Session A41: The Superconducting Order Parameter of UTe2

Single-Component Superconductivity in UTe2 at Ambient Pressure

The microscopic mechanism of Cooper pairing in a superconductor leaves its fingerprint on the symmetry of the order parameter. UTe₂ has been inferred to have a multi-component order parameter that entails exotic effects like time reversal symmetry breaking. However, recent experimental observations in newer-generation samples have raised questions about this interpretation, pointing to the need for a direct experimental probe of the order parameter symmetry. Here, we use pulse echo ultrasound to measure the elastic moduli of samples of UTe₂ that exhibit both one and two superconducting transitions. We demonstrate the absence of thermodynamic discontinuities in the shear elastic moduli of both single- and double-transition samples, providing direct evidence that UTe₂ has a single-component superconducting order parameter. We further show that the superconductivity is highly sensitive to compression strain along the a and c axes, but insensitive to strain along the b axis. This leads us to suggest a single-component, odd-parity order parameter−specifically the B_2u order parameter−as the most likely order parameter in UTe₂.   

Universal nodal behavior in the thermal conductivity of superconducting UTe2

Soon after the discovery of superconductivity in uranium ditelluride (UTe2), evidence began accumulating that the pairing state is odd-parity (triplet). This opens the possibility of topological superconductivity in UTe2 and motivates a careful study of the presence or absence of nodes and their topology/location. In this talk I will present data on the thermal transport of UTe2 in its superconducting state. These data were collected on a series of samples with transition temperatures ranging from 1.6K to 2.1K, and residual resistivity ratios from 30 to 600, synthesized by both chemical vapor transport and flux methods. We will discuss the evolution of the zero-temperature intercept of kappa/T as a function magnetic field applied along the crystal's a-axis and its consistency with a point node gap structure, as well as the significance of these results in the context of other experiments on the superconducting order parameter in UTe2.   

Time-reversal symmetry breaking in UTe2

Topological superconductivity is a long-sought state of matter in bulk materials, and odd-parity superconductor UTe2 is a prime candidate. Several experiments provide evidence that the superconducting order parameter in UTe2 is multicomponent and breaks time-reversal symmetry. Because UTe2 has orthorhombic symmetry, proposals for its superconducting order parameter involve two nearly-degenerate irreducible representations (e.g., B2u + iB3u). By applying symmetry-breaking uniaxial strain, we probe multicomponent superconductivity by looking for a splitting of the transition temperatures. Our findings do not reveal any evidence for two nearly-degenerate order parameters near ambient pressure. Next, we performed Kerr effect measurements on a number of UTe2 samples - grown via both chemical vapor transport and the molten-salt-flux methods - that show a single superconducting transition between 1.6 K and 2.1 K. Our results show no evidence for a spontaneous Kerr signal in zero field measurements. Finally, usr measurements on a sample grown via molten-salt-flux also show no evidence for time-reversal symmetry breaking in the superconducting state. These results imply that the superconducting state of UTe2 does not intrinsically break time-reversal symmetry.   

Theory of the low- and high-field superconducting phases of UTe2

Recent nuclear magnetic resonance and calorimetric experiments have observed that UTe₂ exhibits a transition between two distinct superconducting phases as a function of magnetic field strength for a field applied along the crystalline b axis. To determine the nature of these phases, we employ a microscopic two-band minimal Hamiltonian with the essential crystal symmetries and structural details. We also adopt anisotropic ferromagnetic exchange terms. We study the resulting pairing symmetries and properties of these low- and high-field phases in mean-field theory.    

Fully gapped pairing state in spin-triplet superconductor UTe2

Spin-triplet superconductors provide an ideal platform for realizing topological superconductivity with emergent Majorana quasiparticles. The promising candidate is the recently discovered superconductor UTe₂, but the symmetry of the superconducting order parameter remains highly controversial. Here we determine the superconducting gap structure by the thermal conductivity of ultra-clean UTe₂ single crystals. We find that the a axis thermal conductivity divided by temperature κ/T in zero-temperature limit is vanishingly small for both magnetic fields H||a and H||c axes up to H/H_c2 ~ 0.2, demonstrating the absence of any types of nodes around a axis contrary to the previous belief. The present results, combined with the reduction of the NMR Knight shift in the superconducting state, indicate that the superconducting order parameter belongs to the isotropic A_u representation with a fully gapped pairing state, analogous to the B phase of superfluid ³He. These findings reveal that UTe₂ is likely to be a long-sought three-dimensional (3D) strong topological superconductor characterized by a 3D winding number, hosting helical Majorana surface states on any crystal plane.