Large Nuclear Quadrupole Moments Research Articles

NMR study of quasi-one-dimensional Ta<sub>4</sub>Pd<sub>3</sub>Te<sub>16</sub>

Low-dimensional systems often display rich physical phenomena, such as charge density wave (CDW), spin density wave (SDW), and superconductivity. Many superconductors were found at low dimensions where the superconductivity coexists with or adjoins another ordered state and has an unconventional nature. The high transition temperature (<italic>T</italic>_C) superconductivity has been realized in two-dimensional copper oxides and iron pnictides, where the superconducting phase is located in the vicinity of magnetic and nematic orders. This raises an interest in finding unconventional superconductivity in low-dimensional materials. Ta₄Pd₃Te₁₆ with a quasi-one-dimensional crystal structure was discovered to be superconducting with <italic>T</italic>_C∼4.6 K. It consists of PdTe₂ chains, TaTe₃ chains, and Ta₂Te₄ double chains along the crystallographic <italic>b</italic> axis. Band structure calculations indicate that Ta₄Pd₃Te₁₆ is an s-wave superconductor with pairing from phonons associated with the Te-Te p bonding and a Fermi-surface associated with Te p bands. Although scanning tunneling microscopy (STM) found that the superconducting gap structure in this system is more likely anisotropic without nodes, nodal gap behaviors were claimed by thermal conductivity and specific heat measurements. STM study suggested that the system is in the vicinity of an ordered state that shows a periodic modulation. This suggestion is consistent with the observation that the magnetoresistance shows an <italic>H</italic>-linear behavior without saturation up to 50 T. In this paper, we report nuclear magnetic resonance and nuclear quadrupole resonance investigations on Ta₄Pd₃Te₁₆. The spin-lattice relaxation rate (1/<italic>T</italic>₁) divided by the temperature, 1/<italic>T</italic>₁<italic>T</italic>, of ¹²⁵Te is almost a constant and obeys the Korringa relationship as in a normal metal, but 1/<italic>T</italic>₁<italic>T</italic> of ¹⁸¹Ta increases dramatically below 80 K. These results indicate strong electric-field-gradient (EFG) fluctuations, since ¹⁸¹Ta has a nuclear spin <italic>I</italic>=7/2 with a large nuclear quadrupole moment that couples to EFG, but ¹²⁵Te with spin <italic>I</italic>=1/2 can only relax by magnetic interactions. Upon cooling, both the full width at half maximum of the spectra and 1/<italic>T</italic>₁<italic>T</italic> show a sudden change at <italic>T</italic>=20 K, indicating a CDW transition takes place at <italic>T</italic>_CDW=20 K. This is consistent with the electronic structure calculation which implies that Ta₄Pd₃Te₁₆ prefers the CDW rather than the SDW. In the superconducting state, a Hebel-Slichter coherence peak appears in the temperature dependence of 1/<italic>T</italic>₁ of ¹²⁵Te just below <italic>T</italic>_C, which indicates that Ta₄Pd₃Te₁₆ is a fully gapped superconductor. 1/<italic>T</italic>₁ can be fitted by using a BCS gap with <inline-formula><mml:math xmlns:mml=http://www.w3.org/1998/Math/MathML display=inline><mml:mrow><mml:mi>Δ</mml:mi><mml:mo stretchy=false>(</mml:mo><mml:mn>0</mml:mn><mml:mo stretchy=false>)</mml:mo><mml:mo>=</mml:mo><mml:mn>1.76</mml:mn><mml:msub><mml:mi other=0>k</mml:mi><mml:mtext other=1>B</mml:mtext></mml:msub> <mml:msub><mml:mi other=0>T</mml:mi><mml:mtext other=1>C</mml:mtext> </mml:msub></mml:mrow></mml:math></inline-formula>. Although Ta₄Pd₃Te₁₆ is a multiband system, the main contribution to the DOS is from Te p orbitals. Therefore, the full gap derived from the ¹²⁵Te NMR is consistent with the suggestion from the band calculation that Ta₄Pd₃Te₁₆ should be an s-wave superconductor mediated by phonons associated with the Te-Te bonding if there are no spin fluctuations. Due to coexistence of CDW and superconducting, strong EFG fluctuations eliminate the coherence peak in the temperature dependence of 1/<italic>T</italic>₁ of ¹⁸¹Ta.

Effects of Large Nuclear Quadrupoles on Dielectric Properties of Glasses at Very Low Temperatures

The universal behaviour of amorphous solids at low temperatures, governed by atomic tunneling systems as described by the standard tunneling model, has long been a generally accepted fact. In the last years, however, measurements of dielectric two-pulse polarization echoes have revealed that nuclear quadrupole moments involved in atomic tunneling systems can cause specific material-dependent effects in magnetic fields. We have performed measurements of the dielectric properties of the two multicomponent glasses N-KZFS11 and HY-1, containing several percent of tantalum oxide and holmium oxide respectively. As 181Ta and 165Ho both carry very large nuclear quadrupole moments, these glasses are ideal candidates to study the influence of nuclear quadrupole moments on the properties of glasses at very low temperatures. Our measurements not only show unique dielectric behaviour in both glasses, but also differ significantly from various predictions of the standard tunneling model.

A molecule with small rotational constants containing an atom with a large nuclear quadrupole moment: The microwave spectrum of trans-1-iodoperfluoropropane

Using pulsed jet chirped-pulse, and cavity-based Fourier transform microwave spectroscopies over 900 transitions have been recorded for the title molecule in the 1–4 GHz and 8–18 GHz regions. The C 1 , C 2 and C 3 carbon-13 species have been observed in natural abundance allowing a substitution structure for the CCC backbone to be determined. Nearly all the transitions observed were either a-type R branches or b-type Q branches. No c-type transitions were observed consistent with only the trans conformer being present under our experimental conditions. The χ aa , χ bb , χ cc and χ ab components of the iodine nuclear quadrupole coupling tensor have been determined. Of note, several forbidden, Δ J ± 2 transitions, and one Δ J ± 3 transition were observed with quite reasonable intensity. These observations have been rationalized through considerations of near degeneracies between energy levels connected via a large χ ab value (≈1 GHz).

The chemical nature of nitrogen in native soil organic matter

H. Knicker, R. Frtind and H.-D. Ltidemann Institut for Biophysik und physikalische Biochemie der Universit~it, W-8400 Regensburg, FRG Fossil fuels and soil organic matter (SOM) together contain approximately five times more carbon than the biota and the atmosphere. Of this, soil organic matter accounts for about 30 % of the carbon present. In addition, SOM has an average carbon/nitrogen ratio of 10/1 and contains a huge fraction of the total ni- trogen available for plant growth [1]. Taking into account that the abundance of nitrogen in the earth's crust is much lower than that of carbon, this is a signif- icant fraction of the total nitrogen acces- sible to the biosphere. Under natural soil conditions, without the addition of miner- al fertilizers, SOM provides the majority of the nitrogen necessary for plant growth. It is also thought to be respon- sible for the interaction between agricul- tural biocides and the soil [5-7]. The chemical structure of this ubiquitous material, SOM, and especially the chem- ical nature of the nitrogen are thus of great and general importance. The mo- lecular structure of the nitrogen- containing fraction is, however, still a matter of controversy [2-4]. Structural models based on partial chem- ical analysis claim that a significant part of the nitrogen is present in the form of heteroaromatic structures, while NMR- spectroscopic studies on lSN-enriched composts and recent humic material found approximately 85 % of the signal intensity in the amide/peptide region of chemical shift and no signals in the range typical for heteroaromatic nitrogen. A major fraction of the native soil organic matter has been in the soil for several hundred to several thousand years [8, 9]. Compared to these time spans, laboratory-produced material has been fermented for at most 1 year, and it could be argued that heteroaromatic structures are only produced after much longer fermentation periods. This criti- cism may be overcome by the study of lsN-CPMAS spectra of soil organic mat- ter with natural lSN levels. This has not been achieved hitherto, because the low natural abundance (0.4 %) and the small gyromagnetic ratio of the 15N nucleus and therefore its low sensitivity in NMR experiments appeared to make this experiment an impossible one. The most abundant 14N-isotope (99.6 %) cannot be studied by high-resolution NMR because its large nuclear quadrupole moment leads to very broad and unresolved signals, especially in solid-state NMR [101. In previous systematic studies on 15N- enriched composts and organic soil extracts [11] our group optimized all spectral parameters for the ~sN-CPMAS experiment. A crude estimate showed that it should be possible to obtain 15N spectra with a tolerable signal-to-noise ratio after the accumulation of approximately one million transients. In the present paper we report on the first successful results of such experiments. Six German soils were studied as detailed in Table 1. In Figs. 1 and 2 some of the spectra obtained are shown. They fully corrobo- rate the conclusions drawn from the stud- ies of short-term composting exper-

127I, 185Re and 187Re Solid State NMR Measurements on (CH3)4AsIO4 and (CH3)4AsReO4

Tetramethylarsonium-metaperiodate shows 127I solid state NMR signals without any visible quadrupole splitting. Due to the very large nuclear quadrupole moments of the rhenium isotopes 185,187Re, the analogous Re compound exhibits the expected second order quadrupole NMR effects. The measured 185,187ReRe NMR powder spectra contain superimposed signals arising from two different types of ReO- 4 tetrahedra, indicating that part of the ReO4 - anions deviate only slightly from tetrahedral symmetry, while the other part are nearly undistorted ReO4 - anions. The temperature dependence of the solid state NMR spectra reveals phase transitions for both compounds, which are confirmed also by DSC analyses. Furtherm ore, the DSC analysis of the Re compound again proves the existence of the two different types of ReO4 - tetrahedra in the lattice of the high temperature modification.

ChemInform Abstract: Microwave Spectrum of Methyliodosilane: Determination of the Dipole Moment and eqQ of an Asymmetric Top Molecule Containing an Atom with a Large Nuclear Quadrupole Moment.

Abstract Microwave spectra of methyliodosilane and its deuterated species have been measured. A least-squares analysis for the observed frequencies yielded rotational constants and quadrupole coupling constants of the parent species as: A = 14 747.837 ± 0.054, B = 1893.504 ± 0.003, C = 1728.518 ± 0.003, χ aa = −1012.65 ± 0.43, η a χ aa = χ bb − χ cc = −161.96 ± 0.37, and | χ ab | = 478.29 ± 0.84 (all in MHz). The dipole moment of the parent species was determined from Stark effect measurement of some hyperfine components of the 3 03 -2 02 transition to be μ l = 1.862 ± 0.005, μ a = 1.831 ± 0.005, and μ b = 0.336 ± 0.005 D. Also the product among μ a , μ b , and χ ab was concluded to have a positive sign from the Stark shift observed. A brief discussion has been given on the procedure for determining the dipole moment of a molecule containing an atom with a large nuclear quadrupole coupling moment.

185Re and 187Re solid‐state NMR of metaperrhenates

Abstract127I solid‐state NMR investigations of specific metaperiodates with alkyl‐ or aryl‐substituted cations of main Group V revealed second‐order quadrupole effects, except in the case of tetraethylammonium and tetramethylphosphonium metaperiodate. In these compounds the 127I quadrupole interactions are either very small or vanish. This allowed the approximate evaluation of the electric field gradients of the analogous isomorphous metaperrhenates. Despite the very large nuclear quadrupole moments of the rhenium isotopes, the estimated values of the 185Re and 187Re quadrupole coupling constants of tetraethylammonium and tetramethylphosphonium metaperrhenate have an order of magnitude accessible to solid‐state NMR measurements. The 185Re and 187Re NMR powder spectra of rhenium compounds in non‐cubic lattice structures were measured for the first time. The complete 185Re and 187Re solid‐state NMR spectra of polycrystalline tetramethylphosphonium metaperrhenate were observed, showing temperature‐dependent second‐order quadrupole splittings. In this compound the ReO anions show very small deviations from tetrahedral symmetry, which were estimated from a point charge model and the measured quadrupole splittings. In contrast, the tetrahedral symmetry of the ReO ions of tetraethylammonium metaperrhenate is nearly undistorted, because no quadrupole splittings of the 185Re and 187Re signals were detectable in the powder NMR spectrum.

Microwave spectrum of methyliodosilane: Determination of the dipole moment and eqQ of an asymmetric top molecule containing an atom with a large nuclear quadrupole moment

Microwave spectra of methyliodosilane and its deuterated species have been measured. A least-squares analysis for the observed frequencies yielded rotational constants and quadrupole coupling constants of the parent species as: A = 14 747.837 ± 0.054, B = 1893.504 ± 0.003, C = 1728.518 ± 0.003, χ aa = −1012.65 ± 0.43, η a χ aa = χ bb − χ cc = −161.96 ± 0.37, and | χ ab | = 478.29 ± 0.84 (all in MHz). The dipole moment of the parent species was determined from Stark effect measurement of some hyperfine components of the 3 03-2 02 transition to be μ l = 1.862 ± 0.005, μ a = 1.831 ± 0.005, and μ b = 0.336 ± 0.005 D. Also the product among μ a , μ b , and χ ab was concluded to have a positive sign from the Stark shift observed. A brief discussion has been given on the procedure for determining the dipole moment of a molecule containing an atom with a large nuclear quadrupole coupling moment.

Quadrupole corrections to the rutherford scattering of charged particles

For a nucleus with a static ground state quadrupole moment the elastic scattering amplitude for a charged particle contains a first-order term, proportional to Q, describing a reorientation of the ground state. The term is obtained in perturbation theory together with second-order E1 and E2 terms and resulting cross-sections for unpolarized and polarized targets are given. For the latter there is a first-order quadrupole contribution to the cross-section. The quadrupole term is calculated by an expansion in η −1, where η is the Coulomb parameter, and by an expansion in partial waves. The results of scattering on a target completely polarized along the beam direction are shown to be well-described by a classical theory when η is greater than unity. At θ = π this classical quadrupole cross-section assumes the simple form, Q 10 Z 2 , independent of all characteristics of the bombarding particle, where Z 2 is the charge of the target. In energy regions where nuclear effects are negligible it is found that, although virtual E1 effects are negligible for most charged particles, there are considerable E2 and quadrupole contributions to cross-sections. However first order perturbation theory works only for light incident particles such as muons. The elastic scattering of muons from polarized nuclei would provide a method for the determination of large nuclear quadrupole moments.

On the Quantization of Angular Momenta in Heavy Nuclei

The individual particle model of nuclear structure fails to account for the observed large nuclear quadrupole moments. It is possible, however, to allow for the existence of the quadrupole moments, and still retain the essential features of the individual particle model, by assuming the average field in which the nucleons move to deviate from spherical symmetry. The assumptions underlying such an asymmetric nuclear model are discussed; this model implies, in particular, a quantization of angular momenta in analogy with molecular structure. The asymmetric model appears to account better than the extreme single particle model for empirical data regarding nuclear magnetic moments.