Kazuhisa Kakurai

New Inorganic Spin-Peierls Compound NaV 2O 5 Evidenced by X-Ray and Neutron Scattering

In this letter we report lattice dimerization and spin-gap formation of NaV 2 O 5 below its spin-Peierls transition temperature T sp =35.3 K, in which there are V 4+ ( S =1/2) linear chains along the b axis of its orthorhombic structure. A single-crystal X-ray scattering experiment shows superlattice reflections with a lattice modulation vector q =(1/2, 1/2, 1/4) . Inelastic neutron scattering from a powder sample at T =7 K clearly demonstrates a gap energy of 9.8 meV at a scattering vector | Q |=1.0 A -1 corresponding to the reciprocal lattice point (1, 1/2, 0).

Direct evidence for the localized single-triplet excitations and the dispersive multitriplet excitations in SrCu2(BO3)2.

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.

Observation of Field-Induced Transverse Neel Ordering in the Spin Gap System TlCuCl3.

Neutron elastic scattering experiments have been performed on the spin gap system TlCuCl 3 in magnetic fields parallel to the b -axis. The magnetic Bragg peaks which indicate the field-induced Neel ordering were observed for the magnetic field higher than the gap field H g ≈5.5 T at Q =( h , 0, l ) with odd l in the a * - c * plane. The spin structure in the ordered phase was determined. The temperature and field dependence of the Bragg peak intensities and the phase boundary obtained were discussed in connection with a recent theory which describes the field-induced Neel ordering as a Bose-Einstein condensation of magnons.

Inelastic neutron scattering studies on 1D near‐Heisenberg antiferromagnets: A test of the Haldane conjecture

Our studies of the spin dynamics for two near‐Heisenberg antiferromagnets (AFMs) revealed: In CsNiCl3 (S=1) at low T≪TN we confirm the conclusions given earlier, but disagreement with the spin‐wave calculations used is found for the eigenvectors. For T>TN, the most important result is the observation that all 〈SαSα〉 (α=x,y,z) are identical, demanding another interpretation for the gap observed than the easy axis one. In CsVCl3 (S= (3)/(2) ) we could not observe any inconsistency in the microscopic parameters determined above and below TN. Although there is a striking coincidence with Haldane’s conjecture [unexplained gap for S=1 (CsNiCl3), no such gap for S= (3)/(2) (CsVCl3)], it is not clear whether this can be considered a proof for Haldane’s conjecture.

Strong antiferromagnetic spin fluctuations and the quantum spin-liquid state in geometrically frustrated -Mn, and the transition to a spin-glass state caused by non-magnetic impurity

The magnetic properties of -Mn and the effect of the substitution of non-magnetic Al were systematically investigated by measuring thermodynamic and transport properties, and the NMR and polarized neutron scattering for alloys with -Mn is paramagnetic down to the lowest temperature (1.4 K) but with strong spin fluctuations. Substitution of Al atoms which preferentially occupy one of two crystallographic sites (site II) results in the formation of local moments on Mn atoms, and makes the ground state into a spin-glass-like state. NMR measurements revealed almost independent magnetic behaviour of the two different sites, and static magnetic ordering of Mn at site II for , although the possibility of very weak magnetism of the site-I atoms cannot be excluded. The temperature dependence of the spin - lattice relaxation rate indicates antiferromagnetic correlations of paramagnetic spin fluctuations, and its saturation at higher temperatures for and a critical slowing down of the fluctuations at low temperatures for . Polarized neutron scattering experiments showed directly strong spin fluctuations with antiferromagnetic correlations for both -Mn and . The characteristic energy of fluctuations for -Mn is fairly large even at 7 K , implying a quantum origin of the fluctuations, while the energy spectrum of becomes very sharp at low temperatures, within a resolution-limited energy width, indicating damping of spin fluctuations into a spin-glass-like state. These results can be interpreted in terms of the transition from a spin liquid to a spin glass caused by the substitution of Al that gives rise to the release of the antiferromagnetic frustration of the characteristic crystal lattice. Considerable similarity of the characteristic features to those of the highly frustrated Laves phase compounds is argued. The possible frustration on site II in the -Mn structure, a three-dimensional network of corner-sharing regular triangles, which is similar to the two-dimensional Kagome lattice, is pointed out.

AMATERAS: A Cold-Neutron Disk Chopper Spectrometer

AMATERAS is a new disk-chopper-type spectrometer installed at Materials and Life Science Experimental Facility (MLF) of J-PARC. AMATERAS is equipped with an extra chopper for pulse shaping at the upstream position, in addition to a monochromating chopper, which conventional chopper spectrometers at pulsed source have. Owing to the use of these choppers and the high peak intensity from a coupled moderator source at MLF, the AMATERAS design realizes high-intensity and high-energy-resolution measurements in quasielastic and inelastic neutron scattering experiments. The spectrometer had the first neutron beam in May 2009. During the course of commissioning, the performance of the spectrometer was confirmed by conducting test experiments. AMATERAS is now open to users and is producing scientific outputs.

Magnetic Correlation of NaxCoO2 and Successive Phase Transitions of Na0:5CoO2 —NMR and Neutron Diffraction Studies—

Data taken by various methods including NMR/NQR and neutron scattering are presented for Na x CoO 2 , particularly Na 0.5 CoO 2 . Attention has also been paid to the x dependence of the electronic nature of this system. The pseudo-gap-like behavior observed in the region of x < x c ∼0.6 is emphasized. For samples with x ≤0.6, signals from two kinds of Co sites with different quadrupole frequencies ν Q of ∼4.1 and ∼2.8 MHz were observed, and the former ν Q , which is nearly equal to that of the superconducting system Na 0.3 CoO 2 · y H 2 O, becomes dominant with decreasing x to 0.3. For Na 0.5 CoO 2 , the Co sites with the larger ν Q have larger magnetic moments. They align antiferromagnetically at T c1 ∼87 K with their direction within the plane, while the Co sites with the smaller ν Q have smaller moments. They align in the direction parallel to the c axis. The ordered moments at the two distinct sites exhibit the same T -dependence, indicating that the existence of these sites is not due to macroscopic ...

Neutron Scattering Study of Magnetic Excitations in the Spin S= 1 One-Dimensional Heisenberg Antiferromagnet Y2BaNiO5

Magnetic excitations of the ideal one-dimensional, spin S =1, antiferromagnet Y 2 BaNiO 5 are studied by means of inelastic neutron scattering on single crystal sample. Our experimental results at low temperature indicate that Y 2 BaNiO 5 can be regarded as the best isotropic Haldane-system sofar studied with the intrachain exchange energy J / k B =-24.1 meV and the averaged gap Δ( T =7 K)=8.5 meV. Y 2 BaNiO 5 exhibits a good magnetic one-dimensionality with the inter-/intrachain exchange ratio of | J ′ / J |≤10 -4 and has only very small easy-axis and in-plane single ion anisotropy of | D / J |=0.03 and | E / J |∼0.01. In addition the temperature dependence of the Haldane gap Δ( T ) and its damping rate was determined.

Cu-site doping effects, transport and magnetic properties of high-Tc oxides and their hole concentration dependence

Abstract Doping effects have been studied by using La 2− y Sr y Cu 1−χ M χ O 4 (M = Zn, Ni and Mg) and YBa 2 (Cu 1−χ Zn χ ) 3 O 6+ y . The rate of T c -suppression, |d T c /dχ|, is found to be inversely proportional to the carrier concentration p . The disappearance of the superconductivity with Cu-site doping seems to be primarily due to the loss of the itinerant nature of the holes, which is supported by the variation of the spectral function χ(ω) of the magnetic excitations with temperature, studied by neutron scattering on YBa 2 (Cu 0.965 Zn 0.035 ) 3 O 6.75 . The peak of Hall coefficient versus T curves and the “spin gap” effect found in the T -dependence of the resistivities ϱ become less significant with Zn-doping.

Study of Spin-Gap Formation in Quasi-Two-Dimensional S= 1/2 System CaV 4O 9: Neutron Scattering and NMR

Neutron inelastic scattering study has been performed on aligned single crystals of quasi-two-dimensional spin-gap system CaV 4 O 9 . By analyzing the dispersion curves and the structure factor of the triplet excitation, a physical picture of the present spin system has been constructed: The plaquette singlet units of four S =1/2 V-atoms within four “corner-linked” VO 5 pyramids form the two-dimensional linkage of the spins by the interactions between the V-spins within the “edge-linked” VO 5 pyramids. We have remeasured the NMR relaxation rates and obtained the spin-gap behavior consistent with the neutron results.