37,265 research outputs found
Hardcore bosons on checkerboard lattices near half filling: geometric frustration, vanishing charge order and fractional phase
We study a spinless hardcore boson model on checkerboard lattices by Green
function Monte Carlo method. At half filling, the ground state energy is
obtained up to lattice and extrapolated to infinite size, the
staggered pseudospin magnetization is found to vanish in the thermodynamic
limit. Thus the charge order is absent in this system. Away from
half filling, two defects induced by each hole (particle) may carry fractional
charge (). For one hole case, we study how the defect-defect
correlation changes with , which is the ratio between the hopping integral
and cyclic exchange, equals to when . Moreover, we argue that
these fractional defects may propagate independently when the concentration of
holes (or defects) is large enough
The structure of the magnetic reconnection exhaust boundary
The structure of shocks that form at the exhaust boundaries during
collisionless reconnection of anti-parallel fields is studied using
particle-in-cell (PIC) simulations and modeling based on the anisotropic
magnetohydrodynamic equations. Large-scale PIC simulations of reconnection and
companion Riemann simulations of shock development demonstrate that the
pressure anisotropy produced by counterstreaming ions within the exhaust
prevents the development of classical Petschek switch-off-slow shocks (SSS).
The shock structure that does develop is controlled by the firehose stability
parameter epsilon=1-mu_0(P_parallel-P_perpendicular)/ B^2 through its influence
on the speed order of the intermediate and slow waves. Here P_parallel and
P_perpendicular are the pressure parallel and perpendicular to the local
magnetic field. The exhaust boundary is made up of a series of two shocks and a
rotational wave. The first shock takes epsilon from unity upstream to a plateau
of 0.25 downstream. The condition epsilon =0.25 is special because at this
value the speeds of nonlinear slow and intermediate waves are degenerate. The
second slow shock leaves epsilon=0.25 unchanged but further reduces the
amplitude of the reconnecting magnetic field. Finally, in the core of the
exhaust epsilon drops further and the transition is completed by a rotation of
the reconnecting field into the out-of-plane direction. The acceleration of the
exhaust takes place across the two slow shocks but not during the final
rotation. The result is that the outflow speed falls below that expected from
the Walen condition based on the asymptotic magnetic field. A simple analytic
expression is given for the critical value of epsilon within the exhaust below
which SSSs no longer bound the reconnection outflow.Comment: 13 pages, 5 figure
Comment on ``Validity of certain soft-photon amplitudes''
The criteria suggested by Welsh and Fearing (nucl-th/9606040) to judge the
validity of certain soft-photon amplitudes are examined. We comment on aspects
of their analysis which lead to incorrect conclusions about published
amplitudes and point out important criteria which were omitted from their
analysis.Comment: 6 pages plus 1 postscript figure, Revte
Directional `superradiant' collisions: bosonic amplification of atom pairs emitted from an elongated Bose-Einstein condensate
We study spontaneous directionality in the bosonic amplification of atom
pairs emitted from an elongated Bose-Einstein condensate (BEC), an effect
analogous to `superradiant' emission of atom-photon pairs. Using a simplified
model, we make analytic predictions regarding directional effects for both
atom-atom and atom-photon emission. These are confirmed by numerical mean-field
simulations, demonstrating the the feasibility of nearly perfect directional
emission along the condensate axis. The dependence of the emission angle on the
pump strength for atom-atom pairs is significantly different than for
atom-photon pairs
Tunnelling Effect and Hawking Radiation from a Vaidya Black Hole
In this paper, we extend Parikh' work to the non-stationary black hole. As an
example of the non-stationary black hole, we study the tunnelling effect and
Hawking radiation from a Vaidya black hole whose Bondi mass is identical to its
mass parameter. We view Hawking radiation as a tunnelling process across the
event horizon and calculate the tunnelling probability. We find that the result
is different from Parikh's work because is the function of
Bondi mass m(v)
The temperature dependence of the local tunnelling conductance in cuprate superconductors with competing AF order
Based on the model with proper chosen parameters for describing
the cuprate superconductors, it is found that near the optimal doping at low
temperature (), only the pure d-wave superconductivity (SC) prevails and
the antiferromagnetic (AF) order is completely suppressed. At higher , the
AF order with stripe modulation and the accompanying charge order may emerge,
and they could exist above the SC transition temperature. We calculate the
local differential tunnelling conductance (LDTC) from the local density of
states (LDOS) and show that their energy variations are rather different from
each other as increases. Although the calculated modulation periodicity in
the LDTC/LDOS and bias energy dependence of the Fourier amplitude of LDTC in
the "pseudogap" region are in good agreement with the recent STM experiment
[Vershinin , Science {\bf 303}, 1995 (2004)], we point out that some of
the energy dependent features in the LDTC do not represent the intrinsic
characteristics of the sample
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The septin cytoskeleton facilitates membrane retraction during motility and blebbing.
Increasing evidence supports a critical role for the septin cytoskeleton at the plasma membrane during physiological processes including motility, formation of dendritic spines or cilia, and phagocytosis. We sought to determine how septins regulate the plasma membrane, focusing on this cytoskeletal element's role during effective amoeboid motility. Surprisingly, septins play a reactive rather than proactive role, as demonstrated during the response to increasing hydrostatic pressure and subsequent regulatory volume decrease. In these settings, septins were required for rapid cortical contraction, and SEPT6-GFP was recruited into filaments and circular patches during global cortical contraction and also specifically during actin filament depletion. Recruitment of septins was also evident during excessive blebbing initiated by blocking membrane trafficking with a dynamin inhibitor, providing further evidence that septins are recruited to facilitate retraction of membranes during dynamic shape change. This function of septins in assembling on an unstable cortex and retracting aberrantly protruding membranes explains the excessive blebbing and protrusion observed in septin-deficient T cells
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