6,434 research outputs found
Luttinger liquid superlattices: realization of gapless insulating phases
We investigate Luttinger Liquid superlattices, a periodic structure composed
of two kinds of one-dimensional systems of interacting electrons. We calculate
several properties of the low-energy sector: the effective charge and spin
velocities, the compressibility, various correlation functions, the Landauer
conductance and the Drude weight. The low-energy properties are subsumed into
effective parameters, much like homogeneous one-dimensional systems. A generic
result is the weighted average nature of these parameters, in proportion to the
spatial extent of the underlying subunits, pointing to the possibility of
``engineered'' structures. As a specific realization, we consider a
one-dimensional Hubbard superlattice, which consists of a periodic arrangement
of two long Hubbard chains with different coupling constants and different
hopping amplitudes. This system exhibits a rich phase diagram with several
phases, both metallic and insulating. We have found that gapless insulating
phases are present over a wide range of parameters.Comment: 16 pages, 15 figures, RevTeX
Coherent nu-N scattering and the search for physics beyond the standard model
We focus in future proposals to measure coherent neutrino-nuclei scattering
and we show that such kind of experiments are very sensitive to nonstandard
neutrino interactions with quarks. First in a model independent parametrization
and then we focused in particular models such as leptoquarks and models with
extra neutral gauge bosons and with R-parity breaking interactions. We show
that in all these three different types of new physics it is possible to obtain
competitive bounds to those of future collider experiments. For the particular
case of leptoquarks we found that the expected sensitivity to the coupling and
mass for most of the future experimental setups is quite better than the
current constraints.Comment: 6 pages, 1 Figure, Talk given at 11th Mexican Workshop on Particles
and Fields 2007, Tuxtla Gutierrez, Mexico, 7-12 Nov 200
Revisiting fundamental welding concepts to improve additive manufacturing: From theory to practice
The authors acknowledge Fundacao para a Ciencia e Tecnologia (FCT) for its financial support through the project UID/EMS/00667/2019. Fundo Regional para a Ciencia e Tecnologia and Projeto de I&DT for companies in copromotion SLM-XL, (Ref 3346), funded by Fundo Europeu de Desenvolvimento Regional (FEDER) through Programa Operacional Regional de Lisboa.Additive manufacturing technologies based on melting and solidification have considerable similarities with fusion-based welding technologies, either by electric arc or high-power beams. However, several concepts are being introduced in additive manufacturing which have been extensively used in multipass arc welding with filler material. Therefore, clarification of fundamental definitions is important to establish a common background between welding and additive manufacturing research communities. This paper aims to review these concepts, highlighting the distinctive characteristics of fusion welding that can be embraced by additive manufacturing, namely the nature of rapid thermal cycles associated to small size and localized heat sources, the non-equilibrium nature of rapid solidification and its effects on: internal defects formation, phase transformations, residual stresses and distortions. Concerning process optimization, distinct criteria are proposed based on geometric, energetic and thermal considerations, allowing to determine an upper bound limit for the optimum hatch distance during additive manufacturing. Finally, a unified equation to compute the energy density is proposed. This equation enables to compare works performed with distinct equipment and experimental conditions, covering the major process parameters: power, travel speed, heat source dimension, hatch distance, deposited layer thickness and material grain size.publishersversionpublishe
Modulation of charge-density waves by superlattice structures
We discuss the interplay between electronic correlations and an underlying
superlattice structure in determining the period of charge density waves
(CDW's), by considering a one-dimensional Hubbard model with a repeated
(non-random) pattern of repulsive (U>0) and free (U=0) sites. Density matrix
renormalization group diagonalization of finite systems (up to 120 sites) is
used to calculate the charge-density correlation function and structure factor
in the ground state. The modulation period can still be predicted through
effective Fermi wavevectors, k_F*, and densities, and we have found that it is
much more sensitive to electron (or hole) doping, both because of the narrow
range of densities needed to go from q*=0 to \pi, but also due to sharp
2k_F*-4k_F* transitions; these features render CDW's more versatile for actual
applications in heterostructures than in homogeneous systems.Comment: 4 pages, 5 figures, to appear in Phys Rev
Substrate polarization effects in two-dimensional magnetic arrays
The magnetostatic energy of a two-dimensional (2D) periodic array of magnetic particles (or a thin film with periodic magnetization) is evaluated, including additional energy terms due to a polarizable substrate. The polarization of the substrate is solved self-consistently using surface charges. This requires describing the magnetic potential of the 2D array in terms of an equivalent surface charge distribution. Analytic expressions for the magnetostatic self-energy of the 2D array as well as the energy due to the interaction of the magnetic structure and polarizable substrate are presented. It is shown how substrates with large susceptibility significantly alter the stray-field energy and, hence, the magnetic properties of the array, even promoting a spin-reorientation transition. Our results suggest that system properties can be controlled in a simple way by exploiting substrates with tunable polarizabilityWe acknowledge funding by Consolider-Ingenio en Nanociencia Molecular Ref. No. CSD2007-00010, by the Comunidad de Madrid through Project No. S2009/MAT-1726,
and Project No. FIS 2010-18847 from MICIN
The influence of surface finishing on laser heat treatments of a tool steel
Laser heat treatments (LHT) has received growing attention in the last years because of highly localized precision and manufacturing efficiency related to laser processing of moulds steel. Due of its strong resistance and ability to maintain hardness and strength at high temperatures, AISI P20 steel is one of the most widely used tool steel in the plastics injection mould industry. This work presents an experimental investigation on LHT using P20 mod. steel produced with different surface finishes. After mechanical surface finishing, the diode laser beam with 15 mm width was applied to the P20 specimens at 1060 °C using a feed rate of 8.8 mms-1in an air and argon ambient. The influence of different LHT atmosphere conditions and specimen initial surface finishing on characteristics such as final roughness, microhardness and microstructure were comprehensively analyzed. The use of a controlled atmosphere during processing showed an increment in depth and hardness values of samples. Through 3D profilometer, it was possible to determine the samples roughness. Results showed that lower average roughness leads to higher hardness values close to the surface, while higher average roughness lead to a deeper heat-treated zone. Macroscopical analysis revealed the depth and width afftected by LHT. The microhardness results showed an increment from 300 HV to around 750 HV on laser heat-treated zone using a controlled environment. Optical microscopy analysed the microstructural changes into martensite between LHT and non LHT zones for all samplespublishe
Observational Constraints on Visser's Cosmological Model
Theories of gravity for which gravitons can be treated as massive particles
have presently been studied as realistic modifications of General Relativity,
and can be tested with cosmological observations. In this work, we study the
ability of a recently proposed theory with massive gravitons, the so-called
Visser theory, to explain the measurements of luminosity distance from the
Union2 compilation, the most recent Type-Ia Supernovae (SNe Ia) dataset,
adopting the current ratio of the total density of non-relativistic matter to
the critical density () as a free parameter. We also combine the SNe
Ia data with constraints from Baryon Acoustic Oscillations (BAO) and CMB
measurements. We find that, for the allowed interval of values for ,
a model based on Visser's theory can produce an accelerated expansion period
without any dark energy component, but the combined analysis (SNe Ia + BAO +
CMB) shows that the model is disfavored when compared with CDM model.Comment: 6 pages, 4 figure
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