2,734 research outputs found
Superfluidity of metastable bulk glass para-hydrogen at low temperature
Molecular para-hydrogen has been proposed theoretically as a possible
candidate for superfluidity, but the eventual superfluid transition is hindered
by its crystallization. In this work, we study a metastable non crystalline
phase of bulk p-H2 by means of the Path Integral Monte Carlo method in order to
investigate at which temperature this system can support superfluidity. By
choosing accurately the initial configuration and using a non commensurate
simulation box, we have been able to frustrate the formation of the crystal in
the simulated system and to calculate the temperature dependence of the
one-body density matrix and of the superfluid fraction. We observe a transition
to a superfluid phase at temperatures around 1 K. The limit of zero temperature
is also studied using the diffusion Monte Carlo method. Results for the energy,
condensate fraction, and structure of the metastable liquid phase at T=0 are
reported and compared with the ones obtained for the stable solid phase.Comment: 10 pages, accepted for publication in Phys. Rev.
Origin of Orthorhombic Transition, Magnetic Transition, and Shear Modulus Softening in Iron Pnictide Superconductors: Analysis based on the Orbital Fluctuation Theory
The main features in iron-pnictide superconductors are summarized as (i) the
orthorhombic transition accompanied by remarkable softening of shear modulus,
(ii) high-Tc superconductivity close to the orthorhombic phase, and (iii)
stripe-type magnetic order induced by orthorhombicity. To present a unified
explanation for them, we analyze the multiorbital Hubbard-Holstein model with
Fe-ion optical phonons based on the orbital fluctuation theory. In the
random-phase-approximation (RPA), a small electron-phonon coupling constant
() is enough to produce large orbital (=charge quadrupole)
fluctuations. The most divergent susceptibility is the
-antiferro-quadrupole (AFQ) susceptibility, which causes the s-wave
superconductivity without sign reversal (s_{++}-wave state). At the same time,
divergent development of -ferro-quadrupole (FQ) susceptibility is
brought by the "two-orbiton process" with respect to the AFQ fluctuations,
which is absent in the RPA. The derived FQ fluctuations cause the softening of
shear modulus, and its long-range-order not only triggers the
orthorhombic structure transition, but also induces the instability of
stripe-type antiferro-magnetic state. In other words, the condensation of
composite bosons made of two orbitons gives rise to the FQ order and structure
transition. The theoretically predicted multi-orbital-criticality presents a
unified explanation for abovementioned features of iron pnictide
superconductors.Comment: 19 pages, 15 figure
Light vector meson photoproduction at large t
We have studied in perturbative QCD all independent helicity amplitudes
describing the photoproduction of light vector mesons at large . We found a
new hard production mechanism which is related to the possibility for a real
photon to fluctuate into a massless pair in a chiral-odd spin
configuration. Each helicity amplitude is given as a sum of a usual chiral-even
contribution (when the helicities of quark and antiquark are antiparallel) and
this additional chiral-odd part (where the helicities of quark and antiquark
are parallel). The chiral-odd contribution is large, it leads to a dominance of
the non spin-flip amplitude in a very broad region of intermediately high
. All amplitudes are expressed in terms of short distance asymptotics of
the light-cone wave functions of vector meson (photon). We demonstrate that for
each helicity amplitude there exists a soft non-factorizable contribution. We
give arguments that for dominant non spin-flip helicity amplitude the relative
contribution of the soft nonfactorizable interactions is numerically not large.Comment: 21 pages LATEX, 2 .eps figures, the colour factor of chiral-odd
contributions is corrected, qualitative results remain unchange
The effect of electron beam pitch angle and density gradient on solar type III radio bursts
Copyright 2012 American Institute of Physics. This article may be downloaded for personal use only. Any other use requires prior permission of the author and the American Institute of Physics. This article appeared in Physics of Plasmas 19, 112903 (2012) and may be found at .supplemental material at http://astro.qmul.ac.uk/~tsiklauri/sp.htmlsupplemental material at http://astro.qmul.ac.uk/~tsiklauri/sp.htm
Observation of Coherently-Enhanced Tunable Narrow-Band Terahertz Transition Radiation from a Relativistic Sub-Picosecond Electron Bunch Train
We experimentally demonstrate the production of narrow-band (% at THz) THz transition radiation with tunable
frequency over [0.37, 0.86] THz. The radiation is produced as a train of
sub-picosecond relativistic electron bunches transits at the vacuum-aluminum
interface of an aluminum converter screen. We also show a possible application
of modulated beams to extend the dynamical range of a popular bunch length
diagnostic technique based on the spectral analysis of coherent radiation.Comment: 3 pages, 6 figure
Vortex Origin of Tricritical Point in Ginzburg-Landau Theory
Motivated by recent experimental progress in the critical regime of
high- superconductors we show how the tricritical point in a
superconductor can be derived from the Ginzburg-Landau theory as a consequence
of vortex fluctuations. Our derivation explains why usual renormalization group
arguments always produce a first-order transition, in contrast to experimental
evidence and Monte Carlo simulations.Comment: 4 pages,1 figur
Effective Potential for Scalar Field in Three Dimensions: Ising Model in the Ferromagnetic Phase
We compute the effective potential for one-component real
scalar field in three Euclidean dimensions (3D) in the case of
spontaneously broken symmetry, from the Monte Carlo simulation of the 3D Ising
model in external field at temperatures approaching the phase transition from
below. We study probability distributions of the order parameter on the
lattices from to , at . We find that, in close
analogy with the symmetric case, plays an important role: is very well approximated by the sum of , and
terms. An unexpected feature is the negative sign of the
term. As close to the continuum limit as we can get (), we
obtain
We also compute several universal coupling constants and ratios, including
the combination of critical amplitudes .Comment: 13 pages, 5 Postscript figures, uses epsf.st
Experimental study of magneto-superconductor RuSr2Eu1.5Ce0.5Cu2O10: Effect of Mo doping on magnetic behavior and Tc variation
Mo doped ruthenocuprates Ru1-xMoxSr2Eu1.5Ce0.5Cu2O10 are synthesized for x =
0.0, 0.2, 0.4, 0.6, 0.8 and 1.0, and their magnetic and superconducting
properties are studied. It has been found that the magnetic transition
temperature TZFCpeak, which corresponds to the appearance of weak ferromagnetic
effect, decreases from its value of 75 K for x = 0.0 to 22 K, 25 K and 18 K,
respectively for the x = 0.2, 0.4 and 0.6 samples. Another finding is that the
magnetic susceptibility reduces at TZFCpeak by a factor of about 6, 85 and 413
for x = 0.2, 0.4, and 0.6 respectively. The samples of x = 0.8 and 1.0 are
found to have no magnetic or superconducting effects. The values of the
superconducting transition temperature are obtained from the resistivity versus
temperature data. An important result is that Tc increases by 4.5 K and 7.0 K
for x = 0.2 and 0.4 respectively, and then decreases by 17 K for x = 0.6. The
observed variation of Tc with x has been explained in terms of a theory which
combines the effects of weakening magnetic behavior and reducing carrier
concentration in a phenomenological manner. The resulting theory is found to
provide a good agreement with the observed value of Tc.Comment: 14 pages with Text + Figs. To Appear in PHYS. REV. B, Ist Jan. 2006
issu
Hyperk\"ahler Arnold Conjecture and its Generalizations
We generalize and refine the hyperk\"ahler Arnold conjecture, which was
originally established, in the non-degenerate case, for three-dimensional time
by Hohloch, Noetzel and Salamon by means of hyperk\"ahler Floer theory. In
particular, we prove the conjecture in the case where the time manifold is a
multidimensional torus and also establish the degenerate version of the
conjecture. Our method relies on Morse theory for generating functions and a
finite-dimensional reduction along the lines of the Conley-Zehnder proof of the
Arnold conjecture for the torus.Comment: 13 page
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