Comparison of two superconducting phases induced by a magnetic field in UTe2

Abstract

Superconductivity induced by a magnetic field near metamagnetism is a striking manifestation of magnetically-mediated superconducting pairing. After being observed in itinerant ferromagnets, this phenomenon was recently reported in the orthorhombic paramagnet UTe2. Here we explore the phase diagram of UTe2 under two magnetic-field directions: the hard magnetization axis b, and a direction titled by ≃25-30° from b in the (b,c) plane. Zero-resistivity measurements confirm that superconductivity is established beyond the metamagnetic field Hm in the tilted-field direction. While superconductivity is locked exactly at fields either smaller (for H | | b), or larger (for H tilted by ≃27° from b to c), than Hm, the variations of the Fermi-liquid coefficient in the electrical resistivity and of the residual resistivity are similar for the two field directions. The resemblance of the normal states for the two field directions puts constraints for theoretical models of superconductivity and implies that some subtle ingredients must be in play.

Unconventional Superconductivity in UTe2

“…The shape of Brillouin zone looks similar to that of the well-known ThCr 2 Si 2 -type structure, but it is elongated along a-axis. We further want to emphasise that the [011] direction, corresponds to the direction tilted by 23.7 deg from b to c-axis, at which the field-reentrant superconductivity is observed above the metamagnetic transition field H m (see section 5.5.1 below) [9,10]. It should be stressed that the inversion center of the structure is not located at the U site, but exists at the middle of the U rung in spite of the body-centered orthorhombic structure with the global inversion symmetry.…”

Slow Electronic Dynamics in the Paramagnetic State of UTe$_2$

“…[2][3][4][5] The formation of the spin-triplet pairing was experimentally suggested from tiny decrease in NMR Knight shift below the SC transition temperature T SC ∼ 1.6 K, [6][7][8] and an exceptionally large upper critical field H c2 far exceeding the Pauli-limiting field. 1,[9][10][11][12] The spin-triplet pairing was further supported by the later discovery of a field-induced reentrant SC in high magnetic field (H) over 40 T [10][11][12][13] and multiple SC phases under applied pressure. 14,15) UTe 2 was initially proposed to be located at the paramagnetic end of the uranium-based ferromagnetic (FM) superconductor series, where SC is mediated by FM fluctuations.…”

“…However, this is probably not the case for UTe 2 , because the low-T anomaly in the χ a (T ) has been commonly observed in single crystals grown by different groups. 1,13,20,23) In addition, the anomaly is strongly field-orientation dependent (not observed in H||b and ||c), and suppressed rapidly under applied pressure. 24) Furthermore, we have observed a distinct anomaly in the T dependence of NMR line width and 1/T 2 in the same T region, as described below.…”

“…Figure 1(b) shows the T and H dependences of bulk spinsusceptibility, χ a and the Knight shift K a in H a. For UTe 2 , the susceptibility, and thus the Knight shift, exhibit Ising anisotropy with the a-axis as the magnetic easy axis, and the χ a follows a Curie Weiss behavior with a negative Weiss temperature of θ = −60 K above 150 K. 1,13,22) With further decreasing T , the χ a starts to increase faster than the Curie Weiss behavior, and tends to saturate below 20 K. The χ a becomes strongly H dependent below 20 K, and has an additional upturn below 10 K at lower H ( 2 T). 1,13,20,23) In contrast, we found that the K a is totally H independent over the whole T region.…”

“…For UTe 2 , the susceptibility, and thus the Knight shift, exhibit Ising anisotropy with the a-axis as the magnetic easy axis, and the χ a follows a Curie Weiss behavior with a negative Weiss temperature of θ = −60 K above 150 K. 1,13,22) With further decreasing T , the χ a starts to increase faster than the Curie Weiss behavior, and tends to saturate below 20 K. The χ a becomes strongly H dependent below 20 K, and has an additional upturn below 10 K at lower H ( 2 T). 1,13,20,23) In contrast, we found that the K a is totally H independent over the whole T region. Consequently the linear scaling obtained at higher T between the K a (T ) and χ a (T ) does not work below 20 K. Be- low 10 K, the K a becomes almost T independent, and thus never reproduce the low-T upturn observed in the χ a (T ).…”