Espectroscopías de "Pum & Probe" aplicadas al estudio de aislantes topológicos de floquet. / Pum & Probe spectroscopies applied to the study of floqueet topological insulators.

Peralta Gavensky, Lucila (2016) Espectroscopías de "Pum & Probe" aplicadas al estudio de aislantes topológicos de floquet. / Pum & Probe spectroscopies applied to the study of floqueet topological insulators. Master in Physical Sciences, Universidad Nacional de Cuyo, Instituto Balseiro.

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Abstract in Spanish

La manipulación óptica ultra-rápida y las espectroscopías resueltas en tiempo han avanzado notablemente a lo largo de los últimos años, permitiendo el estudio experimental de sistemas fuera de equilibrio en escalas de tiempo ultra-cortas ( ~ fs). Simultáneamente, la búsqueda de estados topológicos de la materia ha sido de gran interés en la física de la materia condensada desde el descubrimiento de los aislantes topológicos. Combinando estos dos campos, esta tesis tiene como objetivo estudiar materiales de tipo grafeno, donde el acoplamiento de los grados de libertad electrónicos con un campo electromag ético externo produce la aparición de propiedades topológicas de interés. Los materiales donde una excitación continua de frecuencia bien definida produce la aparición de estados de borde clasificados por invariantes topológicos (como los números de Chern) se conocen como aislantes topológicos de Floquet (FTIs). La periodicidad temporal permite, en el marco de la teoría de Floquet, comprender la presencia de estados coherentes luz-materia con un determinado espectro de cuasi-energías. En el mismo se generan brechas prohibidas en las regiones irradiadas con luz circularmente polarizada al romper la simetría de inversión temporal. Las preguntas que surgen naturalmente, a las que intentaremos dar respuesta en esta tesis, están relacionadas a cuánto se puede extender o tomar prestado de los conceptos de topología de un FTI perturbado en forma estacionaria cuando la excitación es reemplazada por un pulso corto (de una duración de unos pocos períodos asociados a la frecuencia del fotón incidente). En este trabajo se estudian en particular la técnica de fotoemisión resuelta tanto en tiempo como angularmente (tr-ARPES) y se realizan cálculos de respuesta Hall en este tipo de sistemas. A través de las simulaciones de fotoemisión se estudia la posibilidad de detectar tanto el espectro de cuasi-energías de Floquet como los cambios en la topología inducidos por la radiación. Se encuentra que la distribución de intensidades de los electrones foto-emitidos posee información de la estructura topológica de las fuciones de onda perturbadas. Asimismo, los cálculos de conductividad Hall demuestran que la misma no se encuentra cuantizada y su relación a los invariantes topológicos no es directa. Aún así, las componentes de Fourier de la respuesta transversal generada por el pulso se encuentran en correspondencia con el espectro de Floquet calculado en forma estacionaria. Por otro lado, se estudia el valor medio de esta magnitud y su relación a las poblaciones en las bandas de Floquet y a los números de Chern asociados a las mismas.

Abstract in English

Ultra-fast optical manipulation and time-resolved spectroscopies have had a remarkable progress over the last few years, allowing the experimental study of out of equilibrium systems in femto-second time scales. At the same time, the quest for novel topological states of matter triggered enormous research activity in condensed matter physics since the discovery of topological insulators. Merging these two felds, this thesis focuses on the study of graphene-like systems where the coupling of the electronic degrees of freedom with an external electromagnetic feld induces topological properties of interest. Materials where a continuous laser driving of well defned frequency produces the appearance of edge states classifed by topological invariants (like Chern numbers) are known as Floquet topological insulators (FTIs). Temporal periodicity allows, in a Floquet theory scheme, the understanding of coherent light-matter states with a quasi-static band structure. Regions irradiated with circularly polarized laser acquire non-equilibrium energy gaps, product of broken time reversal symmetry. Natural follow-up questions, which we will try to give an answer to, are related to what concepts of FTI's topology can be borrowed or extended when continuous-wave driving is replaced by a short laser pulse (with a typical duration of a few periods associated with the incident photon energy). In this work we study both time-resolved and angle-resolved photoemission spectroscopy (tr-ARPES) and the bulk Hall response of these systems. Through the photoemission simulations we study the possibility of detecting the quasi-energy Floquet spectrum and changes of topology induced by the radiation field. We find that the photo-electrons intensity distribution possesses information on the topology of the out of equilibrium wavefunctions. Likewise, the Hall conductance calculations show that this quantity is not quantized and it's not directly related to topological invariants. Nevertheless, the Fourier components of the transverse response generated by the pulse are in correspondence with the stationary Floquet spectrum. On the other hand, we study the mean value of this magnitude and it's relation to population of the Floquet bands and to the Chern numbers associated with them.

Item Type:Thesis (Master in Physical Sciences)
Additional Information:Área Temática: Propiedades electrónicas y dinámicas de sistemas cuánticos topológicos.
Keywords:Spectroscopy; Espectroscopía; [Graphene; Grafeno; Topological insulator; Aislante topológicos; Floquet; Hall conductance; Conductividad hall]
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Subjects:Physics
Divisions:Gcia. de área de Investigación y aplicaciones no nucleares > Gcia. de Física > Materia condensada > Teoría de sólidos
ID Code:592
Deposited By:Tamara Cárcamo
Deposited On:10 May 2017 15:50
Last Modified:10 May 2017 15:50

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