Showing papers on "Nuclear quadrupole resonance published in 1987"

Adiabatic rotational splittings and Berry's phase in nuclear quadrupole resonance.

Abstract: Sample rotation is shown to induce frequency splittings in nuclear-quadrupole-resonance spectra. The splittings are interpreted both as a manifestation of Berry's phase, associated with an adiabatically changing Hamiltonian, and as a result of a fictitious magnetic field, associated with a rotating-frame transformation. Real and fictitious fields are contrasted. Related effects are predicted in other magnetic resonance experiments that involve sample rotation.

Evidence for two pairing energies from nuclear spin-lattice relaxation in superconducting Ba2YCu3O7- delta.

Abstract: We report /sup 63/Cu nuclear spin-lattice relaxation data for Ba/sub 2/YC/sub 3/O/sub 7-//sub delta/ in the superconducting state obtained by nuclear quadrupole resonance in zero applied magnetic field. The temperature dependences of the relaxation rates reveal striking differences in the excitation spectra for quasiparticles on the chain-forming Cu1 and planar Cu2 lattice sites, corresponding to substantially different pairing energies for electrons on the planes and chains. Values of the gap ratio 2..delta../kT/sub c/ 8.3 and 2.4 are obtained for the Cu1 and Cu2 sites, respectively.

Evidence of Successive Magnetic Transition in La2-xBaxCuO4 –NQR Studies of 139La–

Abstract: Nuclear quadrupole resonance (NQR) of 139 La has been performed to study the electronic and magnetic properties of La 2- x Ba x CuO 4 between 1.5 and 240 K. The temperature dependence of the separation of the double peaks in spectra clearly indicates the magnetic ordering in the small x region. The internal fields at La sites due to magnetic order largely decrease with Ba concentration. The divergence of 1/ T 2 indicates that another phase appears at low temperatures. The electric field gradient changes on passing from the normal to the superconducting states, indicating the apparent influence of superconductivity on the electronic state at La sites in this system.

Competition of Magnetic Order and Superconductivity in (La1-xBax)2CuO4 Systems

Abstract: Both the magnetic and superconducting properties of (La 1- x Ba x ) 2 CuO 4 systems have been investigated in terms of 139 La nuclear quadrupole resonance (NQR). For La 2 CuO 4 , it has clearly been demonstrated that the internal field H n is applied on the La site by magnetic ordering below ∼240 K. The observed H n is not consistently explained by the spin structure proposed in the neutron diffraction study. For the doped compound with x =0.01, the internal field at 1.3 K was almost the same as that for La 2 CuO 4 , while for x =0.025, it decreased and was significantly distributed. Furthermore, the superconductivity was found to set in around 2 K. From a microscopic point of view, it is concluded that the magnetic ordering and superconductivity compete around x =0.025.

Nuclear magnetic resonance and nuclear quadrupole resonance study of copper in Ba2YCu

Abstract: Nuclear magnetic resonance (NMR) and nuclear quadrupole resonance (NQR) studies of copper nuclei are reported for high-quality single-phase powder samples of Ba/sub 2/YCu/sub 3/O/sub 7-//sub delta/. The high-field /sup 63/Cu NMR spectrum shows well-defined axial symmetry for the chain-forming Cu(1) site and evidence of appreciable disorder in the Cu(2) planes. Spontaneous oxygen loss was found to degrade the sharp NMR spectrum of the Cu(1) site. Spin-lattice relaxation rates reveal strikingly different electron dynamics for the two sites. For neither site do the rates exhibit conventional Bardeen-Cooper-Schrieffer behavior near T/sub c/.

NQR and NMR of 139La in Antiferromagnetic La2CuO4-δ

Abstract: Rather complex Nuclear Quadrupole Resonance (NQR) spectrum of 139 La in La 2 CuO 4-δ , observed at low temperatures is successfully interpreted as a signal from a single La site which suffers a combined effect of a quadrupole coupling (coupling constant ν Q =6.38±0.02 MHz with asymmetry parameter η=0.01±0.01) and a small Zeeman term where the internal field ( H N =0.997±0.010 kOe) is nearly perpendicular to the direction of the maximum field gradient. The spin structure of Cu at low temperatures is suggested to be a simple antiferromagnet with sublattice moments parallel to the a -axis. The temperature dependences of ν Q and H N have been measured in detail. The fractional decrease of H N which is proportional to the Cu sublattice moments is found to be in general accord with the prediction of spin fluctuations theory for weak itinerant antiferromagnets.

Nuclear quadrupole resonance and nuclear magnetic resonance of copper in the high-Tc superconductor YBa2Cu

Abstract: The /sup 65/Cu and /sup 63/Cu zero-field nuclear quadrupole resonance (NQR), nuclear magnetic resonance (NMR) at 4.7 and 8.5 T and the spin-lattice relaxation time T/sub 1/ at 8.5 T are measured in the superconducting oxide YBa/sub 2/Cu/sub 3/O/sub 7-//sub delta/ between 80 and 300 K. No sign of magnetic order or localized moments are found. The NQR linewidth is strongly dependent on the oxygen content. We assign the /sup 63/Cu NQR line at 22 MHz to the Cu site with oxygen coordination 5. T/sub 1/ is dominated by the quadrupolar relaxation due to spin-phonon coupling.

139 La Pure Quadrupole Resonance of High T c Superconducting Materials (La 1− x M x ) 2 CuO 4 (M=Ba, Sr)

Abstract: Pure Quadrupole Resonance (PQR) of 139La in high Tc superconductor, (La1-xMx)2CuO4 with M=Ba, Sr, has been observed at zero external field between 1.3 K and 4.2 K. In undoped La2CuO4, the PQR spectrum is very complicated, reflecting the orthorhombic structure, while in the superconducting compounds, the spectrum exhibits a drastic change and becomes more simple due to the tetragonal structure. The alloying is suggested to be responsible not only for the change of crystal structure but also for the change of microscopic electronic state.

NQR studies on magnetic properties of La2−xBaxCuO4

Abstract: Nuclear quadrupole resonance (NQR) of 139La has been performed to study magnetic properties of La2-xBaxCuO4 between 1.4 and 250 K. The pronounced enhancement of nuclear relaxation rates of 1/T2 and 1/T1 are obtained for 0.01

Investigation of copper minerals by NQR: Crystallochemistry, electronic structure, lattice dynamics

Abstract: A series of copper minerals has been investigated by NQR (Nuclear Quadrupole Resonance), the most typical cases of Cu(I) environment in oxides, sulphides and sulphosalts being taken into account. The linear complexes CuX2 have the greatest quadrupole splitting. Minimal quadrupole splitting is observed in tetrahedral complexes. Spin-lattice relaxation measurements indicate that the quadrupole relaxation mechanism due to the Raman process predominates.

81Br Nuclear Quadrupole Resonance of Cs2ZnBr4 and Crystal Stability of Compounds of A2MX4 Type

Abstract: The temperature dependences of 81Br nuclear quadrupole resonance frequencies and quadrupolar relaxation times of Cs2ZnBr4 were measured. The librational motion of [ZnBr4]2− about the Zn–Br axis parallel to the a axis in the crystal is more active than those about the other axes. The reason why this compound shows no phase transition can be accounted for by the molecular packing, especially the packing of [ZnBr4]2− along the a axis. General examination of the relative size of cation/anion in the class of compounds of A2MX4 type led to a criterion which may be used to rationalize existence and absence of a phase transition.

Phase transition and anionic motion in solid [(CH3)2NH2]2SnCl6 as studied by the temperature dependence of 35Cl NQR frequencies and chlorine nuclear relaxation times determined with a pulsed NQR spectrometer

Abstract: The temperature dependence of 35Cl NQR frequencies of [(CH3)2NH2]2SnCl6 was precisely determined between 78 and ca. 250 K. by means of an FT NQR spectrometer. The high-frequency line of the two 35Cl NQR signals having the intensity ratio of 2:1 already reported was observed to be split below 102 K. into sharp and broad lines with approximately equal integrated intensify. This clearly indicates the occurrence of a structural phase transition of a second-order nature at this temperature. The presence of phase transition around this temperature has been anticipated from the IR and NQR studies previously done. The temperature dependence of chlorine nuclear spin-lattice relaxation time, T1Q shows a dip with a clear depression at 102 K. The mechanism of the phase transition is discussed from the T1Q results and the fact that the frequency separation of the doublet increases with decreasing temperature. Above 180 K, T1Q of both lines decreased rapidly and the log T1Q vs. T−1 curves were almost linear. This decrease of T1Q was explained through the onset of the C4 reorientation of the anions with the activation energy of ca. 35 kJ mol−1. The fade-out phenomenon of the two NQR lines occurring far below room temperature can be well explained by the onset of the above motion with the very low activation energy.

14N nuclear quadrupole resonance and ab initio MO study of organic azides

Abstract: An ab inito SCF‐MO calculation of the electric field gradient (EFG) using a double zeta basis set with polarization is reported for N−3, HN3 NCN3, and H3CN3. A similar calculation using a Dunning set with polarization is also reported for N−3, HN3, and H3CN3. Excellent correlation was found between the calculated and the components obtained from gas phase microwave data. The calculation showed that the terminal N atom had a positive nuclear quadrupole coupling constant and its pincipal z axis was found to be perpendicular to the NNN direction in substituted azides. The changes in the charge distribution of HN3 and NCN3 obtained from the analysis of the atomic multipoles was shown to be in general agreement with the variations found in the N EFG components calculated for these azides. The 14N nuclear quadrupole resonance frequencies of the terminal and the first N atom of the azide group have been measured at 77 K for phenyl, benzyl, triphenylmethane, and tosyl azide. An analysis based on the Townes and Da...

35 Cl nuclear quadrupole resonance and infrared spectroscopic studies of hydrogen bonding in complexes of trichloroacetic acid with various nitrogen and oxygen bases

Abstract: The 35Cl n.q.r. spectra of 29 trichloroacetic acid complexes with various bases, mainly pyridines and their N-oxides, have been studied. The dependence of the average 35Cl n.q.r. frequencies, text-decoration:overlineνQ, on ΔpKa have been analysed in terms of the proton-transfer equilibrium, and the data can be explained either on the basis of a double-minimum potential function with rapid flipping of the proton between two equilibrium sites or one broad minimum. Infrared spectra of such complexes show very broad absorptions over the whole region, with numerous so-called ‘Evans holes’. Variations of the absorption in complexes from the inversion region are similar to those observed in solution. In solids the range of the inversion is shifted to the lower ΔpKa values. Nitrogen and oxygen bases show similar spectral behaviour. In the vicinity of 50% proton transfer the spectral features are similar for A—H…B and A–…HB+ complexes.

14N and35Cl nuclear quadrupole double resonance study of the structural phase transitions in N-octylammoniumchloride

Abstract: The structural phase transitions between the non-intercalated phases of the layer structure compound N-octylammoniumchloride have been studied via proton-14N and proton-35Cl nuclear quadrupole double resonance. The results have been related to an order-disorder model of the octylammonium chains. The temperature dependence of the order parameters of the −NH3 head groups has been determined.

Theoretical studies on solid ammonia. 1. Electron density

Abstract: Ab initio MO-LCAO-SCF calculations of the electron density in solid ammonia have been carried out, using a 16-atom cluster surrounded by point charges as a model A comparison with gaseous NH/sub 3/ shows that the major changes induced by condensation to the solid state are (a) an increase of the polarity of the N-H bonds and (b) a shift of charge away from the 3a/sub 1/ lone pair orbital toward the regions between the N-H bonds, leading to a more spherical electron density distribution around the nitrogen nucleus, in agreement with experimental evidence from nuclear quadrupole resonance measurements

Evidence of antiferromagnetic ordering in La2CuO4: reinterpretation of 139La nuclear quadrupole resonance (NQR) data

Abstract: A reinterpretation of the results of recent 139La nuclear quadrupole resonance (NQR) measurements on La2CuO4 by Kitaoka et al. (Jpn. J. Appl. Phys. 26 (1987) L397) reveals an antiferromagnetic order in this material at low temperature. The magnitude and the direction of the local field at the La sites are calculated. Supposing localized moments on copper atoms as the source of the magnetic field, it is shown that the ordering of magnetic moments on copper sites is not the simplest "chessboard" antiferromagnetic type.

A Model Calculation of Electric Field Gradients in the Incommensurate Phases of Cs2HgBr4 and Cs2CdBr4

Abstract: Temperature dependence of 81 Br NQR frequencies in the incommensurate phase was measured in Cs 2 HgBr 4 and Cs 2 CdBr 4 . In relation to the analysis of the spectrum, the electric field gradients in the incommensurate phases of both substances were calculated on the basis of a model that rotation of [MBr 4 ] 2- tetrahedron about the a axis occurs and the rotation angle at each site is regularly modulated by incommensurate modulation wave. This model accounted for the temperature dependence of the two NQR lines, i.e., the fact that only two lines are detected in the incommensurate phase.

NQR and NMR of high-Tc superconducting oxides (La1−xBax)2CuO4−y and YBa2Cu3O7−y

Abstract: In (La 1− x Ba x ) 2 CuO 4− y , the antiferromagnetic and superconducting phase diagram have been investigated from both 139 La nuclear quadrupole resonance (NQR) and resistivity measurements. It is concluded that the magnetic phase boundary is around x = 0.025–0.03 where the superconductivity starts to appear around 2 K. In YBa 2 Cu 3 O 7− y , both NQR and NMR of 63 Cu and 65 Cu have been made at zero and high fields up to 135 kOe. Two pairs of Cu NQR lines have been observed at 31.5, 29.5 MHz, and 22.1, 20.4 MHz, corresponding to two inequivalent Cu sites. From the high-field NMR measurement, two Cu sites with the lower and higher NQR frequencies were reasonably assigned to the Cu sites situated in Cu-O chains and planes, respectively.

Zero-field pulsed response and dipolar couplings in systems of spin I= 1 nuclei

Abstract: The interaction representation is used to describe the pulsed response of an ensemble of spin I= 1 nuclei experiencing quadrupolar and dipolar couplings in zero applied magnetic field Particular attention is given to the case of coupling between physically inequivalent spins, the case of equivalent spins being included as a special case Axial symmetry of the electric field gradient tensor is assumed for all spins

A Temperature Dependence Study of 35Cl Nuclear Quadrupole Resonance Frequencies in Some Hexachloroplatinate(IV) Hexahydrates

Abstract: The temperature dependence of 35Cl NQR frequencies was investigated in various hexachloroplatinate( IV) hexahydrates containing the following divalent ions as counter cations: Mn(II), Fe(II), Co(II), Ni(II), Cu(II), Zn(II), Cd(II), and Ba(II). All of the compounds except the Cu(II) and Ba(II) salts yield a single 35Cl NQR line at all temperatures studied. For the Mn(II), Cu(II), and Ba(II) salts, the NQR frequencies are considerably shifted by deuteration. For the deuterated Mn(II) salt, two 35Cl NQR lines were observed whose relative peak-height intensities vary in dependence on the history of the thermal treatment of the sample. The very broad higher frequency line could be observed for a sample which had been at 77 K or below. It could not be observed for a sample which was cooled slowly to room temperature after it had been heated up to ca. 420 K. A structural phase transition was located at 135.6 and 129.4 K for the Cu(II) salt and its deuterated analog, respectively. The frequency shift by deuteration is discussed in relation to the strength of O - H ... CI type hydrogen bonds.