Coexistence and interplay of superconductivity and ferromagnetism in URhGe

Abstract

As ferromagnetism and superconductivity are usually considered to be antagonistic, the discovery of their coexistence in UGe(2), URhGe, UIr and UCoGe has attracted a lot of interest. The mechanism to explain such a state has, however, not yet been fully elucidated. In these compounds superconductivity may be unconventional: Cooper pairs could be formed by electrons with parallel spins and magnetic fluctuations might be involved in the pairing mechanism. URhGe becomes ferromagnetic below a Curie temperature of 9.5 K, with a spontaneous moment aligned to the c-axis. For temperatures below 260 mK and fields lower than 2 T, superconductivity was first observed in 2001. Recently, we discovered a second pocket of superconductivity. This new pocket of superconductivity appears at higher fields applied close to the b-axis, enveloping a sudden magnetic moment rotation transition at H(R) = 12 T. Detailed studies of the field induced metamagnetic transition and superconductivity are presented. The possibility that magnetic fluctuations emerging from a quantum critical point provide the pairing mechanism for superconductivity is discussed.

Wing structure in the phase diagram of the Ising ferromagnet URhGe close to its tricritical point investigated by angle-resolved magnetization measurements

“…This magnetization value is very close to the spontaneous magnetization M c at H = 0, in accordance with a simple picture of the moment reorientation from the easy c axis to the b axis at H R [9,10,14]. Just above H R , there is a small shoulder-like anomaly, which is also seen in dM /dH as a small hump.…”

“…The situation is analogous to an Ising ferromagnet in a transverse magnetic field [15], for which a quantum phase transition (QPT) accompanying a reorientation of the magnetic moment into a state with M H can be expected at a finite critical field H R . Previous studies by transport (T ≥ 0.5 K), magnetic torque (T ≥ 0.1 K) and magnetization (T ≥ 2 K) measurements indicate that the transition occurs at µ 0 H R ∼ 12 T in URhGe for H b and becomes first order at low temperature [9,13,14,16]. Because the critical field H R is very close to the field in which the RSC emerges, it has been argued that the magnetic fluctuations associated with the moment reorientation play an essential role of RSC [9,10,13].…”

“…In contrast, as a magnetic field H b parallel to the b axis being the tuning parameter, URhGe provides a good opportunity to investigate the whole FM phase diagram by various means. Indeed, when a magnetic field is slightly tilted from the b axis towards c axis, the FM wing structure has been observed below TCP in the "T -H b -H c phase diagram" in URhGe [9,28], where H c denotes the c-axis component of the magnetic field, conjugate to the order parameter (OP). The location of TCP has been reported to be ∼ 2 K and > 4 K by thermoelectric power [29] and nuclear magnetic resonance (NMR) [30] experiments, respectively.…”

Superconductivity in uranium compounds

“…13. One must stress that the reentrant superconducting state appears already at field about 8 Tesla that is in significantly lower field H b than the field of the first order transition equal to 12 Tesla reported in papers 23,24 . The magnetic anisotropy in a direction is much harder and a decrease of the Curie temperature in fields H x up to 10-12 Tesla has not yet reported.…”

p-wave superconductivity near a transverse saturation field

“…9 Interestingly, SC reappears above 8 T and shows the highest T sc =0.42 K at 12 T. 9 Theories of ferromagnetic SC have been developed by various groups, [10][11][12][13][14][15] especially for UGe 2 . [16][17][18][19] In this paper, we focus on the reentrant SC in URhGe, which has not been investigated theoretically.…”