Intermediate Exciton Research Articles

Stimulated and Spontaneous Optical Generation of Electron Spin Coherence in Charged GaAs Quantum Dots

We report on the coherent optical excitation of electron spin polarization in the ground state of charged GaAs quantum dots via an intermediate charged exciton (trion) state. Coherent optical fields are used for the creation and detection of the Raman spin coherence between the spin ground states of the charged quantum dot. The measured spin decoherence time, which is likely limited by the nature of the spin ensemble, approaches 10 ns at zero field. We also show that the Raman spin coherence in the quantum beats is caused not only by the usual stimulated Raman interaction but also by simultaneous spontaneous radiative decay of either excited trion state to a coherent combination of the two spin states.

Comparison of HF, HF + MP2, LDA, BLYP, and B3LYP band structures of the homopolypeptides

AbstractAb initio HF, HF + MP2, LDA DFT, BLYP DFT, and B3LYP DFT calculations are compared in the case of 19 homopolypeptides in their β pleated sheet conformation. The results show that the B3LYP method provides good results for the fundamental gaps, as compared with the values estimated on the basis of available UV spectra and intermediate exciton calculations for PolyGly and PolyAla. The HF method gives the best agreement, using Koopman's theorem for the ionization potential, taking the calculated VBmax values in the HF case if one compares them with the experimental ionization potentials of the 19 amino acids measured by mass spectroscopy. Finally, how these methods might be improved to determine the most stable conformations of the homopolypeptides is outlined. © 2004 Wiley Periodicals, Inc. Int J Quantum Chem, 2004

Correlation corrected band structures of homopolypeptides v. B3LYP band structures of 19 homopolypeptides

AbstractAb initio B3LYP crystal orbital (CO) calculations have been performed on the 19 homopolypeptides (PolyGly, PolyAla, PolySer, PolyThre, PolyLeu, PolyiLeu, PolyVal, PolyAspAc, PolyAsp, PolyGlutAc, PolyGlut, PolyHist, PolyProl, PolyCyst, PolyMeth, PolyTyr, PolyPhenAla, PolyArg, and PolyLys) in their β pleated sheet conformation. Keeping the main chain conformation fixed as in PolyGly, the side chain geometries were optimized. For the calculation 2n+1 different k points were used with n = 8 for the case of simpler and n = 10 for more complicated side chains. The basis set applied was the double ζ one of Clementi. According to the results obtained, the conduction bands are shifted upward and the valance band downward, compared with the results of previous BLYP 1 and LDA 7 CO calculations. The bandwidths are similar to the previous cases. The band edges are in many cases not at the endpoints of the first Brillouin zones, causing nonmonotonous dispersion of both the conduction bands (CB) and the valance bands (VB), respectively. The fundamental gap values due to the upward shifts of the CB and downward shifts of the VB are substantially larger than in the case of our previous DFT CO calculations (values 6.0–7.0 eV). They are very close to the gap values, which can be estimated on the basis of experimental ultraviolet (UV) spectra of some homopolypeptides and on the basis of intermediate exciton theoretical calculations (6.5–7.5 eV). These surprisingly good results for the gaps are due to the compensation of errors (LDA or BLYP gives too small and simple HF provides too large gap values) in the B3LYP method. The admixture of the exact HF exchange with a weight of 0.19 obviously compensates the self interaction error occurring in the LDA or BLYP methods. This article discusses whether/how this result could be established by other B3LYP CO calculations on simple polymer chains and on stacked systems (e.g., nucleotide base stacks). Furthermore, a comparative analysis of the ground state DFT methods, the HF method and of the optimized effective potential method could throw more light on our successful theoretical results for the gaps of the homopolypeptides. © 2004 Wiley Periodicals, Inc. Int J Quantum Chem, 2004

Solid state physics of biological macromolecules: the legacy of Albert Szent-Györgyi

A. Szent-Györgyi contributing so much to the citric acid (oxygen metabolism) cycle in which subsequent electron transfers occur in eight proteins has come to the idea already in 1941 that proteins have to be conductors. This hypothesis was first not accepted because of the too large gap and non-periodic nature of a protein chain. With the development of the theory of disordered systems and the occurrence of high-speed computers it was possible to show that both proteins and nucleotide base stacks are good hopping semiconductors, proving the correctness of Szent-Györgyi's original idea. Ab initio Hartree–Fock correlation corrected band structures were computed both for homopolypeptides and periodic nucleotide base stacks. On the basis of these band structures applying the intermediate exciton theory the UV spectra of different biopolymers were calculated. The results for a cytosine stack, polyglycine and polyalanine are in good agreement with experiments. Finally, the expected further development of the quantum theory of biopolymers are discussed.

Vacuum ultraviolet spectroscopy of Pr 3+ in CaAl 4O 7, LaMgAl 11O 19 and SrLaAlO 4

The vacuum ultraviolet spectroscopy of Pr 3+ doped CaAl 4O 7, LaMgAl 11O 19 and SrLaAlO 4 is reported. It appears that whenever the aluminate host lattice is excited directly, mainly exciton and 4f 2–4f 2 [ 3P 0] Pr 3+ emission are observed. When the excitation energy is lower, Pr 3+ ions are excited selectively and 4f5d–4f 2 emission dominates. These observations can be explained by assuming that energy transfer from the host lattice to the Pr 3+ ion takes place preferentially via an intermediate exciton state with an energy too low to reach the energetic Pr 3+ 4f5d excited states.

Correlation corrected band structures of different homopolypeptides

The Hartree–Fock (HF) crystal orbital method for polymers with an arbitrary number of basis functions per unit cell has been is outlined. Starting from the HF energy bands and crystal orbitals it is briefly shown, how one can correct the energy band structure for correlation using the inverse Dyson equation in its diagonal approximation. For the self energy occurring in the Green’s matrix formalism the computationally simple Moeller–Plesset 2 (many body perturbation theory in second order) expression has been used. For seven homopolypeptides, poly(Gly), poly(Ala), poly(Ser), poly(Thre), poly(Leu), poly(iLeu) and poly(Val) the correlation corrected band structure calculations were performed. The results obtained are compared to the previous LDA calculations for the same systems. In agreement with general experience the fundamental gap values are much larger than the too small LDA values. The correlation corrections decrease the gap by 2–3 eV, but they are still larger by ∼3 eV than the values which can be estimated on the basis of intermediate exciton theory.

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.

AgBr nanocrystals in glass: Optical and ODMR investigations

The low-temperature photoluminescence of AgBr nanocrystals (NC's) in glass has been examined. The wavelength of the indirect exciton emission and transmission electron micrographs indicate that various samples were fabricated with nanocrystal radii ranging from 3 to 20 nm. Optically detected magnetic resonance spectra for the larger NC's exhibit the normal donor, acceptor, and intermediate case exciton resonances. Optically detected magnetic resonance spectra of the small NC's displayed only a single resonance at the average of the bulk intermediate case exciton values. This collapse is attributed to a confined wave function of the donor, which has a larger exchange interaction with the acceptor increasing the singlet triplet splitting.

Charge transfer vibronic state formation of localized intermediate Jahn-Teller exciton: Germanium -related centres in α-quartz

It is shown that the origin of triad-type electronic-hole states with dipole moment for a network of Ge-related local centres in α-quartz is connected with Ge-O charge transfer and formation of a Jahn-Teller excition of intermediate radius (Charge Transfer Vibronic Exciton-CTVE). While its electronic part is well localized on the Ge ion, the hole part is delocalized over oxygen ions of the four nearest coordination spheres. The important role of strong T x e Jahn-Teller effect here is in the corresponding tetragonal tension of the first coordination sphere in each Jahn-Teller configuration. As a result, the hole tunneling along Jahn-Teller distortion is eliminated. The hole becomes distributed over two oxygen ions only. The latter leads to triad CTVE formation and explains the EPR results obtained by R. J. McEachern and J. A. Weil [1]. It also manifests its important role in a new long-term memory effect on [GeO4]−Li+ local centres in α-quartz discovered by A. Leyderman, J. A. Weil, and P. Rao [2].

Exciton capture and losses in a stacked submicron array of sidewall quantum wires on patternedGaAs(311)Asubstrates

The optical properties of stacked GaAs/(Al,Ga)As quantum wire arrays, with a 0.5 \ensuremath{\mu}m lateral period fabricated by molecular beam epitaxy on patterned $\mathrm{GaAs}(311)A$ substrates, have been investigated. We observe an unexpectedly high quantum wire related contrast in the lateral distribution of the cathodoluminescence (CL) intensity, in particular at 300 K. The temperature dependence of this contrast as well as of the integrated CL intensities in the quantum wires and connecting quantum wells, reveals the loss mechanisms, which cause a reduction of the exciton transfer efficiency from the well into the wire regions. For low and intermediate temperatures, exciton localization and nonradiative recombination within the quantum well regions contribute to the decrease of the transfer efficiency. Near room temperature, the vertical escape of carriers, in particular out of the quantum well regions into the (Al,Ga)As barriers, is the limiting process. Within the framework of a detailed model, we determined the transfer time and the ratio of the radiative recombination times in the well and wire regions by combining the spatially resolved CL measurements with time-resolved photoluminescence spectroscopy.

Spectroscopy of excitonic Zeeman levels in single quantum dots

Fully confined excitons are investigated in natural quantum dots, which are formed by well-width fluctuations in GaAs/AlAs coupled quantum-well structures. In magnetooptic experiments a population inversion of the Zeeman split levels in the quantum dots is found under the condition of charge injection from the AlAs X-point state. This new phenomenon is explained in terms of spin thermalization in the intermediate indirect exciton state and subsequent tunnelling into the direct quantum-dot state. Population inversion is thereby caused by the associated sign reversal of the effective exciton g-factor.

Resonant Mandelstam-Brillouin light scattering and Raman scattering in semiconductors with intermediate exciton states belonging to discrete exciton bands and the continuous spectrum

The results of a theoretical and experimental investigation of resonant Mandelstam-Brillouin light scattering by thermal acoustic phonons with k=0 near the direct absorption edge (in the case of ZnSe crystals) are analyzed. The appearance of a new type of resonant increase in the intensity of Raman scattering by optical phonons with k≠0, which corresponds to resonance with the scattered light in the output channel, near the indirect absorption edge (in the case of semi-insulating GaP:N crystals) is also reported. The resonant gain reaches ∼4×103 at frequencies corresponding to overtone scattering assisted by LO(X) and LO(L) phonons. Exciton states belonging to both discrete exciton bands and to the continuous spectrum are considered as the intermediate states involved in the scattering processes in calculations of the resonant scattering tensors. In addition, all the intraband transitions, as well as the interband transitions between the conduction band, the valence bands, and the spin-orbit split-off band are taken into account, and good agreement with the experimental results is obtained.

Excitons in electroluminescent polymers: poly( para-phenylene), poly( para-phenylenevinylene) and poly( para-phenylenedivinylene)

We study photoexcited states with large exciton radius in electroluminescent polymers, poly( para-phenylene) (PPP), poly( para-phenylenevinylene) (PPV) and poly( para-phenylenedivinylene) (PPD), by the intermediate exciton theory. We compare properties of excitons in these three polymers. The oscillator strengths of the long-range excitons in PPP are smaller than in PPV, and those of PPD are larger than in PPV. Such relative variations are explained by the differences in the number of vinylene bonds.

Long-range excitons in conjugated polymers with ring torsions

Ring torsion effects on optical excitation properties in poly(para-phenylene) (PPP) and polyaniline (PAN) are investigated by the intermediate exciton formalism. Long-range excitons are characterized, and the long-range component of the oscillator strengths is calculated. We find that ring torsions affect the long-range excitons in PAN more easily than in PPP, due to the larger torsion angle of PAN and the large number of bonds whose hopping integrals are modulated by torsions.

Calculation of the exciton spectrum ofYBa2Cu3O7using the intermediate exciton theory

Starting from the correlation corrected (quasiparticle) band structures of the CuO{sub 2} plane and the CuO chain (always including the apical oxygen atom) of YBa{sub 2}Cu{sub 3}O{sub 7} we have calculated the exciton spectra of both systems. For this the intermediate exciton theory was applied in its first-principles form. This leads to a Green{close_quote}s-matrix equation for the determination of the E{sub {bold q}} exciton energies. We have applied in the denominator of the Green{close_quote}s-matrix quasiparticle one-electron energies instead of the Hartree-Fock ones. In this way we have found very good agreement with experiment for the calculated exciton energies of the CuO{sub 2} plane (theoretical values 4.4{endash}5.7 eV, experimental values 4.5{endash}5.5 eV). Further there is a reflexion spectrum peak of YBa{sub 2}Cu{sub 3}O{sub 6.9} at 8.0 eV which agrees well with our results obtained for the exciton energies of the chain. {copyright} {ital 1997} {ital The American Physical Society}

Long-Range Excitons in Optical Absorption Spectra of Electroluminescent Polymer Poly(para-phenylenevinylene)

The component of photoexcited states with large spatial extent is investigated for poly( para -phenylenevinylene) using the intermediate exciton theory. We find a peak due to long-range excitons at the higher-energy side of the lowest main feature of optical spectra. The fact that the onset of long-range excitons is located near the energy gap is related to the mechanisms of large photocurrents measured in such energy regions. We show that a large value of the hopping integral is realistic for characterizing optical excitations.

Charge-transfer excitons in optical-absorption spectra of C60 dimers and polymers.

Charge-transfer (CT) exciton effects are investigated for the optical absorption spectra of crosslinked C60 systems by using the intermediate exciton theory. We consider the C60-dimers, and the two (and three) molecule systems of the C60-polymers. We use a tight-binding model with long-range Coulomb interactions among electrons, and the model is treated by the Hartree-Fock approximation followed by the single-excitation configuration interaction method. We discuss the variations in the optical spectra by changing the conjugation parameter between molecules. We find that the total CT-component increases in smaller conjugations, and saturates at the intermediate conjugations. It decreases in the large conjugations. We also find that the CT-components of the doped systems are smaller than those of the neutral systems, indicating that the electron-hole distance becomes shorter in the doped C60-polymers.

Relaxation of polarized excitons in asymmetric double quantum wells

We consider the intrawell and interwell relaxation of circularly polarized excitons in an asymmetric double-quantum-well structure. For the intrawell case (photocreated light excitons coupled to the detected heavy excitons by an optical-phonon emission) the polarization changes sign after a fast relaxation. For the interwell case (transfer of polarization from the ground heavy exciton of the narrow well towards the ground heavy exciton of the wide well) we observe that the initial polarization is recovered after energy relaxation only if a fast-relaxation path is available which involves only intermediate heavy exciton states.

Excitonic interaction in the fluorene dimer

The fluorene van der Waals dimer exhibits a complex origin spectrum. This region has been studied by resonance two-photon ionization and by fluorescence excitation spectroscopies. The spectra can be interpreted on the basis of intermediate strength exciton coupling, in which the electronic interaction is comparable to the van der Waals vibrational energies. The spectra are reasonably well described by two distorted adiabatic potential surfaces, which correspond to the two excitonic components of the origin system. A single Franck–Condon active intermolecular mode provides a reasonable description of the system, however the potentials have significant cubic and quartic contributions. Non-Born–Oppenheimer nuclear momentum coupling is present and intermodal (IVR) interactions are observed, even for intermolecular modes as low as v=1. The results are remarkably different from prior observations of excitonic structure in other systems, providing a detailed picture of coupling between electronic and intermolecular motion in a van der Waals dimer.

Approximate calculation of the collective excited states of a cytosine stack

Abstract Using the sum rules of oscillator strengths of different individual transitions of different symmetry, a simple algebraic equation for the frequency of the collective excitation of the π-system, of only the σ electrons, and finally for the whole valence electron system of a cytosine stack was derived. The input data (besides the geometry of DNA B and of the number of different types of electrons in cytosine) for the individual valence and core excitations were estimated on the basis of ab initio Hartree—Fock band structures of a cytosine stack. These bands were corrected for correlation at the MP2 level and the results of an intermediate exciton calculation were taken into account. The obtained frequencies of the different collective excitations (plasmons) are in fairly good agreement with the results of energy loss experiments.