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Resonance from antiferromagnetic spin fluctuations for spin-triplet candidate superconductor UTe2
Speaker: Pengcheng Dai (Rice University)
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, analogous to the equal spin pairing state in the superfluid 3He. Spin-triplet pairing is important because it can host topological states and Majorana fermions relevant for fault tolerant quantum computation. However, spin-triplet pairing is rare and 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 strong 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 at the Brillouin zone (BZ) boundary near AF order, analogous to the resonance seen in high-Tc copper oxides, iron-based, and heavy-fermion superconductors. We find that the resonance in UTe2 occurs below Tc at an energy Er = 7.9kBTc (kB is Boltzmann’s constant) and at the expense of low-energy spin fluctuations. Since the resonance has only been found in spin-singlet superconductors near an AF instability, its discovery in UTe2 suggests that AF spin fluctuations can may also induce spin-triplet pairing for superconductivity  or that electron pairing in UTe2 has a spin-singlet component.
Host: Sang-Wook Cheong