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Electrodynamic response in the electronic nematic phase of BaFe2As2

Speaker: 
Leonardo Degiorgi (ETH-Zürich, Switzerland)
Date: 
Mon, 25/05/2015 - 11:00am to 12:00pm
Location: 
Physics Conference Room (S13-M01-11)
Host: 
CA2DM
Event Type: 
Seminars

Abstract

The ferropnictides harbor a structural tetragonal-to-orthorhombic transition at Ts that may either coincide or precede a transition into a long-range antiferromagnetic order at TN, usually ascribed to a spin-density-wave (SDW) state. We measure the in-plane optical reflectivity of BaFe2As2 over a broad spectral range, covering the energy interval from the far infrared (FIR) to the ultraviolet (UV), at several combinations of uniaxial pressure, used to detwin the specimen, and temperature. Our goal is to probe the anisotropic response in the real part σ1(ω) of the optical conductivity, extracted from the reflectivity data via Kramers-Kronig transformations. We thus elucidate how the anisotropic optical metallic response evolves as a function of stress, considered as an external symmetry breaking field, and across the ferro-elastic structural transition at Ts = TN = 135 K. The infrared response reveals that the dc transport anisotropy in the orthorhombic antiferromagnetic state is determined by the interplay between the Drude spectral weight and scattering rate, but that the dominant effect is clearly associated with the metallic spectral weight. In the paramagnetic tetragonal phase, though, the dc resistivity anisotropy of strained samples is almost exclusively due to stress-induced changes in the Drude weight rather than anisotropy in the scattering rate. This result definitively establishes that the primary effect driving the resistivity anisotropy in the paramagnetic orthorhombic phase (i.e., the electronic nematic state) is the anisotropy of the Fermi surface.

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