Atomic spin sensitive dissipation on magnetic surfaces

We identify the mechanism of energy dissipation relevant to spin-sensitive nanomechanics including the recently introduced magnetic exchange force microscopy, where oscillating magnetic tips approach surface atomic spins. The tip-surface exchange couples spin and atom coordinates, leading to a spin-phonon problem with Caldeira-Leggett type dissipation. In the overdamped regime, that can lead to a hysteretic flip of the local spin with a large spin-dependent dissipation, even down to the very low experimental tip oscillation frequencies, describing recent observations for Fe tips on NiO. A phase transition to an underdamped regime with dramatic drop of magnetic tip dissipation should in principle be possible by tuning tip-surface distance.Comment: 4 pages, 4 figure

Quantum Dissipation at the Nanoscale

The outline of this work will be as follows: in chapter 1, after a brief overview of the systems studied and a preview of the results obtained, we will introduce the general models and methods we are going to use to tackle the systems considered, hopefully providing enough theory to understand our later extensive treatment, without entering in too much detail in the widely studied general theories of our models. In chapter 2 we will show our first application: the description of a spin-sensitive dissipation channel compatible with the experimental findings of an atomic force microscopy experiment. We will describe in some detail the experiment and the system under study and why some direct approaches are unable to account for the observed effect; we will then specialize the previously described path integral technique to obtain a numerical description of the system, highlighting our proposed dissipation mechanism. In chapter 3 we will consider electron current pumping in a threesite system and how it can be affected by the presence of an environment. We will first obtain and solve a simple equation for the isolated system, then couple a bath to this and specialize the master equation theory to obtain an analytical result for the system in presence of an environment, observing some interesting changes it its behavior. The experimental feasibility of the proposed setup will be explored. Finally, in chapter 4 we will briefly present another model for the description of dissipative systems which has been investigated but, for now, is not complete enough to produce interesting results: inspired from another atomic force microscopy experiment where frictional effects of hydrogen atoms on a surface are observed, we will try to investigate the possibility of inherently quantum effects in a similar system, where a light particle is coupled to other heavier atoms, treated classically. We will propose a model and a technique for its simulation, though this will prove too computationally demanding to be of practical use

Dynamics of a Quantum Phase Transition in the XXZ Model

Adiabatic theorem states that for sufficiently slow variations of a parameter, a quantum system starting from the ground state will stay in its instantaneous ground state during the evolution; but if we consider a critical system, as the relaxation time diverges as we approach critical point, during a phase transition the evolution will eventually become non-adiabatic and the system will not be able to relax to its instantaneous ground state: this will result in the formation of imperfections in the final state of the system, such as a kink in a ferromagnetic state. The density of such imperfections can be related to the quench velocity in many (classical or quantum, finite or infinite) systems with the help of the Kibble-Zurek mechanism, which essentially divides the transition in adiabatic and impulse-like regimes when the transition velocity exceeds the relaxation time. The Kibble-Zurek mechanism has been successfully applied to superconductors, superfluids and liquid crystals and it has been shown to overlap very well with the more rigorous Landau-Zener transition probability for avoided level crossing. The XXZ model consists of a chain of spin-1/2 interacting via a Heisenberg coupling with anisotropy in the z direction. By changing the intensity and sign of the z coupling relative to the xy coupling one can modify the ground state properties of the system, ranging from the Ising ferromagnet, to the isotropic ferromagnetic or antiferromagnetic Heisenberg model, to the Ising antiferromagnet. In particular for certain values of the z coupling the system shows a critical behaviour. By gradually changing the value of the z coupling we can make a state of the system evolve through this extended critical region. In this thesis we will describe the phase transition of the XXZ model in the framework of the Kibble-Zurek mechanism (and Landau-Zener theory), to see if and in which way these description can be applied to critical regions instead of critical points; our predictions will be compared with numerical findings obtained using the Density Matrix Renormalization Group algorithm, a very powerful tool for the simulation of one dimensional quantum systems, and in particular its recently developed extension which supports time evolution

Interfaces narrativas. El pasaje de la producción transmediática al diseño de narrativas : Caso de estudio: Muestra interactiva “Lugar a dudas”

Este trabajo de tesis doctoral constituye una reflexión, análisis y síntesis teórica sobre cómo son las construcciones / las lógicas de los contenidos para generar narrativas transmediáticas dentro de espacios interactivos que han sido diseñados para empresas de servicios y, específicamente, en el sector de los museos. Las narrativas transmediáticas pueden ser diseñadas como una estrategia, ser una solución táctica para la comunicación en el devenir de la interacción o, simplemente, ser un derivado sin una intención. El trabajo consta de un análisis de cómo es la construcción de contenidos a través de dos planos: un primer plano sobre la identidad de marca, analizando lo que la organización diseña y quiere que comunique su marca contemplando valores, conceptos y contenidos comunicacionales que serán afectados por factores internos y externos; y un segundo plano sobre la experiencia de usuario, donde se presentan diferentes interacciones sobre las narrativas de los contenidos producidos, con un diseño determinado y con sus relaciones entre los medios físicos y virtuales para generar una intención de experiencia. El trabajo es un aporte teórico al campo dado que se analiza dónde provienen las narrativas transmediáticas, las problemáticas que las rodean y hacia dónde están mutando. Los museos clásicos están cambiando a una nueva concepción de espacios interactivos, donde el diseño del servicio y la generación de los contenidos es lo que prima. Lograr que existan diferentes niveles de lectura, para diferentes tipos de consumidores y que participen en cada una de las propuestas es un gancho diferencial en contraposición a las ofertas tradicionales. El pasaje de la observación o contemplación de un objeto a contar una historia inmersa en una narrativa con la cual se debe interactuar para comprender el mensaje, hace que la participación se convierta en un entretenimiento y por medio de acción lúdica se asimile el contenido. Con el fin de analizar los interrogantes y los objetivos de dicho trabajo, se tomará como caso paradigmático el museo interactivo Lugar a Dudas del Centro Cultural de la Ciencia (C3) perteneciente al Ministerio de Ciencia, Tecnología e Innovación Productiva de la Argentina, en el período 2016-2020.Facultad de Periodismo y Comunicación Socia