Bulletin of the American Physical Society
APS March Meeting 2015
Volume 60, Number 1
Monday–Friday, March 2–6, 2015; San Antonio, Texas
Session A0: Focus Session: Nematicity and Electron Correlations in Fe-Based Superconductors |
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Sponsoring Units: DMP DCOMP Chair: Rafael Fernandes, University of Minnesota Room: Ballroom A |
Monday, March 2, 2015 8:00AM - 8:12AM |
A0.00001: Beyond nematicity: emergent chirality in iron-based superconductors Rafael M. Fernandes, Steven A. Kivelson, Erez Berg In most iron superconductors, the magnetically ordered state is of stripe-type, with an ordering vector $Q_1 = (\pi,0)$ or $Q_2 = (0,\pi)$. One of its hallmarks is the emergence of an Ising-nematic symmetry, whose breaking triggers a vestigial nematic phase that lowers the tetragonal symmetry of the system to orthorhombic. Recent experiments have observed a magnetic state that remains tetragonal, which can be understood only as a double-$Q$ configuration (i.e. simultaneous order at $Q_1$ and $Q_2$) that is either non-uniform or non-collinear. Here we show that these magnetic states also display emergent Ising degrees of freedom that are related not to a rotational, but to a translational symmetry breaking in real space. While in the non-uniform state the Ising symmetry is related to a charge-density wave with ordering vector $Q_1+Q_2 = (\pi,\pi)$, in the non-collinear state it is related to a chiral symmetry arising from a spin-current density-wave with the same ordering vector. We show that, in the presence of a magnetic field, the former becomes a Neel-like magnetic state, while the latter is converted into a staggered charge-current pattern. We discuss the experimental manifestations of these emergent phases and their impact in the phase diagram of the iron superconductors. [Preview Abstract] |
Monday, March 2, 2015 8:12AM - 8:24AM |
A0.00002: Nematic-driven anisotropic electronic properties of underdoped detwinned Ba(Fe$_{1-x}$Co$_x$)$_2$As$_2$ revealed by optical spectroscopy L. Degiorgi We collect optical reflectivity data as a function of temperature across the structural tetragonal-to-orthorhombic phase transition at T$_s$ on Ba(Fe$_{1-x}$Co$_x$)$_2$As$_2$ for x = 0, 2.5\% and 4.5\%, with uniaxial and in-situ tunable applied pressure in order to detwin the sample and to exert on it an external symmetry breaking field. At $T < T_s$, we discover a remarkable optical anisotropy as a function of the applied pressure at energies far away from the Fermi level and very much reminiscent of a hysteretic-like behavior. Such an anisotropy turns into a reversible linear pressure dependence at $T\ge T_s$. Moreover, the optical anisotropy gets progressively depleted with increasing Co-content in the underdoped regime, consistent with the doping dependence of the orthorhombicity but contrary to the non-monotonic behavior observed for the dc anisotropy. Our findings bear testimony for an important anisotropy of the electronic structure and thus underscore an electronic polarization upon (pressure) inducing and entering the nematic phase. [Preview Abstract] |
Monday, March 2, 2015 8:24AM - 8:36AM |
A0.00003: Anisotropic transient reflectivity across optimal doping in the isovalent-doped superconductor BaFe$_2$(As$_{1-x}$P$_x$)$_2$ Eric Thewalt, James Hinton, Joseph Orenstein, Ian Hayes, Toni Helm, James Analytis The isovalent-doped high-$T_c$ superconductor BaFe$_2$(As$_{1-x}$P$_x$)$_2$ is characterized by a rich temperature-doping phase diagram, which includes structural, antiferromagnetic, electron nematic, and superconducting phase transitions. Of particular note is the proposed existence of a quantum critical point at optimal doping. In this work, we use 1.5 eV pump-probe reflectivity measurements to study the recombination dynamics of photoexcited quasiparticles as a function of temperature, doping, and polarization. We find that the low-temperature response is strongly anisotropic across a wide range of dopings, both above and below optimal. This indicates that the anisotropy arises independently of the orthorhombic-tetragonal and antiferromagnetic phase transitions, which occur only on the underdoped side of the phase diagram. [Preview Abstract] |
Monday, March 2, 2015 8:36AM - 8:48AM |
A0.00004: Divergent nematic susceptibility of optimally doped Fe-based superconductors Jiun-Haw Chu, Hsueh-Hui Kuo, Ian Fisher By performing differential elastoresistivity measurements on a wider range of iron based superconductors, including electron doped (Ba(Fe$_{1-x}$Co$_x$)$_2$As$_2$, Ba(Fe$_{1-x}$Ni$_x$)$_2$As$_2$), hole doped (Ba$_{1-x}$K$_x$Fe$_2$As$_2$), isovalent substituted pnictides (BaFe$_2$(As$_{1-x}$P$_x$)$_2$) and chalcogenides (FeTe$_{1-x}$Se$_x$), we show that a divergent nematic susceptibility in the B$_{2g}$ symmetry channel appears to be a generic feature of optimally doped compositions. For the specific case of optimally ``doped'' BaFe$_2$(As$_{1-x}$P$_x$)$_2$, the nematic susceptibility can be well fitted by a Curie-Weiss temperature dependence with critical temperature close to zero, consistent with expectations of quantum critical behavior in the absence of disorder. However for all the other optimal doped iron based superconductors, the nematic susceptibility exhibits a downward deviation from Curie-Weiss behavior, suggestive of an important role played by disorder. [Preview Abstract] |
Monday, March 2, 2015 8:48AM - 9:00AM |
A0.00005: Orbital Nematic Order and Interplay with Magnetism in the Two-Orbital Model for Iron Pnictides Andriy Nevidomskyy, Zhentao Wang Motivated by recent ARPES measurements on FeSe [1] and LiFeAs [2] families of iron-based superconductors, we have studied the orbital nematic order and its interplay with magnetism within random phase approximation, as well as using a non-perturbative variational cluster approximation (VCA). We found that the electron and hole doping affect the two orders differently within the two-orbital Hubbard model. While hole doping tends to suppress both antiferromagnetism and orbital ordering, the electron doping suppresses magnetism faster, so that orbital nematicity is stabilized in the absence of long-range magnetic order for moderately high electron doping. This is reminiscent of the orbital nematic phase observed in FeSe in the absence of magnetism [1,3], as well as in overdoped BaFe$_2$(As$_{1-x}$P$_x$)$_2$ where ARPES finds splitting of $d_{xz}$ and $d_{yz}$ orbitals inside the superconducting phase [4]. This raises the possibility that at least in some cases, the observed electronic nematicity may be primarily due to orbital rather than magnetic fluctuations.\\ * See arXiv:1408.1408 for more details.\\ {[1]} T. Shimojima et al, arxiv:1407.1418.\\ {[2]} H. Miao et al, Phys. Rev. B 89, 220503 (2014).\\ {[3]} S.-H. Baek et al, arXiv:1408.1875.\\ {[4]} T. Sonobe et al, unpublished. [Preview Abstract] |
Monday, March 2, 2015 9:00AM - 9:12AM |
A0.00006: Direct observation of mesoscopic phase separation in K$_x$Fe$_y$Se$_2$ by scanning microwave microscopy Atsutaka Maeda, Hideyuki Takahashi, Yoshinori Imai K$_x$Fe$_y$Se$_2$ is isostructural to 122-FeAs compounds. However, its electronic structure is unique among Fe-based superconductors in the sense that hole Fermi pocket is absent at the center of the Brillouin zone. Therefore, it is important to study this compounds in terms of the mechanism of superconductivity since some pairing (for example, $s\pm$-wave) needs the interaction between hole and electron Fermi pockets. However, the phase separation in this material makes studies using conventional macroscopic measurement techniques very difficult. Scanning near-field microwave microscope (SMM), which can measure local electric property of inhomogeneous conducting samples, should be a powerful tool. Recently we developed the combined instrument of STM and SMM with high sensitivity, and investigated the local electric property of K$_x$Fe$_y$Se$_2$ ($x=$0.8, $y=$1.6$\sim$2, $T_c=$ 31 K) using this scanning tunneling/microwave microscope. The characteristic pattern of mesoscopic phase separation of the metallic and the semiconducting phase was observed. From the comparison with previously reported SEM/EDS result we identified the metallic phase and the semiconducting phase as the minor Fe-rich phase and the major K$_2$Fe$_4$Se$_5$ phase, respectively. [Preview Abstract] |
Monday, March 2, 2015 9:12AM - 9:48AM |
A0.00007: Nematicity without magnetism in FeSe - evidence for orbital ordering? Invited Speaker: Christoph Meingast The origin of electronic nematicity and its relation to superconductivity is one of the hotly debated questions in the field of Fe-based superconductivity. Both spin and orbital degrees of freedom have been invoked with nematicity, as well as with the superconducting pairing itself. It is thus important to find out to which degree spin and/or orbital physics is driving the physics of these materials. FeSe is a particularly interesting material, because it undergoes a similar nematic C4 symmetry breaking transition as found in other Fe-based materials, but without long-range magnetic order. Recent advances in low-temperature vapor growth of FeSe single crystals [1] have led to a wealth of new experimental results. Here we discuss the nature of the nematic phase transition in FeSe in light of recent elastic, NMR, ARPES and quantum oscillations studies [2-4] and compare it to other Fe-based materials [5,6]. . . . . . . . . . . . . . [1] A. E. B\"{o}hmer, et al., Phys. Rev. B 87, 180505(R) (2013). . . . [2] A. E. B\"{o}hmer, et al., arXiv:1407.5497. . . . [3] T. Terashima, et al., Phys. Rev. B 90, 144517 (2014). . . . [4] T. Shimojima, et al., Phys. Rev. B 90, 121111(R) (2014). . . [5] A. E. B\"{o}hmer, et al., PRL 112, 047001 (2014). . . [6] R. M. Fernandes, et al, PRL 111, 137001 (2013). . . . [Preview Abstract] |
Monday, March 2, 2015 9:48AM - 10:00AM |
A0.00008: New tetragonal magnetic phase in (Ba,K)Fe$_2$As$_2$ Anna B\"{o}hmer, Fr\'{e}d\'{e}ric Hardy, Liran Wang, Peter Schweiss, Thomas Wolf, Christoph Meingast The recently discovered C$_4$-symmetric magnetic phase in (Ba,Na)Fe$_2$As$_2$ \footnote{Avci et al., Nature Commun. 5, 3845 (2014)} is a rare exception among iron-based superconductors, which usually display a stripe-type spin-density wave (SDW) ground state at low doping. We re-examine the phase diagram of the closely related (Ba,K)Fe$_2$As$_2$ in great detail, using high-quality single crystals and thermodynamic (thermal-expansion and specific-heat) measurements. We find a small region of a, previously missed, C$_4$-symmetric phase in the ambient-pressure phase diagram, likely related to an unidentified phase transition observed under hydrostatic pressure\footnote{Hassinger et al., Phys. Rev. B 86 140502 (2012)}. We investigate the remarkable interplay of the new C$_4$ phase with superconductivity and with the SDW phase by studying the electronic entropy and the effect of uniaxial pressure on the phase diagram. [Preview Abstract] |
Monday, March 2, 2015 10:00AM - 10:12AM |
A0.00009: Electronic correlations in hole- and electron-doped Fe-based superconductors Frederic Hardy, Anna Boehmer, Peter Schweiss, Thomas Wolf, Rolf Heid, Robert Eder, Robert A. Fisher, Christoph Meingast High-temperature superconductivity in the cuprates occurs at the crossover from a highly-correlated Mott insulating state to a weaker correlated Fermi liquid as a function of hole doping. The iron pnictides were initially thought to be fairly weakly correlated. However, we have recently shown using transport and thermodynamic measurements that KFe$_{2}$As$_{2}$ is strongly correlated. Both the Sommerfeld coefficient and the Pauli susceptibility are strongly enhanced with respect to their bare DFT values. These correlations are even further enhanced in RbFe$_{2}$As$_{2}$ and CsFe$_{2}$As$_{2}$. The temperature dependence of both the susceptibility and the thermal expansion provides strong experimental evidence for the existence of a coherence-incoherence crossover; similar to what is found in heavy-fermion compounds. Whereas the correlations in the cuprates result from a large value of the Hubbard U, recent works have stressed the particular relevance of Hund's coupling in the pnictides. Our data may be interpreted in terms of a close proximity of KFe$_{2}$As$_{2}$ to an orbital-selective Mott transition. We now have good thermodynamic data covering both the hole and electron sides of the BaFe$_{2}$As$_{2}$ system and we will discuss how these correlations are modified by doping. [Preview Abstract] |
Monday, March 2, 2015 10:12AM - 10:24AM |
A0.00010: Preformed pairing in superconducting FeSe in the BCS-BEC cross-over regime Shigeru Kasahara, Y. Shimoyama, R. Kobayashi, T. Yamashita, T. Watashige, Y. Matsuda, T. Shibauchi, T. Wolf, A. E. B\"ohmer, F. Hardy, C. Meingast, H. v. L\"ohneysen The BCS-BEC cross-over bridges the two important theories of bound particles (Bardeen-Cooper-Schrieffer theory and Bose-Einstein condensation) in a unified picture with the ratio of the attractive interaction to the Fermi energy as a tuning parameter. A key issue is to understand the intermediate regime, where new states of matter may emerge. It has been shown that the Fermi energy of FeSe ($T_c \sim 10$ K) is extremely small, with the result that this system is located at the verge of a BCS-BEC cross-over~[1]. Here we show that resistivity, Hall effect, Seebeck and Nernst coefficients all exhibit anomalies at $T\sim 2T_c$, well above the superconducting transition temperature. Moreover, our highly sensitive torque magnetometry shows a suppression of the Pauli susceptibility in the same regime. These anomalies appear to suggest a reduction of the density of states (pseudogap) caused by the onset of pair formation. Based on these results, a new phase diagram of FeSe above $T_c$ is proposed. \\ ~[1] S. Kasahara {\it et al}., Proc.\,Nat.\,Accad.\,Sci.\,(USA), Early Edition, 10.1073/pnas.1413477111 (November 6, 2014). [Preview Abstract] |
Monday, March 2, 2015 10:24AM - 10:36AM |
A0.00011: Impacts of Co-doping on the superconductivity and the orbital ordering state in Fe$_{1-x}$Co$_x$Se single crystal studied by the electrical transport. Takahiro Urata, Yoichi Tanabe, Satoshi Heguri, Katsumi Tanigaki In the FeSe with the simplest crystal structure in the Fe-based superconductor families, although both the superconductivity and the orbital ordering states are investigated, the relation between them is still unclear[1-4]. Here, we report Co doping effects on the superconductivity and the orbital ordering state in Fe$_{1-x}$Co$_x$Se single crystals. The electrical transport measurements demonstrated that the superconductivity vanishes at 4$\%$ Co doping while the orbital ordering state may be robust against Co doping. Present results suggest that the orbital ordering state is not related to the emergence of the superconductivity in FeSe. [1] F. C. Hsu et al., Proc. Nat. Aca. Sci. 105, 14262 (2008). [2] K. Nakayama et al., arXiv:1404.0857v1 [3] T. Shimojima et al., Phys. Rev. B 90, 121111(R) (2014). [4] K. K. Huynh et al., Phys. Rev. B 90, 144516 (2014). [Preview Abstract] |
Monday, March 2, 2015 10:36AM - 10:48AM |
A0.00012: Relationship between structure anisotropy and T$_{\mathrm{C}}$ and phase diagram of AFe$_{\mathrm{2}}$(As$_{\mathrm{1-x}}$P$_{\mathrm{x}})_{2}$ (A$=$Ba, Sr, Ca) Toru Adachi, Tatsuya Kobayashi, Shigeki Miyasaka, Setsuko Tajima, Masayoshi Ichimiya, Masaaki Ashida, Reiji Kumai, Hironori Nakao, Youichi Murakami We investigated how the electronic phase diagram changes when the crystal structure changes in $A$Fe$_{2}$(As$_{\mathrm{1-y}}$P$_{\mathrm{y}})_{2}$ where $A=$Ba/Sr or Sr/Ca. In this study, we synthesized the single crystals of Ba$_{0.5}$Sr$_{0.5}$Fe$_{2}$(As$_{\mathrm{1-x}}$P$_{\mathrm{x}})_{2}$, Sr$_{0.92}$Ca$_{0.08}$Fe$_{2}$(As$_{\mathrm{1-x}}$P$_{\mathrm{x}})_{2}$ and Sr$_{0.84}$Ca$_{0.16}$Fe$_{2}$(As$_{\mathrm{1-x}}$P$_{\mathrm{x}})_{2}$, measured the resistivity and determined precise structure parameters using synchrotron X-ray, then elucidated their phase diagrams and detailed crystal structures. The phase diagram of $A=$Ba$_{0.5}$Sr$_{0.5}$ system is similar to those for $A=$Ba and Sr, while for $A=$Sr/Ca systems the superconducting phase appears at a smaller P content than the case for $A=$Ba and Sr. The important finding is that the maximum $T_{\mathrm{C}}$ values are almost the same in all $A$Fe$_{2}$(As$_{\mathrm{1-y}}$P$_{\mathrm{y}})_{2} $systems. From the X-ray structural analysis, it has been revealed that in the optimally doped crystals, the local structures of FeAs$_{4}$ tetrahedra such as pnictogen heights or bond angles of As-Fe-As are almost the same, whereas the anisotropy of the crystal structures,$ c$/$a$, systematically changes. We conclude that $T_{\mathrm{C}}$ is not affected by the anisotropy ($c$/$a)$ but strongly depends on the local structure such as the pnictogen height. [Preview Abstract] |
Monday, March 2, 2015 10:48AM - 11:00AM |
A0.00013: Electronically driven nematicity in multilayer FeSe Film on SrTiO3 Wei Li, Yan Zhang, J.J. Lee, Hao Ding, Ming Yi, Zhi Li, Sung-Kwan Mo, Makoto Hashimoto, Donghui Lu, R.G. Moore, Xi Chen, Qi-Kun Xue, Zhi-Xun Shen Nematicity in iron-based superconductors is an intensely investigated contemporary subject. Although it is closely connected to the structural transition, it is unclear whether the lattice degree of freedom is responsible for the nematicity. Here we combine molecular beam epitaxy, angle-resolved photoemission spectroscopy and scanning tunneling microscopy together to study the nematicity in multilayer FeSe films on SrTiO3. Our results demonstrate direct connection between electronic anisotropy in momentum space and standing waves in real space. The lifting of orbital degeneracy of dxz/dyz bands causes the unidirectional interference fringes, observed in real space as standing waves produced by scattering electrons off C2 domain walls and Se-defects. On the other hand, the formation of C2 nematic domain walls unexpectedly shows no correlation with lattice strain pattern. Our results establish a clean case that the nematicity is driven by electronic rather than lattice degrees of freedom in FeSe films. [Preview Abstract] |
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