Nuclear Quadrupole Resonance Lineshape Research Articles

Freezing of the Lattice in the Kagome Lattice Heisenberg Antiferromagnet Zn-Barlowite ZnCu_{3}(OD)_{6}FBr.

We use ^{79}Br nuclear quadrupole resonance (NQR) to demonstrate that ultraslow lattice dynamics set in below the temperature scale set by the Cu-Cu superexchange interaction J (≃160 K) in the kagome lattice Heisenberg antiferromagnet Zn-barlowite. The lattice completely freezes below 50K, and ^{79}Br NQR line shapes become twice broader due to increased lattice distortions. Moreover, the frozen lattice exhibits an oscillatory component in the transverse spin echo decay, a typical signature of pairing of nuclear spins by indirect nuclear spin-spin interaction. This indicates that some Br sites form structural dimers via a pair of kagome Cu sites prior to the gradual emergence of spin singlets below ∼30 K. Our findings underscore the significant roles played by subtle structural distortions in determining the nature of the disordered magnetic ground state of the kagome lattice.

14N NQR lineshape in nanocrystals: An ab initio investigation of urea.

14N nuclear quadrupole resonance (NQR) lineshapes mostly contain information of low interest, although in nanocrystals they may display some unexpected behaviour. In this work, we present an ab initio computational study of the 14N NQR lineshapes in urea nanocrystals as a function of the nanocrystal size and geometry, focusing on the surface induced broadening of the lineshapes. The lineshapes were obtained through a calculation of the electric field gradient for each nitrogen site in the nanocrystal separately, taking into account the individual crystal field by embedding the molecule of interest in a suitable lattice of point multipoles representing other urea molecules in the nanocrystal. The small influence of distant molecules is found with a series expansion, using the in-crystal Sternheimer shieldings which we also calculated ab initio. We have considered nanocrystals with two geometries: a sphere and a cube, with characteristic sizes between 5 and 100 nm. Our calculations suggest that there is a dramatic difference between the linewidths for the two geometries. For spheres, we find a steep drop in linewidths at ∼10 nm; at 5 nm the linewidth is ∼11 kHz, whereas for sizes above 20 nm the linewidth is practically negligible (<100 Hz). For cubes, on the other hand, we find a steady 1/size decrease, from 12 kHz at 10 nm to 1.2 kHz at 100 nm. This analysis is important for 14N NQR spectroscopy of crystalline pharmaceuticals, where nanoparticles are increasingly more often embedded in some sort of matrix. Although this is only a theoretical analysis, we believe that this work can serve as a guidance for the forthcoming experimental analysis.

Electron spin echo envelope modulation of molecular motions of deuterium nuclei

Electron Spin Echo Envelope Modulation (ESEEM) spectroscopy is a powerful technique for the study of hyperfine interactions between an unpaired electron and nearby nuclei in solids, and is employed in quantitative structural studies. Here, we describe the use of ESEEM to study the slow motion of deuterium nuclei using their nuclear quadrupole resonance (NQR) line shapes. Two ESEEM techniques were employed: the conventional three-pulse ESEEM experiment, π/2 – τ – π/2 – T– π/2 – τ – echo, and the four-pulse ESEEM, π/2 – τ – π/2 – T/2 – π – T/2 – π/2 – τ – echo, with the time variable T scanned in both cases. The nitroxide free radical 4-tert-butyliminomethyl-2,2,5,5-tetramethyl(d12)-3-imidazoline-1-oxyl with four deuterated methyl groups was investigated in a glassy ortho-terphenyl matrix over a wide temperature range. It was shown that four-pulse ESEEM allowed measurement of the nearly pure 2H NQR line shape. Between 90K and 120K, the ESEEM spectra change drastically. At low temperatures, four-pulse ESEEM spectra show a Pake-like pattern, which evolves into a single line at higher temperatures, which is typical for NQR of rotating methyl CD3 groups. Comparison with literature data on NQR allows estimation of the reorientation rate, k. At ∼100K, where the spectral changes are most pronounced, k was found to be ∼105s−1. The spectral linewidths for the three-pulse ESEEM were found to decrease similarly with increasing temperature; so the three-pulse technique is also capable to detect motion of this type. The ESEEM approach, along with site-directed spin labeling, may be useful for detection of motional transitions near the spin labels in biological systems, when information on motion is required in a wide temperature range.

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.

Nuclear quadrupole resonance study of local structure of the Ge–As–Se ternary chalcogenide glasses

The local bonding structure in the Ge–As–Se ternary glassy system has been investigated by 75As nuclear quadrupole resonance (NQR) at 77K. For the compositions with average coordination number r¯=2.2,2.4,2.54 the NQR measurements yield no evidence for the presence of As–As homopolar bonds. For the composition Ge2As2Se5 (r¯=2.67) the NQR lineshape exhibits two peaks corresponding to arsenic sites with zero (or three) and two As–As bonds. High Ge and As content leads to formation of microclusters, where the predominant structural units are, possibly, As4Se4 molecules or partial molecules with As–As bonds. The NQR spectrum for the composition Ge0.33As0.12Se0.55 (r¯=2.78) shows a weak peak corresponding to an arsenic site with zero (or three) homobonds and two resolved peaks in the high frequency region. Such behavior is consistent with an increase in inhomogenity in the local structure and perhaps nanoscale phase separation.

Photodarkening effect in glassy As2S3 and As2O3

Photodarkening, or the shift of the optical absorption edge to smaller energies after excitation with light whose energy is near that of the optical band edge, has been studied in the chalcogenide glasses for many years. Recently we have conducted high field nuclear magnetic resonance (NMR) studies at 17 T and nuclear quadrupole resonance (NQR) studies of 75As in glassy As2S3. We have compared the 75As NMR and NQR lineshapes in glassy As2S3 and As2O3 before and after irradiation. An increase in the asymmetry parameter would have implied an increase in the departure from cylindrical symmetry in the bonding at the arsenic pyramidal sites. The null result reported here is inconsistent with microscopic models of photodarkening that involve switching of large numbers of As–Chalcogen bonds.

Directional behavior of nuclear quadrupole resonance in YBa2Cu3O6

Abstract Powder samples of YBa 2 Cu 3 O 6 were magnetically aligned and the anisotropies in the systems were studied by means of Cu(1) nuclear quadrupole resonance (NQR) in the absence of external magnetic fields. Our room temperature measurements of the NQR lineshapes and the spin–lattice and spin–spin relaxation times as a function of the aligning magnetic field indicate that full microscopic alignment can be achieved by using a magnetic field of about 4.7 T, for which doublet line patterns arising from a hyperfine splitting were observed.

Glassy slowing of stripe modulation in(La,Eu,Nd)2−x(Sr,Ba)xCuO4: A63Cuand139LaNQR study down to 350 mK

${}^{63}\mathrm{Cu}$ and ${}^{139}\mathrm{La}$ nuclear quadrupole resonance and Zeeman perturbed nuclear magnetic resonance experiments are performed on the striped phase of the high-temperature superconductors ${\mathrm{La}}_{2\ensuremath{-}x}{\mathrm{Ba}}_{x}{\mathrm{CuO}}_{4}$ and ${\mathrm{La}}_{2\ensuremath{-}x\ensuremath{-}y}(\mathrm{N}\mathrm{d},\mathrm{E}\mathrm{u}{)}_{y}{\mathrm{Sr}}_{x}{\mathrm{CuO}}_{4}.$ The first goal of the present study is to utilize the fact that ordered Cu magnetic moments exert a static hyperfine field on the ${}^{63}\mathrm{Cu}$ and ${}^{139}\mathrm{La}$ nuclei to deduce the charge density and ordered moment within the ${\mathrm{CuO}}_{2}$ planes. A hyperfine-broadened nuclear quadrupole resonance (NQR) line shape is observed in both ${\mathrm{La}}_{2\ensuremath{-}x}{\mathrm{Ba}}_{x}{\mathrm{CuO}}_{4}$ and ${\mathrm{La}}_{1.80\ensuremath{-}x}{\mathrm{Eu}}_{0.20}{\mathrm{Sr}}_{x}{\mathrm{CuO}}_{4}$ for $x\ensuremath{\approx}\frac{1}{8}.$ Detailed numerical analysis of the ${}^{63}\mathrm{Cu}$ NQR line shape establishes that widely accepted models of periodic sinusoidal or square-well-shaped modulations of spin density waves with maximum moment $\ensuremath{\sim}0.3 {\ensuremath{\mu}}_{B},$ as inferred from elastic neutron scattering and muon spin rotation $(\ensuremath{\mu}\mathrm{SR})$ measurements, cannot account for the NQR line shape unless we assume a relatively small ordered moment $\ensuremath{\sim}0.15 {\ensuremath{\mu}}_{B}$ with a comparably large distribution. The second goal of the present work is to establish the temperature dependence of the fluctuation frequency scale of stripes. We find that the fraction of missing ${}^{63}\mathrm{Cu}$ NQR intensity below charge ordering temperature ${T}_{\mathrm{charge}}$ accurately tracks the temperature dependence of the charge order parameter as measured by scattering methods. By fitting a single model to the temperature dependences of the wipeout fraction $F(T)$ for ${}^{63}\mathrm{Cu}$ and ${}^{139}\mathrm{La}$ NQR, the spin order parameter measured by elastic neutron scattering, and the $\ensuremath{\mu}\mathrm{SR}$ data, we deduce the spatial distribution of the spin fluctuation frequency scale $\ensuremath{\Gamma}$ and its temperature dependence. These results indicate that as soon as the charge dynamics slow down, the spin fluctuations begin to slow dramatically with spin stiffness $2\ensuremath{\pi}{\ensuremath{\rho}}_{s}^{\mathrm{eff}}\ensuremath{\sim}200 \mathrm{K}.$ By extending the analysis to other hole concentrations, we demonstrate a qualitative difference in the spatial variation of electronic states induced by slowing the charge dynamics above and below $x=\frac{1}{8}.$

Magnetic resonance investigations of phase transitions and modulation wave motions in incommensurate solids

We measured87Rb nuclear magnetic resonance (NMR) and35Cl nuclear quadrupole resonance (NQR) Hahn spin-echo magnetization decays in the incommensurate (I) phase of Rb2ZnCl4 and, in each case, obtained a Hahn echo decay that was shorter than the Carr-Purcell-Meiboom-Gill decay and one which decayed with a time constant proportional to the cube of the echo time. From these measurements we obtained from both the87Rb NMR and35Cl NQR measurements values for the diffusion coefficients that are comparable in magnitude, a fact that strongly supports the existence of slow modulation wave diffusionlike motions in the I phase, since such motions should affect both Rb and Cl ions similarly. In addition, we used87Rb two-dimensional exchange-difference NMR to study atomic motions in the incommensurate (I) and paraelectric (P) phases to elucidate the nature of the I-P transition. We measured as a function of the mixing time the frequency shifts of the cross peaks from the main diagonal and observed a gradual increase towards an asymptotic value in the I phase but a sudden jump to the final value in the P phase. We interpreted the motions observed in the P phase as normal modes arising from simultaneous reorientations of ZnCl4 tetrahedra and corresponding Rb ions displacements between two sites. These normal modes freeze out in the I phase and change to the diffusionlike motion of the modulation wave. We also performed35Cl NQR lineshape andT 1 measurements in K2ZnCl4 and obtained conclusive evidence for the presence of a narrow 1q (singly modulated) I phase between 146 and 149 K.

Cu NQR study of the stripe phase local structure in the lanthanum cuprates

Using Cu nuclear quadrupole resonance (NQR) in Eu-doped La2-xSrxCuO4 we find the evidence of the pinned stripe phase at 1.3 K for 0. 08</=x</=0.18. The pinned fraction increases by 1 order of magnitude near hole doping x = 1/8. The NQR line shape reveals three inequivalent Cu positions: (i) sites in the charged stripe, (ii) nonmagnetic sites outside the stripes, and (iii) sites with a magnetic moment of 0.29&mgr;(B) in the antiferromagnetically correlated regions. A dramatic change of the NQR signal for x>0.18 correlating with the onset of bulk superconductivity corresponds to the depinning of the stripe phase.

Nuclear quadrupole proton magnetic cross relaxation spectra analysis in a smectic A mesophase

In a previous paper, we reported an experimental study of 35Cl quadrupole dips in the proton spin–lattice relaxation dispersion (T1(ν)) of a chlorinated liquid crystal specially synthesized for this work (4ClOB). In contrast with existing literature, our dip shapes were not symmetric. We could successfully fit the experimental data with a model previously used to fit nuclear quadrupole resonance line shapes in solids, based on the existence of a distortion field function (DFF), describing the local order properties of the condensed phase. However, the model allowed us to fit data without defining explicitly such DFF, while the obtained numerical coefficients lacked physical meaning. In the present work, the model is applied to a liquid crystal by defining an orientational DFF for coefficient evaluation. The analysis allowed us to obtain important information about the studied molecule structure and its dynamics in the mesophase. Results are in good agreement with the semiempirical quantum simulations of free molecule structure.

Comparison of 35Cl NQR Spectra between the Mixed Crystals K2Sn1-xRexCl6 and the Al3+ Doped Crystals K2SnCl6:Al3+

Abstract 35Cl Nuclear Quadrupole Resonance (NQR) has provided a valuable tool for investigating the local structure in mixed crystals K2S1-xRexCl6 and Al3+ doped crystals K2SnCl6:Al3+ . The measured NQR line shapes and relaxation times of both kinds of impurity containing crystals in the cubic phase of the host K2SnCl6 show markedly different impurity effects; static impurity effects in K2Sn1-x RexCl6 and dynamic effects in K2SnCl6 :Al3+ . The 35Cl NQR spectra of K2SnCl6:Al3+ near the transition temperature (Tc) indicate the presence of pretransition of the local structure in the high temperature cubic phase.

Nuclear quadrupole resonance line shape study of the orientationally disordered phase of p-chloro nitrobenzene

An experimental study of the nuclear quadrupole resonance (NQR) line shape of p-chloro nitrobenzene in the temperature range 77–150 K is reported. The molecules of p-ClC6H4NO2 in the solid phase are arranged following an orientationally disordered pattern. The experimental NQR spectra at several temperatures are unusually broad for a molecular crystal and show a structure of peaks. Our measurements suggested that the line broadening is due to a static distribution of disordered molecules. In order to explain the nontypical resonance frequency dispersion, a simulation of the NQR line shapes is carried on. The model for the simulated crystal assumes that each molecule is an electric dipole and that the crystal is a disordered binary alloy of dipoles having two possible orientations. By calculating the intermolecular contribution to the electric field gradient (EFG) at the resonant nuclei sites, we get the expected EFG distribution in the sample. The approach we propose here explicitly involves the symmetry of the lattice.

Structural properties of As 2Se 3 fibers

Nuclear quadrupole resonance (NQR) studies of rapidly-drawn fibers of glassy As 2Se 3 reveal that significant structural changes occur as a function of draw rate. Pulsed 75As NQR echo measurements have been performed on freshly prepared and aged samples of glassy As 2Se 3 fibers at three different draw rates (5, 41 and 75 m/min). The 75As NQR lineshapes exhibit systematic trends in both the linewidths and line asymmetrics. Fibers drawn at the most rapid rate exhibit a second resolved peak in the NQR lineshape that is attributed to the presence of AsAs bonds even though the glass is nominally stoichiometric. Presumably the AsAs ‘wrong bonds’ are compensated by the presence of SeSe bonds. Annealing the fibers for several hours near 200°C produces changes in the NQR lineshapes but does not result in NQR lineshapes that are indicative of the bulk glass. These results are consistent with measurements of structural relaxation as obtained from differential scanning calorimetry in glassy As 2Se 3 fibers.

Nuclear-quadrupole-resonance studies of As2Se3 fibers.

Pulsed nuclear-quadrupole-resonance (NQR) experiments have been performed at 77 K on rapidly drawn fibers of ${\mathrm{As}}_{2}$${\mathrm{Se}}_{3}$. The NQR line shape at 77 K was compared to the line shape of bulk glassy ${\mathrm{As}}_{2}$${\mathrm{Se}}_{3}$ and to thin-film ${\mathrm{As}}_{2}$${\mathrm{Se}}_{3}$. The fiber was found to have a line shape that is intermediate between the glassy and thin-film line shapes. The line shape of the fiber was also compared to the line shapes of glassy ${\mathrm{As}}_{0.5}$${\mathrm{Se}}_{0.5}$ at 77 K. The absence of a peak near 78 MHz in the fiber spectrum indicates that there are no As-As bonds in this sample.

Influence of magnetic fields on NQR line shape parameters in oriented YBa 2Cu 3O 7−δ powder samples

63Cu nuclear quadrupole resonance (NQR) experiments on the Cu(1) site of oriented YBa2Cu3O7−δ powder samples are performed at 4.2 K. The application of magnetic fields during the cooling procedure or after zero-field cooling influences the NQR lineshape parameters, i.e. linewidth, intensity, and amplitude. The NQR measurements for different magnetic states of the samples are presented together with corresponding measurements of the remanent magnetic field.

Nuclear quadrupole resonance of 35Cl nuclei in glassy solution of chlorobenzene in pyridine.

Pulsed nuclear quadrupole resonance (NQR) measurements of $^{35}\mathrm{Cl}$ nuclei were performed on 43.5 mol % chlorobenzene solution in pyridine (a) in glassy state (${\mathit{T}}_{\mathit{g}}$=131 K) after quenching in liquid nitrogen and (b) in crystalline precipitate under slow cooling starting from liquid phase. In both cases the NQR line shape, obtained by the fast Fourier transform (FFT) method, consisted of a structure of two peaks, interpreted as contributions of two nonequivalent $^{35}\mathrm{nucleus}$ sites. A theoretical model is proposed for the line-shape analysis and the best-fit parameters yield excellent agreement with experimental spectra. It is suggested that the nonequivalent sites consist of nearly parallel and antiparallel alignment of chlorobenzene and pyridine molecules and that the line-broadening mechanism is due to a random distribution of angular displacements from these positions. Our data are in agreement with Goldstein studies of molecular-relaxation processes related to the changes observed on physical properties of amorphous materials at ${\mathit{T}}_{\mathit{g}}$.

Nuclear-quadrupole-resonance line shapes for spin 1 and the incommensurate structure of the 20r polytype of K3Co(CN)6 at low temperature.

The nuclear quadrupole resonance (NQR) line shape for nuclei with spin I=1 in incommensurate systems is obtained explicitly in terms of the changes produced in the electric field gradient by the displacement wave. We found that generally the ${\ensuremath{\nu}}^{+}$ lines have a different shape from that of the ${\ensuremath{\nu}}^{\mathrm{\ensuremath{-}}}$ lines in the incommensurate phase and that one obtains direct information about the changes introduced in the electric-field-gradient tensor from the NQR line shape in the plane-wave limit. A line-shape study of the $^{14}\mathrm{N}$ spectrum observed in the low-temperature phase of the 20r polytype of ${\mathrm{K}}_{3}$Co(CN${)}_{6}$ showed incommensuration in only one N site. The measurement of the temperature dependence of the complete spectrum of the 20r polytype from room temperature to 77 K showed that the transition was first order and could be interpreted as resulting from the softening of a translational mode that involves the complex ion.

Cubic to tetragonal structural phase transition in K2OsCl6as studied by chlorine nuclear quadrupole resonance: tetragonal phase lineshapes

Chlorine nuclear quadrupole resonance lineshapes are reported for the antifluorite K2OsCl6 for a series of temperatures below the temperature of the cubic to tetragonal structural phase transition. The data serve to clarify the identification of the transition temperature Tc. A detailed analysis of the magnitude spectra shows that: (i) the data are consistent with the occurrence of a commensurate structure phase transition at Tc; (ii) additional strains are introduced into the crystal as a result of the structural alteration.

Chlorine nuclear quadrupole resonance lineshape in the cubic antifluorite K2OsCl6

Chlorine nuclear quadrupole resonance lineshapes are reported for the cubic antifluorite K2OsCl6 in the temperature range 70–300 K. For temperatures above 172 K the spectrum consists of a single symmetric line; for temperatures below 172 K an asymmetric line is observed which can be represented as a sum of two symmetric lines. The symmetric lines are temperature dependent mixtures of Lorentzian and Gaussian profiles. The explanation is that the nuclear quadrupole resonance lines observed in K2OSCl6 are inhomogeneously broadened by temperature dependent local strains caused by point defects and dislocations.