Bulletin of the American Physical Society
APS March Meeting 2023
Volume 68, Number 3
Las Vegas, Nevada (March 5-10)
Virtual (March 20-22); Time Zone: Pacific Time
Session A28: Fe-Based Superconductors: Surface State, Thin Film, Etc. |
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Sponsoring Units: DMP Chair: Lingyuan Kong, California Institute of Technology Room: Room 220 |
Monday, March 6, 2023 8:00AM - 8:12AM |
A28.00001: Decoupling Interfacial Contributions to the Enhanced Superconductivity in FeSe/SrTiO3 Brendan D Faeth, Chad Mowers, Yaoju Tarn, Paul Malinowski, Shuolong Yang, Darrell G Schlom, Kyle M Shen The discovery of greatly enhanced superconductivity at the interface between FeSe and SrTiO3 has attracted enormous interest due to the potential of enhancing superconductivity through interfacial interactions. In our previous work, we confirmed the presence of intrinsic interfacial coupling of electrons in the FeSe layer to optical phonons in the adjacent SrTiO3 substrate [1], but a clear causative connection between this phonon coupling and the apparent enhanced Tc enhancement remains unestablished. In order to better understand the true contribution of interface phonon coupling on the resultant high-Tc state, we consider a highly analogous system: alkali surface-doped FeSe films. This approach produces a controllably electron doped superconducting layer constrained to the film-vacuum interface, analogous to the FeSe/STO interface but lacking any phonon coupling effect. Here, using a simultaneous combination of in situ electrical resistivity and ARPES, we systematically explore the evolution of superconductivity as a function of surface doping concentration at this coupling-free interface. Interestingly, we observe only modest discrepancies in the superconducting Tc and high-temperature pseudogap signatures of optimally surface-doped layers in comparison to interfacial FeSe/STO films. We discuss the implications of this behavior on the broader understanding of FeSe/STO phenomenology. |
Monday, March 6, 2023 8:12AM - 8:24AM |
A28.00002: Towards scanning tunneling spectroscopy of iron-based superconducting van der Waals heterostructures Lingyuan Kong, Hyunjin Kim, Haoxin Zhou, Kenji Watanabe, Takashi Taniguchi, Genda Gu, Stevan Nadj-Perge Iron-based superconductors (FeSCs) set a new paradigm of unconventional superconductivity, owing to their multiorbital nature. Since the discovery of FeSCs, a range of exotic phenomena, including Hund's correlation, magnetism, nematicity, quantum criticality, and non-trivial topology, alongside superconductivity, have been established in this material platform. Despite the significance of both fundamental physics and potential applications on quantum technology, previous measurements were mainly focused on studying bulk crystals, and so far, potentially important quantum effects in finite-size samples have remained to a large degree unexplored. In this talk, we will discuss our efforts to measure exfoliated thin flakes of Fe(Te,Se) using scanning tunneling microscopy and spectroscopy that reveal the presence of multiple well resolved superconducting gaps highlighting high sample quality. Our approach provides a novel opportunity to acquire deeper insights into the nature of the superconductivity of this system and provides a necessary step for potentially controlling Majorana zero mode hosted in superconducting vortices. |
Monday, March 6, 2023 8:24AM - 8:36AM |
A28.00003: ``Universal"Δmax /Tc in Fe-based Superconductors siddhant k panda, Peter Hirschfeld Iron-based superconductors display a large degree of variability in electronic structure at the Fermi surface, resulting in superconducting gap structures, Tc's, and other properties that vary considerably from family to family. Recently it was noted that across many different families of Fe-based systems, the ratio Δmax /Tc is found to be quasi-universal with a large value of ∼3.5 compared to the one-band BCS weak-coupling result of 1.76. Here Δmax is the measured maximum gap across the Fermi surface. This remarkable fact was attributed to strong-coupling effects arising from Hund's metal physics. Here we perform a ``high-throughput" scan across band masses and interaction parameters in a weak-coupling Suhl-Matthias-Walker model. We find that unexpectedly large values of Δmax /Tc can be achieved within weak coupling, and that quasiuniversal behavior similar to experiment emerges for those systems where interband interactions dominate intraband ones. However, within the current framework, a large mass contrast between bands is required. |
Monday, March 6, 2023 8:36AM - 8:48AM |
A28.00004: Thickness-dependent strain patterning in FeSe films and visualization of electronic nematic domains via orbital-selective tunneling Zheng Ren, Hong Li, He Zhao, Shrinkhala Sharma, Ilija Zeljkovic Understanding the interplay of structural and electronic symmetry breaking in Fe-based high temperature superconductors remains of high interest. In this work we grow strain-patterned multilayer FeSe thin films in a range of thicknesses using molecular beam epitaxy. We study the formation of electronic nematic domains and spatially-varying strain using scanning tunneling microscopy and spectroscopy. We directly visualize the formation of edge dislocations that give rise to a two-dimensional edge dislocation network in the films. Interestingly, we observe a 45 degree in-plane rotation of the dislocation network as a function of film thickness, yielding antisymmetric strain along different directions. This results in different coupling ratios between electronic nematic domains and antisymmetric strain. Lastly, we are able to distinguish between different orthogonal nematic domains by revealing a small energy-dependent difference in differential conductance maps between the two regions. This could be explained by orbital-selective tip-sample tunneling. Our observations bring new insights into the dislocation network formation in epitaxial thin films and provide another nanoscale tool to explore electronic nematicity in Fe-based superconductors. |
Monday, March 6, 2023 8:48AM - 9:00AM |
A28.00005: Microwave spectroscopy of Majorana vortex modes Zhibo Ren, Justin Copenhaver, Jukka Vayrynen Majorana zero modes have been predicted to be hosted by vortices of certain iron-based topological superconductors. Recent observations of zero-bias conductance peaks in vortex cores of such superconductors have sparked renewed interest in vortex-bound Majorana states. However, progress toward a vortex-based topological qubit is hindered by our inability to measure the topological quantum state of a non-local vortex Majorana state, i.e., the charge of a vortex pair. In this paper, we theoretically propose a microwave-based charge parity readout of the Majorana vortex pair charge. We study the coupling between electrons in the iron-based superconductor and the photons from a microwave resonator above the such superconductor. The frequency-dependent transmission of the resonator allows for a dispersive readout of the Majorana parity. Our technique may also be used in vortices in conventional superconductors, and allows one to probe the lifetime of vortex-bound quasiparticles, which is currently beyond existing scanning tunneling microscopy capabilities. |
Monday, March 6, 2023 9:00AM - 9:12AM |
A28.00006: Understanding the interplay between superconductivity and atomic-scale interface structure of multilayer FeSe /SrTiO3 Salva Salmani-Rezaie, Brendan D Faeth, Chad Mowers, Yaoju Tarn, Paul Malinowski, Kyle M Shen, David A Muller The discovery of greatly enhanced high-temperature superconductivity in FeSe /SrTiO3 has made this system an attractive platform to explore the science of high-Tc materials. While the exact mechanism of Tc enhancement remains not fully understood, interface structure and interfacial effects are believed to play a critical role. A better understanding of the atomic interface structure of FeSe/SrTiO3 and its correlation with Tc would clarify the role of the interface and constrain theoretical approaches for explaining Tc enhancement. Here, using high-angle annual dark-field scanning transmission electron microscopy and electron energy loss spectroscopy, we directly compare the interface structure of superconducting and non-superconducting FeSe/SrTiO3 films prepared on a range of SrTiO3 surfaces. We find that while a uniform SrTiO3 surface reconstruction tends to improve the quality of growth, superconductivity is not affected by mixed surface termination. We report on the statistical analysis of the distance of FeSe and surface TiO2 for both superconducting and non-superconducting states. We further discuss the role of interface Se on the superconductivity of FeSe in the context of interfacial charge transfer. |
Monday, March 6, 2023 9:12AM - 9:24AM |
A28.00007: Superconductivity and Upper Critical Field Behavior in FeSe Thin Films Max C Stanley, Yanan Li, Johanna Palmstrom, Ross D McDonald, Scott A Crooker, Nandini Trivedi, Nitin Samarth The Fe-chalcogenide superconductors, FeSe and Fe(Se,Te), have attracted significant interest in recent years because of enhanced superconducting critical temperatures in ultrathin films of FeSe on SrTiO3 and the potential for topological superconductivity in Fe(Te,Se). A thorough study of the upper critical field, Hc2, in these materials is important for obtaining a comprehensive understanding of superconductivity in this context. Here, we present an experimental study of the temperature dependence of Hc2, obtained in static magnetic fields up to 14 T and pulsed magnetic fields up to 65 T, in few-layer FeSe and Fe(Te,Se) films grown by molecular beam epitaxy on SrTiO3 substrates. We primarily focus on the measurement and analysis of the temperature dependence of Hc2 in the context of Werthamer-Helfand-Hohenberg (WHH) theory for several FeSe samples with varying degrees of disorder and critical temperatures. We observe a distinct departure from WHH theory at low temperatures and comment on the potential implications of this observation. Finally, we will report on measurements of Hc2 in Fe(Te,Se) as the Te concentration approaches the proposed ideal stoichiometry FeTe0.55Se0.45 for topological superconductivity. |
Monday, March 6, 2023 9:24AM - 9:36AM |
A28.00008: Gapless superconductivity observed in Fe(Te,Se) thin films and heterostructures Jonathan R Stensberg, Xiong Yao, Xiaoyu Yuan, Seongshik Oh, Liang Wu Using time-domain terahertz spectroscopy, we observe a gapless superconducting state in Fe(Te,Se) epitaxial thin films and in Fe(Te,Se) heterostructures with Bi2Te3 and MnTe grown by hybrid symmetry epitaxy. Clear 1/ω behavior arises in the imaginary conductance below the superconducting transition; however, there is no concomitant suppression of the real conductance, indicating a gapless superconducting state. Furthermore, a sharp, low-frequency peak in the real conductance emerges alongside the superconductivity. We model these anomalous features of the superconducting state to determine the origin of this novel behavior. |
Monday, March 6, 2023 9:36AM - 9:48AM |
A28.00009: Determining alloy composition of single-layer iron chalcogenides by machine learning of STM/STS data Basu D Oli, Qiang Zou, Huimin Zhang, Lian Li Chemical pressure is an effective tool to tune the properties of quantum materials, in particular at the single-layer limit. In this work, we investigated the effect of chemical pressure in single-layer FeSe films grown on SrTiO3(001) substrate by molecular beam epitaxy. While both positive and negative pressure can be applied with Se substituted by isovalent S and Te, one of the key parameters that need to be precisely determined is the alloy composition, thus, the magnitude of the pressure. Scanning tunneling microscopy (STM) imaging has been a powerful tool for this task; however, contrasts associated with interfacial inhomogeneity and defects make the determination by visualization challenging, if not impossible. Here, we utilize machine learning to distinguish between S(Te) and Se atoms in STM images. First, defect locations are identified by analyzing spatially dependent dI/dV tunneling spectra using the K-means method. Next, after excluding the defect regions, dI/dV spectra are further analyzed using the singular value decomposition (SVD) method to determine the Se/S(Te) ratio, which was then correlated with the STM images. This approach has been applied to analyze the alloy composition of various Te and S substituted FeSe films, which has played a critical role in the investigation of their superconducting properties. |
Monday, March 6, 2023 9:48AM - 10:00AM |
A28.00010: Calculations of the local electronic structure of a single isoelectronic impurity: boron in gallium arsenide Julian Zanon, Michael E Flatté, Paul M Koenraad The ability to change the band gap and electronic structure with very low doping levels of isoelectronic atoms has led to extensive study of highly mismatched alloys (HMAs) of III-V materials based on nitrides, such as GaAsN in the last few decades [1]. However, HMAs with different group III elements such as BGaAs, which could be used in Si based devices [2], continue to be less investigated. Based on multiband tight-binding Green’s function calculations, successfully used in the past [3] for single isoelectronic atoms in HMAs, here we calculate the local density of states (LDOS) for a single B dopant in GaAs. Comparing with X-STM measurements [4], the LDOS are analyzed as function of the energy and position. The results confirm a principal anion-like contribution to the LDOS and discuss the energy position at which the LDOS peak from the dopant emerges. These results are compared with previous results for N in GaAs, which has a cation-like contribution to the LDOS, highlighting the differences between the two systems. |
Monday, March 6, 2023 10:00AM - 10:12AM |
A28.00011: Charge Transitions in Rare Earth-Vacancy- Defect Complexes in MgO for Optical Memories Swarnabha Chattaraj, Supratik Guha, Giulia Galli The realization of high-density and low power addressable memories is a major goal to accommodate the ever-increasing demands of computational resources. In principle, the use of rare earth (REs) atoms provides a way to increase memory density by exploiting the high degree of wavelength multiplexing within a diffraction limited memory voxel. In this work we theoretically explore a novel class of defect complexes of RE- native vacancies in oxides for creating optically addressable long-lived charged states suitable to build ultra-dense atomic memories. In particular, we address a critical question, namely the influence of the distance between the RE and the vacancy on (1) the charge transition levels of the complex, and (2) the near field non-radiative energy transfer (NRET) occurring via the Forster-Dexter mechanism. We focus on Erbium- VO complexes in MgO and using Density Functional Theory and the Quantum Espresso Code we show that the presence of the Er substitutional sites results in significant (~3%) lattice distortions. In addition it results into charge redistribution affecting charge transition energies and dipoles of the states localized in proximity of the vacancy. Quantum analysis of the NRET process and the optical selection rules at the near field will be discussed, which provide key physical insights towards various processes relevant for classical and quantum optical memory applications. |
Monday, March 6, 2023 10:12AM - 10:24AM |
A28.00012: Ensemble Hubbard-U Correction for Improved Transition Temperatures for Spin-Crossover Materials Angel M Albavera Mata, Richard G Hennig, Samuel B Trickey Thermally induced transitions in spin-crossover condensed phases are attractive for use in data display technologies, actuators [Science 279, 44 (1998)] or quantum gates [Dalton Trans. 44, 17819 (2015)]. The problem at hand is that calculation of transition temperatures and crossover profiles is challenging, even with state-of-the-art density functional approximations. The main difficulty is in computing accurate energetic differences between the meta-stable spin states for these types of metal-organic complexes [Comput. Mat. Sci. 206, 111161 (2022)]. We present a novel ensemble-average determination of a Hubbard-U correction on a reference spin-state and demonstrate that the method improves adiabatic total energy differences and thermodynamic properties of crossover aggregates. We find that the U values using this correction alleviate the overestimation of transition temperatures by as much as 90% with respect to the uncorrected results for a commonly used generalized gradient approximation. |
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