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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Epitaxially strained LaCoO 3 (LCO) thin films were grown with different film thickness, t, on (001) oriented (LaAlO 3 ) 0.3 (SrAl 0.5 Ta 0.5 O 3 ) 0.7 (LSAT) substrates. After initial pseudomorphic growth the films start to relieve their strain partly by the formation of periodic nano-twins with twin planes predominantly along the <100> direction. Nano-twinning occurs already at the initial stage of growth, albeit in a more moderate way. Pseudomorphic grains, on the other hand, still grow up to a thickness of at least several tenths of nanometers. The twinning is attributed to the symmetry lowering of the epitaxially strained pseudo-tetragonal structure towards the relaxed rhombohedral structure of bulk LCO. However, the unit-cell volume of the pseudo-tetragonal structure is found to be nearly constant over a very large range of t. Only films with t > 130 nm show a significant relaxation of the lattice parameters towards values comparable to those of bulk LCO. 1Measurements of the magnetic moment indicate that the effective paramagnetic moment, m eff , and thus the spin state of the Co 3+ ion does not change for films with t ≤ 100 nm. However, the saturated ferromagnetic moment, m s , was found to be proportional only to the pseudo-tetragonal part of the film and decreases with increasing rhombohedral distortion. The measurements demonstrate, that ferromagnetism of LCO is strongly affected by the rhombohedral distortion while the increased unitcell volume mainly controls the effective paramagnetic moment and thus the spin state of the Co 3+ ion. 2 I. IntroductionThe perovskite-type lanthanum cobaltate LaCoO 3 (LCO) has recently attracted much attention due to its unusual electronic and magnetic properties at ambient pressure 1,2 and the observation of ferromagnetism in epitaxially strained thin films. 3,4 At low temperature, T ≤ 35 K, LCO is a nonmagnetic semiconductor with a ground state of Co 3+ ions in a low-spin (LS) configuration (t 2g 6 e g 0 , S = 0). 5,6 This is believed to change to a primarily intermediate-spin (IS) (t 2g 5 e g 1 , S = 1) state 7 in the temperature range 35 K < T < 100 K, and further to a mixture of IS and high-spin (HS) (t 2g 4 e g 2 , S = 2) states in the interval 300 K < T < 600 K. The crossover between spin states with increasing temperature arises from a delicate interplay between the crystal-field splitting, ∆ CF , between the t 2g and e g energy levels, and the intra-atomic exchange interaction (Hund`s rule coupling), ∆ EX . The balance between ∆ CF and ∆ EX can be affected by, e. g., hole or electron doping 8 and by chemical or external pressure. 910 Since ∆ CF is very sensitive to changes of the Co-O bond length, d, 11 and the Co-O-Co bond angle, γ , structural changes with respect to both easily modify the spin state of the Co 3+ ion. We have shown that the population of higher spin states is enhanced in tensile strained LCO films.3Calculations based on the generalized gradient approximation to the density functional theory indeed show that a magnetic state is more stable tha...

The thermoelectric power S of uncompensated Si:P with P concentration N near the metal-insulator transition occurring at N c has been measured at very low temperatures (0.04 < T< 3 K). For Ny>N c , S is negative and shows the linear T dependence of a metal, whereas close to N c an anomalous behavior with a sign change of S at low T is observed. The strong dependence of S on magnetic fields up to 6 T relates the anomaly to magnetic scattering, thus giving the first experimental evidence for localized moments near the metal-insulator transition in a transport property. PACS numbers: 71.30.+h, 72.15.Jf, 72.15.Qm, 72.20.Pa The metal-insulator (MI) transition in disordered systems is one of the central themes in condensed matter physics. Besides disorder, electron-electron interactions are thought to play a major role in this transition. Recent theoretical work emphasizes the role of local moments near the MI transition, possibly even leading to a non-Fermi-liquid behavior with the magnetic susceptibility x an< 3 the linear specific-heat coefficient y diverging for T 7 -• 0 [1-3]. Doped semiconductors like Si:P where the disorder stems from the random distribution of donors constitute a convenient system for studying the MI transition, which occurs as a function of the donor concentration N at a critical concentration N c . In this material, the existence of local moments in the metallic state has been observed in magnetic resonance [4] and static % measurements [5]. Their density as a function of TV has been mapped out in detail with specific-heat measurements [6], The possible influence of local moments on the electrical conductivity a [2,7] is difficult to estimate because of the large T and B dependence of a due to localization effects and electron-electron interactions [8].In this situation, more specific information on transport in disordered materials close to the MI transition is highly desirable.A particularly sensitive transport property is the thermoelectric power S. In view of the giant thermopower observed in Kondo systems, S should be susceptible to local moments near the MI transition. However, generally an accurate analysis of S is difficult, because (in a Fermi-liquid description) the explicit dependence of the scattering time r and density of states at the Fermi level D(Ef) on energy has to be taken into account. Furthermore, at moderate and high temperatures (T^&D, the Debye temperature) S is often dominated by the phonon-drag contribution due to the electron-phonon interaction. Therefore one has to work below 1 K to circumvent this difficultly. Approaching the transition from the metallic side a divergent coefficient S/T is predicted for T-0 [9,10], S/T -(N-N C )~M, where the exponent JA should depend on the significance of the electronelectron interactions or whether symmetry-breaking fields such as an external magnetic field, magnetic impurities, or spin-orbit coupling are present. On the insulating side where a is due to variable-range hopping [11], 5* will also be influenced by correlations....

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