Superconductivity Induced by Longitudinal Ferromagnetic Fluctuations in UCoGe

Abstract

From detailed angle-resolved NMR and Meissner measurements on a ferromagnetic (FM) superconductor UCoGe (TCurie ∼ 2.5 K and TSC ∼ 0.6 K), we show that superconductivity in UCoGe is tightly coupled with longitudinal FM spin fluctuations along the c axis. We found that magnetic fields along the c axis (H c) strongly suppress the FM fluctuations and that the superconductivity is observed in the limited magnetic-field region where the longitudinal FM spin fluctuations are active. These results combined with model calculations strongly suggest that the longitudinal FM spin fluctuations tuned by H c induce the unique spin-triplet superconductivity in UCoGe. This is the first clear example that FM fluctuations are intimately related with superconductivity.PACS numbers: 71.27.+a 74.25.nj, 75.30.Gw The discovery of superconductivity in ferromagnetic (FM) UGe 2 opened up a new paradigm of superconductivity [1,2], since most unconventional superconductivity has been discovered in the vicinity of an antiferromagnetic (AFM) phase [3]. From the theoretical point of view, in an itinerant FM superconductor with the presence of a large energy splitting between the majority and minority spin Fermi surfaces, exotic spintriplet superconductivity is anticipated, in which pairing is between parallel spins within each spin Fermi surface. In addition, it has been argued that critical FM fluctuations near a quantum phase transition could mediate spin-triplet superconductivity [4]. However, there have been no experimental results indicating a relationship between FM fluctuations and superconductivity.Among the FM superconductors discovered so far, UCoGe is one of the most readily explored experimentally, because of its high superconducting (SC) transition temperature (T SC ) and low Curie temperature (T Curie ) at ambient pressure [5]. Microscopic measurements have shown that superconductivity occurs within the FM region, resulting in microscopic coexistence of ferromagnetism and superconductivity [6,7]. 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.Unlike the three dimensional crystal structure, magnetic properties are strongly anisotropic [8]. The magnetization has Ising-like anisotropy with the c axis as a magnetic easy axis, and direction-dependent nuclear-spin lattice relaxation rate (1/T 1 ) measurements on a single crystalline sample have revealed the magnetic fluctuations in UCoGe to be Ising-...

Universality and critical behavior at the critical endpoint in the itinerant-electron metamagnet UCoAl

“…The presence of the 3-D FM fluctuations is also consistent with the resistivity data [ρ(T ) ∝ T 5/3 ] measured in zero field 18 . We comment on that the similar Ising magnetic fluctuations were observed in a ferromagnetic superconductor UCoGe 16,17 .…”

Spin susceptibility in the superconducting state of the ferromagnetic superconductor UCoGe

“…The single crystalline UCoGe was utilized for the measurement, which is the same sample reported previously 5,9 . The sample showed a large residual resistivity ratio (RRR) of approximately 30 along b axis.…”

“…The intimate relationship between ferromagnetic (FM) fluctuations and superconductivity in UCoGe 5 , is a strong experimental suggestion of spintriplet superconductivity. In an itinerant FM superconductor with the presence of a large energy splitting between the majority and minority spin Fermi surfaces, exotic spin-triplet superconductivity is anticipated from a theoretical point of view 6 , in which pairing is between parallel spins within each spin Fermi surface.…”

Phenomenological description of anisotropy effects in some ferromagnetic superconductors

“…The terms with u s and v s represent the anisotropy of spin-triplet Cooper pairs and uniaxial crystal anisotropy, respectively. The strong uniaxial anisotropy of magnetic moment plays an important role for proposed magnetic mediated mechanisms for the appearance of triplet pairing favored by longitudinal magnetic fluctuations, see, for example [11]. The ferromagnetic energy density up to the forth order in magnetization…”