Condensed-Matter Physics & Materials Science Seminar
"Chasing the nematic phase in detwinned Ba(Fe1−xCox)2As2 with optical investigations"
Presented by Prof. Dr. Leonardo Degiorgi, Laboratory for Solid State Physics, ETH Zurich, Switzerland
Thursday, March 14, 2013, 1:30 pm
Small Seminar Room, Bldg. 510
Hosted by: Cedomir Petrovic
A renewed interest in the study of symmetry-breaking competing states in complex interacting systems followed the discovery of a broken rotational symmetry, due to stripe or nematic order, in the pseudogap phase of the copper oxide superconductors. The most recent playground in which to address the competition between structural, magnetic and superconducting phases is provided by the iron-pnictide superconductors. In these systems, the non-superconducting parent compounds undergo an antiferromagnetic transition into a broken-symmetry ground state at TN, which is always preceded by or coincident with a tetragonal-to-orthorhombic structural distortion at Ts ≥TN. This latter transition implies a twofold electronic symmetry, which for a range of dopings coexists with superconductivity and long range magnetic order. Understanding the effects of the structural transition on the charge dynamics and the electronic bands by studying the optical properties of the system is an important step in order to develop a comprehensive theoretical description of these materials. We investigate [1-3] the optical conductivity with light polarized along the in-plane orthorhombic a- and b-axes of Ba(Fe1−xCox)2As2 for x=0, 2.5% and 4.5% (i.e., in the so-called underdoped regime) under tunable uniaxial pressure across their structural and magnetic transitions. On such single-domain and almost totally detwinned specimens, we estimate the dichroism, which extends to high frequencies and temperatures. All together, our results reveal the important role played by a nematic susceptibility as well as the electronic nature of the structural transition and particularly allow us to shed light on the counterintuitive anisotropic behavior ρb>ρa (i.e., along the antiferromagnetic a-axis with respect to the ferromagnetic b-axis) of the dc resistivity.  A. Dusza et al., Europhys. Lett. 93, 37002 (2011) and Europhysics News 42 (3) (2011)  I.R. Fisher, L. Degiorgi