Bulletin of the American Physical Society
APS March Meeting 2022
Volume 67, Number 3
Monday–Friday, March 14–18, 2022; Chicago
Session Y69: Fe-based Superconductors: Nematicity in FeSeFocus Recordings Available
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Sponsoring Units: DMP DCMP DCOMP Chair: Ming Yi, Rice University Room: Hyatt Regency Hotel -Jackson Park A |
Friday, March 18, 2022 8:00AM - 8:36AM |
Y69.00001: Quasiparticle coherence in nematic state studied with strain-dependent ARPES Invited Speaker: Heike Pfau The microscopic mechanism of nematicity in iron-based superconductors is still unresolved. Most models consider either spin or orbital degrees as driving force but typically do not take electronic correlations into account. However, the interplay of Coulomb interactions and Hund’s rule coupling leads to observable bad metallic behavior, orbital-dependent mass renormalizations and an overall suppression of quasiparticle coherence. We studied the influence of nematic order on the quasiparticle coherence in BaFe2As2 using strain-dependent ARPES and compare them to our results on detwinned FeSe. Inside the nematic phase, we find an anisotropy of the spectral weight between the dxz and the dyz orbitals in the coherent quasiparticle peak and in the incoherent Hubbard band. We interpret our results in terms of a more coherent dxz orbital compared to the dyz orbital. Our study highlights the importance of electronic correlations in the description of nematicity in iron-based superconductors. |
Friday, March 18, 2022 8:36AM - 8:48AM |
Y69.00002: The effect of electronic correlations on the nematic electronic phase of FeSe1-xTex Archie B Morfoot, Timur Kim, Matthew D Watson, Amir A Haghighirad, Shiv J Singh, Ryutaro Okuma, Amalia I Coldea Orbitally-dependent electronic correlations can play an important role in the stabilization of the nematic and superconducting states of iron-chalcogenides. In FeSe nematicity is supressed using isoelectronic substitution of sulphur and tellurium, but the increase in bandwidths only occurs with sulphur substitution leading to reduced correlations. Here we will present a study of the electronic structure of the nematic superconductors FeSe1-xTex using ARPES to understand the role of orbitally-dependent correlations on the nematic electronic state with tellurium substitution. We assess the degree of band renormalisation by comparing our ARPES data to DFT band structure calculations. From the linewidths and Fermi velocities of the energy dispersions we access the imaginary part of the self-energy which corresponds to the quasiparticle lifetime. Next, we extract the size of the Fermi surface and its kz-dependence and compare these findings to previous studies on FeSe1-xSx. Lastly, by using deep energy cuts, we discuss the formation of a Hubbard-like band in FeSe1-xTex as compared with FeSe. |
Friday, March 18, 2022 8:48AM - 9:00AM |
Y69.00003: Spontaneous orbital anisotropy in the nematic state of FeSe Connor A Occhialini, Joshua J Sanchez, Gilberto F Fabbris, Yongseong Choi, Jong-Woo Kim, Philip J Ryan, Luiz Gustavo Pimenta Martins, Yonghun Lee, Qian Song, Riccardo Comin Nematicity has played a central role in the phenomenology of iron-based superconductors. FeSe is of unique interest due to the absence of a C4 symmetry-breaking SDW order while preserving robust nematic and superconducting phases. These features have raised intrigue regarding the role of the orbital degrees of freedom and their connection to the pairing mechanism. Here, we report a multi-probe study of the nematic state as a function of in-situ tunable uniaxial strain, using Fe K pre-edge X-ray linear dichroism (XLD) and hard X-ray diffraction measurements combined with simultaneous transport measurement. Together, these probes provide a detailed picture of the anisotropic orbital, transport and structural responses to uniaxial strain in both the high temperature and ordered phases. Strain-detwinning in the nematic phase reveals a sizable ab plane pre-edge XLD which is assigned to orbital anisotropy coupled to the primary nematic order parameter, rather than the secondary lattice orthorhombicity. Strain-induced XLD above Ts is also revealed, exhibiting behavior consistent with a divergent nematic susceptibility that is strongly coupled to an underlying orbital order. Our results shed light on the central role of the Fe 3d orbitals in driving the nematic phase. |
Friday, March 18, 2022 9:00AM - 9:36AM |
Y69.00004: Visualizing the electronic nematic state by laser-photoemission electron microscopy Invited Speaker: Takahiro Shimojima Nematicity is ubiquitous in the electronic phases of iron-based superconductors [1,2]. Previous angle-resolved photoemission spectroscopy revealed the orbital polarization of Fe 3dxz and 3dyz electrons as an order parameter that characterizes the nematic phase [3], but its real-space arrangement remains largely unexplored. We use linear dichroism in a low-temperature laser-photoemission electron microscope [4] to map out the orbital polarization of nonmagentic FeSe and antiferromagnetic BaFe2(As0.87P0.13)2 [5]. In contrast to structural domains, which have atomic-scale domain walls [6], the linear dichroism patterns in both materials show peculiar sinusoidal waves of electronic nematicity with wavelengths more than 1000 times longer than the unit cell. According to the Ginzburg-Landau theory, the sinusoidal waves can be understood by a train of the nematic domain walls with mesoscopic coherence length. These observations suggest the nematic order parameter with high stiffness against real-space modulation and its unusual decoupling from lattice. In this talk, I will also discuss the temperature dependence of the linear dichroism signals. |
Friday, March 18, 2022 9:36AM - 9:48AM |
Y69.00005: The effect of impurity scattering on the electronic and superconducting properties of FeSe via Cu-substitution tuned by applied pressure. Zachary Zajicek, Shiv J Singh, Pascal Reiss, Matthew Bristow, Alix McCollam, Amalia I Coldea An important aspect in understanding the superconducting pair mechanism and the scattering affecting normal electronic properties in iron-based superconductors is via controlled disorder. The substitution of Cu can give rise to intriguing electronic properties compared with other transition metals, like Co and Ni, that often helps to stabilize a high-Tc superconducting state. We present a detailed study on the role of doping and disorder on the superconducting and electronic properties of Cu-FeSe. The nematic and superconducting phases are both quickly suppressed with copper substitution and the carrier mobilities decrease significantly. In addition, studies using applied hydrostatic pressure revealed the suppression of the nematic phase, whilst the superconductivity reaches 22K at 20kbar in a regime where a magnetic phase is stabilized. The pressure-temperature phase diagram reveals that the magnetic phase is highly sensitive to disorder, whereas the superconducting phase remains rather robust. |
Friday, March 18, 2022 9:48AM - 10:00AM |
Y69.00006: Retaining Superconducting Phases Through Low-Temperature Pressure Quenching Trevor Bontke, Liangzi Deng, Rabin Dahal, Yu Xie, Bin Gao, Xue Li, Ketao Yin, Melissa J Gooch, Donald Rolston, Tong Chen, Zheng Wu, Yamming Ma, Pengcheng Dai, Paul C. W Chu In the past 5 years the discovery of superhydride systems, with critical temperatures (Tcs) that approach and exceed room temperature, has pushed the field to new heights. Unfortunately, this novel room-temperature superconductivity (RTS) requires pressures in excess of 260 GPa, inhibiting their application outside academia. One of the greatest challenges remaining in the field of superconductivity is inducing and retaining RTS while lowering or removing the pressure. As a potential solution, we developed a low-temperature, pressure-quenching technique which we successfully used to retain superconducting phases in Bi and also FeSe and CuxFe1-xSe. Quenching at 77 K and 4.2 K from pressures up to 26.6 GPa we retained Bi phases with varying Tcs corresponding to Bi III and V, as well as some ambiguous and/or novel phases. Similarly, we successfully retained superconducting phases with Tcs up to 37 K in FeSe and 27 K in CuxFe1-xSe. Furthermore, the retained superconducting phases of these materials exhibited good stability at low temperature. In particular, CuxFe1-xSe exhibited perfect stability for at least 7 days when quenched and kept at 77 K, retaining a Tc of ~25 K. |
Friday, March 18, 2022 10:00AM - 10:12AM |
Y69.00007: Random strain effects on the coupled magnetic and nematic transitions of iron-based superconductors William J Meese, Thomas Vojta, Rafael M Fernandes Magnetism and nematicity in the iron pnictides provide an interesting example of intertwined phases, with the vestigial nematic phase characterized by an order parameter that is composite in the primary magnetic order parameters. As a result, phenomena impacting one phase will inevitably be manifested in the other phase as well. For instance, structural disorder in the form of random strain, ubiquitously present in quantum materials, act simultaneously as a random nematic field and a random magnetic exchange. Due to its dual character, the impact of random strain is expected to go beyond the random-field Ising model effects that it enforces on the nematic degrees of freedom alone. Here, we propose the random-Baxter-field Ashkin-Teller model as a minimal model to capture the interplay between random strain, magnetism, and nematicity in the iron pnictides. Using Replica-Exchange Wang Landau Monte Carlo simulations, we find that random strain disorder introduces different length and time scales for the magnetic and nematic domains. Moreover, it induces magnetic correlations that are absent in the clean system, manifested by enhanced fluctuations with an emergent enlarged symmetry. Our results demonstrate that random strain disorder in intertwined phases leads to richer physics than in systems where the nematic order is the sole instability of the system. |
Friday, March 18, 2022 10:12AM - 10:24AM |
Y69.00008: Emergent symmetry at transition between intertwined vestigial orders in iron-based superconductors Ning Xi, Yiming Wang, Changle Liu, Jianda Wu, Qimiao Si, Rong Yu The phase diagram of iron-based superconductors consists of various electronic orders, including antiferromagnetic and symmetry related vestigial orders. They are highly entangled with superconductivity. Here we unveil the hidden Lie algebra among the nematic, charge C4, and chiral C4 vestigial orders. We further show that, because of the intertwined nature of these order parameters, the system exhibits an emergent U(1) symmetry at the first-order transition between the nematic and charge C4 phases. This enriched continuous symmetry |
Friday, March 18, 2022 10:24AM - 10:36AM |
Y69.00009: Hallmarks of nematicity in the spectral weight redistribution of Hund's metals Laura Fanfarillo, Angelo Valli, Massimo Capone The theoretical understanding of the nematic state of iron-based superconductors and especially of FeSe is still a puzzling problem. Although a number of experiments call for a prominent role of local correlations and identify iron superconductors as Hund's metals, the effect of the electronic correlations in the nematic state has been theoretically poorly investigated. Recent Fermi-liquid analysis predict that the lifted orbital degeneracy of the nematic state leads to different quasiparticle coherence factor for the yz/xz orbitals at the Fermi energy, however these techniques cannot provide any information about the orbital-dependent spectral weight redistribution at low-energy. In this work we use Dynamical Mean Field Theory to study the nematic phase of a multiorbital system characterized by strong electronic correlations. We distinguish the specific features characterizing the Hund's metal with respect to a standard correlated metal and connect our results with recent ARPES experiments on iron-based superconductors. |
Friday, March 18, 2022 10:36AM - 10:48AM |
Y69.00010: Controlling spin fluctuations and superconducting Tc in FeSe Mark van Schilfgaarde, Swagata Acharya, Dimitar Pashov FeSe is classed as a Hund's metal, with a multiplicity of d bands near the Fermi level. Correlations in Hund's metals mostly originate from the exchange parameter J, which can drive a strong orbital selectivity in the correlations. The Fe-chalcogens are the most strongly correlated of the Fe-based superconductors, with dxy the most correlated orbital. Yet little is understood whether and how such correlations directly affect the superconducting instability in Hund's systems. By applying a recently developed ab initio theory, we show explicitly the connections between correlations in dxy and the superconducting critical temperature Tc. The twin conditions of proximity of the dxy state to the Fermi energy, and the strength of J emerge as the primary criteria for incoherent spectral response and enhanced single- and two-particle scattering that in turn controls Tc. Using constrained RPA, we show further that FeSe in monolayer form (M-FeSe) provides a natural mechanism to enhance J. We explain why M-FeSe/STO has a high Tc, whereas M-FeSe in isolation should not. We also consider the role of nematicity in modifying the spin susceptibility and critical temperature. |
Friday, March 18, 2022 10:48AM - 11:00AM |
Y69.00011: Nematic electronic states under TS and superconducting mechanism near nematic QCP in FeSe family Hiroshi Kontani, Seiichiro Onari, Youichi Yamakawa, Shun Matsubara FeSe is an ideal platform to reveal the mechanism of nematiciy thanks to the rich reliable experimental data. Below TS, (i) the hole-pocket becomes vertically ellipsoidal and the electron pocket around X point becomes horizontally ellipsoidal, due to the orbital polarization Exz(k)-Eyz(k) with sign reversal [1]. In addition, recent ARPES experiments have revealed (ii) the electron pocket around Y point disappears below TS. To explain the whole nematic Fermi surface structure in FeSe, we perform the self-consistent analysis of the the nematic electronic states below TS. It is revealed that both (i) and (ii) are satisfactorily reproduced by the paramagnon interference processes [2]. In addition, enhancement of TS aroung nematic QCP is naturally explained by the present scenario [2]. Thus, both the nematiciy and the superconductivity are uniquely explained based on the paramagnon interference mechanism. |
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