Exciton Kinetic Energy Research Articles

Spatially isolated neutral excitons via clusters on trilayer MoS2.

In spite of having a large exciton binding energy, two-dimensional (2D) transition metal dichalcogenides (TMDs) are limited as light-emitting materials because the spectral weight of neutral excitons decreases exponentially with increasing the excitation density. That is, neutral excitons easily transfer to trions, and exciton-exciton annihilation (EEA) occurs due to the strengthening of exciton kinetic energy in the layered structure. In here, we come up with an isolated neutral exciton system, maintaining its high spectral weight when the carrier density increased, which is achieved via MoS2 clusters on a MoS2 trilayer directly synthesized by metal-organic chemical vapor deposition (MOCVD). While increasing the excitation density, trions are decomposed by spatial confinement at the saturation level of its full width at half maximum (FWHM), and simultaneously the spectral weight of neutral excitons restarts to increase. Furthermore, we reveal the causality relationship between trions and B excitons, providing a keen insight into organic interactions among radiative recombination processes in 2D TMDs.

Giant synthetic gauge field for spinless microcavity polaritons in crossed electric and magnetic fields

The artificial gauge field for electrically neutral exciton polaritons devoid from the polarization degree of freedom can be synthesized by means of applying crossed electric and magnetic fields. The appearance of the gauge potential can be ascribed to the motional (magneto-electric) Stark effect which is responsible for the presence of a linear-in-momentum contribution to the exciton kinetic energy. We study the interplay of this phenomenon with the competing effect which arises from the Rabi-splitting renormalization due the reduction of the electron–hole overlap for a moving exciton. Accounting for this mechanism is crucial in the structures with the high ratio of Rabi splitting and the exciton binding energy. Besides, we propose an approach which boosts the gauge field in the considered system. It takes advantage of the crossover from the hydrogen-like exciton to the strongly dipole-polarized exciton state at a specific choice of electric and magnetic fields. The strong sensitivity of the exciton energy to the momentum in this regime leads to the large values of the gauge field. We consider the specific example of a GaAs ring-shape polariton Berry phase interferometer and show that the flux of the effective magnetic field may approach the flux quantum value in the considered crossover regime.

Observation of moiré excitons in WSe2/WS2 heterostructure superlattices.

Moiré superlattices enable the generation of new quantum phenomena in two-dimensional heterostructures, in which the interactions between the atomically thin layers qualitatively change the electronic band structure of the superlattice. For example, mini-Dirac points, tunable Mott insulator states and the Hofstadter butterfly pattern can emerge in different types of graphene/boron nitride moiré superlattices, whereas correlated insulating states and superconductivity have been reported in twisted bilayer graphene moiré superlattices1-12. In addition to their pronounced effects on single-particle states, moiré superlattices have recently been predicted to host excited states such as moiré exciton bands13-15. Here we report the observation of moiré superlattice exciton states in tungsten diselenide/tungsten disulfide (WSe2/WS2) heterostructures in which the layers are closely aligned. These moiré exciton states manifest as multiple emergent peaks around the original WSe2 A exciton resonance in the absorption spectra, and they exhibit gate dependences that are distinct from that of the A exciton in WSe2 monolayers and in WSe2/WS2 heterostructures with large twist angles. These phenomena can be described by a theoretical model in which the periodic moiré potential is much stronger than the exciton kinetic energy and generates multiple flat exciton minibands. The moiré exciton bands provide an attractive platform from which to explore and control excited states of matter, such as topological excitons and a correlated exciton Hubbard model, in transition-metal dichalcogenides.

Photoreflectance investigation of exciton-acoustic phonon scattering in GaN grown by MOVPE

In this paper, we report a systematic investigation of the near band edge (NBE) excitonic states in GaN using low temperature photoluminescence (PL) and photoreflectance (PR) measurements. For this purpose, GaN films of different thicknesses have been grown on silicon nitride (SiN) treated c-plane sapphire substrates by atmospheric pressure metalorganic vapor phase epitaxy (MOVPE). Low temperature PR spectra exhibit well-defined spectral features related to the A, B and C free excitons denoted by FXA FXB and FXC, respectively. In contrast, PL spectra are essentially dominated by the A free and donor bound excitons. By combining PR spectra and Hall measurements a strong correlation between residual electron concentration and exciton linewidths is observed. From the temperature dependence of the excitonic linewidths, the exciton-acoustic phonon coupling constant is determined for FXA, FXB and FXC. We show that this coupling constant is strongly related to the exciton kinetic energy and to the strain level.

Acoustic and optical phonon assisted formation of biexcitons

Efficient exciton relaxation is required for bounding two cooled excitons to form biexciton. Acoustic and optical phonon scatterings playing key roles in exciton relaxation are responsible for formation of biexcitons at various temperatures. Using ZnO powders, the authors observed a sublinear dependence on excitation power at low temperature, in which the relaxation process involves only emission of acoustic phonons due to the excitons having kinetic energy lower than those of the optical phonons. However, the exponent comes near theoretical value of 2 for participation of optical phonons when the exciton kinetic energy approaches to the energy of the lowest optical phonon.

Exciton dressing and capture by a photonic band edge

We demonstrate electromagnetically induced anomalous quantum dynamics of an exciton in a photonic band gap (PBG)-quantum well (QW) heterostructure. Within the engineered electromagnetic vacuum of the PBG material, the exciton can propagate through the QW by the emission and reabsorption of virtual photons in addition to the conventional electronic hopping mechanism. When the exciton wave vector and recombination energy nearly coincide with a photonic band edge, the exciton kinetic energy is lowered by $1--10\phantom{\rule{0.3em}{0ex}}\mathrm{meV}$ through coherent radiative hopping. This capture of the exciton by the photonic band edge is accompanied by strong electromagnetic dressing in which exciton's renormalized effective mass is 4--5 orders of magnitude smaller than in the absence of the PBG environment. This dressed exciton exhibits a long radiative lifetime characteristic of a photon-atom bound state and is robust to phonon-assisted recombinative decay. By inheriting properties of the PBG electromagnetic vacuum, the bound electron-hole pair becomes a stable, ultramobile quantum excitation.

Direct measurement of acoustic-phonon scattering of hot quantum-well excitons

Due to their high momentum, hot excitons do not couple to photons directly. Thus, the interaction of excitons with phonons is typically studied only for zero-momentum excitons. We show here that the excitonic recombination process assisted by optical phonons provides a direct method to study the acoustic-phonon scattering of hot excitons in semiconductor quantum wells. From the thermal broadening of the hot-exciton luminescence, we obtain the broadening coefficient as a measure of the coupling strength. The coefficient is found to increase with exciton kinetic energy. This finding is in contrast to theoretical predictions taking into account only the direct exciton-phonon scattering process.

Coherence length of excitons in a semiconductor quantum well.

We report on the first experimental determination of the coherence length of excitons in semiconductors using the combination of spatially resolved photoluminescence with phonon sideband spectroscopy. The coherence length of excitons in ZnSe quantum wells is determined to be 300-400 nm, about 25-30 times the exciton de Broglie wavelength. With increasing exciton kinetic energy, the coherence length decreases slowly. The discrepancy between the coherence lengths measured and calculated by considering only the acoustic-phonon scattering suggests an important influence of static disorder.

Scattering of hot excitons due to optical phonons in quantum wells: Multiphonon resonant Raman process

The hot-exciton scattering rate by optical phonons is calculated in the framework of the envelope-function approximation in narrow quantum wells (QWs). The exciton-phonon Fr\ohlich-type interaction is considered and the contributions of the confined phonons and interface modes to the total scattering rate are discussed in terms of the in-plane exciton kinetic energy and quantum-well width. To take into account the full symmetry of the optical phonons a phenomenological approach is employed in which the mechanical and electrostatic matching boundary conditions are fulfilled at QW interfaces. It is shown that the main contribution to the scattering rate comes from intrasubband transition assisted by phonons with even electrostatic potential state. The experimental observation in short period GaAs/AlAs superlattices on the phonon symmetries and their relative intensities of multiphonon resonant Raman process is qualitatively explained in terms of the relative role of the confined and interface optical phonons on the exciton scattering rate.

Confinement of two-dimensional excitons in a nonhomogeneous magnetic field

The effective Hamiltonian describing the motion of an exciton in an external non-homogeneous magnetic field is derived. The magnetic field plays the role of an effective potential for the exciton motion, results into an increment of the exciton mass and modifies the exciton kinetic energy operator. In contrast to the homogeneous field case, the exciton in a non-homogeneous magnetic field can also be trapped in the low field region and the field gradient increases the exciton confinement. The trapping energy and wave function of the exciton in a GaAs two-dimensional electron gas for specific circular magnetic field configurations are calculated. The results show than excitons can be trapped by non-homogeneous magnetic fields, and that the trapping energy is strongly correlated with the shape and strength of the non-homogeneous magnetic field profile.

Free exciton emission in GaN.

We present a detailed study of the free exciton emission in GaN. Photoluminescence and photoluminescence excitation techniques are employed to show the dominant role of exciton-phonon interaction in the creation of the free exciton states and the free exciton emission cascade in GaN. Up to six longitudinal optical (LO) phonon replicas are observed in both photoluminescence and photoluminescence excitation spectra. From an analysis of the free exciton line shape we are able to conclude that free exciton emission and the interaction of free excitons with LO phonons have to be described within the framework of the momentum conservation law. The exciton kinetic energy distribution is found to be a Maxwellian one having the temperature of the lattice. This suggests an enhanced rate of free exciton scattering on the acoustic phonons, in contrast to other wurzite-type semiconductors. \textcopyright{} 1996 The American Physical Society.

Exciton relaxation in AgCl studied by resonant Raman scattering

Resonance Raman scattering in AgCl is investigated at 1.8K for excitation around the indirect exciton absorption edge. The observed scattering processes involve pairs of momentum-conserving TA(L) and LA(L) phonons with energies of 8.2 and 12.9 meV, respectively. The weakness of the scattered intensity is explained by a reduction of free-exciton lifetime due to self-trapping. The quantitative analysis of the scattered intensity as a function of exciton kinetic energy reveals that, besides self-trapping as the most important relaxation mechanism, intravalley scattering by long-wavelength LA and LO phonons and intervalley scattering by TA(X) and LA(X) phonons are effective. Relative exciton-phonon matrix elements and scattering probabilities are determined. An estimate is made for the effective mass and total lifetime of the free exciton giving M*=(2.4+or-1.2)me and tau tot=65 ps, respectively.

Symmetry breaking excitonic instabilities in deformable lattice

Three types of symmetry-breaking instabilities of exciton(s) in the phonon field are described on a unified view-point and in comparison with relevant observations. •(i) Self-trapping — broken translational symmetry — takes place when the exciton-lattice relaxation energy exceeds the exciton kinetic energy for localization.•(ii) Decomposition into a pair of self-trapped particles — broken parity — may arise when the electron and the hole have deformation potentials of opposite sign.•(iii) If the total lattice relaxation energy exceeds the Franck-Condon exciton energy, the relaxed electron-hole pairs spontaneously created everywhere will condense into a new ordered state — bulk phase change.

Phonon‐assisted exciton annihilation in CdSe crystals

AbstractEmission spectra of exciton annihilation are measured in CdSe crystals from 2 to 77 °K. Especially the line‐shape, due to exciton annihilation with simultaneous creation of one or two phonons is considered. This shape can adequately be described on the basis of energy and momentum conservation laws only if the momentum dependence of the exciton‐phonon interaction probability is properly taken into account. As a result analytical expressions E3/2 exp (–E/kT) for the one‐phonon line and E1/2 exp (–E/kT) for the two‐phonon line, where E is the exciton kinetic energy, are obtained. An additional broadening of exciton emission lines, clearly observed at lower temperatures, is assigned to the inherent width of exciton levels.

Free exciton motion in crystals and exciton-phonon interaction

The exciton emission spectrum in CdS single crystals has been investigated. The main feature of this spectrum was manifestation of free exciton kinetic energy in exciton emission lines, which was conditioned by simultaneous emission of photon and phonons. It was shown that excitons are subordinate to the Maxwell distribution at least up to the temperature T = 77° K and interact with phonons in accordance with the momentum conservation law. The characteristic differences between one-phonon and two-phonon processes were noted.