Bulletin of the American Physical Society
2006 APS March Meeting
Monday–Friday, March 13–17, 2006; Baltimore, MD
Session G38: Focus Session: Superconductivity-Carbon Alloying of Magnesium Diboride and Related Compounds |
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Sponsoring Units: DMP Chair: Paul Canfield, Iowa State University Room: Baltimore Convention Center 341 |
Tuesday, March 14, 2006 8:00AM - 8:36AM |
G38.00001: Tuning the Superconducting Properties of MgB$_{2}$ Invited Speaker: The relatively high superconducting transition temperature of 39 K in MgB$_{2}$ has garnered much interest over the past several years in both fundamental and applied research. MgB$_{2}$ is a conventional phonon mediated BCS superconductor with the unconventional property of two superconducting gaps. These gaps ($\sigma $ and $\pi )$ arise from the coupling of boron phonons with two different orthogonal sheets of the Fermi surface. In a conventional single gap superconductor the upper critical field can be tuned by the introduction of nonmagnetic impurities. For MgB$_{2}$ the situation becomes more complex because there are 3 important scattering channels (inter and intra-band). Theoretical calculations predict different developments of the upper critical field and anisotropy ratio if the scattering can be selectively tuned to a specific channel. In this talk I will present data on two different types of perturbations to MgB$_{2}$: carbon doping and neutron irradiation. Low level carbon doping enhances the upper critical field with only a minor decrease in T$_{c}$. Whereas T$_{c}$ suppression is the result of carbon electron doping the system, the enhancement of H$_{c2}$(T=0) is the result of an increase in scattering, with evidence to suggest carbon doping enhances intra-$\pi $-band scattering. In contrast, H$_{c2}$(T=0) values tend to scale with T$_{c}$ in heavily neutron irradiated samples and the superconducting properties can be understood in terms of a decrease in the density of states at the Fermi surface and an increase in interband scattering. [Preview Abstract] |
Tuesday, March 14, 2006 8:36AM - 8:48AM |
G38.00002: Upper critical field and anisotropy in carbon alloyed MgB$_2$ thin films Qi Li, Valeria Ferrando, Jun Chen, Alexej Pogrebnyakov, Joan Redwing, Xiaoxing Xi, Alex Gurevich, David Larbalestier, J. B. Betts, C. H. Mielke We have studied the upper critical field and its anisotropy $\gamma$ of C-alloyed MgB$_2$ thin films grown on (0001) SiC single crystal substrates by hybrid physical-chemical vapor deposition (HPCVD). Different concentrations of C were introduced during the deposition, which increased the residual resistivity systematically but did not affect $T_c$ seriously. The upper critical field was found to increase to above 60 T for $H$ parallel to the ab plane and up to $\sim$ 40T for $H$ parallel to the $c$ axis with even moderate amounts of C doping. Moreover, we show that $H_{c2}$ stays at these levels in a wide range of C doping. The $H_{c2}$ anisotropy was found to decrease as the C concentration increases. The $H_{c2}(T)$ curves for both $H \parallel ab$ and $H \parallel c$ directions were explained by a theoretical model of dirty limit two band superconductivity, which takes into account different scattering rates in $\pi$ and $\sigma$ bands, as well as interband scattering. The differences in the $H_{c2}(T)$ and $\gamma (T)$ for different samples can be explained by the differences of the relative scattering rates in each band which make it possible to adjust the $\pi$ and $\sigma$ scattering such that $H_{c2}$ perpendicular to $ab$ can attain almost 20T at 20K. [Preview Abstract] |
Tuesday, March 14, 2006 8:48AM - 9:00AM |
G38.00003: Accentuated lattice parameters in carbon doped MgB2 thin films with enhanced upper critical fields Jonathan Giencke, Alexej Pogrebnyakov, Ben Senkowicz, Xiaoxing Xi, David Larbalestier, Chang Beom Eom We report a strong correlation between the accentuation of the a- and c-lattice parameters in carbon doped MgB$_{2}$ thin films grown via HPCVD and their enhanced upper critical fields. This phenomenon is in strong contrast to the behavior seen in carbon doped single crystals, where the a-lattice parameter is depressed while the c-lattice parameter remains unchanged. This has lead us to believe that the defects leading to the enhancement of the upper critical fields differ in single crystals and thin films, and is due to the non-thermodynamic equilibrium growth environment unique to thin film processes. In addition, the presence of secondary phases present only in thin films exhibiting the enhanced critical field will be discussed. Understanding the nature of these defects is essential to controlling the superconducting properties of MgB$_ {2}$. [Preview Abstract] |
Tuesday, March 14, 2006 9:00AM - 9:12AM |
G38.00004: Carbon Inhomogeneity and Superconducting Properties of Mg(B,C)$_{2}$ L. D. Cooley, A. J. Zambano, A. R. Moodenbaugh, R. F. Klie Mg and B$_{4}$C mixtures (molecular ratio of [1:2]) were reacted in a single step in stainless steel from 850\r{ }C to 1300\r{ }C to produce nominal MgB$_{1.8}$C$_{0.2}$ as estimated from unit cell volumes. Reactions from 850 to 1100\r{ }C produced an inhomogeneous superconducting phase despite the initial atomic B$_{4}$C compound, suggesting that carbon segregation is an important part of these reactions. For reactions above 1100\r{ }C, the critical temperature T$_{c}$ increased from 18 to 28 K as the strain (as estimated from x-ray diffraction peak broadening) decreased at nominally constant carbon content, suggesting that structural order is also important. These changes will be discussed with regard to the formation and coarsening of amorphous and crystalline impurity phases observed by electron microscopy, which affect electron scattering and connectivity. Since only scattering affects the upper critical field, whereas both scattering and connectivity affect the transport properties, optimization of these reactions is crucial to understanding the potential for Mg(B,C)$_{2}$ as a high-field superconductor. [Preview Abstract] |
Tuesday, March 14, 2006 9:12AM - 9:24AM |
G38.00005: Towards the development of a high quality doped boron precursor and the effect on the superconducting properties of MgB$_{2}$ James Marzik, Albert Kumnick Gas phase plasma synthesis techniques were used to produce nano-sized doped boron powder from vapor phase precursors. The powders were reacted with magnesium to make MgB$_{2}$. The measurement of the resultant superconducting properties suggests that boron made by this synthetic method may result in MgB$_{2}$ superconductors with enhanced critical currents and upper critical fields. The values of J$_{c}$ and H$_{c2}$ obtained using plasma synthesized boron precursors are compared with those values for MgB$_{2}$ obtained using crystalline and amorphous boron powder made by other techniques, as well as boron fiber made by chemical vapor deposition. The roles of processing temperature and time, particle and grain size, purity, dopant concentration, and chemical homogeneity are evaluated in terms of their relative effects on the critical current and upper critical field of MgB$_{2}$. [Preview Abstract] |
Tuesday, March 14, 2006 9:24AM - 9:36AM |
G38.00006: Boron powder purification and its effects on the microstructure and superconducting properties of MgB$_{2}$. Jianyi Jiang, Ben Senkowicz, Xueyan Song, Eric Hellstrom, David Larbalestier MgB$_{2}$ wires can be easily fabricated by reacting Mg powder with amorphous boron powder. But commercial boron powder normally contains impurities such as B$_{2}$O$_{3}$. The presence of B$_{2}$O$_{3}$ could lead to the formation of oxides in MgB$_{2}$ wire, which may reduce the connectivity. In this work, B$_{2}$O$_{3}$ in amorphous boron powder was removed. MgB$_{2}$ samples were made both from purified and unpurified boron powders. X-ray diffraction showed that removing B$_{2}$O$_{3}$ in the boron powder resulted in a significant decrease in the content of MgO in the sample. The effects of boron powder purification on microstructure, critical temperature, critical field and connectivity will be presented. [Preview Abstract] |
Tuesday, March 14, 2006 9:36AM - 9:48AM |
G38.00007: Calorimetric transition widths, microstructure and magnetic relaxation of MgB$_{2}$ with and without additives Carmine Senatore, Paola Lezza, Giuseppe Cusanelli, Shi Xue Dou, Rene Flukiger The addition of nanometric powders of SiC is known to improve the pinning properties of MgB$_{2}$. Nevertheless, the underlying physical mechanism remains yet unclear. In this frame we report on specific heat measurements of sintered polycrystalline samples of MgB$_{2}$ and \textit{in situ} MgB$_{2}$ wires, both pure and with SiC nano-additions. A comparison is made with results obtained from microstructural analysis (X-Ray diffraction, SEM, EDX). Specific heat has been measured as a function of temperature (2-45 K) at zero field and at 14 T. Our goal is to relate the broadening of the superconducting transition observed in the samples with additives to the variation in the lattice structure induced by the nano-additions. Furthermore, magnetic relaxation measurements have been performed in order to investigate the effect of the nano-additions on the pinning mechanism. [Preview Abstract] |
Tuesday, March 14, 2006 9:48AM - 10:00AM |
G38.00008: Critical Current Density and Connectivity Properties of High Jc Ex-situ Mg(B$_{1-X}$C$_{X})_{2}$ B. J. Senkowicz, R. J. Mungall, J. R. Mantei, E. E. Hellstrom, D. C. Larbalestier Using high-energy milling we have obtained J$_{c}$ (4.2K, 8T) $>$ 5*10$^{4}$ A/cm$^{2}$ with H*(4.2K) $>$16T despite high normal state resistivity ( $\rho $(40K) $>$ 100 $\mu \Omega $-cm ). Analyzing resistivity curves by the Rowell method has led to the realization that as a result of overmilling and the presence of unincorporated carbon, our active cross section is only one tenth to one third of our total cross section. Therefore the potential exists to improve J$_{c}$ by a factor of several by increasing connectivity and decreasing grain boundary current obstruction. We examine the effect of composition and milling time on connectivity and superconducting properties. This research was performed under an appointment to the Fusion Energy Sciences Fellowship Program, administered by ORISE under contract between the U.S. DOE and the Oak Ridge Associated Universities. This work was supported by the NSF -- FRG on MgB$_{2}$, and by DOE -- Understanding and Development of High Field Superconductors for Fusion - DE-FG02-86ER52131. [Preview Abstract] |
Tuesday, March 14, 2006 10:00AM - 10:12AM |
G38.00009: Role of SiC Nanoparticles in the Electromagnetic Properties of SiC-doped MgB$_{2}$ Tapes Paul Voyles, Ye Zhu, Matsumoto Akiyoshi, David Larbalestier MgB$_{2}$ tapes with H$_{c2}$(2.3K) = 42 T and J$_{c}$(4.2K) $\sim $ 4$\times $10$^{5}$ A/cm$^{2}$ have recently been demonstrated by Matsumoto et al. These tapes are synthesized by powder-in-tube-processing from MgH$_{2}$, B, and nanoparticle SiC precursors. TEM and scanning TEM analysis of samples as a function of SiC content and reaction temperature shows that the role of SiC is more complicated than simply providing a source of C to dope the MgB$_{2}$. The sample with the highest H$_{c2}$ and J$_{c}$ contains unreacted SiC nanoparticles, which may act as pinning centers directly or may distort the adjoining MgB$_{2}$ lattice, increasing intraband scattering. Compared to pure MgB$_{2}$ samples, samples with SiC contain a significantly higher concentration of oxygen, leading to a higher density of second phases. SiC also tends to reduce the already small grain size of the pure MgB$_{2}$ from $\sim $50 nm to $\sim $20 nm, making grain boundaries another import source of pinning. [Preview Abstract] |
Tuesday, March 14, 2006 10:12AM - 10:24AM |
G38.00010: Enhancement of Critical Current Properties of MgB$_{2}$ Bulks by Controlling Microstructure and Crystallinity Jun-ichi Shimoyama, Akiyasu Yamamoto, Yukari Katsura, Isao Iwayama, Shinya Ueda, Shigeru Horii, Kohji Kishio Critical current properties of MgB$_{2}$ bulks with various grain size, crystallinity and bulk density were systematically studied. Samples with low crystallinity, which was achieved by the low temperature reaction and/or carbon substitution for boron, exhibited high $J_{c}$ in fields and high irreversibility fields, reflecting enhanced $H_{c2}$. In addition, a strong relationship was found between pinning force density and grain size controlled by changing starting particle size of boron, heating conditions and molar ratio between magnesium and boron. The highly dense MgB$_{2 }$bulks prepared by a diffusion method showed excellent $J_{c}$ characteristics particularly in low fields. The carbon substitution for the dense bulk was effective for enhancement of $J_{c}$ under high fields. Essential pinning potential of the MgB$_{2 }$was also evaluated using the dense bulks. Based on these results, a guiding principle to improve critical current properties of practical MgB$_{2}$ conductors will be discussed from various viewpoints. [Preview Abstract] |
Tuesday, March 14, 2006 10:24AM - 10:36AM |
G38.00011: Global and local flux jumps in MgB$_{2}$ films: Magneto-optical imaging and theory D.V. Shantsev, D.V. Denisov, Y.M. Galperin, T.H. Johansen, Sung-Ik Lee, A.L. Rakhmanov, A.V. Bobyl MgB$_{2}$ is one of the most unstable superconducting materials, where flux jumps are commonly observed at low temperatures jeopardizing its potential for applications. We present a detailed MO imaging study of the jumps in MgB$_{2}$ films, where we observe large dendritic flux patterns as well as much smaller jumps down to 50 flux quanta. The large jumps destroy the critical state and dramatically suppress \textbf{\textit{J}}$_{c}$. The small jumps, instead, lead to a new type of the critical state where \textbf{\textit{J}}$_{c}$ is determined not only by the pinning, but also by the jump characteristics. A linear theory assuming a thermal origin of dendritic jumps gives the threshold fields for the instability to start and to form a dendritic pattern and reproduces its experimental temperature dependence. We also determine the size of flux jumps in the adiabatic approximation. Its field dependence as well as the flux density profiles after jumps, are in good agreement with the direct observations, see Denisov et al., cond-mat/0508679, and Shantsev et al. Phys. Rev. B 72, 024541 (2005). [Preview Abstract] |
Tuesday, March 14, 2006 10:36AM - 10:48AM |
G38.00012: Evolution of Anomalous Angular Dependence of $H_{c2}$ in CaAl$_{1-x}$Ga$_{x}$Si Tsuyoshi Tamegai, Kentaro Uozato, Masashi Tokunaga In CaAlSi, the angular dependence of the upper critical field, $H_{c2}(\theta )$, shows an anomalous cusp when the field is applied parallel to the superconducting layer. $H_{c2}(\theta )$ can be reasonably well fitted by the Tinkham model for thin film superconductors. On the other hand, $H_{c2}(\theta )$ in CaGaSi does not show such an anomaly. Concomitant with these features, a lattice modulation along the $c$-axis exists only in CaAlSi. We have grown single crystals of the alloy CaAl$_{1-x}$Ga$_{x}$Si, and studied the evolution of the anomalous angular dependence of $H_{c2}$ and the superstructure. The superstructure.is sustained at least up to x = 0.2, although the period of the modulation changes from $\sim $6$c$ to $\sim $3$c$. On the other hand, the cusp-like feature in $H_{c2}(\theta )$ is strongly suppressed by the Ga substitution, and only evident at low temperatures. We have also investigated the homogeneity of the crystal by inspecting the vortex penetration using magneto-optical technique. We have found an inhomogeneous penetration of vortices along the superconducting plane in the \textit{ac} plane, suggesting the presence of modulation of superconducting parameters. Based on these results, we will discuss the origin of the anomalous $H_{c2}(\theta )$. [Preview Abstract] |
Tuesday, March 14, 2006 10:48AM - 11:00AM |
G38.00013: Anisotropic s-wave superconductivity and peak effect in single crystals CaAlSi R. Prozorov, T. Olheiser, R.W. Giannetta, K. Uozato, T. Tamegai In- and out- of plane London penetration depths were measured in single crystals of CaAlSi by using radio-frequency resonant technique. The obtained temperature variation of the anisotropic superfluid density can be explained by an ellipsoidal s-wave gap with a weak-coupling BCS value in the ab-plane and larger gap amplitude in the c-direction. In a mixed state, similar measurements yield Campbell length, which was used to study the peak effect (PE) in the critical current. The position of the PE line on an H-T phase diagram, the influence of anisotropy and sample parameters are discussed. [Preview Abstract] |
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