Temperature-independent Shift Research Articles

NMR properties of 51V in a complex high-temperature phase of Fe 2VSi

NMR properties of 51V in the high-temperature phase of Fe 2VSi with the α-Mn-type structure (A12) are reported. A free-decay signal with a temperature-independent shift and broad-spin echo signal with a temperature-dependent shift and hyperfine-coupling constant of −3.3 kOe/ μ B have been observed. Both come from nonmagnetic vanadium sites, and paramagnetic moment of 1.6 μ B/f.u. and is suggested to be due to magnetic iron atoms.

Crossover from divalent to valence fluctuating state of Eu in EuCu 2(Ge 1−x Si x) 2 probed by 63,65Cu - NMR

63,65Cu field sweep NMR investigations on a series of EuCu 2(Ge 1− x Si x ) 2 powder samples ( x = 0.3 , 0.7 , 1 ) are presented. EuCu 2(Ge, Si) 2 is unique among Eu-intermetallics because, upon tuning from a divalent antiferromagnetically ordered state for x = 0 to a valence fluctuating state for x = 1 , Heavy Fermion behaviour was found around x≈0.7. The Knight shift 63,65 K is negative for all stoichiometries. Hyperfine fields are largest for the antiferromagnetically ordered Ge-rich compound ( x = 0.3 ) and smallest for the valence fluctuating pure silicon compound ( x = 1 ). For the x = 0.3 compound, above T N≈19 K, 63 K( T) exhibits a Curie–Weiss like behaviour (≈ −10% at 20 K) whereas for EuCu 2Si 2 a temperature-independent shift of 63 K( T)≈−1.5% is found. Our results are discussed in terms of formation of heavy quasiparticles and/or presence of valence fluctuations of Eu.

Mechanisms of femtosecond energy transfer upon strong excitation in crystals

It is shown that the fast temperature-independent shift of the long-wavelength edge of fundamental absorption observed upon electron bombardment of alkali halide, alkaline-earth, and oxygen-containing crystals is caused by the impact mechanism. In the course of the impact shift of lattice ions, short-lived perturbation (mainly, of their valence shells) occurs. The perturbation fields in the vicinity of impact-shifted anions act for no more than 100 fs. As a result, the distribution of the density of states in the valence band of a crystal is distorted, and the long-wavelength shift of the fundamental-absorption edge may increase for less than 100 fs.

Supertransferred Hyperfine Fields at7Li: Variable Temperature7Li NMR Studies of LiMn2O4-Based Spinels

The temperature dependence of the 7Li NMR shift was measured for LiMn2O4, LiMn2-yNiyO4 (y = 0.1, 0.25, 0.33), LiMn2-yCoyO4 (y = 0.25, 0.5, 1.0), Li[Mn2-yLiy]O4 (y = 0.1, 0.33), and λ-MnO2 spinel oxides. The 7Li NMR shift can be separated into temperature-independent and -dependent components. The temperature-dependent shift follows the Curie−Weiss behavior of the bulk magnetic susceptibility. The temperature-independent shift is attributed to contributions from van Vleck and diamagnetic susceptibilities. Pauli susceptibility may also contribute to the temperature-independent shift in the nickel- and cobalt-substituted spinels. Supertransferred hyperfine (STH) coupling constants were derived from the 7Li NMR shifts and bulk magnetic susceptibility data. The progressive increase in average nominal manganese oxidation state from +3.5 to +4 results in an increase in the supertransferred hyperfine field at the 7Li nucleus in the lithium-substituted samples. Replacement of manganese by either cobalt or nickel a...

Comparison of thermopower behaviour in different superconductors

The in-plane normal-state thermoelectric power of the cuprate superconductors, like the resistivity and Hall angle, shows a remarkable systematic pattern as the hole concentration is decreased from the overdoped to the underdoped regime. In the overdoped regime where T C is reduced to small values, the cuprate thermopower is often approximately proportional to temperature, i.e. it resembles conventional metallic diffusion thermopower. Similar proportionality is seen in disordered metal, Chevrel-phase and fullerene superconductors (with a low-temperature anomaly ascribed to a strong electron-phonon interaction). As the hole concentration in the overdoped cuprates is decreased and T C increases to optimal values and then decreases again, there is an approximately temperature-independent shift of thermopower to more positive values. Surprisingly similar shifts are seen in more conventional superconductors such as NbN and Chevrel-phase compounds as the level of disorder is reduced.

Shell-model lattice dynamics and Mössbauer recoilless fraction of SnO.

A lattice-dynamical study of the phonon-dispersion relation in tin monoxide is presented in the framework of the shell model. The M\"ossbauer recoilless fraction f(T) has been evaluated over a wide range of temperatures. Except for a temperature-independent shift, the calculated values of -ln(f) are in good agreement with recent M\"ossbauer spectroscopy experiments on a highly textured powder sample. The phonon density of states shows two prominent bands with practically no mixed Sn and O characters, as identified from the ion-projected densities. The calculated ${\mathrm{\ensuremath{\mu}}}_{\mathrm{\ensuremath{-}}2}$ moment of the spectrum is in accordance with the value obtained from the measurements. \textcopyright{} 1996 The American Physical Society.

Systematic thermopower behaviour in superconductors

The in-plane thermopower of most of the cuprate superconductors (including the new Hg-based materials) shows a systematic pattern as the hole concentration is increased and conductivity increases, namely an approximately temperature-independent shift to more negative values. This pattern is intimately related to the value of T c, and so possibly to the mechanism of the superconductivity. In the overdoped limit where T c is reduced, the thermopower is often approximately proportional to temperature, as for conventional metallic diffusion thermopower, and as seen in fullerene superconductors. A remarkably similar pattern of thermopower shifts (but of smaller magnitude) is seen in NbN films. The similarity of cuprate and non-cuprate thermopower suggests a qualitatively similar mechanism.

Thermopower of an untwinned YBa2Cu3O7- delta crystal.

The thermopower and resistivity of a fully loaded and a partially unloaded crystal of ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7\mathrm{\ensuremath{-}}\mathrm{\ensuremath{\delta}}}$ (Y123) have been measured along the a and b axes. The temperature dependence of the a-axis thermopower in the fully loaded crystal is similar to that observed previously in crystalline and polycrystalline Y123, and rather smaller than is found in other high-${\mathit{T}}_{\mathit{c}}$ cuprates. The thermopower along the b axis, which is affected more strongly by the chains, shows substantial variation between the present and previous data, and we ascribe this to chain disorder. In the unloaded crystal there is a nearly temperature-independent shift to larger thermopower along the a axis, while the b-axis thermopower shows in addition an altered temperature dependence.

A 1H‐NMR method for determining temperature in cell culture perfusion systems

This report describes a noninvasive 1H-NMR method for measuring absolute temperatures (+/- 0.2 degrees C) in biological samples and, in particular, in cell culture perfusion systems, utilizing the linear temperature dependence of the water chemical shift relative to the temperature-independent shift of one of the components of the biological medium, e.g., pyruvate, acetate or lactate. The effects of flow on temperature can be monitored and appropriate adjustment of the temperature controller can be made.

Evidence for disorder-induced enhancement of magnetic interactions in superconducting Zr-Fe metallic glass by neutron irradiation

The electrical resistivity, superconducting Tc, and the normal state magnetic susceptibility have been measured in neutron irradiated Zr75Fe25 metallic glass. With doses up to 7.5*1017 n cm-2 the authors observe small and temperature independent shifts towards increased magnetic interactions in the electron gas magnetic susceptibility and the development of a feature in the normal state resistivity close to Tc which is related to magnetic interactions. These observations give evidence for enhanced magnetic interactions due to increased disorder as predicted by interaction theories.

Variable temperature proton chemical shifts of aqueous beryllium salt solutions

Proton chemical shifts have been measured in ∼1 mol kg–1 aqueous solutions of beryllium nitrate and chloride from 0 to 100°C. The results can be interpreted as arising from rapid proton exchange between bulk water and water in three ionic environments; the cationic complex Be(H2O)2+4 with effectively a temperature independent shift, a second hydration sphere oriented sufficiently by the electric field of the ion to have a small low field chemical shift, and water near the anions which has an upfield shift due to some structural effect. That a known tetrahydrated cation gives results which are consistent with a primary hydration number of four, is a key result and gives strong support to the contention, based on similar results obtained with MI cations, that these are also tetrahyrated.

ChemInform Abstract: PROTON CHEMICAL SHIFTS OF AQUEOUS ALUMINIUM SALT SOLUTIONS

AbstractDie chemischen Verschiebungen werden in wäßrigen 1 m A1‐nitrat‐, Al‐perchlorat‐ und Al‐sulfatlösungen gemessen.

Proton chemical shifts of aqueous aluminium salt solutions

Proton chemical shifts have been measured in 1M-aqueous solutions of aluminium nitrate, perchlorate, and sulphate over a range of temperature. The effects of hydrolysis and of acid addition have been studied in detail and have a marked influence on the shifts which arise as the exchange average between bulk water and three ionic environments: the cationic hydration complex Al(H2O)63+, which has a temperature-independent low-field shift, its second hydration sphere which has a small electric-field-induced shift, and broken water structure around the anion which thus introduces an upfield temperature-independent shift. Exchange averaging is not complete in the region in which measurements were made and this has introduced errors into previous determinations of hydration numbers. The measurements are shown to be consistent with a primary cationic hydration number of six.

Nuclear Magnetic Resonance of Paramagnetic Co3O4. I. Experimental Study

Nuclear magnetic resonance of Co 59 in polycrystalline Co 3 O 4 and ZnCo 2 O 4 , and Al 27 in CoAl 2 O 4 in paramagnetic state was observed. The cobalt signal was identified with the Co 3+ ions at the octahedral sites of the spinel structure. The line shift of Co 59 in Co 3 O 4 was separated into two terms: temperature-independent and temperature-dependent ones. The temperature-independent shift (1.5%) is interpreted as the second order chemical shift due to the mixing of the low lying excited state. The temperature-dependent shift is attributed to the hyperfine coupling between the cobalt nuclear spin and the electron spins on the Co 2+ ions at the tetrahedral sites. The isotropic hyperfine coupling constant A is estimated at 13×10 -4 cm -1 . The results on ZnCo 2 O 4 and CoAl 2 O 4 give supports to the above interpretations. The line shapes were studied and the thermal relaxation time T 1 was estimated to be 1.5×10 -5 sec.