Understanding the microscopic processes affecting the bulk thermal conductivity is crucial to develop more efficient thermoelectric materials. PbTe is currently one of the leading thermoelectric materials, largely thanks to its low thermal conductivity. However, the origin of this low thermal conductivity in a simple rocksalt structure has so far been elusive. Using a combination of inelastic neutron scattering measurements and first-principles computations of the phonons, we identify a strong anharmonic coupling between the ferroelectric transverse optic mode and the longitudinal acoustic modes in PbTe. This interaction extends over a large portion of reciprocal space, and directly affects the heat-carrying longitudinal acoustic phonons. The longitudinal acoustic-transverse optic anharmonic coupling is likely to play a central role in explaining the low thermal conductivity of PbTe. The present results provide a microscopic picture of why many good thermoelectric materials are found near a lattice instability of the ferroelectric type.

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A gradual spin-state transition occurs in LaCoO3 around T approximately 80-120 K, whose detailed nature remains controversial. We studied this transition by means of inelastic neutron scattering and found that with increasing temperature an excitation at approximately 0.6 meV appears, whose intensity increases with temperature, following the bulk magnetization. Within a model including crystal-field interaction and spin-orbit coupling, we interpret this excitation as originating from a transition between thermally excited states located about 120 K above the ground state. We further discuss the nature of the magnetic excited state in terms of intermediate-spin (t(2g)(5)e(g)(1), S=1) versus high-spin (t(2g)(4)e(g)(2), S=2) states. Since the g factor obtained from the field dependence of the inelastic neutron scattering is g approximately 3, the second interpretation is definitely favored.

The design and performance of the new cold neutron chopper spectrometer (CNCS) at the Spallation Neutron Source in Oak Ridge are described. CNCS is a direct-geometry inelastic time-of-flight spectrometer, designed essentially to cover the same energy and momentum transfer ranges as IN5 at ILL, LET at ISIS, DCS at NIST, TOFTOF at FRM-II, AMATERAS at J-PARC, PHAROS at LANSCE, and NEAT at HZB, at similar energy resolution. Measured values of key figures such as neutron flux at sample position and energy resolution are compared between measurements and ray tracing Monte Carlo simulations, and good agreement (better than 20% of absolute numbers) has been achieved. The instrument performs very well in the cold and thermal neutron energy ranges, and promises to become a workhorse for the neutron scattering community for quasielastic and inelastic scattering experiments.

Spin ordering in TbBaCo 2 O 5.5 and its temperature transformation reproducible for differently synthesized samples are studied. First of all, the polymorphism due to the oxygen ordering with the average content close to 5.5 is investigated. One of ceramic samples (I), in addition to the main phase a p × 2a p × 2a p , Pmmm (Z = 2), contained about 25% of the phase a p × a p × 2a p , Pmmm, (Z = 1) with statistical distribution of oxygen over the apical sites, where a p is parameter of perovskite cell. The other sample (II) contained a single phase a p × 2a p × 2a p , Pmmm (Z = 2) with well defined octahedral and pyramidal sublattices. Treatment of neutron diffraction patterns of the sample I itself gives a sophisticated spin structure. Knowing the structure of sample II, one can chose only proper magnetic lines, which give exactly the same results as for sample II. Above the Néel temperature T N ≈ 290 K, there is a structural transition to the phase 2a p × 2a p × 2a p , Pmma. At T N , the spins order with the wave vector k 19 = 0 (phase 1). At T 1 ≈ 255 K, a magnetic transition takes place to the phase 2 with k 22 = b 3 /2. At T 2 ≈ 170 K, the crystal structure changes to 2a p × 2a p × 4a p , Pcca (Z = 4). The wave vector of the spin structure becomes again k 19 = 0 (phase 3). The basis functions of irreducible representations of the group G k have been found. Using results of this analysis, the magnetic structure in all phases is determined. The spins are always parallel to the x axis, and the difference is in the values and mutual orientation of the moments in the ordered non-equivalent pyramidal or octahedral positions. Spontaneous moment M 0 = 0.30(3) µ B /Co at T = 260 K is due to ferrimagnetic ordering of the moments M Py1 = 0.46(9) µ B and M Py2 = −1.65(9) µ B in pyramidal sites (Dzyaloshinskii-Moriya canting is forbidden by symmetry). The moments in the non-equivalent octahedral sites are: M Oc1 = −0.36(9) µ B , M Oc2 = 0.39(9) µ B . At T = 230 K, M Py1 = 0.28(8) µ B , M Py2 = 1.22(8) µ B , M Oc1 = 1.39(8) µ B , M Oc2 = −1.52(8) µ B . At T = 100 K, M Py1 = 1.76(6) µ B , M Py2 = −1.76 µ B , M Oc1 = 3.41(8) µ B , M Oc2 = −1.47(8) µ B . The moment values together with the ligand displacements are used to analyze the picture of spin-state/orbital ordering in each phase.

Inelastic neutron scattering and magnetic susceptibility measurements have been performed on the distorted perovskite NdGaO3. The magnetic susceptibility data show a Curie-Weiss behaviour with an effective magnetic moment close to 3.6 mu B per mole of Nd ions. No long-range magnetic ordering was detected in the temperature range 2-300 K. The inelastic neutron spectra observed at T=12 K exhibit four peaks of magnetic origin between 11 and 7D meV which can be unambiguously assigned to the complete crystalline-electric-field splitting pattern in the ground-state J multiplet 4I9/2 of the Nd3+ ions. We analysed the spectra in terms of geometrical considerations based on the actual C2 site symmetry of Nd3+. The best agreement between the experimental spectra and the calculated level structure was obtained for a model that takes into account the three nearest-neighbouring coordination polyhedra associated with the O2-, Ga3+ and Nd3+ ions as well as J-mixing between all multiplets of the 4I term. We conclude that single-particle crystal-field theory adequately explains the majority of magnetic and crystal-field properties of NdGaO3.

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.

Neutron powder diffraction and inelastic measurements were performed examining the 5d pyrochlore Y2Ir2O7. Temperature dependent measurements were performed between 3.4 K and 290 K, spanning the magnetic transition at 155 K. No sign of any structural or disorder induced phase transition was observed over the entire temperature range. In addition, no sign of magnetic longrange order was observed to within the sensitivity of the instrumentation. These measurements do not rule out long range magnetic order, but the neutron powder diffraction structural refinements do put an upper bound for the ordered iridium moment of ∼ 0.2 µB/Ir (for a magnetic structure with wave vector Q = 0) or ∼ 0.5 µB/Ir (for Q = 0).