89,629 research outputs found
Quantum fluctuations in the spiral phase of the Hubbard model
We study the magnetic excitations in the spiral phase of the two--dimensional
Hubbard model using a functional integral method. Spin waves are strongly
renormalized and a line of near--zeros is observed in the spectrum around the
spiral pitch . The possibility of disordered spiral states is
examined by studying the one--loop corrections to the spiral order parameter.
We also show that the spiral phase presents an intrinsic instability towards an
inhomogeneous state (phase separation, CDW, ...) at weak doping. Though phase
separation is suppressed by weak long--range Coulomb interactions, the CDW
instability only disappears for sufficiently strong Coulomb interaction.Comment: Figures are NOW appended via uuencoded postscript fil
Upflows in the upper transition region of the quiet Sun
We investigate the physical meaning of the prominent blue shifts of Ne VIII,
which is observed to be associated with quiet-Sun network junctions (boundary
intersections), through data analyses combining force-free-field extrapolations
with EUV spectroscopic observations. For a middle-latitude region, we
reconstruct the magnetic funnel structure in a sub-region showing faint
emission in EIT-Fe 195. This funnel appears to consist of several smaller
funnels that originate from network lanes, expand with height and finally merge
into a single wide open-field region. However, the large blue shifts of Ne VIII
are generally not associated with open fields, but seem to be associated with
the legs of closed magnetic loops. Moreover, in most cases significant upflows
are found in both of the funnel-shaped loop legs. These quasi-steady upflows
are regarded as signatures of mass supply to the coronal loops rather than the
solar wind. Our observational result also reveals that in many cases the
upflows in the upper transition region (TR) and the downflows in the middle TR
are not fully cospatial. Based on these new observational results, we suggest
different TR structures in coronal holes and in the quiet Sun.Comment: 4 pages, 4 figures, will appear in the Proceedings of the Solar wind
12 conferenc
Exact one-periodic and two-periodic wave solutions to Hirota bilinear equations in 2+1 dimensions
Riemann theta functions are used to construct one-periodic and two-periodic
wave solutions to a class of (2+1)-dimensional Hirota bilinear equations. The
basis for the involved solution analysis is the Hirota bilinear formulation,
and the particular dependence of the equations on independent variables
guarantees the existence of one-periodic and two-periodic wave solutions
involving an arbitrary purely imaginary Riemann matrix. The resulting theory is
applied to two nonlinear equations possessing Hirota bilinear forms:
and
where , thereby yielding their one-periodic and two-periodic wave
solutions describing one dimensional propagation of waves
On the thermalization of a Luttinger liquid after a sequence of sudden interaction quenches
We present a comprehensive analysis of the relaxation dynamics of a Luttinger
liquid subject to a sequence of sudden interaction quenches. We express the
critical exponent governing the decay of the steady-state propagator as
an explicit functional of the switching protocol. At long distances
depends only on the initial state while at short distances it is also history
dependent. Continuous protocols of arbitrary complexity can be realized with
infinitely long sequences. For quenches of finite duration we prove that there
exist no protocol to bring the initial non-interacting system in the ground
state of the Luttinger liquid. Nevertheless memory effects are washed out at
short-distances. The adiabatic theorem is then investigated with
ramp-switchings of increasing duration, and several analytic results for both
the propagator and the excitation energy are derived.Comment: 7 pages, 4 figure
Magic Wavelengths for Terahertz Clock Transitions
Magic wavelengths for laser trapping of boson isotopes of alkaline-earth Sr,
Ca and Mg atoms are investigated while considering terahertz clock transitions
between the metastable triplet states. Our
calculation shows that magic wavelengths of trapping laser do exist. This
result is important because those metastable states have already been used to
realize accurate clocks in the terahertz frequency domain. Detailed discussions
for magic wavelength for terahertz clock transitions are given in this paper.Comment: 7 page
False discovery rate regression: an application to neural synchrony detection in primary visual cortex
Many approaches for multiple testing begin with the assumption that all tests
in a given study should be combined into a global false-discovery-rate
analysis. But this may be inappropriate for many of today's large-scale
screening problems, where auxiliary information about each test is often
available, and where a combined analysis can lead to poorly calibrated error
rates within different subsets of the experiment. To address this issue, we
introduce an approach called false-discovery-rate regression that directly uses
this auxiliary information to inform the outcome of each test. The method can
be motivated by a two-groups model in which covariates are allowed to influence
the local false discovery rate, or equivalently, the posterior probability that
a given observation is a signal. This poses many subtle issues at the interface
between inference and computation, and we investigate several variations of the
overall approach. Simulation evidence suggests that: (1) when covariate effects
are present, FDR regression improves power for a fixed false-discovery rate;
and (2) when covariate effects are absent, the method is robust, in the sense
that it does not lead to inflated error rates. We apply the method to neural
recordings from primary visual cortex. The goal is to detect pairs of neurons
that exhibit fine-time-scale interactions, in the sense that they fire together
more often than expected due to chance. Our method detects roughly 50% more
synchronous pairs versus a standard FDR-controlling analysis. The companion R
package FDRreg implements all methods described in the paper
Validity of the scattering length approximation in strongly interacting Fermi systems
We investigate the energy spectrum of systems of two, three and four spin-1/2
fermions with short range attractive interactions both exactly, and within the
scattering length approximation. The formation of molecular bound states and
the ferromagnetic transition of the excited scattering state are examined
systematically as a function of the 2-body scattering length. Identification of
the upper branch (scattering states) is discussed and a general approach valid
for systems with many particles is given. We show that an adiabatic
ferromagnetic transition occurs, but at a critical transition point kF a much
higher than predicted from previous calculations, almost all of which use the
scattering length approximation. In the 4-particle system the discrepancy is a
factor of 2. The exact critical interaction strength calculated in the
4-particle system is consistent with that reported by experiment. To make
comparisons with the adiabatic transition, we study the quench dynamics of the
pairing instability using the eigenstate wavefunctions.Comment: 7 pages, 7 figure
Linear spaces with a line-transitive point-imprimitive automorphism group and Fang-Li parameter gcd(k,r) at most eight
In 1991, Weidong Fang and Huiling Li proved that there are only finitely many
non-trivial linear spaces that admit a line-transitive, point-imprimitive group
action, for a given value of gcd(k,r), where k is the line size and r is the
number of lines on a point. The aim of this paper is to make that result
effective. We obtain a classification of all linear spaces with this property
having gcd(k,r) at most 8. To achieve this we collect together existing theory,
and prove additional theoretical restrictions of both a combinatorial and group
theoretic nature. These are organised into a series of algorithms that, for
gcd(k,r) up to a given maximum value, return a list of candidate parameter
values and candidate groups. We examine in detail each of the possibilities
returned by these algorithms for gcd(k,r) at most 8, and complete the
classification in this case.Comment: 47 pages Version 1 had bbl file omitted. Apologie
Fermionic R-operator approach for the small-polaron model with open boundary condition
Exact integrability and algebraic Bethe ansatz of the small-polaron model
with the open boundary condition are discussed in the framework of the quantum
inverse scattering method (QISM). We employ a new approach where the fermionic
R-operator which consists of fermion operators is a key object. It satisfies
the Yang-Baxter equation and the reflection equation with its corresponding
K-operator. Two kinds of 'super-transposition' for the fermion operators are
defined and the dual reflection equation is obtained. These equations prove the
integrability and the Bethe ansatz equation which agrees with the one obtained
from the graded Yang-Baxter equation and the graded reflection equations.Comment: 10 page
A Field Effect Transitor based on the Mott Transition in a Molecular Layer
Here we propose and analyze the behavior of a FET--like switching device, the
Mott transition field effect transistor, operating on a novel principle, the
Mott metal--insulator transition. The device has FET-like characteristics with
a low ``ON'' impedance and high ``OFF'' impedance. Function of the device is
feasible down to nanoscale dimensions. Implementation with a class of organic
charge transfer complexes is proposed.Comment: Revtex 11pages, Figures available upon reques
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