706 research outputs found
Three orbital model for the iron-based superconductors
The theoretical need to study the properties of the Fe-based high-T_c
superconductors with reliable many-body techniques requires us to determine the
minimum number of orbital degrees of freedom that will capture the physics of
these materials. While the shape of the Fermi surface (FS) obtained with the
local density approximation (LDA) can be reproduced by a two-orbital model, it
has been argued that the bands that cross the chemical potential result from
the strong hybridization of three of the Fe 3d orbitals. For this reason, a
three-orbital Hamiltonian obtained with the Slater-Koster formalism by
considering the hybridization of the As p orbitals with the Fe d_xz,d_yz, and
d_xy orbitals is discussed here. This model reproduces qualitatively the FS
shape and orbital composition obtained by LDA calculations for undoped
pnictides when four electrons per Fe are considered. Within a mean-field
approximation, its magnetic and orbital properties in the undoped case are
described. With increasing Coulomb repulsion, four regimes are obtained: (1)
paramagnetic, (2) magnetic (pi,0) spin order, (3) the same (pi,0) spin order
but now including orbital order, and finally (4) a magnetic and orbital ordered
insulator. The spin-singlet pairing operators allowed by the lattice and
orbital symmetries are also constructed. It is found that for pairs of
electrons involving up to diagonal nearest-neighbors sites, the only fully
gapped and purely intraband spin-singlet pairing operator is given by
Delta(k)=f(k)\sum_{alpha} d_{k,alpha,up}d_{-k,alpha,down} with f(k)=1 or
f(k)=cos(k_x)cos(k_y) which would arise only if the electrons in all different
orbitals couple with equal strength to the source of pairing
Coexistence of Pairing Tendencies and Ferromagnetism in a Doped Two-Orbital Hubbard Model on Two-Leg Ladders
Using the Density Matrix Renormalization Group and two-leg ladders, we
investigate an electronic two-orbital Hubbard model including plaquette
diagonal hopping amplitudes. Our goal is to search for regimes where charges
added to the undoped state form pairs, presumably a precursor of a
superconducting state.For the electronic density , i.e. the undoped
limit, our investigations show a robust antiferromagnetic ground
state, as in previous investigations. Doping away from and for large
values of the Hund coupling , a ferromagnetic region is found to be stable.
Moreover, when the interorbital on-site Hubbard repulsion is smaller than the
Hund coupling, i.e. for in the standard notation of multiorbital Hubbard
models, our results indicate the coexistence of pairing tendencies and
ferromagnetism close to . These results are compatible with previous
investigations using one dimensional systems. Although further research is
needed to clarify if the range of couplings used here is of relevance for real
materials, such as superconducting heavy fermions or pnictides, our theoretical
results address a possible mechanism for pairing that may be active in the
presence of short-range ferromagnetic fluctuations.Comment: 8 pages, 4 Fig
Unexpected Conductance Dip in the Kondo Regime of Linear Arrays of Quantum Dots
Using exact-diagonalization of small clusters and Dyson equation embedding
techniques, the conductance of linear arrays of quantum dots is
investigated. The Hubbard interaction induces Kondo peaks at low temperatures
for an odd number of dots. Remarkably, the Kondo peak is split in half by a
deep minimum, and the conductance vanishes at one value of the gate voltage.
Tentative explanations for this unusual effect are proposed, including an
interference process between two channels contributing to , with one more
and one less particle than the exactly-solved cluster ground-state. The Hubbard
interaction and fermionic statistics of electrons also appear to be important
to understand this phenomenon. Although most of the calculations used a
particle-hole symmetric Hamiltonian and formalism, results also presented here
show that the conductance dip exists even when this symmetry is broken. The
conductance cancellation effect obtained using numerical techniques is
potentially interesting, and other many-body techniques should be used to
confirm its existence
Resistivity of Mixed-Phase Manganites
The resistivity of manganites is studied using a
random-resistor-network, based on phase-separation between metallic and
insulating domains. When percolation occurs, both as chemical composition and
temperature vary, results in good agreement with experiments are obtained.
Similar conclusions are reached using quantum calculations and microscopic
considerations. Above the Curie temperature, it is argued that ferromagnetic
clusters should exist in Mn-oxides. Small magnetic fields induce large
changes and a bad-metal state with (disconnected) insulating
domains.Comment: 4 pages, 4 eps figure
Magnetic Domains and Stripes in the Spin-Fermion Model for Cuprates
Monte Carlo simulations applied to the Spin-Fermion model for cuprates show
the existence of antiferromagnetic spin domains and charge stripes upon doping.
The stripes are partially filled, with a filling of approximately 1/2 hole per
site, and they separate spin domains with a phase shift among them. The
stripes observed run either along the x or y axes and they are separated by a
large energy barrier. No special boundary conditions or external fields are
needed to stabilize these structures at low temperatures. When magnetic
incommensurate peaks are observed at momentum and symmetrical
points, charge incommensurate peaks appear at and symmetrical
points, as experimentally observed. The strong charge fluctuations responsible
for the formation of the stripes also induce a pseudogap in the density of
states.Comment: Four pages with four figures embedded in tex
Deviations from Fermi-liquid behavior above in 2D short coherence length superconductors
We show that there are qualitative differences between the temperature
dependence of the spin and charge correlations in the normal state of the 2D
attractive Hubbard model using quantum Monte Carlo simulations. The
one-particle density of states shows a pseudogap above \tc with a depleted
with decreasing . The susceptibility \cs and the low frequency spin
spectral weight track , which explains the spin-gap scaling: 1/T_1T \sim
\cs(T). However the charge channel is dominated by collective behavior and the
compressibility is -independent. This anomalous ``spin-charge
separation'' is shown to exist even at intermediate where the momentum
distribution n(\bk) gives evidence for degenerate Fermi system.Comment: 4 pages (twocolumn format), 5 Postscript figure
Evolution of magnetic polarons and spin-carrier interactions through the metal-insulator transition in EuGdO
Raman scattering studies as functions of temperature, magnetic field, and
Gd-substitution are used to investigate the evolution of magnetic polarons and
spin-carrier interactions through the metal-insulator transition in
EuGdO. These studies reveal a greater richness of phase behavior
than have been previously observed using transport measurements: a
spin-fluctuation-dominated paramagnetic (PM) phase regime for T T
T, a two-phase regime for T T in which magnetic polarons
develop and coexist with a remnant of the PM phase, and an inhomogeneous
ferromagnetic phase regime for T T
Laboratory and telescope demonstration of the TP3-WFS for the adaptive optics segment of AOLI
AOLI (Adaptive Optics Lucky Imager) is a state-of-art instrument that combines adaptive optics (AO) and lucky imaging (LI) with the objective of obtaining diffraction limited images in visible wavelength at mid- and big-size ground-based telescopes. The key innovation of AOLI is the development and use of the new TP3-WFS (Two Pupil Plane PositionsWavefront Sensor). The TP3-WFS, working in visible band, represents an advance over classical wavefront sensors such as the Shack-Hartmann WFS (SH-WFS) because it can theoretically use fainter natural reference stars, which would ultimately provide better sky coverages to AO instruments using this newer sensor. This paper describes the software, algorithms and procedures that enabled AOLI to become the first astronomical instrument performing real-time adaptive optics corrections
in a telescope with this new type of WFS, including the first control-related
results at the William Herschel Telescope (WHT)This work was supported by the Spanish Ministry of Economy under the projects AYA2011-29024, ESP2014-56869-C2-2-P, ESP2015-69020-C2-2-R and DPI2015-66458-C2-2-R, by project 15345/PI/10 from the Fundación Séneca, by the Spanish Ministry of Education under the grant FPU12/05573, by project ST/K002368/1 from the Science and Technology Facilities Council and by ERDF funds from the European Commission. The results presented in this paper are based on observations made with the William Herschel Telescope operated on the island of La Palma by the Isaac Newton Group in the Spanish Observatorio del Roque de los Muchachos of the Instituto de Astrofísica de Canarias. Special thanks go to Lara Monteagudo and Marcos Pellejero for their timely contributions
Disorder induced phase segregation in La2/3Ca1/3MnO3 manganites
Neutron powder diffraction experiments on La2/3Ca1/3MnO3 over a broad
temperature range above and below the metal-insulator transition have been
analyzed beyond the Rietveld average approach by use of Reverse Monte Carlo
modelling. This approach allows the calculation of atomic pair distribution
functions and spin correlation functions constrained to describe the observed
Bragg and diffuse nuclear and magnetic scattering. The results evidence phase
separation within a paramagnetic matrix into ferro and antiferromagnetic
domains correlated to anistropic lattice distortions in the vicinity of the
metal-insulator transition.Comment: 3 pages, 4 figures. Submitted to Phys. Rev. Lett. Figure 1 replace
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