Rotational Anisotropy Research Articles

Polar Kerr Effect and Time Reversal Symmetry Breaking in Bilayer Graphene

The unique sensitivity of optical response to different types of symmetry breaking can be used to detect and identify spontaneously ordered many-body states in bilayer graphene. We predict a strong response at optical frequencies, sensitive to electronic phenomena at low energies, which arises because of nonzero interband matrix elements of the electric current operator. In particular, the polar Kerr rotation and reflection anisotropy provide fingerprints of the quantum anomalous Hall state and the nematic state, characterized by spontaneously broken time-reversal symmetry and lattice rotation symmetry, respectively. These optical signatures, which undergo a resonant enhancement in the near-infrared regime, lie well within reach of existing experimental techniques.

On the Circular Birefringence of Polycrystalline Polymers: Polylactide

The circular birefringence of polycrystalline polymers is invariably obscured by strong linear birefringence. To parse the two mechanisms of light retardation, polycrystalline spherulites of polylactide enantiomers were analyzed by Mueller matrix microscopy. Polymer films are barely optically active in normal incidence, but if illuminated obliquely they become strongly optically active. Opposite hemispheres have oppositely signed circular birefringence. The sign is independent of the enantiomer but dependent on the sense of the sample's tilt. These observations are consistent with light path inhomogeneities resulting from stacked, mis-oriented lamellae. Chiroptical commonalities based on symmetry arguments are discussed among polylactide, a single oriented water molecule, and microfabricated metamaterial arrays, as well as the first physical model of optical activity, Reusch's pile of mica plates. The latter model provides the best explanation of the circular birefringence of polylactide spherulites. The only data on the optical rotation of crystalline polymers to date come from ostensible single crystals of polylactide. The enormous, anisotropic optical rotations observed previously are in quantitative agreement with misoriented lamellae observed here. Limitations of parsing circularly birefringent systems into those showing 'natural optical activity' and those others, somehow 'unnatural', are discussed.

Selective Probing of Photoinduced Charge and Spin Dynamics in the Bulk and Surface of a Topological Insulator

Topological insulators possess completely different spin-orbit coupled bulk and surface electronic spectra that are each predicted to exhibit exotic responses to light. Here we report time-resolved fundamental and second harmonic optical pump-probe measurements on the topological insulator Bi(2)Se(3) to independently measure its photoinduced charge and spin dynamics with bulk and surface selectivity. Our results show that a transient net spin density can be optically induced in both the bulk and surface, which may drive spin transport in topological insulators. By utilizing a novel rotational anisotropy analysis we are able to separately resolve the spin depolarization, intraband cooling, and interband recombination processes following photoexcitation, which reveal that spin and charge degrees of freedom relax on very different time scales owing to strong spin-orbit coupling.

Temperature dependence dynamical permeability characterization of magnetic thin film using near-field microwave microscopy

A temperature dependence characterization system of microwave permeability of magnetic thin film up to 5 GHz in the temperature range from room temperature up to 423 K is designed and fabricated as a prototype measurement fixture. It is based on the near field microwave microscopy technique (NFMM). The scaling coefficient of the fixture can be determined by (i) calibrating the NFMM with a standard sample whose permeability is known; (ii) by calibrating the NFMM with an established dynamic permeability measurement technique such as shorted microstrip transmission line perturbation method; (iii) adjusting the real part of the complex permeability at low frequency to fit the value of initial permeability. The algorithms for calculating the complex permeability of magnetic thin films are analyzed. A 100 nm thick FeTaN thin film deposited on Si substrate by sputtering method is characterized using the fixture. The room temperature permeability results of the FeTaN film agree well with results obtained from the established short-circuited microstrip perturbation method. Temperature dependence permeability results fit well with the Landau-Lifshitz-Gilbert equation. The temperature dependence of the static magnetic anisotropy H(K)(sta), the dynamic magnetic anisotropy H(K)(dyn), the rotational anisotropy H(rot), together with the effective damping coefficient α(eff), ferromagnetic resonance f(FMR), and frequency linewidth Δf of the thin film are investigated. These temperature dependent magnetic properties of the magnetic thin film are important to the high frequency applications of magnetic devices at high temperatures.

Critical thickness investigation of magnetic properties in exchange-coupled bilayers

We present a systematic investigation of the magnetic properties of two series of polycrystalline ferromagnetic-antiferromagnetic bilayers (FM-AF) of Ni${}_{81}$Fe${}_{19}$(10nm)/Ir${}_{20}$Mn${}_{80}$(${t}_{\mathrm{AF}}$) grown by dc magnetron sputtering. One series was grown at an oblique angle of 50\ifmmode^\circ\else\textdegree\fi{} and the other one was grown at 0\ifmmode^\circ\else\textdegree\fi{}. Ferromagnetic resonance (FMR) was used to measure the exchange bias field ${H}_{E}$, the rotatable anisotropy field ${H}_{\mathrm{RA}}$, and the FMR linewidth \ensuremath{\Delta}$H$ as a function of the antiferromagnetic layer thickness ${t}_{\mathrm{AF}}$. Three relaxation channels due to isotropic Gilbert damping, anisotropic two-magnon scattering, and mosaicity effects are simultaneously distinguished through the angular dependence of the FMR linewidth. In the regime of small IrMn layer thicknesses, not enough to establish the exchange bias anisotropy, the FMR linewidth shows a sharp peak due to the contribution of the two-magnon scattering mechanism. The results presented here are of general importance for understanding the dynamics of magnetization in the FM-AF structures.

Influence of ferromagnetic thickness on dynamic anisotropy in exchange-biased MnIr/FeCo multilayered thin films

A comprehensive study of the influence of ferromagnetic thickness on the static and dynamic magnetic properties in exchange-biased FeCo/MnIr multilayers for both strong and weak exchange-bias coupling cases is presented. The results demonstrate that static and dynamic magnetic anisotropy fields decrease with ferromagnetic thickness in both cases. The rising of rotational anisotropy is discussed in conjunction with the enhanced coercivity and exchange bias by taking into account the roles of the rotatable and frozen antiferromagnetic spins in each of the two cases. Due to the contributions of the exchange bias and rotational anisotropy, the resonance frequency can be tailored up to 10 GHz. In addition, the behaviors of the frequency linewidth and the effective damping factor are discussed and ascribed to the dispersion of magnetic anisotropy.

Probing Rotational Viscosity in Synaptic Vesicles

The synaptic vesicle (SV) is a central organelle in neurotransmission, and previous studies have suggested that SV protein 2 (SV2) may be responsible for forming a gel-like matrix within the vesicle. Here we measured the steady-state rotational anisotropy of the fluorescent dye, Oregon Green, within individual SVs. By also measuring the fluorescence lifetime of Oregon Green in SVs, we determined the mean rotational viscosity to be 16.49 ± 0.12 cP for wild-type (WT) empty mice vesicles (i.e., with no neurotransmitters), 11.21 ± 0.12 cP for empty vesicles from SV2 knock-out mice, and 11.40 ± 0.65 cP for WT mice vesicles loaded with the neurotransmitter glutamate (Glu). This measurement shows that SV2 is an important determinant of viscosity within the vesicle lumen, and that the viscosity decreases when the vesicles are filled with Glu. The viscosities of both empty SV2 knock-out vesicles and Glu-loaded WT vesicles were significantly different from that of empty WT SVs (p < 0.05). This measurement represents the smallest enclosed volume in which rotational viscosity has been measured thus far.

Hydrodynamic Interpretation on the Rotational Diffusion of Peroxylamine Disulfonate Solute Dissolved in Room Temperature Ionic Liquids As Studied by Electron Paramagnetic Resonance Spectroscopy

Rotational motion of a nitroxide radical, peroxylamine disulfonate (PADS), dissolved in room temperature ionic liquids (RTILs) was studied by analyzing electron paramagnetic resonance spectra of PADS in various RTILs. We determined physical properties of PADS such as the hyperfine coupling constant (A), the temperature dependence of anisotropic rotational correlation times (τ(∥) and τ(⊥)), and rotational anisotropy (N). We observed that the A values remain unchanged for various RTILs, which indicates negligible interaction between the N-O PADS group and the cation of RTIL. Large N values suggest strong interaction of the negative sulfonyl parts of PADS with the cations of RTILs. Most of the τ(∥), τ(⊥), and (τ(∥)τ(⊥))(1/2) values are within the range calculated on the basis of a hydrodynamic theory with stick and slip boundary conditions. It was deduced that this theory could not adequately explain the measured results in some RTILs with smaller BF(4) and PF(6) anions.

Negative Get-Togethers

Chemistry When transition-metal salts dissolve in water, it's not unusual for a number of the counterions to remain coordinated. In contrast, alkali metal ions such as sodium or potassium weren't traditionally thought to behave this way; dissolution of their salts conjures images of a sea of water molecules keeping anions and cations thoroughly apart. Recently, however, precise spectroscopic studies have been uncovering a more complicated scenario for these simple salts, in which cation and anion continue to influence one another in solution. In one such study, Bian et al. now present evidence that concentrated solutions of alkali thiocyanates (SCN−) exhibit anion clustering. The authors used two-dimensional infrared spectroscopy to measure vibrational energy transfer between isotopically light and heavy SCN− solutes as a probe of their proximity, and they found that in 10 M KSCN, over 90% of the anions gathered in clusters. Rotational anisotropy measurements implicated cluster sizes of approximately 18 anions. Diluting the solution reduced the apparent proportion of anions that gathered in clusters, as did shifting to smaller cations (lithium or sodium); cesium—the largest stable alkali—correspondingly induced the highest clustering proportion. Proc. Natl. Acad. Sci. U.S.A. 108 , 10.1073/pnas.1019565108 (2011).

Strong in-plane anisotropy of magneto-optical Kerr effect in corrugated cobalt films deposited on highly ordered two-dimensional colloidal crystals

The corrugated cobalt films are investigated by magneto-optical Kerr effect and vibrating sample magnetometer (VSM). The films are fabricated by depositing thin cobalt layers on a large area single-domain two-dimensional (2D) hexagonal close-packing colloidal crystal. Strong in-plane anisotropy of the Kerr rotation hysteresis loop is found in sharp contrast to isotropic hysteresis loops obtained by the VSM. The anisotropy of such magneto-optical Kerr rotation disappears when randomly distributed colloidal spheres were used as the substrate. Our observations show that the magneto-optical effect does not completely correspond with the average magnetization state in such a 2D periodic magnetic network.

Rotational Anisotropy Prevents Transition of Tachycardia to Fibrillation in the Ventricular Wall Model with Large Transmural Dispersion of Repolarization: A Simulation Study

Background: We have recently demonstrated in computer simulations that rotational anisotropy of ventricular fiber orientation decreases the sustainability of ventricular fibrillation (VF) even under the large transmural dispersion of repolarization (TDR). However, how the rotational anisotropy restrains VF is still unclear. Methods: To clarify this issue, we repeated simulations of scroll wave (SW) reentry in ventricular wall slab models, incorporating varying degrees of rotational anisotropy. Transmural gradient (electrical heterogeneity through the wall; epi-, midmyo-, and endo-cardial layers) was achieved by modifications of potassium currents. Then, we analyzed the dynamics of both SW and its filament (3-dimensional organizing center of SW). Results: In the control model without rotational anisotropy, larger TDR increased the difference of SW cycle lengths among the layers. Then such asynchronous SW meandering destabilized the transmural I-shaped filament, causing filament fragmentation, i.e., VF. In contrast, as the degree of rotational anisotropy increased, the I-shaped filament became more stable, preventing the degeneration into VF. Conclusions: Large TDR and rotational anisotropy increase independently the SW complexity; however, such combinations might prevent transition of stable SW to chaotic VF via the control of SW filament. Our finding might contribute to clarify the physiological significance of rotational anisotropy in ventricles.

Stability of Ventricular Fibrillation Is Altered by Wall Deformation after the Onset of Fibrillation

Background: There are many studies to investigate mechanisms of induction and maintenance of ventricular fibrillation (VF); however, the mechanisms have not been clarified yet. It is known that the thickness of ventricular wall varies with time during VF; i.e., thickness of left ventricular wall is increased whereas that of right ventricular wall is decreased. We hypothesized that such deformation of ventricular walls during VF alters the stability of VF. To clarify this issue, we conducted computer simulation of VF in the ventricular wall slab models deformed with time. Methods: We constructed slab models with 10-mm thickness for left ventricular wall and with 5-mm thickness for right ventricular wall. Electrical heterogeneity and rotational anisotropy through the ventricular wall were included. After the onset of VF, thicknesses of the slabs were varied dynamically. The scroll wave filament (3-dimensional reentrant center) was expressed as a continuum of phase singularities. Results: In the deforming right ventricular wall, filament trajectory on the epicardial surface was gradually decreased, confirming stability of the scroll wave reentry. In the deforming left ventricular wall, filament was separated into several parts and filament trajectories traced complicated patterns. In contrast, the scroll waves without deformation were terminated by the annihilation of all filaments. Conclusion: Deforming ventricular walls during VF play important roles to maintain VF.

Optical third-harmonic spectroscopy of the magnetic semiconductor EuTe

EuTe possesses the centrosymmetric crystal structure $m3m$ of rocksalt type in which the second-harmonic generation is forbidden in electric dipole approximation but the third-harmonic generation (THG) is allowed. We studied the THG spectra of this material and observed several resonances in the vicinity of the band gap at 2.2--2.5 eV and at higher energies up to 4 eV, which are related to four-photon THG processes. The observed resonances are assigned to specific combinations of electronic transitions between the ground $4{f}^{7}$ state at the top of the valence band and excited $4{f}^{6}5{d}^{1}$ states of ${\text{Eu}}^{2+}$ ions, which form the lowest energy conduction band. Temperature, magnetic field, and rotational anisotropy studies allowed us to distinguish crystallographic and magnetic-field-induced contributions to the THG. A strong modification of THG intensity for the 2.4 eV band and suppression of the THG for the 3.15 eV band was observed in applied magnetic field. Two main features of the THG spectra were assigned to $5d({t}_{2g})$ and $5d({e}_{g})$ subbands at 2.4 eV and 3.15 eV, respectively. A microscopic quantum-mechanical model of the THG response was developed and its conclusions are in qualitative agreement with the experimental results.

Dynamics of Water Clusters Confined in Proteins: A Molecular Dynamics Simulation Study of Interfacial Waters in a Dimeric Hemoglobin

Water confined in proteins exhibits dynamics distinct from the dynamics of water in the bulk or near the surface of a biomolecule. We examine the water dynamics at the interface of the two globules of the homodimeric hemoglobin from Scapharca inaequivalvis (HbI) by molecular dynamics (MD) simulations, with focus on water-protein hydrogen bond lifetimes and rotational anisotropy of the interfacial waters. We find that relaxation of the waters at the interface of both deoxy- and oxy-HbI, which contain a cluster of 17 and 11 interfacial waters, respectively, is well described by stretched exponentials with exponents from 0.1 to 0.6 and relaxation times of tens to thousands of picoseconds. The interfacial water molecules of oxy-HbI exhibit slower rotational relaxation and hydrogen bond rearrangement than those of deoxy-HbI, consistent with an allosteric transition from unliganded to liganded conformers involving the expulsion of several water molecules from the interface. Though the interfacial waters are translationally and rotationally static on the picosecond time scale, they contribute to fast communication between the globules via vibrations. We find that the interfacial waters enhance vibrational energy transport across the interface by ≈10%.

Local mobility in grafted polydimethylsiloxane melts

Spin–lattice relaxation time constants, T 1, were studied for low-molecular-weight linear and grafted polydimethylsiloxane over a wide temperature and frequency range. Quantitative evaluations of proton T 1 measurements indicated two relaxation processes: anisotropic rotation of methyl groups around the Si–C bond (low temperature process) and motions of the PDMS side-chains connected with the glass transition (high temperature process). Additional analyses of the T 1 relaxation dispersion profiles revealed specific local segment fluctuation times, which are characteristic of the coherent motions in the grafted polymer chains.

Magnetic domain crossover in FePt thin films

We have investigated the crossover in the magnetic domain structure of FePt thin films as a function of film thickness. We have directly observed by magnetic force microscopy (MFM) that at a critical thickness ${d}_{cr}\ensuremath{\sim}30\text{ }\text{nm}$ the orientation of the magnetization in the magnetic domains changes from in-plane alignment to a system of stripes in which a component perpendicular to the film plane points alternately in opposite directions. The same critical thickness was also estimated from in-plane magnetization vs field measurements. From the MFM images we have also found that the stripe period is an increasing function of the film thickness following a square root law. These data were interpreted with two different models that yield parameters (magnetization, anisotropy, and exchange stiffness) compatible with those determined from magnetization measurements. Films with thicknesses above ${d}_{cr}$ show a strong dependence of the domain configuration on the magnetic history. Rotatable anisotropy was found in these samples, with a rotational anisotropy field that became stronger with the increase in film thickness. Bubblelike domains could be also observed when the sample is saturated perpendicular to the film plane. All magnetic measurements as a function of film thickness can be interpreted using the same values of magnetization, anisotropy, and exchange stiffness.

A statistical mechanical theory of proton transport kinetics in hydrogen-bonded networks based on population correlation functions with applications to acids and bases

Extraction of relaxation times, lifetimes, and rates associated with the transport of topological charge defects in hydrogen-bonded networks from molecular dynamics simulations is a challenge because proton transfer reactions continually change the identity of the defect core. In this paper, we present a statistical mechanical theory that allows these quantities to be computed in an unbiased manner. The theory employs a set of suitably defined indicator or population functions for locating a defect structure and their associated correlation functions. These functions are then used to develop a chemical master equation framework from which the rates and lifetimes can be determined. Furthermore, we develop an integral equation formalism for connecting various types of population correlation functions and derive an iterative solution to the equation, which is given a graphical interpretation. The chemical master equation framework is applied to the problems of both hydronium and hydroxide transport in bulk water. For each case it is shown that the theory establishes direct links between the defect's dominant solvation structures, the kinetics of charge transfer, and the mechanism of structural diffusion. A detailed analysis is presented for aqueous hydroxide, examining both reorientational time scales and relaxation of the rotational anisotropy, which is correlated with recent experimental results for these quantities. Finally, for OH(-)(aq) it is demonstrated that the "dynamical hypercoordination mechanism" is consistent with available experimental data while other mechanistic proposals are shown to fail. As a means of going beyond the linear rate theory valid from short up to intermediate time scales, a fractional kinetic model is introduced in the Appendix in order to describe the nonexponential long-time behavior of time-correlation functions. Within the mathematical framework of fractional calculus the power law decay ∼t(-σ), where σ is a parameter of the model and depends on the dimensionality of the system, is obtained from Mittag-Leffler functions due to their long-time asymptotics, whereas (stretched) exponential behavior is found for short times.

COSMOLOGICAL SIMULATIONS OF MASSIVE COMPACT HIGH-zGALAXIES

In order to investigate the structure and dynamics of the recently discovered massive (M_* > 10^11 M_sun) compact z~2 galaxies, cosmological hydrodynamical/N-body simulations of a proto-cluster region have been undertaken. At z=2, the highest resolution simulation contains ~5800 resolved galaxies, of which 509, 27 and 5 have M_* > 10^10 M_sun, > 10^11 M_sun and > 4x10^11 M_sun, respectively. Effective radii and characteristic stellar densities have been determined for all galaxies. At z=2, for the definitely well resolved mass range of M_* > 10^11 Msun, the mass-size relation is consistent with observational findings for the most compact z~2 galaxies. The very high velocity dispersion recently measured for a compact z~2 galaxy (~510 km/s; van Dokkum et al 2009) can be matched at about the 1-sigma level, although a somewhat larger mass than the estimated M_* ~ 2 x 10^11 M_sun is indicated. For the above mass range, the galaxies have an average axial ratio <b/a> = 0.64 +/- 0.02 with a dispersion of 0.1, an average rotation to 1D velocity dispersion ratio <v/sigma> = 0.46 +/- 0.06 with a dispersion of 0.3, and a maximum value of v/sigma ~ 1.1. Rotation and velocity anisotropy both contribute in flattening the compact galaxies. Some of the observed compact galaxies appear flatter than any of the simulated galaxies. Finally, it is found that the massive compact galaxies are strongly baryon dominated in their inner parts, with typical dark matter mass fractions of order only 20% inside of r=2R_eff.

Anisotropy of wave propagation in the heart can be modeled by a Riemannian electrophysiological metric

It is well established that wave propagation in the heart is anisotropic and that the ratio of velocities in the three principal directions may be as large as v(f)v(s)v(n) approximately 4(fibers)2(sheets)1(normal). We develop an alternative view of the heart based on this fact by considering it as a non-Euclidean manifold with an electrophysiological(el-) metric based on wave velocity. This metric is more natural than the Euclidean metric for some applications, because el-distances directly encode wave propagation. We develop a model of wave propagation based on this metric; this model ignores higher-order effects like the curvature of wavefronts and the effect of the boundary, but still gives good predictions of local activation times and replicates many of the observed features of isochrones. We characterize this model for the important case of the rotational orthotropic anisotropy seen in cardiac tissue and perform numerical simulations for a slab of cardiac tissue with rotational orthotropic anisotropy and for a model of the ventricles based on diffusion tensor MRI scans of the canine heart. Even though the metric has many slow directions, we show that the rotation of the fibers leads to fast global activation. In the diffusion tensor MRI-based model, with principal velocities 0.25051 m/s, we find examples of wavefronts that eventually reach speeds up to 0.9 m/s and average velocities of 0.7 m/s. We believe that development of this non-Euclidean approach to cardiac anatomy and electrophysiology could become an important tool for the characterization of the normal and abnormal electrophysiological activity of the heart.

A patient specific electro-mechanical model of the heart

This paper presents a patient specific deformable heart model that involves the known electrical and mechanical properties of the cardiac cells and tissue. The whole heart model comprises ten Tusscher's ventricular and Nygren's atrial cell models, the anatomical and electrophysiological model descriptions of the atria (introduced by Harrild et al.) and ventricle (given by Winslow et al.), and the mechanical model of the periodical cardiac contraction and resting phenomena proposed by Moireau et al. During the propagation of the depolarization wave, the kinetic, compositional and rotational anisotropy is handled by the tissue, organ and torso model. The applied patient specific parameters were determined by an evolutionary computation method. An intensive parameter reduction was performed using the abstract formulation of the searching space. This patient specific parameter representation enables the adjustment of deformable model parameters in real-time. The validation process was performed using simultaneously measured ECG and ultrasound image records that were compared with simulated signals and shapes using an abstract, parameterized evaluation criterion.