Bulletin of the American Physical Society
2009 APS March Meeting
Volume 54, Number 1
Monday–Friday, March 16–20, 2009; Pittsburgh, Pennsylvania
Session H35: Focus Session: Iron Pnictides and Other Novel Superconductors VI: Model Hamiltonians |
Hide Abstracts |
Sponsoring Units: DMP Chair: Zlatko Tesanovic, Johns Hopkins Room: 405 |
Tuesday, March 17, 2009 8:00AM - 8:12AM |
H35.00001: Effects of Magnetic Order and Pairing on the Fermi Surface of the Pnictides A. Moreo, M. Daghofer, R. Yu, J. A. Riera, E. R. Dagotto Based on numerical and mean-field calculations performed on models for the FeAs planes of the newly discovered Fe-based superconductors, we present results for the expected shape of the Fermi surface both in the undoped and the doped regime. In the undoped case, numerical studies, Lanczos and VCA, are performed for a two-orbital model, while a mean-field formalism allows us to study a more realistic 4 orbital case. A pocket structure is obtained for the intermediate Hubbard coupling regime of parameters in which the system is magnetically ordered but still metallic [1]. We construct a mean-field model for light electronic doping based on the pairing operator that is found by the unbiased Lanczos calculations in the two orbital model, i.e. a spin singlet, orbital even, operator transforming according to the $B_{2g}$ representation of the group $D_{4h}$.[2] We present the resulting nodal structure [3] and discuss comparisons with ARPES results.\\[0.0001em] [1] R. Yu et al., submitted. [2] M. Daghofer et al., arXiv:0805.0148, to appear in PRL. [3] A. Moreo et al., submitted. [Preview Abstract] |
Tuesday, March 17, 2009 8:12AM - 8:24AM |
H35.00002: Nematic spin order and spin-lattice coupling in Fe-based Superconductors Jiangping Hu, Chen Fang, Wei-Feng Tsai, Hong Yao, Steve Kivelson We show that the structure transitions observed in Fe-based superconductors are magnetically driven. A quantum Heisenberg model ($J_1-J_2-J_z$) exhibits a sequence of two phase transitions: from a high temperature symmetric phase to a narrow region of intermediate ``nematic'' phase, and then to a low temperature spin ordered phase when $J_z$ is small. Identifying phases by their broken symmetries, these phases correspond precisely to the sequence of structural (tetragonal to monoclinic) and magnetic transitions that have been recently revealed in neutron scattering studies of 1111 series of Fe- based superconductors. The structural transition can thus be identified with the existence of incipient (``fluctuating'') magnetic order. We also discuss the effect of spin-lattice coupling on the phase diagram of the model. \\[3pt] Reference: Chen Fang, Hong Yao, Wei-Feng Tsai, JiangPing Hu and Steven A. Kivelson, Phys. Rev. B 77 224509 (2008). [Preview Abstract] |
Tuesday, March 17, 2009 8:24AM - 8:36AM |
H35.00003: Modeling of Fe pnictides: the Magnetic Order and Pairing Channels M. Daghofer, A. Moreo, J. A. Riera, E. Arrigoni, D. J. Scalapino, E. R. Dagotto We use numerical methods - exact diagonalization and the variational cluster approach - to study a two-orbital model for Fe-pnictide superconductors, including onsite Coulomb interaction $U$ and Hund's rule coupling $J$ [1]. Robust next-nearest neighbor hoppings stabilize the spin ``striped'' AF order for undoped clusters, in agreement with neutron scattering data. The ordered magnetic moment depends on $U$ and $J$, and we find a bad metal with small ordered moment at intermediate $U$, as observed experimentally. By adding two electrons to the undoped cluster, we identify three different pairings channels: An inter-orbital triplet at small $U$, which transforms as the $A_{2g}$ representation of the $D_{\rm 4h}$ group, an inter-orbital singlet transforming as $B_{2g}$ at the most realistic intermediate $U$, and an intra-orbital $A_{1g}$ singlet at large $U$. We compare the results to a three-orbital model including the $xy$ orbital in addition to the $xz$ ans $yz$ orbitals. [1] M. Daghofer et al., arXiv:0805.0148, to appear in PRL. [Preview Abstract] |
Tuesday, March 17, 2009 8:36AM - 8:48AM |
H35.00004: Existence of a metallic magnetically ordered state at intermediate Hubbard couplings in multi-orbital models for undoped iron pnictides Rong Yu, Kien Trinh, Adriana Moreo, Maria Daghofer, Jose Riera, Stephan Haas, Elbio Dagotto We present the results of a mean-field study for models that describe undoped iron pnictides. A realistic four-orbital model including iron $d_{xz}$, $d_{yz}$, $d_{xy}$, and $d_{x^2-y^2}$ orbitals is mainly discussed. Results for a two-orbital model with $d_{xz}$ and $d_{yz}$ orbitals are also shown. In both models, we report the existence of a novel intermediate coupling regime where the system is metallic and exhibits a striped spin order. Several properties of this state are discussed. By performing a mean-field study of other models for iron pnictides, we argue that such a metallic striped ordered phase is a general feature of the theoretical models describing iron pnictides.[1] \\[3pt] [1] R. Yu et al., submitted for publication. [Preview Abstract] |
Tuesday, March 17, 2009 8:48AM - 9:00AM |
H35.00005: Low Ordered Magnetic Moment in Fe-As High-T$_{c}$ Superconductors by Violation of Hund's Rule Jose Rodriguez, Edward Rezayi We study by exact diagonalization the J$_{0}$-J$_{1}$-J$_{2}$ model over the square lattice that Si and Abrahams introduced recently to describe magnetism in the newly discovered iron-arsenic class of high-T$_{c}$ superconductors. The case of maximum frustration between the nearest-neighbor and the next-nearest-neighbor Heisenberg exchange terms, J$_{2}=\vert $J$_{1}\vert $/2, over a 4 by 4 square lattice with periodic boundary conditions is focused on. Each site hosts two Fe orbitals. Hidden long-range antiferromagnetic order can appear in the absence of Hund's rule coupling, J$_{0}$ = 0. It shows no net ordered magnetic moment. The ordered collinear/SDW moment steadily increases from zero as Hund's rule coupling turns on: J$_{0} \quad <$ 0. This result compares well with recent determinations of a low ordered magnetic moment for the insulating parent compounds of Fe-As based high-T$_{c}$ superconductors by elastic neutron scattering. Our numerical results are also consistent with a quantum phase transition at intermediate Hund's rule coupling, J$_{0}$ = - 2$\vert $ J$_{1}\vert $, that separates the latter hidden-order state from a more familiar frustrated magnetic state that obeys Hund's rule. [Preview Abstract] |
Tuesday, March 17, 2009 9:00AM - 9:12AM |
H35.00006: Magnetic Excitations in the Iron-based Superconductors Daoxin Yao, JiangPing Hu, Erica W. Carlson We calculate the expected inelastic neutron scattering response based on the spin-orderings found in the iron-based superconductors, using spin-wave theory. For the two-sublattice collinear antiferromagnet, we consider two types of superexchange couplings between Fe atoms: nearest-neighbor coupling J$_1$ and next-nearest-neighbor coupling J$_2$. We show how to distinguish experimentally between ferromagnetic and antiferromagnetic J$_1$. We show the existence of saddlepoints near ($\pi$, $\pi/2$) and ($0$, $\pi/2$), which are expected to give rise to extra scattering intensity. We find that the sublattice magnetization can be reduced by the zero-point motion of spin waves, although not enough to account for the small moments observed in experiment. By comparison with experimental results on SrFe$_2$As$_2$, we estimate that the effective magnetic interlayer coupling is rather large, about $1/8$ the value of the in-plane couplings. References: 1) Phys. Rev. Lett. 101, 167203 (2008); 2) Phys. Rev. B 78, 052507 (2008) [Preview Abstract] |
Tuesday, March 17, 2009 9:12AM - 9:24AM |
H35.00007: Iron pnictides as a model system for heavy fermion behavior: influence of conduction-electron magnetic ordering on Kondo effect Jianhui Dai, Qimiao Si, Jian-Xin Zhu The rare-earth iron pnictides exhibit a number of magnetic ground states besides the unconventional superconductivity. With CeOFeP [1] and CeOFeAs [2] in mind as prototypes, we derive an extended Anderson lattice model which incorporates the hybridizations of the pnictogen 4p (or 3p)- orbitals with both the iron 3d-orbitals and rare earth 4f-orbitals[3]. We show a new type of Kondo lattice physics: Kondo screening of the f-moments are suppressed by the antiferromagnetic ordering of the d-electrons. Inside the d- electron AF state (as in CeOFeAs), the f-moments are dominantly coupled by superexchange with competing components. The resulting magnetic frustration in general favors a helical order. The regime where d-electrons are paramagnetic (including CeOFeP) features the usual RKKY vs. Kondo competition. The implications of our results for heavy fermion physics in general are discussed.\\[3pt] [1] E.M. Br\"uning et al., PRL101, 117206(2008)\\[0pt] [2] G. F. Chen et al., PRL100, 247002(2008). J. Zhao et al., Nat. Mater.(2008)\\[0pt] [3] J. Dai, Q. Si {\&} J-X Zhu, to be published. [Preview Abstract] |
Tuesday, March 17, 2009 9:24AM - 9:36AM |
H35.00008: Tight-binding Hamiltonian for LaFeAsO Dimitrios Papaconstantopoulos, Lane Nixon, Michael Mehl There have been several first-principles calculations reported recently for the superconducting pnictinide LaFeAsO and related compounds. In addition, tight-binding(TB) Hamiltonians for these systems have been constructed with varying degrees of success. In this work we have used the NRL-TB method to fit our LAPW results to a TB basis with the aim of reproducing the band structure very accurately. We have included the s and d orbitals of Fe, the s and p orbitals of As, and the p orbitals of O. We present a study of these TB results in terms of the effect of each of the above orbitals on how accurately the first-principles band structure can be reproduced. Finally, we assess the feasibility of carrying out many-body theory with a Hamiltonian that may contain more than just the d-Fe orbitals. [Preview Abstract] |
Tuesday, March 17, 2009 9:36AM - 9:48AM |
H35.00009: Competing magnetism and superconductivity in two-band metals Anton Vorontsov, Maxim Vavilov, Andrey Chubukov Recently discovered FeAs-based superconductors have a distinct multiple band structure - the hallmark feature of these materials. We consider a simple two-band model for these metals, with one electronic and one hole bands. Within this model, we treat on equal footing magnetic spin density wave (SDW) and superconducting (SC) orders. We find that at low doping, magnetism wins, but at higher dopings superconducting instability comes first. We discuss the type of a transition between the two states, incommensurate SDW order at finite dopings, and co-existence of SDW and SC orders at $T=0$ and finite temperatures. Our results reasonably well explain the phase diagram of LaO$_{1-x}$F$_{x}$FeAs compounds. [Preview Abstract] |
Tuesday, March 17, 2009 9:48AM - 10:00AM |
H35.00010: Superconductivity in the multiband matrix t-J1-J2 model and its implications for the iron pnictides Qimiao Si, Pallab Goswami, Predrag Nikolic, Elihu Abrahams We describe the iron pnictides in terms of an incipient Mott picture. We use local moments with frustrating J1-J2 interactions to model the incoherent electronic excitations, and couple them to the coherent electronic carriers. The resulting multiband matrix t-J1-J2 model is analyzed in terms of a slave boson theory, leading to a superconducting phase diagram as a function of doping and J2/J1 ratio. The different pairing symmetries reflect a competition between the strong coupling effects of the J1-J2 interactions, and the kinematic effects associated with the multiple sheets of Fermi surfaces. [Preview Abstract] |
Tuesday, March 17, 2009 10:00AM - 10:12AM |
H35.00011: Electronic states and material dependences of Fe-based superconductors Michiyasu Mori, Takami Tohyama, Navid Afzal Shooshtary, Sadamichi Maekawa In this study, we will discuss the electronic states of Fe- based superconductors and its material dependences, in particular, by taking account of the bond angle dependences. First, we calculate the crystal field splitting ($\Delta$) of Fe 3d orbital coordinated by four As's. Next, the hopping integrals (t) are estimated by using the Slater and Koster fs method. Note that these parameters, t and $\Delta$ change with $\alpha$. Finally, we can obtain the dispersion relation as a function of $\alpha$. It is found that the spectral weights near the Fermi energy are dominated by yz, zx and x2-y2 orbitals. The yz and zx orbitals are higher in energy around the regular tetragonal geometry, in which $\alpha$ is almost 109$^{\circ}$. On the other hand, those two orbitals become lower in energy for the larger value of $\alpha$. Such an orbital crossing is crucial for the electronic states. The ground state phase diagram is obtained by the Hartree-Fock calculation of multi-band Hubbard model. [Preview Abstract] |
Tuesday, March 17, 2009 10:12AM - 10:24AM |
H35.00012: Minimal two-band model for the superconducting iron oxypnictides Chao-Xing Liu, Srinivas Raghu, Xiao-Liang Qi, Douglas Scalapino, Shoucheng Zhang Following the discovery of the Fe-pnictide superconductors, LDA band structure calculations showed that the dominant contributions to the spectral weight near the Fermi energy came from the Fe 3d orbitals. The Fermi surface is characterized by two hole surfaces around the $\Gamma$ point and two electron surfaces around the M point of the 2 Fe/cell Brillouin zone. Here, we describe a 2-band model that reproduces the topology of the LDA Fermi surface and exhibits both ferromagnetic and $q=(\pi,0)$ spin density wave (SDW) fluctuations. We argue that this minimal model contains the essential low energy physics of these materials. [Preview Abstract] |
Tuesday, March 17, 2009 10:24AM - 10:36AM |
H35.00013: Spin-orbital frustrations and anomalous metallic state in iron-pnictide superconductors Frank Kruger, Sanjeev Kumar, Jan Zaanen, Jeroen van den Brink We develop an understanding of the anomalous metal state of the parent compounds of recently discovered iron based superconductors starting from a strong coupling viewpoint. On the basis of an intermediate-spin ($S=1$) state for the Fe$^{2+}$ ions, we derive a Kugel-Khomskii spin-orbital Hamiltonian for the active $t_{2g}$ orbitals. It turns out to be a highly complex model with frustrated spin and orbital interactions. We compute the classical phase diagrams and provide an understanding for the stability of the various phases by investigating the spin-only and orbital-only limits of the full Hamiltonian. The experimentally observed spin-stripe state is found to be stable over a wide regime of physical parameters and can be accompanied by three different types of orbital orders. Of these the orbital-ferro and orbital-stripe orders are particularly interesting since they break the in-plane lattice symmetry; a robust feature of the undoped compounds. We also compute the magnetic excitation spectra for the $S=1$ Heisenberg model, treating orbital correlations as static. The stable orbital-stripe state provides an explanation for the observed strong reduction of magnetic moment. [Preview Abstract] |
Tuesday, March 17, 2009 10:36AM - 10:48AM |
H35.00014: Theory for the Magnetic Ordering in Fe$_{1+y}$Te$_{1-x}$Se$_{x}$ Chen Fang, B. Andrei Bernevig, Jiangping Hu We develop a local spin model to explain the rich magnetic structures in the iron-based superconductors $Fe_{1+y}Te_{1-x}Se_x$. We show that our model exhibits both commensurate antiferromagnetic and incommensurate magnetic order along the crystal a-axis, the transition between which can be obtained by increasing the concentration of the excess $Fe$ to a critical value. The incommensurate modulation vector is also shown to be $Fe^2$ concentration dependent. Experimentally measurable spin-wave features and Fermi surface properties are calculated and compared to those of other Fe-based superconductors. Our model also suggests the existence of a large quantum critical region due to strong spin frustration upon increasing $Se$ concentration. [Preview Abstract] |
Tuesday, March 17, 2009 10:48AM - 11:00AM |
H35.00015: Global phase diagram and the spin structure in the Fe-based superconductors Kangjun Seo, Chen Fang, Jiangping Hu, B. Andrei Bernevig We study the global phase diagram and calculate the spin susceptivility for different states based on a two-orbital $J_1 - J_2$ model for Fe-based superconductors. Unique features associated with the unconventional $s_{x^2 y^2}\sim \cos (k_x) \cos (k_y)$ wave pairing symmetry are identified. [Preview Abstract] |
Follow Us |
Engage
Become an APS Member |
My APS
Renew Membership |
Information for |
About APSThe American Physical Society (APS) is a non-profit membership organization working to advance the knowledge of physics. |
© 2025 American Physical Society
| All rights reserved | Terms of Use
| Contact Us
Headquarters
1 Physics Ellipse, College Park, MD 20740-3844
(301) 209-3200
Editorial Office
100 Motor Pkwy, Suite 110, Hauppauge, NY 11788
(631) 591-4000
Office of Public Affairs
529 14th St NW, Suite 1050, Washington, D.C. 20045-2001
(202) 662-8700