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...

The superconductor PdTe2 was recently classified as a Type II Dirac semimetal, and advocated to be an improved platform for topological superconductivity. Here we report magnetic and transport measurements conducted to determine the nature of the superconducting phase. Surprisingly, we find that PdTe2 is a Type I superconductor with Tc = 1.64 K and a critical field µ0Hc(0) = 13.6 mT. Our crystals also exhibit the intermediate state as demonstrated by the differential paramagnetic effect. For H > Hc we observe superconductivity of the surface sheath. This calls for a close examination of superconductivity in PdTe2 in view of the presence of topological surface states.Recently the transition metal dichalcogenide PdTe 2 was reported to be a Type II Dirac semimetal [1][2][3]. Topological Dirac semimetals form a new class of topological materials, where non-trivial surface states arise due to the topology of the bulk band structure (for recent reviews see [4][5][6]). Dirac semimetals are the 3D analog of graphene and have a cone-shaped linear energy dispersion around the Dirac point with massless fermions [7].

Recently it was demonstrated that Sr intercalation provides a new route to induce superconductivity in the topological insulator Bi2Se3. Topological superconductors are predicted to be unconventional with an odd-parity pairing symmetry. An adequate probe to test for unconventional superconductivity is the upper critical field, Bc2. For a standard BCS layered superconductor Bc2 shows an anisotropy when the magnetic field is applied parallel and perpendicular to the layers, but is isotropic when the field is rotated in the plane of the layers. Here we report measurements of the upper critical field of superconducting SrxBi2Se3 crystals (Tc = 3.0 K). Surprisingly, field-angle dependent magnetotransport measurements reveal a large anisotropy of Bc2 when the magnet field is rotated in the basal plane. The large two-fold anisotropy, while six-fold is anticipated, cannot be explained with the Ginzburg-Landau anisotropic effective mass model or flux flow induced by the Lorentz force. The rotational symmetry breaking of Bc2 indicates unconventional superconductivity with odd-parity spin-triplet Cooper pairs (Δ4-pairing) recently proposed for rhombohedral topological superconductors, or might have a structural nature, such as self-organized stripe ordering of Sr atoms.

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.

SmB6, a well-known Kondo insulator, has been proposed to be an ideal topological insulator with states of topological character located in a clean, bulk electronic gap, namely the Kondo hybridization gap. Seeing as the Kondo gap arises from many body electronic correlations, this would place SmB6 at the head of a new material class: topological Kondo insulators. Here, for the first time, we show that the k-space characteristics of the Kondo hybridization process is the key to unravelling the origin of the two types of metallic states experimentally observed by ARPES in the electronic band structure of SmB6(001). One group of these states is essentially of bulk origin, and cuts the Fermi level due to the position of the chemical potential 20 meV above the lowest lying 5d-4f hybridization zone. The other metallic state is more enigmatic, being weak in intensity, but represents a good candidate for a topological surface state. However, before this claim can be substantiated by an unequivocal measurement of its massless dispersion relation, our data raises the bar in terms of the ARPES resolution required, as we show there to be a strong renormalization of the hybridization gaps by a factor 2-3 compared to theory, following from the knowledge of the true position of the chemical potential and a careful comparison with the predictions from recent LDA+Gutzwiller calculations. All in all, these key pieces of evidence act as triangulation markers, providing a detailed description of the electronic landscape in SmB6, pointing the way for future, ultrahigh resolution ARPES experiments to achieve a direct measurement of the Dirac cones in the first topological Kondo insulator. * e.frantzeskakis@uva.nl † m.s.golden@uva.nl

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...

We report a high-pressure single crystal study of the topological superconductor Cu{x}Bi{2}Se{3}. Resistivity measurements under pressure show superconductivity is depressed smoothly. At the same time the metallic behavior is gradually lost. The upper-critical field data B{c2}(T) under pressure collapse onto a universal curve. The absence of Pauli limiting and the comparison of B{c2}(T) to a polar-state function point to spin-triplet superconductivity, but an anisotropic spin-singlet state cannot be discarded completely.

We report a high-pressure single-crystal study of the non-centrosymmetric superconductor YPtBi (Tc = 0.77 K). Magnetotransport measurements show a weak metallic behavior with a carrier concentration n ≃ 2.2 × 10 19 cm −3 . Resistivity measurements up to p = 2.51 GPa reveal superconductivity is promoted by pressure. The reduced upper critical field Bc2(T ) curves collapse onto a single curve, with values that exceed the model values for spin-singlet superconductivity. The Bc2 data point to an odd-parity component in the superconducting order parameter, in accordance with predictions for non-centrosymmetric superconductors.