Bulletin of the American Physical Society
APS March Meeting 2021
Monday–Friday, March 15–19, 2021; Virtual; Time Zone: Central Daylight Time, USA
Session A49: Topology and Chiral SuperconductivityInvited Live

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Sponsoring Units: DCMP Chair: Johnpierre Paglione, University of Maryland, College Park 
Monday, March 15, 2021 8:00AM  8:36AM Live 
A49.00001: A Theory from Firstprinciples on Magnetism and Superconductivity in UTe2 Invited Speaker: Youichi Yanase We theoretically study magnetism and superconductivity in UTe2, a recently discovered strong candidate for an oddparity spintriplet superconductor [1,2]. Theoretical studies for this compound faced difficulty because firstprinciples calculations predicted an insulating electronic state, incompatible with superconducting instability. To overcome this problem, we consider electron correlation effects by a GGA+U method and show the insulatormetal transition by Coulomb interaction [1]. Using Fermi surfaces obtained as a function of U, we clarify the topological properties of possible superconducting states. Fermi surface formulas indicate topological superconductivity at an intermediate U for all the oddparity pairing symmetry in the Immm space group. Symmetry and topology of superconducting gap nodes are analyzed, and the gap structure of UTe2 is predicted. A recent ARPES experiment is consistent with the topological superconductivity. 
Monday, March 15, 2021 8:36AM  9:12AM Live 
A49.00002: Multicomponent superconducting order parameter in UTe2 Invited Speaker: Ian Hayes Topological superconductivity holds great promise both as a trove of new physics and a source of powerful applications. However, progress in the field has been limited by a shortage of candidate materials. The recently discovered triplet superconducting state of UTe2 has generated great excitement for this reason, and motivated a careful study of the superconducting order parameter in this system. This talk will present specific heat data that shows that UTe2 actually possesses two nearly coincident superconducting transitions, indicating that the superconducting order parameter is multicomponent. Through a detailed study of the evolution of these two transitions under applied magnetic fields along all three crystal axes, we have been able to deduce important constraints on the symmetry properties of the order parameter. On the basis of this analysis I will discuss the prospects for topological superconductivity in UTe2 as well as possible explanations for the presence of two nearly degenerate superconducting order parameters. 
Monday, March 15, 2021 9:12AM  9:48AM Live 
A49.00003: Even parity superconducting state of the strongly correlated Fermi Liquid Sr_{2}RuO_{4} Invited Speaker: Stuart Brown Unambiguous identification of the superconducting order parameter symmetry of the strongly correlated Fermi Liquid Sr_{2}RuO_{4} has remained elusive for more than a quarter century. While a chiral pwave ground state analogue to superfluid ^{3}HeA was ruled out only very recently [1], other proposed pwave scenarios are still considered viable. We study the fielddependence of the ^{17}O Knight shift at very low temperature, and by way of comparing to corresponding specific heat measurements, conclude that the shift results can be accounted for by the expected fieldinduced quasiparticle response only. This allows us to assign an upper bound for the condensate magnetic response of <10% measured relative to the normal state response [2]. The results are sufficient to exclude oddparity candidates. Implications for these results will be discussed in the broader context of pertinent experimental findings and theoretical proposals. 
Monday, March 15, 2021 9:48AM  10:24AM Live 
A49.00004: Nonunitary Triplet Pairing on the Border of Magnetism and Upper Critical Field in UTe_{2} Invited Speaker: Andriy Nevidomskyy The discovery of superconductivity in the uranium compound UTe_{2} in 2019 and the identification of it likely being a triplet superconductor was one of the most tantalizing recent developments in the field of unconventional superconductivity. Despite many experimental probes, the precise nature of the superconducting order parameter is still under debate. In this talk, I will show, using a combination of phenomenological freeenergy analysis and the weakcoupling BCS theory, coupled with the ab initio band structure calculations, that a timereversal breaking superconducting phase, so called nonunitary state, is stabilized in UTe_{2} [1]. In the past, such nonunitary states have been proposed for ferromagnetic superconductors such as UGe_{2}, whereas in the present proposal, the nonunitary chiral state is predicted to be stable even in the nonmagnetic phase of UTe_{2}. This result has a number of consequences  from chiral edge modes, recently observed in STM, to nonvanishing magnetooptical Kerr effect. Furthermore, we predict the nodal points of the gap to be topologically stable, with associated surface Majorana states and their predicted contribution to thermal Hall effect [1]. 
Monday, March 15, 2021 10:24AM  11:00AM Live 
A49.00005: Topology and superconductivity in UTe_{2}: a microscopic perspective Invited Speaker: Daniel Agterberg Recent Kerr effect and specific heat measurements have placed strong constraints on the pairing symmetry in UTe_{2} [1]. Here we discuss the implications of these measurements on the microscopic descriptions of the superconducting state of UTe_{2}, revealing how topologically protected Weyl nodes naturally arise. We further present a microscopic description of the superconducting state that is motivated by our DFT+U calculations. These calculations reveal an felectron derived topological band near the chemical potential, which illuminates the structure of spinorbit interactions and their role on the superconducting state [2]. 
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