Bulletin of the American Physical Society
APS March Meeting 2011
Volume 56, Number 1
Monday–Friday, March 21–25, 2011; Dallas, Texas
Session L26: Focus Session: Iron Based Superconductors -- ARPES |
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Sponsoring Units: DMP DCOMP Chair: Gabriel Kotliar, Rutgers University Room: D162/164 |
Tuesday, March 22, 2011 2:30PM - 2:42PM |
L26.00001: ABSTRACT WITHDRAWN |
Tuesday, March 22, 2011 2:42PM - 2:54PM |
L26.00002: Angle-resolved photoemission spectroscopy study of Ba(Fe$_{1-x}$Ru$_{x})_{2}$As$_{2}$ Tian Qian, Nan Xu, Pierre Richard, Yingbo Shi, Guanghan Cao, Zu'an Xu, Hong Ding Ru-doped BaFe$_{2}$As$_{2}$ compounds were discovered recently to show superconductivity at a relatively wide doping range. We will present angle-resolved photoemission spectroscopy results of electronic structure and Fermi surface of Ba(Fe$_{1-x}$Ru$_{x})_{2}$As$_{2}$, and discuss implications to its superconductivity. [Preview Abstract] |
Tuesday, March 22, 2011 2:54PM - 3:06PM |
L26.00003: Chiral orbital angular momentum and warping effect in topological insulator Bi2Te3 Won Sig Jung, Y.K. Kwan, B.Y. Kim, J.Y. Kim, B.K. Cho, C. Kim The spin of a topologically protected metallic surface state on topological insulators has a chiral state. The Spin chiral state is aligned with orbital angular momentum of the electron in the surface states. We observe orbital angular momentum direction by using angle resolved photoemission (ARPES) with circularly polarized lights. [Preview Abstract] |
Tuesday, March 22, 2011 3:06PM - 3:18PM |
L26.00004: Pseudogap in non-superconducting pnictides, CaFe$_2$As$_2$ and EuFe$_2$As$_2$ K. Maiti, G. Adhikary, N. Sahadev, D.N. Biswas, R. Bindu, N. Kumar, A. Thamizhavel, S.K. Dhar Superconductivity in Fe-pnictides are studied extensively recently as they provide a non-cuprate domain to study unconventional superconductivity via doping induced suppression of magnetism. In order to study the role of magnetic interactions in the electronic structure and its implication in superconductivity, we probed the electronic structure of the parent compounds CaFe$_2$As$_2$ and EuFe$_2$As$_2$ using high resolution photoemission spectroscopy. Single crystalline samples were prepared by flux method. Photoemission measurements were carried out using a Gammadata Scienta analyzer R4000 and monochromatic photon sources. The high resolution spectra exhibit signature of a pseudogap above the spin density wave (SDW) transition temperature in both CaFe$_2$As$_2$ and EuFe$_2$As$_2$. The intensity at the Fermi level show a sudden decrease across the SDW transition indicating more prominent pseudogap. An additional gap opens up in EuFe$_2$As$_2$ across the antiferromagnetic transition temperature as expected. Interestingly, CaFe$_2$As$_2$ also exhibit signature of another gap opening at low temperatures although no phase transitions observed in this temperature range. [Preview Abstract] |
Tuesday, March 22, 2011 3:18PM - 3:30PM |
L26.00005: Unconventional superconducting gap in NaFe$_{0.95}$Co$_{0.05}$As observed by ARPES Pierre Richard, Z.-H. Liu, K. Nakayama, G.-F. Chen, S. Dong, J.-B. He, D.-M. Wang, T.-L. Xia, K. Umezawa, T. Kawahara, S. Souma, T. Sato, T. Takahashi, T. Qian, Y. Huang, N. Xu, Y. Shi, H. Ding, S.-C. Wang The size, the symmetry and the temperature evolution of the superconducting (SC) gap in a given material are directly related to the SC pairing mechanism. The momentum-resolution capability of angle-resolved photoemission spectroscopy (ARPES) allows precise determination of these key parameters, even for complex multi-band systems such as the iron-based superconductors. We performed an ARPES study of NaFe$_{0.95}$Co$_{0.05}$As. The fermiology of this electron-doped 111-pnictide is similar to that of other pnictides. Similarly, the measured SC gaps are nearly isotropic and their size indicates that the system is in the strong coupling regime. Surprisingly, the SC gaps show little change upon increasing temperature towards $T_{c}$, while coherence vanishes. [Preview Abstract] |
Tuesday, March 22, 2011 3:30PM - 3:42PM |
L26.00006: Studies on the orbital characters and quasi-particle dynamics of LiFeAs Yeongkwan Kim, B.Y. Kim, Chul Kim, D.J. Song, W.S. Kyung, C. Kim, B.S. Lee, K.H. Kim Since the discovery, iron-based superconductors have been intensively and extensively studied by using various techniques including angle resolved photoelectron spectroscopy (ARPES). So far, most of ARPES studies have been performed on 122-compounds and 1111-compounds. However, 122-phase materials do not have neutral cleavage surfaces and have 3-dimensional band structures. These traits of 122- and 1111-phase make the spectral shape generally broad and do not allow investigation of the intrinsic electronic structures in detail. In that respect, LiFeAs is an ideal material with neutral cleavage surfaces and quasi-2 dimensional band structures. In this presentation, our recent ARPES work on the electronic structure of LiFeAs will be presented. We investigated the orbital character of each band by ARPES with various polarizations of the photon. Since the main valence band of LiFeAs comes from iron d-orbitals, pin-pointing the characters of bands should be an important starting point. In addition, we analyzed details of the spectral function in regard to the quasi-particle dynamics.. [Preview Abstract] |
Tuesday, March 22, 2011 3:42PM - 4:18PM |
L26.00007: ARPES studies on the pairing mechanism of iron-based superconductors Invited Speaker: Angle-resolved photoemission spectroscopy (ARPES) has been used extensively in studying electronic structure and superconducting gap of the iron-based high- temperature superconductors (pnictides). In this talk, I will present our recent ARPES results on these pnictide materials, mainly focus on high-resolution measurements of the superconducting gap function of many different pnictide superconductors. Our results strongly suggest that the pairing mechanism of the pnictides is likely to be driven by short-range antiferromagnetic fluctuations. [Preview Abstract] |
Tuesday, March 22, 2011 4:18PM - 4:30PM |
L26.00008: Nodeless superconductivity in the stoichiometric superconductor LiFeAs Hyunsoo Kim, Makariy A. Tanatar, Ruslan Prozorov, Yoo Jang Song, Yong Seung Kwon The in- and out-of-plane London penetration depths were measured in single crystals of the intrinsic LiFeAs superconductor using a tunnel diode resonator (TDR) down to $0.03 T_c$. This compound appears to be in the clean limit with a residual resistivity of $\rho_0 \approx 5~\mu\Omega\cdot$cm and $RRR = 65$; it can be placed at a slightly overdoped value when compared to the charge-doped pnictides. The low-temperature region of the penetration depth, which is sensitive to the superconducting gap symmetry, is exponentially flat implying a nodeless gap. The superfluid density is well described by the self-consistent two-gap $\gamma$-model, where the larger gap is $\Delta_1/T_c\sim2$ and the smaller gap is $\Delta_2/T_c\sim1$. Together with the previous data, our results support the $s_\pm$ symmetry that evolves from nodeless to a nodal gap structure upon departure from optimal doping in Fe-based superconductors. We also conclude that pairbreaking scattering plays an important role in the deviations of the low-temperature behavior from exponential in $\lambda(T)$ of Fe-based compounds. [Preview Abstract] |
Tuesday, March 22, 2011 4:30PM - 4:42PM |
L26.00009: Superfluid Density in the 111 Fe Pnictide Superconductors C. J. Arguello, T. Goko, J.P. Carlo, Y.J. Uemura, A.A. Aczel, T.J. Williams, G.M. Luke, C.Q. Jin We performed muon spin relaxation studies in two kinds of '111' iron pnictides, Li$_{1.1}$FeAs and Li$_{0.9}$FeP. The zero field spectra of the latter show a fast relaxation in a small volume fraction (approximately $13\%$) due probably to magnetism. In the case of the arsenide, the zero field spectra show a completely paramagnetic state. Below Tc, an applied transverse field allowed us to measure the superfluid density (via relaxation rate $\sigma$) for both compounds. We found that both of them have very high superfluid density and low Tc: $\sigma(T=2K)\simeq1.8 \mu s^{-1}$ with Tc$\simeq$18K for Li$_ {1.1}$FeAs, and $\sigma(T=2K)\simeq2.0 \mu s^{-1}$ with Tc$\simeq$4K for Li$_{0.9}$FeP. [Preview Abstract] |
Tuesday, March 22, 2011 4:42PM - 4:54PM |
L26.00010: Spin lattice relaxation rate measurements in Ba$_{0.69}$K$_{0.31}$Fe$_{2}$As$_{2}$ by nuclear magnetic resonance Sangwon Oh, Andrew Mounce, William Halperin, Chenglin Zhang, Pengcheng Dai, Arneil Reyes, Philil Kuhns Magnetic impurities have been a problem with NMR measurements of single crystals in the K doped Ba-122 system because of extremely wide linewidth that can be more than 1MHz at low temperature [1]. We have a high quality single crystal of Ba$_{0.69}$K$_{0.31}$Fe$_2$As$_2$ ($T_c$ = 34K) for which the NMR linewidth does not significantly increase at low temperatures and at very large external magnetic fields. In this sample we measure the spin-lattice relaxation rate, $1/T_1$, from 300 K to 4 K at various magnetic fields 6.4 T, 13 T, and 16 T. The rapid increase of $1/T_1T$ down to $T_c$ on cooling can be attributed to spin fluctuations above $T_c$. In the superconducting state, $1/T_1$ has a kink around 20 K, and below this temperature in a field of 13 T it exhibits a power law dependence, $\backslash$varpropto T$^3$. This behavior can be explained by an impurity effect in a superconductor with extended s-wave symmetry [2]. \\[4pt] [1] S. Mukhopadhyay {\it et al.} New J. Phys. {\bf 11}, 055002 (2009)\\[0pt] [2] Y. Bang {\it et al.} Phys. Rev. B {\bf 79}, 054529 (2009) [Preview Abstract] |
Tuesday, March 22, 2011 4:54PM - 5:06PM |
L26.00011: Frustrated proximity effects between s and s$_{\pm }$ superconductors Valentin Stanev, Alexei E. Koshelev The nature of the superconducting order parameter (OP) in iron pnictides and chalcogenides is a hotly debated issue. It was theoretically proposed that the OP has opposite signs on the hole and the electron bands, i.e., it belongs to the unconventional class of s$_{\pm }$ (or extended s)-wave. There are, however, very few experiments that can directly distinguish this state from the ordinary s-wave OP. One way to address this problem is to study the proximity effects in a sandwich composed of conventional and iron pnictide superconductors (SC). If the pnictides indeed have the s$_{\pm }$ OP this system is intrinsically frustrated. In the case of strong frustration, a time-reversal symmetry-breaking (TRSB) SC state emerges, in which the OP phases in different bands are tilted at an angle, different from $\pi $, and controlled by the coupling strength. Observation of such state in the iron-based SC materials would give definite evidence for the s$_{\pm }$ OP. We present a microscopic, fully self-consistent approach to this problem, based on Usadel equations. We have studied the conditions for existence of the TRSB state and its experimental signatures. [Preview Abstract] |
Tuesday, March 22, 2011 5:06PM - 5:18PM |
L26.00012: Doping dependence of Ba(Fe$_{1-x}$Co$_x$)$_2$As$_2$ thin films by the THz conductivity measurement D. Nakamura, F. Nabeshima, Y. Imai, A. Maeda, T. Katase, H. Hiramatsu, H. Hosono We investigated the THz conductivity for thin films of Fe-based superconductor, Ba(Fe$_{1-x}$Co$_{x}$)$_2$As$_{2}$ with different Co concentrations. For the optimally doped sample, we found a structure corresponding to superconductivity gap, 2 $\Delta$, whose magnitude is 2.8 meV at lowtemperatures, leading to 2 $\Delta/k_BT_c=$ 4.1[1]. This value is in good agreement with the smaller gap found in an ARPES measurement[2]. For the underdoped sample in which the coexistence of antiferromagnetic ordering with superconductivity was observed, we found the strong suppression of the carrier lifetime around the antiferromagnetic phase transition temperature ($T \sim$ 40 K). However, the real part of the complex conductivity did not be clearly suppressed in this temparature regigon. This behavior may be related to the responce of carriers at the Dirac cone, which observed in BaFe$_2$As$_2$[3]. Details will be discussed in the presentation.\\[4pt] [1] D. Nakamura {\it et al.}, arXiv: 0912.4351.\\[0pt] [2] K. Terashima {\it et al.}, PNAS 106 (2009) 7330.\\[0pt] [3] P. Richard {\it et al.}, Phys. Rev. Lett. 104 (2010) 137001. [Preview Abstract] |
Tuesday, March 22, 2011 5:18PM - 5:30PM |
L26.00013: Pair breaking in iron-based superconductors Kevin Kirshenbaum, Shanta Saha, Tyler Drye, Steven Ziemak, Johnpierre Paglione The relative ease of crystal growth combined with the range of elements available for chemical substitution, especially on the transition metal site, has allowed for numerous studies of different iron-based superconductors. There are, however, remaining questions about the pairing symmetry in this system. We present transport scattering rate data for optimally-doped single crystals from several superconducting 122 materials and discuss the relationship between superconducting transition temperature and transport scattering rate in the context of pair breaking. [Preview Abstract] |
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