Effect Of Dipolar Coupling Research Articles

Recoupled separated-local-field experiments and applications to study intermediate-regime molecular motions

A specific separated-local-field NMR experiment, dubbed Dipolar-Chemical-Shift Correlation (DIPSHIFT) is frequently used to study molecular motions by probing reorientations through the changes in XH dipolar coupling and T2. In systems where the coupling is weak or the reorientation angle is small, a recoupled variant of the DIPSHIFT experiment is applied, where the effective dipolar coupling is amplified by a REDOR-like π-pulse train. However, a previously described constant-time variant of this experiment is not sensitive to the motion-induced T2 effect, which precludes the observation of motions over a large range of rates ranging from hundreds of Hz to around a MHz. We present a DIPSHIFT implementation which amplifies the dipolar couplings and is still sensitive to T2 effects. Spin dynamics simulations, analytical calculations and experiments demonstrate the sensitivity of the technique to molecular motions, and suggest the best experimental conditions to avoid imperfections. Furthermore, an in-depth theoretical analysis of the interplay of REDOR-like recoupling and proton decoupling based on Average-Hamiltonian Theory was performed, which allowed explaining the origin of many artifacts found in literature data.

Dipole coupling effect of holographic fermion in charged dilatonic gravity

In this Letter, we study the dipole coupling effect of holographic fermion in a charged dilatonic black hole proposed by Gubser and Rocha (2010) [1]. It is found that the property of Fermi liquid is rigid under perturbation of dipole coupling, and the Fermi momentum is linearly shifted. A gap is dynamically generated as the coupling becomes large enough and the Fermi surface ceases to exist as the bulk dipole coupling further increases.

Magnetization Reversal of Rectangular Particles: Closure States and Effect of Dipolar Coupling

We have investigated the magnetization configurations of ferromagnetic rectangular particles with lateral dimensions 1025 × 450 nm2 and two different thicknesses: 10 and 40 nm. For each thickness, we analyzed both an array consisting of isolated particles and a second one where the nanomagnets are put head-to-tail, with 85 nm interdot spacing, forming long chains of closely-spaced elements. Magneto-optical Kerr effect measurements and in-field magnetic force microscopy experiments were performed to achieve a deep comprehension of the magnetization reversal process. It is shown that inter-particle dipolar coupling substantially modifies the reversal mechanism in the case of the largest thickness, where it can delay or even suppress the occurrence of closure configurations of the magnetization (with one or more vortices), that are typical of isolated nanomagnets.

Homonuclear Dipolar Coupling and CPMG Spin-Echoes in NQR

The nuclear quadrupole resonance signals from both a single crystal and a powder sample of spin-1 nuclei under a Carr–Purcell–Meiboom–Gill are modeled numerically. While the single crystal clearly shows the effects of dipolar coupling for on-resonant pulses, the powder does not. However, for certain off-resonant conditions, the powder sample exhibits the same response as the single crystal. Experimentally, this corresponds to the observation of a rapid decay at these conditions. Using a powder sample of NaNO2, the functional form of the echo train, when the dipolar coupling is not refocused, is clearly different than the decay for an on-resonance sequence, and can be used to characterize the dipolar coupling.

Distance dependence of near-field fluorescence enhancement and quenching of single quantum dots

In fluorescence microscopy and spectroscopy, energy transfer processes between single fluorophores and fluorophore quencher pairs play an important role in the investigation of molecular distances or orientations. At distances larger than about 3 nm these effects originate predominantly from dipolar coupling. As these experiments are commonly performed in homogenous media, effects at the interface boundaries can be neglected. Nevertheless, the combination of such assays with single-molecule manipulation techniques such as atomic force microscopy (AFM) requires a detailed understanding of the influence of interfaces on dipolar coupling effects. In the presented work we used a combined total internal reflection fluorescence microscopy (TIRFM)–AFM setup to elucidate this issue. We measured the fluorescence emission emanating from single quantum dots as a function of distance from the apex of a gold-coated cantilever tip. As well as fluorescence quenching at close proximity to the tip, we found a nonlinear and nonmonotonic distance dependence of the fluorescence emission. To confirm and interpret our findings we performed calculations on the basis of a simplified multiple multipole (MMP) approach, which successfully supports our experimental data. Moreover, we revealed and quantified the influence of interfering processes such as field enhancement confined at interface boundaries, mirror dipoles and (resonant) dipolar coupling.

Effect of aspect ratio on the inter-surface plasmonic coupling of tubular gold nanoparticle

A theoretical study based on discrete dipole approximation (DDA) and coupling effect is presented on the tunable transverse surface plasmon resonance (SPR) in a gold nanotube with varying aspect ratio (AR). Because gold nanotube has the shape features from both rod and shell, both the AR and wall thickness can greatly affect the transverse SPR. It is observed that the maximum red shift can be obtained with small wall thickness and AR. By calculating the local field distribution, the physical mechanism of this multi shape factors controlled plasmon shifting has been illustrated by the coulombic interaction from the charges at the interfaces of gold nanotube. This study indicates that finding the surface charge distribution by calculating the local electric field can be used as an effective method to analyze the plasmonic characters in complicated metallic nanostructure.

Effect of interactions on edge property measurements in magnetic multilayers

This paper reports effects of inter-film interactions on static and dynamic magnetization behavior at film edges in magnetic trilayer stripe arrays under transverse applied fields. The trilayers consist of two magnetic films of Ni80Fe20, 10 nm and 20 nm thick that interact via dipolar fields across thin, nonmagnetic spacers from 1 nm to 20 nm thick. These trilayer films are patterned into stripe arrays by optical interference lithography. Ferromagnetic resonance spectra exhibit separate edge modes that are identified as excitation of edge modes primarily in one magnetic film or the other. When interlayer exchange is negligible, we observe several effects of dipolar edge-edge coupling including shifts of the edge saturation fields and changes in the mode intensity ratio relative to single-film edges. These interaction effects are reproduced by micromagnetic modeling and explained by simple models.

The effect of interelement dipole coupling in patterned ultrathin single crystal Fe square arrays

The correlation between the magnetic properties and the interelement separation in patterned arrays of ultrathin single crystal Fe films of 12 monolayers (ML) grown on GaAs(100) has been studied. The critical condition to form single domain remanent states in the square elements was found to be 10 μm in size and 20 μm for the interelement separation. The coercivity was also found to increase with the increasing interelement separation in the patterned arrays. These results are attributed to the competition between the large in-plane uniaxial anisotropy, the demagnetizing field, and interelement dipole coupling as determined semiqualitatively by the ferromagnetic resonance measurements.

A master-equation approach to the description of proton-driven spin diffusion from crystal geometry using simulated zero-quantum lineshapes

Measurements of proton-driven carbon-13 spin diffusion (PDSD) by NMR spectroscopy are a central component of structural analyses of biomolecules in the solid-state. However, the quantitative link between experimental PDSD data and structural information is difficult to make. Here we observe that a master-equation approach can be used to model full PDSD dynamics accurately in polycrystalline (13)C-labelled L-histidine·HCl·H(2)O under magic-angle spinning. In the master-equation approach, PDSD rates and effective dipolar couplings are related by a function of the carbon-carbon zero-quantum lineshapes; we find that numerical simulations of the zero-quantum lineshapes are sufficiently accurate so as to allow the calculation of PDSD rates that are in good agreement with the measured rates, directly from crystal geometry and with no adjustable parameters. Finally, using carbon-carbon internuclear distances we illustrate the potential of the master-equation approach for structural studies. Generalisation of these results to proton-driven carbon-13 spin diffusion in more complex molecular systems is readily envisaged.

Effect of magnetic dipole coupling on optical binding energies between molecules

Within the framework of molecular quantum electrodynamics, the method of induced multipole moments is used to compute the laser-induced intermolecular energy shift between an electric dipole polarizable molecule and a magnetic dipole susceptible molecule, and between two magnetic dipole polarizable molecules. The physical viewpoint adopted in the calculation is one in which an applied electromagnetic field induces molecular multipole moments in each entity, which couple via the retarded resonant multipole-multipole interaction tensors. Expectation values taken over the ground electronic states of the material system, and a radiation field state containing n photons, leads directly to the coupling energy. Results are obtained for external radiation in a fixed orientation relative to the internuclear axis, with a thorough polarization analysis being carried out for linearly and circularly polarized radiation propagating in directions parallel and perpendicular to the separation distance vector, as well as for a freely tumbling molecular pair. Comparison is made with field-induced discriminatory energy shifts occurring between optically active molecules - which also involve coupling of electric and magnetic dipole moments, and the leading contribution to optical binding forces arising within the electric dipole approximation.

Devices to facilitate magic angle studies in peripheral MRI

Purpose: Four devices are described as variants of a single concept to assist the imaging of tissues with T2 relaxation times substantially influenced by dipolar coupling. The signal levels of such tissues are subjected to “magic angle” effects and determined by their orientation to B0. Methods: The pneumatically actuated devices remotely move patient's anatomy so that the angle of tissues relative to B0 can be easily adjusted within a closed-bore MRI scanner. Images taken with tissues at various orientations are analyzed to study the dipolar coupling effects. The devices include: a small hand unit, a unit for the Achilles tendon, a 3DOF platform for imaging the knee, and an upper limb unit. Results: Results obtained with all four devices are illustrated, demonstrating their utility to manipulate anatomy in an MRI environment. Conclusion: The successful demonstration of the devices can facilitate a fuller clinical evaluation of magic angle-related phenomena. © 2010 Wiley Periodicals, Inc. Concepts Magn Reson Part B (Magn Reson Engineering) 37B: 220–225, 2010.

Spin wave excitations of a magnetic pillar with dipolar coupling between thelayers

It is demonstrated analytically that the spectrum of small-amplitude spatially uniformmagnetization excitations in an in-plane magnetized magnetic pillar with two ferromagneticlayers coupled by dipole–dipole interaction can be approximately described by thetraditional Kittel formula with reduced saturation magnetization and effective anisotropyfield. The spectrum consists of a quasi-symmetric and a quasi-antisymmetric mode, and theapparent reduction of saturation magnetization for the quasi-symmetric mode (≤50%) is much larger than that for the quasi-antisymmetric mode (≤10%). The effect of dynamic dipolar coupling between the nano-pillar layers could be partlyresponsible for the apparent reduction of static magnetization seen in many spin-torqueexperiments performed on magnetic nano-pillars.

Diphosphines with Strongly Polarized P−P Bonds: Hybrids between Covalent Molecules and Donor−Acceptor Adducts with Flexible Molecular Structures

A series of P-phospholyl-substituted N-heterocyclic phosphines was prepared and characterized by single-crystal X-ray diffraction and solution and solid-state (31)P NMR spectroscopy. The molecular structures are distinguished by the presence of P-P bonds of exceptionally variable lengths (2.35-2.70 A) that are all well beyond the standard distance of 2.21 A. The unique flexibility is best illustrated by a specimen 4f where minor conformational changes of remote substituents induce a deviation in P-P bond lengths of some 5 pm between crystallographically independent molecules in the same unit cell. Computational studies suggest to rationalize the bond elasticity as the consequence of a very flat potential energy basin that allows even weak forces to have large impact on bond lengths. Solid-state (31)P NMR studies show that the bond distance variation coincides with substantial changes in the magnitude and sign of (1)J(PP), which is explained in the context of a dominant Fermi contact contribution. A relation between increasing internuclear distance and decreasing magnitude of (1)J(PP) was experimentally proven by determination of effective dipolar coupling constants by the double-quantum dephasing experiment (DoDe) for the crystallographically independent conformers of 4f and further supported by comparison with calculated coupling tensors with inclusion of the anisotropic J-coupling. NMR studies revealed large discrepancies in the values of (1)J(PP) measured in solution and the solid state and a substantial temperature dependence of the former. Interpretation of this behavior was feasible by taking into account that the value of (1)J(PP) in solution is affected by both temperature-dependent equilibria between trans and gauche conformers and additional bond length relaxation that accompanies the dissolution process. Consideration of experimental observations and population analysis of computed electron densities suggested to classify the P-P bonds in the molecules under study as "dative" rather than "normal" covalent bonds and to address the compounds 4 as hybrids between covalent diphosphines and phosphenium-phospholide contact ion pairs.

Simulations of parallel and transverse remanences in textured nano-patterned thin film media

In this work the effects of dipolar coupling on the distribution of effective orientations in textured nano-patterned media is simulated. The modelled films consist of 50×50 cobalt grains of uniform diameters ( D=20 nm) arranged in hexagonal (triangular) arrays. The grains easy axes are distributed according to a Gaussian texture function with a standard deviation of 30° about the texture direction. For different array separations ( d), the distribution of anisotropy orientations is extracted from the simulated parallel M rp( β) and transverse M rt( β) remanence curves where β is the angle by which the film is rotated. For the non-interacting case, predictions show that, M rt( β)=d M rp( β)/d β which is consistent with Shrikman and Treves theory whereas for the interacting case, M rt( β) is deviated from d M rp( β)/d β. The extracted distribution of effective orientations is found to become narrower as the array separation is decreased which is due to dipolar-induced texturing effects.

Synthesis of Diazocine-Containing Poly(arylene ether sulfone)s for Tailored Mechanical and Electrochemical Performance

A series of poly(arylene ether sulfone) copolymers containing the diazocine unit (0, 20, 50, 80, 100 mol %) were synthesized via step-growth polymerization. Copolymer compositions exhibited number-average molecular weights exceeding 30 000 g/mol, and mechanically ductile films were obtained for all compositions. The random copolymers exhibited enhanced thermal stability (Td10% = 500−550 °C) and mechanical properties (tensile stress = 45−65 MPa), and the thermomechanical performance was predictable based on copolymer composition. A secondary transition in the dynamic mechanical analysis appeared at 28 °C, which was attributed to segmental motion of the bulky diazocine and biphenyl units. X-ray single crystallography and 1H NMR spectroscopy as a function of temperature revealed that the diazocine ring existed in the boat-shaped conformation. The effective dipolar couplings of site-specific 1H−13C dipolar couplings in the aromatic rings were measured using DIPSHIFT solid state NMR spectroscopy at various temperatures. The experimental data indicated that the bulkier diazocine group experienced slower ring-flip motion than the smaller phenylene group. Cyclic voltammograms of diazocine-containing poly(arylene ether sulfone)s exhibited two reversible reduction peaks at −0.68 and −1.21 V under inert and basic conditions. UV spectra of the diazocine group in the reduced state indicated an extended π-electron structure, which supported the boat- to planar-conformational change (molecular actuation) via redox reactions.

Probing Heteronuclear 15N−17O and 13C−17O Connectivities and Proximities by Solid-State NMR Spectroscopy

Heteronuclear solid-state magic-angle spinning (MAS) NMR experiments for probing (15)N-(17)O dipolar and J couplings are presented for [(2)H(NH(3)),1-(13)C,(15)N,(17)O(2)]glycine.(2)HCl and [(15)N(2),(17)O(2)]uracil. Two-dimensional (15)N-(17)O correlation spectra are obtained using the R(3)-HMQC experiment; for glycine.(2)HCl, the intensity of the resolved peaks for the CO and C-O(2)H (17)O resonances corresponds to the relative magnitude of the respective (15)N-(17)O dipolar couplings. (17)O-(15)N REDOR curves are presented for glycine.(2)HCl; fits of the initial buildup (DeltaS/S < 0.2) yield effective dipolar couplings in agreement with (+/-20%) the root-sum-squared dipolar couplings determined from the crystal structure. Experimental (15)N-(17)O REAPDOR curves for the (15)N resonances in glycine.(2)HCl and uracil fit well to the universal curve presented by Goldbourt et al. (J. Am. Chem. Soc. 2003, 125, 11194). Heteronuclear (13)C-(17)O and (15)N-(17)O J couplings were experimentally determined from fits of the quotient of the integrated intensity obtained in a heteronuclear and a homonuclear spin-echo experiment, S(Q)(tau) = S(HET)(tau)/S(HOM)(tau). For glycine.(2)HCl, (1)J(CO) was determined as 24.7 +/- 0.2 and 25.3 +/- 0.3 Hz for the CO and C-O(2)H resonances, respectively, while for uracil, the average of the two NH...O hydrogen-bond-mediated J couplings was determined as 5.1 +/- 0.6 Hz. In addition, two-bond intramolecular J couplings, (2)J(OO) = 8.8 +/- 0.9 Hz and (2)J(N1,N3) = 2.7 +/- 0.1 Hz, were determined for glycine.(2)HCl and uracil, respectively. Excellent agreement was found with J couplings calculated using the CASTEP code using geometrically optimized crystal structures for glycine.HCl [(1)J(CO)(CO) = 24.9 Hz, (1)J(CO)(COH) = 27.5 Hz, (2)J(OO) = 7.9 Hz] and uracil [(2h)J(N1,O4) = 6.1 Hz, (2h)J(N3,O4) = 4.6 Hz, (2)J(N1,N3) = 2.7 Hz].

Exchange interaction and magnetic domain formation in periodically inhomogeneous magnetic media

We investigate the formation of magnetic domains in a magnetic trilayer patterned using ion beam bombardment. The system consists of a finite array of in-plane magnetized ferromagnetic Fe elements embedded into an antiferromagnetically coupled $\mathrm{Fe}∕\mathrm{Cr}∕\mathrm{Fe}$ trilayer. Varying the interelement distance, we observe by means of magnetic force microscopy an intriguing transition from individual to collective behavior of the array elements. Above a critical interelement spacing, strong interelement coupling effects are observed, leading to complex correlations between domain structure on individual elements. The mechanism driving these correlations is the formation of domain boundary walls between elements, contrary to the more commonly observed dipolar coupling effects in magnetic arrays fabricated using lithography. Below this critical spacing, the entire array behaves as a single magnetic entity, exhibiting a collective magnetic domain state. The experimental observations can be simulated numerically and explained using an analytical model. The model correctly predicts observed dependencies on interelement distances.

Spin-wave excitations in arrays of asymmetric ferromagnetic nanorings

Calculations are reported for the spin-wave excitations in asymmetric ferromagnetic nanorings using a Hamiltonian-based formalism. Both the exchange and dipole-dipole interactions are included, as well as an external magnetic field and single-ion anisotropy. The equilibrium configurations of the nanorings are found by minimizing numerically the energy functional with respect to the spin orientation. Depending on the geometry, particularly the degree of asymmetry and the in-plane applied magnetic field, the nanorings can be in a vortex, onion, or other inhomogeneous state. Spin-dependent Green’s functions are calculated to deduce the dispersion spectra and the mode profiles of the spin waves, and the dependence of the spin-wave properties on the asymmetry factor is studied. The effects of inter-ring dipolar coupling on the spin-wave spectrum are also investigated by considering small arrays (e.g., a 3×3 square array) of nanorings.

Magnetization curves of dipolar coupled nanomagnets

AbstractInteracting superparamagnet (ISP) model that describes the effects of dipolar coupling on the magnetizing process is examined for assemblies of anisotropic nanomagnets by controlling the coupling strength using two different methods. In the first method, the magnetic moment of each nanomagnet is grown by annealing CoAg nanogranular films. In the second method, the number density of nanomagnets in solidified iron‐nitride magnetic fluids is controlled by the dilution in their melted states. Both the systems show temperature‐independent magnetizing curves at lower temperatures, as predicted by the ISP model. However, in contrast to the prediction, this feature can be primarily attributed not to the dipolar coupling but to magnetic anisotropy of each nanomagnet. (© 2008 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Residual Dipolar Couplings by Off-Magic-Angle Spinning in Solid-State Nuclear Magnetic Resonance Spectroscopy

A solid-state NMR analogue of the solution-state residual dipolar coupling effect is demonstrated. A small misset of the spinning axis by less than 0.5° changes the modulation frequency of spin−echoes, allowing the estimation of internuclear dipole−dipole couplings and hence internuclear distances.