arXiv subjects

Yutaka Ueda

Publications and source records attributed to Yutaka Ueda.

22 records · Page 2Linked to original sources

arXiv2001

Spin Diffusion in the S=1/2 Quasi One-Dimensional Antiferromagnet α-VO(PO_3)_2 via 31P NMR

Low-frequency spin dynamics in the S=1/2 antiferromagnetic spin-chain compound α-VO(PO_3)_2 has been studied by means of 31P NMR. The nuclear spin-lattice relaxation rate 1/T_1 at the P site exhibits H^{-1/2} dependence on the applied magnetic field (H) at temperatures (T) well above the intrachain coupling strength J/k_{B} = 3.50 K indicating one-dimensional diffusive spin dynamics. The diffusive contribution to 1/T_1 decreases on cooling as electronic spins acquire short-range antiferromagnetic correlations within the chain, and vanishes almost entirely around T \approx J/k_{B}. This is accompanied by an apparent increase of the spin-diffusion constant from the value expected in the classical limit. On the other hand, the field-independent part of 1/T_1 increases with decreasing temperature, which may be a precursor for the true long-range antiferromagnetic ordering found below T_{N} = 1.93 \pm 0.01 K.

cond-mat.str-el↗

arXiv2000

Spin Excitations in the Two-leg Ladder Antiferromagnet CaV$_2$O$_5$

The nuclear spin-lattice relaxation rate (1/T1) at the V sites in CaV2O5 was measured. The result was compared with the calculation for the two-leg ladder with the strong-coupling expansion. We found a good agreement on the temperature and magnetic field dependence but a discrepancy between the exchange parameters deduced from 1/T1 and the magnetic susceptibility.

cond-mat.str-el↗

arXiv2000

Zigzag Charge Ordering in alpha'-NaV2O5

23Na NMR spectrum measurements in alpha'-NaV2O5 with a single- crystalline sample are reported. In the charge-ordered phase, the number of inequivalent Na sites observed is more than that expected from the low-temperature structures of space group Fmm2 reported so far. This disagreement indicates that the real structure including both atomic displacement and charge disproportionation is of lower symmetry. It is suggested that zigzag ordering is the most probable. The temperature variation of the NMR spectra near the transition temperature is incompatible with that of second-order transitions. It is thus concluded that the charge ordering transition is first-order.

cond-mat.str-el↗

arXiv1999

Coexistence of double alternating antiferromagnetic chains in (VO)_2P_2O_7 : NMR study

Nuclear magnetic resonance (NMR) of 31P and 51V nuclei has been measured in a spin-1/2 alternating-chain compound (VO)_2P_2O_7. By analyzing the temperature variation of the 31P NMR spectra, we have found that (VO)_2P_2O_7 has two independent spin components with different spin-gap energies. The spin gaps are determined from the temperature dependence of the shifts at 31P and 51V sites to be 35 K and 68 K, which are in excellent agreement with those observed in the recent inelastic neutron scattering experiments [A.W. Garrett et al., Phys. Rev. Lett. 79, 745 (1997)]. This suggests that (VO)_2P_2O_7 is composed of two magnetic subsystems showing distinct magnetic excitations, which are associated with the two crystallographically-inequivalent V chains running along the b axis. The difference of the spin-gap energies between the chains is attributed to the small differences in the V-V distances, which may result in the different exchange alternation in each magnetic chain. The exchange interactions in each alternating chain are estimated and are discussed based on the empirical relation between the exchange interaction and the interatomic distance.

cond-mat.str-el↗