BEGIN:VCALENDAR VERSION:2.0 PRODID:Icfo X-PUBLISHED-TTL:P1W BEGIN:VEVENT UID:69d0d2a155491 DTSTART:20260116T110000Z SEQUENCE:0 TRANSP:OPAQUE DTEND:20260116T120000Z LOCATION:Seminar Room SUMMARY:ICFO | HECTOR OCHOA CLASS:PUBLIC DESCRIPTION:The collective vibrations of moiré\; superlattices or &ld quo\;moiré\; phonons&rdquo\; arise from the relative motion of twist ed van der Waals layers. Among these modes\, there are always two acoustic branches associated with a free-energy invariance of the system against r elative shifts of the layers\; the phasons. The dynamics of these modes is very different compared to conventional acoustic phonons in crystalline s olids\, though. In combination with the inhomogeneous distribution of char ge within the moiré\; supercell\, this dynamics gives rise to otherw ise unexpected responses to electromagnetic fields. I will give three exam ples. First\, in polar multilayers\, like twisted transition-metal dichalc ogenides\, moiré\; phonons can be tuned by the application of electr ic fields\, which couples to the vertical polarization tied to the local s tacking order. For small twist angles\, increasing the electric field lead s to a universal moiré\; phonon spectrum characterized by a substant ially softened longitudinal phason and several flat optical bands. Second\ , in non-polar multilayers\, like twisted bilayer graphene\, moiré\; phonons couple to electromagnetic fields by borrowing optical spectral we ight from the electron-hole continuum via interband matrix elements of the electron-phonon coupling. In particular\, phasons become infrared active by carrying a dipole moment proportional to the amount of free charge adde d to the system\, a response that admits a geometrical interpretation in t erms of a sliding Chern number. Finally\, I will describe how electron sca ttering off phason modes may explain the linear-in-T resistivity of twiste d bilayer graphene and\, in particular\, its survival down to temperatures much lower than the Debye or Bloch-Grü\;neisen scales. I will show ne w numerical and experimental evidence in support of this idea. DTSTAMP:20260404T085809Z END:VEVENT END:VCALENDAR