55 research outputs found
Hubbard Hamiltonian in the dimer representation. Large U limit
We formulate the Hubbard model for the simple cubic lattice in the
representation of interacting dimers applying the exact solution of the dimer
problem. By eliminating from the considerations unoccupied dimer energy levels
in the large U limit (it is the only assumption) we analytically derive the
Hubbard Hamiltonian for the dimer (analogous to the well-known t-J model), as
well as, the Hubbard Hamiltonian for the crystal as a whole by means of the
projection technique. Using this approach we can better visualize the
complexity of the model, so deeply hidden in its original form. The resulting
Hamiltonian is a mixture of many multiple ferromagnetic, antiferromagnetic and
more exotic interactions competing one with another. The interplay between
different competitive interactions has a decisive influence on the resulting
thermodynamic properties of the model, depending on temperature, model
parameters and assumed average number of electrons per lattice site. A
simplified form of the derived Hamiltonian can be obtained using additionally
Taylor expansion with respect to (t-hopping integral between
nearest neighbours, U-Coulomb repulsion). As an example, we present the
expansion including all terms proportional to t and to and we
reproduce the exact form of the Hubbard Hamiltonian in the limit . The nonperturbative approach, presented in this paper, can, in principle, be
applied to clusters of any size, as well as, to another types of model
Hamiltonians.Comment: 26 pages, 1 figure, LaTeX; added reference
The dynamics of the non-heme iron in bacterial reaction centers from Rhodobacter sphaeroides
AbstractWe investigate the dynamical properties of the non-heme iron (NHFe) in His-tagged photosynthetic bacterial reaction centers (RCs) isolated from Rhodobacter (Rb.) sphaeroides. Mössbauer spectroscopy and nuclear inelastic scattering of synchrotron radiation (NIS) were applied to monitor the arrangement and flexibility of the NHFe binding site. In His-tagged RCs, NHFe was stabilized only in a high spin ferrous state. Its hyperfine parameters (IS=1.06±0.01mm/s and QS=2.12±0.01mm/s), and Debye temperature (θD0~167K) are comparable to those detected for the high spin state of NHFe in non-His-tagged RCs. For the first time, pure vibrational modes characteristic of NHFe in a high spin ferrous state are revealed. The vibrational density of states (DOS) shows some maxima between 22 and 33meV, 33 and 42meV, and 53 and 60meV and a very sharp one at 44.5meV. In addition, we observe a large contribution of vibrational modes at low energies. This iron atom is directly connected to the protein matrix via all its ligands, and it is therefore extremely sensitive to the collective motions of the RC protein core. A comparison of the DOS spectra of His-tagged and non-His-tagged RCs from Rb. sphaeroides shows that in the latter case the spectrum was overlapped by the vibrations of the heme iron of residual cytochrome c2, and a low spin state of NHFe in addition to its high spin one. This enabled us to pin-point vibrations characteristic for the low spin state of NHFe
Cathodoluminescence characterization of Ge-doped CdTe crystals
Cathodoluminescence (CL) microscopic techniques have been used to study the spatial distribution of structural defects and the deep levels in CdTe:Ge bulk crystals. The effect of Ge doping with concentrations of 10(17) and 10(19) cm(-3) on the compensation of V-Cd in CdTe has been investigated. Dependence of the intensity distribution of CL emission bands on the dopant concentration has been studied. Ge doping causes a substantial reduction of the generally referred to 1.40 eV luminescence, which is often present in undoped CdTe crystals, and enhances the 0.91 and 0.81 eV emissions
Chemical proprieties of the iron-quinone complex in mutated reaction centers of Rb. sphaeroides
Differential effects of age-structured common carp (Cyprinus carpio) stocks on pond invertebrate communities: implications for recreational and wildlife use of farm ponds
Nanostructure, structural stability, and diffusion characteristics of layered coatings for heat-assisted magnetic recording head media.
Differentiation and integration by using matrix inversion
In the paper certain examples of applications of the matrix inverses for generating and calculating the integrals are presented
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