BEGIN:VCALENDAR VERSION:2.0 PRODID:Data::ICal 0.22 BEGIN:VEVENT DESCRIPTION:Professor Xiaoyang Zhu \, Columbia University\n\n
Lead halide perovsk ites have been demonstrated as high performance materials in solar cells a nd light-emitting devices. These materials are characterized by coherent b and transport expected from crystalline semiconductors\, but dielectric re sponses and phonon dynamics typical of liquids. \; Here we explain the essential physics in this class of materials based on their dielectric fu nctions and dynamic symmetry breaking on nano scales. We show that the die lectric function in the THz region may lead to dynamic and local ordering of polar nano domains by an extra electron or hole\, resulting a quasipart icle which we call a \;ferroelectric large polaron\, a concep t similar to solvation in chemistry. Compared to a conventional large pola ron\, the collective nature of polarization in a ferroelectric large polar on may give rise to order(s)-of-magnitude larger reduction in the Coulomb potential. We show that the shape of a ferroelectric polaron resemble that of a Belgian waffle. Using two-dimensional coherent phonon spectroscopy\, we directly probe the energetics and local phonon responses of the ferroe lectric large polarons. We find that that electric field from a nascent e- h pair drives the local transition to a hidden ferroelectric order on picosecon d time scale.  \;The ferroelectric or Belgian waffle polarons may explain t he defect tolerance and low recombination rates of charge carriers in lead halide perovskites\, as well as providing a design principle of the &ldqu o\;perfect&rdquo\; semiconductor for optoelectronics. p>\n DTSTART:20191004T193000Z LOCATION:Physics Building\, Room 204 SUMMARY:Ferroelectric Polarons\, Belgian Waffles\, and Principles for “Pe rfect” Semiconductors END:VEVENT END:VCALENDAR