Bulletin of the American Physical Society
APS March Meeting 2010
Volume 55, Number 2
Monday–Friday, March 15–19, 2010; Portland, Oregon
Session V1: Structure, Magnetic Properties, and Superconductivity in the Pnictides |
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Sponsoring Units: DCMP Chair: Sergey Budko, Ames Laboratory Room: Oregon Ballroom 201 |
Thursday, March 18, 2010 8:00AM - 8:36AM |
V1.00001: Phase diagram and isotopic effect in high-Tc pnictide superconductors Invited Speaker: We will talk about the discovery of superconductivity with Tc higher than 40 K in Fe-based superconductors SmFeAsO$_{1-x}$F. Tc higher than McMillan limit of 39 K definitely proves pnictide superconductors high-Tc superconductivity$^{1,2}$. In this talk, we present the transport properties: resistivity, Hall coefficient and transport properties under high magnetic field. These results suggest a quantum phase transition around x=0.14 in SmFeAsO$_{1-x}$F$_{x}$ system. A electronic phase diagram is proposed, and coexistence of superconductivity and spin-density-wave is observed in Sm-1111 and Ba-122 system. We discuss the effect of isotopic effect on T$_{C}$ and T$_{SDW}$ in SmFeAsO$_{1-x}$F$_{x}$ and Ba$_{1-x}$K$_{x}$Fe$_{2}$As$_{2 }$systems. Our results show that oxygen isotope effect on T$_{C }$and T$_{SDW }$ is very little, while the iron isotope exponent is about 0.35. Surprisingly, the iron isotope exchange shows the same effect on SDW transition as on superconductivity. Our results indicate that electron-phonon interaction plays some role in the superconducting mechanism, but simple electron-phonon coupling mechanism seems to be rather unlikely because a strong magnon-phonon coupling is included$^{3}$. 1. Chen, X. H. et al. \textit{Nature} 453, 761-762 (2008). 2. Liu, R. H. et al. \textit{Phys. Rev. Lett.} 101, 087001 (2008). 3. R. H. Liu et al., Nature 459, 64-67(2009). [Preview Abstract] |
Thursday, March 18, 2010 8:36AM - 9:12AM |
V1.00002: Structural Distortions under pressure and doping in superconducting BaFe$_{2}$As$_{2}$ Invited Speaker: The discovery of a new family of high-T$_{C}$ materials, the iron arsenides, has led to a resurgence of interest in superconductivity. Several important traits of these materials are now apparent: for example, layers of iron tetrahedrally coordinated by arsenic are crucial structural ingredients. The structure and properties of chemically substituted samples are known to be intimately linked; however, until recently (1), remarkably little was known about this relationship when high pressure is used to induce superconductivity in undoped compounds. Here we show that the key structural features in BaFe$_{2}$As$_{2}$ show the same behaviour under pressure as found in chemically substituted samples. Using experimentally derived structural data, we show that the electronic structure evolves similarly in both cases. Our results show that, in contrast to the cuprates, structural distortions are more important than charge doping in the iron arsenides. This work was performed at the Helmholtz-Zentrum Berlin in collaboration with Ames Laboratory, Goethe-Universit\"{a}t Frankfurt, JCNS J\"{u}lich and the Institute Laue-Langevin. (1) S.A.J. Kimber \textit{et al}, Nature Materials, [Preview Abstract] |
Thursday, March 18, 2010 9:12AM - 9:48AM |
V1.00003: Direct imaging of structural domains in iron pnictides Invited Speaker: The parent compounds of iron-arsenide superconductors undergo first order structural transition between tetragonal and orthorhombic phases at a temperature, $T_S$. In $A$Fe$_2$As$_2$ (122) compounds ($A$=Ca,Sr,Ba) this occurs simultaneously with magnetic transition at $T_M$. Using a combination of polarized light microscopy and spatially-resolved high-energy synchrotron x-ray diffraction we show the orthorhombic distortion leads to the formation of 45$^{\circ}$-type structural domains in both 122 and 1111 single crystals. Domains penetrate through the sample thickness in the $c$-direction and are not affected by crystal imperfections such as growth terraces. The domains form regular stripe patterns in the plane with a characteristic dimension of 10-50 $\mu m$. In a range of low Co-doped compositions structural domains and superconductivity coexist. With the increasing doping level the domain structure becomes more intertwined and fine due to a decrease in the orthorhombic distortion. This results in an energy landscape with maze-like spatial modulations favorable for pinning and intrinsically high critical current densities in the underdoped regime. M.A.Tanatar {\it et al.} Phys. Rev. B {\bf 79}, 180508 (R) (2009). R. Prozorov {\it et al.} arxiv: 0909.0923, Phys. Rev.B accepted. [Preview Abstract] |
Thursday, March 18, 2010 9:48AM - 10:24AM |
V1.00004: Sheets of enhanced diamagnetic susceptibility in pnictide superconductors Invited Speaker: Superconducting quantum interference device (SQUID) microscopy shows stripes of increased diamagnetic susceptibility in underdoped, but not overdoped, single crystals of $Ba(Fe_{1-x} Co_x )_2 As_2 $. These stripes of increased diamagnetic susceptibility are consistent with enhanced superfluid density on twin boundaries. Individual vortices avoid pinning on or crossing the stripes, and prefer to travel parallel to them. These results indicate a relationship between superfluid density, local strain, and frustrated magnetism. The data suggests two mechanisms for enhancing critical currents and hints to an enhanced Tc on the twin boundaries [1]. \\[4pt] [1] B. Kalisky, J.R. Kirtley, J.G. Analytis, J.-H. Chu, A. Vailionis, I.R. Fisher, K.A. Moler, arXiv:0906.5184v2. [Preview Abstract] |
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