Chiral superconductivity in UTe2 probed by anisotropic low-energy excitations

Abstract

Chiral spin-triplet superconductivity is a topologically nontrivial pairing state with broken time-reversal symmetry, which can host Majorana quasiparticles [1,2]. The recently discovered heavy-fermion superconductor UTe 2 [3] exhibits peculiar properties of spin-triplet pairing [3][4][5][6], and the possible chiral state has been actively discussed [7][8][9]. However, the symmetry and nodal structure of its order parameter in the bulk, which determine the Majorana surface states [2],remains controversial [10][11][12][13]. Here we focus on the number and positions of superconducting gap nodes in the ground state of UTe 2 . Our magnetic penetration depth measurements for three field orientations in the Meissner state reveal the power-law temperature dependence with exponents less than 2, which excludes single-component spin-triplet states. The anisotropy of low-energy quasiparticle excitations indicates multiple point nodes near the k y -and k z -axes, evidencing that the order parameter has multiple components in a chiral complex form. We find that most consistent is a chiral B 3u + iA u non-unitary state, which provides fundamentals of the topological properties in UTe 2 .

Spatially inhomogeneous superconductivity in UTe2

“…Polar Kerr effect measurements combined with theoretical modelling revealed that the superconducting order parameter breaks time-reversal symmetry and is likely to contain Weyl nodes 5 . More recently, magnetic penetration depth measurements revealed temperature scaling consistent with a multi-component spin-triplet state 6 . UTe 2 also exhibits striking phase diagrams as a function of applied pressure and magnetic fields.…”

Nuclear spin relaxation rate of nonunitary Dirac and Weyl superconductors

“…These findings suggest that UTe 2 is a potential candidate for odd-parity SCs. In addition, recent experimental studies have reported unconventional superconducting properties [24] such as the existence of point nodes [25,26], time-reversal symmetry breaking [27][28][29], and nonunitary pairing involving multiple components [28,30,31]. Thus, UTe 2 is a promising platform for studying nonunitary odd-parity SCs with point nodes.…”

“…In particular, we found that the point nodes of types (i) and (iv) can be distinguished from others by observing the temperature dependence of (1/T 1 ) /(1/T 1 ) ⊥ , where the subscripts and ⊥ denote the values at θ I = 0 and π/2, respectively. Finally, we discuss the implications of our results for the recently discovered superconductivity in UTe 2 [23] which is a promising material candidate for a nonunitary odd-parity SC with point nodes [24][25][26][27][28][29][30][31].…”

Anomalous acoustoelectric effect induced by clapping modes in chiral superconductors

“…In addition to the quasiparticle current, Eqs. ( 51) and (52) show that the clapping modes (δE ± ) carry the electric current. In particular, the clapping modes (δE ± ) lead to a transverse electric current, flowing perpendicular to the direction of propagation of the acoustic wave.…”

“…The angleresolved NMR measurements in UCoGe suggest non-unitary chiral order with the d-vector represented by d(k) ∼ (a 1 k a + ia 2 k b , a 3 k b + ia 4 k a , 0), where a i (i = 1, 2, 3, 4) are real coefficients [48,49]. Recently, non-unitary chiral superconductivity was also proposed in UTe 2 , where the normal state is paramagnetic, but superconductivity survives even at extremely high magnetic fields over 40 T [50][51][52][53]. The superconducting state from the paramagnetic normal state shares many common features with ferromagnetic superconductors, including strong magnetic Ising anisotropy and the reentrant superconducting transition [54,55].…”