3,604 research outputs found
Interplay of Spin and Lattice Degrees of Freedom in the Frustrated Antiferromagnet CdCr_2O_4: High-field and Temperature Induced Anomalies of the Elastic Constants
Temperature and magnetic field studies of the elastic constants of the
chromium spinel CdCr_2O_4 show pronounced anomalies related to strong
spin-phonon coupling in this frustrated antiferromagnet. A detailed comparison
of the longitudinal acoustic mode propagating along the [111] direction with
theory based on an exchange-striction mechanism leads to an estimate of the
strength of the magneto-elastic interaction. The derived spin-phonon coupling
constant is in good agreement with previous determinations based on infrared
absorption. Further insight is gained from intermediate and high magnetic field
experiments in the field regime of the magnetization plateau. The role of the
antisymmetric Dzyaloshinskii-Moriya interaction discussed and we compare the
spin-phonon coupling in CdCr_2O_4 in both the ordered and disordered states.Comment: 12 pages, 8 figures; Appendix added,To appear in Phys Rev.
Rigidity of escaping dynamics for transcendental entire functions
We prove an analog of Boettcher's theorem for transcendental entire functions
in the Eremenko-Lyubich class B. More precisely, let f and g be entire
functions with bounded sets of singular values and suppose that f and g belong
to the same parameter space (i.e., are *quasiconformally equivalent* in the
sense of Eremenko and Lyubich). Then f and g are conjugate when restricted to
the set of points which remain in some sufficiently small neighborhood of
infinity under iteration. Furthermore, this conjugacy extends to a
quasiconformal self-map of the plane.
We also prove that this conjugacy is essentially unique. In particular, we
show that an Eremenko-Lyubich class function f has no invariant line fields on
its escaping set.
Finally, we show that any two hyperbolic Eremenko-Lyubich class functions f
and g which belong to the same parameter space are conjugate on their sets of
escaping points.Comment: 28 pages; 2 figures. Final version (October 2008). Various
modificiations were made, including the introduction of Proposition 3.6,
which was not formally stated previously, and the inclusion of a new figure.
No major changes otherwis
Optimal Radio Window for the Detection of Ultra-High-Energy Cosmic Rays and Neutrinos off the Moon
When high-energy cosmic rays impinge on a dense dielectric medium, radio
waves are produced through the Askaryan effect. We show that at wavelengths
comparable to the length of the shower produced by an Ultra-High Energy cosmic
ray or neutrino, radio signals are an extremely efficient way to detect these
particles. Through an example it is shown that this new approach offers, for
the first time, the realistic possibility of measuring UHE neutrino fluxes
below the Waxman-Bahcall limit. It is shown that in only one month of observing
with the upcoming LOFAR radio telescope, cosmic-ray events can be measured
beyond the GZK-limit, at a sensitivity level of two orders of magnitude below
the extrapolated values.Comment: Submitted to Astroparticle Physic
Tunneling conductance in strained graphene-based superconductor: Effect of asymmetric Weyl-Dirac fermions
Based on the BTK theory, we investigate the tunneling conductance in a
uniaxially strained graphene-based normal metal (NG)/ barrier
(I)/superconductor (SG) junctions. In the present model, we assume that
depositing the conventional superconductor on the top of the uniaxially
strained graphene, normal graphene may turn to superconducting graphene with
the Cooper pairs formed by the asymmetric Weyl-Dirac electrons, the massless
fermions with direction-dependent velocity. The highly asymmetrical velocity,
vy/vx>>1, may be created by strain in the zigzag direction near the transition
point between gapless and gapped graphene. In the case of the highly
asymmetrical velocity, we find that the Andreev reflection strongly depends on
the direction and the current perpendicular to the direction of strain can flow
in the junction as if there was no barrier. Also, the current parallel to the
direction of strain anomalously oscillates as a function of the gate voltage
with very high frequency. Our predicted result is found as quite different from
the feature of the quasiparticle tunneling in the unstrained graphene-based
NG/I/SG conventional junction. This is because of the presence of the
direction-dependent-velocity quasiparticles in the highly strained graphene
system.Comment: 18 pages, 7 Figures; Eq.13 and 14 are correcte
The distribution function of a semiflexible polymer and random walks with constraints
In studying the end-to-end distribution function of a worm like
chain by using the propagator method we have established that the combinatorial
problem of counting the paths contributing to can be mapped onto the
problem of random walks with constraints, which is closely related to the
representation theory of the Temperley-Lieb algebra. By using this mapping we
derive an exact expression of the Fourier-Laplace transform of the distribution
function, , as a matrix element of an inverse of an infinite rank
matrix. Using this result we also derived a recursion relation permitting to
compute directly. We present the results of the computation of
and its moments. The moments of can be
calculated \emph{exactly} by calculating the (1,1) matrix element of -th
power of a truncated matrix of rank .Comment: 6 pages, 2 figures, added a referenc
Critical dynamics of an isothermal compressible non-ideal fluid
A pure fluid at its critical point shows a dramatic slow-down in its
dynamics, due to a divergence of the order-parameter susceptibility and the
coefficient of heat transport. Under isothermal conditions, however, sound
waves provide the only possible relaxation mechanism for order-parameter
fluctuations. Here we study the critical dynamics of an isothermal,
compressible non-ideal fluid via scaling arguments and computer simulations of
the corresponding fluctuating hydrodynamics equations. We show that, below a
critical dimension of 4, the order-parameter dynamics of an isothermal fluid
effectively reduces to "model A," characterized by overdamped sound waves and a
divergent bulk viscosity. In contrast, the shear viscosity remains finite above
two dimensions. Possible applications of the model are discussed.Comment: 19 pages, 7 figures; v3: minor corrections and clarifications; as
published in Phys. Rev.
Tau Neutrinos in the Auger Observatory : A New Window to UHECR Sources
The cosmic ray spectrum has been shown to extend well beyond 10^{20}eV. With
nearly 20 events observed in the last 40 years, it is now established that
particles are accelerated or produced in the universe with energies near
10^{21}eV. In all production models neutrinos and photons are part of the
cosmic ray flux. In acceleration models (bottom-up models), they are produced
as secondaries of the possible interactions of the accelerated charged
particle; in direct production models (top-down models) they are a dominant
fraction of the decay chain. In addition, hadrons above the GZK threshold
energy will also produce, along their path in the Universe, neutrinos and
photons as secondaries of the pion photo-production processes. Therefore,
photons and neutrinos are very distinctive signatures of the nature and
distribution of the potential sources of ultra high energy cosmic rays. In the
following we describe the tau neutrino detection and identification
capabilities of the Auger observatory. We show that in the range
3x10^{17}-3x10^{20}eV the Auger effective apperture reaches a few tenths of
km^2.sr, making the observatory sensitive to fluxes as low as a few tau
neutrinos per km^2.sr.year. In the hypothesis of nu_mu nu_tau oscillations
with full mixing, this sensitivity allows to probe the GZK cutoff as well as to
provide model independent constraints on the mechanisms of production of ultra
high energy cosmic rays.Comment: 10 pages, 11 figures, accepted by Astroparticle physic
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