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...
We report upper critical field B(c2)(T) measurements on a single-crystalline sample of the ferromagnetic superconductor UCoGe. B(c2)(0) obtained for fields applied along the orthorhombic axes exceeds the Pauli limit for B parallela,b and shows a strong anisotropy B(c2)(a) approximately B(c2)(b)>>B(c2)(c). This provides evidence for an equal-spin pairing state and a superconducting gap function of axial symmetry with point nodes along the c axis, which is also the direction of the uniaxial ferromagnetic moment m(0)=0.07micro(B). An unusual curvature or kink is observed in the temperature variation of B(c2) which possibly indicates UCoGe is a two-band ferromagnetic superconductor.
We report zero-field muon-spin rotation and relaxation measurements on the superconducting ferromagnet UCoGe. Weak itinerant ferromagnetic order is detected by a spontaneous muon-spin precession frequency below the Curie temperature TC=3 K. The micro+ precession frequency persists below the bulk superconducting transition temperature Tsc=0.5 K, where it measures a local magnetic field Bloc=0.015 T. The amplitude of the microSR signal provides unambiguous proof for ferromagnetism present in the whole sample volume. We conclude ferromagnetism coexists with superconductivity on the microscopic scale.
We have investigated the thermal, transport and magnetic properties of URh$_{1-x}$Ru$_x$Ge alloys near the critical concentration $x_{cr} = 0.38$ for the suppression of ferromagnetic order. The Curie temperature vanishes linearly with $x$ and the ordered moment $m_0$ is suppressed in a continuous way. At $x_{cr}$ the specific heat varies as $c \sim TlnT$, the $\gamma$-value $c/T|_{0.5K}$ is maximum and the temperature exponent of the resistivity $\rho \sim T^n$ attains a minimum value $n=1.2$. These observations provide evidence for a ferromagnetic quantum phase transition. Interestingly, the coefficient of thermal expansion and the Gr\"uneisen parameter $\Gamma$ remain finite at $x_{cr}$ (down to $T = 1$ K), which is at odds with recent scaling results for a metallic quantum critical point.Comment: 5 pages; accepted for publication in Phys. Rev.
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