Bulletin of the American Physical Society
APS March Meeting 2010
Volume 55, Number 2
Monday–Friday, March 15–19, 2010; Portland, Oregon
Session P40: Focus Session: Iron Based Superconductors: Physical Propertise II |
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Sponsoring Units: DCMP DMP Chair: Brian Sales, Oak Ridge National Laboratory Room: F151 |
Wednesday, March 17, 2010 8:00AM - 8:36AM |
P40.00001: Muon Spin Relaxation Studies of RFeAsO and MFe$_2$As$_2$ Based Compounds Invited Speaker: Muon spin relaxation measurements of a variety of iron pnictide systems have revealed commensurate long range magnetic order in the parent compounds which can change to incommensurate order with carrier doping. Magnetic order gives way to superconductivity with increased doping; however there are regions of the phase diagrams where the two phenomena co-exist. In the case of Ba$_{1-x}$K$_x$Fe$_2$As$_2$ there is phase separation into superconducting and magnetic domains, whereas in Ba(Fe$_{1-x}$Co$_x$)$_2$As$_2$ the coexistence is apparently microscopic for $x=0.035\rightarrow0.048$. Transverse field muon spin rotation measurements of single crystal Ba(Fe$_{1-x}$Co$_x$)$_2$ and Sr(Fe$_{1-x}$Co$_x$)$_2$ exhibit an Abrikosov vortex lattice from which we are able to determine the magnetic field penetration depth and Ginzburg-Landau parameter. The temperature variation of the superfluid density is well described by a two-gap model. In Ba(Fe$_{1-x}$Co$_x$)$_2$As$_2$, both the superconducting T$_C$ and the superfluid density decrease with increasing doping above $x=0.06$; in all of the pnictides we find that the superfluid density obeys the same nearly linear scaling with T$_C$ as found in the cuprates. [Preview Abstract] |
Wednesday, March 17, 2010 8:36AM - 8:48AM |
P40.00002: Itinerant vs local moment approaches to Fe-pnictides: insight from optical spectroscopy study Nan Lin Wang I present optical spectroscopy investigations on single crystal samples of 4 different Fe-based systems: 1111, 122, 111, and 11. For all FeAs-based parent compounds we observed common spectral features: partial energy-gaps formation along with a removal of a large part of free-carrier spectral weight and a steep reduction of the carrier scattering rate in the magnetic ordered state. However, the 11-type FeTe behaves very differently. No energy gap opens in the magnetic ordered state. We proposed that both the itinerancy and local moment interactions of Fe 3d electrons are present, but they contribute differently to the magnetic instabilities in different systems. Work done with W. Z. Hu, G. Li, Z. G. Chen, J. Dong, P. Zheng, G. F. Chen, and J. L. Luo. [Preview Abstract] |
Wednesday, March 17, 2010 8:48AM - 9:00AM |
P40.00003: Probing superconductivity with polarized neutrons and low-energy muons Vladimir Kozhevnikov, Kristiaan Temst, Margriet Van Bael, Chris Van Haesendonck, Joseph Indekeu A limited depth of magnetic field penetration is one of the most important properties of superconductors. It is usually assumed that in the Meissner state the field $B(z)$ decays exponentially with depth $z$. However, this cannot be the case, unless one deals with conventional type-II superconductors. For example, $B(z)$ is not exponential in nonlocal superconductors, but nonmonotonic and it even changes sign at a certain depth. Recently this nonlocal effect has been confirmed experimentally for a low-\textit{$\kappa $} superconductor. Nonlocality was also predicted for d-wave superconductors, where it can arise from the diverging coherence length near nodal points in momentum space. For such materials and especially for novel superconductors measurements of $B(z)$ may be crucial for interpretation. The $B(z)$ can be measured using Polarized Neutron Reflectivity (\textit{PNR}) and Low-Energy muon Spin Rotation (\textit{LE-$\mu $SR}) techniques. In this talk we will present a critical review of the capabilities of the \textit{PNR }and \textit{LE-$\mu $SR }techniques based on our studies of nonlocality in In. [Preview Abstract] |
Wednesday, March 17, 2010 9:00AM - 9:12AM |
P40.00004: Evidence for dynamic spin fluctuation in specific heat of Sr$_{1-x}$K$_{x}$Fe$_{2}$As$_{2}$ crystals Fengyan Wei, Bing Lv, Feng Chen, YuYi Xue, Arnold Guloy, Ching-Wu Chu We measured the zero-field specific heat in Sr$_{1-x}$K$_{x}$Fe$_{2}$As$_{2}$ system. The sommerfeld coefficients $\gamma$ contracting to the band-structure predict, shows a four-fold jump between the non-superconductive SrFe$_{2}$As$_{2}$ and the optimally doped Sr$_{0.55}$K$_{0.45}$Fe$_{2}$As$_{2}$. We also found that the electronic contribution actually change significantly with temperature, and cannot be attributed to the entropy of the static spin-density-wave (SDW) alone. Together with its unusual doping dependency, the data suggest the existence of dynamic spin fluctuations. The residual electronic specific heat further suggests a possible phase-separation in the region where SC (superconductivity) and SDW coexist. [Preview Abstract] |
Wednesday, March 17, 2010 9:12AM - 9:24AM |
P40.00005: Evolution of London penetration depth in single crystals of Ba(Fe$_{1-x}$T$_{x})_{2}$As$_{2}$ (T=Co, Ni) irradiated with heavy ions H. Kim, R. T. Gordon, N. Ni, M. A. Tanatar, S. L. Bud'ko, P. C. Canfield, J. Hua, U. Welp, W. K. Kwok, R. Prozorov Single crystals of Ba(Fe$_{1-x}$T$_{x})_{2}$As$_{2}$ (T=Co, Ni) were irradiated with 1.4 GeV Pb ions at different fluences to deliberately produce defects of controlled density. The London penetration depth, $\lambda \left( T \right)$, was measured by using the tunnel diode resonator technique. While overall, $\lambda \left( T \right)$ exhibits a power-law behavior at low temperatures, $\lambda \left( T \right)\sim AT^n$ , as was found previously [1], the fitting parameters $A$ and $n$ depend on the irradiation. Furthermore, both transport and magnetic susceptibility measurements show some reduction in the superconducting transition temperature, $T_{c}$, with increasing density of the induced defects. The evolution of the low temperature behavior of the penetration depth and transition temperature with irradiation dose will be analyzed in frameworks of existing theories where scattering plays a significant role due to sign change of the order parameter. The implications for the understanding of the pairing mechanisms in pnictide superconductors will be discussed. [1] R. T. Gordon \textit{et al}., Phys. Rev. Lett. \textbf{102}, 127004 (2009); Phys. Rev. B \textbf{79}, 100506(R) (2009). [Preview Abstract] |
Wednesday, March 17, 2010 9:24AM - 9:36AM |
P40.00006: Evolution of the absolute value of the London penetration depth in single crystals of Ba(Fe$_{1-x}$Co$_{x})_{2}$As$_{2}$ upon cobalt doping Ryan Gordon, Ni Ni, Makariy Tanatar, Vladimir Kogan, Sergey Bud'ko, Paul Canfield, Ruslan Prozorov, Nicholai Salovich, Russell Giannetta The absolute value of the London penetration depth, $\lambda _{0}$, has been measured in single crystals of Ba(Fe$_{1-x}$Co$_{x})_{2}$As$_{2}$ over a wide range of cobalt doping levels using a tunnel diode resonator (TDR) [1]. Normally, the TDR technique can only measure the relative change, $\Delta \lambda $(T), but the absolute value is needed to construct $\lambda $(T) and the superfluid density, $\rho _{s}$=[$\lambda _{0}$/$\lambda $(T)]$^{2}$, which can be compared to theory. A more elaborate approach in which the samples are coated with a thin film of Al allows for a direct estimate of $\lambda _{0}$ by using the same TDR apparatus [2]. The resulting doping-dependent $\lambda _{0}$(x) as well as the calculated $\rho _{s}$(T,x) curves will be discussed within current theoretical models. In particular, attention will be devoted to the effects of possible strong pair breaking predictions which state $\Delta \lambda \propto $T$^{n}$/T$_{c}^{3}$, with n$\approx $2 [3]. [1] R. T. Gordon \textit{et al}., Phys. Rev. B \textbf{79}, 100506(R) (2009). [2] R. Prozorov \textit{et al}., Appl. Phys. Lett. \textbf{77}, 25 (2000). [3] V. G. Kogan, arXiv:0910.4728 [Preview Abstract] |
Wednesday, March 17, 2010 9:36AM - 9:48AM |
P40.00007: Precision microwave electrodynamics of K and Co - doped BaFe$_2$As$_2$ Rinat Ofer, Jake Bobowski, Jordan Baglo, Shun Chi, James Day, Pinder Dosanjh, Brad Ramshaw, Lynne Semple, Ruixing Liang, Walter Hardy, Doug Bonn We present a detailed cavity perturbation measurement of the microwave electrodynamics of single crystal iron-based superconductors. The measurements, down to $\sim$0.4K, were performed on Ba$_{0.72}$K$_{0.28}$Fe$_2$As$_2$ hole-doped with $T_c\sim$30K, and BaFe$_{1.9}$Co$_{0.1}$As$_2$ electron-doped with $T_c\sim$20K. Our results show that the temperature dependence of the in-plane London penetration depth $\Delta\lambda(T)$ does not exhibit the exponential saturation at low temperatures expected from a fully-gapped superconductor. In addition we present broadband surface resistance measurements of two FeAs samples and discuss the behavior of the in-plane superfluid density $\rho_{ab}(T)$ in light of existing theoretical models proposed for the iron pnictides superconductors. [Preview Abstract] |
Wednesday, March 17, 2010 9:48AM - 10:00AM |
P40.00008: The Electronic Specific Heat of Ba$_{1-x}$K$_x$Fe$_2$As$_2$ from 2K to 380K James Storey, John Loram, John Cooper, Zbigniew Bukowski, Janusz Karpinski Using a differential technique, we have measured the specific heats of polycrystalline Ba$_{1-x}$K$_{x}$Fe$_2$As$_2$ samples with $x$ = 0, 0.1 and 0.3, between 2K and 380K and in magnetic fields 0 - 13T. From this data we have determined the electronic specific coefficient $\gamma(\equiv C^{el}/T)$ over the entire range for the three samples. The sample with $x$ = 0.3 exhibits a large SC anomaly $\Delta\gamma(T_c)$ $\sim$ 48 mJ/mol K$^2$ at $T_c$ = 36K, and we determine the energy gap, condensation energy, superfluid density and coherence length. In the normal state for the $x$ = 0.3 sample, $\gamma$ $\sim$ 45 mJ/mol K$^2$ is constant from $T_c$ to 380K. In the parent compound ($x$ = 0) there is a large almost first order anomaly at the SDW transition at $T_o$ = 136K. The corresponding anomaly for the 0.1 sample at $T_o$ $\sim$ 135K is smaller and broader than for $x$ = 0. At low T, $\gamma$ is strongly reduced by the SDW gap for both $x$ = 0 and 0.1, but above $T_o$, $\gamma$ for all three samples are similar. [Preview Abstract] |
Wednesday, March 17, 2010 10:00AM - 10:12AM |
P40.00009: Nernst effect in the pnictide superconductor Ba(Fe$_{1-x}$Co$_x$)$_2$As$_2$ Xigang Luo, H. Shakeripour, M.A. Tanatar, J.-Ph. Reid, J. Chang, F. Lalibert\'e, N. Doiron-Leyraud, N. Ni, S.L. Bud'ko, P.C. Canfield, R. Prozorov, Louis Taillefer In an attempt to shed light on the electronic transformations that pnictide superconductors undergo in different regions of their phase diagram, we have measured the Nernst effect of Ba (Fe$_{1-x}$Co$_x$)$_2$As$_2$ as a function of Co concentration. The data is presented and analyzed in terms of its two contributions, from superconducting fluctuations and normal- state quasiparticles, respectively. [Preview Abstract] |
Wednesday, March 17, 2010 10:12AM - 10:24AM |
P40.00010: Specific Heat vs Field in the 30 K Superconductor BaFe$_{2}$(As$_{0.7}$P$_{0.3})_{2}$ G.R. Stewart, J.S. Kim, P.J. Hirschfeld, S. Kasahara, T. Shibauchi, T. Terashima, Y. Matsuda Recently, superconductivity at 30 K has been reported [1] in P-doped BaFe$_{2}$As$_{2}$, with 1/3 of the As replaced by P. Magnetic penetration and thermal conductivity measurements [2] indicate a nodally gapped superconductor. We report here on measurements of the specific heat divided by temperature, C/T, as a function of field up to 15 T and down to 0.4 K in order to further investigate the nodal structure with another probe. \\[4pt] [1] S. Kasahara, et al., arXiv0905.4427. \\[0pt] [2] K. Hashimoto, et al., arXiv0907.4399. [Preview Abstract] |
Wednesday, March 17, 2010 10:24AM - 10:36AM |
P40.00011: Specific heat studies of the gap structure in iron-arsenide superconductor Ba(Fe$_{1-x}$Co$_{x})_{2}$As$_{2}$ K. Gofryk, A.S. Sefat, M.A. McGuire, B.C. Sales, D. Mandrus, E.D. Bauer, J.D. Thompson, F. Ronning Despite a large theoretical and experimental effort the nature of the superconductivity in FeAs-based materials and the symmetry of the gap remain unknown. Moreover, the experimental results on reported so far are often contradictory, ranging from nodal to fully gapped isotropic superconductivity. We report the doping, field and temperature dependence (down to 0.4 K and in magnetic fields up to 9 T) of the specific heat of Ba(Fe$_{1-x}$Co$_{x})_{2}$As$_{2}$ at under (x=0.038), optimal (x=0.07) and overdoped (x=0.078 and 0.096) regimes. By subtracting the lattice C$_{p}$, the temperature and field dependence of the electronic specific heat has been studied. The temperature and field dependencies of the superconducting part of C$_{p}$ exhibit similar behavior for all doping concentrations. The temperature variation of the electronic C$_{p}$ as well as its field dependence cannot be described by a single isotropic s-wave gap, pointing to complex gap structure in the system. The lack of doping dependence indicates that the gap structure does not change significantly as a function of doping. We also observe a significant residual linear term in the specific heat in the system which suggests that inhomogeneity may be an important factor in Co-doped BaFe$_{2}$As$_{2}$. [Preview Abstract] |
Wednesday, March 17, 2010 10:36AM - 10:48AM |
P40.00012: Heat capacity study of BaFe$_{2}$As$_{2}$ Costel R. Rotundu, Norman E. Phillips, Stephen D. Wilson, Ahram Kim, Giovanni Pinuellas, Byron K. Freelon, Edith Bourret-Courchesne, Robert J. Birgeneau We report heat capacity measurements on high quality single crystalline BaFe$_{2}$As$_{2}$ between 2 and 300 K in magnetic fields to 14 T. The sample, synthesized by a modified self-flux Bridgman method, shows no evidence of magnetic impurities. The low-temperature heat-capacity data give the electron density of states. The relation of the heat-capacity data to resistivity and magnetization measurements near the 140 K magnetic and structural phase transitions of both as-grown and annealed samples are discussed. [Preview Abstract] |
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