Bulletin of the American Physical Society
APS March Meeting 2011
Volume 56, Number 1
Monday–Friday, March 21–25, 2011; Dallas, Texas
Session X2: Coexistence Between Antiferromagnetism and Superconductivity in Fe-pnictides |
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Sponsoring Units: DCMP Chair: Joerg Schmailian, Iowa State University Room: Ballroom A2 |
Thursday, March 24, 2011 2:30PM - 3:06PM |
X2.00001: Competing phases in the iron pnictides Invited Speaker: In this work, we present a theoretical model that consistently describes the interplay between the magnetic, elastic, and superconducting degrees of freedom of the iron pnictides, comparing our results to several experimental observations. First, we show that the outcome of the competition between the antiferromagnetic (AFM) and the superconducting (SC) order depends on the symmetry of the pairing state. In particular, we demonstrate that a conventional phonon-mediated superconducting state cannot coexist microscopically with the itinerant magnetic phase [1,2]. We also show that the magneto-elastic coupling in these materials is mediated by Ising-nematic degrees of freedom, which emerge from the degeneracy of the magnetic ground state. As a result, in the tetragonal phase, nematic fluctuations lead to the \emph{softening} of the lattice in the normal state but to its \emph{hardening} in the SC state, due to the competition between SC and AFM [3]. Accordingly, in the orthorhombic phase, nematic order is suppressed below the SC transition temperature, causing the suppression of the orthorhombic order parameter [4].\\[4pt] [1] R. M. Fernandes \emph{et al.}, Phys. Rev. B \textbf{81}, 140501(R) (2010).\\[0pt] [2] R. M. Fernandes and J. Schmalian, Phys. Rev. B \textbf{82}, 014521 (2010).\\[0pt] [3] R. M. Fernandes \emph{et al.}, Phys. Rev. Lett. \textbf{105}, 157003 (2010).\\[0pt] [4] S. Nandi \emph{et al.}, Phys. Rev. Lett. \textbf{104}, 057006 (2010). [Preview Abstract] |
Thursday, March 24, 2011 3:06PM - 3:42PM |
X2.00002: Superconductivity and Magnetism in iron-pnictides: co-existence or not? Invited Speaker: In this talk I will review the weak-coupling approach to describe the interplay of two electronic orders: superconductivity (SC) in the form of Cooper pairs, and magnetism in the form of the spin-density waves (SDW). The two orders, traditionally thought as incompatible, are close neighbors in magnetically-active Fe-based superconductors with surprisingly high $T_c$. Complex multi-band structure, multiple interactions and many families of these materials create a range of possible states of mingling between superconductivity and magnetism. I will present a list of different parameters, including (a) the Fermi surface shape, (b) the order parameter structure, (c) the relative strength of SC and SDW interactions, (d) the external magnetic field, and describe which properties, or their combinations, lead to co-existence or avoidance of SC and SDW orders, and how transition between the two orders occurs upon doping. [Preview Abstract] |
Thursday, March 24, 2011 3:42PM - 4:18PM |
X2.00003: Superconductivity and magnetism in 111 iron pnictides Invited Speaker: We study different 111 materials at ultra low temperatures by means of angle-resolved photoemission spectroscopy (ARPES). The measurements provide a direct access to the information on the low energy electronic structure, which includes the detailed knowledge of the Fermi surface, band renormalization, electronic self-energy and symmetry of the superconducting order parameter. The results suggest a direct correlation between the fermiology and fundamental physical properties throughout the phase diagram of 111 iron superconductors. In particular, the Van Hove singularity is identified as playing a primary role for the superconductivity. [Preview Abstract] |
Thursday, March 24, 2011 4:18PM - 4:54PM |
X2.00004: Antiferromagnetic phase in iron-based superconductors: selection of magnetic order, spin excitations, competition with superconductivity Invited Speaker: Recent discovery of superconductivity in the iron-based layered pnictides with Tc ranging between 26 and 56K generated enormous interest in the physics of these materials. The superconductivity has been discovered in oxygen containing RFeAsO (R=La, Nd, Sm) as well as in oxygen free AFe2As2 (A=Ba, Sr, Ca). Like the cuprates, the pnictides are quasi-two-dimensional systems, their parent material shows antiferromagnetic long-range order below 150K and superconductivity occurs upon doping of either electrons or holes into the FeAs layers. In my talk I will analyze the properties of the magnetically ordered state. In particular, I will discuss the selection of the stripe magnetic order in the unfolded BZ within itinerant description. Selecting one hole and two electron pockets we find that SDW order is highly degenerate if electron pockets are circular and interactions involved are between holes and electrons only. Repulsive charge interactions between two electrons as well as ellipticity of the electron pockets break the degeneracy and select metallic (0, $\pi )$ [($\pi $,0)] SDW state in the unfolded BZ -- the same order as seen in the experiments. I will argue that the SDW state remains a metal even for the case of a perfect nesting because one combination of the two hole operators and one combination of two electron operators decouple from the SDW mixing. We also demonstrate that the quasi-one-dimensional nanostructure identified in the quasiparticle interference (QPI) is a consequence of the interplay of the magnetic (\textit{$\pi $}, 0) spin-density wave (SDW) order with the underlying electronic structure. Finally, we address the salient experimental features of the magnetic excitations in the spin-density-wave phase of iron-based superconductors. We use a multiband random-phase approximation treatment of the dynamical spin susceptibility. Weakly damped spin waves are found near the ordering momentum and it is shown how they dissolve into the particle-hole continuum. We show that ellipticity of the electron bands accounts for the anisotropy of the spin waves along different crystallographic directions and the spectral gap at the momentum conjugated to the ordering one. *Work done with A. V. Chubukov, J. Knolle, R. Moessner, and A. Akbari. \\[4pt] [1] J. Knolle, I. Eremin, A. Akbari, and R. Moessner, Phys. Rev. Lett. 104, 257001 (2010). \\[0pt] [2] J. Knolle, I. Eremin, A.V. Chubukov, and R. Moessner, Phys. Rev. B 81, 140506(R) (2010) \\[0pt] [3] I. Eremin and A.V. Chubukov, Phys. Rev. B 81, 024511 (2010) [Preview Abstract] |
Thursday, March 24, 2011 4:54PM - 5:30PM |
X2.00005: Optical Investigations of the Superconducting State in 122 Iron-Pnictides Invited Speaker: The new high-Tc iron pnictide superconductors have a pronounced multiband character, which complicates the electronic properties and allows for a variety of possible superconducting ground states. We have used the infrared spectroscopy--one of the most powerful tools to investigate the low-energy electrodynamic properties of superconductors--to study several pnictide families. We made a comparison between them with the aim to answer the following questions: (1) Is it possible to have more than one superconducting gap in iron pnictide? (2) Can their order parameters be distinct from each other? (3) How does the coupling between different bands influence the gap? (4) Do the gaps have a three-dimensional character? (5) Is the gap scenario universal for all the iron pnictides? We could show that the pairing condition depends sensitively on the similarity of geometry and dimension between hole and electron Fermi-surfaces. [Preview Abstract] |
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