Bulletin of the American Physical Society
2009 APS March Meeting
Volume 54, Number 1
Monday–Friday, March 16–20, 2009; Pittsburgh, Pennsylvania
Session H33: Focus Session: Iron Pnictides and Other Novel Superconductors V: Tunneling, PCAR, Josephson |
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Sponsoring Units: DCMP Chair: Laura Greene, University of Illinois Room: 403 |
Tuesday, March 17, 2009 8:00AM - 8:12AM |
H33.00001: Observation of the Josephson effect in Pb/Ba$_{1-x}$K$_{x}$Fe$_{2}$As$_{2}$ single crystal junctions Xiaohang Zhang, Richard Greene, Ichiro Takeuchi, Yoon Seok Oh, Yong Liu, Liqin Yan, Kee Hoon Kim We have fabricated Josephson junctions using single crystals of Ba$_{1-x}$K$_{x}$Fe$_{2}$As$_{2}$ and Pb (or PbIn) as the counter electrode in two geometries. The $c$-plane single crystals of Ba$_{1-x}$K$_{x}$Fe$_{2}$As$_{2}$ were synthesized by the Sn-flux method with the nominal composition of $x$ = 0.4. In one junction geometry, Ag (30 nm) and PbIn (200 nm) were evaporated on the surface of the crystals. In the other geometry, a Pb point contact was used. Both geometry junctions show resistively shunted junction $I-V$ curves below the $T_{C}$ of the counter electrode. Microwave induced steps were observed in the $I-V$ curves, and the critical currents are completely suppressible with applied magnetic field in a manner consistent with a small junction limit. $I_{C}R_{N}$ products of up to 0.3 mV have been observed in these junctions at 4.2 K. The observation of Josephson coupling along the $c$-axis between an iron pnictide superconductor and a conventional superconductor suggests the existence of a non-d-wave superconducting order parameter in iron pnictide superconductors. [Preview Abstract] |
Tuesday, March 17, 2009 8:12AM - 8:24AM |
H33.00002: ABSTRACT WITHDRAWN |
Tuesday, March 17, 2009 8:24AM - 8:36AM |
H33.00003: Multigap superconductivity in pnictides Peter Samuely, Pavol Szabo, Zuzana Pribulova, Gabriel Pristas, Paul Canfield, Sergey Bud'ko Point-contact Andreev reflection studies of the superconducting energy gap on the NdFeAsOF, (Ba,K)Fe$_{2}$As$_{2}$ and Ba(Fe,Co)$_{2}$As$_{2}$ crystals will be presented. The analysis of the data points to a two-gap superconductivity in these materials. Possible existence of the pseudogap in the non-superconducting density of states related to the static or dynamic magnetic order will be discussed as well. [Preview Abstract] |
Tuesday, March 17, 2009 8:36AM - 9:12AM |
H33.00004: Coexistence of two order parameters and a pseudogap in the iron-based superconductors Invited Speaker: The number, the symmetry and the amplitude of the order
parameters (OPs) in the Fe-As superconductors are still open
issues, as well as the origin of the electron pairing. To
address these issues, we performed point-contact Andreev-
reflection measurements in SmFeAsO$_{0.8}$F$_{0.2}$ ($T_c^{on}=53
$ K) and LaFeAsO$_{0.9}$F$_{0.1}$ ($T_c^{on}=27$ K)
polycrystals. In both cases, the low-temperature conductance
curves clearly indicate the presence of two OPs in the
superconducting state. No zero-bias peaks were observed, which
-- considering the non-directional current injection -- clearly
rules out the $d$-wave symmetry. If a superconducting character
is supposed for both the OPs, their amplitudes can be extracted
from a generalized two-band BTK fit (with two s-wave gaps, as in
MgB$_2$) of the normalized conductance curves. The fit is indeed
very good and gives OP amplitudes, $\Delta_1$ and $\Delta_2$,
that lie slightly below and well above the BCS value,
respectively. In Sm-1111, their low-temperature values are
$\Delta_1(0)=6.15\pm0.50 $ meV and $\Delta_2(0)=18\pm3$
meV, which give gap ratios ($2 \Delta/k_BT_c$) of about 2.7 and
8.0. Both $\Delta_1$ and $\Delta_2$ show a BCS-like temperature
dependence and close at the bulk $T_c$. In La-1111 we obtained
point contacts with different local $T_c$ (from 27.3 to 31.0 K)
in crystallites with slightly different doping. Here $\Delta_1$
shows a non-BCS temperature dependence with a high-
temperature ``tail,'' while $\Delta_2$ seems to close at $T |
Tuesday, March 17, 2009 9:12AM - 9:24AM |
H33.00005: Point-contact Andreev reflection tunneling spectroscopic (PCARTS) study on Ba$_{1-x}$K$_{x}$Fe$_{2}$As$_{2}$ and Ba(Fe$_{1-x}$Co$_{x})_{2}$As$_{2}$ Xin Lu, W. K. Park, L. H. Greene, H. Q. Yuan, G. F. Chen, G. L. Luo, N. L. Wang, A. Sefat, M. A. McGuire, R. Jin, B. C. Sales, D. Mandrus PCARTS is applied to investigate the gap structure in the newly-discovered iron pnictide superconductors Ba$_{1-x}$K$_{x}$Fe$_{2}$As$_{2}$ and Ba(Fe$_{1-x}$Co$_{x})_{2}$As$_{2}$. Double peaks due to Andreev reflection with strongly-sloping background are frequently observed in the conductance curves G (V) for the Au-Ba$_{1-x}$K$_{x}$Fe$_{2}$As$_{2}$ point-contact junctions on the freshly-cleaved surface along the c-axis. . If normalized by the background baseline and analyzed by Blonder-Tinkham-Klapwijk model, the data show a gap size $\sim $ 4 meV with 2$\Delta $/k $_{B}$T$_{C }\sim $ 2.6. However, it is observed that, for the Ba(Fe$_{1-x}$Co$_{x})_{2}$As$_{2}$, G(V) curves evolve from V-shape to zero-bias conductance peak with increasing tip pressure, where the tip is likely to penetrate through surface layer. The existence of surface oxide layer is confirmed by comparative XPS characterization on freshly-cleaved and uncleaved surfaces. [Preview Abstract] |
Tuesday, March 17, 2009 9:24AM - 9:36AM |
H33.00006: ABSTRACT WITHDRAWN |
Tuesday, March 17, 2009 9:36AM - 9:48AM |
H33.00007: Probing the Spin-Density-Wave Transition in SmOFeAs using Point-Contact Spectroscopy T. Y. Chen, S.X. Huang, R. H. Liu, X. H. Chen, C. L. Chien In spring 2008, a new family of superconductors with the general composition of SmFeAs(O$_{1-x}$F$_{x})$ has been discovered. The metallic parent compound SmOFeAs has a spin-density-wave (SDW) transition near 150 K with hysteretic temperature dependence in magnetic structure, crystalline structure, and resistance. Doping the parent compound with fluorine results in the suppression of the SDW transition and the emergence of superconductivity with transition temperatures up to 55 K. Some theories suggest that the SDW transition may be intimately related to the superconductivity. In this work, we use point-contact spectroscopy to investigate the SDW transition in the parent SmOFeAs compound. Instead of varying temperature, under a high bias voltage a small region underneath a point contact can be heated up through the SDW transition, resulting in differential conductance peaks. Similar to the temperature dependence of the resistance and structure, the SDW transition is hysteretic with bias voltage at the transition. We further show that this feature in differential conductance, which may be easily mistaken as a pseudo gap of a superconductor, is actually a characteristic of the ballistic heating effect. [Preview Abstract] |
Tuesday, March 17, 2009 9:48AM - 10:00AM |
H33.00008: Theory of Tunneling Spectroscopy in LaFeAsO$_{1-x}$F$_{x}$ Seiichiro Onari, Yukio Tanaka Recent discovery of superconductivity in the iron based LaFeAsO$_{1-x}$F$_{x}$ with Tc = 26K has stimulated great interests as a new class of non-cuprate compound. We calculate the surface density of state of LaFeAsO$_{1-x}$F$_{x}$ superconductor. The gap function is obtained microscopically by solving the Eliashberg equation in a 5-band Hubbard model with the random phase approximation (RPA). Green's function of the surface state is obtained by inserting infinite potential barriers using Matsumoto and Shiba method. Although the gap function has a sign change between Fermi surfaces, we cannot find mid gap Andreev bound state in [100] and [110]-oriented interface. [Preview Abstract] |
Tuesday, March 17, 2009 10:00AM - 10:12AM |
H33.00009: Novel properties in Josephson junctions involving $s_{x^2 y^2}$-pairing state in Iron-Pnictides Wei-Feng Tsai, Dao-Xin Yao, JiangPing Hu, B. Andrei Bernevig We present theoretical results of Andreev bound states in superconductor-normal metal (or insulator)-iron-pnictide junctions. Within the two-orbital exchange coupling model [1], the presence of non-trivial in-gap states, which uniquely appear in the $s$-wave $\cos k_x\cos k_y$ ($s_{x^2 y^2}$) pairing state, can be taken as a sharply distinct feature in contrast to other singlet pairing states. In addition, a proposed novel trilayer $\pi$-junction involving $s_{x^2 y^2}$ superconductivity is also discussed as a new possible signature of such unconventional pairing symmetry.\\[3pt] [1] K. Seo, B. A. Bernevig, and J. Hu, Phys. Rev. Lett. 101, 206404 (2008). [Preview Abstract] |
Tuesday, March 17, 2009 10:12AM - 10:24AM |
H33.00010: Possible phase-sensitive test of pairing symmetry in superconducting pnictides David Parker, Igor Mazin The discovery of the new class of pnictide superconductors has engendered a controversy about their pairing symmetry, with proposals ranging from an extended s-wave or ``s$_{\pm}$'' symmetry to nodal or nodeless d-wave symmetry to still more exotic order parameters such as p-wave. Although there is evidence that a fully gapped state may exist in the pnictides, the symmetry of this state remains indeterminate. Building on the earlier, similar work performed for the cuprates, we propose here a phase-sensitive Josephson interferometry experiment that may allow resolution of the issue, taking into consideration novel features such as the local orbital character (from DFT calculations), and employing a particular potential barrier to restrict the tunneling orientations to favorable directions. [Preview Abstract] |
Tuesday, March 17, 2009 10:24AM - 10:36AM |
H33.00011: Andreev Bound states as a phase sensitive probe of the pairing symmetry of the FeAs superconductors Pouyan Ghaemi, Fa Wang, Ashvin Vishwanath A leading contender for the pairing symmetry in the Fe-pnictide high temperature superconductors is extended s-wave $s_-$, a nodeless state in which the pairing changes sign between Fermi surfaces. Verifying such a pairing symmetry requires a special probe that is sensitive to both phase and magnitude of the order parameter. We show that the sign structure of $s_-$ pairing leads to Andreev bound states at the edge. In the clean limit they only occur when the edge is along the Fe-Fe bond, but not for a diagonal edge. In contrast to d-wave Andreev bound states, they are not at zero energy and, in general, do not produce a zero bias tunneling peak. Consequences for tunneling measurements are derived, within a simplified two band model and also for a more realistic five band model. [Preview Abstract] |
Tuesday, March 17, 2009 10:36AM - 10:48AM |
H33.00012: Examining SrFe$_{2}$As$_{2}$ with a Low Temperature Scanning Tunneling Microscope Francis Niestemski, J. Gillett , Suchitra Sebastian, Vidya Madhavan The new pnictide superconductors have generated huge excitement. These materials are the first to add some chemical diversity to the limited high-Tc list previously exclusive to cuprates. We examine the pnictide material parent compound SrFe$_{2}$As$_{2}$ with a low-temperature ultra-high vacuum scanning tunneling microscope (STM) at 4 K. We find multiple types of topography and spectroscopy with low energy features. We relate our data to results from ARPES and other experiments. [Preview Abstract] |
Tuesday, March 17, 2009 10:48AM - 11:00AM |
H33.00013: Electronic Structure on (001) Surface of Co-doped BaFe$_{2}$As$_{2}$ Studied with Scanning Tunneling Spectroscopy A. Li, D. R. Jayasundara, Y. Xuan, J. P. O'Neal, Y. Chen, W. Kim, C. S. Ting, S. H. Pan, R. Jin, E. W. Plummer, R. Jin, A. S. Sefat, M. A. McGuire, B. C. Sales, D. Mandrus Co-doping makes the pnictide compound BaFe$_{2}$As$_{2}$ superconducting. We cleave the single crystals of this compound in UHV and study their surfaces with a low temperature STM. In this talk, we present the scanning tunneling spectrums obtained on the (001) surface of the optimally doped BaFe$_{2}$As$_{2}$ single crystals (Tc = 23K) and compare these spectrums with the ones obtained on the surface of the parent compound. We have found that the major feature of the spectrums on the superconducting compounds is the opening of a superconducting gap of about 6 meV for the optimally doped one. We have also observed other detailed spectrum features. We will discuss the relation between spectrum features and the local environment and also present some theoretical fit to the superconducting energy gap spectrum. [Preview Abstract] |
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