Superconductivity on the Border of Weak Itinerant Ferromagnetism in UCoGe

Abstract

We report the coexistence of ferromagnetic order and superconductivity in UCoGe at ambient pressure. Magnetization measurements show that UCoGe is a weak ferromagnet with a Curie temperature T C 3 K and a small ordered moment m 0 0:03 B . Superconductivity is observed with a resistive transition temperature T s 0:8 K for the best sample. Thermal-expansion and specific-heat measurements provide solid evidence for bulk magnetism and superconductivity. The proximity to a ferromagnetic instability, the defect sensitivity of T s , and the absence of Pauli limiting, suggest triplet superconductivity mediated by critical ferromagnetic fluctuations. DOI: 10.1103/PhysRevLett.99.067006 PACS numbers: 74.70.Tx, 74.20.Mn, 75.30.Kz In the standard theory for superconductivity (SC) due to Bardeen, Schrieffer, and Cooper ferromagnetic (FM) order impedes the pairing of electrons in singlet states [1]. It has been argued, however, that on the border line of ferromagnetism, critical magnetic fluctuations could mediate SC by pairing the electrons in triplet states [2]. The discovery several years ago of SC in the metallic ferromagnets UGe 2 (at high pressure) [3], URhGe [4], and possibly UIr (at high pressure) [5], has put this idea on firm footing. However, later work provided evidence for a more intricate scenario in which SC in UGe 2 and URhGe is driven by a magnetic transition between two polarized phases [6 -8] rather than by critical fluctuations associated with the zero temperature transition from a paramagnetic to a FM phase. Here we report a novel ambient-pressure FM superconductor UCoGe. Since SC occurs right on the border line of FM order, UCoGe may present the first example of SC stimulated by critical fluctuations associated with a FM quantum critical point (QCP).UCoGe belongs to the family of intermetallic UTX compounds, with T a transition metal and X is Si or Ge, that was first manufactured by Troć and Tran [9]. UCoGe crystallizes in the orthorhombic TiNiSi structure (space group P nma ) [10,11], just like URhGe. From magnetization, resistivity (T 4:2 K) [9,10] and specific-heat measurements (T 1:2 K) [12] it was concluded that UCoGe has a paramagnetic ground state. This provided the motivation to alloy URhGe (Curie temperature T C 9:5 K) with Co in a search for a FM QCP in the series URh 1ÿx Co x Ge (x 0:9) [13]. Magnetization data showed that T C upon doping first increases, has a broad maximum near x 0:6 (T max C 20 K) and then rapidly drops to 8 K for x 0:9 [13]. This hinted at a FM QCP for x & 1:0. In this Letter we show that the end (x 1:0) compound UCoGe is in fact a weak itinerant ferromagnet. Moreover, metallic ferromagnetism coexists with SC below 0.8 K at ambient pressure.Polycrystalline UCoGe samples were prepared with nominal compositions U 1:02 CoGe (sample 2) and U 1:02 Co 1:02 Ge (sample 3) by arc melting the constituents (natural U 99.9%, Co 99.9%, and Ge 99.999%) under a high-purity argon atmosphere in a water-cooled copper crucible. The as-cast samples were annealed for 10 days at 850 C. Sampl...

Effect of Pressure on Magnetism of UIrGe

“…Of particular interest are compounds located close to the empirical Hill criterion, 7) which defines the limit of the distance of nearest uranium ions as dU-U  3.5 Å, at which 5f electrons change from a localized to itinerant character. 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] .…”

Microscopic theory for a ferromagnetic nanowire/superconductor heterostructure: Transport, fluctuations, and topological superconductivity

“…The neutron diffraction measurements in ferromagnetic superconductors ErRh 4 B 4 10 , HoMo 6 S 8 11 and HoMo 6 Se 8 12 indicate an inhomogeneous magnetic order coexisting with superconductivity. More recently, coexistence of these seemingly exclusive states was also seen in Pb/PbO core/shell nanoparticles 13 , and in two-dimensional interfaces between perovskite band insulators LaAlO 3 and SrTiO 3 14,15 .…”

“…In one case, the long-ranged proximity effect is attributed to spin-triplet oddfrequency s-wave pairing which can be induced when magnetization inhomogeneity is present near the interface. 23,24 This beautiful theory, which predicts an exotic odd-frequency (even-momentum) symmetry for the triplet component of the condensate, was originally suggested by Berenzinskii as a possible phase in superfluid 3 He. 25 Other theoretical works have investigated Josephson coupling between two conventional superconductors separated by a half-metallic ferromagnet and studied singlet-triplet conversion due to a spin-active interface.…”

“…36,41 An alternative approach to realizing a one-dimensional topological superconductor is to deposit a ferromagnetic semiconductor wire on top of a ferromagnetic superconductor. [1][2][3][4] A particularly attractive candidate ferromagnetic semiconductor is europium oxide (EuO), which is known to possess nearly spin-polarized bands. 85,86 EuO becomes ferromagnetic below 70K under ambient pressure and the Curie temperature is known to increase with pressure reaching 200K under 1.5 × 10 5 atmospheres.…”

“…Following the first experimental realization a decade ago there are now four such compounds, [1][2][3][4] two of which exhibit the phenomena at ambient pressures. 2,3 We propose that depositing URhGe electrodes on the EuO wire, in the arrangement shown in Fig. 1(a), would be conceptually the simplest structure to realize Majorana fermions that would require neither topological insulators nor control over spin-orbit couplings.…”