When the electrons in a crystal pick out one direction over another that the crystal itself treats as equal. The name comes from liquid crystals, whose rod-shaped molecules line up along one direction while staying free to flow, as in a display screen. In some and related materials the electrons do something similar: a crystal with a square or hexagonal pattern suddenly conducts better along one axis than across it, although its atoms have hardly moved.
Going deeper
In a nematic liquid crystal, rods point the same way without forming a lattice. In an electronic nematic, the electrons of a square or hexagonal crystal start preferring one axis while the lattice barely changes, and the resistance along the two axes splits apart below the transition.
Order without a pattern
Most electronic orders in crystals repeat in space: a piles up electrons in a regular pattern, an alternates its . A nematic state does neither. It keeps the crystal’s repeating unit but treats directions differently – the electrons of a square lattice prefer the x axis to the y axis, say. The name is borrowed from liquid crystals, in which rod-shaped molecules point the same way without forming a lattice.
Because atoms and electrons are coupled, an electronic nematic always pulls the lattice slightly out of shape, and a distorted lattice in turn makes the electrons . The question in each material is which comes first. In the iron-based superconductors the electronic anisotropy is far larger than the tiny distortion of the lattice could explain, and its response to grows without limit as the transition approaches, which identifies the electrons as the driving force.
Where it appears
FeSe is the cleanest example: below about 90 K its square iron layers become slightly rectangular and its electronic properties strongly two-fold, without the magnetic order that accompanies this transition in the other iron-based superconductors – which moved attention to the iron orbitals as the cause. In the CsV3Sb5 and its relatives the charge order that sets in between about 80 and 100 K goes on, at lower temperature, to break the six-fold symmetry as well, and finds a single orientation in each domain.
In graphene and NbSe2 the superconducting state itself responds differently along different directions, for example in its critical field – a property called nematic superconductivity, whose origin in each case is still debated.
How to tell it from strain
Any strain also lowers the symmetry, and real samples are rarely free of it: a on a , a crystal glued to a holder, or a twisted bilayer that has relaxed already has a preferred direction. An intrinsic nematic state shows a at a definite temperature, forms domains of different orientation in an unstrained sample, and responds to a small applied strain far more strongly than an ordinary metal would. Measurements that combine these – elastoresistance, polarised scattering in the symmetry channel concerned, and imaging of domains – are what make the case convincingly.
For specialists
An electronic state that breaks the rotational symmetry of the lattice while keeping its translational symmetry, named by analogy with nematic liquid crystals. Its order parameter is a direction rather than a density wave: in a crystal it lowers four-fold to two-fold symmetry, in a hexagonal one six- or three-fold to two-fold. It appears in the iron-based superconductors – FeSe becomes nematic at about 90 K without magnetic order, with a small orthorhombic distortion that the electrons drive – in kagome metals such as CsV3Sb5 inside the charge-ordered state, and as two-fold anisotropy of the superconducting state itself in magic-angle graphene and few-layer NbSe2. Elastoresistance, polarised Raman scattering, scanning tunnelling microscopy and angle-dependent transport detect it; telling spontaneous nematicity from the effect of strain is the central difficulty.