2,574 research outputs found
On the evaluation of the specific heat and general off-diagonal n-point correlation functions within the loop algorithm
We present an efficient way to compute diagonal and off-diagonal n-point
correlation functions for quantum spin-systems within the loop algorithm. We
show that the general rules for the evaluation of these correlation functions
take an especially simple form within the framework of directed loops. These
rules state that contributing loops have to close coherently. As an application
we evaluate the specific heat for the case of spin chains and ladders.Comment: For publication EPJ
On the evaluation of matrix elements in partially projected wave functions
We generalize the Gutzwiller approximation scheme to the calculation of
nontrivial matrix elements between the ground state and excited states. In our
scheme, the normalization of the Gutzwiller wave function relative to a
partially projected wave function with a single non projected site (the
reservoir site) plays a key role. For the Gutzwiller projected Fermi sea, we
evaluate the relative normalization both analytically and by variational
Monte-Carlo (VMC). We also report VMC results for projected superconducting
states that show novel oscillations in the hole density near the reservoir
site
Spontaneous breaking of the Fermi surface symmetry in the t-J model: a numerical study
We present a variational Monte Carlo (VMC) study of spontaneous Fermi surface
symmetry breaking in the t-J model. We find that the variational energy of a
Gutzwiller projected Fermi sea is lowered by allowing for a finite asymmetry
between the x- and the y-directions. However, the best variational state
remains a pure superconducting state with d-wave symmetry, as long as the
underlying lattice is isotropic. Our VMC results are in good overall agreement
with slave boson mean field theory (SBMFT) and renormalized mean field theory
(RMFT), although apparent discrepancies do show up in the half-filled limit,
revealing some limitations of mean field theories. VMC and complementary RMFT
calculations also confirm the SBMFT predictions that many-body interactions can
enhance any anisotropy in the underlying crystal lattice. Thus, our results may
be of consequence for the description of strongly correlated superconductors
with an anisotropic lattice structure.Comment: 6 pages, 7 figures; final versio
Bosonic resonating valence bond wave function for doped Mott insulators
We propose a new class of ground states for doped Mott insulators in the
electron second-quantization representation. They are obtained from a bosonic
resonating valence bond (RVB) theory of the t-J model. At half filling, the
ground state describes spin correlations of the S=1/2 Heisenberg model very
accurately. Its spin degrees of freedom are characterized by RVB pairing of
spins, the size of which decreases continuously as holes are doped into the
system. Charge degrees of freedom emerge upon doping and are described by
twisted holes in the RVB background. We show that the twisted holes exhibit an
off diagonal long range order (ODLRO) in the pseudogap ground state, which has
a finite pairing amplitude, but is short of phase coherence. Unpaired spins in
such a pseudogap ground state behave as free vortices, preventing
superconducting phase coherence. The existence of nodal quasiparticles is also
ensured by such a hidden ODLRO in the ground state, which is
non-Fermi-liquid-like in the absence of superconducting phase coherence. Two
distinct types of spin excitations can also be constructed. The superconducting
instability of the pseudogap ground state is discussed and a d-wave
superconducting ground state is obtained. This class of pseudogap and
superconducting ground states unifies antiferromagnetism, pseudogap,
superconductivity, and Mott physics into a new state of matter.Comment: 28 pages, 5 figures, final version to appear in Phys. Rev.
Physical implications of INTEGRAL/SPI gamma-ray line measurements of the 2003 October 28 solar flare
The very powerful X-class solar flare of 2003 October 28 was detected with
the INTEGRAL spectrometer as an intense gamma-ray flash of about 15 minutes.
Despite the non-standard incidence of the solar gamma-rays, time-resolved
spectra including several nuclear gamma-ray lines were obtained. Such a
measurement with a high-energy-resolution instrument can provide valuable
information of the isotopic abundances of the ambient solar material, as well
as the composition, directionality and energy spectra of the accelerated
nuclei. First results on the measured gamma-ray line ratios and time history of
the neutron-capture line are presented.Comment: 4 pages, 4 figures (uses vietnam.sty). To appear in the proceedings
of the 5th Rencontres du Vietnam, "New Views on the Universe", Hanoi, Aug
5-11, 200
Properties of the energetic particle distributions during the October 28, 2003 solar flare from INTEGRAL/SPI observations
Analysis of spectra obtained with the gamma-ray spectrometer SPI onboard
INTEGRAL of the GOES X17-class flare on October 28, 2003 is presented. In the
energy range 600 keV - 8 MeV three prominent narrow lines at 2.223, 4.4 and 6.1
MeV, resulting from nuclear interactions of accelerated ions within the solar
atmosphere could be observed. Time profiles of the three lines and the
underlying continuum indicate distinct phases with several emission peaks and
varying continuum-to-line ratio for several minutes before a smoother decay
phase sets in. Due to the high-resolution Ge detectors of SPI and the
exceptional intensity of the flare, detailed studies of the 4.4 and 6.1 MeV
line shapes was possible for the first time. Comparison with calculated line
shapes using a thick target interaction model and several energetic particle
angular distributions indicates that the nuclear interactions were induced by
downward-directed particle beams with alpha-to-proton ratios of the order of
0.1. There are also indications that the 4.4 MeV to 6.1 MeV line fluence ratio
changed between the beginning and the decay phase of the flare, possibly due to
a temporal evolution of the energetic particle alpha-to-proton ratio.Comment: 24 pages, 10 figures, accepted for publication by A&
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