The charge excitation gap for polyacetylene (CH), was calculated using the Lanczos algorithm applied to small chains. The amal value of 4.5 i 0.4eV for lhis gap was obtained using the extended Hubbard model with the parameters fixed from ab inirio calculations (reprled elsewhere). For the second set of model Hamiltonian parameters recommended by many experimenla!isls (those measuring optical gaps or on-site spin densities in plyacetylene) the mmpnding value for the charge transfer gap is 1.6 5 0.8eV. which h reasonably close to what is considered the experimentd value. Our results confirm the earlier finding that the eleclron colrelations favour dimerization in plyacetylene.
Two component Bose-Einstein condensates are considered within a number conserving version of the Bogoliubov theory. We show that the Bogoliubov vacuum state can be obtained in the particle representation in a simple form. We predict considerable density fluctuations in finite systems close to the phase separation regime. We analyze homogeneous condensates and condensates in a double well potential.
We consider a homogeneous Bose gas of particles with an attractive interaction. Mean field theory predicts for this system a spontaneous symmetry breaking at a certain value of the interaction strength. We show that at this point a second-order quantum phase transition occurs. We investigate the system in the vicinity of the critical point using Bogoliubov theory and a continuous description, that allows us to analyze quantum fluctuations in the system even when the Bogoliubov approach breaks down.
We identify regular structures in the globally chaotic spectra of an interacting bosonic quantum gas in tilted periodic potentials. The associated eigenstates exhibit strong localization properties on the lattice, and are dynamically robust against external perturbations. arXiv:1012.4167v2 [cond-mat.quant-gas]
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