Bulletin of the American Physical Society
APS March Meeting 2019
Volume 64, Number 2
Monday–Friday, March 4–8, 2019; Boston, Massachusetts
Session A10: Fe-based Superconductors IFocus
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Sponsoring Units: DMP DCOMP Chair: Alexander Kemper, North Carolina State University Room: BCEC 151B |
Monday, March 4, 2019 8:00AM - 8:36AM |
A10.00001: Orbital-selectivity in iron chalcogenide superconductors Invited Speaker: Ming Yi The electron correlation strength has a systematic spread across the material-base of iron-based superconductors. Near the most correlated end of the spectrum are the iron chalcogenides, where strong orbital-selective correlations are observed. In particular, the dxy orbital exhibits much stronger local behavior than the dxz and dyz orbitals, putting the material in close proximity to an orbital-selective Mott phase (OSMP). In this talk, I will first demonstrate the universality of orbital-selectivity amongst all iron chalcogenide superconductors including both Fe(Te,Se) and heavily electron-doped FeSe systems. For the second part of the talk, I will use the phase diagram of Fe(Te,Se) to illustrate that both Te-substitution and temperature increase act as effective pathways towards the OSMP. In the process, a dramatic increase of the dxy effective mass together with a redistribution of spectral weight on the Fermi surface strongly modify the low energy electronic states of this material family. |
Monday, March 4, 2019 8:36AM - 8:48AM |
A10.00002: A Neutron Total Scattering Study of the Nanoscale Structure of KxFe2-ySe2-zS Panagiotis Mangelis, Hechang Lei, Marshall McDonnell, Mikhail Feygenson, Cedomir Petrovic, Emil Bozin, Alexandros Lappas, Robert Koch A Neutron Total Scattering Study of the Nanoscale Structure of KxFe2-ySe2-zSz |
Monday, March 4, 2019 8:48AM - 9:00AM |
A10.00003: Intertwined spin-orbit coupled orders in the iron-based superconductors Morten Holm Christensen, Jian Kang, William Meier, Andreas Kreyssig, Rafael M Fernandes The phase diagram of the underdoped iron-based superconductors exhibits two electronically ordered phases, nematicity and stripe magnetism. Nematic order breaks the rotational symmetry of the lattice and can be seen as a vestigial phase of stripe magnetism. Here we discuss a similar phenomenon occuring for the tetragonal magnetic orders, the spin-vortex crystal and the charge-spin density-wave phases. These vestigial phases break the glide-plane symmetry and render respectively the As/Se or Fe sites inequivalent. In materials with staggered FeAs/Se layers (e.g. 122), these have ordering vector Q=(0,0,π) while in compounds with no staggering (e.g. 111) these are Q=0 orders. We discuss how these vestigial phases couple to the lattice. Moreover, Kramers degeneracy is broken and Rashba- and Dresselhaus-like spin-orbit couplings are induced. Finally, we discuss the behavior under electromagnetic fields, including magneto-electric effects. |
Monday, March 4, 2019 9:00AM - 9:12AM |
A10.00004: Discovery of a strain-stabilized charge density wave in LiFeAs Chi Ming Yim, Christopher Trainer, Rama K P Aluru, Shun Chi, Walter N Hardy, Ruixing Liang, Douglas Andrew Bonn, Peter Wahl In a number of high TC superconductors, small orthorhombic distortions of the lattice structure result in surprisingly large symmetry breaking of the electronic states and macroscopic properties, an effect often referred to as nematicity. This nematicity has been studied extensively on materials with an orthorhombic crystal structure, where the lattice symmetry is already reduced from four-fold (C4) to two-fold (C2). To directly study the impact of symmetry breaking lattice distortions on the electronic states, we image at the atomic scale the influence of strain-tuned lattice distortions on the correlated electronic states in the iron-based superconductor LiFeAs, a material which in its ground state is tetragonal, with C4 symmetry. Our experiments uncover a new strain-stabilized nematic phase that exhibits a unidirectional charge density wave (CDW) in LiFeAs, an electronic state which not only breaks rotational symmetry but also reduces translational symmetry. We follow the evolution of the superconducting gap from the unstrained material with C4 symmetry through the new nematic phase with C2 symmetry and CDW order to a state where superconductivity is completely suppressed. |
Monday, March 4, 2019 9:12AM - 9:24AM |
A10.00005: Evidence for multi-gap nodal superconductivity in FeS by penetration depth measurements. Sujith Kunniniyil Sudheesh, Tianping Ying, Xiaofang Lai, Jian-Xin Zhu, Shiyan Li, Ee Min Chia We report measurements of London penetration depth and magnetization on single crystals of FeS superconductor with a transition temperature of 4 K. From magnetization measurements, FeS was confirmed to be a type-II superconductor with Hc1(0) = 58 ± 4 Oe, λab(0) = 900 ± 90 nm, and ξ(0) = 90 ± 12 nm. The low temperature (T < 0.3Tc) behavior of penetration depth deviates from a conventional exponential predicted by BCS theory and fits well to a power law with an exponent n = 2.2 ± 0.1 which suggests the presence of nodes in the system. The superfluid density was calculated and was found to fit best to a multi-gap s+d wave model with the d-wave gap having slightly larger weight (p=0.6). Δ(0)/kBTc for the d-wave gap is higher than that of the BCS weak coupling model, implying strong coupling in the system. This gap structure is consistent with recent theoretical and experimental studies. |
Monday, March 4, 2019 9:24AM - 9:36AM |
A10.00006: Influence of Spin-Orbit Coupling in the Iron-Based Superconductors Ryan Day, Giorgio Levy, Matteo Michiardi, Berend Zwartsenberg, Marta Zonno, Shun Chi, Ivana Vobornik, Walter Hardy, Douglas Bonn, Ilya Elfimov, Andrea Damascelli In addition to the remarkable resilience of unconventional superconductivity, the iron-based superconductors (FeSCs) are characterized by a broad phenomenology across their phase diagram. This diversity has compounded the challenge of defining a minimal model for these complex materials. We explore the essential role of spin-orbit coupling (SOC) in the FeSCs via circularly-polarized spin- and angle-resolved photoemission spectroscopy (CPS-ARPES), a technique which provides direct access to both spin and orbital degrees of freedom [1]. Applying this technique to both LiFeAs and FeSe, we establish a strong momentum dependence of the spin-orbital entanglement near the Fermi surface imbued by SOC. Influenced strongly by the details of the crystal field, the relevance of SOC to the states near the Fermi level, and consequently to superconductivity and magnetism is found to be strongly material-dependent. Furthermore, in the case of hole-like carriers in FeSe, SOC is observed to be of comparable strength to orbital order, allowing spin-orbit mixing to persist into the nematic phase. |
Monday, March 4, 2019 9:36AM - 9:48AM |
A10.00007: Infrared Spectroscopic Studies of the Phonon Dynamics in Iron-based Superconductors Xianggang Qiu, Run Yang, Bing XU, Ziyang Qiu, Yaomin Dai The temperature dependent optical reflectivity has been measurement on iron-based superconductors of different families. The optical conductivity has been obtained by using the two-Drude components model. It has been found that the phonons show red- or blue-shift in different samples. Interestingly, the phonon conductivity exhibits a Fano lineshape, suggesting possible coupling between phonon and electrons. Based on the temperature evolution of the lineshape and peak shift, we discuss the possible role played by electron-phonon coupling and spin fluctuation in the occurrence of superconductivity in iron-based superconductors. |
Monday, March 4, 2019 9:48AM - 10:00AM |
A10.00008: Spin anisotropy in the stoichiometric iron-based superconductor CaKFe4As4 Tao Xie, Huiqian Luo, Shiliang Li In the stoichiometric iron pnictide superconductor CaKFe4As4, we have discovered triple spin resonance modes with strong L-modulation by unpolarized neutron scattering experiments. The two odd modes appear around 9.5 meV and 13 meV, and the even mode locates around 18.3 meV. [1] To study the spin anisotropy of the three modes, we performed polarized inelastic neutron scattering experiments on the same CaKFe4As4 sample. Here, I will present our recent results. These results will give better understanding on the spin-orbit coupling in iron-based superconductors, which can also enlighten us about the nature of magnetism and its relation with unconventional superconductivity. |
Monday, March 4, 2019 10:00AM - 10:12AM |
A10.00009: Study of CaK(Fe1-xAx)4As4 (A = Mn and Cr) single crystals Mingyu Xu, William Meier, Li Xiang, Sergey Budko, Paul Canfield Stoichiometric CaKFe4As4 is a Fe-based superconductor which has a critical temperature at 35 K. Substituting Fe with Ni or Co suppresses superconductivity and stabilizes an unusual hedgehog spin-vortex crystal magnetic structure [1]. There is a range of concentrations where superconductivity and magnetism coexist [2,3,4]. Having in mind different effect of [Fe, Co] and [Mn, Cr] substitutions in BaFe2As2, we grew single crystals of CaK(Fe1-xAx)4As4 (A = Cr and Mn) and characterized them by magnetic and transport measurements. Phase diagrams of CaK(Fe1-xAx)4As4 (A = Fe, Co, Mn, Cr) will be compared and the observed trends will be discussed in the context of Fe-based superconductors. |
Monday, March 4, 2019 10:12AM - 10:24AM |
A10.00010: The FFLO State in Unconventional Superconductors Charles Agosta, Raju Ghimire, Camille E Bales, Makariy A Tanatar Recent experiments suggest that inhomogeneous superconductivity (the FFLO state) exists in organic1, pnictide2, and heavy Fermion3 superconductors. The FFLO state is a type of superconducting pair density wave, although unlike the proposed pair density wave in cuprates, the FFLO state is accompanied by a magnetized charge density wave (CDW) of the left over (un-condensed) quasiparticles. We will review the data from each of the families of superconductors above and present new data for the BaxK1-xFe2As2 family of pnictide superconductors via measurements of the rf penetration depth using a tunnel diode oscillator, down to 40 mK. We will also discuss early results of experiments to measure the q-vector of a CDW in organic conductors with x-rays. This CDW is known to compete with the superconducting state rather than coexist with the FFLO state referred to above. Additionally, this technique may be a way to measure the q-vector of the FFLO state directly. Understanding these competing and intertwined orders in unconventional superconductors will further our knowledge of quantum mechanical ground states in electronic materials. |
Monday, March 4, 2019 10:24AM - 10:36AM |
A10.00011: Enhancement of Jc at around 1/3BΦ in Iron-based Superconductors with Splayed Columnar Defects Tsuyoshi Tamegai, Ayumu Takahashi, Nozomu Ito, Sunseng Pyon, Atsushi Yoshida, Tadashi Kambara, Satoru Okayasu, Ataru Ichinose Columnar defects (CDs) are known to trap vortices very effectively, leading to a significant increase in the critical current density (Jc). In contrast to the case of parallel CDs along the c-axis, splaying the direction of columnar defects enhances Jc even further due to the suppression of kink motions. In (Ba,K)Fe2As2 with splayed CDs introduced by irradiating 2.6 GeV U ions, we found a novel enhancement of Jc at around 1/3BΦ, (BΦ: matching field) for a range of splay angle of CDs. In the present study, we compare the enhancement of Jc in (Ba,K)Fe2As2 with splayed CDs created by 2.6 GeV U ions with those created by 320 MeV Au ions. We find out that the enhancement of Jc always occurs at ~1/3BΦ, while the degree of enhancement and optimum splay angle depends on the morphology of splayed CDs. We also investigate the effect of the symmetry breaking of the two sets of splayed CDs, namely for the case where the average direction of two sets of CDs is different from the c-axis and another case where doses of two sets of splayed CDs are different. |
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