Electric Field Gradient Asymmetry Research Articles

Probing electronic environment in gamma irradiated sodium borosilicate glass and simulated waste glass: a perturbed angular correlation spectroscopy study

Time Differential Perturbed Angular Correlation (TDPAC) has been used to investigate the local structural changes (if any) in Sodium borosilicate (NBS) Glass and Simulated Waste Oxide loaded sodium borosilicate glass (SNBS) used for immobilization of high-level liquid waste at Trombay, Mumbai. The glasses were doped with 181Hf tracer as TDPAC probe and irradiated with varying dose (1–50 MGy) of gamma radiation using 60Co (average γ energy 1.25 MeV) source. Hyperfine interaction parameters (quadrupole interaction frequency, its distribution and asymmetry of electric field gradient) were obtained for irradiated NBS and SNBS glasses from fitting of TDPAC data using DPAC software. The data was compared with the corresponding unirradiated glasses. Results do not show any micro-crystallinity upon irradiation in both the NBS and SNBS glass even at the highest gamma dose of 50 MGy even though small changes were observed in quadrupole interaction frequency at doses beyond 30 MGy. This suggests that the sodium borosilicate glass used for immobilization of high-level waste at Trombay is stable towards gamma radiation at least up to a gamma dose of 50 MGy.

Local nematic susceptibility in stressed BaFe2As2 from NMR electric field gradient measurements

The electric field gradient (EFG) tensor at the $^{75}$As site couples to the orbital occupations of the As p-orbitals and is a sensitive probe of local nematicity in BaFe$_2$As$_2$. We use nuclear magnetic resonance to measure the nuclear quadrupolar splittings and find that the EFG asymmetry responds linearly to the presence of a strain field in the paramagnetic phase. We extract the nematic susceptibility from the slope of this linear response as a function of temperature and find that it diverges near the structural transition in agreement with other measures of the bulk nematic susceptibility. Our work establishes an alternative method to extract the nematic susceptibility which, in contrast to transport methods, can be extended inside the superconducting state.

Systemic Study of Orbital and Spin Nematicity in NaFe1−x Co x As by NMR

Nematic order, a self-organized state with rotational symmetry broken, has been observed in both copper-oxide and iron-pnictide high temperature superconductors. However, its origin is still a mystery in the iron pnictides although it is considered as a key to understand the mechanism of superconductivity. Here, we report a systemic nuclear magnetic resonance (NMR) study on NaFe1−xCoxAs (0 ≤x ≤ 0.042) that an orbital order, accompanied by an instant spin nematicity, occurs at at a temperature T∗ far above structural transition temperature Ts in the tetragonal phase. We show that the observed NMR spectra splitting and its evolution is due to an incommensurate orbital order that sets in below T∗ and becomes commensurate below Ts. We show that the electric field gradient asymmetry parameter is a good measure for the orbital order parameter which undergoes a Landau-like 2nd-order phase transition. We further show that the spin nematicity is well accounted for by the observed orbital order.

Dependence of nuclear quadrupole resonance transitions on the electric field gradient asymmetry parameter for nuclides with half-integer spins

Allowed transition energies and eigenstate expansions have been calculated and tabulated in numerical form as functions of the electric field gradient asymmetry parameter for the zero field Hamiltonian of quadrupolar nuclides with I=3/2,5/2,7/2, and 9/2. These results are essential to interpret nuclear quadrupole resonance (NQR) spectra and extract accurate values of the electric field gradient tensors. Applications of NQR methods to studies of electronic structure in heavy element systems are proposed.

Solid-state NMR/NQR and first-principles study of two niobium halide cluster compounds

Two hexanuclear niobium halide cluster compounds with a [Nb6X12]2+ (X=Cl, Br) diamagnetic cluster core, have been studied by a combination of experimental solid-state NMR/NQR techniques and PAW/GIPAW calculations. For niobium sites the NMR parameters were determined by using variable Bo field static broadband NMR measurements and additional NQR measurements. It was found that they possess large positive chemical shifts, contrary to majority of niobium compounds studied so far by solid-state NMR, but in accordance with chemical shifts of 95Mo nuclei in structurally related compounds containing [Mo6Br8]4+ cluster cores. Experimentally determined δiso(93Nb) values are in the range from 2400 to 3000ppm. A detailed analysis of geometrical relations between computed electric field gradient (EFG) and chemical shift (CS) tensors with respect to structural features of cluster units was carried out. These tensors on niobium sites are almost axially symmetric with parallel orientation of the largest EFG and the smallest CS principal axes (Vzz and δ33) coinciding with the molecular four-fold axis of the [Nb6X12]2+ unit. Bridging halogen sites are characterized by large asymmetry of EFG and CS tensors, the largest EFG principal axis (Vzz) is perpendicular to the X-Nb bonds, while intermediate EFG principal axis (Vyy) and the largest CS principal axis (δ11) are oriented in the radial direction with respect to the center of the cluster unit. For more symmetrical bromide compound the PAW predictions for EFG parameters are in better correspondence with the NMR/NQR measurements than in the less symmetrical chlorine compound. Theoretically predicted NMR parameters of bridging halogen sites were checked by 79/81Br NQR and 35Cl solid-state NMR measurements.

Study of the EFG tensor at 75As nuclei in Ge‐As‐Se chalcogenide glasses

Asymmetry parameters of the electric field gradient tensor at (75) As nuclei were determined for chalcogenide glassy semiconductors (CGS) of the Ge-As-Se system by comparing the experimental and simulated (75) As nuclear quadrupole resonance nutation interferograms. The electric field gradient asymmetry in CGS was analyzed, and it is believed that a structural change in these glassy semiconductors takes place at r¯ = 2.425. Electron paramagnetic resonance spectra of the Ge-As-Se system were obtained for the first time. A comparison was made between the results of analysis of the Ge-As-Se system by nuclear quadrupole resonance and electron paramagnetic resonance methods, and this allowed us to make the supposition that a structural phase transition occurs at r¯ = 2.4 from two-dimensional to three-dimensional CGS structure.

Angular dependent 75As NMR study of the electric field gradient in CaFe2As2

The electric field gradient (EFG) at the As nuclei in the BaFe2As2 and SrFe2As2 iron arsenide compounds becomes axially asymmetric in the low temperature orthorhombic phase, but little has been reported about the EFG in the CaFe2As2 analog. We present As nuclear magnetic resonance (NMR) spectra as a function of orientation in this phase and extract the EFG tensor. The EFG asymmetry is significantly less in this material, which is a result of the unusually strong c-axis hybridization.

Nuclear magnetic relaxation induced by exchange-mediated orientational randomization: Longitudinal relaxation dispersion for spin I = 1

The frequency dependence of the longitudinal relaxation rate, known as the magnetic relaxation dispersion (MRD), can provide a frequency-resolved characterization of molecular motions in complex biological and colloidal systems on time scales ranging from 1 ns to 100 μs. The conformational dynamics of immobilized proteins and other biopolymers can thus be probed in vitro or in vivo by exploiting internal water molecules or labile hydrogens that exchange with a dominant bulk water pool. Numerous water (1)H and (2)H MRD studies of such systems have been reported, but the widely different theoretical models currently used to analyze the MRD data have resulted in divergent views of the underlying molecular motions. We have argued that the essential mechanism responsible for the main dispersion is the exchange-mediated orientational randomization (EMOR) of anisotropic nuclear (electric quadrupole or magnetic dipole) couplings when internal water molecules or labile hydrogens escape from orientationally confining macromolecular sites. In the EMOR model, the exchange process is thus not just a means of mixing spin populations but it is also the direct cause of spin relaxation. Although the EMOR theory has been used in several studies to analyze water (2)H MRD data from immobilized biopolymers, the fully developed theory has not been described. Here, we present a comprehensive account of a generalized version of the EMOR theory for spin I = 1 nuclides like (2)H. As compared to a previously described version of the EMOR theory, the present version incorporates three generalizations that are all essential in applications to experimental data: (i) a biaxial (residual) electric field gradient tensor, (ii) direct and indirect effects of internal motions, and (iii) multiple sites with different exchange rates. In addition, we describe and assess different approximations to the exact EMOR theory that are useful in various regimes. In particular, we consider the experimentally important dilute regime, for which approximate analytical results are derived. As shown by the analytical expressions, and confirmed by exact numerical calculations, the dispersion is governed by the pure nuclear quadrupole resonance frequencies in the ultraslow-motion regime, where the relaxation rate also exhibits a much stronger dependence on the electric field gradient asymmetry than in the motional-narrowing regime.

First-Principle Calculations on the Microscopic 57Fe Electric-Field-Gradient Tensor of Ferrous Chloride Tetrahydrate: A Prototypical Mössbauer Species

Since the pioneering work on the theoretical description of Mössbauer quadrupole line intensities for a single-crystal to elucidate information on the electric-field-gradient (EFG) tensor, ferrous chloride tetrahydrate, FeCl(2)·4H(2)O, has represented a prototypical Mössbauer species. In addition, this species also typifies a so-called ambiguous, low symmetry, iron center where traditionally only macroscopic tensors have been assumed available for determination. Recent experiments on FeCl(2)·4H(2)O have successfully determined physically meaningful spin-Hamiltonian parameters for the (57)Fe microscopic (local) EFG and mean-squared displacement tensors. This paper reports a density functional theory investigation that finds good agreement with experiment. Assuming a gas-phase scaffold cluster approach to describe the crystalline geometry, the sign of the EFG was determined to be positive, in agreement with earlier magnetically perturbed experiments, and the EFG asymmetry parameter η was calculated to be 0.21-0.23 depending on the density functional used, which is in excellent accord with experiment at 0.25(2). By virtue of theoretical and experimental agreement, this work indicates that simultaneous electric-field-gradient-mean-square-displacement Mössbauer determinations can resolve the apparent ambiguity associated with monoclinic, 2/m or 1 Laue class, sites provided there is sufficient anisotropy in the Lamb-Mössbauer recoilless fraction.

Dynamic lattice distortions in Sr2RuO4: microscopic studies by perturbed angular correlation spectroscopy and ab initiocalculations

Applying time differential perturbed angular correlation (TDPAC) spectroscopyand ab initio calculations, we have investigated possible lattice instabilities inSr2RuO4 by studying the electric quadrupole interaction of a 111Cd probe at the Ru site. We find evidence for a dynamic lattice distortion, revealed from theobservations of: (i) a rapidly fluctuating electric-field gradient (EFG) tensor showingnon-Arrhenius relaxation, (ii) an anomalous temperature dependence of the quadrupoleinteraction frequency, and (iii) a monotonic increase of the EFG asymmetry (η) below 300 K. We argue that the observed dynamic lattice distortion is causedby strong spin fluctuations associated with the inherent magnetic instability inSr2RuO4.

Stereoelectronic srtructure of γ-chloropropyltrichlorostannane. Ab initio calculation

The quantum-chemical calculations by the RHF/3-21G* and MR2/3-21G* methods of the Cl(CH2)3SnCl3 and ClCH2SnCl3 molecules with full optimization of their geometry was performed. The Cl(CH2)3SnCl3 molecule was also calculated by the RHF/3-21G* method at different distances between the Sn atom and the Cl atom in the C-Cl fragment. From the results of the calculations 35Cl NQR frequencies were estimated and the electric field gradient asymmetry parameters at the 35Cl nuclei of these molecules. The abnormally high electron density on the Cl atom of the C-Cl fragment in the Cl(CH2)3SnCl3 molecule is due mainly to the polarization of this compound under the influence of significant positive charge on the Sn atom directly through the field. The polarization of the C-Cl bond in the molecule is of the opposite direction to the polarization of the related bond in the ClCH2SnCl3 molecule because of the peculiarities of their geometries. The transfer of the electron density from the Cl atom of the C-Cl fragment to the Sn atom in the Cl(CH2)3SnCl3 molecule is not observed.

Nutation versus angular-dependent NQR spectroscopy and impact of underdoping on charge inhomogeneities inYBa2Cu3Oy

We describe two different nuclear quadrupole resonance (NQR) based techniques, designed to measure the local asymmetry of the internal electric field gradient, and the tilt angle of the main NQR principal axis z from the crystallographic axis c. These techniques use the dependence of the NQR signal on the duration of the radio frequency (rf) pulse and on the direction of the rf field H1 with respect to the crystal axis. The techniques are applied to oriented powder of YBa$_{2}$Cu$%_{3}$O$_{y}$ fully enriched with 63Cu. Measurements were performed at different frequencies, corresponding to different in-plane copper sites with respect to the dopant. Combining the results from both techniques, we conclude that oxygen deficiency in the chain layer lead to a rotation of the NQR main principal axis at the nearby Cu on the CuO2 planes by 20+-degrees. This occurs with no change to the asymmetry. The axis rotation associated with oxygen deficiency means that there must be electric field inhomogeneities in the CuO2 planes only in the vicinity of the missing oxygen.

Convergence of electric field and electric field gradient versus atomic basis sets in all‐siliceous and Mg substituted phillipsites

Electrostatic potential (EP), electric field (EF), and electric field gradient (EFG) values are calculated in periodic models of magnesium substituted phillipsite (MgPHI) zeolite forms using periodic DFT (PDFT) hybrid B3LYP level with fourteen different basis sets. Relative root mean square differences between the EP, EF, or EFG values estimated between different basis sets are evaluated in several spatial domains available for adsorbate molecules in the zeolite. In these areas, the EF increase in MgPHI is evaluated relative to all-siliceous PHI types. The EP is interpreted in terms of framework ionicity for MgPHI and all-siliceous PHI models. Angular Si-O-Si dependence of the EFG asymmetry at (17)O atoms in all-siliceous zeolites is discussed.

Manifestation of Berry’s Phase in Nuclear Quadrupole Resonance Spectra of Rotating Powder Samples

The effect of Berry’s phase on the nuclear quadrupole resonance (NQR) spectra of rotating powder samples is studied experimentally, and its application for the determination of the electric field gradient asymmetry η is demonstrated. The NQR frequency splittings, which are observed for the spin 3/2 nucleus of 35Cl in powder samples of p-dichlorobenzene (C6H4Cl2) and cyanuric chloride (C3N3Cl3), are interpreted as a manifestation of Berry’s phase, associated with the adiabatically changing Hamiltonian due to sample rotation. The accumulation of Berry’s phase during the rotation process is responsible for the observed dependence of the NQR line shape on the rotation frequency and the asymmetry parameter. The proposed method for the determination of η involves the analysis of the NQR powder patterns of the rotating samples.

Determination of the electric field gradient asymmetry from 2D nutation NQR spectra of 75As nuclei in oriented samples of As 2Se 3 semiconductor

The method of two-dimensional nutation nuclear quadrupole resonance in time domain is used to study the chalcogenide semiconductor As 2Se 3. In this system, the nuclear quadrupole resonance (NQR) resonance line width is as large as 10 MHz; therefore, the radiofrequency field produced by a pulsed NQR spectrometer can excite only a portion of the nuclear spins. The proposed method relies on polarizing the melted glass specimen in a strong magnetic field so that orientational disorder is partially removed. After hardening the sample is placed in a spectrometer r.f. coil in such a way that that the axes of sample polarization and r.f. coil coincide. We demonstrate the application of this method to determine η in glassy As 2Se 3.

Unconventional Superconductivity Induced by Quantum Critical Fluctuations in Hydrate Cobaltate Nax(H3O)zCoO2·yH2O –Relationship between Magnetic Fluctuations and Superconductivity Revealed by Co Nuclear Quadrupole Resonance–

Co nuclear-quadrupole-resonance (NQR) measurements were performed on various bilayered hydrate cobaltate Na_x(H_3O)_zCoO_2\cdot yH_2O with different values of the superconducting and magnetic-ordering temperatures, T_c and T_M, respectively. From measurements of the temperature and sample dependence of the NQR frequency, it was revealed that the NQR frequency is changed by the change of the electric field gradient (EFG) along the c axis \nu_{zz} rather than the asymmetry of EFG within the ab-plane. In addition, it is considered that the change of \nu_{zz} is gaverned mainly by the trigonal distortion of the CoO_2 block layers along the c axis, from the relationships between \nu_{zz} and the various physical parameters. We found the tendency that samples with \nu_{zz} larger than 4.2 MHz show magnetic ordering, whereas samples with lower \nu_{zz} show superconductivity. We measured the nuclear spin-lattice relaxation rate 1/T_1 in these samples, and found that magnetic fluctuations depend on samples. The higher-\nu_{zz} sample has stronger magnetic fluctuations at T_c. From the relationship between \nu_{zz} and T_c or T_M, we suggest that the NQR frequency can be regarded as a tuning parameter to determine the ground state of the system, and develop the phase diagram using \nu_{zz}. This phase diagram shows that the highest-T_c sample is located at the point where T_M is considered to be zero, which suggests that the superconductivity is induced by quantum critical fluctuations. We strongly advocate that the hydrate cobaltate superconductor presents an example of the magnetic-fluctuation-mediated superconductivity argued in the heavy-fermion compounds. The coexistence of superconductivity and magnetism observed in the sample with the highest \nu_{zz} is also discussed on the basis of the results of our experiments.

Determination of the electric field gradient asymmetry from Berry’s phase in NQR of powder samples

An effect of Berry’s phase on the NQR spectrum of the rotating powder sample is described and applied for the determination of the electric field gradient asymmetry. The proposed method involves the analysis of the frequency singularities in the NQR powder patterns of the rotating samples. The Berry’s phases for the eigenstates, associated with an adiabatically changing quadrupole hamiltonian, are calculated for nuclei with a spin I = 3/2 and I = 1 as a function of the asymmetry parameter.

Two-Dimensional Nutation NQR Broad-Line Spectra in Oriented Samples

The NQR nutation method to determine the electric field gradient asymmetry parameter η in systems, where the resonance line is so broad that the radio frequency field can excite only a portion of the nuclear spins, is presented. In this situation, the recently developed spectroscopic methods are not applicable. Two-dimensional nutation NQR spectra of oriented powders are calculated and used to determine η at particular frequencies along a broad NQR line. The proposed method is useful for single crystals, oriented powders, glasses, and disordered systems even for small values of the asymmetry parameter. Therefore it can be used to evaluate fluctuations in η and the quadrupole coupling constant e2qQ due to inhomogeneities. We demonstrate the application of this method to oriented chalcogenide semiconducting glasses.

Layered Inclusion Compounds Containing Aniline and Polyaniline Studied by NQR and IR Spectroscopy

Temperature dependence of the 127I NQR frequencies in the intercalates of PbI2 with aniline were measured. Unusual temperature coefficients of frequency and the electric field gradient asymmetry parameters for these compounds were discussed, taking into account the structure peculiarities and the amplitude of iodine oscillations.

Pure nuclear quadrupole resonance determination of the electric field gradient asymmetry for broad lines

We present an angle dependent nuclear quadrupole resonance (ADNQR) method to determine the electric field gradient asymmetry parameter η in systems where the resonance line is so broad that the radio frequency field can excite only a portion of the nuclear spins. In this situation, the recently developed spectroscopic methods are not applicable. ADNQR is useful for single crystals and oriented powders, and, for small η determines η 4. Therefore, it can be used to evaluate fluctuations in η due to inhomogeneities. We demonstrate the application of ADNQR experimentally to oriented superconducting YBa 2Cu 3O 7 powder.