1. The new iron-based superconductor Hao Hu The University of Tennessee Department of Physics and Astronomy, Knoxville Course: Advanced Solid State Physics II (Spring 2009) Instructor: Elbio Dagotto (Dated: March 22, 2009)

2. Outline Introduction Typical iron based superconductor ‘111’ and ‘122’ Experiment results Summary

3. Introduction Hideo Hosono, at the Tokyo Institute of Technology, and colleagues found the first compound, fluorine-doped lanthanum oxygen iron arsenide (LaO 1–x F x FeAs), as they reported online 23 February 2008 in the Journal of the American Chemical Society. It weighed in with a Tc of 26 Kelvin. Y. Kamihara, et al, J. Am. Chem. Soc. 130, 3296 (2008).

4. Introduction Four Chinese groups quickly pushed the critical temperatures higher by replacing the lanthanum with other rare earth elements. Xianhui Chen reported on the arXiv preprint server (www.arXiv.org) a Tc=43K Four days later, Zhong- Xian Zhao reported on the server that praseodymium oxygen fluorine iron arsenide has a Tc=52K 55 kelvin for the samarium compound grown under pressure

5. Crystal Structure of LaOFeAs Schematic crystal structure of LaOFeAs. Electron carriers generated by F- doping into oxygen sites are injected into FeAs metallic layers as a result of the large energy offset between these two layers. Note that the carrier doping layer is spatially separated from the conduction layer. They got Tc=43k at 4GPa on this F-doped LaOFeAs Hiroki Takahashi, et al, Nature 06972 (2008) Fe is sitting on a tetrahedral coordination. And Fe is formally divalent.

6. Magnetic Structure Plainly similar. The old and the new superconductors both contain planes of ions magnetized in opposite directions. In the older ones, electrons hop from copper to copper (arrow). For Fe-based SC, early results shows the As-Fe-As layer is responsible for the superconductivity. Adrian Cho, Science 320, 870(2008)

7. Crystal and magnetic Structure of BaFe 2 As 2 Marianne Rotter, et al, PRL 101 107006 (2008) The structural and electronic properties of the parent compound BaFe 2 As 2 are closely related to LaFeAsO. By hole doping potassium in this barium compound they get a Tc=38K

8. Phase diagram Cuprate The red circles indicate the onset temperature of the P4=nmm to Cmma phase transition. The black squares and green triangles designate the Ne´ el temperatures of Fe, TN (Fe), and Cerium, TN (Ce), respectively, as determined from neutron measurements J.Zhao, et.al, Nature Mater. 7, 953(2008)

9. Simplified doping dependent phase diagrams of iron-based superconductors for both Ln-1111 and Ba-122 materials. The phases shown are the antiferromagnetic/spin density wave (AF/SDW) phase close to zero doping and the superconducting phase around optimal doping. the superconducting phase is close to or overlapping with the magnetic phase.

10. Fermi surface topology of Ba 1-x K x Fe 2 As 2 ARPES measurement: Fermi surface maps of Ba 1-x K x Fe 2 As 2 measured using respective excitation energies of hν=80 and 50 eV at T=14K A sharp disagreement with band structure calculation this paper reported that the electronic structure of Ba 1-x K x Fe 2 As 2 revealed a reconstruction characterized by a (π, π) wave vector. This electronic order coexist with superconductivity and persists up to room temperature. V.B.Zabolotnyy, et.al, Nature 457, 569(2009) T=300k

11. STM measurement Boyer et al, STM of (Sr 1-x K x )Fe 2 As 2 arXiv:0806.4400 Topography reveals stripes. (a) 300 Å constant current topography reveals a stripe ordered region of the strontium sample, bounded by unit cell height step edges in the upper left and lower right leading to non-atomically resolved regions. (b) 30 Å field of view more clearly showing resolution of atoms

12. Summary Compare to cuprate Fe- based SC have a lower Tc up to now The magnetic moment in Fe-based SC are aligned with in the FeAs plane. In cuprate, it is up and down. For the magnetic ordered state in the cuprate, the AFM phase parent is Mott insulator whose electrons are localized. For the Fe-based superconductors, the AFM phase or SDW phase remains metallic. Its Superconductivity is competing with the spin wave density state. SC and SDW may coexist in underdoped samples.

13. Reference Adrian Cho, Science 320, 870(2008) J.Zhao, et.al, Nature Mater. 7, 953(2008) Marianne Rotter, et al, PRL 101 107006 (2008) Y. Kamihara, et al, J. Am. Chem. Soc. 130, 3296 (2008) Hiroki Takahashi, et al, Nature 06972 (2008) Boyer et al, STM of (Sr 1-x K x )Fe 2 As 2 arXiv:0806.4400 V.B.Zabolotnyy, et.al, Nature 457, 569(2008) A. Sefat et al., Phys. Rev. B 77, 174503 (2008) A. D. Christianson et al., Phys. Rev. Lett. 101 157004 (2008) L. Boeri et al., Phys. Rev. Lett. 101, 026403 (2008). C. de la Cruz et al., Nature (London) 453, 899 (2008). J. Dong et al., Europhys. Lett. 83, 27 006 (2008). A. I. Goldman et al., Phys. Rev. B 78, 100506 (2008); C. Krellner et al., Phys. Rev. B 78, 100504 (2008). M. Daghofer et al. Phys. Rev. Lett. 101, 237004 (2008) C. Cao et al., Phys. Rev. B 77, 220506 (2008). K. Haule et al., Phys. Rev. Lett. 100, 226402 (2008). S. Raghu et al., Phys. Rev. B 77, 220503 (2008). Z.P. Yin et al., Phys. Rev. Lett. 101 047001 (2008) Q. Han et al., Europhys. Lett. 82, 37 007 (2008); T. Li, J. Phys. Condens. Matter 20, 425203 (2008).