15,760 research outputs found
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
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
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
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
Perturbative QCD Forbidden Charmonium Decays and Gluonia
We address the problem of observed charmonium decays which should be
forbidden in perturbative QCD. We examine the model in which these decays
proceed through a gluonic component of the and the , arising
from a mixing of and glueball states. We give some bounds on the
values of the mixing angles and propose the study of the reaction, at GeV, as an independent test of the
model.Comment: 8pages, lateX, DFTT 64-9
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
Open Luttinger liquids
We study the problem of Luttinger liquids interacting with an active
environment. We are particularly interested in how dissipation affects the
response and correlation functions of non-isolated Luttinger liquids. We show
that the exchange of particles, energy, and momentum lead to changes in the
exponents characterizing the various correlations functions. We discuss the
importance of the zero mode physics in this context.Comment: Revtex, 4 pages. Final version published in PR
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
The 1989 and 2015 outbursts of V404 Cygni: a global study of wind-related optical features
The black hole transient V404 Cygni exhibited a bright outburst in June 2015
that was intensively followed over a wide range of wavelengths. Our team
obtained high time resolution optical spectroscopy (~90 s), which included a
detailed coverage of the most active phase of the event. We present a database
consisting of 651 optical spectra obtained during this event, that we combine
with 58 spectra gathered during the fainter December 2015 sequel outburst, as
well as with 57 spectra from the 1989 event. We previously reported the
discovery of wind-related features (P-Cygni and broad-wing line profiles)
during both 2015 outbursts. Here, we build diagnostic diagrams that enable us
to study the evolution of typical emission line parameters, such as line fluxes
and equivalent widths, and develop a technique to systematically detect outflow
signatures. We find that these are present throughout the outburst, even at
very low optical fluxes, and that both types of outflow features are observed
simultaneously in some spectra, confirming the idea of a common origin. We also
show that the nebular phases depict loop patterns in many diagnostic diagrams,
while P-Cygni profiles are highly variable on time-scales of minutes. The
comparison between the three outbursts reveals that the spectra obtained during
June and December 2015 share many similarities, while those from 1989 exhibit
narrower emission lines and lower wind terminal velocities. The diagnostic
diagrams presented in this work have been produced using standard measurement
techniques and thus may be applied to other active low-mass X-ray binaries.Comment: Accepted for publication in MNRAS. 23 pages paper, plus a 9 pages
appendix with extra tables and figures. 18 figures are included in the paper
and 8 in the appendi
Effect of Coulomb interactions on the physical observables of graphene
We give an update of the situation concerning the effect of electron-electron
interactions on the physics of a neutral graphene system at low energies. We
revise old renormalization group results and the use of 1/N expansion to
address questions of the possible opening of a low-energy gap, and the
magnitude of the graphene fine structure constant. We emphasize the role of
Fermi velocity as the only free parameter determining the transport and
electronic properties of the graphene system and revise its renormalization by
Coulomb interactions in the light of recent experimental evidence.Comment: Proceedings of the Nobel Symposium on graphene 2010, to appear as a
special issue in Physica Script
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