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Time and momentum resolved electron spectroscopy of strongly correlated systems

English title Time and momentum resolved electron spectroscopy of strongly correlated systems
Applicant Carbone Fabrizio
Number 200331
Funding scheme Project funding
Research institution Laboratoire pour la microscopie et la diffusion d'électrons EPFL - SB - ICMP - LUMES
Institution of higher education EPF Lausanne - EPFL
Main discipline Condensed Matter Physics
Start/End 01.10.2021 - 30.09.2025
Approved amount 322'684.00
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Keywords (6)

Electron Energy Loss Spectroscopy; Graphite; Transmission Electron microscopy; Ultrafast ; Superconductivity; valleytronics

Lay Summary (Italian)

Lead
La maggiorate dei dispositivi elettronici funziona tramite la manipolazione del corrente in un semiconduttore tramite una tensione. Per migliorare le funzionalità di tali dispositivi, in passato si sono sviluppate nuove tecniche di nanofabbricazione e nuovi materiali. Recentemente, I solidi a bassa dimensionali hanno offerto prospettive interessanti per la prossima generazione di dispositivi elettronici grazie alla loro maggiore mobilità, cioè la facilità di trasportare corrente, e la potenziale miniaturizzazione fino alla scala atomica. Per raggiungere questi obiettivi, serve una comprensione microscopica di tali nuovi meccanismi di trasporto della corrente, e nuoe strategie per controllarli, ad esempio tramite fasci di luce.
Lay summary
L'obiettivo principale di questo progetto è di creare dei portatori di carica in materiali a bassa dimensionali, cone dei film sottili di grafite, tramite impulsi di luce laser ultraveloci (femtosecondi), per poi seguire la loro evoluzione nello spazio e nel tempo tramite una tecnica avanzata di microscopia elettronica risolta nel tempo sviluppate nel nostro laboratorio. 
Questo approccio è unico perchè permette di combinare una risoluzione in energia, spazio e tempo dei meV, nanometri e femtosecondi, impossibile da raggiungere con altre tecniche. Grazie a questo livello di dettaglio, potremo sviluppare protocolli per manipolare il passaggio di corrente nei nuovi materiali a scale di lunghezza atomiche, abilitando una nuova frontiera per i dispositivi elettronici futuri. 
Questo studio appartiene al contesto scientifico che oggi si chiama valleytronics, che si riferisce alla capacità di manipolare i segnali elettronici attraverso l'ingegnerizzazione delle proprietà microscopiche dei materiali (in gergo, le valli). Se funzionerà, il nostro studio restituirà un metodo unico per caratterizzare questi nuovi segnali elettronici, e ancora meglio per manipolarli usando impulsi ultraveloci di luce. 

In prospettiva, dispositivi piu' piccoli, piu' veloci e che consumano meno energia avranno un ovvio impatto sulla società tramite una riduzione del bisogno di energia e di batterie costose. Inoltre, l'aumentata capacità computazione derivante da queste tecnologie aumenterà la connettività dei dispositivi oltre che le loro performances generali. 
Direct link to Lay Summary Last update: 29.03.2021

Lay Summary (English)

Lead
Most electronic devices function through the manipulation of a current in a semiconductor using voltages. Key to their performance evolution, in terms of speed and energy consumption, and to their miniaturisation have been both the development of nano fabrication tools and the engineering of the underlying materials properties. Recently, new materials based on low-dimensional solids such as graphene and other layered systems have shown promise for the next generation of electronics thanks to their increased carriers mobility, i.e. ease to transport current, and potential for atomic-level miniaturisation. To achieve these goals, a detailed understanding of the microscopic mechanisms responsible for the current transport as well as new tools to manipulate them, such as light for example, have to be developed.
Lay summary
The main goal of this project is to create charge carriers in low-dimensional systems, such as a thin film of graphite, using very fast light pulses (femtoseconds) and map their subsequent evolution in space and time using an advanced method for time-resolved electron microscopy developed in our laboratory. 
Our approach is unique because it offers a combination of energy, space and time resolution in the range of the meV, nm, and fs which is unmatched by other techniques. Thanks to this, we intend to develop new protocols for manipulating the current flow in new materials at a time and length scale which would provide a new frontier for novel electronic devices.

This study is in the scientific context of what today is called valleytronics, which refers to a new way of manipulating electronic signals through the engineering of subtle microscopic properties of low-dimensional materials (in jargon called valleys). If successful, our study will provide a unique tool to characterise such novel electronic signal and better, manipulate them using a potentially very fast handle such as light. 

In perspective, smaller, faster and less consuming electronics have obvious impact on society via the reduction of energy consumption, in particular that stored in precious and expensive batteries, and via the improvement of connectivity and overall computing power that will be made available by the new devices based on these technologies.
Direct link to Lay Summary Last update: 29.03.2021

Responsible applicant and co-applicants

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Abstract

In this project, we propose to characterize and control the generation and decay of electron-hole pairs generated by laser pulses in specific regions of a material’s reciprocal space. In particular, we will show that tailored light excitation can populate different valleys in the band structure of graphite and that their decay can be controlled via a sequence of pulses. Furthermore, the same method will allow us to perform a unique analysis of the momentum-dependent energy transfers accompanying the onset of superconductivity in cuprates, providing a crucial feedback for the understanding of the thermodynamics of such phase transition. To achieve this, we will develop a new method to obtain combined temporal, energy and momentum resolution in Electron Energy Loss Spectroscopy (EELS) performed in an ultrafast Transmission Electron Microscope (TEM).Historically, to investigate the low energy electronic structure of materials, Angle Resolved Photoemission Spectroscopy (ARPES) and optical spectroscopy played a major role. In simple terms, the former provides momentum and energy resolved maps of the single particle spectral function, directly yielding information on a material’s band structure, while the latter delivers more direct information on collective modes by probing the solid’s joint density of states. A critical limitation of optical spectroscopy is its restriction to the observation of vertical transitions, i.e. dipole-excitation with a null momentum exchange. In the case of strongly correlated systems, this is particularly important as very often the electronic structure of these solids is strongly anisotropic and so are their emergent properties. A good example of such a situation is the d-wave nature of the superconducting gap in high-critical-temperature cuprates. In these materials, optical spectroscopy could provide insights into the thermodynamics of the phase transition by monitoring the evolution of the optical spectral weight while crossing the critical temperature; however, it has become increasingly evident that the full picture of how the balance between kinetic energy and Coulomb energy evolves during the formation of the Cooper pairs could only be obtained by following the redistribution of the optical spectral weight with momentum resolution covering the entire Brillouin zone. The crucial role of collective modes for the physics of strongly correlated materials has also been highlighted by out of equilibrium experiments revealing the possibility to manipulate them to alter their interaction with the low energy electronic structure, literally creating new states of matter with exotic properties. Also in these cases, ARPES and optical spectroscopy played a major role in uncovering interesting out-of-equilibrium phenomena, with the latter having the same limitations of its static counterpart in terms of momentum resolution. Besides the possibility to characterize better the ground state of exotic materials such as superconductors, observing and controlling their momentum-dependent electronic structure was also shown to hold promise for implementing new logic gates and signal manipulation tools in 2D materials, a branch of solid-state physics research also termed valleytronics. Our plan is to photo-induce the superconducting to normal phase transition in high temperature superconductor and investigate the redistribution of the momentum-dependent loss function to deduce the kinetic and potential energy changes induced by the onset of superconductivity in these materials. Very recent theoretical predictions highlighted the important role of the potential energy changes induced by the onset of superconductivity, but could not be verified experimentally due to the lack of momentum resolution in optical spectroscopy. In our laboratory, we host a femtosecond-resolved TEM that has been recently upgraded to detect individual electrons. In this instrument, it will be possible to carry out momentum-resolved EELS experiments in out-of-equilibrium conditions. Specifically, we plan to selectively photo-excite carriers in different regions of the Brillouin zone of a graphite thin films by using two laser pulses of different energies, and monitor the intervalley interactions and the consequent decay of the out-of-equilibrium electronic structure via ultrafast momentum-resolved EELS using a third pulse made of electrons, the details of this method will be described below. Such a seminal experiment is highly feasible as the first femtosecond-resolved EELS experiments were carried out by the PI himself on a graphite thin film and therefore the ideal conditions to go beyond those results and access the momentum resolved loss functions are a specific competence of our group. Preliminary results were recently obtained and will be described later in the proposal. Interestingly, these experiments will demonstrate the possibility to observe and control intervalley scattering mechanisms in 2D materials in a very direct fashion, having direct access to the electronic structure itself, but also the lattice structure and the material morphology via imaging in the TEM.
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