20,023 research outputs found
The diamagnetism above the superconducting transition in underdoped La(1.9)Sr(0.1)CuO(4) revisited: Chemical disorder or phase incoherent superconductivity?
The interplay between superconducting fluctuations and inhomogeneities
presents a renewed interest due to recent works supporting an anomalous [beyond
the conventional Gaussian-Ginzburg-Landau (GGL) scenario] diamagnetism above Tc
in underdoped cuprates. This conclusion, mainly based in the observation of new
anomalies in the low-field isothermal magnetization curves, is in contradiction
with our earlier results in the underdoped La(1.9)Sr(0.1)CuO(4) [Phys. Rev.
Lett. 84, 3157 (2000)]. These seemingly intrinsic anomalies are being presented
in various influential works as a 'thermodynamic evidence' for phase incoherent
superconductivity in the pseudogap regime, this last being at present a central
and debated issue of the cuprate superconductors' physics. Here we have
extended our magnetization measurements in La(1.9)Sr(0.1)CuO(4) to two samples
with different chemical disorder, in one of them close to the one associated
with the random distribution of Sr ions. For this sample, the corresponding
Tc-distribution may be approximated as symmetric around the average Tc, while
in the most disordered sample is strongly asymmetric. The comparison between
the magnetization measured in both samples provides a crucial check of the
chemical disorder origin of the observed diamagnetism anomalies, which are
similar to those claimed as due to phase fluctuations by other authors. This
conclusion applies also to the sample affected only by the intrinsic-like
chemical disorder, providing then a further check that the intrinsic
diamagnetism above the superconducting transition of underdoped cuprates is not
affected by the opening of a pseudogap in the normal state. It is also shown
here that once these disorder effects are overcome, the remaining precursor
diamagnetism may be accounted at a quantitative level in terms of the GGL
approach under a total energy cutoff.Comment: 13 pages, 7 figures. Minor corrections include
Tensor network states and algorithms in the presence of a global SU(2) symmetry
The benefits of exploiting the presence of symmetries in tensor network
algorithms have been extensively demonstrated in the context of matrix product
states (MPSs). These include the ability to select a specific symmetry sector
(e.g. with a given particle number or spin), to ensure the exact preservation
of total charge, and to significantly reduce computational costs. Compared to
the case of a generic tensor network, the practical implementation of
symmetries in the MPS is simplified by the fact that tensors only have three
indices (they are trivalent, just as the Clebsch-Gordan coefficients of the
symmetry group) and are organized as a one-dimensional array of tensors,
without closed loops. Instead, a more complex tensor network, one where tensors
have a larger number of indices and/or a more elaborate network structure,
requires a more general treatment. In two recent papers, namely (i) [Phys. Rev.
A 82, 050301 (2010)] and (ii) [Phys. Rev. B 83, 115125 (2011)], we described
how to incorporate a global internal symmetry into a generic tensor network
algorithm based on decomposing and manipulating tensors that are invariant
under the symmetry. In (i) we considered a generic symmetry group G that is
compact, completely reducible and multiplicity free, acting as a global
internal symmetry. Then in (ii) we described the practical implementation of
Abelian group symmetries. In this paper we describe the implementation of
non-Abelian group symmetries in great detail and for concreteness consider an
SU(2) symmetry. Our formalism can be readily extended to more exotic symmetries
associated with conservation of total fermionic or anyonic charge. As a
practical demonstration, we describe the SU(2)-invariant version of the
multi-scale entanglement renormalization ansatz and apply it to study the low
energy spectrum of a quantum spin chain with a global SU(2) symmetry.Comment: 32 pages, 37 figure
Asymptotic entanglement capacity of the Ising and anisotropic Heisenberg interactions
We compute the asymptotic entanglement capacity of the Ising interaction ZZ,
the anisotropic Heisenberg interaction XX + YY, and more generally, any
two-qubit Hamiltonian with canonical form K = a XX + b YY. We also describe an
entanglement assisted classical communication protocol using the Hamiltonian K
with rate equal to the asymptotic entanglement capacity.Comment: 5 pages, 1 figure; minor corrections, conjecture adde
Scattering of surface plasmons by one-dimensional periodic nanoindented surfaces
In this work, the scattering of surface plasmons by a finite periodic array
of one-dimensional grooves is theoretically analyzed by means of a modal
expansion technique. We have found that the geometrical parameters of the array
can be properly tuned to achieve optimal performance of the structure either as
a Bragg reflector or as a converter of surface plasmons into light. In this
last case, the emitted light is collimated within a few degrees cone.
Importantly, we also show that a small number of indentations in the array are
sufficient to fully achieve its functional capabilities.Comment: 5 pages, 5 figures; changed sign convention in some definition
Numerical study of the hard-core Bose-Hubbard Model on an Infinite Square Lattice
We present a study of the hard-core Bose-Hubbard model at zero temperature on
an infinite square lattice using the infinite Projected Entangled Pair State
algorithm [Jordan et al., Phys. Rev. Lett. 101, 250602 (2008)]. Throughout the
whole phase diagram our values for the ground state energy, particle density
and condensate fraction accurately reproduce those previously obtained by other
methods. We also explore ground state entanglement, compute two-point
correlators and conduct a fidelity-based analysis of the phase diagram.
Furthermore, for illustrative purposes we simulate the response of the system
when a perturbation is suddenly added to the Hamiltonian.Comment: 8 pages, 6 figure
Observation of anisotropic diamagnetism above the superconducting transition in iron-pnictide Ba_(1-x)K_xFe2As2 single crystals due to thermodynamic fluctuations
High resolution magnetization measurements performed in a high quality
Ba_(1-x)K_xFe2As2 single crystal allowed to determine the diamagnetism induced
above the superconducting transition by thermally activated Cooper pairs. These
data, obtained with magnetic fields applied along and transverse to the crystal
ab layers, demonstrate experimentally that the superconducting transition of
iron pnictides may be explained at a phenomenological level in terms of the
Gaussian Ginzburg-Landau approach for three-dimensional anisotropic
superconductors.Comment: Final version with minor corrections. 6 pages, 4 figure
Non-linear optics with two trapped atoms
We show theoretically that two atomic dipoles in a resonator constitute a
non-linear medium, whose properties can be controlled through the relative
position of the atoms inside the cavity and the detuning and intensity of the
driving laser. We identify the parameter regime where the system operates as a
parametric amplifier, based on the cascade emission of the collective dipole of
the atoms, and determine the corresponding spectrum of squeezing of the field
at the cavity output. This dynamics could be observed as a result of
self-organization of laser-cooled atoms in resonators.Comment: 11 pages, 8 figure
Quantum mechanical analysis of the elastic propagation of electrons in the Au/Si system: application to Ballistic Electron Emission Microscopy
We present a Green's function approach based on a LCAO scheme to compute the
elastic propagation of electrons injected from a STM tip into a metallic film.
The obtained 2D current distribution in real and reciprocal space furnish a
good representation of the elastic component of Ballistic Electron Emission
Microscopy (BEEM) currents. Since this component accurately approximates the
total current in the near threshold region, this procedure allows --in contrast
to prior analyses-- to take into account effects of the metal band structure in
the modeling of these experiments. The Au band structure, and in particular its
gaps appearing in the [111] and [100] directions provides a good explanation
for the previously irreconcilable results of nanometric resolution and
similarity of BEEM spectra on both Au/Si(111) and Au/Si(100).Comment: 12 pages, 9 postscript figures, revte
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