Huamán Gutiérrez, Angiolo M. (2022) Ingeniería de Floquet para el transporte de carga en sistemas bidimensionales / Floquet engineering for charge transport in two-dimensional systems. Tesis Doctoral en Física, Universidad Nacional de Cuyo, Instituto Balseiro.
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Resumen en español
En este trabajo se estudian algunos efectos producidos por la interacción entre ciertos materiales bidimensionales y la radiación electromagnética. Más precisamente, estudiaremos el disulfuro de tungsteno WS2 (como representante de la familia de dicalcogenuros de metales de transición) y el grafeno en el régimen Hall. El primero de estos materiales es un aislante ordinario, mientras que el segundo exhibe características de un aislante topológicamente no trivial. Observaremos que, sometidos a iluminación, el primero adquiere propiedades de no equilibrio equivalentes a las de un aislante topológico. En particular, en ambos casos se forman brechas de cuasienergía (una extensión del concepto de energía válida para este tipo de sistemas) que pueden albergar estados de borde conductores en su interior. Estos estados, como se puede mostrar en el caso del grafeno irradiado, son quirales y robustos: poseen un sentido de propagación bien definido y que no cambia si la muestra posee imperfecciones, es decir, son robustos frente al desorden. La interacción entre el láser y estos materiales se introduce en los elementos de salto entre los sitios de la red. Esto puede hacerse tanto en un modelo continuo cuanto en la aproximación de enlace fuerte. De esta forma se introduce una dependencia temporal en el hamiltoniano de estos sistemas que podemos suponer periódica en el tiempo con el mismo período del láser. Con esta suposición el nuevo hamiltoniano puede tratarse usando el formalismo de Floquet. Los objetos de nuestros estudio serán dos: las bandas de cuasienergía o de Floquet, que son una extensión para sistemas dependientes del tiempo de las bandas de Bloch usuales, y la conductancia o transmitancia dc (en el formalismo que Landauer-Büttiker que usaremos ambas cantidades son equivalentes). Esta última será calculada en dispositivos de dos (grafeno y disulfuro de tungsteno) y seis (grafeno) terminales. Notaremos que, mientras que en sistemas estáticos la correlación entre bandas de energía y conductancia es bastante clara, este no es el caso en sistemas iluminados (o en general, forzados). El caso del grafeno iluminado en el régimen Hall es de especial interés ya que un campo magnético determina un régimen aislante particular o topológicamente distinto de un aislante ordinario, y que se modifica al someter la muestra a iluminación. Por otra parte, el hecho de que en el régimen Hall el transporte de carga se dé principalmente en los bordes, hace este problema cualitativamente distinto al del WS2, donde hay que separar las contribuciones de estados localizados en los bordes y lejos de estos. Además, la conductancia Hall muestra una dependencia con la helicidad del láser incidente, invirtiendo su signo cuando la helicidad del láser cambia. Esto permite la manipulación de la señal Hall simplemente modificando el sentido de polarización del láser. Además, discutiremos algunos resultados concernientes a sistemas con desorden en grafeno, con el objetivo de determinar en qué medida los resultados anteriores se mantienen o se modifican. Finalmente presentaremos las conclusiones a las que se ha arribado en esta tesis.
Resumen en inglés
In this work, we study some effects produced by the interaction between certain two-dimensional materials and electromagnetic radiation. More precisely, we will study tungsten disulfide WS2 (as a representative of the family of transition metal dicalcogenides) and graphene in the Hall regime. The first of these materials is an ordinary insulator, while the second exhibits characteristics of a topologically non-trivial insulator. We will observe that, when exposed to illumination, the first acquires non-equilibrium properties equivalent to those of a topological insulator. In particular, in both cases quasienergy (an extension of the concept of energy valid for this type of systems) gaps are formed that can harbor conducting edge states inside. These states, as can be shown in the case of irradiated graphene, are chiral and robust: they have a well-defined velocity that does not change if the sample has imperfections, that is, they are robust against disorder. The interaction mechanism between the laser and these materials is through the hopping elements between the lattice sites. This can be done either in a continuous model or in the tight binding approximation. In this way, a time dependence is introduced in the Hamiltonian of these systems, which we can assume to be periodic in time with the same period of the laser. With this assumption, the new Hamiltonian can be treated within the Floquet formalism. The objects of our study will be two: the quasienergy or Floquet bands, which are an extension for time-dependent systems of the usual Bloch bands, and the dc conductance or transmittance (in the Landauer-Büttiker formalism that we will use both quantities are equivalent). The latter will be calculated in devices with two (graphene and tungsten disulfide) and six (graphene) terminals. We will note that, while in static systems the correlation between energy bands and conductance is quite clear, this is not the case in illuminated systems (or in general, forced ones). The case of illuminated graphene in the Hall regime is of special interest since a magnetic field defines a particular insulating regime topologically different from an ordinary insulator, and that is modified when the sample is subjected to illumination. On the other hand, the fact that in the Hall regime charge transport occurs mainly at the edges makes this problem qualitatively different from that of theWS2, where it is necessary to separate the contributions of edge and bulk states. Furthermore, the Hall conductance shows a dependence on the helicity of the incident laser, changing its sign when the helicity of the laser changes. This allows manipulation of the Hall signal simply by changing the direction of the polarization of the laser. In addition, we will discuss some results concerning graphene systems with disorder, with the aim of determining to what extent the previous results hold or are modified. Finally, we will present the conclusions reached in this thesis.
| Tipo de objeto: | Tesis (Tesis Doctoral en Física) |
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| Palabras Clave: | Graphene; Grafeno; Transport; Transporte; [Floquet theory; Teoría de floquet; Edge states; Estado de borde; Quasienergy; Cuasienergía] |
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| Materias: | Física > Sistema Nanoscópico |
| Divisiones: | Gcia. de área de Investigación y aplicaciones no nucleares > Gcia. de Física > Materia condensada > Teoría de sólidos |
| Código ID: | 1076 |
| Depositado Por: | Tamara Cárcamo |
| Depositado En: | 14 Jul 2022 11:31 |
| Última Modificación: | 14 Jul 2022 11:31 |
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