DIAGRAMMATIC VIBRATIONAL COUPLED-CLUSTER

A diagrammatic vibrational coupled-cluster method for calculation of zero-point energies and an equation-of-motion coupled-cluster method for calculation of anharmonic vibrational frequencies are developed. The methods, which we refer to as XVCC and EOM-XVCC respectively, rely on the size-extensive vibrational self-consistient field (XVSCF) method for reference wave functions. The methods retain the efficiency advantages of XVSCF making them suitable for applications to large molecules and solids, while they are numerically shown to accurately predict zero-point energies and frequencies of small molecules as well. In particular, EOM-XVCC is shown to perform well for modes which undergo Fermi resonance where traditional perturbative methods fail. Rules for the systematic generation and interpretation of the XVCC and EOM-XVCC diagrams to any order are presented

Plasmonically Enhanced Reflectance of Heat Radiation from Low-Bandgap Semiconductor Microinclusions

Increased reflectance from the inclusion of highly scattering particles at low volume fractions in an insulating dielectric offers a promising way to reduce radiative thermal losses at high temperatures. Here, we investigate plasmonic resonance driven enhanced scattering from microinclusions of low-bandgap semiconductors (InP, Si, Ge, PbS, InAs and Te) in an insulating composite to tailor its infrared reflectance for minimizing thermal losses from radiative transfer. To this end, we compute the spectral properties of the microcomposites using Monte Carlo modeling and compare them with results from Fresnel equations. The role of particle size-dependent Mie scattering and absorption efficiencies, and, scattering anisotropy are studied to identify the optimal microinclusion size and material parameters for maximizing the reflectance of the thermal radiation. For composites with Si and Ge microinclusions we obtain reflectance efficiencies of 57 - 65% for the incident blackbody radiation from sources at temperatures in the range 400 - 1600 {\deg}C. Furthermore, we observe a broadbanding of the reflectance spectra from the plasmonic resonances due to charge carriers generated from defect states within the semiconductor bandgap. Our results thus open up the possibility of developing efficient high-temperature thermal insulators through use of the low-bandgap semiconductor microinclusions in insulating dielectrics.Comment: Main article (8 Figures and 2 Tables) + Supporting Information (8 Figures

Elucidating the real-time Ag nanoparticle growth on a-Ag2WO4 during electron beam irradiation: experimental evidence and theoretical insights

Why and how Ag is formed when electron beam irradiation takes place on α-Ag2WO4 in a vacuum transmission electron microscopy chamber? To find an answer, the atomic-scale mechanisms underlying the formation and growth of Ag on α-Ag2WO4 have been investigated by detailed in situ transmission electron microscopy (TEM) and field emission scanning electron microscopy (FE-SEM) studies, density functional theory based calculations and ab initio molecular dynamics simulations. The growth process at different times, chemical composition, size distribution and element distribution were analyzed in depth at the nanoscale level using FE-SEM, operated at different voltages (5, 10, 15, and 20 kV), and TEM with energy dispersive spectroscopy (EDS) characterization. The size of Ag nanoparticles covers a wide range of values. Most of the Ag particles are in the 20–40 nm range. The nucleation and formation of Ag on α-Ag2WO4 is a result of structural and electronic changes in the AgOx (x = 2,4, 6, and 7) clusters used as constituent building blocks of this material, consistent with metallic Ag formation. First principle calculations point out that Ag-3 and Ag-4-fold coordinated centers, located in the sub-surface of the (100) surface, are the most energetically favorable to undergo the diffusion process to form metallic Ag. Ab initio molecular dynamics simulations and the nudged elastic band (NEB) method were used to investigate the minimum energy pathways of these Ag atoms from positions in the first slab layer to outward sites on the (100) surface of α-Ag2WO4. The results point out that the injection of electrons decreases the activation barrier for this diffusion step and this unusual behavior results from the presence of a lower energy barrier process.Generalitat-Valenciana: Prometeo/2009/053 Ministerio de Economia y Competitividad (Spain): CTQ2012-36253-C03-02 Spanish Brazilian program: PHB2009-0065-PC FAPESP : 2013/07296-2, 2012/14468-1, 2010/16970-0, 2013/02032-7. CAPES CNPq : 573636/2008-7, 150753/2013-6. CAPES : 088/201

Diagrammatic theories for the vibrational many-body problem

Anharmonic vibrational many-body methods are developed for and applied to small molecules and extended systems in a bound potential energy surface (PES). Diagrammatically size-consistent and basis-set-free vibrational coupled-cluster (XVCC) theory for both zero-point energies and transition frequencies, the latter through the equation-of-motion (EOM) formalism, is defined for an $n$th-order Taylor-series PES. Quantum-field-theoretical tools (the rules of normal-ordered second quantization and Feynman--Goldstone diagrams) for deriving their working equations are established. The equations of XVCC and EOM-XVCC are derived and implemented with the aid of computer algebra. Algorithm optimizations known as strength reduction, intermediate reuse, and factorization are carried out before code generation,producing algorithms with optimal cost scaling. A similarity-transformed equation-of-motion vibrational coupled-cluster (STEOM-XVCC) method is introduced as a one-mode theory with an effective vibrational Hamiltonian, which is similarity transformed twice so that its lower-order operators are dressed with higher-order anharmonic effects. From diagonalization of this doubly similarity-transformed Hamiltonian in the small one-mode excitation space, the method simultaneously computes accurate anharmonic vibrational frequencies of all fundamentals, which have unique significance in vibrational analyses. We establish a diagrammatic method of deriving the working equations of STEOM-XVCC and prove their connectedness and thus size-consistency as well as the exact equality of its frequencies with the corresponding roots of EOM-XVCC. An extended STEOM-XVCC (Ext-STEOM-XVCC) method is defined as an $m$th order configuration interaction method with the doubly similarity-transformed Hamiltonian including up to $m$th-order excitation operators. Because the doubly transformed Hamiltonian is dressed with higher-order anharmonic effects, the frequencies of overtones and combinations obtained are different and superior to the corresponding EOM-XVCC method. We compare and contrast the Ext-STEOM-XVCC method to its electronic counterpart. We apply the previously established second-order size-extensive vibrational many-body perturbation (XVMP2) method to the anharmonic phonon dispersion curves of a model two-mass system and the optical phonons of polyethylene. We obtain accurate results despite the presence of multiple Fermi-resonances in the crystalline systems

Plasmons in Photocharged ZnO Nanocrystals Revealing the Nature of Charge Dynamics

Localized surface plasmon resonances (LSPRs), known for their fascinating optical properties, have thus far been limited to nanostructures of materials with high steady-state charge carrier densities. Here, we show that even a nonequilibrium charge population can support a LSPR mode. Photocharged zinc oxide (ZnO) nanocrystals show an infrared LSPR, which can be dynamically turned off by discharging via redox activity. It is deduced that the photoinduced LSPR is a collective mode of as few as four conduction band electrons, the least observed thus far. The sustenance of a free-electron plasma in charged ZnO, supported by the LSPR observation, leads us to propose the existence of a many-body excitonic state and suggest a mechanism for previously unresolved charge trapping dynamics in ZnO. The LSPR, which serves as an optical signature of a charged state of the nanoparticle, is also demonstrated as a useful probe of surface redox reactions

Unified Theoretical Framework for Realizing Diverse Regimes of Strong Coupling between Plasmons and Electronic Transitions

Strong coupling between light and quantum mechanical transitions historically observed in atomic optics is now being realized in the solid state using plasmon resonances. Recent experiments on hybrid plasmonic/excitonic systems have reported varied observations such as excitonic splitting, asymmetric line shapes, and dips in scattering spectra. Here, we unite these seemingly disparate empirical observations under a single theoretical framework, illustrating that the same generalized hybrid system allows access to diverse forms of coupling between plasmons and molecular transitions. Simply by modifying the damping rate of the plasmon resonance, it is possible to transition from one regime of coupling to another (e.g., from Rabi splitting to Fano interference). Common experimental handles such as size, shape, and nature of the metal can be varied to tune the regime of coupling, as shown by electrodynamic simulations. We also show that strong coupling can be achieved using simple nanostructure configurations such as a plasmonic core/excitonic shell geometry without the necessity of sophisticated design of near-field hotspots. The unified model developed here will allow rational predictive design of hybrid plasmonic systems for achieving unique control of light on the nanoscale