Chirality Transitions Research Articles

Bloch points and topological dipoles observed by X-ray vector magnetic tomography in a ferromagnetic microstructure

Bloch points are small 3D magnetic textures with unit topological charge that require the use of advanced vector imaging techniques for their direct experimental characterization. Here we show results from the reconstruction of the magnetization field {{{{{bf{m}}}}}}left({{{{{bf{r}}}}}}right) of an elongated permalloy microstructure by X-ray vector magnetic tomography. A central asymmetric Bloch domain wall is observed, decorated by Bloch points arranged in several dipoles and a triplet. The analysis of the {{{{{bf{m}}}}}}left({{{{{bf{r}}}}}}right) map in terms of topological concepts provides a quantitative description of the Bloch points as topological monopoles connected by bundles of emergent magnetic field lines carrying a fractional topological flux. It also reveals the topological constraints that determine chirality transitions of the central domain wall in the vicinity of Bloch points, independent of specific material properties. This approach could be readily extended to the study of magnetic microstructures in arbitrary remanent configurations.

Variational analysis in one and two dimensions of a frustrated spin system: chirality and magnetic anisotropy transitions

<abstract><p>We study the energy of a ferromagnetic/antiferromagnetic frustrated spin system where the spin takes values on two disjoint circles of the 2-dimensional unit sphere. This analysis will be carried out first on a one-dimensional lattice and then on a two-dimensional lattice. The energy consists of the sum of a term that depends on nearest and next-to-nearest interactions and a penalizing term related to the spins' magnetic anisotropy transitions. We analyze the asymptotic behaviour of the energy, that is when the system is close to the helimagnet/ferromagnet transition point as the number of particles diverges. In the one-dimensional setting we compute the $ \Gamma $-limit of scalings of the energy at first and second order. As a result, it is shown how much energy the system spends for any magnetic anistropy transition and chirality transition. In the two-dimensional setting, by computing the $ \Gamma $-limit of a scaling of the energy, we study the geometric rigidity of chirality transitions.</p></abstract>

Chirality transitions in a system of active flat spinners

We study in this work the 2D dynamics of an experimental system of disk-shaped rotors, fluidized by turbulent upflow. Contrary to previous knowledge, our experiments show the same particle chiral geometry can produce flows with different chiralities. In particular, we unveil a conspicuous complex chiral flow, which displays multiple persistent vortexes with either sign, located randomly in the system. This peculiar phase mediates a continuous transition, which takes place as the kinetic energy input increases, from a flow with positive chirality (one vortex rotating in the same direction as particles spin) to a flow with negative chirality (one vortex in opposite sense to particle spin). We find that these surprising transitions are determined by the specific state of the statistical correlations between particle spin and translational velocity. We discuss how these correlations are determined in turn by the combined action of a series of mechanisms (for instance, heat dissipation at the boundaries, particle activity, average kinetic energy...), several of which are unveiled here.

Exchange-stiffness measurement scheme based on domain-wall chirality transition

Herein, we propose an experimental scheme to determine the strength of the Heisenberg exchange interaction in ultrathin magnetic films. In this scheme, the chirality transition between the Bloch- and Néel-type domain walls is analyzed under an in-plane magnetic field. Subsequently, the exchange stiffness constant is estimated based on the strength of the magnetic field for chirality transitions, as proposed by an analytical theory pertaining to chirality transitions, and confirmed based on micromagnetic prediction. Using the magneto-optical Kerr effect with high sensitivity on surface atomic layers, the present scheme is applicable to ultrathin magnetic films down to a few atomic layers, whose sensitivity is well below the lowest sensitivity limit of conventional measurement schemes. Hence, the present scheme is useful for extending the experimental range to investigate the exchange stiffness of few-atomic-layer-thick magnetic films.

Accelerating All-Atom Simulations and Gaining Mechanistic Understanding of Biophysical Systems through State Predictive Information Bottleneck.

An effective implementation of enhanced sampling algorithms for molecular dynamics simulations requires a priori knowledge of the approximate reaction coordinate describing the relevant mechanisms in the system. In this work, we focus on the recently developed artificial intelligence-based State Predictive Information Bottleneck (SPIB) approach and demonstrate how SPIB can learn such a reaction coordinate as a deep neural network even from undersampled trajectories. We exemplify its usefulness by achieving more than 40 times acceleration in simulating two model biophysical systems through well-tempered metadynamics performed by biasing along the SPIB-learned reaction coordinate. These include left- to right-handed chirality transitions in a synthetic helical peptide (Aib)9 and permeation of a small benzoic acid molecule through a synthetic, symmetric phospholipid bilayer. In addition to significantly accelerating the dynamics and achieving back and forth movement between different metastable states, the SPIB-based reaction coordinate gives mechanistic insights into the processes driving these two important problems.

The antiferromagnetic xy model on the triangular lattice: topological singularities

We study the discrete-to-continuum variational limit of the antiferromagnetic XY model on the two-dimensional triangular lattice in the vortex regime. Within this regime, the spin system cannot overcome the energetic barrier of chirality transitions, hence one of the two chirality phases is prevalent. We find the order parameter that describes the vortex structure of the spin field in the majority chirality phase and we compute explicitly the $\Gamma$-limit of the scaled energy, showing that it concentrates on finitely many vortex-like singularities of the spin field.

The antiferromagnetic XY model on the triangular lattice: chirality transitions at the surface scaling

We study the discrete-to-continuum variational limit of the antiferromagnetic XY model on the two-dimensional triangular lattice. The system is fully frustrated and displays two families of ground states distinguished by the chirality of the spin field. We compute the Gamma -limit of the energy in a regime which detects chirality transitions on one-dimensional interfaces between the two admissible chirality phases.

Diameter, strength and resistance tuning of double-walled carbon nanotubes in a transmission electron microscope

We report an atomic structure and property tailoring method that enables precise control of the diameter of individual double-walled carbon nanotubes (DWNTs), as well as the corresponding tuning of their tensile strength and electrical resistance. As demonstrated in a transmission electron microscope (TEM), this facile method involves the electron irradiation of DWNTs, followed by a thermal annealing process. The former process is responsible for the random atom loss and tube size reduction, whereas the latter allows for a complete structural recovery of defective nanotubes. The regarded DWNTs thus experience repeated disorder-order structural transitions, leading to the stepwise shrinkage in tube size, until the desired diameter is reached. Accordingly, these disorder-order transitions allow individual DWNTs to be controllably weakened and then strengthened, accompanied with the reversible changes in their breaking modes, as revealed by direct in-situ tensile tests. Differently, the resistance tuning of DWNTs upon irradiation/annealing does not follow the same trend, but depends significantly on the initial tube electrical characteristics and subsequent chirality transitions. These experimental results, combined with first-principles calculations, suggest that the change of tube chirality involved in the disorder-order transitions profoundly influences DWNT transport properties, but has a little impact on their mechanical strength.

Variational Analysis of a Two-Dimensional Frustrated Spin System: Emergence and Rigidity of Chirality Transitions

We study the discrete-to-continuum variational limit of the $J_{1}$-$J_{3}$ spin model on the square lattice in the vicinity of the ferromagnet/helimagnet transition point as the lattice spacing vanishes. Carrying out the $\Gamma$-convergence analysis of proper scalings of the energy, we prove the emergence and characterize the geometric rigidity of the chirality phase transitions.

Chirality transitions and transport properties of individual few-walled carbon nanotubes as revealed by in situ TEM probing

Physical properties of carbon nanotubes (CNTs) are closely related to the atomic structure, i.e. the chirality. It is highly desirable to develop a technique to modify their chirality and control the resultant transport properties. Herein, we present an in situ transmission electron microscopy (TEM) probing method to monitor the chirality transition and transport properties of individual few-walled CNTs. The changes of tube structure including the chirality are stimulated by programmed bias pulses and associated Joule heating. The chirality change of each shell is analyzed by nanobeam electron diffraction. Supported by molecular dynamics simulations, a preferred chirality transition path is identified, consistent with the Stone–Wales defect formation and dislocation sliding mechanism. The electronic transport properties are measured along with the structural changes, via fabricating transistors using the individual nanotubes as the suspended channels. Metal-to-semiconductor transitions are observed along with the chirality changes as confirmed by both the electron diffraction and electrical measurements. Apart from providing an alternative route to control the chirality of CNTs, the present work demonstrates the rare possibility of obtaining the dynamic structure-properties relationships at the atomic and molecular levels.

Chirality Transitions in Frustrated Ferromagnetic Spin Chains: A Link with the Gradient Theory of Phase Transitions

We study chirality transitions in frustrated ferromagnetic spin chains, in view of a possible connection with the theory of Liquid Crystals. A variational approach to the study of these systems has been recently proposed by Cicalese and Solombrino, focusing close to the helimagnet-ferromagnet transition point corresponding to the critical value of the frustration parameter $\alpha=4$. We reformulate this problem for any $\alpha\geq0$ in the framework of surface energies in nonconvex discrete systems with nearest neighbors ferromagnetic and next-to-nearest neighbors antiferromagnetic interactions and we link it to the gradient theory of phase transitions, by showing a uniform equivalence by $\Gamma$-convergence on $[0,4]$ with Modica-Mortola type functionals.

Chirality transitions in frustrated S2-valued spin systems

We study the discrete-to-continuum limit of the helical XY [Formula: see text]-spin system on the lattice [Formula: see text]. We scale the interaction parameters in order to reduce the model to a spin chain in the vicinity of the Landau–Lifschitz point and we prove that at the same energy scaling under which the [Formula: see text]-model presents scalar chirality transitions, the cost of every vectorial chirality transition is zero. In addition we show that if the energy of the system is modified penalizing the distance of the [Formula: see text]-field from a finite number of copies of [Formula: see text], it is still possible to prove the emergence of nontrivial (possibly trace-dependent) chirality transitions.

Superhelices with Designed Helical Structures and Temperature-Stimulated Chirality Transitions

The synthesis of multicomponent superhelices with determined chirality has been realized. By adding water to polymer solution in organic solvents (tetrahydrofuran/N,N′-dimethylformamide, THF/DMF), poly(γ-benzyl l-glutamate)-block-poly(ethylene glycol) (PBLG-b-PEG) are able to pack orderly around the surface of PBLG homopolymer bundles with designed helical structures, e.g., right-handed and left-handed, depending on THF/DMF ratio and temperature. A systematic investigation leads to the construction of a temperature–organic solvent composition phase diagram. Before the organic solvents were totally removed, the chirality of the assemblies can be reversibly switched using temperature stimulus. This temperature-stimulated chirality transition of polypeptide superhelices is unprecedented. Circular dichroism (CD) experiments indicated that the packing mode of pending phenyl groups from PBLG chain is responsible for the determination of the helical morphologies.

Chirality transitions in frustrated ferromagnetic chains

AbstractWe consider the Γ‐limit, as the number of particles diverges, of the energy per particle of a one‐dimensional ferromagnetic/anti‐ferromag frustrated S1 (or S2) spin chain close to the helimagnet/ferromagnet transition point discussing the emergence of chirality transitions. (© 2015 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Frustrated Ferromagnetic Spin Chains: A Variational Approach to Chirality Transitions

We study the energy per particle of a one-dimensional ferromagnetic/antiferromagnetic frustrated spin chain with nearest and next-to-nearest interactions close to the helimagnet/ferromagnet transition point as the number of particles diverges. We rigorously prove the emergence of chiral ground states, and we compute, by performing the $$\Gamma $$ -limits of proper renormalizations and scalings, the energy for a chirality transition.

Shape and chirality transitions in off-axis twist nematic elastomer ribbons

Using both experiments and finite element simulations, we explore the shape evolution of off-axis twist nematic elastomer ribbons as a function of temperature. The elastomers are prepared by cross-linking the mesogens with planar anchoring of the director at top and bottom surfaces with a 90° left-handed twist. Shape evolution depends sensitively on the off-axis director orientation at the sample midplane. Both experiments and theoretical studies show that when the director at midplane is parallel to either the ribbon's long or short axes, ribbons form either helicoids or spirals depending on aspect ratio and temperature. Simulation studies show that if the director at midplane is off-axis, ribbons never form helicoids, instead evolving to distorted spiral shapes. Experimental studies for two samples with off-axis geometry show agreement with this prediction. Samples in all these geometries show a remarkable transition from right- to left-handed chiral shapes on change of temperature. Simulations predict off-axis samples also change their macroscopic chirality at fixed temperature, depending on the angular offset. These results provide insight into the mechanisms driving shape evolution and macroscopic chirality, enabling engineering design of these materials for future applications.

Hot quark matter with an axial chemical potential

We analyze the phase diagram of hot quark matter in presence of an axial chemical potential, ${\ensuremath{\mu}}_{5}$. The latter is introduced to mimic the chirality transitions induced, in hot quantum chromodynamics, by the strong sphaleron configurations. In particular, we study the curvature of the critical line at small ${\ensuremath{\mu}}_{5}$, the effects of a finite quark mass and of a vector interaction. Moreover, we build the mixed phase at the first order phase transition line and draw the phase diagram in the chiral density and temperature plane. We finally compute the full topological susceptibility in presence of a background of topological charge.

Magnetic Chirality Transitions of D- and L-Alanine

A chiral spin state of (N +H) in D- and L-alanine was established by monitoring the temperature dependence of dc-magnetic susceptibility (dc: direct current) under the external magnetic field of 1 T. An intrinsic spin chirality of electrons in the atomic magnetic dipole moment of (N +H) was also supported by polarized Raman spectroscopy. Magnetic chirality was associated with a strongly correlated electron system that was related to spin rigidity. Raman vibrational spectra were unrelated to structural chirality but could reflect spin chirality due to the reversal of motion breaking. The spin transition of (N +H) occured at 270 K without bond breaking but was assisted by an intermediate hydrogen bond elongation, splitting and reformation with NH 3 + torsion. The energy difference of spin chirality transitions between D- and L-alanine was around 10 −4–10 −5 eV·molecule −1.

Neutrino helicity and chirality transitions in Schwarzschild space-time

We study the helicity and chirality transitions of a high-energy neutrino propagating in a Schwarzschild space-time background. Using both traditional Schwarzschild and isotropic spherical co-ordinates, we derive an ultrarelativistic approximation of the Dirac Hamiltonian to first-order in the neutrino's rest mass, via a generalization of the Cini-Touschek transformation that incorporates non-inertial frame effects due to the noncommutative nature of the momentum states in curvilinear co-ordinates. Under general conditions, we show that neutrino's helicity is not a constant of the motion in the massless limit due to space-time curvature, while the chirality transition rate still retains an overall dependence on mass. We show that the chirality transition rate generally depends on the zeroth-order component of the neutrino's helicity transition rate under the Cini-Touschek transformation. It is also shown that the chiral current for high-energy neutrinos is altered by corrections due to curvature and frame-dependent effects, but should have no significant bearing on the chiral anomaly in curved space-time. We determine the upper bound for helicity and chirality transitions near the event horizon of a black hole. The special case of a weak-field approximation is also considered, which includes the gravitational analogue of Berry's phase first proposed by Cai and Papini. Finally, we propose a method for estimating the absolute neutrino mass and the number of right-handed chiral states from the expectation values of the helicity and chirality transition rates in the weak-field limit.

Triangle anomaly in triple-Regge limits

Reggeized gluon interactions due to a single quark loop are studied in the full triple-regge limit and in closely related helicity-flip helicity-pole limits. Triangle diagram reggeon interactions are generated that include local axial-vector effective vertices. It is shown that the massless quark triangle anomaly is present as a chirality-violating infra-red divergence in the interactions generated by maximally non-planar Feynman diagrams. An asymptotic dispersion relation formalism is developed which provides a systematic counting of anomaly contributions. The asymptotic amplitude is written as a sum over dispersion integrals of triple discontinuities, one set of which is unphysical and can produce chirality transitions. The physical-region anomaly appears in the generalized real parts, determined by multi-regge theory, of the unphysical discontinuities. The amplitudes satisfy a signature conservation rule that implies color parity is not conserved by vertices containing the anomaly. In the scattering of elementary quarks or gluons the signature and color parity of the exchanged reggeon states are such that the anomaly cancels. At lowest-order, it cancels in individual diagrams after the transverse momentum integrations are performed.