Bulletin of the American Physical Society
APS March Meeting 2021
Monday–Friday, March 15–19, 2021; Virtual; Time Zone: Central Daylight Time, USA
Session M46: Topological Superconductivity: TheoryLive

Hide Abstracts 
Sponsoring Units: DCMP Chair: YiTing Hsu 
Wednesday, March 17, 2021 11:30AM  11:42AM Live 
M46.00001: Z2 Topologically Obstructed Superconducting Order Canon Sun, Yi Li We propose a class of topological superconductivity in which the pairing order is Z2 topologically obstructed in a timereversal invariant system in three dimensions. When two Fermi surfaces are related by timereversal and mirror symmetries, such as those in a Z2 Dirac semimetal, the interFermisurface pairing in the weakcoupling regime inherits the band topological obstruction. As a result, the pairing order cannot be welldefined over the entire Fermi surface and forms a timereversal invariant generalization of U(1) monopole harmonic pairing. A tightbinding model of the Z2 topologically obstructed superconductor is constructed based on a doped Z2 Dirac semimetal and exhibits nodal gap function. At an open boundary, the system exhibits a timereversal pair of topologically protected surface states. 
Wednesday, March 17, 2021 11:42AM  11:54AM Live 
M46.00002: Pairing Obstructions in Topological Superconductors Frank Schindler, Barry Bradlyn, Mark Fischer, Titus Neupert The modern understanding of topological insulators is based on Wannier obstructions in position space. This insight suggests that there should also be a position space perspective on topological superconductors. For a proximitized nanowire, I will explain in what sense particlehole symmetry implies that Cooper pairs are weakly paired in the topological phase. At the same time, this regime is captured by a quantized separation of Majorana pairing centers from the atomic positions. I will apply our results to diagnose secondorder topological superconductors in two dimensions. Our work establishes a vantage point for the generalization of topological quantum chemistry to superconductivity. 
Wednesday, March 17, 2021 11:54AM  12:06PM Live 
M46.00003: HigherOrder Topological Superconductivity with Swave Pairing in Twisted Bilayer Graphene Aaron Chew, Yijie Wang, Andrei B Bernevig, Zhida Song We demonstrate that introducing swave superconductivity into twisted bilayer graphene induces symmetryprotected higherorder topological superconductivity. Multiple Majorana zero modes are bound to $C_{2x}$protected corners of the sample. Using the Dirac Hamiltonian for twisted bilayer graphene, we explicitly construct Hamiltonians describing the edge and demonstrate the existence of corner modes at domain walls, provided the appropriate symmetries are preserved. We characterize the superconductor using the Wilson loop formalism and prove it is anomalous, and verify the presence of corner states with numerics. Finally, we study the fate of the Majorana zero modes in the presence of interactions, and suggest experimental setups to detect the topological superconductivity. 
Wednesday, March 17, 2021 12:06PM  12:18PM Live 
M46.00004: Oddfrequency Cooper pairing in chiral symmetric systems Shun Tamura, Yukio Tanaka, Shintaro Hoshino Oddfrequency Cooper pairs emerge at the edge of topological superconductors. We study chiral symmetric systems, where the Hamiltonian anticommutes with a chiral operator and the topological number called a winding number can be defined in the bulk. We found that the oddfrequency Cooper pair at the edge is related to an extended version of the winding number we call this relation as spectral bulk boundary correspondence (SBBC) [1]. Then the total amount of the oddfrequency Cooper pair at the edge is predicted from the bulk value through the SBBC. From the SBBC with low frequency, oddfrequency Cooper pair diverges in the topologically nontrivial phase and it is linear in the trivial phase. We will also discuss the robustness of this relation against impurity. 
Wednesday, March 17, 2021 12:18PM  12:30PM Live 
M46.00005: Faithful derivation of symmetry indicators: A case study for topological superconductors with timereversal and inversion symmetries ShengJie Huang, YiTing Hsu We propose a protocol for deriving symmetry indicators that depend on a minimal set of necessary symmetry data of the bulk bands and can diagnose boundary features. Specifically, to obtain indicators manifesting clear bulkboundary correspondence, we combine the topological crystal classification scheme in the real space and a twisted equivariant K group analysis in the momentum space. The key step is to disentangle the generally mixed strong and weak indicators through a systematic basismatching procedure between our realspace and momentumspace approaches. We demonstrate our protocol using an example of twodimensional timereversal oddparity superconductors, where the inversion symmetry is known to protect a higherorder phase with corner Majoranas. Symmetry indicators derived from our protocol can be readily applied to ab initio database and could fuel material predictions for strong and weak topological crystalline superconductors with various boundary features. 
Wednesday, March 17, 2021 12:30PM  12:42PM Live 
M46.00006: Multiorbital superconductivity in 4HbTaS2 in the dirty limit: Part I Ezra DayRoberts, David Dentelski, Turan Birol, Jonathan Ruhman, Rafael Fernandes The transition metal dichalcogenide 4HbTaS_{2} consists of alternating single layers of octahedrally oriented T structure and trigonally oriented H structure. Separately, in their bulk form, these two structures have very different ground states: while 1TTaS_{2} is a strongly correlated insulator and a spinliquid candidate, 2HTaS_{2} is a superconductor. The 4Hb compound also has a superconducting ground state, which has been argued to be chiral due to the onset of a μSR signal concomitant to the superconducting transition. 
Wednesday, March 17, 2021 12:42PM  12:54PM Live 
M46.00007: Multiorbital superconductivity in 4HbTaS2 in the dirty limit: Part II David Dentelski, Ezra DayRoberts, Turan Birol, Rafael Fernandes, Jonathan Ruhman <div style="direction: ltr;">The transition metal dichalcogenide 4HbTaS_{2} consists of alternating single layers of octahedrally oriented T structure and trigonally oriented H structure. Separately, in their bulk form, these two structures have very different ground states: while 1TTaS_{2} is a strongly correlated insulator and a spinliquid candidate, 2HTaS_{2} is a superconductor. The 4Hb compound also has a superconducting ground state, which has been argued to be chiral due to the onset of a μSR signal concomitant to the superconducting transition. </div> <div style="direction: ltr;">At the same time however, measurements of the heat capacity below Tc indicate the existence of a large density of gapless states. In part II of this talk, we show how a residual Fermi surface can naturally emerge in a multipocket superconductor due to pairbreaking impurities with pocketdependent scattering rates. Based on the DFT calculations and effective tightbinding model introduced in part I, we show how a strong pocketdependent coupling to magnetic moments arising from the 1T layers naturally forms in this system. We then compute the tunneling DOS and heat capacity, which agree well with the data. </div> 
Wednesday, March 17, 2021 12:54PM  1:06PM Live 
M46.00008: Linking structure of centrosymmetric superconductors in 3D momentum space Sunje Kim, BohmJung Yang In topological semimetals and nodal superconductors, band crossings between occupied and unoccupied bands form stable nodal points/lines/surfaces carrying quantized topological charges. In particular, in centrosymmetric systems, some nodal structures at the Fermi energy carry two distinct topological charges, and thus they are called doubly charged nodes. We show that doubly charged nodal surfaces of centrosymmetric superconductors in threedimensions always develop peculiar linking structures with nodal points or lines formed between occupied bands below the Fermi energy. Such linking structures can naturally explain the inherent relationship between the charge of the node below the Fermi energy and the two charges of the nodal surfaces at the Fermi energy. Based on the AltlandZirnbauer (AZ)type tenfold classification of nodes with additional inversion symmetry I, which is called the AZ+I classification, we provide the complete list of linking structures of doubly charged nodes in centrosymmetric systems. The linking structures of doubly charged nodes clearly demonstrate that not only the local band structure around the node but also the global band structure play a critical role in characterizing the nodes of gapless topological phases. 
Wednesday, March 17, 2021 1:06PM  1:18PM Live 
M46.00009: Optical Ninvariant of graphene's viscous Hall fluid Todd Van Mechelen, Wenbo Sun, Zubin Jacob Over the past three decades, graphene has become the prototypical platform for discovering unique phases of topological matter. Both the Chern and Z2 insulator were first predicted in graphene, which led to a veritable explosion of research in topological materials. Here, we introduce a new topological classification of twodimensional matter  the optical Nphases. The Chern and Z2 phases are related to charge and spin transport respectively, whereas the Nphases are connected to polarization transport. We prove that graphene's viscous Hall fluid is optically nontrivial with the underlying physical mechanism being Hall viscosity. We discover spin1 Neeltype skyrmions in the bulk magnetoplasma and a deep subwavelength phenomenon reminiscent of the Meissner effect. We also propose a novel probe of topological matter, evanescent magnetooptic Kerr effect (eMOKE) spectroscopy. Lastly, we rigorously analyze the boundary physics which reveals gapless edge magnetoplasmons that are immune to backscattering and navigate sharp defects with impunity. 
Wednesday, March 17, 2021 1:18PM  1:30PM Live 
M46.00010: Optically induced topological superconductivity via Floquet interaction engineering Pouyan Ghaemi, Hossein Dehghani, Mohammad Hafezi We study the photoinduced superconductivity in a twovalley semiconductor with a massive Dirac type band structure. The superconducting phase results from the outofequilibrium excitation of carriers in the presence of Coulomb repulsion and is stabilized by coupling the driven semiconductor to a bosonic or fermionic thermal bath. We consider a circularlypolarized light pump and show that by controlling the detuning of the pump frequency relative to the band gap, different types of chiral superconductivity would be induced. The emergence of novel superconducting states, such as the chiral pwave pairing, results from the Floquet engineering of the interaction. This is realized by modifying the form of the Coulomb interaction by projecting it into the states that are resonant with the pump frequency. We discuss a promising experimental platform to realize our proposal. 
Wednesday, March 17, 2021 1:30PM  1:42PM Live 
M46.00011: Driven superconductors with anomalous Floquet higherorder topology DinhDuy Vu, Ruixing Zhang, Zhicheng Yang In this work, we classify periodically driven class D superconductors in two dimensions with higherorder topologies that are statically impossible. While hosting trivial Floquet Bogoliubovde Gennes (BdG) bands in the bulk, such an anomalous Floquet higherorder topological superconductor (AFHOTSC) features corner Majorana zero and π modes that are protected by crystalline symmetries. Unlike its static counterpart, the presence of corner Majorana modes in an AFHOTSC is indicated by robust nodal structures in the eigenspectrum of the timeevolution operator. By proposing a set of higherorder topological indices, we achieve a systematic classification of all 2D AFHOTSCs protected by rotation group symmetries in class D. Concrete lattice models of AFHOTSCs are constructed to demonstrate applications of our theory. Experimental realizations and detections of AFHOTSCs are also discussed. 
Wednesday, March 17, 2021 1:42PM  1:54PM Live 
M46.00012: Z_{2}enriched symmetry indicators for topological superconductors in the 1651 magnetic space groups Seishiro Ono, Hoi Chun Po, Ken Shiozaki Symmetrybased diagnosis of topological insulators and semimetals has enabled largescale discovery of topological materials candidates. One important new ingredient in the analysis of topological superconductivity is the presence of Z_{2}valued Pfaffian invariants associated with certain highsymmetry momenta. Such topological invariants lie beyond the conventional scope of symmetry representation theory for band structures, and as such they are nontrivial to incorporate into the systematic calculations of the symmetry indicators of band topology. Here, we overcome this challenge and report the full computation of the Z_{2}enriched symmetry indicators for SCs in all symmetry settings. Our framework can also be readily integrated with DFT calculations to elucidate on the possible properties of unconventional superconductivity in a given compound. As a demonstration, we analyze the interplay between pairing symmetry and topologies in the recently discovered SCs CaPtAs and CaSb_{2}. 
Wednesday, March 17, 2021 1:54PM  2:06PM Live 
M46.00013: Topological skyrmion phases of matter Ashley Cook We introduce topological phases of matter defined by skyrmions in the ground state spinor pseudospinexpectation value textures in the Brillouin zone, the chiral and helical topological skyrmion phases of matter. These phases are protected by a symmetry present in all centrosymmetric superconductors and detectable with ARPES techniques. We demonstrate the physics of the chiral topological skyrmion phase first in simple four band models and then consider a tightbinding model for spintriplet superconductivity in transition metal oxides and find it realizes each of the chiral and helical topological skyrmion phases of matter. The chiral phase is furthermore realized for a parameter set characterizing Sr_{2}RuO_{4} with spintriplet superconductivity. We also find two types of topological phase transitions by which the skyrmion number can change. The second type occurs without the closing of energy gaps in a system described by a quadratic Hamiltonian without breaking the protecting symmetries when spinorbit coupling is nonnegligible. This contradicts the ``flat band'' limit assumption of the entanglement spectrum, Wilson loops, and the tenfold way classification scheme of topological phases of matter. 
Wednesday, March 17, 2021 2:06PM  2:18PM Live 
M46.00014: NonBloch band theory in bosonic Bogoliubovde Gennes systems Kazuki Yokomizo, Shuichi Murakami In our previous work, we established a nonBloch band theory in onedimensional tightbinding nonHermitian systems [1,2]. We show a way to determine the generalized Brillouin zone for the complex Bloch wave number in a large open chain. In our recent work, we construct the nonBloch band theory in bosonic Bogoliubovde Gennes (BdG) systems. By using our theory, one can investigate general nonHermitian properties in bosonic BdG systems with open boundary conditions in the thermodynamic limit. In this story, we find rich aspects of the nonHermitian skin effect, i.e. infinitesimal instability and reentrant behavior in the bosonic KitaevMajorana chain. [1] K. Yokomizo and S. Murakami, Phys. Rev. Lett. 123, 066404 (2019). [2] K. Yokomizo and S. Murakami, Phys. Rev. Research 2, 043045 (2020). 
Follow Us 
Engage
Become an APS Member 
My APS
Renew Membership 
Information for 
About APSThe American Physical Society (APS) is a nonprofit membership organization working to advance the knowledge of physics. 
© 2021 American Physical Society
 All rights reserved  Terms of Use
 Contact Us
Headquarters
1 Physics Ellipse, College Park, MD 207403844
(301) 2093200
Editorial Office
1 Research Road, Ridge, NY 119612701
(631) 5914000
Office of Public Affairs
529 14th St NW, Suite 1050, Washington, D.C. 200452001
(202) 6628700