Bulletin of the American Physical Society
APS March Meeting 2014
Volume 59, Number 1
Monday–Friday, March 3–7, 2014; Denver, Colorado
Session S48: Invited Session: Multi-orbital Effects and Pairing Symmetry in Iron-Based Superconductors |
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Sponsoring Units: DCMP Chair: Andrey Chubukov, University of Wisconsin-Madison Room: Mile High Ballroom 1A-1B |
Thursday, March 6, 2014 8:00AM - 8:36AM |
S48.00001: Impact of nematicity on the competing superconducting instabilities of the iron pnictides Invited Speaker: Rafael Fernandes Magnetic fluctuations have been proposed not only to give rise to unconventional pairing states in iron pnictides and chalcogenides, but also to be responsible for an emergent electronic nematic transition that breaks the tetragonal symmetry of the system down to orthorhombic. In this talk, we discuss the interplay between nematicity and superconductivity using both a phenomenological approach and a microscopic electronic model. When only the $s^{+-}$ superconducting instability is present (i.e. gaps with different signs on electron and hole pockets), nematic order competes with superconductivity [1], resulting in a suppression of $T_{c}$ and in a hardening of the shear modulus across the superconducting transition. However, this scenario changes dramatically when a competing d-wave superconducting instability is also present [2], as it has been suggested in several iron-based compounds. In this case, an unusual tri-linear coupling between the superconducting and nematic order parameters arises in the free energy, strongly impacting the phase diagram [3]. On the one hand, nematic order now leads to an increase of $T_{c}$, and the shear modulus is softened across the superconducting transition. On the other hand, nematic fluctuations promote an effective attraction between the $s^{+-}$ and d-wave states, favoring a mixed phase that does not break time-reversal symmetry, but instead spontaneously breaks the tetragonal symmetry of the system. Our findings offer a new perspective on how $T_{c}$ can be enhanced in the iron pnictides, and demonstrate that nematicity can be used as a diagnostic tool to probe exotic pairing states in these materials.\\[5pt] [1] R. M. Fernandes, S. Maiti, P. W\"olfle, and A. V. Chubukov, Phys. Rev. Lett. \textbf{111}, 057001 (2013).\\[0pt] [2] R. M. Fernandes and A. J. Millis, Phys. Rev. Lett. \textbf{110}, 117004 (2013).\\[0pt] [3] R. M. Fernandes and A. J. Millis, Phys. Rev. Lett. \textbf{111}, 127001 (2013). [Preview Abstract] |
Thursday, March 6, 2014 8:36AM - 9:12AM |
S48.00002: Study of the Nematic State of Pnictides using the Spin Fermion model with Spin, Orbital, and Lattice Degrees of Freedom Invited Speaker: Adriana Moreo The anisotropic behavior of the resistivity above the Ne\'el temperature in several iron-pnictides has been explained in terms of a nematic phase whose origin is currently under heated debate. In some scenarios the leading role is attributed to the magnetic degrees of freedom while in others the orbitals act as triggers, and the lattice is always assumed to be a follower. To analyze these issues a three-orbital ($xz$, $yz$, $xy$) Spin-Fermion model was studied via Monte Carlo simulations [1,2]. Our main result is that in order to reproduce the experiments, including a separation between the structural critical temperature ($T_S$) and the magnetic Ne\'el temperature ($T_N$) both the lattice-orbital and lattice-spin couplings are needed. In general, the Ne\'el temperature increases with the spin-lattice constant while the separation between the structural and the N\'eel transition temperatures is controlled by the orbital-lattice coupling [2,3]. Experimental results for the anisotropic behavior of the resistivity, the ARPES orbital spectral weight varying temperature, and the neutron scattering weights at ($\pi$,0) and (0,$\pi$) are captured by the numerical simulations [2]. Calculations of the nematic susceptibility, which is proportional to the elastorresistivity coefficient $m_{66}$, will be presented [3] and contrasted against experimental results by the Stanford group. \\[4pt] [1] S.Liang et al., Phys.Rev.Lett.109, 047001 (2012).\\[0pt] [2] S.Liang et al., Phys.Rev.Lett.111, 047004 (2013).\\[0pt] [3] S.Liang et al., in preparation. [Preview Abstract] |
Thursday, March 6, 2014 9:12AM - 9:48AM |
S48.00003: ARPES studies of the superconducting gap in highly hole-doped (Ba,K)Fe2As2 Invited Speaker: Hong Ding |
Thursday, March 6, 2014 9:48AM - 10:24AM |
S48.00004: Pairing symmetry in strongly hole-doped iron-based superconductors Invited Speaker: Fazel Fallah Tafti The fabric of superconductivity in the multiband iron-based superconductors is woven out of inter-band and intra-band interactions. By tuning the relative strength of different pairing interactions via external parameters such as pressure we can tune the pairing symmetry of these multiband superconductors. I will present experimental evidence for a pressure induced change of pairing state in the fully hole-doped iron-based superconductor KFe2As2. Our main experimental finding is a sharp reversal in the pressure dependence of Tc at a critical pressure Pc $=$ 18 kbar [1]. Compared to previous reports on two separate superconducting domes in fully electron-doped chalcogenides, our discovery points to several novel aspects: (a) Pc is very low, meaning structural changes are negligible; (b) Tc remains finite through the transition, suggesting the phase transition is confined within the superconducting state; (c) No anomalies are observed in the normal state properties, ruling out the possibility of a Lifshitz transition; (d) The two superconducting states manifest a different sensitivity to disorder. These observations lead us to conclude that the sharp reversal of Tc at the critical pressure signals a phase transition between two different pairing symmetries in KFe2As2: a transition which leaves no traces in the normal state properties. Theoretical calculations formulate such a phase transition between different pairing states favored by different inelastic scattering processes [2]. We explore this hypothesis by tracing Tc versus inelastic scattering and demonstrate that below the critical pressure, Tc correlates with inelastic scattering but above the critical pressure, Tc anticorrelates with inelastic scattering. This is consistent with different channels of interactions giving rise to different pairing symmetries and pressure simply tunes the relative strength of these interactions. \\[4pt] [1] F. F. Tafti \textit{et al}., Nature Physics \textbf{9}, 349 (2013).\\[0pt] [2] R. M. Fernandes and A. J. Millis, Physical Review Letters \textbf{110}, 117004 (2013). [Preview Abstract] |
Thursday, March 6, 2014 10:24AM - 11:00AM |
S48.00005: Peculiarities under the Superconducting Dome in Iron Based Superconductors Invited Speaker: Saurabh Maiti Pairing symmetry in Fe based superconductors is a heavily investigated topic. Of the many materials across several families, the K-doped (hole doped) BaFe$_2$As$_2$ is one of the most investigated ones. Upon heavy hole doping this material undergoes a change in the Fermi Surface topology which opens up discussion on its effect on the symmetry and structure of the superconducting order parameter. I will highlight some of the important consequences this has in the context of KFe$_2$As$_2$ (the end member of the above family) such as nodal s-wave gap structure with higher harmonics and also a possibility of a superconducting state that spontaneously breaks time reversal symmetry. [Preview Abstract] |
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