Optical absorption preceding resonant double photoionization of aromatic hydrocarbons hydrocarbons

We analyze resonances in the double photoionization of a variety of aromatic hydrocarbons. The resonances reflect the breakup of quasi-bound electron pairs. The basic premise of this paper is that there is a direct connection between the quasi-bound pairs and resonant peaks in the optical absorption that are associated with doubly occupied sites on the perimeter and inside the perimeter of the molecule. The optical absorption leading to the high-energy resonance (approximately 40 eV), calculated from a many-site one-dimensional Hubbard model, has a peak at U, the electrostatic interaction energy for two electrons with antiparallel spins on the same carbon atom. In the model, there are also two satellites whose separation from the main resonance is approximately +/-10 eV suggesting that unresolved satellite structure may be contributing to the linewidth of the resonant peak. The low energy resonances (approximately 10 eV) involve carbon atoms located inside the perimeter, a configuration present only in pyrene and coronene (among the hydrocarbons studied). In the case of pyrene, which has two carbon atoms inside the perimeter, we employ a two-site Hubbard model to characterize the absorption leading to the quasi-bound state. A brief analysis of the double photoionization resonance of the heterocyclic inorganic molecule 1,3,5-triazine presented. We also discuss recent results for the double photoionization of the cyclic inorganic molecule tribromoborazine and the organic molecules furan, pyrrole, selenophene, and thiophene where the 2+ ion concentration varies linearly with the difference between the photon energy and the threshold energy. A theory for the linear behavior is outlined

Coulomb Pairing and Double Photoionization in Aromatic Hydrocarbons

Recently reported anomalies in the double-photonionization spectra of the aromatic molecules partially deuterated benzene, naphthalene, anthracene, pentacene, azulene, phenanthrene, pyrene and coronene are attributed to Coulomb-pair resonances of pi electrons. The properties of the resonance in benzene are investigated in detail. The linear behavior in the 2+/1+ ion ratio above the resonance is attributed to a two-electron transition associated with excitation from the ground state to a two-electron continuum. A similar explanation accounts for the linear behavior seen in the pentagonal rings pyrrole, furan, selenophene and thiophene which do not display resonance peaks.Comment: 6 pages. arXiv admin note: substantial text overlap with arXiv:1312.049

Comments on Coulomb pairing in aromatic hydrocarbons

Recently reported anomalies in the double-photonionization spectra of aromatic molecules such as benzene, naphthalene, anthracene and coronene are attributed to Coulomb-pair resonances of pi electrons.Comment: 5 page

Linear temperature dependence of electron spin resonance linewidths in La0.7Ca0.3MnO3 and YBaMn2O6

We analyze recent electron spin resonance (ESR) experiments in La0.7Ca0.3MnO3 and YBaMn2O6 focusing on the behavior of the linewidth at high temperatures where it is a linear function of the temperature. Noting that the g-factors of the resonances are characteristic of the Mn4+ ion in a cubic environment, we make the assumption that the linewidth involves the static susceptibility of the Mn4+ spins which we analyze in the molecular field approximation. We conclude that the linear dependence on temperature is associated with the susceptibility having a Curie or Curie-Weiss form while the temperature-dependent relaxation mechanism has a microscopic rate proportional to the temperature. In La0.7Ca0.3MnO3, the Mn4+ susceptibility has the ferromagnetic Curie-Weiss form, and the static contribution to the linewidth arising from distortions of the oxygen octahedra is absent due to motional narrowing brought on by the rapid hopping of the eg polarons. In YBaMn2O6 either of two scenarios is possible. The Mn4+ susceptibility above 520 K is Curie-like and the static term is present, or the susceptibility has the antiferromagnetic Curie-Weiss form and the static term is absent due to motional narrowing. It is concluded that the Curie model, with offsetting double exchange and and superexchange Curie-Weiss parameters, is the more likely scenario. It is suggested that the linear-T variation of the linewidth in both materials arises from either a Korringa-like mechanism involving interactions with mobile carriers or from a spin-phonon process coming from interactions between the Mn4+ ions and the lattice vibrations

Dynamics of an Ensemble of Noisy Bistable Elements with Global Time-Delayed Coupling

The dynamics of an ensemble of bistable elements with global time-delayed coupling under the influence of noise is studied analytically and numerically. Depending on the noise level the system undergoes ordering transitions and demonstrates multi-stability. That is, for a strong enough positive feedback it exhibits a non-zero stationary mean field and a variety of stable oscillatory mean field states are accessible for positive and negative feedback. The regularity of the oscillatory states is maximal for a certain noise level, i.e., the system demonstrates coherence resonance. While away from the transition points the system dynamics is well described by a Gaussian approximation, near the bifurcation points a description in terms of a dichotomous theory is more adequate.Comment: 4 pages, 3 figures. Accepted for publication in Phys. Rev. Let

The effect of anisotropy on the absorption spectrum and the density of states of two-dimensional Frenkel exciton systems with Gaussian diagonal disorder

On the optical absorption and the density of states of Frenkel exciton systems on square, rectangular, and triangular lattices with nearest-neighbor interactions and a Gaussian distribution of transition frequencies. The analysis is based on an elliptic integral approach that gives results over the entire spectrum. It is found that the absorption is weakly affected by the anisotropy in contrast to the density of states where the effects can be much stronger. The results for the square lattice are in good agreement with the finite array calculations of Schreiber and Toyozawa. Our findings suggest that the coherent potential approximation can be useful in interpreting the optical properties of two-dimensional systems with dominant nearest-neighbor interactions and Gaussian diagonal disorder where the optically excited states are Frenkel excitons.Comment: one pdf fil

Equilibrium Properties of Mixtures of Bosons and Fermions

Partial Quantum Nearest Neighbor Probability Density Functions (PQNNPDF's) are formulated for the purpose of determining the behavior of quantum mixed systems in equilibrium in a manner analogous to that provided for classical multi-component systems. Developments in partial quantum m-tuplet distribution functions, a generalization of the partial quantum radial distribution function, along with their relationship to PQNNPDF's, are briefly elucidated. The calculation of statistical thermodynamic properties of quantum mixtures is presented for arbitrary material systems. Application to the limiting case of dilute, weakly correlated quantum gas mixtures has been outlined and the second virial coefficient is derived. The case of dilute strongly degenerate mixtures is also addressed, providing an expression for the PQNNPDF applicable in this thermodynamic regime.Comment: 32 pages including abstract and title page

The classical Generalized Constant Coupling method for Geometrically Frustrated Magnets revisited: Microscopic formulation and effect of perturbations beyond nearest neighbor interactions

A microscopic derivation of the classical Generalized Constant Coupling (GCC) model for geometrically frustrated magnets is presented. Starting from the classical Heisenberg Hamiltonian, the partition function for clusters with p = 2, 3, 4 ,...spins in the presence of the inhomogeneous symmetry breaking fields (SBF) created by spins outside the unit is calculated. The effective fields characterizing the interaction between units naturally arise as averages over the SBF. These effective fields are fixed by a self-consistency condition. In the paramagnetic regime, we recover all the results previously obtained in a more phenomenological way, which were shown to be in excellent agreement with Monte Carlo calculations for these lattices. In the absence of applied magnetic field, it is found that, for antiferromagnetic interactions, the equilibrium configuration is a non-collinear configuration in which the total magnetization of the unit is zero, and the condition under which such an ordered state occurs is also obtained from the calculation. However, frustration inhibits the formation of such an state, and the system remains paramagnetic down to 0 K, if only nearest neighbor interactions are taken into account, for all the systems considered. Furthermore, we also study the effect of next nearest neighbor interactions (NNN) and site dilution by non-magnetic impurities for the pyrochlore lattice. It is found that NNN interactions can stabilize a non-collinear ordered state, or ferromagnetic one, depending on the relation between NN and NNN interactions, and the phase diagram is calculated. However, site dilution is not enough by itself to form such an ordered state.Comment: 26 pages, 9 figures, RevTeX

Accuracy of the coherent potential approximation for a one-dimensional array with a Gaussian distribution of fluctuations in the on-site potential

We investigate the accuracy of the coherent potential approximation (CPA) for a one-dimensional array with nearest-neighbor interactions and a Gaussian distribution of fluctuations in the on-site potential. The CPA values of the integrated density of states and the inverse localization length are compared with the results of mode-counting studies carried out on arrays of 107 - 108 sites. Good agreement is obtained suggesting that the CPA may be exact for this model. We also consider the asymptotic behavior of the inverse localization length and show that it can be approximated by the reciprocal of the decay length of a state localized about a single, strongly perturbed site in an otherwise perfect lattice.Comment: 14 pages, 5 figures

Classification of Mixed State Topology in One Dimension

We show how to generalize the concepts of identifying and classifying symmetry protected topological phases in 1D to the case of an arbitrary mixed state. The pure state concepts are reviewed using a concrete spin-1 model. For the mixed state setup we demonstrate our findings numerically using matrix product state algorithms. Starting from the ground state and applying various types of noise sources we find a transient regime where the system is driven out of equilibrium while retaining its topological properties.Comment: 4 pages + appendi