Resonant inelastic x-ray scattering study of charge excitations in La(2)CuO(4)

We report a resonant inelastic x-ray scattering study of the dispersion relations of charge-transfer excitations in insulating La(2)CuO(4).. These data reveal two peaks, both of which show two-dimensional characteristics. The lowest energy excitation has a gap energy of approximately 2.2 eV at the zone enter, and a dispersion of approximately 1 eV. The spectral weight of this mode becomes dramatically smaller around (pi, pi). The second peak shows a smaller dispersion ( approximately 0.5 eV) with a zone-center energy of approximately 3.9 eV. We argue that these are both highly dispersive exciton modes damped by the presence of the electron-hole continuum.     

Determination of the exciton binding energy in single-walled carbon nanotubes

We report that measurements of the Raman intensity versus applied voltage are sensitive to filling of the density of states and enable us to measure the second band gap in specific semiconducting single-walled carbon nanotubes (SWNTs). Raman scattering preferentially selects sets of SWNTs whose excitonic transitions are resonant with the incident or scattered photon energies. Simultaneous measurement of the electronic gap and exciton resonance allows us to infer binding energies for the exciton of 0.49+/-0.05 and 0.62+/-0.05 eV for tubes of (10, 3) and (7, 5), respectively. Metallic SWNTs exhibit no excitonic feature.     

Electronic absorption spectrum of Cs<Subscript>2</Subscript>ZnI<Subscript>4</Subscript> thin films

The absorption spectrum of Cs₂ZnI₄ thin films in the energy range 3–6 eV at temperatures from 90 to 340 K has been investigated. It is established that this compound belongs to direct-gap insulators. Low-frequency exciton excitations are localized in ZnI₄ structural elements of the lattice. Phase transitions at 280 K (paraelectric phase ? incommensurate phase), 135 K (incommensurate phase ? monoclinic ferroelastic phase), and 96 K (monoclinic phase ? triclinic ferroelastic phase) have been found from the temperature dependences of the spectral position and halfwidth of the low-frequency exciton band. Additional broadening of the exciton band is observed for ferroelastic phases; it is likely to be due to exciton scattering from strain fluctuations near domain walls.     

Fabrication and low-temperature photoluminescence spectra of Mn-doped ZnO nanowires

Mn-doped ZnO nanowires have been fabricated through a high temperature vapor-solid deposition process. The low-temperature photoluminescence spectra of the samples show that there are multipeak emissions at the ultraviolet (UV) region (about 3.4?C3.0?eV). The excitonic and phonon-assisted transitions in Mn-doped ZnO nanowires were investigated. The results show that there is an obvious oscillatory structure emission at the UV region under low temperature from 12?C125?K. The oscillatory structure has an energy periodicity about 70?meV and the oscillatory structure is mainly attributed to longitudinal optical (LO) phonon replicas of free excitons?(FX). The multipeak emissions at 12?K are attributed to a donor-bound exciton (DBX, 3.3617?eV), 1LO-phonon replicas of a free exciton (FX-1LO, 3.3105?eV), 2LO-phonon replicas of a free exciton (FX-2LO, 3.2396?eV), and 3LO-phonon replicas of a free exciton (FX-3LO, 3.1692?eV), respectively. The intensity of UV emission and the efficiency of emission from the Mn-doped ZnO nanowires are improved.     

Resonant Raman effect in TlBr: Forbidden one-phonon and intervalley scattering at the direct exciton

Resonant Raman scattering at the direct exciton in TlBr at 1.8K is reported. The cross-section for forbidden 1LO (Γ) scattering shows resonances at both the 1s and 2s exciton. For the first time strongly resonant forbidden 1TO (Γ) scattering is observed for which possible mechanisms are discussed. Several two-phonon Raman processes involving M-point phonons confirm by direct observation the phonon intervalley scattering between non-equivalent X-points.     

Excitons perturbed by self-trapped excitons in alkali iodine crystals

A strong transient optical absorption band on the exciton shoulder has been observed in pure alkali iodine crystals following pulsed electron irradiation. The ultraviolet transition is ascribed to creation of a second exciton in the vicinity of a self-trapped exciton. The perturbation caused by the proximity of the self-trapped exciton lowers the energy needed to form the second exciton by 0.14 eV for KI and 0.12 eV for RbI.     

The exciton absorption in deformed germanium

Absorption spectra at 77° K near the direct (κ = 0) exciton transition are reported for deformed and undeformed single-crystal films of n-type Ge oriented on (111); Elliott's theory is applied. The optical width of the forbidden band for this transition is found as E_{g 0} = (0.8821 ±±0.0002) eV, while the exciton binding energy is found as E_ex(0) = = (0.0016±0.0003) eV for undeformed Ge at 77 ° K. The mean temperature coefficient of E_g for κ = 0 in the range 77 °–297 ° K is (dE_g/ /dT)_p =?3.50 · 10^?4 eV/deg. The effects of thermoelastic deformation on the exciton spectrum give (dE_g/dT)_d = (?1.5±0.1) · 10^?4 eV/deg. The half-width σ ≈ 5 · 10^?4 eV of the exciton peak gives the exciton lifetime as gt ≥ 10^?12 sec.     

Resonant Brillouin scattering by acoustoelectrically amplified phonons in CdSe

The dispersion of Brillouin scattering efficiency from the acoustoelectrically amplified phonon flux in CdSe has been measured in the range 1.37 to 1.65 eV. The dispersion curve shows enhancement and cancellation similar to those of GaAs, CdS and ZnO, and is in a good agreement with modified Loudon theory including exciton effects.     

Exciton dispersion and electronic excitations in hcp 4He

We present first measurements of the dispersion of excitons in solid helium, taken on a single hcp 4He crystal along the c axis. In agreement with studies on helium clusters, the major energy-loss peak can be interpreted as an intermediate molecular-type exciton, as we do not observe Wannier-like excitations. The measurements are in the (0 0 2) periodic zone, with the exciton energy dispersing along the c axis with a minimum at the gamma point. A calculated conduction band minimum at 31.0 eV above the valence band at gamma is supported by our data at energies above the exciton energy, leading to an exciton binding energy of 8.4 eV.     

Photoluminescence properties of exciton–exciton scattering in a GaAs/AlAs multiple quantum well

We report on the photoluminescence (PL) properties of a GaAs (20 nm)/AlAs (20 nm) multiple quantum well under high-density-excitation conditions at excitation energies near the fundamental exciton energies. The biexciton-PL band is dominant in a relatively low-excitation-power region. The PL originating from exciton–exciton scattering, the so-called P emission, suddenly appears with an increase in excitation power. The excitation-energy dependence of the intensity of the P-PL band indicates that the excitation energy higher than the fundamental heavy-hole exciton by the energy of the longitudinal optical (LO) phonon is the most efficient for the P PL. This suggests that the LO-phonon scattering plays an important role in the relaxation process of excitons leading to the P PL. The appearance of the P-PL band remarkably suppresses the intensity of the biexciton-PL band; namely, the exciton–exciton scattering process prevents the formation of biexcitons. Furthermore, we have confirmed the existence of optical gain due to the exciton–exciton scattering process with use of a variable-stripe-length method.     

Luminescence of ZnO nanopowders

The luminescence of ZnO nanocrystals prepared by different methods was studied under pulsed electron beam excitation. It is shown that the luminescence intensity depends on the nanocrystal sintering conditions and does not depend on the nanocrystal size within the range 10–50 nm. The relative luminescence intensities for the 3.32 eV (free exciton) and 3.20 eV (bound exciton) bands showed a dependence on nanocrystal size. The role of the nanocrystal surface in excitonic luminescence is discussed.     

Two-phonon resonant Raman scattering at the indirect exciton in silver chloride

Resonant Raman scattering in AgCl is reported for the first time. With excitation in the indirect absorption edge at 1.8K the observed scattering processes involve pairs of momentum-conserving TA(L) and LA(L) phonons with energies of 8.2 meV and 12.9 meV, respectively. The dependence of the scattered intensity on excitation energy suggests that, in contrast to AgBr, intravalley scattering by long wavelength acoustic phonons and intervalley scattering are negligibly small, the main relaxation mechanism of the free exciton being self-trapping. Some enhancement in the 2LO scattering intensity observed is probably due to resonance with the lowest direct exciton.     

Magnetoluminescence studies of excitonic scattering processes in high purity GaAs

The broad emission line at about 1.512 eV, which dominates the emission spectra of pure GaAs at intermediate excitation levels (1–300 kW/cm²) and low temperatures (<40 K) is investigated in magnetic fields up to magnetic flux densities of 10 T. The shift of the emission maximum in the magnetic field is exactly the same as recently reported for the free exciton. This demonstrates that at high excitation levels exciton-electron scattering is the dominant mechanism for the radiative decay of free excitons.     

Backscattering of light by a finite two-dimensional crystal plate in the region of exciton resonance: Antispecular reflection

It is shown that scattering of a plane monochromatic wave by a finite two-dimensional crystal (quantum well or Langmuir-Blodgett film) in the region of exciton resonance shows, along with the peak of specular reflection, a peak of comparable magnitude corresponding to backward scattering propagating toward the incident beam (antispecular reflection). The effect is related to reflection of the exciton by the boundary of the two-dimensional crystal. The solution of the scattering problem for a simple model system with a Frenkel exciton, demonstrating this phenomenon, is given.     

Influence of electron-electron scattering on the luminescence quantum efficiency in solid argon

The quantum efficiency of the intrinsic luminescence of solid Ar has been investigated for excitation energies between 15 and 55 eV. The spectral dependence of the quantum efficiency yields a stepwise increase with increasing excitation energies. The threshold energies for the steps closely correspond to the sum of the gap energy and multiples of the energy of the n = 1 exciton. The results are discussed in the framework of electron-electron scattering and give evidence that excitation of free electronic polaron complexes occurs in solid rare gases at higher excitation energies.     

Exciton relaxation by intervalley scattering in AgBr

By resonant Raman measurements at low temperatures intervalley scattering is demonstrated to be an efficient relaxation mechanism of the free indirect exciton in AgBr. The observed strongly enhanced scattering lines are attributed to TA(X) and LA(X) phonons connecting different L-points in the Brillouin zone. Analyzing the energy dependence of the line intensities the relative contributions of different exciton relaxation processes to the total exciton lifetime are estimated.     

Resonant inelastic X-ray scattering of the holon-antiholon continuum in SrCuO2

We report a resonant inelastic x-ray scattering study of charge excitations in the quasi-one-dimensional Mott insulator SrCuO2. We observe a continuum of low-energy excitations, the onset of which exhibits a small dispersion of approximately 0.4 eV. Within this continuum, a highly dispersive feature with a large sinusoidal dispersion (approximately 1.1 eV) is observed. We have also measured the optical conductivity, and studied the dynamic response of the extended Hubbard model with realistic parameters, using a dynamical density-matrix renormalization group method. In contrast to earlier work, we do not find a long-lived exciton, but rather these results suggest that the excitation spectrum comprises a holon-antiholon continuum together with a broad resonance.     

Theory of resonant Raman enhancement of a forbidden phonon in Cu2O

A theoretical calculation is given of the frequency dependence of resonant enhancement of Raman scattering by the normally forbidden 109 cm^?1 (Γ12-) optic phonon for incident frequencies near the 1s Yellow exciton energy. The theoretical results are in good agreement with recent experiments. This gives support to a quadrupole-dipole mechanism with Yellow-Blue exciton scattering in the intermediate state.     

正硅酸锌中能量传递的激子机理

在α型正硅酸锌中,用接近吸收边(Eg～5.5eV)的光能激发,发现了激子能量传递到发光中心的某些特征。在一个弛豫状态上,激子扩散的激活能估计为0.075eV,激子在自局域化前通过的扩散体积为0.5×10-19cm-3。实验确定的磷光体中能量传递的参数可实用于选择最佳颗粒尺寸分布。