BEGIN:VCALENDAR VERSION:2.0 PRODID:researchseminars.org CALSCALE:GREGORIAN X-WR-CALNAME:researchseminars.org BEGIN:VEVENT SUMMARY:Raquel Queiroz (Weizmann Institute of Science) DTSTART:20200727T160000Z DTEND:20200727T170000Z DTSTAMP:20260423T024747Z UID:QM3/13 DESCRIPTION:Title: Bou ndary Obstructed Topological Phases\nby Raquel Queiroz (Weizmann Insti tute of Science) as part of Quantum Matter meets Maths (IST\, Lisbon)\n\n\ nAbstract\nSymmetry protected topological (SPT) phases are gapped phases o f matter that cannot be deformed to a trivial phase without breaking the s ymmetry or closing the bulk gap. Here\, we introduce a new notion of a top ological obstruction that is not captured by bulk energy gap closings in p eriodic boundary conditions. More specifically\, given a symmetric boundar y termination we say two bulk Hamiltonians belong to distinct bound­ary o bstructed topological phases (BOTPs) if they can be deformed to each other on a system with periodic boundaries\, but cannot be deformed to each oth er in the open system without closing the gap at at least one high symmetr y surface. BOTPs are not topological phases of matter in the standard sens e since they are adiabat­ically deformable to each other on a torus but\, similar to SPTs\, they are associated with boundary signatures in open sy stems such as surface states or fractional corner charges. In contrast to SPTs\, these boundary signatures are not anomalous and can be removed by s ymmetrically adding lower dimensional SPTs on the boundary\, but they are stable as long as the spectral gap at high-symmetry edges/surfaces remains open. We show that the double-mirror quadrupole model of [Science\, 357(6 346)\, 2018] is a prototypical example of such phases\, and present a deta iled analysis of several aspects of boundary obstructions in this model. I n addition\, we intro­duce several three-dimensional models having bounda ry obstructions\, which are characterized either by surface states or frac tional corner charges. We also provide a general framework to study bounda ry obstructions in free-fermion systems in terms of Wannier band represent ations (WBR)\, an extension of the recently-developed band representation formalism to Wannier bands. WBRs capture the notion of topological obstruc tions in the Wannier bands which can then be used to study topological obs tructions in the boundary spectrum by means of the correspondence between the Wannier and boundary spectra. This establishes a form of bulk-boundary correspondence for BOTPs by relating the bulk band representation to the boundary topology.\n LOCATION:https://researchseminars.org/talk/QM3/13/ END:VEVENT END:VCALENDAR