Bulletin of the American Physical Society
2008 APS March Meeting
Volume 53, Number 2
Monday–Friday, March 10–14, 2008; New Orleans, Louisiana
Session U11: Focus Session: MgB2-like: Enhancement of Superconducting Properties |
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Sponsoring Units: DMP Chair: Alex Koshelev, Argonne National Laboratory Room: Morial Convention Center RO9 |
Thursday, March 13, 2008 8:00AM - 8:12AM |
U11.00001: Introduction of Carrier Scattering in MgB2, and its Effect on both Normal and Superconducting Properties, especially Hc2 N. Newman, Y. Shen, R. Singh, J. Rowell, D. Larbalestier, F. Hunte The low Hc2 values seen in pure and well ordered MgB2 can be raised dramatically, to 35 T or more, by introducing carrier scattering by native and impurity defects. We describe three means to do this.~ First, He ion irradiation is used to tune Tc from 39K to less than 10K, while at Tc near 33K, Hc2 reached a maximum value. Similar behavior has been reported for neutron damage and carbon doping. Second, we introduced oxygen in the films, either in-situ or ex-situ, and again, high Hc2 values were seen and in these films, very high Jc values as well. Finally, a novel route has been investigated. We deposited MgB2 films on room temperature substrates, then annealed at temperatures just sufficient to produce crystallinity, giving Tcs in the range of 10 to 30K. Such films exhibit large dHc2/dT values near Tc, sometimes larger than 2 T/K. This work is of practical importance and gives an improved understanding of how intraband and interband carrier scattering in the ``2-gap'' superconductor MgB2 determine its Hc2, resistivity and Tc. [Preview Abstract] |
Thursday, March 13, 2008 8:12AM - 8:24AM |
U11.00002: Upper critical field enhancements of TMB HPCVD magnesium diboride F. Hunte, J. Jaroszynski, A. Gurevich, D.C. Larbalestier, Y. Zhu, P.M. Voyles, R.H. Wilke, X.X. Xi The $H_{c2}$ of four well textured carbon-doped MgB$_2$ films grown by HPCVD from tri-methyl boron (TMB) at flow rates from 2.5 to 10 sccm was measured in fields up to 45T. $H_{c2}$ derived from low- current, four-point magnetoresistance clearly increases with increasing TMB flow rate. TMB appears to be a more uniform dopant than the (C$_6$H$_7$)$_2$Mg used earlier. These earlier films exhibited $H_{c2}^{\parallel}(0)$ up to 70 T but also were imperfectly connected ($\rho(50\mbox{ K})\sim200-800$~$\mu\Omega$cm) due to excess amorphous C-rich phases observed between the MgB$_2$ grains. In strong contrast, $\rho(50\mbox{ K})$ was only $10-20$~$\mu\Omega$cm for the TMB films. When first measured, the linearly extrapolated $H_{c2}^{\parallel}(0)$ reached $\sim$40 T for the film with the highest TMB flow rate, but after about 3 months of aging, this value rose to $\sim50$~T. The angular dependence of $H_{c2} $ for this sample was measured up to 45~T yielding $H_{c2}^{\parallel}(4.2\mbox{ K}) = 45.8$~T and showing the good Ginzburg-Landau scaling with an $H_{c2}$ anisotropy of 2.88 at 4.2 K. These results are discussed in terms of the theory of dirty two-gap superconductors as a part of an in-depth study of the effect of ternary doping of magnesium diboride. [Preview Abstract] |
Thursday, March 13, 2008 8:24AM - 8:36AM |
U11.00003: Kinetic roughening of magnetic flux penetration in MgB$_{2}$ thin films Andrea Lucarelli, Gunter Luepke, Brian Moeckly, Yue Zhao, Shi Dou Mg$_{\mbox{B2}}$thin films exhibit pronounced instabilities such as finger-like structures, flux jumps or dendritic patterns, which endanger electronic devices and lead to energy dissipation. We investigated the magnetic flux behavior of Mg$_{\mbox{B2}}$ thin films samples grown by in situ pulsed laser deposition and in situ reactive deposition technique on different substrates. We performed time-resolved magneto-optical imaging (TRMOI) measurements as a function of applied static field and for a static filed plus ac current to visualize the kinetic roughening of the flux penetration front. The TRMOI images are analyzed by employing dynamic scaling concepts used in the studies of interface roughening of stochastic systems. For both static field and ac current the resulting critical state shows self-affine structure characterized by universal exponents. [1] Dynamic scaling-laws determined in both cases are consistent with the directed percolation depinning model, placing the vortex dynamics of Mg$_{\mbox{B2}}$ in the same universality class as YBCO and Nb. \newline [1] Lucarelli et al. Appl. Phys. Lett. 91, 22 (2007) [Preview Abstract] |
Thursday, March 13, 2008 8:36AM - 8:48AM |
U11.00004: Evidence of new pinning centers in irradiated MgB$_{2}$ C. Tarantini, A. Martinelli, P. Manfrinetti, A. Palenzona, I. Pallecchi, M. Putti, C. Ferdeghini, M.R. Cimberle It has been shown that C or SiC addictions can strongly enhance upper critical field of MgB$_{2}$, leading to an in-field increase of critical current, but without introducing pinning centers other than grain boundaries. On the contrary neutron irradiation introduces new pinning centers, as highlighted by a significant shift of the maximum of pinning force and by a strong improvement of J$_{c}$ at high field. This effect can be correlated to the defects that neutron irradiation produces. In fact TEM images show the presence of nanometric amorphous regions whose sizes are compatible with the coherence length and such as to act as pinning centers through two different mechanisms. The influence that neutron irradiation induces on MgB$_{2}$ is also confirmed by magnetization decays that, differently by doped samples, show an important enhancement of pinning energies at high field. These measurements highlight as the increase of pinning energy with irradiation fluence is strongly correlated with J$_{c}$ improvement. [Preview Abstract] |
Thursday, March 13, 2008 8:48AM - 9:00AM |
U11.00005: Flux Pinning and Connectivity in MgB2 M.D. Sumption, M. Susner, M. Bhatia, E.W. Collings The transport and pinning properties of in-situ MgB2 bulks and strands are discussed. The influence of SiC, excess Mg, B4C, TiC, and their combination on Birr and Bc2 as distinct from connectivity and flux pinning is the focus of the work. SiC dopants increase Bc2 and Birr predominantly, with little influence on connectivity or flux pinning. Excess Mg improves the transport current, changes the grain microstructure, and also leads local maxima in Bc2 and Birr at excess Mg levels of 15\% mol fraction. Fp curves are consistent with grain boundary pinning for the binary materials over the whole temperature range. This is also true for SiC and TiC doped materials at lower fields and temperatures, while higher temperatures show a deviation from surface pinning. These higher temperature deviations are consistent with the size and distribution of these nanoparticulate additions. Normal state resistivity measurements and models are used to extract residual resistivity values, percent connectivity, and Debye temperatures. Debye temperatures are seen to be depressed by SiC doping, an effect which is confirmed by heat capacity measurements. Residual resistivity values are seen to correlate with Bc2 and Birr enhancements, consistent with B site substitution with C as evidenced by XRD extracted lattice parameter shifts. [Preview Abstract] |
Thursday, March 13, 2008 9:00AM - 9:12AM |
U11.00006: Upper critical field study of MBE grown MgB$_2$ thin films J. Jaroszynski, F. Hunte, A. Gurevich, D. C. Larbalestier, Y. Zhu, P. M. Voyles, Y. Shen, R. Gandikota, R. Singh, J. Rowell, N. Newman "Normal" alloying of MgB$_2$ enhances the electron scattering, as does radiation damage. As a result, the upper critical field $H_{c2}^{\parallel}(0)$ parallel to the ab planes doubles from about 18 to 35 T. ASU has been growing films by non-equilibrium MBE methods and either intentionally doping with oxygen during growth, or by deposition at room temperature with subsequent annealing {\em ex situ} at rather low temperature: {\em e.g.} 350 $^\circ$C for 36 hrs followed by 600 $^\circ$C for 30 min. The resistive transitions of the films have been measured in fields up to 45 T at different temperatures. The measurements revealed strong enhancement of $H_{c2}^{\parallel}(0)$. In particular, the cold-deposited film remains superconducting at 45 T at 5 K, while extrapolation yields $H_{c2}^{\parallel}(0)$ higher than 65 T, almost as high as the best C-doped HPCVD films. At the same time, the film is strongly inhomogeneous, the resistivity is as high as ~30 m$\Omega$cm, while the transitions are very broad, and the critical temperature is lowered to 24 K. However, $dH_{c2}/dT$ at $T_c$ reaches a record high value of 2.7 T/K. These observations open up another way to get exceptional $H_{c2}$ values in MgB$_2$ films. [Preview Abstract] |
Thursday, March 13, 2008 9:12AM - 9:48AM |
U11.00007: Novel properties in the normal state and the mixed state due to multiband effect in MgB$_{2}$ Invited Speaker: Based on high quality MgB$_{2}$ thin films, we measured the longitudinal and transverse resistivity of seven different samples with variable disorders characterized by the residual resistance ratio (\textit{RRR}) ranging from 4.0 to 33.3. Strong nonlinear Hall effect and strong magnetoresistance have been found in clean samples and they decrease gradually with the increase of scattering centers or temperature. By fitting to the theoretical model for a four-band system, for the first time, we derived the scattering rates for each band. Nontrivial difference between the transport properties of these four bands are unraveled. In the mixed state, a non-vanishing dissipation has been observed in the low temperature regime. The Hall Effect measurement confirms that it is induced by the vortex motion. Together with the data of I-V curves, point-contact tunneling and the magnetization relaxation, we conclude that this non-vanishing dissipation in the zero temperature limit is induced by the proliferation of the pi-band quasiparticles, in association with the multigap feature. In collaboration with H. Yang, Y. Jia, J. R. Shi, L. Shan, C. Ren and Y. Z. Zhang at IOP, CAS and C. G. Zhuang, Z. K. Liu, Qi Li, Yi Cui, and X. X. Xi at Penn State University. [Preview Abstract] |
Thursday, March 13, 2008 9:48AM - 10:00AM |
U11.00008: Structural-Microstructural Characteristics and its Correlations with the Superconducting Properties of in-situ PIT Processed MgB$_{2}$ Tapes with Ethyltoulene and SiC Powder added Anjana Asthana, H. Yamada, N. Uchiyama, A. Matsumoto, H. Kitaguchi, Y. Matsui, H. Kumakura The structure and microstructures of pure MgB$_{2}$, ethyltoulene and ethyltoulene + SiC added MgB$_{2}$ tapes have been investigated by using selected area electron diffraction, bright field, dark field and high resolution electron microscopy. As reported, the Jc values of the ethyltoluene and ethyltoulene + SiC added MgB$_{2}$ tapes are much higher than the pure MgB$_{2}$ tape sample. Analysis of the microstructures shows that pure MgB$_{2}$ tape~ sample consist of grains of 100-200nm. With the addition of ethyltoulene and ethyltoulene + SiC to the starting powder of \textit{in situ} processed MgB$_{2}$ tapes, the grain size decreases drastically to an average size of about 20-50nm. The higher Jc value of~ the ethyltoulene and ethyltoulene + SiC added MgB$_{2}$ tapes as compared to the pure MgB$_{2}$ tapes has been attributed~to the decrease in grain size and better connectivity of the grains and also presence of pinning centers as some precipitates and Mg$_{2}$Si particles of size less than 100nm. [Preview Abstract] |
Thursday, March 13, 2008 10:00AM - 10:12AM |
U11.00009: Properties of MgB$_{2}$ Thin Films Grown at Different Temperatures by Hybrid Physical-Chemical Vapor Deposition Menno Veldhorst, Ke Chen, Che-Hui Lee, Qi Li, Xiaoxing Xi MgB$_{2}$ films grown by Hybrid Physical-Chemical Vapor Deposition (HPCVD) at high temperature excel in $T_{c}$, cleanness, and crystallinity. MgB$_{2}$ films have been grown at temperatures from 350$^{^{\circ}}$C to 750$^{^{\circ}}$C by a HPCVD system with separate Mg and substrate heaters. The 100 nm MgB$_{2}$ film grown on a (001) SiC substrate at 350$^{^{\circ}}$C has a $T_{c0}$ of about 36K and a residual resistance ratio of about 1.4. X-ray diffraction and atomic force microscopy show that the film is polycrystalline. The low-temperature grown MgB$_{2}$ films are promising as the top electrode for sandwich-type all-MgB$_{2}$ junctions to preserve the integrity of the barrier layer. [Preview Abstract] |
Thursday, March 13, 2008 10:12AM - 10:24AM |
U11.00010: Superconductivity in Sc, Y, Lu, and CaLi$_{2}$ under Exreme Pressures M. Debessai, J.J. Hamlin, A.K. Gangopadhyay, J.S. Schilling, T. Matsuoka, K. Shimizu Following the first experiments by Sizoo and Onnes in 1925 on Sn, studies of superconductivity under high pressures have made important contributions by furthering our understanding of this exotic state and creating many new and novel superconducting materials, including O, Si, Fe, I, and Cs. Indeed, the number of elemental superconductors across the periodic table has almost doubled through the application of extreme pressures. MgB$_{2}$ exhibits the highest value of T$_{c}$ of any known binary compound, but the T$_{c}$ values of elemental superconductors under extreme pressures are not far behind. We have recently used pressures as high as nearly 2 Mbar to induce superconductivity in Sc, Y, and Lu, as well as to search for superconductivity in CaLi$_{2}$. T$_{c}$ values as high as 20 K are obtained, comparable to the highest values observed for the A15 compounds. These studies thus allow an investigation into the question: what is the maximum possible value of T$_{c}$ in a phonon-mediated superconductor? [Preview Abstract] |
Thursday, March 13, 2008 10:24AM - 10:36AM |
U11.00011: ABSTRACT WITHDRAWN |
Thursday, March 13, 2008 10:36AM - 10:48AM |
U11.00012: Superconductivity in MgCNi$_{3}$.Tunneling and heat capacity on single crystals Peter Samuely, Zuzana Pribulova, Jozef Kacmarcik, Pavol Szabo, Christophe Marcenat, Thierry Klein, D.-J. Jang, H.-G. Lee, H.-S. Lee, S.I. Lee MgCNi$_{3}$ reveals superconductivity despite a large molar volume of Ni atoms. The origin of superconductivity in this material has not yet been clarified. There is a lot of discrepancy in experimental results and physical interpretation where even unconventional pairing or a two-band model have been proposed for the system, but these suggestions are based on measurements on polycrystalline samples. Here we present the point-contact tunneling spectroscopy and ac-calorimetry measurements on single crystals of very good quality. Measurements have been performed in the temperature range from 0.7 K and in magnetic fields up to 8 T. The temperature dependence of the energy gap of the system is presented and compared to the BCS model. [Preview Abstract] |
Thursday, March 13, 2008 10:48AM - 11:00AM |
U11.00013: No Evidence for Spin Density Waves in Pb using Phonon Imaging Timothy Head, James Wolfe Phonon-imaging in superconducting Pb has proven to be a sensitive probe of quasiparticle density due to the highly anisotropic absorption of ballistic phonons by quasiparticles. Slower than expected temperature dependences of quasiparticle density observed previously by Wolfe and Short (\textit{Physica B} \textbf{316,} 107 (2002)) are explained here by taking into account the effects of nonequilibrium quasiparticles. Minimizing the effects of nonequilibrium quasiparticles enables us to extract a value of the zero-temperature superconducting gap parameter. We measure $\Delta $=1.32 $\pm $0.07 meV consistent with tunneling measurements in Pb and the conventional BCS picture, and inconsistent with earlier specific heat data that motivated the proposal of a spin-density-wave ground state in Pb (Overhauser and Daemen, \textit{PRL} \textbf{61}, 1885 (1988)). [Preview Abstract] |
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