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)

Outline

• Introduction• Typical iron based superconductor ‘111’ and

‘122’• Experiment results• Summary

Introduction• Hideo Hosono, at the Tokyo Institute of Technology, and colleagues found

the first compound, fluorine-doped lanthanum oxygen iron arsenide (LaO1–xFxFeAs), 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).

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

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.

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)

Crystal and magnetic Structure of BaFe2As2

Marianne Rotter, et al, PRL 101 107006 (2008)

The structural and electronic properties of the parent compound BaFe2As2 are closely related to LaFeAsO.By hole doping potassium in this barium compound they get a Tc=38K

Phase diagram

CuprateThe 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)

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.

Fermi surface topology of Ba1-xKxFe2As2

ARPES measurement:Fermi surface maps of Ba1-xKxFe2As2 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 Ba1-xKxFe2As2 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

STM measurement

Boyer et al, STM of (Sr1-xKx)Fe2As2 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

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.

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