Nuclear quadrupole resonance studies of transparent conducting oxides

We report ^63,65Cu spin–lattice relaxation rates measured by nuclear quadrupole resonance (NQR) in the delafossite compound CuYO₂ and CuYO₂:Ca over a temperature range from 200 to 450 K. CuYO₂:Ca is a prototype transparent oxide exhibiting p-type electrical conductivity. Relaxation rates in CuYO₂:Ca are enhanced by one to two orders of magnitude relative to undoped material, exhibit much stronger temperature dependence, and contain contributions from magnetic and quadrupolar relaxation mechanisms with roughly equal strengths. Relaxation in undoped CuYO₂ is of purely quadrupolar origin and is attributed to interactions with lattice phonons. The main focus of this paper is the magnetic contribution to the relaxation rate in CuYO₂:Ca which is attributed to the hyperfine fields of carriers. It is argued that the dynamics of the hyperfine field are dominated by the hopping rate for carrier transfers between neighboring atoms in the copper planes of the delafossite structure. Comparison of the magnetic relaxation rates with the DC conductivity permits an estimate of the carrier concentration and mobility.     

Nuclear orientation and resonance studies of96TcFe

Low temperature nuclear orientation and nuclear magnetic resonance (NMRON) have been used to measure the magnetic hyperfine interaction in⁹⁶TcFe and the decay scheme properties of⁹⁶Tc. The spin of the⁹⁶Tc ground state is confirmed as 7. Its magnetic moment μ(⁹⁶Tc)=5.37±0.17 n.m. The magnetic hyperfine field for⁹⁶TcFe is ?298±10 kOe. The anisotropies of the 778, 813, 850 and 1126 keV transitions in⁹⁶Mo agree with pure E2 multipolarity assignments. The spin-parity of the 2876 keV level is 7⁺.     

Nuclear resonance fluorescence in94Mo,95Mo and96Mo

Using gaseous sources of Tc₂O₇ containing the radioactive isotopes⁹⁴Tc,⁹⁵Tc and⁹⁶Tc, levels at 871.0keV (⁹⁴Mo), 765.8, 820.6, 947.8, 1074.0keV (⁹⁵Mo) and 778.3keV (⁹⁶Mo) have been excited. From the effective cross sections for nuclear resonance scattering and from the lifetimes of the 947.8, 1074.0 and 778.3keV levels known from Coulomb excitation experiments the profiles of theγ-lines have been determined. A broadening of theγ-lines due to Coulomb explosion of the molecules has been observed. Making use of the line profiles, lifetimes ofΤ=(6.4±1.0) ps andΤ=(0.90 ± 0.20) ps have been determined for the 765.8 and 820.6keV levels, respectively. The angular distribution of the resonantly scattered radiation yields an amplitude ratioδ for the mixed M1 E2 765.8keV transition ofδ=0.14 _?0.009 ^+0.08 . TheB(E2) from a Coulomb excitation experiment and the lifetimeΤ from the present experiment yield ¦δ¦=0.07±0.01 for the 820.6keV transition.     

Nuclear magnetic resonance approaches to brain function studies

The fast developing and newly emerging nuclear magnetic resonance methods for functional brain imaging are discussed. The main features of blood-oxygen-level-dependent (BOLD) imaging and perfusion imaging are exposed together with their limitations. The combination of localized spectroscopy with magnetic resonance imaging (MRI) and neuronal activation provide the crucial metabolic information, which is complementary to the physiological data given by BOLD and perfusion imaging. Finally, the diffusion tensor imaging and the nonlinear MRI, with the perspective of getting information about the “architecture” of the axonal connections and of the macrostructures are discussed.     

Nuclear magnetic resonance studies in rare earth ternary phosphides

The results of³¹P Knight shift (KS) and spin-lattice relaxation time (T ₁) measurements in the temperature interval 4.2–300 K are reported for the compounds RENi₂P₂(RE=Ce, Eu, Yb) in order to understand the nature of the ground state in these compounds. KS results conclusively establish that all these compounds exhibit non-magnetic ground states. The temperature dependence of spin-fluctuation temperature (T _sf) in each case is estimated from the measured data. For EuNi₂P₂ the values ofT _sf above 77 K qualitatively agree with those obtained from Mössbauer and susceptibility data employing ionic interconfigurational fluctuation model, but disagree at lower temperatures.     

Nuclear magnetic resonance studies of vortices in high temperature superconductors

The distinct distribution of local magnetic fields due to superconducting vortices can be detected with nuclear magnetic resonance (NMR) and used to investigate vortices and related physical properties of extreme type II superconductivity. This review summarizes work on high temperature superconductors (HTS) including cuprates and pnictide materials. Recent experimental results are presented which reveal the nature of vortex matter and novel electronic states. For example, the NMR spectrum has been found to provide a sharp indication of the vortex melting transition. In the vortex solid a frequency dependent spin–lattice relaxation has been reported in cuprates, including YBa₂Cu₃O_7-x, Bi₂SrCa₂Cu₂O_8+δ, and Tl₂Ba₂CuO_6+δ. These results have initiated a new spectroscopy via Doppler shifted nodal quasiparticles for the investigation of vortices. At very high magnetic fields this approach is a promising method for the study of vortex core excitations. These measurements have been used to quantify an induced spin density wave near the vortex cores in Bi₂SrCa₂Cu₂O_8+δ. Although the cuprates have a different superconducting order parameter than the iron arsenide superconductors there are, nonetheless, some striking similarities between them regarding vortex dynamics and frequency dependent relaxation.     

Nuclear magnetic resonance studies of surface interactions between trifluoromethanesulfonic acid and silica

The nature of the interaction between trifluoromethanesulfonic acid (triflic acid) and the surface groups of silica as a support has been investigated by¹H and¹⁹F-NMR studies. The¹⁹F-NMR spin-lattice relaxation time (T ₁) dependence on temperature suggests that the dominant relaxation mechanism for¹⁹F is a combination of spin-rotation (SR) interactions, one of which seems to be quenched by cage-like structures. This mechanism is activated by the treatment followed to stabilize the acid on silica. Hydrogen proton relaxation seems instead to be driven by dipolar interactions and the formation of hydrogen bridges. Activation energies have been estimated for various cases, assuming an Arrhenius-type temperature dependence for the correlation times of molecular motion. A linear dependence of¹H chemical shift on inverse temperature was also found, which is interpreted as thermal weakening of hydrogen bonding. The results support the conclusion that the stability of triflic acid on silica is due to the formation of strong hydrogen bridges between the sulfonic groups of triflic acid and the silica surface hydroxyls, provided a network structure similar to that observed for pure acid is obtained in which strong interactions between acid molecules occur. The acid seems to be organized as solid-like “drops” on the silica surface, which accounts for the long residence time on the silica surface.     

Nuclear stochastic resonance

Quantitative theory of the effect of nuclear ferromagnetism on the superconductivity of metals is proposed taking into account the electron-nuclear spin-spin interactions. At negative nuclear temperatures, when the nuclear magnetization is in opposition to an external magnetic field, nuclear ferromagnetism is favorable to superconductivity rather than suppressing it. The critical magnetic field in Be and TiH_2.07 hydrate metals may exceed the critical field of a nonmagnetic superconductor by an order of magnitude.     

Nuclear resonance scattering

Nuclear resonance scattering is an atomistic spectroscopy sensitive to magnetic and electronic properties as well as slow and fast structural dynamics. Applications, which take advantage of both the outstanding properties of third generation synchrotron radiation sources and those of the Mössbauer effect, benefit most. Examples resulting from investigations at the ESRF will be given in applications to high pressure and low temperatures, nano-scale materials, and dynamics of disordered systems. To cite this article: R. Rüffer, C. R. Physique 9 (2008).     

Logarithmic Fermi-liquid breakdown in NbFe2

The d-electron low temperature magnet NbFe2 is poised near the threshold of magnetism at ambient pressure, and can be tuned across the associated quantum critical point by adjusting the precise stoichiometry within the Nb1-yFe2+y homogeneity range. In a nearly critical single crystal (y= -0.01), we observe a T3/2 power-law dependence of the resistivity rho on temperature T and a logarithmic temperature dependence of the Sommerfeld coefficient gamma=C/T of the specific heat capacity C over nearly 2 orders of magnitude in temperature, extending down to 0.1 K.     

Nuclear magnetic resonance cryoporometry

Nuclear Magnetic Resonance (NMR) cryoporometry is a technique for non-destructively determining pore size distributions in porous media through the observation of the depressed melting point of a confined liquid. It is suitable for measuring pore diameters in the range 2 nm–1 μm, depending on the absorbate. Whilst NMR cryoporometry is a perturbative measurement, the results are independent of spin interactions at the pore surface and so can offer direct measurements of pore volume as a function of pore diameter. Pore size distributions obtained with NMR cryoporometry have been shown to compare favourably with those from other methods such as gas adsorption, DSC thermoporosimetry, and SANS. The applications of NMR cryoporometry include studies of silica gels, bones, cements, rocks and many other porous materials. It is also possible to adapt the basic experiment to provide structural resolution in spatially-dependent pore size distributions, or behavioural information about the confined liquid.     

Nuclear quadrupole resonance of norephedrine

Toward searching for illegal drugs, we investigated the pulsed nuclear quadrupole resonance (NQR) response of ¹⁴N in (1R,2S)-(-)-norephedrine, based on the predictions of quantum chemical calculations. Two pairs of spectral lines (ν₊=3.089, 3.093 MHz and ν₋=2.594, 2.608 MHz) were observed despite its molecule structure having only a single nitrogen atom. This indicates that the molecular crystal has two nonequivalent nitrogen atoms in the unit cell. The temperature dependence of the NQR frequencies and relaxation properties were investigated for the purpose of accurate remote sensing of the drugs. The NQR frequency shift was approximately 0.23 kHz/K around room temperature. The spin-lattice relaxation and spin-phase memory times were 5.2–10.2 ms and 0.6–1.5 ms, respectively.     

Chemically mediated quantum criticality in NbFe2

Laves-phase Nb(1+c)Fe(2-c) is a rare itinerant intermetallic compound exhibiting magnetic quantum criticality at c(cr)～1.5%Nb excess; its origin, and how alloying mediates it, remains an enigma. For NbFe(2), we show that an unconventional band critical point above the Fermi level E(F) explains most observations and that chemical alloying mediates access to this unconventional band critical point by an increase in E(F) with decreasing electrons (increasing %Nb), counter to rigid-band concepts. We calculate that E(F) enters the unconventional band critical point region for c(cr) > 1.5%Nb and by 1.74%Nb there is no Nb site-occupation preference between symmetry-distinct Fe sites, i.e., no electron-hopping disorder, making resistivity near constant as observed. At larger Nb (Fe) excess, the ferromagnetic Stoner criterion is satisfied.