Tetsuya Yokoo

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.

Observation of the Haldane gap in Y2BaNiO5 single crystal

Magnetization measurement and inelastic neutron scattering have been performed on a single crystal of a new Haldane system Y 2 BaNiO 5 . Magnetic susceptibility shows a weak anisotropy and decreases as temperature decreases, reflecting the singlet ground state at low temperature. The energy gaps were found to be Δ // / k B =118 K and Δ ⊥ / k B =104 K, parallel and perpendicular to the chain, respectively. Using inelastic neutron scattering magnetic excitations along the chain ( a * -) and perpendicular ( c * -) directions up to 30 meV energy transfer are studied. Energy gap of about 10 meV (116 K) is observed along the one-dimensional, antiferromagnetic zone center, in good agreement with the magnetization measurement. The observed dispersion relation in the chain direction with the gap can be well described by the exact numerical results of S =1, Heisenberg antiferromagnetic chain yielding an intrachain exchange of J / k B =280±10 K.

POLARIZED-NEUTRON OBSERVATION OF LONGITUDINAL HALDANE-GAP EXCITATIONS IN ND2BANIO5

Polarized and unpolarized inelastic neutron scattering is used to study Haldane-gap excitations in the mixed-spin linear-chain antiferromagnet Nd2BaNiO5. The longitudinal mode, polarized along the direction of ordered moments, is observed for the first time. The model of isolated Haldane chains in a static staggered exchange field, that is known to work very well for static properties and transverse spin excitations in R2BaNiO5 compounds, fails to explain new results for the longitudinal spin gap.

S = 1/2 Degrees of Freedom in Haldane System Y2BaNiO5 Observed by Submillimeter Wave ESR

We have performed submillimeter wave ESR measurement of Haldane system Y 2 BaNiO 5 in the frequency region from 50 to 383 GHz using the pulsed magnetic field up to 16 T. Due to the excess oxygens, a few percent of Ni 2+ ions of Y 2 BaNiO 5 , which we used, are substituted by the Ni 3+ ion of which spin is S =1/2. The ESR signals coming from the S =1/2 degrees of freedom originated by the Ni 3+ ion at the end of the finite chain, are observed at 4.2 K. The frequency-field relations of these ESR signals are analyzed by the model based on the VBS model, which is used in the case of Cu 2+ doped NENP. The signs of the single ion anisotropies D and E are determined from the analysis.

Resistivity and thermopower of CaB6 single crystal

Abstract The resistivity ρ and thermopower S of CaB6 single crystal have been measured at temperatures from 2 to 1100 K in magnetic fields up to 15 T . Temperature dependence of ρ is expressed as [ρ(T)−ρ(0)]∝ exp [−(T ∗ /T) 1/2 ] for T⩽90 K and T1.7 for 90 K ⩽T⩽1100 K , where ρ 0 =232 μΩ cm . The magnitude of ρ and S is very large suggesting thin concentration of carriers. The temperature dependence of them suggests the conduction is metallic and electron type. ρ and S can be fitted on the basis of free electron model and obtained the Fermi level is situated near the bottom of an electron band with the Fermi energy of 80 K . The experimental and fitting results show this compound is semi-metallic. No scattering anomalies in temperature dependence of ρ were observed and also no sudden change of the Fermi surface accompanied with magnetic transition in S above 100 K including the region where it might be considered to have ferromagnetic transition.

Quantum and classical dynamics in mixed-spin one-dimensional antiferromagnets

Mixed-spin systems composed of quantum spin chains with gapped magnetic excitations interacting through `auxiliary magnetic ions show an effective separation between low-frequency classical and high-frequency quantum spin correlations. This phenomenon is realized in a family of rare-earth nickelates. Studies of these materials enable experimental measurements of some previously inaccessible fundamental properties of quantum spin chains to be made. The non-trivial intrinsic dynamics of the auxiliary spins gives rise to peculiar excitations of a mixed nature, and in certain cases may play a key role in long-range magnetic ordering.

ESR STUDY OF THE HALDANE GAP SYSTEM Y2BANIO5 IN HIGH MAGNETIC FIELDS

Abstract We performed the ESR experiments in Y2BaNiO5, which has been proposed as a good candidate for the Haldane gap system, over a wide magnetic field range up to 100xa0T. We observed a relatively broad EPR signal at temperatures above the gap energy and another resonance peak which predominates the spectra below 100xa0K. We also found some more resonant peaks, which exhibit an infinite energy when extrapolated to zero magnetic field. The gap energy is estimated to be 5.22xa0meV. This resonance seems to correspond to the transition between the singlet state and the triplet state, because the intensity of this peak becomes stronger as the temperature is lowered. However, the gap energy obtained in the present experiment is smaller than the Haldane gap obtained from the neutron inelastic scattering.

ESR study of Haldane system Y2BaNiO5 in submillimeter wave region

Abstract Submillimeter wave ESR of Haldane system Y 2 BaNiO 5 has been performed. The ESR absorption lines, which were observed in the temperature range above 15 K, are attributed to the transition between the excited triplet states. Frequency-field relations of these absorption lines are analyzed by the theory which explained those observed in NENP as the transitions between the excited triplet states near k = π.

µSR Studies on Spin-Peierls System Cu1-xZnxGeO3 (x=0, 0.01, 0.03, and 0.07)

The zero-field and the longitudinal field spin-relaxations of positive muon (µ + ) were measured in Cu 1- x Zn x GeO 3 ( x =0, 0.01, 0.03, and 0.07) to study the spin-Peierls system. The obtained relaxation functions were separated into two components below T SP in CuGeO 3 . We conclude that the fast relaxation is originated from a few Cu 2+ spins being not coupled with spin-singlet state, and that the slow relaxation is due to the nuclear dipolar fields. The fast relaxation function shape changes to the Gaussian at 5 K in the sample of x =0.07. The system changes from the static diluted to the densed one. We also confirmed the appearances of the spin-glass-like relaxations at 5 K in the samples of x =0.03 and 0.07.

Spatially Modulated Antiferromagnetism in Sr 2.5Ca 11.5Cu 24O 41 Probed by Muon Spin Relaxation

A strongly modulated Neel order has been inferred from muon spin relaxation (µSR) measurement in Sr 2.5 Ca 11.5 Cu 24 O 41 below T N = 2.2 ±0.1 K in conjunction with the previous result of neutron ...