High-energy, local multiplet excitations of the d-electrons are revealed in our inelastic neutron scattering measurements on the prototype magnetic insulator NiO. These become allowed by the presence of both non-zero crystal field and spin-orbit coup
ling. The observed excitations are consistent with optical, x-ray, and EELS measurements of d-d excitations. This experiment serves as a proof of principle that high-energy neutron spectroscopy is a reliable and useful technique for probing electronic excitations in systems with significant crystal field and spin-orbit interactions.
We report a comprehensive x-ray scattering study of charge density wave (stripe) ordering in $rm La_{2-x}Ba_xCuO_4 (x approx 1/8)$, for which the superconducting $T_c$ is greatly suppressed. Strong superlattice reflections corresponding to static ord
ering of charge stripes were observed in this sample. The structural modulation at the lowest temperature was deduced based on the intensity of over 70 unique superlattice positions surveyed. We found that the charge order in this sample is described with one-dimensional charge density waves, which have incommensurate wave-vectors (0.23, 0, 0.5) and (0, 0.23, 0.5) respectively on neighboring $rm CuO_2$ planes. The structural modulation due to the charge density wave order is simply sinusoidal, and no higher harmonics were observed. Just below the structural transition temperature, short-range charge density wave correlation appears, which develops into a large scale charge ordering around 40 K, close to the spin density wave ordering temperature. However, this charge ordering fails to grow into a true long range order, and its correlation length saturates at $sim 230AA$, and slightly decreases below about 15 K, which may be due to the onset of two-dimensional superconductivity.
We present neutron scattering data on two single crystals of the high temperature superconductor La2-x(Ca,Sr)xCaCu2O6+delta. The Ca0.1-doped crystal exhibits a long-range antiferromagnetically ordered ground state. In contrast, the Sr0.15-doped cryst
al exhibits short-range antiferromagnetic order as well as weak superconductivity. In both crystals antiferromagnetic correlations are commensurate; however, some results on the Ca0.1-doped crystal resemble those on the spin-glass phase of La2-xSrxCuO4, where magnetic correlations became incommensurate. In addition, both crystals show a structural transition from tetragonal to orthorhombic symmetry. Quite remarkably, the temperature dependence and correlation length of the magnetic order is very similar to that of the orthorhombic distortion. We attribute this behavior to an orthorhombic strain-induced inter-bilayer magnetic coupling, which triggers the antiferromagnetic order. The large size of the crystals made it also possible to study the magnetic diffuse scattering along rods perpendicular to the CuO2 planes in more detail. For comparison we show X-ray diffraction and magnetization data. In particular, for the Ca0.1-doped crystal these measurements reveal valuable information on the spin-glass transition as well as a second anomaly associated with the Neel transition.
