Bulletin of the American Physical Society
APS March Meeting 2020
Volume 65, Number 1
Monday–Friday, March 2–6, 2020; Denver, Colorado
Session F61: Fe-Based Superconductors - Inhomogeneous Order ParameterFocus
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Sponsoring Units: DMP DCMP DCOMP Chair: Elena Gati, Ames Lab Room: Mile High Ballroom 4B |
Tuesday, March 3, 2020 8:00AM - 8:36AM |
F61.00001: A strongly inhomogeneous superfluid in an iron-based superconductor Invited Speaker: Milan Allan Although the possibility of spatial variations in the superfluid of unconventional, strongly correlated superconductors has been suggested, it is not known whether such inhomogeneities—if they exist—are driven by disorder, strong scattering or other factors. In this seminar, I will present our recent atomic-resolution Josephson scanning tunneling microscopy study that reveal a strongly inhomogeneous superfluid in an iron-based superconductor. By simultaneously measuring the topographic and electronic properties, we find that the inhomogeneity in the order parameter is not caused by structural disorder or strong inter-pocket scattering, and does not correlate with variations in the energy of the Cooper pair-breaking gap. Instead, we see a clear spatial correlation between the order parameter and the quasiparticle strength, defined as the height of the coherence peak, on a local scale. his result places iron-based superconductors on equal footing with copper oxide superconductors, where a similar relation has been observed on the macroscopic scale. When repeated at different temperatures, our technique could further help elucidate what local and global mechanisms limit the critical temperature in unconventional superconductors. Time permitting, I will further discuss our recent atomic-scale noise spectroscopy studies in conventional and unconventional superconductors [2,3]. |
Tuesday, March 3, 2020 8:36AM - 8:48AM |
F61.00002: Superconducting Fluctuations and Pairing Enhancement in Ultra-Thin FeSe/SrTiO3 Brendan Faeth, Shuolong Yang, Jason Kawasaki, Jocienne Nelson, Pramita Mishra, Chen Li, Darrell Schlom, Kyle M Shen A central challenge in understanding the Tc enhancement mechanism in FeSe/SrTiO3 monolayer films has been to decouple the myriad effects that the underlying substrate imposes on the superconductivity. Alkali surface-dosed multilayer films provide a natural comparison where the heavily electron-doped superconducting layer is constrained to the film-vacuum interface, analogous to the monolayer FeSe/STO interface but lacking any STO phonon contribution. To better understand the influence of the STO phonon contribution, we systematically explore the evolution of superconductivity as measured unambiguously by in situ electrical resistivity under varied conditions of film thickness, surface doping concentration, and substrate interface condition. In contrast to observations from spectroscopic probes which indicate a substantial enhancement in the pairing Tc, we observe only modest discrepancies in the zero-resistance temperature for surface doped layers in comparison to monolayer films. We demonstrate that this behavior is due to the shared influence of 2D fluctuation effects which act to suppress the resistive transition well below the cooper pair formation temperature, and discuss the implications of our results on the broader understanding of the FeSe/STO enhancement phenomenology. |
Tuesday, March 3, 2020 8:48AM - 9:00AM |
F61.00003: Nonlocal Correlations in Iron Pnictides and Chalcogenides Kristofer Bjornson, Shinibali Bhattacharyya, Karim Zantout, Andreas Kreisel, Roser Valenti, Brian M Andersen, Peter Hirschfeld Deviations of low-energy electronic structure of iron-based superconductors from density functional theory predictions have been parameterized in terms of band-dependent mass renormalizations and energy shifts. Theoretically, these have typically been described either in terms of a local self-energy within the framework of Dynamical Mean Field Theory, or in terms of nonlocal effects due to interband scattering. By calculating the renormalized bandstructure in both random phase approximation and the two-particle self-consistent approximation, we show that correlations in pnictide systems like LaFeAsO and LiFeAs can be described rather well by nonlocal correlations. In particular, Fermi pocket shrinkage as seen in experiment occurs along the paradigm of interband scattering scenario. We also show agreement with experimentally observed non-local scattering lifetime data for LiFeAs. Next, we compare with the canonical iron chalcogenide system FeSe in its bulk tetragonal phase, and show that band renormalizations are completely inconsistent with this picture. We discuss possible reasons for this discrepancy. |
Tuesday, March 3, 2020 9:00AM - 9:12AM |
F61.00004: Tunable impurity states in the unconventional superconductor FeTe0.55Se0.45 Damianos Chatzopoulos, Doohee Cho, Koen M Bastiaans, Genda Gu, Milan P. Allan When a single impurity interacts with an unconventional superconductor, localized bound states appear inside its superconducting gap. Interestingly, these states can be used as a tool to bring insight into the electronic states of the material at stake. In this study, we use a low temperature scanning tunneling microscope operating at ultra-high vacuum, to find impurity states in the iron-based unconventional superconductor FeTe0.55Se0.45 (Tc=14.5 K). In order to enhance the energy resolution, a superconducting tip (Pb-coated) was employed. We find that the energy of the states shifts spatially and can be tuned by changing the distance between the sample and our scanning probe at the impurity site. In this talk I will discuss possible mechanisms that can lead to this dispersive behavior of the impurity energy state in space. |
Tuesday, March 3, 2020 9:12AM - 9:24AM |
F61.00005: kz selective scattering within Quasiparticle Interference measurements of FeSe Luke Rhodes, Matthew D. Watson, Timur Kim, Matthias Eschrig Quasiparticle interference (QPI) provides a wealth of information relating to the electronic structure of a material. However, it is often assumed that this information is constrained to two-dimensional electronic states. Here, we show that this is not necessarily the case. For FeSe, a system dominated by surface defects, we show that it is actually all electronic states with negligible group velocity in the z-axis that are contained within the experimental data. By using a three-dimensional tight-binding model of FeSe, fit to photoemission measurements, we directly reproduce the experimental QPI scattering dispersion, within a T-matrix formalism, by including both kz = 0 and kz = π electronic states. This result unifies both tunnelling and photoemission based experiments on FeSe and highlights the importance of kz within surface-sensitive measurements of QPI. |
Tuesday, March 3, 2020 9:24AM - 9:36AM |
F61.00006: STM studies on FeSexTe1-x thin films grown on Bi2Te3 single crystal Guannan Chen, Lin Jiao, Jorge Olivares Rodriguez, Anuva Aishwarya, Lianyang Dong, Stephen Wilson, Vidya Madhavan The iron-based superconductor FeSexTe1-x has attracted intensive interest for the fundamental understanding of unconventional superconductivity. Here we report nanoscale studies on FeSexTe1-x films on Bi2Te3 single crystal. Three different kinds of heterostructures: fully embedded monolayer; half-embedded monolayer; and top monolayer were studied systematically using Scanning Tunneling Microscopy/Spectroscopy (STM/STS). The studies show different superconducting behaviors for the three different monolayers. In the half-embedded monolayer, we find an identical statistical distribution of superconducting gaps with similar gap size values to the ones in the bulk. Furthermore, on Bi2Te3 surface, a proximitized gap is observed with a uniform distribution. If time permits, we will present our measurements of Fourier transform quasiparticle interference (QPI) for studying the order parameter. |
Tuesday, March 3, 2020 9:36AM - 9:48AM |
F61.00007: Visualization of impurity-induced magnetism in FeSe using scanning tunneling microscopy Sang Yong Song, Jungpil Seo High temperature superconductivity (HTSC) is typically found in the vicinity of the magnetic ordering. As the magnetic ordering is reduced by various means including chemical doping, intercalation and applying pressure, spin fluctuations (SF) emerge which are believed to mediate the Cooper pairing between electrons. Therefore, understanding of emergence of magnetism in HTSC is one of the fundamental keys to reveal the secret of the unconventional superconductivity. For most iron-based superconductors, the parent state of materials shows stripe antiferromagnetic (AFM) ordering which breaks the C4 symmetry of the lattice. However, such long range magnetism is absent in FeSe which has been one of mysteries in iron-based superconductors. Recently, theories suggest FeSe is close to the magnetic quantum critical point and thus weak impurities could induce local magnetism in FeSe. Here, we show the local magnetism is directly induced by impurities of FeSe using scanning tunneling microscopy (STM). More interestingly, the induced magnetism was consistent with the (pi,0) magnetic ordering, showing that this material falls into the category of Hund’s metal in which orbital selectivity plays an important role. |
Tuesday, March 3, 2020 9:48AM - 10:00AM |
F61.00008: High-field superconducting phase in FeSe investigated by spectroscopic-imaging scanning tunneling microscopy Tetsuo Hanaguri, Tadashi Machida, Yuki Sato, Shigeru Kasahara, Takasada Shibauchi, Yuji Matsuda The iron-based superconductor FeSe is characterized by its small Fermi energy that is only several times larger than the superconducting gap amplitude. In such a situation, so-called Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) state with periodic nodal planes in the order parameter is expected near the upper critical field at low temperatures. In FeSe, magnetic-field dependence of thermal conductivity exhibits a cusp-like anomaly below the upper critical field, which is argued as a signature of the transition from the low-field normal vortex state to the high-field FFLO state [1]. We performed high-field spectroscopic-imaging scanning tunneling microscopy at an ultra-low temperature below 90 mK to investigate the change in the electronic state across the phase boundary. We found that the vortex image diminishes at about 14 T where the thermal conductivity shows a cusp. This result suggests that the nodal plane is pinned at the surface, if the high-field phase is an FFLO state. |
Tuesday, March 3, 2020 10:00AM - 10:12AM |
F61.00009: Probing antiferromagnetic order of iron-based superconductors through the surface Zhuozhi Ge, Qiang Zou, Mingming Fu, Shivani Rajput, Liurukara D Sanjeewa, Athena S. Sefat, Lian Li, Zheng Gai Understanding the surface structural and electronic properties of iron-based superconductors (FeSCs) is of great significance, as it can reveal the bulk superconducting properties. Previous study reveals a state near 0.2eV below Fermi level on √2×√2 or 2×1 reconstructed surface of Ba(Fe1-xCox)2As2, which is identified as a surface state by ARPES and DFT calculations. This state has also been observed on the surfaces of other 122 FeSCs with different doping levels, alkaline earth metal elements, and surface reconstructions. Its common presence suggests an origin other than simple surface state. Here, we systematically investigated low-temperature cleaved parent and slightly doped BaFe2As2 superconductors by scanning tunneling microscopy/spectroscopy (STM/S). We observed the same state on all √2×√2, 2×1 and even mixed surfaces. However, it vanishes when heating the sample above the Neel temperature (TN), and recovers after cooling down below TN, suggesting it is related to the bulk antiferromagnetic order. |
Tuesday, March 3, 2020 10:12AM - 10:24AM |
F61.00010: Investigation of the FeSe/SrTiO3 interface using Atom Probe Tomography Samantha O'Sullivan, Ruizhe Kang, Christian Matt, Jason Hoffman, Jennifer E. Hoffman Monlayer FeSe on SrTiO3 (FeSe/STO) is a high temperature superconductor with Tc above 100 K. Although scanning tunneling microscopy (STM) imaging has characterized its surface in detail, the content and behavior in the FeSe/STO interface remains unclear. Characterizing the interface is important to gain further understanding on the mechanism for the unexpected high-temperature superconductivity in this system. Techniques such as scanning transmission electron microscopy (STEM) have been used to demonstrate that the interface structure is correlated with the material’s superconducting properties, however much remains unclear about the specific composition and behavior of the interface. Here we characterize the interface using atom probe tomography (APT), which reveals the structural and chemical properties of the FeSe/STO interface and provides insight into the origin of the material’s superconductivity. |
Tuesday, March 3, 2020 10:24AM - 10:36AM |
F61.00011: Imaging Nematic Transitions in Iron-Pnictide Superconductors with a Quantum Gas Fan Yang, Stephen Taylor, Stephen D Edkins, Johanna Palmstrom, Ian Fisher, Benjamin L Lev The SQCRAMscope is a recently realized Scanning Quantum CRyogenic Atom Microscope that utilizes an atomic Bose-Einstein condensate to measure magnetic fields emanating from solid-state samples. Here, we combine the SQCRAMscope with an in situ microscope that measures optical birefringence near the surface of a sample to study iron-pnictide superconductors, where the relationship between electronic and structural symmetry-breaking resulting in a nematic phase is under debate. We conduct simultaneous and spatially resolved measurements of both bulk and surface manifestations of nematicity via transport and structural deformation channels, respectively. By performing the first local measurement of emergent resistivity anisotropy in iron pnictides, we observe a spatially inhomogeneous increase in the temperature at which optical birefringence appears near the surface over that at which anisotropic local transport appears within the bulk. This is consistent with the existence of a higher-temperature surface nematic transition, albeit one that emerges inhomogeneously. More broadly, these measurements demonstrate the SQCRAMscope's ability to reveal important insights into the physics of complex quantum materials. |
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F61.00012: Manipulating superconductivity of monolayer FeSe on graphene/SiC(0001) surface Wan-tong Huang, Haicheng Lin, Cheng Zheng, Yuguo Yin, Xinqiang Cai, Shuai-Hua Ji, Xi Chen Compared with those of bulk FeSe, the sizes of electron and hole pockets become smaller in monolayer FeSe because the absence of inter-layer coupling tends to narrow the energy band and decrease the overlap between electron and hole pockets in energy. For FeSe monolayer grown on graphene/SiC(0001) surface, we find the Fermi energy is in the range of a few meV. In addition, we demonstrate an approach to tuning the Fermi energy by the thickness of graphene layers. At the base temperature of STM, the film grown on trilayer graphene becomes superconducting with Δ=0.62meV. In contrast, no signature of superconductivity has been detected in the dI/dV spectra of FeSe monolayer on bilayer graphene. The FeSe monolayer on graphene/SiC(0001) has the potential to study the intriguing physics where the Fermi energy, the superconducting gap and the Zeeman energy in magnetic field are in the same order of magnitude. |
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