Bulletin of the American Physical Society
2009 APS March Meeting
Volume 54, Number 1
Monday–Friday, March 16–20, 2009; Pittsburgh, Pennsylvania
Session P35: Focus Session: Iron Pnictides and Other Novel Superconductors IX: Mostly Transport |
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Sponsoring Units: DCMP Chair: Sergey Budko, Ames Laboratory Room: 405 |
Wednesday, March 18, 2009 8:00AM - 8:12AM |
P35.00001: Low-temperature thermal transport properties of BaNi$_2$As$_2$ Nobuyuki Kurita, Filip Ronning, Yoshifumi Tokiwa, Eric D. Bauer, Alaska Subedi, David J. Singh, Joe D. Thompson, Roman Movshovich Low-temperature specific heat $C$($T$) and thermal conductivity $\kappa$($T$) measurements under magnetic field have been performed on a recently discovered Ni-based superconductor BaNi$_2$As$_2$ ($T_{\mathrm{c}}$\,=\,0.7\,K). In zero field, $\kappa$($T$) shows $T$-linear behavior in the normal state and exhibits a BCS-like exponential decrease below $T_{\mathrm{c}}$. The field dependence of the residual thermal conductivity extrapolated to zero temperature is indicative of a fully gapped superconductor. This conclusion is supported by the analysis of $C$($T$) data, which are well fit based on the BCS theory. [Preview Abstract] |
Wednesday, March 18, 2009 8:12AM - 8:24AM |
P35.00002: Anisotropy of electronic transport in the iron-pnictide superconductor Ba(Fe,Co)$_2$As$_2$ Makariy A. Tanatar, N. Ni, C. Martin, R.T. Gordon, H. Kim, V.G. Kogan, G.D. Samolyuk, S.L. Bud'ko, P.C. Canfield, R. Prozorov The anisotropy of electrical resistivity in the iron pnictide superconductor Ba(Fe,Co)$_2$As$_2$ has been studied using the Montgomery technique and direct transport measurements in single crystals cut along principal directions of the conductivity tensor (tetragonal c-axis and a-direction perpendicular to it). A good quantitative agreement is found between two sets of data, with a $\rho_c/ \rho_a$ anisotropy of 5 $\pm$ 1 just above the superconducting transition temperature. This is in very good agreement with expectations based on the anisotropy of the critical fields, suggestive of orbital limiting of superconductivity at T$_c$. [Preview Abstract] |
Wednesday, March 18, 2009 8:24AM - 8:36AM |
P35.00003: C-axis transport of pnictide single crystals. Yuri Koval, Paul Mueller, Guenter Behr, Bernd Buechner Mesa-type structures of $\sim $1 $\mu $m$^{2}$ area were fabricated on the (ab) plane of small LaO$_{0.9}$F$_{0.1}$FeAs single crystals. Resistance vs. temperature measurements showed a metallic behavior with a residual resistance ratio higher than 10. Both magnetic susceptibility and transport measurements showed the same value for the critical temperature, i.e. $\sim $20K. Current-voltage characteristics are typical for overdamped Josephson junctions with a critical current density of $\sim $10$^{5}$ A/cm$^{2}$. Moreover, the critical current vs. temperature dependence follows the Ambegaokar-Baratoff relation for the maximum dc Josephson current. One possible explanation could be that we have observed an intrinsic Josephson effect in $<$c$>$-direction. This is supported by recent measurements of radiation emission between 11 and 12 GHz. Finally, we discuss current injection effects on Josephson critical current and T$_{c}$. [Preview Abstract] |
Wednesday, March 18, 2009 8:36AM - 8:48AM |
P35.00004: Thermoelectric Power of RO$_{1-x-y}$F$_{x}$FeAs Feng Chen, Kalyan Sasmal, Melissa Gooch, Fengyan Wei, Bernd Lorenz, Yuyi Xue, C.W. Chu, Bing Lv, Zhongjia Tang, Arnold Guloy The thermoelectric power $S$($T$) has been measured on the RO$_ {1-x-y}$F$_{x}$FeAs samples with $0 \leq x \leq 0.3$ and $0 \leq y \leq 0.5$ and with the rare earth R = La, Ce, Sm and Pr. Together with Hall and lattice parameter studies, systematical $x$-dependency is observed, although rather weak for samples with $x > 0$ and $y = 0$. The $S$($T$) of the undoped samples with $x = y = 0$, however, appears to have rather different shape and amplitude. By comparing with the resistivity drop around 150 K, the change seems to be associated with the spin/lattice instability previously proposed. [Preview Abstract] |
Wednesday, March 18, 2009 8:48AM - 9:00AM |
P35.00005: A novel non-Fermi-liquid state in the iron-pnictide FeCrAs Wenlong Wu, Alix McCollam, Ian Swainson, Patrick Rourke, Denis Rancourt, Stephen Julian We report transport and thermodynamic properties of stoichiometric single crystals of the hexagonal iron-pnictide FeCrAs. The in-plane resistivity shows an unusual ``non-metallic" dependence on temperature $T$, rising continuously with decreasing $T$ from $\sim$ 800~K tobelow 100 mK. The $c$-axis resistivity is similar, except for a sharp drop upon entry into an antiferromagnetic state at $T_N \sim 125$~K. Below 10~K the resistivity follows a non-Fermi-liquid power law, $\rho (T) =\rho_0 -AT^x$ with $x<1$. The specific heat, on the other hand, shows typical Fermi liquid behaviour with a linear temperature dependence and a large Sommerfeld coefficient, $ \gamma \sim 30 {\rm\ mJ/mol\,K^2}$. The magnetic susceptibility does not follow Curie-Weiss law and it is rather weakly temperature dependent at low temperature. The high temperature properties of FeCrAs are reminiscent of those of the parent compounds of the new layered iron-pnictide superconductors, however the $T \rightarrow 0{\rm ~K}$ properties suggest a new class of non-Fermi liquid. This low temperature state has some features expected of a fractionalized electron system, in which conduction electrons break up into a charge carrying part that scatters anomalously and a spin part that has the thermodynamic properties of a Fermi liquid. [Preview Abstract] |
Wednesday, March 18, 2009 9:00AM - 9:12AM |
P35.00006: Large quasiparticle thermal Hall conductivity in the superconductor $\rm Ba_{1-x}K_xFe_2As_2$ Joseph Checkelsky, Lu Li, G.F. Chen, J.L. Luo, N.L. Wang, N.P. Ong We have measured the thermal conductivity $\kappa_{xx}$ and thermal Hall conductivity $\kappa_{xy}$ in single-crystal $\rm Ba_{1-x}K_xFe_2As_2$. Below the superconducting transition temperature $T_{c}$ ($\sim$ 30 K), we observe a large peak in the weak-field $\kappa_{xy}$ . A corresponding peak in the zero-field thermal conductivity $\kappa_{xx}$ is also observed. Together, these imply the existence of a large population of hole-like quasiparticles below $T_{c}$ . In magnetic fields $H$ approaching 35 T, the peaks in $\kappa_{xx}$ are strongly suppressed. A fit of the $\kappa_{xx}$ vs. $H$ curves shows that the data are consistent with the scattering of long-lived quasiparticles by vortices. Using these fits, we have extracted estimates of the quasiparticle mean-free-path, and separated the zero-field electronic and phononic terms $\kappa_{e}$ and $\kappa_{ph}$, respectively. We discuss the origin of the large quasiparticle population in terms a strongly anisotropic gap parameter or a gap with nodes. [Preview Abstract] |
Wednesday, March 18, 2009 9:12AM - 9:48AM |
P35.00007: Fe pnictides in high magnetic fields. Invited Speaker: High magnetic fields provide invaluable tool in probing such complex and phase-rich materials as novel pnictide superconductors, where upper critical field estimates surpass 100 Tesla for some of the compositions. We have investigated a number of tertiary and quaternary iron pnictides in pulsed magnetic fields exceeding 60 T and will present the latest data on their superconducting and normal state properties at high fields. We probe the importance of reduced dimensionality for high temperature superconductivity across different families of pnictides, where we find contrasting behavior between 122 and 1111 compounds. We find clear signature of the Fermi surface reconstruction in magnetotransport properties of the field-induced normal state, which coincides with reported structural and magnetic phase transitions. [Preview Abstract] |
Wednesday, March 18, 2009 9:48AM - 10:00AM |
P35.00008: Spin density wave instability and pseudogap formation in EuFe2As2 Jong Hoon Shin, Soon Jae Moon, Ju-Young Kim, Seung Hyun Kim, Woo Seok Choi, Byung Chul Jeon, Yun Sang Lee, Beong Ki Cho, Kee Hoon Kim, Tae Won Noh Recently, iron arsenide superconductors have aroused great amount of interest. In these compounds, by doping electron or hole, the superconductivity arises with the suppression of spin density wave (SDW) order. The close relation between the superconducting state and SDW instability suggests that the magnetic fluctuation might play an important role. Therefore, it is imperative to study the magnetic ground state of the parent compounds to understand the mechanism of the superconductivity. We investigated optical conductivity spectra of EuFe2As2 single crystals, which showed SDW order below about 190 K. Across the transition temperature, the optical spectral weight transferred from low energy (below 900cm-1) to higher energy (above 900 cm-1), forming a pseudogap. In the SDW phase, the sharp Drude-like response still remained. Our results indicate that the SDW formation induce the partial gap opening in the Fermi surface of EuFe2As2. [Preview Abstract] |
Wednesday, March 18, 2009 10:00AM - 10:12AM |
P35.00009: Electronic anisotropy from magneto-transport near T$_{c}$ in SmFeAs(O$_{0.7}$F$_{0.25})$ and (Ba,Rb)Fe$_{2}$As$_{2}$ single crystals Philip Moll, Karsten Kunze, Zbigniew Bukowski, Nikolai Zhigadlo, Janusz Karpinski, Bertram Batlogg We derived thermally activated flux flow (TAFF) activation energies E$_{a}$(H) and the upper critical fields \textbf{Hc}$_{2}$(T) parallel to the c-axis and in the Lorentz-force free configuration (\textbf{H} $\vert \vert $ \textbf{ab} $\vert \vert $ \textbf{j}) of SmFeAs(O$_{0.7}$F$_{0.25})$ and (Ba,Rb)Fe$_{2}$As$_{2}$ single crystals from resistance measurements and compare them to the ones reported for other REFeAs(OF). A perfectly rectangular rod (67x11x4 $\mu $m), aligned with the crystal axes, was cut from a larger SmFeAs(O$_{0.7}$F$_{0.25})$ single crystal ($\sim $ 200 $\mu $m) by a Focused Ion Beam (FIB) which allowed us to precisely control its geometry factor L/A = 0.89 1/$\mu $m. The FIB was also used to deposit 4 Pt contacts. We found a slope of \textbf{H}$_{c2, 50\% }$ (T), parallel to the c-axis, of 1.9 T/K for SmFeAs(O$_{0.7}$F$_{0.25})$ and 3.7 T/K for (Ba,Rb)Fe$_{2}$As$_{2}$ near T$_{c}$. The electronic anisotropy, derived from magneto-transport, is significantly larger in the REFeAs(OF) crystals than in (Ba,Rb)Fe$_{2}$As$_{2}$. [Preview Abstract] |
Wednesday, March 18, 2009 10:12AM - 10:24AM |
P35.00010: Quantum critical regime in the phase diagram of K$_x$Sr$_{1-x}$Fe$_2$As$_2$ Bernd Lorenz, Melissa Gooch, Bing Lv, Arnold M. Guloy, Ching-Wu Chu The electrical and thermoelectric properties of K$_x$Sr$_{1-x}$Fe$_2$As$_2$ are investigated. While the temperature dependence of the resistivity of SrFe$_2$As$_2$ (x=0) and KFe$_2$As$_2$ (x=1) is strongly nonlinear over a large temperature range it becomes surprisingly linear for x close to x$_c$ = 0.4 above the superconducting transition. This apparent deviation from the Fermi liquid behavior is similar to the high-T$_c$ cuprate superconductors and may indicate the existence of a quantum critical regime above the superconducting dome. We show that the temperature dependence of the thermoelectric power S follows a logarithmic scaling, S/T = const.*log(T) at the critical value x$_c$. The experimental results are consistent with a Ginzburg-Landau model for FeAs compounds predicting quantum critical scaling with a dynamical exponent z=2 and an effective dimension d+z=4. [Preview Abstract] |
Wednesday, March 18, 2009 10:24AM - 10:36AM |
P35.00011: Low temperature thermal conductivity of single crystal Ba$_{1-x}$K$_x$Fe$_2$As$_2$ J.-P. Reid $^1$ , M.A. Tanatar $^2$ , X. Luo $^1$ , N. Doiron-Leyraud $^1$ , N. Ni $^3$ , S.L. Bud'ko $^{2,3}$ , P.C. Canfield $^{2,3}$ , H. Lup $^4$ , Z. Wang $^4$ , H.H. Wen $^4$ , R. Prozorov $^{2,3}$ , Louis Taillefer $^1$ Novel iron-arsenic based superconductors with T$_c$ $>$ 50 K form a good reference for studying physical processes, underlyning high-T$_c$ superconductivity. Indeed, the importance of unique features of the superconductivity in cuprates, like proximity to Mott insulating state in the phase diagram, magnetism, d-wave superconducting pairing can be tested experimentally. Here we report a study aiming at understanding the symmetry of the superconducting gap in iron-arsenic compounds. Temperature and magnetic field dependence of low temperature thermal conductivity of Ba$_{1-x}$K$_x$Fe$_2$As$_2$ single crystals was studied down to 60 mK (T$_c$/500) and in magnetic fields up to 15 T (H/H$_{c2}\cong$~ 0.25 ). We find no residual linear term in zero magnetic field and can therefore exclude a superconducting gap with a line of nodes. [Preview Abstract] |
Wednesday, March 18, 2009 10:36AM - 10:48AM |
P35.00012: Effects of Co substitution on thermodynamic and transport properties and anisotropic H$_{c2}$ in Ba(Fe$_{1-x}$Co$_{x})_{2}$As$_{2}$. Ni Ni, M. E. Tillman, J-Q. Yan, A. Kracher, S. L. Bud'ko, P. C. Canfield, S. T. Hannahs Single crystal samples of Ba(Fe$_{1-x}$Co$_{x})_{2}$As$_{2 }$, x$<$ 0.12, have been characterized by microscopic, thermodynamic and transport measurements. With increasing Co concentration, the features of the structural and magnetic transitions are suppressed at a rate of roughly 15K per percent of Co. Superconductivity is stabilized at low temperatures for 0.038 $\le $x and up through our highest doping level of x = 0.114. The superconducting region has a dome like appearance with maximum T$_{c}$ values ($\sim $ 23 K) found near x $\sim $ 0.07. The T -- x phase diagram shows that either the existence of superconductivity in both the tetragonal and the orthorhombic (AFM) phase or there is a structural phase separation. Anisotropic H$_{c2}$ data clearly show that the superconductivity which occurs in samples that show features associated with the transition to the low temperature orthorhombic state is 50{\%} smaller than that found in samples that remain in the tetragonal phase. These data show that the superconductivity is sensitive to the suppression of the higher temperature phase transition. [Preview Abstract] |
Wednesday, March 18, 2009 10:48AM - 11:00AM |
P35.00013: Log-T upturn in resistivity of lightly-doped oxypnictide superconductor in an intense magnetic field Z. Stegen, S.C. Riggs, J.B. Kemper, Y. Jo, L. Balicas, G.S. Boebinger, F.F. Balakirev, A. Migliori, H. Chen, R.H. Liu, X.H. Chen We report the resistivity of a series of fluorine-doped SmFeAsO$_{1-x}$F$_x$ polycrystalline superconductors in magnetic fields up to 60 T. For underdoped samples ($x < 0.15 $), the low-temperature resistive state is characterized by pronounced magneto-resistance including an upturn in the resistivity at low temperatures. The ``insulating behavior'' is characterized by a log-T divergence observed over a decade in temperature. In contrast, samples with doping $x > 0.15$ display metallic behavior with little magnetoresistance, where intense magnetic fields serve to broaden the superconducting transition rather than significantly suppress T$_c$. The doping regime for the log-T behavior coincides with the spin density wave (SDW) in the phase diagram for SmFeAsO$_{1-x}$F$_x$. [Preview Abstract] |
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