Bulletin of the American Physical Society
APS March Meeting 2011
Volume 56, Number 1
Monday–Friday, March 21–25, 2011; Dallas, Texas
Session Y26: Focus Session: Iron Based Superconductors -- Orbital Order |
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Sponsoring Units: DMP DCOMP Chair: Clarina dela Cruz, Oak Ridge National Laboratory Room: D162/164 |
Friday, March 25, 2011 8:00AM - 8:12AM |
Y26.00001: Effect of doping on the crystalline structure and superconductivity properties in Ca$_{1-x}$Na$_{x}$Fe$_{2}$As$_{2 }$ single crystals Leonardo Civale, Nestor Haberkorn, Boris Maiorov, Marcelo Jaime, G. Chen, W. Yu We have studied the crystalline structure and the superconducting properties of Ca$_{1-x}$Na$_{x}$Fe$_{2}$As$_{2}$ single crystals for various levels of the chemical doping (x). We performed a comparative analysis of the angular dependent $H_{c2}(\Theta )$ (where $\Theta $ is the angle between the magnetic field and the c axis) for x$\sim $0.5 and x $\sim $ 0.75, corresponding to T$_{c} \quad \sim $ 19 K and 33 K, respectively. We found that in both cases $H_{c2}(\Theta )$ near T$_{c}$ exhibits a single band character with the same anisotropy $\gamma \quad \sim $ 1.8. In the crystal with T$_{c} \quad \sim $ 33 K we detected the presence of a narrow vortex-liquid phase, in agreement with the expectation from estimates based on the Lindemann criterion and the Ginzburg number. We also found large and anisotropic flux creep rates, with temperature dependences that indicate glassy relaxation. We analyzed those results in terms of single vortex and collective pinning regimes associated with random and correlated disorder. [Preview Abstract] |
Friday, March 25, 2011 8:12AM - 8:24AM |
Y26.00002: First-principles study of orbital-selective magnetism in FeAs-based superconductors Hyungju Oh, Donghan Shin, Hyoung Joon Choi LaFeAsO$_{1-x}$F$_{x}$ and related compounds show unconventional superconductivity (SC) in the vicinity of the antiferromagnetism (AFM). These compounds are featured with multiple Fermi surfaces with strong orbital characters. We perform first-principles calculations of the electronic and magnetic properties in LnFeAsO (Ln=La, Ce, Pr, Nd, Sm, and Gd) as a function of Fe magnetic moment to study material-dependent interplay between orbitals and magnetic moments. With this approach, we show orbital-selective magnetic phases in small-Fe-moment regime: $d_{xy}$ magnetic phase, which is itinerantly driven by orbital selection of Fermi-surface nesting, and $d_{yz}$ magnetic phase, which is driven by local interactions. The Fe magnetic moments in the two phases show different coupling strengths to Fermi-surface electrons orbital-selectively, suggesting different roles in SC and in AFM, and making orbital characters of the Fe magnetic moment resolvable by measuring the electronic structures. This work was supported by the NRF of Korea (Grant No. 2009-0081204). Computational resources have been provided by KISTI Supercomputing Center (Project No. KSC-2008-S02-0004) [Preview Abstract] |
Friday, March 25, 2011 8:24AM - 8:36AM |
Y26.00003: Orbital characters of the iron-pnictides Donglai Feng The orbital degree of freedom plays an important role in the physics of iron-based high-temperature superconductors and their parent compounds. For example, possible orbital ordering has been associated with the spin density wave, and we recently found that the superconducting gap sizes are different at the same Fermi momentum for two bands with different spatial symmetries. We studied the orbital characters of the electronic structure in optimally electron-doped BaFe$_{1.85}$Co$_{0.15}$As$_{2}$ by exploiting the polarization-sensitivity of the orbitals in angle resolved photoemission spectroscopy. The orbital characters of the low energy electronic structure and Fermi surface in three dimensional momentum space are determined. Our results indicate that the previous orbital assignments of band structure calculations are just partially correct. Particularly, the contributions of the $d_{xy}$ and $d_{x2-y2}$ orbitals were not right. Our results lay the foundation for constructing realistic microscopic models of iron-based superconductors. Furthermore, we studied the transport properties and electronic structure of magnetically detwinned NaFeAs, and AEFe$_{2}$As$_{2}$. We identify the roles of various orbitals in the spin density wave formation. [Preview Abstract] |
Friday, March 25, 2011 8:36AM - 8:48AM |
Y26.00004: Dichotomy between Large Local and Small Ordered Magnetic Moments in Iron-Based Superconductors Philipp Hansmann, Ryotaro Arita, Alessandro Toschi, Shiro Sakai, Giorgio Sangiovanni, Karsten Held We study a four-band model for iron-based superconductors within the local density approximation combined with dynamical mean-field theory (LDA+DMFT). This successfully reproduces the results of models which take As p degrees of freedom explicitly into account and has several physical advantages over the standard five d-band model. Our findings reveal that the new superconductors are more strongly correlated than their single-particle properties suggest. Two-particle correlation functions unveil the dichotomy between local and ordered magnetic moments in these systems, calling for further experiments to better resolve the short time scale spin dynamics. [Preview Abstract] |
Friday, March 25, 2011 8:48AM - 9:00AM |
Y26.00005: High-energy x-ray diffraction studies of \textit{AE}Fe$_{2}$As$_{2}$ compounds D.K. Pratt, A. Kreyssig, M.G. Kim, S. Ran, A. Thaler, S.L. Bud'ko, R.J. McQueeney, P.C. Canfield, A.I. Goldman The relationship between structure, magnetism and superconductivity has become a major theme in studies of the iron arsenide family of superconductors. We have used high-energy x-ray diffraction, together with two-dimensional area detectors, to image large regions of reciprocal space in order to gain further insight into the structural transitions in the \textit{AE}Fe$_{2}$As$_{2}$ (\textit{AE} = Ca, Sr, Ba) compounds. Here we present results of our study of the impact of annealing and temperature on the structure of these materials. [Preview Abstract] |
Friday, March 25, 2011 9:00AM - 9:12AM |
Y26.00006: High Resolution X-ray Scattering Studies of Structural Phase Transitions in BaFe$_{2-x}$Cr$_x$As$_2$ B.D. Gaulin, J.P. Clancy, J.J. Wagman, A.S. Sefat While the effects of electron-doping on the parent compounds of the 122 family of Fe-based superconductors have been extremely well-studied in recent years, far less is known about the influence of hole-doping in compounds such as BaFe$_{2-x}$Cr$_x$As$_2$. In contrast to the electron-doped 122 systems, the hole-doped compounds do not become superconducting. Furthermore, while the hole-doped compounds exhibit similar structural and magnetic phase transitions, they appear to be much less sensitive to dopant concentration. We have performed high resolution x-ray scattering and magnetic susceptibility measurements on single crystal samples of BaFe$_{2-x}$Cr$_x$As$_2$ for Cr concentrations ranging from 0 $\le$ x $\le$ 0.67. These measurements allow us to determine the magnetic and structural phase transitions for this series and map out the low temperature phase diagram as a function of doping. In particular, we have carried out detailed measurements of the tetragonal (I4/mmm) to orthorhombic (Fmmm) structural phase transition which reveal how the orthorhombicity of the system evolves with increasing Cr concentration and how this correlates with the values of T$_s$ and T$_m$. [Preview Abstract] |
Friday, March 25, 2011 9:12AM - 9:48AM |
Y26.00007: Orbital Order and Spontaneous Orthorombicity in Iron Pnictides Invited Speaker: Phase diagram of the Iron Pnictide families of superconductors show a tetragonal to orthorhombic transition, sometimes coincident with the antiferromagnetic phase transition and sometimes at temperatures clearly above the antiferromagnetic phase transition. Inelastic neutron scattering spectra show exchange constants with strong spatial anisotropy. Recent photoemission measurements in mechanically detwinned samples show clear evidence of unequal orbital occupation and strikingly different spectra for dxz and dyz iron orbitals. Scanning Tunneling Microscopy and transport measurements have also shown substantial orthorhombicity in these materials. We discuss a simple microscopic picture for coupled spin and orbital degrees of freedom as the root cause for such an anisotropy and discuss the extent to which this picture can be distinguished from spin-frustration induced Ising nematic fluctuations as being the dominant driver for this phenomena. [Preview Abstract] |
Friday, March 25, 2011 9:48AM - 10:00AM |
Y26.00008: Commensurate antiferromagnetic ordering in Ba(Fe$_{1-x}$Co$_{x})_{2}$As$_{2}$ determined by x-ray resonant magnetic scattering at the Fe $K$ edge M.G. Kim, A. Kreyssig, Y.B. Lee, J.W. Kim, D.K. Pratt, A. Thaler, S.L. Bud'ko, P.C. Canfield, B.N. Harmon, R.J. McQueeney, A.I. Goldman We describe x-ray resonant magnetic diffraction measurements at the Fe $K$ edge of both the parent BaFe$_{2}$As$_{2}$ and superconducting Ba(Fe$_{0.953}$Co$_{0.047})_{2}$As$_{2}$ compounds. From these high-resolution measurements we conclude that the magnetic structure is commensurate for both compositions. The energy spectrum of the resonant scattering is in reasonable agreement with theoretical calculation using the full-potential linear augmented plane-wave method with a local density functional. The calculation suggests that the resonant scattering at the Fe $K$ edge in the $\sigma $-to-$\pi $ scattering channel arises from dipole allowed transitions from the core 1$s$ states to the unoccupied 4$p$ states that are spin polarized due to hybridization with the 3$d$ states close to the Fermi energy. [Preview Abstract] |
Friday, March 25, 2011 10:00AM - 10:12AM |
Y26.00009: Antiferromagnetic ordering in the absence of a structural distortion in Ba(Fe$_{1-x}$Mn$_{x})_{2}$As$_{2}$ A.I. Goldman, M.G. Kim, A. Kreyssig, A. Thaler, D.K. Pratt, W. Tian, J.L. Zarestky, M.A. Green, S.L. Bud'ko, P.C. Canfield, R.J. McQueeney Neutron and x-ray diffraction studies of
Ba(Fe$_{1-x}$Mn$_{x})_{2}$As$_{2}$ for low doping concentrations
($x \le $ 0.176) reveal that at a critical concentration,
0.102$ |
Friday, March 25, 2011 10:12AM - 10:24AM |
Y26.00010: An empirical method to account for spin-fluctuation suppression of magnetism in Fe pnictides Peter Blaha, Igor Mazin, Michelle Johannes Parent materials of Fe-based superconductors, such as BaFe$_2$As$_2$, are itinerant antiferromagnets, and as such should be better described by LDA calculations than are strongly-correlated cuprates. To an extent, this is true, but LDA, being a mean-field approximation, underestimates the suppression of the long-range magnetism due to spin fluctuations. These can be accounted for within Moria's self-consistent renormalization theory, which, however, includes unknown parameters such as the mean amplitude of the spin-fluctuations. We propose to include Moria's renormalization empirically, through a scaling of the LDA exchange-correlation magnetic field by a uniform constant factor, tuned so as to reproduce the observed phase diagram. This is a much more physical method to produce electronic bands with a proper exchange splitting, than adding an artificial ``negative-U'' term within an LDA+U formalism, a technique used now. We will report the results of such renormalized calculations for BaFe$_2$As$_2$ and, for comparison, for a prototypical itinerant magnet, ZrZn$_2$. [Preview Abstract] |
Friday, March 25, 2011 10:24AM - 10:36AM |
Y26.00011: In-plane resistivity anisotropy in underdoped Ba(Fe$_{1-x}$Ni$_x$)$_2$As$_2$ and Ba(Fe$_{1-x}$Cu$_x$)$_2$As$_2$ Hsueh-Hui Kuo, Jiun-Haw Chu, James Analytis, Leo Yu, Kristiaan De Greve, Peter McMahon, Yoshihisa Yamamoto, Ian Fisher Underdoped Fe arsenide superconductors suffer a structural transition that is either coincident with, or precedes the onset of long range antiferromagnetic order. Crystals tend to form a dense array of twins upon cooling through the structural transition, but uniaxial pressure can be used to almost completely detwin samples, enabling measurement of the associated in-plane electronic anisotropy. Initial experiments on detwinned samples of Ba(Fe$_{1- x}$Co$_x$)$_2$As$_2$ revealed a large in-plane resistivity anisotropy which varied non-monotonically with cobalt concentration. Here we present data extending the initial study to include detwinned samples of Ba(Fe$_{1-x}$Ni$_x$)$_2$As$_2$ and Ba(Fe$_{1-x}$Cu$_x$)$_2$As$_2$. The composition-dependence of the resistivity anisotropy $\rho_b/\rho_a$ reveals a striking correlation with that of the Hall coefficient for all three substitution series. [Preview Abstract] |
Friday, March 25, 2011 10:36AM - 10:48AM |
Y26.00012: Quasiparticle interference of C$_{2}$-symmetric surface states in LaOFeAs parent compound Xiaodong Zhou, Cun Ye, Peng Cai, Xiangfeng Wang, Xianhui Chen, Yayu Wang We present scanning tunneling microscopy studies on the LaOFeAs parent compound of iron pnictide superconductors [1]. High resolution spectroscopic imaging reveals strong standing wave patterns induced by quasiparticle interference of two-dimensional surface states. Fourier analysis shows that the distribution of scattering wavevectors exhibits pronounced two-fold (C$_{2})$ symmetry, strongly reminiscent of the nematic electronic state found in CaFe$_{1.94}$Co$_{0.06}$As$_{2}$ [2]. The implications of these results to the electronic structure of the pnictide parent states will be discussed. \\[4pt] [1] X.Zhou \textit{et al.} arXiv:1008.2642 \\[0pt] [2] T.M.Chuang \textit{et al.} Science \textbf{327}, 181 (2010) [Preview Abstract] |
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