150 research outputs found
Effect of dopant atoms on local superexchange in cuprate superconductors: a perturbative treatment
Recent scanning tunneling spectroscopy experiments have provided evidence
that dopant impurities in high- Tc superconductors can strongly modify the
electronic structure of the CuO2 planes nearby, and possibly influence the
pairing. To investigate this connection, we calculate the local magnetic
superexchange J between Cu ions in the presence of dopants within the framework
of the three-band Hubbard model, up to fifth-order in perturbation theory. We
demonstrate that the sign of the change in J depends on the relative
dopant-induced spatial variation of the atomic levels in the CuO2 plane,
contrary to results obtained within the one-band Hubbard model. We discuss some
realistic cases and their relevance for theories of the pairing mechanism in
the cupratesComment: 5 pages, 4 figures, revised versio
Analysis of spin density wave conductivity spectra of iron pnictides in the framework of density functional theory
The optical conductivity of LaFeAsO, BaFeAs, SrFeAs, and
EuFeAs in the spin-density wave (SDW) state is investigated within
density functional theory (DFT) in the framework of spin-polarized generalized
gradient approximation (GGA) and GGA+U. We find a strong dependence of the
optical features on the Fe magnetic moments. In order to recover the small Fe
magnetic moments observed experimentally, GGA+ with a suitable
choice of negative on-site interaction was considered. Such
an approach may be justified in terms of an overscreening which induces a
relatively small U compared to the Hund's rule coupling J, as well as a strong
Holstein-like electron-phonon interaction. Moreover, reminiscent of the fact
that GGA+ with a positive is a simple approximation
for reproducing a gap with correct amplitude in correlated insulators, a
negative can also be understood as a way to suppress magnetism
and mimic the effects of quantum fluctuations ignored in DFT calculations. With
these considerations, the resulting optical spectra reproduce the SDW gap and a
number of experimentally observed features related to the antiferromagnetic
order. We find electronic contributions to excitations that so far have been
attributed to purely phononic modes. Also, an orbital resolved analysis of the
optical conductivity reveals significant contributions from all Fe 3d orbitals.
Finally, we observe that there is an important renormalization of kinetic
energy in these SDW metals, implying that the effects of correlations cannot be
neglected.Comment: 8 pages, 4 figures; recalculated spectra for U_eff=-1.9 eV for better
comparison to experimental results, added discussion of the role of U and J
in LDA+
Anodización del titanio para la mejora ante la degradación quÃmica de material quirúrgico
The titanium alloys used in orthopaedic and dental implants are corrosion-resistant and compatible with the human body due to the titanium oxid film which forms spontaneously on the implant surface. However, the utilization of more than one type of alloy or metal on the same patient and the fragility and the thin thickness of the oxid layer, can give rise to the appearance of the corossion phenomena and/or ion release. It is well known that enough concentration of metallic particles are toxic, and are able to produce lnflammation, allergy genetic mutations or carcinogenic processes. With the purpose of minimize the corrossion in the implants and the metallic ion release to the physiological environment it is possible to grow up the titanium oxid layer in a controlled way to have a tough, homogeneous and stable layer with the anodization technique. In this work it is evaluated the improvements in corrosion resistance and the decrease of the metallic ion release due to the anodized layer
Determination of effective microscopic models for the frustrated antiferromagnets CsCuCl and CsCuBr by density functional methods
We investigate the electronic and magnetic properties of the frustrated
triangular-lattice antiferromagnets CsCuCl and CsCuBr in the
framework of density functional theory. Analysis of the exchange couplings J
and J' using the available X-ray structural data corroborates the values
obtained from experimental results for CsCuBr but not for
CsCuCl. In order to understand this discrepancy, we perform a detailed
study of the effect of structural optimization on the exchange couplings of
CsCuCl employing different exchange-correlation functionals. We find
that the exchange couplings depend on rather subtle details of the structural
optimization and that only when the insulating state (mediated through spin
polarization) is present in the structural optimization, we do have good
agreement between the calculated and the experimentally determined exchange
couplings. Finally, we discuss the effect of interlayer couplings as well as
longer-ranged couplings in both systems.Comment: Phys. Rev. B in pres
Effects of Lifshitz Transition on Charge Transport in Magnetic Phases of Fe-Based Superconductors
The unusual temperature dependence of the resistivity and its in-plane
anisotropy observed in the Fe-based superconducting materials, particularly
Ba(FeCo)As, has been a longstanding puzzle. Here we
consider the effect of impurity scattering on the temperature dependence of the
average resistivity within a simple two-band model of a dirty spin density wave
metal. The sharp drop in resistivity below the N\'eel temperature in the
parent compound can only be understood in terms of a Lifshitz transition
following Fermi surface reconstruction upon magnetic ordering. We show that the
observed resistivity anisotropy in this phase, arising from nematic defect
structures, is affected by the Lifshitz transition as well.Comment: 8 pages, 5 figure
Signatures of a gearwheel quantum spin liquid in a spin- pyrochlore molybdate Heisenberg antiferromagnet
We theoretically investigate the low-temperature phase of the recently
synthesized LuMoON material, an extraordinarily rare
realization of a three-dimensional pyrochlore Heisenberg
antiferromagnet in which Mo are the magnetic species. Despite a
Curie-Weiss temperature () of K, experiments have
found no signature of magnetic ordering spin freezing down to
K. Using density functional theory, we find that the compound
is well described by a Heisenberg model with exchange parameters up to third
nearest neighbors. The analysis of this model via the pseudofermion functional
renormalization group method reveals paramagnetic behavior down to a
temperature of at least , in agreement with the
experimental findings hinting at a possible three-dimensional quantum spin
liquid. The spin susceptibility profile in reciprocal space shows
momentum-dependent features forming a "gearwheel" pattern, characterizing what
may be viewed as a molten version of a chiral noncoplanar incommensurate spiral
order under the action of quantum fluctuations. Our calculated reciprocal space
susceptibility maps provide benchmarks for future neutron scattering
experiments on single crystals of LuMoON.Comment: Published version. Main paper (6 pages, 3 figures) + Supplemental
Material (4 pages, 3 figures, 1 table
TiOCl, an orbital-ordered system?
We present first principles density functional calculations and downfolding
studies of the electronic and magnetic properties of the layered quantum spin
system
TiOCl. We discuss explicitely the nature of the exchange pathes and attempt
to clarify the concept of orbital ordering in this material. An analysis of the
electronic structure of slightly distorted structures according to the phononic
modes allowed in this material suggests that this system is subject to large
orbital fluctuations driven by the electron-phonon coupling. Based on these
results, we propose a microscopic explanation of the behavior of TiOCl near the
phase transition to a spin-gapped system.Comment: Some figures are compressed, for higher quality please contact the
author
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