328 research outputs found
Comparison of non-crossing perturbative approach and generalized projection method for strongly coupled spin-fermion systems at low doping
We analyze the two-dimensional spin-fermion model in the strong coupling
regime relevant to underdoped cuprates. We recall the set of general sumrules
that relate moments of spectral density and the imaginary part of fermion
self-energy with static correlation functions. We show that two-pole
approximation of projection method satisfies the sumrules for first four
moments of spectral density and gives an exact upper bound for quasiparticle
energy near the band bottom. We prove that non-crossing approximation that is
often made in perturbative consideration of the model violates the sumrule for
third moment of spectral density. This leads to wrong position of lowest
quasiparticle band. On the other hand, the projection method is inadequate in
weak coupling limit because of approximate treatment of kinetic energy term. We
propose a generalization of projection method that overcomes this default and
give the fermion self-energy that correctly behaves both in weak and strong
coupling limits.Comment: 9 pages, 4 EPS figures, RevTe
Many-Body Approch to Spin-Dependent Transport in Quantum Dot Systems
By means of a diagram technique for Hubbard operators we show the existence
of a spin-dependent renormalization of the localized levels in an interacting
region, e.g. quantum dot, modeled by the Anderson Hamiltonian with two
conduction bands. It is shown that the renormalization of the levels with a
given spin direction is due to kinematic interactions with the conduction
sub-bands of the opposite spin. The consequence of this dressing of the
localized levels is a drastically decreased tunneling current for
ferromagnetically ordered leads compared to that of paramagnetically ordered
leads. Furthermore, the studied system shows a spin-dependent resonant
tunneling behaviour for ferromagnetically ordered leads.Comment: 8 pages, 5 figure
The J_1-J_2 antiferromagnet with Dzyaloshinskii-Moriya interaction on the square lattice: An exact diagonalization study
We examine the influence of an anisotropic interaction term of
Dzyaloshinskii-Moriya (DM) type on the groundstate ordering of the J_1-J_2
spin-1/2-Heisenberg antiferromagnet on the square lattice. For the DM term we
consider several symmetries corresponding to different crystal structures. For
the pure J_1-J_2 model there are strong indications for a quantum spin liquid
in the region of 0.4 < J_2/J_1 < 0.65. We find that a DM interaction influences
the breakdown of the conventional antiferromagnetic order by i) shifting the
spin liquid region, ii) changing the isotropic character of the groundstate
towards anisotropic correlations and iii) creating for certain symmetries a net
ferromagnetic moment.Comment: 7 pages, RevTeX, 6 ps-figures, to appear in J. Phys.: Cond. Ma
Performances and stability of a 2.4 ton Gd organic liquid scintillator target for antineutrino detection
In this work we report the performances and the chemical and physical
properties of a (2 x 1.2) ton organic liquid scintillator target doped with Gd
up to ~0.1%, and the results of a 2 year long stability survey. In particular
we have monitored the amount of both Gd and primary fluor actually in solution,
the optical and fluorescent properties of the Gd-doped liquid scintillator
(GdLS) and its performances as a neutron detector, namely neutron capture
efficiency and average capture time. The experimental survey is ongoing, the
target being continuously monitored. After two years from the doping time the
performances of the Gd-doped liquid scintillator do not show any hint of
degradation and instability; this conclusion comes both from the laboratory
measurements and from the "in-tank" measurements. This is the largest stable
Gd-doped organic liquid scintillator target ever produced and continuously
operated for a long period
Hole motion in the Ising antiferromagnet: an application of the recursion method
We study hole motion in the Ising antiferromagnet using the recursion method.
Using the retraceable path approximation we find the hole's Green's function as
well as its wavefunction for arbitrary values of . The effect of small
transverse interaction also is taken into account. Our results provide some
additional insight into the self-consistent Born approximation.Comment: 8 pages, RevTex, no figures. Accepted for publication in Phys.Rev.
Optical-Model Description of Time-Reversal Violation
A time-reversal-violating spin-correlation coefficient in the total cross
section for polarized neutrons incident on a tensor rank-2 polarized target is
calculated by assuming a time-reversal-noninvariant, parity-conserving
``five-fold" interaction in the neutron-nucleus optical potential. Results are
presented for the system for neutron incident energies
covering the range 1--20 MeV. From existing experimental bounds, a strength of
keV is deduced for the real and imaginary parts of the five-fold
term, which implies an upper bound of order on the relative -odd
strength when compared to the central real optical potential.Comment: 11 pages (Revtex
Quantum Phase Transition in the Frustrated Heisenberg Antiferromagnet
Using the J_1-J_2 model, we present a description of quantum phase transition
from Neel ordered to the spin-liquid state based on the modified spin wave
theory. The general expression for the gap in the spectrum in the spin-liquid
phase is presented.Comment: 8 pages of REVTeX 3.0, one PostScript file appended (Eq. 15
corrected, two recent references added, + some minor changes
Renormalization of the weak hadronic current in the nuclear medium
The renormalization of the weak charge-changing hadronic current as a
function of the reaction energy release is studied at the nucleonic level. We
have calculated the average quenching factors for each type of current (vector,
axial vector and induced pseudoscalar). The obtained quenching in the axial
vector part is, at zero momentum transfer, 19% for the sd shell and 23% in the
fp shell. We have extended the calculations also to heavier systems such as
Ni and Sn, where we obtain stronger quenchings, 44% and 59%,
respectively. Gamow--Teller type transitions are discussed, along with the
higher order matrix elements. The quenching factors are constant up to roughly
60 MeV momentum transfer. Therefore the use of energy-independent quenching
factors in beta decay is justified. We also found that going beyond the zeroth
and first order operators (in inverse nucleon mass) does not give any
substantial contribution. The extracted renormalization to the ratio
at q=100 MeV is -3.5%, -7.1$%, -28.6%, and +8.7% for mass 16, 40, 56, and 100,
respectively.Comment: 28 pages, 6 figure
Two-hole problem in the t-J model: A canonical transformation approach
The t-J model in the spinless-fermion representation is studied. An effective
Hamiltonian for the quasiparticles is derived using canonical transformation
approach. It is shown that the rather simple form of the transformation
generator allows to take into account effect of hole interaction with the
short-range spin waves and to describe the single-hole groundstate. Obtained
results are very close to ones of the self-consistent Born approximation.
Further accounting for the long-range spin-wave interaction is possible on the
perturbative basis. Both spin-wave exchange and an effective interaction due to
minimization of the number of broken antiferromagnetic bonds are included in
the effective quasiparticle interaction. Two-hole bound state problem is solved
using Bethe-Salpeter equation. The only d-wave bound state is found to exist in
the region of 1< (t/J) <5. Combined effect of the pairing interactions of both
types is important to its formation. Discussion of the possible relation of the
obtained results to the problem of superconductivity in real systems is
presented.Comment: 19 pages, RevTeX, 12 postscript figure
Measurement of the solar neutrino capture rate with gallium metal
The solar neutrino capture rate measured by the Russian-American Gallium
Experiment (SAGE) on metallic gallium during the period January 1990 through
December 1997 is 67.2 (+7.2-7.0) (+3.5-3.0) SNU, where the uncertainties are
statistical and systematic, respectively. This represents only about half of
the predicted Standard Solar Model rate of 129 SNU. All the experimental
procedures, including extraction of germanium from gallium, counting of 71Ge,
and data analysis are discussed in detail.Comment: 34 pages including 14 figures, Revtex, slightly shortene
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