Showing papers on "Nuclear quadrupole resonance published in 1993"

Cu NMR and NQR Studies of Impurities-Doped YBa2(Cu1-xMx)3O7 (M=Zn and Ni)

Abstract: The normal and superconducting properties in impurity-doped YBa 2 (Cu 1- x M x ) 3 O 7 (M=Zn and Ni) have been investigated by 63 Cu NMR and NQR techniques. The nonmagnetic Zn-doping into the CuO 2 plane in YBa 2 Cu 3 O 7 (YBCO 7 ) causes a local collapse of antiferromagnetic (AF) spin correlation near Zn impurities and induces a gapless superconductivity with a finite density of states at the Fermi surface. On the other hand, in the case of Ni doping, there is no appreciable modification except for a small decrease of T c , although the Ni spins behave actually as local moments. The gapless feature in Zn-doped YBCO 7 is shown to be consistently interpreted within the framework of the d -wave model with gap zeros of lines at the Fermi surface as argued in heavy fermion superconductors. It is pointed out that the AF spin fluctuation-induced superconductivity is applicable to high- T c cuprates.

NMR and NQR studies of magnetism and superconductivity in the antiferromagnetic heavy fermion superconductors UM2Al3 (M=Ni and Pd)

Abstract: Both magnetic and superconducting characteristics of the new heavy fermion superconductors (HFS), UNi 2 Al 3 and UPd 2 Al 3 have been investigated extensively by means of 27 Al NMR and NQR. Systematic studies of the nuclear-spin-lattice relaxation time T 1 , the Knight shift and the spectrum of 27 Al have revealed different types of magnetic fluctuations and spin structure between these two compounds. UNi 2 Al 3 shows a band-type itinerant antiferromagnetic behavior, while UPd 2 Al 3 belongs to a heavy fermion antiferromagnet with an ordinary size of U-derived moment. In the superconducting state, the relaxation behavior in UPd 2 Al 3 is similar to those observed in the other HFS reported so far, providing an evidence for an unconventional anisotropic superconductivity with line of gap zeros on the Fermi surface as well. Knight shift has been found to decrease considerably below T c , showing that the pseudo spin is well defined, not affected by the impurity scattering.

Rotational spectrum, structure, and chlorine nuclear quadrupole coupling constants of the van der Waals complex Ar–Cl2

Abstract: The pure rotational spectrum of the van der Waals complex Ar–Cl2 has been observed between 5 and 14 GHz using a Balle–Flygare type pulsed molecular beam microwave Fourier transform spectrometer. Ten a‐type rotational transitions of Ar–35Cl2, as well as five a‐type rotational transitions of the mixed isotopomer Ar–35Cl37Cl, have been assigned. The rotational constants and quartic centrifugal distortion constants have been determined. Unlike its isovalent linear isomer Ar–ClF, Ar–Cl2 has been found to be a T‐shaped complex. This confirms the result from electronic spectroscopy and is in accord with the atom–atom additive model. The distance from the Ar nucleus to the center of mass of the Cl2 subunit has been calculated to be 3.7190 A for Ar–35Cl2 and 3.7184 A for Ar–35Cl37Cl, respectively. The van der Waals stretch and bend force constants, as well as their corresponding harmonic vibrational frequencies, have been derived and compared with the theoretically predicted values. The nuclear hyperfine splitting...

NMR study of local structure in metallic La2CuO4+ delta.

Abstract: We employ [sup 139]La NMR spectroscopy as a structural probe of metallic La[sub 2]CuO[sub 4+[delta]]. We find a distribution of lateral displacements of the oxygen atoms which form the apices of the oxygen octahedra. We directly observe a wide distribution of structures of the La-O layer; this distribution is strongly temperature dependent. We present copper NQR spectra which reveal a second, unexplained, copper site in the CuO[sub 2] planes. We show that these features are not manifestations of disturbances of the lattice by interstitial dopants, but are the intrinsic response of La[sub 2]CuO[sub 4+[delta]] to doped holes.

63 Cu NQR/NMR Study of Zn-Substituted YBa 2 Cu 4 O 8 —Effect of Impurity on Spin-Gap-Like Behavior—

Abstract: The nuclear-spin-lattice relaxation rate, 1/ T 1 , and the Knight shift, K a b , of 63 Cu in the plane have been measured for YBa 2 ( Cu 1- x Zn x ) 4 O 8 ( x =0.01 and 0.02) in order to unravel th...

Magnetic-field dependence of planar copper and oxygen spin-lattice relaxation rates in the superconducting state of YBa2Cu3O7.

Abstract: The authors report nuclear spin-lattice relaxation rates, [ital W][sub 1], for planar copper and oxygen sites in the superconducting state of YBa[sub 2]Cu[sub 3]O[sub 7]. These measurements were made with the static magnetic field along the [ital c] axis at a variety of static magnetic-field strengths. The data show that the relaxation rate of [sup 17]O depends on magnetic-field strength at low temperature and confirm a similar result for [sup 63]Cu. The importance of these measurements is that they give the zero-field limit necessary for comparison with theory. The weak-field data at low temperature for both copper and oxygen have a temperature dependence close to [ital T][sup 3], which is the result expected for spin-singlet, orbital [ital d]-wave pairing and does not seem compatible with orbital [ital s]-wave pairing.

Deuterium quadrupole coupling constants and ionic bond character in transition metal hydride complexes from 2H NMR T1 relaxation data in solution

Abstract: 2 H T 1 NMR relaxation times have been measured in solution for the D ligands of some transition metal hydride complexes and used for the calculation of deuterium quadrupole coupling constants (DQCC) and of the ionic contribution to the M-D bond

Observation of 55 Mn NMR and NQR Signals from Site II in βMn Metal

Abstract: NMR and NQR signals of 55 Mn in site II were observed. The electric quadrupole coupling constant and the asymmetry parameter were estimated to be e 2 q Q / h =30 MHz and η∼0, respectively. The nuclear spin lattice relaxation rate, 1/ T 1 , is proportional to \(\sqrt {T}\), which indicates β Mn to be an itinerant antiferromagnetic system with T N ∼0 K. The value of 1/ T 1 at site II is about 20 times larger than the value at site I. The electronic state of Mn at site II is more magnetic than that at site I.

17O NMR study of La2−xSrxCuO4 in the lightly-and heavily-doped regions

Abstract: A 17 O NMR study has been carried out for La 2− x Sr x CuO 4 in the lightly- ( x =0.075) and heavily-doped ( x =0.24) regions in the normal state. Together with the data previously reported for x =0.15, it was found that the nuclear quadrupole frequency ν Q in the apical oxygen (O(1)) does not change with x , suggesting that the doped hole does not go into the LaO layer at all. On the contrary, ν Q in the planar oxygen (O(2)) increases appreciably with Sr doping. By analyzing ν Q of Cu and O(2), it was shown that the increase of holes on the planar O 2p σ orbit is more appreciable than that in the Cu 3d orbit. From the ratio of the hole density in Cu to that in oxygen, and the spin fluctuation probed by 63 ( T 1 T ), it is suggested that the depression of T c in the “heavily-doped” region is neither due to the over-doping nor to the collapse of the two-dimensionality, but to other causes. The temperature dependences of the Knight shift 17 K in two samples are in sharp contrast with each other. In the lightly-doped region, 17 K decreases with decreasing temperature. In the heavily-doped region, however, 17 K increases slightly with decreasing temperature, becoming almost T -independent below 100 K. An increase of the anisotropic part of spin Knight shift 17 K s,ani with x also indicates that the hole density at the O 2p σ orbit increases with Sr doping.

On the Electric Field Gradient at Copper Nuclei in Oxides

Abstract: A useful interpretation is presented of the material dependence of Cu electric field gradient (EFG) in a great variety of insulating and superconducting copper oxides. The present study is concerned only with copper sites in nearly tetragonal symmetry and in stoichiometric compositions. The experimental data of Cu EFGs have been analyzed in terms of ionic picture. The analysis has revealed for the first time a systematic correlation between the observed Cu EFG and the ionic contribution to the EFG. By using the correlation, we have extracted empirical values of the Sternheimer antishielding factor γ ∞ and the hyperfine constant for Cu 2+ and Cu 1+ ions. Those values are somewhat different from the traditional ones of the results of unrestricted Hartree-Fock (UHF) calculations for free ions.

Coordination of ortho chlorines in nickel and zinc 4-substituted 2,6-dichlorophenolates. Crystal and molecular structures of (N,N,N',N'-tetramethyl-1,2-ethanediamine)bis(2,4,6-trichlorophenolato-O,Cl)nickel(II) and tris(pyridine)bis(2,4,6-trichlorophenolato-O)nickel(II)

Abstract: Nickel(II) and zinc(II) complexes of the 2,4,6-trichlorophenolate (tcp), 2,6-dichlorophenolate, and 4-bromo-2,6-dichlorophenolate ions were examined for organochlorine-metal secondary bonding by 35 Cl nuclear quadrupole resonance (NQR) spectroscopy and compared to analogous compounds with other transition metals.

Spin dynamics in CuO and Cu1-xLixO from 63Cu and 7Li nuclear relaxation.

Abstract: [sup 63]Cu nuclear quadrupole resonance (NQR), nuclear antiferromagnetic resonance (AFNMR), and spin-lattice relaxation, as well as [sup 7]Li NMR and relaxation measurements in CuO and in Cu[sub 1[minus][ital x]]Li[sub [ital x]]O, are used to study the spin dynamics in the paramagnetic and in the antiferromagnetic (AF) phase, and in particular the effects induced by lithium doping. It is argued that Li enters in CuO lattice up to a doping [ital x] around 4%; from a comparison of the electric-field gradient (EFG) and of the dipolar field at the nucleus with theoretical estimates, it appears that Li[sup +] is slightly displaced from the Cu lattice site. From NQR, NMR, and AFNMR spectra, information on the Cu hyperfine interaction and the EFG are derived. The Neel temperature [ital T][sub [ital N]] is affected by Li doping to a moderate extent, decreasing from 226 K in CuO to [ital T][sub [ital N]]=183 K for [ital x]=3.7%. The relaxation rates, driven by the time-dependent part of the magnetic electron-nucleus Hamiltonians, indicate that the Cu[sup 2+] spin dynamics is almost unaffected by Li doping for [ital T][much gt][ital T][sub [ital N]] and possibly controlled by valence fluctuations. The Li doping modifies in a dramatic waymore » the spin fluctuations in the AF phase.« less

Polycrystalline cobalt-59 NMR studies of metal-ligand interaction in axially symmetric diamagnetic cobalt(III) complexes. Correlation of .delta.(59Co) with NQCC/.DELTA.Eav

Abstract: The polycrystalline 59 Co NMR spectra (1/2↔-1/2 transition) of the diamagnetic cobalt(III) complexes Na 3 [Co(NO 2 ) 6 ] and trans-[Co(en) 2 (NO 2 ) 2 ]NO 3 were measured at 7.1 and 9.5 T. It is demonstrated that for axially symmetric diamagnetic cobalt(III) complexes the 59 Co chemical shielding anisotropy (CSA) is semiquantitatively correlated (r=0.998) with the product of the nuclear quadrupole coupling constant (NQCC) and the inverse of the weighted averate of the first d-d electronic transition energies (ΔE av ) through the similarity of the d-orbitals population imbalance in the expressions for the CSA and the NQCC

Copper-63,65 nuclear quadrupole resonance of complexes of copper(I) halides with phosphorus-containing ligands

Abstract: The 63Cu NQR spectra have been recorded for ten complexes of Cu1 with phosphine and halide ligands that, either on the basis of their previously determined crystal structures or of their analogy to complexes of known structure, were believed to contain three-co-ordinated copper atoms. The NQR resonance frequencies support these attributions and the structure determination of two of the complexes, [{CuX(L)}2][X = Cl or Br, L = P(C6H4Me-o)3], confirms the co-ordination number of three. The NQR results indicated that the previously reported structure for [Cu4Br4(Ph2PCH2PPh2)2] was incorrect and a redetermination of the structure confirmed this conclusion.

Al nuclear-quadrupole-resonance studies of CeAl2.

Abstract: We report 27 Al nuclear-quadrupole-resonance (NQR) studies on CeAl 2 in the temperature range between 0.09 and 4.2 K. Below T N =3.45 K CeAl 2 orders antiferromagnetically. The spin-lattice relaxation rate (T 1 -1 ) shows a sharp peak at T N , and the NQR linewidth increases from 27±3 kHz in the paramagnetic state to 270±15 kHz at 2 K. Below 1.5 K, in the low-temperature regime of the magnetically ordered state, the temperature dependence of the spin-lattice relaxation rate follows a Korringa law with (T 1 T) -1 =7.9±0.8 (K sec) -1

Chlorine-nuclear-quadrupole-resonance study of the neutral-to-ionic transition in tetrathiafulvalene-chloranil.

Abstract: The temperature dependence of the chlorine-35 nuclear-quadrupole-resonance (NQR) parameters has been studied in the mixed-stack charge-transfer complex tetrathiafulvalene-chloranil and discussed with reference to spectroscopic and neutron-diffraction results. The NQR spectrum changes conspicuously at the neutral-to-ionic transition temperature, ${\mathit{T}}_{\mathit{N}\mathrm{\ensuremath{-}}\mathit{I}}$=79.2 K. The changes have been analyzed in terms of the symmetry breaking occurring at the transition. Below, coexistence of the neutral and ionic phases has been observed down to the limit of metastability of the neutral phase (${\mathit{T}}_{\mathit{c}}$\ensuremath{\sim}69 K). Spin-lattice relaxation rates reveal the existence of highly anisotropic critical fluctuations with a predominantly one-dimensional character.

Theory of pulses in nuclear quadrupole resonance spectroscopy

Abstract: The effect of radiofrequency pulses in nuclear quadrupole resonance (NQR) spectroscopy is examined. The description is different from that needed in nuclear magnetic resonance (NMR) spectroscopy. In particular, both rotating and counter-rotating radiofrequency field components are retained. Pulses in NQR spectroscopy are selective and cause transitions between two pairs of levels (±M) → ±(M + 1), other transitions are not normally excited. The formulation of pulses is then described for any spin I by two 2 × 2 rotation matrices, one causing an M → M + 1 transition and the other causing a - (M + 1) → -M transition. Calculations on resonance for spins with an axially symmetric nuclear quadrupole for up to three pulses are presented for spins I = 1 and I = 3/2. The results agree with the work of Reddy, R. and Narasimhan, P. T., 1991, Molec. Phys., 72, 491, in the appropriate limits.

Analysis of the Electric Field Gradients and the Knight Shifts at All Cu and O Nuclei in YBa2Cu3O7

Abstract: Electronic configurations and local susceptibilities at all copper and oxygen sites in YBa 2 Cu 3 O 7 are determined from the analysis of the electric field gradients (EFG's) and the Knight shifts (shifts). Besides main contributions from 3 d holes for Cu sites and 2 p σ holes for O sites, it is necessary to consider 4 p electrons of Cu and 2 p π holes of O for accurate analysis of the EFG's, and 4 s electrons of Cu and 2 s holes of O for the shifts. Difference in the observed EFG's between Cu(1) and Cu(2) is due to the 4 p -electron contributions, and the 3 d -electron states are determined to be quite similar. Each site of Cu and O shares almost same amount of holes. The local spin susceptibilities, however, are fairly different in order of magnitude as Cu(2), Cu(1), O(2, 3), O(4), implying a strong correlation in Cu(2).

Thermal history-dependent superconductivity and local structure in La2CuO4+δ

Abstract: DC magnetic susceptibility measurements reveal a dependence of the superconducting critical temperature ( T c ) on the cooling rate ( R ) in super-oxygenated La 2 CuO 4+δ Bulk T c 's vary by ≈ 4 K depending on whether the sample is “quenched” or slow cooled through a narrow temperature range around 195 ± 10 K 139 La nuclear quadrupole resonance spectroscopy (NQR) shows the appearance of a feature indicating a distribution of local structures in the metallic phase below ≈ 195 K, the same temperature regime where R is critical for T c The spectral weight associated with the metallic phase appears to increase for rapid cooling

Remote nuclear quadrupole resonance in solids

Abstract: A theory is developed for remote nuclear quadrupole resonance (NQR) in solid dielectrics with transient remote excitation of crystals for various materials and various conditions of observation. Experimental studies are made to assess the possibilities of using remote NQR and NQR introscopy. Data are presented and recommendations based on theoretical and experimental investigations are formulated for the development of devices for various purposes.

Copper NQR study of the La 214 system from an aspect of microscopic environment of copper

Abstract: The nuclear quadrupole resonance (NQR) has been investigated for Cu in La2−x A x CuO y (A=Sr and Ba). Three Cu NQR lines, A, B and C, have been observed, which correspond to three different Cu sites. From the analysis of these NQR intensities, site assignments are possibly as follows. These lines A, B and C are attributed to CuO6 octahedral, CuO5 pyramidal and CuO4 planar sites, respectively. The apical oxygen O(2) is induced to transfer to the interstitial O(3) site between LaO planes when two or more Sr2+ ions are located in its neighboring La sites. The excess holes doped into the CuO2 plane are provided mainly by O(3) as well as the unpaired Sr2+ ion.

Copper-63,65 nuclear quadrupole resonance studies of copper(I) complexes with sulfur-containing ligands. Part 2. Bis(1-alkylimidazolidine-2-thione)copper(I) iodides and related compounds

Abstract: The 63Cu NQR resonances frequencies of ten three-co-ordinated copper(I) complexes, in each of which at least two of the ligands are thiones, have been determined. The Zeeman splitting of the 63Cu NQR spectrum of a single crystal of bis(tetrahydro-1H-pyrimidine-2-thione)copper(I) chloride yields the value of 0.36(2) for the asymmetry parameter with the field-gradient z axis perpendicular to the CuS2Cl plane and the y axis along the Cu–Cl bond. The crystal structure of a representative member of these complexes, bis(1-propylimidazolidine-2-thione)copper(I) iodide, confirms the three-co-ordination of the CuI. When one of the ligands is a halogen atom the results confirm the previously reported trend that, for a given ligand, the 63Cu NQR resonance frequencies of the complexes [CuL2X] are in the order Cl > Br > I. For two of the complexes the temperature dependence of the NQR frequency indicates that a phase change occurs in the range 77–300 K.

New nuclear quadrupole resonance technique in β-NMR

Abstract: The β-NMR technique has been modified in order to detect efficiently the nuclear quadrupole effects in the NMR spectra. β-NMR is detected as a function of coupling frequency; all RFs that correspond to the coupling frequency were applied simultaneously.

Nuclear-quadrupole double-resonance study of RbH2PO4 in the supercooled high-temperature monoclinic phase.

Abstract: Rubidium nuclear-quadrupole-resonance (NQR) spectra have been measured in the high-temperature monoclinic phase of RbH 2 PO 4 above T p =86 o C and also in the supercooled high-temperature phase below T p . At the transition into the high-temperature phase, the rubidium quadrupole-coupling constants drop by a factor of 5. The high-temperature phase is metastable below T p . The kinetics of the transformation of the metastable phase into the stable tetragonal phase in a polycrystalline sample is determined at room temperature from the time dependence of the rubidium NQR spectra

A study of activated molecular motion in 2-nitrobenzene sulphonyl chloride by NQR

Abstract: A comprehensive chlorine nuclear quadrupole resonance study of the temperature and pressure dependences of frequency, line width and spin-lattice relaxation time in 2-nitrobenzene sulphonyl chloride is reported. The results show that each of the two substituted groups on the benzene ring contributes to the spin-lattice relaxation via a different and independent mechanism. In the high-temperature regime, the data are successfully explained assuming reorientation of the SO2Cl group between unequal potential wells and field gradient fluctuations resulting from reorientations of nearby NO2 groups. The activation energy of both motions and the fraction of the electric field gradient influenced by the motion of nitro groups have been estimated, as well as the pressure dependence of the activation enthalpy and the activation volume for these motions.