoted by a low-frequency phonon mode.     

Carrier capture and relaxation through a continuum background in InAs quantum dots

Carrier capture and relaxation in self-assembled InAs/GaAs quantum dots (QDs) have been studied, using bleaching rise time measurements for both the ground state (GS) and the first excited state (ES) transition, as a function of temperature (5, 77 and 293 K) and excitation density. We surprisingly observe that the bleaching rise time is longer for the ES than for the GS, indicating that the ES does not act as an intermediate state. At intermediate excitation density where the carrier relaxation is usually explained by Auger scattering, we still observe a temperature dependence pointing towards a single phonon emission process. For high excitation density, we observe a temperature-dependent plateau in the initial bleaching rise time, contradicting an Auger scattering-based relaxation model. Both these experimental results point towards a relaxation through the continuum background, followed by a single LO-phonon emission towards the QD GS.     

Relaxation and deorientation of110Ag(T 1/2=24.4s) nuclei produced by capture of polarized neutrons in the silver halides

Polarized¹¹⁰Ag nuclei are produced in the silver halides by capture of polarized neutrons at temperatures below 30 K and magnetic field strengths up to 6 kOe. The depolarization process is studied by observation of the β decay asymmetry as a function of magnetic field, temperature and of the radio frequency field strength in NMR signals. The depolarization is caused by a field dependent deorientation process and by temperature dependent spin-lattice relaxation. The deorientation is due to a succession of coupling steps of the nuclear spin with electromagnetic fields of defects generated as a consequence of the capture process, and the field dependence of the polarization can be understood as a decoupling curve. The temperature dependence of the spin-lattice relaxation is in accordance with the theory of quadrupolar relaxation above 18 K if an empirical phonon spectrum is used for the calculation. At lower temperatures the experimental relaxation rate is anomalously high, which may be due to resonance modes connected with recoil lattice defects.     

Polaron relaxation dynamics in InAs/GaAs self-assembled quantum dots

Polaron decay in n-type InAs quantum dots has been investigated using energy dependent, mid-infrared pump–probe spectroscopy. By studying samples with differing ground state to first excited state energy separations the relaxation time has been measured between 40 and 60 meV. The low-temperature decay time increases with increasing detuning between the pump energy and the optical phonon energy and is maximum (55 ps) at 56 meV. From the experimentally determined decay times we are able to extract a low-temperature optical phonon lifetime of 13 ps for InAs QDs. We find that the polaron decay time decreases by a factor of 2 at room temperature due to the reduction of the optical phonon lifetime.     

Translational oscillations of a microsphere in superfluid helium

The translational motion of a microsphere (radius 100 μm) in liquid helium is investigated. The sphere is levitating inside a superconducting capacitor and oscillates about its equilibrium position. The velocity amplitude and the resonance frequency are measured as a function of driving force and temperature (0.35 K up to 2.2 K). By increasing the driving force we first find a linear regime (laminar flow) which changes abruptly into a nonlinear one (turbulent flow). For temperatures below 0.7 K the linear drag is given by ballistic roton and phonon scattering whereas for temperatures above 1.1 K the hydrodynamic force on the sphere is described by Stoke's solution. In the turbulent regime, above a temperature independent threshold velocity, we find the drag force to be given by turbulence in the superfluid component plus an essentially laminar drag by the normal component.     

Photoemission studies of core exciton decay in KI

The decay of K⁺ 3p-core excitons in KI was studied by using new methods for obtaining photoemission spectroscopy data with synchrotron radiation. The Auger and non-radiative direct-recombination decay processes are separately identified in the data and a measure for the Auger energy distribution is obtained.