Probing orientation information using 3-dimensional reciprocal space volume analysis.

The crystallographic texture of polycrystalline materials is the result of how these materials are processed and what external forces materials have experienced. Neutron and X-ray diffraction are standard methods to characterize global crystallographic textures. However, conventional neutron and X-ray texture analyses rely on pole figure inversion routines derived from intensity analysis of individual reflections or powder Rietveld analysis to reconstruct and model the orientation distribution from slices through reciprocal space. In this work, we describe an original approach to directly probe the crystallographic texture information of rolled aluminum from the intensity distribution in 3-dimensional reciprocal space volumes measured simultaneously. Using the TOPAZ time-of-flight Laue neutron diffractometer, reciprocal space analysis allowed determination of "pole spheres" with <1° angular resolution. These pole spheres are compared with reconstructed pole figures from classic texture analysis.

Tomonaga-Luttinger liquid behavior and spinon confinement in YbAlO3.

Low dimensional quantum magnets are interesting because of the emerging collective behavior arising from strong quantum fluctuations. The one-dimensional (1D) S = 1/2 Heisenberg antiferromagnet is a paradigmatic example, whose low-energy excitations, known as spinons, carry fractional spin S = 1/2. These fractional modes can be reconfined by the application of a staggered magnetic field. Even though considerable progress has been made in the theoretical understanding of such magnets, experimental realizations of this low-dimensional physics are relatively rare. This is particularly true for rare-earth-based magnets because of the large effective spin anisotropy induced by the combination of strong spin-orbit coupling and crystal field splitting. Here, we demonstrate that the rare-earth perovskite YbAlO3 provides a realization of a quantum spin S = 1/2 chain material exhibiting both quantum critical Tomonaga-Luttinger liquid behavior and spinon confinement-deconfinement transitions in different regions of magnetic field-temperature phase diagram.

A suite-level review of the neutron single-crystal diffraction instruments at Oak Ridge National Laboratory.

The nascent suite of single-crystal neutron diffractometers at the Oak Ridge National Laboratory has no equal at any other neutron scattering facility worldwide and offers the potential to re-assert single-crystal diffraction using neutrons as a significant tool to study nuclear and magnetic structures of small unit cell crystals, nuclear structures of macromolecules, and diffuse scattering. Signature applications and features of single-crystal neutron diffraction are high resolution nuclear structure analysis, magnetic structure and spin density determinations, contrast variation (particularly D2O/H2O) for nuclear structural studies, lack of radiation damage when using crystals of biological molecules such as proteins, and the fidelity to measure nuclear and magnetic diffuse scattering with elastic discrimination.

WAND2-A versatile wide angle neutron powder/single crystal diffractometer.

Wide Angle Neutron Diffractometer Squared is a high-flux versatile diffractometer with a 2-Dimensional Position Sensitive Detector at the High Flux Isotope Reactor. The instrument has strengths in both powder and single crystal diffraction. It is a unique instrument in the neutron scattering landscape of North America, and its capabilities are at least equal to similar instruments in the world.

A suite-level review of the neutron powder diffraction instruments at Oak Ridge National Laboratory.

The suite of neutron powder diffractometers at Oak Ridge National Laboratory (ORNL) utilizes the distinct characteristics of the Spallation Neutron Source and High Flux Isotope Reactor to enable the measurements of powder samples over an unparalleled regime at a single laboratory. Full refinements over large Q ranges, total scattering methods, fast measurements under changing conditions, and a wide array of sample environments are available. This article provides a brief overview of each powder instrument at ORNL and details the complementarity across the suite. Future directions for the powder suite, including upgrades and new instruments, are also discussed.

Pauling Entropy, Metastability, and Equilibrium in Dy_{2}Ti_{2}O_{7} Spin Ice.

Determining the fate of the Pauling entropy in the classical spin ice material Dy_{2}Ti_{2}O_{7} with respect to the third law of thermodynamics has become an important test case for understanding the existence and stability of ice-rule states in general. The standard model of spin ice-the dipolar spin ice model-predicts an ordering transition at T≈0.15 K, but recent experiments by Pomaranski et al. suggest an entropy recovery over long timescales at temperatures as high as 0.5 K, much too high to be compatible with the theory. Using neutron scattering and specific heat measurements at low temperatures and with long timescales (0.35 K/10^{6} s and 0.5 K/10^{5} s, respectively) on several isotopically enriched samples, we find no evidence of a reduction of ice-rule correlations or spin entropy. High-resolution simulations of the neutron structure factor show that the spin correlations remain well described by the dipolar spin ice model at all temperatures. Furthermore, by careful consideration of hyperfine contributions, we conclude that the original entropy measurements of Ramirez et al. are, after all, essentially correct: The short-time relaxation method used in that study gives a reasonably accurate estimate of the equilibrium spin ice entropy due to a cancellation of contributions.

Evidence for a spinon Fermi surface in a triangular-lattice quantum-spin-liquid candidate.

A quantum spin liquid is an exotic quantum state of matter in which spins are highly entangled and remain disordered down to zero temperature. Such a state of matter is potentially relevant to high-temperature superconductivity and quantum-information applications, and experimental identification of a quantum spin liquid state is of fundamental importance for our understanding of quantum matter. Theoretical studies have proposed various quantum-spin-liquid ground states, most of which are characterized by exotic spin excitations with fractional quantum numbers (termed 'spinons'). Here we report neutron scattering measurements of the triangular-lattice antiferromagnet YbMgGaO4 that reveal broad spin excitations covering a wide region of the Brillouin zone. The observed diffusive spin excitation persists at the lowest measured energy and shows a clear upper excitation edge, consistent with the particle-hole excitation of a spinon Fermi surface. Our results therefore point to the existence of a quantum spin liquid state with a spinon Fermi surface in YbMgGaO4, which has a perfect spin-1/2 triangular lattice as in the original proposal of quantum spin liquids.

The low-temperature magnetostructure and magnetic field response of Pr0.9Ca0.1MnO3: the roles of Pr spins and magnetic phase separation.

With the goal of elucidating the background of photoinduced ferromagnetism phenomena observed in the perovskite structured (Pr,Ca) manganites, the low-temperature magnetostructure of the material Pr0.9Ca0.1MnO3 was revised using cold neutron powder diffraction, SQUID magnetometry and ab initio calculations. Particular emphasis was placed on determining the presence of nanoscale magnetic phase separation. Previously published results of a canted A-AFM average ground state were reproduced to a good precision both experimentally and theoretically, and complemented by investigating the effects of an applied magnetic field of 2.7 T on the magnetostructure. Explicit evidence of nanoscale magnetic clusters in the material was obtained based on high-resolution neutron diffractograms. Along with several supporting arguments, we present this finding as a justification for extending the nanoscale magnetic phase separation model of manganites to the material under discussion despite its very low Ca doping level in the context of the model. In the light of the new data, we also conclude that the low temperature magnetic moment of Pr must be ca. 300% larger than previously thought in this material, close to the high spin value of 2µB per formula unit.

Damped spin waves in the intermediate ordered phases in Ni3V2O8.

Spin dynamics in the intermediate ordered phases (between 4 and 9 K) in Ni3V2O8 have been studied with inelastic neutron scattering. It is found that the spin waves are very diffuse, indicative of short lived correlations and the coexistence of paramagnetic moments with the long-range ordered state.

A detailed study of the magnetic phase transition in CuCrO2.

The phase transition in CuCrO2 to an ordered magnetic state is studied with bulk measurements and elastic and inelastic neutron scattering techniques. The reported onset of spontaneous electric polarization at T = 23.5 K coincides with the appearance, on cooling, of elastic magnetic scattering. At higher temperatures long range magnetic correlations gradually develop but they are dynamic. The ground state is characterized by three-dimensional long range magnetic ordering but along the c direction the correlation length remains limited to ∼200 Å.

Magnetic structure of CuCrO2: a single crystal neutron diffraction study.

This paper presents results of a recent study of multiferroic CuCrO(2) by means of single crystal neutron diffraction. This system has two close magnetic phase transitions at T(N) = 24.2 K and T(mf) = 23.6 K. The low temperature magnetic structure below T(mf) is unambiguously determined to be a fully three-dimensional proper screw. Between T(N) and T(mf) antiferromagnetic order is found that is essentially two-dimensional. In this narrow temperature range, magnetic near neighbor correlations are still long range in the (H,K) plane, whereas nearest neighbors along the L direction are uncorrelated. Thus, the multiferroic state is realized only in the low temperature three-dimensional state and not in the two-dimensional state.

Electronic structure and nesting-driven enhancement of the RKKY interaction at the magnetic ordering propagation vector in Gd2PdSi3 and Tb2PdSi3.

Measurements of the low-energy electronic structure in Gd2PdSi3 and Tb2PdSi3 by means of angle-resolved photoelectron spectroscopy reveal a Fermi surface consisting of an electron barrel at the Gamma point surrounded by spindle-shaped electron pockets originating from the same band. The calculated momentum-dependent RKKY coupling strength is peaked at the 1/2GammaK wave vector, which coincides with the propagation vector of the low-temperature in-plane magnetic order observed by neutron diffraction, thereby demonstrating the decisive role of the Fermi surface geometry in explaining the complex magnetic ground state of ternary rare earth silicides.