44 research outputs found
Stability of metallic stripes in the extended one-band Hubbard model
Based on an unrestricted Gutzwiller approximation (GA) we investigate the
stripe orientation and periodicity in an extended one-band Hubbard model. A
negative ratio between next-nearest and nearest neighbor hopping t'/t, as
appropriate for cuprates, favors partially filled (metallic) stripes for both
vertical and diagonal configurations. At around optimal doping diagonal
stripes, site centered (SC) and bond centered (BC) vertical stripes become
degenerate suggesting strong lateral and orientational fluctuations. We find
that within the GA the resulting phase diagram is in agreement with experiment
whereas it is not in the Hartree-Fock approximation due to a strong
overestimation of the stripe filling. Results are in agreement with previous
calculations within the three-band Hubbard model but with the role of SC and BC
stripes interchanged.Comment: 10 pages, 8 figure
Stripes in cuprate superconductors: Excitations and dynamic dichotomy
We present a short account of the present experimental situation of stripes
in cuprates followed by a review of our present understanding of their ground
state and excited state properties. Collective modes, the dynamical structure
factor, and the optical conductivity of stripes are computed using the
time-dependent Gutzwiller approximation applied to realistic one band and three
band Hubbard models, and are found to be in excellent agreement with
experiment. On the other hand, experiments like angle-resolved photoemission
and scanning tunneling microscopy show the coexistence of stripes at high
energies with Fermi liquid quasiparticles at low energies. We show that a
phenomenological model going beyond mean-field can reconcile this dynamic
dichotomy.Comment: 20 pages, 14 figures. Review paper for a Special Issue of Physica C
on "Stripes and Electronic Liquid Crystals in Strongly Correlated Systems
Stripes and Superconductivity in Cuprates
Holes doped into the CuO2 planes of cuprate parent compounds frustrate the
antiferromagnetic order. The development of spin and charge stripes provides a
compromise between the competing magnetic and kinetic energies. Static stripe
order has been observed only in certain particular compounds, but there are
signatures which suggest that dynamic stripe correlations are common in the
cuprates. Though stripe order is bad for superconducting phase coherence,
stripes are compatible with strong pairing. Ironically, magnetic-field-induced
stripe order appears to enhance the stability of superconducting order within
the planes.Comment: 6 pages, submitted to proceedings of ECRYS-201
A Flavor Symmetry Model for Bilarge Leptonic Mixing and the Lepton Masses
We present a model for leptonic mixing and the lepton masses based on flavor
symmetries and higher-dimensional mass operators. The model predicts bilarge
leptonic mixing (i.e., the mixing angles theta_{12} and theta_{23} are large
and the mixing angle theta_{13} is small) and an inverted hierarchical neutrino
mass spectrum. Furthermore, it approximately yields the experimental
hierarchical mass spectrum of the charged leptons. The obtained values for the
leptonic mixing parameters and the neutrino mass squared differences are all in
agreement with atmospheric neutrino data, the Mikheyev--Smirnov--Wolfenstein
large mixing angle solution of the solar neutrino problem, and consistent with
the upper bound on the reactor mixing angle. Thus, we have a large, but not
close to maximal, solar mixing angle theta_{12}, a nearly maximal atmospheric
mixing angle theta_{23}, and a small reactor mixing angle theta_{13}. In
addition, the model predicts theta_{12} ~= pi/4 - theta_{13}.Comment: 41 pages, 4 figures, Elsevier LaTeX. Final version to be published in
Nucl. Phys.
Understanding high pressure hydrogen with a hierarchical machine-learned potential
The hydrogen phase diagram has a number of unusual features which are
generally well reproduced by density functional calculations. Unfortunately,
these calculations fail to provide good physical insights into why those
features occur. In this paper, we parameterize a model potential for molecular
hydrogen which permits long and large simulations. The model shows excellent
reproduction of the phase diagram, including the broken-symmetry Phase II, an
efficiently-packed phase III and the maximum in the melt curve. It also gives
an excellent reproduction of the vibrational frequencies, including the maximum
in the vibrational frequency and negative thermal expansion. By
detailed study of lengthy molecular dynamics, we give intuitive explanations
for observed and calculated properties. All solid structures approximate to
hexagonal close packed, with symmetry broken by molecular orientation. At high
pressure, Phase I shows significant short-ranged correlations between molecular
orientations. The turnover in Raman frequency is due to increased coupling
between neighboring molecules, rather than weakening of the bond. The liquid is
denser than the close-packed solid because, at molecular separations below
2.3\AA, the favoured relative orientation switches from
quadrupole-energy-minimising to steric-repulsion-minimising. The latter allows
molecules to get closer together, without atoms getting closer but this cannot
be achieved within the constraints of a close-packed layer
Magnetic interactions in iron superconductors: A review
High temperature superconductivity in iron pnictides and chalcogenides
emerges when a magnetic phase is suppressed. The multi-orbital character and
the strength of correlations underlie this complex phenomenology, involving
magnetic softness and anisotropies, with Hund's coupling playing an important
role. We review here the different theoretical approaches used to describe the
magnetic interactions in these systems. We show that taking into account the
orbital degree of freedom allows us to unify in a single phase diagram the main
mechanisms proposed to explain the (\pi,0) order in iron pnictides: the
nesting-driven, the exchange between localized spins, and the Hund induced
magnetic state with orbital differentiation. Comparison of theoretical
estimates and experimental results helps locate the Fe superconductors in the
phase diagram. In addition, orbital physics is crucial to address the magnetic
softness, the doping dependent properties, and the anisotropies.Comment: Invited review article for a focus issue of Comptes Rendus Physique:
26 pages, 10 figures. Revised version, as accepted. Small changes throughout
the text plus new subsection (Sec. IIIE
Nanosecond x-ray diffraction from polycrystalline and amorphous materials in a pinhole camera geometry suitable for laser shock compression experiments.
Nanosecond pulses of quasimonochromatic x-rays emitted from the K shell of ions within a laser-produced plasma are of sufficient spectral brightness to allow single-shot recording of powder diffraction patterns from thin foils of order millimeter diameter. Strong diffraction signals have been observed in a cylindrical pinhole camera arrangement from both polycrystalline and amorphous foils, and the experimental arrangement and foil dimensions are such that they allow for laser shocking or quasi-isentropic loading of the foil during the diffraction process
In situ diffraction measurements of lattice response due to shock loading, including direct observation of the alpha-epsilon phase transition in iron
In situ diffraction is a technique to probe directly the lattice response of materials during the shock loading process. It is used to record diffraction patterns from multiple lattice planes simultaneously. The application of this technique is described for laser-based shock experiments. The approach to analyze in situ wide-angle diffraction data is discussed. This is presented in the context of single crystal [001] iron shock experiments where uniaxial compression of the bee lattice by up to 6% was observed. Above the alpha-epsilon transition pressure, the lattice showed a collapse along the [001] direction by 15-18%. Additional diffraction lines appear that confirm the transformation of the iron crystal from the initial bee phase to the hcp phase. (C) 2006 Elsevier Ltd. All rights reserved