Bulletin of the American Physical Society
APS March Meeting 2022
Volume 67, Number 3
Monday–Friday, March 14–18, 2022; Chicago
Session S45: Magnetism and the Anomalous Properties of Unconventional Superconductor UTe2Invited Live Streamed
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Sponsoring Units: DCMP Chair: Johnpierre Paglione, University of Maryland, College Park Room: McCormick Place W-375D |
Thursday, March 17, 2022 8:00AM - 8:36AM |
S45.00001: Anomalous normal fluid response in a chiral superconductor UTe2 Invited Speaker: Seokjin Bae Recently reported evidence of two-component order parameters and broken time-reversal symmetry of UTe2 suggests that UTe2 could be an example of chiral superconductor [1]. The observation raises a question about if there exists a surface normal fluid which corresponds to the non-trivial topology of the system. In this talk, we report evidence of a significant surface normal fluid response from a chiral spin-triplet superconductor UTe2 from a microwave surface impedance study [2]. The surface impedance of UTe2 crystals is measured and converted to complex conductivity. The real part of conductivity in the zero temperature limit shows anomalous residual value supporting the presence of a significant normal fluid response in the ground state. The imaginary part of conductivity follows the low temperature behavior predicted for the spin-triplet state with point nodes, consistent with the scenario of the two-component chiral order parameter. Possible extrinsic origins of residual normal fluid such as high impurity scattering rate, pair-breaking rough surface, and microwave-excited quasiparticles are examined and excluded, putting more weight on intrinsic origin. Candidate intrinsic origin such as a surface Majorana normal fluid predicted for chiral triplet superconductor is discussed. A proposal experiment for further verifying the topological origin of the surface normal fluid is also discussed. |
Thursday, March 17, 2022 8:36AM - 9:12AM |
S45.00002: Anomalous c-axis Transport Response of UTe2 Invited Speaker: Yun Suk Eo We study the temperature dependence of electrical resistivity for currents directed along all crystallographic axes of the spin-triplet superconductor UTe2. We focus particularly on an accurate determination of the resistivity along the c-axis (ρc) by using transport geometries that allow extraction of two resistivities along with the primary axes directions. In contrast to the existence of the highly anisotropic band structure, our measurement of the absolute values of resistivities in all current directions reveals a surprisingly nearly isotropic transport behavior. The temperature dependence of ρc exhibits a peak associated with the temperature much lower than the onset of Kondo coherence observed in ρa and ρb, consistent with features in magnetotransport and magnetization that point to a magnetic origin. A comparison to the temperature-dependent evolution of scattering observed in angle-resolved photoemission experiments also provides important insights into the underlying electronic structure necessary for building a microscopic model of UTe2. |
Thursday, March 17, 2022 9:12AM - 9:48AM |
S45.00003: Chiral superconductivity in UTe2 probed by anisotropic low-energy excitations Invited Speaker: Kota Ishihara The new uranium‐based superconductor UTe2 attracts much interest as a paramagnetic analog of ferromagnetic superconductors. The extremely high upper critical field along the magnetic hard axis, reentrant superconductivity, and only a small reduction of the Knight shift in UTe2 indicate the spin‐triplet superconducting state likely mediated by ferromagnetic fluctuations. Furthermore, scanning tunneling spectroscopy and optical Kerr effect measurements suggest a chiral superconducting state with topologically nontrivial surface states. Focusing on the superconducting gap, previous experimental studies reported the gap structure with point nodes which is consistent with the spin‐triplet superconductivity. However, the positions of the point nodes, which are quite important to confirm the multicomponent superconducting order parameter and superconducting symmetry, are still unrevealed. Thus, we performed the magnetic penetration depth measurements with the magnetic field along each crystallographic axis to detect the anisotropy of the low‐energy excitations, from which we can estimate the positions of the point nodes. Based on our results, we can rule out the single component order parameters, and we find that the most plausible superconducting state is a chiral B3u+iAu state, which provides fundamentals of the topological properties in UTe2. |
Thursday, March 17, 2022 9:48AM - 10:24AM |
S45.00004: Resonance from antiferromagnetic spin fluctuations for spin-triplet superconductivity in UTe2 Invited Speaker: Chunruo Duan Superconductivity has its universal origin in the formation of bound (Cooper) pairs of electrons that can move through the lattice without resistance below the superconducting transition temperature Tc. While electron Cooper pairs in most superconductors form anti-parallel spin-singlets with total spin S = 0, they can also form parallel spin-triplet Cooper pairs with S = 1 and an odd parity wavefunction. Spin-triplet pairing is important because it can host topological states and Majorana fermions relevant for fault tolerant quantum computation. However, spin-triplet pairing has not been unambiguously identified in any solid state systems. Since spin-triplet pairing is usually mediated by ferromagnetic (FM) spin fluctuations, uranium based heavy-fermion materials near a FM instability are considered ideal candidates for realizing spin-triplet superconductivity. Indeed, UTe2, which has a Tc _ 1.6 K, has been identified as a candidate for chiral spin-triplet topological superconductor near a FM instability, although the system also exhibits antiferromagnetic (AF) spin fluctuations. Here we use inelastic neutron scattering (INS) to show that superconductivity in UTe2 is coupled with a sharp magnetic excitation, termed resonance, at the Brillouin zone (BZ) boundary near AF order. Since the resonance has only been found in spin-singlet unconventional superconductors near an AF instability, its discovery in UTe2 suggests that AF spin fluctuations may also induce spin-triplet pairing or that electron pairing in UTe2 has a spin-singlet component. |
Thursday, March 17, 2022 10:24AM - 11:00AM |
S45.00005: Low-dimensional antiferromagnetic fluctuations in the superconductor UTe2 Invited Speaker: William Knafo We present inelastic-neutron-scattering measurements performed on a large single crystal (mass of 241 mg) of the new heavy-fermion superconductor UTe2 above [1] and below [2] its superconducting temperature. We confirm the presence of antiferromagnetic fluctuations with the incommensurate wavevector k1 = (0 0.57 0). We emphasize the low-dimensionality of the magnetic fluctuations, consequence of the ladder structure of the U atoms, which is indicated by weak correlations along the direction c. The absence or weakness of ferromagnetic fluctuations in our data is discussed. A phenomenological magnetic-exchange scheme, with intra-ladder ferromagnetic coupling and inter-ladder antiferromagnetic coupling along b, is proposed. A gap-opening of the antiferromagnetic fluctuations is observed in the superconducting phase. Our neutron-scattering data obtained from a unique single crystal are compared with data obtained using large assemblies of co-aligned single crystals [3,4]. A discussion about the possible signatures of the observed antiferromagnetic fluctuations in bulk electronic-transport and thermodynamic measurements, but also from NMR spectroscopy, is also proposed. These new data may stimulate the elaboration of models based on magnetically-mediated pairing to describe superconductivity in UTe2. |
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