12,641 research outputs found
From d- to p-wave pairing in the t-t' Hubbard model at zero temperature
We develop a DCA(PQMC) algorithm which employs the projective quantum Monte
Carlo (PQMC) method for solving the equations of the dynamical cluster
approximation (DCA) at zero temperature, and apply it for studying pair
susceptibilities of the two-dimensional Hubbard-model with next-nearest
neighbor hopping. In particular, we identify which pairing symmetry is dominant
in the U-n parameter space (U: repulsive Coulomb interaction; n: electron
density). We find that p_{x+y}- (d_{x^2-y^2}-) wave is dominant among triplet
(singlet) pairings -at least for 0.3<n<0.8 and U<=4t. The crossover between
d_{x^2-y^2}-wave and p_{x+y}-wave occurs around n~0.4.Comment: 5 pages 5 figures; two additional panels in Fig. 2; as to appear in
Phys. Rev.
An interactive graphics system to facilitate finite element structural analysis
The characteristics of an interactive graphics systems to facilitate the finite element method of structural analysis are described. The finite element model analysis consists of three phases: (1) preprocessing (model generation), (2) problem solution, and (3) postprocessing (interpretation of results). The advantages of interactive graphics to finite element structural analysis are defined
Thermodynamic and spectral properties of compressed Ce calculated by the merger of the local density approximation and dynamical mean field theory
We have calculated thermodynamic and spectral properties of Ce metal over a
wide range of volume and temperature, including the effects of 4f electron
correlations, by the merger of the local density approximation and dynamical
mean field theory (DMFT). The DMFT equations are solved using the quantum Monte
Carlo technique supplemented by the more approximate Hubbard I and Hartree Fock
methods. At large volume we find Hubbard split spectra, the associated local
moment, and an entropy consistent with degeneracy in the moment direction. On
compression through the volume range of the observed gamma-alpha transition, an
Abrikosov-Suhl resonance begins to grow rapidly in the 4f spectra at the Fermi
level, a corresponding peak develops in the specific heat, and the entropy
drops rapidly in the presence of a persistent, although somewhat reduced local
moment. Our parameter-free spectra agree well with experiment at the alpha- and
gamma-Ce volumes, and a region of negative curvature in the correlation energy
leads to a shallowness in the low-temperature total energy over this volume
range which is consistent with the gamma-alpha transition. As measured by the
double occupancy, we find a noticeable decrease in correlation on compression
across the transition; however, even at the smallest volumes considered, Ce
remains strongly correlated with residual Hubbard bands to either side of a
dominant Fermi-level structure. These characteristics are discussed in light of
current theories for the volume collapse transition in Ce.Comment: 19 pages including 14 eps figure
Dynamical Mean-Field Theory - from Quantum Impurity Physics to Lattice Problems
Since the first investigation of the Hubbard model in the limit of infinite
dimensions by Metzner and Vollhardt, dynamical mean-field theory (DMFT) has
become a very powerful tool for the investigation of lattice models of
correlated electrons. In DMFT the lattice model is mapped on an effective
quantum impurity model in a bath which has to be determined self-consistently.
This approach lead to a significant progress in our understanding of typical
correlation problems such as the Mott transition; furthermore, the combination
of DMFT with ab-initio methods now allows for a realistic treatment of
correlated materials. The focus of these lecture notes is on the relation
between quantum impurity physics and the physics of lattice models within DMFT.
Issues such as the observability of impurity quantum phase transitions in the
corresponding lattice models are discussed in detail.Comment: 18 pages, 5 figures, invited paper for the Proceedings of the "3rd
International Summer School on Strongly Correlated Systems, Debrecen, 2004
Orbital-selective Mott-Hubbard transition in the two-band Hubbard model
Recent advances in the field of quantum Monte Carlo simulations for impurity
problems allow --within dynamical mean field theory-- for a more thorough
investigation of the two-band Hubbard model with narrow/wide band and
SU(2)-symmetric Hund's exchange. The nature of this transition has been
controversial, and we establish that an orbital-selective Mott-Hubbard
transition exists. Thereby, the wide band still shows metallic behavior after
the narrow band became insulating -not a pseudogap as for an Ising Hund's
exchange. The coexistence of two solutions with metallic wide band and
insulating or metallic narrow band indicates, in general, first-order
transitions.Comment: 4 pages, 3 figures; 2nd version as published in Phys. Rev. B (R);
minor corrections, putting more emphasis on differences in spectra when
comparing SU(2) and Ising Hund's exchang
Cryptic mitochondrial lineages in the widespread pycnogonid Colossendeis megalonyx Hoek, 1881 from Antarctic and Subantarctic waters
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