Bulletin of the American Physical Society
APS March Meeting 2016
Volume 61, Number 2
Monday–Friday, March 14–18, 2016; Baltimore, Maryland
Session Y20: Correlations and Superconductivity in Fe-chalcogenides IIIFocus
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Sponsoring Units: DMP Chair: Robert Moore, SLAC Room: 319 |
Friday, March 18, 2016 11:15AM - 11:51AM |
Y20.00001: Unraveling the electron pairing mechanism of FeSe by MBE and STM Invited Speaker: CanLi Song Studies of high-transition-temperature superconductivity usually suffer from various imperfections in materials. Here we apply the state-of-the-art molecular beam epitaxy (MBE) to prepare controllably high-quality FeSe films on various substrates, and explore their superconducting properties using cryogenic scanning tunneling microscope [1,2]. Single impurities, twin boundaries as well as strain are found in the MBE-grown FeSe films on graphene, and invariably suppress the superconductivity [1, 3, 4]. Meanwhile, electronic nematicity and signatures of a bosonic mode, whose energy also decreases with strain [4], were identified. More significantly, we observed two disconnected superconducting domes at alkali-metal potassium (K)-dosed FeSe surface, stepping towards the mechanistic understanding of superconductivity in FeSe-derived superconductors. Our results are clarifying the secret of high-Tc superconductivity in FeSe-related superconductors, and by implications, in other unconventional superconductors, and guiding how to enhance Tc by interface engineering. References: [1] Can-Li Song et al., Science 332, 1410 (2011). [2] Q. Y. Wang et al., Chin. Phys. Lett. 29, 037402 (2012). [3] C. L. Song et al., Phys. Rev. Lett. 109, 137004 (2012). [4] C. L. Song et al., Phys. Rev. Lett. 112, 057002 (2014). [Preview Abstract] |
Friday, March 18, 2016 11:51AM - 12:03PM |
Y20.00002: In-plane Resistivity Anisotropy in Mechanically De- twinned Single Crystals FeSe Erik Timmons, Makariy Tanatar, Anna BOHMER, Gil DRACHUCK, Valentin Taufour, Sergey Bud'ko, Paul Canfield, Ruslan Prozorov, Michael Schuett, Rafael Fernandes The in-plane resistivity anisotropy was studied in stress-detwinned vapor transport grown single crystals of FeSe, which exhibit the tetragonal-orthorhombic structural transition temperature at $T_s \sim $ 90 K in unstrained samples, but no long-range magnetic order. Direct transport and elastoresistivity measurements show a significant in-plane resistivity anisotropy above $T_s$ induced by a very moderate mechanical stress. This anisotropy peaks slightly below $T_s$ and decreases to nearly zero on cooling to base temperature, while the degree of orthorhombic distortion grows monotonically before saturating at low temperatures. We explain the non-monotonic temperature dependence of the resistivity anisotropy as a result of the inelastic scattering of electrons by anisotropic spin fluctuations. Experimental work was supported by the U.S. DOE/OS/MSED and was performed at the Ames Laboratory, Iowa State University under contract DE-AC02-07CH11358. M.S. acknowledges the support from the Humboldt Foundation. R.M.F. is supported by the U.S. DOE, Office of Science, Basic Energy Sciences, under Award No. DE-SC0012336. [Preview Abstract] |
Friday, March 18, 2016 12:03PM - 12:15PM |
Y20.00003: Nematic quantum criticality in FeSe$_{1-}_{x}$S$_{x}$ revealed by elastoresistance measurements Suguru Hosoi, Kohei Matsuura, Hao Wang, Kousuke Ishida, Yuta Mizukami, Tatsuya Watashige, Shigeru Kasahara, Yuji Matsuda, Takasada Shibauchi Electronic nematicity and its connection to the high-temperature superconductivity is one of the central issues in iron-based superconductors. Among them, FeSe is unique in that it exhibits a tetragonal-to-orthorhombic structural transition but no antiferromagnetic order, which enables us to study the nematicity without the effect of magnetism. Here we report on elastoresistance measurements in FeSe$_{1-}_{x}$S$_{x}$ evidencing a nonmagnetic nematic quantum critical point near $x\sim $ 0.2. When the Se site is substituted by the isovalent S, the structural transition temperature is reduced gradually and it vanishes above $x\sim $ 0.2. From the changes in in-plane resistivity induced by anisotropic strain, we evaluate the nematic susceptibility which shows Curie-Weiss-like temperature dependence. We find that with increasing $x$ the Weiss temperature changes its sign indicating a quantum critical point, while there is no sign of antiferromagnetism for all samples. The superconducting transition temperature does not show a significant change with S concentration, suggesting that the nonmagnetic nematic quantum criticality does not help to enhance superconductivity in this system. [Preview Abstract] |
Friday, March 18, 2016 12:15PM - 12:27PM |
Y20.00004: Nematic magnetoelastic effect contrasted between $\mathrm{Ba(Fe_{1-x}Co_x)_2As_2}$ and FeSe Yuwen Hu, Xiao Ren, Rui Zhang, Huiqian Luo, Shigeru Kasahara, Tatsuya Watashige, Takasada Shibauchi, Pengcheng Dai, Yan Zhang, Yuji Matsuda, Yuan Li Whether the nematic order ubiquitously found in Fe-based superconductors is driven by the spin or the charge or orbital degree of freedom is currently under heated debate. To elucidate its microscopic origin, we report a Raman scattering study of lattice dynamics, which quantify the extent of $C_4$-symmetry breaking, in $\mathrm{BaFe_2As_2}$ and FeSe. FeSe possesses a nematic ordering temperature $T_{\mathrm{s}}$ and orbital-related band-energy split below $T_{\mathrm{s}}$ that are similar to those in $\mathrm{BaFe_2As_2}$, but unlike $\mathrm{BaFe_2As_2}$ it has no long-range magnetic order. We find that the $E_g$ phonon-energy split in FeSe sets in only well below $T_{\mathrm{s}}$, and its saturated value is substantially smaller than that in $\mathrm{BaFe_2As_2}$. Together with reported results for the $\mathrm{Ba(Fe_{1-x}Co_{x})_2As_2}$ family, the data suggest that magnetism exerts a major influence on the lattice. [Preview Abstract] |
Friday, March 18, 2016 12:27PM - 12:39PM |
Y20.00005: Berry's phase observed in the ordered state of Fe(Se,S) Shigeru Kasahara, T. Yamashita, Y. Shimoyama, T. Watashige, Y. Matsuda, J. B\'{e}ard, M. Nardone, W. Knafo, M.D. Watson, N.R. Davis, A.I. Coldea, M. Suzuki, R. Arita, H. Ikeda, T. Shibauchi Among iron-based superconductors, FeSe offers a unique platform in that it exhibits a nematically ordered phase without long-range magnetic ordering. Several experiments have shown that the low-temperature Fermi surface of FeSe consists only of very small, shallow pockets~[1-3]. Tuning the ground state via isoelectronic chemical substitution provides an ideal way to solve the puzzles regarding the nematic ordering in this material. Here, by using ultra-high magnetic fields up to $\sim$\,90\,T, we report observations of Shubnikov-de Haas (SdH) oscillations in isoelectronically substituted Fe(Se,S). For the smallest pocket of $\sim0.2$\% of the Brillouin-zone, we observe non-zero $\pi$ Berry's phase shift in the SdH oscillations. Our results indicate presence of Dirac cone, which would be a key to understand the mechanism of the nematic ordering in this system. \\ ~[1] S. Kasahara {\it et al}., Proc.\,Natl.\,Acad.\,Sci. U.\,S.,A. {\bf 111}, 16309 (2014). \\ ~[2] T. Terashima {\it et al}., Phys. Rev. B {\bf 90}, 144517 (2014). \\ ~[3] M.W. Watson {\it et al}., Phys. Rev. Lett. {\bf 115}, 027006 (2015). [Preview Abstract] |
Friday, March 18, 2016 12:39PM - 12:51PM |
Y20.00006: \textbf{Nematicity in FeSe single crystals probed by pump-probe spectroscopy} C. W. Luo, P. C. Cheng, K. H. Wu, J. Y. Juang, S.-H. Wang, J.-C. Chiang, J.-Y. Lin, D. A. Chareev, O. S. Volkova, A. N. Vasiliev The anisotropic quasiparticle dynamics in FeSe single crystals have been studied by polarized pump-probe spectroscopy. Two distinguishable relaxation components were unambiguously observed in transient reflectivity changes ($\Delta R$/$R)$. The orientation-dependent fast component with the timescale of 0.1-1.5 ps associated with the electronic structure clearly shows two-fold symmetry, which further reveals the gap opening along k$_{\mathrm{y}}$ below the temperature of structure phase transition ($T_{\mathrm{s}})$ and the electronic nematicity can persist up to 200 K. For the slow component with the timescale of 8-25 ps, it is assigned to the energy relaxation through spin sub-system and also shows a two-fold symmetry below $T_{\mathrm{s}}$. However, this two-fold symmetry is dramatically weakened above $T_{\mathrm{s}}$ and surprisingly persists up to at least 200 K. Consequently, the high-temperature nematic fluctuations in FeSe may be driven by the order parameters which associated with both charge (orbital) and spin sub-systems. [Preview Abstract] |
Friday, March 18, 2016 12:51PM - 1:03PM |
Y20.00007: Nematic quantum paramagnet and possible application to FeSe Fa Wang, Steven A. Kivelson, Dung-Hai Lee The nematic phases in iron pnictides are in close proximity to the stripe antiferromagnetic order, suggesting that magnetism is the driving force for the spontaneous 4-fold crystal rotation symmetry breaking. In contrast, bulk FeSe shows a nematic phase below 90K at ambient pressure, but has no magnetic long range order down to very low temperature. This prompts suggestions that the nematicity in FeSe is driven by some other mechanism. We argue that magnetic correlation can still drive nematic order in the absence of magnetic order. By field theoretical considerations and exact diagonalization results on finite size lattices, we conclude that the paramagnetic phase in frustrated spin-1 $J_1$-$J_2$ models on square lattice is such a "nematic quantum paramagnet", which breaks only the crystal 4-fold rotation symmetry. The prototype wavefunctions of such quantum ground states are horizontal(vertical) aligned spin-1 AKLT chains. We suggest that the local spins in FeSe may form this phase due to strong frustration. One unique consequence of this proposal is that the nematic paramagnetic phase will be close to both stripe and Neel antiferromagnetic order, and will thus host low but finite energy spin fluctuations at both ordering wavevectors. [Preview Abstract] |
Friday, March 18, 2016 1:03PM - 1:15PM |
Y20.00008: Sign-Reversing Orbital Polarization in FeSe Driven by the Nematic Symmetry Breaking Self-Energy Seiichiro Onari, Youichi Yamakawa, Hiroshi Kontani Novel \textbf{\textit{k}}-dependence of the orbital polarization in the orthorhombic phase in FeSe has been observed by the ARPES measurement [1], where the sign-reversal of the orbital splitting appears between hole pockets and electron pockets. We analyze the multiorbital Hubbard models in the orbital-ordered state by extending the orbital-spin fluctuation theory [2]. The present theory describes the spontaneous symmetry breaking with respect to the orbital polarization and spin susceptibility self-consistently. In the orbital-ordered state, we obtain the two Dirac cone Fermi pockets in addition to the sign-reversing orbital polarization, consistently with experiments. The orbital-order originates from the strong orbital-spin interplay due to the Aslamazov-Larkin processes. [1] Y. Suzuki \textit{et al}., arXiv:1504.00980. [2] S. Onari, Y. Yamakawa, and H. Kontani, arXiv:1509.01172. [Preview Abstract] |
Friday, March 18, 2016 1:15PM - 1:27PM |
Y20.00009: Nematicity and magnetism in FeSe and other families of Fe-based superconductors Youichi Yamakawa, Seiichiro Onari, Hiroshi Kontani We investigate the emergence of the nematic orbital order ($n_{xz} \neq n_{yz}$) in various Fe-based superconductors based on the first-principles Hubbard models [1]. In Fe-based superconductors, spin-fluctuation-mediated large orbital-fluctuations appear because of the strong orbital-spin interplay due to the many-body effect. This effect is very significant in FeSe due to the small ratio between the Hund's and Coulomb interactions ($\bar{J}/\bar{U}$) and large $d_{xz},d_{yz}$-orbitals weight at the Fermi level. For this reason, in FeSe, orbital order is established by weak spin fluctuations, so the magnetism is absent. In contrast, in LaFeAsO, the magnetic order appears just below the structural transition temperature both experimentally and theoretically. Thus, the orbital-spin interplay is the key ingredient of the wide variety of the normal-state phase diagram in Fe-based superconductors. [1] Y. Yamakawa, S. Onari, and H. Kontani, arXiv:1509.01161. [Preview Abstract] |
Friday, March 18, 2016 1:27PM - 1:39PM |
Y20.00010: Determinant Quantum Monte Carlo Study of a Multi-orbital Electronic Model: Application to Nematic and Superconducting Order in FeSe Philipp Dumitrescu, Maksym Serbyn, Richard Scalettar, Ashvin Vishwanath The iron chalcogenide FeSe has attracted much recent interest due to a high superconducting transition in monolayer samples. In bulk samples, nematic order is seen without the presence of magnetic order, hinting at the importance of nematic order in determining the monolayer properties. We study an effective two band model of the iron-pnictides with interactions that capture the nematic ordering arising from spontaneous symmetry breaking between the two orbitals. These models are sign-problem free and can be simulated in an unbiased fashion using Determinant Quantum Monte Carlo. We find a variety of unexpected orders and consider the effects of the nematic fluctuations on superconductivity. [Preview Abstract] |
Friday, March 18, 2016 1:39PM - 1:51PM |
Y20.00011: Alternating-Sign S-Wave Superconductivity in Single-Layer FeSe from the Local Moment Limit Jose Rodriguez We obtain the exact low-energy spectrum of two mobile electrons roaming over a 4 by 4 lattice of iron atoms governed by a t-J model for a monolayer of FeSe. Each iron atom contains the minimum $d_{xz}$ and $d_{yz}$ orbitals. The hopping parameters (t) account only for electron bands centered at wave vectors $(\pi,0)$ and $(0,\pi)$, while the Heisenberg exchange parameters (J) imply a quantum-critical point (QCP) at half-filling that separates a commensurate spin-density wave (cSDW) at strong Hund coupling from a hidden-order antiferromagnet at weak Hund coupling. The hidden-order antiferromagnet has ordering wavevector $(\pi,\pi)$. After tuning the Hund coupling near the QCP, we find an $S^{+-}$ ground state and a $D^{+-}$ excited state that are separated in energy from the edge of a quasi-particle continuum. Both bound states alternate in sign between electron pairs at cSDW momenta and electron pairs at emergent electronic structure with zero 2D momentum. Exact calculations for a single electron with the same t-J model parameters find that the emergent electronic structure at zero 2D momentum moves off the Fermi level as Hund coupling weakens below the QCP. We therefore suggest that the above $S^{+-}$ groundstate describes Coopers pairs in a monolayer of FeSe. [Preview Abstract] |
Friday, March 18, 2016 1:51PM - 2:03PM |
Y20.00012: Comparing the anomalous phonons in Fe(Te,Se) and (Fe,Ni)(Te,Se) via neutron scattering John Schneeloch, Zhijun Xu, Genda Gu, Igor Zaliznyak, Barry Winn, Jose Rodriguez-Rivera, Robert Birgeneau, Guangyong Xu, John Tranquada We studied the anomalous acoustic-type phonons in the Fe(Te,Se) iron-based superconductor family that arise from the (100) Bragg peak, which is forbidden according to the reported crystal structure for these materials. Inelastic neutron scattering was performed on superconducting and non-superconducting crystals of various compositions. The (100) phonons were much weaker in a non-superconducting nickel-doped crystal than in a superconducting crystal with similar selenium fraction, but comparison with another non-superconducting crystal suggests the difference is not simply related to superconductivity. This composition dependence was observed for both transverse and longitudinal phonons. The temperature dependences of the (100) phonons resembled those of conventional phonons. We will discuss these results and possible explanations for the relation between composition and lattice dynamics in this system. [Preview Abstract] |
(Author Not Attending)
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Y20.00013: Elucidating the magnetic and superconducting phases in Rb$_{x}$Fe$_{y}$Se$_{2-z}$S$_{z}$. Meng Wang, Ming Yi, Wei Tian, Edith Bourret-Courchesne, Robert Birgeneau The complex interdigitated phases have greatly frustrated attempts to document the basic features of the superconductivity in the $A_{x}$Fe$_{y}$Se$_{2-z}$S$_{z}$ ($A=$ alkali metals) system. We have employed elastic neutron scattering, energy-dispersive x-ray spectroscopy, and resistivity measurements to elucidate the relations of these phases in Rb$_{x}$Fe$_{y}$Se$_{2-z}$S$_{z}$.We find: i) the iron content is the driving parameter in stabilizing the structural separated phases; ii) the existence of~the~245 antiferromagnetic phase stabilizes the iron vacancy free phase; iii) the sulfur substitutions progressively tune the iron vacancy free phase from a superconductor in Rb$_{x}$Fe$_{2}$Se$_{2}$ to a metallic phase in Rb$_{x}$Fe$_{2}$S$_{2}$. Several phase diagrams as functions of the iron content and the Se:S ratio will be provided. [Preview Abstract] |
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