8,018 research outputs found
Superconductivity in striped and multi-Fermi-surface Hubbard models: From the cuprates to the pnictides
Single- and multi-band Hubbard models have been found to describe many of the
complex phenomena that are observed in the cuprate and iron-based
high-temperature superconductors. Simulations of these models therefore provide
an ideal framework to study and understand the superconducting properties of
these systems and the mechanisms responsible for them. Here we review recent
dynamic cluster quantum Monte Carlo simulations of these models, which provide
an unbiased view of the leading correlations in the system. In particular, we
discuss what these simulations tell us about superconductivity in the
homogeneous 2D single-orbital Hubbard model, and how charge stripes affect this
behavior. We then describe recent simulations of a bilayer Hubbard model, which
provides a simple model to study the type and nature of pairing in systems with
multiple Fermi surfaces such as the iron-based superconductors.Comment: Published as part of Superstripes 2011 (Rome) conference proceeding
Pseudogap and antiferromagnetic correlations in the Hubbard model
Using the dynamical cluster approximation and quantum monte carlo we
calculate the single-particle spectra of the Hubbard model with next-nearest
neighbor hopping . In the underdoped region, we find that the pseudogap
along the zone diagonal in the electron doped systems is due to long range
antiferromagnetic correlations. The physics in the proximity of is
dramatically influenced by and determined by the short range correlations.
The effect of on the low energy ARPES spectra is weak except close to the
zone edge. The short range correlations are sufficient to yield a pseudogap
signal in the magnetic susceptibility, produce a concomitant gap in the
single-particle spectra near but not necessarily at a location in
the proximity of Fermi surface.Comment: 5 pages, 4 figure
[3+2] Fragmentation of a Pentaphosphido Ligand by Cyanide
The activation of white phosphorus (P4) by transitionâmetal complexes has been studied for several decades, but the functionalization and release of the resulting (organo)phosphorus ligands has rarely been achieved. Herein we describe the formation of rare diphosphanâ1âide anions from a P5 ligand by treatment with cyanide. Cobalt diorganopentaphosphido complexes have been synthesized by a stepwise reaction sequence involving a lowâvalent diimine cobalt complex, white phosphorus, and diorganochlorophosphanes. The reactions of the complexes with tetraalkylammonium or potassium cyanide afford a cyclotriphosphido cobaltate anion 5 and 1âcyanodiphosphanâ1âide anions [R2PPCN]â (6âR). The molecular structure of a related product 7 suggests a novel reaction mechanism, where coordination of the cyanide anion to the cobalt center induces a ligand rearrangement. This is followed by nucleophilic attack of a second cyanide anion at a phosphorus atom and release of the P2 fragment
Îą-Diimine Ferrates and Cobaltates as Highly Reactive Complex Fragments in Synthesis and Catalysis
Diese Dissertation handelt von der Synthese niedervalenter a-Diimineisen- und Cobaltkomplexe, sowie deren Anwendung in der reduktiven Katalyse. Besondere Aufmerksamkeit wurde dabei auf (De)Hydrierungsreaktionen gelegt und der Untersuchung der zugrundeliegenden Reaktionsmechanismen
New Records for the Arctic Shrew, Sorex arcticus and the Newly Recognized Maritime Shrew, Sorex maritimensis
We report the first record for the Arctic Shrew (Sorex arcticus) in the state of Montana, USA. We also report range extensions for the closely related Maritime Shrew (Sorex maritimensis) in New Brunswick and Nova Scotia, Canada. These collections augment our limited knowledge of the ranges and habitat associations of these rarely collected shrews, and highlight the need for a careful assessment of the status of S. maritimensis in Canada
Pairing in the Two-Dimensional Hubbard Model from Weak to Strong Coupling
The Hubbard model is the simplest model that is believed to exhibit
superconductivity arising from purely repulsive interactions, and has been
extensively applied to explore a variety of unconventional superconducting
systems. Here we study the evolution of the leading superconducting
instabilities of the single-orbital Hubbard model on a two-dimensional square
lattice as a function of onsite Coulomb repulsion and band filling by
calculating the irreducible particle-particle scattering vertex obtained from
dynamical cluster approximation (DCA) calculations, and compare the results to
both perturbative Kohn-Luttinger (KL) theory as well as the widely used random
phase approximation (RPA) spin-fluctuation pairing scheme. Near half-filling we
find remarkable agreement of the hierarchy of the leading pairing states
between these three methods, implying adiabatic continuity between weak- and
strong-coupling pairing solutions of the Hubbard model. The -wave
instability is robust to increasing near half-filling as expected. Away
from half filling, the predictions of KL and RPA at small for transitions
to other pair states agree with DCA at intermediate as well as recent
diagrammatic Monte Carlo calculations. RPA results fail only in the very dilute
limit, where it yields a ground state instead of a -wave state
established by diagrammatic Monte Carlo and low-order perturbative methods, as
well as our DCA calculations. We discuss the origins of this discrepancy,
highlighting the crucial role of the vertex corrections neglected in the RPA
approach. Overall, comparison of the various methods over the entire phase
diagram strongly suggests a smooth crossover of the superconducting interaction
generated by local Hubbard interactions between weak and strong coupling.Comment: 9 pages, 5 figure
Long-term stable compressive elastocaloric cooling system with latent heat transfer
Elastocaloric cooling systems can evolve into an environmentally friendly alternative to compressor-based cooling systems. One of the main factors preventing its application is a poor long-term stability of the elastocaloric material. This especially applies to systems that work with tensile loads and which benefit from the large surface area for heat transfer. Exerting compressive instead of tensile loads on the material increases long-term stability-though at the expense of cooling power density. Here, we present a heat transfer concept for elastocaloric systems where heat is transferred by evaporation and condensation of a fluid. Enhanced heat transfer rates allow us to choose the sample geometry more freely and thereby realize a compression-based system showing unprecedented long-term stability of 10 cycles and cooling power density of 6270 W kg
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