Search

We have performed copper nuclear quadrupole resonance experiments in high-temperature superconductors ${\mathrm{YBa}}_{2}{\mathrm{Cu}}_{4}{\mathrm{O}}_{8},$ ${\mathrm{YBa}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7},$ and ${\mathrm{La}}_{2\ensuremath{-}x}{\mathrm{Sr}}_{x}{\mathrm{CuO}}_{4}$ $(x=0.12$ and $x=0.15),$ using the stimulated echo technique which utilizes the rf-pulse sequence $\ensuremath{\pi}/2\ensuremath{-}(\ensuremath{\tau})\ensuremath{-}\ensuremath{\pi}/2\ensuremath{-}(T\ensuremath{-}\ensuremath{\tau})\ensuremath{-}\ensuremath{\pi}/2.$ The $\ensuremath{\tau}$ and T dependences of the stimulated echo intensity is analyzed by a model that includes the spin-lattice relaxation process ${(T}_{1}$ process) and the fluctuating local field due to nuclear spin-spin coupling. The model gives quantitative account of the experimental results in ${\mathrm{YBa}}_{2}{\mathrm{Cu}}_{4}{\mathrm{O}}_{8}$ and ${\mathrm{YBa}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7},$ using the known values of ${1/T}_{1}$ and ${1/T}_{2G},$ the Gaussian decay rate of the spin-echo intensity. The same model applied to ${\mathrm{La}}_{2\ensuremath{-}x}{\mathrm{Sr}}_{x}{\mathrm{CuO}}_{4}$ enables us to extract the value of ${T}_{2G}.$ Our results indicate that ${T}_{1}{T/T}_{2G}^{z}$ shows significant temperature dependence for $z=2$ but is approximately independent of temperature for $z=1.$ This implies that the dynamic exponent is close to one in ${\mathrm{La}}_{2\ensuremath{-}x}{\mathrm{Sr}}_{x}{\mathrm{CuO}}_{4}.$

X-ray absorption (XAS) and emission (XES) spectroscopy near B K and C K edges have been performed on metallic ($\ensuremath{\sim}0.1\phantom{\rule{0.3em}{0ex}}\mathrm{at.}\phantom{\rule{0.2em}{0ex}}%$ B, B-diamond) and semiconducting ($\ensuremath{\sim}0.03\phantom{\rule{0.3em}{0ex}}\mathrm{at.}\phantom{\rule{0.2em}{0ex}}%$ B and N, BN-diamond) doped diamond films. Both B K XAS and XES spectra show a metallic partial density of states (PDOS) with the Fermi energy of $185.3\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$, and there is no apparent boron-concentration dependence in contrast to the different electric property. In C K XAS spectrum of B-diamond, the impurity state ascribed to boron is clearly observed near the Fermi level. The Fermi energy is found to be almost same with the top of the valence band of nondoped diamond: ${E}_{V}=283.9\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$. C K XAS of BN-diamond shows both the B-induced shallow level and N-induced deep and broad levels as the in-gap states, in which the shallow level is in good agreement with the activation energy $({E}_{a}=0.37\phantom{\rule{0.3em}{0ex}}\mathrm{eV})$ estimated from the temperature dependence of the conductivity; namely, the change in $\mathrm{C}\phantom{\rule{0.2em}{0ex}}2p$ PDOS of impurity-induced metallization is directly observed. The electric property of this diamond is ascribed mainly to the electronic structure of $\mathrm{C}\phantom{\rule{0.2em}{0ex}}2p$ near the Fermi level. The observed XES spectra are compared with the discrete variational $\mathrm{X}\ensuremath{\alpha}$ ($\mathrm{DVX}\ensuremath{\alpha}$) cluster calculation. The $\mathrm{DVX}\ensuremath{\alpha}$ result supports the strong hybridization between $\mathrm{B}\phantom{\rule{0.2em}{0ex}}2p$ and $\mathrm{C}\phantom{\rule{0.2em}{0ex}}2p$ observed in XAS and XES spectra, and suggests that the small amount of boron $(\ensuremath{\leqslant}0.1\phantom{\rule{0.3em}{0ex}}\mathrm{at.}\phantom{\rule{0.2em}{0ex}}%)$ in diamond occupies the substitutional site rather than interstitial site.

The effect of electron correlation (EC) on the electronic structure in MgB 2 , AlB 2 , and ZrB 2 , is studied by examining the partial density of states (PDOS) of B 2pσ and pπ orbitals using the polarization dependence of x-ray emission and absorption spectra. The discrepancies between observed and calculated PDOS's cannot be attributed to EC effects. The present results suggest that the EC effect is less than the experimental error (∼0.2 eV), which indirectly supports a scenario that electron-phonon interaction plays an essential role in the occurrence of superconductivity.

Results of the nuclear resonance experiments for the planar Cu sites in PrBa 2 Cu 4 O 8 are presented. The nuclear magnetic resonance spectrum at 1.5 K in zero magnetic field revealed an internal field of 6.1 T, providing evidence for an antiferromagnetic order of the planar Cu spins. This confirms that the CuO 2 planes are insulating; therefore, the metallic conduction in this material is entirely due to the one-dimensional zigzag Cu 2 O 2 chains. The results of the spin-lattice relaxation rates measured by zero-field nuclear quadrupole resonance above 245 K in the paramagnetic state are explained by the theory for a Heisenberg model on a square lattice.

Soft X-ray absorption and emission measurements are performed for the K- edge of B in MB$_2$ (M=Mg, Al, Ta and Nb). Unique feature of MgB$_2$ with a high density of B 2$p_{xy}(\sigma)$-state below and above the Fermi edge, which extends to 1 eV above the edge, is confirmed. In contrast, the B 2$p$ density of states in AlB$_2$ and TaB$_2$, both of occupied and unoccupied states, decreased linearly towards the Fermi energy and showed a dip at the Fermi energy. Furthermore, there is a broadening of the peaks with $p\sigma$-character in XES and XAS of AlB$_2$, which is due to the increase of three dimensionality in the $p\sigma$-band in AlB$_2$. The DOS of NbB$_2$ has a dip just below the Fermi energy. The present results indicate that the large DOS of B-2$p\sigma$ states near the Fermi energy are crucial for the superconductivity of MgB$_2$.