Bulletin of the American Physical Society
APS March Meeting 2011
Volume 56, Number 1
Monday–Friday, March 21–25, 2011; Dallas, Texas
Session J26: Focus Session: Iron Based Superconductors -- Vortices & High Fields |
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Sponsoring Units: DMP DCOMP Chair: Stephen Wilson, Boston College Room: D162/164 |
Tuesday, March 22, 2011 11:15AM - 11:27AM |
J26.00001: Anomalous High-Field Meissner Effect in Pnictide Superconductors Ruslan Prozorov, Makariy A. Tanatar, Sergey L. Bud'ko, Paul C. Canfield, Bing Shen, Peng Cheng, Hai-Hu Wen The Meissner effect has been studied in Ba(Fe$_{0.926}$Co$_ {0.074}$)$_2$As$_2$ and Ba$_{0.6}$K$_{0.4}$Fe$_2$As$_2$ single crystals and compared to well known, type-II superconductors LuNi$_2$B$_2$C and V$_3$Si. Whereas flux penetration is mostly determined by the bulk pinning (and, perhaps, surface barrier) resulting in a large negative magnetization, the flux expulsion upon cooling in a magnetic field is very small, which could also be due to pinning and/or surface barrier effects. However, in stark contrast with the expected behavior, the amount of the expelled flux increases almost linearly with the applied magnetic field, at least up to our maximum field of 5.5 T, which far exceeds the upper limit for the surface barrier. One interpretation of the observed behavior is that there is a field-driven suppression of magnetic pair-breaking. [Preview Abstract] |
Tuesday, March 22, 2011 11:27AM - 11:39AM |
J26.00002: Flux Creep associated with Strong Pinning in Isovalently Doped Iron-Based Superconductors M. Konczykowski, Sultan Demirdis, C.J. van der Beek, R. Prozorov, M. Tanatar, P.C. Canfield, S. Kasahara, T. Shibauchi, Yuji Matsuda Strong pinning in Iron-Based Superconductors leads to the ubiquitous central peak of the irreversible magnetization. Notably, isovalently doped materials such as BaFe$_2$(As$_{1-x}$P$_x$)$_2$ and Ba(Fe$_{1-x}$Ru$_x$)$_2$As$_2$ offer a paradigm for the study of strong pinning because it is the only contribution to the critical current density $j_c$. We have studied flux creep rates as function of field and temperature in the low- and high field regimes in which $j_c$ is limited by the line tension of a single pinned vortex, and by vortex interactions, respectively. For $T < \frac{1}{2}T_c$, screening currents $j$ are of the order of $10^9$ Am$^{-2}$, in spite of a creep rate $d \ln j / d \ln t \sim 0.02$. Creep is initially Anderson Kim-like, \em i.e. \rm, creep barriers $U$ depend on $j$ as $U \propto (1 - j / j_c )$ over an order of magnitude in $j$, before crossing over to a nonlinear behavior. $j_c$ is easily extracted from the high-current, short-time part of the magnetic relaxation. The results cast doubt on the range of applicability of the often-used ''interpolation formula'' $j \propto [1 + (k_BT/U_c) \ln( t + t_0 / \tau )]^{-1/\mu}$ for weak collective pinning. [Preview Abstract] |
Tuesday, March 22, 2011 11:39AM - 11:51AM |
J26.00003: Strong Pinning and Nonlinear Creep Barriers in Iron-Pnictide Superconductors S. Demirdis, M. Konczykowski, C.J. van der Beek, R. Prozorov, S. Kasahara, T. Shibauchi, Yuji Matsuda The irreversible magnetization of Iron-Based Superconductors is characterized by the presence of an ubiquitous peak of the critical current density $j_c$, centered around zero field. Closer examination shows that the field-dependence of $j_c$ corresponds, in all cases, to a low-field plateau, followed by a power-law decrease, $j_c \propto B^{-\alpha}$ (with $\alpha \sim \frac{5}{8}$) above a cross-over field $B^{*}$. This strongly suggests that vortex pinning at low magnetic field is due to strong pinning by nanometer-scale defects. In isovalently doped materials such as BaFe$_2$(As$_{1-x}$P$_{x}$)$_2$, strong pinning is the only contribution to the critical current. The analysis of $j_c$ allows one to extract, without a priori assumptions, the elementary pinning force and the defect density. In BaFe$_2$(As$_{1-x}$P$_{x}$)$_2$, the latter quantity is in qualitative agreement with that obtained by H. Shishido \em et al.\rm [Phys. Rev. Lett. {\bf 104}, 057008 (2010)]. The temperature dependence of the screening current above $B^{*}$ is strongly affected by flux creep. The current decays as $j \sim [ (k_BT/U) \ln( t + t_0 / \tau )]^{-1/\mu}$, with $\mu \sim 1.6$, showing that nonlinear creep barriers are not an exclusive feature of weak collective pinning. [Preview Abstract] |
Tuesday, March 22, 2011 11:51AM - 12:03PM |
J26.00004: Model of vortex states in hole-doped iron-pnictide superconductors Yi Gao, Huai-Xiang Huang, Chun Chen, C.S. Ting, Wu-Pei Su Based on a phenomenological model with competing spin-density- wave (SDW) and extended $s-$wave superconductivity, the vortex states in Ba$_{1-x}$K$_{x}$Fe$_{2}$As$_{2}$ are investigated by solving Bogoliubov-de Gennes equations. Our result for the optimally doped compound without induced SDW is in qualitative agreement with recent scanning tunneling microscopy experiment. We also propose that the main effect of the SDW on the vortex states is to reduce the intensity of the in-gap peak in the local density of states and transfer the spectral weight to form additional peaks outside the gap. [Preview Abstract] |
Tuesday, March 22, 2011 12:03PM - 12:15PM |
J26.00005: Angular dependence of the critical current and vortex phase diagram in Co-doped BaFe2As2 films with strong pinning B. Maiorov, T. Katase, H. Hiramatsu, H. Hosono, L. Civale Studying the angular dependence of the critical current density (Jc) as a function of temperature in superconductors with complex pinning landscapes is very important both from the technical and fundamental points of view. The low anisotropy found in the Ba122 family tightened with strong naturally grown pinning make Ba122 films very attractive. It is also interesting to understand the different factors affecting vortex pinning in different regimes of temperature (T) and magnetic field (H). We present results on iron-arsenide superconducting films with varied pinning landscapes composed of columnar defects and nanoparticles. We analyze different field and angular regimes. We find that the naturally grown correlated defects found in Co-doped BaFe2As2 films are effective up to very high fields (m0H=15T) affecting a very wide region o the angular phase diagram. We also investigate the effects of film's thickness and the addition of defects produced by irradiation [Preview Abstract] |
Tuesday, March 22, 2011 12:15PM - 12:27PM |
J26.00006: Comparative study of flux pinning characteristics of Ba(Fe$_{1-x}$Co$_x$)$_2$As$_2$ and BaFe$_2$(As$_{1-x}$P$_x)_2$ single crystals Noriko Chikumoto, Wataru Hirata, Shigeki Miyasaka, Setsuko Tajima, Keiichi Tanabe We have studied the magnetization behavior of iron-pnictide superconductor, Ba(Fe$_{1-x}$Co$_x$)$_2$As$_2$ (Co-doped Fe122) with various Co doping and BaFe$_2$(As$_{0.65}$P$_{0.35})_2$ (P-doped Fe122) single crystals. All of the Co-doped Fe122 crystals showed a very pronounced ``peak effect'' in all the temperature range, irrespective of doping state. It is important to mention that a similar peak effect was previously reported for REBa$_2$Cu$_3$O$_y$. In order to get further insight into the pinning mechanism of the present system, we analyzed the pinning force density $F_p=J_cB$. A good scaling of the $F_p$ versus the reduced field, $b=B/B_{irr}$, was established for all the Co-doped Fe122 crystals and the scaling curves were well fitted with the function given by $F_p/F_{p,max} =Ab^p(1-b)^q$, where $A$ is a numerical parameter, $p$ and $q$ are describing the actual pinning mechanism. It was found that $p$ value monotonically increases with x, while $q$ value decreases with x. On the other hand, P-doped Fe122 did not show ``peak effect''. We will discuss about the possible pinning mechanism causing the peak effect. [Preview Abstract] |
Tuesday, March 22, 2011 12:27PM - 12:39PM |
J26.00007: Dynamics of Vortices in Heavy-ion Irradiated Co-doped BaFe$_2$As$_2$ Tsuyoshi Tamegai, Toshihiro Taen, Hidenori Yagyuda, Tomotaka Taniguchi, Shyam Mohan, Yasuyuki Nakajima, Satoru Okayasu, Masato Sasase, Hisashi Kitamura, Takeshi Murakami, Tadashi Kambara, Yasuyuki Kanai Effects of heavy-ion irradiation on the critical current density, $J_c$, and vortex dynamics are investigated in Ba(Fe$_{1-x}$Co$_x$)$_2$As$_2$ irradiated by heavy- ions of various kinds. Irradiations induce a large enhancement of $J_c$ in the case of 200 MeV Au creating nearly continuous columnar tracks. On the other hand, in the case of 800 MeV Xe irradiation, despite the enhancement of $J_c$, clear columnar defects are not observed. In the case of 2.6 GeV U irradiation, new types of structure appears in the $M$-$H$ loop at high matching field. We also discuss the behavior of vortex dynamics in the irradiated Ba(Fe$_{1-x}$Co$_x$)$_2$As$_2$. [Preview Abstract] |
Tuesday, March 22, 2011 12:39PM - 12:51PM |
J26.00008: High critical current density in BaAs$_{2}$(Fe,Co)$_{2}$ thin films up to 35 T J. Jiang, C. Tarantini, J.D. Weiss, J. Jaroszynsky, E.E. Hellstrom, D.C. Larbalestier, S. Lee, C.W. Bark, H.W. Jang, C.M. Folkman, S.H. Baek, J.W. Park, C.B. Eom, Y. Zhang, C.T. Nelson, X.Q. Pan In the Co-doped BaFe$_{2}$As$_{2}$ thin films we intensively investigated field and angular dependences of J$_{c}$ down 4.2 K in high field. We found a strong correlated c-axis pinning and Jc for field along the c-axis exceeds J$_{c}$ for H//ab plane up to $\sim $20T, inverting the expectation of the Hc2 anisotropy. As a consequence the angular dependence is very weak and J$_{c}$ is still over 10$^{5}$ A/cm$^{2}$ at 20T. Moreover the maximum pinning force F$_{p}$(4.2K) reaches 35-40 GN/m$^{3}$ at 15-20T depending on the field configuration, indicative of strong high-field vortex pinning. High resolution transmission electron microscopy reveals that the strong vortex pinning is due to a high density of non-superconducting Ba-Fe-O nanocolumnar defects whose diameter is $\sim $2$\xi $, perfect conditions for a strong pinning. [Preview Abstract] |
Tuesday, March 22, 2011 12:51PM - 1:03PM |
J26.00009: Critical current in disordered iron-pnictide superconducting wires Dushko Kuzmanovski, Maxim Vavilov, Anton Vorontsov We evaluate the critical current in narrow wires of disordered iron-based pnictide superconductors. We present the Eilenberger and Usadel equations for a two-band model of a pnictide superconductor which take into account both inter and intra-band scattering events. The intra-band scattering events are responsible for the momentum relaxation of charged excitations, but do not suppress the homogeneous superconducting state. On the contrary, the inter-band scattering acts as the depairing mechanism. We apply the Usadel equation to analyze the dependence of the critical current on the strength of disorder in narrow iron-pnictide wires. [Preview Abstract] |
Tuesday, March 22, 2011 1:03PM - 1:15PM |
J26.00010: Upper critical field measured up to 85T in different iron-based compounds C. Tarantini, J. Jaroszynsky, A. Gurevich, D.C. Larbalestier, F. Balakirev, H.H. Wen, E. Bellingeri, I. Pallecchi, C. Ferdeghini We report magneto-transport measurements of H$_{c2}$(T) at very high dc and pulsed magnetic field up to 85T on different families of iron-based single crystals and thin film superconductors with different doping levels and stress. Some of these materials show high H$_{c2}$ extrapolating to $\sim $100T and extremely high slopes up to 20 T/K for H//c and over 200 T/K for H//ab, indicating significant Pauli pair breaking and a possibility of the Fulde-Ferrel-Larkin-Ovchinnikov state. The superconducting transitions remain sharp also at the highest field showing an irreversibility field close to H$_{c2}$. [Preview Abstract] |
Tuesday, March 22, 2011 1:15PM - 1:27PM |
J26.00011: Flux Pinning and Quasi-particle Scattering in Charge- Doped Iron-Based Superconductors Kees van der Beek, S. Demirdis, M. Konczykowski, S. Kasahara, T. Terashima, R. Okazaki, T. Shibauchi, Yuji Matsuda Whereas isovalently doped iron-based superconductors, such as BaFe$_2$(As$_{1-x}$P$_x$)$_2$ and Ba(Fe$_{1-x}$Ru$_x$)$_2$As$_2$ show only strong, ''individual-defect'' vortex pinning due to nanometer-sized defects, charge-doped iron-pnictide superconductors show a low-field, field-independent contribution to the critical current density $j_c$ that is well described by the collective pinning theory. Quantitative analysis of the magnitude, temperature, and field-dependence of $j_c$ in the PrFeAsO$_{1-y}$ compound shows that the behavior of $j_c$ can be fully explained, if one assumes the oxygen vacancies in this material to be responsible for quasi-particle scattering in the vortex cores. Analysis of $j_c$ of this and other charge-doped compounds such as NdFeAs(O,F), (Ba,K)Fe$_2$As$_2$, and Ba(Fe,Co)$_2$As$_2$ yields estimates for the transport scattering cross-section of the dopant impurities in all these materials. We find scattering to be in the Born limit, with a scattering phase angle $\delta_0$ such that $\sin \delta_0 \sim 0.2 - 0.3$. [Preview Abstract] |
Tuesday, March 22, 2011 1:27PM - 1:39PM |
J26.00012: Nearly Isotropic Critical Currents in SmFeAs(O,F) in High Magnetic Fields Bertram Batlogg, Philip Moll, Roman Puzniak, Fedor Balakirev, Luis Balicas, Janusz Karpinski, Nikolai Zhigadlo The layered structure of SmFeAs(O,F) naturally raises questions about the electronic anisotropy of this 55K superconductor. To investigate the transport anisotropy, we performed electric 4-probe measurements on Focused Ion Beam (FIB) cut single crystals with sub-$\mu$m$^2$ cross-section, with current along and perpendicular to the FeAs layers. The normal state resistivity is indeed anisotropic ($\rho_c$/$\rho_{ab} \approx$ 2 at RT, $\approx$ 10 at 50K) and consistent with the calculated Fermi velocity anisotropy. In contrast, the dissipation in high fields below $T_c$ is more isotropic. The critical current densities at 4K are nearly isotropic and very high ($>$ 2 10$^6$ A/cm$^2$), and up to 14 T, they are almost independent of the field orientation and strength. These values agree well with magnetization measurements. Additional measurements to much higher fields are presented. [Preview Abstract] |
Tuesday, March 22, 2011 1:39PM - 1:51PM |
J26.00013: Low-energy muons and polarized neutrons for studying superconductivity Vladimir Kozhevnikov, Kristiaan Temst, Andreas Suter, Timothy Charlton, Helmut Fritzsche, Thomas Prokscha, Elvezio Morenzoni, Margriet Van Bael, Christian Van Haesendonck, Joseph Indekeu The penetration of the magnetic field into superconductors (SC) in the Meissner state is one of the major resources for studying SC. The merit of this resource will be greatly enhanced by quantitative measurements of the field distribution over the penetration layer, which, in particular, may lead to new insights for unconventional SC. We will report on measurements of the magnetic field profile in In and Sn using low-energy muons (LE-mSR) and polarized neutrons (PNR). The results solidly demonstrate nonexponential decay of the magnetic induction in accord with the Pippard and BCS theories. However, in contrast to In, results on which were reported last year, the data for Sn show significant differences in the values of the London penetration depth and the Pippard coherent length with those known from the literature. [Preview Abstract] |
Tuesday, March 22, 2011 1:51PM - 2:03PM |
J26.00014: Incoherent Interplane Response of FeTe$_{0.55}$Se$_{0.45}$ S.J. Moon, C.C. Homes, A. Akrap, Z.J. Xu, J.S. Wen, Z.W. Lin, Q. Li, G.D. Gu, D.N. Basov We investigated the interplane $c$ axis electronic response of iron-chalcogenide superconductor FeTe$_{0.55}$Se$_{0.45}$ using infrared spectroscopy. We found that the normal-state $c$ axis electronic response of FeTe$_{0.55}$Se$_{0.45}$ is incoherent. The $c$ axis optical conductivity does not display well-defined Drude response and it becomes further suppressed with decreasing temperature. This normal-state $c$ axis optical response is remarkably similar to that of the mildly underdoped cuprates but is in sharp contrast to the coherent $c$ axis response of Ba$_{1-x}$K$_{x}$Fe$_{2}$As$_{2}$. From the analyses on the electronic anisotropy of various layered superconductors, we found a close correlation between the degree of the coherence in the $c$ axis transport and the strength of the dissipation in the \textit{ab} plane response. [Preview Abstract] |
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