Enhancement of Superconductivity near the Ferromagnetic Quantum Critical Point in UCoGe

Abstract

We report a high-pressure single crystal study of the superconducting ferromagnet UCoGe. Acsusceptibility and resistivity measurements under pressures up to 2.2 GPa show ferromagnetism is smoothly depressed and vanishes at a critical pressure pc = 1.4 GPa. Near the ferromagnetic critical point superconductivity is enhanced. Upper-critical field measurements under pressure show Bc2(0) attains remarkably large values, which provides solid evidence for spin-triplet superconductivity over the whole pressure range. The obtained p − T phase diagram reveals superconductivity is closely connected to a ferromagnetic quantum critical point hidden under the superconducting 'dome'. PACS numbers: 74.70.Tx, 75.30.Kz, 74.62.Fj The recent discovery of superconductivity in itinerantelectron ferromagnets tuned to the border of ferromagnetic order [1,2,3,4] disclosed a new research theme in the field of magnetism and superconductivity. Notably, superconducting ferromagnets provide a unique testing ground [1,5] for superconductivity not mediated by phonons, but by magnetic interactions associated with a magnetic quantum critical point (QCP) [6,7,8]. In the 'traditional' model for spin-fluctuation mediated superconductivity [6] a second-order ferromagnetic quantum phase transition takes place when the Stoner parameter diverges, and near the critical point the exchange of longitudinal spin fluctuations stimulates spin-triplet superconductivity. Superconductivity is predicted to occur in the ferromagnetic as well as in the paramagnetic phase, while at the critical point the superconducting transition temperature T s → 0. Research into ferromagnetic superconductors will help to unravel how magnetic fluctuations can stimulate superconductivity. This novel insight might turn out to be crucial in the design of new superconducting materials.High-pressure experiments have been instrumental in investigating the interplay of magnetism and superconductivity. In the case of UGe 2 [1] superconductivity is found only in the ferromagnetic phase under pressure close to the critical point and at the critical pressure, p c , ferromagnetism and superconductivity disappear simultaneously. The ferromagnetic transition becomes first order for p → p c = 1.6 GPa [9]. Moreover, a field-induced first-order transition between two states with different polarizations was found in the ferromagnetic phase [10]. Superconductivity is attributed to critical magnetic fluctuations associated with this first order metamagnetic transition [11], rather than with critical spin fluctuations near p c . In UIr the ferro-to-paramagnetic phase transition remains second order under pressure all the way to p c = 2.8 GPa [3,12]. Superconductivity appears in the ferromagnetic phase in a small pressure range close to p c , however, it is not observed for p ≥ p c , which is at variance with the 'traditional' spin-fluctuation model [6]. In URhGe [2] ferromagnetism and superconductivity are observed at ambient pressure. Pressure raises the Curie temperature, T C , and drives the sy...

Effect of Pressure on Magnetism of UIrGe

“…The surprising analogy between the hydrostatic and chemical pressure by Si substitution can be found in all three isoelectronic compounds. The response of UCoGe is similar to that of UIrGe 18,19,43) . Hydrostatic pressure increases TC only in URhGe 17) .…”

“…However, the shapes of their phase diagrams are markedly different. The steep drop to zero from a relatively high TN just above pc in UIrGe is in contrast to the gradual vanishing of the FM in UCoGe up to the intersection with the summit of the SC dome 18,19,43) . The sudden drop in TN at pc and the resulting rectangular p-T phase diagram of UIrGe have the signature of a first-order transition.…”

“…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.…”

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

“…These systems include several U-based systems, We also discuss the temperature-pressure (T -P ) and chemical composition (T -x) phase diagrams of various f -electron compounds that exhibit superconductivity in the vicinity of magnetic quantum critical points that are accessed through the application of an external pressure. These include the antiferromagnetic compounds CeIn 3 [9,18], CeRhIn 5 [19], and the ferromagnetic compounds UGe 2 [20,21], URhGe [22,23], and UCoGe [24][25][26].…”

“…Fig. 18 (a) Pressure dependence of the Curie temperature T C and superconducting critical temperature T c for both polycrystalline [25] and single crystalline [26] UCoGe. Panels (b) and (c) show the results of quadratic T 2 fits to the electrical resistivity over the narrow temperature window between T c and 1.7 K. (d) Magnitude of the feature at T C in the ac magnetic susceptibility signal, which may be related to the size of the ordered magnetic moment…”