Spin Dipole-dipole Interactions Research Articles

Anatomy of large perpendicular magnetic anisotropy in free-standing Co/Ni (1 1 1) multilayer

We investigated the magnetic anisotropy energy (MAE) in the free-standing Co/Ni (111) multilayer as both Ni thickness dependence (tNi = 1-4MLs) and number of multilayer repetition times (N = 1–3) by means of a first-principles electronic structure calculation based on spin density functional theory. We included both contributions to the MAE from magnetocrystalline anisotropy energy (MCAE) originating from spin-orbit coupling and shape magnetic anisotropy energy (SMAE) originating from spin dipole-dipole interaction. The MCAE part was evaluated from both methods of total energy (TE) and grand-canonical force theorem (GCFT). The SMAE part was calculated by using a spin density approach (SDA). All MCAE values from the TE are well reproduced by those from the GCFT method. In N = 1, the total MAE (MCAE + SMAE) for tNi showed a perpendicular MAE (PMAE) with a maximum value of 1.67 mJ/m2 at tNi = 2MLs. The PMAE increases with increasing N. The series of tNi = 3MLs showed a linear behavior as N dependence with an increasing ratio of 0.68 mJ/m2, which is in good agreement with experimental measurement. By using the GCFT, we evaluated the atom-resolved and k-resolved MCAEs. The atom-resolved MCAE indicates that the Co/Ni interface is the main origin of PMAE. The PMAE is mainly located at Γ¯-K¯ line in the two dimensional Brillouin zone. This is attributed to large components of the d-orbitals extending along the multilayer plane on Co and Ni near the Fermi energy. We also calculated the SMAE using a discrete approach (DA) and found that there is a reduction of SMAE part in the SDA, compared to the DA. This reduction originates from a prolate quadrupole component of spin density distribution. The present comprehensive study may provide a better understanding of magnetic properties in Co/Ni multilayers as widely used in spintronic devices.

Tunneling spectroscopy and Majorana modes emergent from topological gapless phases in high-Tccuprate superconductors

We explore possible signatures for observing Majorana Fermions in the tunneling spectroscopy of high-Tc cuprate superconductors. We find that as long as the Rashba spin orbit interaction is in presence either through proximity effect due to an electrode made by heavy metal or by the intrinsic nature of cuprates, in addition to the Heisenberg spin exchange interaction, the Dzyaloshinskii-Moriya and spin dipole-dipole interactions are induced. As a result, $p$-wave superconductivity is induced with the gap function $d$-vector being not aligned with the internal magnetic field of the spin-orbit interaction. Most importantly, the ground state goes through transitions into gapless phases with split nodal points. The split nodal structure always results in Majorana modes for any interfaces that are not exactly in (100) or (010) directions. Hence for general interfaces, existence of Majorana bound edge states is a robust feature. Our results indicate that these Majorana modes would result in a small plateau in tunneling spectrum near zero bias peak and in 4 $\pi$ periodicity in typical SIS$'$ junctions . As a result, it is easy for a $\pi$-ring in tricrystal experiments to hold Majorana Ferimions and exhibit periods of two flux quanta in external magnetic fields. These phenomena may have been already observed in experiments and their connections to experimental results are discussed.

1H relaxation dispersion in solutions of nitroxide radicals: Influence of electron spin relaxation

The work presents a theory of nuclear ((1)H) spin-lattice relaxation dispersion for solutions of (15)N and (14)N radicals, including electron spin relaxation effects. The theory is a generalization of the approach presented by Kruk et al. [J. Chem. Phys. 137, 044512 (2012)]. The electron spin relaxation is attributed to the anisotropic part of the electron spin-nitrogen spin hyperfine interaction modulated by rotational dynamics of the paramagnetic molecule, and described by means of Redfield relaxation theory. The (1)H relaxation is caused by electron spin-proton spin dipole-dipole interactions which are modulated by relative translational motion of the solvent and solute molecules. The spectral density characterizing the translational dynamics is described by the force-free-hard-sphere model. The electronic relaxation influences the (1)H relaxation by contributing to the fluctuations of the inter-molecular dipolar interactions. The developed theory is tested against (1)H spin-lattice relaxation dispersion data for glycerol solutions of 4-oxo-TEMPO-d16-(15)N and 4-oxo-TEMPO-d16-(14)N covering the frequency range of 10 kHz-20 MHz. The studies are carried out as a function of temperature starting at 328 K and going down to 290 K. The theory gives a consistent overall interpretation of the experimental data for both (14)N and (15)N systems and explains the features of (1)H relaxation dispersion resulting from the electron spin relaxation.

1H NMR relaxation in glycerol solutions of nitroxide radicals: Effects of translational and rotational dynamics

(1)H spin-lattice relaxation rates in glycerol solutions of selected nitroxide radicals at temperatures between 200 K and 400 K were measured at 15 MHz and 25 MHz. The frequency and temperature conditions were chosen in such a way that the relaxation rates go through their maximum values and are affected by neither the electron spin relaxation nor the electron-nitrogen nucleus hyperfine coupling, so that the focus could be put on the mechanisms of motion. By comparison with (1)H spin-lattice relaxation results for pure glycerol, it has been demonstrated that the inter-molecular electron spin-proton spin dipole-dipole interactions are affected not only by relative translational motion of the solvent and solute molecules, but also by their rotational dynamics as the interacting spins are displaced from the molecular centers; the eccentricity effects are usually not taken into account. The (1)H relaxation data have been decomposed into translational and rotational contributions and their relative importance as a function of frequency and temperature discussed in detail. It has been demonstrated that neglecting the rotational effects on the inter-molecular interactions leads to non-realistic conclusions regarding the translational dynamics of the paramagnetic molecules.

Interactions of 2,2,2‐trifluoroethanol with melittin

Melittin dissolved in 42% trifluoroethanol-water at pH 2 has been shown to be alpha-helical between residues 6 and 12 and between residues 13 and 25, with the two helical regions separated by a bend at the Leu13 residue. The inter-helix angle was found to be 154 +/- 3 degrees at 0 degrees C and 135 +/- 3 degrees at 25 degrees C. The dominant conformation of the peptide is thus similar to those observed by previous workers for the peptide in a variety of media. At 25 degrees C, intermolecular nuclear Overhauser effects arising from nuclear spin dipole-dipole interactions between melittin hydrogens and fluorines of the solvent are essentially those expected for a system that is homogeneous as regards concentration and translational diffusion of the peptide and fluoroalcohol components. However, at 0 degrees C, peptide-trifluoroethanol cross-relaxation terms are negative, a result consistent with the conclusion that fluoroalcohol molecules associate with the peptide for times (approximately 1 ns) that are long compared to the time of a typical peptide-fluoroalcohol diffusive encounter (approximately 0.2 ns). Such interactions may be responsible for the reduction of the translational diffusion coefficient of trifluoroethanol produced by dissolved peptides.

Dynamic structures of motor proteins myosin and kinesin, and switch protein troponin as detected by SDSL-ESR

We have studied biological nano-machines, motor and switch proteins operating as supramolecular complexes by electron spin resonance (ESR) and found key features of their molecular movements. In all the systems, the specific movements of elements or domains were detected and quite dynamic at nanometer scale. We have observed two broad but distinct orientations, separated by a 25 degrees axial rotation, of a spin label attached specifically to the light chain (LC) domain of myosin motor in the muscle fibers. The distribution became only narrower upon muscle activation. ESR spectrum from the spin label of the neck-linker of dimeric kinesin motor consisted of immobilized and mobilized components and did not exhibit nucleotide-dependent mobility change. The distance between two labels of kinesin dimer was also measured by spin dipole-dipole interaction, showing a broad distribution and a nucleotide-dependent change on the nanometer scale (>1.5 nm). These results suggest that two LC domains of myosin and two neck linkers of kinesin play a similar role for sliding movement using two conformations. The spin label of the skeletal (Tn)-I regulatory domain (TnIreg) showed a large mobility change by Ca2+ ion suggesting a Ca-induced switch movement of TnIreg. Spin dipole-dipole interaction showed that in reconstituted muscle fibers both skeletal and cardiac TnC undergo Ca2+-induced structural change that is thought to be essential for TnIreg movement. We also succeeded in fixing the newly-synthesized bifunctional spin label rigidly on the TnC molecule in solution, indicating that we can determine the precise coordinate of the spin principal axis of troponin on the oriented filament.

Orientation and motion of myosin light chain and troponin in reconstituted muscle fibers as detected by ESR with a new bifunctional spin label.

Using electron spin resonance, we have studied dynamic structures of myosin neck domain and troponin C by site-directed spin labeling. We observed two broad but distinct orientations of a spin label attached specifically to a single cysteine (cys156) on the regulatoy light chain (RLC) of myosin in relaxed skeletal muscle fibers. The two probe orientations, separated by a 25 degrees axial rotation, did not change upon muscle activation, but orientational distributions became narrower substantially, indicating that a fraction of myosin heads undergoes a disorder-to-order transition of the myosin light chain domain upon force generation and muscle contraction. These results provide insight into the mechanism how myosin heads move their domains to translocate an actin filament. Site-directed spin-labeling was achieved by cysteine residues of human cardiac troponin C (TnC). Spin dipole-dipole interaction showed that free TnC undergoes a global structural change (extended-to-compact) by Ca2+ or Mg2+. The spectra from the spin labels at N-terminal half domain were broad and almost identical in parallel and perpendicular orientations of fiber, suggesting that the N-terminal of TnC molecule is flexible or disoriented with respect to the filament axis. We also succeeded, for the first time, in fixing the newly-synthesized bifunctional spin label rigidly on TnC molecule in solution (either in +/- Ca2+), giving a promise that we can determine the precise coordinate of the spin principal axis on protein surface.

Temperature dependence of solid-liquid scalar interactions in aqueous char suspensions by nonstationary DNP at low magnetic field

The temperature dependence of DNP enhancements in aqueous suspensions of hardwood chars by nonstationary DNP at low magnetic field has been investigated. With increasing temperature, the sign of the water proton DNP enhancement changed from negative to positive, indicating a change in the relative contributions from exchange (scalar) and dipole-dipole nuclear-electron coupling. A cross-over temperature was found at which the contributions of the dipole-dipole and exchange interactions canceled each other and the DNP effect disappeared. A model of molecular migration at the solid-liquid interface is constructed to account for these observations. Correlation of experimental and theoretical results has been achieved through consideration of the proton-electron spin dipole-dipole interaction and chemical exchange of solvent molecules. The increase with temperature in the contribution of the contact interaction to proton relaxation is a result mainly of a decrease in the lifetime τ h of water in the short-lived hydrated complex at the hardwood char interface. The enthalpy of activation E h = 13.5 kcal/mol for surface exchange diffusion as well as the lifetime of water molecules on the surface τ h(300K) = 1.9 × 10 −4s and hyperfine scalar constant a = 5.3 × 10 −3MHz are estimated. The results are characteristic of weak short-lived chemisorption bonds at the solid-liquid interface.

Intermolecular Dipole-Dipole Spin Relaxation of Xenon-129 Dissolved in Benzene. A Molecular-Dynamics Simulation Study

By use of molecular-dynamics simulations for xenon dissolved in liquid benzene, it is confirmed that the intermolecular dipole-dipole spin-relaxation mechanism contributes significantly to the relaxation of 129Xe. The calculated dipolar relaxation time (750 ± 160 s) lies in the range of the experimental determinations, which are 390 and 870 s. The time correlation function of the XeH spin dipole-dipole interaction displays a fast decay between 0 and 5 ps, decreases exponentially between 5 and 15 ps, and follows a t −3/2 power law at longer times. The XeH correlation function can be, in very good approximation, factored into distance and angular contributions; the latter is found to be the major contribution. The influence of the rotational dynamics of the solvent molecules is investigated on the basis of comparisons between the XeH time correlation functions and the corresponding functions for spins located at the center of mass of the benzene molecules. Finally, the relaxation of 129Xe and 131Xe are compared. It is shown that the quadrupole relaxation of 131Xe depends essentially on the rotational motions of the solvent molecules while the dipolar relaxation of 129Xe is largely dependent on the translational motions.

Dynamic imaging and spatially localized ESR spectroscopy of oriented phospholipid multilayers

Dynamic ESR imaging of various patterns (i.e., concentration profiles) of spin-labeled lipids in dimyristoylphosphatidylcholine multilayers provides a novel technique for macroscopic lateral-diffusion measurements at a spatial resolution of ∼50 μm. It is shown in this study that the use of the modulated-field-gradient method allows the simultaneous detection of one-dimensional spatial images and spatially localized ESR spectra of different regions of the sample. The spectral effects of local dehydration at the rim of multilayered samples is discussed, separating it from the fully hydrated region in the center and assigning the broad spectrum component to strong spin dipole-dipole interaction in this region. A new method is recommended for the analysis of dynamically rearranging images and a lipid lateral-diffusion coefficient of ∼6 × 10 −8 cm 2 s −1 is obtained from the filling of narrow concentration gaps.

AB initio calculation of the spin dipole-dipole interaction in ground-state triplets

Ab initio results are presented for the spin dipole-dipole interaction in the ground-state linear triplet radicals NCN, CNN and CCO. The relative contributions from the first-order spin dipole-dipole interaction and the second-order spin-orbit interaction, to the observed spin coupling constant in these systems, are discussed.