Interconnected Renormalization of Hubbard Bands and Green's Function Zeros in Mott Insulators Induced by Strong Magnetic Fluctuations

We analyze the role of spatial electronic correlations and, in particular, of the magnetic fluctuations in Mott insulators. A half-filled Hubbard model is solved at large strength of the repulsion U on a two-dimensional square lattice using an advanced diagrammatic non-perturbative approach capable of going beyond Hartree-Fock and single-site dynamical mean-field theories. We show that at high temperatures the magnetic fluctuations are weak, and the electronic self-energy of the system is mainly local and is well reproduced by the atomic (Hubbard-I) approximation. Lowering the temperature toward the low-temperature magnetically ordered phase, the non-locality of the self-energy becomes crucial in determining the momentum-dispersion of the Hubbard bands and the Green's function zeros. We therefore establish a precise link between Luttinger surface, non-local correlations and spectral properties of the Hubbard bands

Fluctuating local field approach to free energy of 1D molecules with strong collective electronic correlations

The impact of leading collective electronic fluctuations on a free energy of a prototype 1D model for molecular systems is considered within the recently developed Fluctuating Local Field (FLF) approach. The FLF method is a non-perturbative extension of a mean-field theory, where a self-consistent effective constant field is replaced by a fluctuating one. Integrating the fluctuating field out numerically exactly allows to account for collective electronic fluctuations mediated by this field without any assumptions on their magnitude, degree of nonlinearity, etc. Using a half-filled Hubbard ring as a benchmark system, we find that the FLF method noticeably improves a mean-field estimation for the free energy, in particular below the mean-field Ne\'el temperature. We further demonstrate that the mean-field result can be even more improved introducing a multi-mode FLF scheme that additionally takes into account sub-leading fluctuations. Possible applications for the thermodynamics of real molecules are also discussed.Comment: 14 pages, 2 figure

Affective Behaviour Analysis of On-line User Interactions: Are On-line Support Groups more Therapeutic than Twitter?

The increase in the prevalence of mental health problems has coincided with a growing popularity of health related social networking sites. Regardless of their therapeutic potential, On-line Support Groups (OSGs) can also have negative effects on patients. In this work we propose a novel methodology to automatically verify the presence of therapeutic factors in social networking websites by using Natural Language Processing (NLP) techniques. The methodology is evaluated on On-line asynchronous multi-party conversations collected from an OSG and Twitter. The results of the analysis indicate that therapeutic factors occur more frequently in OSG conversations than in Twitter conversations. Moreover, the analysis of OSG conversations reveals that the users of that platform are supportive, and interactions are likely to lead to the improvement of their emotional state. We believe that our method provides a stepping stone towards automatic analysis of emotional states of users of online platforms. Possible applications of the method include provision of guidelines that highlight potential implications of using such platforms on users' mental health, and/or support in the analysis of their impact on specific individuals

Optical absorption of BaF[2] crystals with different prehistory when irradiated by high-energy electrons

The spectra of stable optical absorption of BaF[2] crystals containing uncontrollable impurities after irradiation with 3 MeV electrons are studied at room temperature. The dependence of the efficiency of stable color accumulation in the region of emerging cross-luminescence on the absorption coefficients measured near the fundamental absorption edge in unirradiated crystals of various prehistory is traced