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The New Fe-based Superconductor Hao Hu [email protected] The University of Tennessee Department of Physics and Astronomy, Knoxville, TN 37922 Course: Advanced Solid State Physics II (Spring 2009) Instructor: Elbio Dagotto (Dated: March 3, 2009) Abstract The discovery in fluorine doped LaFeAsO superconductor with TC = 26 K was reported in February of 2008[1]. This discovery inspired researchers around the globe to study this unusual family of materials, which contain as their essential feature a square net of iron in the formal oxidation state Fe2+. In the following month of 2008, hundreds of papers were published on these and related compounds and they have been analyzed with almost every imaginable experimental technique. A group in Beijing reported their ReFeAsO1−δ (Re =Sm, Nd, Pr, Ce, La) superconductor which is prepared by high-pressure synthesis and without fluorine doping. The highest TC they obtained so far is 55K in SmFeAsO1−δ which suggests an unconventional pairing mechanism may be at work [3][4]. Several related families of materials with different crystal structures (still containing the square nets of formally divalent iron) were examined, and superconductivity has been found in almost every case. These fascinating families of compounds will be introduced in this paper and their crystal structure, physical properties of the superconducting and undoped “parent” phases will be discussed. Introduction Iron-based superconductors contain layers of iron and a pnictogen such as arsenic orphosphorus, or chalcogens. Most undoped iron-based superconductors show a tetragonal-orthorhombic structural phase transition followed at lower temperature by magnetic ordering, similar to the cuprate superconductors. The magnetic parent compounds of the iron arsenide family of superconductors crystallize in tetragonal structures, with LaFeAsO [1] forming in the tetragonal ZrCuSiAs structure. As Y. Kamihara’s report, Fig1. (a) Crystal structure of LaOFeAs. (b) Tc, Tonset, and Tmin in the F-T curves as a function of F- content (x) for La[O1-xFx]FeAs. Tc is defined as the temperature where the F value becomes half of that at Tonset. Tanom values for the undoped and LaO0.97F0.03- FeAs are also shown. Dotted curves are guides for eyes. They discovered this iron-based compound which undergoes superconducting transition under doping with F- ions at the O2- site. As shown by Fig1[1], its Tc exhibits a trapezoidal shape dependence on F- content, with the highest Tc of ~26 K at 5-11 atom %. The analysis of undoped compounds, such as LaOFeAs, is expected to provide important information toward the understanding of the superconducting state reached by ~10% F doping. People have studied the phonon density of states (PDOS) in LaFeAsO1_xFx with inelastic neutron scattering methods [3]. The PDOS of the parent compound (x= 0) is very similar to the PDOS of samples optimally doped with fluorine to achieve the maximum Tc (x ~0.1).This is unlike copper based superconductor. Neutron scattering experiments have provided evidence of magnetic order in LaOFeAs at 134 K: Fe spins order into ferromagnetic ‘‘stripes’’ that are aligned antiferromagnetically [5–7]. In the two-dimensional (2D) square lattice notation, the LaOFeAs magnetic structure factor has peaks at wave vectors q ~(0,π), (π, 0) [5–7]. Assuming a smooth continuity between the undoped and F-doped compounds, the pairing mechanism could be magnetic in origin and triggered by this unusual magnetic state [8]. More evident shows that Superconductivity in LaFeAsO emerges from specific structural and electronic conditions in the (FeAs)δ- layer.However, if only the iron arsenide layer is essential, also other structure types could serve as parent compounds. Another important compound in iron based family is BaFe2As2, which is called the “122” superconductor (see Fig2)[14]. The undoped parent phases LaFeAsO and BaFe2As2 have FeAs square-lattice sheets, and itinerant electrons in these layers undergo antiferromagnetic long range order (AFLRO) below modest temperatures, TSDW ~140 K[5]; electron or hole doping suppresses the AFLRO and induces superconductvity. Fig.2 Crystal structure of BaFe2As2 (ThCr2Si2-type structure, space group I4=mmm) Fig.3. 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.Iron based superconductor are poor metals rather than Mott insulators and have five bands at the Fermi surface rather than one. The phase diagram emerging as the iron-arsenide layers are doped is remarkably similar, with the superconducting phase close to or overlapping the magnetic phase. Strong evidence that the Tc value varies with the As-Fe-As bond angles has already emerged and shows that the optimal Tc value is obtained with undistorted FeAs4 tetrahedra. Theoretical Calculations Many theoretical calculations have been done to understand the properties of the iron based superconductor. Such as band structure calculations that have shown the relevance of the 3d levels of Fe [9,10]. A metallic state involving a Fermi surface (FS) made out of disconnected small pieces (‘‘pockets’’) was predicted [9]. To understand some of the properties of the undoped limit, electron correlations appear to be important [10]. In K. Haule’s paper, they compute the electronic structure, momentum resolved spectral function and optical conductivity of the new superconductor LaO1_xFxFeAs within the combination of the density functional theory and dynamical mean field theory. They find that the compound in the normal state is a strongly correlated metal and the parent compound is a bad metal at the verge of the metal insulator transition. And they argue that the superconductivity is not phonon mediated. And some calculation showed that the optimized position of As atom doesn’t agree with experimental structure [12].Fe magnetic moment vary sensitively to As position. Two-orbitals descriptions [11, 13] and other models have been proposed, and a variety of approximations have led to several unconventional pairing channel proposals [8]. Experimental Method and Result Many experiments have been done on the iron based superconductor to investigate its properties. In this paper I’ll give some experimental results which are quite


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UTK PHYS 672 - The New Fe-based Superconductor

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