Katsuaki Kodama

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.

Direct observation of PrCuO2 hybridization in PrBa2Cu3O7−δ by the MEM charge density study

Abstract In order to prove the existence of hybridization between the Pr and CuO2 conduction planes, a charge density study of PrBa2Cu3O6.87(PrBCO) and YBa2Cu3O6.97(YBCO) has been carried out by the maximum entropy method (MEM) using synchrotron radiation (SR) powder data. In the charge density of PrBCO obtained by MEM, there can be found directional robes of the charge density from the Pr atom toward the O atoms in the CuO2-planes. For YBCO there have not been observed appreciable charge densities in the interatomic region around the Y atom. The distinct difference of the charge densities between PrBCO and YBCO presents clear experimental evidence of the hybridization between Pr 4f-O 2pπ orbitals which supports the idea that the hole trapping by the hybridized states suppresses the superconductivity in PrBCO.

Superconductivity-induced B2u phonon anomalies of YBa2Cu3O6 + x and symmetry of the order parameter. Neutron inelastic scattering studies

Abstract Superconductivity-induced phonon anomalies have been observed by neutron scattering for a c -axis polarized B 2u mode with out-of-phase motions of O(2) and O(3) within a plane in YBa 2 Cu 3 O 6 + x ( x = 1.0 and 0.7). A mode which has in-phase motions of these oxygens with respect to Cu atoms within a CuO 2 -plane does not show any appreciable anomaly. Based on these results and recent theoretical arguments, the symmetry of the superconducting order parameter is pointed out to be ( x 2 − y 2 )-like. In YBa 2 Cu 3 O 6.7 the B 2u mode also exhibits phonon anomalies associated with the so-called spin-gap phenomena.

The nuclear density of YBCO by the maximum entropy method using neutron powder data

Abstract In order to examine the nature of thermal motions of the YBCO superconductor, particularly in the CuO 2 and CuO planes, the nuclear density distribution was observed by the maximum entropy method (MEM) using neutron powder diffraction data measured at 15 K. The characteristic squarish deformation of the nuclear density due to anharmonic thermal motion was found even at such a low temperature for the Cu atom in the CuO 2 plane but not in the CuO plane. The obtained MEM nuclear density is consistent with the existence of strong CuO bonds in the CuO 2 plane and not in the CuO plane.

Orbital Fluctuations in Ground State of YTiO3: 47,49Ti NMR Study

47,49 Ti NMR spectra of a single crystal of YTiO 3 have been obtained at 1.5 K in the ferromagnetically ordered state under an external field of 3 T. With rotation of the sample in the magnetic fie...

Pseudo gap formation and related phenomena in Cu-oxides and other systems

Abstract Detailed process of the singlet formation in high- T c oxides and quantum spin systems with a spin gapped ground state have been studied by various kinds of measurements including neutron inelastic scattering. The results can provide a physical picture how the anomalous metallic phase is formed and how the superconductivity appears in high- T c oxides as the temperature is lowered.

NMR study of orbital ordering in RTiO3 (R=Y, Gd, and La)

Abstract Recently, the orbital degree of freedom has attracted renewed interest in strongly correlated electron systems. The RTiO3 (R: rare earths) system is one of the attracting systems in the study of the orbital degree of freedom. While an orbital ordering state is observed in a ferromagnet YTiO3, no orbital ordering has yet been observed in an antiferromagnet LaTiO3. In this study, we have measured 47,49 Ti NMR spectra of RTiO3 (R=Y, Gd, and La). We investigate the orbital state by analyzing the NMR spectra of RTiO3.

Tc-suppression mechanism of Pr-doping in (R, Pr)Ba2Cu3O7

Abstract Neutron inelastic measurements on Y0.52Pr0.48Ba2Cu3O7 have been carried out. From the magnetic excitation spectra χ″(q, ω), it is found that the reduction of the hole-carrier concentration p and the randomness effects introduced by the substitution of Y with Pr suppress the superconductivity. Based on the hole-trapping model, the origin of the randomness effects can be explained. The R-ion-size dependence of the suppression rate of the superconducting transition temperature, | d T c d x | , can also be explained qualitatively. By measuring the Hall coefficients RH of R1−xPrxBa2Cu3O7 (R = Yb, Y, Gd and Sm), | d p d x | is found to be insensitive to the species of R. For RBa2Cu3O7 (without Pr atoms), p decreases with increasing R-ion-size, which presents another possible explanation of the R-ion-size dependence of | d T c d x | .

MAGNETIC EXCITATIONS OF CUNB2O6

Abstract Dynamical magnetic properties of the quasi-one-dimensional spin-gap system CuNb 2 O 6 have been studied by means of neutron inelastic scattering. The dispersion curve of the singlet–triplet excitation indicates that the system has ferromagnetic–antiferromagnetic (F–AF) alternating bond chains, which is the origin of the spin gap. The magnetic excitation and the macroscopic properties are analyzed by the F–AF alternating bond model. The impurity effect of this system is also studied.

Structural Transitions and Magnetic Structure in NH4CuCl3 via 14N-NMR

We report results of 14 N-NMR experiments on NH 4 CuCl 3 at the magnetic field of 7 T, where the 1/4-magnetization plateau is observed at low temperatures. The quadrupole coupling parameter v z splits below 70 K, indicating a structural phase transition. At 4.2 K, eight N sites with distinct values of both v z and the magnetic hyperfine shift K z are resolved in the NMR spectrum for general field directions. We then conclude that the magnetic structure in the 1/4-plateau does not break the symmetry of the crystal. Based on the NMR and the recent neutron scattering results by Riiegg et al. [Phys. Rev. Lett. 93 (2004), 037207], we propose that. triplet dimers in the 1/4-plateau are formed not between the nearest neighbor pairs but over different chains.