Showing papers on "Nuclear quadrupole resonance published in 2014"

Optically Polarized Atoms: Understanding light-atom interactions

Abstract: I: INTRODUCTION TO LIGHT-ATOM INTERACTIONS 1. Introduction 2. Atomic States 3. A bit of angular-momentum theory 4. Atoms in external electric and magnetic fields 5. Polarized atoms 6. Polarized light 7. Atomic transitions 8. Coherence in atomic systems 9. Optical pumping 10. Light-atom interaction observed in transmitted light II: ADVANCED TOPICS 11. Nonlinear magneto-optical rotation 12. Perturbative analysis of light-atom interactions 13. Polarization effects in transitions with partially resolved hyperfine structure 14. The effect of hyperfine splitting on nonlinear magneto-optical rotation 15. Spectral properties of excitation light ground-state coherence effects revisited 16. Collapse and revival in quantum beats 17. Nuclear quadrupole resonance and alignment-to-orientation conversion 18. Selective addressing of high-rank polarization moments 19. Tensor structure of the DC- and AC-Stark polarizabilities 20. Photoionization of polarized atoms with polarized light Appendix A Constants, units and notations Appendix B Units of energy, frequency and wavelength Appendix C Reference data for hydrogen and the alkali atoms Appendix D Nonlinear magneto-optical rotation with hyperfine structure Appendix E The Atomic Density Matrix software package

Oxidation and magnetic states of chalcopyrite CuFeS2: a first principles calculation

Abstract: The ground state band structure, magnetic moments, charges and population numbers of electronic shells of Cu and Fe atoms have been calculated for chalcopyrite CuFeS2 using density functional theory. The comparison between our calculation results and experimental data (X-ray photoemission, X-ray absorption and neutron diffraction spectroscopy) has been made. Our calculations predict a formal oxidation state for chalcopyrite as Cu1+Fe3+S22−. However, the assignment of formal valence state to transition metal atoms appears to be oversimplified. It is anticipated that the valence state can be confirmed experimentally by nuclear magnetic and nuclear quadrupole resonance and Mossbauer spectroscopy methods.

Spin-singlet superconductivity with a full gap in locally noncentrosymmetric SrPtAs

Abstract: We report Pt-195-NMR and As-75 nuclear quadrupole resonance measurements for the locally noncentrosymmetric superconductor SrPtAs where the As-Pt layer breaks inversion symmetry while globally the compound is centrosymmetric. The nuclear spin-lattice relaxation rate 1/T-1 shows a well-defined coherence peak below T-c and decreases exponentially at low temperatures. The spin susceptibility measured by the Knight shift also decreases below Tc down to T < T-c/6. These data, together with the penetration depth obtained from the NMR spectra, can be consistently explained by a spin-singlet superconducting state with a full gap. Our results suggest that the spin-orbit coupling due to the local inversion-symmetry breaking is not large enough to bring about an exotic superconducting state, or the interlayer hopping interaction is larger than the spin-orbit coupling.

Suppression of electron correlations in the collapsed tetragonal phase of CaFe2As2 under ambient pressure demonstrated by As75 NMR/NQR measurements

Abstract: The static and the dynamic spin correlations in the low-temperature collapsed tetragonal and the high-temperature tetragonal phase in $\mathrm{Ca}{\mathrm{Fe}}_{2}{\mathrm{As}}_{2}$ have been investigated by $^{75}\mathrm{As}$ nuclear magnetic resonance (NMR) and nuclear quadrupole resonance (NQR) measurements. Through the temperature ($T$) dependence of the nuclear spin lattice relaxation rates (1$/{T}_{1}$) and the Knight shifts, although stripe-type antiferromagnetic (AFM) spin correlations are realized in the high-temperature tetragonal phase, no trace of the AFM spin correlations can be found in the nonsuperconducting, low-temperature, collapsed tetragonal (c$\mathcal{T}$) phase. Given that there is no magnetic broadening in $^{75}\mathrm{As}$ NMR spectra, together with the $T$-independent behavior of magnetic susceptibility $\ensuremath{\chi}$ and the $T$ dependence of 1$/{T}_{1}T\ensuremath{\chi}$, we conclude that Fe spin correlations are completely quenched statically and dynamically in the nonsuperconducting c$\mathcal{T}$ phase in $\mathrm{Ca}{\mathrm{Fe}}_{2}{\mathrm{As}}_{2}$.

Multigap superconductivity in locally noncentrosymmetric SrPtAs: An As 75 nuclear quadrupole resonance investigation

Abstract: We report detailed $^{75}\mathrm{As}$ nuclear quadrupole resonance investigations of the locally noncentrosymmetric superconductor SrPtAs. The spin-lattice relaxation studies prove weakly coupled multigap superconductivity. A retardation of the decay in $1/{T}_{1}T$ evidences a nodeless (fully gapped) superconducting state on the complex multipocket Fermi surface, which is consistent with an anisotropic $s$-wave order parameter and with proposed unconventional $f$-wave and chiral $d$-wave symmetries. A quantitative analysis of these models favors the unconventional $f$-wave state.

An Overview of NQR Signal Detection Algorithms

Abstract: Nuclear quadrupole resonance (NQR) is a solid-state radio frequency spectroscopic technique that can be used to detect the presence of quadrupolar nuclei, that are prevalent in many narcotics, drugs, and explosive materials. Similar to other modern spectroscopic techniques, such as nuclear magnetic resonance, and Raman spectroscopy, NQR also relies heavily on statistical signal processing systems for decision making and information extraction. This chapter provides an overview of the current state-of-the-art algorithms for detection, estimation, and classification of NQR signals. More specifically, the problem of NQR-based detection of illicit materials is considered in detail. Several single- and multi-sensor algorithms are reviewed that possess many features of practical importance, including (a) robustness to uncertainties in the assumed spectral amplitudes, (b) exploitation of the polymorphous nature of relevant compounds to improve detection, (c) ability to quantify mixtures, and (d) efficient estimation and cancellation of background noise and radio frequency interference.

A Low-Power Compact NQR Based Explosive Detection System

Abstract: Nuclear quadruple resonance (NQR) has been studied for explosive detection for years. Some prototype NQR-based detection systems have been reported, but were all designed from the laboratory perspective, with bulky sizes and high power consumption. To achieve a portable NQR-based detection system for field applications, a low-power compact system utilizing state-of-the-art semiconductor technologies is presented in this paper. Several novel circuits for the key modules are proposed, including a Class-D type power amplifier, a power multiplexing and matching network, a customized IC with integrated analog signal processing chain, and a microcontroller-based logic control unit. An advanced digital signal processor platform is employed for data collecting and processing. The customized IC is fabricated in 0.18 μm CMOS process. Test results on the prototype system show the effectiveness of the proposed solution and low power consumption as 2.12 W in the receiving state. The power efficiency of the proposed transmitting section is higher than 60%.

Tautomeric transformations and reactivity of isoindole and sila-indole: A computational study

Abstract: In this work, the tautomeric transformations and reactivity of isoindole and sila-isoindole molecules has been explored using the B3LYP/6-311G(d,p) level of theory in gas and solution phases. These calculations show that isoindole isomer has more stability rather than 1-h-isoindole. There is identical trend in silated species. The frontier molecular orbitals (FMO) and band gap energy calculations were performed at the B3LYP/6-311G(d,p) level in gas and various solvent. Solvent effects have been analyzed by using the self-consistent reaction field (SCRF) method based on polarizable continuum model (PCM) in chloroform, chlorobenzene, dichloromethane and tetrahydrofurane. Thermodynamic parameters calculated at room temperature have been analyzed. Also, electron affinities were computed. Local reactivity descriptors as Fukui functions local softness and electrophilicity indices analyses are performed to find out the reactive sites within molecule. Density functional theory (DFT) calculations were performed to compute nitrogen-14 nuclear quadrupole resonance (NQR) spectroscopy parameters.

Mechanism of metallization and superconductivity suppression in YBa$_2($Cu$_{0.97}$Zn$_{0.03})_3$O$_{6.92}$ revealed by $^{67}$Zn NQR

Abstract: We measure the nuclear quadrupole resonance (NQR) signal on the Zn site in nearly optimally doped YBa$_2$Cu$_3$O$_{6.92}$, when Cu is substituted by 3\% of isotopically pure $^{67}$Zn. We observe that Zn creates large insulating islands, confirming two earlier conjectures: that doping provokes an orbital transition in the CuO$_2$ plane, which is locally reversed by Zn substitution, and that the islands are antiferromagnetic. Also, we find that the Zn impurity locally induces a breaking of the D$_4$ symmetry. Cluster and DFT calculations show that the D$_4$ symmetry breaking is due to the same partial lifting of degeneracy of the nearest-neighbor oxygen sites as in the LTT transition in La$_{2-x}$Ba$_x$CuO$_4$, similarly well-known to strongly suppress superconductivity. These results show that in-plane oxygen $2p^5$ orbital configurations are principally involved in the metallicity and superconductivity of all high-T$_c$ cuprates, and provide a qualitative symmetry-based constraint on the SC mechanism.

Rapid detection of arsenic minerals using portable broadband NQR

Abstract: The remote real-time detection of specific arsenic species would significantly benefit in minerals processing to mitigate the release of arsenic into aquatic environments and aid in selective mining. At present, there are no technologies available to detect arsenic minerals in bulk volumes outside of laboratories. Here we report on the first room-temperature broadband 75As nuclear quadrupole resonance (NQR) detection of common and abundant arsenic ores in the Earth crust using a large sample (0.78 L) volume prototype sensor. Broadband excitation aids in detection of natural minerals with low crystallinity. We briefly discuss how the proposed NQR detector could be employed in mining operations.

The T 1 and T 2 Relaxation Times Distribution for the 35 Cl and 14 N NQR in Micro-composites and in Porous Materials

Abstract: The paper describes the results of an experimental study of the 35Cl and 14N nuclear quadrupole resonance in composite and porous materials, the influence of the environment and the crystallite size of powder on the nuclear quadrupole resonance line widths, as well as on the spin–spin and spin–lattice relaxation times. It is established that the spin–lattice relaxation time has a unimodal distribution, and the spin–spin relaxation time—bimodal distributions for all the investigated samples. The idealized distribution function of the relaxation times is obtained on the assumption that the concentration of inhomogeneities and relaxativity decreases with an increasing distance from the surface into the interior of the crystallite exponentially. It is shown that the model with the spin diffusion explains the shortening of the decay signal with decreasing grain size, but this is not confirmed by the experimental distribution of relaxation times obtained by means of the inverse Laplace transformation.

Weak itinerant antiferromagnetism in PuIn3 explored using 115In nuclear quadrupole resonance.

Abstract: The results of 115In nuclear quadrupole resonance (NQR) measurements on PuIn3 are reported. Three of the four NQR lines of 115In expected for nuclear spin I = 9/2 are observed. The equal spacing of these lines at 20 K yields the NQR frequency of νQ = 10.45 MHz, and the asymmetry parameter of the electric field gradient η = 0. The NQR line profile and the nuclear spin–lattice relaxation rate 1/T1 display an abrupt change at 14 K, which is associated with the onset of long-range antiferromagnetic order. The temperature dependences of the staggered magnetization MQ(T), extracted from the NQR spectra, and 1/T1 below TN = 14 K are well explained by the self-consistent renormalization (SCR) theory for spin fluctuations. In addition, the scaling between T1T and MQ(T)/MQ(0) is also consistent with the predictions of SCR theory, providing further evidence that PuIn3 is a weak itinerant antiferromagnet in which spin fluctuations around the antiferromagnetic wavevector play a major role in the system's behavior at finite temperatures.

Microscopic properties of the heavy-fermion superconductor PuCoIn 5 explored by nuclear quadrupole resonance

Abstract: We report 115In nuclear quadrupolar resonance (NQR) measurements on the heavy-fermion superconductor PuCoIn5, in the temperature range 0.29K ? T ? 75K. The NQR parameters for the two crystallographically inequivalent In sites are determined, and their temperature dependence is investigated. A linear shift of the quadrupolar frequency with lowering temperature below the critical value Tc is revealed, in agreement with the prediction for composite pairing. The nuclear spin?lattice relaxation rate T1?1(T) clearly signals a superconducting (SC) phase transition at Tc ? 2.3 K, with strong spin fluctuations, mostly in-plane, dominating the relaxation process in the normal state near to Tc. Analysis of the T1?1 data in the SC state suggests that PuCoIn5 is a strong-coupling d-wave superconductor.

High precision measurement of the 11Li and 9Li quadrupole moment ratio using zero-field β-NQR

Abstract: The ratio of electric quadrupole moments of 11 Li and 9 Li was measured using the zero-field β-detected nuclear quadrupole resonance technique at TRIUMFISAC. The precision on the ratio Q11/Q9 = 1.0775(12) was improved by more than one order of magnitude and an absolute value for the quadrupole moment of 11 Li was inferred. Systematic effects, as argued here, are not expected to contribute to the ratio on this scale. The zero-field spin-lattice relaxation time for 8 Li implanted within SrTiO3 at 295K in zero-field was found to be T1 = 1.73(2)s. A comparison of the quadrupole moments of 9,11 Li and their ratio is made with the latest models, however, no conclusion may yet be drawn owing to the size of the theoretical uncertainties.

Hydrogen bonding and proton transfer in cocrystals of 4,4′-bipyridyl and organic acids studied using nuclear quadrupole resonance

Abstract: Cocrystals of 4,4′-bipyridyl and several carboxylic acids were grown from the methanol solution of the cocrystal formers. Complete 14N NQR spectra of these cocrystals have been measured using 1H–14N nuclear quadrupole double resonance. The principal values of the quadrupole coupling tensor are calculated from the 14N NQR frequencies. A large variation in the 14N quadrupole coupling constant between 1.3 MHz and 4.7 MHz is observed. A very low 14N quadrupole coupling constant, characteristic for proton transfer O–H⋯N → O−⋯H–N+, is observed in 4,4′-bipyridyl–oxalic acid (1 : 1). In 4,4′-bipyridyl–5-chlorosalycilic acid (1 : 1) the 14N NQR data show the presence of a short, strong N⋯H⋯O hydrogen bond. A correlation of the principal values of the 14N quadrupole coupling tensor is observed. The correlation is analyzed in the model, where the deformation of the lone pair electron orbital and the change of the population of the π-electron orbital produce the variation of the 14N quadrupole coupling tensor in the hydrogen bonded 4,4′-bipyridyl. The temperature variation of the 14N quadrupole coupling tensor in 4,4′-bipyridyl–5-chlorosalycilic acid (1 : 1) is analyzed. Proton displacement within the N⋯H⋯O hydrogen bond and the change of the population of the π-electron orbital at the two nitrogen positions in a 4,4′-bipyridyl molecule in the temperature interval between 157 K and 323 K are determined.

Anisotropic Uniaxial Pressure Response in UCoAl Studied by Nuclear Magnetic Resonance Measurement

Abstract: We have performed nuclear quadrupole resonance and nuclear magnetic resonance measurements on UCoAl with strong Ising-type anisotropy under b- and c-axes uniaxial pressure. In the b-axis uniaxial pressure (P||b) measurement, we observed an increase in the metamagnetic transition field with increasing P||b. In the c-axis uniaxial pressure (P||c) measurement, on the other hand, we observed a ferromagnetic transition in zero magnetic field along the c-axis above P||c = 0.08 GPa. The anomaly of the nuclear spin–lattice relaxation rate divided by the temperature [(T1T)−1] at T = 20 K is suppressed by P||b and slightly enhanced by P||c. The anisotropic uniaxial pressure response indicates that uniaxial pressure is a good parameter for tuning the Ising magnetism in UCoAl.

Radio-frequency atom magnetometer and method for measuring nuclear magnetic resonance (NMR) signal by same

Abstract: The invention discloses a radio-frequency atom magnetometer and a method for measuring a nuclear magnetic resonance (NMR) signal by the same. The atom magnetometer comprises an NMR system, a laser source, a probe and a detection device, wherein the laser source, the probe and the detection device are connected through a laser path so as to form a detection part of the atom magnetometer. The method comprises the following steps: under the action of the NMR system, a sample generates an NMR or nuclear quadrupole resonance (NQR) free induction decay signal; a pump light generated by the laser source enters a rubidium atom gas chamber after being subjected to beam expansion and is used for pumping rubidium atom vapor; under the action of the free induction decay signal, the polarization angle of an incident pump light is modulated, the modulation of the polarization angle of an emergent pump light is detected, and the frequency constituent of the free induction decay signal can be determined by analyzing the modulation information of the polarization angle, so that the information of the sample can be obtained. The magnetometer is simple in structure, high in sensitivity, low in technology realization requirement, low in power consumption, and capable of detecting a radio-frequency magnetic field generated by the NMR system.

Improved modeling and bounds for NQR spectroscopy signals

Abstract: Nuclear Quadrupole Resonance (NQR) is a method of de- tection and unique characterization of compounds containing quadrupolar nuclei, commonly found in many forms of ex- plosives, narcotics, and medicines. Typically, multi-pulse sequences are used to acquire the NQR signal, allowing the resulting signal to be well modeled as a sum of exponentially damped sinusoidal echoes. In this paper, we improve upon the earlier used NQR signal model, introducing an observed amplitude modulation of the spectral lines as a function of the sample temperature. This dependency noticeably af- fects the achievable identification performance in the typical case when the substance temperature is not perfectly known. We further extend the recently presented Cramer-Rao lower bound to the more detailed model, allowing one to determine suitable experimental conditions to optimize the detection and identifiability of the resulting signal. The theoretical results are carefully motivated using extensive NQR measurements. (Less)

Further Improvement of NQR Technique for Detection of Illicit Substances

Abstract: The nuclear quadrupole resonance (NQR) method has been used for the detection of explosives and other illicit materials in a number of applications, including baggage screening and landmine detection. This method provides automated detection of explosives types in configurations missed by the imaging X-ray method. This paper describes the continuing efforts to improve the function of the NQR baggage scanner by increasing detection sensitivity, and the use of better methods of reducing false alarm events generated from the RF excitation and various types of interference. Based on the current research findings and a review of the previous NQR based scanners, the key design objectives have been identified and discussed.

β-detected nuclear quadrupole resonance and relaxation of 8Li+ in sapphire

Abstract: We report detailed behaviour of low energy 8Li implanted near the surface of α- Al2O3 single crystal, as revealed by beta-detected NQR of 8Li. We find that the implanted 8Li occupies at least two sites with non-cubic symmetry in the Al2O3 lattice. In both sites the 8Li experiences axially symmetric electric field gradient, with the main principal axis along the c-crystallographic direction. The temperature and field dependence of the spin lattice relaxation of 8Li in α-Al2O3, indicate that the 8Li diffusion is negligible on the scale of its lifetime, 1.21 s.

Theoretical 14N and 17O nuclear quadrupole resonance parameters for tirapazamine and related metabolites

Abstract: Density functional theory (DFT) calculations were performed to realize the effects of the N–O group on the reactivity and electronic properties of 3-amino-1,2,4-benzotriazines. The electric field gradient, EFG, tensors of 14N and 17O nuclei and natural bond orbital (NBO) analysis in the tirapazamine (TPZ) and its four derivatives were calculated at the B3LYP/6-311+G(d,p)//B3LYP/6-31G(d) method in the gas phase. The NBO analysis reveal that the bond strength, proton affinity and position of N–O group in the heterocyclic ring have major influence on the reactivity of considered molecules. Accordingly, we suggest that the TPZ and 4-oxide (d) structures due to having a weaker N–O bond with larger negative charge on the oxygen atom at the 4-position are more active than the other ones. Calculated 14N and 17O EFG tensors were used to evaluate nuclear quadrupole coupling tensors, χ, and asymmetry parameters, η Q . Results showed that oxidation of a nitrogen atom at any position have significant influence on its 14N nuclear quadrupole resonance (NQR) parameters. Also, the occupancy of nitrogen lone pair plays an important role in determination of the q zz and χ values of mentioned nuclei. It is found that the η Q and χ are appropriate parameters to study the contribution of lone pair electrons of nitrogen atom in the formation of chemical bond or conjugation with the aromatic system. Finally, a linear correlation is observed between the χ(14N) and χ(17O) values in the N–O bond which may be associated with the reactivity of these compounds.

75As NQR signature of the isoelectronic nature of ruthenium for iron substitution in LaFe1−xRuxAsO

Abstract: We have investigated the effects of the isovalent ruthenium substitution in LaFeRuAsO, by extensive 75As NQR (nuclear quadrupole resonance) measurements, supported by DFT (density functional theory) calculations, in order to characterize both the lattice and electronic structure details. The evidence for five different local configurations around the arsenic site emerges upon increasing ruthenium for iron substitution. DFT calculations confirm the attribution of the measured electric field gradients (EFGs) to ruthenium atom occupancies (0, 1, 2, 3, and 4) on the nearest-neighbour sites of arsenic. It is found that the low-frequency (Ru-free) NQR peak remains almost unaffected upon ruthenium substitution, providing an experimental confirmation that ruthenium does not introduce delocalized carriers in the iron plane.

Size Effect in 14N Nuclear Quadrupole Resonance Spectroscopy

Abstract: The influence of the size effect of the crystallites in powders on the form and width of spectral lines, on the spin-spin and spin-lattice relaxation parameters of the nuclear quadrupole resonance (NQR) of 14N nuclei in sodium nitrite was studied. It was established that a decrease of the average crystallite size produces the widening of the NQR lines and the shortening of the relaxation times in the direct method of NQR detection. It was supposed that these are the results of the spin-spin diffusion process. A multi-exponential inversion of the decays of the longitudinal and transverse components of the nuclear magnetization was used to obtain the distribution of relaxation times.

Unconventional Superconductivity in the Vicinity of Strong First-Order Helimagnetic Transition in CrAs: ^75As-Nuclear Quadrupole Resonance Study

Abstract: Pressure-induced superconductivity was recently discovered in the binary helimagnet CrAs. We report the results of measurements of nuclear quadrupole resonance for CrAs under pressure. In the vicinity of the critical pressure P_c between the helimagnetic (HM) and paramagnetic (PM) phases, a phase separation is observed. The large internal field remaining in the phase-separated HM state indicates that the HM phase disappears through a strong first-order transition. This indicates the absence of a quantum critical point in CrAs; however, the nuclear spin-lattice relaxation rate 1/T_1 reveals that substantial magnetic fluctuations are present in the PM state. The absence of a coherence effect in 1/T_1 in the superconducting state provides evidence that CrAs is the first Cr-based unconventional superconductor.