Stability of a Nonunitary Triplet Pairing on the Border of Magnetism in UTe$_2$

Abstract

Motivated by the recent discovery of superconductivity in UTe2, we analyze the stability and nodal structure of various triplet superconducting order parameters. Using a combination of symmetry group-theoretical analysis, phenomenological Landau free energy and weak-coupling BCS theory, we show that chiral nonunitary superconducting order can be stabilized on the border of ferromagnetism in UTe2, even in the absence of long-range magnetic order. We further perform first principles density functional theory (DFT) calculations of the so-called "small" Fermi surface, excluding the contribution of U f -electrons, and find it to be in excellent agreement with the recent angular resolved photoemission study and DFT+DMFT calculations. This permits us to elucidate the nodal structure of the superconducting gap, which we find generically to possess point nodes along the crystallographic a direction, in agreement with experiments. The topological stability of these point nodes and their associated Majorana surface states is analyzed. The nonunitary structure of the predicted superconducting state supports chiral edge modes observed in recent scanning tunneling microscopy (STM) data and is predicted to result in a non-vanishing magneto-optical Kerr effect.

Unconventional Superconductivity in UTe2

“…4(d)] resemble the results of the DFT+DMFT calculation which assumes a large U [15,17]. Thus, the rectangular Fermi surfaces are consistently obtained by the DFT+U and DFT+DMFT calculations, and they are similar to the Fermi surfaces of ThTe 2 without f -electrons [17,18,19]. Combining these results, we suppose that the f -electrons are almost localized in the large Coulomb interaction region.…”

“…Accordingly, theories proposed various superconducting states. Most of them assume purely odd-parity spintriplet superconductivity, such as the B 3u + iB 2u state [108] and the B 3u + iB 1u state [19]. Assuming that long-lived ferromagnetic fluctuation plays the same role as the ferromagnetic long-range order and ignoring the spin-orbit coupling, a spin-triplet pairing state with d(k) = (k b + ik c )( b + iĉ) was also proposed [109,106,197].…”

Chiral superconductivity in UTe2 probed by anisotropic low-energy excitations

“…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 .…”

Interplay between magnetism and superconductivity in UTe2

“…UTe 2 is an example of superconductivity in the presence of ferromagnetic fluctuations that persist to temperatures approaching zero, suggesting a magnetic quantum critical phenomenon. The observations of two successive superconducting transitions in specific heat and spontaneous polar Kerr effect (PKE) [1] along the crystallographic c-axis led to the conclusion that superconductivity in UTe 2 is characterized by a two-component order-parameter that breaks time reversal symmetry [1][2][3]. While several possibilities were considered for the symmetry representation, the one that seemed to agree with the data was (ψ 1 ∈ B 3u , ψ 2 ∈ B 2u ) [1,3], implying a non-unitary order-parameter, which we previously argued can be stabilized by magnetic fluctuations [1].…”

“…While two superconducting states have been confirmed, the nature of the magnetic interaction associated with each of the states could be different [7,8]. Nevertheless, extrapolation from other similar uraniumbased superconductors including URhGe, UCoGe and UGe 2 strongly support a spin triplet state mediated by ferromagnetic fluctuations [1][2][3]. Focusing on the superconducting state at ambient pressure, one way to resolve the magnetic nature of the superconducting state of UTe 2 is to test what magnetic state the superconducting orderparameter nucleates upon interaction with the magnetic fluctuations in both, the Meissner and vortex states.…”