Fragile antiferromagnetism in the heavy-fermion compound YbBiPt

We report results from neutron scattering experiments on single crystals of YbBiPt that demonstrate antiferromagnetic order characterized by a propagation vector, $\tau_{\rm{AFM}}$ = ($\frac{1}{2} \frac{1}{2} \frac{1}{2}$), and ordered moments that align along the [1 1 1] direction of the cubic unit cell. We describe the scattering in terms of a two-Gaussian peak fit, which consists of a narrower component that appears below $T_{\rm{N}}~\approx 0.4$ K and corresponds to a magnetic correlation length of $\xi_{\rm{n}} \approx$ 80 $\rm{\AA}$, and a broad component that persists up to $T^*\approx$ 0.7 K and corresponds to antiferromagnetic correlations extending over $\xi_{\rm{b}} \approx$ 20 $\rm{\AA}$. Our results illustrate the fragile magnetic order present in YbBiPt and provide a path forward for microscopic investigations of the ground states and fluctuations associated with the purported quantum critical point in this heavy-fermion compound.Comment: 5 pages, 3 figure

Competing magnetic phases and itinerant magnetic frustration in SrCo2 As2

Whereas magnetic frustration is typically associated with local-moment magnets in special geometric arrangements, here we show that SrCo2As2 is a candidate for frustrated itinerant magnetism. Using inelastic neutron scattering (INS), we find that antiferromagnetic (AF) spin fluctuations develop in the square Co layers of SrCo2As2 below T approximate to 100 K centered at the stripe-type AF propagation vector of (1/2, 1/2), and that their development is concomitant with a suppression of the uniform magnetic susceptibility determined via magnetization measurements. We interpret this switch in spectral weight as signaling a temperature-induced crossover from an instability toward ferromagnetism ordering to an instability toward stripe-type AF ordering on cooling, and show results from Monte-Carlo simulations for a J(1)-J(2) Heisenberg model that illustrates how the crossover develops as a function of the frustration ratio -J(1)/(2J(2)). By putting our INS data on an absolute scale, we quantitatively compare them and our magnetization data to exact-diagonalization calculations for the J(1)-J(2) model [N. Shannon et al., Eur. Phys. J. B 38, 599 (2004)1, and show that the calculations predict a lower level of magnetic frustration than indicated by experiment. We trace this discrepancy to the large energy scale of the fluctuations (J(avg) greater than or similar to 75 meV), which, in addition to the steep dispersion, is more characteristic of itinerant magnetism

Survival of a Diffusing Particle in a Transverse Shear Flow: A First-Passage Problem with Continuously Varying Persistence Exponent

We consider a particle diffusing in the y-direction, dy/dt=\eta(t), subject to a transverse shear flow in the x-direction, dx/dt=f(y), where x \ge 0 and x=0 is an absorbing boundary. We treat the class of models defined by f(y) = \pm v_{\pm}(\pm y)^\alpha where the upper (lower) sign refers to y>0 (y<0). We show that the particle survives with probability Q(t) \sim t^{-\theta} with \theta = 1/4, independent of \alpha, if v_{+}=v_{-}. If v_{+} \ne v_{-}, however, we show that \theta depends on both \alpha and the ratio v_{+}/v_{-}, and we determine this dependence.Comment: 4 page

High-energy X-ray diffraction studies of i-Sc12Zn88

Although quasicrystals form in a wide variety of ternary and quaternary metallic alloys, examples of stable binary icosahedral quasicrystals are quite rare. Indeed, it has been a decade since the discovery of icosahedral phases in Yb–Cd and Ca–Cd. We have discovered millimeter-sized facetted grains of i-Sc12Zn88 with icosahedral (pentagonal dodecahedral and rhombic triacontahedral) morphologies in solution-grown samples. Structural characterization of the bulk icosahedral phase was accomplished through single-grain high-energy X-ray diffraction. For both growth morphologies, all diffraction peaks could be indexed by a primitive (P-type) icosahedral phase. The two types of morphology do, however, present interesting differences in their respective degrees of quasicrystalline order

In situ high energy x-ray synchrotron diffraction study of the synthesis and stoichiometry of LaFeAsO and LaFeAsO1−xFy

The reaction path for the synthesis of LaFeAsO and LaFeAsO1−xFy by solid state reaction was studied by in situ high temperature x-ray diffraction technique and differential thermal analysis in the temperature interval 100 °C ⩽ T ⩽ 1150 °C. Starting with LaAs, Fe2O3, Fe, and LaF3 as precursors, the results show that the synthesis is characterized by three temperature intervals: (1) Below 500 °C the sequential reduction of Fe2O3 and Fe3O4takes place through the oxidization of LaAs. Below 400 °C, Fe2O3 is reduced to Fe3O4 by LaAs and then at 400 °C\u3cT\u3c500 °C Fe3O4 is further reduced to Fe. (2) In the temperature interval 500 °C\u3cT\u3c800 °C, multiple intermediate reactions take place resulting in the formation of FeAs and La2O3. (3) The formation of LaFeAsO based phase could be unambiguously resolved above 800 °C. For both LaFeAsO and LaFeAsO1−xFy, FeAs is a primary impurity at high temperatures that melts at ∼ 1040 °C. Possible reaction pathways and the difference between F-free and F-doped samples are discussed

Spin-flop transition in Gd5Ge4 observed by x-ray resonant magnetic scattering and first-principles calculations of magnetic anisotropy

X-ray resonant magnetic scattering was employed to study a fully reversible spin-flop transition in orthorhombic Gd5Ge4 and to elucidate details of the magnetic structure in the spin-flop phase. The orientation of the moments at the three Gd sites flop 90° from the c axis to the a axis when a magnetic field, Hsf=9 kOe, is applied along the c axis at T=9 K. The magnetic space group changes from Pnm′a to Pn′m′a′ for all three Gd sublattices. The magnetic anisotropy energy determined from experimental measurements is in good agreement with the calculations of the magnetic anisotropy based on the spin-orbit coupling of the conduction electrons and an estimation of the dipolar interactions anisotropy. No significant magnetostriction effects were observed at the spin-flop transition

Control of magnetic, nonmagnetic, and superconducting states in annealed Ca(Fe1-xCox)(2)As-2

We have grown single-crystal samples of Co substituted CaFe2As2 using an FeAs flux and systematically studied the effects of annealing/quenching temperature on the physical properties of these samples. Whereas the as-grown samples (quenched from 960 degrees C) all enter the collapsed tetragonal phase upon cooling, annealing/quenching temperatures between 350 and 800 degrees C can be used to tune the system to low-temperature antiferromagnetic/orthorhomic or superconducting states as well. The progression of the transition temperature versus annealing/quenching temperature (T-T-anneal) phase diagrams with increasing Co concentration shows that, by substituting Co, the antiferromagnetic/orthorhombic and the collapsed tetragonal phase lines are separated and bulk superconductivity is revealed. We established a 3D phase diagram with Co concentration and annealing/quenching temperature as two independent control parameters. At ambient pressure, for modest x and T-anneal values, the Ca(Fe1-xCox)(2)As-2 system offers ready access to the salient low-temperature states associated with Fe-based superconductors: antiferromagnetic/orthorhombic, superconducting, and nonmagnetic/collapsed tetragonal

Effect of Biaxial Strain on the Phase Transitions of Ca(Fe1−xCox)2As2

We study the effect of applied strain as a physical control parameter for the phase transitions of Ca(Fe1−xCox)2As2 using resistivity, magnetization, x-ray diffraction, and 57Fe Mössbauer spectroscopy. Biaxial strain, namely, compression of the basal plane of the tetragonal unit cell, is created through firm bonding of samples to a rigid substrate via differential thermal expansion. This strain is shown to induce a magnetostructural phase transition in originally paramagnetic samples, and superconductivity in previously nonsuperconducting ones. The magnetostructural transition is gradual as a consequence of using strain instead of pressure or stress as a tuning parameter

Activation and Oxidation of Mesitylene C−H Bonds by (Phebox)Iridium(III) Complexes

A pincer iridium(III) complex, (Phebox)Ir(OAc)_2OH_2 (1) (Phebox = 3,5-dimethylphenyl-2,6-bis(oxazolinyl)), selectively cleaves the benzylic C–H bond of mesitylene to form an isolable iridium mesityl complex, (Phebox)Ir(mesityl)(OAc) (3), in >90% yield. The trifluoroacetate analogue, (Phebox)Ir(OCOCF_3)_2OH_2 (2), was synthesized to compare with complex 1 for C–H activation, and (Phebox)Ir(mesityl)(OCOCF_3) (4) was synthesized by ligand exchange of complex 3. Both complexes 1 and 2 catalyze H/D exchange between mesitylene and D2O at 180 °C, exclusively at the benzylic position; 2 gave a higher turnover number (11 TO) than 1 (6 TO) in 12 h. Using d4-acetic acid as the source of deuterium, up to 92 turnovers of benzylic H/D exchange of mesitylene were obtained with complex 1. (Phebox)Ir(OCOCF3)_2OH_2 catalyzed the benzylic C–H oxidation of mesitylene using Ag_2O as a terminal oxidant at 130 °C, to form 3,5-dimethylbenzaldehyde and 3,5-dimethylbenzoic acid in 35% ± 4% yield (5.1 ± 0.6 TO). DFT calculations were used to investigate two possible pathways for the catalytic oxidation of mesitylene: (1) C–H activation followed by oxy-functionalization and (2) Ir-oxo formation followed by outer-sphere C–H hydroxylation. Results of calculations of the C–H activation pathway appear to be the more consistent with the experimental observations