Toshihiko Fukamachi

Spin-gap behavior of CuNb2O6

The magnetic properties of two polymorphs of CuNb 2 O 6 with quasi-one-dimensional zig-zag spin ( S =1/2) chains of Cu 2+ ions are studied. The magnetic excitation spectra of Cu spins measured by neutron scattering, and the T -dependence of the longitudinal spin relaxation rate of 93 Nb nuclei by NMR indicate the existence of the spin gap in the monoclinic phase whose magnitude is consistent with the one deduced by the detailed analyses of uniform magnetic susceptibility at low temperatures. The behavior in the monoclinic phase is in clear contrast to that found in the orthorhombic phase.

Specific heat of quasi-one-dimensional spin 1/2 system CuNb2O6

The specific heat C of CuNb 2 O 6 with quasi-one-dimensional zig-zag spin S =1/2 chains has been measured in the temperature ( T ) range between 1.6 K and 40 K. For the orthorhombic phase, a sharp anomaly of C is observed at the antiferromagnetic transition temperature T N ∼7.3 K. The T -dependence of C of the monoclinic phase, whose ground state has a spin gap, can be well described by the exchange parameters determined by fitting the magnetic susceptibility χ calculated for a model of a ferromagnetic-antiferromagnetic alternating bond to the experimentally obtained χ. The gap-value estimated from the low temperature behavior of C is about 21 K, which is also consistent with the ones obtained by other measurements. Results of the measurements of the magnetic susceptibilities χ and the specific heats C of M–Cu 1- x Zn x Nb 2 O 6 (0≤ x ≤0.5) and their structural parameters are also presented.

NMR Studies on Spin-Gap Behavior of Monoclinic CuNb2O6.

The NMR Knight shift and the spin-lattice relaxation rate 1/ T 1 have been measured at Nb sites in two polymorphs of quasi one-dimensional zig-zag spin chain compounds CuNb 2 O 6 . The temperature ( T ) dependence of NMR-1/ T 1 of the monoclinic CuNb 2 O 6 indicates the existence of spin excitation gap (spin gap) whose magnitude (∼20 K) is consistent with the result of other kinds of measurements. The difference of the T -dependences between 1/ T 1 T and K is similar to that well-known for high- T c oxides. A simultaneous growing of the antiferromagnetic and the singlet correlation has been observed with decreasing T at around the temperature of the susceptibility maximum (≥20 K). Results of the NMR measurements for the orthorhombic CuNb 2 O 6 , which undergoes an antiferromagnetic transition at about 7.3 K, are also presented.

On the Magnetic Properties of Systems with Low Dimensional Linkage of VO5 Pyramids.

[N(CH 3 ) 4 ]V 3 O 7 , (en) 2 ZnV 6 O 14 (en=ethylenediamine) and (C 6 H 14 N 2 )V 6 O 14 ·H 2 O with edge-sharing VO 5 chains have been prepared hydrothermally and studied by magnetic susceptibility measurements and by NMR. Observed magnetic properties, which indicate the existence of the spin gap, can be explained by a model of isolated spin dimers of V 4+ ions, where a very large exchange interaction J exists only between the electrons at V sites within two edge-sharing VO 5 pyramids whose apical oxygens are on the same side of the basal planes. It can be understood by considering the kinetic exchange mechanism acting between the electrons in the x y orbitals.

Magnetization measurements and 87Rb-NMR on Rb2M3S4 (M=Ni and Pd) with the Kagome lattice

We report results of magnetization measurements and 87 Rb-NMR on Rb 2 Ni 3 S 4 and Rb 2 Pd 3 S 4 which have the Kagome lattice. The anomalous increase of the magnetization found in the former system at low temperatures with decreasing temperature is not intrinsic and possibly caused by magnetic parts induced by lattice imperfections within the lattice. The NMR spectra of these systems measured for aligned sheets are successfully assigned.

Cu NQR Study on the “Wipeout” Phenomenon in YBa2Cu3Oy

We have investigated if the wipeout phenomenon observed for the NQR signal of La 2- x Sr x CuO 4 in the region x ∼1/8 at low temperatures, can also be observed in YBa 2 Cu 3 O y system, and found no evidence in the 60 K plateau and higher carrier concentration regions of y . Only in the region of y <6.5, where at least part of hole carriers is localized, the reduction of Cu NQR spectra at low temperatures has been found. The inhomogeneous distribution of these localized carriers seems to cause the wipeout phenomenon. The wipeout phenomenon observed for YBa 2 (Cu 0.98 Zn 0.02 ) 3 O y in the wide region of y can also be understood by the same mechanism.

Absence of the “Wipeout” Phenomenon in the NQR Spectra of YBa2Cu3O6.61

Nuclear quadrupole resonance of the in-plane Cu of YBa 2 Cu 3 O y ( y =6.61) has been carried out to investigate if the “wipeout” phenomenon, which is observed in La 2- x - y Nd y Sr x CuO 4 and La 2- x Sr x CuO 4 and is considered to be related to the slowdown of the charge fluctuation and possibly to the “stripe” formation, can also be found for the system in the 60 K plateau region of the oxygen number y . No evidence for such a phenomenon has been found, indicating that the origin of the plateau cannot easily be connected to the existence of the fluctuating “stripe” in the system.

51V-NMR studies on the spin dynamics of S = 1/2 zig-zag chains of VO5 pyramids

NMR Knight shifts K and nuclear relaxation rates 1/ T 1 have been studied on the V 4+ sites of two compounds [N(CH 3 ) 4 ]V 3 O 7 and (en) 2 ZnV 6 O 14 which are known to exhibit spin-gap behavior from their susceptibility data. The activation-type behavior has been observed in K and 1/ T 1 , confirming the existence of the spin-gap in these systems. The frequency (ω N ) dependence of 1/ T 1 of [N(CH 3 ) 4 ]V 3 O 7 has been found to be \(1/\sqrt {\omega _{\rm N}}\)-type one and 1/ T 1 of (en) 2 ZnV 6 O 14 is linear in -ln (ω N ), indicating the diffusive and ballistic characters of the triplet excitations, respectively. The results are discussed in relation to the theory recently developed by Sachdev and Damle [Phys. Rev. B 57 (1998) 8307].

59Co-NMR Studies on La4Co3O10+δ

Samples of La 4 Co 3 O 10+δ with δ∼0.1, 0.3 and 0.6 have been prepared. For δ∼0.1, the crossover-like changes of the spin state at ∼100 K and ∼550 K have been confirmed by the measurements of the magnetic susceptibility. At ∼550 K, we have observed for the sample the anomalous T -dependence of the electrical resistivity and the thermoelectric power. Its NMR spectra exhibit the very large broadening at low temperatures due to the antiferromagnetic ordering. NMR spectra of the sample with δ∼0.6 show that there exist two kinds of Co atoms: The NMR signal intensity of one of the two with the larger relaxation rate 1/ T 2 decreases, while the intensity of the other with the smaller 1/ T 2 increases, with decreasing T below 100 K, which indicates that the spectra of the latter are from the Co 3+ in the low spin state. The T -dependence of 1/ T 1 shows the existence of the energy difference between the low spin (spin S =0) and intermediate spin ( S =1) states of the order of 100 K.

Studies on Successive Electronic State Changes in Systems with NiO2 Planes –139La-NMR/NQR–

139 La-NMR/NQR measurements of La 3 Ni 2 O 7-δ (δ∼0.0 and δ∼0.08), and La 4 Ni 3 O 10 have been performed. 139 La-NMR and transport and magnetic studies have also been carried out for Tl(La 2 Sr 2 )Ni 2 O 9 . Anomalous temperature ( T ) dependence of the longitudinal relaxation rates 1/ T 1 has been found at temperatures T A ∼(140–150 K) for all the systems. In Tl(La 2 Sr 2 )Ni 2 O 9 , there exists a transition to a magnetically ordered state at T ∼20 K. The large broadening of the NMR spectra observed for Tl(La 2 Sr 2 )Ni 2 O 9 below 20 K indicates that the system is in the charge ordered state in the temperature region between 20 K and T A with localized magnetic moments at Ni sites. The NQR intensity of La 4 Ni 3 O 10 begins to decrease rapidly with decreasing T at T A ∼140 K and almost disappears at T ∼120 K (wipeout). By arguing results of the present experimental studies, we propose that in all the systems with NiO 2 planes studied here exhibit similar type transitions to the charge ordered states a...