Bulletin of the American Physical Society
APS March Meeting 2016
Volume 61, Number 2
Monday–Friday, March 14–18, 2016; Baltimore, Maryland
Session H11: Correlations and Superconductivity in Fe Chalcogenides IFocus

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Sponsoring Units: DMP Chair: Amalia Coldea, Oxford University Room: 307 
Tuesday, March 15, 2016 2:30PM  3:06PM 
H11.00001: Systematic investigation of the electron Fermi surface dominated FeSe based systems Invited Speaker: Donglai Feng FeSebased superconductors exhibit very rich phenomena. We systematically investigate the behavior of isovalent doping and electron carrier doping effects in a variety of FeSebased superconductors with angleresolved photoemission spectroscopy (ARPES). For the isovalentdoped A$_{\mathrm{x}}$Fe$_{\mathrm{2}}$(Se, S/Te)$_{\mathrm{2}}$, a continuous decreasing bandwidth is observed from the heavily S doped side to the highly Te doping side, while the system ground state evolves from a metal to a superconductor, and eventually to a correlationdriven insulator. On the other hand, when electron doping is introduced by surface K dosing on bulk FeSe or thick FeSe films, the system ground state evolves from a nematic metal to a superconductor, and then an insulator before it becomes a metal again. Correlation is found to increase with electron doping as well. These two intriguing phase diagrams of FeSebased superconductors highlight the important role of correlations on the ground state, and provide a microscopic understanding of various FeSebased superconductors, including the recently discovered (Li,Fe)OHFeSe. Reference [1] X. H. Niu et al. Phys. Rev. B 92, 060504(R) (2015). [2] C. H. P. Wen et al. arXiv:1508.05848. [3] X. H. Niu et al. arXiv:1506.04018. [Preview Abstract] 
Tuesday, March 15, 2016 3:06PM  3:18PM 
H11.00002: Observation of two distinct dxz/dyz band splittings in FeSe Peng Zhang, Tian Qian, Pierre Richard, Xiaoping Wang, Hu Miao, Baiqing Lv, Binbin Fu, Thomas Wolf, Christoph Meingast, Xianxin Wu, Ziqiang Wang, Jiangping Hu, Hong Ding We report the temperature evolution of the detailed electronic band structure in FeSe single crystals measured by angleresolved photoemission spectroscopy (ARPES), including the degeneracy removal of the dxz and dyz orbitals at the \textunderscore \textunderscore Gamma/Z and M points, and the orbitalselective hybridization between the dxy and dxz/yz orbitals. The temperature dependences of the splittings at the Gamma/Z and M points are different, indicating that they are controlled by different order parameters. The splitting at the M point is closely related to the structural transition and is attributed to orbital ordering defined on FeFe bonds with a dwave form in the reciprocal space that breaks the rotational symmetry. In contrast, the band splitting at the \textunderscore \textunderscore Gamma/Z points remains at temperature far above the structural transition. Although the origin of this latter splitting remains unclear, our experimental results exclude the previously proposed ferroorbital ordering scenario. [Preview Abstract] 
Tuesday, March 15, 2016 3:18PM  3:30PM 
H11.00003: The effects of local correlations on the electronic structure of FeSe Matthew Watson, Timur Kim, Amir Haghighirad, Amalia Coldea FeSe is structurally the simplest of Febased superconductors, but its complex and unique properties pose important theoretical questions. One important aspect of the physics of FeSe is the understanding of the strength and effects of electronic correlations. In order to explore this, we have performed angleresolved photoemission spectroscopy (ARPES) measurements on high quality bulk single crystals of FeSe over a wide range of binding energies, in different scattering geometries and with varying incident photon energies, analysing the quasiparticle renormalisations, scattering rates and degree of coherence. We find that FeSe exhibits moderately strong, orbitaldependent correlation effects which are understood to arise primarily due to local electronelectron interactions on the Fe sites. We conclude that electronic correlations constitute a key ingredient in understanding the electronic structure of FeSe. [Preview Abstract] 
(Author Not Attending)

H11.00004: Magnetic interactions in FeSe studied by first principle calculations Shuai Wang, Fa Wang Based on first principle calculations we have investigated the evolution of magnetism in freestanding monolayer FeSe with respect to lattice constant and magnetism in bulk FeSe. The computational results show that the magnetic order in freestanding monolayer FeSe will change from antiferromagnetic paircheckboard order to stripe collinear order along with enlarging lattice constant. The magnetic order in bulk FeSe will change from stripe collinear order to antiferromagnetic paircheckboard order only if structure reconstruction is allowed. We use J$_1$J$_2$K$_1$ model to fit the calculated total energies of different magnetic orders to study magnetic interaction strengths in FeSe. The fitting results of J$_1$J$_2$K$_1$ indicate that magnetic interactions in FeSe are quite strong and highly frustrated, and increase slowly with enlarging lattice parameter. [Preview Abstract] 
Tuesday, March 15, 2016 3:42PM  3:54PM 
H11.00005: Highly accurate calculation of the magnetic properties of FeSe and FeTe using first principles quantum Monte Carlo Brian Busemeyer, Mario Dagrada, Sandro Sorella, Michele Casula, Lucas K. Wagner While the origin of superconductivity in the ironbased materials is still controversial, the proximity of the superconductivity to magnetic order is suggestive that magnetism may be important. We use first principles quantum Monte Carlo (QMC) techniques to obtain an accurate microscopic picture of the effects of magnetic configurations on the electronic structure of FeSe under pressure. The QMC calculations reproduce experimental values for bulk modulus, lattice constants, bandwidth, and lowest energy magnetic ordering. The relative energy of magnetic orderings is rather different from standard density functional theory calculations, which may warrant a reassessment of the role of magnetism in this material. We analyze the correlated wave functions to obtain a microscopic explanation for the spontaneous S4 symmetrybreaking in this material, and contrast these results with the nonsuperconducting case of FeTe. [Preview Abstract] 
Tuesday, March 15, 2016 3:54PM  4:06PM 
H11.00006: Quadrupolar Spin Orders in FeSe Zhentao Wang, Andriy Nevidomskyy Motivated by the absence of longrange magnetic order and the strong spin fluctuations observed in the Febased superconductor FeSe, we study spin1 model on a square lattice up to nextnearest neighbor Heisenberg and biquadratic spin exchanges. The zerotemperature variational phase diagram gives the conventional antiferromagnetic order and also more exotic quadrupolar spin phases. These quadrupolar phases do not host longrange magnetic order and preserve timereversal symmetry, but break the spin SU(2) symmetry. In particular, we observe a robust ferroquadrupolar order (FQ) in immediate proximity to the columnar AFM phase. We envision that FeSe may be positioned within the FQ phase close to the phase boundary. Using the flavorwave technique, we calculate the structure factor inside the FQ phase and find a Goldstone mode emerging from $Q=(0,0)$, which however bears zero spectral weight at $\omega=0$ due to time reversal symmetry. At the same time, we observe strong spin fluctuations near $(\pi,0)/(0,\pi)$, which agrees with the recent neutron scattering experiments. Further, we calculate the higher order interactions between the $(\pi,0)$ and $(0,\pi)$ spin fluctuations inside the FQ phase, which may shed light on the $C_4$ symmetry breaking in the nematic phase of FeSe. [Preview Abstract] 
Tuesday, March 15, 2016 4:06PM  4:18PM 
H11.00007: Electronic correlations and topological Fermi surface transition in the ironbased chalcogenides S. Skornyakov, I. Leonov, V.I. Anisimov, D. Vollhardt We present results of a theoretical investigation of the electronic structure and phase stability of paramagnetic FeSe obtained within a combination of $ab~initio$ methods for calculating band structure and dynamical meanfield theory. Our results reveal an entire reconstruction of the Fermi surface topology upon a moderate expansion of the lattice (Lifshitz transition), with a change of magnetic correlations from the inplane magnetic wave vector $(\pi,\pi)$ to $(\pi,0)$. We attribute this behavior to a correlationinduced shift of the Van Hove singularity originating from the $xy$ and $xz/yz$ bands at the Mpoint across the Fermi level. Our results predict a structural transition of FeSe upon a ca. 10 $\%$ expansion of the lattice volume as well as a topological change of the Fermi surface of FeSe upon partial substitution Se by Te, which is accompanied with a sharp increase of the local moments. We expect that these changes are responsible for the experimentally observed increase of T$_{c}$ in FeSe upon doping with Te. The microscopic origin for superconductivity in this system is then due to a Van Hove singularity close to the Fermi level. This identification may open a new route to increase T$_{c}$ even further. [Preview Abstract] 
Tuesday, March 15, 2016 4:18PM  4:30PM 
H11.00008: Effect of hydrostatic pressure on the structural and magnetic transitions in FeSe K. Kothapalli$^{*}$, A. E. B\"{o}hmer$^{*}$, W. T. Jayasekara$^{*}$, P. Das$^{*}$, A. Sapkota$^{*}$, B. G. Ueland$^{*}$, V. Taufour$^{*}$, S. L. Bud'ko$^{*}$, P. C. Canfield$^{*}$, Y. Xiao$^{+}$, A. I. Goldman, A. Kreyssig$^{*}$ The phase diagram of FeSe is unique among all the ironbased superconductors. At ambient pressure, FeSe undergoes a tetragonaltoorthorhombic structural phase transition at $T_s$ = 90 K, and becomes superconducting below $T_c$ = 8 K. Unlike other ironbased materials, it does not magnetically order down to the lowest measured temperature ($T$). However, under the application of hydrostatic pressure ($p$), a new magnetic phase is stabilized starting from $\sim$1 GPa. Higher pressure increases $T_c$, whose maximum onset reaches a surprising 37 K at $\sim$7 GPa. We investigate the $p$$T$ phase diagram using highquality vaporgrown single crystals, which shows features not seen previously in powder and mixedphase samples. Specifically, using highpressure lowtemperature diffraction and synchrotron M\"ossbauer we elucidate the effect of pressure  evolution of orthorhombic distortion and emergence of magnetic ordering  in the vicinity of the crossover region of the structural, magnetic and superconducting transitions.\\ Work at Ames Lab. was supported by the DOE, BES, Division of Materials Sciences \& Engineering, under Contract No. DEAC0207CH11358. This research used resources at Argonne National Lab. [Preview Abstract] 
Tuesday, March 15, 2016 4:30PM  4:42PM 
H11.00009: Neutron Scattering Study of Low Energy Magnetic Excitation in superconducting Tevapor annealed underdoped FeTeSe Zhijun Xu, Ming Yi, Guangyong Xu, J. A. Shneeloch, Masaaki Matsuda, Songxue Chi, Genda Gu, J. M. Tranquada, R.J. Birgeneau To study the interplay between magnetism and superconductivity, we have performed neutron scattering and magnetization measurements on a Te vapor annealed single crystal Fe1$+$yTe0.8Se0.2 (Tc\textasciitilde 13K) sample. Te vapor annealed process is found to reduce/remove the excess Fe in the asgrown sample and make the underdoped originally nonsuperconducting sample become good superconducting sample. Our neutron scattering studies show both spin gap and spin resonance found in the Te vapor annealed superconducting sample. Comparing to commensurate spin resonance in asgrown optimaldoped sample, the spin resonance of Te annealed sample only shows up at the clearly incommensurate positions. The temperature and energy dependence of low energy magnetic excitations are also measured in the sample. This work is supported by the Office of Basic Energy Sciences, DOE. [Preview Abstract] 
Tuesday, March 15, 2016 4:42PM  4:54PM 
H11.00010: Thermal evolution of antiferromagnetic correlations and tetrahedral bond angles in superconducting FeTe$_{1x}$Se$_x$ Guangyong Xu, Zhijun Xu, John Schneeloch, Jinsheng Wen, Emil Bozin, Barry Winn, M. Feygenson, R. J. Birgeneau, Genda Gu, Igor Zaliznyak, John Tranquada We will present neutron scattering measurements of low energy magnetic excitations from superconducting FeTe$_{1x}$Se$_x$ samples. A model with shortrange correlated spin plaquettes characterized by particular antiferromagnetic wave vectors is used to describe the measured magnetic scattering data in the (HK0) plane. We show that the characteristic antiferromagnetic wave vector evolves from that characteristic of the bicollinear structure characteristic of FeTe$_{1x}$Se$_x$ (at high temperature) to that associated with the stripe structure of antiferromagnetic iron arsenides (at low temperature). We also present powder neutron diffraction results for lattice parameters in FeTe$_{1x}$Se$_x$ indicating that the tetrahedral bond angle tends to increase towards the ideal value on cooling, with a corresponding reduction in crystalfield splitting of the Fe 3d orbitals. We suggest that the thermal change in spin correlations implies a relative change among the exchange couplings, and that this is associated with changes in orbital occupancies. Finally, while the magnitude of the low energy magnetic spectral weight is substantial at all temperatures, it actually weakens somewhat at low temperature, where the charge carriers become more itinerant. [Preview Abstract] 
Tuesday, March 15, 2016 4:54PM  5:06PM 
H11.00011: Physical properties of superconducting single crystal iron sulfide Efrain E. Rodriguez, Christopher K. H. Borg, Xiuquan Zhou, Johnpierre Paglione Recently, the simple binary tetragonal iron sulfide, FeS, was found to be a superconductor with a $T_c =5$K.[1] We have prepared single crystals of tetragonal iron sulfide through hydrothermal deintercalation of K${_x}$Fe${_{2y}}$S${_2}$. The K${_x}$Fe${_{2y}}$S${_2}$ single crystal precursors were grown by slow cooling of stoichiometric melts of K, Fe and S. The silver, platelike FeS single crystals were highly crystalline with a superconducting transition temperature ($T_c$) of 4 K. The high quality of the FeS crystals revealed highly anisotropic nature of the magnetic and electronic properties intrinsic to FeS. The physical properties and thermal stability of single crystal FeS will be discussed in detail. [1]Lai X, et al., \textit{JACS} \textbf{2015} 137 (32) [Preview Abstract] 
Tuesday, March 15, 2016 5:06PM  5:18PM 
H11.00012: Structure and Magnetic Interactions in FeS: A lowT$_c$ superconductor S. J. Kuhn, M. R. Eskildsen, L. DebeerSchmitt, L. Li, C. de la Cruz, A. S. Sefat Tetragonalphase iron sulfide (FeS), with the same structure as the wellknown superconductor FeSe (T$_c$ $\sim$ 8 K), was recently discovered as a superconductor with a T$_c$ of $\sim$ 5 K [1]. Although it has been difficult to synthesize this binary in pure tetragonal, crystalline, and superconducting form by various methods (e.g.[2]), the simple lowtemperature hydrothermal method yields pure FeS products. Careful composition and particle size analyses, in addition to the results of neutron diffraction and magnetization across transition temperature(s), will be presented. Preliminary results show high sensitivity of pure products to synthesis procedure, particle sizes of $\sim$40 nm, and phase transitions in addition to T$_c$. We explain reasons for superconductivity. [1]Lai, X. et al, Jour. Amer. Chem. Soc, 137. 10148 (2015). [2] Sines, I. T. et al. Jour. Sol. Stat. Chem, 196. 17 (2012). The work of AS, LL, and SJK is supported by the U.S. Department of Energy (DOE), Office of Science, Basic Energy Sciences (BES); SJK is supported by the DOE, Office of Science Graduate Student Research (SCGSR) Award. MRE is funded by the Office of BES (DEFG0210ER46783). The work at ORNLâ€™s High Flux Isotope Reactor (HFIR) was sponsored by the Scientific User Facilities Div., BES (LDS, CC) [Preview Abstract] 
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