Bulletin of the American Physical Society
APS March Meeting 2013
Volume 58, Number 1
Monday–Friday, March 18–22, 2013; Baltimore, Maryland
Session B37: Focus Session: Fe-based Superconductors: Impurity Effects |
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Sponsoring Units: DMP DCOMP Chair: Ruslan Prozorov, Ames Lab Room: 345/346 |
Monday, March 18, 2013 11:15AM - 11:51AM |
B37.00001: Effect of heavy-ion and electron irradiation on properties of Fe-based superconductors Invited Speaker: Marcin Konczykowski The introduction of defects by particle irradiation is used to reveal the role of disorder in matter, which is unavoidable in all crystalline solids. In superconductors defects introduce flux pinning, controlling critical current, $J_c$; as well as pair-breaking scattering, limiting the critical temperature, $T_c$. To elucidate defect related properties of Fe-based superconductors (FBS) we precede in two types of irradiation: heavy ion (6GeV Pb) to create disorder in the form of amorphous tracks and low temperature electron irradiation (2.5MeV at 20K) to create point like defects. Substantial increase of irreversible magnetization and an upward shift of the irreversibility line are observed after heavy ion irradiation of all FBS investigated to date. In $BaK122$, signatures of a Bose-glass vortex state; angular dependence and variable-range hopping flux creep are revealed. Remarkably, heavy ion irradiation does not depress $T_c $, however, point-like disorder introduced by electron irradiation does substantially. In isovalently substituted $Ba(FeAs_{1-x}P_{x})_{2}$ and $Ba(Fe_{1-x}Ru_{x}As)_{2}$ crystals, $T_c $ decreases linearly with dose. Suppression to $40\%$ of initial value of $T_c $ was achieved in $Ba(FeAs_{1-x}P_{x})_2$. An increase of normal state resistivity is observed and correlated to depression of $T_c $. Change of superconducting gap structure with disorder was determined from penetration depth measurements, $\lambda(T)$ dependence, at various stages of irradiation. Linear in $T$ variation of pristine samples, indicative of the presence of nodes in gap, turned at low irradiation dose to exponential $T$ variation, indicative of a fully gaped state. $T^{2}$ variation of $\lambda$ is observed at higher doses. This behaviour is incompatible with symmetry-imposed nodes of d-wave pairing but consistent with $S+/-$, $S+/+$ mechanisms. This is the first observation of the impurity-induced node lifting expected in anisotropic s-wave superconductors [Preview Abstract] |
Monday, March 18, 2013 11:51AM - 12:03PM |
B37.00002: Impurity-induced changes in the superconducting order parameter in iron-based superconductors BaFe$_2$(As$_{1-x}$P$_x)_2$ Yuta Mizukami, Y. Kawamoto, K. Hashimoto, S. Kasahara, M. Konczykowski, C.J. van der Beek, B. Boizot, R. Prozorov, Y. Wang, A. Kreisel, P.J. Hirschfeld, V. Mishra, Y. Matsuda, T. Shibauchi To determine the symmetry and structure of superconducting order parameter in iron-based superconductors is one of the prime challenges in strongly correlated electron systems. A systematic study on the effect of impurity scattering on the structure of superconducting order parameter can be used to distinguish S$++$ and S$+$- symmetry. We introduced the point defects on iron-based superconductors BaFe$_2$(As$_{1-x}$P$_x)_2$ by electron irradiation which does not change lattice constants and carrier density, and performed magnetic penetration depth measurements on those samples. Here, we report on the suppression of the critical temperature and the change of the superconducting gap structure in iron-based superconductors BaFe$_2$(As$_{1-x}$P$_x)_2$ with increasing the defect density, from which we discuss the superconducting symmetry in this system. [Preview Abstract] |
Monday, March 18, 2013 12:03PM - 12:15PM |
B37.00003: Theoretical study of impurity effects in iron-based superconductors Maria Navarro Gastiasoro, Peter Hirschfeld, Brian Andersen Several open questions remain unanswered for the iron-based superconductors (FeSC), including the importance of electronic correlations and the symmetry of the superconducting order parameter. Motivated by recent STM experiments which show a fascinating variety of resonant defect states in FeSC, we adopt a realistic five-band model including electronic Coulomb correlations to study local effects of disorder in the FeSC. In order to minimize the number of free parameters, we use the pairing interactions obtained from spin-fluctuation exchange to determine the homogeneous superconducting state. The ability of local impurity potentials to induce resonant states depends on their scattering strength $V_{imp}$; in addition, for appropriate $V_{imp}$, such states are associated with local orbital- and magnetic order. We investigate the density of states near such impurities and show how tunneling experiments may be used to probe local induced order. In the SDW phase, we show how C2 symmetry-breaking dimers are naturally formed around impurities which also form cigar-like (pi,pi) structures embedded in the (pi,0) magnetic bulk phase. Such electronic dimers have been shown to be candidates for explaining the so-called nematogens observed previously by QPI in Co-doped CaFe$_2$As$_2$. [Preview Abstract] |
Monday, March 18, 2013 12:15PM - 12:27PM |
B37.00004: Enhancement of $T_c$ by impurity scattering in underdoped iron-arsenide superconductors Maxim G. Vavilov, Rafael M. Fernandes, Andrey V. Chubukov When analyzing the effects of disorder on the superconducting transition temperature $T_c$ of the iron pnictides, the conventional wisdom is that inter-band impurity scattering is quite harmful to the $s^{\pm}$ state. In this talk, we show that this is the case only in the overdoped region of the phase diagram. In the underdoped region, impurity scattering gives rise to two opposite effects due to the competition between superconductivity and a pre-existing magnetic state. The first effect is the direct reduction of $T_c$ due to the pair-breaking contribution coming from inter-band impurity scattering. The second effect is an indirect increase in $T_c$ due to the suppression of long-range magnetic order by both intra-band and inter-band impurity scattering. We show that for a wide range of parameters the second effect overcomes the first, leading to an overall enhancement of $T_c$ by disorder. Our results explain recent puzzling experimental observations on the impact of disorder on $T_c$ of the iron pnictides, providing further evidence in favor of an $s^{\pm}$ pairing state. [Preview Abstract] |
Monday, March 18, 2013 12:27PM - 12:39PM |
B37.00005: Using controlled disorder to distinguish $s_\pm$ and $s_{++}$ gap structure in Fe-based superconductors Yan Wang, Andreas Kreisel, Peter Hirschfeld, Vivek Mishra We reconsider the effect of disorder on the properties of a superconductor characterized by a sign-changing order parameter appropriate for Fe-based materials. Within a simple two band model, we calculate simultaneously $T_c$, the change in residual resistivity $\Delta \rho_0$, and the zero-energy density of states, and show how these results change for various types of gap structures and assumptions regarding the impurity scattering. The rate of $T_c$ suppression is shown to vary dramatically according to details of the impurity model considered. We search therefore for a practical, experimentally oriented signature of a gap of the $s_\pm$ type, and propose that observation of particular evolution of the penetration depth, thermal conductivity or NMR temperature dependence with disorder would suffice. [Preview Abstract] |
Monday, March 18, 2013 12:39PM - 12:51PM |
B37.00006: Single nonmagnetic impurity resonance in FeSe-based 122-type superconductors as a probe for pairing symmetry Qian-En Wang, Zi-Jian Yao, Fu-Chun Zhang The effect of a single nonmagnetic impurity in A$_{y}$Fe$_{2-x}$Se$_{2}$ (A=K, Rb, or Cs) superconductors has been studied based on a three-orbital model. The local density of states on and near the impurity site has been calculated by solving the Bogoliubov-de Gennes equations self-consistently. Both repulsive and attractive impurity scattering potential are considered in our calculations. The impurity-induced in-gap bound states are found only for attractive scattering potential in the state of $d_{x^2-y^2}$ wave pairing, and it turns out that they are very sensitive to the magnitude of the scattering potential. The emergence of the impurity-induced bound states in the vicinity of the Fermi level demonstrates a strong violation of the electron-hole symmetry which is originated from the nodeless $d_{x^2-y^2}$ wave pairing state. The results obtained in our calculation, which simulate the doping of Co and Ni in FeSe-based 122-type superconductors, as an approach to examine the pairing symmetry of this novel superconducting material, can be a proposal of STM observation. [Preview Abstract] |
Monday, March 18, 2013 12:51PM - 1:03PM |
B37.00007: Effect of electron irradiation on superconductivity in isovalently substituted Ba(Fe$_{1-x}$Ru$_x$)$_2$As$_2$ Ruslan Prozorov, M.A. Tanatar, A. Thaler, S.L. Bud'ko, P.C. Canfield, M. Konczykowski, T. Shibauchi Single crystals of isovalently substituted Ba(Fe$_{1-x}$Ru$_{x}$)$_{2}% $As$_{2}$ were irradiated at $23$ K by $2.5$ MeV electrons with a total fluence up to $2\times10^{19}$ electrons per cm$^{2}$. The resistance was measured both in situ at $23$ K during irradiation, and as a function of temperature in a separate set-up, between the irradiation runs while the sample warmed to room temperature. We found that $\Delta\rho_{0}/\rho_{0}\approx0.2$ change in the residual resistivity, reached at the maximum irradiation dose, led to about a $\Delta T_{c}/T_{c0}\approx0.35$ decrease of $T_{c}$. This trend is universal in samples of different doping levels with different initial $T_{c0}$. The in-situ measurements also allowed us to understand the effects of room temperature annealing on the point-like defects induced by irradiation. The annealing results in a decrease of about a 20\% of the total increase in resistance achieved due to irradiation. However, residual 80\% remain stable at least one month after irradiation. We compare our results with theoretical predictions for different pairing scenarios, including extended $s_{\pm}$. Work in Ames was supported by the Department of Energy Office of Science, Basic Energy Sciences under Contract No. DE-AC02-O7CH11358. [Preview Abstract] |
Monday, March 18, 2013 1:03PM - 1:15PM |
B37.00008: Theoretical visualization of atomic-scale impurity states in Fe-based superconductors Peayush Choubey, Peter Hirschfeld, Tom Berlijn, Chao Cao We study the impurity induced local density of states (LDOS) in Fe-based superconductors, incorporating Wannier functions to obtain a higher resolution derived from a downfolding of density functional theory bands onto a 10-Fe tight-binding model. This enables us to compare our results with those obtained experimentally using STM. We solve the ten orbital Bogoliubov-de Gennes (BdG) equations for the single impurity problem and obtain the superconducting state lattice space Green's function, which is then transformed to the Wannier basis. The utility and limitations of this approximation are discussed. [Preview Abstract] |
Monday, March 18, 2013 1:15PM - 1:27PM |
B37.00009: Effects of disordered Ru substitution in BaFe$_2$As$_2$: possible superdiffusion mechanism in real materials Limin Wang, Tom Berlijn, Yan Wang, Chia-Hui Lin, Peter Hirschfeld, Wei Ku An unexpected insensitivity of the Fermi surface to impurity scattering is found in Ru substituted BaFe$_2$As$_2$ from first-principles theory, offering a natural explanation of the unusual resilience of transport and superconductivity to a high level of disordered substitution in this material. This robustness is shown to originate from a coherent interference of correlated on-site and inter-site impurity scattering, similar in spirit to the microscopic mechanism of superdiffusion in one dimension. Our result also demonstrates a strong substitution dependence of the Fermi surface and carrier concentration, and provides a natural resolution to current discrepancies in recent photoelectron spectroscopy. These effects offer a natural explanation of the diminishing long-range magnetic, orbital and superconducting order with high substitution. [Preview Abstract] |
Monday, March 18, 2013 1:27PM - 1:39PM |
B37.00010: Electron irradiation of iron-based superconductors S. Demirdis, C.J. van der Beek, M. Konczykowski, S. Kasahara, T. Terashima, R. Okazaki, T. Shibauchi, Y. Matsuda, D. Colson, P. Gierlowski, R. Prozorov The premise of $\mbox{s}_{\pm } $ superconductivity in the multiband iron-based superconductors, with a sign-changing order parameter between the electron-like and hole-like Fermi-surface sheets, has raised the question of the effect of atomic-scale point-like disorder on superconductivity in these materials. In this contribution, we compare the effect of the controlled introduction of point-like defects in different 122-type iron-based superconductors by 2.5 MeV electron irradiation at 20 K. Preliminary data reveal that the effect point-like defects on the critical temperature of isovalently doped materials vastly outweighs that on the charge-doped compounds. The weak collective contribution to $J_{c}$ in Co-doped 122 compounds is found to clearly increase. Moreover this contribution appears after irradiation of the P-doped compound in which it was previously absent. [Preview Abstract] |
Monday, March 18, 2013 1:39PM - 1:51PM |
B37.00011: Impurity substitution effect on Dirac cone in Ba(FeAs)$_2$ studied by magnetoresistance Takahiro Urata, Yoichi Tanabe, Khuong Huynh, Satoshi Heguri, Hidetoshi Oguro, Kazuo Watanabe, Katsumi Tanigaki In iron pnictide superconductors, the three dimensional Dirac cones (DCs) are created as a node of spin-density-wave gap [1]. Due to the pseudospin chirality, these DCs are robust for both nonmagnetic and magnetic impurities. Here we report Ru and Mn substitution effects on DCs in Ba(FeAs)$_{2}$ studied by magnetoresistance [2]. We assume that Ru is the nonmagnetic impurity while Mn is the magnetic one due to the stability of the Mn$^{\mathrm{2+}}$(3d$^{5})$ state. A linear magnetoresistance (LMR) against magnetic field (B) was observed above the certain magnetic field strength of B* for both cases. LMR in Ba(Fe$_{\mathrm{1-x}}$Ru$_{\mathrm{x}}$As)$_{2}$ is consistent with the quantum limit of DC [3,4]. Temperature (T) dependence of B* deviated from the theoretical model at low T for Ba(Fe$_{\mathrm{1-x}}$Mn$_{\mathrm{x}}$As)$_{2}$. This can be understood in term of the decrease of Fermi velocity of DCs, being associated with the magnetic scattering effect on DCs. [1] Y. Ran et al., Phys. Rev. B 79, 014505 (2009).[2]Y. Tanabe et al., Phys. Rev. B 84, 100508(R) (2011). /Phys. Rev. B 86, 094510 (2012). [3]A. A. Abrikosov, Phys. Rev. B 58, 2788 (1998).[4]K. K. Huynh et al., Phys. Rev. Lett. 106, 217004 (2011). [Preview Abstract] |
Monday, March 18, 2013 1:51PM - 2:03PM |
B37.00012: Effect of electron irradiation on superconductivity in isovalently substituted Ba(Fe$_{1-x}$Ru$_{x}$)$_{2}$As$_2$ and SrFe$_2$(As$_{1-x}$P$_x$)$_2$ C.P. Strehlow, A. Thaler, M.A. Tanatar, S.L. Bud'ko, P.C. Canfield, R. Prozorov, M. Koczykowski, S. Miyasaka Single crystals of isovalently substituted Ba(Fe$_{1-x}$Ru$_{x}$)$_{2}$ As$_{2}$ and SrFe$_{2}$(As$_{1-x}$P$_{x}$)$_{2}$ were irradiated at $23$ K by $2.5$ MeV electrons with a total fluence up to $2\times10^{19}$ electrons per cm$^{2}$. Both the resistivity and Hall coefficient were measured before and after irradiation using the van der Pauw method. Irreversible vortex properties were probed using miniature Hall-probe arrays. We correlate the change in resistivity due to irradiation with changes in flux pinning, relaxation rate and irreversibility line. We compare the results with theoretical predictions for different pairing scenarios, including extended $s_{\pm}$. [Preview Abstract] |
Monday, March 18, 2013 2:03PM - 2:15PM |
B37.00013: Understanding the role of disorder in Fe-arsenide superconductors James Analytis, Hsueh-Hui Kuo, Ian Fisher Disorder has a profound affect on iron-pnictide superconductors, changing the transport anisotropy, the magneto-elastic coupling, and the properties of the superconductivity itself. In the 122 structural motifs, the parent compounds have a folded band structure with compensated hole and electron pockets. Some of these pockets are thought to be protected from disorder by the topological properties of the band structure. However, the influence of disorder on each pocket is in general very difficult to reveal because transport properties will in general measure an average of all Fermi surfaces, and other Fermi surface probes (such as ARPES) will not be sensitive to subtle changes in the dynamical properties of each Fermi surface. We present results of detailed quantum oscillation studies which aim to understand how the dynamics of each Fermi surface pocket is affected by disorder. [Preview Abstract] |
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