Charge disproportionation and nano phase separation in $$\textit{R}\mathrm{SrNiO}_{4}$$

We have successfully grown centimeter-sized layered $$\textit{R}\mathrm{SrNiO}_{4}$$ R SrNiO 4 single crystals under high oxygen pressures of 120–150 bar by the floating zone technique. This enabled us to perform neutron scattering experiments where we observe close to quarter-integer magnetic peaks below $$\sim 77~\mathrm{K}$$ ∼ 77 K that are accompanied by steep upwards dispersing spin excitations. Within the high-frequency Ni–O bond stretching phonon dispersion, a softening at the propagation vector for a checkerboard modulation can be observed. We were able to simulate the magnetic excitation spectra using a model that includes two essential ingredients, namely checkerboard charge disproportionation and nano phase separation. The results thus suggest that charge disproportionation is preferred instead of a Jahn–Teller distortion even for this layered $$\mathrm{Ni}^{3+}$$ Ni 3 + system.

Composite Spin and Quadrupole Wave in the Ordered Phase of Tb2+xTi2-xO7+y

The hidden ordered state of the frustrated pyrochlore oxide Tb 2+x Ti 2-x O 7+y is possibly one of the two electric multipolar, or quadrupolar, states of the effective pseudospin-1/2 Hamiltonian derived from crystal-field ground state doublets of non-Kramers Tb 3+ ions. These long-range orders are antiparallel or parallel alignments of transverse pseudospin components representing electric quadrupole moments, which cannot be observed as magnetic Bragg reflections by neutron scattering. However, pseudospin waves of these states are composite waves of the magnetic-dipole and electric-quadrupole moments, and can be partly observed by inelastic magnetic neutron scattering. We calculate these spin-quadrupole waves using linear spin-wave theory and discuss previously observed low-energy magnetic excitation spectra of a polycrystalline sample with x = 0.005(T c = 0.5 K).