309 research outputs found
Magnetic properties of the 2D t-t'-Hubbard model
The two-dimensional (2D) t-t'-Hubbard model is studied within the slave-boson
(SB) theory. At half-filling, a paramagnetic to antiferromagnetic phase
transition of first order at a finite critical interaction strength U_c(t'/t)
is found. The dependences on U/t and t'/t of the sublattice magnetization and
of the local magnetic moment are calculated. Our results reasonably agree with
recent (Projector) Quantum Monte Carlo data. The SB ground-state phase diagram
reveals a t'-induced electron-hole asymmetry, and, depending on the ratio t'/t,
the antiferromagnetic or ferromagnetic phases are stable down to U=0 at a
critical hole doping.Comment: 2 pages, 3 Postscript figure, submitted to Int. Conf. M2S-HTSC-V
Beijing 97, to appear in Physica
Non-spherical shapes of capsules within a fourth-order curvature model
We minimize a discrete version of the fourth-order curvature based Landau
free energy by extending Brakke's Surface Evolver. This model predicts
spherical as well as non-spherical shapes with dimples, bumps and ridges to be
the energy minimizers. Our results suggest that the buckling and faceting
transitions, usually associated with crystalline matter, can also be an
intrinsic property of non-crystalline membranes.Comment: 6 pages, 4 figures (LaTeX macros EPJ), accepted for publication in
EPJ
First-Order Vortex Lattice Melting and Magnetization of YBaCuO$_{7-\delta}
We present the first non-mean-field calculation of the magnetization
of YBaCuO both above and below the flux-lattice melting
temperature . The results are in good agreement with experiment as a
function of transverse applied field . The effects of fluctuations in both
order parameter and magnetic induction are included in the
Ginzburg-Landau free energy functional: fluctuates within the
lowest Landau level in each layer, while fluctuates uniformly according to
the appropriate Boltzmann factor. The second derivative is predicted to be negative throughout the vortex liquid state and
positive in the solid state. The discontinuities in entropy and magnetization
at melting are calculated to be per flux line per layer and
~emu~cm at a field of 50 kOe.Comment: 11 pages, 4 PostScript figures in one uuencoded fil
Molecular cytogenetic characterization of a critical region in bands 7q35-q36 commonly deleted in malignant myeloid disorders
Loss of chromosome 7 (-7) or deletion of the long arm (7q-) are recurring chromosome abnormalities in myeloid leukemias. The association of - 7/7q- with myeloid leukemia suggests that these regions contain novel tumor suppressor gene(s), whose loss of function contribute to leukemic transformation or tumor progression. Based on chromosome banding analysis, two critical regions have been identified, one in band q22 and another in bands q32-q35. Presently there are no data available on the molecular delineation of the distal critical region. In this study we analyzed bone marrow and blood samples from 13 patients with myeloid leukemia (de novo myelodysplastic syndrome [MDS], n=3; de novo acute myeloid leukemia [AML], n=9; therapy-related (t-) AML, n=1) which, on chromosome banding analysis, exhibited deletions (n=12) or in one case a balanced translocation involving bands 7q31-qter using fluorescence in situ hybridization (FISH). As probes we used representative clones from a contig map of yeast artificial chromosome (YAC) clones that spans chromosome bands 7q31.1-qter. In the 12 cases with loss of 7q material, we identified a commonly deleted region of approximately 4 to 5 megabasepairs in size encompassing the distal part of 7q35 and the proximal part of 7q36. Furthermore, the breakpoint of the reciprocal translocation from the patient with t-AML was localized to a 1,300-kb sized YAC clone that maps to the proximal boundary of the commonly deleted region. Interestingly, in this case both homologs of chromosome 7 were affected: one was lost (-7) and the second exhibited the t(7q35). The identification and delineation of translocation and deletion breakpoints provides the first step toward the identification of the gene(s) involved in the pathogenesis of 7q35-q36 aberrations in myeloid disorders.link_to_OA_fulltex
Magnetization Jump in a Model for Flux Lattice Melting at Low Magnetic Fields
Using a frustrated XY model on a lattice with open boundary conditions, we
numerically study the magnetization change near a flux lattice melting
transition at low fields. In both two and three dimensions, we find that the
melting transition is followed at a higher temperature by the onset of large
dissipation associated with the zero-field XY transition. It is characterized
by the proliferation of vortex-antivortex pairs (in 2D) or vortex loops (in
3D). At the upper transition, there is a sharp increase in magnetization, in
qualitative agreement with recent local Hall probe experiments.Comment: updated figures and texts. new movies available at
http://www.physics.ohio-state.edu:80/~ryu/jj.html. Accepted for publication
in Physical Review Letter
First-Order Melting of a Moving Vortex Lattice: Effects of Disorder
We study the melting of a moving vortex lattice through numerical simulations
with the current driven 3D XY model with disorder. We find that there is a
first-order phase transition even for large disorder when the corresponding
equilibrium transition is continuous. The low temperature phase is an
anisotropic moving glass.Comment: Important changes from original version. Finite size analysis of
results has been added. Figure 2 has been changed. There is a new additional
Figure. To be published in Physical Review Letter
Flux-line entanglement as the mechanism of melting transition in high-temperature superconductors in a magnetic field
The mechanism of the flux-line-lattice (FLL) melting in anisotropic high-T_c
superconductors in is clarified by Monte Carlo
simulations of the 3D frustrated XY model. The percentage of entangled flux
lines abruptly changes at the melting temperature T_m, while no sharp change
can be found in the number and size distribution of vortex loops around T_m.
Therefore, the origin of this melting transition is the entanglement of flux
lines. Scaling behaviors of physical quantities are consistent with the above
mechanism of the FLL melting. The Lindemann number is also evaluated without
any phenomenological arguments.Comment: 10 pages, 5 Postscript figures, RevTeX; changed content and figures,
Phys. Rev. B Rapid Commun. in pres
Vortices and 2D bosons: A Path-Integral Monte Carlo Study
The vortex system in a high-T_c superconductor has been studied numerically
using the mapping to 2D bosons and the path-integral Monte Carlo method. We
find a single first-order transition from an Abrikosov lattice to an entangled
vortex liquid. The transition is characterized by an entropy jump dS = 0.4 k_B
per vortex and layer (parameters for YBCO) and a Lindemann number c_L = 0.25.
The increase in density at melting is given by d\rho = 6.0*10^{-4} /
\lambda(T)^2. The vortex liquid corresponds to a bosonic superfluid, with
\rho_s = \rho even in the limit \lambda -> \infty.Comment: 9 pages, RevTeX, 4 PostScript figures. The entropy jump at the
transition has been recomputed and is now in agreement with experiments on
YBCO. Some minor modifications were made in the tex
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