168 research outputs found
Fermi Surface of The One-dimensional Kondo Lattice Model
We show a strong indication of the existence of a large Fermi surface in the
one-dimensional Kondo lattice model. The characteristic wave vector of the
model is found to be , being the density of the
conduction electrons. This result is at first obtained for a variant of the
model that includes an antiferromagnetic Heisenberg interaction between
the local moments. It is then directly observed in the conventional Kondo
lattice , in the narrow range of Kondo couplings where the long
distance properties of the model are numerically accessible.Comment: 11 pages, 6 figure
Evidence for the Coexistence of Anisotropic Superconducting Gap and Nonlocal Effects in the Non-magnetic Superconductor LuNi2B2C
A study of the dependence of the heat capacity Cp(alpha) on field angle in
LuNi2B2C reveals an anomalous disorder effect. For pure samples, Cp(alpha)
exhibits a fourfold variation as the field H < Hc2 is rotated in the [001]
plane, with minima along (alpha = 0). A slightly disordered sample,
however, develops anomalous secondary minima along for H > 1 T, leading
to an 8-fold pattern. The anomalous pattern is discussed in terms of coexisting
superconducting gap anisotropy and non-local effects.Comment: 5 pages, 4 figure
Towards a Tetravalent Chemistry of Colloids
We propose coating spherical particles or droplets with anisotropic
nano-sized objects to allow micron-scale colloids to link or functionalize with
a four-fold valence, similar to the sp3 hybridized chemical bonds associated
with, e.g., carbon, silicon and germanium. Candidates for such coatings include
triblock copolymers, gemini lipids, metallic or semiconducting nanorods and
conventional liquid crystal compounds. We estimate the size of the relevant
nematic Frank constants, discuss how to obtain other valences and analyze the
thermal distortions of ground state configurations of defects on the sphere.Comment: Replaced to improve figures. 4 figures Nano Letter
Theory of Cylindrical Tubules and Helical Ribbons of Chiral Lipid Membranes
We present a general theory for the equilibrium structure of cylindrical
tubules and helical ribbons of chiral lipid membranes. This theory is based on
a continuum elastic free energy that permits variations in the direction of
molecular tilt and in the curvature of the membrane. The theory shows that the
formation of tubules and helical ribbons is driven by the chirality of the
membrane. Tubules have a first-order transition from a uniform state to a
helically modulated state, with periodic stripes in the tilt direction and
ripples in the curvature. Helical ribbons can be stable structures, or they can
be unstable intermediate states in the formation of tubules.Comment: 43 pages, including 12 postscript figures, uses REVTeX 3.0 and
epsf.st
Phenomenological Models for the Gap Anisotropy of Bi-2212 as Measured by ARPES
Recently, high resolution angle-resolved photoemission spectroscopy has been
used to determine the detailed momentum dependence of the superconducting gap
in the high temperature superconductor Bi-2212. In this paper, we first
describe tight binding fits to the normal state dispersion and superlattice
modulation effects. We then discuss various theoretical models in light of the
gap measurements. We find that the simplest model which fits the data is the
anisotropic s-wave gap , which within a one-band BCS frame-
work suggests the importance of next near neighbor Cu-Cu interactions. Various
alternative interpretations of the observed gap are also discussed, along with
the implications for microscopic theories of high temperature superconductors.Comment: 14 pages, revtex, 9 uuencoded postscript figure
Computational Nuclear Physics and Post Hartree-Fock Methods
We present a computational approach to infinite nuclear matter employing
Hartree-Fock theory, many-body perturbation theory and coupled cluster theory.
These lectures are closely linked with those of chapters 9, 10 and 11 and serve
as input for the correlation functions employed in Monte Carlo calculations in
chapter 9, the in-medium similarity renormalization group theory of dense
fermionic systems of chapter 10 and the Green's function approach in chapter
11. We provide extensive code examples and benchmark calculations, allowing
thereby an eventual reader to start writing her/his own codes. We start with an
object-oriented serial code and end with discussions on strategies for porting
the code to present and planned high-performance computing facilities.Comment: 82 pages, to appear in Lecture Notes in Physics (Springer), "An
advanced course in computational nuclear physics: Bridging the scales from
quarks to neutron stars", M. Hjorth-Jensen, M. P. Lombardo, U. van Kolck,
Editor
Renormalization of Hamiltonian Field Theory; a non-perturbative and non-unitarity approach
Renormalization of Hamiltonian field theory is usually a rather painful
algebraic or numerical exercise. By combining a method based on the coupled
cluster method, analysed in detail by Suzuki and Okamoto, with a Wilsonian
approach to renormalization, we show that a powerful and elegant method exist
to solve such problems. The method is in principle non-perturbative, and is not
necessarily unitary.Comment: 16 pages, version shortened and improved, references added. To appear
in JHE
Comparison of techniques for computing shell-model effective operators
Different techniques for calculating effective operators within the framework
of the shell model using the same effective interaction and the same excitation
spaces are presented. Starting with the large-basis no-core approach, we
compare the time-honored perturbation-expansion approach and a model-space
truncation approach. Results for the electric quadrupole and magnetic dipole
operators are presented for Li. The convergence trends and dependence of
the effective operators on differing excitation spaces and Pauli Q-operators is
studied. In addition, the dependence of the electric-quadrupole effective
charge on the harmonic-oscillator frequency and the mass number, for A=5,6, is
investigated in the model-space truncation approach.Comment: 18 pages. REVTEX. 4 PostScript figure
Correlation effects in ionic crystals: I. The cohesive energy of MgO
High-level quantum-chemical calculations, using the coupled-cluster approach
and extended one-particle basis sets, have been performed for (Mg2+)n (O2-)m
clusters embedded in a Madelung potential. The results of these calculations
are used for setting up an incremental expansion for the correlation energy of
bulk MgO. This way, 96% of the experimental cohesive energy of the MgO crystal
is recovered. It is shown that only 60% of the correlation contribution to the
cohesive energy is of intra-ionic origin, the remaining part being caused by
van der Waals-like inter-ionic excitations.Comment: LaTeX, 20 pages, no figure
Linear Field Dependence of the Normal-State In-Plane Magnetoresistance of Sr2RuO4
The transverse and longitudinal in-plane magnetoresistances in the normal
state of superconducting Sr2RuO4 single crystals have been measured. At low
temperatures, both of them were found to be positive with a linear
magnetic-field dependence above a threshold field, a result not expected from
electronic band theory. We argue that such behavior is a manifestation of a
novel coherent state characterized by a spin pseudo gap in the quasi-particle
excitation spectrum in Sr2RuO4.Comment: 4 pages + 5 figure
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