2,164 research outputs found
Spin-phonon induced magnetic order in Kagome ice
We study the effects of lattice deformations on the Kagome spin ice, with
Ising spins coupled by nearest neighbor exchange and long range dipolar
interactions, in the presence of in-plane magnetic fields. We describe the
lattice energy according to the Einstein model, where each site distortion is
treated independently. Upon integration of lattice degrees of freedom,
effective quadratic spin interactions arise. Classical MonteCarlo simulations
are performed on the resulting model, retaining up to third neighbor
interactions, under different directions of the magnetic field. We find that,
as the effect of the deformation is increased, a rich plateau structure appears
in the magnetization curves.Comment: 7 pages, 8 figure
Synchronization of the Frenet-Serret linear system with a chaotic nonlinear system by feedback of states
A synchronization procedure of the generalized type in the sense of Rulkov et
al [Phys. Rev. E 51, 980 (1995)] is used to impose a nonlinear Malasoma chaotic
motion on the Frenet-Serret system of vectors in the differential geometry of
space curves. This could have applications to the mesoscopic motion of
biological filamentsComment: 12 pages, 7 figures, accepted at Int. J. Theor. Phy
Magnetic-field effects on transport in carbon nanotube junctions
Here we address a theoretical study on the behaviour of electronic states of
heterojunctions and quantum dots based on carbon nanotubes under magnetic
fields. Emphasis is put on the analysis of the local density of states, the
conductance, and on the characteristic curves of current versus voltage. The
heterostructures are modeled by joining zigzag tubes through single
pentagon-heptagon pair defects, and described within a simple tight binding
calculation. The conductance is calculated using the Landauer formula in the
Green functions formalism. The used theoretical approach incorporates the
atomic details of the topological defects by performing an energy relaxation
via Monte Carlo calculation. The effect of a magnetic field on the conductance
gap of the system is investigated and compared to those of isolated constituent
tubes. It is found that the conductance gap of the studied CNHs exhibits
oscillations as a function of the magnetic flux. However, unlike the pristine
tubes case, they are not Aharonov-Bohm periodic oscillations
Yangians, finite W-algebras and the Non Linear Schrodinger hierarchy
We show an algebra morphism between Yangians and some finite W-algebras. This
correspondence is nicely illustrated in the framework of the Non Linear
Schrodinger hierarchy. For such a purpose, we give an explicit realization of
the Yangian generators in terms of deformed oscillators.Comment: LaTeX2e, 10 pages, Talk presented by E. Ragoucy at ACTP-Nankai
Symposium on Yang-Baxter systems, non linear models and their applications,
Seoul (Korea) October 20-23, 199
Intermediate magnetization state and competing orders in Dy2Ti2O7 and Ho2Ti2O7
We thank R. Moessner, C. Castelnovo and M. Gingras for helpful discussions, and the financial support of ANPCYT through PICT 2013-2004 and PICT 2014-2618 and CONICET (Argentina), the EPSRC and the Royal Society (UK).Among the frustrated magnetic materials, spin-ice stands out as a particularly interesting system. Residual entropy, freezing and glassiness, Kasteleyn transitions and fractionalization of excitations in three dimensions all stem from a simple classical Hamiltonian. But is the usual spin-ice Hamiltonian a correct description of the experimental systems? Here we address this issue by measuring magnetic susceptibility in the two most studied spin-ice compounds, Dy2Ti2O7 and Ho2Ti2O7, using a vector magnet. Using these results, and guided by a theoretical analysis of possible distortions to the pyrochlore lattice, we construct an effective Hamiltonian and explore it using Monte Carlo simulations. We show how this Hamiltonian reproduces the experimental results, including the formation of a phase of intermediate polarization, and gives important information about the possible ground state of real spin-ice systems. Our work suggests an unusual situation in which distortions might contribute to the preservation rather than relief of the effects of frustration.Publisher PDFPeer reviewe
Central star formation and metallicity in CALIFA interacting galaxies
We use optical integral-field spectroscopic (IFS) data from 103 nearby
galaxies at different stages of the merging event, from close pairs to merger
remnants provided by the CALIFA survey, to study the impact of the interaction
in the specific star formation and oxygen abundance on different galactic
scales. To disentangle the effect of the interaction and merger from internal
processes, we compared our results with a control sample of 80 non-interacting
galaxies. We confirm the moderate enhancement (2-3 times) of specific star
formation for interacting galaxies in central regions as reported by previous
studies; however, the specific star formation is comparable when observed in
extended regions. We find that control and interacting star-forming galaxies
have similar oxygen abundances in their central regions, when normalized to
their stellar masses. Oxygen abundances of these interacting galaxies seem to
decrease compared to the control objects at the large aperture sizes measured
in effective radius. Although the enhancement in central star formation and
lower metallicities for interacting galaxies have been attributed to tidally
induced inflows, our results suggest that other processes such as stellar
feedback can contribute to the metal enrichment in interacting galaxies.Comment: 9 pages, 9 figures. Accepted for publication in Astronomy &
Astrophysic
First survey of Wolf-Rayet star populations over the full extension of nearby galaxies observed with CALIFA
The search of extragalactic regions with conspicuous presence of Wolf-Rayet
(WR) stars outside the Local Group is challenging task due to the difficulties
in detecting their faint spectral features. In this exploratory work, we
develop a methodology to perform an automated search of WR signatures through a
pixel-by-pixel analysis of integral field spectroscopy (IFS) data belonging to
the Calar Alto Legacy Integral Field Area survey, CALIFA. This technique
allowed us to build the first catalogue of Wolf-Rayet rich regions with
spatially-resolved information, allowing to study the properties of these
complexes in a 2D context. The detection technique is based on the
identification of the blue WR bump (around He II 4686 {\AA}, mainly associated
to nitrogen-rich WR stars, WN) and the red WR bump (around C IV 5808 {\AA} and
associated to carbon-rich WR stars, WC) using a pixel-by-pixel analysis. We
identified 44 WR-rich regions with blue bumps distributed in 25 galaxies of a
total of 558. The red WR bump was identified only in 5 of those regions. We
found that the majority of the galaxies hosting WR populations in our sample
are involved in some kind of interaction process. Half of the host galaxies
share some properties with gamma-ray burst (GRB) hosts where WR stars, as
potential candidates to being the progenitors of GRBs, are found. We also
compared the WR properties derived from the CALIFA data with stellar population
synthesis models, and confirm that simple star models are generally not able to
reproduce the observations. We conclude that other effects, such as the binary
star channel (which could extend the WR phase up to 10 Myr), fast rotation or
other physical processes that causes the loss of observed Lyman continuum
photons, are very likely affecting the derived WR properties, and hence should
be considered when modelling the evolution of massive stars.Comment: 33 pages, accepted for publication in A&
Self-passivating W-Cr-Y alloys: characterization and testing
The use of self-passivating tungsten alloys for the first wall armor of future fusion reactors is advantageous
concerning safety issues in comparison with pure tungsten. Bulk W-10Cr-0.5Y alloy manufactured by mechanical
alloying followed by HIP resulted in a fully dense material with grain size around 100 nm and a dispersion of Y-rich
oxide nanoparticles located at the grain boundaries. An improvement in flexural strength and fracture toughness was
observed with respect to previous works. Oxidation tests under isothermal and accident-like conditions revealed a very
promising oxidation behavior for the W-10Cr-0.5Y alloy. Thermo-shock tests at JUDITH-1 to simulate ELM-like loads
resulted in a crack network at the surface with roughness values lower than those of a pure W reference material. An
additional thermal treatment at 1550 °C improves slightly the oxidation and thermo-shock resistance of the alloy
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