Bulletin of the American Physical Society
2009 APS March Meeting
Volume 54, Number 1
Monday–Friday, March 16–20, 2009; Pittsburgh, Pennsylvania
Session Y35: Focus Session: Iron Pnictides and Other Novel Superconductors XVI: Hc2 and Vortex Dynamics |
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Sponsoring Units: DMP Chair: Ruslan Prozorov, Ames Laboratory Room: 405 |
Friday, March 20, 2009 8:00AM - 8:12AM |
Y35.00001: Fluorine Doping Effect On H$_{c2}$ and R$_{h}$ In LaFeAsO$_{1-x}$F$_{x}$ Y. Kohama, S. Riggs, F. Balakirev, M. Jaime, Y. Kamihara, T. Atake, M. Hirano, H. Hosono The iron arsenide superconductors discovered earlier this year have attracted much interest, and some families showing high-$T_{c}$ have been identified. \textit{Ln}FeAsO (\textit{Ln}; lanthanide) is the first copper-free family of compounds with $T_{c}$ exceeding 50 K. Here, we present the first systematic study of $H_{c2}$ and $R_{H}$ in the wide fluorine doping region (LaFeAsO$_{1-x}$F$_{x}$; $x$ = 0, 0.25, 0.05, 0.07, 0.11 and 0.14). We found that $H_{c2}$ increases monotonically with decreasing $x$, while the superconducting phase diagram ($T_{c}$, $x)$ displays the classic dome-shaped structure. Furthermore, the shape of $H_{c2}(T)$ depends strongly on $x$. This, according to Gurevich's model, suggests a multiband electronic structure. The Hall resistivity for non-superconducting samples $x$ = 0, 0.025 show a non-linear magnetic field dependence, which also supports a multiband electronic structure interpretation. In addition, the estimated low-field limit of the $R_{H}$ for $x$ = 0, 0.025 detect a gap opening at the structural transition and magnetic transition. The evidence for multiband electronic structure links these materials to famous multiband superconductor MgB$_{2}$ rather than high-$T_{c}$ cuprates. [Preview Abstract] |
Friday, March 20, 2009 8:12AM - 8:24AM |
Y35.00002: Upper critical fields of NdFeAsO$_{0.7}$F$_{0.3}$ single crystal J. Jaroszynski, F. Hunte, L. Balicas, Youn-jung Jo, Ivana Raicevic, A. Gurevich, D.C. Larbalestier, F.F. Balakirev, L. Fang, P. Cheng, Y. Jia, H.H. Wen We present measurements of the resistivity and the upper critical field $H_{c2}$ of NdFeAs O$_{0.7}$F$_{0.3}$ single crystals in strong DC and pulsed magnetic fields up to 45 T and 60 T, respectively. We found that the field scale of $H_{c2}$ is comparable to $\sim100$~T of high $T_c$ cuprates. $H_{c2}(T)$ parallel to the c-axis exhibits a pronounced upward curvature similar to what was extracted from earlier measurements on polycrystalline samples. Thus this behavior is indeed an intrinsic feature of oxypnictides, rather than manifestation of vortex lattice melting or granularity. The orientational dependence of $H_{c2}$ shows deviations from the one-band Ginzburg-Landau scaling. The mass anisotropy decreases as $T$ decreases, from 9.2 at 44 K to 5 at 34 K. We discuss to what extent different pairing scenarios can manifest themselves in the observed behavior of $H_{c2}$, using the two-band model of superconductivity. The results indicate the importance of paramagnetic effects on $H_{c2}(T)$, which may significantly reduce $H_{c2}(0)$ as compared to $H_{c2}(0)\sim200-300$ T based on extrapolations of $H_{c2}(T)$ near $T_c$ down to low temperatures. [Preview Abstract] |
Friday, March 20, 2009 8:24AM - 8:36AM |
Y35.00003: Determination of anisotropic $H_{c2}$ in (Ba$_{0.55}$K$_{0.45}$)Fe$_2$As$_2$ single crystals C. H. Mielke, M. M. Altarawneh, K. Collar, N. Ni, S. L. Bud'ko, P. C. Canfield The radio frequency penetration depth was measured in the superconductor (Ba$_{0.55}$K$_{0.45}$)Fe$_{2}$As$_{2}$ under pulsed magnetic fields extending to 60 tesla and down to 14 K. Using these data we are able to infer a $H_{c2}(T)$, $H-T$ phase diagram, for applied fields parallel and perpendicular to the crystallographic $c$-axis. The upper critical field curvature is different for the respective orientations but they each remain positive down to 14 K. The upper critical field anisotropy is moderate, $\approx 3.5$ close to $T_c$, and drops with the decrease of temperature, reaching $\approx 1.2$ at $14 K$. These data and analysis indicate that (i) (Ba$_{0.55}$K$_{0.45}$)Fe$_{2}$As$_{2}$ anisotropy diminishes with temperature and has an unusual temperature dependence, (ii) $H_{c2} (T=0)$ for this compound may easily approach fields of 75 tesla. [Preview Abstract] |
Friday, March 20, 2009 8:36AM - 8:48AM |
Y35.00004: Thermodynamic estimation of the upper critical field slope of doped SmFeAsO from fluctuation conductivity in the critical regime Marina Putti, Ilaria Pallecchi, Carlo Fanciulli, Matteo Tropeano, Maurizio Ferretti, Alberto Martinelli, Andrea Palenzona, Carlo Ferdeghini We measure magnetotransport in SmFeAs(O$_{1-x}$F$_{x})$ polycrystalline samples up to 28T and we extract the upper critical fields, using different criteria. Due to fluctuation effects, not negligible magnetoresistance and resistivity not saturating to a residual value at Tc, H$_{c2}$ values turn out to be strongly criterion-dependent. In order to circumvent this problem, we propose a thermodynamic estimation of the upper critical field slope dH$_{c2}$/dT based on the analysis of conductivity fluctuations in the critical regime at high fields. Indeed, in this regime we find evidence of a two-dimensional lowest Landau level (LLL) scaling for applied fields larger than $\mu _{0}$H$_{LLL}\sim $8T, which allows to extract a high field slope as large as -12T/K for the optimally doped sample SmFeAs(O$_{0.85}$F$_{0.15})$. A comparison of the fluctuation behavior with that of high-T$_{c}$ cuprates indicates that this H$_{LLL}$ value may be related to the H$_{c2}$ and $\kappa $ values higher than those of cuprates.. [Preview Abstract] |
Friday, March 20, 2009 8:48AM - 9:00AM |
Y35.00005: Upper critical field in Ba(Fe$_{1-x}$Co$_{x})_{2}$As$_{2}$ and FeSe$_{1-x}$ Te$_{x}$ compounds Chiara Tarantini, Jan Jaroszynski, Jianyi Jiang, Alex Gurevich, David C. Larbalestier, Rongyin Jin, Athena S. Sefat, Michael A. McGuire, Brian C. Sales, David G. Mandrus We report H$_{c2}$ measurements in high magnetic field up to 31 T on Ba(Fe$_{1-x}$Co$_{1-x})_{2}$As$_{2}$ and FeSe$_{1-x}$ Te$_{x}$\textbf{ }pnictide compounds for different doping levels. Both materials exhibit a very high upper critical fields and unconventional temperature dependencies of H$_{c2}$(T) with the extremely high slopes dH$_{c2}$/dT from 10 to 30 T/K near T$_{c}$ and a relatively low anisotropy: $\gamma ={H_{c2} ^{//}} \mathord{\left/ {\vphantom {{H_{c2} ^{//}} {H_{c2} ^\bot }}} \right. \kern-\nulldelimiterspace} {H_{c2} ^\bot }$ for the doped ternary compound and $\gamma \approx 1.1\div 1.2$ for the binary one. The observed temperature dependences of H$_{c2}$(T) and the high H$_{c2}$ values well above the BCS paramagnetic limit indicate the importance of the Zeeman pairbreaking effects in these compounds. [Preview Abstract] |
Friday, March 20, 2009 9:00AM - 9:12AM |
Y35.00006: Thermodynamic Determination of the Upper Critical Field and Anisotropy of Ba$_{0.6}$K$_{0.4}$Fe$_{2}$As$_{2}$ Single Crystals Wai -K. Kwok, Ulrich Welp, Ruobing Xie, Alexei Koshelev, John Schlueter, Jiong Hua, Hui-qian Luo, Zhao-sheng Wang, Gang Mu, Hai-hu Wen We present anisotropic heat capacity measurements of the upper critical field of Ba$_{0.6}$K$_{0.4}$Fe$_{2}$As$_{2}$ single crystals in fields up to 8 Tesla. In zero-magnetic field a clear step in the heat capacity is observed at T$_{c} \sim $36K. Using an entropy conserving construction we determined the transition temperatures in applied fields and the upper critical field slopes dH$_{c2 \vert \vert c}$/dT = -6.5 T/K and dH$_{c2 \vert \vert ab}$/dT -17.4 T/K, the latter showing record high critical field slope near T$_{c}$. The temperature dependence of the specific heat of Ba$_{0.6}$K$_{0.4}$Fe$_{2}$As$_{2}$ indicates strong coupling effects. Based on the experimental values of the upper critical field slopes, we determined the Ginzburg parameter, coherence and penetration lengths, anisotropy and thermodynamic critical fields. We also present magnetization measurements and discuss their implications on the nature of critical currents in this material. [Preview Abstract] |
Friday, March 20, 2009 9:12AM - 9:24AM |
Y35.00007: Inter- and intra-granular current properties of iron pnictide superconductors Akiyasu Yamamoto, Anatolii Polyanskii, Jianyi Jiang, Fumitake Kametani, Marina Putti, Chiara Tarantini, Frank Hunte, Jan Jaroszynski, Eric Hellstrom, Alex Gurevich, David Larbalestier The iron pnictide superconductors have very high upper critical field $B_{c2}$(0) of possibly over 100 T for 1111 and 50-70 T for 122. We have recently shown [1,2] that polycrystalline 1111 samples exhibit electromagnetic granular behavior, perhaps in an analogous way to that seen now to be intrinsic to the HTS cuprates. Detailed investigation is proceeding in parallel with serious efforts to make more single phase samples, since it appears that all present polycrystalline oxypnictides are multi-phase. In particular we are using magneto-optical imaging to study the local variation of current density and then performing detailed microstructural analysis by SEM, TEM and orientation analysis to understand intergranular current flow. At the present time we see that samples are multi-phase, often with a grain boundary wetting phase, but even so the global $J_{c}$ attains 1000-4000 A/cm$^{2}$, some 10-40 times that seen in single phase YBCO randomly oriented polycrystalline. On the other hand, very high intra-grain critical current owing to the strong pinning reminiscent of Nb-Ti is observed in the Co doped Ba122 pnictide. We will report on our latest results on the inter- and intra-granular current properties in the high-$T_{c}$ pnictides. [1] A. Yamamoto et al., Appl. Phys. Lett. \textbf{92}, 252501 (2008). [2] A. Yamamoto et al., Supercond. Sci. Technol. \textbf{21}, 095008 (2008). [Preview Abstract] |
Friday, March 20, 2009 9:24AM - 9:36AM |
Y35.00008: Magneto-optical imaging of flux distribution in single crystals Ba(Fe$_{1-x}$Co$_x$)$_2$As$_2$ of various doping level $x$. Erick C. Blomberg, P. Prommapan, M. A. Tanatar, V. G. Kogan, N. Ni, S. L. Bud'ko, P. C. Canfield, R. Prozorov Near optimal doping of Ba(Fe$_{0.93}$Co$_{0.07}$)$_2$As$_2$ crystals exhibit uniform superconductivity and vortex properties similar to high-T$_c$ cuprates \footnote{R. Prozorov \textit{et al.}, Phys. Rev. B in print (2008). arXiv:0810.1338}. In this contribution, single crystals of Ba(Fe$_{1-x}$Co$_x$)$_2$As$_2$ with the measured doping level x=3.8\%, 4.7\%, 5.8\%, 7.4\%, 10\%, and 11.8\%, covering all regimes - from underdoped to overdoped were studied using real-time magneto-optical imaging. Inhomogeneity of the superconducting state as well as field and temperature dependencies of the magnetic induction distribution were analyzed. Superconductivity is homogeneous at all except for the highest doping level. The results are correlated with macroscopic transport and magnetic measurements. [Preview Abstract] |
Friday, March 20, 2009 9:36AM - 9:48AM |
Y35.00009: ABSTRACT WITHDRAWN |
Friday, March 20, 2009 9:48AM - 10:00AM |
Y35.00010: Similarity Between Fe-based Pnictide Superconductors and High-T$_c$ Cuprates Revealed by the Irreversible Magnetic Behavior in the Vortex State Ruslan Prozorov, M.A. Tanatar, C. Martin, R.T. Gordon, E.C. Blomberg, P. Prommapan, V.G. Kogan, N. Ni, M.E. Tillman, S.L. Bud'ko, P.C. Canfield Static and dynamic measurements of DC magnetization and direct real-time magneto-optical imaging have revealed unconventional vortex behavior in NdFeAsO$_{0.9}$F$_{0.1}$\footnote{R. Prozorov \textit{et al.} arXiv:0805.2783 (2008)} and Ba(Fe$_{0.93}$Co$_{0.07}$)$_2$As$_2$\footnote{R. Prozorov \textit{et al.}, Phys. Rev. B in print (2008). arXiv:0810.1338} single crystals. In particular, the fishtail effect, giant magnetic relaxation and a crossover from elastic to plastic vortex creep, reminiscent of similar features found in pure single crystals of Y-Ba-Cu-O and Nd-Ce-Cu-O cuprates. Moreover, despite weak anisotropy of the London penetration depth and second critical field, vortex properties of the pnictides are shown to be very anisotropic. We discuss possible explanations of the observed behavior and conclude that pnictide superconductors bridge a gap between s-wave multiband MgB$_2$ and highly anisotropic d-wave cuprates. [Preview Abstract] |
Friday, March 20, 2009 10:00AM - 10:12AM |
Y35.00011: Flux Pinning and Structural Inhomogeneity in Superconducting BaFe$_{1.8}$Co$_{0.2}$As$_{2}$ Single Crystals Qiang Li, Juan Zhou, Jiufeng Tu, Yuhang Ren, Linjun Li, Yongkang Luo, Hang Chen, Guanghan Cao, Zhu'an Xu We report coordinated studies of flux pinning behavior and structural inhomogeneity in BaFe$_{1.8}$Co$_{0.2}$As$_{2}$ single crystals with superconducting transition temperature at 24 K, in order to understand the flux pinning mechanism in the iron-based superconducting materials. Static and dynamic behavior of vortices are investigated by transport, bulk magnetization, and quantitative magneto-optical imaging techniques, while high resolution analytical TEM is used to investigate the structural inhomogeneity down to atomistic level. Frequent flux jump, and enhanced flux pinning at elevated magnetic field, corresponding to the ``fish tail'' in magnetization hysteresis, are often observed. We will discuss the relationship between the flux pinning behavior and detailed structural properties in this and other related superconducting materials. [Preview Abstract] |
Friday, March 20, 2009 10:12AM - 10:24AM |
Y35.00012: Thermally-activated and temperature-independent magnetic relaxation in aligned grains of NdFeAsO(F) James R. Thompson, Y. L. Zuev, D. K. Christen, E. D. Specht, R. Jin, B.C. Sales, M. A. McGuire, A. Sefat, D. G. Mandrus We have studied flux creep in a magnetically-aligned powder of NdFeAsOF and found it to be strikingly similar to the situation in cuprates. The magnetic relaxation rate S=-dlnM$_{irr}$/dlnt is linear in temperature at low temperatures. There is an extrapolated~finite creep rate of about S=-0.02 at T=0, indicative of a quantum tunneling of vortices under energy barriers. This quantum creep rate is field-independent. From the temperature-dependent creep data we have obtained activation energy as a function of persistent current density, U(J). Comparison with existing creep theories will be made. [Preview Abstract] |
Friday, March 20, 2009 10:24AM - 10:36AM |
Y35.00013: Magnetic imaging of vortices and inhomogeneity in Ba(Fe,Co)$_2$As$_2$ by magnetic force microscopy Weida Wu, S. Park, Linjun Li, Yongkang Li, Hang Chen, Guanghan Cao, Zhu'an Xu Single crystals of BaFe$_{2-x}$Co$_x$As$_2$ synthesized by FeAs flux method were studied by variable temperature magnetic force microscopy (VT-MFM). The nominal Co doping concentrations range from underdoped region (x$\sim$0.1) to optimum doped region (x$\sim$0.2) of the superconducting dome. Sharp superconducting transitions ($\sim$1K) indicate good sample quality. Individual Abrikosov vortices were visualized by VT-MFM below Tc at low magnetic field. The temperature dependence of vortex configuration indicates a strong pinning effect, which is supported by Bean-model behavior\footnote{C.P. Bean, PRL, 8, 250 (1962).} observed at high magnetic field. Results of magnetic inhomogeneity of underdoped samples will be discussed. [Preview Abstract] |
Friday, March 20, 2009 10:36AM - 10:48AM |
Y35.00014: Isotropic low-temperature upper critical field in (Ba,K)Fe$_2$As$_2$ Huiqiu Yuan, Scot Baily, John Singleton, Fedor Balakirev, G.F. Chen, J.L. Luo, N.L. Wang Furious activity has been generated by the discovery of superconductivity in the iron-arsenic-based compounds ReFeAs(O,F) (Re = lanthanide) and (A,K)Fe$_2$As$_2$ (A=Ba, Sr). Superconducting $T_{\rm c}$s as high as 55~K have been observed, provoking comparisons with the ``high $T_{\rm c}$'' cuprates. The layered crystal structure of the cuprates led to speculations that reduced dimensionality is necessary for ``high-temperature'' superconductivity; at first sight, the iron-arsenic compounds, which also possess layered structures, give additional credence to this idea. However, we report measurements in magnetic fields of up to 60~T, necessary to overcome the large upper critical fields, that demonstrate that the superconducting properties of single crystals of (Ba,K)Fe$_2$As$_2$ are in fact rather three dimensional. This is markedly different from the highly anisotropic properties of all previously-known layered superconductors (e.g. the cuprates and the crystalline organic metals); we suggest that it is attributable to the distinctive electronic structure of the iron-arsenide compounds. Our measurements indicate that in contrast to the assumptions based on the cuprates, reduced dimensionality is not a prerequisite for ``high-temperature'' superconductivity. [Preview Abstract] |
Friday, March 20, 2009 10:48AM - 11:00AM |
Y35.00015: Unconventional magnetic field-temperature superconducting phase-diagram in Co doped BaFe$_2$As$_2$ Younjung Jo, R.H. Liu, H. Chen, X.H. Chen, L. Balicas Here, we report electrical transport and preliminary torque magnetometry measurements in underdoped Ba(Fe$_{1-x}$Co$_2$As$_2$ single crystals under high magnetic fields. For fields perpendicular to the conducting planes one observes an upward curvature for the temperature dependence of the upper critical field $H_{C2} (T)$ which contrasts markedly with one expects for conventional superconductors. While for fields along the conducting planes $H_{C2} (T)$ behaves nearly linearly in temperature down to low temperatures. For both configurations $H_{C2} (T)$ is found to surpass the weak coupling Pauli limiting field e.g., for a sample displaying a superconducting transition temperature $T_c \simeq 9.7$ K one observes $H_{C2} (T \rightarrow 0$ K $\geq 35$ T for fields along an in-plane direction. This value surpasses the expected Pauli limiting field by a factor of two. Thus, the resulting phase-diagram presents strong similarities with the one reported for poly-crystalline LaFeASO$_{1-x}$F$_x$ and which was recently claimed to be described by a model based on a two-gap strong coupled superconducting state. Our on-going torque magnetometry measurements are expected to check the validity of this interpretation. [Preview Abstract] |
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