BEGIN:VCALENDAR VERSION:2.0 PRODID:Icfo X-PUBLISHED-TTL:P1W BEGIN:VEVENT UID:69d4afc37c852 DTSTART:20240523T080000Z SEQUENCE:0 TRANSP:OPAQUE LOCATION:ICFO Auditorium and Online (Teams) SUMMARY:ICFO | JAIME DÍEZ MÉRIDA CLASS:PUBLIC DESCRIPTION:In 2018\, following a theoretical prediction from 2011\, it was found that stacking two layers of graphene with a relative twist angle of 1.1°\; between them leads to multiple new properties. At this so-calle d magic angle\, the electronic band structure of the material reconstructs \, creating a narrow flat band at the Fermi level. The formation of a flat band enhances electron-electron interactions\, resulting in the emergence of states of matter not present in the original graphene layers\, includi ng correlated insulators\, superconductivity\, ferromagnetism and non-triv ial topological states. The understanding of the origin of these correlate d states could help unravel the physics of highly correlated flat band sys tems which could potentially provide key technological developments.\nThe main objective of this thesis is to study magic-angle twisted bilayer grap hene (MATBG) by creating monolithic gate-defined Josephson junctions. By e xploiting the rich phase space of the material\, we can create a Josephson junction by independently tuning the superconductor and the weak link sta te. Studying the Josephson effect is a first step towards understanding fu ndamental properties of a superconductor\, such as its order parameter.\nF irst\, we have optimized the fabrication of these gate-defined junctions m ade of all van der Waals materials. We have made double-graphite-gated hBN encapsulated MATBG devices where the top gate is split into two parts via nanolithography techniques. This configuration allows to independently co ntrol the three regions of the Josephson junction (superconductor\, weak-l ink and superconductor). Then\, we have studied the gate-defined Josephson junctions via low-temperature transport measurements. After demonstrating the Josephson effect in the fabricated devices\, we focus on the behavior of one of these junctions in great detail.\nIn particular\, we have obser ved an unconventional behavior when the weak link of the junction is set c lose to the correlated insulator at half-filling of the hole-side flatband . We have observed a phase shifted Fraunhofer pattern with a pronounced ma gnetic hysteresis\, characteristic of magnetic Josephson junctions. To und erstand the origin of the signals\, we have performed a critical current d istribution Fourier analysis as well as a tight binding calculation of a M ATBG Josephson junction. Our theoretical calculations with a valley polari zed state as the weak link can explain the key signatures observed in the experiment. Lastly\, the combination of magnetization and its current-indu ced magnetization switching has allowed us to realize a programmable zero- field superconducting diode.\nFinally\, we have shown the flexibility of t hese devices by studying a MATBG p-n junction under light illumination. We have studied the relaxation dynamics of hot electrons using time and freq uency-resolved photovoltage measurements. The measurements have revealed a n ultrafast cooling in MATBG compared to Bernal-bilayer from room temperat ure down to 5 K. The enhanced cooling in MATBG can be explained by the pre sence of the moiré\; pattern and corresponding mini-Brillouin zone.\ nIn summary\, we have demonstrated that by integrating various MATBG state s within a single device\, we can gain a deeper insight into the system's properties and can engineer innovative\, complex hybrid structures\, such as magnetic Josephson junctions and superconducting diodes.\n \;\nThur sday May 23\, 10:00 h. ICFO Auditorium and online via Teams\nThesis Direct or: Prof Dr. Dmitri K. Efetov and Prof. Dr. Maciej Lewenstein DTSTAMP:20260407T071827Z END:VEVENT END:VCALENDAR