Bulletin of the American Physical Society
APS March Meeting 2016
Volume 61, Number 2
Monday–Friday, March 14–18, 2016; Baltimore, Maryland
Session K11: Electronic Structure and Magnetism in Fe-based Superconductors IIFocus
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Sponsoring Units: DMP GMAG Chair: Emilia Morosan, Rice University Room: 307 |
Wednesday, March 16, 2016 8:00AM - 8:36AM |
K11.00001: Orbital-dependent electron correlation effects in iron-based superconductors Invited Speaker: Ming Yi The iron chalcogenide superconductors constitute arguably one of the most intriguing families of the iron-based high temperature superconductors given their ability to superconduct at comparable temperatures as the iron pnictides, despite the lack of similarities in their magnetic structures and Fermi surface topologies. In particular, the lack of hole Fermi pockets at the Brillouin zone center posts a challenge to the previous proposal of spin fluctuation mediated pairing via Fermi surface nesting. In this talk, using angle-resolved photoemission spectroscopy measurements, I will present evidence that show that instead of Fermi surface topology, strong electron correlation observed in electron bandwidth is an important ingredient for superconductivity in the iron chalcogenides. Specifically, I will show i) there exists universal strong orbital-selective renormalization effects and proximity to an orbital-selective Mott phase in Fe$_{\mathrm{1+y}}$Te$_{\mathrm{1-x}}$Se$_{\mathrm{x}}$, A$_{\mathrm{x}}$Fe$_{\mathrm{2-y}}$Se$_{\mathrm{2}}$, and monolayer FeSe film on SrTiO$_{\mathrm{3}}$ [1,2], and ii) in Rb$_{\mathrm{x}}$Fe$_{\mathrm{2}}$(Se$_{\mathrm{1-z}}$S$_{\mathrm{z}})_{\mathrm{2}}$, where sulfur substitution for selenium continuously suppresses superconductivity down to zero, little change occurs in the Fermi surface topology while a substantial reduction of electron correlation is observed in an expansion of the overall bandwidth, implying that electron correlation is one of the key tuning parameters for superconductivity in these materials. [1] M. Yi et al. Phys. Rev. Lett. 110, 067003 (2013). [2] M. Yi et al. Nat. Comm. 6, 7777 (2015). [3] M. Yi et al. arXiv: 1505.06636. [Preview Abstract] |
Wednesday, March 16, 2016 8:36AM - 8:48AM |
K11.00002: Bandwidth-controlled metal-superconductor-insulator phase diagram in iron-chalcogenides Xiaohai Niu, Sudi Chen, Juan Jiang, Zirong Ye, Tianlun Yu, Difei Xu, Min Xu, Yu Feng, Yajun Yan, Binping Xie, Jun Zhao, Dachun Gu, Liling Sun, Qianhui Mao, Hangdong Wang, Minghu Fang, C. J. Zhang, J. P. Hu, Zhe Sun, Donglai Feng Using angle- resolved photoemission spectroscopy, we studied isovalently doped K\textunderscore {\$}x{\$}Fe\textunderscore {\$}\textbraceleft 2$-$y\textbraceright {\$}Se\textunderscore {\$}\textbraceleft 2$-$z\textbraceright {\$}S\textunderscore {\$}z{\$}, Rb\textunderscore {\$}x{\$}Fe\textunderscore {\$}\textbraceleft 2$-$y\textbraceright {\$}Se\textunderscore {\$}\textbraceleft 2$-$z\textbraceright {\$}Te\textunderscore {\$}z{\$} and (Tl,K)\textunderscore {\$}x{\$}Fe\textunderscore {\$}\textbraceleft 2$-$y\textbraceright {\$}Se\textunderscore {\$}\textbraceleft 2$-$z\textbraceright {\$}S\textunderscore {\$}z{\$}, in which the superconducting transition temperature decreases with either positive or negative chemical pressures. The bandwidths of Fe 3d bands in the energy window of [0, -0.5] eV in these materials change systematically with doping: with the decreasing of bandwidth, the ground state evolves from a metal to a superconductor, and eventually to an insulator. This systematic study of electronic structures discovered the correlation-driven insulator state by tuning the bandwidth, which is independent with carrier density. The results also indicate that moderate correlation strength is beneficial to enhance superconductivity. [Preview Abstract] |
Wednesday, March 16, 2016 8:48AM - 9:00AM |
K11.00003: ARPES investigation of heavily hole-doped Fe-based superconductor (Ba,K)Fe$_2$As$_2$ Xun Shi, Pierre Richard, Peng Zhang, Ambroise van Roekeghem, Tian Qian, Jiangping Hu, Hong Ding, Delong Fang, Haihu Wen, Nan Xu, Ming Shi, Timur Kim, Moritz Hoesch, Xianhui Chen A Lifshitz transition occurs in the (Ba,K)Fe$_2$As$_2$ family upon K doping and electron pocket are absent in the heavily doped compounds, including KFe$_2$As$_2$. The pairing symmetry is argued to undergoes a phase transition due to the existence of gap node(s) reported in various experiments. In this work we present angle-resolved photoemission spectroscopy and scanning tunneling spectroscopy studies of KFe$_2$As$_2$. We observe a van Hove singularity (vHs) in proximity of the Fermi level ($E_F$), which locates in the middle of the principal axes of the first Brillouin zone. The density-of-states at $E_F$ mainly comes from the vHs whereas it is non-gapped in the superconducting state. Our observation provides natural explanations for many novel behaviors in this material. In particular, it is consistent with our measurements of the gap structure in Ba$_{0.1}$K$_{0.9}$Fe$_2$As$_2$. All these results suggest that Cooper pairing is induced by a strong-coupling mechanism. [Preview Abstract] |
Wednesday, March 16, 2016 9:00AM - 9:12AM |
K11.00004: Persistence of Dirac Node near Antiferromagnetic-to-Superconducting Phase Boundary in Ba(Fe$_{1-x}$Co$_x$)$_2$As$_2$ Hitoshi Takita, Naoya Kishimoto, Yousuke Nakashima, Akihiro Ino, Masashi Arita, Hirohumi Namatame, Masaki Taniguchi, Yoshihiro Aiura, Izumi Hase, Hiroshi Eisaki, Kunihiro Kihou, Chul-Ho Lee, Akira Iyo, Masamichi Nakajima, Shin-ichi Uchida Since the ground state of iron-pnictides changes from an antiferromagnetic (AF) phase to a superconducting (SC) phase, the evolution of electronic structure has attracted much attention. However, systematic investigation has been hindered by the intricate multiple bands arising from the orbital degree of freedom of iron $3d$ states. Here we performed a polarization-dependent ARPES study of Ba(Fe$_{1-x}$Co$_x$)$_2$As$_2$ across the AF-SC phase boundary. The doping-dependence of ARPES spectra has shown that the Dirac node reported in the AF phase of BaFe$_2$As$_2$ persists in $x=0.04$ near the AF-SC phase boundary, and that it disappears in the SC phase of $x=0.05$. We parametrized the cone-like dispersion in $x=0.04$. The polarization-dependence of our ARPES spectra is consistent with the view that the Dirac node is protected by Berry phase arising from orbital degree of freedom under the inversion symmetry. [Preview Abstract] |
Wednesday, March 16, 2016 9:12AM - 9:24AM |
K11.00005: orbital selective correlation reduce in collapse tetragonal phase of CaFe$_2$(As$_{0.935}$P$_{0.065}$)$_2$ and electronic structure reconstruction studied by angel resolved photoemission spectroscopy Lingkun Zeng We performed an angle-resolved photoemission spectroscopy (ARPES) study of the CaFe$_2$(As$_{0.935}$P$_{0.065}$)$_2$ in the collapse tetragonal(CT) phase and uncollapse tetragonal(UCT) phase. We find in the CT phase the electronic correlation dramatically reduces respective to UCT phase. Meanwhile, the reduction of correlation in CT phase show an orbital selective effect: correlation in \textit{d$_{xy}$} reduces the most, and then \textit{d$_{xz/yz}$}, while the one in \textit{d$_{z^2 - r^2}$} almost keeps the same. In CT phase, almost all bands sink downwards to higher binding energy, leading to the hole like bands around Brillouin zone(BZ) center sink below \textit{E$_F$} compared with UCT phase. However, the electron pocket around Brillouin Zone(BZ) corner(M) in UCT phase, forms a hole pocket around BZ center(Z point) in CT phase. Moreover, the \textit{d$_{xy}$} exhibits larger movement down to higher binding energy, resulting in farther away from \textit{d$_{yz/xz}$} and closer to \textit{d$_{xy}$}.We propose the electron filling ,namely high spin state in UCT phase to low spin state in CT phase(due to competing between crystal structure field and Hund's coupling), other than the Fermi surface nesting might be responsible for the absent of magnetic ordering. [Preview Abstract] |
Wednesday, March 16, 2016 9:24AM - 9:36AM |
K11.00006: Effect of directional strain on the phase diagram of Ca(Fe$_{1-x}$Co$_x$)$_2$As$_2$ A. E. B\"ohmer, G. Drachuk, M. A. Tanatar, S. L. Bud{'}ko, R. Prozorov, P. C. Canfield The iron-based superconductor Ca(Fe$_{1-x}$Co$_x$)$_2$As$_2$ is exceptionally sensitive to directional stress with $ab$-plane compression stabilizing and $c$-axis compression de-stabilizing the orthorhombic antiferromagnetic phase [1]. Due to differential thermal expansion between a sample and a substrate, an effective in-plane compressive strain can be exerted on it upon cooling. We found that this strain induces a phase transition even in overdoped compositions where the usual magneto-structural transition, observed in underdoped compounds, does not occur in the unstrained state. The induced transition has been characterized by 4-probe resistivity, elastoresistivity (the derivative of resistivity with respect to deformations), polarized light microscopy and M\"ossbauer spectroscopy. We found a pronounced increase of the resistivity and a divergence of the elastoresistivity coefficients, which is a signature of the tetragonal-to-orthorhombic transition in other iron-based superconductors. The polarized light images directly show the formation of a particularly rich domain pattern below the transition in these samples. This work was supported by the Ames Laboratory, US DOE, under Contract No. DE-AC02-07CH11358. [1] Bud{'}ko et al., PRB 88,064513 (2013). [Preview Abstract] |
Wednesday, March 16, 2016 9:36AM - 9:48AM |
K11.00007: Quantum fluctuations in iron-pnictide superconductor BaFe$_2$(As$_{1-x}$P$_x$)$_2$ Lei Shu, Z. F. Ding, J. Zhang, C. Tan, K. Huang, L. Liu, S. Cheung, Y. J. Uemura, D. E. MacLaughlin, O. O. Bernal, P.-C. Ho, D. Hu, P.C. Dai Muon-spin-relaxation/rotation ($\mu$SR) experiments were performed on single crystals of iron-pnictide superconductors BaFe$_2$(As$_{1-x}$P$_x$)$_2$ ($x = 0.28, 0.30$, and 0.33). Our preliminary results reveal that the static muon relaxation rate from ZF-$\mu$SR measurements is temperature independent through $T_c$, suggesting that time reversal symmetry is preserved in the superconducting state. Above $T_c$, the field dependence of muon relaxation rate shows NFL behaviors for optimal composition $x=0.3$. A maximum of zero temperature penetration depth at $x=0.3$ is also observed. [Preview Abstract] |
Wednesday, March 16, 2016 9:48AM - 10:00AM |
K11.00008: Soft-mode transitions of alkaline-earth 122 pnictides Michael Widom, Khandker Quader $A$-122 pnictides ($A$=Ca, Sr, Ba) exhibit three pressure-driven transitions: a first order enthalpic transition at $P_H$ from the striped AFM orthorhombic (OR) to a tetragonal (T) or a collapsed tetragonal (cT) phase; a transition at $P_M > P_H$ from the metastable AFM OR to a T or cT phase; a Lifshitz transition at $P_L$ that causes T to collapse to a cT phase. Transitions at $P_H$ and $P_L$ were previously examined through total energy and band structure calculations \footnote{M. Widom and K. Quader, Phys. Rev. B 88, 045117 (2013)}\footnote{K. Quader and M. Widom, Phys. Rev. B 90, 144512 (2014)}. Here we address the transition at $P_M$, beyond which the metastable AFM OR state ceases to exist. We show this transition occurs through a loss of elastic stability caused by softening of a shear mode associated with stretching along the c-axis. Simultaneously, magnetism and orthorhombicity approach limiting values with an approximately square-root singularity \footnote{M. Widom and K. Quader, arXiv:1508.07932}. Together these suggest a strong magneto-elastic coupling that may be relevant to a further understanding of the $A$-122-pnictides under pressure. [Preview Abstract] |
Wednesday, March 16, 2016 10:00AM - 10:12AM |
K11.00009: Correlation driven dimensional reduction in a two orbital Hubbard model Anamitra Mukherjee, Niravkumar D. Patel, Adrianna Moreo, Elbio Dagotto We apply a recently developed many-body technique that allows for the incorporation of thermal effects, to a two orbital Hubbard model of relevance for the pnictides. In this ‘Mean Field-Monte Carlo’ (MF-MC) approach, we first perform a mean field (MF) decomposition of the Hubbard model and then treat the mean field parameters via the standard finite-temperature classical Monte Carlo (MC). We have earlier established [1] that for the one orbital Hubbard model, this MF-MC approach provides remarkable improvement over simple finite-temperature mean field methods and is in good agreement with Determinantal Quantum Monte Carlo results. In this talk we will discuss our MC-MF results applied to the two orbital Hubbard model with degenerate dxz and dyz orbitals for the undoped pnictides [2]. The onsite repulsion strength $U$ vs. temperature phase diagram is rich and has a narrow window of nematicity above the N’eel temperature. Our main result is the discovery of a novel intermediate coupling regime characterized by an unexpected spontaneous dimensional reduction that renders one direction insulating and the other metallic. [1] A. Mukherjee, et. al. Phys. Rev. \textbf{B} 90, 205133 (2014). [2] A. Mukherjee, et. al. arXiv: 1510.04902 [Preview Abstract] |
Wednesday, March 16, 2016 10:12AM - 10:24AM |
K11.00010: Testing The Constrained-Path Quantum Monte Carlo Method Using A One Dimensional Three Orbital Hubbard Model. Guangkun Liu, Nitin Kaushal, Chris Bishop, Shuhua Liang, Shaozhi Li, Steve Johnston, Elbio Dagotto The ``sign problem'' usually prevents the large scale quantum Monte Carlo simulations of the multi-orbital Hubbard models. Projecting from a variety of initial states constructed via the Hartree-Fock technique, a constrained-path quantum Monte Carlo [1] (CPQMC) simulation has been carried out for the full one-dimensional three-orbital Hubbard model [2] and also for the same model but neglecting the pair-hopping and spin-flip interactions. The corresponding phase diagrams varying electronic density n and Hubbard U are constructed. Extensive comparisons with density matrix renormalization group and determinant quantum Monte Carlo results demonstrate that CPQMC is capable of capturing the physics of the orbital-selective Mott phase [2,3]. Our results also suggest that the spin-flip and pair-hopping interactions only have a limited effect on multi-orbital Hubbard model phase diagrams. [1] Guangkun Liu, Zhongbing Huang, and Yongjun Wang, J. Phys.: Condens. Matter 26, 325601(2014) [2] Julian Rincon, Adriana Moreo, Gonzalo Alvarez, and Elbio Dagotto, Phys. Rev. Lett. 112 106405 (2014) [3] Julian Rincon, Adriana Moreo, Gonzalo Alvarez, and Elbio Dagotto, Phys. Rev. B 90 241105(R)(2014) [Preview Abstract] |
Wednesday, March 16, 2016 10:24AM - 10:36AM |
K11.00011: Density Matrix Renormalization Group Study of a One Dimensional Three-Orbital Hubbard Model: The role of pair hopping and spin-flip interactions. Nitin Kaushal, Guangkun Liu, Chris Bishop, Shuhua Liang, Shaozhi Li, Steve Johnston, Elbio Dagotto Using the Density Matrix Renormalization Group technique, we extensively study a three-orbital Hubbard model in one dimension without pair hopping and spin-flip Hund interactions. The phase diagram varying the electronic density $n$ and Hubbard $U$ is constructed and compared against previous results obtained using the full interaction Hamiltonian [1]. Our results suggest that spin-flip and pair hopping terms are not crucially important to address the orbital-selective Mott phase [1]. This analysis paves the way to study multiorbital Hubbard models using techniques such as the Constrained-Path Quantum Monte Carlo (CPQMC) and Determinant Quantum Monte Carlo (DQMC) methods since they perform better, reducing for instance the severity of the ``sign problem", in the absence of pair hopping and spin flip terms in the interaction.\\ [4pt] [1] Julian Rincon et al., \textit{Phys. Rev. Lett. \textbf{112}, 106405 (2014)}, Julian Rincon et al., \textit{Phys. Rev. B. \textbf{90}, 241105 (2014)}\\ [Preview Abstract] |
Wednesday, March 16, 2016 10:36AM - 10:48AM |
K11.00012: Doping Evolution of Electronic Structure and Superconductivity in FeSe/SrTiO3 Films Bing Shen, Yong Hu, Defa Liu, Jianwei Huang, Aiji Liang, Yu Xu, Lin Zhao, Shaolong He, Lili Wang, Xucun Ma, Qikun Xue, Chuangtian Chen, Zuyan Xu, Xingjiang Zhou The latest discovery of high temperature superconductivity in FeSe/SrTiO3 film has attracted extensive attention. Our previous ARPES studies on the single-layer and double-layer FeSe/SrTiO3 films showed that, it is possible to transform single-layer FeSe/SrTiO3 films from N-phase to S phase by vacuum annealing and get superconductivity with a high Tc over 65K. We also showed that it is hard to get double-layer FeSe/SrTiO3 films doped and it remains in the semiconducting/insulating state under an extensive annealing condition. In this talk, we will present our new ARPES results on the FeSe/SrTiO3 films with different layers (1UC, 2UC, 3UC and 20UC), especially for the 3UC-FeSe films. We use another method, potassium deposition, to increase the carrier concentration. As the result, we observed N phase to S phase transformation in FeSe/SrTiO3 films with different layers. This systematic study will provide insight in understanding the evolution of electronic structure and superconductivity from the single-layer FeSe film, to multiple-layer FeSe film and eventually to the bulk FeSe superconductor. [Preview Abstract] |
Wednesday, March 16, 2016 10:48AM - 11:00AM |
K11.00013: Electronic structure and lattice dynamics at the interface of single layer FeSe and SrTiO$_{\mathrm{3}}$ Towfiq Ahmed, Alexander Balatsky, Jian-Xin Zhu Recent discovery of high-temperature superconductivity with the superconducting energy gap opening at temperatures close to or above the liquid nitrogen boiling point in the single-layer FeSe grown on SrTiO$_{\mathrm{3}}$ has attracted significant interest. ~It suggests that the interface effects can be utilized to enhance the superconductivity. It has been shown recently that the coupling between the electrons in FeSe and vibrational modes at the interface play an important role. Here we report on a detailed study of electronic structure and lattice dynamics in the single-layer FeSe/SrTiO$_{\mathrm{3}}$ interface by using the state-of-art electronic structure method within the density functional theory. The nature of the vibrational modes at the interface and their coupling to the electronic degrees of freedom are analyzed. In addition, the effect of hole and electron doping in SrTiO$_{\mathrm{3}}$ on the electron-mode coupling strength is also considered.~[1]~Q. Y. Wang et al., Chin. Phys. Lett. 29, 037402 (2012); [2]~Jian-Feng Ge et al., Nature Mater. 14, 285 (2014). [Preview Abstract] |
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