Bulletin of the American Physical Society
APS March Meeting 2015
Volume 60, Number 1
Monday–Friday, March 2–6, 2015; San Antonio, Texas
Session Z51: Invited Session: FeSe/Oxide Interface Superconductivity |
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Sponsoring Units: DCMP Chair: Qimiao Si, Rice University Room: Grand Ballroom C1 |
Friday, March 6, 2015 11:15AM - 11:51AM |
Z51.00001: Significant Tc enhancement in FeSe films on SrTiO3 due to interfacial mode coupling Invited Speaker: Robert Moore The enhanced superconductivity in monolayer FeSe films grown on SrTiO3 (STO) substrates has generated a lot of attention. Several previous studies have shown a dramatic difference between the superconducting monolayer and non-superconducting bilayer films suggesting the substrate or interface plays a critical role in the enhancement of superconductivity. Utilizing Molecular Beam Epitaxy (MBE) and in situ Angle Resolved Photoemission Spectroscopy (ARPES), we explore the role of the substrate on the electronic structure of superconducting monolayer FeSe films. The electronic structure at the Fermi surface of the monolayer film consists of two electron pockets at the M point of the Brillouin zone, in contrast to bilayer and thicker films that show an electronic structure similar to bulk FeSe. The orbital character of the two pockets has been resolved which shows two distinct gaps and places strict constraints on the possible pairing symmetry. Surprisingly, we observe exact replicas of the FeSe bands which are attributed quantum shakeoffs arising from strong electron phonon coupling. Shakeoff bands have never been resolved with such clarity in a solid before and are only observed in the monolayer films. The coupling is between a high energy oxygen phonon in the STO substrate and electrons in the FeSe monolayer. Oxide MBE has been used to grow STO with oxygen 18 to explore the isotope effect on the electron phonon coupled shakeoff bands and will be discussed. Theoretical investigations show how these distinct features result from strong forward scattering which enhances superconductivity. This enhancement does not depend on the origins of superconductivity as it exists in all channels and is responsible for the increase in Tc. These results suggest a possible avenue for engineering superconductors with higher Tc. [Preview Abstract] |
Friday, March 6, 2015 11:51AM - 12:27PM |
Z51.00002: Interface-induced high-temperature superconductivity in FeSe/TiO$_2$(001) heterostructure Invited Speaker: Hao Ding The recently discovered high transition temperature ($T_c$) superconductivity at the interface of single unit-cell (UC) FeSe films on SrTiO$_3$(001) has generated considerable excitement [1,2], which may eventually lead to the discovery of a new family of high-$T_c$ superconductors at many different interfaces. In this talk, we will present our recent work on a new interfacial system with high-$T_c$ superconductivity, 1 UC FeSe films on anatase TiO$_2$(001). By using molecular beam epitaxy (MBE) techniques, we have successfully prepared high-quality 1 UC FeSe films on anatase TiO$_2$(001) formed on SrTiO$_3$. \textit{In situ} scanning tunneling spectroscopy (STS) reveals large superconducting gap ($\Delta$) ranging from 17 meV to 22 meV, which is nearly one order of magnitude larger than $\Delta$ = 2.2 meV of bulk FeSe with $T_c$ = 9.4 K, indicating the signature of high-$T_c$ superconductivity. The superconductivity of this heterostructure system is further verified by imaging vortex lattice under external magnetic field. By examining the distinct properties of anatase TiO$_2$ from SrTiO$_3$, as well as their influences on superconductivity, we will also discuss about the possible pairing mechanism of this system. Together with our previous work of 1 UC FeSe/SrTiO$_3$ [1,2], this work demonstrates that interface engineering is a powerful way to fabricate new high-$T_c$ superconductors and investigate the mechanism of high-$T_c$ superconductivity.\\[4pt] [1] Q.-Y. Wang \textit{et al.}, \textit{Chin. Phys. Lett.} \textbf{29,} 037402 (2012).\\[0pt] [2] W.-H. Zhang \textit{et al.}, \textit{Chin. Phys. Lett.} \textbf{31,} 017401 (2014). [Preview Abstract] |
Friday, March 6, 2015 12:27PM - 1:03PM |
Z51.00003: Tuning the superconductivity in single-layer FeSe/oxides by interface engineering Invited Speaker: Rui Peng The discovery of high T$_c$ in single-layer FeSe films has enormous implications for both searching new high Tc superconductors and exploring the important factors for high temperature superconductivity. In this talk, I will show our recent angle-resolved photoemission studies on various FeSe-based heterostructures grown by molecular beam epitaxy. We systematically studied the electronic structures and superconducting properties of FeSe with varied strain, different interfacial oxide materials and different thicknesses, and uncover that electronic correlations and superconducting gap-closing temperatures are tuned by interfacial effects. We exclude the direct relation between superconductivity and tensile strain, or the energy of an interfacial phonon mode, and demonstrate the crucial and non-trivial role of FeSe/oxide interface on the high pairing temperature. By tuning the interface, superconducting pairing temperature reaches up to 75K in FeSe/Nb:BaTiO$_3$/KTaO$_3$ with the in-plane lattice of 3.99~\AA~, which sets a new superconducting-gap-closing temperature record for iron-based superconductors, and may paves the way to more cost-effective applications of ultra-thin superconductors. Besides, in extremely tensile-strained single-layer FeSe films, we found that the Fermi surfaces consist of two elliptical electron pockets at the zone corner, without detectable hybridization. The lifting of degeneracy is clearly observed for the first time for the iron-based superconductors with only electron Fermi surfaces. Intriguingly, the superconducting gap distribution is anisotropic but nodeless around the electron pockets, with minima at the crossings of the two pockets. Our results provide important experimental foundations for understanding the interfacial superconductivity and the pairing symmetry puzzle of iron-based superconductors, and also provide clues for further enhancing T$_c$ through interface engineering. [Preview Abstract] |
Friday, March 6, 2015 1:03PM - 1:39PM |
Z51.00004: First-principles study of the electron/spin-phonon interaction in compressed FeSe crystal and FeSe/STO system Invited Speaker: Zhong-Yi Lu By using the first-principles calculations with van der Waals corrections, we have studied the electronic structures, lattice dynamics, and magnetic properties of crystal FeSe under hydrostatic pressure [1] and FeSe/STO system [2,3]. For bulk FeSe, the frequencies of all optical phonon modes increase with pressure. In a range between 5 and 6 GPa, the frequency of the A$_{\mathrm{1g}}$ mode, which relates to the Se height from the Fe-Fe plane, shows a sudden jump. This is also the pressure range within which the highest superconducting transition temperature Tc of FeSe is reached in experiments. In comparison with the other phonon modes, the zero-point atomic displacement of the A$_{\mathrm{1g}}$ mode also induces the strongest variation of local magnetic moment on Fe, which reaches the maximum around 5 GPa. These results suggest that the effect of phonon via spin-phonon coupling could not be omitted. For monolayer FeSe epitaxial film on SrTiO$_{\mathrm{3}}$, the combined effect of electron doping and phonon readily leads to magnetic frustration between the collinear antiferromagnetic state and checkerboard antiferromagnetic Neel state. For bilayer FeSe epitaxial film on SrTiO$_{\mathrm{3}}$, such a magnetic frustration is much easier induced by electron doping in its bottom layer than its top layer. The underlying physics is that the doped electrons are accumulated at the interface between the FeSe layers and the substrate. These results are consistent with the existing experimental studies. [1] Q.-Q. Ye, K. Liu, and Z.-Y. Lu, Influence of spin-phonon coupling on antiferromagnetic spin fluctuations in FeSe under pressure: First-principles calculations with van derWaals corrections. Phys. Rev. B \textbf{88}, 205130 (2013). [2] K. Liu, Z.-Y. Lu, and T. Xiang, Atomic and electronic structures of FeSe monolayer and bilayer thin films on SrTiO3 (001):First-principles study. Phys. Rev. B \textbf{85}, 235123 (2012). [3] K. Liu, B.-J.Zhang, and Z.-Y. Lu, First-principles study of magnetic frustration in FeSe epitaxial films on SrTiO3. Submitted. [Preview Abstract] |
Friday, March 6, 2015 1:39PM - 2:15PM |
Z51.00005: Density Functional Plus Dynamical Mean Field Theory of Correlated Oxides Invited Speaker: Andrew Millis The density functional plus dynamical mean field method is outlined and a few recent successes including applications to spin crossover molecules, oxide superlattices and metal-insulator transitions in bulk transition metals are outlined. Insights from the method into the essential role played by lattice distortions (both rotations and bond length changes) in determining the phase diagrams of correlated materials are presented. The key theoretical issue of the double counting correction is outlined, different approaches are compared, and a connection to the energy level differences between strongly and weakly correlated orbitals is presented. Charge transfer across oxide interfaces shown to depend crucially on the double counting correction, suggesting that experiments on oxide superlattices may provide insights into this important problem. Future directions are discussed. This work is performed in collaboration with Jia Chen, Hung Dang, Hyowon Park and Chris Marianetti. [Preview Abstract] |
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