Papers by Javier Villegas
Madera y Bosques, 2013
Effect of mycorrhyzation in the establishment of Pinus pseudostrobus in gullies RESUMEN Se evaluó... more Effect of mycorrhyzation in the establishment of Pinus pseudostrobus in gullies RESUMEN Se evaluó el establecimiento de Pinus pseudostrobus Lindl., en tres condiciones de propagación y trasplante en cárcavas desprovistas de vegetación en la localidad de Atécuaro, municipio de Morelia, Michoacán. La primera condición corresponde a plantas de procedencia conocida, germinadas y propagadas en condiciones controladas (estratificadas a 4 °C, germinadas a 25 °C con 12 h luz y propagadas con riego cada tercer día), para contar con dos lotes de plantas, uno con inóculo y otro sin inóculo del hongo ectomicorrícico Pisolithus tinctorius (Pers.) Coker et Couch. Como una segunda condición, ambos lotes fueron trasplantados al sitio de plantación a los 15 meses de edad con adición de sustrato fértil (fibra de coco, corteza de árbol y agrolita) en las cepas. En otro sitio muy cercano, una tercera condición fue una reforestación convencional con plantas de la misma especie pero sin controlar las variables antes descritas. Se evaluó altura, diámetro a la altura de la base, cobertura, número de ramas y porcentaje de supervivencia. Los resultados muestran que los pinos que fueron plantados bajo las primeras condiciones presentaron mayor crecimiento y supervivencia que los pinos de la segunda reforestación (tercera condición) (P < 000,1). La supervivencia de los pinos inoculados fue de 86%, la de la misma plantación sin inóculo 62% y la de la segunda reforestación 30%. La inoculación se plantea como una estrategia a aplicar en proyectos de restauración de sitios severamente degradados y erosionados, planteándose la necesidad de una inversión que incremente el éxito en los programas de reforestación.
Vortex-lattice dynamics can be studied using regular arrays of nanometric pinning centers obtaine... more Vortex-lattice dynamics can be studied using regular arrays of nanometric pinning centers obtained by e-beam lithography techniques. We have explored the effect of these periodic pinning centers on the vortex lattice direction of motion. A measuring geometry, in which two in-plane orthogonal (crossing) electrical currents are injected through the sample, allows selecting the direction and intensity of the resultant driving
We have studied angular dependent vortex lattice dynamics in Nb films with nanostructured pinning... more We have studied angular dependent vortex lattice dynamics in Nb films with nanostructured pinning potentials. We have investigated guided vortex motion regimes, that are limited by vortex lattice velocity, driving force and applied magnetic field values. We have found enhanced transverse effects due to the channel landscapes.
We study hyperbolic partial differential equations on a one dimensional spatial domain with contr... more We study hyperbolic partial differential equations on a one dimensional spatial domain with control and observation at the boundary. Using the idea of feedback we show these systems are well-posed in the sense of Weiss and Salamon if and only if the state operator generates a C 0 -semigroup. Furthermore, we show that the corresponding transfer function is regular, i.e., has a limit for s going to infinity. keywords: Infinite-dimensional systems, hyperbolic boundary control systems, C 0 -semigroup, well-posedness. * file:artikel/Dirac/well-posedness 4.tex
Superconducting/Ferromagnetic YBCO/LCMO heterostructures exhibit a number of novel behaviors, suc... more Superconducting/Ferromagnetic YBCO/LCMO heterostructures exhibit a number of novel behaviors, such as unexpected long-range proximity effects, and giant magnetoresistance. The microscopic mechanisms behind these effects are under debate and include, among others, different types of charge and spin transport processes across the YBCO/LCMO interface. Characterizing these and determining the electronic density of states nearby the interfaces is key to understand the nature of the S/F interactions. With this motivation, we have fabricated vertical YBCO/LCMO and YBCO/LCMO/YBCO junctions (of areas down to 8 μm^2) using optical lithography and ion etching. The current-perpendicular-to-plane differential conductance shows salient features such as i) a sizable interface resistivity ˜ 10-3 φ.cm^2, ii) a prominent zero-bias conductance peak, and iii) a symmetric series of resonances at quasiparticle sup-gap energies, reminiscent of Tomasch and McMillan-Rowell oscillations. We will discuss the origin of these behaviors and their implications in regard of proximity effects.
La caducidad o transitoriedad de los "métodos)) o ((estrategias)~ de análisis con respecto a lqs ... more La caducidad o transitoriedad de los "métodos)) o ((estrategias)~ de análisis con respecto a lqs textos literarlo~ o teatr:al~J ~u qmtinua sustitución por otros es una constante incitación a -considerar sus fundamentos, sus supuestos, su validez o adecuación para los objetos culturales examinados en cada ocasión. Pocos niegan las coÍlllotaciones ideológicas de las diferentes estrategias o las implicaciones socioestéticas de los códigos estéticos y culturales. Los desplazamientos o sustituciones se producen ya sea por una transformación de los fundamentos ideológicos y estéticos de los discursos críticos o por la conciencia de su insatisfactoriedad con respecto a sus objetos. En otras ocasiones, la resemantización del objeto conduce necesariamente a un cambio en la estrategia para su estudio. Generalmente, estas sustituciones están asociadas a transfor~ nuestro tiempo, las teorías 50bre la posmoderuidad.y Améri~a Larina, por ejemplo, evidencian el carácter "moderno" de la mayor par.(.e de las historias
The transition from positive to negative exchange bias can be systematically tuned with antidot a... more The transition from positive to negative exchange bias can be systematically tuned with antidot arrays artificially introduced into Ni/ Fe F2 ferromagnetic (FM)/antiferromagnetic (AF) heterostructures. This is a consequence of the energy balance between the Zeeman coupling of the AF spins to the cooling field, and the AF exchange coupling at the FM/AF interface. The nanostructure plays a key role in the formation of pinned uncompensated spins in the AF: the antidot carving produces regions of locally pinned uncompensated spins throughout the antidot faces of the Fe F2 and these non interfacial magnetic moments favor the onset of positive exchange bias at lower cooling fields, by increasing the Zeeman energy of AF domains and favoring the alignment with the latter. Those non interfacial AF spins, and the pinned uncompensated interfacial AF spins responsible for the exchange bias (loop shift), align simultaneously with the cooling field since they belong to the same AF domain and become pinned below the Néel temperature.
Curr Sociol, May 11, 2007
The research described in this thesis was undertaken at the Department of Applied Mathematics, in... more The research described in this thesis was undertaken at the Department of Applied Mathematics, in the Faculty EWI, Universiteit Twente, Enschede. The funding of the research was provided by the NWO Grant through project number 613.000.223 (Energy-based ...
Aps Meeting Abstracts, Mar 1, 2008
We constructed a superconducting/ferromagnetic hybrid system in which the ordering of the pinning... more We constructed a superconducting/ferromagnetic hybrid system in which the ordering of the pinning potential landscape for flux quanta can be manipulated. Flux pinning is induced by an array of magnetic nanodots in the ``magnetic vortex'' state, and controlled by the magnetic history. This allows switching on and off the collective pinning of the flux-lattice. In addition, we observed field-induced superconductivity that originates from the annihilation of flux quanta induced by the stray fields from the ``magnetic vortices.''
Bulletin of the American Physical Society, Mar 5, 2014
Neuromodulation, 2006
Well-posedness and regularity for a class of hyperbolic boundary ... Hans Zwart, Yann Le Gorrec, ... more Well-posedness and regularity for a class of hyperbolic boundary ... Hans Zwart, Yann Le Gorrec, Bernhard Maschke and Javier Villegas ... AbstractWe study a class of hyperbolic partial differential equations on a one dimensional spatial domain with control and observation ...
Bulletin of the American Physical Society, Mar 17, 2009
Superconducting/Ferromagnetic (S/F) hybrids exhibit a plethora of induced effects and novel physi... more Superconducting/Ferromagnetic (S/F) hybrids exhibit a plethora of induced effects and novel physical properties, due to the interplay between the competing S and F orders. We will show a few examples of those, in a series of experiments on a simple hybrid system: a S thin film with an array of F nanodots. Changing the array geometry, the nanodots size or their magnetic-state allows to investigate a large variety of physical phenomena. We will focus on two of them: flux pinning effects and stray-magnetic-field induced manipulation of superconductivity. We will firstly consider geometry induced effects; in particular, we will compare the pinning properties of periodic, quasiperiodic, and fractal arrays [1]. Secondly, we will discuss the effects induced by particular nanodot magnetic-states. We will show experiments on the interaction between flux quanta and nanodot magnetic vortices, which can be used to obtain switchable flux pinning potentials [2]. Finally, we will describe an experiment in which the magnetic reversal events of the nanodot magnetic vortices are imprinted into the transport properties of a superconducting thin film [3]. This yields a very unusual hysteretic magnetoresistance. This effect is induced by the stray magnetic fields from the nanodots, which drive the superconducting-to-normal transition of the hybrid depending on the magnetic history. [4pt] [1] J.E. Villegas et al., Phys. Rev. Lett. 97, 027002 (2006). [0pt] [2] J.E. Villegas et al., Phys. Rev. B 77, 134510 (2008). [0pt] [3] J.E. Villegas et al., Phys. Rev. Lett. 99, 227001 (2007).
Scientia Agropecuaria, Apr 3, 2014
Recibido 17 noviembre 2013. Aceptado 18 marzo 2014.
ABSTRACT The electrostatic tuning of physical properties in materials offers significant potentia... more ABSTRACT The electrostatic tuning of physical properties in materials offers significant potential in a large variety of systems. For example, the application of an electric field allows depressing or enhancing superconductivity in certain oxides. Using heterostructures that combine a large-polarization ferroelectric (BiFeO3) and a high-temperature superconductor (YBa2Cu3O7-x), we demonstrate here the nanoscale modulation of the superconducting condensate via ferroelectric field effects [1]. The ability to design the ferroelectric domain structure at will enables us to create nanoscale ``patterns&#39;&#39; of normal regions within the superconductor, in a reversible and modifiable way. This produces an energy landscape for magnetic flux quanta and, in turn, couples the local polarization in the ferroelectric to the local magnetic induction in the superconductor. This new form of magnetoelectric coupling allows the electrostatic manipulation of magnetic flux quanta. [4pt] [1] A. Crassous et al., Phys. Rev. Lett. in press (2011)
Superconductor Science and Technology, Jan 20, 2011
The magneto-transport of a superconducting/ferromagnetic hybrid structure, consisting of a superc... more The magneto-transport of a superconducting/ferromagnetic hybrid structure, consisting of a superconducting thin film in contact with an array of magnetic nanodots in the so-called 'magnetic vortex state', exhibits interesting properties. For certain magnetic states, the stray magnetic field from the vortex array is intense enough to drive the superconducting film into the normal state. In this fashion, the normal-to-superconducting phase transition can be controlled by the magnetic history. The strong coupling between superconducting and magnetic subsystems allows characteristically ferromagnetic properties, such as hysteresis and remanence, to be dramatically transferred into the transport properties of the superconductor.
Http Dx Doi Org 10 1080 00018732 2010 534865, Jan 6, 2011
Page 1. Advances in Physics Vol. 60, No. 1, JanuaryFebruary 2011, 584 REVIEW ARTICLE Ultrathin ... more Page 1. Advances in Physics Vol. 60, No. 1, JanuaryFebruary 2011, 584 REVIEW ARTICLE Ultrathin oxide films and interfaces for electronics and spintronics Manuel Bibes*, Javier E. Villegas and Agnès Barthélémy Unité ...
Bulletin of the American Physical Society, Mar 1, 2007
We investigated experimentally the magnetotransport properties of superconducting Al thin films o... more We investigated experimentally the magnetotransport properties of superconducting Al thin films on top of sub-100 nm Fe nanodot arrays. Samples with arrays of single-domain magnetic dots behave similar to plain Al films. A dramatically different behavior is observed for arrays of dots in the so-called magnetic vortex-state. For these, the details of the magnetic reversal are "imprinted" into the superconductor and show up in the magnetotransport properties below the superconducting T C . The resulting hybrid system shows a giant (up to 10 5 %) hysteretic magnetoresistance with different reversible/irreversible regimes related to the array's magnetic state. Such controllable effects originate from the stray fields produced by the magnetic vortex cores in the nanodots, which induce a normal/superconducting transition depending on the distribution of the vortex cores polarities.
Control Engineering Practice, Sep 1, 2009
In this paper, a distributed parameter approach is used to model and control a batch fluidised be... more In this paper, a distributed parameter approach is used to model and control a batch fluidised bed dryer. Electric capacitance tomography (ECT) is used as a sensor to measure and control the distributed permittivity in the bed. First, a lumped model is developed to describe the heat and mass transfer between solid, gas and bubble phases in the bed. Then,
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Papers by Javier Villegas