7,536 research outputs found
Multifractality and scale invariance in human heartbeat dynamics
Human heart rate is known to display complex fluctuations. Evidence of
multifractality in heart rate fluctuations in healthy state has been reported
[Ivanov et al., Nature {\bf 399}, 461 (1999)]. This multifractal character
could be manifested as a dependence on scale or beat number of the probability
density functions (PDFs) of the heart rate increments. On the other hand, scale
invariance has been recently reported in a detrended analysis of healthy heart
rate increments [Kiyono et al., Phys. Rev. Lett. {\bf 93}, 178103 (2004)]. In
this paper, we resolve this paradox by clarifying that the scale invariance
reported is actually exhibited by the PDFs of the sum of detrended healthy
heartbeat intervals taken over different number of beats, and demonstrating
that the PDFs of detrended healthy heart rate increments are scale dependent.
Our work also establishes that this scale invariance is a general feature of
human heartbeat dynamics, which is shared by heart rate fluctuations in both
healthy and pathological states
Passive Scalar: Scaling Exponents and Realizability
An isotropic passive scalar field advected by a rapidly-varying velocity
field is studied. The tail of the probability distribution for
the difference in across an inertial-range distance is found
to be Gaussian. Scaling exponents of moments of increase as
or faster at large order , if a mean dissipation conditioned on is
a nondecreasing function of . The computed numerically
under the so-called linear ansatz is found to be realizable. Some classes of
gentle modifications of the linear ansatz are not realizable.Comment: Substantially revised to conform with published version. Revtex (4
pages) with 2 postscript figures. Send email to [email protected]
Excitonic Instabilities and Insulating States in Bilayer Graphene
The competing ground states of bilayer graphene are studied by applying
renormalization group techniques to a bilayer honeycomb lattice with nearest
neighbor hopping. In the absence of interactions, the Fermi surface of this
model at half-filling consists of two nodal points with momenta ,
, where the conduction band and valence band touch each other,
yielding a semi-metal. Since near these two points the energy dispersion is
quadratic with perfect particle-hole symmetry, excitonic instabilities are
inevitable if inter-band interactions are present. Using a perturbative
renormalization group analysis up to the one-loop level, we find different
competing ordered ground states, including ferromagnetism, superconductivity,
spin and charge density wave states with ordering vector
, and excitonic insulator states. In
addition, two states with valley symmetry breaking are found in the excitonic
insulating and ferromagnetic phases. This analysis strongly suggests that the
ground state of bilayer graphene should be gapped, and with the exception of
superconductivity, all other possible ground states are insulating.Comment: 17 pages, 6 figures, 2 Tables, Added reference
Interlayer Exchange Coupling Beyond the Proximity Force Approximation
Ion bombardment has been shown to be capable of enhancing the interlayer
exchange coupling in a trilayer system that exhibits giant magnetoresistance.
We demonstrate that this phenomenon can be derived from the phase coherence
among scattered paths within the two rough interfaces when their topographies
are correlated. In the case of mild corrugations, our method reproduces the
predictions by the proximity force approximation which does not consider the
interference. When the characteristic Fourier conjugate of the tomography
becomes large and comparable to the Fermi momentum, interesting new features
arise and can only be captured by our more general approach. Among our
findings, the scenario of an enhanced interlayer exchange coupling due to the
interface roughness is explained, along with how it depends on the sample
parameters. An additional channel for the resonant transmission is identified
due to extra scattering paths from the roughness.Comment: 9 pages, 7 figures, submitted to PRB (2010
Homotopy nilpotent groups
We study the connection between the Goodwillie tower of the identity and the
lower central series of the loop group on connected spaces. We define the
simplicial theory of homotopy n-nilpotent groups. This notion interpolates
between infinite loop spaces and loop spaces. We prove that the set-valued
algebraic theory obtained by applying is the theory of ordinary
n-nilpotent groups and that the Goodwillie tower of a connected space is
determined by a certain homotopy left Kan extension. We prove that n-excisive
functors of the form have values in homotopy n-nilpotent groups.Comment: 16 pages, uses xy-pic, improved exposition, submitte
Active and Passive Fields in Turbulent Transport: the Role of Statistically Preserved Structures
We have recently proposed that the statistics of active fields (which affect
the velocity field itself) in well-developed turbulence are also dominated by
the Statistically Preserved Structures of auxiliary passive fields which are
advected by the same velocity field. The Statistically Preserved Structures are
eigenmodes of eigenvalue 1 of an appropriate propagator of the decaying
(unforced) passive field, or equivalently, the zero modes of a related
operator. In this paper we investigate further this surprising finding via two
examples, one akin to turbulent convection in which the temperature is the
active scalar, and the other akin to magneto-hydrodynamics in which the
magnetic field is the active vector. In the first example, all the even
correlation functions of the active and passive fields exhibit identical
scaling behavior. The second example appears at first sight to be a
counter-example: the statistical objects of the active and passive fields have
entirely different scaling exponents. We demonstrate nevertheless that the
Statistically Preserved Structures of the passive vector dominate again the
statistics of the active field, except that due to a dynamical conservation law
the amplitude of the leading zero mode cancels exactly. The active vector is
then dominated by the sub-leading zero mode of the passive vector. Our work
thus suggests that the statistical properties of active fields in turbulence
can be understood with the same generality as those of passive fields.Comment: 13 pages, 13 figures, submitted to Phys. Rev.
Co-expression of Gbeta 5 Enhances the Function of Two Ggamma Subunit-like Domain-containing Regulators of G Protein Signaling Proteins
Regulators of G protein signaling (RGS) stimulate the GTPase activity of G protein Galpha subunits and probably play additional roles. Some RGS proteins contain a Ggamma subunit-like (GGL) domain, which mediates a specific interaction with Gbeta 5. The role of such interactions in RGS function is unclear. RGS proteins can accelerate the kinetics of coupling of G protein-coupled receptors to G-protein-gated inwardly rectifying K+ (GIRK) channels. Therefore, we coupled m2-muscarinic acetylcholine receptors to GIRK channels in Xenopus oocytes to evaluate the effect of Gbeta 5 on RGS function. Co-expression of either RGS7 or RGS9 modestly accelerated GIRK channel kinetics. When Gbeta 5 was co-expressed with either RGS7 or RGS9, the acceleration of GIRK channel kinetics was strongly increased over that produced by RGS7 or RGS9 alone. RGS function was not enhanced by co-expression of Gbeta 1, and co-expression of Gbeta 5 alone had no effect on GIRK channel kinetics. Gbeta 5 did not modulate the function either of RGS4, an RGS protein that lacks a GGL domain, or of a functional RGS7 construct in which the GGL domain was omitted. Enhancement of RGS7 function by Gbeta 5 was not a consequence of an increase in the amount of plasma membrane or cytosolic RGS7 protein
- …