Bulletin of the American Physical Society
APS March Meeting 2020
Volume 65, Number 1
Monday–Friday, March 2–6, 2020; Denver, Colorado
Session J61: Fe-Based Superconductors - Nematicity III / ARPESFocus
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Sponsoring Units: DMP DCMP DCOMP Chair: Vitalii Vlasko-Vlasov, Argonne Natl Lab Room: Mile High Ballroom 4B |
Tuesday, March 3, 2020 2:30PM - 3:06PM |
J61.00001: A local structure perspective on iron pnictides and chalcogenides: Insights gained from pair distribution function analysis Invited Speaker: Benjamin Frandsen The field of iron-based superconductivity, now well into its second decade of existence, continues to challenge and stimulate condensed matter physicists. One perennial issue of debate is the origin and implications of the electronic nematic phase. This state is characterized by a spontaneous breaking of four-fold rotational symmetry, which manifests as unequal occupations of the dxz and dyz orbitals, a tetragonal-to-orthorhombic structural phase transition, and (in many cases) stripe-type magnetic order. In addition to the statically ordered nematic phase observed widely in various families of iron-based superconductors, strong nematic fluctuations have also been found in broad regions of parameter space outside the ordered nematic state. Pair distribution function (PDF) analysis of x-ray and neutron total scattering data is a proven method of studying local, short-range structural correlations that deviate from the average structure, such as the orthorhombic distortions associated with nematic fluctuations in iron-based superconductors. Here, I will present recent PDF investigations of several representative families of iron-based superconductors. The results reveal the presence of local orthorhombic distortions across large regions of temperature-composition space and provide a detailed look at the temperature dependence and characteristic length scale of these distortions. The deeper understanding of the local structure of iron-based superconductors enabled by these PDF experiments will help clarify some of the outstanding questions relating to these fascinating materials. |
Tuesday, March 3, 2020 3:06PM - 3:42PM |
J61.00002: Time Reversal Symmetry Breaking in the FeTe1-xSex family of high Tc superconductors. Invited Speaker: Peter Johnson Laser-based ARPES with variable light polarization offers a powerful probe of the electronic structure near the center of the Brillouin zone. Here the technique is used to examine the Fe-based superconductor family, FeTe1-xSex. At the zone center we observe the presence of Dirac cones with helical spin structure as expected for topological surface states and as previously reported in the related FeTe0.55Se0.45.[1] These studies are compared with theoretical studies that take account of the disordered local magnetic moments related to the paramagnetism observed in this system. Indeed including the magnetic contributions in the theoretical description is necessary to bring the chemical potential of the calculated electronic band structure into alignment with experimental observation. In the bulk superconducting state for FeTe0.7Se0.3 the topological state appears to acquire mass below Tc. With a single state at the center of the zone, mass acquisition is indicative of time reversal symmetry breaking which in turn suggests the potential formation of ferromagnetism in the surface layer. |
Tuesday, March 3, 2020 3:42PM - 3:54PM |
J61.00003: Orbital transmutation in the nematic state of FeSe: the consequences for Raman and ARPES experiments Andrey Chubukov, Mattia Udina, Marco Grilli, Lara Benfatto, Morten Holm Christensen, Rafael Fernandes In this talk I will discuss nematicityinduced change of the orbital composition of low-energy excitations in FeSe, and how it affects Raman and ARPES probes. I will argue that deep in the nematic state, hole and electron pockets become nearly mono-orbital, consistent with polarized ARPES measurements. I show that this leads to strong reduction of Raman intensity in B1g channel (in 1Fe zone), as observed in the measurements. The reduction comes about because for nearly mono-orbital pockets, B1g Raman response gets reduced by vertex corrections, which enforce charge conservation. I further discuss ARPES experiments at the M point (π,π). Deep in the nematic state, the two excitations, best visible in ARPES, have been identified as having xz and yz orbital character. However, they remain split above the nematic transition, in apparent contradiction to the fact that in the tetragonal phase the xz and yz orbitals are degenerate. We show that these experimental data are naturally explained by orbital transmutation of the excitations at M between tetragonal and nematic phases. |
Tuesday, March 3, 2020 3:54PM - 4:06PM |
J61.00004: Time-reversal symmetry-breaking superconductivity in FeSe1-xSx Mingwei Qiu, Kohei Matsuura, Takaaki Takenaka, Yuichi Sugimura, Takasada Shibauchi, Qi Sheng, Kohtaro Yamakawa, Yasutomo J Uemura, Yipeng Cai, Andrea Damascelli, Ryan P Day, Kenji Kojima, James W Beare, Graeme Luke, Zhao Guo Qiang, Changqing Jin, Yilun Gu, LiCheng Fu, Fanlong Ning, Mikihiko Saito The FeSe1-xSx superconductors involving non-magnetic nematic phase and its quantum criticality provide a unique platform to investigate the relationship between nematicity and superconductivity. The lifting of superconducting gap nodes due to twin boundaries has been observed experimentally, which suggests indirect evidence for time reversal symmetry breaking (TRSB). It is consistent with the theoretical prediction that the superconducting order parameter breaks the time-reversal symmetry near the nematic twin boundaries. We have performed the muon spin rotation (μSR) measurements on FeSe and observed the spontaneous internal field below the superconducting transition temperature Tc, providing evidence for TRSB state in FeSe. Here we extend zero-field μSR studies to tetragonal FeSe1-xSx (x=0.18, 0.2) without nematicity. We find that the μSR relaxation rate starts to grow below Tc in these crystals. This indicates that weak but finite internal magnetic field is induced in the superconducting state, providing strong evidence for TRSB state not only near twin boundaries but also inside the bulk of FeSe1-xSx. |
Tuesday, March 3, 2020 4:06PM - 4:18PM |
J61.00005: An ARPES and STM/S investigation of the interplay between nematicity, orbital order, and superconductivity in FeSe Morgan Walker, Timothy Boyle, Zitong Zhao, Eduardo H Da Silva Neto, Journey Byland, Jackson R Badger, Valentin Taufour, Ryan P Day, Sergey Zhdanovich, Andrea Damascelli, Tor Pedersen, Sergey Gorovikov Nematic order coexists with a myriad of iron-based superconductors, suggesting it is an important actor in the mechanism behind the superconductivity in these compounds. Many of these materials also exhibit magnetism near the nematic and superconducting phases, complicating the quest to understand their relationship. Fortunately, FeSe has no magnetic ground state and is therefore a great candidate for disentangling the interplay between nematicity and superconductivity. We performed detailed scanning tunneling microscopy and spectroscopy (STM/S) and angle-resolved photoemission spectroscopy (ARPES) studies on single crystals of FeSe to investigate the interplay between nematicity, orbital order and superconductivity. |
Tuesday, March 3, 2020 4:18PM - 4:30PM |
J61.00006: Nematicity studied with strain-dependent ARPES Heike Pfau, Su-Di Chen, Ming Yi, Makoto Hashimoto, Costel R. Rotundu, Johanna Palmstrom, Tong Chen, Peng-Cheng Dai, Joshua A Straquadine, Alexander T Hristov, Robert J Birgeneau, Donghui Lu, Ian Fisher, Zhixun Shen Nematicity is a phenomenon found in an increasing number of strongly correlated materials in close proximity to other quantum phases such as unconventional superconductivity. It is therefore important to understand its microscopic origin. The phase diagram of most iron-based superconductors contains a nematic phase. It breaks rotational symmetry and involves an in-plane anisotropy of lattice, spin and charge degrees of freedom. It is currently debated whether spin or orbital degrees of freedom are the driving force and whether there is a common microscopic mechanism in iron-based superconductors with and without magnetic order. We were able to combine angel-resolved photoemission spectroscopy (ARPES) with in-situ tuneable uniaxial strain and determine the momentum dependence of the nematic order parameter in FeSe and BaFe2As2. The order parameter has the same non-trivial symmetry in both compounds suggesting a universal behavior and a common microscopic mechanism. |
Tuesday, March 3, 2020 4:30PM - 4:42PM |
J61.00007: Unprecedented Optical Anisotropy in Optimally Doped Iron-Based Superconductor Leonardo Degiorgi, Anirban Pal, Manuel Chinotti The divergent nematic susceptibility, obeying a simple Curie-Weiss power law over a large temperature interval, is empirically found to be a ubiquitous signature in several iron-based materials across their doping-temperature phase diagram. The composition at which the associated Weiss temperature extrapolates to zero is found to be close to optimal doping, boosting the debate to what extent nematic fluctuations contribute to the pairing-mechanism and generally affect the electronic structure of iron-based superconductors. Here, we offer a comprehensive optical investigation [1] of the optimally hole-doped Ba0.6K0.4Fe2As2 over a broad spectral range, as a function of temperature and of tunable applied stress, which acts as an external symmetry breaking field. We show that the stress-induced optical anisotropy in the infrared spectral range is reversible upon sweeping the applied stress and occurs only below the superconducting transition temperature. These findings demonstrate that there is a large electronic nematicity at optimal doping which extends right under the superconducting dome. |
Tuesday, March 3, 2020 4:42PM - 4:54PM |
J61.00008: Strain effects in Fe-based superconductors: Nematicity, exotic magnetism and superelasticity Roser Valenti, Vladislav Borisov, Karim Zantout, Sananda Biswas, Stephen Winter In this talk we will analyze the role of strain |
Tuesday, March 3, 2020 4:54PM - 5:06PM |
J61.00009: Bilayer Spliting and Unusual Superconducting Gap in KCa2Fe4As4F2 Superconductor with Double Fe2As2 Layers Dingsong Wu, Wenshan Hong, Chengxiao Dong, Jiangping Hu, Shiliang Li, Huiqian Luo, Lin Zhao, Xingjiang Zhou We report the first high resolution laser Angle-Resolved Photoemission Spectroscopy(ARPES) measurements on a newly discovered iron based superconductor, KCa2Fe4As4F2 (K12442) which has the K+-cations connecting double Fe2As2 layers separated by the insulating Ca2F2 block with a high TC of 34 K at stoichiometric composition. Total five hole like Fermi surfaces are clearly identified around Brillouin zone center Γ(0,0) point and four of them are possible from the bilayer splitting of two Fermi surfaces. A tiny electron like pocket is shown around M(±π,±π) points surrounded by four strong spots with their top just below Fermi level. All the hole like pockets around Γ(0,0) point exhibit nearly isotropic nodeless superconducting gap symmetry but with different gap sizes. Their gap sizes can neither be understood in the simple │cos(kx)+cos(ky)│ functional form due to the biggest gap size from one middle Fermi surface, nor be consistent with the so-called quasi-nesting between the Fermi pockets around Γ and M points because of mismatch of Femri pocket size. Our observations provide new information and require new insight to understand the superconductivity mechanism in iron based superconductor. |
Tuesday, March 3, 2020 5:06PM - 5:18PM |
J61.00010: Pairing symmetry and capping layer influence of the superconductivity at FeSe/SrTiO3 interface Yanan Li, Timothy Pillsbury, Grant Smith, Erzsebet Vincent, Anthony R. Richardella, David Awschalom, Nitin Samarth Single unit cell FeSe films on SrTiO3 have attracted growing attention due to the enhancement of the superconducting critical temperature (Tc) over its bulk value and the possibility of topological superconductivity. The pairing symmetry and mechanisms underlying the superconducting gap have been studied via angle-resolved photoemission spectroscopy (ARPES) and scanning tunneling spectroscopy (STS). Here, we report the synthesis and characterization of ultrathin FeSe films grown on SrTiO3 by molecular beam epitaxy. We gain insight into the symmetry of the gap by measuring the angular dependence of the in-plane critical magnetic field. Additionally, to understand the discrepancy between Tc measured by in-situ techniques (such as STS and ARPES) and ex-situ transport, we investigate how Tc is influenced by varying the capping layer and Fermi energy. Supported by the University of Chicago, the Vannevar Bush Fellowship, and 2DCC-MIP/NSF DMR-1539916. |
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