Bulletin of the American Physical Society
APS March Meeting 2015
Volume 60, Number 1
Monday–Friday, March 2–6, 2015; San Antonio, Texas
Session S5: Focus Session: Spin Fluctuations and Pairing Symmetry in Fe-based Superconductors |
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Sponsoring Units: DMP DCOMP Chair: Wei-Cheng Lee, Binghamton University Room: Juan Gorman Room 005 |
Thursday, March 5, 2015 8:00AM - 8:12AM |
S5.00001: Study on the correlation between s$\pm$ pairing and intra-orbital spin fluctuations in 1111 iron based superconductors with isovalent doping Hidetomo Usui, Katsuhiro Suzuki, Kazuhiko Kuroki Recently, 1111 iron based superconductors with isovalent doping have been experimentally investigated in LnFeAs$_{x}$P$_{1-x}$O$_{1-y}$F$_{y}$ (Ln$=$La, Nd, Pr) [1-4]. Interestingly, it was found that $T_{c}$ takes its local maximum in the intermediate regime of arsenic/phosphorous ratio, which indicates that the superconductivity is locally optimized at a certain Fe-Pn-Fe (Pn$=$Pnictogen) bond angle larger than 109 deg. Given this background, we study the correlation between the local lattice structure, the orbital character of the Fermi surface, and $T_{c}$ in 1111 system with isovalent doping. We calculate the band structure of LnFeAs$_{x}$P$_{1-x}$O$_{1-y}$F$_{y}$ and construct effective five orbital models. To our surprise, it is found that superconductivity is indeed locally optimized in the intermediate arsenic doping regime. The origin of this local optimization is traced back to the gradual variation of the orbital character and the density of states of the hole Fermi surfaces around the $\Gamma $ point, which is controlled by the bond angle. The consistency with the experiment strongly indicates the importance of the spin fluctuation played in this series of superconductors.\\[4pt] [1] S. Miyasaka \textit{et al}., J. Phys. Soc. Jpn.~\textbf{82}~124706 (2013).\\[0pt] [2] S. Kitagawa \textit{et al}., J. Phys. Soc. Jpn. \textbf{83}, 023707 (2014).\\[0pt] [3] H. Mukuda \textit{et al}., Phys. Rev. B \textbf{89}, 064511 (2014).\\[0pt] [4] K. T. Lai \textit{et al}., Phys. Rev. B \textbf{90}, 064504 (2014). [Preview Abstract] |
Thursday, March 5, 2015 8:12AM - 8:24AM |
S5.00002: Observation of Momentum-Confined In-Gap Impurity State in Ba$_{0.6}$K$_{0.4}$Fe$_{2}$As$_{2}$: Evidence for Antiphase s$+$- Pairing Peng Zhang, Pierre Richard, Tian Qian, Xun Shi, Jun Ma, Lingkun Zeng, Xiaoping Wang, Emile Rienks, Chenglin Zhang, Pengcheng Dai, Yizhuang You, Zhengyu Weng, Xianxin Wu, Jiangping Hu, Hong Ding We report the observation by angle-resolved photoemission spectroscopy of an impurity state located inside the superconducting gap of Ba$_{0.6}$K$_{0.4}$Fe$_{2}$As$_{2}$ and vanishing above the superconducting critical temperature, for which the spectral weight is confined in momentum space near the Fermi wave-vector positions. We demonstrate, supported by theoretical simulations, that this in-gap state originates from weak scattering between bands with opposite sign of the superconducting-gap phase. This weak scattering, likely due to off-plane nonmagnetic (Ba, K) disorder, occurs mostly among neighboring Fermi surfaces, suggesting that the superconducting-gap phase changes sign within holelike (and electronlike) bands. Our results impose severe restrictions on the models promoted to explain high-temperature superconductivity in these materials. [Preview Abstract] |
Thursday, March 5, 2015 8:24AM - 8:36AM |
S5.00003: Anisotropic staggered magnetization in electron-doped Fe-based superconductors Hong-Yi Chen, C.S. Ting Based upon a two-orbital model with competing collinear antiferromagnetism and $s\pm$ pairing superconductivity, the electronic structures are investigated by solving Bogoliubov-de Gennes equations. Our results for the optimally electron-doped compound exhibit the anisotropic staggered magnetization. The Fourier transformation of the staggered magnetization shows two uneven peaks at $Q_x=(\pi,0)$ and $Q_y=(0,\pi)$. The spatial variation of the s? pairing superconductivity and the electronic charge distribution show a checkerboard-like pattern with $C_2$ symmetry. Finally, in the calculation of the local density of states, we found that the anisotropic staggered magnetization does not open a gap. Our results are good in agreement with the recently reported experimental results. [Preview Abstract] |
Thursday, March 5, 2015 8:36AM - 9:12AM |
S5.00004: Enhanced spin fluctuations and $s\pm$ pairing by diagonal electron hopping in Fe-based superconductors Invited Speaker: Kazuhiko Kuroki In the itinerant spin picture of the iron-based superconductors, the nesting between electron and hole Fermi surfaces is usually considered to be the origin of the spin fluctuation and thus the pairing glue. However, there have appeared some experimental observations suggesting absence of Fermi surface nesting. For instance, in the 1111 materials $Ln$FeAsO$_{1-x}$H$_x$ ($Ln$=La,Sm, etc.) [1], electron doping rate $x$ reaches up to 50\%, which in a rigid band picture would wipe out the hole Fermi surfaces. Still, superconductivity not only survives, but is even enhanced in the largely doped regime, in contradiction to the expectation from the bad nesting. Another example is K$_x$Fe$_{2-y}$Se$_2$, where the ARPES experiments show the absence of hole Fermi surfaces[2-5]. In the present talk, we first focus on $Ln$FeAsO$_{1-x}$H$_x$, where the band structure is actually not rigid against doping, and the hole Fermi surface originating from the $d_{xy}$ orbital remains nearly unchanged. The origin of this can be traced back to real space, where the nearest neighbor hopping $t_1$ within the $d_{xy}$ orbital is found to be strongly suppressed with doping [6]. Although the nesting itself is degraded, the spin fluctuation in the largely electron doped regime is enhanced due to $t_2>t_1$, where $t_2$ is the 2nd neighbor diagonal hopping. This re-enhances s$\pm$ pairing superconductivity, and explains the double dome $x$-$T_c$ phase diagram of LaFeAsO$_{1-x}$H$_x$[1]. From this viewpoint, it is also interesting to look into the relation between $t_1$ and $t_2$ in other materials. For instance, our first principles estimation for KFe$_2$Se$_2$ gives $t_1=-0.008$ eV and $t_2=0.056$ eV, and from this strong reduction of $t_1$, both electron and hole Fermi surfaces are expected to be present around the $\Gamma$ point, in contradiction to previous experimental observations. Results of a recent ARPES experiment will be discussed from this viewpoint. References [1] S. Iimura et al., Nat. Commun. {\bf 3}, 943 (2012). [2] T. Qian et al., Phys. Rev. Lett. {\bf 106}, 187001 (2011). [3] Y. Zhang et al., Nature Mat. {\bf 10}, 273 (2011). [4] D. Mou et al., Phys. Rev. Lett. {\bf 106}, 107001 (2011). [5] M. Yi et al., Phys. Rev. Lett. {\bf 110}, 067003 (2013). [6] K. Suzuki et al., Phys. Rev. Lett. {\bf 113}, 027002 (2014). [Preview Abstract] |
Thursday, March 5, 2015 9:12AM - 9:24AM |
S5.00005: Anomalous scaling behavior of the Specific Heat jump $\Delta $C vs. T$_{\mathrm{c}}$ in the Fe-based superconductors Yunkyu Bang, G.R. Stewart So called BNC scaling ($\Delta$C$\sim$T$_{\mathrm{c}}^{3}$) -- Bud'ko, Ni, and Canfield, PRB, 79, 220516 (2009)) -- has been observed in a wide range of the Fe-based superconducting compounds such as Ba(Fe$_{\mathrm{1-x}}$,Co$_{\mathrm{x}}$)$_{2}$As$_{2}$, Ba(Fe$_{\mathrm{1-x}}$,Ni$_{\mathrm{x}}$)$_{2}$As$_{2}$, BaFe$_{2}$(AsP)$_{2}$, and Ba$_{\mathrm{1-x}}$Na$_{\mathrm{x}}$Fe$_{2}$As$_{2}$. More recently, however, Canfield and coworkers reported that the Ba$_{\mathrm{1-x}}$K$_{\mathrm{x}}$Fe$_{2}$As$_{2}$ compound severely deviates from this scaling when x\textgreater 0.7 and argued that this is an indication of the Lifshitz transition in the Ba$_{\mathrm{1-x}}$K$_{\mathrm{x}}$Fe$_{2}$As$_{2}$ compound at higher hole-doping. In this presentation, we propose a theory that the BNC scaling as well as its strong deviation, as observed in Ba$_{\mathrm{1-x}}$K$_{\mathrm{x}}$Fe$_{2}$As$_{2}$, are an intrinsic property of the multiband superconductor mediated by a dominant interband pairing potential as realized in the sign-changing S-wave state. [Preview Abstract] |
Thursday, March 5, 2015 9:24AM - 9:36AM |
S5.00006: The effects of Coulomb interactions on the superconducting gaps in iron-based superconductors Zhidong Leong, Philip Phillips Recent ARPES measurements on Co-doped LiFeAs report a large and robust superconducting gap on a band below the chemical potential. We will show that, unlike a conventional BCS theory, a multiband system with strong interband Coulomb interactions can explain the observations. We use a two-band model consisting of a superconducting electron band and a hole band that is below the chemical potential. The two bands are coupled via interband Coulomb interactions. Using Eliashberg theory, we found that superconductivity in the electron band induces a large superconducting gap in the hole band. Furthermore, the repulsive nature of the Coulomb interactions gives the induced gap an opposite sign, corresponding to an $s_{\pm}$ gap symmetry. Unlike other families of iron pnictides, the gap symmetry of LiFeAs has not been ascertained experimentally. The implications for the superconducting mechanism in iron pnictides will be discussed. [Preview Abstract] |
Thursday, March 5, 2015 9:36AM - 9:48AM |
S5.00007: Doping-induced crossover of the pairing symmetry in iron- pnictide superconductors Mahmoud Abdel-Hafiez, Zheng He, Xingye Lu, Huiqian Luo, Pengcheng Dai, Xiao-Jia Chen Iron pnictides present a rich phase diagram and superconductivity coexists and competes with the spin density wave and the nematic order, giving unconventional pairing mechanisms. Although various efforts have put forth to clarify pairing symmetry of the Cooper pair, experimental confirmations of the precise symmetry of the superconducting (SC) order parameter and its evolution with doping remains highly controversial. Here we present the investigation results of the low-temperature specific heat down to 70 mK for high-quality BaFe$_{\mathrm{2-x}}$Ni$_{\mathrm{x}}$As$_{\mathrm{2}}$ single crystals. For $x \quad \le $ 0.12, the temperature dependence of the SC-state specific heat provides strong evidence for a two-band $s$-wave order parameter. Upon doping for $x \ge $ 0.15, we find the temperature and magnetic field contributions to the specific heat in $T^{\mathrm{2}}$ and $H^{\mathrm{0.5}}$, respectively, which strongly indicate the nodes in the SC gap. [Preview Abstract] |
Thursday, March 5, 2015 9:48AM - 10:00AM |
S5.00008: Leggett modes and vortex dynamics in time-reversal symmetry breaking multiband superconductors Mikhail Silaev, Egor Babaev In the framework of quasiclassical kinetic theory we study the spectrum of collective excitations and vortex dynamics in multiband superconductors. We show that the existence of mixed phase-density modes in multiband superconductors with broken time-reversal symmetry generates a new contribution to the viscosity of magnetic flux flow. Near the time reversal symmetry breaking phase transition this new contribution dominates over the usual Tinkham and Bardeen-Stephen mechanisms and provides a peculiar temperature dependence of the vortex viscosity. The results could be relevant for three band superconductor Ba$_{\mathrm{1-x}}$K$_{\mathrm{x}}$Fe$_{\mathrm{2}}$As$_{\mathrm{2}}$. [Preview Abstract] |
Thursday, March 5, 2015 10:00AM - 10:12AM |
S5.00009: Spontaneous currents in a superconductor with s+is symmetry Manfred Sigrist, Saurabh Maiti, Andrey Chubukov We analyze s + is state proposed as a candidate superconducting state for strongly hole-doped Ba-122. Such a state breaks time-reversal symmetry (TRS) but does not break any other discrete symmetry. We address the issue whether TRS breaking alone can generate spontaneous currents near impurity sites, which could be detected in, e.g., ?SR experiments. We argue that there are no spontaneous currents if only TRS is broken. However, supercurrents do emerge if the system is put under external strain and C4 lattice rotation symmetry is externally broken. [Preview Abstract] |
Thursday, March 5, 2015 10:12AM - 10:24AM |
S5.00010: Gap structure in Fe-based superconductors with accidental nodes Alberto Hinojosa Alvarado, Andrey Chubukov We study Fe-based superconductors with $s$-symmetry and accidental gap nodes on electron pockets. We consider ellipticity, hybridization and an additional inter-pocket pairing interaction and analyze their effect on the gap structure and on the existence and location of nodal points in the quasiparticle dispersions. Depending on these parameters the gap functions at the Fermi surface may be uniform, have nodal points, or vary their phase continuously. In the quasiparticle dispersions, there exist nodal points only if the phase difference between the hybridization and inter-pocket pairing parameters is a multiple of $\pi$/2. The two parameters tend to shift the position of the nodes in the same or in opposite directions depending on their relative phases. When the parameters reach a critical value the nodes disappear. [Preview Abstract] |
Thursday, March 5, 2015 10:24AM - 10:36AM |
S5.00011: Glide plane symmetry and gap structure in the iron-based superconductors Yan Wang, Tom Berlijn, Peter Hirschfeld, Douglas Scalapino, Thomas Maier We consider the effect of glide plane symmetry of the Fe-pnictogen/chalcogen layer in Fe-based superconductors for pairing in spin fluctuation models. Recent theories have proposed that so-called $\eta$-pairing states with nonzero total momentum can be realized and possess exotic properties such as odd parity spin singlet symmetry and time-reversal symmetry breaking. Here we show that $\eta$-pairing is inevitable when there is orbital weight at the Fermi level from orbitals with even and odd mirror reflection symmetry in $z$; however, by explicit calculation, we conclude that the gap function that appears in observable quantities is identical to that found in earlier, 1 Fe per unit cell pseudo-crystal momentum calculations. P.J.H. and Y.W. were supported by Grant No. DOE DE-FG02-05ER46236 and T.B. was supported as a Wigner Fellow at the Oak Ridge National Laboratory. A portion of this research was conducted at the Center for Nanophase Materials Sciences, which is sponsored at Oak Ridge National Laboratory by the Scientific User Facilities Division, Office of Basic Energy Sciences, US Department of Energy. [Preview Abstract] |
Thursday, March 5, 2015 10:36AM - 10:48AM |
S5.00012: Enhanced $s\pm$ pairing due to prioritized diagonal motion of electrons in the iron-based superconductors Kazuhiko Kuroki, Katsuhiro Suzuki, Hidetomo Usui In the itinerant spin picture of the iron-based superconductors, the nesting between electron and hole Fermi surfaces is usually considered to be the origin of the spin fluctuation. However, there are now some experimental results suggesting that the nesting is not important for superconductivity. An example is the 1111 materials LnFeAsO$_{1-x}$H$_x$ (Ln=La,Sm etc.), where over 50\% of electron doping can be accomplished. Superconductivity not only survives, but is even enhanced in the largely electron doped regime, in contradition to the expectation from the bad nesting. In LaFeAsO$_{1-x}$H$_x$ in particular, the x vs. $T_c$ phase diagram exhibits a double dome feature, suggesting a possible difference in the pairing mechanism between the lightly doped and the heavily doped regimes. In the present study, we analyze the five orbital model of this system, and show that a peculiar relation among the real space hoppings is realized in the largely electron doped regime, namely, the next nearest neighbor hopping dominates over the nearest one within the $d_{xy}$ orbitals. We argue that this enhances the $s\pm$ pairing, which is a next nearest neighbor pairing in real space, despite the degraded nesting. We also discuss about some other materials having similar real space hoppings. [Preview Abstract] |
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