4,528 research outputs found
Competing Pairing Symmetries in a Generalized Two-Orbital Model for the Pnictides
We introduce and study an extended "t-U-J" two-orbital model for the
pnictides that includes Heisenberg terms deduced from the strong coupling
expansion. Including these J terms explicitly allows us to enhance the strength
of the (pi, 0)-(0, pi) spin order which favors the presence of tightly bound
pairing states even in the small clusters that are here exactly diagonalized.
The A1g and B2g pairing symmetries are found to compete in the realistic
spin-ordered and metallic regime. The dynamical pairing susceptibility
additionally unveils low-lying B1g states, suggesting that small changes in
parameters may render any of the three channels stable.Comment: submitted PRL 10/5/1
Superconductivity from spin fluctuations and long-range interactions in magic-angle twisted bilayer graphene
Magic-angle twisted bilayer graphene (MATBG) has been extensively explored
both theoretically and experimentally as a suitable platform for a rich and
tunable phase diagram that includes ferromagnetism, charge order, broken
symmetries, and unconventional superconductivity. In this work, we investigate
the intricate interplay between long-range electron-electron interactions, spin
fluctuations, and superconductivity in MATBG. By employing a low-energy model
for MATBG that captures the correct shape of the flat bands, we explore the
effects of short- and long-range interactions on spin fluctuations and their
impact on the superconducting (SC) pairing vertex in the Random Phase
Approximation (RPA). We find that the SC state is notably influenced by the
strength of long-range Coulomb interactions. Interestingly, our RPA
calculations indicate that there is a regime where the system can traverse from
a magnetic phase to the SC phase by \emph{increasing} the relative strength of
long-range interactions compared to the on-site ones. These findings underscore
the relevance of electron-electron interactions in shaping the intriguing
properties of MATBG and offer a pathway for designing and controlling its SC
phase.Comment: 9 pages, 5 figure
Static Holes in the Geometrically Frustrated Bow Tie Ladder
We investigate the doping of a geometrically frustrated spin ladder with
static holes by a complementary approach using exact diagonalization and
quantum dimers. Results for thermodynamic properties, the singlet density of
states, the hole-binding energy and the spin correlations will be presented.
For the undoped systems the ground state is non-degenerate, with
translationally invariant nearest-neighbor spin correlations. In the doped
case, we find that static holes polarize their vicinity by a localization of
singlets in order to reduce the frustration. This polarization induces short
range repulsive forces between two holes and an oscillatory behavior of the
long range two-hole energy. For most quantities investigated, we find very good
agreement between the quantum dimer approach and the results from exact
diagonalization.Comment: 7 pages, 9 eps figure
Kondo effect under the influence of spin–orbit coupling in a quantum wire
The analysis of the impact of spin–orbit coupling (SOC) on the Kondo state has generated considerable controversy, mainly regarding the dependence of the Kondo temperature T K on SOC strength. Here, we study the one-dimensional (1D) single impurity Anderson model (SIAM) subjected to Rashba (α) and Dresselhaus (β) SOC. It is shown that, due to time-reversal symmetry, the hybridization function between impurity and quantum wire is diagonal and spin independent (as it is the case for the zero-SOC SIAM), thus the finite-SOC SIAM has a Kondo ground state similar to that for the zero-SOC SIAM. This similarity allows the use of the Haldane expression for T K, with parameters renormalized by SOC, which are calculated through a physically motivated change of basis. Analytic results for the parameters of the SOC-renormalized Haldane expression are obtained, facilitating the analysis of the SOC effect over T K. It is found that SOC acting in the quantum wire exponentially decreases T K while SOC at the impurity exponentially increases it. These analytical results are fully supported by calculations using the numerical renormalization group (NRG), applied to the wide-band regime, and the projector operator approach, applied to the infinite-U regime. Literature results, using quantum Monte Carlo, for a system with Fermi energy near the bottom of the band, are qualitatively reproduced, using NRG. In addition, it is shown that the 1D SOC SIAM for arbitrary α and β displays a persistent spin helix SU(2) symmetry similar to the one for a 2D Fermi sea with the restriction α = β.VL acknowledges a PhD scholarship from the Brazilian Agency Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq), process 160071/2015-1, and financial support from the Generalitat Valenciana through Grant reference Prometeo 2017/139. MM acknowledges a PhD scholarship from the Brazilian Agency Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES). GBM acknowledges financial support from CNPq, processes 424711/2018-4 and 305150/2017-0. EVA acknowledges financial support from CNPq, process 306000/2017-2
2-Methyl-5-(4-tolyl)-7-(trifluoromethyl)pyrazolo[1,5-a]pyrimidine
In the title compound, C15H12F3N3, the pyrazolo[1,5-a]pyrimidine system ring is essentially planar with a maximum deviation from the mean plane of 0.014 (1) Å. The 4-tolyl group makes a dihedral angle of 14.1 (1)° with the pyrazolo[1,5-a]pyrimidine ring system. The crystal packing is stabilized mainly by van der Waals forces
A model for predicting the Ms temperatures of steels.
Using neural networks in a Bayesian framework, a model has been derived for the Ms temperature of steels over a
wide range of compositions. By its design and by use of a more extensive database, this model improves over existing
ones, by its accuracy and its ability to avoid wild predictions.NPL for provision of
MTDATA and Neuromat for provision of the
Model Manager.Peer reviewe
Results of two multi-chord stellar occultations by dwarf planet (1) Ceres
We report the results of two multi-chord stellar occultations by the dwarf
planet (1) Ceres that were observed from Brazil on 2010 August 17, and from the
USA on 2013 October 25. Four positive detections were obtained for the 2010
occultation, and nine for the 2013 occultation. Elliptical models were adjusted
to the observed chords to obtain Ceres' size and shape. Two limb fitting
solutions were studied for each event. The first one is a nominal solution with
an indeterminate polar aspect angle. The second one was constrained by the pole
coordinates as given by Drummond et al. Assuming a Maclaurin spheroid, we
determine an equatorial diameter of 972 6 km and an apparent oblateness
of 0.08 0.03 as our best solution. These results are compared to all
available size and shape determinations for Ceres made so far, and shall be
confirmed by the NASA's Dawn space mission.Comment: 9 pages, 6 figures. Accepted for publication in MNRA
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