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We performed inelastic neutron scattering on the 2D Shastry-Sutherland system SrCu2(11BO3)2 with an exact dimer ground state. Three energy levels at around 3, 5, and 9 meV were observed at 1.7 K. The lowest excitation at 3.0 meV is almost dispersionless with a bandwidth of 0.2 meV at most, showing a significant constraint on a single-triplet hopping owing to the orthogonality of the neighboring dimers. In contrast, the correlated two-triplet excitations at 5 meV exhibit a more dispersive behavior.

We measured two magnetic modes with finite and discrete energies in an antiferromagnetic ordered phase of a geometrically frustrated magnet MgCr2O4 by single-crystal inelastic neutron scattering, and clarified the spatial spin correlations of the two levels: one is an antiferromagnetic hexamer and the other is an antiferromagnetic heptamer. Since these correlation types are emblematic of quasielastic scattering with geometric frustration, our results indicate instantaneous suppression of lattice distortion in an ordered phase by spin-lattice coupling, probably also supported by orbital and charge. The common features in the two levels, intermolecular independence and discreteness of energy, suggest that the spin molecules are interpreted as quasiparticles (elementary excitations with energy quantum) of highly frustrated spins, in analogy with the Fermi liquid approximation.

Neutron diffraction measurements have been performed on the cubic compound PrPb3 in a [001] magnetic field to examine the quadrupolar ordering. Antiferromagnetic components with q=(1/2+-d 1/2 0), (1/2 1/2+-d 0) (d~1/8) are observed below the transition temperature TQ (0.4 K at H=0) whose amplitudes vary linear with H and vanish at zero field, providing the first evidence for a modulated quadrupolar phase. For H, Physical Review Letters, in press. 4 pages, 4 figures

The magnetic structure of the localized-5f uranium intermetallic compound U3Pd20Si6 has been determined by means of a neutron diffraction experiment. Our data demonstrate that this compound has a collinear coupling of the sublattice ordering of the uranium spins on the 4a and 8c sites. We conclude that higher-order exchange and/or quadrupole interactions are necessary to stabilize this unique collinear structure. We discovered a new type of spin-flop transition against the uniaxial anisotropy induced by this collinear coupling.

We report ${}^{27}\mathrm{Al}$ Knight shift ( $^{27}K$) measurement on a single-crystal ${\mathrm{UNi}}_{2}{\mathrm{Al}}_{3}$ that reveals a coexistence of superconductivity and a spin-density-wave (SDW) type of magnetic ordering ( ${T}_{\mathrm{SDW}}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}4.5\mathrm{K}$). The spin part of $^{27}K$, ${}^{27}{K}_{\mathrm{s}}$, does not change down to 50 mK across the superconducting (SC) transition temperature ${T}_{c}\ensuremath{\sim}0.9\mathrm{K}$. In contrast with the isostructural compound ${\mathrm{UPd}}_{2}{\mathrm{Al}}_{3}$ ( ${T}_{c}\ensuremath{\sim}2\mathrm{K}$), which was identified to be a spin-singlet $d$-wave superconductor, the behavior of $^{27}K$ strongly supports that ${\mathrm{UNi}}_{2}{\mathrm{Al}}_{3}$, like ${\mathrm{UPt}}_{3}$ and ${\mathrm{Sr}}_{2}{\mathrm{RuO}}_{4}$, belongs to a class of spin-triplet SC pairing state superconductors.

The Dzyaloshinski-Moriya interaction partially lifts the magnetic frustration of the spin-1/2 oxide SrCu(2)(BO(3))(2). It explains the fine structure of the excited triplet state and its unusual magnetic field dependence, as observed in previous ESR and new neutron inelastic scattering experiments. We claim that it is mainly responsible for the dispersion. We propose also a new mechanism for the observed ESR transitions forbidden by standard selection rules, which relies on an instantaneous Dzyaloshinski-Moriya interaction induced by spin-phonon couplings.