BEGIN:VCALENDAR VERSION:2.0 PRODID:Icfo X-PUBLISHED-TTL:P1W BEGIN:VEVENT UID:6ab2a232a8098 DTSTART:20261019T130000Z SEQUENCE:0 TRANSP:OPAQUE LOCATION:ICFO Auditorium SUMMARY:ICFO | RICCARDO BERTINI CLASS:PUBLIC DESCRIPTION:Two-dimensional moiré\; superlattices have opened a new e ra in quantum materials\, as they provide a versatile platform for enginee ring electronic bands and accessing correlated and topological phases. The ir intrinsically narrow minibands\, achieved by twisting or aligning atomi cally thin two-dimensional crystals\, favor strong electronic interactions . Additionally\, electrostatic gating further enables in situ control of c arrier density\, bandwidth\, and bandgaps\, stabilizing a wide range of qu antum phases\, including superconductivity\, correlated insulating states\ , ferromagnetism\, and Chern and fractional Chern insulators.\nMost experi mental knowledge of these systems has come from quantum transport\, scanni ng-probe\, or optical experiments in the linear-response regime. This work extends the experimental toolbox by employing cryogenic nonlinear optoele ctronic methods. By applying such techniques to graphene moiré\; mat erials\, we probe two regimes that remain difficult to access with convent ional experiments: the quantum geometry of interacting flat bands and the out-of-equilibrium states generated by large in-plane currents.\nUsing ter ahertz radiation resonant with the flat bands of magic-angle twisted bilay er graphene (MATBG)\, we observe a polarization-resolved photocurrent whos e principal axis rotates abruptly whenever the carrier density reaches an integer filling of the moiré\; Brillouin zone. We identify this resp onse as a shift current\, a second-order photocurrent governed by the inte rband quantum geometry. Microscopically\, the effect arises from the inter play between extrinsic symmetry breaking of the moiré\; crystal\, at tributed to alignment with hexagonal boron nitride or strain\, and Hartree -driven reconstruction of the MATBG flat bands. We further present prelimi nary mid-infrared photocurrent measurements aimed at resolving the spatial distribution of these broken-symmetry states.\nThe second part of this wo rk investigates the nonlinear current&ndash\;voltage response of bilayer g raphene aligned to hexagonal boron-nitride under large in-plane bias. Usin g dual-gated devices\, we map the density and displacement-field dependenc e of the out-of-equilibrium critical transition that occurs when single-ba nd transport breaks down\, accompanied by negative differential conductanc e (NDC). The critical current scales with the miniband bandwidth\, a relat ion verified across multiple devices and moiré\; systems. By combini ng temperature-dependent measurements with Boltzmann-transport simulations \, we find evidence for strong electronic overheating at the transition\, followed by the formation of an electron&ndash\;hole plasma in which elect ron&ndash\;hole collisions dominate the transport response. Scanning photo current microscopy at 4 K reveals that the transition is localized in a bu lk hotspot\; at the bistable threshold near the NDC regime\, this hotspot also acts as the active region for single-photon detection.\nTogether\, th ese results demonstrate the value of optoelectronic probes for investigati ng moiré\; quantum materials\, and open multiple directions for futu re study. Polarization-resolved terahertz photocurrent spectroscopy provid es access to the quantum geometry of correlated moiré\; bands\, wher eas high-bias transport offers a direct handle on superlattice dispersion and nonequilibrium carrier dynamics. These techniques are readily applicab le to a vast range of materials. Additionally\, our findings pave the way for novel optoelectronic devices based on moiré\; materials operatin g at long wavelengths.\nThesis Director: Prof. Dr. Frank Koppens and Dr. R oshan Krishna Kumar DTSTAMP:20260922T154346Z END:VEVENT END:VCALENDAR