Bulletin of the American Physical Society
APS March Meeting 2010
Volume 55, Number 2
Monday–Friday, March 15–19, 2010; Portland, Oregon
Session P1: Superconductivity and Magnetism of Iron Chalcogenides |
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Sponsoring Units: DCMP Chair: Collin Broholm, John Hopkins University Room: Oregon Ballroom 201 |
Wednesday, March 17, 2010 8:00AM - 8:36AM |
P1.00001: Structural Stability and Superconductivity in the Iron Chalcogenides Invited Speaker: We have developed processes to grow Fe$_{1+x}$Se single crystals and epitaxial films. X-ray diffraction measurements show that the plate side thin films of the crystal is tetragonal $\beta $-FeSe. The as grown crystals show a superconducting transition T$_{c}$ at 8 K. In addition, superconducting Fe$_{1+x}$(Se$_{1-y}$Te$_{y})$ thin films have also been fabricated by pulsed laser deposition on MgO. All Fe$_{1+x}$(Se$_{1-y}$Te$_{y})$ films show preferred orientation and smooth surface morphology. However, a strong orientation and thickness dependence of Tc was found in Fe$_{1+x}$Se thin films deposits at low substrate temperature. Detailed x-ray structural studies on both the single crystal and epitaxial thin films show that the existence of a low temperature structural distortion is essential for the occurrence of superconductivity. [Preview Abstract] |
Wednesday, March 17, 2010 8:36AM - 9:12AM |
P1.00002: Structure, Chemistry and Property Correlations in FeSe and 122 Pnictides Invited Speaker: Determining how crystal structure and chemical bonding influence the properties of solids is at the heart of collaborative research programs between materials physicists and solid state chemists. In some materials, the high Tc copper oxides and colossal magnetoresistance manganates, for example, the subtleties of how structure, bonding and properties are coupled yields an almost baffling complexity, while in others, such as many classical intermetallic superconductors, the properties are more easily understood, with bonding and structure playing a less profound role. The new superconducting pnictides appear to fall somewhere between these two limits, and have so far been the subject of relatively little study by solid state chemists. Here I will describe some of our recent work on superconducting FeSe and superconductor-related ``122'' (ThCr$_{2}$Si$_{2}$-type) solid solution phases as examples of the kinds of insights that structural and chemical studies can contribute to understanding these important materials. [Preview Abstract] |
Wednesday, March 17, 2010 9:12AM - 9:48AM |
P1.00003: Phase diagram of Fe$_{1+y}$(Te$_{1-x}$Se$_{x}$): evolution from antiferromagnetism to superconductivity Invited Speaker: Iron chalcogenide Fe$_{1+y}$(Te$_{1-x}$Se$_{x}$) is the simplified version of Fe-based superconductors [1,2] and has a unique antiferromagnetic (AFM) structure in the parent compound Fe$_{1+y}$Te [3,4]. In iron pnictide superconductor parent compounds, the AFM wavevector $Q_{AF}$ is along the FS nesting direction [5-7], while in Fe$_{1+y}$Te, $Q_{AF}$ is rotated 45$^{\circ}$ from the FS nesting direction. Understanding the magnetic and superconducting properties of this system is considered critical [8]. In this talk I will discuss the phase diagram of Fe$_{1+y}$(Te$_{1-x}$Se$_{x}$) that we recently established. We found that long-range AFM order is gradually suppressed by Se substitution, disappearing near 9\% Se, above which short-range AFM order coexists with non-bulk superconductivity (NBSC). Bulk superconductivity (BSC) does not appear until the Se content is greater than \~30\%. The normal state exhibits distinct properties between the NBSC and BSC regions: metallic behavior is observed above $T_{c}$ for the BSC region, while the NBSC region exhibits weak localization behavior above $T_{c}$. These observations, together with our results of neutron scattering studies, suggest that the short-range magnetic order near $Q_{AF}$ leads to weak charge carrier localization, and is thus unfavorable to superconducting pairing. \\[4pt] [1] F.C. Hsu \textit{et al.}, Proc. Natl. Acad. Sci. USA. \textbf{105}, 14262 (2008).\\[0pt] [2] M.H. Fang \textit{et al.}, Phys. Rev. B \textbf{78}, 224503 (2008). \\[0pt] [3] W. Bao \textit{et al.}, Phys. Rev. Lett. \textbf{102}, 247001 (2009). \\[0pt] [4] S.L. Li \textit{et al.}, Phys. Rev. B \textbf{79}, 054503 (2009).\\[0pt] [5] C. Cruz \textit{et al.}, Nature \textbf{453}, 899 (2008).\\[0pt] [6] Q. Huang \textit{et al.}, Phys. Rev. Lett \textbf{101}, 257003 (2008). \\[0pt] [7] J. Zhao \textit{et al.}, Phys. Rev. Lett. \textbf{101}, 167203 (2008).\\[0pt] [8] A.V. Balatsky and D. Parker, Viewpoint, Physics \textbf{2}, 59 (2009). [Preview Abstract] |
Wednesday, March 17, 2010 9:48AM - 10:24AM |
P1.00004: Structure, magnetic order and excitations of the Fe(Se,Te) superconductor system Invited Speaker: The Fe(Se,Te) material, instead of being chalcogen deficient as initially proposed, in general has excessive Fe at an interstitial site [1]. It shares the similar ferminology of the LaFeAsO and BaFe$_2$As$_2$ types of Fe-based superconducting materials. Therefore, the same spin-density-wave (SDW) antiferromagnetic (AF) order at the nesting wave-vector textbf{Q}$_N=(pi,0)$ was expected for the parent compound Fe$_{1+y}$Te. However, we observe using neutron scattering technique that the AF order is characterized by a completely different propagating vector textbf{Q}$_M=(deltapi,deltapi)$, which is along the diagonal direction of the Fe ``square''-lattice instead of along one of its edges, and the $delta$ is tunable from an incommensurate value to a commensurate 1/2 by excessive Fe $y$ [1]. This result casts doubt on the prevailing SDW picture for magnetic transition in these Fe-based materials, and prevents premature exclusion of other alternative mechanism such as on-site correlations or orbital ordering. Even with a textbf{Q}$_M$-type AF order in the parent compound, a spin resonance excitation mode is observed in superconducting FeSe$_{0.4}$Te$_{0.6}$ at the energy $\hbar{\Omega}_0$=5.3 $k_{BT{_c}}$ and around $\mathbf{Q}_N$ [2]. Our results support the theoretical proposal that the superconducting symmetry of the Fe-based superconductors is of the $s^{pm}$ type, with the gap function of opposite signs on the hole and electron Fermi surfaces respectively [2]. Interestingly, while the intensity of the resonance mode behaves like an order parameter, the gap energy itself looks temperature independent. This is corroborated by the temperature dependence of the superconducting gap observed in ARPES measurements. \\[4pt] [1] W. Bao, Y. Qiu, Q. Huang et al., Phys. Rev. Lett. textbf{102}, 247001 (2009).\\[0pt] [2] Y. Qiu, W. Bao, Y. Zhao et al., Phys. Rev. Lett. textbf{103}, 067008 (2009). [Preview Abstract] |
Wednesday, March 17, 2010 10:24AM - 11:00AM |
P1.00005: NMR investigation of iron-based superconductors Invited Speaker: We report NMR investigation of the electronic properties of iron-based superconductors with primary focus on the 11 (FeSe) and 122 (Co-doped BaFe$_{2}$As$_{2})$ systems. From the $^{77}$Se and $^{75}$As NMR Knight shift $K$ measurements, we will deduce the intrinsic temperature and concentration dependences of the uniform spin susceptibility, $\chi _{spin}$, in these systems. We will also demonstrate the evolution of antiferromagnetic spin fluctuations (AFSF) as a function of pressure (in FeSe) or the doping level (in Ba[Fe$_{1-x}$Co$_{x}$]$_{2}$As$_{2})$. Our results show that the optimal superconducting phase exists in close proximity with SDW order; superconductivity sets in only after AFSF grow toward $T_{c}$. This work was carried out in collaboration with F.L. Ning and K. Ahilan (McMaster), T. McQueen and R.J. Cava (Princeton), A.S. Sefat, M.A. McGuire, B. C. Sales, and D. Mandrus (Oak Ridge), P. Cheng, B. Shen, and H.-H Wen (Chinese Academy of Sciences). The work at McMaster was supported by NSERC, CIFAR, and CFI. [Preview Abstract] |
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