Electronic nematicity in iron-based superconductors
2014
Emerging evidence for the presence of strongly
anisotropic electronic states in the underdoped regime of both
cuprate and iron-based high temperature superconductors suggests
the possibility of an important role for electronic nematic order
in these materials. The central theme of my thesis work has been
the experimental study of electronic nematicity in iron-based
superconductors via measurement of resistivity anisotropy. To do
this, I have developed several new experimental techniques, on the
one hand enabling detwinning of sub-mm size single crystals in the
broken-symmetry orthorhombic state, and on the other hand revealing
the nematic susceptibility in the high-symmetry tetragonal state. A
major part of my thesis work has involved measurement of the
elastoresistance; that is, the change in the resistance of a
material as a consequence of the strains that it experiences. In
this thesis, I will show how differential elastoresistance
measurements can directly reveal the nematic susceptibility of a
material in the tetragonal state. I will introduce the appropriate
tensor formalism necessary to describe these measurements, and
describe an experimental technique to determine these coefficients
using piezoelectric stacks to provide anisotropic bi-axial strain.
Results in the tetragonal state of various underdoped families
based on the parent compound BaFe2As2 explicitly demonstrate that
the tetragonal-to-orthorhombic structural transition in these
materials is fundamentally driven by an electronic nematic
instability. These results also suggest that the resistivity
anisotropy in the paramagnetic orthorhombic state is dominated by
the Fermi surface anisotropy, rather than an anisotropy in the
scattering rate. Finally, similar measurements of a wide variety of
optimally doped iron-pnictides and iron-chalcogenides reveal that a
divergence of the nematic susceptibility in the B2g symmetry
channel appears to be a generic feature of optimally-doped
iron-based superconductors. In addition to the above, I also employ
a mechanical detwinning technique to reveal the resistivity
anisotropy in the orthorhombic state of the same Fe-based
superconductors. For the isovalently-substituted material
BaFe2(As1-xPx)2, these measurements reveal a strong coupling
between external stress and both the Neel temperature and the
superconducting critical temperature.
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