Moohee Lee

Suppression of antiferromagnetic spin fluctuation in Zn-substituted YBa2Cu3O7

Abstract We have prepared Zn-substituted YBa2Cu3−xZnxO7 (YBCO, x=0.0–0.09) and performed 63,65 Cu nuclear quadrupole resonance (NQR) measurements for the plane site at 300 and 100 K as a function of Zn concentration. The substitutional effects are observed in resonant frequencies and linewidths of spectra, and relaxation times as well as in the superconducting transition temperature. The spin–lattice relaxation rate 1/T1 is reduced for the higher Zn concentration and the reduction is more significant at 100 K. The ratio of 63,65 Cu spin–lattice relaxation rates suggests that a magnetic contribution due to the antiferromagnetic spin fluctuation becomes weak as the Zn concentration increases. These effects confirm that the antiferromagnetic spin fluctuation of Cu 3d spins is suppressed by the Zn substitution due to the absence of local moment at the zinc site.

Vortex dynamics in YNi2B2C single crystal by 11B NMR

11B pulsed nuclear magnetic resonance (NMR) measurements have been performed on a single crystal of YNi2B2C superconductor to investigate the vortex structure and dynamics. As temperature decreases, 11B NMR spectrum, shift, linewidth, and , exhibit distinct features, from which we are able to identify three vortex phases, namely, vortex liquid, vortex glass, and vortex lattice at 1.8T parallel to the c-axis. Also we has observed thermal hysteresis of saddle-point field, motional narrowing of linewidth and the double peak structures of , all of which indicate unexpectedly significant thermal motion of vortices in this low Tc and nealy isotropic 3D superconductor.

11B NMR study of TbNi2B2C

Abstract 11 B pulsed NMR measurements have been performed to investigate local electronic structure and 4f spin dynamics for TbNi 2 B 2 C single crystal. 11 B NMR shift and linewidth are huge and strongly temperature dependent. In addition, both are proportional to magnetic susceptibility, indicating that the hyperfine field at the boron site originates from the 4f spins of Tb. The shift and the spin-lattice relaxation rate show high anisotropy for field parallel and perpendicular to the c -axis. Anisotropy of the shift and the relaxation rates suggests that the hyperfine field perpendicular to the c -axis is larger.

63,65Cu nuclear quadrupole resonance study of Ni-doped YBa2Cu3O7

Abstract We have prepared pure YBCO (YBa 2 Cu 3 O 7− δ ) and Ni-doped YBCO (YBa 2− x La x Cu 3− x Ni x O 7− δ , x =0.1) samples and performed 63,65 Cu nuclear quadrupole resonance (NQR) measurements. La substitution introduced for charge compensation, as well as Ni doping, severely distorts the electric field gradient at the copper sites inducing extra 63,65 Cu NQR peaks other than the main peaks, for both the plane and the chain sites. 63,65 Cu NQR linewidths for the plane site increased significantly whereas those for the chain site remained similar. Furthermore, both the spin–lattice and the spin–spin relaxation times for the plane copper decreased whereas those for the chain copper increased. The Ni substitutional effects on linewidths and relaxation times confirm that Ni carrying a local moment substitutes for the plane copper.

11B NMR measurement of vortex dynamics in YNi2B2C

Abstract 11 B pulsed nuclear magnetic resonance (NMR) measurements have been performed on a single crystal of YNi 2 B 2 C superconductor to investigate vortex structures and dynamics. With lowering temperature below T c , we have observed distinct features of NMR data; namely, thermal hysteresis of 11 B NMR shift, motional narrowing of line width and a double-peak structure of T 2 −1 . These features indicate significant thermal motion of vortices in spite of low T c and nearly isotropic 3D superconductor.

Nuclear Magnetic Resonance Study of YMn4Al8

We have performed 27Al and 55Mn nuclear magnetic resonance (NMR) measurements on a single crystal of YMn4Al3 at 8 T. The 27Al NMR spectrum showed two sets of five satellites from two different Al sites. Based on the difference in NMR shift, linewidth and relaxation rates, two sets of NMR resonance peaks are assigned to the respective Al sites. The NMR shift and spin‐lattice and transverse relaxation rates, 1 T1 and 1 T2, were measured down to 4 K at 8 T. The shift and linewidth for the 27Al and 55Mn NMR followed the temperature dependence of the susceptibility, confirming that the hyperfine field from the Mn 3d spins dominates the local electronic structures at both the Al and Mn sites. The 1 T1 of the 27Al and 55Mn NMR rapidly decrease at low temperature, which is indicative of a gap opening.

11B Nuclear Magnetic Resonance Study of Spin Structure and Dynamics in Gadolinium Tetraboride

11B nuclear magnetic resonance (NMR) measurements were performed on single crystals of GdB4 to investigate local electronic structure and 4f spin dynamics. The 11B NMR spectrum, Knight shift, spin‐lattice and spin‐spin relaxation rates, 1/T1 and 1/T2, were measured down to 4 K at 8 T. The 11B NMR shift and linewidth are huge and strongly temperature‐dependent due to the 4f moments. In addition, both are proportional to the magnetic susceptibility, indicating that the hyperfine field at the boron site originates from the 4f spins of Gd. Below TN, the single broad resonance peak of 11B NMR splits into several peaks reflecting the local magnetic fields due to antiferromagnetic spin arrangements. The longitudinal and the transverse relaxation rates, 1/T1 and 1/T2, which are independent of temperature above TN, both decrease tremendously below TN confirming the suppression of spin fluctuations.

13C NMR Study of Vortex Dynamics in LuNi2B2C

13C pulsed nuclear magnetic resonance (NMR) measurements have been performed on LuNi2B2C single crystals to investigate the vortex structure and dynamical behavior. The spectrum in the superconducting state significantly broadens by local field inhomogeneity due to a vortex lattice and then evolves to double peaks at low temperature. Both peaks shift toward low magnetic field due to imperfect field penetration whereas linewidth increases because of larger field inhomogeneity. Both the linewidth, reflecting vortex fluctuation, and the transverse relaxation rate 1/T2, probing slow motion of vortices, indicates significant thermal fluctuation of vortices in this low Tc and isotropic superconductor.

11B NMR measurements of vortex dynamics for YNi2B2C superconductor

Abstract 11 B Pulsed nuclear magnetic resonance (NMR) measurements have been performed on a single crystal of YNi 2 B 2 C superconductor to investigate the vortex structure and dynamical behavior. Spectrum, line width and transverse relaxation rate of 11 B NMR are measured at 1.8 and 3.0 T. The spectrum below 4 K exhibits a typical local field distribution for a square lattice at 3.0 T parallel to the c -axis. However, the shape of the spectrum at higher temperature changes to a symmetric narrow line, which indicates significant motion of vortices. Furthermore, thermal hysteresis of the saddle-point field, motional narrowing of line width, and enhancement of transverse relaxation rate also confirm substantial distortion and fluctuation of vortices in this low T c and nearly isotropic 3D superconductor.

11B NMR study of vortex dynamics in LuNi2B2C

Abstract 11 B NMR measurements have been performed on single crystals of LuNi2B2C superconductor to investigate vortex lattice structure and dynamical behavior. The spectrum in the superconducting state is significantly broadened by local field inhomogeneity due to the vortex lattice and the peak point of the spectrum shifts toward low magnetic field due to the imperfect field penetration. The linewidth of the spectrum reflecting local field variation is much smaller than expected for conventional vortex lattices and shows a peculiar increase at low temperature. Furthermore, the transverse relaxation rate, 1/T2, probing slow motion of vortices, exhibits a peak as temperature decreases. These results highlight significant fluctuation of vortices even for this low Tc and nearly isotropic 3D superconductor.