Bulletin of the American Physical Society
APS March Meeting 2020
Volume 65, Number 1
Monday–Friday, March 2–6, 2020; Denver, Colorado
Session X67: New Developments in Higher Order TopologyInvited

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Sponsoring Units: DCMP Room: Four Seasons 23 
Friday, March 6, 2020 11:15AM  11:51AM 
X67.00001: Higher order topology in superconducting and interacting electronic systems Invited Speaker: Titus Neupert Topological states with hinge and corner modes, socalled higher order topological insulators, have been predicted to exist in various crystalline materials such as elementary bismuth and WTe2. I will start with a review of the theoretical foundations of higherorder topology, introducing topological invariants to identify such phases as well as an explanation for their generalized topological bulkboundary correspondence. Second, I will discuss venues for realizing higher order topology in superconductors, magnetic, and strongly interacting systems. To define (crystalline) topology in superconductors, I will discuss the question how theoretically a trivial reference state has to be defined (an atomic limit), and how a topological classification can be built up on this. For magnetic systems, the possibility of topological Kondo insulators hosting a phase with hinge states upon undergoing a magnetic instability is outlined. Finally, the generalization of the bulkboundary correspondence of higherorder topology will be elevated to the interacting case, including the possibility of surface topological order. 
Friday, March 6, 2020 11:51AM  12:27PM 
X67.00002: Probing the topological properties of Bismuth, a second order topological insulator Invited Speaker: Sophie Gueron Higher Order Topological Insulators are a new family of materials whose conduction paths are located not at the crystal boundaries but at the boundary of the boundaries: Thus, three dimensional Second Order Topological Insulators should conduct via one dimensional paths at the crystal hinges. In the talk I will demonstrate how mesoscopic physics experiments (along with some theory!) led to the discovery that Bismuth was the first example of a solid state Second Order Topological Insulators [1]. I will show how we detected such hinge states in monocrystalline nanowires connected to superconducting contacts, and further used the supercurrentversusphase relation [2,3] to demonstrate their ballistic nature. I will also present our recent measurement of the nanowire’s linear response to an ac phase difference excitation. I will argue that the response’s sharp dissipation peak when the superconducting phase difference is π is a telltale sign of topological protection at the hinges [4,5]. 
Friday, March 6, 2020 12:27PM  1:03PM 
X67.00003: HigherOrder Topology in Febased Superconductors and Related Materials Invited Speaker: Sankar Das Sarma

Friday, March 6, 2020 1:03PM  1:39PM 
X67.00004: Partial Lattice Defects in HigherOrder Topological Insulators Invited Speaker: Raquel Queiroz Nonzero weak topological indices are thought to be a necessary condition to bind a single helical mode to lattice dislocations. In this work we show that higherorder topological insulators (HOTIs) can, in fact, host a single helical mode along screw or edge dislocations (including step edges) in the absence of weak topological indices. When this occurs, the helical mode is necessarily bound to a dislocation characterized by a fractional Burgers vector, macroscopically detected by the existence of a stacking fault. The robustness of a helical mode on a partial defect is demonstrated by an adiabatic transformation that restores translation symmetry in the stacking fault. We present two examples of HOTIs, one intrinsic and one extrinsic, that show helical modes at partial dislocations. Since partial defects and stacking faults are commonplace in bulk crystals, the existence of such helical modes can measurably affect the expected conductivity in these materials. 
Friday, March 6, 2020 1:39PM  2:15PM 
X67.00005: Resolving the topological classification of bismuth using topological defects Invited Speaker: Haim Beidenkopf The growing diversity of topological classes leads to ambiguity between classes that share similar boundary phenomenology. This is the status of bulk bismuth. Recent studies have classified it as either a strong or a higherorder topological insulator, both of which host helical modes on their boundaries. We resolve the topological classification of bismuth by spectroscopically mapping the response of its boundary modes to a screwdislocation. We find that the onedimensional mode, on stepedges, extends over a wide energy range and does not open a gap near the screwdislocations. This signifies that this mode binds to the screwdislocation, as expected for a material with nonzero weak indices. We argue that the small energy gap, at the time reversal invariant momentum L, positions bismuth within the critical region of a topological phase transition between a higherorder topological insulator and a strong topological insulator with nonzero weak indices. 
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