2,238 research outputs found
On localization effects in underdoped cuprates
We comment on transport experiments in underdoped LaSrCuO in the
non-superconducting phase. The temperature dependence of the resistance
strongly resembles what is expected from standard localization theory. However
this theory fails, when comparing with experiments in more detail.Comment: 8 pages, to be published in J. of Superconductivit
Joint superexchange--Jahn-Teller mechanism for A-type antiferromagnetism in
We propose a mechanism for A-type antiferromagnetism in orthorombic LaMnO_3,
compatible with the large Jahn-Teller splitting inferred from structural data.
Orbital ordering resulting from Jahn-Teller distortions effectively leads to
A-type ordering (antiferromagnetic in the c axis and ferromagnetic in the ab
plane) provided the in-plane distorsion Q_2 is large enough, a condition
generally fulfilled in existing data.Comment: 4 pages Late
The Electron-Phonon Interaction in the Presence of Strong Correlations
We investigate the effect of strong electron-electron repulsion on the
electron-phonon interaction from a Fermi-liquid point of view: the strong
interaction is responsible for vertex corrections, which are strongly dependent
on the ratio. These corrections generically lead to a strong
suppression of the effective coupling between quasiparticles mediated by a
single phonon exchange in the limit. However, such effect
is not present when . Analyzing the Landau stability
criterion, we show that a sizable electron-phonon interaction can push the
system towards a phase-separation instability. A detailed analysis is then
carried out using a slave-boson approach for the infinite-U three-band Hubbard
model. In the presence of a coupling between the local hole density and a
dispersionless optical phonon, we explicitly confirm the strong dependence of
the hole-phonon coupling on the transferred momentum versus frequency ratio. We
also find that the exchange of phonons leads to an unstable phase with negative
compressibility already at small values of the bare hole-phonon coupling. Close
to the unstable region, we detect Cooper instabilities both in s- and d-wave
channels supporting a possible connection between phase separation and
superconductivity in strongly correlated systems.Comment: LateX 3.14, 04.11.1994 Preprint no.101
Optical conductivity near finite-wavelength quantum criticality
We study the optical conductivity sigma(Omega) of an electron system near a
quantum-critical point with finite-wavelength ordering. sigma(Omega) vanishes
in clean Galilean-invariant systems, unless electrons are coupled to dynamical
collective modes, which dissipate the current. This coupling introduces a
nonuniversal energy scale. Depending on the parameters of each specific system,
a variety of responses arise near criticality: scaling peaks at a temperature-
and doping-dependent frequency, peaks at a fixed frequency, or no peaks to be
associated with criticality. Therefore the lack of scaling in the far-infrared
conductivity in cuprates does not necessarily call for new concepts of quantum
criticality.Comment: 4 pages, 4 figures; version as publishe
Disorder effects in the quantum Heisenberg model: An Extended Dynamical mean-field theory analysis
We investigate a quantum Heisenberg model with both antiferromagnetic and
disordered nearest-neighbor couplings. We use an extended dynamical mean-field
approach, which reduces the lattice problem to a self-consistent local impurity
problem that we solve by using a quantum Monte Carlo algorithm. We consider
both two- and three-dimensional antiferromagnetic spin fluctuations and
systematically analyze the effect of disorder. We find that in three dimensions
for any small amount of disorder a spin-glass phase is realized. In two
dimensions, while clean systems display the properties of a highly correlated
spin-liquid (where the local spin susceptibility has a non-integer power-low
frequency and/or temperature dependence), in the present case this behavior is
more elusive unless disorder is very small. This is because the spin-glass
transition temperature leaves only an intermediate temperature regime where the
system can display the spin-liquid behavior, which turns out to be more
apparent in the static than in the dynamical susceptibility.Comment: 15 pages, 7 figure
3D Modeling of the Magnetization of Superconducting Rectangular-Based Bulks and Tape Stacks
In recent years, numerical models have become popular and powerful tools to
investigate the electromagnetic behavior of superconductors. One domain where
this advances are most necessary is the 3D modeling of the electromagnetic
behavior of superconductors. For this purpose, a benchmark problem consisting
of superconducting cube subjected to an AC magnetic field perpendicular to one
of its faces has been recently defined and successfully solved. In this work, a
situation more relevant for applications is investigated: a superconducting
parallelepiped bulk with the magnetic field parallel to two of its faces and
making an angle with the other one without and with a further constraint on the
possible directions of the current. The latter constraint can be used to model
the magnetization of a stack of high-temperature superconductor tapes, which
are electrically insulated in one direction. For the present study three
different numerical approaches are used: the Minimum Electro-Magnetic Entropy
Production (MEMEP) method, the -formulation of Maxwell's equations and the
Volume Integral Method (VIM) for 3D eddy currents computation. The results in
terms of current density profiles and energy dissipation are compared, and the
differences in the two situations of unconstrained and constrained current flow
are pointed out. In addition, various technical issues related to the 3D
modeling of superconductors are discussed and information about the
computational effort required by each model is provided. This works constitutes
a concrete result of the collaborative effort taking place within the HTS
numerical modeling community and will hopefully serve as a stepping stone for
future joint investigations
Hidden Ferronematic Order in Underdoped Cuprates
We study a model for low doped cuprates where holes aggregate into oriented
stripe segments which have a vortex and an antivortex fixed to the extremes. We
argue that due to the interaction between segments a state with macroscopic
polarization is stabilized, which we call a ferronematic. This state can be
characterized as a charge nematic which, due to the net polarization, breaks
inversion symmetry and also exhibits an incommensurate spin modulation. Our
calculation can reproduce the doping dependent spin structure factor of
lanthanum cuprates in excellent agreement with experiment and allows to
rationalize experiments in which the incommensurability has an order
parameter-like temperature dependence.Comment: 5 pages, 4 figure
Dynamical charge and spin density wave scattering in cuprate superconductor
We show that a variety of spectral features in high-T_c cuprates can be
understood from the coupling of charge carriers to some kind of dynamical order
which we exemplify in terms of fluctuating charge and spin density waves. Two
theoretical models are investigated which capture different aspects of such
dynamical scattering. The first approach leaves the ground state in the
disordered phase but couples the electrons to bosonic degrees of freedom,
corresponding to the quasi singular scattering associated with the closeness to
an ordered phase. The second, more phenomological approach starts from the
construction of a frequency dependent order parameter which vanishes for small
energies. Both theories capture scanning tunneling microscopy and angle-resoved
photoemission experiments which suggest the protection of quasiparticles close
to the Fermi energy but the manifestation of long-range order at higher
frequencies.Comment: 27 pages, 13 figures, to appear in New J. Phy
Effectiveness of Climbing Lanes for Slow-Moving Vehicles When Riding Uphill: A Microsimulation Study
Long uphill stretches of single-carriageway rural roads with one lane per travel direction may reduce the Level of Service (LoS), due to the decreased speed of heavy vehicles. In those circumstances, a slowdown of traffic, resulting in the formation of platoons, may be generated due to the difficulty of performing overtaking maneuvers safely. To solve this critical issue, an additional (climbing) lane for slow vehicles may be included in the road platform. This study aims to evaluate the effectiveness of such climbing lanes in a real case in Italy (National Road n. 4 “Via Salaria”— around 44+000 km). Using a microsimulation model implemented in VISSIM, the study analyzes speeds and travel times, delays, and queuing waiting times, comparing the Actual Scenario (AS) without climbing lanes, with two counterfactual scenarios: the first one (CS1) with three stretches of climbing lanes, and the second one (CS2), with just two stretches, in which the first two additional lanes of CS1 are merged together. The obtained results confirm the effectiveness of installing climbing lanes on road sections with the described characteristics, and the potential of microsimulation models also to carry out such kind of evaluations
Wave-breaking and generic singularities of nonlinear hyperbolic equations
Wave-breaking is studied analytically first and the results are compared with
accurate numerical simulations of 3D wave-breaking. We focus on the time
dependence of various quantities becoming singular at the onset of breaking.
The power laws derived from general arguments and the singular behavior of
solutions of nonlinear hyperbolic differential equations are in excellent
agreement with the numerical results. This shows the power of the analysis by
methods using generic concepts of nonlinear science
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