Bulletin of the American Physical Society
APS March Meeting 2013
Volume 58, Number 1
Monday–Friday, March 18–22, 2013; Baltimore, Maryland
Session F37: Focus Session: Fe-based Superconductors: Order Parameter Symmetry/Fe(Te,Se) Films |
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Sponsoring Units: DMP DCOMP Chair: Qiang Li, Brookhaven Natl Lab Room: 345/346 |
Tuesday, March 19, 2013 8:00AM - 8:12AM |
F37.00001: Symmetry measurements of the order parameter of BaFe$_2$As$_2$ superconductors Juan Atkinson, Dale Van Harlingen, Paul Canfield, Duck Chung Since the discovery of the Fe-pnictide superconductors, extensive efforts have been directed toward understanding the symmetry and mechanism of the superconducting pairing. Extended s-wave models, predominately the s$\pm$ model, are predicted by many theories, but a definitive experimental verification has been elusive. We are using phase-sensitive Josephson interferometry to test for magnitude and phase anisotropy in electron (Co-doped) and hole (K-doped) BaFe$_2$As$_2$ single crystals. In particular, we are looking in the heavily K-doped regime that is predicted to exhibit d-wave symmetry characterized by a sign change in the order parameter. We are also searching for proximity-induced structure in the density-of-states of an s-wave superconductor proximity-coupled to an Fe-pnictide superconductor that is predicted to arise for s$\pm$ pairing (Koshelev, 2012). [Preview Abstract] |
Tuesday, March 19, 2013 8:12AM - 8:24AM |
F37.00002: Sign-changing nodal $s$-wave gap in heavily over doped (Ba$_{1-x}$K$_x$)Fe$_2$As$_2$ evidenced from thermal-transport measurement Daiki Watanabe, Shigeru Kasahara, Takuya Yamashita, Takumi Ota, Takasada Shibauchi, Yuji Matsuda, Minoru Yamashita, Hideto Fukazawa, Taku Saito, Yoh Kohori, Shigeyuki Ishida, Kunihiro Kiho, Chul-Ho Lee, Akira Iyo, Hiroshi Eisaki, Anton Vorontsov The superconducting state of hole-doped Fe-based superconductors, (Ba$_{1-x}$K$_x$)Fe$_2$As$_2$, changes from a fully-gapped state near the optimally doping ($x\sim$0.5) to a nodal one at the end material ($x$= 1)[1,2]. Here we report the results of thermal-transport measurements for heavily overdoped $x$=1, 0.93, 0.88, 0.76 crystals and discuss the doping evolution of the superconducting gap. For $x$=0.88, 0.93 and 1, the $T$-dependence of thermal conductivity in zero field shows a finite $\kappa _0/T$ in the zero-temperature limit. In low magnetic fields, $\kappa/T$ ($T\to 0$ K) increases as $\propto \sqrt{H}$. These results indicate the presence of gap nodes in the gap function. We find that the residual $\kappa _0/T$ exhibits a non-monotonic $x$-dependence, which is inconsistent with $d$-wave symmetry. We show that the observed $x$-dependence can be explained by nodal $s$-wave pairing with sign change between zone centered hole pockets. [1] K. Hashimoto {\it et al.}, Phys. Rev. B {\bf 82}, 014526 (2010). [2] K. Okazaki {\it et al.}, Science {\bf 337}, 1314 (2012). [Preview Abstract] |
Tuesday, March 19, 2013 8:24AM - 8:36AM |
F37.00003: Angle Dependent Specific Heat Study of BaFe$_2$(As$_{0.7}$P$_{0.3}$)$_2$ Liam Malone, Yuta Mizukami, Philip Walmsley, S. Kasahara, T. Shibauchi, Y. Matsuda, Antony Carrington The structure of the superconducting gap of the pncitide superconductors is an unresolved but crucial issue to understanding their mechanism of superconductivity. While some experiments and theories support a fully gapped s+/s- state, several experiments have revealed evidence for nodes in some families of pnictides. Detailed knowledge of the superconducting gap structure and how it varies between different families can be useful in helping to decide between microscopic theories. BaFe$_2$(As$_x$P$_{1-x}$)$_2$ is a pnictide family where penetration depth and thermal conductivity measurements show evidence for nodes [1]. We have measured the specific heat of a single crystal of BaFe$_2$(As$_{0.7}$P$_{0.3}$)$_2$ (T$_c\sim$29\,K) at low fields and as a function of applied field angle. The angle dependence of specific heat at low fields and low temperature is expected to show minima whenever it is along a nodal direction and can be used to differentiate between gap symmetries [2]. Our results show a clear angle dependent component consistent with the presence of nodes and we discuss the implications on the gap structure of BaFe$_2$(As$_{0.7}$P$_{0.3}$)$_2$.\\[4pt] [1] K. Hashimoto et al, Phys. Rev. B, 81, 220501R,(2010).\\[0pt] [2] A. B. Vorontsov et al, Phys. Rev. Lett. 105 ,187004 (2010) [Preview Abstract] |
Tuesday, March 19, 2013 8:36AM - 8:48AM |
F37.00004: Isotropic superconducting gap structure in BaFe1.90Pt0.10As2 from low temperature thermal conductivity Kevin Kirshenbaum, Shanta Saha, Steven Ziemak, Rongwei Hu, Jean-Philippe Reid, Ryan Gordon, Louis Taillefer, Johnpierre Paglione In this study we present measurements of thermal transport down to 50 mK in single crystals of the iron-based superconductor BaFe1.90Pt0.10As2 with Tc $=$ 23 K [1]. Magnetic fields up to 15 T were applied along the c-axis of the crystal as well as along the basal plane direction to probe the anisotropy of the superconducting gap. The lack of any significant residual electronic term in thermal conductivity for all field directions and values confirms the absence of nodes and places limits on the depth of gap minima in this system.\\[4pt] [1] S.R. Saha et al, JPCM 22 072204 (2010). [Preview Abstract] |
Tuesday, March 19, 2013 8:48AM - 9:00AM |
F37.00005: Investigation of Pairing Symmetry in Pt-Substituted BaFe2As2 S. Ziemak, K. Kirshenbaum, S.R. Saha, R. Hu, J. Paglione, J.-Ph. Reid, R. Gordon, L. Taillefer, A. Ignatov, D. Kolchmeyer, G. Blumberg, D. Evtushinsky, S. Thirupathaiah, S.V. Borisenko We present results from several measurements on BaFe1.9Pt0.1As2 single crystals designed to measure the superconducting gap structure. Low temperature thermal conductivity was measured in magnetic fields up to 15 T and will be compared to other materials. Point-contact Andreev reflection spectroscopy measurements were made using the needle-anvil technique and the spectra analyzed using BTK theory. Raman spectroscopy was used to probe excitations in multiple channels. Finally, angle-resolved photoemission spectroscopy was used to provide further information about the band structure and superconducting gap. We will discuss the implications that the combination of these results reveal about the superconducting order parameter in this system. [Preview Abstract] |
Tuesday, March 19, 2013 9:00AM - 9:12AM |
F37.00006: In-plane transport anisotropy study of oxygen doped iron telluride MBE thin films on LaAlO3 substrate Can Zhang, Huihuo Zheng, Mao Zheng, Brian Mulcahy, Xiaoxiao Wang, Ying Jia, Ulrich Welp, James Eckstein FeTe is well known for its simple crystal structure in the 11 family iron-based high temperature superconductors. We have observed two distinct in-plane transport phenomena in MBE grown FeTe thin films on LaAlO3 substrates. The first one is an unexpected global alignment of the in-plane transport anisotropy. A low temperature resistivity upturn feature has been observed in the neighborhood of the superconducting transition temperature as a function of transport direction. The resistivity upturn feature emerges from 8K to 20K. The second one is the coexistence of superconductivity with the low temperature resistivity upturn. We will report our studies of these distinct transport anisotropy experimental results comparing them with first principle simulations. [Preview Abstract] |
Tuesday, March 19, 2013 9:12AM - 9:24AM |
F37.00007: Electric transport properties in FeSe0.3Te0.7 / Au c-axis tunneling junctions Yu Tien Shen, You Sheng Li, Cheng Chung Chi Currently favored pairing symmetry in the iron-based superconductor is the nodaless S$+$- wave. Based on this theory, the conductance spectra of the normal metal to superconductor tunneling junctions do not exhibit ZBCP. We report the fabrication and the transport properties of c-axis tunneling junctions formed by FeSe0.3Te0.7 (FeSeTe) and Au. When FeSeTe is in its normal state, the conductance spectrum shows a V-shape background; while when FeSeTe becomes superconducting, the conductance spectrum shows some remarkable features: First, a pronounced ZBCP was obtained as temperature is just below Tc, and when the temperature was further decreased to below 4K, a clear double peak structure appears; Second, there were two dip structures at around 4 and 20mV. We found out that there is a linear dependence of the voltage difference between the double peaks versus applied field. Though the origin of the ZBCP, the double peak structure, and the dip structures were unclear and still under investigation, we believe that they all related to the superconducting gaps due to their dependence on applied magnetic fields and temperature. The existence of the prominent ZBCP is not consistent with the proposed S$+$- wave symmetry of the superconducting gap. [Preview Abstract] |
Tuesday, March 19, 2013 9:24AM - 9:36AM |
F37.00008: New bi-epitaxial Grain boundary Josephson Junction of FeSe$_{0.3}$Te$_{0.7}$ You-Sheng Li, Y.T. Shen, M.J. Wang, M.K. Wu, C.C. Chi We have successfully fabricated epitaxial FeSe$_{0.3}$Te$_{0.7}$films on MgO substrate with its in-plane crystalline axis either parallel to or rotated 45$^{\circ}$ with respect to the MgO lattice. We use this technique to fabricate the 45$^{\circ}$ grain-boundary Josephson junction. The IV-curve measured at 4.2 K can be fitted with the RSJ model, and the measured I$_{c}$R$_{n}$value is 9.24$\mu$V, which is in general agreement with the values obtained by previous results for Fe-based grainboundary junctions on bi-crystal substrates. We have also measured the dc Josephson current as a function of applied magnetic fields, which shows a clear Fraunhofer-like pattern. Hence we can rule out the possibility of d-wave symmetry in FeSe$_{0.3}$Te$_{0.7}$ superconductor. Upon applying 6 GHz microwave irradiation, the junction IV curve exhibits clear Shapiro steps. Thus we have demonstrated our ability to fabricate high quality grain-boundary Josephson junctions of this new class of material. Further physical properties, such as the noise power spectrum, are currently being investigated. [Preview Abstract] |
Tuesday, March 19, 2013 9:36AM - 9:48AM |
F37.00009: The tunneling spectra and superconducting gaps observed by using scanning tunneling microscopy near the (100)/(110) grain-boundary of FeSe0.5Te0.5 films Kuang Cheng Lin, You-Sheng Li, Cheng-Chung Chi We have found that, using PLD method, the a- and b-axis of the FeSe$_{0.5}$Te$_{0.5}$ film deposited on pristine MgO substrate are parallel to those of MgO, while these axes of the film grown on MgO substrate treated with Ar-ion milling rotate 45$^{\circ}$ along its c-axis. Here, we prepared such film with two kinds of orientations (0$^{\circ}$ and 45$^{\circ}$ ab plane with respect to the substrate axis) on MgO substrate with the connection between them form a ramp at an angle about 30$^{\circ}$ to the substrate plane. We used STM to study the tunneling spectra of two orientations of c-axis planes and the connection ramp between them. In the planar region with different orientation, we have observed similar tunneling spectra with a superconducting gap about 5 meV. This gap value is consistent with the previous studies of a variety of FeSeTe samples. However, a much larger gap about 18 meV is observed in the ramp region. The only paper we found to have such a large gap in the family of Fe-based superconductors is the one by Xue et.al. They have shown a gap of 20 meV in one unit-cell thick of FeSe on STO substrate. Furthermore, we have also observed a small ZBCP inside the large gap at 4.3K. The ZBCP becomes smaller with increasing temperature and disappears near and above TC. [Preview Abstract] |
Tuesday, March 19, 2013 9:48AM - 10:00AM |
F37.00010: Transport properties of transition-metal substituted FeTe0.65Se0.35 single crystals Valeriy L. Bezusyy, Dariusz J. Gawryluk, Artur Malinowski, Marek Berkowski, Marta Z. Cieplak We use the ab-plane resistivity and Hall effect measurements to evaluate the influence of substitutions on the superconductivity and normal-state transport in Fe$_{\mathrm{1-y}}$M$_{\mathrm{y}}$Te$_{\mathrm{0.65}}$Se$_{0.35}$ single crystals, where M$=$Co, Ni or Cu. The crystals, with 0 \textless\ y \textless\ 0.11, are grown by Bridgman's method. We find that the Co impurity induces markedly different effects than the other two impurities. Superconducting transition temperature (T$_{\mathrm{c}})$ is suppressed with the rate of about 1.3 K per at.{\%} of Co impurity, while the rate is about 3.5 and 4 times larger in case of Ni and Cu, respectively. The resistivity at the T$_{\mathrm{c}}$ onset remains almost unaffected by Co doping, while it increases substantially for Ni and Cu. The Hall constant (R$_{\mathrm{H}})$ is positive for all samples, indicating that hole carriers dominate the transport. However, while the R$_{\mathrm{H}}$ is gradually suppressed towards zero with increasing Co content suggesting that electron doping occurs, it remains almost unchanged by Ni or Cu doping, suggesting that these impurities are rather of isovalent nature. The implications of these results will be discussed. [Preview Abstract] |
Tuesday, March 19, 2013 10:00AM - 10:12AM |
F37.00011: FeSe$_{0.5}$Te$_{0.5}$ thin film Josephson junction on SrTiO$_{3}$ bicrystal substrates Weidong Si, Cheng Zhang, Xiaoya Shi, Qiang Li Josephson junctions were fabricated in the epitaxial FeSe$_{0.5}$Te$_{0.5}$ thin films on [100] tilted SrTiO$_{3}$ bicrystal substrates with a CeO$_{2}$ buffer layer. These junctions with a 24 degree of grain boundary misorientation show a typical resistive-shunt-junction like current-voltage behavior. Critical current densities across the grain boundary in these junctions were observed to be remarkably suppressed and modulated by the magnetic field. Films without the grain boundary show a critical current density much higher than those with the grain boundary. These results indicate a Josephson Effect in those grain boundary junctions. [Preview Abstract] |
Tuesday, March 19, 2013 10:12AM - 10:24AM |
F37.00012: Planar tunnel junction on oxygen doped iron telluride thin films Mao Zheng, Han Zhao, Can Zhang, Gustaf Olson, Brian Mulcahy, Valentin Stanev, Alexei Koshelev, Laura Greene, James Eckstein Since its discovery, iron based superconductivity has garnered much interest from the research community for its potential in both application and fundamental science. One of the questions awaiting an answer is the pairing symmetry of this new class of superconductors. Recently, Koshelev and Stanev proposed a fingerprint of s$+$- symmetry in the NIS tunneling spectrum where the iron based superconductor is proximity-coupled to a thin s-wave superconductor[1]. We have prepared oxygen doped iron telluride (FeTe:Ox) thin films, along with an in-situ grown tunnel barrier and top electrode by Molecular Beam Epitaxy (MBE). We have fabricated them into planar tunnel junction and will report the temperature dependence of both tunneling and point contact spectra. [1]. A. E. Koshelev and V. Stanev, EPL (Europhysics Letters) \textbf{96} (2), 27014 (2011). [Preview Abstract] |
Tuesday, March 19, 2013 10:24AM - 10:36AM |
F37.00013: Non-Fermi liquid behavior in overdoped iron-pnictide Ba(Fe,Co,Ni)2As2 Alex Hughes, Yasuyuki Nakajima, Kevin Kirshenbaum, Shanta R. Saha, Tyler Drye, Johnpierre Paglione Very low-temperature specific heat was used to study a series of single-crystal iron-based intermetallic compounds with the ThCr2Si2 structure with transition metal substitution used to heavily over-dope the system. This system has been found to exhibit non-Fermi liquid characteristics in transport, magnetic and thermodynamic properties. We will present low-temperature specific heat capacity measurements of this compound in order to elucidate the non-Fermi liquid nature of the ground state and to help elucidate the origin of these properties and their relation to superconductivity. [Preview Abstract] |
Tuesday, March 19, 2013 10:36AM - 10:48AM |
F37.00014: Observation of a c-axis collapse in superconducting FeTeO$_{\mathrm{x}}$ films below T$_{\mathrm{c}}$ Lahiru Narangammana, Xuerong Liu, Yuefeng Nie, Joseph Budnick, Christof Niedermayer, John Hill, Genda Gu, Barrett Wells We compared the temperature dependent crystal structure of superconducting FeTeO$_{\mathrm{x}}$ films and non superconducting Fe$_{1.02}$Te single crystals. The primary difference between the two is that the superconducting FeTeO$_{\mathrm{x}}$ films show a collapse of the c-axis below the superconducting transition temperature 13K. No such transition occurs in the single crystal. The room temperature structures of the two are similar and both show a tetragonal to monoclinic transition near 60K. Preliminary neutron diffraction studies indicate a suppression in antiferromagnetic order below T$_{\mathrm{c}}$ on superconducting FeTeO$_{\mathrm{x}}$ thin films. [Preview Abstract] |
Tuesday, March 19, 2013 10:48AM - 11:00AM |
F37.00015: Effect of doping on the specific heat jump in iron-based superconductors Dushko Kuzmanovski, Saurabh Maiti, Maxim Vavilov, Andrey Chubukov, Frederic Hardy In this talk we present a theoretical description of the jump of the specific heat at the transition to a superconducting phase of iron-based pnictides. We discuss both the overdoped regime, when the transition occurs between non-magnetic and superconducting phases, and the underdoped regime, when superconductivity emerges from a pre-emptive SDW phase. Both effects lead to a qualitatively similar phase diagram as a function of doping, but details differ. We presume that doping simultaneously modifies the Fermi surface of pnictides and introduces disorder. By fitting the transition temperatures for the SDW and SC phases, we establish the relative strengths of the the rigid band shift caused by doping and doping-induced disorder. We then evaluate the specific heat jump as a function of doping. Our theory is consistent with measurements made by Karlsruhe group of the specific heat jump in $\mathrm{BaFe}_{2}\mathrm{As}_{2}$ compounds with $\mathrm{K}$- and $\mathrm{Co}$-doping. [Preview Abstract] |
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