Bulletin of the American Physical Society
APS March Meeting 2014
Volume 59, Number 1
Monday–Friday, March 3–7, 2014; Denver, Colorado
Session G13: Focus Session: Fe-based Superconductors-Pairing Symmetry and Damage |
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Sponsoring Units: DMP Chair: Roman Movshovich, Los Alamos National Laboratory Room: 207 |
Tuesday, March 4, 2014 11:15AM - 11:27AM |
G13.00001: Symmetry imposed line nodes in the superconducting gap of KFe$_2$As$_2$ Hyunsoo Kim, Makariy A. Tanatar, Yong Liu, Thomas A. Lograsso, Ruslan Prozorov, Chenglin Zhang, Pengcheng Dai The variation with temperature of the London penetration depth, $\Delta\lambda(T)$, was measured in single crystals of K$_{1-x}$Na$_x$Fe$_2$As$_2$ ($x=0,~0.07$) by using a tunnel diode resonator down to 50 mK. Electrical resistivity measurements on the same samples show that isoelectronic Na-substitution significantly increases residual resistivity $\rho_0$ (from 0.2 to 1 $\mu\Omega$), without changing the shape of $\rho(T)$, and changes $T_c$ from 3.6 K to 2.9 K for $x=0$ and 0.07 samples. In both pure and doped compounds, the penetration depth follows the power-law function, $\Delta\lambda(T)=AT^n$ below $0.3\,T_c$ with $n\approx 1.5$ and 2.0 for $x=0$ and 0.07 samples, respectively. This behavior is consistent with presence of line nodes in the superconducting gap with moderate scattering for $x=0$ evolving into dirty limit for $x=0.07$. The normalized superfluid density, $\rho_s=\lambda^2(0)/\lambda^2(T)$ was calculated with $\lambda(0)=200$ nm and 500 nm for $x=0$ and $x=0.07$, respectively. Detailed investigation of the calculated $\rho_s$ strongly supports the existence of symmetry imposed line nodes in KFe$_2$As$_2$ superconductor and is consistent with thermal conductivity data that concluded $d-$wave pairing in this compound. [Preview Abstract] |
Tuesday, March 4, 2014 11:27AM - 11:39AM |
G13.00002: Local disorder and superconductivity in K$_{x}$Fe$_{2-y}$Se$_{2}$ Despina Louca, Keeseong Park, Bing Li, Y.-Q. Yan, Joerg Neuefeind In the K$_{x}$Fe$_{2-y}$Se$_{2}$ family of Fe-based superconductors, superconductivity is observed between a semi-metallic region below x $\sim$ 0.7 and an insulating and antiferromagnetic region above x $\sim$ 0.85. By probing the local structure we observe that superconductivity emerges in a locally distorted Fe sublattice that accommodates two kinds of bonding environments, forming a double-well distribution. Consisting of short bonds which are metallic in nature and of long ones which are insulating and antiferromagnetic, their distribution changes with x. Even though crystallographically the atomic structure changes slowly on average by adding K, a continuous transition from the short to the long Fe bonds is observed across this region. What is unique to this system's superconducting state is the presence of the double-well distribution in equal proportions, in contrast to other Fe-based materials where only one kind of Fe bond is present. This suggests that superconductivity is intertwined with magnetism, appearing at the crossover from metallic to insulating conditions and is not due to phase separation. [Preview Abstract] |
Tuesday, March 4, 2014 11:39AM - 11:51AM |
G13.00003: Systematics of the temperature dependent inter-plane resistivity in (Ba$_{1-x}$K$_x$)Fe$_2$As$_2$ Makariy Tanatar, Yong Liu, T.A. Lograsso, B. Jensen, K.W. Dennis, R.W. McCallum, Ruslan Prozorov The single crystals of hole-doped iron-based superconductor (Ba$_{1-x}$K$_x$)Fe$_2$As$_2$ were studied in great details in underdoped to optimally - doped compositions. In contrast, the overdoped side was practically inaccessible due to the difficulty in crystals growth. Only recently high quality single crystals have become available, and here we report the temperature dependent inter-plane resistivity for the whole doping range $x$=0 to $x$=1. In a parent compound, BaFe$_2$As$_2$, inter-plane resistivity shows a broad cross-over maximum at a characteristic temperature $T_{max} \sim$200~K. With K doping this maximum first shifts to higher temperatures, reaches a maximum 250~K close to the optimal doping, $x=0.4$ and then decreases towards the K-rich compositions. In pure KFe$_2$As$_2$ the maximum shows clear correlation with features in magnetization measurements. [Preview Abstract] |
Tuesday, March 4, 2014 11:51AM - 12:27PM |
G13.00004: Universal Heat Conduction in the Iron-Arsenide Superconductor KFe$_2$As$_2$: Evidence of a $d$-wave State Invited Speaker: Jean Philippe Reid The thermal conductivity $\kappa$ of the iron-arsenide superconductor KFe$_2$As$_2$ was measured down to 50 mK for a heat current parallel and perpendicular to the tetragonal $c$ axis. A residual linear term at $T \to 0$, $\kappa_0/T$, is observed for both current directions, confirming the presence of nodes in the superconducting gap. Our value of $\kappa_0/T$ in the plane is equal to that reported by Dong et al. [Phys. Rev. Lett. 104, 087005 (2010)] for a sample whose residual resistivity $\rho_0$ was ten times larger. This independence of $\kappa_0/T$ on impurity scattering is the signature of universal heat transport, a property of superconducting states with symmetry-imposed line nodes. This argues against an $s$-wave state with accidental nodes. It favours instead a $d$-wave state, an assignment consistent with five additional properties: the magnitude of the critical scattering rate $\Gamma_c$ for suppressing $T_c$ to zero; the magnitude of $\kappa_0/T$, and its dependence on current direction and on magnetic field; the temperature dependence of $\kappa(T)$. [Preview Abstract] |
Tuesday, March 4, 2014 12:27PM - 12:39PM |
G13.00005: Resistive Losses in Single-Crystal Ba$_{0.6}$K$_{0.4}$Fe$_{2}$As$_{2}$ Brendan Benapfl, Chenglin Zhang, Pengcheng Dai, H.A. Blackstead Recently, we conducted surface resistance measurements using electron spin resonance techniques on single-crystal Ba$_{0.6}$K$_{0.4}$Fe$_{2}$As$_{2}$ samples (\textit{rf} frequency = 20.3 GHz), testing temperature and field dependence. In the superconducting state, the samples exhibit dissipative losses which increase monotonically as a function of applied field for fixed temperature. The level of field-dependent dissipation increases as $T$ approaches $T_C$ from below, and vanishes at the transition. The dissipation is also dependent on the angle between the \textit{rf} field and the static field, consistent with flux-flow models of other high-temperature superconductors, such as YBCO. [Preview Abstract] |
Tuesday, March 4, 2014 12:39PM - 12:51PM |
G13.00006: Multiband, Strong coupling and Strong Correlations in Ba$_{1-x}$K$_{x}$Fe$_{2}$As$_{2}$ single crystals. Mind the (nodal) gap! Fr\'ed\'eric Hardy, Dai Aoki, Thomas Wolf, Carley Paulsen, Robert Eder, Anna B\"ohmer, Rolf Heid, Martin Jackson, Robert A. Fisher, Christoph Meingast We report an exhaustive thermodynamic study (heat capacity, magnetization, thermal expansion) of the normal- and superconducting-state properties of the entire Ba$_{1-x}$K$_{x}$Fe$_{2}$As$_{2}$ series. We show that strong correlations develop with increasing hole concentration indicating the possible proximity to an orbital-selective Mott transition predicted by theory. In the superconducting state, these single crystals exhibit strong multiband and paramagnetic effects and we give evidence of a strong-to-weak-coupling crossover that occur near the concentration where the electron sheets disappear. Our data show no evidence for a symmetry change, from {\it s} to {\it d}-wave, of the superconducting state. Dissimilarities with the Ba(Fe$_{1-x}$Co$_{x}$)$_{2}$As$_{2}$ series are emphasized. [Preview Abstract] |
Tuesday, March 4, 2014 12:51PM - 1:03PM |
G13.00007: High-field critical current enhancement by irradiation induced correlated and random defects in (Ba0.6K0.4)Fe2As2 Karen Kihlstrom, Lei Fang, Ying Jia, Bing Shen, Alexei Koshelev, Ulrich Welp, George W. Crabtree, Wai Kwong Kwok, Asghar Kayani, Shaofei Zhu, Hai Hu Wen Mixed pinning landscapes in superconductors are emerging as an effective strategy to achieve high critical currents in high, applied magnetic fields. Here, we use heavy-ion and proton irradiation to create correlated and point defects to explore the vortex pinning behavior of each and combined constituent defects in the iron-based superconductor Ba$_{\mathrm{0.6}}$K$_{\mathrm{0.4}}$Fe$_{\mathrm{2}}$As$_{\mathrm{2}}$ and find that the pinning mechanisms are non-additive. The major effect of p-irradiation in mixed pinning landscapes is the generation of field-independent critical currents in very high fields. At 7T and 5K, the critical current density exceeds 5\textunderscore MA/cm2. This work supported by the Center for Emergent~Superconductivity, an Energy Frontier Research Center funded by the U.S. D.O.E., Office of Science, Office of Basic Energy Sciences and by the D.O.E, Office of Basic Energy Sciences, under Contract No. DE-AC02-06CH11357. The operation of the ATLAS facility was supported by the U.S. D.O.E., Office of Nuclear Physics, under Contract No. DE-AC02-06CH11357. The work in China was supported by the NSF of China, the MOST of China (2011CBA00102 and 2012CB821403) and PAPD. [Preview Abstract] |
Tuesday, March 4, 2014 1:03PM - 1:15PM |
G13.00008: Effect of electron irradiation on magnetic and superconducting transitions in underdoped (Ba$_{1-x}$K$_x$)Fe$_2$As$_2$ Ruslan Prozorov, M. A. Tanatar, M. Ko\'{n}czykowski, R. Fernandes, B. Shen, Hai-Hu Wen Single crystals of (Ba$_{0.8}$K$_{0.2}$)Fe$_2$As$_2$ ($T_{c0} =$ 17 K) were irradiated by $2.5$ MeV electrons in several steps up to a total fluence of $2\times10^{19}$ electrons per cm$^{2}$. The sample resistance was measured both in situ at $23$ K during the irradiation, and as a function of temperature in a separate set-up, between the irradiation runs. Annealing of the induced defects by warming the sample up to different temperatures showed that the defects are stable as long as sample temperature remains at or below the highest temperature the sample was subject to. We found that both superconducting and magnetic transition temperatures decrease linearly with the increase of the residual resistivity. Surprisingly, both transitions are supressed at the same rate of 0.1 K/$\mu \Omega$cm. For the highest dose, the residual resistivity changed by $\Delta \rho(0) = 85~ \mu \Omega$cm, whereas $T_c$ changed from 17 K to 8 K and $T_N$ changed from 102 to 93 K. Our results provide a strong evidence that both superconductivity and magnetism of iron - based superconductors are derived from the interband nesting - like interactions. [Preview Abstract] |
Tuesday, March 4, 2014 1:15PM - 1:27PM |
G13.00009: Study of magnetic fluctuations and nematic phase transition in BaFe$_{2}$As$_{2}$ Xu Luo, Valentin Stanev, Bing Shen, XinSheng Ling, Wai-Kwong Kwok, Ulrich Welp We used high resolution AC micro-calorimetry and SQUID magnetometry to study the phase transitions in BaFe$_{2}$As$_{2}$ single crystals. A pronounced peak observed in the specific heat at T$_{\mathrm{N}}$ $\sim$ 132K, together with a step in the magnetization at the same temperature signifies the AFM/Structural transition in BaFe$_{2}$As$_{2}$. Annealing treatment of the sample shifts the peak in specific heat to 137K and reduces its width to $\sim$ 0.8K (FWHM). A thorough investigation of the specific heat up to 220K revealed no discernible, additional phase transitions above a typical background noise level of 5 to 10 $\times$ 10$^{-4}$ for $\Delta $C/C. However, strong in-plane magnetic fluctuations were observed to persist to as high as 180K in magnetization, which provide a possible explanation for previous reports of a nematic phase transition. [Preview Abstract] |
Tuesday, March 4, 2014 1:27PM - 1:39PM |
G13.00010: Ising-nematic spin correlations in the tetragonal state of uniaxial strained BaFe$_{2-x}$Ni$_x$As$_2$ Xingye Lu, Jitae Park, Rui Zhang, Huiqian Luo, A.H. Nevidomskyy, Qimiao Si, Pengcheng Dai Superconductivity in iron pnictides can be derived from electron or hole-doping to their antiferromagnetic (AF) ordered parent compounds. In the normal state above the superconducting transition temperature (Tc), an in-plane resistivity anisotropy has been observed in uniaxial strained iron pnictides BaFe$_{2-x}$T$_x$As$_2$ (T $=$ Co, Ni) near the AF ordered state. This anisotropy has been suggested as a signature of the spin Ising-nematic phase that breaks the in-plane four-fold rotational symmetry of the underlying tetragonal lattice, but direct evidence for a spin nematic state is lacking. Here we use inelastic neutron scattering (INS) to show that low-energy spin excitations in uniaxial strained BaFe$_{2-x}$Ni$_x$As$_2$ (x $=$ 0, 0085) change from four-fold symmetry to two-fold symmetry in the tetragonal phase at temperatures corresponding to the onset of the in-plane resistivity anisotropy. Our results thus indicate that spin excitation anisotropy plays a crucial role in the electronic nematic behavior of iron pnictides. [Preview Abstract] |
Tuesday, March 4, 2014 1:39PM - 1:51PM |
G13.00011: Evolution of the in-plane resistivity anisotropy in isovalenly substituted Ba(Fe$_{1-x}$Ru$_x$)$_2$As$_2$ Erick C. Blomberg, M.A. Tanatar, S. Ran, S.L. Bud'ko, P.C. Canfield, R. Prozorov, A. Thaler Recent studies of electronic anisotropy in iron-based superconductors have revealed a dramatic asymmetry between electron and hole doped compounds [1]. A natural question is: What effect would isovalent substitution have? The BaFe$_2$As$_2$ system shows little change in its Fermi surface and carrier concentration upon Ru-doping, even at the levels far beyond the point of total suppression of the AFM state, making it a valuable system to compare against the hole and electron doped system. Here we study in-plane resistivity anisotropy in detwinned single crystals of Ba(Fe$_{1-x}$Ru$_x$)$_2$As$_2$. Polarized optical imaging was used to confirm detwinning. A quantitative comparison of our new results with the isovalently substituted BaFe$_2$(As$_{1-x}$P$_x$)$_2$ [2] system, as well as the charge doped Ba(Fe$_{1-x}$Co$_x$)$_2$As$_2$ and Ba$_{1-x}$K$_x$Fe$_2$As$_2$ systems will be discussed. This work was supported by the Department of Energy Office of Science, Basic Energy Sciences under Contract No. DE-AC02-O7CH11358.\\[4pt] [1] E. C. Blomberg, \textit{et al},. Nat. Comm. \textbf{4}, 1914 (2013).\\[0pt] [2] H-.H. Kuo, \textit{et al}., Phys. Rev. B \textbf{86}, 134507 (2012). [Preview Abstract] |
Tuesday, March 4, 2014 1:51PM - 2:03PM |
G13.00012: Influence of proton irradiation on the magnetotransport properties of BaFe$_{2}$As$_{2}$ and BaFe$_{1.985}$Co$_{0.015}$As$_{2}$ Dominc Moseley, Karen Yates, Will Branford, Lesley Cohen, David Mandrus, Athena Sefat Since their discovery in 2008, the iron-based superconductors have provided a new and unexpected system to probe the superconducting quantum phenomena. They have also presented the opportunity to investigate the intriguing interplay between magnetism, structure and transport properties. An additional complexity is the existence of Dirac Cones (DCs) within the electronic structure of the underdoped ferropnictides. In an attempt to elucidate the effects of these competing influences in the ferropnictides, we have performed a series of magnetoresistance (MR) experiments on proton irradiated undoped BaFe$_{2}$As$_{2}$ and sub-optimally doped BaFe$_{1.985}$Co$_{0.015}$As$_{2}$. Our findings show a non-saturating (up to 7T) linear MR above a temperature dependent critical magnetic field; in agreement with previous studies. Invoking the quantum linear magnetoresistance (QLM) model, it has been suggested that this infers the carrier transport dominance of DCs within the ferropnictides. Controlled proton irradiation allows us to test this concept by introducing random point defects into these materials. The conclusions drawn from this study will be discussed. [Preview Abstract] |
Tuesday, March 4, 2014 2:03PM - 2:15PM |
G13.00013: Magneto-transport and magnetization studies on thermally activated flux flow in iron-based superconductors Martin Nikolo, Xiaoyan Shi, Eun Sang Choi, Jianyi Jiang, Jeremy Weiss, Eric Hellstrom We study the magneto-transport properties of three iron-based high temperature superconductors, polycrystalline samples, Ba(Fe$_{0.95}$ Ni$_{0.05})_{2}$As$_{2}$ ($T_{c} =$ 20.4 K), Ba(Fe$_{0.94}$ Ni$_{0.06})_{2}$As$_{2}$ ($T_{c} =$ 18.5 K), and Ba(Fe$_{0.91}$Co$_{0.09})_{2}$As$_{2}$ ($T_{c} =$ 25.3 K) in magnetic fields of up to 18 T. The thermally activated magnetic flux behavior was retrieved by plotting ln$\rho$ vs. 1/$T$ ($\rho$ and $T$ are resistivity and temperature, respectively) and obtaining the activation energies $U_{0}$ for flux motion near $T_{c}$. We show a 3-D plot of the distribution of $U_{0}$ as a function of $T$ and magnetic field $H$. We apply the WHH model by measuring \textit{dH}$_{c2}$\textit{/dT} at $T_{c}$ to estimate the upper critical field $H_{c2}(T=$ 0); we estimate the coherence length $\xi (T=$ 0). We study the broadening of resistive transition as a function of the applied magnetic field and compare it to Tinkham's prediction for high-T$_{c}$ materials. [Preview Abstract] |
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