66 research outputs found
Electronic Correlations within Fermionic Lattice Models
We investigate two-site electronic correlations within generalized Hubbard
model, which incorporates the conventional Hubbard model (parameters:
(hopping between nearest neighbours), (Coulomb repulsion (attraction))
supplemented by the intersite Coulomb interactions (parameters:
(parallel spins), (antiparellel spins)) and the hopping of
the intrasite Cooper pairs (parameter: ). As a first step we find the
eigenvalues and eigenvectors of the dimer and we
represent each partial Hamiltonian
() in the second quantization with the use of the Hubbard
and spin operators. Each dimer energy level possesses its own Hamiltonian
describing different two-site interactions which can be active only in the case
when the level will be occupied by the electrons. A typical feature is the
appearence of two generalized interactions ascribed to two different
energy levels which do not vanish even for and their
coupling constants are equal to in this case. The competition between
ferromagnetism, antiferromagnetism and superconductivity (intrasite and
intersite pairings) is also a typical feature of the model because it persists
in the case and . The same types of the
electronic, competitive interactions are scattered between different energy
levels and therefore their thermodynamical activities are dependent on the
occupation of these levels. It qualitatively explains the origin of the phase
diagram of the model. We consider also a real lattice as a set of interacting
dimers to show that the competition between magnetism and superconductivity
seems to be universal for fermonic lattice models.Comment: 12 page
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
Infekcyjne zapalenie wsierdzia zastawki aortalnej i płucnej u 20-letniego pacjenta z dwujamową prawą komorą i ubytkiem przegrody międzykomorowej
W niniejszej pracy przedstawiono przypadek 20-letniego pacjenta z dwujamową prawą komorą
i z ubytkiem przegrody międzykomorowej (VSD), powikłanym infekcyjnym zapaleniem
wsierdzia. U chorego zastosowano terapię przeciwbakteryjną i przeciwgrzybiczą, a następnie
zakwalifikowano go do leczenia operacyjnego. Korekcja obejmowała zamknięcie ubytku przegrody
międzykomorowej łatą goreteksową, resekcję drogi odpływu prawej komory oraz wszczepienie
homogaftu aortalnego i płucnego. W pracy przedstawiono problemy związane z diagnostyką,
farmakoterapią oraz leczeniem operacyjnym infekcyjnego zapalenia wsierdzia. (Folia
Cardiol. 2004; 11: 681-686
Infekcyjne zapalenie wsierdzia zastawki aortalnej i płucnej u 20-letniego pacjenta z dwujamową prawą komorą i ubytkiem przegrody międzykomorowej
W niniejszej pracy przedstawiono przypadek 20-letniego pacjenta z dwujamową prawą komorą
i z ubytkiem przegrody międzykomorowej (VSD), powikłanym infekcyjnym zapaleniem
wsierdzia. U chorego zastosowano terapię przeciwbakteryjną i przeciwgrzybiczą, a następnie
zakwalifikowano go do leczenia operacyjnego. Korekcja obejmowała zamknięcie ubytku przegrody
międzykomorowej łatą goreteksową, resekcję drogi odpływu prawej komory oraz wszczepienie
homogaftu aortalnego i płucnego. W pracy przedstawiono problemy związane z diagnostyką,
farmakoterapią oraz leczeniem operacyjnym infekcyjnego zapalenia wsierdzia. (Folia
Cardiol. 2004; 11: 681-686
Indirect Exchange in Magnetic Semiconductors. III. Effect of Alloying
We apply the extended s-f model to magnetic pseudobinary alloys. We calculate the Curie temperature and magnetic moment in the paramagnetic state as functions of concentration x. The calculated quantities are in good agreement with the quantities experimentally measured for EuGdS, EuGdSe, ReGdAl (Re = Dy, Ho, Er, La, Lu, Y), Gd(AlMe) (Me = Cu, Ag, Pd, In, Sn),..
Chemical Potential Evidence for Phase Transitions in Magnetic and Superconducting Compounds and Alloys
We announce that all phase transitions (induced by temperature or concentration) including structural ones and transitions between metastable or "exotic" states can be detected by the chemical potential critical behaviour, as well as, from the average occupation numbers of the electronic system (critical electron redistribution)
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