The property of an object that cannot be turned into its own mirror image, like a left and a right hand or a corkscrew that turns one way. Molecules can be chiral, and so can crystals: in tellurium the atoms form screw-shaped chains that wind either to the left or to the right. Electrons, and vibrations can take on a handedness too, and chiral materials can treat left- and right-turning light, or spins pointing one way or the other, differently.
Going deeper
A chiral object differs from its mirror image, as a left hand from a right. Tellurium crystals are built from screw-shaped chains that wind one way, and spins, currents and vibrations can choose a sense of rotation too – which is what lets chiral materials tell left from right.
Hands and mirrors
Hold a left hand up to a mirror and the image is a right hand; no amount of turning makes the two coincide. Anything with this property is chiral – the word comes from the Greek for hand. A screw, a spiral staircase and many molecules of life are chiral; a sphere, a cube and most simple crystals are not, because they contain a or a centre of symmetry that turns them into themselves.
Crystals can be chiral too. In tellurium and selenium the atoms bond in screw-shaped chains that wind either clockwise or anticlockwise along the axis, and a crystal grows in one sense or the other. Such crystals rotate the polarisation of light passing through them, and a current running along the screw makes them slightly magnetic, because ties the electrons’ spins to their motion along the helix.
Handedness beyond the lattice
In physics the word is used more widely. in Weyl semimetals come in two chiralities, and parallel electric and magnetic fields pump electrons from one kind to the other, which shows up as a negative – the chiral anomaly. In magnets the interaction makes spin spirals and skyrmions turn in one sense only. In the lattice vibrations at the corners rotate, carrying angular momentum as chiral . And in some materials the electrons themselves may order in a chiral pattern – loop currents proposed in such as CsV3Sb5, and chiral orbital currents in Mn3Si2Te6 that a current can switch.
Telling true from apparent
Not everything that seems to have a handedness is chiral in the strict sense. Parallel electric and magnetic fields, for instance, are turned into their mirror image by reversing time rather than by any rotation – ‘false’ chirality in Laurence Barron’s terms – whereas a helix is ‘truly’ chiral. The distinction decides which effects a handedness can produce on its own, such as rotating light or favouring one spin. Claims of chiral order in quantum materials have to show which kind they mean, usually with , the or measured in both senses.
For specialists
The absence of improper symmetry operations (mirror planes, inversion, rotoreflections), so that an object and its mirror image are distinct enantiomorphs. Chiral crystals belong to the 65 Sohncke , such as trigonal tellurium (P3121 or P3221), whose helical chains give natural optical activity, current-induced magnetisation and radial through spin–orbit coupling. Chirality also describes Weyl fermions, whose chirality sets the chiral anomaly and the chiral magnetic effect; that propagate one way; spin spirals and skyrmions with a fixed sense of rotation from the Dzyaloshinskii–Moriya interaction; phonons carrying angular momentum in the valleys of TMDCs; and proposed chiral orders such as loop currents in kagome metals and Mn3Si2Te6. Barron’s distinction between true and false chirality decides which physical effects a given handedness allows.