Showing papers on "Nuclear quadrupole resonance published in 2006"

Subfemtotesla radio-frequency atomic magnetometer for detection of nuclear quadrupole resonance

Abstract: A radio-frequency tunable atomic magnetometer is developed for detection of nuclear quadrupole resonance (NQR) from room temperature solids. It has a field sensitivity 0.24fT∕Hz1∕2 at the 423kHz N14 NQR frequency of ammonium nitrate. A potential application of the magnetometer is detection of nitrogen-containing explosives which is difficult with conventional tuned copper coils due to a poor signal-to-noise ratio (SNR) below a few megahertz. The NQR signal from 22g of powdered ammonium nitrate located 2cm away from the sensor is detected with a SNR of 9 in a 4.4-s-long multiple echo sequence, which represents an estimated order-of-magnitude improvement in sensitivity over the pickup coil detection.

Exploiting temperature dependency in the detection of NQR signals

Abstract: Nuclear quadrupole resonance (NQR) offers an unequivocal method of detecting and identifying land mines. Unfortunately, the practical use of NQR is restricted by the low signal-to-noise ratio (SNR), and the means to improve the SNR are vital to enable a rapid, reliable, and convenient system. In this paper, an approximate maximum-likelihood detector (AML) is developed, exploiting the temperature dependency of the NQR frequencies as a way to enhance the SNR. Numerical evaluation using both simulated and real NQR data indicate a significant gain in probability of accurate detection as compared with the current state-of-the-art approach.

Near-surface structural phase transition of SrTiO3 studied with zero-field beta-detected nuclear spin relaxation and resonance.

Abstract: We demonstrate that zero-field beta-detected nuclear quadrupole resonance and spin relaxation of low energy (8)Li can be used as a sensitive local probe of structural phase transitions near a surface. We find that the transition near the surface of a SrTiO(3) single crystal occurs at T(c) approximately 150K, i.e., approximately 45K higher than T(c)bulk, and that the tetragonal domains formed below T(c) are randomly oriented.

Subfemtotesla radio-frequency atomic magnetometer for nuclear quadrupole resonance detection

Abstract: A radio-frequency tunable atomic magnetometer for detection of nuclear quadrupole resonance (NQR) from room temperature solids, including detection of nitrogen-containing explosives placed external to a sensor unit. A potassium radio-frequency magnetometer with sensitivity of 0.24 fT/Hz1/2 operating at 423 kHz is provided. The magnetometer detected a 14N NQR signal from room temperature ammonium nitrate (NH4NO3) in the zero-applied field limit. Results demonstrate first time detection of NQR with an atomic magnetometer, providing that a cryogen-free atomic magnetometer, with intrinsically frequency-independent sensitivity and easy tuning capabilities, can be an attractive new tool for detecting magnetic resonance signals in the kHz to MHz range. Low intensity magnetic fields are measured using an alkali metal vapor, by increasing magnetic polarization of the vapor to increase its sensitivity, then probing the magnetic polarization of the vapor to receive an output, and determining characteristics of the low intensity magnetic field from the output.

Multiband Superconductivity in Filled-Skutterudite Compounds (Pr1-xLax)Os4Sb12: An Sb Nuclear-Quadrupole-Resonance Study

Abstract: We report on the systematic evolution of normal-state properties and superconducting characteristics in filled-skutterudite compounds (Pr 1- x La x )Os 4 Sb 12 determined using Sb nuclear-quadrupol...

Possible spin triplet superconductivity in NaxCoO2?yH2O ?59Co NMR studies

Abstract: We report 59Co NMR studies on magnetically oriented powder samples of Co oxide superconductors, NaxCoO2·yH2O, with Tc~4.7 K. From the two-dimensional powder pattern in the NMR spectrum, the ab-plane Knight shift in the normal state was estimated from the magnetic field dependence of the second-order quadrupole shifts at various temperatures. Below 50 K, the Knight shift shows a Curie–Weiss-like temperature dependence, similarly to the bulk magnetic susceptibility χ. From the analysis of the so-called K–χ plot, the spin and the orbital components of K and the positive hyperfine coupling constant were estimated. The onset temperature of the superconducting transition in the Knight shift does not change much in an applied magnetic field up to 7 T, which is consistent with the reported high upper critical field Hc2. The Knight shift at 7 T shows an invariant behaviour below Tc. No coherence peak just below Tc was observed in the temperature dependence of the nuclear spin–lattice relaxation rate 1/T1 in either case (NMR, nuclear quadrupole resonance). We conclude that the invariant behaviour of the Knight shift below Tc and unconventional behaviours of 1/T1 may indicate spin triplet superconductivity with p- or f-wave symmetry.

Charge Disproportionation in Highly One-Dimensional Molecular Conductor TPP[Co(Pc)(CN)2]2

Abstract: We investigated the ground state of a highly one-dimensional conductor, TPP[Co(Pc)(CN) 2 ] 2 (TPP = tetraphenylphosphonium and Pc = phthalocyanine), by the measurement of the X-ray diffraction, electron spin resonance, nuclear quadrupole resonance, and magnetoresistance. An increase of the magnetic fluctuations was observed below 20 K, where no structural deformation was detected. In the 59 Co nuclear quadrupole resonance, we found an asymmetric broadening of the spectra owing to the intrinsic inhomogeneity of the molecular charge. We propose that the ground state is characterized by a weak charge disproportionation with antiferromagnetic fluctuations due to the high one dimensionality. A large magnetoresistance was observed under a high magnetic field. Spin effects are dominant at low fields. The anisotropic magnetoresistance suggests a change in the ground-state nature above 10 T.

59Co Nuclear Quadrupole Resonance Studies of Superconducting and Nonsuperconducting Bilayer Water Intercalated Sodium Cobalt Oxides NaxCoO2·yH2O

Abstract: We report 59 Co nuclear quadrupole resonance (NQR) studies of bilayer water intercalated sodium cobalt oxides Na x CoO 2 · y H 2 O (BLH) with the superconducting transition temperatures, 2 K < T c ≤4.6 K, as well as a magnetic BLH sample without superconductivity. We obtained a magnetic phase diagram of T c and the magnetic ordering temperature T M against the peak frequency ν 3 of the 59 Co NQR transition I z = ±5/2 ↔±7/2 and found a dome-shaped superconducting phase. The 59 Co NQR spectrum of the nonsuperconducting BLH shows a broadening below T M without the critical divergence of 1/ T 1 or 1/ T 2 , suggesting an unconventional magnetic ordering. The degree of enhancement of 1/ T 1 T at low temperatures increases with the increase of ν 3 though the optimal ν 3 of approximately 12.30 MHz. In the Na x CoO 2 · y H 2 O system, the optimal- T c superconductivity emerges close to the magnetic instability. T c is suppressed near the phase boundary at ν 3 ∼12.50 MHz, which is not a conventional magnetic quantu...

Na content dependence of superconductivity and the spin correlations in NaxCoO2·1.3H2O

Abstract: We report systematic measurements using the 59Co nuclear quadrupole resonance (NQR) technique on the cobalt oxide superconductors NaxCoO2·1.3H2O over a wide Na content range x = 0.25–0.34. We find that Tc increases with decreasing x but reaches a plateau for x≤0.28. In the sample with x~0.26, the spin–lattice relaxation rate 1/T1 shows a T3 variation below Tc and down to T~Tc/6, which unambiguously indicates the presence of line nodes in the superconducting (SC) gap function. However, for larger or smaller x,1/T1 deviates from the T3 variation below T~2 K even though the Tc (~4.7 K) is similar, which suggests an unusual evolution of the SC state. In the normal state, the spin correlations at a finite wavevector become stronger upon decreasing x, and the density of states at the Fermi level increases with decreasing x, which can be understood in terms of a single-orbital picture suggested on the basis of LDA calculation.

59Co Nuclear Quadrupole Resonance Studies of Superconducting and Non-superconducting Bilayer Water Intercalated Sodium Cobalt Oxides NaxCoO2.yH2O

Abstract: We report 59Co nuclear quadrupole resonance (NQR) studies of bilayer water intercalated sodium cobalt oxides NaxCoO2.yH2O (BLH) with the superconducting transition temperatures, 2 K +-7/2 and found a dome shape superconducting phase. The 59Co NQR spectrum of the non-superconducting BLH shows a broadening below T_M without the critical divergence of 1/T_1 and 1/T_2, suggesting an unconventional magnetic ordering. The degree of the enhancement of 1/T_1T at low temperatures increases with the increase of nu_3 though the optimal nu_3~12.30 MHz. In the NaxCoO2.yH2O system, the optimal-T_c superconductivity emerges close to the magnetic instability. T_c is suppressed near the phase boundary at nu_3~12.50 MHz, which is not a conventional magnetic quantum critical point.

A systematic investigation of hydrogen-bonding effects on the 17O, 14N, and 2H nuclear quadrupole resonance parameters of anhydrous and monohydrated cytosine crystalline structures: a density functional theory study.

Abstract: A systematic computational study was carried out to characterize the 17O, 14N, and 2H nuclear quadrupole resonance (NQR) parameters in the anhydrous and monohydrated cytosine crystalline structures. To include the hydrogen-bonding effects in the calculations, the most probable interacting molecules with the central molecule in the crystalline phase were considered in the pentameric clusters of both structures. To calculate the parameters, couples of the methods B3LYP and B3PW91 and the basis sets 6-311++G** and CC-pVTZ were employed. The mentioned methods calculated reliable values of 17O, 14N, and 2H NQR tensors in the pentameric clusters, which are in good agreements with the experiment. The different influences of various hydrogen-bonding interactions types, N−H···N, N−H···O, and O−H···O, were observed on the 17O, 14N, and 2H NQR tensors. Lower values of quadrupole coupling constants and higher values of asymmetry parameters in the crystalline monohydrated cytosine indicate the presence of stronger hyd...

Evidence for Ferromagnetic Fluctuations in Filled Skutterudite LaFe4Sb12:Sb-NQR and La-NMR

Abstract: We report the results of 121,123 Sb-nuclear quadrupole resonance (NQR) and 139 La-NMR measurements on a filled skutterudite antimonide LaFe 4 Sb 12 in order to investigate the magnetic properties at low temperatures from a microscopic point of view. The nuclear spin–lattice relaxation time T 1 of Sb nuclei deviates from the relation T 1 T = constant above 4.2 K, where 1/ T 1 T has a Curie–Weiss temperature dependence 1/ T 1 T = C /( T +θ) with θ∼30 K. The temperature dependence of the Knight shift of 139 La nuclei, which is related to the susceptibility at q = 0, is scaled to that of 1/ T 1 T above 40 K. This relation strongly suggests that ferromagnetic fluctuations are predominant in LaFe 4 Sb 12 . We also point out that LaFe 4 Sb 12 is situated close to the ferromagnetic instability due to the small Weiss temperature in the Curie–Weiss behavior of 1/ T 1 T and the Knight shift.

Scanning a band of frequencies using an array of high temperature superconductor sensors tuned to different frequencies

Abstract: The methods of the invention for scanning a band of frequencies using a nuclear quadrupole resonance detection system with an array of high temperature superconductor sensors to detect nuclear quadrupole resonance signals improve the nuclear quadrupole resonance detection system performance.

Polarization enhanced “single shot” N14 nuclear quadrupole resonance detection of trinitrotoluene at room temperature

Abstract: The authors show that a combination of proton-nitrogen level crossing polarization transfer with a pulsed spin-locking sequence makes N14 nuclear quadrupole resonance (NQR) fast and sensitive enough to be used in routine explosive detection as well as in the pharmaceutical industry for nondestructive chemical analysis of solid samples and polymorph determination. As an example we present “single shot” measurements of the N14 NQR spectra of 15g of trinitrotoluene at room temperature with a total measuring time of 20s.

Unconventional Superconductivity Induced by Quantum Critical Fluctuations in Hydrate Cobaltate Na$_{x}$(H$_3$O)$_{z}$CoO$_{x}\cdot$ $y$H$_{2}$O -- Relationship between Magnetic Fluctuations and the Superconductivity Revealed by a Co Nuclear Quadrupole Resonance --

Abstract: 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 u_{zz} rather than the asymmetry of EFG within the ab-plane. In addition, it is considered that the change of u_{zz} is gaverned mainly by the trigonal distortion of the CoO_2 block layers along the c axis, from the relationships between u_{zz} and the various physical parameters. We found the tendency that samples with u_{zz} larger than 4.2 MHz show magnetic ordering, whereas samples with lower u_{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- u_{zz} sample has stronger magnetic fluctuations at T_c. From the relationship between u_{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 u_{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 u_{zz} is also discussed on the basis of the results of our experiments.

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–

Abstract: Co nuclear-quadrupole-resonance (NQR) measurements were performed on various bilayered hydrate cobaltate Na x (H 3 O) z CoO 2 · y H 2 O 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 ν z z rather than the asymmetry of EFG within the a b -plane. In addition, it is considered that the change of ν z z is gaverned mainly by the trigonal distortion of the CoO 2 block layers along the c axis, from the relationships between ν z z and the various physical parameters. We found the tendency that samples with ν z z larger than 4.2 MHz show magnetic ordering, whereas samples with lower ν z z 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-ν z...

Magnetic properties of bismuth(III) oxy compounds

Abstract: Nuclear quadrupole resonance and SQUID-magnetometric studies on magnetic properties of bismuth(III) oxy compounds comprising neither d- nor f-elements are surveyed and systematised. The magnetic properties, not conventionally expected in the Main Group element substances, were found to exist in most of the compounds discussed. The results of the analysis of the zero-field 209Bi NQR line shapes, the Zeeman-perturbed patterns in external fields of various orientations with respect to the crystal axes, and the spin-echo envelopes suggest the existence in α-Bi2O3 , Bi3O4Br, Bi2Al4O9 , Bi2Ge3O9 , Bi4Ge3O12 , BaBiO2Cl, of ordered magnetic fields ranging from 30 to 250 G, which notably exceed the dipole nuclear magnetic fields. The magnetoelectric effect was observed in the α-Bi2O3 single crystal, and the paramagnetism depending on the thermal prehistory of the crystal was found using a SQUID-technique. The questions concerning the origin of unusual magnetic and electronic properties of the title compounds are discussed.

Spin dynamics in the pulsed spin locking of nuclear quadrupole resonance

Abstract: We theoretically examine the perturbative effects of a series of radio-frequency (rf) pulses, electric field gradient inhomogeneity, and dipole-dipole coupling on the spin dynamics of spin-1 nuclei dominated by the quadrupole interaction. The dipole-dipole coupling is between neighboring spin-1 nuclei with identical nuclear quadrupole resonance frequencies, but the principal axes frames of the electric field gradient at each nucleus are not aligned. Such a comprehensive treatment is necessary to determine the optimal sequence of rf pulses which maximizes the echoes in the detection of the substance of interest, for example, an explosive or other contraband material. We confirm our theoretical model using the nitrogen in powder samples of $p$-chloroaniline and sodium nitrite.

Charge-inhomogeneity doping relations in Y Ba 2 Cu 3 O y detected by angle-dependent nuclear quadrupole resonance

Abstract: The origin of charge inhomogeneity in $\mathrm{Y}{\mathrm{Ba}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{y}$ is investigated using an experimental method designed to determine the nuclear quadrupole resonance (NQR) asymmetry parameter $\ensuremath{\eta}$ for very wide NQR lines at different positions on the line. The method is based on the measurement of the echo intensity as a function of the angle between the radio frequency field ${\mathbf{H}}_{1}$ and the principal axis of the electric field gradient. Static charge inhomogeneity deduced from $\ensuremath{\eta}g0$ is found in this compound, but only in conjunction with oxygen deficiency. This limits considerably the possible forms of charge inhomogeneity in bulk $\mathrm{Y}{\mathrm{Ba}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{y}$.

Improving the sensitivity of a high-Tc SQUID at MHz frequency using a normal metal transformer

Abstract: Superconducting quantum interference devices (SQUIDs) can be used to detect the signals of nuclear quadrupole resonance (NQR). The NQR frequencies of some interesting materials are in the order of MHz. However, the sensitivity of a high-Tc SQUID is normally not enough to detect the weak NQR signals. To improve the sensitivity of a high-Tc SQUID at MHz frequency, we used a transformer made of normal copper wire. The transformer was composed of a pickup coil, an input coil and a capacitor. The pickup coil was used to detect the magnetic field; the input coil was used to couple the field to the SQUID; and the capacitor was used to create a resonant frequency. By using the normal metal transformer, the magnetic field resolution of the high-Tc dc SQUID was improved by about 38.8 times (from 220 to 5.67 fT Hz−1/2) at 954 kHz.

14N Pulsed nuclear quadrupole resonance. 2. Effect of a single radio-frequency pulse in the general case

Abstract: A novel theory, based on density operator calculations, is provided for assessing the nuclear quadrupole resonance behaviour of a spin 1 (14N) subjected to a single radio-frequency pulse. It is for a powder sample in zero magnetic field for an electric field gradient tensor without symmetry. A complete set of equations is obtained for the quantities of interest. It is derived from the general Liouville–von Neumann equation and from a proper basis on which the density operator is expanded. Theoretical results, in terms of signal evolution as a function of the pulse length (nutation experiments), show that the same nutation curve is expected for the three different transitions which exist when the electric field gradient tensor is without symmetry. This latter nutation curve is, however, different from that which prevails in the case of an axially symmetric tensor, this apparent discrepancy being easily resolved on theoretical grounds. Experimental data (for NaNO2, electric field gradient tensor without sym...

Hidden order and disorder effects in URu2Si2

Abstract: NMR experiments at ambient pressure in URu2Si2 demonstrate a linewidth enhancement effect below the hidden order transition temperature T 0 We present single-crystal 29Si NMR parameters for various temperatures and for an applied magnetic field perpendicular to the crystal c-axis By comparing oriented-powder and single-crystal data, we observe that the size of the linewidth enhancement below T 0 correlates with the size of the high-T broadening We measure a 29Si up-field line shift below T 0 which indicates the presence of an internal-field average for the entire crystal This shift also correlates with the high-temperature width The 101Ru NQR frequency as a function of temperature was also measured No strong effect on the NQR frequency is observed at T 0 Both NMR and NQR measurements suggest a connection between linewidth/disorder effects and the transition to hidden order

Detection of Landmines Using Nuclear Quadrupole Resonance (NQR): Signal Processing to Aid Classification

Abstract: Nuclear quadrupole resonance (NQR) is a sensor technology that measures a signature unique to the explosive contained in the mine, thus providing a means of efficiently detecting landmines. Unfortunately, the measured signals are inherently weak and therefore detection times are currently too long (especially for TNT-based landmines) to implement in a man-portable detection system. However, the NQR hardware is light enough to be integrated into a robot based system. This paper investigates several power spectrum estimation algorithms applied to NQR signals in order to distinguish between data containing signals from explosive and data that does not.

Nuclear quadrupole resonance study of local bonding in glassy As{sub x}Se{sub 1-x}

Abstract: Nuclear quadrupole resonance (NQR) experiments were performed on glassy As{sub x}Se{sub 1-x} to study the local structural order. The bonding in As{sub x}Se{sub 1-x} is governed by preferential bonding (chemical ordering) between arsenic and selenium at arsenic concentrations x{ =}0.45 the glasses are inhomogeneous with the presence of local regions of different composition. Measurements of the NQR spin echo intensity identify regions formed by arsenic atoms bonded to zero, one, two, or three selenium atoms. The NQR spectral line shapes and the longitudinal relaxation times suggest that these regions have a more ordered structure as compared to the homogeneous samples with low arsenic content. The existence of a small concentration of crystalline inclusions in amorphous As{sub 0.60}Se{sub 0.40} is also suggested.