Detection of a Pair Density Wave State in UTe2

Abstract

Although UTe2 is a very promising candidate material to embody bulk topological superconductivity, its superconductive order-parameter ∆(k) remains unknown. Many diverse forms for ∆(k) are physically possible because, in uranium-based heavy fermion materials, strongly hybridized flat bands of composite fermions generate highly complex interactions. Moreover, in such materials intertwined density waves of spin (SDW), charge (CDW) and pairs (PDW) may interpose, with the latter state exhibiting spatially modulating, superconductive order-parameter ∆(r), electron pair density and pairing energy gap. Hence, the newly discovered CDW state in UTe2 motivates the exciting prospect that a PDW state may exist in this material. To search for a PDW in UTe2, we visualize the pairing energy-gap with μV-scale energy-resolution made possible by superconductive STM tips at subkelvin temperatures. We detect three PDWs, each with peak-peak gap modulations circa 10 μeV and at incommensurate wavevectors P_(i=1,2,3) that are indistinguishable from the wavevectors Q_(i=1,2,3) of the prevenient CDW. Concurrent visualization of the UTe2 superconductive PDWs and the non-superconductive CDWs reveals that every Pi: Qi pair is registered to each other spatially, but with a relative phase δϕ≈π. From these observations, and given UTe2 as a spin-triplet superconductor, the PDW state detected here should be a spin-triplet pair density wave. While such states do exist in superfluid 3He, for superconductors they are unprecedented.

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