Exciton Phonon Research Articles

Quasi one‐dimensional chains and exciton–phonon interactions in TiSe2

AbstractIn charge ordered materials such as the transition metal dichalcogenides, in which the lattice modes are strongly coupled to the charges, the critical fluctuations surrounding a quantum critical point will consist of slow charge modulations as well as soft phonons. The presence of both these types of fluctuations offers an excellent opportunity for novel phases and unconventional forms of supercondictivity to arise. One such material, TiSe2, has been found to superconduct upon intercalation of copper [Morosan et al., Nature Phys. 2, 544 (2006)], with a concomitant suppression of CDW order. The transition is driven by an imbalance of charge carriers, leading to conventional superconductivity. Very recently however, Kusmartseva et al. [Phys. Rev. Lett. 103, 236401 (2009)] also discovered superconductivity in the pure system under pressure. This finding cannot be explained solely by the an imbalance of charge carriers and hints at the possibility of exciton‐mediated superconductivity made manifest in a clean experimental system. Here we analyze the electronic structure of TiSe2 using a tight binding model to fit its band structure. The resulting values found for the various overlap integrals in this material strongly suggest that TiSe2 is best viewed as a system of weakly linked one‐dimensional chains. We argue that in these chains excitons and Jahn–Teller phonons will strongly interact, and that it is therefore likely that both the charge density wave transition and the superconductivity which arises upon suppression of the CDW phase are driven by excitonic as well as phononic modes.

DEPENDENCE OF THE DAVYDOV'S SOLITON BEHAVIOR IN THE LINEAR POLYMER CHAIN ON THE EXCITON–PHONON COUPLING

The Davydov's soliton propagation in the linear polymer chain is analyzed numerically by solving the discrete equations of motion for exciton and phonon amplitudes. The main difference with respect to the results of continual approximation is that two exciton–phonon coupling constants influence separately the soliton behavior. Their influence is studied both in the ideal chain and the chain with single impurity.

The dynamics of a coherently driven localized exciton in a semiconducting single-wall carbon nanotube

The dynamics of a coherently driven localized exciton in a semiconducting single-wall carbon nanotube (SWNT) is investigated theoretically in terms of the perturbation treatment based on a unitary transformation. The decoherence rate of the exciton is obtained analytically and is compared with the one obtained with Born–Markov approximation. The results indicate that the decoherence strongly depends on the exciton–phonon coupling constant and the Rabi frequency of the coherently driving field. Furthermore, since the exciton–phonon coupling for SWNT is relatively strong and Born–Markov approximation becomes increasingly unreliable, we do need the numerically precise approach to treat the dynamics of the exciton in SWNT.

Photoluminescence properties of PbSe/PbS core‐shell quantum dots

AbstractThe present study describes the investigation of the ground‐state exciton emission in the PbSe/PbS core‐shell colloidal quantum dots (CQDs) with different core‐radius/shell‐thickness ratio, by recording temperature dependence of ground‐state exciton photoluminescence (PL) properties, including line‐width, integrated PL intensity, and the temperature coefficient of energy band‐gap, over a temperature range from 1.4 K to 300 K. The obtained data reveal a reduction of the temperature coefficient of the energy band‐gap as well as slightly decrease of exciton and optical phonon coupling in the PbSe/PbS core‐shell CQDs with respect to that in the corresponding PbSe core. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

A tunable phonon–exciton Fano system in bilayer graphene

Fano resonances are features in absorption, scattering or transport spectra resulting from the interaction of discrete and continuum states. They have been observed in a variety of systems. Here, we report a many-body Fano resonance in bilayer graphene that is continuously tunable by means of electrical gating. Discrete phonons and continuous exciton (electron-hole pair) transitions are coupled by electron-phonon interactions, yielding a new hybrid phonon-exciton excited state. It may also be possible to control the phonon-exciton coupling with an optical field. This tunable phonon-exciton system could allow novel applications such as phonon lasers.

Temperature dependent luminescence from quantum dot arrays: phonon-assisted line broadening versus carrier escape-induced narrowing

The paper presents a theoretical model describing the temperature dependence of the photoluminescence spectrum of self-ordered quantum dots arrays taking into account exciton–phonon interaction and thermal carriers transfer. This model is applied to the photoluminescence behaviour of InAs quantum dots grown on GaAs vicinal substrates. It allows distinguishing between effects caused by the different temperature-induced mechanisms and thus provides information about the physical and electronic structure of the quantum dot arrays.

Exciton–phonon luminescence and Raman scattering in CuGaS 2 crystals

Photoluminescence and resonance Raman scattering spectra of CuGaS 2 crystals are investigated at low temperature (10 K) under the excitation with the radiation from a spectral interval obtained by passing the radiation of an incandescent or xenon lamp through a monochromator. The thermalized luminescence of the Γ 4 and Γ 5 excitons was revealed under the excitation by an interval of photon energies higher than the energy of the ground state of long-wavelength Γ 4 excitons ( ω Т(Γ 4) + 1 E LO 1 ). The energy conversion between the polariton modes in luminescence and resonance Raman scattering spectra is considered.

Interface phonons and dielectric enhancement effects on excitons in quantum well

The theory of exciton polaron in quantum well is developed. The interaction with symmetric interface phonons is shown to contribute significantly to polaron exciton binding energy. As a result, this energy depends both on effective masses of charge particles in the quantum well and on polarization properties of the barriers. The conditions are found for strong exciton–phonon coupling in quantum well.

Exciton-phonon coupling in molecular crystals: Synergy between two intramolecular vibrational modes in quaterthiophene single crystals

Exciton-phonon (EP) coupling in molecular crystals is investigated in the case where two intramolecular vibrational modes are involved and a theoretical model is presented which applies when one of the modes is strongly coupled to crystal excitons. The model is used to simulate the low energy portion of the absorption spectra of quaterthiophene (4T) single crystals, for which we find it appropriate to consider a low energy vibrational mode at 161 cm(-1) and an effective strongly coupled high energy mode at 1470 cm(-1). Our numerical results demonstrate that the high energy mode renormalizes the excitonic band, thereby strongly affecting the environment seen by the low energy mode and the overall EP coupling regime. Numerical simulations also confirm the existence of the new coupling regimes "intermediate-I" and "strong-I" already introduced for oligothiophene aggregates [Spano et al., J. Chem. Phys. 127, 184703 (2007)], which arise as a consequence of the large effective mass of low energy excitons in 4T crystals. Comparison with experimental high resolution absorption spectra is also reported and shown to support the model predictions.

Exciton spectrum in hexagon nanotube accounting exciton–phonon interaction

The theory of exciton spectrum in the array of hexagon nanotubes is developed within the Bethe method and Green functions taking into account the exciton–phonon interaction. The theory is based at the effective masses model for the excitons’ electron and hole and dielectric continuum model for the interface and confined phonons. For the InAs hexagon nanotubes embedded into InP it is proven that the calculated magnitudes of energies and intensities of quantum transitions correlate to the experimental data.

Characterization of cadmium sulfide nanorods prepared by the solvothermal process

Cadmium sulfide nanorods were successfully prepared from cadmium nitrate tetrahydrate and thiourea in ethylenediamine by 200 °C solvothermal reactions using hydroxyethyl cellulose (HEC) as a capping material. X-ray diffraction (XRD), selected area electron diffraction (SAED), and Raman spectroscopy showed that the products were hexagonal wurtzite CdS with the 1st and 2nd harmonic modes at 303.5 and 593.0 cm − 1 , respectively. The intensity ratios of the 2nd to 1st harmonic modes were increased with their aspect ratios, due to the great strength of exciton–phonon coupling. By using scanning electron microscopy (SEM), transmission electron microscopy (TEM), high resolution TEM (HRTEM) and fast Fourier transformation (FFT), the products were in the shape of nanoparticles in the HEC-free solution, and became nanorods with higher aspect ratios in the HEC-added solutions — especially with longer reaction time. These nanorods were single crystals with growth in the [001] direction.

Temperature dependent high resolution resonant spectroscopy on a charged quantum dot

AbstractWe present temperature dependent high resolution resonant optical spectroscopy on a single, negatively charged InGaAs quantum dot. We performed laser transmission measurements yielding the natural linewidth of the excitonic ground state transition of a quantum dot in a temperature range from 4.2 K up to 25 K. Here, we describe the linewidth evolution and the temperature induced red shift of the resonance energy with simple models based on the exciton–phonon coupling in the quantum dot. The resonant spectroscopy measurements are complemented with results from non‐resonant PL measurements on the very same quantum dot. Here we observe a simple linear behavior of the linewidth according to an effect of a fluctuating environment. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Temperature dependent analysis of three classes of fluorescence spectra from p-6P nanofiber films

Discontinuous nanofiber films of para-hexaphenylene molecules can be routinely fabricated via vacuum deposition on muscovite mica. The fibers emit upon UV illumination blue fluorescence with excitonic spectral peaks. Their intense fluorescence makes them very attractive for the use in photonic devices, given that the spectra are reproducible for varying surface temperatures. A detailed investigation as a function of surface temperature variation from 300 to 30 K reveals three classes of spectra: (a) spectra with well resolved excitonic peaks, which shift 35 meV to the blue with decreasing temperature, (b) similar spectra with an additional intermediate broadening around 150 K, and (c) excitonic spectra similar to (b), but with a green defect emission band. Quantitative fitting of type (a) spectra results in an exciton–phonon coupling factor of 80 ± 10 meV and an average phonon temperature of Θ = 670 ± 70 K . The Huang–Rhys factor decreases linearly from 1.2 at 300 K to 1.0 at 30 K. Fitting of type (b) spectra reveals that the apparent intermediate temperature broadening is due to additional fluorescence peaks, the relative importance of which increasing with decreasing temperature.

Enhancement of Exciton–Phonon Interaction in InGaN Quantum Wells Induced by Electron-Beam Irradiation

InGaN/GaN multiple quantum wells grown by metal–organic chemical vapor deposition were irradiated with the electron-beam from a low energy accelerator. The electron irradiation induced a redshift by 50 meV in the photoluminescence spectra of the electron-irradiated InGaN/GaN quantum wells, irrespective of the exposure time to the electron beam, which ranges from 10 to 1000 s. The localization parameter extracted from the temperature-dependent photoluminescence spectra was found to increase in the irradiated samples. Analysis of the intensity of the longitudinal optical phonon sidebands showed the enhancement of exciton–phonon coupling, indicating that the excitons are more strongly localized in the irradiated InGaN wells. The changes in photoluminescence spectra in the irradiated InGaN/GaN quantum wells were explained in terms of the increase of indium concentration in indium rich clusters induced by the electron irradiation.

Spin injection studies into GaAs quantum wells in the presence of confined electrons

We compare the electroluminescence spectra (spectral composition and polarization characteristics) of two types of Fe-based AlGaAs/GaAs n-i-p spin light emitting diodes (spin LEDs). In type A spin LEDs the GaAs quantum well (QW) does not contain any confined carriers, while in type B LEDs the GaAs QW is occupied by confined electrons generated by excess n-type doping in the AlGaAs(n) barrier. Type B LEDs show a significantly smaller circular polarization at the e1h1 feature than type A devices. Other differences include the presence of the e1ℓ1 exciton as well as excitonic phonon replicas in type B LEDs. Possible mechanisms for these differences are discussed.

Phase‐relaxation processes of excitons in semiconducting single‐walled carbon nanotubes

AbstractWe have investigated the phase‐relaxation process of excitons in semiconducting single‐walled carbon nanotubes (SWNTs) by a time‐resolved degenerate four‐wave mixing (DFWM) method. The phase‐relaxation time (T2) estimated from the time evolution of the diffracted DFWM signals strongly depends on the temperature and excitation intensity, shorter T2 with higher temperature and excitation intensity. The experimental trend can be explained by the enhancement of phase‐relaxation rate due to frequent exciton–phonon scattering at high temperature and exciton–exciton scattering under high‐density excitation conditions. The T2 value at the weak excitation limit measured for semiconducting SWNT with a tube diameter of 1.1 nm at 2 K is estimated to be 350 fs. This T2 value is in good agreement with the homogeneous width estimated from microluminescence measurements. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Well-width dependence of exciton–longitudinal-optical-phonon coupling in MgZnO/ZnOsingle quantum wells

The well-width dependence of the exciton–phonon interaction inMg0.1Zn0.9O /ZnO single quantum wells (QWs) with a wedged structure was investigated by photoluminescence(PL) measurements at 4 K. Within the Frank–Condon approximation, the Huang–Rhys factorS, as a measure of the coupling strength between the exciton and the longitudinal-optical(LO) phonon, was extracted from the relative intensities between the first-orderphonon replica and the zero-phonon peak. It was found that the value ofS increased monotonically with the increase of the well width(LW). By studying the excitation-density-dependent PL spectra, this result was successfullyexplained by taking into account the internal electric field originating from the spontaneousand piezoelectric polarizations. The electric field is expected to push electrons andholes to the opposite sides in the well, and consequently results in the increase ofS withincreasing LW.

Excitonic polarons in quasi-one-dimensional LH1 and LH2 bacteriochlorophyll a antenna aggregates from photosynthetic bacteria: A wavelength-dependent selective spectroscopy study

Spectral characteristics of the optically excited states in the ring-shaped quasi-one-dimensional aggregates comprising 18 and 32 tightly coupled bacteriochlorophyll a molecules have been investigated using selective spectroscopy methods and theoretical modelling of the data. Distinguished by the lowest electronic transition energies in the LH2 and LH1 antenna complexes these aggregates govern the functionally important ultrafast funneling of solar excitation energy in the photosynthetic membranes of purple bacteria. It was found by using a sophisticated differential fluorescence line narrowing method that exciton–phonon coupling in terms of the dimensionless Huang–Rhys factor is strong in these systems, justifying an excitonic polaron theoretical approach for the data analysis. Although we reached this qualitative conclusion already previously, in this work essential dependence of the exciton–phonon coupling strength and reorganization energy on excitation wavelength as well as on excitation light fluence has been established. We then show that these results corroborate with the properties of excitonic polarons in diagonally disordered ensembles of the aggregates. Furthermore, the weighted density of states of the phonon modes, which is an important characteristic of dynamical systems interacting with their surroundings, was derived. Its shape, being similar for all studied circular aggregates, deviates significantly from a reference profile describing local response of a protein to the Q y electronic transition in a single bacteriochlorophyll a molecule. Similarities of the data for regular and B800 deficient mutant LH2 complexes indicate that the B800 pigments have no direct influence on the electronic states of the B850 aggregate system. Consistent set of model parameters was determined, unambiguously implying that excitonic polarons, rather than bare excitons are proper lowest-energy optical excitations in the LH1 and LH2 antenna complexes.

Exciton–phonon coupling and exciton thermalization in Mg xZn 1− xO thin films

Temperature dependent transmission measurements have been performed on Mg x Zn 1− x O thin layers on sapphire with x in the range from 0 to 0.33. Multiple minima can be observed in the spectra from samples with low Mg content. These can be attributed to exciton–phonon complexes. To determine the exciton transition energy, the transmission spectra have been fitted using the model dielectric function of Adachi with Gaussian-like broadening. The exciton transition energy is compared to the photoluminescence maximum in dependence on temperature and Mg content.

Stoichiometry enhanced exciton–phonon interactions in ZnO epilayers

Stoichiometry enhanced exciton–phonon couplings have been studied in ZnO epilayers grown on Al2O3 substrates by pulsed laser deposition. Rutherford backscattering spectroscopy indicated the stoichiometry, while clear hexagonal morphology revealed the Zn-polar growth of ZnO epilayers. From these high-quality and single-crystalline ZnO epilayers free-exciton emissions were observed at 3.30–3.31 eV with a low resistivity of ⩽10−2 Ω cm. Theoretical models were considered for fitting the experimental data to estimate the exciton–phonon interactions in ZnO epilayers. The enhanced exciton–phonon coupling, effective phonon energy and electron–phonon interaction were found to be 680 meV, 65.5 meV and 0.093 meV K−1, respectively. These larger interactions between excitons and phonons even with the lattice dilation have been attributed to the higher Fröhlich constant due to the strong localization energy in the stoichiometric ZnO epilayers.