12,199 research outputs found
Magnetic incommensurability and fluctuating charge density waves in the repulsive Hubbard model
Magnetic and charge susceptibilities of the two-dimensional repulsive Hubbard
model are investigated applying a strong coupling diagram technique in which
the expansion in powers of the hopping constants is used. For small lattices
and high temperatures results are in agreement with Monte Carlo simulations.
With the departure from half-filling the low-frequency magnetic
susceptibility becomes incommensurate and the incommensurability parameter
grows with . The incommensurability, its dependence on frequency and on
resemble experimental results in lanthanum cuprates. Also for finite sharp
maxima appear in the static charge susceptibility. The maxima are finite which
points to the absence of the long-range charge ordering (static stripes).
However, for the maxima are located near the momenta
, . In this case an interaction of carriers with
tetragonal distortions can stabilize stripes with the wavelength of four
lattice spacings, as observed in the low-temperature tetragonal phase of
cuprates. As follows from the obtained results, the magnetic incommensurability
is not a consequence of the stripes.Comment: 4 pages, 3 figures, manuscript for proceefings of LT2
The t-J model on a semi-infinite lattice
The hole spectral function of the t-J model on a two-dimensional
semi-infinite lattice is calculated using the spin-wave and noncrossing
approximations. In the case of small hole concentration and strong
correlations, , several near-boundary site rows appear to be depleted
of holes. The reason for this depletion is a deformation of the magnon cloud,
which surrounds the hole, near the boundary. The hole depletion in the boundary
region leads to a more complicated spectral function in the boundary row in
comparison with its bulk shape.Comment: 8 pages, 5 figure
Near-Boundary and Bulk Regions of a Semi-Infinite Two-Dimensional Heisenberg Antiferromagnet
Using the spin-wave approximation elementary excitations of a semi-infinite
two-dimensional Heisenberg antiferromagnet are considered. The
spectrum consists of bulk modes -- standing spin waves and a
quasi-one-dimensional mode of boundary spin waves. These latter excitations
eject bulk modes from two boundary rows of sites, thereby dividing the
antiferromagnet into two regions with different dominant excitations. As a
result absolute values of nearest-neighbor spin correlations on the edge exceed
the bulk value.Comment: 8 pages, 3 figure
Coherent Excitation of the 6S1/2 to 5D3/2 Electric Quadrupole Transition in 138Ba+
The electric dipole-forbidden, quadrupole 6S1/2 5D3/2 transition in Ba+
near 2051 nm, with a natural linewidth of 13 mHz, is attractive for potential
observation of parity non-conservation, and also as a clock transition for a
barium ion optical frequency standard. This transition also offers a direct
means of populating the metastable 5D3/2 state to measure the nuclear magnetic
octupole moment in the odd barium isotopes. Light from a diode-pumped, solid
state Tm,Ho:YLF laser operating at 2051 nm is used to coherently drive this
transition between resolved Zeeman levels in a single trapped 138Ba+ ion. The
frequency of the laser is stabilized to a high finesse Fabry Perot cavity at
1025 nm after being frequency doubled. Rabi oscillations on this transition
indicate a laser-ion coherence time of 3 ms, most likely limited by ambient
magnetic field fluctuations.Comment: 5 pages, 5 figure
Chromatic Ramsey number of acyclic hypergraphs
Suppose that is an acyclic -uniform hypergraph, with . We
define the (-color) chromatic Ramsey number as the smallest
with the following property: if the edges of any -chromatic -uniform
hypergraph are colored with colors in any manner, there is a monochromatic
copy of . We observe that is well defined and where
is the -color Ramsey number of . We give linear upper bounds
for when T is a matching or star, proving that for , and where
and are, respectively, the -uniform matching and star with
edges.
The general bounds are improved for -uniform hypergraphs. We prove that
, extending a special case of Alon-Frankl-Lov\'asz' theorem.
We also prove that , which is sharp for . This is
a corollary of a more general result. We define as the 1-intersection
graph of , whose vertices represent hyperedges and whose edges represent
intersections of hyperedges in exactly one vertex. We prove that for any -uniform hypergraph (assuming ). The proof uses the list coloring version of Brooks' theorem.Comment: 10 page
Low-Frequency Quantum Oscillations due to Strong Electron Correlations
The normal-state energy spectrum of the two-dimensional - model in a
homogeneous perpendicular magnetic field is investigated. The density of states
at the Fermi level as a function of the inverse magnetic field
reveals oscillations in the range of hole concentrations . The
oscillations have both high- and low-frequency components. The former
components are connected with large Fermi surfaces, while the latter with van
Hove singularities in the Landau subbands, which traverse the Fermi level with
changing . The singularities are related to bending the Landau subbands due
to strong electron correlations. Frequencies of these components are of the
same order of magnitude as quantum oscillation frequencies observed in
underdoped cuprates.Comment: 10 pages, 3 figures, Proc. NSS-2013, Yalta. arXiv admin note: text
overlap with arXiv:1308.056
Aquila X--1: a low inclination soft X-ray transient
We have obtained I-band photometry of the neutron star X-ray transient Aql
X--1 during quiescence. We find a periodicity at 2.487 cd-1, which we interpret
as twice the orbital frequency (19.30+/-0.05 h). Folding the data on the
orbital period, we model the light curve variations as the ellipsoidal
modulation of the secondary star. We determine the binary inclination to be
20--31 degrees (90 per cent confidence) and also 95 per cent upper limits to
the radial velocity semi-amplitude and rotational broadening of the secondary
star to be 117 kms-1 and 50 kms-1 respectively.Comment: 4 pages text, 3 figures, to appear in MNRA
Human Perception and the Color of Flavor
Human taste perception can be analyzed in different areas of study. Physiology and psychology work together to construct the way we taste, and our sense of taste is not obtained merely from the tongue. The process of tasting involves olfaction, vision, and texture reception to form our overall perception of taste. The present study involved 25 participants who tasted and rated multiple samples of flavored gelatin. Half of the gelatin samples were unlikely color/flavor combinations, and half were unlikely flavor/scent combinations. Responses to the flavors as perceived were collected and used to gain insight into the interactions among sight, smell, and taste perception
Resonance peak in underdoped cuprates
The magnetic susceptibility measured in neutron scattering experiments in
underdoped YBaCuO is interpreted based on the self-consistent
solution of the t-J model of a Cu-O plane. The calculations reproduce correctly
the frequency and momentum dependencies of the susceptibility and its variation
with doping and temperature in the normal and superconducting states. This
allows us to interpret the maximum in the frequency dependence -- the resonance
peak -- as a manifestation of the excitation branch of localized Cu spins and
to relate the frequency of the maximum to the size of the spin gap. The
low-frequency shoulder well resolved in the susceptibility of superconducting
crystals is connected with a pronounced maximum in the damping of the spin
excitations. This maximum is caused by intense quasiparticle peaks in the hole
spectral function for momenta near the Fermi surface and by the nesting.Comment: 9 pages, 6 figure
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