Bulletin of the American Physical Society
APS March Meeting 2015
Volume 60, Number 1
Monday–Friday, March 2–6, 2015; San Antonio, Texas
Session J16: Focus Session: Theory of Magnetism and Correlation in Fe-Based Superconductors |
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Sponsoring Units: DMP DCOMP Chair: IIya Eremin, Theory Physics III University Room: 101AB |
Tuesday, March 3, 2015 2:30PM - 2:42PM |
J16.00001: Itinerancy enhanced quantum fluctuation of magnetic moments in iron-based superconductors Yu-Ting Tam, Dao-Xin Yao, Wei Ku We investigate the influence of itinerant carriers on dynamics and fluctuation of local moments in Fe-based superconductors, via linear spin-wave analysis of a spin-fermion model containing both itinerant and local degrees of freedom. Surprisingly against the common lore, instead of enhancing the ($\pi$,0) order, itinerant carriers with well nested Fermi surfaces is found to introduce significant amount of spatial and temporal quantum fluctuation that leads to the observed small ordered moment. Interestingly, the underlying mechanism is shown to be nesting-associated long-range coupling, rather than the previously believed ferromagnetic double-exchange effect. This talk will also address the strongly asymmetric suppression of magnetic order via electron- and hole-doping. [Preview Abstract] |
Tuesday, March 3, 2015 2:42PM - 2:54PM |
J16.00002: Spin fluctuations-corrected DFT for Fe-based superconductors Luciano Ortenzi, Hlynur Gretarsson, S. Kasahara, Y. Matsuda, T. Shibauchi, K.D. Finkelstein, W. Wu, S.R. Julian, Young-June Kim, I.I. Mazin, Lilia Boeri Albeit density functional theory (DFT) is, at the moment, the most appropriate tool for treating itinerant magnetism, its mean field implementations -local spin denstity approximation (LSDA) with or without gradient corrections- underestimate the effect of non local spin fluctuations. As a result DFT fails in reproducing, at the same time, the crystal structure and the amplitude of local moment in near critical systems. In this talk I will present a simple method for correcting the magnetic properties of itinerant systems in LSDA. The method is called reduced Stoner theory (RST). I will apply this method to study the ferromagnetic-paramagnetic transition under pressure in Ni$_3$Al itinerant ferromagnet and for describing the puzzling temperature behavior of the local moment found in doped-CaFe$_2$As$_2$ pnictides. [Preview Abstract] |
Tuesday, March 3, 2015 2:54PM - 3:06PM |
J16.00003: Understanding the Origin of Magnetism in Various Iron-based Superconductors from Itinerant Limit Yu-Zhong Zhang, Ming-Cui Ding, Hai-Qing Lin By investigating the bare susceptibilities from first principles which quantify the tendency of itinerant electrons towards magnetically ordered states, we find that the physical properties of various iron-based superconductors can be well understood by the relative strength of the particle-hole excitations at ($\pi$,$\pi$). Though the excitations in a few compounds show anomalous behaviors, they are not the counterexamples against the itinerant scenario. As long as the orbital degrees of freedom, which may lead to competing tendencies towards different magnetically ordered states, and the interlayer couplings are taken into account, these anomalies can be naturally accounted for from the itinerant limit. Moreover, we find that the particle-hole excitations away from the Fermi level are more relevant to the physical properties of iron-based superconductors than those close to the Fermi surfaces, which resolves the long-standing problem of why the Fermi surfaces alone can hardly explain various magnetic states observed experimentally in different iron-based superconductors. Finally, We predict based on our first principles calculations that K-doped BaFe$_2$P$_2$ and La- or Al-doped MgFeGe may be a possible iron-based superconductor. [Preview Abstract] |
Tuesday, March 3, 2015 3:06PM - 3:42PM |
J16.00004: Magnetic, structural and superconducting phase diagram in bulk Fe chalcogenides: role of nematic fluctuations and biquadratic exchange Invited Speaker: Igor Mazin It has been recently realized that even the bulk FeSe is distinctly unusual, compared to ``old'' pnictogen-based Fe based superconductors (FeBS), which may be a clue to understanding more exotic FeSe-derivatives. The mystery starts with the FeSe phase diagram: numerous pnictides experience an orthorhombic transition, likely of ``spin-nematic'' nature, followed by a magnetic transition; external pressure favors superconductivity if the starting phase is magnetic, and suppresses it otherwise, consistent with pressure suppressing spin fluctuations. FeSe, however, experiences an orthorhombic transition with no apparent sign of magnetic ordering, and its Tc raises rapidly with pressure, before switching to the usual, opposite trend. In this talk I will revisit, based on DFT calculations, magnetic interactions in chalcogenides, and show that they, unlike pnictides, demonstrate unusual (and unanticipated) frustration, which suppresses magnetic, but not nematic order, and fully explain the non-monotonic $T_c(P)$. Specifically, after the discovery of FeBS multiple attempts have been made to map the magnetic interactions in these systems (deemed to be crucial for superconductivity) onto a set of short range pairwise exchange interactions, initially in terms of the $J_1-J_2$ Heisenberg model HM. This approach failed to explain the double-stripe magnetism in FeTe, so the model was extended to include $J_3$. However, it was soon realized that this HM contradicts both $ab$ $initio$ calculations and neutron experiments in the magnetically ordered state of Fe pnictides. Thus the model was augmented to include a nearest neighbor biquadratic exchange $K$. It was also appreciated that the same interaction is essential for explaining the splitting between antiferromagnetic and orthorhombic phase transition in Fe pnictides. What has not been appreciated though was that (1) the double-stripe order is never a ground state of the HM, independent of the values of $J_{1,2,3}$; it can be stabilized only through $K$, (2) the HM model has, in addition to usually considered in FeBS phases, a highly competitive novel antiferromagnetic ``staggered stripes'' phase, which appears to be the ground state in ab initio calculations for FeSe (but not FeTe or for FeSe under pressure). Applying the full $J_{1,2,3}+K$ model to the Fe(Se,Te) system demonstrates unusual frustration, not relevant for As-based FeBS, which can explain the phase diagram of the system, nonmonotonic behavior if Tc under pressure and unexpectedly large orthorhombic ``nematic'' region in the FeSe phase diagram. [Preview Abstract] |
Tuesday, March 3, 2015 3:42PM - 3:54PM |
J16.00005: An energetically competitive $\tau_3$ $B_{1g}$ pairing in a $t-J_{1}-J_{2}$ model with orbital differentiated exchange couplings: implications for superconductivity in alkaline iron selenides Rong Yu, Emilian Marius Nica, Qimiao Si The pairing state in the alkaline iron selenides remains a challenge to our understanding. We address this issue in the incipient Mott picture based on the bad-metal behavior [1] of these materials. In conjunction with this picture, the multi-orbital effect is amplified, with two studied possibilities being the orbital-selective Mott transition [2] and the orbital-dependent pairing [3]. Here we carry out calculations in a five-orbital $t-J_{1}-J_{2}$ model, in which the orbital-dependent correlations are directly encoded in the exchange couplings. Specifically, we consider intra-orbital exchange couplings for $d_{xz}$, $d_{yz}$, and $d_{xy}$ but allow varying the ratio $r_o=J_{xz,xz}/J_{xy,xy}$. For $r_o<1$, we find a regime in the phase diagram where the leading pairing channel is an unusual $\tau_3$ $s_{x^2y^2}^{B_{1g}}$. This state has a full gap on the Fermi surfaces at both the zone boundary and center, with the pairing function changing sign between the two electron pockets. We propose this pairing state as a viable candidate for superconducting alkaline iron selenides. [1] R. Yu et al.,Nat. Commun. 4:2783 doi:10.1038/ncomms3783 (2013). [2] R.Yu and Q.Si, Phys. Rev. Lett. 110, 146402 (2013). [3] R. Yu, J.-X. Zhu, and Q. Si, Phys. Rev. B 89, 024509 (2014). [Preview Abstract] |
Tuesday, March 3, 2015 3:54PM - 4:06PM |
J16.00006: Effective Exchange Interactions for Bad Metals and Implications for Iron-based Superconductors Wenxin Ding, Rong Yu, Qimiao Si, Elihu Abrahams The experimentally observed bad metal behavior in parent iron pnictides and chalcogenides suggests that these systems contain strong electronic correlations and are on the verge of a metal-to-insulator transition. The magnetic excitations in this bad-metal regime mainly derive from the incoherent part of the electronic spectrum away from the Fermi energy. We present a microscopic study of the exchange interactions in such a regime within a slave rotor approach. Generalizations to the multi-orbital case are discussed, as are the implications for the strength of superconducting pairing amplitude in the iron-based superconductors. [Preview Abstract] |
Tuesday, March 3, 2015 4:06PM - 4:18PM |
J16.00007: Comparative study of pure and Co doped-BaFe2As2 Jacques Soullard, Ilya G. Kaplan, Raul Perez-Enriquez We present a comparative study of the high critical temperature superconductor $Co$ doped-$BaFe_{2}As_{2}$ at the electron correlation level by the embedded cluster method; the electron correlation is calculated through the second order M{\o}ller Plesset perturbation theory. We study successively the pure compound, the $Co$ doped-compound in the antiferromagnetic state and in the non-magnetic state. The $Co$ doping introduces a strong modification of the spin distribution in its neighboring atoms. The analysis of the orbital population reveals that the spin density of the $Co$ impurity becomes 3 times greater than that of the central $Fe$ of the pure compound, increase attributed to a corresponding increase of the $d_{z^{2}}$ orbital population; a local antiferromagnetic order along the {\bf b} axis of the crystal structure appears. The formation mechanism of the local magnetic moments implies a spin transfer from the (n.n.) and (n.n.n) atoms to the central $Co$ and is relevant to the $J_{1}-J_{2}$ Heisenberg model. The orbital population analysis reveals also that, in the doped compound and in both magnetic cases, the electron charge is associated to a singlet state and may correspond to a holon. [Preview Abstract] |
Tuesday, March 3, 2015 4:18PM - 4:30PM |
J16.00008: Study of multi-orbital Hubbard model at finite temperature using the Monte Carlo-Mean Field approach Anamitra Mukherjee, Niravkumar D. Patel, Shuai Dong, Steve Johnston, Adriana Moreo, Elbio Dagotto The phenomenology of iron-based superconductors suggest the need to use multi-orbital Hubbard models. For this reason, here we apply a recently developed technique, the ``Monte Carlo-Mean Field'' (MC-MF) method, to single and multiband Hubbard models [1]. In this approach, first a mean field approximation is used. The MF parameters are then treated via a finite-temperature classical MC as opposed to usual self consistency. In this talk, we show that the MC-MF results substantially improve on the naive finite-temperature MF approach and are in very good agreement with Determinantal Quantum Monte Carlo (DQMC) data for the single orbital case, both in weak and strong Hubbard U coupling. In the case of multiorbital models, phase diagrams for the parent compounds will be presented, varying U at fixed Hund coupling. Region of preformed local moments above the ordering temperatures will be discussed. Results for dynamical quantities such as the orbital resolved single particle spectral function $A(\vec{k},\omega)$, optical conductivity, and real space charge/spin/orbital density maps are also presented. \\[4pt] [1] A. Mukherjee et al., arXiv:1409.6790, to appear in PRB [Preview Abstract] |
Tuesday, March 3, 2015 4:30PM - 4:42PM |
J16.00009: Monte Carlo-Mean Field approach for the one band Hubbard model Niravkumar D. Patel, Anamitra Mukherjee, Shuai Dong, Steve Johnston, Adriana Moreo, Elbio Dagotto Multiorbital model Hamiltonians are crucial to understand iron-based superconductors. We employ a recently developed ``Monte Carlo-Mean Field'' (MC-MF) method [1] to study single and multiband Hubbard models. The focus here is on the single band case at half filling. We start with a mean-field (MF) decomposition of the Hubbard hamiltonian and then promote the MF parameters to classical variables studied via MC simulations, while fermions are exactly diagonalized in the background of those classical variables. We present the Hubbard $U$ vs. temperature phase diagram on large three and two dimensional clusters. Our MC-MF method can capture the nonmonotonicity of $T_N$ with $U$, local moment physics above $T_N$, and the two peak behavior of specific heat, as compared with Determinantal Quantum Monte Carlo (DQMC). Results for the $t - t'$ Hubbard model in two dimensions show that our approach can capture ground state and finite temperature properties reliably where DQMC fails due to sign problems. These one-band results set the stage for extending the MC-MF method to multiband Hubbard models of relevance to the Pnictide superconductors. \\[4pt] [1] Mukherjee et.al. arXiv:1409.6790 (to appear in Phys. Rev. B) [Preview Abstract] |
Tuesday, March 3, 2015 4:42PM - 4:54PM |
J16.00010: Constrained-path Quantum Monte Carlo study of the three-orbital Hubbard model for iron-based superconductors Guangkun Liu, Shuhua Liang, Chris Bishop, Elbio Dagotto Pairing and magnetic properties of the three-orbital Hubbard model for iron-based superconductors are systematically studied via the constrained-path quantum Monte Carlo method. We further develop the algorithm for multi-orbital Hubbard models in general and introduce parallel techniques to allow for large-lattice simulations. In addition, the effects of the additional $d_{xy}$ orbital are studied by comparing with our previous simulation on the two-orbital model [1].\\[ 4pt] [1] G.-K. Liu et al., J. Phys.: Condens. Matter 26, 325601 (2014) [Preview Abstract] |
Tuesday, March 3, 2015 4:54PM - 5:06PM |
J16.00011: Quantum oscillations near quantum critical point Arkady Shekhter Extensive experimental investigations of quantum oscillations in high temperature superconducting cuprates and pnictides suggest significant increase of quasiparticle mass approaching critical doping. We report theoretical analysis of quantum oscillation amplitude in a metal near quantum critical point which suggest dynamic, rather than thermodynamic, origin of the observed increase in mass. Direct thermodynamic measurements are discussed to substantiate this analysis. [Preview Abstract] |
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