Nematicity and Strain
We have developed new techniques for performing magnetic resonance in crystals under uniaxial stress. Our work has uncovered a piezomagnetic effect, in which the direction of the ordered magnetic moments in an antiferromagnet changes in response to stress. NMR is inherently very sensitive to these effects, and we are actively investigating how applied stress in different crystals affects the electronic properties via the NMR response.
Applying stress along particular directions in a crystal can alter the fundamental properties of a material. Not only do the atoms in the lattice move, distorting the crystal symmetry, but these changes can also alter the electronic system in dramatic fashion. Materials that have high symmetries and degeneracy of electronic orbitals, for example, can be strongly affected by the application of stress, even though the applied forces shift the lattice positions by less than a percent.
The iron-based superconductors are a paradigmatic example of this effect. The relative energies of the electrons in the 3dxz and 3dyz orbitals of the iron shift dramatically under strain, even though the atoms themselves change position by a fraction of a percent.
In some cases, a crystal can spontaneously develop strain in order to minimize its free energy. This phenomenon is known as nematicity. A nematic phase breaks the underlying symmetry of the crystal lattice, and gives rise to anisotropic behavior. The word ‘nematic‘ comes from the Greek word for ‘needle‘, and is widely used to describe a phase of liquid crystals in which needle-like molecules tend to align with one another in a given direction.
Relevant Publications
Anisotropic nematic fluctuations above the ferroquadrupolar transition in TmVO$_4$ Journal Article
In: Phys. Rev. B, vol. 104, no. 20, pp. 205137, 2021.
Uniaxial strain control of spin-polarization in multicomponent nematic order of BaFe$_2$As$_2$ Journal Article
In: Nat. Commun., vol. 9, no. 1, pp. 1058, 2018.
Nuclear magnetic resonance probe head design for precision strain control Journal Article
In: Rev. Sci. Instrum., vol. 88, no. 10, pp. 103902, 2017.
NMR study of nematic spin fluctuations in a detwinned single crystal of underdoped Ba(Fe$_1-x$Co$_x$)$_2$As$_2$ Journal Article
In: Phys. Rev. B, vol. 94, pp. 165123, 2016.
NMR Evidence for Inhomogeneous Nematic Fluctuations in BaFe$_2$As$_2-x$P$_x$ Journal Article
In: Phys. Rev. Lett., vol. 116, pp. 107202, 2016.
NMR evidence for inhomogeneous glassy behavior driven by nematic fluctuations in iron arsenide superconductors Journal Article
In: Phys. Rev. B, vol. 92, no. 16, pp. 165116, 2015, ISSN: 1550-235X.