838 research outputs found
Four particle exchange in solid He-3
Calculations which demonstrate a physically important four-atom exchange process in bodycentered cubic He crystal and thus an important four-spin term in the exchange Hamiltonian are discussed. A simple, mean-field analysis of this Hamiltonian appears to account for a number of the perplexing properties of bodycentered cubic He crystal. It is suggested that an understanding of other properties may require treatment of the exact four-spin term
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
The electronic Hamiltonian for cuprates
A realistic many-body Hamiltonian for the cuprate superconductors should include both copper d and oxygen p states, hopping matrix elements between them, and Coulomb energies, both on-site and inter-site. We have developed a novel computational scheme for deriving the relevant parameters ab initio from a constrained occupation local density functional. The scheme includes numerical calculation of appropriate Wannier functions for the copper and oxygen states. Explicit parameter values are given for La2CuO4. These parameters are generally consistent with other estimates and with the observed superexchange energy. Secondly, we address whether this complicated multi-band Hamiltonian can be reduced to a simpler one with fewer basis states per unit cell. We propose a mapping onto a new two-band effective Hamiltonian with one copper d and one oxygen p derived state per unit cell. This mapping takes into account the large oxygen-oxygen hopping given by the ab initio calculations
Periodic Anderson model with electron-phonon correlated conduction band
This paper reports dynamical mean field calculations for the periodic
Anderson model in which the conduction band is coupled to phonons. Motivated in
part by recent attention to the role of phonons in the -
transition in Ce, this model yields a rich and unexpected phase diagram which
is of intrinsic interest. Specifically, above a critical value of the
electron-phonon interaction, a first order transition with two coexisting
phases develops in the temperature-hybridization plane, which terminates at a
second order critical point. The coexisting phases display the familiar Kondo
screened and local moment character, yet they also exhibit pronounced polaronic
and bipolaronic properties, respectively.Comment: 4 pages, 6 figure
The agrin gene codes for a family of basal lamina proteins that differ in function and distribution
We isolated two cDNAs that encode isoforms of agrin, the basal lamina protein that mediates the motor neuron-induced aggregation of acetylcholine receptors on muscle fibers at the neuromuscular junction. Both proteins are the result of alternative splicing of the product of the agrin gene, but, unlike agrin, they are inactive in standard acetylcholine receptor aggregation assays. They lack one (agrin-related protein 1) or two (agrin-related protein 2) regions in agrin that are required for its activity. Expression studies provide evidence that both proteins are present in the nervous system and muscle and that, in muscle, myofibers and Schwann cells synthesize the agrin-related proteins while the axon terminals of motor neurons are the sole source of agrin
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