Quadrupole Frequency νQ Research Articles

First-Order Metal–Semiconductor Transition Triggered by Rattling Transition in Tetrahedrite Cu12Sb4S13: Cu-Nuclear Magnetic Resonance Studies

Tetrahedrite Cu12Sb4S13 shows a metal–semiconductor transition (MST) at TMST = 85 K. We have studied the mechanism of the MST by measurements of Cu-NMR. Above TMST, the Cu-NMR spectrum consists of signals from a tetrahedral Cu(1) 12d site and a trigonally coordinated Cu(2) 12e site. Analyses of the spectra at 95 and 20 K yield NMR and nuclear quadrupole resonance (NQR) parameters above and below TMST. The Cu(1) signal does not show clear quadrupole splitting, which remains unchanged on cooling across TMST. On the other hand, the Cu(2) signal at T > TMST clearly shows the quadrupole splitting characterized by a quadrupole frequency νQ(2) ≈ 18.6 MHz and an asymmetry parameter η(2) ≈ 0.03. Below TMST, the values of νQ(2) and η(2) change markedly. Such marked changes in NQR parameters provide evidence of a remarkable change in the local electronic structure around the Cu(2) site. Together with νQ from NMR spectra and that from first-principles calculation, we conclude that the Cu(2) atoms are displaced from the sulfur triangles below TMST. We also found that the NMR Knight shift and the spin–lattice relaxation rate divided by temperature, 1/T1T, for the Cu(1) signal markedly change below TMST, which is ascribable to the reduction in the electronic density of states at the Fermi level. We further discuss the relationship among the MST, structural transformation, and crystal structure below TMST.

TOPOLOGICAL FEATURES OF THE ELECTRON DENSITY DISTRIBUTION IN COVELLITE: POSITION Cu1.

The synthesized sample of covellite was investigated by NQR 63,65Cu methods. The experimentally determined values of the quadrupole frequency νQ and the asymmetry parameter η have been used to study the distribution of electron density in the region of the quadrupole nuclei in position Cu1. It is found that Cu-S bond in position Cu1 is ionic, and the gap LUMO-HOMO decreases with increasing temperature.

17O NMR study of the triangular lattice antiferromagnet CuCrO2

The single crystalline and powder samples of multiferroic CuCrO2 have been investigated by 17O NMR techniques in the paramagnetic and magnetically ordered states. The unusual negative hyperfine field on oxygen nuclei, Hhf = −5.2 kOe/µB, has been determined in paramagnetic state. The 17O NMR spectrum in ordered state has a shape typical for incommensurate magnetic structure. It can be well described taking into account only dipole fields which are produced by Cr3+-ions with magnetic moment μ = 2.2 μB. The parameters of the EFG (electric field gradient) tensor – the quadrupole frequency (νQ) and the asymmetry parameter, have been determined. The value of νQ increases monotonously with decreasing temperature in the range 90 K > T > 1.5 K. The estimates of the population of the ligands p – orbitals have been obtained.

Investigation of the Temperature Dependence of the NQR Parameters in Covellite

By a method NQR 63,65Cu the synthesized standard of covellite was studied. The experimental values of quadrupole frequency νQ and parameter of asymmetry η were used for research of distribution of electronic density and Laplacian of the electron density in area of quadrupole kernel in position of Cu1. It is set that connection of Cu - S in position of Cu1 is mainly ionic tipe, and size of gap of LUMO - HOMO diminishes with the increase of temperature.

Features of Electron Density Distribution in Delafossite Cualo2

We have used pulsed 63,65Cu nuclear quadrupole resonance at room temperature to study the semiconductor compound CuAlO2 with a delafossite crystal structure, and we have determined the quadrupole frequency νQ = 28.12 MHz and the asymmetry parameter η ~ 0, which we used to study the features of the electron density distribution in the vicinity of the quadrupolar nucleus. In order to take into account the influence of correlation effects on the electric field gradient, we carried out ab initio calculations within the density functional theory (DFT) approximation using a set of correlation functionals VWN1RPA, VWN5, PW91LDA, CPW91, and B3LYP1. We mapped the electron density distribution in the vicinity of the quadrupolar copper nucleus for the Cu7Al6o 14 ‐ 1 cluster and we calculated the size of the LUMO–HOMO gap, Δ ~ 3.33 eV. We established the anisotropy of the spatial electron density distribution. Based on analysis of the electron density distribution obtained, we suggest that the bond in CuAlO2 is not purely covalent.

Themal Expansion and Magnetostriction of YbAuCu4

Precise thermal expansion and magnetostriction measurements were performed on the heavyfermion compound YbAuCu4 in order to examine the crossover valence transition at TV which was proposed by the nuclear magnetic resonance measurements. The temperature dependence of the thermal expansion coefficient α under magnetic fields shows a broad peak, which shifts to higher temperatures with increasing magnetic fields. The corresponding linear thermal expansion Δℓ/.ℓ parallel to the magnetic field of 7.0 T shows a marked decrease below about 10K, indicating a contraction of sample length at low temperatures. These results are discussed in relation to the observed temperature dependence of the nuclear quadrupole frequency νQ under magnetic fields.

High field nuclear magnetic resonance in transition metal substituted BaFe2As2

We report high field 75As nuclear magnetic resonance (NMR) measurements on Co and Cu substituted BaFe2As2 single crystals displaying same structural/magnetic transition T0≃128 K. From our anisotropy studies in the paramagnetic state, we strikingly found virtually identical quadrupolar splitting and consequently the quadrupole frequency νQ≃2.57(1) MHz for both compounds, despite the claim that each Cu delivers 2 extra 3d electrons in BaFe2As2 compared to Co substitution. These results allow us to conclude that a subtle change in the crystallographic structure, particularly in the Fe–As tetrahedra, must be the most probable tuning parameter to determine T0 in this class of superconductors rather than electronic doping. Furthermore, our NMR data around T0 suggest coexistence of tetragonal/paramagnetic and orthorhombic/antiferromagnetic phases between the structural and the spin density wave magnetic phase transitions, similarly to what was reported for K-doped BaFe2As2 [Urbano et al., Phys. Rev. Lett. 105, 107001 (2010)].

Electron-density distribution in CuFeS2 as determined by 63,65Cu NMR in an internal magnetic field

NMR spectra of 63,65Cu in an internal magnetic field were studied experimentally. The electric field gradient (EFG) at Cu nuclei in chalcopyrite CuFeS2 was evaluated ab initio by using a cluster approach. Calculations were carried out in the framework of the self-consistent field restricted open-shell Hartree–Fock method (SCF-LCAO-ROHF). The largest cluster for which calculations were carried out had the formula Cu9Fe10S28 n (R ~ 6 A, 47 atoms), where n is the cluster charge. The best agreement of the quadrupole parameters (quadrupole frequency νQ and EFG tensor asymmetry parameter η) that were determined experimentally (νQ = 1.29 MHz, η = 0.34) and were calculated (νQ = 1.40 MHz, η = 0.50) was obtained for the cluster Cu9Fe10S28 −4. Maps of electron-density distribution in the neighborhood of the Cu quadrupolar nucleus were built for the cluster Cu9Fe10S28 −4. It was suggested based on an analysis of the obtained electron-density distribution that the bond in chalcopyrite is not covalent. The energy-level diagram that was calculated in the ROHF high-spin approximation defined rather well chalcopyrite as a semiconductor with a very narrow LUMO–HOMO gap and was consistent with the notion of chalcopyrite as a gapless semiconductor.

111Cd-TDPAC study of pressure effect on the valence of Yb in the YbGe2.85 cubic phase

New cubic phases of YbGe2.85, TbGe2.85 and DyGe2.85 crystallized in the AuCu3 structure are synthesized at a pressure of 8GPa. Using the differential perturbed angular γγ-correlation method (TDPAC) we measure the electric field gradient Vzz (the quadrupole frequency νQ=eQVZZ/h) at 111Cd nuclei probes inserted in Ge vacancy lattice sites for all these compounds. In YbGe2.85 the electric field gradient (EFG) is obtained as a function of pressure (up to 8GPa) at room temperature and as a function of temperature (down to 4K) at normal pressure. In TbGe2.85 and DyGe2.85 EFG is measured only as a function of temperature (down to 77K) at normal pressure. In YbGe2.85 the change of EFG and νQ with pressure indicate a change of Yb valance from 2.46 at normal pressure to 2.89 at 8GPa. In all compounds EFG and νQ are found to be practically independent of temperature.

NMR study of the electric field gradient in the paramagnetic phase of M3V2O8 (M = Co, Ni) compounds

The NMR spectra and the decay of a spin echo signal from 51V nuclei in Kagome-staircase Co3V2O8 (CVO) and Ni3V2O8 (NVO) single crystals are measured in the temperature range 30–300 K and a magnetic field H0 = 20 kOe. The orientation dependences of the 51V NMR line shape are used to determine the electric field gradient (EFG) parameters, namely, quadrupole frequency νQ and asymmetry parameter η. These parameters for NVO and CVO are νQ = 180(10) kHz, η = 0.5(1) and νQ = 130(10) kHz, η = 0.6(1), respectively. A comparison of the results of calculating EFG tensors with a point charge model and the NMR data indicates that the crystallographically equivalent vanadium atoms in the Ni3V2O8 and Co3V2O8 compounds differ in the EFG axis orientation. M3V2O8 crystals are found to have vanadium positions (V1, V2) with different orientations of the z axis, which specifies the direction of the principal value of EFG (Vzz): these orientations lie in the bc plane and make an angle of either +51(5)° (V1) or −51(5)° (V2) with axis c. In the temperature range 30–300 K, the EFG tensor components and the local symmetry of the charge surrounding of the vanadium positions in NVO and CVO oxides are found to change insignificantly.

Nuclear magnetic relaxation in a quasi-one-dimensional conductor β-Ag0.33V2O5

51V nuclear-magnetic-resonance (NMR) measurements were performed on β-Ag0.33V2O5, a quasi-one-dimensional conductor with three crystallographically inequivalent V sites (V1, V2, and V3). We observed the site difference between the V1 and V2 sites. The nuclear quadrupole frequency (νQ) shows different temperature dependence between the V1 and V2 sites, and the values of the relaxation rate (1/T1) are also different between the two sites in the metallic phase. These facts suggest that a non-uniform metallic state is realized: a 3d electron shared within a V1–V3 or V2–V2 rung in the CO phase could move in the quasi-one-dimensional direction in the metallic phase.

105Pd NQR study on NpPd5Al2 and CePd5Al2

We report the results of 105Pd NQR experiments on NpPd5Al2 and CePd5Al2. In the normal state at 6 K, the 105Pd NQR spectrum consists of four lines in both systems. These lines can be assigned to two sets of ±1/2 ↔ ±3/2 and ±3/2 ↔ ±5/2 NQR transitions arising from two crystallographically inequivalent Pd sites. From the analysis of the 105Pd NQR spectrum the nuclear quadrupole frequency νQ, asymmetry parameter η and electric field gradient Vzz have been deduced.

Relationship between magnetic fluctuations and superconductivity in Nax(H3O)zCoO2 · yH2O revealed by a Co nuclear quadrupole resonance

Nuclear quadrupole frequency (νQ) and spin-lattice relaxation rate 1/T1 of Co were measured on various bilayered hydrate (BLH) NaxCoO2·yH2O (y∼1.3) with different values of superconducting (SC) and magnetic-transition temperatures, Tc and TM, together with non-SC monolayered hydrate (MLH) NaxCoO2·yH2O (y∼0.7). In the temperature range above 70K, 1/T1T in all samples follows the similar temperature dependence as each other. In the BLH compounds, 1/T1T is enhanced with decreasing temperature below 70K, and the values of 1/T1T at Tc are larger in the higher-ν3 sample, where ν3 is the peak frequency of the NQR spectrum arising from ±5/2↔±7/2 transitions. The magnetic ordering is indicated from the prominent divergence of 1/T1T at TM and the presence of the internal field at the Co nuclear site in the samples with ν3 higher than 12.5MHz. We analyze the temperature dependence of 1/T1T in all samples on the basis of identical formula, and discuss the relationship between magnetic fluctuations and superconductivity in the BLH cobaltate.

Studies of graphite intercalation compounds using time-differential perturbed angular distributions

The technique of time-differential perturbed angular distributions (TDPAD) was used to investigate the structure of graphite samples intercalated by AsF5 vapor. Following population of the 198 keV state (τ=125 ns, Q=0.072b) of 19F by the 19F(p, p′γ) reaction, measurements of the γ-ray angular distribution were performed to give information on quadrupole interactions. Theoretical fits reveal the presence of two static electric field gradient (efg) interactions, the principal one corresponding to the formation of CF bonds, with quadrupole frequency νQ=57.4±0.8MH z. Although information on graphite intercalation chemistry was not forthcoming from the measurements, it is shown that the strong texture exhibited in the perturbation spectra can be explained in terms of the angular dependence of the efg, suggesting a novel method for the characterization of graphite intercalation compounds.

NQR Investigations of High-Tc-Superconducting Oxides

NQR measurements on 139La and 63,65Cu in powdered La–Ba–Cu–O and Ba–Y–Cu–O compounds are reported. The temperature dependence and the distribution of the quadrupole frequency νQ is investigated for both the normal and superconducting state. In Ba2YCu3O7 a steep decrease of νQ at Tc is observed followed by an increase with decreasing temperature.

Orientation of an oxygen-induced non-axially symmetric electric field gradient at Ta in Nb

The interaction of dilute Hf in Nb with interstitially dissolved oxygen has been investigated via the γ-γ-TDPAC-technique applied to the 482 keV state of181Ta. After melting the parent radioactivity181Hf with Nb the trapping of oxygen at the probe atoms leads to a quadrupole interaction frequency ν Q =268(1) MHz with an asymmetry parameter of η=0.68(1). The temperature dependence of this frequency between 17 K and 293 K was found to be very weak whereas η varies considerably. In a single crystal experiment the orientation of the three principal axes of the electric field gradient tensor leading to ν Q has been determined. TheV zz -axis points into a 〈110〉, theV yy -axis into a 〈100〉 and theV x -axis into a 〈110〉 direction. The results can be explained in the point charge model by assuming that one oxygen atom is trapped on the second nearest neighbour octahedral position next to the substitutional atom and the covalent bonds between the daughter Ta and the oxygen lead to a localization of charge near the probe atom.

Impurity Quadrupole Interaction in Light Rare Earth Metals: 111Cd in La, Pr and Nd

The nuclear quadrupole interaction of dilute light rare earth (RE) metals La, Pr and Nd by time differential perturbed angular correlation measurements. The quadrupole frequency νQ has been determined in Pr as a function of temperature and pressure, in Nd as a function of temperature, and in La at 19 K. The pressure dependence of νQ of 111Cd in Pr ((d In νQ/d P)290 K = + 2.7 (2.3) · 10-3 Kbar-1) is a factor of 3 - 4 weaker than in the heavy RE metals Gd to Er. At the same time the temperature dependence of νQ in the light RE is up to a factor of 3 stronger than in the heavy RE. Between 300 K and 900 K. the quadrupole frequency is a linear function of temperature with the same slope (d In νQ/dT)290 k = - 8.4(2) · 10-4 K-1 both in Pr and Nd. Between 25 K and 300 K the decrease is stronger than linear, and more pronounced in Pr than in Nd. The temperature coefficient (d In νQ/dT)290 k is found to decrease linearly with the RE atomic number across both the series of the fight and the heavy RE metals.

NMR study of CuAl2: 27Al, 63Cu, and 65Cu quadrupole and anisotropic shift interactions

The nuclear magnetic resonance spectra of 27Al, 63Cu, and 65Cu have been studied in a polycrystalline specimen of the intermetallic compound CuAl2 of the C16 structure type at room temperature between 3 and 40 MHz. The 27Al spectra yield to interpretation in terms of a completely anisotropic Knight shift and an asymmetric nuclear electric quadrupole interaction. The observed field separations of the several 27Al quadrupole satellite spectral features at fixed frequencies yield values of the lowest quadrupole frequency νQ = 420.2(8) kHz and asymmetry parameter η = 0.413(9). The 27Al satellite features also yield the three Knight shift parameters Kx = 0.159(1) %, Ky = 0.153(2) %, and Kz = 0.148(10) %. Excellent agreement has also been obtained between observed 27Al central line shape and the theoretical shapes of J. F. Baugher, H. M. Kriz, P. C. Taylor, and P. J. Bray, J. Magn. Reson. 3, 415 (l970). The 63Cu and 65Cu central lines exhibit a strong second and a fourth order quadrupole splitting with νQ(63Cu) = 7.54(17) MHz and νQ(65Cu) = 7.05(18) MHz. An analysis of the copper Knight shift is also presented.

NMR in the quadrupolar regime: Chemical shifts, asymmetry parameter, and angular orientation

Degenerate perturbation theory, exact analytic solutions, and numerical calculations were used to determine the frequency of transition between the lowest two energy levels of a spin I = 3/2 nucleus experiencing a large quadrupole interaction in an applied field in a single crystal. It is shown how the field dependence of the appropriate resonance experiment may be used to extract the characteristic quadrupole frequency νQ and the components of the chemical shift tensor σ, and how the resonance position as a function of the angle φ′ of orientation of the crystal about the axis of the NMR receiver coil depends on the orientational Euler angles as well as on the quadrupolar asymmetry parameter η and the shift components. This theory is applied in our experiments to α-para-dichlorobenzene. In a combined self-consistent least-squares fit of the exact expressions to the field dependences and the φ′ dependence of the resonance position, using the value νQ = 34.262 MHz from the literature, the chemical shift components were found to be σx=0.000± 0.001. σy=0.003± 0.001, and σz=−0.001± 0.001, assuming that the principal axes of the chemical shift tensor are coincident with those of the quadrupole tensor. The asymmetry parameter was found to be η = 0.0712± 0.0005: this and our own value for νQ are in excellent agreement with values determined by other methods. Additionally, the Euler angles of orientation of the single crystal in the mounting were determined (within an eightfold ambiguity) to within ± 0.8°.

NMR Study of ThAl2: 27Al, Dipolar Doublet, Quadrupole, and Anisotropic Shift Interactions

The nuclear magnetic resonance spectra of powdered intermetallic compounds MX2 of the C32 (AlB2) structure type lend themselves to analysis and interpretation in terms of nuclear dipole doublet interaction, nuclear electric quadrupole, and anisotropic Knight shift interactions. Within the hexagonal plane of the C32 structure three Pake pairs may be formed with each of the X nearest neighbors. The observed doublets in the NMR spectra of X nuclei are of the Pake type when the magnetic moment of the N nuclear species is small. Examples are 11B in ZrB2 and TiB2 and 27Al in ThAl2. An extensive analysis of the ThAl2 NMR spectra is presented which yields values for the lowest pure quadrupole frequency νQ=230.5± 2.0 kHz and Knight shift parameters Kiso=(0.042± 0.003)% and Ksx=−(0.012± 0.003)% The nearest-neighbor distance obtained was in agreement with spacings determined by x-ray techniques.