NQR Measurements Research Articles

INTENSITY OF NMR SIGNAL IN La1.90Sr0.10CuO4: CHARGE vs. SPIN FREEZING

We report on 139 La NMR and 63 Cu NQR measurements in La 1.90 Sr 0.10 CuO 4 above its superconducting transition temperature. We find that the intensity of the signal integrated on frequencies decreases significantly on cooling from 60 K to 28 K for 63 Cu nuclei (wipeout effect), while it remains constant for 139 La nuclei in this T-range. The distribution of 139 La spin-lattice relaxation time (T1) values in the "wipeout regime" reveals a spread of electronic fluctuation frequencies in CuO 2 planes. A simple calculation, for spin fluctuation induced relaxation, shows that 63 Cu nuclei at sites where electronic fluctuations are slowest should not be observable because of too short relaxation times. This means that the 63 Cu NQR wipeout effect may be explained primarily by slow magnetic fluctuations.

INVESTIGATION OF THE CLUSTER SPIN-GLASS TRANSITION IN La2–xSrxCuO4 BY MEANS OF ANELASTIC SPECTROSCOPY

A step-like increase of the acoustic absorption (Q-1) is found in La 2-x Sr x CuO 4 (x = 0.019, 0.025, 0.032 and 0.064) below the temperature (Tg) at which NQR and μSR measurements evidenced the formation of a cluster spin-glass (CSG) state. The sharp maxima appearing in the 139 La NQR and μSR relaxation rates indicate the slowing down of antiferromagnetic fluctuations below the measuring frequency (107-108 Hz in those experiments). The acoustic absorption does not appear to be due to the freezing of magnetic fluctuations, for which one expects a narrow peak; rather, it is attributed to the size change of the antiferromagnetically correlated clusters, produced through magnetoelastic coupling by the vibration stress.

NMR [formula omitted]NQR and specific heat studies on the [formula omitted]-type [formula omitted] system

The magnetic properties of the ThMn 12 -type CeCu 4+x Al 8−x (0⩽x⩽0.55) system have been investigated by specific heat and electrical resistivity, 27 Al NMR and 63,65 Cu NQR measurements. The lattice constants ( a and c) showed a monotonic decrease for x>0.3, whereas they remained nearly constant x⩽0.3. It has been revealed in the present work that this system is the heavy-fermion-type antiferromagnet and the Néel temperature T N shows a monotonic decrease with increasing x for x⩾0.4. The x and temperature dependences of the NMR /NQR spectra and the relaxation rate 1/T 1 have been studied from a microscopic standpoint.

Electronic structure and electric field gradient for Sr 2RuO 4 and La 2CuO 4

We have calculated the electronic structures and the electric field gradients for layered oxides, Sr 2RuO 4 and La 2CuO 4 by using a full-potential LAPW method and compared the results with the NMR and NQR measurements. The results are fairly good especially for O-sites in both compounds.

Chlorine-35 NQR and 1H NMR study of guest structure anddynamics in the thiourea-hexachloroethane inclusion compound

Atom-atomic potential calculation of the channel non-stoichiometricthiourea-hexachloroethane inclusion compound shows that the structure of theguest sublattice comprises two types of finite molecular chain, havingdifferent structure and separated by domain walls. In the present paper wepresent results of 35Cl NQR and 1H NMR measurements ofthiourea-hexachloroethane, [2.95(NH2)2CS]C2Cl6,in the temperature range from 7.5 to 90 K, which confirm this model and showthe existence of such a state at least below 60 K. Two resonances in the NQRspectra were assigned to the two nearly commensurate regions, while the thirdresonance, showing an anomalous behaviour, was attributed to the guestmolecules in the domain wall. The observed structure results from thedifferent periodicity of the guest and host substructures and shows adifference from conventional continuum models of the incommensurate state.Propagation motion of the domain wall over the channel is discussed.

NMR and NQR studies of impurities in high-temperature superconducting cuprates

The substitution of non-magnetic Zn and magnetic Ni for Cu in the superconducting CuO 2 plane leads to a rapid decline in T c and localised behaviour near the impurities. There is no consensus about the mechanism for the supression of superconductivity or the origin of the localized behaviour. Some discussion of these subjects is given based on NMR and NQR measurements.

Electronic structure and electric field gradient of UCoAl

We have carried out a full-potential LAPW band structure calculation including the spin–orbit interactions for UCoAl. From the obtained charge density distributions, the electric field gradients have been calculated and compared with the recent NMR and NQR measurements. The theoretical values agree well with the experimental ones for each site.

NMR and NQR studies of URu 2Si 2

We have performed 29Si NMR and 101Ru NQR measurements on a single crystalline URu 2Si 2. From the 29Si NMR measurements, any appreciable internal field at Si site was not observed in the antiferromagnetic state, indicating that tiny magnetic moments of U are not static but fluctuating faster than the NMR frequency. From the 101Ru NQR line which is much narrower than that of polycrystalline powder, the electric quadrupole interaction parameter ( ν Q) was precisely evaluated to be 5.713 MHz at 4.2 K.

The NQR picture of electronic phase separation in the lanthanum–barium cuprates

We report the results of Cu and La NQR investigations of the spin and charge separation in La 2− x Ba x CuO 4 compounds. For Ba doping x= 1 8 inducing the low-temperature tetragonal phase our Cu–NQR measurements revealed three inequivalent Cu sites in CuO 2 planes. In terms of the stripe-phase picture we assign them to the charged stripe, to its neighborhood and to the AF correlated regions free of holes correspondingly. The last site corresponds to the ordered Cu magnetic moment of 0.29 μ B.

Effect of Deuteration on Ammonium Motion and Structural Phase Transition Studied by Nuclear Quadrupole Resonance

35Cl NQR measurements revealed two phase transitions at 14.5 K and 70 K in NH4ICI4. By deuteration of the ammonium ions, the respective transition temperature increased to 27 K and 74 K. In the lowest-temperature phase of NH4ICI4, by the deuteration a remarkable change of the temperature dependence of the 35Cl NQR frequencies was observed. The NQR spin-lattice relaxation-time minimum, ascribable to the modulation effect of the electric field gradient (EFG) due to a motion of the ammonium ions, was observed at 7 K and 11 K in NH4ICl4 and ND4ICI4, respectively. The activation energy of the ammonium motion and the fluctuation fraction q' /q of the EFG were determined to be 0.32 kJ mol-1 and 1.3x10-3 for NH4ICI4, and 0.30 kJ mol-1 and 0.88x10-3 for ND4ICI4. The experimental result suggests that the inter-ionic interaction between the ammonium ion and the ICl4 - complex anion is weakened by the deuteration. However, the frequency shift observed in the lowest-temperature phase is much larger in ND4ICI4 than in NH4ICI4. This suggests that the lattice distortion in NH4ICI4 is suppressed to a great deal by the tunneling rotation of NH4 + ions. The effective inter-ionic interaction responsible for the lattice distortion, the transition temperature and the enhancement of tunneling of NH4 + as compared with ND4 + is discussed for NH4ICI4 as well as for previously reported NH4AuCI4 and α-NH4HgCl3.

A Pulsed Spectrometer Designed for Feedback NQR

A pulsed NQR spectrometer specifically designed to facilitate real-time tuning of pulse sequence parameters is described. A modular approach based on the interconnection of several rack-mounted blocks provides easy access to all spectrometer signals and simplifies the task of modifying the spectrometer design. We also present experimental data that demonstrates the ability of the spectrometer to increase the signal to noise ratio of NQR measurements by automatically adjusting the pulse width in the strong off-resonant comb pulse sequence.

35Cl NQR in Glassy Crystal of 2-chlorothiophene

Stable crystalline 2-chlorothiophene has two glass transitions at 164 and 186 K. 35CI NQR measurements were carried out between 77 and 200 K. Two NQR signals with full widths of about 100 kHz at half maximum were observed in this temperature range. The spin-lattice relaxation times T1 were measured at the two peak frequencies. The activation energy obtained from the results of the T1 measurements showed a fairly good agreement with those estimated from calorimetric measurements.

Muon spin relaxation in spin gap state of BaVS 3

Positive muon spin relaxation measurements were performed on a spin-1/2 system BaVS3, which shows a metal-insulator transition at TMI= 70 K. We found a marked muon-spin depolarization below TX= 30 K without appreciable critical divergence. The possibility of muonium formation in the insulating state rather than electron spin freezing is discussed taking into account the quenching of V spins evidenced by 51V NMR and NQR measurements.

105Pd NMR/NQR studies in UPd 2Al 3

105Pd NMR and NQR measurements have been performed both in the normal and in the superconducting states for UPd 2Al 3. In the antiferromagnetically (AF) ordered state, the small internal field of about 3 kOe (at 4.2 K), which is perpendicular to the c-axis, appears below 14.5 K and does not decrease in the superconducting state. The nuclear spin-lattice relaxation rate, 1/ T 1, in UPd 2Al 3 is nearly temperature, T, independent in the paramagnetic state up to 60 K, successively diverges at Néel temperature associated with the critical slowing down of U magnetic moments, and then decreases markedly in the AF ordered state. In the superconducting state, 1/ T 1 decreases in proportion to T 3 down to 0.4 K with no coherence peak just below T C, which indicates the occurrence of anisotropic superconductivity having line nodes in the superconducting energy gap.

Phase transitions in the (Cs1−x Rbx)2ZnI4 system

(Cs1−xRbx)2ZnI4 crystals were grown by two different methods with Rb concentrations varying from x=0 to 2.5%. 127I NQR and calorimetric measurements showed that crystals grown by the Bridgman technique contain residual impurities (∼0.5%) for all x. While x=0 crystals grown from solution do not contain detectable impurities, they allow incorporation only of a low Rb concentration, not above 0.5%. A transition-temperature-concentration (x) phase diagram has been constructed for Bridgman-grown crystals from NQR data. Rb doping shifts the normal-incommensurate and incommensurate-ferroelastic phase-transition points toward higher temperatures with different rates. The P21/m↔P1 first-order transition shifts toward lower temperatures. The region of low Rb concentrations lies closest to the critical point.

Pressure dependence of the electric-field gradient in semimetallic arsenic and antimony: NQR measurements and full-potential linearized augmented-plane-wave calculations

The pressure variation of the electric-field gradient (efg) at nuclear sites in powder samples of semimetallic arsenic and antimony has been investigated up to 2.0 GPa at 293 K using pulsed nuclear quadrupole resonance to measure the nuclear resonance frequency. Theoretical calculations of both the total efg and the ionic contributions have been carried out. Full-potential linearized augmented plane wave (FLAPW) calculations of the total efg give values that are approximately 70% of the measured efg. The comparison between experimental and theoretical magnitudes of the efg is, however, to some extent limited by the accuracy of the values used for the quadrupole moments of ${}^{75}\mathrm{As}$ and ${}^{121}\mathrm{Sb}.$ Additional uncertainties arise from the pressure dependence of the lattice parameters. FLAPW calculations of the dependence of the internal z parameter on the $c/a$ ratio suggest an inconsistency in available data for the pressure dependence of the lattice parameters. This inconsistency is particularly marked in the arsenic case, where very little high-pressure data are available. In the antimony case, our FLAPW estimate of the relative pressure variation of the efg at the nuclear sites, using experimentally determined lattice parameters, shows good agreement with the nuclear quadrupole resonance measurements. The pressure variation of the efg in arsenic is well reproduced by the FLAPW calculation when the experimental $c/a$ ratio and theoretically estimated z parameter are used.

Electrodynamics of the Superconducting State in CeRu2

We have studied the complex conductivity in superconducting and normal metallic states of CeRu 2 by the surface impedance measurements at millimeter-wave frequency of 45.4 GHz. The temperature dependence of the complex conductivity below T c = 6.2 K exhibits the characteristic behaviors as seen in a conventional superconductor. The coherence peak observed in the real part σ 1 provides a strong evidence for a conventional s -wave superconductivity as well as the previous NQR measurements. The temperature dependence of the penetration depth evaluated from the surface reactance at low temperatures has revealed the ratio of 2 Δ(0)/ k B T c = 4.4. The strong-coupling theories suggest the electron-phonon coupling strength with λ ep < 2. These results indicate that CeRu 2 is an intermediate strong-coupling superconductor in a clean limit.

Cu-NQR Study in the Paramagnetic State in YBa2Cu3O6

The NQR measurements for the Cu(2) and the Cu(1) sites in antiferromagnetic insulator YBa 2 Cu 3 O 6 with T N =415 K were carried out in the paramagnetic state for the respective temperature ranges of 475–575 K and of 295–775 K. For the Cu(2) sites, the nuclear spin-lattice relaxation rate 1/ T 1 and the second moment of the indirect nuclear spin-spin interactions 1/ T 2G exhibit a similar behavior to those in La 2 CuO 4 not only in the temperature dependence but also in the magnitude at least below 550 K. These results suggest a possibility that the renormalized classical description for two-dimensional quantum Heisenberg antiferromagnet is also applicable to YBa 2 Cu 3 O 6 as to La 2 CuO 4 and that the upper limit of the bilayer coupling J ''/ J is as large as 10 -2 , where J and J '' are the intra-layer and bilayer exchange couplings, respectively. The Cu(1) NQR does not probe any spin dynamics in the CuO 2 plane except for the temperatures below 600 K. The rapid increase in 1/ T 1 above 600 K suggests...

Novel incommensurate phase in Cs 3Bi 2I 9

The incommensurate modulation has been found in the hexagonal Cs 3Bi 2I 9, belonging to the A 3B 2X 9 family of the layered compounds. The second order normal-incommensurate transition at T i = 224 K is accompanied by the symmetry change P6 3 mmc ↔ P2 1 m . The transition into a ferroelastic incommensurate phase occurs at the zone center, according to the NQR and ultrasonic measurements.

105Pd NMR and NQR studies in heavy-fermion superconductor UPd 2Al 3

105Pd NMR and NQR measurements have been performed both in the normal and in the superconducting states for UPd 2Al 3. In the antiferromagnetically (AF) ordered state, a small internal field of about 3 kOe (at 4.2 K), which is perpendicular to the c-axis, appears below 14.5 K and does not decrease in the superconducting state. The nuclear spin-lattice relaxation rate, 1/T 1, in UPd 2Al 3 is nearly temperature, T, independent in the paramagnetic state up to 60 K, successively diverges at the Néel temperature associated with the critical slowing down of U magnetic moments, and then decreases markedly in the AF ordered state. In the superconducting state, 1/T 1 decreases in proportion to T 3 down to 0.5 K with no coherence peak just below T C , which indicates the occurrence of anisotropic superconductivity having line nodes in the superconducting energy gap.