There are two main theoretical descriptions of antiferromagnets. The first arises from atomic physics, which predicts that atoms with unpaired electrons develop magnetic moments. In a solid, the coupling between moments on nearby ions then yields antiferromagnetic order at low temperatures. The second description, based on the physics of electron fluids or 'Fermi liquids' states that Coulomb interactions can drive the fluid to adopt a more stable configuration by developing a spin density wave. It is at present unknown which view is appropriate at a 'quantum critical point' where the antiferromagnetic transition temperature vanishes. Here we report neutron scattering and bulk magnetometry measurements of the metal CeCu(6-x)Au(x), which allow us to discriminate between the two models. We find evidence for an atomically local contribution to the magnetic correlations which develops at the critical gold concentration (x(c) = 0.1), corresponding to a magnetic ordering temperature of zero. This contribution implies that a Fermi-liquid-destroying spin-localizing transition, unanticipated from the spin density wave description, coincides with the antiferromagnetic quantum critical point.

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 new measurements of the electrical conductivity σ of the canonical three-dimensional metal-insulator system Si:P under uniaxial stress S. The zero-temperature extrapolation of σ(S, T → 0) ∼| S − Sc | µ shows an unprecidentedly sharp onset of finite conductivity at Sc with an exponent µ = 1. The value of µ differs significantly from that of earlier stress-tuning results. Our data show dynamical σ(S, T ) scaling on both metallic and insulating sides , viz. σ(S, T ) = σc(T ) · F(| S − Sc | /T y ) where σc(T ) is the conductivity at the critical stress Sc. We find y = 1/zν = 0.34 where ν is the correlation-length exponent and z the dynamic critical exponent. 71.30.+h, 71.55.Cu, 72.80.Cw Quantum phase transitions have become of steadily increasing interest in recent years [1]. These continuous transitions ideally occur at temperature T = 0 where quantum fluctuations play the role corresponding to thermal fluctuations in classical phase transitions. In particular, certain types of metal-insulator transitions (MIT) such as localization transitions have been studied extensively. Experimentally, the MIT may be driven by an external parameter t such as carrier concentration N , uniaxial stress S, or electric or magnetic fields. Generally, electron localization might arise from disorder (Anderson transition) or from electron-electron (e-e) interactions (Mott-Hubbard transition) [2]. In Nature, these two features go hand in hand. For instance, the disorderinduced MIT occurring as a function of doping in threedimensional (d = 3) semiconductors where the disorder stems from the statistical distribution of dopant atoms in the crystalline host, bears signatures of e-e interactions as evidenced from the transport properties in both metallic [3] and insulating regimes [4]. This makes a theoretical treatment of the critical behavior of a MIT exceedingly difficult. Even for purely disorder-induced transitions, the critical behavior of the zero-temperature dc conductivity, σ(0) ∼| t − t c | µ where t c is the critical value of t, is not well understood. Theoretically, µ is usually inferred from the correlation-length critical exponent ν via Wegner scaling µ = ν(d−2). Numerical values of ν range between 1.3 and 1.6 [5,6].Experimentally, it has long been suggested that the critical behavior of the conductivity falls into two classes: µ ≈ 0.5 for uncompensated semiconductors and µ ≈ 1 for compensated semiconductors and amorphous metals [7]. However, there appears to be no clear physical distinction between these materials that would justify different universality classes. While many different materials were reported to show µ ≈ 1, the exponent µ ≈ 0.5 was largely based on the very elegant experiments by Paalanen and coworkers [8][9][10], where uniaxial stress was used to drive an initially insulating uncompensated Si:P sample metallic. This allows to fine-tune the MIT since the stress can be changed continuously at low T thus eliminating geometry errors incurring when different samples are employed in concentration tuning ...

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