15,575 research outputs found
Super Five Brane Hamiltonian and the Chiral Degrees of Freedom
We construct the Hamiltonian of the super five brane in terms of its physical
degrees of freedom. It does not depend on the inverse of the induced metric.
Consequently, some singular configurations are physically admissible, implying
an interpretation of the theory as a multiparticle one. The symmetries of the
theory are analyzed from the canonical point of view in terms of the first and
second class constraints. In particular it is shown how the chiral sector may
be canonically reduced to its physical degrees of freedom.Comment: 16 pages, typos correcte
Minimum Conductivity and Evidence for Phase Transitions in Ultra-clean Bilayer Graphene
Bilayer graphene (BLG) at the charge neutrality point (CNP) is strongly
susceptible to electronic interactions, and expected to undergo a phase
transition into a state with spontaneous broken symmetries. By systematically
investigating a large number of singly- and doubly-gated bilayer graphene (BLG)
devices, we show that an insulating state appears only in devices with high
mobility and low extrinsic doping. This insulating state has an associated
transition temperature Tc~5K and an energy gap of ~3 meV, thus strongly
suggesting a gapped broken symmetry state that is destroyed by very weak
disorder. The transition to the intrinsic broken symmetry state can be tuned by
disorder, out-of-plane electric field, or carrier density
Suppression of static stripe formation by next-neighbor hopping
We show from real-space Hartree-Fock calculations within the extended Hubbard
model that next-nearest neighbor (t') hopping processes act to suppress the
formation of static charge stripes. This result is confirmed by investigating
the evolution of charge-inhomogeneous corral and stripe phases with increasing
t' of both signs. We propose that large t' values in YBCO prevent static stripe
formation, while anomalously small t' in LSCO provides an additional reason for
the appearance of static stripes only in these systems.Comment: 4 pages, 5 figure
Evidence for Fermi surface reconstruction in the static stripe phase of LaEuSrCuO,
We present a photoemission study of LaEuSrCuO
with doping level =1/8, where the charge carriers are expected to order
forming static stripes. Though the local probes in direct space seem to be
consistent with this idea, there has been little evidence found for such
ordering in quasiparticle dispersions. We show that the Fermi surface topology
of the 1/8 compound develops notable deviations from that observed for LaSrCuO in a way consistent with the FS reconstruction expected for
the scattering on the antiphase stripe order
Coulomb-driven broken-symmetry states in doubly gated suspended bilayer graphene
The non-interacting energy spectrum of graphene and its bilayer counterpart
consists of multiple degeneracies owing to the inherent spin, valley and layer
symmetries. Interactions among charge carriers are expected to spontaneously
break these symmetries, leading to gapped ordered states. In the quantum Hall
regime these states are predicted to be ferromagnetic in nature whereby the
system becomes spin polarized, layer polarized or both. In bilayer graphene,
due to its parabolic dispersion, interaction-induced symmetry breaking is
already expected at zero magnetic field. In this work, the underlying order of
the various broken-symmetry states is investigated in bilayer graphene that is
suspended between top and bottom gate electrodes. By controllably breaking the
spin and sublattice symmetries we are able to deduce the order parameter of the
various quantum Hall ferromagnetic states. At small carrier densities, we
identify for the first time three distinct broken symmetry states, one of which
is consistent with either spontaneously broken time-reversal symmetry or
spontaneously broken rotational symmetry
Herschel/HIFI observations of molecular emission in protoplanetary nebulae and young planetary nebulae
We performed Herschel/HIFI observations of intermediate-excitation molecular
lines in the far-infrared/submillimeter range in a sample of ten protoplanetary
nebulae and young planetary nebulae. The high spectral resolution provided by
HIFI yields accurate measurements of the line profiles. The observation of
these high-energy transitions allows an accurate study of the excitation
conditions, particularly in the warm gas, which cannot be properly studied from
the low-energy lines.
We have detected FIR/sub-mm lines of several molecules, in particular of
12CO, 13CO, and H2O. Emission from other species, like NH3, OH, H2^{18}O, HCN,
SiO, etc, has been also detected. Wide profiles showing sometimes spectacular
line wings have been found. We have mainly studied the excitation properties of
the high-velocity emission, which is known to come from fast bipolar outflows.
From comparison with general theoretical predictions, we find that CRL 618
shows a particularly warm fast wind, with characteristic kinetic temperature Tk
>~ 200 K. In contrast, the fast winds in OH 231.8+4.2 and NGC 6302 are cold, Tk
~ 30 K. Other nebulae, like CRL 2688, show intermediate temperatures, with
characteristic values around 100 K. We also discuss how the complex structure
of the nebulae can affect our estimates, considering two-component models. We
argue that the differences in temperature in the different nebulae can be due
to cooling after the gas acceleration (that is probably due to shocks); for
instance, CRL 618 is a case of very recent acceleration, less than ~ 100 yr
ago, while the fast gas in OH 231.8+4.2 was accelerated ~ 1000 yr ago. We also
find indications that the densest gas tends to be cooler, which may be
explained by the expected increase of the radiative cooling efficiency with the
density.Comment: 24 pages, 31 figure
Superconducting phase coherence in striped cuprates
We study the problem of phase coherence in doped striped cuprates. We assume
the stripes to form a network of one-dimensional Luttinger liquids which are
dominated by superconducting fluctuations and pinned by impurities. The problem
of phase coherence is discussed. We study the dynamics of the superconducting
phase using a model of resistively shunted junctions which leads to a
Kosterlitz-Thouless transition. We show that our results are consistent with
recent experiments in Zn-doped cuprates. We also explain the scaling of the
superconducting critical temperature with the incommensurability as seen
in recent neutron scattering experiments and predict the behavior of
in the underdoped region.Comment: Final version to appear in Physical Review Letters with a new
reference to an earlier work of F.Guinea and G.Zymanyi on Luttinger network
Effect of disorder on the ground-state properties of graphene
We calculate the ground-state energy of Dirac electrons in graphene in the
presence of disorder. We take randomly distributed charged impurities at a
fixed distance from the graphene sheet and surface fluctuations (ripples) as
the main scattering mechanisms. Mode-coupling approach to scattering rate and
random-phase approximation for ground-state energy incorporating the many-body
interactions and the disorder effects yields good agreement with experimental
inverse compressibility.Comment: Extended introduction and discussion. To appear in Phys. Rev.
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