371 research outputs found
Peierls-like transition induced by frustration in a two-dimensional antiferromagnet
We show that the introduction of frustration into the spin-1/2
two-dimensional (2D) antiferromagnetic Heisenberg model on a square lattice via
a next-nearest neighbor exchange interaction can lead to a Peierls-like
transition, from a tetragonal to an orthorhombic phase, when the spins are
coupled to adiabatic phonons. The two different orthorhombic ground states
define an Ising order parameter, which is expected to lead to a finite
temperature transition. Implications for , the first
realization of that model, will be discussed.Comment: 4 pages, to be published on Physical Review Letter
Realization of a large J_2 quasi-2D spin-half Heisenberg system: Li2VOSiO4
Exchange couplings are calculated for Li2VOSiO4 using LDA. While the sum of
in-plane couplings J_1 + J_2 = 9.5 \pm 1.5 K and the inter-plane coupling
J_{perp} \sim 0.2 - 0.3 K agree with recent experimental data, the ratio
J_2/J_1 \sim 12 exceeds the reported value by an order of magnitude. Using
geometrical considerations, high temperature expansions and perturbative mean
field theory, we show that the LDA derived exchange constants lead to a
remarkably accurate description of the properties of these materials including
specific heat, susceptibility, Neel temperature and NMR spectra.Comment: 4 two-column pages, 4 embedded postscript figure
Instant recovery of shape from spectrum via latent space connections
We introduce the first learning-based method for recovering shapes from Laplacian spectra. Our model consists of a cycle-consistent module that maps between learned latent vectors of an auto-encoder and sequences of eigenvalues.
This module provides an efficient and effective linkage between Laplacian spectrum and geometry. Our data-driven approach replaces the need for ad-hoc regularizers required by prior methods, while providing more accurate results at a fraction of the computational cost. Our learning model applies without modifications across different dimensions (2D and 3D shapes alike), representations (meshes, contours and point clouds), as well as across different shape classes, and admits arbitrary resolution of the input spectrum
without affecting complexity. The increased flexibility allows us to address notoriously difficult tasks in 3D vision and geometry processing within a unified framework, including shape generation from spectrum, mesh superresolution, shape exploration, style transfer, spectrum estimation from point clouds, segmentation transfer and pointtopoint matching
The role of inflammation in patients with intraductal mucinous neoplasm of the pancreas and in those with pancreatic adenocarcinoma
Background: There are very few data regarding inflammation in patients with intraductal papillary mucinous neoplasms (IPMNs) of the pancreas. Aim: To evaluate the circulating concentrations of placental growth factor (PlGF), transforming growth factor-alpha (TGF-\u3b1), transforming growth factor-beta 1 (TGF-\u3b21), tumour necrosis factor receptor 1 (TNF-R1) and matrix metalloproteinase-2 (MMP-2) in patients with IPMNs and in those with pancreatic adenocarcinomas. Patients and Methods: Sixty-nine patients were enrolled: 23 (33.3%) had IPMNs and 46 (66.7%) had histologically confirmed pancreatic adenocarcinomas. Thirteen healthy subjects were also studied. PlGF, TGF-\u3b1, TGF-\u3b21, TNF-R1 and MMP-2 were determined using commercially available kits. Results: TNF-R1 (p=0.003) was the only protein significantly different among the three groups. Conclusion: Serum TNF-R1 was elevated in patients with IPMNs and in those with pancreatic adenocarcinomas, suggesting a high apoptotic activity in both groups of patients studied
Resonating Valence Bond Wave Functions for Strongly Frustrated Spin Systems
The Resonating Valence Bond (RVB) theory for two-dimensional quantum
antiferromagnets is shown to be the correct paradigm for large enough ``quantum
frustration''. This scenario, proposed long time ago but never confirmed by
microscopic calculations, is very strongly supported by a new type of
variational wave function, which is extremely close to the exact ground state
of the Heisenberg model for .
This wave function is proposed to represent the generic spin-half RVB ground
state in spin liquids.Comment: 4 Pages, 5 figures, accepted for publication in PR
Suppression of Dimer Correlations in the Two-Dimensional - Heisenberg Model: an Exact Diagonalization Study
We present an exact diagonalization study of the ground state of the
spin-half model. Dimer correlation functions and the susceptibility
associated to the breaking of the translational invariance are calculated for
the and the clusters. These results -- especially when
compared to the one dimensional case, where the occurrence of a dimerized phase
for large enough frustration is well established -- suggest either a
homogeneous spin liquid or, possibly, a dimerized state with a rather small
order parameter
Chiral critical behavior in two dimensions from five-loop renormalization-group expansions
We analyse the critical behavior of two-dimensional N-vector spin systems
with noncollinear order within the five-loop renormalization-group
approximation. The structure of the RG flow is studied for different N leading
to the conclusion that the chiral fixed point governing the critical behavior
of physical systems with N = 2 and N = 3 does not coincide with that given by
the 1/N expansion. We show that the stable chiral fixed point for ,
including N = 2 and N = 3, turns out to be a focus. We give a complete
characterization of the critical behavior controlled by this fixed point, also
evaluating the subleading crossover exponents. The spiral-like approach of the
chiral fixed point is argued to give rise to unusual crossover and
near-critical regimes that may imitate varying critical exponents seen in
numerous physical and computer experiments.Comment: 17 pages, 12 figure
Preliminary evaluation of the performance of a new highly sensitive commercial immunoassay for serum ferritin determination
We evaluated the analytical performance of a new, commercial, fully automated immunoturbidimetric assay for the determination of ferritin [FERLatex(X2)CN SEIKEN, Denka Seiken, Japan] in serum on the Olympus AU2700 analyzer. The new assay is a latex-enhanced turbidimetric immunoassay with an analysis time of 10 min. The linearity of the assay was confirmed up to 2505 pmol/L (R-2 =0.999). The detection limit and the functional sensitivity were both 4.5 pmol/L. The intra- and inter-assay imprecision (CV) at 67, 506, 2186 pmol/L was < 1.8% and < 2.5%, respectively. Verification of the traceability to a WHO standard (80/578) showed a recovery of 102.6% (target value 449 pmol/L). No hook effect was observed in samples containing up to 33,705 pmol/L. The assay showed good correlation with the Beckman Immage nephelometric system (r=0.999). Hemoglobin <= 9.8 g/L), total bilirubin (<= 113 [Lmol/L), conjugated bilirubin (<= 109 mu mol/L) and rheumatoid factor (<= 5.2 x 10(5) IU/L) did not interfere with the assay. The reference interval (2.5-97.5 percentile) was 72-521 pmol/L for men and 27-267 pmol/L for women. The reference interval in patients with anemia, malignant tumors and hemochromatosis was 5.6-52, 130-2436 and 1465-2903 pmol/L, respectively. On the basis, of the receiver operating characteristic curve, the 90% sensitivity cut-off value to distinguish between patients with and without iron deficiency was 40 pmol/L. The new latex turbidimetric procedure for ferritin assay is an attractive alternative that avoids the need for dedicated instrumentation
Critical Dynamics of Singlet Excitations in a Frustrated Spin System
We construct and analyze a two-dimensional frustrated quantum spin model with
plaquette order, in which the low-energy dynamics is controlled by spin
singlets. At a critical value of frustration the singlet spectrum becomes
gapless, indicating a quantum transition to a phase with dimer order. This T=0
transition belongs to the 3D Ising universality class, while at finite
temperature a 2D Ising critical line separates the plaquette and dimerized
phases.
The magnetic susceptibility has an activated form throughout the phase
diagram, whereas the specific heat exhibits a rich structure and a power law
dependence on temperature at the quantum critical point.
We argue that the novel quantum critical behavior associated with singlet
criticality discussed in this work can be relevant to a wide class of quantum
spin systems, such as antiferromagnets on Kagome and pyrochlore lattices, where
the low-energy excitations are known to be spin singlets, as well as to the
CAVO lattice and several recently discovered strongly frustrated square-lattice
antiferromagnets.Comment: 5 pages, 5 figures, additional discussion and figure added, to appear
in Phys. Rev.
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