Bulletin of the American Physical Society
APS March Meeting 2018
Volume 63, Number 1
Monday–Friday, March 5–9, 2018; Los Angeles, California
Session A14: Fe-based Superconductors -- SpectroscopyFocus
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Sponsoring Units: DMP Chair: Jinguang Cheng, Chinese Academy of Sciences (CAS) Room: LACC 304B |
Monday, March 5, 2018 8:00AM - 8:36AM |
A14.00001: Abstract Withdrawn Invited Speaker:
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Monday, March 5, 2018 8:36AM - 8:48AM |
A14.00002: Nodal Superconductivity in Single-crystalline Tetragonal FeS: A Muon Spin Relaxation and Rotation
Study Cheng Tan, Tianping Ying, Lei Shu We report the muon spin relaxation and rotation (μSR) measurements on hydrothermal grown singlecrystalline tetraganol FeS, which is much debated recently on its magnetic state and superconducting gap symmetry. We measured the zero-field relaxation rate λZF down to 0.025 K, which saturates below Tsat ≈ 4 K. The temperature dependence of λZF can be well explained by thermal excitation of electronic fluctuations, demonstrating FeS is a paramagnet. Our transverse field measurements give the in-plane penetration depth λab(0) = 312(5) nm. The linear low-T dependence of depolarization rate σsc ∝ λab-2 reveals the nodal superconductivity in tetragonal FeS. The s+d-waves model gives the best fit to our data at the fields we applied (μ0H = 7.5 mT, 30 |
Monday, March 5, 2018 8:48AM - 9:00AM |
A14.00003: Temperature-dependent optical properties of Sr2VO3FeAs Jungseek Hwang, Seokbae Lee, Jong Ok, Jun Sung Kim, Kwan-Woo Lee We performed infrared spectroscopic study on a single crystal of Sr2VO3FeAs grown by a self-flux method. This layered material system consists of two alternative layers of Sr2VO3 and FeAs. Since the typical size of single crystalline Sr2VO3FeAs samples is 200 x 200 x 10 μm3 optical study on this material is challenging. We could measure reflectance spectra of ab-plane at various temperatures from 8 K to 300 K using an in-situ gold evaporation technique in far- and mid-infrared regions. This material shows dc resistivity and magnetic susceptibility anomalies near 155 K. We observed that a Fano-shaped phonon appears below 200 K and shows a strong blue-shift with lowering the temperature. We also observed an additional interband transition around 1000 cm-1, which is absent in other doped Ba-122 Fe-pnictides. We will discuss about further issues on this material. |
Monday, March 5, 2018 9:00AM - 9:12AM |
A14.00004: Intrinsic charge dynamics in high-Tc AFeAs(O,F) superconductors Aliaksei Charnukha, Daniel Pröpper, N. Zhigadlo, Michio Naito, Michael Schmidt, Zhe Wang, Joachim Deisenhofer, Alois Loidl, Bernhard Keimer, Alexander Boris, Dimitri Basov We report the first determination of the in-plane complex optical conductivity of 1111 high-Tc superconducting iron oxypnictide single crystals PrFeAs(O,F) and thin films SmFeAs(O,F) by means of conventional and micro-focused infrared spectroscopy, ellipsometry, and time-domain THz transmission spectroscopy. A strong itinerant contribution is found to exhibit a dramatic difference in coherence between the crystal and the film. Using extensive temperature-dependent measurements of THz transmission we identify a previously undetected 2.5-meV collective mode in the optical conductivity of SmFeAs(O,F), which is strongly suppressed at Tc and experiences an anomalous T-linear softening and narrowing below T*≈110K»Tc. The suppression of the infrared absorption in the superconducting state reveals a large optical superconducting gap with a similar gap ratio 2Δ/kBTc≈7 in both materials, indicating strong pairing. |
Monday, March 5, 2018 9:12AM - 9:24AM |
A14.00005: Optical properties for superconducting parent compound Ca10(Pt4As8)(Fe10As10) single crystal. Yuil Seo, Woo-Jae Choi, Shinichi Kimura, YongSeung Kwon We have measured the reflectivity of the parent compound Ca10(Pt4As8)(Fe10As10) single crystal (Tc =32.6 K) over the broad frequency range from 20 cm−1 to 12000 cm−1 and for temperatures from 8 K to 300 K. The optical conductivity spectra of the low frequency region (< 1000 cm−1) in the normal state (38 K T 300 K) is well fitted with two Drude forms, which indicates the presence of multiple bands at the Fermi level. When the superconducting state in 8 K, the real part optical conductivity 1 was not completely suppressed. This indicates that the superconducting gap of this compound is not fully open in Fermi surface. So we can think about the possibility of a nodal point in the superconducting gap of this compound. The extracted SC gap size is = 2.85 meV and the scattering rate 1/= 34.71 meV, suggesting Ca10(Pt4As8)(Fe10As10) as a single-gap superconductor with a nodal point possibility in superconducting gap. |
Monday, March 5, 2018 9:24AM - 9:36AM |
A14.00006: Energy Gap Measurements of K-doped, Co-doped, and P-doped Ba(1-x)KxFe2As2 Superconductors using Soft Point Contact Spectroscopy Oberon Wackwitz, Luke Conover, Caitlyn McConnell, Brett Conti, Guotai Tan, Yu Song, Chenglin Zhang, Rui Zhang, Ding Hu, Pengcheng Dai, Roberto Ramos Iron-based superconductors provide a new platform where multi-band superconductivity can be studied. Multiple energy gaps in these samples can be measured, depending on the way the crystal has been grown and how the tunneling directions are accessed through fabrication and the way electrical contacts are made. These energy gaps are anisotropic relative to the crystal lattice, with gaps sensitive to tunneling that is parallel or perpendicular to the c-axis of the lattice. In this presentation, we report our progress in ongoing measurements of the energy gaps of K-doped Ba(1-x)KxFe2As2, Co-doped and P-doped BaFe2As2. We report the observation of peaks and broad shoulders; some correlate well with existing data from literature while other new peaks are unexplained. These measurements were performed fully by undergraduates. |
Monday, March 5, 2018 9:36AM - 9:48AM |
A14.00007: Phonon anomalies in AEFe2As2 (AE = Ca, Sr and Ba) Christopher Homes, Yaomin Dai, Ana Akrap, Sergey Bud'ko, Paul Canfield The infrared vibrational properties of AEFe2As2 (AE = Ca, Sr and Ba) have been measured above and below the structural and magnetic transition at TN = 172, 195 and 138 K [1], and compared to ab initio calculations of the frequency and atomic character at the center of the Brillouin zone. In the Ca compound both of the infrared-active Eu modes split below TN. However, while the low-frequency mode splits in the Sr compound, the high-frequency mode appears to only have a lower branch which displays an anomalous increase in strength below TN. In the Ba compound, the high-frequency mode again fails to split and the oscillator strength increases dramatically below TN [2]; furthermore, there is a net decrease in the strength of the low-frequency modes at low temperature. Ab initio calculations indicate that the nature of the high-frequency mode does not change across this series of compounds. The presence of SDW order along the b axis below TN suggests the nature of the bonding has been altered due to orbital ordering, possibly explaining the lack of splitting and the increase in strength of the high-frequency mode. |
Monday, March 5, 2018 9:48AM - 10:00AM |
A14.00008: Nodal Superconductivity in FeSe single crystals from heat capacity Frédéric Hardy, Mingquan He, Liran Wang, Thomas Wolf, peter schweiss, Michael Merz, robert eder, Christoph Meingast The superconducting gap in vapor-grown single-crystalline FeSe is studied via heat-capacity measurements down to 0.4 K and up to 14 Tesla. In our best crystals, i.e. with the highest Tc and RRR values, we consistently find a linear C/T term at low temperatures indicative of a nodal superconducting gap. This is supported by the magnetic field dependence of C/T. The electronic specific heat curve up to Tc is shown to be consistent with a recent gap determination using quasi particle interference only if one forces the gap to change sign at the minimum of one of the Fermi surface pockets. Finally, the specific heat of FeSe crystals grown under different conditions are compared to various C/T data taken from the literature. |
Monday, March 5, 2018 10:00AM - 10:12AM |
A14.00009: Broadband microwave spectroscopy of FeSe Graham Baker, James Day, Shun Chi, Doug Bonn The large enhancements of Tc that occur when FeSe is pressurized or grown as a single layer on an insulating substrate have generated immense current interest in this superconductor. Measurements of the electrodynamic response of FeSe at microwave frequencies provide some of the key means of probing its gap structure and superconducting charge dynamics. Recently, precision measurements of FeSe's microwave surface impedance were reported in which a quasiparticle mean free path in excess of 50 µm was found at 0.1 K, a record for a compound superconductor [1]. Additionally, strong evidence for a non-zero energy-gap minimum was presented, in contention with previous reports of line nodes in FeSe [2]. These measurements were made using cavity perturbation of a self-resonant coil and were limited to two discrete frequencies. Here we present measurements of surface resistance continuously from 1 to 20 GHz using a home-built broadband bolometric microwave spectrometer, and discuss their bearing on the superconducting gap structure of FeSe. |
Monday, March 5, 2018 10:12AM - 10:24AM |
A14.00010: The Study of Electron-Phonon Coupling at the FeSe/SrTiO3 Interface. Shuyuan Zhang, Xuetao Zhu, Jiandong Guo The significant role of interfacial coupling in the superconductivity enhancement in FeSe films on SrTiO3 (STO) has been widely recognized. But the explicit origination of this coupling is yet to be identified. Using high resolution electron energy loss spectroscopy, both phonons and plasmons of FeSe/STO were measured and studied. We show that, the Fe- and Se-derived phonons do not exhibit differences between films with different thicknesses. Theoretical calculations indicate the antiferromagnetic correlation is uniform in both FeSe films and bulk. However, the growth of FeSe films leads to a significant change of Fuchs-Kliewer (F-K) phonons and plasmons in STO. Thus, the interfacial coupling between the electrons in FeSe film and the F-K phonons from substrate is essential for the superconductivity enhancement. Combining the electronic structures and polaron excitations observed in experiments, we find that this electron-phonon coupling at FeSe/STO interface belongs to the non-retarded regime, resulting the formation of interfacial polarons. We propose that the formation of interfacial polaron directly relates to the superconductivity enhancement and the corresponding interaction locates the superconductivity inside the BSC-BEC crossover regime. |
Monday, March 5, 2018 10:24AM - 10:36AM |
A14.00011: Spin order and fluctuations in FeSe Rudolf Hackl, Andreas Baum, Harrison Ruiz, Nenad Lazarevic, Yao Wang, Thomas Boehm, Ramez Hosseinian Ahangharnejhad, Peter Adelmann, Thomas Wolf, Zoran Popovic, Brian Moritz, Thomas Devereaux FeSe is the simplest Fe-based system. Depending on the environment, the superconducting transition temperature ranges between 9 and approximately 100 K. As opposed to the Fe pnictides, long range magnetic order is not found in FeSe. Yet, the charge and spin dynamics of FeSe may hold key information on the physics of high temperature superconductors in general. We report results of light scattering experiments as a function of polarization and temperature and simulate the spectra using exact diagonalization. With the parameters of a frustrated spin 1 Heisenberg model the experiment can be reproduced for all Raman active symmetries. We find a low energy peak in B1g symmetry and assign this feature to excitations of nearly localized spins. Comparable agreement between theory and experiment can be achieved for neutron scattering data. The study furnishes evidence for nearly frustrated spin order and critical fluctuations of spin stripes with an ordering vector close to or at (π,0). |
Monday, March 5, 2018 10:36AM - 10:48AM |
A14.00012: Polaronic behavior in highly two-dimensional iron-based superconductor Ca10(Pt3As8)(Fe1-xPtxAs)10 Run Yang, Yaomin Dai, Xianggang Qiu, Christopher Homes In the phase diagram of Ca10(Pt3As8)(Fe1-xPtxAs)10, above the superconducting (SC) and antiferromagnetic (AFM) dome, there exists a semiconducting-like behavior, which mimics the pseudogap in cuprates. Investigating the origin of this behavior would provide us an important clue for the pairing mechanism. In this work, we have synthesized the parent compound and the optimal doped Ca10(Pt3As8)(Fe1-xPtxAs)10 and measured their optical properties. From their optical conductivity, we found that the diminishing of the low-energy spectral weight in the normal state, corresponding to the semiconducting-like behavior, is related to the AFM fluctuation. Furthermore, we’ve realized the magnetic-enhanced electron-phonon coupling and that the far-infrared absorption peak in the optimal doped sample could be well described by the large polaron model, indicating the formation of large polaron in the semiconducting dome. Below Tc~12K, the polaron peak collapsed into the superfluid. Thus, we propose that the magnetism induced polaron may contribute the unconventional pairing. |
Monday, March 5, 2018 10:48AM - 11:00AM |
A14.00013: SQUID imaging of edge magnetization and weak local diamagnetism in un-doped Ba122 Ruby Shi, Johanna Palmstrom, John Kirtley, Hilary Noad, Kathryn Moler, Ian Fisher Motivated by reports of filamentary superconductivity in the Iron-based Ba pnictide superconductors (Ba122), we used a scanning SQUID (Superconducting Quantum Interference Device), with sub-micron resolution, to detect local magnetization and susceptibility variation of Ba122 single crystals. We found evidence of both weak local diamagnetism and static magnetization. Intriguingly, the magnetization seems to appear on certain facets of the crystal. |
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