Bulletin of the American Physical Society
2009 APS March Meeting
Volume 54, Number 1
Monday–Friday, March 16–20, 2009; Pittsburgh, Pennsylvania
Session T33: Focus Session: Iron Pnictides and Other Novel Superconductors XI: Penetration Depth and Magnetic Anisotropy |
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Sponsoring Units: DMP Chair: Myron Salomon, University of Texas at Dallas Room: 403 |
Wednesday, March 18, 2009 2:30PM - 2:42PM |
T33.00001: Temperature Dependence of the Penetration Depth of LaFePO from Scanning SQUID Susceptometry Cliffird Hicks, Thomas Lippman, Martin Huber, James Analytis, Jiun-Haw Chu, Ian Fisher, Kathryn Moler We use a scanning SQUID susceptometer to measure locally the temperature dependence of the penetration depth of the superconductor LaFePO. We observe a linear temperature dependence down to 850 mK, with a slope that varies with position on the sample. This is in contrast to recent measurements on the related iron arsenide family of superconductors, which indicate fully-gapped superconductivity. We also report on our ability to measure T$_c$ locally, and observe local regions with weak superconductivity at temperatures well above the dominant T$_c$. [Preview Abstract] |
Wednesday, March 18, 2009 2:42PM - 2:54PM |
T33.00002: Broadband microwave absorption of Fe-based superconductors Jake Bobowski, Pinder Dosanjh, James Day, Doug Bonn, Walter Hardy Preliminary microwave measurements of the penetration depth in single crystals of the hole-doped iron-based superconductor Ba$_{1-x}$K$_{x}$Fe$_{2}$As$_{2}$, using cavity perturbation and ac susceptometry, have been performed by our group. These results hint at a gap with nodes plus the presence of scattering. As a complement to these techniques, we have also undertaken broadband microwave absorption (i.e., surface resistance) measurements on the same pnictide samples. Our initial results of the microwave conductivity are consistent with a system possessing a finite density of states at low temperature due to impurity scattering which then evolves with temperature above about 5 K. [Preview Abstract] |
Wednesday, March 18, 2009 2:54PM - 3:06PM |
T33.00003: Lower Critical Fields and the Anisotropy in PrFeAsO$_{1-y}$ Single Crystals Ryuji Okazaki, Marcin Konczykowski, C.J. van der Beek, Tomonari Kato, Ken-ichiro Hashimoto, Masaaki Shimozawa, Hiroaki Shishido, Minoru Yamashita, Takasada Shibauchi, Motoyuki Ishikado, Shin-ichi Shamoto, Hijiri Kito, Akira Iyo, Hiroshi Eisaki, Yuji Matsuda By utilizing miniature Hall-sensor array, we evaluated the lower critical fields $H_{c1}$ in Fe-based oxipnictide PrFeAsO$_{1-y}$ single crystals for {\boldmath $H$}$\parallel c$ and {\boldmath $H$}$\parallel ab$-planes. The temperature dependence of $H_{c1}$ for {\boldmath $H$}$\parallel c$ is well scaled by the in-plane penetration depth and is consistent with a full-gap superconducting state. The anisotropy of penetration depths at low temperatures is estimated to be $\simeq$ 3, which is much smaller than that of coherence lengths. This indicates the multiband superconductivity, in which the active band for the superconductivity is more anisotropic. [Preview Abstract] |
Wednesday, March 18, 2009 3:06PM - 3:18PM |
T33.00004: Temperature Dependent Anisotropy of Oxypnictide Superconductors Studied by Torque Magnetometry Stephen Weyeneth, Roman Puzniak, Nikolai D. Zhigadlo, Sergiy Katrych, Zbigniew Bukowski, Janusz Karpinski, Urs Mosele, Stefan Kohout, Josef Roos, Hugo Keller Single crystals of different oxypnictide superconductors of the family ReFeAsO$_{1-x}$F$_{y}$ (Re $=$ Sm, Nd, Pr) with various carrier dopings and with masses $m\simeq100$ ng have been investigated by means of torque magnetometry. We present most recent data, obtained by using highly sensitive piezoresistive torque sensors from which the superconducting anisotropy parameter $\gamma$ and the in-plane magnetic penetration depth $\lambda_{ab}$ were extracted. As an important result $\gamma$ was found to increase strongly as the temperature is decreased from $T_c$ down to low temperatures. This unconventional temperature dependence of $\gamma$ is similar to that observed in the two-band superconductor MgB$_2$ and cannot be explained within the classical Ginzburg-Landau model. This scenario strongly suggests a new multi-band mechanism in the novel class of oxypnictide high-temperature superconductors. [Preview Abstract] |
Wednesday, March 18, 2009 3:18PM - 3:30PM |
T33.00005: Magnetic penetration depth and critical current density in aligned grains of NdFeAsO(F) Yuri L. Zuev, E. D. Specht, D. K. Christen, J. R. Thompson, R. Jin, B. C. Sales, A. Sefat, M. A. McGuire, D. G. Mandrus We have prepared a powder sample of NdFeAsOF, where most crystallite particles are aligned with their c-axis along a common direction. We have measured magnetic penetration depth and a critical current density in the basal-plane. The penetration depth shows a fully gapped superconducting state with two energy gaps, one roughly twice as large as the other. The in-plane critical current density is as high as several MA/cm$^{2}$ at low temperatures, zero field, which may be promising for applications. [Preview Abstract] |
Wednesday, March 18, 2009 3:30PM - 3:42PM |
T33.00006: Low temperature penetration depth of Fe-based superconductors James Day, Brad Ramshaw, Jake Bobowski, Pinder Dosanjh, Doug Bonn, Walter Hardy The recent discovery of superconductivity in layered iron-based pnictide compounds has generated levels of excitement comparable to the early days of the copper-based oxides. While connections may be drawn between the cuprates and the pnictides, there exist important differences between the two materials. We report high-precision microwave measurements of the penetration depth in single crystals of the hole-doped iron-based superconductor Ba$_{1-x}$K$_{x}$Fe$_{2}$As$_{2}$. Using cavity perturbation techniques in conjunction with ac susceptometry, we find that the low temperature penetration depth does not fit well to a simple s-wave model. Furthermore, our initial results suggest that the temperature dependence of the superfluid density is consistent with a gap with nodes plus the presence of scattering. [Preview Abstract] |
Wednesday, March 18, 2009 3:42PM - 3:54PM |
T33.00007: Penetration Depth and Quasiparticle Conductivity in Iron-based High-Temperature Superconductors Ken-ichiro Hashimoto, Takasada Shibauchi, Tomonari Kato, Kosuke Ikada, Ryuji Okazaki, Hiroaki Shishido, C.J. van der Beek, M. Konczykowski, Hijiri Kito, Akira Iyo, Hiroshi Eisaki, Motoyuki Ishikado, Shin-ichi Shamoto, H. Takeya, K. Hirata, Shigeru Kasahara, Takahito Terashima, Yuji Matsuda We measure the in-plane penetration depth and the quasiparticle conductivity in the newly discovered Fe-based high-temperature superconductors PrFeAsO$_{1-y}$[1] and Ba$_{1-x}$K$_{x}$Fe$_{2}$As$_{2}$[2], using a sensitive superconducting resonator. We show that the penetration depth exhibits a very flat behavior at low temperatures in both compounds, which indicates that the superconducting gap opens up all over the Fermi surface. The temperature dependence of the superfluid density is well fitted with the two gap model, suggesting the multi-gap nature of superconductivity in this system. Moreover, the observed large enhancement of the quasiparticle conductivity suggests a suppression of the quasiparticle scattering, reminiscent of the superconductors in strongly correlated electronic systems.[1] K. Hashimoto et al., arXiv:0806.3149., [2] K. Hashimoto et al., arXiv:0810.3506. [Preview Abstract] |
Wednesday, March 18, 2009 3:54PM - 4:06PM |
T33.00008: London Penetration Depth in Single Crystals of Nd(Fe,Co)As(O,F) Superconductors. H. Kim, C. Martin, R.T. Gordon, M.A. Tanatar, M.E. Tillman, S. Kim, S.L. Bud'ko, P.C. Canfield, R. Prozorov Iron arsenide superconductors, with the general formula RFeAsO$_ {1-x}$F$_x$ (R=Nd,Sm,Pr,Gd), exhibit the highest transition temperatures among the compounds of the pnictide family, in excess of 50 K. London penetration depth studies performed on single crystals, grown under high pressure with nominal fluorine content x=0.1, have revealed a nodeless superconducting state with a modestly anisotropic gap\footnote {C. Martin \textit{et al.}, arXiv:0807.0876}. Since doping is one of the most efficient ways to perturb the superconductivity, we explore here the evolution of the London penetration depth with doping, achieved by F substitution on O sites and by Co substitution on Fe sites. Notable differences in the superconducting transition temperatures for the two sets of crystals suggests the importance of impurity scattering for superconductivity in these compounds. Systematic variation of the London penetration depth with doping will be discussed. [Preview Abstract] |
Wednesday, March 18, 2009 4:06PM - 4:18PM |
T33.00009: Co-concentration dependence of the London penetration depth in single crystals of Ba(Fe$_{1-x}$Co$_x$)$_2$As$_2$ R.T. Gordon, C. Martin, H. Kim, M.A. Tanatar, N. Ni, S.L. Bud'ko, P.C. Canfield, J. Schmalian, R. Prozorov The in-plane London penetration depth, $\lambda_{ab}$(T), has been measured in single crystals of Ba(Fe$_{1-x}$Co$_x$)$_2$As$_2$ for several values of $x$ using a tunnel diode resonator (TDR) technique. The low temperature behavior of the variation of the penetration depth follows a clear power law behavior, $\Delta\lambda$(T) $\propto$ T$^n$, for all measured Co concentrations with n varying between 2 on the underdoped side to about 2.5 at the optimal doping. This non-exponential behavior in $\Delta\lambda$ indicates the existence of a significant number of superconducting quasiparticles, which is characteristic of a superconducting gap function possessing some type of nodal structure. The overal change in the superfluid density as a function of doping will be discussed. R. T. Gordon $\textit{et al.}$ arXiv:0810.2295 (2008). [Preview Abstract] |
Wednesday, March 18, 2009 4:18PM - 4:30PM |
T33.00010: Magnetic force microscopy study of a Ba$_{0.55}$K$_{0.45}$Fe$_{2}$As$_{2}$ single crystal: Local penetration depth and flux pinning Lan Luan, Ophir M. Auslaender, Kathryn A. Moler, James G. Analytis, Jiun-Haw Chu, Ian R. Fisher We use a magnetic force microscope (MFM) to study both the penetration depth ($\lambda _{ab} )$ and flux pinning in the iron-pnictide superconductor Ba$_{0.55}$K$_{0.45}$Fe$_{2}$As$_{2}$ by imaging and manipulating vortices. We observe the same regular signature from all vortices in a 50$\mu m$ x 50$\mu m$ field of view, implying little or no inhomogeneity of the superfluid density down to the sub-micron scale. Quantitative analysis of images of isolated vortices and of the Meissner repulsion of the magnetic tip from the sample gives the temperature-induced change of $\lambda _{ab} $ and an estimate for its absolute value. We detect no long-range order in the vortex positions, suggesting the absence of correlated pinning in the material. We measure the force required to depin individual vortices and the force required to drag them across the entire sample. This allows us to characterize the pinning potential in the material and its distribution as well as to set bounds on the local critical current. [Preview Abstract] |
Wednesday, March 18, 2009 4:30PM - 4:42PM |
T33.00011: Evidence for Two Energy Gaps in Superconducting Ba$_{0.6}$K$_{0.4}$Fe$_2$As$_2$ Single Crystals and Breakdown of the Uemura Plot Cong Ren, Zhao-Sheng Wang, Hui-Qian Luo, Huan Yang, Lei Shan, Hai-Hu Wen We report a detailed investigation on the lower critical field $H_{c1}$ of the superconducting Ba$_{0.6}$K$_{0.4}$Fe$_2$As$_2$ (FeAs-122) single crystals. A pronounced kink is observed on the $H_{c1}(T)$ curve, which is attributed to the existence of two superconducting gaps. By fitting the data $H_{c1}(T)$ to the two-gap BCS model in full temperature region, a small gap of $\Delta_a(0)=2.0\pm 0.3$ meV and a large gap of $\Delta_b(0) =8.9 \pm 0.4$ meV are obtained. The in-plane penetration depth $\lambda_{ab}(0)$ is estimated to be 105 nm corresponding to a rather large superfluid density, which points to the breakdown of the Uemura plot in FeAs-122 superconductors. [Preview Abstract] |
Wednesday, March 18, 2009 4:42PM - 4:54PM |
T33.00012: Anisotropic London Penetration depth in NdFeAsO$_{0.9}$F$_{0.1}$ and Ba$_{0.55}$K$_{0.45}$Fe$_{2}$As$_{2}$. Catalin Martin, R.T. Gordon, M.A. Tanatar, H. Kim, M.E. Tillman, N. Ni, P.C. Canfield, V.G. Kogan, R. Prozorov, H. Lou, Z. Wang, H.H. Wen In and out of plane London penetration depth was measured in single crystals of NdFeAsO$_{0.9}$F$_{0.1}$ (Nd-1111) and Ba$_{0.55}$K$_{0.45}$Fe$_{2}$As$_{2}$ (BaK-122) as a function of temperature by using an rf-resonator technique. In Nd-1111, penetration depth shows exponential behavior at low temperature, implying a fully-gapped Fermi surface. Superfluid density is best described in the full temperature range by a slightly anisotropic order parameter. In contrast, penetration depth of BaK-122 shows power-law temperature dependence ($\Delta\lambda(T)\propto T^{n}$, n$\approx$2) down to T$approx$0.02T$_{c}$ and anisotropy $\gamma_{\lambda}$ varies from 2 at T$_{c}$ to about 4 at 0.5 K. Possible symmetries of the gap consistent with such behavior and comparison with results from other techniques and with proposed theoretical models will be discussed. Also, the temperature dependence of the anisotropy $gamma_{\lambda}=\lambda_{c}/\lambda_{ab}$, for both compounds, will be compared to that of $\gamma_{H_{c2}}=H_{c2}^{c}/H_{c2}^ {ab}$, in connection with possible multiband superconductivity. [Preview Abstract] |
Wednesday, March 18, 2009 4:54PM - 5:06PM |
T33.00013: Muon Spin Relaxation Study of Ba(Fe$_{0.93}$Co$_{0.07})_{2}$As$_{2}$ Graeme Luke, A. Aczel, J.P. Carlo, T. Goko, T.J. Williams, N. Ni, S.L. Bud'ko, P.C. Canfield, Y.J. Uemura We have performed muon spin relaxation ($\mu $SR) measurements of superconducting Ba(Fe$_{0.93}$Co$_{0.07})_{2}$As$_{2}$. Zero field measurements show that static magnetism is absent for this Co concentration. In the mixed state, we observe a well-formed vortex lattice which results in an anisotropic $\mu $SR lineshape. We have fit the spectra to a microscopic model for the vortex state to obtain the magnetic field penetration depth. The penetration depth is about 2000{\AA} in low (200G) fields and increases with applied field. The temperature dependence of the superfluid density is well described by a power law; behaviour which is consistent with gap nodes, substantial gap anisotropy or multi-band superconductivity. Research at McMaster University is supported by NSERC and CIFAR. Work at the Ames Laboratory was supported by the Department of Energy, Basic Energy Sciences under Contract No. DE-AC02-07CH11358. [Preview Abstract] |
Wednesday, March 18, 2009 5:06PM - 5:18PM |
T33.00014: Anisotropic superconducting properties of aligned Sm$_{0.95}$La$_{0.05}$FeAsO$_{0.85}$F$_{0.15}$ microcrystalline powder H.C. Ku, B.C. Chang, C.H. Hsu, Y.Y. Hsu, Z. Wei, K.Q. Ruan, X.G. Li The Sm$_{0.95}$La$_{0.05}$FeAsO$_{0.85}$F$_{0.15}$ compound is a quasi-2D layered superconductor with a superconducting transition temperature T$_c$ = 52 K. Due to the Fe spin-orbital related anisotropic exchange coupling (antiferromagnetic or ferromagnetic fluctuation), the tetragonal microcrystalline powder can be aligned at room temperature using the field- rotation method where the tetragonal $\it{ab}$-plane is parallel to the aligned magnetic field B$_{a}$ and $\it{c}$- axis along the rotation axis. Anisotropic superconducting properties with anisotropic diamagnetic ratio $\chi_{c}$/$\chi_ {ab}\sim$ 2.4 + 0.6 was observed from low field susceptibility $\chi$(T) and magnetization M(B$_{a}$). The anisotropic low- field phase diagram with the variation of lower critical field gives a zero-temperature penetration depth $\lambda_{c}$(0) = 280 nm and $\lambda_{ab}$(0) = 120 nm. The magnetic fluctuation used for powder alignment at 300 K may be related with the pairing mechanism of superconductivity at lower temperature. [Preview Abstract] |
Wednesday, March 18, 2009 5:18PM - 5:30PM |
T33.00015: Diminishing anisotropy of PrFeAsO$_{1-y}$ single crystal as temperature goes zero Daichi Kubota, Takekazu Ishida, Motoyuki Ishikado, Shin-ichi Shamoto, Hiroshi Eisaki, Hijiri Kito, Akira Iyo The magnetic torque of a high-quality PrFeAsO$_{1-y}$ single crystal has been investigated at temperatures from 10 K to 45 K in magnetic fields from 5 kG and 50 kG. The torque curves of the PrFeAsO$_{1-y}$ single crystal have been measured systematically. The superconducting anisotropy of PrFeAsO$_{1-y}$ is determined by analyzing the torque curves. The temperature and field dependence of $\gamma$ reflect that the Fe-oxypnictide superconductivity has the rather three- dimensional isotropic nature of Fe-As layer network. We consider that this is good evidence for the multi-band superconductivity in Fe-oxypnictide. We find that the superconducting anisotropy $\gamma$ in PrFeAsO$_{1-y}$ can be approximated by $\gamma = 1.08 + 0.0068 T$ in all fields by employing the Kogan model. We find it surprising to have such a low anisotropy parameter $\gamma$ even for a layered superconductor. Such a small anisotropic parameter in Fe-pnictides is preferential to ensure the intergrain connectivity and the resulting high critical current density $J_c$. [Preview Abstract] |
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