887 research outputs found
Structural properties in Sr0.61a0.39Nb2O6 in the temperature range 10 K to 500 K investigated by high-resolution neutron powder diffraction and specific heat measurements
We report high-resolution neutron powder diffraction on Sr0.61Ba0.39Nb2O6,
SBN61, in the temperature range 15-500 K. The results indicate that the
low-temperature anomalies (T<100K) observed in the dielectric dispersion are
due to small changes in the incommensurate modulation of the NbO6-octahedra, as
no structural phase transition of the average structure was observed. This
interpretation is supported by specific heat measurements, which show no latent
heat, but a glass-like behavior at low temperatures. Furthermore we find that
the structural changes connected with the ferroelectric phase transition at Tc
approx. 350K start already at 200K, explaining the anisotropic thermal
expansion in the temperature range 200-300K observed in a recent x-ray
diffraction study.Comment: Accepted by PRB (2006
Dimensional reduction by pressure in the magnetic framework material CuF(DO)pyz: from spin-wave to spinon excitations
Metal organic magnets have enormous potential to host a variety of electronic
and magnetic phases that originate from a strong interplay between the spin,
orbital and lattice degrees of freedom. We control this interplay in the
quantum magnet CuF(DO)pyz by using high pressure to drive the
system through a structural and magnetic phase transition. Using neutron
scattering, we show that the low pressure state, which hosts a two-dimensional
square lattice with spin-wave excitations and a dominant exchange coupling of
0.89 meV, transforms at high pressure into a one-dimensional spin-chain
hallmarked by a spinon continuum and a reduced exchange interaction of 0.43
meV. This direct microscopic observation of a magnetic dimensional crossover as
a function of pressure opens up new possibilities for studying the evolution of
fractionalised excitations in low dimensional quantum magnets and eventually
pressure-controlled metal--insulator transitions
Magnetic inversion symmetry breaking and ferroelectricity in TbMnO3
TbMnO3 is an orthorhombic insulator where incommensurate spin order for
temperature T_N < 41K is accompanied by ferroelectric order for T < 28K. To
understand this, we establish the magnetic structure above and below the
ferroelectric transition using neutron diffraction. In the paraelectric phase,
the spin structure is incommensurate and longitudinally-modulated. In the
ferroelectric phase, however, there is a transverse incommensurate spiral. We
show that the spiral breaks spatial inversion symmetry and can account for
magnetoelectricity in TbMnO3.Comment: 4 pages revtex, accepted by Phys. Rev. Lett. on June 21, 200
Filmes utilitĂĄrios, amadores, ĂłrfĂŁos e efĂ©meros: repensando o cinema a partir dos âoutros filmesâ
Os estudos de cinema tĂȘm vindo a construir-se em torno do filme de ficção, dentro de um paradigma predominantemente estĂ©tico, autoral e nacional. Apesar de constituĂrem a maior parte da produção mundial, os filmes de nĂŁo-ficção â uma categoria alargada, de metragem variada, que inclui diferentes gĂ©neros â tĂȘm sido marginalizados ou excluĂdos das vĂĄrias histĂłrias e historiografias do cinema, nacionais e internacionais. Esta introdução ao dossier âOutros Filmesâ, começa por delinear o surgimento, na dĂ©cada de 1990, dentro e fora da academia, de perspectivas crĂticas que permitiram a valorização destes filmes e, por conseguinte, o seu estudo. Ă tendo em conta este contexto internacional que se apresenta um breve balanço do panorama portuguĂȘs e brasileiro. O objectivo do dossier, a partir dos seus 6 artigos (a maior parte dos quais sobre o filme amador e o filme de actualidades), Ă© lançar as bases para uma discussĂŁo mais vasta sobre o modo como os âoutros filmesâ podem (devem?) levar-nos a repensar os estudos de cinema e das imagens em movimento.info:eu-repo/semantics/publishedVersio
Search for excited Ï-leptons and leptoquarks in the final state with Ï-leptons and jets in pp collisions at âs = 13 TeV
A search is reported for excited Ï-leptons and leptoquarks in events with two hadronically decaying Ï-leptons and two or more jets. The search uses proton-proton (pp) collision data at âs = 13 TeV recorded by the ATLAS experiment during the Run 2 of the Large Hadron Collider in 2015â2018. The total integrated luminosity is 139 fbâ1. The excited Ï-lepton is assumed to be produced and to decay via a four-fermion contact interaction into an ordinary Ï-lepton and a quark-antiquark pair. The leptoquarks are assumed to be produced in pairs via the strong interaction, and each leptoquark is assumed to couple to a charm or lighter quark and a Ï-lepton. No excess over the background prediction is observed. Excited Ï-leptons with masses below 2.8 TeV are excluded at 95% CL in scenarios with the contact interaction scale Î set to 10 TeV. At the extreme limit of model validity where Î is set equal to the excited Ï-lepton mass, excited Ï-leptons with masses below 4.6 TeV are excluded. Leptoquarks with masses below 1.3 TeV are excluded at 95% CL if their branching ratio to a charm quark and a Ï-lepton equals 1. The analysis does not exploit flavour-tagging in the signal region
Search for vector-boson resonances decaying into a top quark and a bottom quark using pp collisions at âs = 13 TeV with the ATLAS detector
A search for a new massive charged gauge boson, WâČ, is performed with the ATLAS detector at the LHC. The dataset used in this analysis was collected from proton-proton collisions at a centre-of-mass energy of âs = 13 TeV, and corresponds to an integrated luminosity of 139 fbâ1. The reconstructed tb invariant mass is used to search for a WâČ boson decaying into a top quark and a bottom quark. The result is interpreted in terms of a WâČ boson with purely right-handed or left-handed chirality in a mass range of 0.5â6 TeV. Different values for the coupling of the WâČ boson to the top and bottom quarks are considered, taking into account interference with single-top-quark production in the s-channel. No significant deviation from the background prediction is observed. The results are expressed as upper limits on the WâČ â tb production cross-section times branching ratio as a function of the WâČ-boson mass and in the plane of the coupling vs the WâČ-boson mass
Classical generalized constant coupling model for geometrically frustrated antiferromagnets
A generalized constant coupling approximation for classical geometrically
frustrated antiferromagnets is presented. Starting from a frustrated unit we
introduce the interactions with the surrounding units in terms of an internal
effective field which is fixed by a self consistency condition. Results for the
magnetic susceptibility and specific heat are compared with Monte Carlo data
for the classical Heisenberg model for the pyrochlore and kagome lattices. The
predictions for the susceptibility are found to be essentially exact, and the
corresponding predictions for the specific heat are found to be in very good
agreement with the Monte Carlo results.Comment: 4 pages, 3 figures, 2 columns. Discussion about the zero T value of
the pyrochlore specific heat correcte
Search for direct production of electroweakinos in final states with one lepton, jets and missing transverse momentum in pp collisions at âs = 13 TeV with the ATLAS detector
Search for magnetic monopoles and stable particles with high electric charges in âs = 13 TeV pp collisions with the ATLAS detector
We present a search for magnetic monopoles and high-electric-charge objects using LHC Run 2 âs = 13 TeV proton-proton collisions recorded by the ATLAS detector. A total integrated luminosity of 138 fbâ1 was collected by a specialized trigger. No highly ionizing particle candidate was observed. Considering the Drell-Yan and photon-fusion pair production mechanisms as benchmark models, cross-section upper limits are presented for spin-0 and spin-1/2 magnetic monopoles of magnetic charge 1gD and 2gD and for high-electric-charge objects of electric charge 20 †|z| †100, for masses between 200 GeV and 4000 GeV. The search improves by approximately a factor of three the previous cross-section limits on the Drell-Yan production of magnetic monopoles and high-electric charge objects. Also, the first ATLAS limits on the photon-fusion pair production mechanism of magnetic monopoles and high-electric-charge objects are obtained
Searches for lepton-flavour-violating decays of the Higgs boson into eÏ and ÎŒÏ in âs=13 TeV pp collisions with the ATLAS detector
This paper presents direct searches for lepton flavour violation in Higgs boson decays, H â eÏ and H â ÎŒÏ, performed using data collected with the ATLAS detector at the LHC. The searches are based on a data sample of proton-proton collisions at a centre-of-mass energy âs = 13 TeV, corresponding to an integrated luminosity of 138 fbâ1. Leptonic (Ï â âÎœâÎœÏ) and hadronic (Ï â hadrons ÎœÏ) decays of the Ï-lepton are considered. Two background estimation techniques are employed: the MC-template method, based on data-corrected simulation samples, and the Symmetry method, based on exploiting the symmetry between electrons and muons in the Standard Model backgrounds. No significant excess of events is observed and the results are interpreted as upper limits on lepton-flavour-violating branching ratios of the Higgs boson. The observed (expected) upper limits set on the branching ratios at 95% confidence level, ÎČ(H â eÏ) < 0.20% ÎČ(0.12%) and (H â ÎŒÏ ) < 0.18% (0.09%), are obtained with the MC-template method from a simultaneous measurement of potential H â eÏ and H â ÎŒÏ signals. The best-fit branching ratio difference, ÎČ(H â ÎŒÏ) â ÎČ(H â eÏ), measured with the Symmetry method in the channel where the Ï-lepton decays to leptons, is (0.25 ± 0.10)%, compatible with a value of zero within 2.5Ï
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