External Field Gradient Research Articles

Electrical influence on molecular conformation in glyoxal and glycine

Ab initio calculations have been performed to find the internal rotation potentials of glyoxal and glycine under the influence of different external electric fields and electric field gradients. Not only is the interconversion barrier height directly affected by both fields and field gradients, but also important is the reshaping of the internal rotational potentials. In certain regions, a field gradient introduces a particularly sharp dependence of the potential on the internal rotation angle. These results serve to give a quantitative assessment of the field and field gradient strengths necessary for influencing conformational energetics and dynamics. We also find that there is mostly a small interplay between electron correlation effects and the conformational effects of the electric field and field gradient, including those from sizable fields and field gradients.

Measurement of an electron’s electric dipole moment using Cs atoms trapped in optical lattices

We propose to measure the electron's permanent electric dipole moment (EDM) using cesium atoms trapped in a sparsely populated, trichromatic, far blue-detuned three-dimensional (3D) optical lattice. In the proposed configuration, the atoms can be strongly localized near the nodes of the light field and isolated from each other, leading to a strong suppression of the detrimental effects of atom-atom and atom-field interactions. Three linearly polarized standing waves with different frequencies create an effectively linearly polarized 3D optical lattice and lead to a strong reduction of the tensor light shift, which remains a potential source of systematic error. Other systematics concerning external field instability and gradients and higher-order polarizabilities are discussed. Furthermore, auxiliary atoms can be loaded into the same lattices as effective ``comagnetometers'' to monitor various systematic effects, including magnetic-field fluctuations and imperfect electric-field reversal. We estimate that a sensitivity 100 times higher than the current upper bound for the electron's EDM of $4\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}27}e\mathrm{cm}$ can be achieved with the proposed technique.

External-Field Shifts of the (199)Hg(+) Optical Frequency Standard.

Frequency shifts of the 199Hg+ 5d106s 2S1/2 (F = 0, MF = 0) to 5d96s2 2D5/2 (F = 2, MF = 0) electric-quadrupole transition at 282 nm due to external fields are calculated, based on a combination of measured atomic parameters and ab initio calculations. This transition is under investigation as an optical frequency standard. The perturbations calculated are the quadratic Zeeman shift, the scalar and tensor quadratic Stark shifts, and the interaction between an external electric field gradient and the atomic quadrupole moment. The quadrupole shift is likely to be the most difficult to evaluate in a frequency standard and may have a magnitude of about 1 Hz for a single ion in a Paul trap.

Coupled cluster investigation of Sternheimer shieldings and electric field gradient polarizabilities

A coupled cluster (CC) investigation is presented for the (generalized) Sternheimer shieldings and the electric field gradient (EFG) polarizabilities which describe the effect of external electric fields and field gradients on the electric field gradient at the nuclei. Calculations are performed for the linear molecules N2, CO, HF, C2H2, HCl, HCN, and HNC. Correlation effects are monitored by employing a hierarchy of CC models consisting of CCS, CC2, CCSD, and CC3. The effect of tight basis functions and core correlation is investigated by carrying out CCSD calculations with core-valence basis sets. Accurate theoretical estimates for EFGs, Sternheimer shieldings, and EFG polarizabilities are given and the effects of vibrational corrections are discussed. Our final estimates for the considered EFG properties can be used, for example, in simulations of electric field effects on the EFG at the nuclei in interacting molecules.

Low-frequency oscillations of Fermi-liquid domain wall in normal 3He

We studied low-frequency oscillations of a coherently precessing two-domain spin state in normal 3He. Three modes of oscillations were observed and the dependence of their frequencies on external field gradient were measured. It was found that two of these modes correspond to oscillation modes predicted earlier theoretically. The origin of the third mode is not yet clear.

Proton magnetic relaxation study of pretransitional phenomena in the isotropic phase of a nematic liquid crystal II. Presence of inner magnetic field gradients as revealed by self-diffusion study

Above the clearing point of a nematic LC self-diffusion was studied by the Hahn pulse sequence (90-t-180) in the presence of a weak (up to 4 Gs cm-1) permanent external magnetic field gradient G = ∂H/∂z. The non-linearity of spin-echo decays was interpreted as a result of the inner magnetic field gradient Gamma, indicating the magnetically non-homogeneous nature of the pretransitional zone of the LC. The pretransitional zone was considered to be biphasic (locally ordered clusters ⇔ isotropic surrounding). Due to the orientation of clusters in the magnetic field and the anisotropy of the diamagnetic susceptibility of their molecules, the diamagnetic susceptibility of clusters in the z direction should be different from that of their disordered surroundings. Therefore, clusters behave as specific filled particles, their diamagnetic response in the presence of G being different from that of their isotropic surroundings, i.e. clusters can experience a translational motion in the external field gradient. This leads to the peculiar diamagnetic separation in space, accompanied by an increase of the field gradient. The inner field gradient was shown to be proportional to G2t3 and to decrease with temperature.

Coronal Plasmoid Dynamics

AbstractA possible acceleration mechanism by the magnetic force is suggested for a plasmoid, modeled as a spherical body with a magnetic dipole situated in its center of mass. The governing equations of the motion of a dipole in an inhomogeneous magnetic field are solved analytically and numerically. Both methods show the possibility for the plasmoid to be accelerated in the direction of the external field gradient.

Cochlear outer hair cell bending in an external electric field

We have used a high-resolution motion analysis system to reinvestigate shape changes in isolated guinea pig cochlear outer hair cells (OHCs) evoked by low-frequency (2-3 Hz) external electric stimulation. This phenomenon of electromotility is presumed to result from voltage-dependent structural changes in the lateral plasma membrane of the OHC. In addition to well-known longitudinal movements, OHCs were found to display bending movements when the alternating external electric field gradients were oriented perpendicular to the cylindrical cell body. The peak-to-peak amplitude of the bending movement was found to be as large as 0.7 microm. The specific sulfhydryl reagents, p-chloromercuriphenylsulfonic acid and p-hydroxymercuriphenylsulfonic acid, that suppress electrically evoked longitudinal OHCs movements, also inhibit the bending movements, indicating that these two movements share the same underlying mechanism. The OHC bending is likely to result from an electrical charge separation that produces depolarization of the lateral plasma membrane on one side of the cell and hyperpolarization on the other side. In the cochlea, OHC bending could produce radial distortions in the sensory epithelium and influence the micromechanics of the organ of Corti.

129Xe and 131Xe NMR of Xenon in Mesophases of Cetyltrimethylammonium Bromide in Formamide

For the first time, noble gas NMR has been applied to a study of a lyotropic liquid crystal. 129Xe and 131Xe NMR spectra of natural xenon gas dissolved in pure formamide and three cetyltrimethylammonium bromide (CTAB)/formamide (FA) mixtures were recorded over wide temperature ranges. The CTAB/FA system forms micelles at low concentrations of CTAB and mesophases at higher concentrations and elevated temperatures. The phase transitions are observed in the series of xenon spectra. In addition, the free xenon gas signal is seen in the spectra at low temperature, when the CTAB/FA system has a coexisting solid/liquid phase. The gas signal indicates that Xe is not soluble in the solid but accumulates in macroscopic bubbles. The intensity variation of the central peak of the 131Xe NMR spectra shows the presence of a static external electric field gradient (EFG). However, the spectral satellites are not observed and, therefore, the magnitude of the EFG cannot be determined. The obvious reason for the missing 131Xe satellites is the orientation distribution of the EFG tensor leading to broadening of the satellites.

Drift and nucleation of magnetic domains in weak ferromagnet DyFeO3

The behavior of weak ferromagnetic strip domain structures is studied in ac and pulse magnetic fields in the presence of an external field gradient. Directed domains motion and the related process of nucleation of new domains are observed as the magnetic field reaches its threshold value.

Anab initiostudy of molecular hydrogen and its deuterated analogues partially oriented in nematic liquid crystals

Thermal expectation values for nuclear magnetic resonance (NMR) observables for molecular hydrogen and its isotopically substituted analogues partially oriented in nematic liquid crystal solvents are calculated from first principles. High-quality nuclear wavefunctions and available data for highly correlated electronic wavefunctions at many different geometries are used from the literature. In the present description vibrational anharmonicity, centrifugal distortion, the changes of the nuclear quadrupole coupling with internuclear distance, and the dependence of the molecular quadrupole moment on the rotational quantum number have been included. The results strongly support previous conclusions that hydrogens in nematic phases are predominantly oriented through the interaction between their molecular quadrupole moment and the non-zero average external electric field gradient present in these solvents. In addition, the present theoretical results provide the basis for obtaining reliable estimates of such a...

Derivative Hartree–Fock approach to molecular Sternheimer shielding. Calculation of intermolecular influence on nuclear quadrupole coupling

Molecular Sternheimer shielding constants, γ, the proportionality constants relating the electric field gradient at a quadrupolar nucleus to an external electric field gradient are usually introduced phenomenologically. In this report, we take a comprehensive view of the sensitivity of the electric field gradient at a nucleus to arbitrary external electrical potentials and we show how the response can be obtained from analytically determined properties via derivative Hartree–Fock theory. From application of this ab initio technique, values have been obtained for the first and second order changes in nuclear quadrupole coupling with respect to external fields and field gradients, as well as nearby ideal multipole moments, for HCN and HCl. These values have been used to evaluate the change in the nuclear quadrupole coupling for several weakly bound complexes and to provide a nonempirical approach to relative effects on Sternheimer shielding. In weak molecular complexes, the effect of uniform fields can be as sizable as the effect of external field gradients in the overall change in nuclear quadrupole coupling, and so the underlying issue of convergence of multipolar expansions is considered over a range of geometries. This is important for structural interpretations of both nuclear magnetic resonance (NMR) and microwave data, and a simple formula, representing a practical point of truncation, is presented for quadrupole coupling analysis.

Temperature dependence of nuclear shielding and quadrupolar coupling of noble gases in liquid crystals

A theoretical model for nuclear magnetic resonance (NMR) observables of noble gas atoms in nematic and smectic A liquid crystal environments is developed. It is used to account for the behavior of the nuclear shielding of 129Xe (σ) and the quadrupolar coupling of 21Ne (B) measured in 1-butyl-c-4-(4′-octylbiphenyl-4-yl)-r-1-cyclo-hexan-carbonitrile (NCB 84). The analysis provides detailed information on the orientational and positional order of the liquid crystal solvent and of the solute atoms. In the nematic phase, the decrease of σ with decreasing temperature is attributed to the increase of the density and orientational order parameter S of the solvent. The former factor determines the variation of the isotropic part of σ, while the latter governs the change in the anisotropic part (the anisotropy of the shielding tensor Δσ is of the order of −10 ppm). The prominent increase of σ in the smectic A phase is ascribed to the tendency of the atoms to occupy the interspaces between the smectic layers rather than their interiors (the deviation from the uniform distribution of the atoms is of the order of 5%). The main result is that the isotropic part of σ increases as the density of the solvent in the immediate neighborhood of the solute decreases. In the case of the quadrupolar coupling, the effect of the redistribution of the atoms is small, as there is no isotropic contribution to B. The behavior of the quadrupolar coupling of 21Ne in the nematic phase indicates that there are two distinct contributions to B. One is due to the distortion of the electron cloud from the spherical symmetry and the other arises directly from the external electric field gradient produced by the neighboring molecules. The temperature dependence of the latter contribution is not determined by the variation of S alone.

Anisotropic solute–solvent interactions in liquid crystals: An analysis of dipolar and quadrupolar couplings of methane in nematic solvents

The most complete sets of available dipolar and quadrupolar couplings obtained from the NMR spectra of methane and its deuterated analogs dissolved in two different liquid crystals are analyzed. The analyses are based on a generalized theory for the correlation between the vibration and rotation of partially oriented molecules, which retains the quadratic terms in the Taylor series describing the functional dependence of the orienting forces on the vibrational normal coordinates. These quadratic terms are essential for the cases where vibrational effects (e.g., isotope effects) on the molecular orientation are of importance. In both liquid crystals all the 17 couplings can be reproduced by three adjustable parameters which describe the forces acting on the CH or CD bonds. However, the resulting forces are unexpectedly strong, and physically more plausible solutions are obtained by allowing for an additional parameter Bdp+Bext. Bdp and Bext are contributions to quadrupolar couplings arising from direct perturbations of the molecular electron distribution and external electric field gradient due to the neighboring molecules, respectively. The values of Bdp+Bext corresponding to plausible solutions can be rationalized in terms of (1) orientation dependent direct perturbations of the order of 0.5% on the quadrupole coupling constant of the deuteron, or (2) an external field gradient produced by the quadrupole moments of the liquid crystal molecules surrounding a cylindrical cavity which encloses the methane molecule. Independent values of Bext, estimated previously by the quadrupolar couplings of the hydrogen molecule, are strikingly similar to the present values of Bdp+Bext. The results appear to be consistent with two different physical interpretations: the anisotropic forces experienced by the methane molecule may be dominated by (1) the van der Waals forces acting on its atoms, or (2) the interaction between its quadrupole moment and the external field gradient. The results also lend support to the assumption that the molecular distortions due to a liquid crystal environment are normally dominated by bond-bending deformations.

The suppression of hysteresis losses by an external nonuniform field — A quantitative phenomenological model

Suppression of hysteresis losses by an external field gradient is described quantitatively and compared with a recent experiment. By fitting the theoretical curves to the experimental data conclusions are drawn on the distribution and shape of the energy-wells (microdefects) responsible for the hysteresis of the domain wall motion. The microdefects are shown to give rise to localized wall-pinning points, spaced a few micrometers from each other in the investigated LPE garnet film.

131Xe NMR in probing electric field gradients in thermotropic nematogens

131Xe NMR spectra of natural xenon gas dissolved in three nematic liquid crystals were recorded at various temperatures. The spectra in the nematic phases of the liquid crystals display quadrupole couplings of 162 to 495 kHz depending upon liquid crystal and temperature. The coupling arises from the interaction of the 131Xe nuclear quadrupole moment with the average electric field gradient at the nucleus. The external electric field gradient introduced earlier in the literature to explain the orientation and solvent dependence of the deuteron quadrupole couplings in methane and molecular hydrogen in liquid crystals does not alone explain the observed results, but at least two, and most probably more, different contributions must be considered.

An Instrument For Measuring the Axial Remanent Magnetization of Long Sediment Cores

Summary This paper describes how DIGICO or MOLSPIN apparatus can form the basis of a new magnetometer for measuring vertical components of remanence along complete sediment cores. the core oscillates as a horizontal pendulum while longitudinal external field gradients are detected using a shielded fluxgate array. A deconvolution procedure recovers the vertical magnetization profile from the signal. These data can then be combined with horizontal component information to obtain records of palaeomagnetic inclination without recourse to subsampling. the instrument has been evaluated using a synthetic core.

Acetylene in nematic liquid crystals: A vibrational analysis of the observed dipolar couplings

Experimental dipole couplings for several isotopically substituted modifications of acetylene dissolved in the nematic liquid crystals 1132 (Merck ZLI 1132), EBBA [N-(4-ethoxybenzylidene)-2, 6-dideutero-4-n-butylaniline] and a 55 wt. % 1132/EBBA mixture are obtained from NMR measurements. The dipolar couplings are calculated using a model for the solvent–solute interaction. The interaction is taken to be of second rank tensorial form and the effects of the coupling between the vibrations and rotations are taken into account. The calculated values are in good agreement with the experimental results although some discrepancies exist. These discrepancies are discussed in terms of a possible contribution to the observed carbon–carbon coupling from the anisotropy in the indirect coupling. The results show that the interaction between the vibrations and rotations of the solute plays an important role in determining the observed couplings and that specific interactions with the liquid crystal need not be invoked to explain the results. Previous studies have shown that molecular hydrogen experiences an external electric field gradient due to its liquid crystal environment and that in 55 wt. % 1132/EBBA the value of this field gradient is zero at 301.4 K. It has also been shown that the interaction between this field gradient and the solute molecular quadrupole moment dominates the ordering of molecular hydrogen in liquid crystals, such as 1132 and EBBA, where the field gradient is large. A qualitative analysis indicates that this interaction is important for the ordering of acetylene and that other mechanisms also play a role.

Deep penetration fuelling of reactor-grade tokamak plasmas with accelerated compact toroids

The application of a novel but practical technique for central fuelling of reactor-grade tokamak plasmas by compact torus (CT) plasma rings driven by a coaxial accelerator is examined. It is emphasized that this is the only advanced method of controlled and localized deep penetration fuelling which can be developed in the near term and which is already applied in a currently operating experimental system. Assessment of the dynamics of the CT as it traverses the plasma requires quantification of several interrelated constraints, including ring decay, tilting, field line reconnection, deceleration in the external field gradient and ring expansion/contraction. A self-consistent, radial zoning scheme is employed to model the transport of the CT to the desired plasma deposition point. It is demonstrated that the injection velocity requirements are usually dominated by CT tilting and reconnection with the external toroidal field of the tokamak. The implications for CT fuel transport around the reconnection point are then examined. The application of this formalism to the TIBER Engineering Test Reactor permits the parameterization of fuelling requirements in terms of injection velocity, fuel mass, penetration distance, CT dimensions and repetition rate. The hardware implications for near-term applications are also assessed.

Molecular hydrogen in nematic liquid crystals. Unusual isotope effects

Dipolar and quadrupolar couplings measured from n.m.r. experiments are reported for H2, HD and D2 dissolved in several nematic liquid-crystal mixtures. These results are discussed in terms of the solute–solvent interactions. It has been shown that the interaction between the solute molecular quadrupole moment and a mean external electric field gradient due to the liquid crystal accounts for most but not all of the ordering of these solutes. In some of the liquid-crystal mixtures used the external field gradient is very small. The results indicate that at least one other mechanism is involved in the solute ordering. This mechanism makes a negative contribution to the order parameter of molecular hydrogen and has an unusual isotope dependence. This second contribution is discussed in terms of van der Waals forces which include both attractive and repulsive interactions between the solute and liquid crystal. It is also shown that this additional contribution is observed in the experimental couplings obtained in other nematic liquid crystals.