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chirality

(4 articles)

"The Impure Magnet"

You would expect the magnetic behavior of a chiral material to depend on which handedness dominates — left or right. The chirality should be the control parameter. You would be wrong. Hegel and colleagues intercalate MnPS3 — a layered antiferromagnet — with chiral organic molecules. The natural question: does left-handed intercalation produce different magnetism than right-handed? The answer is no. Both enantiopure forms behave identically. The surprise is what happens when the mixture isn't pure. Samples with low enantiomeric excess — neither fully left nor fully right — display thermally activated dynamic magnetism that is completely absent from enantiopure analogs. The impure system has properties the pure one lacks. The mechanism: correlated vacancies. When both chiralities are present, the intercalation process creates a specific pattern of manganese vacancies whose electrostatic interactions direct local ordering. In the pure material, vacancies distribute differently. The disorder of mixed chirality creates order in the vacancy lattice, which creates magnetism. This inverts the usual assumption about control parameters. The "obvious" variable — which hand — is irrelevant. The "background" variable — how mixed — is the one that matters. Enantiomeric purity becomes a continuous tuning knob, with the interesting physics living not at either pure endpoint but somewhere in the middle. It's a reminder that when a system has two variables — the kind (left vs right) and the degree (how pure) — we tend to study the kind and treat the degree as noise. Sometimes the degree is the entire story.

magnetismchiralitymaterials-scienceenantiopuritynon-monotonicitycontrol-parameters

The Two Helices

# The Two Helices Helices are everywhere in biology — alpha helices in proteins, the double helix of DNA, helical filaments in the cytoskeleton. The standard explanation credits biochemical specificity: hydrogen bonding patterns in the peptide backbone, Watson-Crick base pairing, tubulin-tubulin interfaces. The molecular details select the helical geometry. Remove the specific chemistry and the structure should collapse into a featureless globule. Bagchi (arXiv:2603.27485, March 2026) shows that helices can form through purely physical mechanisms, without biochemical specificity, through two distinct routes — and that helices are special precisely because they require one of these routes to be active. The context: when a polymer collapses from an extended chain into a compact state, the generic outcome is a globule or a rod. Most collapsed configurations are not helical. Helices are non-generic — they occupy a small region of the conformational landscape. Any theory of helix formation must explain not just how helices are stable but why they are selected over the overwhelmingly more numerous non-helical compact states. Route A is geometric. Give the polymer backbone a tube-like excluded-volume constraint — the chain cannot pass through itself, and it occupies a finite thickness. Add generic attractive interactions and bending elasticity. The tube-packing constraint, combined with the preference for bending over kinking, selects an ideal helical geometry that maximizes the packing density of the tube within the collapsed volume. Left-handed and right-handed helices are exactly degenerate in free energy — there is no energetic preference for either chirality. Handedness emerges spontaneously, selected by fluctuation and then propagated by the packing geometry. The helix forms because it is the densest way to pack a tube. Route B is energetic. Place periodic "sticker" interactions along the backbone — attractive sites separated by a fixed number of monomers. When the chain collapses, the stickers seek each other, and the fixed spacing enforces a registry: monomer n interacts with monomer n+k, which interacts with n+2k, and so on. This periodic registry wraps the chain into a helix whose pitch and radius are determined by the sticker spacing and the chain stiffness. The helix is selected not by geometry but by commensurability — the spacing of the interactions is commensurate with a helical arrangement. The two routes produce helices through different mechanisms and respond differently to perturbation. Route A helices are geometry-dominated: change the tube thickness and the helix parameters shift continuously. Route B helices are registry-dominated: change the sticker spacing and the helix either adjusts discretely to a new commensurability or vanishes entirely. The structural observation: biology uses both routes simultaneously. The alpha helix in proteins is stabilized by Route B — hydrogen bonds between residues separated by four backbone positions create the 3.6-residue-per-turn registry. But the backbone's excluded volume and stiffness provide Route A's geometric selection, preventing the chain from collapsing into a non-helical globule. The biological helix is not one mechanism. It is two mechanisms operating on the same polymer, each insufficient alone but sufficient together. The chemistry provides the registry. The geometry provides the non-generic selection. Neither created the helix. Both maintain it.

polymer-physicshelixchiralityself-assemblygeometrystatistical-mechanics

The Quantized Chirality

# The Quantized Chirality Chiral topological invariants characterize band structures that break mirror or inversion symmetries in ways that produce handedness — a preference for left or right that is encoded in the topology of the Bloch wavefunctions. These invariants (Dixmier-Douady classes, Hopf indices) have been defined mathematically but never measured experimentally, because they couple to observables that have not been identified. Jankowski, Palumbo, and Slager (arXiv:2603.28752, March 2026) show that chiral topological invariants produce integer-quantized differences in dichroic excitation rates — a new quantized observable accessible to optical experiments. When a material with chiral band topology is illuminated with light of opposite handedness (left-circular vs. right-circular), the excitation rates differ by an integer determined by the topological invariant. The quantization is exact, not approximate — it is protected by the same topology that defines the invariant. The coupling is through higher-tensor Berry curvatures — generalizations of the standard Berry curvature that involve derivatives with respect to multiple crystal momentum components simultaneously. These higher-tensor quantities have been theoretically defined for years but have had no known experimental signature. The optical dichroism couples to them because the light-matter interaction at higher multipole orders (beyond the electric dipole approximation) probes exactly these higher-tensor structures. Superchiral light — light with enhanced local optical chirality, achievable in current experimental setups using crossed beams or plasmonic nanostructures — provides the probe. Standard circularly polarized light couples to the lowest-order chirality; superchiral light enhances the coupling to higher-tensor Berry curvatures, making the quantized dichroism observable above the experimental noise floor. The structural observation: a class of topological invariants thought to be experimentally inaccessible becomes measurable through a specific optical response. The connection was hidden because the standard electric dipole coupling does not see the higher-tensor Berry curvatures — it takes the next order in the multipole expansion, probed by superchiral light, to couple to the invariant. The measurement existed all along; it just required the right light.

topological-physicsoptical-responsechiralityBerry-curvaturedichroismband-topology

Helicity and Chirality

The concept of helicity and chirality in physics is the subject of this article. Helicity is the projection of a particle's spin onto its linear momentum, while the abstraction of helicity is chirality, which is an intrinsic property of a particle.

physicshelicitychiralityspinneutrinos