Chiral States Research Articles

Creation of a Chiral Bobber Lattice in Helimagnet-Multilayer Heterostructures.

A chiral bobber is a localized three-dimensional magnetization configuration, terminated by a singularity. Chiral bobbers coexist with magnetic skyrmions in chiral magnets, lending themselves to new types of skyrmion-complementary bits of information. However, the on-demand creation of bobbers, as well as their direct observation remained elusive. Here, we introduce a new mechanism for creating a stable chiral bobber lattice state via the proximity of two skyrmion species with comparable size. This effect is experimentally demonstrated in a Cu_{2}OSeO_{3}/[Ta/CoFeB/MgO]_{4} heterostructure in which an exotic bobber lattice state emerges in the phase diagram of Cu_{2}OSeO_{3}. To unambiguously reveal the existence of the chiral bobber lattice state, we have developed a novel characterization technique, magnetic truncation rod analysis, which is based on resonant elastic x-ray scattering.

Interconversion between Möbius chiroptical states sustained by hexaphyrin dynamic coordination.

Harnessing the chiroptical properties of molecular Möbius rings is motivated by fundamental aspects while challenged by synthetic difficulties. Focusing on Möbius aromatic Zn(ii) hexaphyrin complexes, interconversion between two chiral states was achieved through binding and release of an amino ligand (forward/backward stimuli), leading to different chiroptical switching phenomena (amplification, on-off, inversion). The amine either supplies the chirality or behaves as an achiral effector regulating the Zn(ii)-binding of a second (chiral) carboxylato ligand. These results highlight the Möbius [28]hexaphyrin scaffold as an attractive chiral switchable unit.

Programmable chiral states in flocks of active magnetic rollers.

Inspired by nature, active matter exemplified by self-organization of motile units into macroscopic structures holds great promise for advanced tunable materials capable of flocking, shape-shifting, and self-healing. Active particles driven by external fields have repeatedly demonstrated potential for complex self-organization and collective behavior, yet how to guide the direction of their collective motion largely remains unexplored. Here, we report a system of microscopic ferromagnetic rollers driven by an alternating magnetic field that demonstrates programmable control of the direction of a self-organized coherent vortical motion (i.e., chirality). Facilitated by a droplet confinement, the rollers get synchronized and display either right- or left-handed spontaneous vortical motion, such that their moving direction determines the vortex chirality. We reveal that one can remotely command a flock of magnetic rollers to switch or maintain its chiral state by modulating a phase shift of the sinusoidal magnetic field powering the active rollers. Building on our findings, we realize a self-assembled remotely controlled micro-pump architecture capable of switching the fluid transport direction on demand. Our studies may stimulate new design strategies for directed transport and flocking robotics at the microscale based on active colloids.

Physical problems and experimental progress in layered magnetic topological materials

The intersection between layered magnetic materials and topological materials combines the advantages of the two, forming a material system with both the magnetic orders and topological properties within the minimum two-dimensional unit, i.e. layered magnetic topological materials. This type of material may host Dirac points, Weyl points, nodal lines, etc. which are associated with helical or chiral electronic states ranging from insulator, semimetal to metal. This results in lots of novel physical problems and effects, which attract much attention of scientists. In this paper, we focus our attention on intrinsic magnetic topological insulator, magnetic Weyl semimetal, magnetic Dirac semimetal, and take them for example to briefly review the interplay between magnetic orders and topological orders and recent experimental results. This emergent area requires further studies to explore more new material candidates, which is a challenging frontier of condensed matter physics.

Nanoarchitectonics for Coordination Asymmetry and Related Chemistry

Abstract Nanoarchitectonics is a concept envisioned to produce functional materials from nanoscale units through fusion of nanotechnology with other scientific disciplines. For component selection, coordination complexes with metallic elements have a wider variety of element selection because metallic elements cover ca. 80% of the periodic table of the elements. Application of nanoarchitectonics approaches to coordination chemistry leads to huge expansion of this concept to a much wider range of elements. Especially, coordination asymmetry strategy architects asymmetrical and/or chiral structures and/or electronic states through formation of metal coordination complexes, leading to functional material systems in certain anisotropy and selectivity. This review article presents expansion of the nanoarchitectonics concept to coordination asymmetry through collecting recent examples in the field of coordination asymmetry. Introduced examples are classified into several categories from various viewpoints: (i) basic molecular and material designs; (ii) specific features depending on interfacial media, space and contact with bio-functions; (iii) functions; (iv) supporting techniques such as analyses and theory.

Soliton locking phenomenon over finite magnetic field region in the monoaxial chiral magnet CrNb3S6

Previous magnetic measurements for the chiral magnet CrNb3S6 have shown that the soliton penetration and soliton dissipation in the chiral soliton lattice state strongly depend on the crystal's size and shape. We performed magnetic measurements for a thin single crystal of CrNb3S6 with a thickness 3 μm, permitting the existence of approximately 60 solitons, and observed the magnetization (M)—magnetic field (H) curve with a remarkable full M–H loop. By investigating the minor loop of the M–H curve in detail, we found that the inside of the full M–H loop allows an infinite number of quasi-stable states for each eigen number of solitons, utilizing a non-negligible distribution of the thickness around 3 μm. This study reveals that in a microcrystal with a chiral axis length of a few micrometers, any magnetic state with an arbitrary soliton number can be created in a certain H range within the full M–H loop.

Tunable Lattice Reconstruction, Triangular Network of Chiral One-Dimensional States, and Bandwidth of Flat Bands in Magic Angle Twisted Bilayer Graphene.

The interplay between interlayer van der Waals interaction and intralayer lattice distortion can lead to structural reconstruction in slightly twisted bilayer graphene (TBG) with the twist angle being smaller than a characteristic angle θ_{c}. Experimentally, the θ_{c} is demonstrated to be very close to the magic angle (θ≈1.08°). Here we address the transition between reconstructed and unreconstructed structures of the TBG across the magic angle by using scanning tunneling microscopy (STM). Our experiment demonstrates that both structures are stable in the TBG around the magic angle. By using a STM tip, we show that the two structures can be changed to each other and a triangular network of chiral one-dimensional states hosted by domain boundaries can be switched on and off. Consequently, the bandwidth of the flat band, which plays a vital role in the emergent strongly correlated states in the magic angle TBG, is tuned. This provides an extra control knob to manipulate the exotic electronic states of the TBG near the magic angle.

Spin\u2013valley Hall phenomena driven by Van Hove singularities in blistered graphene

Using first-principles calculations, we investigate the possibility of realizing valley Hall effects (VHE) in blistered graphene sheets. We show that the Van Hove singularities (VHS) induced by structural deformations can give rise to interesting spin–valley Hall phenomena. The broken degeneracy of spin degree of freedom results in spin-filtered VH states and the valley conductivity have a Hall plateau of ±e2/2h, while the blistered structures with time-reversal symmetry show the VHE with the opposite sign of sigma _{xy}^{K/K^{prime}} (e2/2h) in the two valleys. Remarkably, these results show that the distinguishable chiral valley pseudospin state can occur even in the presence of VHS induced spin splitting. The robust chiral spin–momentum textures in both massless and massive Dirac cones of the blistered systems indicate significant suppression of carrier back-scattering. Our study provides a different approach to realize spin-filtered and spin-valley contrasting Hall effects in graphene-based devices without any external field.

Chiral Instability of the Homogeneous State of a Ferromagnetic Film on a Magnetic Substrate

The conditions of formation of chiral magnetization distributions in the systems ferromagnet/superconductor and ferromagnet/paramagnet are theoretically determined. The formation of chiral states is caused by the magnetostatic interaction in inhomogeneous magnetic systems. The estimates performed demonstrate that the predicted effects can be experimentally observed.

Optical Hall response of bilayer graphene: Manifestation of chiral hybridized states in broken mirror symmetry lattices

The authors study the optical Hall response of bilayer graphene under sliding and twisting and, based on symmetry arguments, explain the effects of Kerr and Faraday rotations as well as the circular dichroism.

Theory of inversion- Z4 protected topological chiral hinge states and its applications to layered antiferromagnets

We study positions of chiral hinge states in higher-order topological insulators (HOTIs) with inversion symmetry. First, we exhaust all possible configurations of the hinge states in the HOTIs in all type-I magnetic space groups with inversion symmetry by studying dependence of the sign of the surface Dirac mass on surface orientations. In particular, in the presence of glide symmetry, for particular surface orientations, the surface Dirac mass changes sign by changing the surface terminations. By applying this result to a layered antiferromagnet (AFM), we find a difference in the hinge states between the cases with an even and odd number of layers. In the case of an even number of layers, which does not preserve inversion symmetry, positions of hinge states are not inversion symmetric. Nonetheless, these inversion-asymmetric hinge states result from the bulk topology. We show that their inversion-asymmetric configurations are uniquely determined from the symmetries and the topological invariant.

Reduction of the Spin Susceptibility in the Superconducting State of Sr_{2}RuO_{4} Observed by Polarized Neutron Scattering.

Recent observations [A. Pustogow et al., Nature (London) 574, 72 (2019).NATUAS0028-083610.1038/s41586-019-1596-2] of a drop of the ^{17}O nuclear magnetic resonance (NMR) Knight shift in the superconducting state of Sr_{2}RuO_{4} challenged the popular picture of a chiral odd-parity paired state in this compound. Here we use polarized neutron scattering (PNS) to show that there is a 34±6% drop in the magnetic susceptibility at the Ru site below the superconducting transition temperature. We measure at lower fields H∼1/3H_{c2} than a previous PNS study allowing the suppression to be observed. The PNS measurements show a smaller susceptibility suppression than NMR measurements performed at similar field and temperature. Our results rule out the chiral odd-parity d=z[over ^](k_{x}±ik_{y}) state and are consistent with several recent proposals for the order parameter including even-parity B_{1g} and odd-parity helical states.

Oscillatory chiral flows in confined active fluids with obstacles

An active colloidal fluid comprised of self-propelled spinning particles injecting energy and angular momentum at the microscale demonstrates spontaneous collective states that range from flocks to coherent vortices. Despite their seeming simplicity, the emergent far-from-equilibrium behavior of these fluids remains poorly understood, presenting a challenge to the design and control of next-generation active materials. When confined in a ring, such so-called polar active fluids acquire chirality once the spontaneous flow chooses a direction. In a perfect ring, this chirality is indefinitely long-lived. Here, we combine experiments on self-propelled colloidal Quincke rollers and mesoscopic simulations of continuum Toner-Tu equations to explore how such chiral states can be controlled and manipulated by obstacles. For different obstacle geometries three dynamic steady states have been realized: long-lived chiral flow, an apolar state in which the flow breaks up into counter-rotating vortices and an unconventional collective state with flow having an oscillating chirality. The chirality reversal proceeds through the formation of intermittent vortex chains in the vicinity of an obstacle. We demonstrate that the frequency of collective states with oscillating chirality can be tuned by obstacle parameters. We vary obstacle shapes to design chiral states that are independent of initial conditions. Building on our findings, we realize a system with two triangular obstacles that force the active fluid towards a state with a density imbalance of active particles across the ring. Our results demonstrate how spontaneous polar active flows in combination with size and geometry of scatterers can be used to control dynamic patterns of polar active liquids for materials design.

Chiral flat band superconductivity from symmetry-protected three-band crossings

We show that chiral (nearly) flat band superconductivity can develop and host novel Majorana fermions at a time-reversal pair of symmetry-protected three-band crossing points. Based on symmetry analysis, mean-field study, and superfluid stiffness calculation, we determine and analyze the irreducible pairing channels with flat band pairings in the low-energy spin-$1$ fermion theory. Flat band pairing can enhance superconductivity dramatically, where the critical temperature scales linearly in the interaction strength. While fully gapped flat band pairing states develop in the single-component pairing channels, we find chiral $\bar p\pm i\bar p$ flat band superconductivity in the multicomponent pairing channels. Three-dimensional itinerant Majorana fermions arise at the bulk nodal points, whereas Majorana arcs appear on the surface.

Mixed topology ring states for Hall effect and orbital magnetism in skyrmions of Weyl semimetals

Skyrmion lattices as a novel type of chiral spin states are attracting increasing attention, owing to their peculiar properties stemming from real-space topological properties. At the same time, the properties of magnetic Weyl semimetals with complex $k$-space topology are moving into the focus of research in spintronics. We consider the Hall transport properties and orbital magnetism of skyrmion lattices imprinted in topological semimetals, by employing a minimal model of a 2D mixed Weyl semimetal which, as a function of the magnetization direction, exhibits two Chern insulator phases separated by a Weyl state for an an in-plane magnetization direction. We find that while the orbital magnetization is topologically robust and Hall transport properties are very sensitive to the details of the spin distribution in accordance to the behavior expected from the recently discovered chiral Hall effect[1], their behavior in the region of the Chern insulator gap is largely determined by the properties of the so-called mixed topology ring states, emerging in domain walls that separate the skyrmion core from the ferromagnetic background. In particular, we show that these localized ring states possess a given orbital chirality which reverses sign as a function of the skyrmion radius, thereby mediating a smooth switching dynamics of the orbital magnetization of the skyrmion lattice. We speculate that while the emergent ring states can possibly play a role in the physics of Majorana states, probing their properties experimentally can provide insights into the details of skyrmionic spin structures.

Metallic states beyond the Tomonaga-Luttinger liquid in one dimension

In this paper, we propose some new strongly correlated gapless states (or critical states) of spin-1/2 electrons in 1+1-dimensions, such as doped anti-ferromagnetic spin-1/2 Ising chain. We find doped anti-ferromagnetic Ising chain to be a different metallic phase from the doped ferromagnetic Ising chain, despite the two have identical symmetry. The doped anti-ferromagnetic Ising chain has a finite energy gap for all charge-1 fermionic excitations even without pairing caused by attractive interactions, resembling the pseudo-gap phase of underdoped high Tc superconductors. Applying a transverse field to the ferromagnetic and anti-ferromagnetic metallic phases can restore the $Z_2$ symmetry, which gives rise to two distinct critical points despite that the two transitions have exactly the same symmetry breaking pattern. We also propose new chiral metallic states. All those new gapless states are strongly correlated in the sense that they do not belong to the usual Tomonaga-Luttinger phase of fermions, i.e., they cannot be smoothly deformed into the non-interacting fermion systems of the same symmetry. Our non-perturbative results are obtained by noticing that gapless quantum systems have emergent categorical symmetries, i.e., non-invertible gravitational anomalies), which are described by multi-component partition functions that are modular covariant. This allows us to calculate the scaling dimensions and quantum numbers of all the low energy operators for those strongly correlated gapless states. This demonstrates an application of emergent categorical symmetries in determining low energy properties of strongly correlated gapless states, which are hard to obtain otherwise.

Reconfigurable metamaterial for chirality switching and selective intensity modulation.

A reconfigurable metamaterial for chirality switching and selective intensity modulation is demonstrated experimentally. Through simple folding strategy, nonchiral state, single-band chiral states and dual-bands chiral states can be switched. Circular dichroism up to 0.94 is measured with folding angles of 70°. Meanwhile, selective intensity modulation is realized by the combined effect of folding angle and incident angle. The transmission intensity of circularly polarized waves can be modulated by more than 90% at any selected resonating frequency between 8.97 and 10.73 GHz. This work will benefit the researches of foldable metamaterials and have potential applications in the field of reconfigurable devices.

Anomalous Hall transport in tilted multi-Weyl semimetals

We study the effect of a perpendicular magnetic field B on a multinode Weyl semimetal (mWSM) of arbitrary integer monopole charge n, with the two Weyl multinodes separated in k-space. Besides type-I mWSMs, there exist type-II mWSMs which are characterized by the tilted minimal dispersion for low-energy excitations; the Weyl points in type-II mWSMs are still protected crossings but appear at the contact of the electron and hole pockets, after the Lifshitz transition. We find that the presence of a perpendicular magnetic field quantizes the occupation pockets due to the presence of Fermi tubes. In this theory, the Hilbert space is spanned by a set of n chiral degenerate ground states, and a countably infinite number of particle-hole symmetric Landau levels (LLs). We calculate the Hall conductivity for the tilt-symmetric case of type-I mWSM using the Kubo formula, in the zero-frequency (DC) limit, and recover the well-known vacuum contribution. We compute the Fermi surface corrections and show that the expression generalizes from the formula for elementary (n = 1) type-I WSMs. We derive an expression for the type-II mWSM Hall conductivity, which is bounded by a LL cutoff introduced on physical grounds. Interestingly, we find that the anomalous vacuum Hall conductivity is vanishing in the type-II phase at all temperatures. The corresponding thermal Hall and Nernst conductivities are evaluated and characterized for both phases. The qualitative and quantitative observations presented here may serve in the characterization of generic mWSMs of both types.

Dirac quantum well engineering on the surface of a topological insulator

We investigate a quantum well that consists of a thin topological insulator sandwiched between two trivial insulators. More specifically, we consider smooth interfaces between these different types of materials such that the interfaces host not only the chiral interface states, whose existence is dictated by the bulk-edge correspondence, but also massive Volkov-Pankratov states. We investigate possible hybridization between these interface states as a function of the width of the topological material and of the characteristic interface size. Most saliently, we find a strong qualitative difference between an extremely weak effect on the chiral interface states and a more common hybridization of the massive Volkov-Pankratov states that can be easily understood in terms of quantum tunneling in the framework of the model of a (Dirac) quantum well we introduce here.

Self-assembly of plasmonic chiral superstructures with intense chiroptical activity

Chiral nanostructures are asymmetric nanoarchitectures that cannot be superimposed with their mirrored-symmetric counterparts, which have attracted considerable attention due to their special photophysical properties and potential applications in plasmonics, spectroscopy and nanosensors. In particular, Self-Assembly of chiral nanostructures with symmetric or asymmetric objects might exhibit exceptional optical activity because those chiral superstructures can manipulate chiral states of light that leads to circular dichroism (CD) effect. This review highlights recent advances on the self-assembly of plasmonic chiral superstructures from simpler dimeric, and trimeric chiral nanoassemblies to complicated chiral nanoarchitectures, especially emphasizes the resulted superior optical activity and the corresponding principles.