Arlette Trokiner

Selective and non-selective NMR excitation of quadrupolar nuclei in the solid state

Abstract Spectral intensities of quadrupolar nuclei and the radiofrequency (rf) pulse lengths for selective and non-selective excitation of any single-quantum NMR transition in the solid state are calculated. When nuclei of the same element occupy sites which experience different electric field gradients, the spectra are quantitatively reliable only when the length of the rf pulse is much smaller than π/2ω rf √ I ( I +1)− m ( m +1). By using hydrated samples and short rf pulses, quantitative determination of aluminium by 27 Al NMR in thermally treated (ultrastable) zeolite Y has been achieved. Quadrupole nutation NMR reveals novel information about the various aluminium species present.

Influence of the doping level on the charge distribution among the inequivalent CuO2 layers in Tl2Ba2Ca2Cu3O10−δ : a NMR study

Abstract 63 Cu and 17 O NMR measurements in the normal and superconducting states of Tl2Ba2Ca2Cu3O10−δ with different δ are reported. In the overdoped Tl2223 sample with Tc=117 K (Tcopt=123 K) and δ1 17 K are revealed for inequivalent CuO2 layers. For the inner CuO2 layer with the square oxygen coordination of Cu the decrease of 17 K with temperature is more gradual. In going towards the underdoped Tl2223 with Tc=104 K and δ2>δopt the changes of 63,17 K with temperature are found to be the same for both types of copper layers. The quadrupole coupling constants for copper and oxygen from different CuO2 layers were obtained. From the variations with doping of the valence contribution to the electric field gradient at copper sites, we estimate both the hole numbers at Cu and oxygen sites and the real concentration of mobile hole carriers nh in each of inequivalent CuO2 layers. In the overdoped Tl2223 sample the charge density in the inner layer differs from the one in the outer plane (with five-fold oxygen coordination for Cu). Our results show that the inhomogeneity of the charge distribution disappears in the underdoped regime. The results are compared with calculations of the charge distribution among the CuO2 planes in multilayered cuprates reported by Haines and Tallon [E.M. Haines, J.L. Tallon, Phys. Rev. B 45 (1992) 3127].

Structural properties of CaBaLaCu3Oy studied by NMR/NQR method

Abstract The first NMR/NQR study of the high- T c superconductor CaBaLaCu 3 O y has been carried out on a sample enriched with 43 Ca and 135 Ba which allowed one to observe signals from all cations. Direct evidence was obtained that the superconducting compound contains less Ca than the starting composition and a large part of Ca and Cu forms a spurious dielectric phase. The composition of the superconducting phase is close to (Ca 0.5 La 0.5 )(Ba 1.25 La 0.75 )Cu 3 O y as was proposed for the first time by Keller-Belest et al. A highly asymmetric electric-field gradient observed at the Ba and La sites in the Ba/La plane indicates that the local environment of these ions is not tetragonal in spite of the overall tetragonal crystal symmetry. A symmetric field gradient, however, is found for the cations in the central Ca/La plane. No evidence for cation ordering or a special superstructure of the oxygen could be detected.

Isotopic disorder in Ge single crystals probed with73GeNMR

NMR spectra of 7 3 Ge (nuclear spin I=9/2) in germanium single crystals with different isotopic compositions have been measured at the frequency of 17.4 MHz at room temperature. Due to the small concentration (∼0.1%) of the magnetic ( 7 3 Ge) isotope, the magnetic dipole-dipole interaction is negligible in the samples studied, and the observed specific features of the resonance line shapes (a narrow central peak and a wide plateau) are determined mainly by the quadrupole interaction of magnetic nuclei with the random electric-field gradient (EFG) induced by the isotopic disorder. The second and fourth moments of the distribution function of the EFG are calculated taking into account local lattice deformations due to mass defects in the close neighborhood of the magnetic nuclei, as well as charge-density redistributions and lattice strains induced by distant impurity isotopes. The simulated line shapes, represented by a superposition of Gaussians corresponding to individual transitions between nuclear Zeeman sublevels, agree reasonably well with the measured spectra.

73Ge NMR spectra in germanium single crystals with different isotopic composition

We have studied the influence of isotopic disorder on the local deformations in Ge single crystals from both experimental and calculation points of view. The nuclear magnetic resonance (NMR) spectra of73Ge nuclei (the nuclear spin equals 9/2) in perfect single crystals of germanium with different isotopic content were measured at temperatures 80, 300 and 450 K. Abnormal broadening of the spectrum was found to occur when the magnetic field was aligned along the [111] axis of a crystal. The observed specific angular dependence of the quadrupole broadening was attributed to isotopic disorder among atoms of germanium sited around the73Ge NMR probe. Local lattice deformations in germanium crystal lattice due to isotopic impurity atoms were calculated in the framework of the adiabatic bond charge model. The results obtained were applied to study random noncubic crystal field interactions with the nuclear quadrupole moments and corresponding effects in NMR spectra. Simulated second and fourth moments of resonance frequency distributions caused by the magnetic dipole-dipole and electric quadrupole interactions are used to analyze the lineshapes, theoretical predictions agree qualitatively with the experimental data.

Charge and orbital ordering in Pr0.5Ca0.5MnO3 studied by O-17 NMR

The charge and orbital ordering in Pr_{0.5} Ca_{0.5} MnO_3 is studied for the first time by ^{17}O NMR. This local probe is sensitive to spin, charge and orbital correlations. Two transitions exist in this system: the charge and orbital ordering at T_{CO} = 225 K and the antiferromagnetic (AF) transition at T_N = 170 K. Both are clearly seen in the NMR spectra measured in a magnetic field of 7T. Above T_{CO} there exists only one NMR line with a large isotropic shift, whose temperature dependence is in accordance with the presence of ferromagnetic (FM) correlations. This line splits into two parts below T_{CO}, which are attributed to different types of oxygen in the charge/orbital ordered state. The interplay of FM and AF spin correlations of Mn ions in the charge ordered state of Pr_{0.5} Ca_{0.5} MnO_3 is considered in terms of the hole hopping motion that is slowed down with decreasing temperature. The developing fine structure of the spectra evidences, that there still exist charge-disordered regions at T_{CO}>T>T_N and that the static (t>10^{-6}s) orbital order is established only on approaching T_N. The CE-type magnetic correlations develop gradually below T_{CO}, so that at first the AF correlations between checkerboard ab-layers appear, and only at lower temperature - CE correlations within the ab-planes.

Nuclear magnetic resonance of 11B in sodium borohydrides, NaBH4 and NaBD4, in the structural transition region

11B spin‐lattice relaxation time, T1, has been measured in NaBH4 and NaBD4, in the near vicinity of the structural phase transition. The experimental results have confirmed the first order nature of the phase transtion, showing up a thermal hysteresis, phases coexistence and a discontinuity of the spin‐lattice relaxation time. We have explained the discontinuity, experimentally found for T1 at the phase transition, by a theory which only takes into account the order–disorder nature of the phase transition.

The 63Cu and 17O NMR studies of spin susceptibility in differently doped Tl2Ba2CaCu2O8−δ compounds

Abstract The NMR line shifts, spin–lattice relaxation rate T 1 −1 of 63 Cu and 17 O and 63 Cu spin–spin relaxation rate 63 T 2 −1 were measured in normal and superconducting states of the magnetically oriented ceramics of Tl 2 Ba 2 CaCu 2 O 8− δ with different hole concentrations ( T c =102 K, T c =112 K, T c =104 K) involving the optimally doped composition ( T c =112 K). The magnetic hyperfine fields on the Cu and O atoms in the CuO 2 layer were estimated using the temperature dependences of the Knight shift. The temperature dependences of magnetic correlation length ξ and characteristic energy Γ Q of antiferromagnetic (AF) spin fluctuations were analyzed using the 63,17 T 1 −1 and 63 T 2 −1 data obtained. In going to overdoped state of cuprate the crossover in temperature between different regimes of spin fluctuations is discussed taking into account the magnetic phase diagram proposed for superconducting cuprates by Barzykin and Pines [V. Barzykin, D. Pines, Phys. Rev. B 52 (1995) 13585].

17O NMR investigation of the properties of the various types of CuO2 planes in (Bi,Pb)2Sr2Can−1CunOy

Abstract In the normal state of the (2223) phase of bismuth cuprates the 17 O Knight shift of the CuO 2 planes with pyramidal coordination for copper is nearly temperature independent except in the vicinity of Tc where it presents a steep decrease. The 17 O Knight shift of the CuO 2 planes with square plane coordination for copper is significantly lower and presents a continuous decrease with decreasing temperature which is already detected just below room-temperature. We discuss. these results on the basis of a recent theoretical prediction for the existence of an inhomogeneous charge carriers distribution among the two types of CuO 2 planes, and also on the basis of similar available data, for other high Tc superconductors.

Study of the structural phase transition of the fluoroperovskite RbCaF3 by nmr techniques.

Abstract The cubic to tetragonal phase transition occurring at 198K in the fluoro-perovskite RbCaF3 has been studied by the 87Rb nuclei magnetic resonance. In the tetragonal phase, the CaF6 octahedra tilt angle Φ, which is the order-parameter, has been deduced from the quadrupolar coupling constant. Its temperature dependence is described by a power-law with a cross-over from a Landau-type exponent to an exponent β = 0,32 at 150 K. In the cubic phase, the intensity of the Larmor frequency signal deviates from a Curie law twenty degrees above Tc; this can be attributed to the existence of tetragonal domains.