Detection of Orbital Fluctuations Above the Structural Transition Temperature in the Iron Pnictides and Chalcogenides

Note: Special Day; Joint with MSE Pizza served at 11:45 AM
Speaker: Laura H. Greene, University of Illinois, Urbana-Champaign

When: February 13, 2012 (Mon), 12:00PM to 01:00PM (add to my calendar)
Location: SCI 352
Hosted by: David Bishop

This event is part of the Biophysics/Condensed Matter Seminar Series.

Abstract: The role of quantum criticality in Fe-based superconductors is studied using quasiparticle scattering spectroscopy (QPS), also called point contact spectroscopy [1].  With this technique, we probe the electronic structure of the parent, electron-doped, and hole-doped Fe-pnictides AEFe2As2 (AE=Ca,Sr,Ba), and Fe-chalcogenides Fe1+yTe.  For AE=Sr,Ba and Fe1+yTe.  The onset of a conductance enhancement is reproducibly observed at To, well above the structural (Ts) and magnetic (TN) transition temperatures.  For Ba(Fe1-xCox)2 As2,the conductance enhancement exists only in the underdoped regime, allowing us to add a new region of strong correlations in the phase diagram:  For x = 0 to 5.5, To ~ 175 K to ~ 150K, respectively.  The onset of these strong correlations has been identified as arising from orbital fluctuations:  As To is crossed, the dzx and dzy orbital degeneracy is broken by a Pomeranchuk instability, causing an increase in the zero-energy density of states; hence the excess conductance [2].  We associate this orbital ordering with a resistance anisotropy observed in detwinned crystals above Ts [3].  The prediction that the orbital ordering fluctuations above Ts would not be observed in detwinned crystals that do not exhibit this resistance anisotropy above Ts (CaFe2As2 and BaxK1-x Fe2 As2) [2] was verified [1]. Due to inherent difficulties in detwinning Fe1+yTe, it has not been tested for resistive anisotropy, but since we observe conductance enhancement above TS, we predict an in-plane resistive anisotropy of Fe1+yTe above TS.  We discuss the role of orbital fluctuations and nematicity in the quantum criticality of these materials. [4]   This work is supported by the Center for Emergent Superconductivity, an Energy Frontier Research Center funded by the US DOE, Office of Science, Award No. DE-AC0298CH1088. Work at Cambridge is supported by EPSRC, and work at Ames Lab is supported by the US DOE under Award No. DE-AC02-07CH11358. 
  

  1. H. Z. Arham et al, arXiv:1201.2479.
  2. W.-C. Lee and P. Phillips, arXiv:1110.5917.
  3. I. R. Fisher et al, Rep. Prog. Phys. 74, 124506 (2011) and references therein.
  4. S. Kasahara et al, Phys. Rev. B 81, 184519 (2010); S. Kasahara et al., Bull. Am. Phys. Soc. 56, Z26.00010 (2011); Y. Matsuda, private commun.