1,025 research outputs found
Effective action for the Kondo lattice model. New approach for S=1/2
In the partition function of the Kondo lattice, spin matrices are exactly
replaced by bilinear combinations of Fermi operators with the purely imaginary
chemical potential lambda=-i.pi.T/2 (Popov representation). This new
representation of spin operators allows one to introduce new Green's functions
with Matsubara frequencies 2.pi.T(n+1/4) for S=1/2. A simple temperature
diagram technique is constructed with the path integral method. This technique
is standard and does not contain the complicated combinatoric rules
characteristic of most of the known variants of the diagram techniques for spin
systems. The effective action for the almost antiferromagnetic Kondo lattice is
derived.Comment: 7 pages, Proceedings of SCES98/Paris; one reference adde
Understanding the Heavy Fermion Phenomenology from Microscopic Model
We solve the 3D periodic Anderson model via two impurity DMFT. We obtain the
temperature v.s. hybridization phase diagram. In approaching the quantum
critical point (QCP) both the Neel and lattice Kondo temperatures decrease and
they do not cross at the lowest temperature we reached. While strong
ferromagnetic spin fluctuation on the Kondo side is observed, our result
indicates the critical static spin susceptibility is local in space at the QCP.
We observe in the crossover region logarithmic temperature dependence in the
specific heat coefficient and spin susceptibility
Enhancement of superconductive critical temperatures in almost empty or full bands in two dimensions: possible relevance to beta-HfNCl, C60 and MgB2
We examine possibility of enhancement of superconductive critical temperature
in two-dimensions. The weak coupling BCS theory is applied, especially when the
Fermi level is near the edges of the electronic bands. The attractive
interaction depends on due to screening. The density of states(DOS)
does not have a peak near the bottom of the band, but -dependent
contribution to DOS (electron density on the Fermi surface) has a diverging
peak at the bottom or top. These features lead to significant enhancement of
the critical temperatures. The results are qualitatively consistent with the
superconductive behaviors of HfNCl (\Tc \le 25K) and ZrNCl(\Tc \le 15K),
C with a field-effect transistor configuration (\Tc = 52K), and
MgB (\Tc \approx 40K) which have the unexpectedly high superconductive
critical transition temperatures.Comment: 5 pages,4 figure
Glass Model, Hubbard Model and High-Temperature Superconductivity
In this paper we revisit the glass model describing the macroscopic behavior
of the High-Temperature superconductors. We link the glass model at the
microscopic level to the striped phase phenomenon, recently discussed widely.
The size of the striped phase domains is consistent with earlier predictions of
the glass model when it was introduced for High-Temperature Superconductivity
in 1987. In an additional step we use the Hubbard model to describe the
microscopic mechanism for d-wave pairing within these finite size stripes. We
discuss the implications for superconducting correlations of Hubbard model,
which are much higher for stripes than for squares, for finite size scaling,
and for the new view of the glass model picture.Comment: 7 pages, 7 figures (included), LaTex using Revtex, accepted by Int.
J. Mod. Phys.
Nature of Chemisorption on Titanium Carbide and Nitride
Extensive density-functional calculations are performed to understand atomic
chemisorption on the TiC(111) and TiN(111) surfaces, in particular the
calculated pyramid-shaped trends in the adsorption energies for second- and
third-period adatoms. Our previously proposed concerted-coupling model for
chemisorption on TiC(111) is tested against new results for adsorption on
TiN(111) and found to apply on this surface as well, thus reflecting both
similarities and differences in electronic structure between the two compounds.Comment: 7 pages, 4 figures, conference proceeding presented at IWSP-2005
(Polanica Zdoj, Poland, 2005), submitted to Surf. Sci. (2005
Symplectic quaternion scheme for biophysical molecular dynamics
Massively parallel biophysical molecular dynamics simulations, coupled with efficient methods, promise to open biologically significant time scales for study. In order to promote efficient fine-grained parallel algorithms with low communication overhead, the fast degrees of freedom in these complex systems can be divided into sets of rigid bodies. Here, a novel Hamiltonian form of a minimal, nonsingular representation of rigid body rotations, the unit quaternion, is derived, and a corresponding reversible, symplectic integrator is presented. The novel technique performs very well on both model and biophysical problems in accord with a formal theoretical analysis given within, which gives an explicit condition for an integrator to possess a conserved quantity, an explicit expression for the conserved quantity of a symplectic integrator, the latter following and in accord with Calvo and Sanz-Sarna, Numerical Hamiltonian Problems (1994), and extension of the explicit expression to general systems with a flat phase space
Parallelization of the exact diagonalization of the t-t'-Hubbard model
We present a new parallel algorithm for the exact diagonalization of the
-Hubbard model with the Lanczos-method. By invoking a new scheme of
labeling the states we were able to obtain a speedup of up to four on 16 nodes
of an IBM SP2 for the calculation of the ground state energy and an almost
linear speedup for the calculation of the correlation functions. Using this
algorithm we performed an extensive study of the influence of the next-nearest
hopping parameter in the -Hubbard model on ground state energy and
the superconducting correlation functions for both attractive and repulsive
interaction.Comment: 18 Pages, 1 table, 8 figures, Latex uses revtex, submitted to Comp.
Phys. Com
Extended Dynamical Mean Field Theory Study of the Periodic Anderson Model
We investigate the competition of the Kondo and the RKKY interactions in
heavy fermion systems. We solve a periodic Anderson model using Extended
Dynamical Mean Field Theory (EDMFT) with QMC. We monitor simultaneously the
evolution of the electronic and magnetic properties. As the RKKY coupling
increases the heavy fermion quasiparticle unbinds and a local moment forms. At
a critical RKKY coupling there is an onset of magnetic order. Within EDMFT the
two transitions occur at different points and the disapparence of the magnetism
is not described by a local quantum critical point.Comment: 4 pages, 4 figure
Anomalous Pressure Dependence of Kadowaki-Woods ratio and Crystal Field Effects in Mixed-valence YbInCu4
The mixed-valence (MV) compound YbInCu4 was investigated by electrical
resistivity and ac specific heat at low temperatures and high pressures. At
atmospheric pressure, its Kadowaki-Woods (KW) ratio, A/\gamma ^2, is 16 times
smaller than the universal value R_{KW}(=1.0 x 10^-5 \mu \Omega cm mol^2 K^2
mJ^-2), but sharply increases to 16.5R_{KW} at 27 kbar. The pressure-induced
change in the KW ratio and deviation from R_{KW} are analyzed in terms of the
change in f-orbital degeneracy N and carrier density n. This analysis is
further supported by a dramatic change in residual resistivity \rho_0 near 25
kbar, where \rho_0 jumps by a factor of 7.Comment: 4pages, 3figure
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