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Quantum embedding theories model a collection of interacting molecules as a set of subsystems, where each can be treated with a particular electronic structure method (wave function or density functional theory, for example); these theories can lead to computationally efficient and accurate algorithms. Motivated by challenges in the field, we previously described a formalism (that models two subsystems), which we call ``locally coupled open subsystems'' (LCOS), for the computation of ground-state energies, fractional electron-occupation numbers of the subsystems, and the size-consistent limit of subsystem dissociation. In this work we present the full (nonrelativistic) LCOS theory and the following extensions of our previous work: (i) the framework to study systems composed of multiple subsystems and a procedure to spin-adapt the auxiliary wave function that describes the partitioned system, so that its spin state matches that of the real system of interest; (ii) potential functionals and ideas to employ machine learning for the computation of ground-state densities and energies; (iii) formulation of two LCOS ground-state approaches where the fragments are assigned Kohn-Sham wave functions; and (iv) a time-dependent (TD) extension of these two ground-state formalisms in which the state of the subsystems evolves according to a unitary propagation; from this evolution we can extract TD electron populations of the fragments, for instance. We also discuss potential applications of the TD LCOS theory to linear photoabsorption and Raman spectroscopy. The developments presented in this work can lead to ground-state and TD electronic structure calculations where the computational scaling can be controlled, depending on the level of theory and the accuracy desired to model each one of the subsystems and their coupling.

Ratchets rectify the motion of randomly moving particles, which are driven by isotropic sources of energy such as thermal and chemical energy, without applying a net, time-averaged force between source and drain. This paper describes the behavior of a damped electron, modeled by a quantum Lindblad master equation, within a flashing ratchet (a one-dimensional potential that oscillates between a flat surface and a periodic asymmetric surface). By examining the complete space of all biharmonic potential shapes and a large range of oscillation frequencies, two modes of ratchet operation, differentiated by their oscillation frequencies (relative to the rate of electron relaxation), are identified. Slow-oscillating, strong friction ratchets operate by a classical, overdamped mechanism. In fast-oscillating, weak friction ratchets, current is primarily produced when the frequency of the oscillating potential is resonant with the beating of the electron wave function in the potential well. The shape of the ratchet potential determines the direction of the current (and, in some cases, straightforwardly accounts for current reversals), but the maximum achievable current at any shape is controlled by the degree of friction applied to the electron.

We investigate the local electron temperature distribution in graphene nanoribbon and graphene junctions subject to an applied thermal gradient. Using a realistic model of a scanning thermal microscope, we predict quantum temperature oscillations whose wavelength is related to that of Friedel oscillations. Experimentally this wavelength can be tuned over several orders of magnitude by gating or doping, bringing quantum temperature oscillations within reach of the spatial resolution of existing measurement techniques.

We present a description of a short-range high-density hole in a polarized electron gas. Two leading derivatives of the same-spin correlation hole in a dense electron gas near the origin are derived analytically. They agree well with published numerical results, provided that they are renormalized appropriately. Combined with the two leading derivatives of the opposite-spin correlation hole calculated earlier, they give a complete description of the correlation hole in a dense electron gas near the origin. The spin resolution of the correlation hole in the dense gas is also discussed.

We consider the quantum evolution and relaxation of an electron or hole which is coupled to a set of bath modes. In most applications the bath modes would be the vibronic coordinates but the model considered applies to any type of dynamic boson environment. The method is developed specifically for the problem of dynamic polaron formation in small nonperiodic systems. It can describe a broad group of experimental situations, including in particular electron localization in organics and polymeric materials and devices. The immediate bath is allowed to dissipate energy to a secondary bath. The bath obeys classical dynamics which puts some restriction on the range of validity of this approach. Using the density matrix formalism on a tight binding model consisting of a linear chain coupled to vibronic coordinates, we demonstrate in real time how the interaction with a dissipative bath makes the initial quantum distribution reach a steady-state population. This calculation is based on the Ehrenfest dynamics approximation. As an example we consider coupling at a single impurity site and find that for given parameters (bath coupling, site energy, and relaxation rate), the particle becomes dynamically localized in space on a particular time scale. This localized particle can be called a polaron. We define a population formation time in the same way as done in the experimental measurement. This formation time is studied as a function of the coupling strength, bandwidth, and energy dissipation rate. Energy dissipation plays a crucial role in the spatial localization process. The formation time shortens as the electron-vibration coupling increases, and as the intersite tunneling increases, but lengthens with impurity trap depth. Polaron formation is suppressed for sufficiently wide electronic bands.

We study the time dependence of the energy redistribution between an initially localized electron and the vibrations of a one-dimensional anharmonic lattice. Numerical solutions show the periodic electron-lattice energy exchange, in contrast to the usual irreversible gradual decrease of the electronic energy. This energy trapping is shown to occur due to the appearance of localized vibrations, induced by strong enough electron-vibrational coupling in the anharmonic lattice. For these strong-coupling situations, we observed both localized (trapped) and conductive electronic charge.

We consider the extension of ultrafast laser alignment schemes to surface-adsorbed molecules, where the laser field coerces the molecule to reorient itself relative to the surface. When probed by a scanning tunneling microscope tip, this reorientation modifies the tip-molecule distance, and thus the tunneling current, suggesting a route to an ultrafast, nanoscale current switch. In addition to exploring the controllability of adsorbed molecules by moderately intense laser fields and discussing the fundamental differences of alignment behavior between surface-adsorbed molecules and gas phase molecules, we computationally investigate the quality of orientation with respect to field intensity, field duration, and the location of the tip. Overall, the molecule moves directly to its oriented configuration, which is reasonably insensitive to the tip location. These results collectively suggest the efficacy of using laser alignment schemes to control electron transport through junctions.

A closed-form algorithm for computing subsurface Green functions—the blocks of a material’s Green function between the surface and the bulk—is presented, where we assume the system satisfies a common principal-layer approximation. By exploiting the block tridiagonal and nearly block Toeplitz structure of the Hamiltonian and overlap matrices, this method scales independently of the system size (constant scaling), allowing studies of large systems. As a proof-of-concept example, we investigate the decay of surface effects in an armchair graphene nanoribbon, demonstrating the persistence of surface effects hundreds of atomic layers (∼0.5 μm) away from a surface. We finally compare the surface-to-bulk transitions of finite and semi-infinite systems, finding that finite systems exhibit amplified surface effects.

Both the energy differences between metastable and stable phases and the energy barriers separating these phases decrease with decreasing particle number. Then, for small enough systems, random heterophasic fluctuations of the entire system become an observable form of thermal motion. We discuss mechanisms and observation conditions for these random transitions between phases.

A method developed by Sandalov et al. [Int. J. Quantum Chem. 94, 113 (2003)] is applied to inelastic transport in the case of strong correlations on the molecule, which is relatively weakly coupled to contacts. The ability of the approach to deal with the transport in the language of many-body molecular states as well as to take into account charge-specific normal modes and nonadiabatic couplings is stressed. We demonstrate the capabilities of the technique within simple model calculations and compare it to previously published approaches.

In this paper, a simple spin-spin Ising interaction model for the surface ferromagnetism is combined with the bulk Au diamagnetic response to model the size dependence of the magnetization of a Au nanoparticle. Using the maximum entropy formalism, we obtain the average temperature dependent magnetization within a mean field model. Our results qualitatively reproduce recent experimental observations of size-dependent magnetization of Au nanoparticles in which the ferromagnetic moment of thiol-capped nanoparticles is seen to increase for diameters larger than $0.7\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$, peaking at approximately $3\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$, and subsequently decreasing as the particle diameter increases further.

A light-sensitive molecular switch based on single azobenzene molecule has been proposed recently C. Zhang, M. H. Du, H. P. Cheng, X. G. Zhang, A. E. Roitberg, and J. L. Krause, Physical Review Letters 92, 158301 2004 . Here we investigate the stability of the molecular switch under finite bias. Using a firstprinciples method that combines the nonequilibrium Green’s function technique and density functional theory, we compute the current-voltage curves for both trans and cis configurations of the azobenzene molecule connected to two gold leads between bias voltages of 0 and 1 V. We find that the current through the trans configuration is significantly higher than that through the cis configuration for most biases, suggesting that the molecular switch proposed previously is stable under the finite bias. A negative differential conductance NDR is found for the cis configuration at 0.8 V. Analysis of the band structure of the leads and the molecular states reveals that the transmission through the highest occupied molecular orbital state of the molecule is suppressed significantly at this bias voltage, which causes the NDR.

We use a computational method based on the discrete-dipole approximation (DDA) to calculate the gradient and nongradient contributions to optical forces on nanometer sized silver particles in water. We find that, due to a contribution that is usually neglected, nongradient forces are often non-negligible. This result is not a consequence of an approach to the dipole limit. We suggest that this method could provide useful input for a more detailed understanding of the physics relevant to optical trapping and binding phenomena.

A Keldysh nonequilibrium Green's function approach is used in an analytic treatment of transport in oligomeric molecular wire junctions in the presence of dephasing effects. Both the dephasing and molecule-electrode interactions are treated using the self-energy formalism, and limiting analytic forms are obtained for the current as a function of dephasing strength, length, injection energy, molecular hopping integral, and temperature. Dephasing due to the interaction between electrons and thermal phonons is investigated in detail, and we relate the phenomenological dephasing parameter to the inner-sphere reorganization energy. In the single-site limit, we observe transitions in the spectral function from Lorentzian to semicircular as the dephasing strength increases, while for the extended chain we observe both tunneling and hopping behavior. In the limit of strong dephasing, the overall resistance can be expressed as the sum of the contact resistance and a chain-length-dependent hopping term.

We present a scaling analysis of electronic and transport properties of metal-semiconducting carbon nanotube interfaces as a function of the nanotube length within the coherent transport regime, which takes fully into account atomic-scale electronic structure and three-dimensional electrostatics of the metal-nanotube interface using a real-space Green's function based self-consistent tight-binding theory. As the first example, we examine devices formed by attaching finite-size single-wall carbon nanotubes (SWNT) to both high- and low- work function metallic electrodes through the dangling bonds at the end. We analyze the nature of Schottky barrier formation at the metal-nanotube interface by examining the electrostatics, the band lineup and the conductance of the metal-SWNT molecule-metal junction as a function of the SWNT molecule length and metal-SWNT coupling strength. We show that the confined cylindrical geometry and the atomistic nature of electronic processes across the metal-SWNT interface leads to a different physical picture of band alignment from that of the planar metal-semiconductor interface. We analyze the temperature and length dependence of the conductance of the SWNT junctions, which shows a transition from tunneling- to thermal activation-dominated transport with increasing nanotube length. The temperature dependence of the conductance is much weaker than that of the planar metal-semiconductor interface due to the finite number of conduction channels within the SWNT junctions. We find that the current-voltage characteristics of the metal-SWNT molecule-metal junctions are sensitive to models of the potential response to the applied source/drain bias voltages., Minor revision to appear in Phys. Rev. B. Color figures available in the online PRB version or upon request to: yxue@uamail.albany.edu

We present an atomistic self-consistent tight-binding study of the electronic and transport properties of metal-semiconducting carbon nanotube interfaces as a function of the nanotube channel length when the end of the nanotube wire is buried inside the electrodes. We show that the lineup of the nanotube band structure relative to the metal Fermi-level depends strongly on the metal work function but weakly on the details of the interface. We analyze the length-dependent transport characteristics, which predicts a transition from tunneling to thermally-activated transport with increasing nanotube channel length., Comment: To appear in Phys.Rev.B Rapid Communications. Color figures available in PRB online version

We investigate the effect on molecular transport due to the different structural aspects of metal-molecule interfaces. The example system chosen is the prototypical molecular device formed by sandwiching the phenyl dithiolate molecule (PDT) between two gold electrodes with different metal-molecule distance, atomic structure at the metallic surface, molecular adsorption geometry and with an additional hydrogen end atom. We find the dependence of the conductance on the metal-molecule interface structure is determined by the competition between the modified metal-molecule coupling and the corresponding modified energy level lineup at the molecular junction. The results of the detailed microscopic calculation can all be understood qualitatively from the equilibrium energy level lineup and the knowledge of the voltage drop across the molecular junction at finite bias voltages., Comment: Submitted to Phys.Rev.B

We present the first in a series of microscopic studies of electrical transport through individual molecules with metallic contacts. We view the molecules as ``heterostructures'' composed of chemically well-defined atomic groups, and analyze the device characteristics in terms of the charge and potential response of these atomic-groups to the perturbation induced by the metal-molecule coupling and the applied electrical field, which are modeled using a first-principles based self-consistent matrix Green's function (SCMGF) method. As the first example, we examine the devices formed by attaching two benzene-based molecular radicals--phenyl dithiol (PDT) and biphenyl dithiol (BPD)--symmetrically onto two semi-infinite gold electrodes through the end sulfur atoms., Submitted to Phys.Rev.B

Monte Carlo simulation has been used to characterize the capillary force due to the condensation of a liquid meniscus between a tip with a nanoscale asperity and a flat surface. To consider both hydrophobic and hydrophilic molecules coating the tip as a model of dip-pen nanolithography, tips with various wettabilities are studied. The capillary force due to the meniscus is calculated for various saturations (humidities). We have implemented a thermodynamic integration technique that can project the force into energetic and entropic contributions. In most cases, the force is mainly energetic in origin. At the snap-off separation where the meniscus disappears, the tip feels a significant entropic force at high saturation. Our calculation shows nonmonotonic behavior of the pull-off force as a function of saturation, which is in qualitative accord with experiments.

This paper presents a detailed experimental study of random lasers with resonant feedback. The dependences of the lasing threshold and the number of lasing modes on the transport mean free path, the pump area, and the sample size are measured. An analytical model based on the concept of quasistates is developed to explain the behaviors of random lasers.

Utilisation des energies vibrationnelles des modes de valence de CO 2 et une generalisation de la methode d'inversion de Rydberg-Klein-Rees pour les molecules polyatomiques pour determiner la surface de potentiel 2D de CO 2 sans deformation