37 research outputs found
Effective Hamiltonians for some highly frustrated magnets
In prior work, the authors developed a method of degenerate perturbation
theory about the Ising limit to derive an effective Hamiltonian describing
quantum fluctuations in a half-polarized magnetization plateau on the
pyrochlore lattice. Here, we extend this formulation to an arbitrary lattice of
corner sharing simplexes of sites, at a fraction of the
saturation magnetization, with . We present explicit effective
Hamiltonians for the examples of the checkerboard, kagome, and pyrochlore
lattices. The consequent ground states in these cases for are also
discussed.Comment: 10 pages, 2 figures,. Conference proceedings for Highly Frustrated
Magnetism 200
Inducing spin-dependent tunneling to probe magnetic correlations in optical lattices
We suggest a simple experimental method for probing antiferromagnetic spin
correlations of two-component Fermi gases in optical lattices. The method
relies on a spin selective Raman transition to excite atoms of one spin species
to their first excited vibrational mode where the tunneling is large. The
resulting difference in the tunneling dynamics of the two spin species can then
be exploited, to reveal the spin correlations by measuring the number of doubly
occupied lattice sites at a later time. We perform quantum Monte Carlo
simulations of the spin system and solve the optical lattice dynamics
numerically to show how the timed probe can be used to identify
antiferromagnetic spin correlations in optical lattices.Comment: 5 pages, 5 figure
Quantum Critical Point of the XY Model and Condensation of Field-Induced Quasiparticles in Dimer Compounds
The quantum critical point of the three-dimensional XY model in a
symmetry-preserving field is investigated. The results of Monte Carlo
simulations with the directed-loop algorithm show that the quantum critical
behavior is characterized by the mean-field values of critical exponents. The
system-size dependence of various quantities is compared to a simple
field-theoretical argument that supports the mean-field scaling
Phase diagram of a Bose-Fermi mixture in a one-dimensional optical lattice in terms of fidelity and entanglement
We study the ground-state phase diagram of a Bose-Fermi mixture loaded in a
one-dimensional optical lattice by computing the ground-state fidelity and
quantum entanglement. We find that the fidelity is able to signal quantum phase
transitions between the Luttinger liquid phase, the density-wave phase, and the
phase separation state of the system; and the concurrence can be used to signal
the transition between the density-wave phase and the Ising phase.Comment: 4 pages 3 figure
Quantum and thermal ionic motion, oxygen isotope effect, and superexchange distribution in La<sub>2</sub>CuO<sub>4</sub>
We study the zero-point and thermal ionic motion in LaCuO by means of
high-resolution neutron diffraction experiments. Our results demonstrate
anisotropic motion of O and to a lesser extent of Cu ions, both consistent with
the structure of coupled CuO octahedra, and quantify the relative effects
of zero-point and thermal contributions to ionic motion. By substitution of
O, we find that the oxygen isotope effect on the lattice dimensions is
small and negative (), while the isotope effect on the ionic
displacement parameters is significant ( to ). We use our results as
input for theoretical estimates of the distribution of magnetic interaction
parameters, , in an effective one-band model for the cuprate plane. We find
that ionic motion causes only small () effects on the average value
, which vary with temperature and O isotope, but results in
dramatic (-) fluctuations in values that are subject to
significant (-) isotope effects. We demonstrate that this motional
broadening of can have substantial effects on certain electronic and
magnetic properties in cuprates.Comment: 13 pages, 9 figure
The Short Range RVB State of Even Spin Ladders: A Recurrent Variational Approach
Using a recursive method we construct dimer and nondimer variational ansatzs
of the ground state for the two-legged ladder, and compute the number of dimer
coverings, the energy density and the spin correlation functions. The number of
dimer coverings are given by the Fibonacci numbers for the dimer-RVB state and
their generalization for the nondimer ones. Our method relies on the recurrent
relations satisfied by the overlaps of the states with different lengths, which
can be solved using generating functions. The recurrent relation method is
applicable to other short range systems. Based on our results we make a
conjecture about the bond amplitudes of the 2-leg ladder.Comment: REVTEX file, 32 pages, 10 EPS figures inserted in text with epsf.st
Anomaly and a QCD-like phase diagram with massive bosonic baryons
We study a strongly coupled lattice gauge theory with two flavors of
quarks, invariant under an exact symmetry which is the same as QCD with
two flavors of quarks without an anomaly. The model also contains a coupling
that can be used to break the symmetry and thus mimic the QCD
anomaly. At low temperatures and small baryon chemical potential
the model contains massless pions and massive bosonic baryons similar to QCD
with an even number of colors. In this work we study the phase
diagram of the model and show that it contains three phases : (1) A chirally
broken phase at low and , (2) a chirally symmetric baryon superfluid
phase at low and high , and (3) a symmetric phase at high . We
find that the nature of the finite temperature chiral phase transition and in
particular the location of the tricritical point that seperates the first order
line from the second order line is affected significantly by the anomaly.Comment: 22 pages, 16 figures, 5 tables, references adde
Diagonal Ladders: A New Class of Models for Strongly Coupled Electron Systems
We introduce a class of models defined on ladders with a diagonal structure
generated by plaquettes. The case corresponds to the necklace
ladder and has remarkable properties which are studied using DMRG and recurrent
variational ansatzes. The AF Heisenberg model on this ladder is equivalent to
the alternating spin-1/spin-1/2 AFH chain which is known to have a
ferrimagnetic ground state (GS). For doping 1/3 the GS is a fully doped (1,1)
stripe with the holes located mostly along the principal diagonal while the
minor diagonals are occupied by spin singlets. This state can be seen as a Mott
insulator of localized Cooper pairs on the plaquettes. A physical picture of
our results is provided by a model of plaquettes coupled diagonally
with a hopping parameter . In the limit we recover the
original model on the necklace ladder while for weak hopping parameter
the model is easily solvable. The GS in the strong hopping regime is
essentially an "on link" Gutzwiller projection of the weak hopping GS. We
generalize the model to diagonal ladders with and the 2D
square lattice. We use in our construction concepts familiar in Statistical
Mechanics as medial graphs and Bratelli diagrams.Comment: REVTEX file, 22 pages (twocolumn), 35 figures inserted in text. 12
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Elevated Levels of the Polo Kinase Cdc5 Override the Mec1/ATR Checkpoint in Budding Yeast by Acting at Different Steps of the Signaling Pathway
Checkpoints are surveillance mechanisms that constitute a barrier to oncogenesis by preserving genome integrity. Loss of checkpoint function is an early event in tumorigenesis. Polo kinases (Plks) are fundamental regulators of cell cycle progression in all eukaryotes and are frequently overexpressed in tumors. Through their polo box domain, Plks target multiple substrates previously phosphorylated by CDKs and MAPKs. In response to DNA damage, Plks are temporally inhibited in order to maintain the checkpoint-dependent cell cycle block while their activity is required to silence the checkpoint response and resume cell cycle progression. Here, we report that, in budding yeast, overproduction of the Cdc5 polo kinase overrides the checkpoint signaling induced by double strand DNA breaks (DSBs), preventing the phosphorylation of several Mec1/ATR targets, including Ddc2/ATRIP, the checkpoint mediator Rad9, and the transducer kinase Rad53/CHK2. We also show that high levels of Cdc5 slow down DSB processing in a Rad9-dependent manner, but do not prevent the binding of checkpoint factors to a single DSB. Finally, we provide evidence that Sae2, the functional ortholog of human CtIP, which regulates DSB processing and inhibits checkpoint signaling, is regulated by Cdc5. We propose that Cdc5 interferes with the checkpoint response to DSBs acting at multiple levels in the signal transduction pathway and at an early step required to resect DSB ends