Bulletin of the American Physical Society
APS March Meeting 2015
Volume 60, Number 1
Monday–Friday, March 2–6, 2015; San Antonio, Texas
Session D0: Focus Session: Mostly Monolayer FeSe |
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Sponsoring Units: DMP DCOMP Chair: Peter Hirschfeld, University of Florida Room: Ballroom A |
Monday, March 2, 2015 2:30PM - 2:42PM |
D0.00001: Influence of ion irradiation on iron-chalcogenide superconducting films Toshinori Ozaki, Weidong Si, Cheng Zhang, Lijun Wu, Qiang Li Iron-chalcogenide superconductors have rather simple crystal structure and no charge reservoir. They also exhibit remarkable properties including small anisotoropy, high upper critical fields, a significant pressure effect on superconductivity. We have grown iron-chalcogenide FeSe0.5Te0.5 (FST) superconducting films on various substrate by pulsed laser deposition [Rep. Prog. Phys. 74, 124510 (2011)]. The FST films on CeO2 buffer layer exhibit enhanced Tc (Tc$^{\mathrm{onset}}$ \textgreater 20 K, Tc$^{\mathrm{zero}}$ $=$ 18.0 K), which is about 30{\%} higher than that found in the bulk materials and superior high field performance over the low temperature superconductors. [Nature Commun. 4, 1347 (2013)]. Recently, we were successful in further enhancement of Jc without Tc degradation by ion irradiation, especially, at high temperature and high magnetic field. The low-energy proton irradiation produces a Jc enhancement of one order of magnitude over the field of 6T//c at 12 K. Extensive TEM studies of the irradiated FST films have been carried out, which revealed an intriguing defect morphology provided by the irradiation. We will discuss the relationship between the superconducting properties and the created defects of the iron-chalcogenide films. [Preview Abstract] |
Monday, March 2, 2015 2:42PM - 2:54PM |
D0.00002: Strong electron-phonon interaction in an FeSe monolayer Sinisa Coh, Marvin L. Cohen, Steven G. Louie We show that the electron-phonon coupling in an FeSe monolayer on a SrTiO$_3$ substrate is significantly larger than in earlier theoretical estimates. The role of the SrTiO$_3$ substrate is two-fold. First, the interaction of the FeSe and TiO$_2$ terminated face of SrTiO$_3$ prevents the FeSe monolayer from undergoing a shearing-type (orthorhombic) structural phase transition. Second, the substrate allows an anti-ferromagnetic ground state of FeSe which opens certain electron-phonon coupling channels within the monolayer that are prevented by symmetry in the non-magnetic phase. The spectral function for the electron-phonon coupling ($\alpha^2F$) in our calculations agrees well with inelastic tunneling data. [Preview Abstract] |
Monday, March 2, 2015 2:54PM - 3:06PM |
D0.00003: STM investigation of nanoscale inhomogeneity in single-layer FeSe/SrTiO$_{3}$ Dennis Huang, Tatiana A. Webb, Can-Li Song, Cui-Zu Chang, Jagadeesh Moodera, Jennifer E. Hoffman FeSe possesses the simplest stoichiometry within the family of iron-based high-$T_{c}$ superconductors. The ability to grow high quality films layer-by-layer using molecular beam epitaxy (MBE) yields opportunities to engineer high-$T_c$ superconducting heterostructures with novel behaviors. In particular, single-layer FeSe deposited on SrTiO$_{3}$ exhibits an order of magnitude increase in $T_c$ from its bulk value, possibly due to strain, charge doping or enhanced electron-phonon coupling from the substrate. We use a combined scanning tunneling microscope (STM) and MBE system to examine single-layer FeSe/SrTiO$_{3}$ at the atomic scale. Using real-space spectroscopic imaging, we investigate the superconducting states in this material, and their perturbation by various nanoscale disorders within the FeSe film and its substrate. [Preview Abstract] |
Monday, March 2, 2015 3:06PM - 3:18PM |
D0.00004: Fermi surface topology and gap anisotropy in monolayer FeSe thin film Yan Zhang, James Lee, Robert Moore, Wei Li, Ming Yi, Makoto Hashimoto, Donghui Lu, Zahid Hussain, Tom Devereaux, Dung-Hai Lee, Zhi-Xun Shen The discovery of superconductivity in monolayer FeSe thin film has generated great interests. The superconducting transition temperature (Tc) was reported to be over 65 K, which holds the record in iron-based superconductors. More intriguingly, the superconductivity was found only in one monolayer (1ML) film, while the films thicker than 1ML are non-superconducting. Utilizing the angle-resolved photoemission spectroscopy (ARPES), we studied the Fermi surface topology and superconducting gap anisotropy in 1ML FeSe. We resolved two ellipse-like electron pockets at the zone corner overlapping with each other. No hybridization between these two electron pockets was observed, which indicates that the glide mirror symmetry breaking due to the substrate is extremely weak in 1ML FeSe. Multi-gap behavior and gap anisotropy were further observed on the electron pockets. The superconducting gap minimums locate along the M-X direction for both inner and outer electron pockets. The observed Fermi surface topology and gap distribution provide a good starting point for constructing theoretical models and put strong constrains on determining the pairing symmetry in 1ML FeSe. [Preview Abstract] |
Monday, March 2, 2015 3:18PM - 3:30PM |
D0.00005: STM investigation of FeSe/SrTiO$_3$ band structure Tatiana A. Webb, Dennis Huang, Can-Li Song, Cui-Zu Chang, Jagadeesh Moodera, Jennifer E. Hoffman Growing a single unit cell of FeSe on a SrTiO$_3$ substrate (1 u.c. FeSe/STO) enhances the superconducting transition temperature (T$_c$) by an order of magnitude. While the dramatic effect of the interface is evident, a mechanism is not. ARPES studies have revealed that the band structure differs significantly from bulk FeSe and the majority of other Fe-based superconductors, most notably in lacking a hole pocket at the Fermi level. ARPES, however, is limited to probing the filled electron states. STM/STS has access to the band structure, both above and below the Fermi level, with spatial resolution, and the data encodes electronic properties including orbital character and interactions. We present an STM/STS study of 1 u.c. FeSe/STO grown by molecular beam epitaxy (MBE), focusing on the empty-state band structure of this new high T$_c$ superconductor. [Preview Abstract] |
Monday, March 2, 2015 3:30PM - 3:42PM |
D0.00006: Investigation of superconductivity in single layer FeSe on SrTiO3 (001) by quasi-particle interference and impurity states Tong Zhang, Qin Fan, Wenhao Zhang, Xi Liu, Miao Xia, Hongyan Chen, Rui Peng, Haichao Xu, Binping Xie, Donglai Feng Recently, single layer FeSe films on SrTiO3 (001) were discovered to have much enhanced superconductivity [1]. Here by using scanning tunneling microscopy/spectroscopy, we investigated the superconductivity of single layer FeSe through quasi-particle interference (QPI), magnetic vortex mapping and impurity induced bound states. The films were grown by MBE and transfer to STM in-situ. The magnetic vortex lattice was observed in dI/dV mappings in the field. QPI mappings show that intra-band and inter-band scattering of superconducting quasi-particles have significant anisotropy. Single atom impurities were introduced on the surface by in-situ deposition. We found that nonmagnetic impurities (Zn, Ag, K) do not induce bound states in the superconducting gap, but the magnetic ones (Cr, Mn) do. Upon these observations, the paring symmetry of single layer FeSe will be discussed. \\[4pt] [1] Q. Wang, et al., Chin. Phys. Lett. 29, 037402 (2012). [Preview Abstract] |
Monday, March 2, 2015 3:42PM - 4:18PM |
D0.00007: Interface enhanced superconductivity in one unit-cell FeSe films grown on SrTiO3 Invited Speaker: Xucun Ma Heterostructure based interface engineering has been proved an effective method for finding new superconducting systems and raising superconducting transition temperature (Tc). Recently discovered high temperature superconductivity in one unit-cell (UC) FeSe films on SrTiO3 (STO) substrate grown by molecular beam epitaxy has attracted intensive attention. In sharp contrast to FeSe films on graphene where a 2.2 meV superconducting gap is observed on thick films and no superconducting gap on 1-UC FeSe down to 2.3 K, 1-UC FeSe films on STO substrate exhibit unexpected large superconducting gaps of 15-20 meV. Interestingly, the anomalously large superconducting gap is only found in the first UC FeSe but not on 2-UC or thicker layers, indicating that interface plays a crucial role in the enhanced superconductivity in 1-UC FeSe films on STO substrate. Another interesting point of this system is its simple band structure that consists only of electron Fermi pockets at M points, which is different from that of bulk FeSe. In this talk, a comprehensive study of 1-UC FeSe films by in situ scanning tunneling microscopy/spectroscopy (STM/STS) and angle-resolved photoemission spectroscopy (ARPES) and ex situ transport measurements will be presented to discuss the possible superconducting mechanism in this well-defined heterostructure. \\[4pt] Collaborators: Qi-Kun Xue, Lili Wang, Yayu Wang (Tsinghua University); Xingjiang Zhou (Institute of Physics, CAS); Jian Wang (Peking University) [Preview Abstract] |
Monday, March 2, 2015 4:18PM - 4:30PM |
D0.00008: Phase separation or not in K$_{x}$Fe$_{2-y}$Se$_{2}$ Despina Louca, Junjie Yang The coexistence of insulating and superconducting phases in the K$_{x}$Fe$_{2-y}$Se$_{2}$ family of Fe-based superconductors is investigated using neutron scattering on samples grown under different conditions. In this family, three scenarios have so far been proposed regarding the superconducting phase. In the first, a superconducting minority phase with the 122 composition is separated from the insulating and majority 245 phase. Under this scenario several phase diagrams have been developed in which the superconducting phase is sandwiched between semiconducting and insulating, antiferromagnetic phases. In the second, the superconducting phases exists in an inhomogeneous structure, hence no phase separation. And in the third, a purely superconducting phase of the alkali intercalated FeSe can be made with the 122 structure that has no other phases. By probing the local structure, we previously observed that superconductivity emerges in a locally distorted Fe sublattice that accommodates two kinds of bonding environments, forming a double-well distribution that changes with the concentration of K. In addition, the Fe bond distribution changes with the annealing treatment. Implications to the coexistence of the two phases will be discussed. [Preview Abstract] |
Monday, March 2, 2015 4:30PM - 4:42PM |
D0.00009: Suppression of Phase Separation and Enhanced Superconducting Transition Temperature of FeSe$_{1-x}$Te$_x$ Thin Films Fuyuki Nabeshima, Yuichi Sawada, Yoshinori Imai, Atsutaka Maeda To clarify the mechanism of superconductivity of Fe-based superconductors, it is crucial to investigate superconductivity of FeSe$_{1-x}$Te$_x$, which has the simplest crystal structure. There is, however, a serious obstacle to the understanding of its superconductivity; phase separation by spinodal decomposition occurs in the region of $0.1 < x < 0.4$ and thus a whole phase diagram has not been available. A useful method to fabricate metastable materials is thin-film deposition because of its thermodynamically non-equilibrium growth. In the presentation we will report the first demonstration of the suppression of the phase separation of FeSe$_{1-x}$Te$_x$ thin films on CaF$_2$ substrates[1]. Surprisingly the optimal composition to achieve the highest superconducting transition temperature, $T_{\mathrm c}$, was found in this phase separation region; $T_{\mathrm c}^{\mathrm {onset}}$ reaches $\sim $23 K. A whole phase diagram we will present provides a new perspective for the superconductivity of this material. [1] F. Nabeshima $et\ al$., Appl. Phys. Lett. \textbf{103} (2013) 172602. [Preview Abstract] |
Monday, March 2, 2015 4:42PM - 4:54PM |
D0.00010: Fermi surface deformation in a simple iron-based superconductor, FeSe Amalia Coldea, Matthew Watson, Timur Kim, Amir Haghighirad, Alix McCollam, Moritz Hoesch, Andrew Schofield One of the outstanding problems in the field superconductivity is the identification of the normal state out of which superconductivity emerges. FeSe is one of the simplest and most intriguing iron-based superconductors, since in its bulk form it undergoes a structural transition before it becomes superconducting, whereas its single-layer form is believed to be a high-temperature superconductor. The nature of the structural transition, occurring in the absence of static magnetism, is rather unusual and how the electronic structure is stabilized by breaking of the rotational symmetry is the key to understand the superconductivity in bulk FeSe. Here we report angle-resolved photoemission spectroscopy measurements on FeSe that gives direct access to the band structure and orbital-dependent effects. We complement our studies on bulk FeSe with low-temperature angular-dependent quantum oscillation measurements using applied magnetic fields that are sufficiently strong to suppress superconductivity and reach the normal state. These studies reveal a strong deformation of Fermi surface through the structural transition driven by electronic correlations and orbital-dependent effects. . [Preview Abstract] |
Monday, March 2, 2015 4:54PM - 5:06PM |
D0.00011: Electronically-driven orthorhombic distortion in FeSe Matthew Watson, Nathaniel Davies, Amir Haghighirad, Arjun Narayanan, Timur Kim, Moritz Hoersch, Samuel Blake, Amalia Coldea FeSe is structurally the simplest of Fe-based superconductors, and exhibits a tetragonal-to-orthorhombic structural transition at $\sim$ 90 K, but no long-range magnetism at any temperature. We report measurements of the resistivity anisotropy in FeSe above Ts finding a large and divergent response to an applied strain, with a comparable magnitude and temperature-dependence to measurements in Ba(Fe$_{1-x}$Co$_x$)$_2$As$_2$, but opposite sign. We compare this data with literature reports on NMR and our own ARPES data, which taken together indicate that the structural transition is electronically-driven with orbital degrees of freedom playing a central role. [Preview Abstract] |
Monday, March 2, 2015 5:06PM - 5:18PM |
D0.00012: The superconductivity in single-layer FeTe$_{\mathrm{1-x}}$Se$_{\mathrm{x}}$ films on SrTiO$_{3}$ Lili Wang, Xucun Ma, Qikun Xue For bulk FeSe, the highest transition temperature \textit{Tc} $=$ 9 K has been observed for the composition with stoichiometry Fe$_{1.1}$Se. The tetragonal-orthorhombic structural transition observed in FeSe is suppressed with Te substitution and the superconducting transition temperature reaches a maximum of \textit{Tc} $=$15.2 K at about 50{\%} Te substitution. For single-layer FeSe films on SrTiO$_{3}$, \textit{ in situ} scanning tunneling microscopy and angle resolved photoemission spectroscopy have revealed a superconducting gap as large as 20 meV, and \textit{ex situ} transport measurements have confirmed the interface enhanced superconductivity with $T_{C}$ above 55 K. Here we report a detailed \textit{in situ} scanning tunneling microscopy and transport study of the single-layer FeTe$_{\mathrm{1-x}}$Se$_{\mathrm{x}}$ films on SrTiO$_{3}$. We found that Te substitution in the single-layer FeSe films doesn't induce further increase of the transition temperature \textit{Tc}, which is in contrast to the results for the corresponding bulk materials. This implies that the SrTiO$_{3}$ substrates play important role in the interfacial superconductivity.\\[4pt] [1] F. C. Hsu, \textit{et al.} Proc.Natl. Acad. Sci. U.S.A. \textbf{105} 14262 (2008).\\[0pt] [2] Y. Mizuguchi, \textit{et al.}, J. Phys. Soc. Jpn. \textbf{78} 074712 (2009).\\[0pt] [3] Q. Y. Wang \textit{et al.}, Chin Phys Lett, \textbf{29}, 037402 (2012).\\[0pt] [4] S. L. He, \textit{et al.} Nature Mater. \textbf{12,} 605 (2013).\\[0pt] [5] W. H. Zhang, \textit{et al.}, Chin Phys Lett, \textbf{31}, 017401 (2014).\\[0pt] [6] W. H. Zhang, \textit{et al.}, Phys. Rev. B \textbf{89}, 060506 (2014). [Preview Abstract] |
Monday, March 2, 2015 5:18PM - 5:30PM |
D0.00013: Theoretical studies of the effects of orbital ordering on spin fluctuations and superconductivity in FeSe Shantanu Mukherjee, Andreas Kreisel, Peter J. Hirschfeld, Brian M. Andersen FeSe is currently one of the most hotly debated iron-based systems due in part to its very high T$_{c}$ when monolayers are placed on STO substrates, and in part due to the fact that the material exhibits a structural distortion near T$_{S}$, $\sim$ 90K without any concomitant magnetic order. In addition, undoped bulk FeSe samples, which become superconducting below T $\sim$ 8K, display evidence of orbital ordering setting in near T$_{S}$. We discuss the normal and superconducting properties of FeSe using a ten orbital tight-binding model, and include the effect of ferro-orbital ordering. The model reproduces the essential features of FeSe band structure seen in ARPES [1] and quantum oscillation experiments [3]. Using this model, the spin lattice relaxation rate is calculated and the results are compared with recent NMR experiments [2]. We next discuss the consequences of a spin fluctuation mediated superconducting pairing in FeSe and the resulting gap structure. Finally, the local density of states derived from our calculations is compared to STM experiments [4]. \\[4pt] [1] T. Shimojima et al., Phys. Rev. B \textbf{90}, 121111(R) (2014).\\[0pt] [2] S.-H. Baek, et al., ArXiv:1408.1875.\\[0pt] [3] T. Terashima et al., Phys. Rev. B \textbf{90}, 144517 (2014).\\[0pt] [4] C. L. Song et al$.,$ Science \textbf{332}, 1410 (2010). [Preview Abstract] |
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