Sub-Ohmic spin-boson model with off-diagonal coupling: Ground state properties

We have carried out analytical and numerical studies of the spin-boson model in the sub-ohmic regime with the influence of both the diagonal and off-diagonal coupling accounted for via the Davydov D1 variational ansatz. While a second-order phase transition is known to be exhibited by this model in the presence of diagonal coupling only, we demonstrate the emergence of a discontinuous first order phase transition upon incorporation of the off-diagonal coupling. A plot of the ground state energy versus magnetization highlights the discontinuous nature of the transition between the isotropic (zero magnetization) state and nematic (finite magnetization) phases. We have also calculated the entanglement entropy and a discontinuity found at a critical coupling strength further supports the discontinuous crossover in the spin-boson model in the presence of off-diagonal coupling. It is further revealed via a canonical transformation approach that for the special case of identical exponents for the spectral densities of the diagonal and the off-diagonal coupling, there exists a continuous crossover from a single localized phase to doubly degenerate localized phase with differing magnetizations.Comment: 11 pages, 7 figure

Disorder influenced absorption line shapes of a chromophore coupled to two-level systems

We have carried out a theoretical and numerical study of disorder-induced changes in the absorption line shape of a chromophore embedded in a host matrix. The stochastic sudden jump model is employed wherein the host matrix molecules are treated as noninteracting two-level systems (TLSs) occupying points on a three-dimensional lattice with randomly oriented dipole moments. By systematically controlling the degree of positional disorder (α) attributed to them, a perfectly crystalline (α = 0) or a glassy environment (α = 1) or a combination of the two is obtained. The interaction between the chromophore and the TLSs is assumed to be of the dipole–dipole form. With an increase in α, the broadening of the absorption line shape was found to follow a power-law behavior. More importantly, it is revealed in the long-time limit that the resultant line shape is Gaussian in the absence of disorder but transforms to Lorentzian for a completely disordered environment. For an arbitrarily intermediate value of α, the resultant line shape can be approximately fitted by a linear combination of Gaussian and Lorentzian components. The Lorentzian profile for the disordered medium is attributed to the chomophore–TLS pairs with vanishingly small separation between them.Accepted versio

Disorder and spectral line shapes in two-level systems

Using a stochastic model, we examine disorder-induced changes in the absorption line shape of a chromophore embedded in a matrix of noninteracting two-level systems (TLSs) with randomly oriented dipole moments. By systematically controlling the degree of TLS positional disorder, a perfectly crystalline, glassy or a combination of the two environments is obtained. The chromophore is assumed to interact with TLSs via long-range dipole–dipole coupling. At long times and in the absence of disorder, Gaussian line shapes are found, which morph into Lorentzian for a completely disordered environment owing to strong coupling between the chromophore and a TLS in close vicinity.Accepted versio

Internal Conversion and Vibrational Energy Redistribution in Chlorophyll A

We have computationally investigated the role of intramolecular vibrational modes in determining nonradiative relaxation pathways of photoexcited electronic states in isolated chlorophyll A (ChlA) molecules. To simulate the excited state relaxation from the initially excited Soret state to the lowest excited state Qy, the approach of nonadiabatic excited state molecular dynamics has been adopted. The intramolecular vibrational energy relaxation and redistribution that accompany the electronic internal conversion process is followed by analyzing the excited state trajectories in terms of the ground state equilibrium normal modes. The time dependence of the normal mode velocities is determined by projecting instantaneous Cartesian velocities onto the normal mode vectors. Our analysis of the time evolution of the average mode energies uncovers that only a small subset of the medium-to-high frequency normal modes actively participate in the electronic relaxation processes. These active modes are characterized by the highest overlap with the nonadiabatic coupling vectors(NACRs) during the electronic transitions. Further statistical analysis of the nonadiabatic transitions reveals that the electronic and vibrational energy relaxation occurs via two distinct pathways with significantly different time scales on which the hopping from Soret to Qx occurs thereby dictating the overall dynamics. Furthermore, the NACRs corresponding to each of the transitions have been consistently found to be predominantly similar to a set of normal modes that vary depending upon the transition and the identified categories. Each pathway exhibits a differential time scale of energy transfer and also a differential set of predominant active modes. Our present analysis can be considered as a general approach allowing identification of a reducedsubset of specific vibrational coordinates associated with nonradiative relaxation pathways. Therefore, it represents an adequate prior strategy that can particularly facilitates mixed quantum-classical approaches.Fil: Shenai, Prathamesh M.. Nanyang Technological University; SingapurFil: Fernández Alberti, Sebastián. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina. Universidad Nacional de Quilmes; ArgentinaFil: Bricker, William P.. University of Washington; Estados UnidosFil: Tretiak, Sergei. Los Alamos National High Magnetic Field Laboratory; Estados UnidosFil: Zhao, Yang. Nanyang Technological University; Singapu

Unraveling the Activity of Iron Carbide Clusters Embedded in Silica for Thermocatalytic Conversion of Methane

Isolated Fe-sites on silica substrate have recently been reported for direct and non-oxidative conversion of gaseous methane with high selectivity. The activated catalyst was proposed to be FeC2 cluster embedded in silica. Using a combination of density-functional theoretic methods and micro-kinetic modeling, we show that under the same reaction conditions (1223 K , 1 atm) FeC2 sites convert to FeC3 and the latter is instead responsible for the observed activity. We investigate the detailed mechanism of conversion of methane to methyl radical and hydrogen on FeC3@SiO2 under different conditions of methane partial pressure. We find that methyl radical evolution is the rate-determining step for the overall conversion. Our calculations also indicate that the conversion of embedded FeC3 to FeC4 competes with methyl radical evolution from the active catalyst. However, due to the higher stability of FeC3 sites, we anticipate that formation of higher carbides can be inhibited by controlling the hydrogen partial pressure.</p

Non-radiative relaxation of photoexcited chlorophylls: Theoretical and experimental study

Nonradiative relaxation of high-energy excited states to the lowest excited state in chlorophylls marks the first step in the process of photosynthesis. We perform ultrafast transient absorption spectroscopy measurements, that reveal this internal conversion dynamics to be slightly slower in chlorophyll B than in chlorophyll A. Modeling this process with non -Adiabatic excited state molecular dynamics simulations uncovers a critical role played by the different side groups in the two molecules in governing the intramolecular redistribution of excited state wavefunction, leading, in turn, to different time-scales. Even given smaller electron-vibrational couplings compared to common organic conjugated chromophores, these molecules are able to efficiently dissipate about 1 eV of electronic energy into heat on the timescale of around 200 fs. This is achieved via selective participation of specific atomic groups and complex global migration of the wavefunction from the outer to inner ring, which may have important implications for biological light-harvesting function.Fil: Bricker, William P.. University of Washington; Estados UnidosFil: Shenai, Prathamesh M.. Nanyang Technological University; SingapurFil: Ghosh, Avishek. Nanyang Technological University; SingapurFil: Liu, Zhengtang. Nanyang Technological University; SingapurFil: Enriquez, Miriam Grace M.. Nanyang Technological University; SingapurFil: Lambrev, Petar H.. Nanyang Technological University; Singapur. Hungarian Academy of Sciences; HungríaFil: Tan, Howe-Siang. Nanyang Technological University; SingapurFil: Lo, Cynthia S.. Nanyang Technological University; SingapurFil: Tretiak, Sergei. Los Alamos National Laboratory; Estados UnidosFil: Fernández Alberti, Sebastián. Universidad Nacional de Quilmes; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; ArgentinaFil: Zhao, Yang. Nanyang Technological University; Singapu