Bulletin of the American Physical Society
APS March Meeting 2019
Volume 64, Number 2
Monday–Friday, March 4–8, 2019; Boston, Massachusetts
Session F10: Fe-based Superconductors -- Magnetic properties |
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Sponsoring Units: DMP GMAG DCOMP Chair: Minjun Lee, Seoul National University Room: BCEC 151B |
Tuesday, March 5, 2019 11:15AM - 11:27AM |
F10.00001: Hedgehog spin-vortex crystal antiferromagnetic quantum criticality in CaK(Fe1-xNix)4As4 revealed by NMR Qing-Ping Ding, William Meier, JINFANG CUI, Mingyu Xu, Anna Boehmer, Sergey Budko, Paul Canfield, Yuji Furukawa Two degrees of freedom, antiferromagnetism and nematicity, have been observed in most iron-based superconductors (SCs). In contrast to those compounds, the newly discovered SC CaK(Fe1-xNix)4As4 exhibits an antiferromagnetic (AFM) state, called hedgehog spin-vortex crystal (SVC) structure, without the orthorhombic distortion [1,2]. This system provides an opportunity to investigate he relationship between magnetic fluctuation and SC in a system that remains tetragonal. Here, we report an 75As nuclear magnetic resonance (NMR) study of CaK(Fe1-xNix)4As4 (0 ≤ x ≤ 0.049). We will show the experimental evidence which suggests that, based on NMR data [3] , a hedgehog SVC quantum-critical point (QCP) would exist around x = 0 in the normal state, but is avoided due to the onset of superconductivity. We will discuss the relationship of spin fluctuations and superconductivity in this system. Furthermore, the advantage of CaKFe4As4 over other iron-based SCs with QCP will also be discussed. |
Tuesday, March 5, 2019 11:27AM - 11:39AM |
F10.00002: Unusually stronger quantum fluctuation with larger spins: Novel phenomena revealed by emergent magnetism in pressurized high-temperature superconductor FeSe Wei Ku, Yuting Tan, Tianyu Zhang, Tao Zou, Antonio M. dos Santos, Jin Hu, Dao-Xin Yao, Zhiqiang Mao, Xianglin Ke A counter-intuitive enhancement of quantum fluctuation with larger spins, together with a few novel physical phenomena, is discovered in studying the recently observed emergent magnetism in high-temperature superconductor FeSe under pressure. Starting with experimental crystalline structure from our high-pressure X-ray refinement, we analyze theoretically the stability of the magnetically ordered state with a realistic spin-fermion model. We find surprisingly that in comparison with the magnetically ordered Fe-pnictides, the larger spins in FeSe suffer even stronger long-range quantum fluctuation that diminishes their ordering at ambient pressure. This "fail-to-order" state then develops into an ordered state above 1GPa due to weakened fluctuation accompanying the reduction of anion height and carrier density. We further clarify the controversial nature of magnetism and its interplay with nematicity in FeSe in the same unified picture for all Fe-based superconductors. Our study establishes a generic exceptional paradigm of stronger quantum fluctuation with larger spins that complements the standard knowledge of insulating magnetism. |
Tuesday, March 5, 2019 11:39AM - 11:51AM |
F10.00003: Tuning the Magnetic and Superconducting Properties of Hole Doped BaFe2As2: A Universal Scaled Phase Diagram Omar Chmaissem, Ryan Stadel, Daniel Bugaris, Raymond Osborn, Stephan Rosenkranz Embarking on an extensive effort in which more than fifty hole-doped BaFe2As2 samples have been produced and characterized by neutron diffraction allowed us to extract universal behaviors and properties of this rich system. All the measured magnetic and superconducting transitions values collapse into a single phase diagram. Strontium, calcium, and sodium subsitutions at the barium site enabled the production of many samples in which both the re-entrant tetragonal double-Q magnetic C4 and the orthorhombic SDW magnetic C2 phases coexist and compete with superconductivity just before reaching the peak of the superconductivity-only region of the phase diagram. The evolution of TN, Tr and the internal structual parameters are in agreement with recent theoretical models that correlate the stabiity of these observed phases with approaching a perfect tetrahedral FeAs4 angle and perfect Fermi surface nesting. Our results separate the individual roles played by either the As-Fe-As bond angles or the Fe-As bond lengths and demonstrate their combined effects in tuning the magnetic and superconducting properties. |
Tuesday, March 5, 2019 11:51AM - 12:03PM |
F10.00004: Possible multiple modes of neutron spin resonance in Ba1-xKxFe2As2 and BaFe2-xNixAs2 Huiqian Luo, Tao Xie, Shiliang Li, Uwe Stuhr, Tom Fennell, Sergey Danilkin Motivated by the triple modes of neutron spin resonance with both odd and even L-symmetries in CaKFe4As4[1],we use inelasitc neutron scattering to re-investigate the spin resonance in 122 system of iron-based superconductors. In the optimally hole doped Ba1-xKxFe2As2, the spin resonance has very weak L-dependence both for intensity and center energy, while the H-dependence show a combination with an incommensurate mode at high energy and a commensurate mode at low energy. In the optimally electron doped BaFe2-xNixAs2, the spin resonance becomes L-dependent at low energy. A very weak even mode (L=2,4, 6...) probably exist around 14 meV. Therefore, in contrast to the explaination based on weak L-dispersion on single spin resonance mode, we propose an alternative explanation based on the multiply Fermi surface nesting induced multi-modes with different symmetries, which give Q-dependent intensity either for out-of-plane or in-plane spin excitations. |
Tuesday, March 5, 2019 12:03PM - 12:15PM |
F10.00005: Multiple Distinct Magnetic and Superconducting Phases in LaFeAs1-xPxO 1111 Characterized Via X-ray and Neutron Diffraction Ryan Stadel, Matthew Krogstad, Stephan Rosenkranz, Raymond Osborn, Keith Taddei, Dmitry Khalyavin, Jinke Bao, Duck Young Chung, Omar Chmaissem Following the discovery of the microscopic co-existence of antifermagnetic spin density waves and superconductivity in Ba1-xKxFe2As2 and the low temperature structural re-entrance to the novel magnetic C4 tetragonal phase in Ba1-xNaxFe2As2, an effort was made to seek similar phases in the related LaFeAs1-xPxO 1111 system. Previous work had reported two superconducting domes as well as two distinct magnetic regions. While the low doping magnetic phase had been well characterized by neutron diffraction, the higher doped magnetic region had been deduced from short range 31P-NMR measurements. We present a detailed characterization of the nuclear and magnetic structures based on high resolution and high intensity x-ray and neutron powder and single crystal diffraction. Our work demonstrates distinct magnetic symmetries for the two magnetic regions of the phase diagram. |
Tuesday, March 5, 2019 12:15PM - 12:27PM |
F10.00006: Anisotropic spin excitations in detwinned parent iron pnictides Qimiao Si, Changle Liu, Xingye Lu, Pengcheng Dai, Rong Yu Motivated by recent inelastic neutron scattering measurement on the fully detwinned BaFe2As2, we study the spin excitations of a local-moment model in the (π,0) collinear antiferromagnetically ordered phase. We show that the spin quadrupole modes can provide significant contributions to the spin excitation spectrum at high energies, and demonstrate this effect in an S=1 frustrated spin model with bilinear-biquadratic interactions using a flavor wave approach in an SU(3) representation. As a combined effect of the magnons and quadrupole modes, the spin excitation anisotropy drops with increasing energy. Our results are in good agreement with the salient features of the experimental observation. |
Tuesday, March 5, 2019 12:27PM - 12:39PM |
F10.00007: Breaking of four-fold rotational symmetry driven by stripe-type magnetism in semiconducting KFe0.8Ag1.2Te2 Yu Song, Huibo Cao, Bryan C. Chakoumakos, Yang Zhao, Aifeng Wang, Hechang Lei, Cedomir Petrovic, Robert J Birgeneau Superconductivity in the iron pnictides and chalcogenides emerges in the vicinity of an electronic nematic state, whose driving force remains controversial. We use X-ray and neutron scattering to study the semiconducting alkaline metal iron chalcogenide KFe0.8Ag1.2Te2, that is structurally analogous to the prototypical iron pnictide BaFe2As2. We find that KFe0.8Ag1.2Te2 realizes isolated 2x2 Fe blocks, separated by nonmagnetic Ag atoms. Long-range magnetic order sets in below TN = 35 K, with magnetic moments within each Fe block ordering into a stripe-type configuration. A structural transition that breaks four-fold rotational symmetry of the lattice accompanies the magnetic transition, resulting in different lattice spacings along the two orthogonal Fe-Fe bond directions. This difference in lattice spacings is similar to that in BaFe2As2 in magnitude, and like BaFe2As2, the lattice spacing is longer along the antiferromagnetically aligned Fe-Fe direction. Since KFe0.8Ag1.2Te2 is a semiconductor, local-moment magnetism is likely responsible for driving the breaking of four-fold rotational symmetry, and similar magnetic interactions may play an important role in the superconducting alkaline metal iron chalcogenides. |
Tuesday, March 5, 2019 12:39PM - 12:51PM |
F10.00008: Study of the pressure on magnetism in spin-ladder superconductor BaFe2Se3 Shan Wu, Robert J Birgeneau, Junjie Yin, Meng Wang, Benjamin Frandsen, Tom Forrest, Craig L. Bull, Nick Funnell, Gediminas Simutis, Rustem Khasanov, Thomas Smart, Martin Kunz, Andrew Doran Magnetic fluctuations were proposed as important for the pairing glue to the unconventional superconductivity (SC). The proximity of an antiferromagnetic (AFM) ordered has been found ubiquitously, with the majority of the iron-based superconductors exhibiting the same two-dimensional square lattice structural motif. Optimal SC typically appears when the magnetic order is suppressed by applying pressure or doping carriers. Pressure-induced SC in the BaFe2X3 (X=Se,S) introduces a quasi-one-dimensional prototype for the studies of the unconventional SC[1,2]. BaFe2Se3 shows a block-type AFM transition below TN = 256 K. By applying pressure, the Mott insulating state is gradually suppressed and displays a SC dome above 10 Gpa similar to the BaFe2S3. In this talk, I will present a systematic study of the magnetic and structural properties under hydrostatic pressure in BaFe2Se3 using neutron diffraction, x-ray diffraction and muon spin relaxation techniques. These provide the information on how the magnetic phase evolves approaching the SC and compares to the case of BaFe2S3.[1]H. Takahashi,et al.,Nat. Mater.(2018).[2]J.J. Ying,et al.,PRB (2017). |
Tuesday, March 5, 2019 12:51PM - 1:03PM |
F10.00009: Quantum critical point in Ba(Fe1-xCox)2As2 from London penetration depth measured by optical NV magnetometer Kamal Joshi, Naufer M Nusran, Kyuil Cho, Makariy Tanatar, Sergey L. Bud'ko, Paul Canfield, Alex Levchenko, Ruslan Prozorov Lower critical field, Hc1, allows direct access to London penetration depth, λ, which provides an insight into the normal-state electronic properties inside the dome of superconductivity in temperature-doping phase diagram. Using recently developed minimally invasive optical NV-centers magnetometry and the revised demagnetization corrections, Hc1 was measured in single crystals of electron-doped Ba(Fe1-xCox)2As2 for several x- values. The measurements reveal a sharp peak in λ(x) near x=0.057, exactly the composition at which the long-range magnetic order becomes incommensurate. Together with previous observation of quantum critical point (QCP) under the dome of superconductivity in isovalent AsP122, our results provide strong evidence for a ubiquitous nature of QCP in iron pnictides at the disappearance of the long-range ordered magnetism at T=0. |
Tuesday, March 5, 2019 1:03PM - 1:15PM |
F10.00010: Effect of variable-energy electron irradiation on superconductivity in Ba1-xKxFe2As2 crystals Kyuil Cho, Makariy A Tanatar, Yong Liu, Thomas Antony Lograsso, Ruslan Prozorov, Marcin Konczykowski Point-like defects produced by MeV-range electron irradiation can be used as a phase-sensitive probe of the energy gap structure in superconductors. In complex compounds, the probability of knocking out an ion from a particular lattice site depends on the type of the ion and the incoming electron energy. Here we examine the effect of irradiation on superconducting Tc and electrical resistivity of in Ba1-xKxFe2As2 single crystals using electrons of different energies, from 1.0 MeV to 2.5 MeV. From the resistivity measurements in-situ (during irradiation) and ex-situ (in a separate cryostat), the relation between the changes in Tc and residual resistivity as a function of electron energy was determined. The results will be discussed in a broader context of the connection between particular crystal structure and superconductivity. |
Tuesday, March 5, 2019 1:15PM - 1:27PM |
F10.00011: Mapping the superfluid density in an Iron-based Superconductor Doohee Cho, Koen Bastiaans, Damianos Chatzopoulos, Genda Gu, Milan P Allan Cooper pairs can tunnel through the vacuum barrier between two superconducting electrodes, known as a Josephson junction. Depending on the properties of the junction and the environment, one can extract the superconducting order parameter from the current-voltage spectra. Here, I will present the Josephson tunneling spectroscopic results of an unconventional Fe-based superconductor acquired with a superconducting STM tip. We demonstrate the Josephson tunneling induced characteristic current-voltage curves close to zero bias on a Pb(111) surface and a Fe-based superconductor. I will discuss the spatial variations of the superfluid density in this correlated superconductor and reveal their correlation with an inhomogeneous quasiparticle density of states. |
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