1、Yuen Yiu University of Tennessee, Department of Physics and Astronomy Knoxville, TN 37922,An Introduction to Fe-based superconductors,Overview,Brief history: Discovery and progress Material variations: 4 types of material: “1111”, “122”, “111” and “11” 10 Experiments and physical properties:Transpor
2、t properties, magnetic properties, crystal structures, phase diagram Theoretical models,Brief History,Reported by Kamihara et al. on 19rd March 2008, paper titled “Iron based superconductor LaO1-xFxFeAsO with Tc=26K” 12has been cited for 1,117 times as of 4th March 2010! Only non-cuprate high Tc sup
3、erconductors (Tc20K) Very popular at the moment: There is at least one presentation session EVERY DAY at the upcoming APS March meeting,Table 1 Maximum Tc in each RFeAs(O1-xFx). The F concentration x, which gives the maximum Tc is shown 10,Material variations,“1111” family,“122” family,RFeAsO (R can
4、 be but not limited to: Ce, Pr, Nd, Sm, La) Superconductivity induced by Oxygen site electron doping (usually with F), or simply creating oxygen deficiency Iron site electron doping has also been reported,AFe2As2(A = Ba, Sr, Ca, etc.) SC induced by A-site doping with monovalent B+ (i.e. K, Cs, Na, e
5、tc.) Iron site doping with Co has been reported,Material variations,“111” family (?),“11” family,Li-deficient LiFeAs superconducts Superconductivity very sensitive to sample preparation 10 Parent compounds superconducts (?) Not as popular,Simplest structure Se-deficient FeSe superconducts up to 8K 1
6、0NOTE: the PARENT COMPOUNDS DO NOT SUPERCONDUCT UNDER AMBIENT PRESSURE!,Crystal structure,“1111”: layers of FeAs and LaO “122”: layers of FeAs and K/Sr “111”: layers of FeAs and Li “11”: layers of FeSe,Figure 1 (a) Crystal structure of LaOFeAs; 2(b) Crystal structure of (K/Sr)Fe2As2 and (Cs/Sr)Fe2As
7、2 2,Sample Synthesis,EXAMPLE: Polycrystals: conventional solid state reaction. PrFeAsO: start with PrAs, Fe2O3 and Fe powders. Ground up stoichiometric mixtures in glovebox, pressed into pellets, sealed in silica tubes in argon, and then heated at 1200oC for 30 hrs. 11,Figure 2 A picture of what PrF
8、eAsO powder looks like,Transport properties,Figure 3 Temperature dependence of electrical resistivity of LaFeAsO1-xFx. The inset is a phase diagram constructed with the data. 9,Broad peak at T150K is generally associated with the SDW phase transition The transition is suppressed and shifted to a low
9、er temperature as doping level increases Superconductivity emerges at x=0.03,Neutron Powder Diffraction,Figure 4, 5, 6 (left) NPD data for PrFeAsO 5; (center) Lattice parameter v. Temperature data showing structural transition Yiu Y. et al., unpublished; (right) Lattice parameters v. doping level 8,
10、Peak splitting: tetragonal-orthorhombic structural transition (Extra) Magnetic peaks found at 5K,Behold! The General “1111” Phase Diagram,Figure 7 The structural, magnetic, and superconducting phase diagram of PrFeAsO1xFx 3,Suppress the following and SC will EMERGE! Structural transition, magnetic p
11、hase transition (Nave, experimentalist point of view) Can be done by chemical doping or applied pressure,Behold! The General “122” Phase Diagram,Figure 8 The structural, magnetic, and superconducting phase diagram of BaFe2-xCoxAs2 a member from the122 family 14,Superconductivity coexists with antife
12、rromagnetism!? Interesting,Theoretical models,Non BCS theory of superconductivity!? Works suggesting the s-pairing state Unconventional and mediated by (nesting-related) antiferromagnetic spin fluctuations 13 First example of multigap superconductivity with a discontinuous sign change between the ba
13、nds 13 Similar but different from the famous superconducting MgB2 13 No common consensus yet,Conclusions,“Fe-based superconductors” is a new and exciting field 4 different types of crystal structure found in this group The parent compounds do not superconduct, but undergo a structural distortion, a
14、SDW phase transition and magnetic ordering instead. These transitions can be suppressed by chemical doping or applied pressure (unexplored today) and superconductivity will emerge No universally accepted theoretical model yet,References,1 Liu R. H. et al, Phy. Review Letters, 101, 087001 (2008) 2 Sa
15、smal K. et al, Phy. Review Letters, 101, 107007 (2008) 3 Rotundu C. R. et al, Phy. Review B, 80, 144517 (2009) 4 Ren Z. A, Materials Research Innovations 12, 1 (2008) 5 Zhao J. et al, Phy. Review B, 78, 132504 (2008) 6 Qi Y. P. et al, Phy. Review B, 80, 054502 (2009) 7 Wang et al, Phy, Review B, 78,
16、 054521 (2009) 8 Han F. et al, Phy, Review B, 80 024506 (2009) 9 Dong J. et al, Europhys. Letter, 83, 27006 (2008) 10 Ishida k. et al, Journal of the Phy. Socity of Japan, 78, 062001 (2009) 11 McGuire M. A. et al, Journal of Solid State Chemistry, 182, 8, p 2326-2331 (2009) 12 Kamihara et al., Journal of American Chemical Society, 130, 11, p. 3296+ (2008) 13 Mazin I. I. et al., Phys. Rev. Lett., 101 057003 (2008) 14 Wang X. F. et al., arXiv:0811.2920.,
