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To explore energy transfer in the nonequilibrium spin-boson model (NESB) from weak to strong system-bath coupling regimes, we propose a polaron-transformed nonequilibrium Green's function (NEGF) method. By combining the polaron transformation, we are able to treat the system-bath coupling nonperturbatively, thus in direct contrast to conventionally used NEGF methods which take the system-bath coupling as a perturbation. The Majorana-fermion representation is further utilized to evaluate terms in the Dyson series. This method not only allows us to deal with weak as well as strong coupling regimes but also enables an investigation on the role of bias in the energy transfer. As an application of the method, we study an Ohmic NESB. For an unbiased spin system, our energy current result smoothly bridges predictions of two benchmarks, namely, the quantum master equation and the nonequilibrium noninteracting blip approximation, a considerable improvement over existing theories. In case of a biased spin system, we found a bias-induced nonmonotonic behavior of the energy conductance in the intermediate coupling regime, resulting from the resonant character of the energy transfer. This finding may offer a nontrivial quantum control knob over energy transfer at the nanoscale.

We have synthesized novel crystalline ropes of "polygonized" single-wall carbon nanotubes (SWCNTs). The tubes exhibit rounded-hexagonal cross sections in contrast to the earlier observations of nearly circular tubes. To investigate the structural characteristics of the lattice of SWCNTs we have performed extensive molecular-dynamics simulations. We find several metastable structures of the lattice characterized by different tube cross sections, hexagonal, rounded-hexagonal, and circular, and increasing cell volume. The competition between different tube shapes as a function of tube diameter is analyzed and compared to experiments.

Surface relaxations and electronic structures of (001) surfaces of cubic ${\mathrm{SrTiO}}_{3}$ have been studied by first-principles total-energy calculations with the generalized-gradient approximation. Both ${\mathrm{TiO}}_{2}$-terminated (surface-I) and SrO-terminated (surface-II) surfaces are investigated and the atomic configurations have been fully relaxed. We show that both surfaces show a first-layer buckling, with oxygen ions being pulled out of the surface and a relaxation of the interlayer distances.

The excited states of poly(p-phenylene vinylene) (PPV) are studied in the Su-Schrieffer-Heeger model appended by a long-range Coulomb interaction. We have calculated the energy levels of singlet and triplet excitons with odd and even parity, the biexciton state, and the onset of the continuum band in PPV; these results can comprehensively interpret relevant experimental data. More significantly, we have constructed the wave function of the biexciton by using the Heitler-London method and calculated the binding energy and radius of the biexciton in conjugated polymers.

We investigate the Pomeranchuk instability (PI) in the two-dimensional extended Hubbard model with frustration using the variational cluster approach. The system shows a tendency toward the PI which breaks the tetragonal symmetry of the Fermi surface. However, a pure superconducting state with isotropic underlying lattice is more favored for weak nearest-neighbor repulsion $V$. As $V$ is increased beyond the critical value, superconductivity is suppressed and the system undergoes a first-order transition to the PI. We find that the local density of states (LDOS) shows interesting behavior in the presence of Fermi-surface symmetry breaking. The low-energy feature of LDOS is sharp and the spectral weight remarkably transfers from low-energy peaks to the local minimum near the Fermi level. Meanwhile, the high-energy feature remains almost unchanged. Based on these findings, we can establish a potential relationship between the PI and metamagnetic transition in the bilayer ruthenate ${\text{Sr}}_{3}{\text{Ru}}_{2}{\text{O}}_{7}$.

The state-resolved differential cross sections for both the $1{s}^{2}$ ${}^{1}{S}_{0}\ensuremath{\rightarrow}1s2s$ ${}^{1}{S}_{0}$ monopolar transition and the $1{s}^{2}$ ${}^{1}{S}_{0}\ensuremath{\rightarrow}1s2p$ ${}^{1}{P}_{1}$ dipolar transition of atomic helium had been measured over a large momentum transfer region by high-resolution inelastic x-ray scattering (IXS). The almost-perfect match of the present measurement with the theoretical calculations gives a stringent test of the theoretical method and the calculated wave functions. Our results demonstrate that high-resolution IXS is a powerful tool for studying the excitations in atoms and molecules.

We study heat transport in quantum spin systems analytically and numerically. First, we demonstrate that heat current through a two-level quantum spin system can be modulated from zero to a finite value by tuning a magnetic field. Second, we show that a spin system, consisting of two dissimilar parts---one is gapped and the other is gapless---exhibits current rectification and negative differential thermal resistance. Possible experimental realizations by using molecular junctions or magnetic materials are discussed.

The normal modes and the melting character of a bilayer system consisting of binary charged particles with different charge and/or different mass, interacting through a Coulomb potential and confined in a parabolic trap are investigated. The normal mode spectrum is discussed as a function of the charge ratio (CR) and mass ratio (MR) of the two kinds of charged particles as well as the interlayer separation. We show that the dependence of the normal modes on the excited states can be tuned by varying the CR, the MR, and the interlayer distance. Once the interlayer distance is larger than a critical value, the first excited state corresponds only to the intershell rotation mode. In addition, the intershell rotation melting temperature is discussed as a function of the CR and MR as well as the interlayer separation.

The motion of polarons, which serve as charge carriers in conjugated polymers, is of fundamental importance for understanding transport properties of organic optoelectronic devices. We investigate the dynamics of a charged polaron in the presence of both electron-phonon and electron-electron interactions under the influence of an external electric field, which is modeled by the one-dimensional tight-binding Su-Schrieffer-Heeger (SSH) model supplemented with a Hubbard on-site repulsion term. For this many-body dynamical evolution problem, we develop an adaptive time-dependent density matrix renormalization group ($t$-DMRG) method in combination with a Newtonian equation of motion for atomic displacements. Our results show that the velocity of the polaron is suppressed by the on-site Coulomb interaction $U$. The polaron moves with a supersonic velocity, about four times the sound velocity at the small $U$ limit, and approaches the sound velocity at the large $U$ limit. Furthermore, the dependence of the polaron velocity and the polaron effective mass on the lattice structures are discussed.

We have studied the effects of uniaxial pressure on the geometric structure and the electronic structure of single-walled carbon nanotube bundles theoretically. The local-density approximation in the density-functional theory has been applied to three types of carbon nanotube bundles, made up of the (8, 0), (10, 0), and (11, 0) tubes under uniaxial pressure perpendicular to the tubule axis. In all these types of bundles, an abrupt change is observed in the deformation of the tubes and their configuration at a certain pressure. It is also found that, despite a similar change of the lattice constants of the bundle, the deformation and the configuration of the tubes depend strongly on their types: While the (8, 0) tube bundle has a dense structure at high pressures, larger tube bundles prefer a loose one. All types of bundles, which are calculated to be semiconducting, exhibit a semiconductor-metal transition before or at the beginning of the abrupt change of the lattice constants when they are deformed by the uniaxial pressure. The pressure effect on the energy gap, however, is not monotonous: a decrease and an upturn followed by its disappearance. By analyzing the atomic arrangement, the band structure, and the wave functions of the three types of carbon nanotube bundles, the relationship between them is also established.

In order to clarify the coexistence of a Fermi edge and the steplike multiphonon structure, recently observed in the photoemission spectra PES of the boron-doped diamond, we apply a path-integral theory to calculate the PES, using the many-impurity Holstein model in a simple cubic lattice. Being lightly doped by boron as an acceptor, the diamond shows p-type character with an activation energy gap of about 0.37 eV. We find that, due to the electron-phonon coupling and the increase of the dopant concentration, the impurity band extends up to the top of valence band, and fills the gap gradually. The emergence of a clear Fermi edge is theoretically demonstrated, indicating the strong itineracy of electrons from one impurity atom to another through those intermediate carbon atoms. Simultaneously, the multiphonon satellite structure, a little below the Fermi level, is also theoretically reproduced in the doped site PES, denoting the localization of electrons through the coupling with Einstein phonons. Although we have used a simpler lattice structure than the real diamond one, our exploration of the coexistence of the two intrinsic properties of electrons: itineracy and localization, well agrees with the experimental findings. DOI: 10.1103/PhysRevB.77.045207

On considere des systemes electrons-reseau unidimensionnels, a l'aide de l'approche de Genkin-Mednis. En se fondant sur le modele de Su-Schrieffer-Heeger, on etablit une expression analytique de la generation de la troisieme harmonique de ces polymeres conducteurs

The multicarrier hopping process in disordered organic materials is studied via Monte Carlo simulations taking into consideration both the site exclusion effect and Coulomb interaction. The carrier mobility in materials with Gaussian energetic disorder is found to depend heavily on carrier density. At high carrier densities, the Coulomb interaction is found to reduce the carrier mobility in materials with low intrinsic disorder or at high temperatures, and to enhance the mobility in materials with high intrinsic disorder or at low temperatures.

The relaxation process of the electron and phonon coupling systems in three-dimensional $\mathrm{Sr}\mathrm{Ti}{\mathrm{O}}_{3}$ crystal is investigated by a nonadiabatic molecular-dynamics method. We show, with the ultraviolet illuminations, that the formation of the ferroelectric domains is an ultrafast process.

In connection with the recent experimental discovery on photoenhancements of the electronic conductivity and the quasi-static electric susceptibility in $\mathrm{Sr}\mathrm{Ti}{\mathrm{O}}_{3}$, we theoretically study a photogeneration mechanism of charged and conductive ferroelectric domains in this perovskite type quantum dielectric. The photo-generated electron, being quite itinerant in the $3d$ band of ${\mathrm{Ti}}^{4+}$, is assumed to couple weakly but quadratically with soft-anharmonic ${T}_{1u}$ phonons in this quantum dielectric, in view of the parity of this lattice vibration. The photo-generated electron is also assumed to couple strongly but linearly with the breathing type high energy phonons. Using a tight-binding model for electrons, we will show that this dual electron-phonon coupling results in two types of polarons, a ``super-para-electric (SPE) large polaron'' with a quasi-globle parity violation, and an ``off-center type self-trapped polaron'' with only a local parity violation. This SPE large polaron is shown to be equal to a singly charged $({e}^{\ensuremath{-}})$ and conductive ferroelectric domain with a quasi-macroscopic range. Two of such large polarons are shown to aggregate and form an SPE large bipolaron, which is still conductive. Various other bipolaron clusters are also shown to be formed in this electron-phonon coupled system. These large polarons have a high mobility and an enhanced quasi-static dielectric susceptibility. Effect of adulteration is also discussed.

Density matrix renormalization group methods are used to investigate the quantum phase diagram of a one-dimensional half-filled ionic Hubbard model with bond-charge attraction, which can be mapped from the Su-Schrieffer-Heeger-type electron-phonon coupling at the antiadiabatic limit. A bond order wave (dimerized) phase which separates the band insulator from the Mott insulator always exists as long as electron-phonon coupling is present. This is qualitatively different from that at the adiabatic limit. Our results indicate that electron-electron interaction, ionic potential and quantum phonon fluctuations combine in the formation of the bond-order wave phase.

The atomic structures of 124 single-walled carbon nanotubes, described by their diameter and helicity or equivalently by the two chiral indices $(u,v)$ that define the perimeter of each nanotube, have been determined unambiguously by nanobeam electron diffraction. A mapping of 70 possible nanotubes in the range of 1.20--1.65 nm in diameter with all possible helicities has been constructed experimentally for a carbon nanotube sample produced by arc discharge. Among the total 124 nanotubes characterized experimentally, 58 nanotubes of different structure have been identified. By examining the histogram of occurring helicities, we find that, while certain nanotubes were observed slightly more often than others, the overall feature showed a rather uniform distribution in occurrence. Basing on a nucleation-and-growth model, we suggest that the uniform distribution of helicity be originated from the weak dependence on helicity of the formation energy of carbon nanotubes, while the growth prefers slightly the structure with helicity 15\ifmmode^\circ\else\textdegree\fi{}--30\ifmmode^\circ\else\textdegree\fi{} for which the addition of carbon dimers would facilitate the growth of carbon nanotubes.

The one-electron spectral function of the one-dimensional spin-1/2 Holstein model at half filling is computed by use of the cluster perturbation theory. The cluster Green's function is obtained by the Lanczos exact diagonalization method within an optimized phonon approach. It is shown that the method allows reliable calculations using a relatively small size cluster and a few optimal phonon bases for the system from weak to strong electron-phonon coupling. In the strong-coupling limit, the spectral function shows the excitation behavior of a bipolaron state with a large gap at the Fermi surface. However, the obtained spectral function displays a metallic character in the weak-coupling regime, which is in accord with the suggestion that the Peierls gap is suppressed by quantum fluctuation of the phonons.

By considering a metal/polymer/metal structure within a tight-binding one-dimensional model, we have investigated the polaron formation in the presence of an electric field. When a sufficient voltage bias is applied to one of the metal electrodes, an electron is injected into the polymer chain, and then a self-trapped polaron is formed at a few hundreds of femtoseconds while it moves slowly under a weak electric field (not larger than $1.0\ifmmode\times\else\texttimes\fi{}{10}^{4}\mathrm{V}/\mathrm{c}\mathrm{m}).$ At an electric field between $1.0\ifmmode\times\else\texttimes\fi{}{10}^{4}\mathrm{V}/\mathrm{c}\mathrm{m}$ and $8.0\ifmmode\times\else\texttimes\fi{}{10}^{4}\mathrm{V}/\mathrm{c}\mathrm{m},$ the polaron is still formed, since the injected electron is bounded between the interface barriers for quite a long time. It is shown that the electric field applied at the polymer chain reduces effectively the potential barrier in the metal/polymer interface.

Density-matrix renormalization-group methods are used to investigate the quantum phase transitions in a Hubbard chain with an alternating on-site energy $\ensuremath{\Delta}$ at half filling. Our results support the existence of a bond-order wave phase, which separates the band insulator from the Mott insulator even in the absence of electron-phonon coupling. The critical points are determined accurately at fixed staggered potentials. Our results indicate a slightly smaller size of the bond-order wave phase than that of previous results.

By using the density-matrix renormalization-group numerical method, we investigate the bond order wave ~BOW! and energy gap ~EG! in a one-dimensional half-filling bond-charge attraction model, which can be mapped from the electron-phonon interacting Su-Schrieffer-Heeger ~SSH! Hamiltonian at the antiadiabatic limit. The obtained results are compared with that of the SSH model at the adiabatic limit as well as that within the mean-field approximation. It is shown that a simple linear relation between the BOW and the EG is kept even at the antiadiabatic limit while the reduction of the BOW is much smaller than that of the EG.

Hydrostatic pressure up to 10 GPa has been applied to determine the lattice compressibility and the dependence of transition temperature ${T}_{c}$ of an ${\mathrm{MgB}}_{2}$ superconducting material that shows a very narrow transition range when prepared by high-pressure sintering. The transition temperature ${T}_{c}$ decreases with increasing pressure at a rate of 1.03 K/GPa. The material has a bulk modulus 172 GPa and the unit cell of the ${\mathrm{MgB}}_{2}$ crystals shows an anisotropic compressibility with its c axis appearing more compressible than the $a/b$ axis. The experimental results are discussed within the framework of the BCS theory, which gives rise to estimates in good agreement with the experimental results.