Bulletin of the American Physical Society
2005 APS March Meeting
Monday–Friday, March 21–25, 2005; Los Angeles, CA
Session J12: Superconducting Properties of MgB2 |
Hide Abstracts |
Sponsoring Units: DCMP Chair: Allen Hermann, University of Colorado Room: LACC 402A |
Tuesday, March 22, 2005 11:15AM - 11:27AM |
J12.00001: Superconducting Mg(B$_{1-x}$C$_x$)$_2$ with Titanium Additions D.K. Finnemore, R.H.T. Wilke, S.L. Bud'ko, P.C. Canfield, Raymond J. Suplinskas, S.T. Hannahs We have studied the superconducting properties of Mg(B$_{1-x}$C$_x$)$_2$ up to x$~=~0.021$ with and without 0.5$\% $ titanium impurities to determine the feasibility of simultaneously enhancing both the upper critical field and critical current density via chemical additions. Carbon substitutes for boron, increasing H$_{c2}$ by increasing scattering within the $\pi$ band. Titanium forms inter and intragranular precipitates of either TiB or TiB$_2$, which enhance flux pinning and J$_c$. The two effects appear additive and result in an increase in J$_c$ and H$_{c2}$ values. [Preview Abstract] |
Tuesday, March 22, 2005 11:27AM - 11:39AM |
J12.00002: Effects of Carbon Doping and Neutron Irradiation on MgB2 R.H.T. Wilke, S.L. Bud'ko, P.C. Canfield, D.K. Finnemore, Raymond J. Suplinskas, S.T. Hannahs, J. Farmer We have irradiated Mg(B$_{0.962}$C$_{0.038}$)$_2$ with thermal neutrons and performed post exposure annealing studies to probe the interplay between two different sources of defects. Carbon is believed to act as a point defect, enhancing H$_{c2}$ due to an increase in scattering in the $\pi$ band, without significantly enhancing flux pinning. Neutron damage studies on pure MgB$_2$ wire segments show a suppression of H$_{c2}$ that approximately scales with T$_c$, and an increase in the critical current density at low fields. Irradiation of carbon doped fibers results in a similar scaling of the enhanced H$_ {c2}$ with T$_c$, and the cumulative effects enhance J$_c$ at intermediate fields. [Preview Abstract] |
Tuesday, March 22, 2005 11:39AM - 11:51AM |
J12.00003: Effect of Al doping on the upper critical field of MgB$_{2}$ single crystals Sung-Ik Lee, Byeongwon Kang, Heon-Jung Kim, Hyun-Sook Lee, Thomas Dahm We use magnetization measurements to investigate the effect of Al substitution on the temperature dependence of the upper critical field, H$_{c2}$(T), of MgB$_{2}$ single crystals. We find that as the Al concentration is increased, the shape of H$_ {c2}$(T) changes from that for dirty $\sigma$ bands to that for dirty $\pi$ bands, which verifies that Al doping enhances intraband scattering mainly in the $\pi$ bands. Thus, one of the characteristics of the two-gap nature of MgB$_{2}$, i.e., the strong temperature dependence of the H$_{c2}$(T) anisotropy $\gamma_{H} = H_{c2}^{ab}/H_{c2}^{c}$ in pure MgB$_{2}$, is drastically affected by Al doping. [Preview Abstract] |
Tuesday, March 22, 2005 11:51AM - 12:03PM |
J12.00004: The Specific Heat of Mg(B$_{1-x}$C$_{x}$)$_{2}$: Two-Gap Superconductors N.E. Phillips, R.A. Fisher, N. Oeschler, W.E. Mickelson, A. Zettl Two polycrystalline samples of Mg(B$_{1-x}$C$_{x})_{2}$ were measured in magnetic fields (B) to 9 T. The samples show no evidence of magnetic impurities and have only small non-superconducting fractions. For x = 0.1, T$_{c}$ = 32 K and for x = 0.2, T$_{c}$ = 20 K. The specific heats of both samples can be fit with two superconducting energy gaps as was the case for MgB$_{2}$, although the magnitudes and fractions for each are different. For the two carbon-substituted samples the evolution of $\gamma $(B) with B, the normal state $\gamma $ values, and the Debye thetas will be compared to those of MgB$_{2}$. [Preview Abstract] |
Tuesday, March 22, 2005 12:03PM - 12:15PM |
J12.00005: Magnetic Field and Carbon-doping Dependent Far-infrared Studies of Epitaxial MgB$_{2}$ Films Jiufeng Tu, G.L. Carr, Qiang Li, L. Myhaly, P. Origiani, X.X. Xi Magnetic field dependent (up to 10T) far-infrared transmission studies have been carried out for a series of pure and carbon- doped epitaxial MgB$_{2}$ films (between 30-40nm) on SiC substrates. While the carbon-doped film (30nm) exhibits the typical characteristics for a dirty BCS superconductor in the T$_{S}$ /T$_{N}$ ratio, the pure MgB$_{2}$ films show anomalous behavior: the peak in the T$_{S}$ /T$_{N}$ ratio is broad in frequency; and as the film thickness increases the peak height decreases. Both of these observations could be the direct results of multi-gap nature of the superconducting state in MgB$_{2}$. As a function of magnetic field, the pure MgB$_{2}$ films show saturation behavior beyond 8T which gives the value of H$_{c2}$ at 8T for these films. Interestingly, the carbon- doped film also exhibits saturation behavior at around 8T, suggesting that the carbon impurities are not effective pinning centers for the vortex flux. Reflectivity measurements are underway to deduce the absolute scattering rates for both pure and carbon-doped films in order to construct an effective BCS model with multiple gaps based on the infrared data. [Preview Abstract] |
Tuesday, March 22, 2005 12:15PM - 12:27PM |
J12.00006: Probing the Superconducting Gaps of Carbon Substituted MgB2 Films using SIS Tunnel Junctions Tom Heitmann, Mark Rzchowski, Pasquale Orgiani, Xiaoxing Xi, Derek Wilke, Douglas Finnemore, Paul Canfield We report scanning tunneling spectroscopy of superconducting-insulating-superconducting (SIS) tunnel junctions consisting of a polycrystalline MgB$_{2}$ micro-wire as the probe tip and a series of C-substituted thin films as samples. The MgB$_{2}$ micro-wire is affixed to a 250$\mu $m Pt-Ir wire and approached to each C-substituted film using an Oxford CryoSXM in STM mode. We are then able to scan over the film surface and probe different areas of the film locally using this SIS configuration. The micro-wire tip, for which we have previously characterized the gaps, is used as a reference for the C-substituted films and to enhance energy resolution by pushing the spectral features to higher energies. The films were grown by hybrid physical-chemical vapor deposition and extensive magnetic and structural characterization of identical films have been reported previously. We investigate the evolution of $\Delta _{\pi }$ and $\Delta _{\sigma }$ with carbon substitution and correlate these results with T$_{C}$ and H$_{C2}$ results. [Preview Abstract] |
Tuesday, March 22, 2005 12:27PM - 12:39PM |
J12.00007: Observation of Non-Rigid-Band Alloying of Al- and C-doped MgB$_2$ by Electron Energy-Loss Spectroscopy Robert Klie, Lance Cooley, Qiang Li, Arnold Moodenbaugh, Robert Sabatini, Antonio Zambano, Yimei Zhu Angular-resolved electron energy loss spectroscopy in a scanning transmission electron microscope was used to study the B K-edge in pure and doped MgB$_{2}$. We have shown that the p$_{xy}$ states have a high density up to 0.8 eV above the Fermi level before dropping to near zero and then starts to rise again 5 eV above the Fermi level; the density of p$_{z}$-states changes only very little over the first 10 eV. The incompletely filled p$_{xy}$ states at the Fermi level are believed to play an important role in the superconductivity of MgB$_{2}$. Samples with carbon substituted for boron display a shift of the B K-edge towards lower energy but leave the pre-peak structure undisturbed, thus suggesting that carbon's extra electron enters mainly the $\sigma $ band and has little effect on the $\pi $-band states.~ Samples with aluminum substituted for magnesium display dramatic changes in the B K-edge pre-peak, which implies that the Mg site dopant contribute strongly to $\pi $-band states and only weakly to the $\sigma $-band states. These site-specific changes in local density of states are discussed in terms of intra- and interband scattering scenarios. [Preview Abstract] |
Tuesday, March 22, 2005 12:39PM - 12:51PM |
J12.00008: Effect of carbon doping on pinning force in MgB2 films Qi Li, Jun Chen, V. Ferrando, P. Orgiani, A. Pogrebniakov, Yi Cui, X. Xi, J. Redwing MgB$_{2}$ thin films were doped with carbon by adding carbon containing bis(cyclopentadienyl) magnesium to the carrier gas during the hybrid physical-chemical vapor deposition process. The superconducting properties were studied by transport and magnetization measurements. It was observed that the carbon doping greatly increased upper critical field Hc2 and the irreversibility field Hirr, and significantly improved the magnetic field dependence of critical current Jc. For example, Jc in carbon doped films could reach 3$\times $10$^{5}$A/cm$^{2}$ at 5T and 4.2K when the field was applied perpendicular the film plane, while Jc in the undoped film was already diminished at 5T of field. This indicated that extra stronger pinning centers were introduced into the samples by carbon doping. Analysis of the pinning scaling law indicated that the extra pinning mechanism could be normal core pinning and large precipitates pinning depending on the doping level. The effect of carbon doping on the flux dynamics will also be discussed. [Preview Abstract] |
Tuesday, March 22, 2005 12:51PM - 1:03PM |
J12.00009: Probing 2-band superconductivity of Al and C-substituted MgB2 with heat capacity measurements Antonio J. Zambano, Lance Cooley, Arnold Moodenbaugh We study the changes in the heat capacity Cp(T) in Mg$_{1-x} $Al$_{x}$B$_{2}$ (x $\le $ 0.19) and Mg(B$_{1{\-}y}$C$_{y})_{2}$ (y $\le $ 0.08). The two band model is used to fit Cp(T) and extract the two energy gaps and electron-phonon coupling matrix for the different dopant concentrations. These fitting routines are sensitive to background subtraction, and we discuss what constraints this places on sample quality and preparation. Like previous results, fits for Al doping do not indicate merging of the gaps, suggesting Al does not increase interband scattering. However, we also notice trends that are different from those seen in previous experiments, which we also discuss. For instance, the main peak due to the sigma band does not smear with increasing Al content, but remains fairly abrupt. Other characterization suggests the Al doped samples are of very high quality. Results for C doping are also discussed in terms of filling the sigma band hole states with electrons and increased interband scattering. [Preview Abstract] |
Tuesday, March 22, 2005 1:03PM - 1:15PM |
J12.00010: Synthesis and Superconducting Properties of the Yb Intercalated MgB$_2$ Bora Kalkan, Engin Ozdas The effect of the lattice parameters on the superconducting transition temperature (T$_{c})$ of MgB$_{2}$ was studied as a function of the Yb metal doping concentration. The intercalated MgB$_{2}$ samples, Mg$_{1-x}$Yb$_{x}$B$_{2}$, were prepared in a relatively small range of Yb concentration, 0.02$\le $x$\le $0.10. The phase analysis and characterization of the superconductivity were carried out by X-ray diffraction (XRD) and AC susceptibility measurements. The results show that a 0.15{\%} increase in the $c$-parameter for a starting composition of Mg$_{0.95}$Yb$_{0.05}$B$_{2}$ samples. [Preview Abstract] |
Tuesday, March 22, 2005 1:15PM - 1:27PM |
J12.00011: Effect of ion damage on the electrical properties of MgB2 Raghuram Gandikota, Rakesh Singh, Jihoon Kim, Barry Wilkens, Nathan Newman, John Rowell, Alexej Pogrebnyakov, Xiaoxing Xi, Joan Redwing, Shengyong Xu, Qi Li, Brian Moeckly The effect of point defects introduced by 2MeV alpha particles on Tc, resistivity and Hc2 of MgB2 films produced was studied. The damage controllably alters the material so the physical mechanisms that determine the widely varying electrical properties of the MgB2 samples and, in particular, the role of the 2 superconducting gaps can be investigated. The residual resistivity increases linearly by a factor of 50 as Tc decreases with damage to below 10K, while the change in resistivity from 300K to 40K decreases by less than 50{\%}. Our results indicate damage increases the residual resistivity of the grains themselves, but has no significant effect on connectivity. These results, and the common resistivity at which Tc extrapolates to 0K, indicate a direct correlation between Tc and intra-grain resistivity. dHc/dT near Tc increases with damage, reaches a maximum for Tc of 30K and then decreases. Hc2 extrapolated to 0K are as high as 57T for fields perpendicular to c-axis. [Preview Abstract] |
Tuesday, March 22, 2005 1:27PM - 1:39PM |
J12.00012: A Study of the Electronic Structure and the Effects of Oxygen on the Superconducting Properties of MgB$_2$ by Electron Energy Loss Spectroscopy Juan C. Idrobo, Serdar Ogut, Taner Yildirim, Nigel Browning The transport properties of MgB$_2$ have a strong dependence on the incorporation of impurities such as oxygen. Different phases of oxygen precipitates have been characterized and it has been found that the presence of oxygen in MgB$_2$ systematically changes the electronic structure of the boron atoms. Among the different phases of oxygen precipitates found in MgB$_2$, those forming coherent superlattice structures of MgB$_2$- MgB$_x$O$_y$ were studied in detail by first principles calculations. This kind of precipitate has been reported to increase the upper critical fields and critical current density without decreasing the critical temperature of MgB$_2$. This effect is reflected by the low critical temperatures calculated for coherent oxygen precipitates with different concentrations of oxygen using density functional theory. These low critical temperatures explain the behavior of the oxygen precipitates as pinning centers and highlight the importance of oxygen on the superconducting properties of MgB$_2$. Additionally, due to the presence of two carrier species given by the B and Mg states in MgB$_2$, a low energy plasmon mode was theoretically proposed. This work presents the first experimental evidence of this plasmon mode, which has a quadratic dispersion, $\omega_p\propto q^2$. [Preview Abstract] |
Tuesday, March 22, 2005 1:39PM - 1:51PM |
J12.00013: Study on the formation of MgB2 and other phases in Cu-sheathed /MgB2 wires. Gan Liang, Hui Fang, David Katz, Kemal Salama, Zhongjia Tang We report the results from a systematic x-ray diffraction, SEM, and critical current density measurements for Cu-sheathed MgB$_{2 }$wires fabricated using the powder-in-tube method and ultra-fine Mg and B precursor prepared by high-energy ball milling. The samples were sintered at temperatures ranging from 450 \r{ }C to 900 \r{ }C for 5 minutes. It is found that MgB$_{2}$ phase can be formed in this whole temperature range. Below 550 \r{ }C, the weight of the Mg$_{2}$Cu phase increases with sintering temperature while the Mg-phase decreases. Between 550 \r{ }C and 725 \r{ }C, the Mg$_{2}$Cu phase disappears, only MgB$_{2}$ and MgCu$_{2}$ phase co-exist. At or above 725 C, MgB$_{2}$, MgB$_{4}$, and Mg$_{1+y}$Cu$_{32+\delta }$ phase coexist, and the fraction of the Mg$_{1+y}$Cu$_{32+\delta }$ phase increases with sintering temperature while the other two phases decrease. These results are supported by our SEM and $J_{c}$ measurement results. [Preview Abstract] |
Tuesday, March 22, 2005 1:51PM - 2:03PM |
J12.00014: Processing, Microstructure, and Properties of Doped and Undoped MgB2 Fibers John DeFouw, James Marzik, Raymond Suplinskas, David Dunand A composite consisting of continuous MgB$_{2}$ fibers within an Mg matrix is synthesized by reacting CVD boron fibers with liquid magnesium. Over the reaction temperature range of 700 - 1100 \r{ }C, the reaction time varies by several orders of magnitude. Much slower kinetics are observed for reaction of B fibers doped with C or Ti by co-deposition during the CVD process, which has been shown to improve superconducting properties after reaction to MgB$_{2}$. The reaction times, resulting microstructures, and superconducting properties are compared for both doped and undoped fibers. [Preview Abstract] |
Follow Us |
Engage
Become an APS Member |
My APS
Renew Membership |
Information for |
About APSThe American Physical Society (APS) is a non-profit membership organization working to advance the knowledge of physics. |
© 2024 American Physical Society
| All rights reserved | Terms of Use
| Contact Us
Headquarters
1 Physics Ellipse, College Park, MD 20740-3844
(301) 209-3200
Editorial Office
100 Motor Pkwy, Suite 110, Hauppauge, NY 11788
(631) 591-4000
Office of Public Affairs
529 14th St NW, Suite 1050, Washington, D.C. 20045-2001
(202) 662-8700