Extremely Large and Anisotropic Upper Critical Field and the Ferromagnetic Instability in UCoGe

Abstract

Magnetoresistivity measurements with fine tuning of the field direction on high quality single crystals of the ferromagnetic superconductor UCoGe show anomalous anisotropy of the upper critical field Hc2. Hc2 for H b-axis (H b c2 ) in the orthorhombic crystal structure is strongly enhanced with decreasing temperature with an S-shape and reaches nearly 20 T at 0 K. The temperature dependence of H a c2 shows upward curvature with a low temperature value exceeding 30 T, while H c c2 at 0 K is very small (∼ 0.6 T). Contrary to conventional ferromagnets, the decrease of the Curie temperature with increasing field for H b-axis marked by an enhancement of the effective mass of the conduction electrons appears to be the origin of the S-shaped H b c2 curve. These results indicate that the field-induced ferromagnetic instability or magnetic quantum criticality reinforces superconductivity.The coexistence of superconductivity (SC) and ferromagnetism (FM) has attracted much attention, since the exotic SC state based on spin-triplet pairing mediated by longitudinal spin fluctuations is expected. 1 The first example was discovered in UGe 2 under pressure, where T sc is much lower than T Curie . 2 The SC phase exists only in the FM phase, and SC disappears in the paramagnetic (PM) phase above the critical pressure P c . Soon after that, SC was found at ambient pressure in the weak ferromagnet URhGe. 3 T sc (= 0.25 K) is much lower than T Curie (= 9.5 K). The upper critical field H c2 exceeds the Pauli paramagnetic limit, thus it is believed that the spin-triplet state with equal-spin pairing is realized. Recently, reentrant superconductivity (RSC) was found between 8 and 13 T when the field is applied along the b-axis of the orthorhombic TiNiSi-type structure (space group: Pnma). 4 With increasing field, the magnetic moment gradually tilts from the c-axis (easy-magnetization axis) to b-axis. Finally the moment is suddenly aligned to the b-axis at the spin reorientation field H R . The fieldinduced critical magnetic fluctuations induce RSC. Recently, we have observed the enhancement of effective mass in the RSC phase, and explained the emergence of RSC. 5,6 A newcomer, UCoGe, which crystallizes in the same structure as URhGe, was recently reported. 7 UCoGe is a weak ferromagnet with T Curie ∼ 3 K and the ordered moment µ 0 = 0.07 µ B /U. T sc (∼ 0.6 K) is larger than that in URhGe. Since T Curie is low, one can naively consider that UCoGe is close to the quantum critical point. Indeed, our previous measurement shows that T Curie is immediately suppressed by applying a small pressure (P c ∼ 1 GPa). 8 Contrary to the case of UGe 2 where SC exists only in the FM domain, SC survives even in the PM phase with the maximum T sc (≃ 0.75 K) around P c . A new theory * E-mail address: aokidai@gmail.com from symmetry considerations was developed in order to explain the temperature-pressure phase diagram. 9 H c2 at ambient pressure shows strong anisotropy. H c2 for H a (H a c2 ) and b-axis (H b c2 ) reveal almost the same temperature depend...

Effect of Pressure on Magnetism of UIrGe

“…Isostructural compounds URhGe and UCoGe, which crystallize in a TiNiSi-type structure naturally fulfill this condition, and the coexistence of ferromagnetism (FM) and SC has been discovered here 8,9) . Both FM and SC can be affected by alloying [10][11][12][13][14] , the application of a magnetic field 15,16) , and the external pressure [17][18][19] . The resulting H-T-p-x phase diagrams corroborate the close connection between FM and SC.…”

Superconductivity Induced by Longitudinal Ferromagnetic Fluctuations in UCoGe

“…Studies of the SC upper critical field (H c2 ) and its angle dependence along each crystalline axis have reported remarkable enigmatic behavior [8,9]: superconductivity survives far beyond the Pauli-limiting field along the a and b axes, whereas H c2 for fields along the c direction (H c c2 ) is as small as 0.5 T. Colossal H c2 for fields along the a and b axes seems to suggest spin triplet pairing. In addition, a steep angle dependence of H c2 was reported when the field was tilted slightly from the a axis toward the c axis [9]. The observed characteristic H c2 behavior is one of mysterious features of SC UCoGe and its origin can be related to the mechanism of the superconductivity.…”

Non-Fermi Liquid Regimes and Superconductivity in the Low Temperature Phase Diagrams of Strongly Correlated d- and f-Electron Materials

“…However, because T C and T c are rather close together, the temperature interval over which the fitting is performed is very small (this is also an issue for the power law fits described below for UCoGe under pressure and Si-doped UCoGe). For fields along the a-and c-axes, the coefficient A is gradually suppressed, while for fields applied along the b axis, A passes through a maximum near the field (15 T) at which FM order is destroyed [231] ( Fig. 17(b)).…”

“…17 (a) Curie temperature and superconducting critical temperature of UCoGe single crystals as a function of magnetic field applied along the b-axis, perpendicular to the easy magnetization axis [231]. (b) A coefficient of quadratic T 2 fits to the electrical resistivity as a function of fields applied along different crystallographic axes [231] Like URhGe, the magnetic easy axis for the FM in UCoGe is along the c-axis. For magnetic fields applied along the a-axis of UCoGe, large zero temperature critical fields, H c2 (0) ∼ 20-30 T, are found [231].…”

“…(b) A coefficient of quadratic T 2 fits to the electrical resistivity as a function of fields applied along different crystallographic axes [231] Like URhGe, the magnetic easy axis for the FM in UCoGe is along the c-axis. For magnetic fields applied along the a-axis of UCoGe, large zero temperature critical fields, H c2 (0) ∼ 20-30 T, are found [231]. For fields aligned to the b-axis, hints of re-entrant superconductivity are evinced by an "S"-shaped bend in the upper critical field curve near T /T c ∼ 0.65 ( Fig.…”