Bulletin of the American Physical Society
APS March Meeting 2015
Volume 60, Number 1
Monday–Friday, March 2–6, 2015; San Antonio, Texas
Session F16: Focus Session: Search for New Fe-based Superconductors I |
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Sponsoring Units: DMP DCOMP Chair: Qiang Li, Brookhaven National Laboratory Room: 101AB |
Tuesday, March 3, 2015 8:00AM - 8:36AM |
F16.00001: Why are the T$_{\mathrm{c}}$s so high in rare-earth doped CaFe$_{2}$As$_{2}$ single crystals and ultrathin FeSe epi-films? Invited Speaker: C.W. Chu Recent reports of non-bulk superconductivity with unexpectedly high onset-T$_{\mathrm{c}}$s up to 49 K in the Pr-doped CaFe$_{2}$As$_{2}$ [(Ca,Pr)122] single crystals [1] and up to 100 K in one-unit-cell (1UC) FeSe epi-films [2], respectively, offer an unusual opportunity to seek an answer to the question posed in the title. Through systematic compositional, structural, resistive, and magnetic investigations on (Ca,R)122 single crystals with R $=$ La, Ce, Pr, and Nd, we have observed a doping-level-independent T$_{\mathrm{c}}$, a large magnetic anisotropy, and the existence of mesoscopic-2D structures in these crystals, thus providing evidence consistent with the proposed interface-enhanced T$_{\mathrm{c}}$ in these naturally assembled Fe-based superconductors. Similar resistive and magnetic measurements were also made on the 1-4UC FeSe ultra thin epi-films. We have detected a Meissner state below 1 Oe with extensive weak-links up to $\sim$ 20 K, unconnected small superconducting patches up to $\sim$ 40 K, and an unusual dispersion of diamagnetic moment with frequency up to 80 K. The unusual frequency dependences of the diamagnetic moment observed in the films at different temperature ranges suggest that collective excitations of electron and/or spin nature may exist in the FeSe films below 20 K and 40-80 K. The experimental results will be presented and the implications discussed. \\[4pt] [1] Q. Y. Wang et al., Chin. Phys. Lett. 29, 037402 (2012); J. F. Ge et al., arXiv:1406.3435 [cond-mat.supr-con] (2014). \\[0pt] [2] L. Z. Deng et al., arXiv:1311.6459 [cond-mat.supr-con] (2013); B. Lv et al. PNAS USA 108, 15705 (2011); B. Lv. et al., MRS Symp. Proc. 1684 (2014). [Preview Abstract] |
Tuesday, March 3, 2015 8:36AM - 8:48AM |
F16.00002: Probing the coexistence of superconductivity and ferromagnetism Christian Urban, Ali Basaran, Ursula Pachmayr, Dirk Johrendt, Ivan K. Schuller We present magnetic field modulated microwave spectroscopy (MFMMS) and VSM data of a lithiated iron selenide hydroxide, [(Li$_{\mathrm{1-x}}$Fe$_{\mathrm{x}})$OH](Fe$_{\mathrm{1-y}}$Li$_{\mathrm{y}})$Se (x$=$0.2, y$=$0.08). This compound is one of the few claimed to exhibit superconductivity below 43 K together with ferromagnetism below 10 K. MFMMS is a unique technique which is highly sensitive and selective to electronic phase transistions. It allows us to probe the superconductivity and ferromagnetism at the same time in the lithiated iron selenide hydroxide. This is the first time that a microwave signature of this class of materials is presented. We compare our results to standard magnetometry measurements. This study is financially supported by the AFOSR Grant No. FA9550-12-1-0381. [Preview Abstract] |
Tuesday, March 3, 2015 8:48AM - 9:00AM |
F16.00003: La$_{0.5-x}$Na$_{0.5+x}$Fe$_{2}$As$_{2}$: electron and hole doping in the spacing layer Jiaqiang Yan, S. Nandi, B. Sales, D. Mandrus The electron-hole asymmetry in the phase diagram of iron-based superconductors is well illustrated in doped BaFe$_{2}$As$_{2}$ by comparing hole-doped Ba$_{\mathrm{1-x}}$K$_{\mathrm{x}}$Fe$_{2}$As$_{2}$ and electron doped BaFe$_{\mathrm{2-x}}$Co$_{\mathrm{x}}$As$_{2}$, mainly due to the availability of high quality single crystals which enable systematic studies using various probes. In Ba$_{\mathrm{1-x}}$K$_{\mathrm{x}}$Fe$_{2}$As$_{2}$, K-doping takes place at the spacing layer while FeAs layers remain intact. In contrast, Co substitution in BaFe$_{\mathrm{2-x}}$Co$_{\mathrm{x}}$As$_{2}$ disturbs the contiguity of the [FeAs$_{4}$] tetrahedra and interferes with superconductivity in the FeAs layers. This effect coming from substitution at different crystallographic sites has been suggested to contribute to the electron-hole asymmetry. In this talk, I will present the magnetic and structural transitions of La$_{\mathrm{0.5-x}}$Na$_{\mathrm{0.5+x}}$Fe$_{2}$As$_{2}$. Our results show that La$_{\mathrm{0.5-x}}$Na$_{\mathrm{0.5+x}}$Fe$_{2}$As$_{2}$, or even compounds with other rare earth and alkali ions in the spacing layer, provides a new material platform for the study of iron-based superconductors. The material could be tuned from electron-doped to hole-doped by varying the ratio between the alkali metal and rare earth ions. [Preview Abstract] |
Tuesday, March 3, 2015 9:00AM - 9:12AM |
F16.00004: The structural and magnetic phase transitions in a ``parent'' Fe pnictide compound Ni Ni, Jared Allred, Huibo Cao, Wei Tian, Lian Liu, Kyuil Cho, Matthew Krogstad, Jie Ma, Keith Taddei, Makariy Tanatar, Ruslan Prozorov, Masaaki Matsuda, Stephan Rosenkranz, Yasutomo Uemura, Shan Jiang We will present transport, thermodynamic, synchrotron X-ray, neutron diffraction, $\mu $SR, ARPES and polarized optical image measurements on the ``parent'' compound of the 112 high Tc superconducting Fe pnictide family. Structural and magnetic phase transitions are revealed. Detailed magnetic structure was solved by single crystal neutron diffraction. We will discuss the similarity and difference of these transitions comparing to the parent compounds of other Fe pnictide superconductors. [Preview Abstract] |
Tuesday, March 3, 2015 9:12AM - 9:24AM |
F16.00005: Study on the non-bulk superconductivity in CaFe$_{2}$As$_{2}$ single crystals under different annealing conditions. K. Zhao, B. Lv, L.Z. Deng, Y.Y. Xue, C.W. Chu The interplay between SDW and tetragonal/collapsed tetragonal(T/cT) structural transition in CaFe$_{2}$As$_{2}$ single crystals under different annealing conditions has been thoroughly investigated, while the occurrence of filamentary superconductivity with T$_{c}$ $\sim$ 10K remains mysterious. In this talk, I will present our studies on the interplay of magnetism, structural transition, and superconductivity in the undoped CaFe$_{2}$As$_{2}$ single crystals obtained under different synthetic conditions and annealing procedures. The diamagnetic shift has been detected for the first time and taken as an evidence of superconductivity. High temperature annealing and low temperature annealing drive CaFe$_{2}$As$_{2}$ into the cT ground state and the T ground state, respectively. The superconductivity signal is maximized in the condition where the sample reaches the unstable region at the border of these two. The detailed results and their implications will be presented and discussed. [Preview Abstract] |
Tuesday, March 3, 2015 9:24AM - 9:36AM |
F16.00006: First-principles study on the specific heat of optimally hole-doped BaFe2As2 compound Hyungju Oh, Sinisa Coh, Marvin L. Cohen We present density functional calculation of optimally K-doped BaFe2As2, including a modification of the GGA potential by adding a repulsive term (GGA$+$A). We tune the additional repulsive term until the occupied bandwidth of the M-point electron pocket agrees with experimental data. The calculated Sommerfeld coefficient and electron-phonon coupling constant yield a theoretical specific heat coefficient comparable to the experimental one. In addition, overall band structure and Fermi surface topology are improved with respect to the experiment. This work was supported by NSF Grant No. DMR10-1006184 and the U.S. Department of Energy under Contract No. DE-AC02-05CH11231. Computational resources have been provided by the DOE at Lawrence Berkeley National Laboratory's NERSC facility. [Preview Abstract] |
Tuesday, March 3, 2015 9:36AM - 9:48AM |
F16.00007: High-temperature superconductivity from fine-tuning of Fermi-surface singularities in iron oxypnictides Aliaksei Charnukha In the family of iron-based superconductors, 1111-type materials exhibit superconductivity with the highest transition temperature Tc=55K. Early theoretical predictions of their electronic structure revealed multiple large circular sheets of the Fermi surface. Here we use ARPES to show that two prototypical compounds of the 1111 type are at odds with this description. Their low-energy band structure is formed by the edges of several bands, which are pulled to the Fermi level from the depths of the theoretically predicted band structure by strong electronic interactions. We further demonstrate that although their low-energy electronic looks remarkably similar, the Tc differs by a factor of 2. Upon closer examination we uncover that one of the bands in the higher-Tc compound sinks to 2.3meV below the Fermi level and thus does not produce a Fermi surface. And yet we find that it hosts a superconducting energy gap 10x larger than the same band in the lower-Tc sister compound. Our results show that the Fermi-surface singularities in the iron-oxypnictides dramatically affect their low-energy electronic properties, including superconductivity, and must therefore be explicitly taken into account in any attempt to understand the pairing mechanism. [Preview Abstract] |
Tuesday, March 3, 2015 9:48AM - 10:00AM |
F16.00008: Mott Kondo Insulating Behavior in Iron-Oxychalcogenides Byron Freelon, YuHao Liu, Jung-Leng Chen, Luis Craco, Mukul Laad, Stefano Leoni, Jiaqi Chen, Li Tao, Minghu Fang, Roxana Flauca, Zahra Yamani, Yi-sheng Liu, Chinglin Chang, J.-H. Guo, Zahid Hussain The findings of unconventional high-Tc superconductivity (HTSC) in Fe arsenides and selenides have reinvigorated HTSC research. The outstanding debate over how the normal state gives way to HTSC in cuprates has emerged in discussions on possible HTSC mechanisms in Fe-based superconductors. Does Fe-based HTSC result as a pairing instability of a conventional Landau-Fermi liquid (LFL), or as one of a non-LFL, akin to the cuprates? One possible way to address this important question is to investigate Fe-based materials that exhibit a Mott insulating parent phase as in the cuprates. Here we present a study of such materials. We performed a combined experimental-theoretical investigation of the Fe-oxychalcogenides (FeO\textit{Ch}) series La$_{\mathrm{2}}$O$_{\mathrm{2}}$Fe$_{\mathrm{2}}$O$M_{\mathrm{2}}$ ($M=$S, Se), the latest among the Fe-based materials with the potential to show unconventional high-Tc superconductivity (HTSC). [Preview Abstract] |
Tuesday, March 3, 2015 10:00AM - 10:12AM |
F16.00009: Correlation effects in KFe$_2$As$_2$, RbFe$_2$As$_2$ and CsFe$_2$As$_2$ Steffen Backes, Harald O. Jeschke, Roser Valenti We perform a systematic LDA+DMFT study of the iron-pnictide series KFe$_2$As$_2$\footnote{Backes {\it et al.}, New J. Phys. \textbf{16}, 083025 (2014)}, RbFe$_2$As$_2$ and CsFe$_2$As$_2$ and compare with available experiments. We find not only strong orbital-dependent renormalizations and Fermi surface topology changes compared to the local-density approximation but also interesting features at higher binding energies. We discuss the observation of a possible orbitally-selective instability as a function of isoelectronic doping\footnote{Backes {\it et al.}, in preparation}. [Preview Abstract] |
Tuesday, March 3, 2015 10:12AM - 10:24AM |
F16.00010: Electronic Properties of Fe-based Ladder Compounds Fei Du, Kenya Ohgushi, Yutaka Ueda The crystal structure of Fe-based superconductors found so far have two-dimensional conducting planes composed of a square lattice of Fe atoms coordinated tetrahedrally by pnictogens or chalcogens. Although there is no report on the discovery of superconductivity in Fe-based materials with one-dimensional structures, elucidating electronic states of such compounds is expected to give an important clue to the mechanism of superconductivity as well as a strong hint for searching new superconductors. We here report on electronic properties of a series of quasi-one-dimensional spin-ladder compounds AFe$_{2}$X$_{3}$ (A $=$ K, Cs, Ba; X $=$ S, Se) with a special focus on a solid-solution AFe$_{2}$(S$_{1-x}$Se$_{x})_{3}$. We demonstrate that fruitful electronic states emerge as a consequence of the strong electron correlation effect and quantum fluctuations in a low dimensional crystal structure. The following is the list of papers directly related to this talk: [1] Y. Nambu, et al., Phys. Rev. B, 85 064413 (2012). [2] F. Du, et al, Phys. Rev. B 85, 214436 (2012). [3] F. Du, et al, Phys. Rev. B 90, 085143 (2014). [Preview Abstract] |
Tuesday, March 3, 2015 10:24AM - 10:36AM |
F16.00011: H$_{\mathrm{c2}}$(0) and the Kondo Effect in FeSe$_{0.1}$Te$_{0.9}$ Epitaxial Films Nicholas Cornell, Anvar Zakhidov, Marcelo Jaime, Jijie Huang, Hayan Wang, Myron Salamon High-quality, [001]-oriented epitaxial films of FeSe$_{0.1}$Te$_{0.9}$ have been grown on SrTiO$_{3}$. They are found to have increased critical temperatures and critical fields relative to both bulk samples and thin films of the sister compound, FeSe$_{0.5}$Te$_{0.5}$. Critical field values in excess of 114 T have been reported based on WHH theory. In addition to these improved properties, most samples show resistance minima above T$_{\mathrm{c}}$, reminiscent of the Kondo effect, presumably from excess Fe. We report results of a high field investigation of these thin films that reveals an empirical zero-temperature value of H$_{\mathrm{c2}}$(0) $\approx $ 46 T along [001], significantly less than the~ WHH estimate, but still exceeding the maximum strong coupling correction to the Pauli limit. Large negative magnetoresistance above the critical field confirms the presence of Kondo behavior in the normal state and persists without saturation up to 60 T. Why the measured critical field exceeds the paramagnetic limit remains a question. However, a Kondo temperature that exceeds the superconducting T$_{\mathrm{c}}$ can lead to overestimated WHH upper critical fields and could explain the wide variation in T$_{\mathrm{c}}$ and H$_{\mathrm{c2}}$ among the ``11'' iron chalcogenides. [Preview Abstract] |
Tuesday, March 3, 2015 10:36AM - 10:48AM |
F16.00012: High Critical Field Superconductivity in FeSe0.1Te0.9 Coated Carbon Nanotubes Haiyan Wang, Nicholas Cornell, Jijie Huang, Myron Salamon, Anvar Zakhidov Thin films of FeSe0.1Te0.9, grown on SrTi03, have been shown to possess an increased critical temperature, field, and current relative to both bulk samples of FeSe0.1Te0.9 and thin films of the related compound FeSe0.5Te0.5. Empirical measurement of FeSe0.1Te0.9 thin films reveal a zero temperature Hc2(0) $\sim$ 45T. Carbon nanotubes are a promising lightweight flexible material for superconducting applications and have proven a robust substrate when conformally coated by superconducting MgB2. Thin film coatings of FeSe0.1Te0.9 have been deposited via pulsed laser deposition on dry- drawn multiwall carbon nanotube sheets drawn from CVD grown forests. While true zero resistance isn't achieved due to inter-connectivity issues or junction effects in multiwall CNT case, clear superconducting transitions with R reaching zero can be seen on other single wall CNT, and non-oriented carbon nanotube substrates. Properties of these superconducting FeSe0.1Te0.9@SWCNT thin films are discussed. [Preview Abstract] |
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