Bulletin of the American Physical Society
APS March Meeting 2014
Volume 59, Number 1
Monday–Friday, March 3–7, 2014; Denver, Colorado
Session G52: Focus Session: Films and Crystals: Understanding Causes of Superconductivity |
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Sponsoring Units: DMP Chair: Minghu Pan, Oak Ridge National Laboratory Room: Mile High Ballroom 1F |
Tuesday, March 4, 2014 11:15AM - 11:51AM |
G52.00001: Infrared probe of the pseudogap in the BaFe$_{2}$As$_{2}$ high-$T_{c}$ superconductors Invited Speaker: Soonjae Moon The nature of the pseudogap of high-$T_{c}$ cuprate superconductors is one of the most significant problems in condensed matter physics. The central issue concerns whether the pseudogap is associated with the precursor superconductivity or other broken symmetry state. Regardless of the origin, the pseudogap is considered as an essential part of physics of unconventional cuprate superconductors. There is mounting evidence that the superconductivity of the Fe-based materials is also quite exotic. However, the spectroscopic manifestations of the pseudogap in the Fe-based superconductors remained elusive. We present the \textit{ab}-plane and $c$-axis infrared data of the prototypical pnictide system: the BaFe$_{2}$As$_{2}$ family. Our experiments have identified the hallmarks of the pseudogap that mirror the manifestations of the pseudogap in the underdoped cuprates. The evolution of the charge dynamics across the phase diagram suggests that the pseudogap is not directly related to precursor superconductivity but may be linked to antiferromagnetism. [Preview Abstract] |
Tuesday, March 4, 2014 11:51AM - 12:03PM |
G52.00002: Pressure induced metallic state in LaMnPO revealed via Infrared spectroscopy Kirk Post, Alex Goncharov, Jack Simonson, Daniel McNally, Zhiping Yin, Brian Chapler, Gabriel Kotliar, Meigan Aronson, Dimitri Basov We investigated the energy gap (E$_{Gap}$) of the antiferromagnetic insulator LaMnPO$_{1-x}$F$_{x}$ for x = 0.0, 0.04, as a function of temperature and pressure using infrared spectroscopy. The results obtained from these measurements show that the band gap shows no discontinuous change upon crossing the Ne\'el temperature of 375 K and is therefore, likely unrelated to the antiferromagnetic ordering. Despite the resilience of the band gap to temperature, the band gap is dramatically reduced with the application of pressure and fully collapses by 28 GPa for both samples. These measurements confirm theoretical work predicting the collapse of the band gap with pressure. [Preview Abstract] |
Tuesday, March 4, 2014 12:03PM - 12:15PM |
G52.00003: Strain and strain relaxation analysis of superconducting Ba(Fe$_{0.92}$Co$_{0.08}$)$_{2}$As$_{2}$ films on various substrates Artemis Rafti, Q.Y. Lei, M. Golalikhani, W.K. Withanage, J. Qiu, M. Hambe, F. Williams, Q. Yang, D. Temple, E.D. Bauer, F. Ronning, Q.X. Jia, X.F. Wang, X.H. Chen, J.D. Weiss, E.E. Hellstrom, X.X. Xi We have grown high quality, optimally doped superconducting Ba(Fe$_{0.92}$Co$_{0.08})_{2}$As$_{2}$ films on SrTiO$_{3}$, (La, Sr)(Al, Ta)O$_{3}$, LaAlO$_{3}$, CaF$_{2}$ and BaF$_{2}$ substrates. The variation in lattice mismatch allows the study of epitaxial strain effects on the structural and transport properties of the films. Reciprocal space mapping has been employed for detailed strain and strain relaxation analysis of the Ba(Fe$_{0.92}$Co$_{0.08}$)$_{2}$As$_{2}$ films on the different substrates. We observed large substrate dependant changes in both in plane and out of plane lattice parameters. Furthermore, the crystallinity of the grown films, the lattice constant and lattice volume evolution with strain and strain relaxation were investigated, revealing an epitaxial strain and strain relaxation dependence on the superconducting transition temperature. [Preview Abstract] |
Tuesday, March 4, 2014 12:15PM - 12:27PM |
G52.00004: ARPES investigations of single unit cell iron selenide James Lee, Felix Schmitt, Robert Moore, Steve Johnston, Yongtao Cui, Wei Li, Ming Yi, Zhongkai Liu, Makoto Hashimoto, Yan Zhang, Donghui Lu, Tom Devereaux, Dung-Hai Lee, Zhi-Xun Shen Recent spectroscopic measurements on single unit cell iron selenide (1UC FeSe) films have indicated the opening of a superconducting-like gap at temperatures near 65K. ~A current goal is to understand the cause of such a high gap-opening temperature in this system and its relation to superconductivity. Here we present in-situ angle-resolved photoemission studies of 1UC FeSe films grown via molecular beam epitaxy. We find signatures of strong coupling between the electrons in the FeSe and the phonons in the substrate, which manifest as replica bands in the spectra. The implications of this electron-phonon coupling on the Cooper-pairing interaction strength are discussed. [Preview Abstract] |
Tuesday, March 4, 2014 12:27PM - 12:39PM |
G52.00005: Interfacial Interplay Between Superconducting FeSe Films and Underlying Substrate Rob Moore The recent discovery of superconducting single unit cell iron selenide (FeSe) films on strontium titante (STO) substrates with significantly enhanced transition temperatures (Tc) has created a flurry of activity. Understanding the influence of the underlying substrate is paramount for fundamental understanding of the superconducting phenomena with a potential for breaking current Tc records. We have investigated the influence of different substrates and the interfacial structure utilizing thin films grown via molecular beam epitaxy (MBE) with in situ angle-resolved photoemission spectroscopy (ARPES) and low energy electron diffraction (LEED-IV) characterization. We will discuss the implications of substrates on the electronic and crystalline structure of the single FeSe unit cell. Our results help illuminate the interfacial coupling between these degrees of freedom and suggest mechanisms for Tc enhancement. [Preview Abstract] |
Tuesday, March 4, 2014 12:39PM - 12:51PM |
G52.00006: RIXS-probed spin excitations in one individual unit cell of ``214'' and ``123'' cuprate superconductors Giacomo Ghiringhelli, G. Dellea, L. Braicovich, M. Minola, M. Le Tacon, F. Baiutti, G. Cristiani, G. Logvenonv, B. Keimer, M. Salluzzo High quality ultrathin epitaxial films of nominally optimally doped cuprates preserve their transport properties down to few unit cells (uc). However at 2 uc the critical temperature ($T_c$) drops, and at 1 uc superconductivity is lost. The substrate and the protecting overlayer can induce strain and structural modifications, doping can be modified by charge transfer across the interfaces and oxygen content can be altered. We have used Cu L$_3$ resonant inelastic x-ray scattering (RIXS) to map the spin excitation spectra of optimally doped La$_{2-x}$Sr$_x$CuO$_4$ and NdBa$_2$Cu$_3$O$_7$ ultrathin individual films down to 1 uc. Paramagnons are present even in the thinnest films, but their energy and dispersion are significantly different than in thick films and bulk crystals. These results complete the recent findings on the robustness of paramagnons up to very high doping levels in Y123, Tl2201 [1] and La214 [2], and in CaCuO$_2$/SrTiO$_3$ superconducting superlattices [3].\\[4pt] [1] M. Le Tacon et al, Phys. Rev. B 88, 020501 (2013).\\[0pt] [2] P.M.P. Dean et al, Nat. Mater. 12, 1019 (2013).\\[0pt] [3] G. Dellea et al, unpublished. [Preview Abstract] |
Tuesday, March 4, 2014 12:51PM - 1:27PM |
G52.00007: Superconductivity and Critical Current of Iron-Based Superconductors in High Field Invited Speaker: Qiang Li Although high-temperature superconducting cuprates have been discovered for more than 26 years, high-field applications are still based on low-temperature superconductors (LTS), such as Nb$_{\mathrm{3}}$Sn. The high anisotropies, brittle textures and high manufacturing costs limit the applicability of the cuprates. Recently, we demonstrated that the iron superconductors, without most of the drawbacks of the cuprates, have a superior high-field performance over LTS at 4.2 K [Nat. Commun. \textbf{4}:1347 (2013); Rep. Prog. Phys. \textbf{74} 124510 (2011)]. In this presentation, I will discuss recent progress aimed at understanding the relationships between superconductivity, critical current, and nano-scaled structure defects in iron-based superconductors, with emphasis on the properties of superconducting iron chalcogenide films. Critical current densities $J_{\mathrm{c}}$ $\sim$ 10$^{\mathrm{7}}$ A/cm$^{\mathrm{2}}$ were observed in FeSe$_{\mathrm{0.5}}$Te$_{\mathrm{0.5}}$ films grown on CeO$_{\mathrm{2}}$ buffered single-crystalline and flexible metal substrates. These films are capable of carrying $J_{\mathrm{c}}$ exceeding 10$^{\mathrm{5}}$ A/cm$^{\mathrm{2}}$ under 30 T magnetic fields. Furthermore, we found that these films have significantly higher $T_{\mathrm{c}}$ (\textgreater 20K) as compared to bulk samples (bulk $T_{\mathrm{c}}$ $\sim$ 15 K) for the entire doping regime of FeSe$_{\mathrm{1-x}}$Te$_{\mathrm{x}}$. Structural analysis revealed that these films generally have significantly smaller c-axis and a-axis lattice constant than the bulk value, suggesting that the crystal structure changes have a dominating impact on the superconducting transition in iron-based superconductors. Large $J_{\mathrm{c}}$ enhancement can also be realized in iron based superconductors by irradiation with proton and heavy ions that opens a new avenue for a tailored landscape of effective vortex pinning defects. [Preview Abstract] |
Tuesday, March 4, 2014 1:27PM - 1:39PM |
G52.00008: Superconductivity in ultra-thin FeSe films L.Z. Deng, Y.Y. Xue, B. Lv, Z. Wu, L.L. Wang, X.C. Ma, Q.K. Xue, C.W. Chu The recent discovery of a high Tc above 50 K in FeSe unit-cell film in comparison with the 8 K in bulk FeSe has attracted much attention, which is proposed to be related to interface superconductivity. Meissner effect and zero resistivity are two critical evidence for the existence of superconductivity. Unfortunately, the Tc has mostly indirectly obtained from the energy gap measurements, and preliminary resistive and magnetic measurements. There has not been report of the observation of Meissner effect to provide the sufficient proof of superconductivity to date. This motivates our systematic magnetic investigation here. We have observed in the 1-4 unit-cell FeSe-films: 1) Meissner effect with extensive weak-links up to $\sim$ 20 K; 2) unconnected small superconducting patches up to $\sim$ 40 K; and 3) an unusual relaxation of the diamagnetic signal of unknown nature up to 80 K, all are consistent with our resistance results. Their implications on the high Tc superconductivity and the film growth will be discussed. [Preview Abstract] |
Tuesday, March 4, 2014 1:39PM - 1:51PM |
G52.00009: A transparent superconductor: LiTi$_{2}$O$_{4}$ epitaxial films Taro Hitosugi, Takeo Ohsawa, Tsutomu Nojima, Ryota Shimizu, Naoomi Yamada, Susumu Shiraki A framework is presented for the transparent conducting mechanism of transparent conductor LiTi$_{2}$O$_{4}$. Within the Bardeen-Cooper-Schrieffer (BCS) theory, achieving high superconducting transition temperature ($T_{\mathrm{c}})$ requires large carrier density at Fermi energy. This requirement prohibits the emergence of transparent superconductivity at high temperature, since the large carrier density leads to the optical absorption in visible. However, we here demonstrate high optical transmittance in superconducting LiTi$_{2}$O$_{4}$(111) epitaxial with $T_{\mathrm{c}}$ exceeding 13 K. Photoemission studies, electron transport measurements and optical analysis reveal the key role of electron effective mass, shifting a plasma frequency to infrared region. [Preview Abstract] |
Tuesday, March 4, 2014 1:51PM - 2:03PM |
G52.00010: Fabrication and Transport Properties of FeSe Thin Films on CaF$_2$ Substrates with Increased $T_{\rm c}$ Fuyuki Nabeshima, Yoshinori Imai, Masafumi Hanawa, Ataru Ichinose, Ichiro Tsukada, Atsutaka Maeda Fe(Se,Te) has the simplest crystal structure among Fe-based superconductors. Superconducting transition temperature, $T_{\rm c}$, is strongly dependent on the applied pressure. Indeed, strained thin films of FeSe$_{0.5}$Te$_{0.5}$ have higher $T_{\rm c}$ than that of bulk crystals[1,2]. On the other hand, an end member, FeSe, shows large increase in $T_{\rm c}$ under pressure compared with Te-doped ones. However there is no report on increased $T_{\rm c}$ of FeSe thin films except for the interface-induced superconductivity[3]. In the presentation we will report on the first successful introduction of compressive strain in FeSe thin films using CaF$_2$ substrates. As a result, $T_{\rm c}^{\rm {zero}}$ reaches 11.4 K, which is about 1.5 times higher than that of bulk crystals[4]. We will also report on the transport properties of FeSe thin films on CaF$_2$ in the normal state including the THz conductivity and the Hall resistivity comparing them with the results of FeSe$_{0.5}$Te$_{0.5}$ films. [1] E. Bellingeri $et\ al$., APL {\bf 96} (2010) 102512. [2] I. Tsukada $et\ al$., APEX {\bf 4} (2011) 053101. [3] Q.-Y. Wang $et\ al$., Chin. Phys. Lett. {\bf 29} (2012) 037402. [4] F. Nabeshima $et\ al$., APL {\bf 103} (2013) 172602. [Preview Abstract] |
Tuesday, March 4, 2014 2:03PM - 2:15PM |
G52.00011: Magnetotransport and structure of Ba(Fe $_{\mathrm{1-x}}$ Co$_{\mathrm{x}})_{2}$As$_{2}$ ultrathin films Adele Ruosi, Sanghan Lee, T. Hernandez, Yanjun Ma, M.S. Rzchowski, C.B. Eom Since the discovery of superconductivity in iron-based materials significant progress has been made in the fabrication of high quality bulk and thin film materials to explore their intrinsic properties and evaluate novel device applications. For both pathways, the best crystalline quality and optimal superconducting properties are required. Here Co-doped Ba-122 thin films grown on various substrates and thicknesses down to 6 nm, have been investigated. Crystal structure analysis was used to investigate the Fe-As-Fe bond angle and the Fe-As distance, and magnetotransport measurements were used to evaluate the electronic characteristics of the thin films. In particular, we observe an anomalous Hall effect that depends on temperature and film thickness. Success in very thin film fabrication involving pnictides will serve to spur progress in heterostructured systems exhibiting novel interfacial phenomena and device applications. [Preview Abstract] |
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