11 research outputs found
Josephson lattice model for phase fluctuations of local pairs in copper-oxide superconductors
We derive an expression for the effective Josephson coupling from the
microscopic Hubbard model. It serves as a starting point for the description of
phase fluctuations of local Cooper pairs in -wave superconductors
in the framework of an effective model of plaquettes, the Josephson
lattice. The expression for the effective interaction is derived by means of
the local-force theorem, and it depends on local symmetry-broken correlation
functions that we obtain using the cluster dynamical mean-field theory.
Moreover, we apply the continuum limit to the Josephson lattice to obtain an
expression for the gradient term in the Ginzburg-Landau theory and compare
predicted London penetration depths and Kosterlitz-Thouless transition
temperatures with experimental data for YBaCuO.Comment: 13 pages, 13 figure
Electronic correlations and competing orders in multiorbital dimers: a cluster DMFT study
We investigate the violation of the first Hund's rule in 4 and 5
transition metal oxides that form solids of dimers. Bonding states within these
dimers reduce the magnetization of such materials. We parametrize the dimer
formation with realistic hopping parameters and find not only regimes, where
the system behaves as a Fermi liquid or as a Peierls insulator, but also
strongly correlated regions due to Hund's coupling and its competition with the
dimer formation. The electronic structure is investigated using the cluster
dynamical mean-field theory for a dimer in the two-plane Bethe lattice with two
orbitals per site and -filling, that is three electrons per dimer. It
reveals dimer-antiferromagnetic order of a high-spin (double exchange) state
and a low-spin (molecular orbital) state. At the crossover region we observe
the suppression of long-range magnetic order, fluctuation enhancement and
renormalization of electron masses. At certain interaction strengths the system
becomes an incoherent antiferromagnetic metal with well defined local moments.Comment: 11 pages, 10 figure
Exactly solvable model of strongly correlated d-wave superconductivity
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Detecting quantum critical points in the t-t ' Fermi-Hubbard model via complex network theory
A considerable success in phenomenological description of high-Tc superconductors has been achieved within the paradigm of Quantum Critical Point (QCP)-a parental state of a variety of exotic phases that is characterized by dense entanglement and absence of well-defined quasiparticles. However, the microscopic origin of the critical regime in real materials remains an open question. On the other hand, there is a popular view that a single-band t-t ' Hubbard model is the minimal model to catch the main relevant physics of superconducting compounds. Here, we suggest that emergence of the QCP is tightly connected with entanglement in real space and identify its location on the phase diagram of the hole-doped t-t ' Hubbard model. To detect the QCP we study a weighted graph of inter-site quantum mutual information within a four-by-four plaquette that is solved by exact diagonalization. We demonstrate that some quantitative characteristics of such a graph, viewed as a complex network, exhibit peculiar behavior around a certain submanifold in the parametric space of the model. This method allows us to overcome difficulties caused by finite size effects and to identify precursors of the transition point even on a small lattice, where long-range asymptotics of correlation functions cannot be accessed
Environmental changes during the Paleocene–Eocene Thermal Maximum in Spitsbergen as reflected by benthic foraminifera
The study deals with environmental changes during the Paleocene–Eocene Thermal Maximum (PETM) and its background conditions in Spitsbergen through analysis of benthic foraminiferal assemblages (FA) in a section drilled in the Paleogene Central Basin. The impact of this extreme global warming occurs here in prodelta shelf mudstones composing the lower part of the Gilsonryggen Member (Frysjaodden Formation). The start of the PETM perturbation is marked by a faunal turnover, in which the medium-diversity circumpolar Reticulophragmium assemblage was replaced by a low-diversity Trochammina fauna. During the hyperthermal period, benthic foraminiferal diversity decreased severely, while the dominance of small-sized taxa with epifaunal morphology strongly increased. This low-diversity fauna occurs in sediments with a reduced thorium/uranium ratio (proxy for oxygenation) and kaolinite enrichment (proxy for high humidity). The faunal changes were thus caused by the combined effects of hypoxic and hyposaline conditions in a stratified water column, due to extreme warming with its accompanying intensified hydrologic cycle. The PETM acme coincides with the maximum flooding surface (MFS) of the Gilsonryggen depositional sequence, composed of the Gilsonryggen Member and the overlying Battfjellet and Aspelintoppen formations. The transgressive phase of the sequence was initiated by local tectonics, while the eustatic sea-level rise of the PETM was superimposed on this transgression.To access the supplementary material for this article, please see supplementary files under Article Tools online