Nuclear Quadrupole Resonance Experiments Research Articles

Ferroelastic phase transition inCs3Bi2I9: A neutron diffraction study

Crystalline ${\mathrm{Cs}}_{3}{\mathrm{Bi}}_{2}{\mathrm{I}}_{9},$ with the ${P6}_{3}/mmc$ symmetry at room temperature was studied by Laue and four-circle neutron diffractometry from room temperature down to 50 K. At ${T}_{0}=220 \mathrm{K}$ the crystal undergoes a second-order proper ferroelastic phase transition to a polydomain structure with a nonprimitive monoclinic $C12/m1$ space group. Satellites were not found below ${T}_{0},$ indicating that the continuous distribution of sites observed in previous ${}^{127}\mathrm{I}$ nuclear quadrupole resonance experiments is due either to an undetected incommensurate phase characterized by a very small displacement amplitude, or due to the fraction of the crystal volume occupied by domain walls. We argue that thick domain walls are expected in the present structure due to the absence of mechanical compatibility between domains.

NMR evidence for Kondo disorder inUCu3.5Pd1.5

We report on Cu nuclear quadrupole resonance (NQR) and nuclear magnetic resonance experiments probing the cubic non-Fermi-liquid compound ${\mathrm{UCu}}_{3.5}{\mathrm{Pd}}_{1.5}.$ The NQR spectrum indicates a partial disorder in the occupation of Cu and Pd sites, with a preferential occupation of the cubic c site by Pd. Our results for the temperature dependence of the nuclear magnetization recovery show unusual features that cannot be reconciled with common expectations for a simple metal, but they are well accounted for in a description using the Kondo disorder model.

Role of bromine in restoring superconductivity inYBa2Cu3Oy

${}^{63,65}\mathrm{Cu}$ nuclear quadrupole resonance (NQR), ${}^{63,65}\mathrm{Cu}$ and ${}^{79,81}\mathrm{Br}$ nuclear magnetic resonance (NMR), and Br K-edge x-ray-absorption fine-structure (XAFS) measurement techniques have been used to study the local structures of Cu and Br in well-characterized samples of deoxygenated and brominated ${\mathrm{YBa}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{y}$ (YBCO). The combined results provide a detailed picture of the role of bromine in reoxygenating the YBCO structure and an explanation for the partial restoration of superconductivity in the YBCO system. Characterization of the powder samples, with particle sizes of 1, 20, 30, and 40 \ensuremath{\mu}m, included x-ray diffraction, thermal gravimetric analysis, energy dispersive x-ray analysis, and magnetometry. From the XAFS and NMR results, it is concluded that, upon bromination at 260 \ifmmode^\circ\else\textdegree\fi{}C, Br does not enter the YBCO lattice either substitutionally or interstitially. Instead, there is compelling evidence for the formation of nanoscale ${\mathrm{BaBr}}_{2}$ precipitates which result from the local destruction of the ${\mathrm{YBa}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{y}$ phase. Furthermore, on the basis of the NQR and NMR experiments, it is concluded that through this decomposition into an inhomogeneous material, oxygen is liberated which repopulates the nearby O(4) chain sites, thus restoring superconductivity.

Metastable, drawing-induced crystallization in As 2Se 3 fibers

Pulsed 75 As nuclear quadrupole resonance (NQR) experiments performed on rapidly-drawn (>100 m/min) fibers of As 2Se 3 reveal structure which differs from the bulk, well-annealed glass. In addition to the presence of As–As bonds in the nominally stoichiometric fibers, a distorted `crystalline' phase occurs at the greatest draw rates. Like the Ge–Se–S system, this phase probably involves more ordered local molecular clusters whose sizes are not yet well-determined. Because these crystalline regions are highly distorted compared to bulk, crystalline As 2Se 3, they are not observable by X-ray spectroscopy except in a few cases. On the other hand, the NQR experiments show the presence of crystalline regions. In all cases, the crystalline phase makes up less than 5% of the volume. In addition to being strained, the imbedded crystallites partially `anneal' to the amorphous phase over time when stored at 300 K. This annealing behavior is similar to that which exists in the Ge–Se–S system, but different from the `photo-induced amorphization' recently reported for evaporated AsSe films.

Nuclear quadrupole resonance spectra of drawing-induced crystallization in As 2Se 3 fibers

Pulsed 75As nuclear quadrupole resonance (NQR) experiments performed on fibers of As 2Se 3 drawn at various rates (> 100 m/min) reveal structural changes from the bulk, well-annealed glass. In addition to the presence of AsAs bonds in the nominally stoichiometric fibers, a distorted crystalline phase (< 10% by volume) occurs at the largest draw rates. Because they are highly distorted, these crystallites are not observable by X-ray diffraction spectroscopy in almost all cases. On the other hand, the NQR experiments show these crystalline regions very clearly as small width components in the NQR spectra between 56 and 60 MHz. In addition to being strained, the imbedded crystallites are dominated by surface effects because they partially revert to the amorphous phase over time when stored at 300 K. The nature of the distortions in these crystallites and the kinetics of this counter-intuitive annealing behavior are not well determined. Because large draw rates are important in commercial production of fibers, the drawing-induced crystallization in this prototypical chalcogenide glass may have implications for the use of these materials in infrared transmission applications.

A low-frequency instrument for remote nuclear quadrupole resonance experiments

A special pulsed instrument has been constructed for short-range remote nuclear quadrupole resonance experiments. The instrument was designed for detection of nitrogen-14 chemical compounds (in the low-frequency band 0.5 - 10 MHz). High sensitivity has been achieved by using a special construction of main sub-units and optimum experimental techniques. A description of the construction and performance of this instrument is given. As an example the results of the short-range remote detection of RDX are presented.

One-dimensional nutation nuclear quadrupole resonance spectroscopy for measurement of the electric field gradient tensor-asymmetry parameter

A novel one-dimensional nuclear quadrupole resonance experiment to measure the asymmetry parameter of the electric field gradient tensor in polycrystalline specimens using nutation spectroscopy is reported. The complete data set, necessary for the reconstruction of the powder nutation lineshape, is recorded in a single scan provided by a train of short and intense on-resonance radiofrequency pulses. A modified scheme is presented to deal with multiple well-resolved resonances to provide a frequency selective nutation method. The technique is successfully demonstrated for the two 35Cl resonances in cyanuric chloride. A reduction of 1 or 2 orders of magnitude in the data acquisition time, relative to the usual two-dimensional method, is achieved.

Magnetic-field fluctuations and the NQR relaxation rate in high-temperature superconductors

In high-temperature superconductors, fluctuating vortices are present even in the absence of an external magnetic field. These vortices lead to the relaxation of nuclear magnetic moments. Nuclear quadrupole resonance (NQR) experiments may thus yield information about the density and dynamics of vortices. To determine their contribution, the longitudinal and transverse NQR relaxation rates are expressed in terms of the magnetic-field correlation function for vanishing external field. The concept of a spin temperature is not applicable in this case. Instead, we employ the Bloch-Wangsness-Redfield theory to obtain the rates. The expressions for the rates are significantly different from the case of NMR (large external field). The correlation function is determined for bound vortex pairs subject to diffusive dynamics.

Two-dimensional on- and off-resonance nutation FFT/MEM NQR spectroscopy

The new two-dimensional nuclear quadrupole resonance experiments based on the principle of nutation spectroscopy, which can be used to determine the asymmetry parameter, and thus the full quadrupolar tensor of spin-3/2 nuclei at zero applied magnetic field are discussed. The problems of reconstructing 2D-nutation NQR spectra using conventional methods and the advantages of using implementation of the maximum entropy method (MEM) are analyzed. Use of the MEM in 2D-NQR spectroscopy can lead to sensitivity improvement, reduction of instrumental artifacts and truncation errors, shortened data acquisition times and automatic suppression of noise, while at the same time increasing the resolution. The possibilities of off-resonance irradiation in nutation experiments are demonstrated experimentally and theoretically. It is shown that the off-resonance nutation spectroscopy is a useful extension of the conventional on-resonance experiments thus facilitating the determination of asymmetry parameters in multiple spectrum. The methods have been successfully demonstrated for the35Cl on-and off-resonance 2D-nutation spectra in polycrystalline 2,4,6-trichloro-1,3,5-triazine.

Asymmetry parameter studies for systems containing the 35Cl nucleus from Zeeman NQR data

A review of Zeeman nuclear quadrupole resonance (NQR) work on systems containing the 35Cl nucleus is presented. In the case of the 35cl nucleus with spin I = 3 2 , owing to the existence of ± m degeneracy in the absence of a magnetic field (Kramer's degeneracy), there exists only one pure NQR frequency which is related to the two electric field gradient (EFG) parameters, η and e 2 qQ, by the formula, v = ( e 2qQ 2h )(1 + ( η 2 3 )) 1 2 . Hence it is necessary to study the Zeeman effect to obtain both these parameters. Zeeman NQR studies have been carried out by many workers using single crystals and powder specimens, and these are discussed here. The computer simulation method for powder η values is also presented in this paper. The numerical computation program for an IBM 370-158 computer used to simulate the powder Zeeman spectrum is also discussed briefly. The paper also presents two-dimensional Zeeman NQR and Zeeman perturbed spin-echo envelope modulation (ZSEEM) studies on 35Cl systems. Very recently, two-dimensional NQR experiments based on the principle of nutation spectroscopy have been carried out with zero applied magnetic field on 3 2 spin nuclei. This method is also discussed in this review. The relationship between the asymmetry parameter (η) and bond properties is also discussed.

209Bi NMR and NQR investigation of the small-gap semiconductor Ce3Bi4Pt3.

209Bi NMR and NQR investigation of the small-gap semiconductor Ce3Bi4Pt3.

A broadband nuclear magnetic resonance spectrometer: Digital phase shifting and flexible pulse programmer

A simple, low-cost pulse nuclear magnetic resonance (NMR) spectrometer is described. There are two key features of the instrument: (1) all radio frequency phase shifting is done digitally with a new-generation frequency synthesizer and (2) the pulse programmer is based on a high-speed 32-bit parallel interface card in a desktop computer. The spectrometer is well suited for single-frequency channel experiments such as nuclear quadrupole resonance and deuterium quadrupole echo NMR experiments. Because the rf phase shifting is done digitally, the spectrometer operates easily over a wide frequency range, here 10–300 MHz. The pulse programmer and other software functions are written in LabView, a graphical programming language.

Bromine Resonance in PrBr 3 as an Example of Mixed Spin Echoes in Pure Nuclear Quadrupole Resonance

The density-operator formalism is used to obtained an expression for the nuclear spin response to a general two-pulse echo sequence in a pure nuclear quadrupole resonance experiment. The derivation is for a spin- 3 2 system experiencing a non-axial electric field gradient and subjected to electric quadrupolar and magnetic dipole-dipole couplings. Explicit expressions for the spin response are given for four commonly used sequences in terms of the van Vleck moments. The analysis of bromine NQR data for PrBr 3 provides a demonstration that quadrupolar, homonuclear dipolar and heteronuclear dipolar contributions to the van Vleck moments can be separately determined by applying appropriate spin-echo sequences.

NMR evidence for metallic-to-antiferromagnetic phase separation in cuprate superconductors

Phase separation into metallic and antiferromagnetic regions in cuprate superconductors was proposed from theoretical models recently. In this contribution we want to provide some experimental evidence for phase separation from nuclear magnetic resonance (NMR), nuclear quadrupole resonance (NQR) and microwave absorption (mw) experiments. We distinguish "chemical" and "electronic" phase separation. Furthermore a distinction between "static" and "dynamic" phase separation is made.

Nqr experiments on Sc-Y alloys at very low temperatures

We have performed Nuclear Quadrupole Resonance (NQR) experiments on an alloy of scandium and yttrium. We find that the characteristic resonance frequencies in zero external field are 50 percent larger than in pure scandium. This increase in frequency makes the Sc-Y alloy a better candidate for absolute thermometry below 500 µK. However, the spin-lattice relaxation time of this alloy is more than an order of magnitude longer than in the pure scandium. In addition, we have observed an abrupt increase in the Korringa constant for temperatures below 5mK.

Erratum: Coexisting static magnetic order and superconductivity in CeCu2Si2 found by nuclear quadrupole resonance

$^{63}\mathrm{Cu}$ nuclear quadrupole resonance (NQR) has been observed in a heavy-fermion system ${\mathrm{CeCu}}_{2}$${\mathrm{Si}}_{2}$. In the superconducting ${\mathrm{CeCu}}_{2}$${\mathrm{Si}}_{2}$ specimens (${\mathit{T}}_{\mathit{c}}$\ensuremath{\simeq}0.6 K), NQR experiments have revealed an onset of a phase transition below \ensuremath{\sim}1 K. Zero- and longitudinal-field muon-spin-relaxation measurements suggest a static magnetic ordering below \ensuremath{\sim}0.8 K, either in the spin-glass or in incommensurate spin-density-wave states. The exotic transition observed in NQR measurements should be closely related to the static magnetic ordering in muon-spin-relaxation measurements. NQR data from a nonsuperconducting ${\mathrm{CeCu}}_{2}$${\mathrm{Si}}_{2}$ sample are consistent with extensive disorder in Cu site occupation. Therefore, the spin-density wave may be depressed in the nonsuperconducting ${\mathrm{CeCu}}_{2}$${\mathrm{Si}}_{2}$. The superconducting transition temperature ${\mathit{T}}_{\mathit{c}}$ can be enhanced by the presence of spin-density waves such that, if the spin-density wave is depressed, ${\mathrm{CeCu}}_{2}$${\mathrm{Si}}_{2}$ will lose its superconducting properties.

Coexisting static magnetic order and superconductivity in CeCu2Si2 found by nuclear quadrupole resonance.

$^{63}\mathrm{Cu}$ nuclear quadrupole resonance (NQR) has been observed in a heavy-fermion system ${\mathrm{CeCu}}_{2}$${\mathrm{Si}}_{2}$. In the superconducting ${\mathrm{CeCu}}_{2}$${\mathrm{Si}}_{2}$ specimens (${\mathit{T}}_{\mathit{c}}$\ensuremath{\simeq}0.6 K), NQR experiments have revealed an onset of a phase transition below \ensuremath{\sim}1 K. Zero- and longitudinal-field muon-spin-relaxation measurements suggest a static magnetic ordering below \ensuremath{\sim}0.8 K, either in the spin-glass or in incommensurate spin-density-wave states. The exotic transition observed in NQR measurements should be closely related to the static magnetic ordering in muon-spin-relaxation measurements. NQR data from a nonsuperconducting ${\mathrm{CeCu}}_{2}$${\mathrm{Si}}_{2}$ sample are consistent with extensive disorder in Cu site occupation. Therefore, the spin-density wave may be depressed in the nonsuperconducting ${\mathrm{CeCu}}_{2}$${\mathrm{Si}}_{2}$. The superconducting transition temperature ${\mathit{T}}_{\mathit{c}}$ can be enhanced by the presence of spin-density waves such that, if the spin-density wave is depressed, ${\mathrm{CeCu}}_{2}$${\mathrm{Si}}_{2}$ will lose its superconducting properties.

The random phase approximation and crystal field effects in magnetism

A novel solution to the problem of incorporating a strong axial crystal field interaction into the RPA model of a Heisenberg ferromagnet is presented and discussed. The method is based on technique widely used in the interpretation of nuclear quadrupole resonance (NQR) experiments, where the Hamiltonian is first transformed into the interaction picture and terms which oscillate rapidly in time are subsequently dropped. It is shown that when this method is applied to an S=1 Heisenberg ferromagnet with an axially symmetric quadrupole interaction, unique solutions can be obtained for the ensemble averages (Szn). The results are compared with those of earlier workers and it is shown that (i) in the limit T to 0 K and D to 0 the usual spin wave result is obtained, (ii) both excitation branches exhibit dispersion at finite temperatures, and (iii) a unique solution can be obtained for the Curie temperature Tc in the presence of crystal fields, in contrast to previous work.

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.

Two-dimensional Nutation Echo Nuclear Quadrupole Resonance Spectroscopy

Abstract We discuss two new two-dimensional nuclear quadrupole resonance experiments, both based on the principle of nutation spectroscopy, which can be used to determine the asymmetry parameter, and thus the full quadrupolar tensor, of spin-3/2 nuclei at zero applied magnetic field. The first experiment is a simple nutation pulse sequence in which the first time period (t1) is the duration of the radiofrequency exciting pulse; and the second (t2) is the normal free-precession of a quadrupolar nucleus at zero-field. After double Fourier-transformation, the result is a 2 D spectrum in which the first frequency dimension is the nutation spectrum for the quadrupolar nucleus at zero-field. For polycrystalline samples this sequence generates powder lineshapes; the position of the singularities, in these lineshapes can be used to determine the asymmetry parameters η in a very straightforward manner, η has previously only been obtainable using Zeeman perturbed NQR methods. The second sequence is the same nutation experiment with a spin-echo pulse added. The virtue of this refocussing pulse is that it allows acquisition of nutation spectra from samples with arbitrary inhomogeneous linewidth; thus, asymmetry parameters can be determined even where the quadrupolar resonance is wider than the bandwidth of the spectrometer. Experimental examples of 35Cl, 81Br and 63Cu nutation and nutation-echo spectra are presented.