Bulletin of the American Physical Society
APS March Meeting 2018
Volume 63, Number 1
Monday–Friday, March 5–9, 2018; Los Angeles, California
Session H14: Fe-based Superconductors -- Topological Superconductivity and New FrontiersFocus
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Sponsoring Units: DMP Chair: Ming Yi, University of California - Berkeley Room: LACC 304B |
Tuesday, March 6, 2018 2:30PM - 3:06PM |
H14.00001: Superconductivity and topology of monolayer Fe(Se,Te) Invited Speaker: Yujie Sun The origin of enhanced superconductivity over 50 K in the recently discovered FeSe monolayer films grown on SrTiO3 (STO), as compared to 8 K in bulk FeSe, is intensely debated. Many mechanisms, like lattice strain, interfacial coupling, electron doping, have been discussed as possible reasons which can change the superconductivity. Using molecular beam epitaxy (MBE), we are able to manipulate several parameters in monolayer Fe(Se, Te)/STO. In-situ high-resolution angle-resolved photoemission spectroscopy (ARPES) measurement displays how the lattice strain, electron, and chemical doping affect physical properties of monolayer FeSe. We observed enhanced superconductivity accompanying Lifshitz transition in both FeSe and Fe(Se, Te). Moreover, our results provide a clear experimental indication that the Fe(Se, Te) materials are high-temperature topological superconductors in which band topology and superconductivity are integrated intrinsically. |
Tuesday, March 6, 2018 3:06PM - 3:18PM |
H14.00002: Point Contact Study of FeTe1-xSex in Magnetic Fields Mason Gray, Ryan O'Connor, Ruidan Zhong, Genda Gu, Kenneth Burch The material FeTe1-xSex provides an easy platform to study the Iron-based superconductors due to its simple crystal structure and relatively high superconducting transition temperature. In addition, some recent ARPES and STM studies suggest that FeTe0.55Se0.45 may be topologically non-trivial. With this in mind, we use Point-Contact Differential Conductance studies to further investigate this material. We find a “Cusp-like” Zero Bias Conductance Peak (ZBCP) emerges below Tc. The magnetic field dependence of this ZBCP is investigated under different field orientations in order to determine its physical origin. |
Tuesday, March 6, 2018 3:18PM - 3:30PM |
H14.00003: Discrete energy levels of Caroli-de Gennes-Martricon states in quantum limit due to small Fermi energy in FeTe0.55Se0.45 Chen Mingyang, Chen Xiaoyu, Huan Yang, Zengyi Du, Zhu Xiyu, Wang Enyu, Hai-Hu Wen Caroli-de Gennes-Martricon (CdGM) states were predicted as low energy excitations within vortex cores in type-II superconductors. In the quantum limit, namely T/Tc<<Δ/EF, the energy levels of these states were predicted to be discrete with the basic levels at Eμ = ± μΔ2/EF (μ = 1/2, 3/2, 5/2, …). However, due to the small ratio of Δ/EF in most type-II superconductors, it is very difficult to observe the discrete CdGM states, but rather a symmetric peak appears at zero-bias at the vortex center. Here we report a clear observation of these discrete energy levels of CdGM states in FeTe0.55Se0.45. The rather stable energies of these states versus space clearly validates our conclusion. Analysis based on the energies of these CdGM states indicates that the Fermi energy in the system is very small. |
Tuesday, March 6, 2018 3:30PM - 3:42PM |
H14.00004: Superconductivity switched by spin-current and enhanced by forward-scattering interfacial phonons in heterostructure iron-based superconductor Jhinhwan Lee, Seokhwan Choi, Jinoh Jung, Yeonghoon Lee, SungBin Lee, Yannis Semertzidis, Jong Ok, Jun Sung Kim, Steven Johnston, Hyoung Joon Choi, Yunkyu Bang, Sang-Wook Cheong, Andreas Heinrich, Won-Jun Jang, Alex Lee Heterostructure iron-based superconductor made by self-assembly of FeAs monolayers atomically coupled with perovskite bilayers have attracted significant interest due to the possibilities of magnetic frustrations and existence of forward-scattering interfacial phonons. Using variable temperature and magnetic field spin-polarized scanning tunneling microscopy [1] and density-functional-theory calculations, we show that the magnetic symmetries in the Fe-layer of Sr2VO3FeAs are indeed metastable and as a result the superconductivity can be switched in nanometer-scale by changing the magnetic symmetry with spin-polarized or unpolarized tunneling current below the bulk Fe magnetic ordering temperature [2]. This may introduce possibilities of fast spintronic transistors controlling iron-based superconductivity. We also performed high-resolution quasiparticle interference (QPI) measurements and self-consistent BCS-theory-based QPI simulations to provide direct atomic-scale proofs of enhancement of iron-based superconductivity due to the BCS mechanism based on forward-scattering interfacial phonons [3]. |
Tuesday, March 6, 2018 3:42PM - 3:54PM |
H14.00005: Switching Magnetism and Superconductivity with Spin-Polarized Current in Iron-Based Superconductor Seokhwan Choi, Hyoung Joon Choi, Jong Ok, Yeonghoon Lee, Won-Jun Jang, Alex Lee, Young Kuk, SungBin Lee, Andreas Heinrich, Sang-Wook Cheong, Yunkyu Bang, Steven Johnston, Jun Sung Kim, Jhinhwan Lee We have explored a new mechanism for switching magnetism and superconductivity in a magnetically frustrated iron-based superconductor using spin-polarized scanning tunneling microscopy (SPSTM). Our SPSTM study on single crystal Sr2VO3FeAs shows that a spin-polarized tunneling current can switch the Fe-layer magnetism into a non-trivial C4 (2×2) order, which cannot be achieved by thermal excitation with unpolarized current. Our tunneling spectroscopy study shows that the induced C4 (2×2) order has characteristics of plaquette antiferromagnetic order in the Fe layer and strongly suppresses superconductivity. Also, thermal agitation beyond the bulk Fe spin ordering temperature erases the C4 state. These results suggest a new possibility of switching local superconductivity by changing the symmetry of magnetic order with spin-polarized and unpolarized tunneling currents in iron-based superconductors. |
Tuesday, March 6, 2018 3:54PM - 4:06PM |
H14.00006: Emergent Odd-frequency Superconductivity on the Edge of Pnictides Cody Youmans, Areg Ghazaryan, Pouyan Ghaemi Despite the presumed constructive coexistence of extended s-wave superconductivity and spin density wave (SDW) in certain Fe-pnictides, experiments show signatures of enhanced superconductivity at defects in the SDW. This motivates the analysis of pairing channels, beyond extended s-wave, that can emerge as a consequence of spatial inhomogeneity in conjunction with the superconductivity and spin-rotational symmetry breaking. Using a two-orbital model, we investigate the hypothesis that defects can enhance unconventional odd-frequency pairing within this coexistence phase. As the simplest example, we studied possible unconventional pairings on the edge and showed that certain types of anomalous odd frequency correlations are enhanced at edges of pnictides in the coexistence phase of SDW and superconductivity. Our results show that emergence of such superconducting states could potentially explain the enhancement of superfluid density in the defects of SDW in pnictides. |
Tuesday, March 6, 2018 4:06PM - 4:18PM |
H14.00007: Structure-Based Phase Diagram of LaFeAsxP1-xO 1111 System Ryan Stadel, Omar Chmaissem, Stephan Rosenkranz, Raymond Osborn, Matthew Krogstad In this talk, I will be reporting the structural and physical properties of high quality powder samples of LaFeAsxP1-xO from x=0 to x=1 examined using high resolution x-ray and neutron diffraction. This system is of interest due to similarities to the 122 system where novel magnetic phases were discovered in co-existence with superconductivity. To investigate such similarities, multiple samples bordering reported antiferromagnetic and superconducting regions were prepared. While preliminary analysis confirms the existence of AFM properties with low P doping, neutron data revealed no evidence for the AFM2 phase that was previously reported in the literature centered around x=0.5. I will be discussing the full details of the current state of the phase diagram. |
Tuesday, March 6, 2018 4:18PM - 4:30PM |
H14.00008: Anisotropic Superconductivity and Magnetism in RbEuFe4As4 Ulrich Welp, Matthew Smylie, Kristin Willa, Wai-Kwong Kwok, Jinke Bao, Duck Young Chung, Mercouri Kanatzidis, Zahirul Islam, Zhu Diao, Andreas Rydh RbEuFe4As4 is a stoichiometric compound that undergoes a superconducting transition at Tc = 37 K and a magnetic transition at Tn = 15 K in which Eu-moments order into a type–A antiferromagnetic structure. Thus, an interleaved stacking of ferromagnetic Eu-layers and superconducting Fe2As2 layers arises. Using magnetoresistance, magnetization and specific heat measurements we construct the magnetic and superconducting phase diagrams and find that the anisotropy Γ of the upper critical field Hc2 is surprisingly low, Γ ~ 1.8. No re-emergent resistive state associated with the onset of magnetic order is seen below Tc for current flow in or out of plane. The saturation field for the Eu-moments is ~2 kG for in-plane fields and ~20 kG for out-of-plane magnetization, consistent with an in-plane moment orientation. Specific heat measurements reveal that in magnetic fields, magnetic fluctuations become pronounced extending to temperatures well above Tc. We attribute a negative normal-state magnetoresistance for in-plane as well as c-axis currents to scattering by these magnetic fluctuations. |
Tuesday, March 6, 2018 4:30PM - 4:42PM |
H14.00009: Coexistence of magnetic order and superconductivity in RbEuFe4As4 according to high-resolution magnetic x-ray scattering studies Zahirul Islam, Kristin Willa, Matthew Smylie, Zhu Diao, Andreas Rydh, Jong Woo Kim, Jinke Bao, Duck Young Chung, Mercouri Kanatzidis, Wai-Kwong Kwok, Ulrich Welp Superconductivity (SC) and the presence of ordered magnetic moments are in general antagonistic. Nonetheless, when a material is forced to accommodate these apparently incompatible order parameters it may exhibit subtle organization of these phases, in the form of "cryptomagnetism", FFLO, and spontaneous vortex states. In-depth experimental studies of such inhomogeneous phases have been rather challenging. Here we present a superconductor, RbEuFe4As4, with intertwined magnetic and superconducting properties ideally suited for magnetic x-ray scattering (MXS) studies. These diffraction studies revealed a long-range, fully coherent, and three-dimensionally ordered magnetic phase. This phase corresponds to a long-range order of Eu moments below the Néel temperature of ~15K. Magnetization measurements confirmed a full ionic moment of Eu coexisting with SC. MXS results of the magnetic structure, its correlation, and order parameter will be presented. |
Tuesday, March 6, 2018 4:42PM - 4:54PM |
H14.00010: Hedgehog spin-vortex crystal magnetic order in superconducting CaK(Fe1-xMx)4As4 (M=Co, Ni) William Meier, Q.-P. Ding, Andreas Kreyssig, Sergey Bud'ko, Aashish Sapkota, Karunakar Kothapalli, Vladislav Borisov, Roser Valenti, Cristian Batista, Peter Orth, Rafael Fernandes, Alan Goldman, Yuji Furukawa, Anna Boehmer, Paul Canfield Antiferromagnetism in Fe-based superconductors is characterized by a pair of propagation vectors which may support a number of magnetic phases. Ni- and Co-doping in CaKFe4As4 suppresses superconductivity and stabilizes a new antiferromagnetic phase. Thermodynamic, transport, structural and local magnetic probes reveal this phase to be consistent with the hedgehog spin-vortex crystal. Ab initio calculations also indicate a preference for this non-collinear antiferromagnetism which is stabilized by the reduced symmetry of the CaKFe4As4 structure. |
Tuesday, March 6, 2018 4:54PM - 5:06PM |
H14.00011: Magnetic order in CaK(Fe1-xNix)4As4 and its interplay with superconductivity Andreas Kreyssig, John Wilde, Anna Böhmer, Wei Tian, William Meier, Mingyu Xu, Sergey Bud'ko, Paul Canfield, Robert McQueeney, Alan Goldman CaK(Fe1-xNix)4As4 shows a phase diagram remarkably similar to other Fe-based superconductors - by changing the Ni concentration the properties can be tuned from purely superconducting at low temperatures to superconducting and antiferromagnetically ordered. Here we present a microscopic study of the magnetism employing neutron diffraction measurements on single crystals. Although the antiferromagnetic order adopts a non-collinear structure conserving tetragonal symmetry, its propagation vectors are the same as for the stripe-like magnetic order that is frequently present in Fe-based superconductors. This points to a similar underlying coupling mechanism that forsters the observed strong interplay between magnetism and superconductivity. |
Tuesday, March 6, 2018 5:06PM - 5:18PM |
H14.00012: Thermal conductivity of KFe2As2 crystals in precisely oriented vector magnetic fields Makariy Tanatar, Erik Timmons, Yong Liu, T. A. Lograsso, Ruslan Prozorov, Volodymyr Bondarenko, Tristin Metz, Johnpierre Paglione Temperature and magnetic field dependent thermal conductivity, κ(T,H), was measured in high quality single crystals of KFe2As2 in precisely oriented magnetic fields in 2D vector magnet for current along the tetragonal b-direction. For fields along a- and c-tetragonal axes we observed monotonic quasi-linear increase of κ(H) followed by a rapid step-wise increase towards the upper critical field, Hc2, consistent with expectations for clean nodal superconductors. Anomalous, increase of κ(T)/T on cooling is found in H∥a configuration for magnetic fields just below Hc2=5 T. These observations are contrasted with the previous heat capacity measurements. |
Tuesday, March 6, 2018 5:18PM - 5:30PM |
H14.00013: Effects of swift-particle irradiation on CaKFe4As4 single crystals Tsuyoshi Tamegai, Sunseng Pyon, Nozomu Ito, Ayumu Takahashi, Shigeyuki Ishida, Hiroshi Eisaki, Akira Iyo, Motoharu Imai, Hideki Abe, Taichi Terashima CaKFe4As4 is a newly found iron-based superconductor with Tc~36 K, which has a similar crystal structure to BaFe2As2 with ordered Ca and K layers. We study effects of artificial defects created by heavy-ion irradiation on the critical current (Jc) in CaKFe4As4 single crystals that are irradiated by 800 MeV Xe ions up to the matching field (BΦ) of 64 T. At 2 K, the self-field Jc for the pristine crystal is over ~1.5 MA/cm2. However, it is reduced by a factor of ten at 50 kOe. We occasionally observe non-monotonic temperature dependence of Jc at high fields, which could be explained by the presence of field-induced pinning centers. Jc is enhanced monotonically with increasing BΦ, and it reaches over 10 MA/cm2 at 2 K and BΦ = 16 T. The general behavior of Jc-BΦ in CaKFe4As4 is similar to that in Xe-irradiated (Ba,K)Fe2As2. |
Tuesday, March 6, 2018 5:30PM - 5:42PM |
H14.00014: NMR Study of Over-doped NaFe1-xCuxAs Single Crystals Yizhou Xin, Jeongseop Lee, Ingrid Stolt, William Halperin, Yu Song, Weiyi Wang, Pengcheng Dai, Chongde Cao, Arneil Reyes We present our NMR study of over-doped pnictide NaFe1-xCuxAs, which has been demonstrated to be a possible Mott insulator near x ≈ 0.5 [1,2]. Our NMR spectra show two inequivalent 23Na sites due to Cu dopants. Both 23Na and 75As NMR spectra show a possible formation of stripe-like ordering of Fe and Cu in the 35% and 50% Cu-doped samples. The large Curie-Weiss-like magnetization due to impurity effects results in strongly inhomogeneous field distribution through out the heavily Cu-doped samples, significantly broadening both 23Na and 75As spectra. Our 23Na spin-lattice relaxation data shows an enhanced relaxation rate in regions close to Cu dopants. |
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