Multiple Superconducting Phases and Unusual Enhancement of the Upper Critical Field in UTe₂

Abstract

We performed AC calorimetry and magnetoresistance measurements under pressure for H a-axis (easymagnetization axis) in the novel heavy-fermion superconductor UTe 2 . Thanks to the thermodynamic information, multiple superconducting phases have been revealed under pressure and magnetic field. The (H, T ) phase diagram of superconductivity under pressure displays an abrupt increase of the upper critical field (H c2 ) at low temperature and in the high field region, and a strong convex curvature of H c2 at high temperature. This behavior of H c2 and the multiple superconducting phases require a state for the spin-triplet superconducting order parameter more complex than an equal spin pairing. Above the superconducting critical pressure, P c , we find strong indications that the possible magnetic order is closer to antiferromagnetism than to ferromagnetism.The recently discovered heavy-fermion superconductivity (SC) in UTe 2 attracts much attention, 1, 2) because spin-triplet SC is most likely realized in this system, lying at the proximity of ferromagnetic (FM) order. SC coexisting microscopically with long-range FM order is already well studied 3, 4) in three uranium compounds, UGe 2 , 5) URhGe 6) and UCoGe. 7) Because of the strong internal field due to the FM moment, spin-triplet state with equal-spin pairing (ESP) is favoured. In this case, SC can survive even under strong internal exchange field. Furthermore SC can be even reinforced at high magnetic field (H), pressure (P) and uniaxial stress by tuning the FM fluctuations. When the field is applied along the intermediate hard-magnetization axis (b-axis) in URhGe and UCoGe, the FM Curie temperature is suppressed, and the FM fluctuations are remarkably enhanced. Then field-reentrant (-reinforced) SC is observed at high fields, which highly exceeds the socalled Pauli limit. 8,9) A quite similar situation might be also realized in UTe 2 . UTe 2 is a paramagnet with a body-centered orthorhombic crystal structure (space group: Immm, #71, D 25 2h ). The large Sommerfeld coefficient, γ ∼ 120 mJ K −2 mol −1 , indicates strong correlations in the electronic states. 10) The magnetization curves show a relatively large anisotropy between the easy-magnetization axis (a-axis) and the hard-magnetization axes (b and c-axes). The b-axis is the hardest magnetization axis at low temperatures; the magnetization curve shows sharp 1st order metamagnetic transition at H m = 35 T, where the effective mass is strongly enhanced. [11][12][13] The value of H m is well scaled by T χ max (∼ 35 K), at which a broad maximum of susceptibility χ is observed at low field.The SC properties of UTe 2 are spectacular. The SC transition occurs at T sc = 1.6 K with the large specific heat jump. The residual γ-value amounts to ∼ 40 % against the normal state γ-value for the best quality sample, 14) and the entropy balance is not satisfied, assuming a constant γ-value extrapolated from the normal state above T sc . Strong electronic correlations are dominant in this system, which is also indirectly con...

Evidence for a pressure-induced antiferromagnetic quantum critical point in intermediate-valence UTe 2

“…Figure 1C shows that at 1.45 GPa, a new peak appears in heat capacity at T m1 = 3.8 K. This temperature has been previously associated with the onset of magnetic order. Notably, the signature of bulk superconductivity is still clearly observed in heat capacity at T c1 = 1.8 K. The observation of bulk superconductivity occurring below magnetic order is in contrast to prior reports (8,(10)(11)(12)(13). Compared to 1.40 GPa, however, the superconducting peak is notably broadened and has less entropy.…”

“…This change in character is also consistent with antiferromagnetic order but with pressure-dependent Q (25). Further evidence for antiferromagnetism has been reported in (12), in which it was claimed that T m2 is suppressed with applied magnetic field for all field directions. It was also argued that that field-temperature phase diagram under pressure is similar to other heavy fermion antiferromagnets.…”

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

“…Rich-phase diagrams were obtained for UTe 2 under different field directions and pressures. Although the measurements presented here and in other works 33,[37][38][39]41,61,71 start to bring a clear picture of the complex phase diagram of UTe 2 , which includes multiple superconducting and magnetic phases, we are still far from a deep understanding of its electronic properties. A target is now to perform microscopic studies to identify the nature of the magnetic fluctuations and their change through H m .…”

Orientation of point nodes and nonunitary triplet pairing tuned by the easy-axis magnetization in UTe2

“…Unfortunately, the specific heat at an ambient pressure shows an unusual enhancement at low temperatures, hindering a possible weak anomaly associated with this second transition. However, a two-step specific-heat anomaly was found under hydrostatic pressure [41,42], suggesting an occurrence of multiple superconducting phases. In order to lift the degeneracy of the multiple order parameters in zero field, a spontaneous magnetization or very slow longitudinal spin fluctuations are needed.…”