Effects of Orbital Ordering on Superconductivity and Spin Fluctuations in FeSe

<strong>Seminar of the School of Physical Sciences -------------------------------------------------</strong> <strong>Title: Effects of Orbital Ordering on Superconductivity and Spin Fluctuations in FeSe</strong> <strong>Speaker: Shantanu Mukherjee</strong> (Niels Bohr Institute, Copenhagen) Date:<strong> September 9, 2015</strong> <strong>Abstract:</strong> FeSe is currently one of the most hotly debated iron-based systems due in part to its very high Tc when monolayers are placed on STO substrates, and in part due to the fact that the material exhibits a structural distortion near TS ~ 90K without any concomitant magnetic order. In addition, undoped bulk FeSe samples, which become superconducting below T~8K, display evidence of orbital ordering setting in near TS. I will discuss the normal and superconducting properties of FeSe using a tight-binding model [1], and include the effect of orbital ordering. The model reproduces the essential features of FeSe band structure seen in ARPES and quantum oscillation experiments [2]. Using this model, the spin lattice relaxation rate is calculated and the results are compared with recent NMR experiments [3]. We next discuss the structure of the spin fluctuations seen in inelastic neutron scattering experiments [4] and consequences of a spin fluctuation mediated superconducting pairing in FeSe and the resulting gap structure. Finally, the density of states derived from our calculations is compared to STM experiments [5]. [1] Shantanu Mukherjee et al., Phys. Rev. Lett. 115, 026402 (2015). [2] Watson et al., Phys. Rev. B 91, 155106 (2015). [3] S.-H. Baek, et al., Nature Materials 14, 210 (2014). [4] Rahn et al., Phys. Rev. B 91, 180501 (2015). [6] C. L. Song et al., Science 332, 1410 (2010).