Bulletin of the American Physical Society
APS March Meeting 2017
Volume 62, Number 4
Monday–Friday, March 13–17, 2017; New Orleans, Louisiana
Session Y26: Fe-based Superconductivity: FeSe Monolayers |
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Sponsoring Units: DMP DCMP DCOMP Chair: Brian Andersen, Niels Bohr Institute Room: 289 |
Friday, March 17, 2017 11:15AM - 11:27AM |
Y26.00001: Abstract Withdrawn
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Friday, March 17, 2017 11:27AM - 11:39AM |
Y26.00002: First-principles study of magnetic interactions in monolayer FeSe Tatsuya Shishidou, Michael Weinert, Daniel Agterberg Due to its high-temperature superconductivity, the monolayer FeSe has received considerable attention. Although long-range magnetic order has not been observed, magnetic interactions among local moments of Fe may play important role in the superconducting mechanism. Based on the density-functional theory (DFT) calculations, a large number of magnetic configurations are explored by using supercell and reciprocal-space approaches. Spin models for the monolayer and bulk FeSe, which reasonably account for the DFT results, are constructed. [Preview Abstract] |
Friday, March 17, 2017 11:39AM - 11:51AM |
Y26.00003: ARPES Study of Interfacial Superconductivity of Monolayer FeSe Tao Jia, Slavko Rebec, Chaofan Zhang, Makoto Hashimoto, Donghui Lu, Robert Moore, Zhi-Xun Shen Ever since the first realization of interfacial superconductors, the low dimensionality and interface effect have created new possibilities to the research of superconductivity. In this talk, I will talk about the mechanism for the enhanced superconductivity of monolayer FeSe on different substrates. We analyze the effects of substrates to the superconductivity of monolayer FeSe by substrate engineering in the molecular beam epitaxy (MBE) growth and angle-resolved photoemission spectroscopy (ARPES) measurement. [Preview Abstract] |
Friday, March 17, 2017 11:51AM - 12:03PM |
Y26.00004: Abstract Withdrawn
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Friday, March 17, 2017 12:03PM - 12:15PM |
Y26.00005: Enhanced superconductivity accompanying a Lifshitz transition in electron-doped FeSe monolayer Xun Shi, Pierre Richard, Zhiqing Han, Xiliang Peng, Tian Qian, Xianxin Wu, Mingwei Qiu, Shancai Wang, Jiangping Hu, Yujie Sun, Hong Ding The origin of enhanced superconductivity over 50 K in the recently discovered FeSe monolayer films grown on SrTiO$_3$ (STO), as compared to 8 K in bulk FeSe, is intensely debated. As with the ferrochalcogenides A$_x$Fe$_{2-y}$Se$_2$ and potassium doped FeSe, which also have a relatively high superconducting critical temperature ($T_c$), the Fermi surface (FS) of the FeSe/STO monolayer films is free of hole-like FS, suggesting that a Lifshitz transition by which these hole FSs vanish may help increasing $T_c$. However, the fundamental reasons explaining this increase of $T_c$ remain unclear. Here we report a 15 K jump of $T_c$ accompanying a second Lifshitz transition characterized by the emergence of an electron pocket at the Brillouin zone (BZ) centre, which is triggered by high electron doping following \textit{in-situ} deposition of potassium on FeSe/STO monolayer films. Our results suggest that the pairing interactions are orbital-dependent with the $d_{xy}$ orbital playing a determining role in generating enhanced superconductivity in FeSe. [Preview Abstract] |
Friday, March 17, 2017 12:15PM - 12:27PM |
Y26.00006: Screening effects in FeSe/SrTiO$_3$ Yuanjun Zhou, Andrew Millis Monolayer films of FeSe grown on SrTiO$_3$ substrates exhibit a significantly higher superconducting transition temperatures ($T_c$). The enhancement of $T_c$ has been proposed to arise from an interaction of electrons in the FeSe layer with SrTiO$_3$ phonons. We systematically study the electron-phonon interaction in the FeSe/SrTiO$_3$ system, investigate the SrTiO$_3$ longitudinal optical phonon mediated attractive potential, and calculate the electronic screening effect to the random phase approximation level. We find that since the longitudinal phonons live in a wide depletion region in SrTiO$_3$ substrate, they are not effectively screened by the electron fluctuations in the FeSe film. This unscreened attractive potential may induce a high $T_c$ superconductivity. The plasmon and collective mode spectrum is also calculated. [Preview Abstract] |
Friday, March 17, 2017 12:27PM - 12:39PM |
Y26.00007: Nematic Order, Superconductivity and their Evolution with Doping in Single-Layer FeSe/SrTiO$_{\mathrm{\mathbf{3}}}$\textbf{ Films} Yong HU, Xingjiang Zhou, Qikun Xue, Zuyan Xu, Chuangtian Chen The discovery of high temperature superconductivity with a T$_{\mathrm{C}}$ higher than 65K in single-layer FeSe films epitaxially-grown on SrTiO$_{\mathrm{3}}$ substrate (FeSe/STO films) has attracted much attention. The origin of such a dramatic Tc enhancement remains to be further investigated. While the nematic state has been reported in bulk FeSe and multi-layer FeSe/STO films, its presence in the single-layer FeSe/STO films remains under debate. In this talk, we will present our high-resolution angle-resolved photoemission investigations on the electronic structure evolution of single-layer FeSe/STO films with electron doping by \textit{in-situ} potassium deposition. We have confirmed the existence of a nematic order in single-layer FeSe/STO films at low electron doping. By increasing electron doping, the nematic order is suppressed and superconductivity emerges. Our results establish a unified picture about the nematicity in bulk FeSe, multi-layer FeSe/STO films, and single-layer FeSe/STO films. The relation between nematicity and superconductivity in single-layer FeSe/STO films will be discussed. [Preview Abstract] |
Friday, March 17, 2017 12:39PM - 12:51PM |
Y26.00008: Studies of single layer FeSe nanoribbons on SrTiO$_{\mathrm{\mathbf{3}}}$ Zhuozhi Ge, Chenhui Yan, Daniel Agterberg, Michael Weinert, Lian Li Nanoribbons of single layer FeSe are grown on SrTiO$_{\mathrm{3}}$ substrates by molecular beam epitaxy, and their topographic and electronic properties studied by in situ scanning tunneling microscopy/spectroscopy. Systematic tunneling spectroscopy investigation carried out at 6K shows a superconducting gap of 20 meV for ribbons larger than 5 nm. Furthermore, edge states are also detected at the edges of these ribbons, suggesting the coexistence of topological and superconducting states in single layer FeSe/ SrTiO$_{\mathrm{3}}$. [Preview Abstract] |
Friday, March 17, 2017 12:51PM - 1:03PM |
Y26.00009: Study of the Superconductivity Enhancement at FeSe/SrTiO$_{\mathrm{3}}$ Interface from the phonon perspective. Shuyuan Zhang, Jiaqi Guan, Xun Jia, Bing Liu, Weihua Wang, Jiandi Zhang, Ward Plummer, Xuetao Zhu, Jiandong Guo The significant role of interfacial coupling in the superconductivity enhancement in FeSe films on SrTiO$_{\mathrm{3}}$ has been widely recognized. But the explicit origination of this coupling is yet to be identified. By surface phonon measurements using high resolution electron energy loss spectroscopy, both FeSe and SrTiO$_{\mathrm{3}}$ phonons were observed and studied. The observation of SrTiO$_{\mathrm{3}}$ surface phonons indicates the electric field generated by Fuchs-Kliewer (F-K) phonon modes of SrTiO$_{\mathrm{3}}$ can penetrate into FeSe films and strongly interact with electrons therein. With increasing FeSe thickness, the penetrating field intensity decays exponentially, which matches well the observed exponential decay of the superconducting gap. The surface Debye temperature (\textasciitilde 250 K) and FeSe phonons do not exhibit difference between films with different thicknesses. Thus FeSe phonons do not participate in the interfacial enhancement of superconductivity. It is unambiguously shown that, besides charge transfer, the SrTiO$_{\mathrm{3}}$ F-K phonon penetrating into FeSe is also essential in the interfacial superconductivity enhancement. [Preview Abstract] |
Friday, March 17, 2017 1:03PM - 1:15PM |
Y26.00010: Orbitally resolved superconductivity within FLEX: doping evolution of the FeSe monolayer Andreas Linscheid, Yan Wang, Saurabh Maiti, Steven Johnston, Peter Hirschfeld FeSe-derived materials have been studied by several recent experiments concerning a number of phenomena that still lack a concise explanation. First, the nature of the pairing in materials with only electron pockets at the Fermi level is still under debate, with proposed explanations by both electronic $s_{++}$ and $s_\pm$ and/or phononic pairing. Second, the effective quasi particle mass at the Fermi level is very orbital dependent which likely affects the pairing and may require to solve the superconducting (SC) pairing in the space of orbitals. The effective mass is bound to have consequences for the spin- and charge fluctuations and the problem should therefore be solved self-consistently. In this work, we extend our previous study [PRL 117, 077003] and describe the electron doped FeSe in an orbitally resolved microscopic model. Starting from the normal state DFT band structure, we apply orbitally resolved FLEX to study the system in the SC state, as well as in the magnetic phase. By retaining full momentum resolution, we can also include strong forward scattering by phonons in the FeSe/STO system and discuss the self-consistent influence of the orbitally-resolved quasi particle mass on SC as a function of doping. [Preview Abstract] |
Friday, March 17, 2017 1:15PM - 1:27PM |
Y26.00011: Effect of nonmagnetic impurities on $s_\pm$ superconductivity in the presence of incipient bands Xiao Chen, Vivek Mishra, Saurabh Maiti, Peter Hirschfeld Several Fe chalcogenide superconductors without hole pockets at the Fermi level display high temperature superconductivity, in apparent contradiction to naive spin fluctuation pairing arguments. Recently, scanning tunneling microscopy measurements have measured the influence of impurities on some of these materials, and claimed that non-magnetic impurities do not create in-gap states, leading to the conclusion that the gap must be $s_{++}$, i.e. conventional $s$ wave with no gap sign change. Here we present various ways sign-changing gaps can be consistent with the absence of such bound states. In particular, we calculate the bound states for an $s_\pm$ system with a hole pocket below the Fermi level, and show that the nonmagnetic impurity bound state energy generically tracks the gap edge in the system, thereby rendering it unobservable. A failure to observe a bound state in the case of a nonmagnetic impurity can therefore not be used as an argument to exclude sign-changing pairing states. [Preview Abstract] |
Friday, March 17, 2017 1:27PM - 1:39PM |
Y26.00012: Sensitivity of gap symmetry to an incipient band: Application to iron based superconductors vivek mishra, Douglas Scalapino, Thomas Maier Observation of high temperature superconductivity in iron-based superconductors with a submerged hole band has attracted wide interest. A spin fluctuation mediated pairing mechanism has been proposed as a possible explanation for the high transition temperatures observed in these systems. Here we discuss the importance of the submerged band in the context of the gap symmetry. We show that the incipient band can lead to an attractive pairing interaction and thus have significant effects on the pairing symmetry. We propose a framework to include the effect of the incipient band in the standard multi-orbital spin-fluctuation theories which are widely used for studying various iron-based superconductors. [Preview Abstract] |
Friday, March 17, 2017 1:39PM - 1:51PM |
Y26.00013: Non-equilibrium dynamics of forward scattering superconductors Alexander Kemper Recent experiments have observed a surprisingly high critical temperature in a monolayer of FeSe on SrTiO3 an substrate. Among the proposed origins of this effect is enhancement or determination of the superconducting phase by a phonon within the SrTiO3 substrate, whose effect on the FeSe monolayer is to provide a source of forward scattering. It remains an open question whether such a \textit{forward scattering superconductor} is realized in the FeSe/SrTiO3 system. Using a non-equilibrium Keldysh approach, we study the effect of a pump laser field on superconducting phase of a model forward scattering superconductor. We compare the resulting temporal dynamics to those of a more typical BCS-like \textit{isotropic} superconductor, where the phonon scattering is not as limited. We demonstrate how pump-probe experiments can be used to distinguish the two types of superconductors under consideration. [Preview Abstract] |
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