M. Mali

Copper NQR and NMR in the superconductor YBa2Cu4O8+x

Abstract The frequencies νQ, the linewidth and the spin-lattice and spin-spin relaxation times T1 and T2, respectively, of the 63Cu nuclear quadrupole resonance (NQR) signals for chain and planar copper sites in YBa2Cu4O8 were measured between 6 and 750 K; in addition nuclear magnetic resonance (NMR) spectra were taken at 150 and 300 K. The results are compared with data known for the related structure YBa2Cu3O7. Above Tc the tensors of the electric field gradient and the anisotropic Knight shift at the copper sites differ in a subtle way from those of YBa2Cu3O7. At the Cu2 site, the derivative of νQ with respect to temperature changes its sign around 200 K. No indication for magnetic ordering at the Cu sites was found. Below Tc there is no evidence for BCS type relaxation and the Cu2 Tc rates vary like exp( T T 0 ) and the Cu1 rates like Tn with n≈1.35. Above Tc the Cu1 rates suggest the presence of quasiparticles (and phonon interactions) while the Cu2 rates can perhaps be explained by a model of Monien et al. postulating the coupling of Cu nuclei to itinerant interacting quasiparticles.

NMR, DSC, and conductivity study of a poly(ethylene oxide) complex electrolyte : PEO(LiClO4)x

Abstract The phase diagram of the P(EO) (LiClO 4 ) x system has been determined by DSC and NMR and confirm the existence of two distinct stoichiometric crystalline complexes, the stoichiometry of which are respectively x −1 = 3 and 6. The diffusion constant of Li 7 D has been determined by the Pulsed Magnetic Field Gradient NMR technique on the same membranes studied for conductivity for the three concentrations x = 0.05, 0.167 and 0.125. We deduced that t + the cationic transference number, is temperature independent and equal to 0.18 for x = 0.125 and to 0.23 for x = 0.167

Direct Observation of the Oxygen Isotope Effect on the In-Plane Magnetic Field Penetration Depth in Optimally Doped YBa~2Cu~3O~7~-~d~e~l~t~a

We report the first direct observation of the oxygen-isotope ((16)O/(18)O) effect on the in-plane penetration depth lambda(ab) in a nearly optimally doped YBa(2)Cu(3)O(7-delta) film using the novel low-energy muon-spin rotation technique. Spin-polarized low-energy muons are implanted in the film at a known depth z beneath the surface and process in the local magnetic field B(z). This feature allows us to measure directly the profile B(z) of the magnetic field inside the superconducting film in the Meissner state and to make a straightforward determination of lambda(ab). A substantial isotope shift Delta lambda(ab)/lambda(ab)=2.8(1.0)% at 4 K is observed, implying that the in-plane effective supercarrier mass m*(ab) is oxygen-isotope dependent with Delta m*(ab)/m*(ab)=5.5(2.0)%. These results are in good agreement with magnetization measurements on powder samples.

Mixed magnetic and quadrupolar relaxation in the presence of a dominant static Zeeman Hamiltonian

We discuss the multi-exponential nuclear magnetization recovery which occurs in spin-lattice relaxation when NMR lines are split by quadrupole interaction. We treat the general situation in which both magnetic and quadrupolar fluctuations are present and consider three cases differing in their initial conditions: (1) a short radio-frequency pulse is applied selectively to one of the transitions; (2) all lines are saturated at once; (3) a selected line is saturated by continuous waves or by means of a long comb of pulses. Exact solutions are presented for spin I = 1 and I = 3/2, whereas for spin I = 5/2, exact solutions are given for special cases and approximate solutions for the general case. The spin I = 7/2 case is treated for magnetic fluctuations only. The detailed analysis reveals that the form of the recovery law is surprisingly insensitive to an additional relaxation channel, e.g. quadrupolar fluctuations in the presence of predominantly magnetic fluctuations or vice versa.

109Ag NMR investigations of the superionic glasses (AgI)x·(Ag2O·2B2O3)1−x

Abstract The first measurements of the 109 Ag NMR relaxation times and linewidths and of the self-diffusion coefficient in ( AgI ) x ·( Ag 2 O ·2 B 2 O 3 ) 1− x are reported. The initial findings indicate that at lower temperatures (which decreases with increasing x) two distinct Ag + populations, mobile and static, can be detected. At high temperatures all cations appear to be mobile. For the x =0.65 sample, the diffusion measurements give an activation energy of 0.36±0.11 eV . The Ag local motions as probed by relaxation and linewidth measurements are associated with activation energies of about 0.10 eV.

The Knight shift and diffusion in solid lithium and sodium at pressures up to 8 GPa studied by NMR

The authors have measured the pressure dependence of the Knight shift and the linewidth of the nuclear magnetic resonance (NMR) signals of 7Li (up to 8 GPa) and 23Na (up to 6 GPa) in metallic lithium and sodium, respectively, at room temperature using a diamond anvil cell (DAC). It is concluded that diffusion is hindered by pressure; the diffusion coefficients and the activation volumes are estimated. In Li, diffusion increases at the BCC-FCC phase transition. The Li Knight shift increases with rising pressure up to the phase transition, where a marked reduction is observed which might be due to a drastic decrease of the s character of the conduction electron wave function. The Na shift decreases up to about 1.7 GPa and then increases. The Knight shift data are compared with theories that exist for pressures up to 1 GPa. By extrapolating existing values for the Pauli susceptibility, the pressure dependence of the spin density at the nuclear site is estimated.

A diamond anvil cell for high‐pressure NMR investigations

A diamond anvil cell suitable for high‐pressure NMR experiments in a cryostat is described. Inside a rhenium gasket pressures up to 8.3 GPa could be generated. With this apparatus the pressure dependence of the Knight shift and of the self‐diffusion coefficient in metallic lithium and sodium has been measured (as published elsewhere).

Spin susceptibility and spin dynamics at Cu sites in Y Ba2Cu4O8

Abstract New measurements of the Knight shift tensor K at both Cu sites in the normal conducting state of the 82 K superconductor 1-2-4 are reported. The data are in accordance with a picture where at both Cu sites the Cu valence is close to 2+ and the hole on the plane site is quasi-localized. A significant temperature dependence at plane Cu site is found which is associated with temperature dependent antiferromagnetic fluctuations. Knight shift and spin-lattice relaxation times at Cu plane site are interpreted to be dominated by antiferromagnetic correlations with a temperature dependent correlation length.

High‐pressure NMR probe head for low temperatures

A nuclear‐magnetic‐resonance probe head for investigations under hydrostatic pressures up to 7 kbar is described. Helium gas is used as the pressure transmitting medium allowing low‐temperature measurements. Compact design allows for use in small bore superconducting magnets. As a first result the discovery of a new high‐pressure phase in the superionic conductor RbAg5I5 is presented.

17O NMR study in aligned YBa2Cu4O8 powder

We present 17 O nuclear magnetic resonance (NMR) experiments in YBa 2 Cu 4 O 8 . Using an uniaxially oriented powder we were able to distinguish the four inequivalent oxygen sites in YBa 2 Cu 4 O 8 . The electric field gradient and the magnetic shift tensors have been determined at 100 K for all oxygen sites. Good agreement has been found between the experimental electric field gradient tensor and the one obtained from linearized augmented plane wave calculations. We further report on the temperature dependence of the in-plane and out-of-plane magnetic shift of the plane oxygens in the temperature range of 85–350 K. The results are compared with related data from YBa 2 Cu 3 O 7 and YBa 2 Cu 3 O 6.63 .