215 research outputs found
Staggered flux and stripes in doped antiferromagnets
We have numerically investigated whether or not a mean-field theory of spin
textures generate fictitious flux in the doped two dimensional -model.
First we consider the properties of uniform systems and then we extend the
investigation to include models of striped phases where a fictitious flux is
generated in the domain wall providing a possible source for lowering the
kinetic energy of the holes. We have compared the energetics of uniform systems
with stripes directed along the (10)- and (11)-directions of the lattice,
finding that phase-separation generically turns out to be energetically
favorable. In addition to the numerical calculations, we present topological
arguments relating flux and staggered flux to geometric properties of the spin
texture. The calculation is based on a projection of the electron operators of
the model into a spin texture with spinless fermions.Comment: RevTex, 19 pages including 20 figure
A Monte Carlo study of O(3) antiferromagnetic models in three dimensions
We study three antiferromagnetic formulations of the O(3) spin model in three
dimensions by means of Monte Carlo simulations: 1. a two parameter
model with nearest and next to nearest neighbors couplings in a cubic lattice;
2. a face centered cubic lattice with nearest neighbors interaction; 3. a cubic
lattice with a set of fully frustrating couplings. We discuss in all cases the
vacua properties and analyze the phase transitions. Using Finite Size Scaling
analysis we conclude that all phase transitions found are of first order.Comment: 24 pages, uuencoded gzipped postscript file. 13 figures include
Ground state parameters, finite-size scaling, and low-temperature properties of the two-dimensional S=1/2 XY model
We present high-precision quantum Monte Carlo results for the S=1/2 XY model
on a two-dimensional square lattice, in the ground state as well as at finite
temperature. The energy, the spin stiffness, the magnetization, and the
susceptibility are calculated and extrapolated to the thermodynamic limit. For
the ground state, we test a variety of finite-size scaling predictions of
effective Lagrangian theory and find good agreement and consistency between the
finite-size corrections for different quantities. The low-temperature behavior
of the susceptibility and the internal energy is also in good agreement with
theoretical predictions.Comment: 6 pages, 8 figure
Finite-Size Scaling of the Ground State Parameters of the Two-Dimensional Heisenberg Model
The ground state parameters of the two-dimensional S=1/2 antiferromagnetic
Heisenberg model are calculated using the Stochastic Series Expansion quantum
Monte Carlo method for L*L lattices with L up to 16. The finite-size results
for the energy E, the sublattice magnetization M, the long-wavelength
susceptibility chi_perp(q=2*pi/L), and the spin stiffness rho_s, are
extrapolated to the thermodynamic limit using fits to polynomials in 1/L,
constrained by scaling forms previously obtained from renormalization group
calculations for the nonlinear sigma model and chiral perturbation theory. The
results are fully consistent with the predicted leading finite-size corrections
and are of sufficient accuracy for extracting also subleading terms. The
subleading energy correction (proportional to 1/L^4) agrees with chiral
perturbation theory to within a statistical error of a few percent, thus
providing the first numerical confirmation of the finite-size scaling forms to
this order. The extrapolated ground state energy per spin, E=-0.669437(5), is
the most accurate estimate reported to date. The most accurate Green's function
Monte Carlo (GFMC) result is slightly higher than this value, most likely due
to a small systematic error originating from ``population control'' bias in
GFMC. The other extrapolated parameters are M=0.3070(3), rho_s = 0.175(2),
chi_perp = 0.0625(9), and the spinwave velocity c=1.673(7). The statistical
errors are comparable with those of the best previous estimates, obtained by
fitting loop algorithm quantum Monte Carlo data to finite-temperature scaling
forms. Both M and rho_s obtained from the finite-T data are, however, a few
error bars higher than the present estimates. It is argued that the T=0
extrapolations performed here are less sensitive to effects of neglectedComment: 16 pages, RevTex, 9 PostScript figure
Theory of anyon excitons: Relation to excitons of nu=1/3 and nu=2/3 incompressible liquids
Elementary excitations of incompressible quantum liquids (IQL's) are anyons,
i.e., quasiparticles carrying fractional charges and obeying fractional
statistics. To find out how the properties of these quasiparticles manifest
themselves in the optical spectra, we have developed the anyon exciton model
(AEM) and compared the results with the finite-size data for excitons of nu=1/3
and nu=2/3 IQL's. The model considers an exciton as a neutral composite
consisting of three quasielectrons and a single hole. The AEM works well when
the separation between electron and hole confinement planes, h, is larger than
the magnetic length l. In the framework of the AEM an exciton possesses
momentum k and two internal quantum numbers, one of which can be chosen as the
angular momentum, L, of the k=0 state. Existence of the internal degrees of
freedom results in the multiple branch energy spectrum, crater-like electron
density shape and 120 degrees density correlations for k=0 excitons, and the
splitting of the electron shell into bunches for non-zero k excitons. For h
larger than 2l the bottom states obey the superselection rule L=3m (m are
integers starting from 2), all of them are hard core states. For h nearly 2l
there is one-to-one correspondence between the low-energy spectra found for the
AEM and the many- electron exciton spectra of the nu=2/3 IQL, whereas some
states are absent from the many-electron spectra of the nu=1/3 IQL. We argue
that this striking difference in the spectra originates from the different
populational statistics of the quasielectrons of charge conjugate IQL's and
show that the proper account of the statistical requirements eliminates
excessive states from the spectrum. Apparently, this phenomenon is the first
manifestation of the exclusion statistics in the anyon bound states.Comment: 26 pages with 9 figures, typos correcte
Tumor markers in breast cancer - European Group on Tumor Markers recommendations
Recommendations are presented for the routine clinical use of serum and tissue-based markers in the diagnosis and management of patients with breast cancer. Their low sensitivity and specificity preclude the use of serum markers such as the MUC-1 mucin glycoproteins ( CA 15.3, BR 27.29) and carcinoembryonic antigen in the diagnosis of early breast cancer. However, serial measurement of these markers can result in the early detection of recurrent disease as well as indicate the efficacy of therapy. Of the tissue-based markers, measurement of estrogen and progesterone receptors is mandatory in the selection of patients for treatment with hormone therapy, while HER-2 is essential in selecting patients with advanced breast cancer for treatment with Herceptin ( trastuzumab). Urokinase plasminogen activator and plasminogen activator inhibitor 1 are recently validated prognostic markers for lymph node-negative breast cancer patients and thus may be of value in selecting node-negative patients that do not require adjuvant chemotherapy. Copyright (C) 2005 S. Karger AG, Basel
Berry's phase and Quantum Dynamics of Ferromagnetic Solitons
We study spin parity effects and the quantum propagation of solitons (Bloch
walls) in quasi-one dimensional ferromagnets. Within a coherent state path
integral approach we derive a quantum field theory for nonuniform spin
configurations. The effective action for the soliton position is shown to
contain a gauge potential due to the Berry phase and a damping term caused by
the interaction between soliton and spin waves. For temperatures below the
anisotropy gap this dissipation reduces to a pure soliton mass renormalization.
The gauge potential strongly affects the quantum dynamics of the soliton in a
periodic lattice or pinning potential. For half-integer spin, destructive
interference between soliton states of opposite chirality suppresses nearest
neighbor hopping. Thus the Brillouin zone is halved, and for small mixing of
the chiralities the dispersion reveals a surprising dynamical correlation: Two
subsequent band minima belong to different chirality states of the soliton. For
integer spin, the Berry phase is inoperative and a simple tight-binding
dispersion is obtained. Finally it is shown that external fields can be used to
interpolate continuously between the Bloch wall dispersions for half-integer
and integer spin.Comment: 20 pages, RevTex 3.0 (twocolumn), to appear in Phys. Rev. B 53, 3237
(1996), 4 PS figures available upon reques
Pairing and Density Correlations of Stripe Electrons in a Two-Dimensional Antiferromagnet
We study a one-dimensional electron liquid embedded in a 2D antiferromagnetic
insulator, and coupled to it via a weak antiferromagnetic spin exchange
interaction. We argue that this model may qualitatively capture the physics of
a single charge stripe in the cuprates on length- and time scales shorter than
those set by its fluctuation dynamics. Using a local mean-field approach we
identify the low-energy effective theory that describes the electronic spin
sector of the stripe as that of a sine-Gordon model. We determine its phases
via a perturbative renormalization group analysis. For realistic values of the
model parameters we obtain a phase characterized by enhanced spin density and
composite charge density wave correlations, coexisting with subleading triplet
and composite singlet pairing correlations. This result is shown to be
independent of the spatial orientation of the stripe on the square lattice.
Slow transverse fluctuations of the stripes tend to suppress the density
correlations, thus promoting the pairing instabilities. The largest amplitudes
for the composite instabilities appear when the stripe forms an antiphase
domain wall in the antiferromagnet. For twisted spin alignments the amplitudes
decrease and leave room for a new type of composite pairing correlation,
breaking parity but preserving time reversal symmetry.Comment: Revtex, 28 pages incl. 5 figure
The spin-1/2 J1-J2 Heisenberg antiferromagnet on the square lattice: Exact diagonalization for N=40 spins
We present numerical exact results for the ground state and the low-lying
excitations for the spin-1/2 J1-J2 Heisenberg antiferromagnet on finite square
lattices of up to N=40 sites. Using finite-size extrapolation we determine the
ground-state energy, the magnetic order parameters, the spin gap, the uniform
susceptibility, as well as the spin-wave velocity and the spin stiffness as
functions of the frustration parameter J2/J1. In agreement with the generally
excepted scenario we find semiclassical magnetically ordered phases for J2 <
J2^{c1} and J2 > J2^{c2} separated by a gapful quantum paramagnetic phase. We
estimate J2^{c1} \approx 0.35J1 and J2^{c2} \approx 0.66J1.Comment: 16 pages, 2 tables, 11 figure
Bile Acid Sequestrants for Lipid and Glucose Control
Bile acids are generated in the liver and are traditionally recognized for their regulatory role in multiple metabolic processes including bile acid homeostasis, nutrient absorption, and cholesterol homeostasis. Recently, bile acids emerged as signaling molecules that, as ligands for the bile acid receptors farnesoid X receptor (FXR) and TGR5, activate and integrate multiple complex signaling pathways involved in lipid and glucose metabolism. Bile acid sequestrants are pharmacologic molecules that bind to bile acids in the intestine resulting in the interruption of bile acid homeostasis and, consequently, reduction in low-density lipoprotein cholesterol levels in hypercholesterolemia. Bile acid sequestrants also reduce glucose levels and improve glycemic control in persons with type 2 diabetes mellitus (T2DM). This article examines the mechanisms by which bile acid–mediated activation of FXR and TGR5 signaling pathways regulate lipid and glucose metabolism and the potential implications for bile acid sequestrant–mediated regulation of lipid and glucose levels in T2DM
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