The Ni1+/Ni2+ states of nickelates have the identical (3d(9)/3d(8)) electronic configuration as Cu2+/Cu3+ in the high temperature superconducting cuprates, and are expected to show interesting properties. An intriguing question is whether mimicking the electronic and structural features of cuprates would also result in superconductivity in nickelates. Here we report experimental evidence for a bulklike magnetic transition in La4Ni3O8 at 105 K. Density functional theory calculations relate the transition to a spin density wave nesting instability of the Fermi surface.

We present 75As nuclear magnetic resonance data from measurements of a series of Ba(Fe(1-x)Co(x))2As2 crystals with 0.00≤x≤0.075 that reveals the coexistence of frozen antiferromagnetic domains and superconductivity for 0.060≤x≤0.071. Although bulk probes reveal no long range antiferromagnetic order beyond x=0.06, we find that the local spin dynamics reveal no qualitative change across this transition. The characteristic domain sizes vary by more than an order of magnitude, reaching a maximum variation at x=0.06. This inhomogeneous glassy dynamics may be an intrinsic response to the competition between superconductivity and antiferromagnetism in this system.

The heavy electron Kondo liquid is an emergent state of condensed matter that displays universal behavior independent of material details. Properties of the heavy electron liquid are best probed by NMR Knight shift measurements, which provide a direct measure of the behavior of the heavy electron liquid that emerges below the Kondo lattice coherence temperature as the lattice of local moments hybridizes with the background conduction electrons. Because the transfer of spectral weight between the localized and itinerant electronic degrees of freedom is gradual, the Kondo liquid typically coexists with the local moment component until the material orders at low temperatures. The two-fluid formula captures this behavior in a broad range of materials in the paramagnetic state. In order to investigate two-fluid behavior and the onset and physical origin of different long range ordered ground states in heavy electron materials, we have extended Knight shift measurements to URu 2 Si 2 , CeIrIn 5 , and CeRhIn 5 . In CeRhIn 5 we find that the antiferromagnetic order is preceded by a relocalization of the Kondo liquid, providing independent evidence for a local moment origin of antiferromagnetism. In URu 2 Si 2 the hidden order is shown to emerge directly from the Kondo liquid and so is not associated with local moment physics. Our results imply that the nature of the ground state is strongly coupled with the hybridization in the Kondo lattice in agreement with phase diagram proposed by Yang and Pines.nuclear magnetic resonance | heavy fermion | hyperfine couplings C ompetition between different energy scales gives rise to a rich spectrum of emergent ground states in strongly correlated electron materials. In the heavy fermion compounds, a lattice of nearly localized f electrons interacts with a sea of conduction electrons, and depending on the magnitude of this interaction different types of long-range order may develop at low temperatures (1). The Kondo lattice model strives to capture the essential physics of heavy fermion materials by considering the various magnetic interactions between the conduction electron spins, S c , and the local moment spins, S f (2). Different ground states can emerge depending on the relative strengths of the interaction, J, between S c and S f and the intersite interaction, J f f , between the S f spins (3, 4). Much of the physics of the phase diagram is driven by a quantum critical point, which separates long-range ordered ground states from those in which the local moments have fully hybridized with the conduction electrons to form itinerant states with large effective masses and large Fermi surfaces (5). In several materials quantum critical fluctuations give rise to anomalous non-Fermi liquid behavior in various bulk transport and thermodynamic quantities (6-8).In recent years, evidence has emerged that in the high temperature disordered phase the electronic degrees of freedom simultaneously exhibit both itinerant and localized behavior (9). Below a temperature T Ã that marks the onset...

We report 75 As NMR measurements in BaFe2As2 doped with Ni. Like Co, Ni doping suppresses the antiferromagnetic and structural phase transitions and gives rise to superconductivity for sufficiently large Ni doping. The spin lattice relaxation rate diverges at TN, with a critical exponent consistent with 3D ordering of local moments. In the ordered state the spectra quickly broaden inhomogeneously with doping. We extract the average size of the ordered moment as a function of doping, and show that a model in which the order remains commensurate but with local amplitude variations in the vicinity of the dopant fully explains our observations. PACS numbers: 76.60. Gv, 71.27.+a, 75.50.Ee, 75.30.Mb The recent discovery of the iron arsenide superconductors has reignited interest in the interplay of superconductivity and magnetism in condensed matter.1,2 There are several iron arsenide families that exhibit superconductivity either under pressure or with chemical doping. 3Each family, however, contains a common structural element consisting of FeAs planes, in which the Fe 3d orbitals hybridize with the As p orbitals, giving rise to multiple bands that cross the Fermi energy.4,5 In the parent state (either undoped or at ambient pressure) the nesting of two of these Fermi surfaces gives rise to a spin density wave instability.6 It is believed that doping (both inplane and out-of plane) and/or pressure tunes the chemical potential and modifies the nesting condition, which consequently suppresses the long range antiferromagnetic order. For sufficiently large electron or hole doping superconductivity emerges.7 Although it appears that the superconductivity may be intimately related to the antiferromagnetic fluctuations present in these systems, details of how these fluctuations emerge and are related to the superconductivity remain clouded.8 Furthermore, it is unclear how the superconducting condensate can remain intact in the presence of a high concentration of inplane impurities.9 Detailed experimental studies of the local effects of dopants therefore are crucial to understand the physics that drive the evolution of long range order in these materials.In order to shed light on the influence of dopants on the antiferromagnetic order, we have conducted 75 As Nuclear Magnetic Resonance (NMR) studies in a series of Ni doped Ba(Fe 1−x Ni x ) 2 As 2 crystals. The advantage of this particular system is that large high quality single crystals are available, and Ni has the greatest effect of any transition metal dopants so that superconductivity is reached at only ∼2%.7,10 We find that the As NMR spectrum in the antiferromagnetic phase broadens inhomogeneously very quickly with doping, reflecting a large distribution of local hyperfine fields. This distribution can be understood by realizing that the local spin polarization surrounding the dopant site is not only reduced on-site (at the Ni), but also extends to several of the surrounding Fe sites. In the paramagnetic state, we find that the FIG. 1. (color online)The local stru...

Resistivity, magnetization and microscopic 75 As nuclear magnetic resonance (NMR) measurements in the antiferromagnetically ordered state of the iron-based superconductor parent material CaFe2As2 exhibit anomalous features that are consistent with the collective freezing of domain walls. Below T * ≈ 10 K, the resistivity exhibits a peak and downturn, the bulk magnetization exhibits a sharp increase, and 75 As NMR measurements reveal the presence of slow fluctuations of the hyperfine field. These features in both the charge and spin response are strongly field dependent, are fully suppressed by H * ≈ 15 T, and suggest the presence of filamentary superconductivity nucleated at the antiphase domain walls in this material.

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We report 29 Si NMR measurements in single crystals and aligned powders of URu2Si2 in the hidden order and paramagnetic phases. The spin-lattice-relaxation data reveal evidence of pseudospin fluctuations of U moments in the paramagnetic phase. We find evidence for partial suppression of the density of states below 30 K, and analyze the data in terms of a two component spin-fermion model. We propose that this behavior is a realization of a pseudogap between the hidden order transition THO and 30 K. This behavior is then compared to other materials that demonstrate precursor fluctuations in a pseudogap regime above a ground state with long-range order. 75.30.Mb, 75.25.Dk, 76.60.Es Despite more than twenty years since its discovery, URu 2 Si 2 continues to attract considerable interest in the condensed matter community. 1,2 This heavy fermion system develops a "hidden" order phase below T HO = 17.5 K, and an unconventional superconducting state below 1.5 K. 1,3 The nature of the hidden order (HO) phase remains controversial, but it clearly does not involve magnetic ordering of dipole moments. 4 It may involve order of higher order multipoles, 5 exotic spin, orbital or spinorbital density waves, 6-10 or hybridization between local moments and conduction electrons. 11-13 Extensive neutron scattering and angle-resolved photoemission work has suggested that it has an itinerant nature and involves some type of Fermi surface instability. 4, [14][15][16][17][18] Recent evidence has suggested that the hidden order is intimately connected with the onset of coherence of the Kondo lattice. [19][20][21][22] In Kondo lattice systems the 5f electrons of the U are partially screened by the conduction electrons, leading to a renormalization of the electronic dispersion near the Fermi level. At high temperatures the 5f electrons remain localized and scatter the itinerant conduction electrons. [19][20][21][22] Below a coherence temperature, T coh , the f electrons hybridize with the conduction electrons, and the electronic dispersion reflects renormalized heavy quasiparticles. In other Kondo lattice systems coherence emerges as a crossover, but recent scanning tunneling microscopy (STM) results suggest that T coh coincides with T HO , and thus the HO parameter is in fact the hybridization gap. 13,21,22 A re-examination and re-analysis of previous thermodynamic and neutron scattering measurements under the context of HO gap fluctuations have revealed the possible existence of a pseudogap occurring before the HO state and starting around 30 K. 23 This analysis highlighted the presence of anomalies in magnetic susceptibility, 6 point contact spectroscopy (PCS), 24 and neutron scattering measurements. 14,25 Other thermodynamic probes (heat capacity, thermal expansion and ultrasound velocity) also register an anomaly between 25-30 K, where these are sensitive to changes in the elastic constants of the crystal lattice. 26-29 A similar temperature scale has been observed in ultrafast and conventional optical spectroscopy, which found a ...