Bulletin of the American Physical Society
APS March Meeting 2017
Volume 62, Number 4
Monday–Friday, March 13–17, 2017; New Orleans, Louisiana
Session H41: Fe-based Superconductors: FeSeFocus
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Sponsoring Units: DMP Chair: Maxim Korshunov, Kirensky Institute of Physics, Federal Research Center KSC SB RAS Room: 388 |
Tuesday, March 14, 2017 2:30PM - 3:06PM |
H41.00001: TBD - Fe-based Superconductors Invited Speaker: Bernd Buechner While there is broad consensus that superconductivity in Fe based superconductors is due to an unconventional, most likely purely electronic pairing, many important aspects of both, normal and superconducting state are still unexplored. For example the role of orbital degrees of freedom for the normal state electronic properties, nematicity, and pairing is discussed very controversial. It is not clear whether nesting or the proximity to Lifshitz points are crucial for superconductivity. Moreover, the broad variety of properties found for different systems raises the question about the generic phase diagram of these systems. In my talk I will focus on anomalous state properties of Fe-based superconductors. Based on new NMR data I will discuss the origin of nematic order addressing again the apparent differences in FeSe and BaFe$_{\mathrm{2}}$As$_{\mathrm{2}}$. The role of orbitals will be emphasized including the possibility of formation of ``orbital polarons''. Moreover, highly unusual temperature dependencies of the electronic structure as revealed from our recent ARPES measurements will be presented. A possible connection to well-known anomalous T dependencies of both, magnetic and structural properties will be discussed. Work done in collaboration with S-H. Baek, S. Borisenko [Preview Abstract] |
Tuesday, March 14, 2017 3:06PM - 3:18PM |
H41.00002: Quasi-two-dimensional superconductivity in FeSe$_{\mathrm{0.3}}$Te$_{\mathrm{0.7}}$ thin films and electric-field modulation of superconducting transition Chenguang Mei, Zhu Lin, Linlin Wei, Zhangao Sun, Shilong Wu, Haoliang Huang, Shu Zhang, Chang Liu, Yang Feng, Huanfang Tian, Huaixin Yang, Jianqi Li, Yayu Wang, Guangming Zhang, Yalin Lu, Yonggang Zhao Heterostructures composed of superconductors and ferroelectrics are important for studying the coupling between superconductivity and ferroelectricity, especially the modulation of superconductivity by ferroelectricity. We study the structural and superconducting properties of FeSe$_{\mathrm{0.3}}$Te$_{\mathrm{0.7}}$ (FST) thin films with different thicknesses grown on ferroelectric Pb(Mg$_{\mathrm{1/3}}$Nb$_{\mathrm{2/3}})_{\mathrm{0.7}}$Ti$_{\mathrm{0.3}}$O$_{\mathrm{3}}$ substrates. The FST films undergo biaxial tensile strains which are fully relaxed for films above 200 nm, and the ultrathin and thicker films exhibit an insulating behavior and superconductivity, respectively. The current-voltage curves around the superconducting transition follow the Berezinskii-Kosterlitz-Thouless (BKT) transition behavior, revealing quasi-two-dimensional phase fluctuation. Upon applying electric field to the heterostructure, Tc of FST thin film increases due to the reduction of the tensile strain in FST. This work sheds light on the superconductivity, strain effect as well as electric-field modulation of superconductivity in FST films. [Preview Abstract] |
Tuesday, March 14, 2017 3:18PM - 3:30PM |
H41.00003: Superconductivity fluctuation of FeSe$_{1-x}$Te$_x$ as a function of x measured by microwave conductivity A. MAEDA, F. NABESHIMA, K. NAGASAWA, D. ASAMI, M. KAWAI, Y. IMAI Iron chalcogenides, Fe$_{1-x}$Se$_x$, have attracted much attention because of many reasons. One of these is that superconductivity fluctuation can be seen up to rather high temperatures probably because of the small Fermi surface effect, and detailed investigation of the superconductivity fluctuation contributes to the understanding of many anomalous and characteristic aspects of superconductivity in these materials. We investigated superconductivity fluctuation of Fe$_{1-x}$Se$_x$ as a function of $x$ (including $x$=0), by measuring complex conductivity in a nonresonant microwave technique. In all materials investigated, superconductivity fluctuation was observed up to higher temperatures than in NbN, which was measured as a conventional standard superconductor. In all samples, however, the fluctuation was not the BKT like. At least, the $x$=0.5 sample shows the 3D-XY behavior, which is popular for many superconductors. With increasing x, the temperature range of prominent superconductivity fluctuation decreases. This might be due to the increase of the Fermi surface volume by the introduction of Te. We will discuss more detailed feature of the fluctuation, such as the critical exponent for other samples etc. [Preview Abstract] |
Tuesday, March 14, 2017 3:30PM - 3:42PM |
H41.00004: Structure and magnetic properties of K$_{1-}$$_{\delta}$(Cu$_{1-x}$Fe$_{1+x}$)Se$_{2}$ Elizabeth Chavira, Pierre Toulemonde, Pierre Strobel, Andres Sulpice, Sebastiane Pairis, Christophe Lepoittevin In our attempt to synthesize new layered K$_{2-x}$Ba$_{x}$CuO$_{2}$Fe$_{2}$Se$_{2}$ hybrid materials [1] we have a new “122” selenide. Started from KO$_ {2}$, FeSe and a precursor of Ba$_{2}$CuO$_{3}$, put as pellets into alumina crucible and reacted at temperature 400-810 °C in sealed quartz tube under vacuum. The XRD show the presence of a tetragonal I4/mm phase, similar to the KFe$_{2-z}$Se$_{2}$ (K122) reported [2] mixed with Fe$_{3}$O$_{4}$ magnetite impurity. The a-axis is increased and c-axis decreased compared to K122, an indication of partial substitution of the cationic site. Our SEM and TEM study coupled to EDX confirm the I4/mm lattice and gives and average (K$_{0.63}$Ba$_{0.28}$)(Fe$_{0.46}$Cu$_{0.30}$)$_{2}$Se$_{2}$ composition. This compound is stable in air, on the contrary of the pure K-based 122 phase: it means that the Cu substitution and/or Ba substitution for K and Fe sites respectively stabilize the structure. In addition the electron diffraction patterns do not show signatures of Cu/Fe orderings, in similarity with K(MnAg)Se$_{2}$ compound [3]. The magnetization measurements show two transitions in addition to the one around 125K due to magnetite, a first one around 250K and another one at 50K. Superconductivity does not seem to be present in this selenide [Preview Abstract] |
Tuesday, March 14, 2017 3:42PM - 3:54PM |
H41.00005: Fully - gapped anisotropic superconductivity robust against point - like disorder in FeSe single crystals. Serafim Teknowijoyo, Kyuil Cho, Makariy A. Tanatar, Anna E. Boehmer, Sergey L. Bud'ko, Paul C. Canfield, Ruslan Prozorov, Olivier Cavani, Vivek Mishra, Peter J. Hirschfeld A highly anisotropic but finite superconducting gap is found in single crystals of FeSe by studying the London penetration depth $\lambda (T)$ measured down to 50 mK in samples before and after 2.5 MeV electron irradiation. The gap minimum increases with the introduced point-like disorder, indicating the absence of symmetry-imposed nodes. Surprisingly, the superconducting transition temperature $T_c$ increases by 0.4 K from $T_{c0} \approx$ 8.8 K while the structural transition temperature $T_s$ decreases by 0.9 K from $T_{s0} \approx$ 91.2 K after electron irradiation. We discuss several scenarios for the $T_c$ enhancement and propose that local strengthening of the pair interaction by irradiation-induced Frenkel defects most likely explains the phenomenon. [Preview Abstract] |
Tuesday, March 14, 2017 3:54PM - 4:06PM |
H41.00006: The gap structure and temperature-pressure phase diagram of FeS superconductor Tianping Ying, Jun Zhang, Fengliang Liu, Xiaofang Lai, Xiaochen Hong, Wenge Yang, Fuqiang Huang, Shiyan Li We report low-temperature heat transport measurements on both single crystal and foil of iron sulfide FeS ($T_c \approx$ 4.5 K), a sulfide counterpart of FeSe. In zero magnetic field, a significant residual linear term $\kappa_0/T$ is observed. At low field, $\kappa_0/T$ increases rapidly with the increase of field. These results suggest a nodal superconducting gap in FeS. The origin of this nodal superconductivity in FeS is discussed, by comparing with other iron-based superconductors with nodal gap. Further, we perform high-pressure measurements on the FeS single crystals. A rapid suppression of $T_c$ and vanishing of superconductivity at 4.0 GPa are observed, followed by a second superconducting dome from 5.0 to 22.3 GPa with a 30\% enhancement in maximum $T_c$. An onsite tetragonal to hexagonal phase transition occurs around 7.0 GPa, followed by a broad pressure range of phase coexistence. The residual deformed tetragonal phase is considered as the source of second superconducting dome. The observation of two superconducting domes in iron-based superconductors poses great challenges for understanding their pairing mechanism. \bf Reference Phys. Rev. B {\bf 94}, 100504(R) (2016). arXiv:1604.05254. [Preview Abstract] |
Tuesday, March 14, 2017 4:06PM - 4:18PM |
H41.00007: In situ electrical resistivity of FeSe thin films Brendan Faeth, Jocienne Nelson, Jason Kawasaki, Shuolong Yang, Chen Li, Darrell Schlom, Kyle Shen Thin films of FeSe grown on SrTiO3 substrates have recently attracted great attention due to the dramatic superconducting Tc enhancement (70-100K) observed in monolayer films in comparison to bulk (9K). Despite substantial research efforts the transport evidence of superconductivity and its systematics remain difficult to obtain, as the extremely air-sensitive nature of these thin films precludes direct observation of superconductivity via traditional ex situ transport measurements without the addition of a potentially damaging capping layer. To circumvent this issue we construct an \textit{in situ} UHV four-point electrical contact probe in conjunction with angle-resolved photoemission spectroscopy to directly study the intrinsic macroscopic superconducting behavior of FeSe thin films. In particular, we investigate the thickness-dependence of the FeSe superconducting Tc as manifested in both transport and photoemission experiments. [Preview Abstract] |
Tuesday, March 14, 2017 4:18PM - 4:30PM |
H41.00008: Optical observation of intercalation-induced symmetry breaking in Fe$_{\mathrm{x}}$-TaS$_{\mathrm{2}}$ Shiyu Fan, Amal Wahish, Kevin Smith, Kenneth O'Neal, Ian Manuel, Jason Haraldsen, Yoichi Horibe, Sang Cheong, Janice Musfeldt Fe$_{\mathrm{\thinspace }}$intercalated TaS$_{\mathrm{2}}$ single crystals exhibit interesting domain patterns at room temperature with the concentration of 1/4 and 1/3. In this work, we measured the optical spectrum and Raman response of Fe$_{\mathrm{1/4}}$TaS$_{\mathrm{2\thinspace }}$and Fe$_{\mathrm{1/3}}$TaS$_{\mathrm{2}}$, and compared this data with the parent compound 2H-TaS$_{\mathrm{2.\thinspace }}$The optical conductivity data indicates a strong free carrier response, and Fe intercalation induced both $d-d$ excitations and new phonon modes compared to the parent compound that are strongly related to the symmetry breaking of the parent compound. Our findings were in good agreement with first principle calculations. [Preview Abstract] |
Tuesday, March 14, 2017 4:30PM - 4:42PM |
H41.00009: Variation of transition temperatures and residual resistivity ratio in vapor-grown FeSe and Fe(Se,S) A. E. B\"ohmer, V. Taufour, L. Xiang, U. S. Kaluarachchi, M. A. Tanatar, B. G. Ueland, A. Kreyssig, W. E. Straszheim, T. Wolf, R. Prozorov, A. I. Goldman, S. L. Bud{'}ko, P. C. Canfield We report on the vapor growth and physical properties of single-crystalline FeSe and Fe(Se,S). Significant variations of sample morphology resulting from slight modifications of the growth conditions are observed. The superconducting transition temperature, $T_{\mathrm c}$, of different FeSe samples varies between 8.8 K and 3 K, and the structural transition temperature, $T_{\mathrm s}$, varies between 90 K and 72 K, respectively. We find that those variations cannot be correlated with the sample composition, measured by WDS. Instead, we point out a clear correlation between $T_{\mathrm s}$, $T_{\mathrm c}$ and disorder, as measured by the residual resistivity ratio. Notably, $T_{\mathrm s}$ and $T_{\mathrm c}$ are linearly correlated with each other\footnote{B\"ohmer et al., PRB 94, 024526 (2016).}. On the other hand, a small amount of sulfur-substitution suppresses $T_{\mathrm s}$ but enhances $T_{\mathrm c}$. We will compare and discuss the effects of various perturbations---disorder, chemical substitution and hydrostatic pressure---on the transition temperatures and properties of FeSe. [Preview Abstract] |
Tuesday, March 14, 2017 4:42PM - 4:54PM |
H41.00010: Suppression of phase separation and whole phase diagram in FeSe$_{1-x}$Te$_x$ thin films on LaAlO$_3$ substrates Yoshinori Imai, Fuyuki Nabeshima, Yuichi Sawada, Daisuke Asami, Masataka Kawai, Atsutaka Maeda In FeSe$_{1-x}$Te$_{x}$, it is known that the phase separation occurs at $x=0.1-0.4$ for bulk samples, and it prevents the complete understanding of this system. In previous meeting, we reported the fabrication of FeSe$_{1-x}$Te$_{x}$ films on CaF$_2$ with with whole range of $x$ by a pulsed laser deposition [1]. Surprisingly, the optimal composition was found in the phase-separation region; $T_c$ reaches 23 K at $x=0.2$. In this presentation, we report the successful fabrication of FeSe$_{1-x}$Te$_{x}$ films with $x=0-0.7$, which includes the phase-separation region, on LaAlO$_3$ substrates. The highest $T_c$ of the films on LaAlO$_3$ substrates reaches 19 K, which is also higher than that of bulk samples. The doping-temperature ($x-T$) phase diagram of FeSe$_{1-x}$Te$_{x}$ films clearly shows that $T_c$ suddenly changes at the Te content where the structural transition disappears, which is commonly observed for both films on LaAlO$_3$ and CaF$_2$[2,3]. Our result indicates that one of the key factors to realize a further increase of $T_c$ in iron chalcogenides is the control of the structural transition. $[1]$ Y. Imai $et$ $al.$, Proc. Natl. Acad. Sci. USA. 112 (2015) 1937. $[2]$ Y. Sawada $et$ $al.$, J. Phys. Soc. Jpn. 85 (2016) 073703. $[3]$ Y. Imai $et$ $al.$, $submitting$. [Preview Abstract] |
Tuesday, March 14, 2017 4:54PM - 5:06PM |
H41.00011: Material transitions and phonon softening correlations in FeSe Adrian Merritt, Alfred Baron, Thomas Wolf, Dmitry Reznik In Ba(Fe1-xCox)2As2 , cooling through the structural transition leads to a softening and then a hardening of the transverse acoustic phonons with small wave vector q, which reflect nematic fluctuations with the same q. These \textbar q\textbar \textgreater 0 fluctuations build up below the superconducting transition in contrast with q$=$0 where nematic fluctuations compete with superconductivity. I will discuss these effects in FeSe. We tracked the transverse acoustic phonon dispersion of single-crystal FeSe using inelastic X-ray scattering at SPring-8 and the Advanced Photon Source. We found a similar result where the acoustic phonon softens at low q through the structural transition and gradually recovers its previous energy upon further cooling. These results as well as the behavior across the superconducting transition will be presented and the implications for our understanding of Fe-based superconductors will be discussed. [Preview Abstract] |
Tuesday, March 14, 2017 5:06PM - 5:18PM |
H41.00012: Quantifying Electron-Phonon Coupling in FeSe by Tracking Coherent Phonons P.S. Kirchmann, S. Gerber, S.-L. Yang, H. Soifer, D. Zhu, J.A. Sobota, S. Rebec, J.J. Lee, T. Jia, B Moritz, C. Jia, Y. Li, D. Leuenberger, Y. Zhang, H. Jang, J.-S. Lee, S. Song, J.M. Glownia, S. Nelson, K.W. Kim, Y.-D. Chuang, R.G. Moore, T.P. Devereaux, W.-S. Lee, Z.-X. Shen The superconductor FeSe is a good prototype for studying how correlation effects impact the electron-phonon coupling. Here, we present a new time-domain method to quantify the electron-phonon coupling strength: An optical femtosecond pump pulse excites a coherent A1g phonon; time-resolved x-ray diffraction tracks the coherent lattice motion while ultrafast photoemission monitors the related changes of the electronic bands. We combine the oscillatory responses from both experiments in a high-precision ‘coherent lock-in’ measurement of the electron-phonon coupling strength. Comparison with theory reveals a strong enhancement of the coupling strength in FeSe due to correlation effects. [Preview Abstract] |
Tuesday, March 14, 2017 5:18PM - 5:30PM |
H41.00013: Reentrant high-magnetic field superconductivity in a clean two-dimensional superconductor with shallow band Alexei E. Koshelev, Kok Wee Song We investigate the superconducting instability in the magnetic field for a clean two-dimensional multiple-band superconductor in the vicinity of the Lifshitz transition when one of the bands is very shallow. Due to a small number of carriers in this band, the quasiclassical Werthamer-Helfand approximation breaks down and Landau quantization has to be taken into account. We found that the transition temperature $T_{c2}(H)$ has giant oscillations and is resonantly enhanced at the magnetic fields corresponding to full occupancy of the Landau levels in the shallow band. This enhancement is especially pronounced for the lowest Landau level. As a consequence, the reentrant superconducting regions in the temperature-field phase diagram emerge at low temperatures near the magnetic fields at which the chemical potential matches the Landau levels. These regions may be disconnected from the main low-field superconducting region. The specific behavior depends on the relative strength of the intraband and interband coupling constants and the effect is most pronounced when the interband coupling dominates. The Zeeman spin splitting reduces sizes of the reentrant regions and changes their location in the parameter space. The predicted behavior may realize in the gate-tuned FeSe monolayer. [Preview Abstract] |
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