1,570 research outputs found
Demonstration of a robust pseudogap in a three-dimensional correlated electronic system
We outline a partial-fractions decomposition method for determining the
one-particle spectral function and single-particle density of states of a
correlated electronic system on a finite lattice in the non self-consistent
T-matrix approximation to arbitrary numerical accuracy, and demonstrate the
application of these ideas to the attractive Hubbard model. We then demonstrate
the effectiveness of a finite-size scaling ansatz which allows for the
extraction of quantities of interest in the thermodynamic limit from this
method. In this approximation, in one or two dimensions, for any finite lattice
or in the thermodynamic limit, a pseudogap is present and its energy diverges
as Tc is approached from above; this is an unphysical manifestation of using an
approximation that predicts a spurious phase transition in one or two
dimensions. However, in three dimensions one expects the transition predicted
by this approximation to represent a true continuous phase transition, and in
the thermodynamic limit any pseudogap predicted by this formulation will remain
finite. We have applied our method to the attractive Hubbard model on a
three-dimensional simple cubic lattice, and find that for intermediate coupling
a prominent pseudogap is found in the single-particle density of states, and
this gap persists over a large temperature range. In addition, we also show
that for weak coupling a pseudogap is also present. The pseudogap energy at the
transition temperature is almost a factor of three larger than the T=0 BCS gap
for intermediate coupling, whereas for weak coupling the pseudogap and BCS gap
energies are essentially equal.Comment: 28 pages, 9 figure
Spin dynamics in the antiferromagnetic phase for electron-doped cuprate superconductors
Based on the --- model we have calculated the dynamical spin
susceptibilities in the antiferromagnetic (AF) phase for electron-doped
cuprates, by use of the slave-boson mean-field theory and random phase
approximation. Various results for the susceptibilities versus energy and
momentum have been shown at different dopings. At low energy, except the
collective spin-wave mode around and 0, we have primarily observed
that new resonance peaks will appear around and equivalent
points with increasing doping, which are due to the particle-hole excitations
between the two AF bands. The peaks are pronounced in the transverse
susceptibility but not in the longitudinal one. These features are predicted
for neutron scattering measurements.Comment: 5 pages, 3 figures, published version with minor change
Direct-laser writing for subnanometer focusing and single-molecule imaging
Two-photon direct laser writing is an additive fabrication process that utilizes two-photon absorption of tightly focused femtosecond laser pulses to implement spatially controlled polymerization of a liquid-phase photoresist. Two-photon direct laser writing is capable of nanofabricating arbitrary three-dimensional structures with nanometer accuracy. Here, we explore direct laser writing for high-resolution optical microscopy by fabricating unique 3D optical fiducials for single-molecule tracking and 3D single-molecule localization microscopy. By having control over the position and three-dimensional architecture of the fiducials, we improve axial discrimination and demonstrate isotropic subnanometer 3D focusing (<0.8 nm) over tens of micrometers using a standard inverted microscope. We perform 3D single-molecule acquisitions over cellular volumes, unsupervised data acquisition and live-cell single-particle tracking with nanometer accuracy
Magnetic susceptibility of a CuO2 plane in the La2CuO4 system: I. RPA treatment of the Dzyaloshinskii-Moriya Interactions
Motivated by recent experiments on undoped La2CuO4, which found pronounced
temperature-dependent anisotropies in the low-field magnetic susceptibility, we
have investigated a two-dimensional square lattice of S=1/2 spins that interact
via Heisenberg exchange plus the symmetric and anti-symmetric
Dzyaloshinskii-Moriya anisotropies. We describe the transition to a state with
long-ranged order, and find the spin-wave excitations, with a mean-field
theory, linear spin-wave analysis, and using Tyablikov's RPA decoupling scheme.
We find the different components of the susceptibility within all of these
approximations, both below and above the N'eel temperature, and obtain evidence
of strong quantum fluctuations and spin-wave interactions in a broad
temperature region near the transition.Comment: 20 pages, 2 column format, 22 figure
Unifying the Phase Diagrams of the Magnetic and Transport Properties of La_(2-x)Sr_xCuO_4, 0 < x < 0.05
An extensive experimental and theoretical effort has led to a largely
complete mapping of the magnetic phase diagram of La_(2-x)Sr_xCuO_4, and a
microscopic model of the spin textures produced in the x < 0.05 regime has been
shown to be in agreement with this phase diagram. Here we use this same model
to derive a theory of the impurity-dominated, low temperature transport. Then,
we present an analysis of previously published data for two samples: x = 0.002
data from Chen et. al., and x = 0.04 data from Keimer et. al. We show that the
transport mechanisms in the two systems are the same, even though they are on
opposite sides of the observed insulator-to-metal transition. Our model of
impurity effects on the impurity band conduction, variable-range hopping
conduction, and coulomb gap conduction, is similar to that used to describe
doped semiconductors. However, for La_(2-x)Sr_xCuO_4 we find that in addition
to impurity-generated disorder effects, strong correlations are important and
must be treated on a equal level with disorder. On the basis of this work we
propose a phase diagram that is consistent with available magnetic and
transport experiments, and which connects the undoped parent compound with the
lowest x value for which La_(2-x)Sr_xCuO_4 is found to be superconducting, x
about 0.06.Comment: 7 pages revtex with one .ps figur
Sr impurity effects on the magnetic correlations of LaSrCuO
We examine the low-temperature magnetic properties of moderately doped
LaSrCuO paying particular attention to the spin-glass (SG) phase and the C-IC
transition as they are affected by Sr impurity disorder. New measurements of
the low-temperature susceptibility in the SG phase show an increase of an
anomalously small Curie constant with doping. This behaviour is explained in
terms of our theoretical work that finds small clusters of AFM correlated
regions separated by disordered domain walls. The domain walls lead to a
percolating sequence of paths connecting the impurities. We predict that for
this spin morphology the Curie constant should scale as , a
result that is quantitatively in agreement with experiment. Also, we find that
the magnetic correlations in the ground states in the SG phase are
commensurate, and that this behaviour should persist at higher temperatures
where the holes should move along the domain walls. However, our results show
that incommensurate correlations develop continuously around 5 % doping,
consistent with recent measurements by Yamada.Comment: 30 pages, revtex, 8 .ps format figures (2 meant to be in colour), to
be published in Physical Review B
Critiquing Variational Theories of the Anderson-Hubbard Model: Real-Space Self-Consistent Hartree-Fock Solutions
A simple and commonly employed approximate technique with which one can
examine spatially disordered systems when strong electronic correlations are
present is based on the use of real-space unrestricted self-consistent
Hartree-Fock wave functions. In such an approach the disorder is treated
exactly while the correlations are treated approximately. In this report we
critique the success of this approximation by making comparisons between such
solutions and the exact wave functions for the Anderson-Hubbard model. Due to
the sizes of the complete Hilbert spaces for these problems, the comparisons
are restricted to small one-dimensional chains, up to ten sites, and a 4x4
two-dimensional cluster, and at 1/2 filling these Hilbert spaces contain about
63,500 and 166 million states, respectively. We have completed these
calculations both at and away from 1/2 filling. This approximation is based on
a variational approach which minimizes the Hartree-Fock energy, and we have
completed comparisons of the exact and Hartree-Fock energies. However, in order
to assess the success of this approximation in reproducing ground-state
correlations we have completed comparisons of the local charge and spin
correlations, including the calculation of the overlap of the Hartree-Fock wave
functions with those of the exact solutions. We find that this approximation
reproduces the local charge densities to quite a high accuracy, but that the
local spin correlations, as represented by , are not as well
represented. In addition to these comparisons, we discuss the properties of the
spin degrees of freedom in the HF approximation, and where in the
disorder-interaction phase diagram such physics may be important
Elliptical supra-cellular topographies regulate stem cells migratory pattern and osteogenic differentiation
In living systems, the extracellular environment is structured in a hierarchal order assembling into tissues in a myriad of shapes and complex geometries. Residing within the extracellular matrix, cells are presented and influenced by geometrical cues at several scales. While there is an emerging body of evidence that substrate with symmetric supra-cellular scale geometries (e.g. cylinders and spheres) influence the cell behavior, the effect of physiologically relevant, non-symmetric geometries with varying mean curvatures remain unexplored. In this study, we systematically explore the migratory and differentiation behavior of adipose derived stem cells (ADSCs) on arrays of elliptical cylinders (up to 80 × cell size) with varying mean curvature made from hydroxyapatite. Here, we report a new substrate-driven cell response, which we term “ridge-effect” that leads to osteogenic differentiation and nuclear deformation of cells adhered on regions of highest mean curvature at the ridge of the elliptical cylinders. This phenomenon is observed in both expansion and osteogenic medium. Live imaging combined with functional analysis shows that cells travel along-side the zero mean curvature direction on elliptical cylinders and significantly promote expression of collagen I and osteocalcin compared to a flat surface, in the absence of osteogenic supplements. Altogether, this work identifies supra-cellular scale topographies, and suggest the “ridge-effect” as a physical cue for guiding cellular mechanoresponse and promoting osteogenic differentiation. This knowledge could be utilized as an important biomaterial design parameter for the development of biomedical interfaces and bone scaffolds in tissue engineering and regenerative medicine
A reliable Pade analytical continuation method based on a high accuracy symbolic computation algorithm
We critique a Pade analytic continuation method whereby a rational polynomial
function is fit to a set of input points by means of a single matrix inversion.
This procedure is accomplished to an extremely high accuracy using a novel
symbolic computation algorithm. As an example of this method in action we apply
it to the problem of determining the spectral function of a one-particle
thermal Green's function known only at a finite number of Matsubara frequencies
with two example self energies drawn from the T-matrix theory of the Hubbard
model. We present a systematic analysis of the effects of error in the input
points on the analytic continuation, and this leads us to propose a procedure
to test quantitatively the reliability of the resulting continuation, thus
eliminating the black magic label frequently attached to this procedure.Comment: 11 pages, 8 eps figs, revtex format; revised version includes
reference to anonymous ftp site containing example codes (MapleVr5.1
worksheets) displaying the implementation of the algorithm, including the
padematinv.m library packag
An Exact Diagonalization Demonstration of Incommensurability and Rigid Band Filling for N Holes in the t-J Model
We have calculated S(q) and the single particle distribution function
for N holes in the t - J model on a non--square sqrt{8} X sqrt{32} 16--site
lattice with periodic boundary conditions; we justify the use of this lattice
in compariosn to those of having the full square symmetry of the bulk. This new
cluster has a high density of vec k points along the diagonal of reciprocal
space, viz. along k = (k,k). The results clearly demonstrate that when the
single hole problem has a ground state with a system momentum of vec k =
(pi/2,pi/2), the resulting ground state for N holes involves a shift of the
peak of the system's structure factor away from the antiferromagnetic state.
This shift effectively increases continuously with N. When the single hole
problem has a ground state with a momentum that is not equal to k =
(pi/2,pi/2), then the above--mentioned incommensurability for N holes is not
found. The results for the incommensurate ground states can be understood in
terms of rigid--band filling: the effective occupation of the single hole k =
(pi/2,pi/2) states is demonstrated by the evaluation of the single particle
momentum distribution function . Unlike many previous studies, we show
that for the many hole ground state the occupied momentum states are indeed k =
(+/- pi/2,+/- pi/2) states.Comment: Revtex 3.0; 23 pages, 1 table, and 13 figures, all include
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