Bulletin of the American Physical Society
APS March Meeting 2013
Volume 58, Number 1
Monday–Friday, March 18–22, 2013; Baltimore, Maryland
Session C37: Focus Session: Fe-based Superconductors: DFT and DMFT |
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Sponsoring Units: DMP DCOMP Chair: Wei Ku, Brookhaven National Laboratory Room: 345/346 |
Monday, March 18, 2013 2:30PM - 2:42PM |
C37.00001: First principles investigation of Sr and P doping of CaFe$_2$As$_2$ Harald O. Jeschke, Milan Tomic, Roser Valenti The doping-temperature-pressure phase diagrams of the 122 family of superconductors have been discussed intensively due to electronic nematicity above the structural and superconducting transition and the complex coupling between electronic and lattice degrees of freedom. We employ density functional theory to determine the structure of supercells of Ca$_{1-x}$Sr$_x$Fe$_2$As$_2$ and CaFe$_2$(As$_{1-x}$P$_x$)$_2$. We then predict structural transitions of the doped compounds under pressure and compare to the parent compound. We carefully analyze the changes in the electronic structure caused by doping and stress. [Preview Abstract] |
Monday, March 18, 2013 2:42PM - 2:54PM |
C37.00002: Effect of uniaxial stress on structural and electronic properties of BaFe$_2$As$_2$ and CaFe$_2$As$_2$ Milan Tomic, Harald O. Jeschke, Roser Valenti We investigate the effects of the uniaxial tensile and compressive stresses applied along {\bf a}, {\bf b} and {\bf a}+{\bf b} directions in BaFe$_2$As$_2$ and CaFe$_2$As$_2$ in the framework of ab initio density functional theory calculations. While the systems remain in the orthorhombic phase at moderate pressures, we observe an inversion of magnetism at a critical strain happening when the {\bf a} and {\bf b} axes approach the tetragonal condition. We discuss our results in view of recent reports of modified magnetic and structural transitions in BaFe$_2$As$_2$ under externally applied uniaxial strain and make a connection to phenomenological models proposed for these transitions. [Preview Abstract] |
Monday, March 18, 2013 2:54PM - 3:06PM |
C37.00003: Electronic band structure of BaCo$_{2}$As$_2$: a fully-doped ferropnictide with reduced electronic correlations Pierre Richard, N. Xu, A. van Roekeghem, P. Zhang, H. Miao, W.-L. Zhang, T. Qian, M. Ferrero, A. S. Sefat, S. Biermann, H. Ding We report an angle-resolved photoemission spectroscopy investigation of the Fermi surface and electronic band structure of BaCo$_{2}$As$_2$. Although its quasi-nesting-free Fermi surface differs drastically from that of its Fe-pnictide cousins, we show that the BaCo$_{2}$As$_2$ system can be used as an approximation to the bare unoccupied band structure of the related BaFe$_{2-x}$Co$_x$As$_2$ and Ba$_{1-x}$K$_x$Fe$_2$As$_2$ compounds. However, our experimental results, in agreement with dynamical mean field theory calculations, indicate that electronic correlations are much less important in BaCo$_{2}$As$_2$ than in the ferropnictides. Our findings suggest that this effect is due to the increased filling of the electronic 3$d$ shell in the presence of significant Hund's exchange coupling. [Preview Abstract] |
Monday, March 18, 2013 3:06PM - 3:18PM |
C37.00004: First-principle studies of electronic structure and magnetic excitations in FeSe monolayer Timur Bazhirov, Marvin L. Cohen Recent experimental advances made it possible to study single-layered superconducting systems of iron-based compounds. The results show evidence of significant enhancement of superconducting properties compared to the bulk case. We use first-principle pseudopotential density functional theory techniques and the local spin-density approximation to study the electronic properties of an FeSe monolayer in different spin configurations. The results show that the experimental shape of the Fermi surface is best described by a checkerboard antiferromagnetic (AFM) spin arrangement. To explore the underlying pairing mechanism, we study the evolution of the non-magnetic to the AFM-ordered structures under constrained magnetization, and we estimate the electronic coupling to magnetic excitations involving transfer and increase of iron magnetic moments and compare it to the electron-phonon coupling. Finally, we simulate the substrate-induced interaction by using uniform charge doping and show that the latter can lead to an increase in the density of states at the Fermi level and possibly produce higher superconducting transition temperatures. [Preview Abstract] |
Monday, March 18, 2013 3:18PM - 3:30PM |
C37.00005: Glide symmetry of iron-based superconductors: Electronic structure and superconductivity Chung-Pin Chou, Chia-Hui Lin, Wei Ku The alternating positioning of As/Se atoms above and below the Fe plane is known to alter the in-plane translational symmetry of Fe-based high-temperature superconductors, into a glide symmetry: translation followed by z-inversion. The proper descriptions of this symmetry and its consequences on the electronic structure and superconductivity have become a focus of recent intense theoretical studies [1-3]. In this talk, we will explicitly incorporate the glide symmetry in our first-principles studies of electronic structure, and discuss its consequence in superconductivity and other correlation in this class of materials. [1] Chia-Hui Lin, et al., Phys. Rev. Lett. 107, 257001 (2011). [2] Jiangping Hu and Ningning Hao, Phys. Rev. X 2, 021009 (2012). [3] M. Khodas and A. V. Chubukov, Phys. Rev. Lett. 108, 247003 (2012). [Preview Abstract] |
Monday, March 18, 2013 3:30PM - 3:42PM |
C37.00006: Density functional study on anisotropic magnetic exchange couplings in iron-based superconductors Hyungju Oh, Hyoung Joon Choi Iron-based superconductors exhibit many different antiferromagnetically ordered ground states. An understanding of the magnetic exchange couplings and ground-state Hamiltonian in the parent compounds of these materials is important because such an information could be the basis to unveil the mechanism of unconventional superconductivity. By performing first-principles calculations of the electronic and magnetic properties with non-collinear spin configurations, we study the anisotropic magnetic exchange couplings in iron-based superconductors. We obtain magnetic excitations using the anisotropic magnetic exchange couplings, and compare the results with published inelastic neutron scattering data. This work was supported by the NRF of Korea (Grant No. 2011-0018306). Computational resources have been provided by KISTI Supercomputing Center (Project No. KSC-2012-C2-14). [Preview Abstract] |
Monday, March 18, 2013 3:42PM - 4:18PM |
C37.00007: Impact of Hund's rule on the physics of the Fe-based superconductors Invited Speaker: Zhiping Yin The Hund's rule coupling J$_H$, as opposed to the Coulomb interaction (Hubbard) U, plays a dominating role in the Fe-based superconductors. The strong Hund's rule coupling combined with the multi-orbital nature and special valence of the Fe $3d$ shell, as well as the small crystal fields from the surroundings of an Fe atom, lead to many experimental consequences. In this talk, I will discuss the insights from first-principles calculations based on a combination of density functional theory and dynamical mean field theory. I will demonstrate the observable effects of \textit{Hundsness} on transport, optical conductivity, X-ray spectroscopy, angle-resolved photoemission spectroscopy, spin/magnetic excitations, and so on.\\[4pt] References:\\[0pt] [1] Z. P. Yin, K. Haule, and G. Kotliar,\textit{Nat. Mater.} {\bf 10}, 932-935 (2011).\\[0pt] [2] Z.P. Yin, K. Haule, and G. Kotliar,\textit{Nat. Phys.} {\bf 7}, 294-297 (2011).\\[0pt] [3] Z.P. Yin, K. Haule, and G. Kotliar, arXiv:1206.0801.\\[0pt] [4] Z.P. Yin, K. Haule, and G. Kotliar, unpublished.\\[0pt] [5] K. Haule and G. Kotliar,\textit{New J. Phys.} {\bf 11}, 025021 (2009).\\[0pt] [6] A. Kutepov, K. Haule, S. Y. Savrasov, and G. Kotliar, \textit{Phys. Rev. B} {\bf 82}, 045105 (2010).\\[0pt] [7] H. Park, K. Haule, and G. Kotliar, \textit{Phys. Rev. Lett.} {\bf107}, 137007 (2011). [Preview Abstract] |
Monday, March 18, 2013 4:18PM - 4:30PM |
C37.00008: Many-body effects in iron pnictides and chalcogenides -- non-local vs dynamic origin of effective masses Jan M. Tomczak, Mark van Schilfgaarde, Gabriel Kotliar We apply the quasi-particle self-consistent GW (QS{\it GW}) approximation to some of the iron pnictide and chalcogenide superconductors. We compute Fermi surfaces and density of states, and find excellent agreement with experiment, substantially improving over standard band-structure methods. Analyzing the QS{\it GW} self-energy we discuss non-local and dynamic contributions to effective masses. We present evidence that these two contributions are mostly separable. Indeed the quasi-particle weight is found to be essentially independent of momentum. The main effect of non-locality is captured by the static but non-local QSGW effective potential. Moreover, these non-local self-energy corrections, absent in e.g.\ dynamical mean field theory (DMFT), can be relatively large. We show, on the other hand, that QS{\it GW} only partially accounts for dynamic renormalizations at low energies. These findings suggest that QS{\it GW} combined with DMFT will capture most of the many-body physics in the iron pnictides and chalcogenides.\\[4pt] Reference: Jan M. Tomczak, M. van Schilfgaarde, G. Kotliar, PRL accepted, preprint: arXiv:1209.2213 [Preview Abstract] |
Monday, March 18, 2013 4:30PM - 4:42PM |
C37.00009: Effect of Pressure on the Electronic Structure and Optical Properties of FeSe: A DFT+DMFT Study Subhasish Mandal, R.E. Cohen, K. Haule Superconductivity in the iron-chalcogenide superconductors is extremely sensitive to pressure; $T_c$ increases from 8 K to 37 K upon applying pressure[1]. In order to better understand the sensitivity of $T_c$ to pressure in FeSe we have performed fully self-consistent Density Functional Theory - Dynamical Mean Field Theory (DMFT) (DFT-DMFT) computations with a continuous quantum Monte Carlo impurity solver as a function of compression and temperature. Using analytic continuation, we have computed spectral functions, Fermi surfaces, and the optical properties for comparison with experiments. Our preliminary results, obtained at room temperature show a $\sim$ 43\% reduction of the dc conductivity upon increasing the pressure from 0 to 2.6 GPa.\\[4pt] [1] S. Medvedev {\it et al.} Nat. Mater. {\bf 8}, 630 (2009). [Preview Abstract] |
Monday, March 18, 2013 4:42PM - 4:54PM |
C37.00010: Fermi surface topology of LaFePO, LiFeP and LiFeAs Roser Valenti, Johannes Ferber, Harald O. Jeschke We performed charge self-consistent LDA+DMFT (density functional theory combined with dynamical mean field theory) calculations to study correlation effects on the Fermi surfaces of the iron pnictide superconductors LaFePO, LiFeP an LiFeAs. We find a distinctive change in the topology of the Fermi surface in LaFePO and LiFeP where a hole pocket with Fe dz$^2$ orbital character changes its geometry from a closed shape in LDA to an open shape upon inclusion of correlations. In LiFeAs correlations influence mostly the shape of the hole pockets. We discuss our results in the context of angle-resolved photoemission spectroscopy and de Haas van Alphen observations. [Preview Abstract] |
Monday, March 18, 2013 4:54PM - 5:06PM |
C37.00011: Development of a two-particle self-consistent method for multi-orbital systems and its application to unconventional superconductors Hideyuki Miyahara, Ryotaro Arita, Hiroaki Ikeda We extend the two-particle self-consistent method proposed by Vilk and Tremblay [1] to multi-orbital systems. Starting with the sum rules for the spin and charge susceptibilities, we derive self-consistent equations to determine the renormalized effective interactions. We apply this method to LaFeAsO. In the former, we study the two-orbital model for the La$_{2-x}$(Sr/Ba)$_x$CuO$_4$ system. FLEX underestimated the pairing instability for it.[2] We show that, in our TPSC, the inter-orbital scattering enhances the d-wave instability. In the latter, we investigate a five-orbital d-model for LaFeAsO. This model has been extensively studied by RPA [3]. There, it has been shown that strong spin fluctuation mediates the s$_{+-}$ superconductivity. On the other hand, it has been pointed out that vertex corrections can enhance orbital fluctuations, which mediate s$_{++}$ superconductivity [4]. Finally, we show that orbital fluctuations can be enhanced in TPSC, while the dominant pairing symmetry is still s$_{+-}$ superconductivity when the system resides.\\[4pt] [1] Y.M. Vilk, A.-M.S. Tremblay, J. Phys. I France 7, 13091368.\\[0pt] [2] H. Sakakibara, et al, Phys. Rev. Lett. 105, 057003.\\[0pt] [3] K. Kuroki, et al, Phys. Rev. Lett. 101, 087004.\\[0pt] [4] H. Kontani and S. Onari, Phys. Rev. Lett. 104, 157001. [Preview Abstract] |
Monday, March 18, 2013 5:06PM - 5:18PM |
C37.00012: Impact of Dynamic Orbital Correlations on Magnetic Excitations in the Normal State of Iron-Based Superconductors Wei-Cheng Lee, Weicheng Lv, John Tranquada, Philip Phillips We show here that orbital degrees of freedom produce a distinct signature in the magnetic excitation spectrum of iron-based superconductors above the magnetic ordering temperature. Because d$_{\mathrm{xz}}$ and d$_{\mathrm{yz}}$ orbitals are strongly connected with Fermi surface topology, the nature of magnetic excitations can be modified significantly due to the presence of either static or fluctuating orbital correlations. Within a five-orbital itinerant model, we show that static orbital order generally leads to an enhancement of commensurate magnetic excitations even when the original Fermi surface lacks nesting at commensurate wavevectors. When long-range orbital order is absent, Gaussian fluctuations beyond the standard random-phase approximation capture the effects of fluctuating orbital correlations on the magnetic excitations. We find that commensurate magnetic excitations can also be enhanced if the orbital correlations are strong. We propose that this unusual incommensurate-to-commensurate transformation is an important signature to distinguish orbital from spin physics in the normal state of iron-based superconductors. [Preview Abstract] |
Monday, March 18, 2013 5:18PM - 5:30PM |
C37.00013: Orbital differentiation and the role of orbital ordering in the magnetic state of Fe superconductors Elena Bascones, Belen Valenzuela, Maria J. Calderon There is increasing evidence for orbital differentiation and a possible coexistence of itinerant and localized electrons in Fe superconductors. In order to shed light on the role of the different orbitals on the magnetic state of these superconductors we analyze the metallic $(\pi,0)$ antiferromagnetic state as a function of the interactions treated within mean field. We find that with increasing interactions the system does not evolve trivially from the pure itinerant to the pure localized regime. Instead we find a region with a strong orbital differentiation between $xy$ and $yz$, which are half-filled gapped states at the Fermi level, and itinerant $zx$, $3z^2 - r^2$ and $x^2 - y^2$. We argue that orbital ordering between the $yz$ and $zx$ orbitals arises as a consequence of the interplay of the exchange energy in the antiferromagnetic $x$ direction and the kinetic energy gained by the itinerant orbitals along the ferromagnetic $y$ direction with an overall dominance of the kinetic energy gain. We indicate that iron superconductors may be close to the boundary between the itinerant and the orbital differentiated regimes and that it could be possible to cross this boundary with doping [arXiv: 1208.1917. Phys. Rev. B, November 2012]. [Preview Abstract] |
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