Coherent Hole Research Articles

Surface Bound States and Spin Currents in Noncentrosymmetric Superconductors

We investigate the ground state properties of a noncentrosymmetric superconductor near a surface. We determine the spectrum of Andreev bound states due to surface-induced mixing of bands with opposite spin helicities for a Rashba-type spin-orbit coupling. We find that the order parameter suppression qualitatively changes the bound state spectrum. The spin structure of Andreev states leads to a spin supercurrent along the interface, which is strongly enhanced compared to the normal state spin current. Particle and hole coherence amplitudes show Faraday-like rotations of the spin along quasiparticle trajectories.

Intramolecular charge transfer in the dye molecules containing bis-dimethylfluoreneaniline

Two novel metal-free organic dyes (JK-1, JK-2), composed of bis-dimethylfluoreneaniline moiety as the electron donor and cyanoacrylic acid moiety as the electron acceptor, are theoretically investigated with quantum chemical methods. The energies and densities of highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO), as well as the binding energies, are compared between the two dyes. The HOMO → LUMO electronic transition describes their lowest singlet excited states. The orientation and strength of transition dipole moment are revealed visually with transition density. We also show the orientation and results of the intramolecular charge transfer with charge difference density. Transition density matrices provide information about the electron–hole coherence and excitation delocalization. Theoretical results reveal that exciton size and transition dipole moment increase with the expansion of the π conjugation and the charge transfer occurs from donor unit across the π bridge to the acceptor unit.

Slow light by coherent hole burnings

We show that the simultaneous application of a copropagating saturating pump and a counterpropagating coherent beam can be used to burn a narrow spectral hole within the absorption line of the optical transition in a Doppler-broadened medium. The large index of refraction of this hole slows down a light pulse by a factor of about ${10}^{4}$. In addition, we propose a method to create two-color slow light pulses with simultaneous gain by employing a bichromatic field to saturate the medium.

Static gain saturation in quantum dot semiconductor optical amplifiers

Measurements of saturated amplified spontaneous emission-spectra of quantum dot semiconductor optical amplifiers demonstrate efficient replenishment of the quantum-dot ground state population from excited states. This saturation behavior is perfectly modeled by a rate equation model. We examined experimentally the dependence of saturation on the drive current and the saturating optical pump power as well as on the pump wavelength. A coherent noise spectral hole is observed with which we assess dynamical properties and propose optimization of the SOA operating parameters for high speed applications.

Spectral properties of a hole coupled to optical phonons in the generalizedt−Jmodel

We present an efficient numerical method for the description of ground state and spectral properties of a doped hole in generalized $t\text{\ensuremath{-}}J$ models coupled to optical phonons. In the weak-coupling regime, coherent hole motion is less affected by the electron-phonon (EP) interaction in the electron than in the hole-doped system. Increasing the strength of EP coupling leads to a vanishing low-energy spectral weight around $\ensuremath{\Gamma}$ point. Spectral properties are in the strong coupling regime consistent with recent high-resolution angle resolved photoemission spectra (ARPES) [K. M. Shen et al., Phys. Rev. Lett. 93, 267002 (2004)]. A waterfall-like feature appears connecting the dispersive low-energy band with the incoherent high-energy peak resembling ARPES data [F. Ronning et al., Phys. Rev. B 71, 094518 (2005)].

Mechanism of Förster-type hopping of charge transfer and excitation energy transfer along blocked oligothiophenes by Si-atoms

The ground and excited-state properties of oligothiophenes connected by Si-atoms have been studied theoretically, based on recent experimental reports [M. Fujitsuka, D.W. Cho, J. Ohshita, A. Kunai, T. Majima, J. Phys. Chem. C 111 (2007) 1993]. Herein, we have employed a density-functional theory (DFT) approach toward examining the influence of the number of oligothiophenes on molecular ground-state properties, focusing on the density of state and the density distribution on the subunit of the oligothiophenes. Furthermore, we have investigated several excited-state properties of these oligothiophene. We discuss absorption with transition densities, which reveal the orientations and strengths of transition dipole moments, charge difference densities, which allow for the study of transition dipole moments and charge transfer in the absorption processes, and transition density matrices, which provide information about the electron–hole coherence and excitation delocalization. All of these properties were studied by employing time-dependent density functional theory (TD-DFT). Our theoretical results indicate that there are not only localized excited states, but also inter-branched charge transfer excited states in absorption for block copolymers of the oligothiophenes. In all, the theoretical analyses provide insight into the ground- and excited-state properties of the polymers, notably on the hopping mechanism of charge transfer in blocked oligothiophenes by Si atoms.

Absence of Hole Confinement in Transition-Metal Oxides with Orbital Degeneracy

We investigate the spectral properties of a hole moving in a two-dimensional Hubbard model for strongly correlated t(2g) electrons. Although superexchange interactions are Ising-like, a quasi-one-dimensional coherent hole motion arises due to effective three-site terms. This mechanism is fundamentally different from the hole motion via quantum fluctuations in the conventional spin model with SU(2) symmetry. The orbital model describes also propagation of a hole in some e(g) compounds, and we argue that orbital degeneracy alone does not lead to hole self-localization.

Direct observation of the coherent spectral hole in the noise spectrum of a saturated InAs/InP quantum dash amplifier operating near 1550 nm

We demonstrate a direct observation of the coherent noise spectral hole in a saturated quantum dash amplifier. Its width 500-600 GHz is determined by the response time and is responsible for high speed regeneration properties.

First-principles experimental observation of coherent hole burnings in atomic rubidium vapor

We demonstrate in experiment that the simultaneous use of two laser fields, a saturating and a coupling field, allows us to observe three or four coherent hole burnings with controllable widths, depths, and positions in a single Doppler absorption profile. Appropriate theoretical simulations and analysis show that these correlated coherent hole burnings result from the combination of strong saturating excitation and dynamically induced quantum coherence.

Excited state properties of the p- and n-type semiconductors of thiazolothiazole derivative having thiophene and trifluormethylphenyl rings

Excited state properties of novel p- and n-type organic semiconductors with a thiazolothiazole unit are theoretically investigated with quantum chemical methods. The calculated absorption frequencies of them are consistent with the experimental data. The dihedral angles between the thiazolothiazole unit and the trifluoromethylphenyl (or thiophene) are examined from the optimized geometries at ground states. To study the influence of the individual units of the derivatives to the excited state properties of them, the energies and densities of frontier orbital HOMOs and LUMOs of the individual unit and the derivatives are investigated in the absorption processes. The excited properties of the two derivatives are studied with 2D and 3D real-space analysis methods, which are employed to study the electron–hole coherence and the excitation delocalization (with transition density matrix method), and charge and energy transfer (with transition and charge difference density method). The insights of the optical electron properties of the semiconductor in the absorption are revealed theoretically.

Control of Emission by Intermolecular Fluorescence Resonance Energy Transfer and Intermolecular Charge Transfer

Control of emission by intermolecular fluorescence resonant energy transfer (IFRET) and intermolecular charge transfer (ICT) is investigated with the quantum-chemistry method using two-dimensional (2D) and three-dimensional (3D) real space analysis methods. The work is based on the experiment of tunable emission from doped 1,3,5-triphenyl-2-pyrazoline (TPP) organic nanoparticles (Peng, A. D.; et al. Adv. Mater. 2005, 17, 2070). First, the excited-state properties of the molecules, which are studied (TPP and DCM) in that experiment, are investigated theoretically. The results of the 2D site representation reveal the electron-hole coherence and delocalization size on the excitation. The results of 3D cube representation analysis reveal the orientation and strength of the transition dipole moments and intramolecular or intermolecular charge transfer. Second, the photochemical quenching mechanism via IFRET is studied (here "resonance" means that the absorption spectrum of TPP overlaps with the fluorescence emission spectrum of DCM in the doping system) by comparing the orbital energies of the HOMO (highest occupied molecular orbital) and the LUMO (lowest unoccupied molecular orbital) of DCM and TPP in absorption and fluorescence. Third, for the DCM-TPP complex, the nonphotochemical quenching mechanism via ICT is investigated. The theoretical results show that the energetically lowest ICT state corresponds to a pure HOMO-LUMO transition, where the densities of the HOMO and LUMO are strictly located on the DCM and TPP moieties, respectively. Thus, the lowest ICT state corresponds to an excitation of an electron from the HOMO of DCM to the LUMO of TPP.

Magnetic order in lightly doped cuprates: Coherent versus incoherent hole quasiparticles and nonmagnetic impurities

We investigate magnetic properties of lightly doped antiferromagnetic Mott insulators in the presence of non-magnetic impurities. Within the framework of the t-J model we calculate the doping dependence of the antiferromagnetic order parameter using the self-consistent diagrammatic techniques. We show that in the presence of non-magnetic impurities the antiferromagnetic order is more robust against hole doping in comparison with the impurity-free host, implying that magnetic order can re-appear upon Zn doping into lightly hole-doped cuprates. We argue that this is primarily due to the loss of coherence and reduced mobility of the hole quasiparticles caused by impurity scattering. These results are consistent with experimental data on Zn-doped LaSrCuO.

Excited state properties of novel p- and n-type organic semiconductors with an anthracene unit

Photoinduced dynamics of novel p- and n-type organic semiconductors with an anthracene unit are theoretically investigated with quantum chemistry methods. The calculated vertical absorption and fluorescence frequencies of them are consistent with the experimental data. The changing tendencies of the dihedral angles between anthracene unit and trifluoromethylphenyl (or thiophene) in the photoinduced dynamics processes (vertical absorption and vertical fluorescence) are examined from the geometries of optimized ground and excited states. To study the influence of the individual units of the derivatives to the excited state properties of the derivatives, the energies and densities of frontier orbital HOMOs and LUMOs of the individual unit and the derivatives are studied in the processes of vertical absorption and fluorescence. The excited state properties of the two derivatives in the processes of vertical absorption and fluorescence are studied with 2D and 3D real space analysis methods, which are employed to study the electron–hole coherence and the excitation delocalization (with transition density matrix method), and charge and energy transfer (with transition and charge difference density method). Overall, the computed results remain in good agreement with the relevant experimental data, and the theoretical results promote deeper understanding to the optical and electronic properties of the semiconductor in the process of photoinduced dynamics.

Enhanced Pairing in Doped Quantum Magnets with Frustrated Hole Motion

Evidence for strong pairing at arbitrarily small J/t is provided in a t-J model on the checkerboard lattice for a specific sign of the hopping amplitude. Destructive quantum interferences suppress Nagaoka ferromagnetism when J/t-->0 and drastically reduce coherent hole motion in the fluctuating singlet background. It is shown that, by pairing in various orbital symmetry channels, holes can benefit from a large gain of kinetic energy.

On the nature of the magnetic transition in a Mott insulator

Using a combination of exact enumeration and the dynamical mean-field theory (DMFT) we study the drastic change of the spectral properties, obtained in the half-filled two-dimensional Hubbard model at a transition from an antiferromagnetic to a paramagnetic Mott insulator, and compare it with the results obtained using the quantum Monte Carlo method. The coherent hole (electron) quasiparticle spin-polaron subbands are gradually smeared out when the AF order disappears, either for increasing Coulomb repulsion U at fixed temperature T, or for increasing T at fixed U. Within the DMFT we present numerical evidence (a continuous disappearence of the order parameter) suggesting that the above magnetic transition is second order both in two and in three dimensions.

Comparison of coherent induced hole-burning between Λ, V and ladder systems

In this Letter, we investigate the novel coherent hole-burning phenomena in three different three-level Doppler broadening atomic Rb 87 systems. In the Λ model, coherent burning holes depend on Rabi frequency and detuning of the saturating laser as well as the nonradiative decay rate between ground levels. Comparing with V and Ladder models, we find that coherent burning holes in the Λ model are the most obvious.

Optimal electronic transitions in a simple two-band model

A quantum-mechanical problem of coherent control of charge-carrier transfer dynamics between two parabolic and spherical energy bands using optical electric fields was solved within the effective-mass approximation. Starting from the Luttinger-Kohn Hamiltonian, the problem at first was reduced to a single time-dependent Schr\"odinger-like equation for coherent hole transitions between two bands. Then, the obtained equation and the functional that minimizes the energy of optical pulse are used to deduce the Euler equation for optimal control. The multiplicity of the quantum-control problem is demonstrated explicitly. An example of interband excitation with an optimal $\ensuremath{\pi}$-type monopulse is presented.

Origins of the energy distribution structure in coherent pion production at intermediate energies

New calculations have been done that now cover the intermediate energy range from 100 MeV/nucleon to 2.0 GeV/nucleon incident energy for the reaction 12C + 12C [Formula: see text] 12C + 12C (15.11 MeV) +π0, where constructive, coherent Δ-hole states are excited in either nucleus while the companion nucleus is excited to a coherent nucleon–hole state describing the spin–isospin, giant resonant state at 15.11 MeV. The Δ (1232 MeV) isobar then decays to a nucleon and pion. Theoretical pion energy distributions are calculated and, for the first time, results above 400 MeV/nucleon are shown. A theoretical basis for understanding how the shapes of the pion distributions change as a function of incident energy is described. The fundamental shape of the Δ-production amplitude as a function of momentum transfer is discussed and the effects of the energy-dependent nuclear width are examined. Furthermore, the connection between the origins of the pion distribution to the final pion shapes is made and the importance of the giant resonance in providing an important signature is pointed out. By pushing the calculations above 400 MeV/nucleon, it was discovered that sliding kinematics and kinematic turnarounds occur due to the two-to-three-body sequential nature of the reactions and these effects determine the final structure of the pion distributions at higher incident energies. PACS Nos.: 24.10Cn, 24.30Cz, 25.70-z, 25.80-e

Charge hopping in DNA.

The efficiency of charge migration through stacked Watson-Crick base pairs is analyzed for coherent hole motion interrupted by localization on guanine (G) bases. Our analysis rests on recent experiments, which demonstrate the competition of hole hopping transitions between nearest neighbor G bases and a chemical reaction of the cation G(+) with water. In addition, it has been assumed that the presence of units with several adjacent stacked G bases on the same strand leads to the additional vibronic relaxation process (G(+)G...G) --> (GG...G)(+). The latter may also compete with the hole transfer from (G(+)G...G) to a single G site, depending on the relative positions of energy levels for G(+) and (G(+)G...G). A hopping model is proposed to take the competition of these three rate steps into account. It is shown that the model includes two important limits. One corresponds to the situation where the charge relaxation inside a multiple guanine unit is faster than hopping. In this case hopping is terminated by several adjacent G bases located on the same strand, as has been observed for the GGG triple. In the opposite, slow relaxation limit the GG...G unit allows a hole to migrate further in accord with experiments on strand cleavage exploiting GG pairs. We demonstrate that for base pair sequences with only the GGG triple, the fast relaxation limit of our model yields practically the same sequence- and distance dependencies as measurements, without invoking adjustable parameters. For sequences with a certain number of repeating adenine:thymine pairs between neighboring G bases, our analysis predicts that the hole transfer efficiency varies in inverse proportion to the sequence length for short sequences, with change to slow exponential decay for longer sequences. Calculations performed within the slow relaxation limit enable us to specify parameters that provide a reasonable fit of our numerical results to the hole migration efficiency deduced from experiments with sequences containing GG pairs. The relation of the results obtained to other theoretical and experimental studies of charge transfer in DNA is discussed. We propose experiments to gain a deeper insight into complicated kinetics of charge-transfer hopping in DNA.

Features of the dissipative electron transport near the boundary in the system Al(N)-In(S)

The temperature behavior of the coherent electron transport that arises in the presence of Andreev reflections is studied near the boundary of a hybrid system consisting of aluminum (N) and indium (S). The qualitative change of the temperature behavior upon changes in the electron mean free path lel as a result of the elastic deformation of the sample is observed for the first time on the same sample, at probes mounted in the normal region at fixed distances and from the boundary. As the temperature is lowered, the measured effective resistance decreases for lel ≪ L1, 2 and increases in a certain temperature region in which lel ∼ L1, 2. It is shown that phenomena of this kind correspond to quantum-interference features in the scattering of coherent electronic and Andreev hole excitations on elastic centers.