Bulletin of the American Physical Society
APS March Meeting 2020
Volume 65, Number 1
Monday–Friday, March 2–6, 2020; Denver, Colorado
Session L61: Fe-Based Superconductors - Nematicity IVFocus
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Sponsoring Units: DMP DCMP DCOMP Chair: Benjamin Frandsen, Brigham Young Univ - Provo Room: Mile High Ballroom 4B |
Wednesday, March 4, 2020 8:00AM - 8:36AM |
L61.00001: Electronic Nematicity in FeSe Invited Speaker: Ming Yi Superconductivity emerges in proximity to a nematic phase in most iron-based superconductors. It is therefore important to understand the impact of nematicity on the electronic structure. Orbital assignment and tracking across the nematic phase transition proved to be challenging due to the multiband nature of iron-based superconductors and twinning effects. Here we report a detailed study of the electronic structure of fully detwnned FeSe across the nematic phase transition using angle-resolved photoemission spectroscopy. We clearly observe a nematicity-driven band-reconstruction involving dxz, dyz and dxy orbitals. The nematic energy scale between dxz and dyz bands reach a maximum of 50meV at the Brillouin zone corner. We are also able to track the dxz electron pocket across the nematic transition and explain its absence in the nematic state. Our comprehensive data of the electronic structure provide an accurate basis for theoretical models of the superconducting pairing in FeSe. |
Wednesday, March 4, 2020 8:36AM - 8:48AM |
L61.00002: Evolution of soft phonons in FeSe under pressure Adrian Merritt, Frank Weber, Anna Boehmer, Kaushik Sen, John-Paul Castellan, Thomas Wolf, Sofia Michaela Souliou, Ahmet Alatas, Ayman Said, Wenli Bi, Rafael Fernandes, Dmitry Reznik FeSe, as the simplest iron-based superconductor (Fe-SC), is of great interest for furthering our understanding of the Fe-SC materials. It shares a common tetragonal-to-orthorhombic phase transition at 90 K with many of the Fe-SCs, and becomes superconducting at 9 K, but in contrast displays no magnetic order. Previous studies have shown an enhancement of the superconducting transition temperature with increasing pressure, with a maximum of 37 K at approximately 6.3 GPa, and magnetic order concurrent with the structural transition above 1.6 GPa. Here we present IXS measurements of TA phonon dispersion from which we can extract the nematic correlation length. The phonon renormalization effects observed in our experiments have important implications for nematicity and superconductivity. This includes a significant reduction in the phonon softening at the high-pressure structural-magnetic transition and a new phonon hardening effect at the superconducting transition. We will discuss the implications of these results in the context of pressure- and doping-dependence in the Fe-SCs. |
Wednesday, March 4, 2020 8:48AM - 9:00AM |
L61.00003: Observation of intra-unit-cell nematic order in epitaxial bilayer FeSe films on SrTiO3(001) Huimin Zhang, Zhuozhi Ge, Qiang Zou, Michael Weinert, Lian Li Epitaxial FeSe thin films provide an ideal platform to probe the interplay of superconductivity and nematicity, due to the absence of long-range magnetic order. Here, we systematically investigate the nematic order in high quality bilayer FeSe/SrTiO3 films grown by molecular beam epitaxy. Using low temperature scanning tunneling microscopy/spectroscopy, we observe features associated with Se atoms to be elongated along the Fe-Fe lattice direction within a specific energy window, demonstrating symmetry breaking from fourfold to twofold. Detailed analysis of Fourier transformation of the STM images reveals that the intensity of Fe Bragg peak breaks rotational symmetry within each Fe unit cell, indicative of an intra-unit-cell nematicity. Our results provide critical information on nematicity in Fe-based superconductors, an essential element in understanding superconducting transition in these materials. |
Wednesday, March 4, 2020 9:00AM - 9:12AM |
L61.00004: Phonon spectroscopy in FeSe using high-resolution inelastic x-ray scattering Naoki Murai, Tatsuo Fukuda, Masamichi Nakajima, Mitsuaki Kawamura, Daisuke Ishikawa, Setsuko Tajima, Alfred Baron We report an inelastic x-ray scattering investigation of phonons in FeSe superconductor. Comparing the experimental phonon dispersion with density functional theory (DFT) calculations in the non-magnetic state, we found a significant disagreement between them. Improved overall agreement was obtained by allowing for spin-polarization in the DFT calculations, despite the absence of magnetic order in the experiment. This calculation gives a realistic approximation of the disordered paramagnetic state of FeSe, in which strong spin fluctuations are present. We will discuss the current state of our analysis relating magnetism to the phonon dispersion. |
Wednesday, March 4, 2020 9:12AM - 9:24AM |
L61.00005: Room temperature local nematicity in FeSe superconductor Robert Koch, Tatiana Konstantinova, Milinda Abeykoon, Aifeng Wang, Cedomir Petrovic, Yimei Zhu, Emil Bozin, Simon J L Billinge We report pair distribution function measurements of the iron-based superconductor FeSe above and below the structural transition temperature. Structural analysis reveals a local orthorhombic distortion with a correlation length of about 4 nm at temperatures where an average tetragonal symmetry is observed. The analysis further demonstrates that the local distortion is larger than the global distortion at temperatures where the average observed symmetry is orthorhombic. Our results suggest that the low-temperature macroscopic nematic state in FeSe forms from an imperfect ordering of orbital-degeneracy-lifted nematic fluctuations which persist up to at least 300 K. |
Wednesday, March 4, 2020 9:24AM - 9:36AM |
L61.00006: Observation of layer-dependent dxz/dyz band splittings in the nematic phase of FeSe/SrTiO3 films Xiliang Peng, Kun Jiang, Xianxin Wu, Yonghao Yuan, Yaobo Huang, Tian Qian, Wei Li, Jiangping Hu, Ziqiang Wang, Yujie Sun, Hong Ding The effect of interlayer coupling on physical properties of layered materials such as iron-based superconductors is an interesting question. Here, we use angle-resolved photoemission spectroscopy to directly observe the electronic structure of FeSe films with different layers grown on SrTiO3(001) substrates by in situ molecular beam epitaxy. We find two distinct layer-dependent band splittings in the Brillouin zone center and corner, respectively. In the Brillouin zone corner, the band splitting caused by the nematic phase decreases with the increase in the number of layers. On the contrary, the degeneracy of dxz/dyz bands in double-layer FeSe is lifted in multi-layer films and the band splitting in the Brillouin zone center becomes larger with the increase of film thickness. The results of double-layer FeSe grown on SrTiO3 favor the proposal of d-wave nematic order. Our layer-dependent experiments provide limitations for the theoretical study of the nematic phase in FeSe and shed insights for the study of the interplay between nematicity and superconductivity. |
Wednesday, March 4, 2020 9:36AM - 9:48AM |
L61.00007: Nematicity from spin correlations in iron-based superconductors Rong Yu, Yiming Wang, Wenjun Hu, Qimiao Si Electronic nematic order has been the topic of considerable interest in the area of iron-based superconductors. Motivated by recent experimental works, we study the connection between nematic order and magnetic fluctuations. We show, based on a symmetry analysis, that the spin correlations in the system allow for a variety of nematic orders, in particular an unusual B2g nematicity. Using qualitative considerations as well as microscopic calculations, we discuss the types of magnetic fluctuations that stabilize this B2g nematicity and how our proposed mechanism provides a natural understanding of the recent experimental observations in the heavily hole doped iron pnictides (Rb,Cs)Fe2As2. We also discuss the effects associated with the nematic transition. |
Wednesday, March 4, 2020 9:48AM - 10:00AM |
L61.00008: Spin nematicity in the local-moment Fe-chalcogenide KFe0.8Ag1.2Te2 Yu Song Iron-based superconductors appear in the vicinity of a nematic quantum critical point, and due to the presence of spin, orbital and charge degrees of freedom, nature of the nematic order has been the focus of research in recent years. One route to address this question is to look for clues in related materials that exhibit similar physics. In semiconducting KFe0.8Ag1.2Te2, an analogue of the archetypical iron pnictide BaFe2As2, we found simultaneous stripe-type magnetic order and nematic order below TS,N=35 K, with striking similarities between the two systems. Above TS,N, a sizable spin anisotropy develops under a small strain and increase upon cooling towards TS,N, indicative of a divergent nematic susceptibility. Since the magnetic susceptibility of KFe0.8Ag1.2Te2 is well described by the Curie-Weiss law with S≈1, it is a local-moment magnet with orbital degeneracy. The similarities between nematic orders in KFe0.8Ag1.2Te2 and BaFe2As2 suggest our findings to be relevant for iron-based superconductors, and establish KFe0.8Ag1.2Te2 as a model system to investigate electronic nematicity in the localized limit. |
Wednesday, March 4, 2020 10:00AM - 10:12AM |
L61.00009: Exploring the electronic structure of BaFe2As2 using resonant Raman scattering Rudolf Hackl, Andreas Baum, Ying Li, Nenad Lazarevic, Daniel Jost, Jiun-Haw Chu, Ian Fisher, Roser Valenti, Igor Mazin Iron-based materials are characterized by the competition of various phases including magnetism, nematicity, and superconductivity. The interrelation of these instabilities is a key question of both materials science and theoretical modelling. In this study we present light scattering data on twin-free BaFe2As2 as a function of the energy of the incident photons. The phonon intensities display an anisotropy for the polarizations oriented along either the ferro- or the antiferromagnetically ordered direction. This anisotropy makes the fully symmetric As phonon to appear in crossed polarizations. The anomalous intensity and the anisotropy of the spectral weight increase substantially in the blue-green spectral range. The experimental results are analyzed using density functional theory. Qualitative agreement between model calculations and experiment is found if the magnetic order is properly taken into account. The anomalies are fully developed only in the presence of long range magnetic order and are not directly linked to nematicity. Rather they are a high-energy phenomenon indicating that magnetism and nematicity are interrelated. |
Wednesday, March 4, 2020 10:12AM - 10:24AM |
L61.00010: Hidden antiferro-nematic order in BaFe2As2 and NaFeAs above TS Seiichiro Onari, Hiroshi Kontani In some Fe-based superconductors such as BaFe2As2 and NaFeAs, C4 symmetry breaking emerges at T*,which is tens of Kelvin higher than the structural transition temperature TS [1,2]. In this "hidden nematic state" below T*, the orthorhombicity is very tiny. To explain this long-standing mystery, we propose the emergence of antiferro-bond order with antiferro wavevector q=(0,π) at T*, by solving the linearized density-wave equation based on the vertex correction theory [3,4]. This antiferro-bond order originates from the inter-orbital nesting between the dxy-orbital hole-pocket and the electron-pocket, and naturally explains the pseudogap, band-folding, and tiny nematicity linear in T*-T. |
Wednesday, March 4, 2020 10:24AM - 10:36AM |
L61.00011: Quantum Critical Enhancement of Nematic-Resistivity Anisotropy in Underdoped Ba(Fe1-xCox)2As2 Shua Sanchez, Paul Malinowski, Jong Woo Kim, Philip Ryan, Jiun-Haw Chu The proportionality constant between the thermodynamic order parameter of a symmetry breaking phase and its associated transport coefficient often contains important information about the underlying electronic structure of a material. For instance, the anomalous Hall effect has been used to probe the Berry curvature of the band structures. Here we show how the proportionality constant between resistivity anisotropy and orthorhombicity in the nematic ordered phase is directly related to the elastoresistivity coefficient measured above the phase transition temperature. Using this relation, we discovered that this proportional constant increases significantly as doping approaches optimal, indicating the enhanced coupling between nematicity and conducting electrons near the quantum critical point. |
Wednesday, March 4, 2020 10:36AM - 10:48AM |
L61.00012: Non-linear elasto-Hall measurements in iron-based superconductors Paul Malinowski, Qianni Jiang, Zhaoyu Liu, Yue Shi, Shua Sanchez, Jiun-Haw Chu It is well established that a rotational symmetry breaking electronic nematic phase is ubiquitous in the phase diagrams of iron-based superconductors and that anisotropic strain is a uniquely powerful in-situ probe of electronic anisotropy in these systems. In addition to the long-held belief that magnetic/nematic fluctuations are essential to understanding the pairing mechanism of the high Tc superconductivity, they also drastically modify the normal state properties, evidenced by non-Fermi liquid behavior and strongly temperature dependent transport coefficients. In this work, we present the observation of the non-linear elasto-Hall effect in Co doped and P doped BaFe2As2. Our measurement reveals a large quadratic term of the Hall coefficient as a function of B2g anisotropic strain. For the optimally doped composition, the nonlinear elasto-resistivity diverges as temperature decreases. Combined with elastoresistivity, our result provides crucial insight into how nematicity and nematic fluctuations couple to conducting electrons. |
Wednesday, March 4, 2020 10:48AM - 11:00AM |
L61.00013: Investigation of neamtic phase in LaFeAsO Soonsang Huh, Younsik Kim, Wonshik Kyung, Jongkeun Jung, Saicharan Aswartham, Bernd Buechner, Chengxiao Dong, Jiangping Hu, Soohyun Cho, Dawei Shen, Jonathan Denlinger, Changyoung Kim The nematic phase is belived to play a key role in iron based superconductor (IBSC), but it is not fully understood. LaFeAsO, known to have nematic phase but its electronic structure is not well understood, is a good system to get new clues about the nematic phase. To investigate the electronic structure of LaFeAsO nematic phase, we performed angle resolved phtoemisson spectroscopy (APRES) experiment. We report temperature dependent band splitting behavior between dxz and dyz hole band near Γ and M point |
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