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 present magnetotransport and 209 Bi nuclear magnetic resonance (NMR) data on a series of single crystals of Bi2Se3, Bi2Te2Se and CuxBi2Se3 with varying carrier concentrations. The Knight shift of the bulk nuclei is strongly correlated with the carrier concentration via a hyperfine coupling of 27 µeV, which may have important consequences for scattering of the protected surface states. Surprisingly we find that the NMR linewidths and the spin lattice relaxation rate appear to be dominated by the presence of localized spins, which may be related to the presence of Se vacancies.

The effects of uniaxial compressive stress on the normal state 17 O nuclear magnetic resonance properties of the unconventional superconductor Sr2RuO4 are reported. The paramagnetic shifts of both planar and apical oxygen sites show pronounced anomalies near the nominal a-axis strain εaa ≡ εv, that maximizes the superconducting transition temperature, Tc. The spin susceptibility weakly increases on lowering the temperature below T 10 K, consistent with an enhanced density of states associated with passing the Fermi energy through a van Hove singularity. Although such a Lifshitz transition occurs in the γ band, formed by the Ru dxy states hybridized with in-plane O pπ orbitals, the large Hund's coupling renormalizes the uniform spin susceptibilty, which, in turn, affects the hyperfine fields of all nuclei. We estimate this "Stoner" renormalization, S, by combining the data with first-principles calculations and conclude that this is an important part of the strain effect, with implications for superconductivity.

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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