Bulletin of the American Physical Society
2007 APS March Meeting
Volume 52, Number 1
Monday–Friday, March 5–9, 2007; Denver, Colorado
Session H8: Focus Session: Novel Superconductors III: Mostly MgB2 |
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Sponsoring Units: DMP Chair: Anthony Carrington, Bristol University Room: Colorado Convention Center Korbel 1C |
Tuesday, March 6, 2007 8:00AM - 8:36AM |
H8.00001: Large anisotropic normal-state magnetoresistance in clean MgB$_2$ thin films Invited Speaker: MgB$_2$ is arguably the first material which shows clear multiband superconductivity and two energy gaps. The interplay between the interband and intraband scattering as well as electron-phonon coupling has manifested in many physical properties. In this talk, I will review the magnetoresistance measurements of the normal state of MgB$_2$. We have shown large normal-state magnetoresistance with temperature-dependent anisotropy in very clean epitaxial MgB$_2$ thin films (residual resistivity much smaller than 1 $\mu \Omega$cm) grown by hybrid physical-chemical vapor deposition.[1] The magnetoresistance shows a complex dependence on the orientation of the applied magnetic field, with a maximum magnetoresistance (MR~136\%) observed at the field $H$ parallel to $ab$ plane at low temperature. However, the angular dependence changes dramatically as the temperature is increased, and at high temperatures ($T >$ 100 K), the magnetoresistance maximum changes to the $H$ perpendicular to $ab$ direction. We attribute the large magnetoresistance and the evolution of its angular dependence with temperature to the multiple bands with different Fermi surface topology in MgB$_2$ and the relative scattering rates of the $\sigma$ and $\pi$ bands, which vary with temperature due to stronger electron-phonon coupling for the sigma bands. The change of anisotropy with disorder has also been reported in neutron irradiated MgB$_2$ thin films. \newline Work done in collaboration with B. T. Liu, Y. F. Hu, J. Chen, H. Gao, L. Shan, H. H. Wen, A.V. Pogrebnyakov, J. M. Redwing, and X. X. Xi. \newline \newline [1] Qi Li et al., Phys. Rev. Lett., 96, 167003 (2006). [Preview Abstract] |
Tuesday, March 6, 2007 8:36AM - 8:48AM |
H8.00002: Tunneling study of two-band superconductivity in MgB$_{2}$ Ke Chen, Chenggang Zhuang, Yi Cui, Qi Li, Xiaoxing Xi, Zi-Kui Liu Thin film Pb/barrier/MgB$_{2}$ sandwich-type tunnel junctions have been fabricated. The MgB$_{2}$ films were epitaxially grown on (211) MgO substrates with c-axis tilted by 19.5 degrees with respect to the substrate normal by hybrid physical-chemical vapor deposition. Tunneling from both $\sigma $ and $\pi $ bands of MgB$_{2}$ to Pb is observed from the current-voltage characteristics (CVCs). The temperature dependence and the magnetic field dependence of the two gaps were obtained by fitting the CVCs in the framework of two band superconductivity. It shows that at zero temperature and zero field, $\Delta _{\sigma } \quad \approx $ 2.3 mV and $\Delta _{\pi } \quad \approx $ 7.4 mV. As the applied magnetic field normal to the substrate increases, the contribution from the $\pi $ gap is suppressed much more quickly than the $\sigma $ gap, which is in agreement with lower critical field corresponding to the $\pi $ gap. [Preview Abstract] |
Tuesday, March 6, 2007 8:48AM - 9:00AM |
H8.00003: Mixed State Dissipation in Zero Temperature Limit Enhanced by Two Gap Effect in MgB$_{2}$ Thin Films Ying Jia, Yan Huang, Huan Yang, Lei Shan, Cong Ren, Hai-Hu Wen, Chenggang Zhuang, Yi Cui, Qi Li, Xiaoxing Xi Through the measurements of resistive transition, point contact tunneling spectrum and Hall effect on crystalline MgB$_2$ thin films, the dissipation in the mixed state has been exclusively investigated. It is found that the resistive transition broadens monotonously with the magnetic field leading to a non-vanishing mixed state dissipation in zero temperature limit. Hall effect and point contact tunneling measurements indicate that this dissipation is contributed by the vortex motion and associated with the losing of long range phase coherence induced by the proliferation of the quasiparticles from the $\pi$-band. These results suggest the existence of the vortex quantum liquid enhanced by the two gap effect. Some preliminary results on mesoscopic-bridges will also be reported showing the interesting interplay of the two band superconductivity. [Preview Abstract] |
Tuesday, March 6, 2007 9:00AM - 9:12AM |
H8.00004: Observation of the Leggett's collective mode in MgB$_2$ two-band superconductor G. Blumberg, A. Mialitsin, B. S. Dennis, N. D. Zhigadlo, J. Karpinski We report observation of novel collective mode in the multi-band MgB$_2$ superconductor by resonant electronic Raman spectroscopy. The mode appears below T$_c$ in the $A_1g$ scattering channel at 9.2 meV, which is in-between the two gap values, 4.5 meV for the fundamental gap in the $\pi$-band and 13.5 meV for the gap in the $\sigma$-band. We attribute this excitation to collective mode first discussed by Leggett: If a system contains two coupled superfluids a simultaneous cross tunneling of a pair of electrons become possible. The mode is caused by dynamical oscillations of the pairs between the two superfluids leading to small fluctuations of the relative phase of two superconducting condensates. For MgB$_2$ the the oscillations between the condensates involve scattering of a pair of $\sigma$-band electrons with momentum $(k, -k)$ into a pair of $\pi$-band electrons with momentum $(k^\prime, -k^\prime)$. The symmetry and energy of observed mode is consistent with theoretical predictions. [Preview Abstract] |
Tuesday, March 6, 2007 9:12AM - 9:24AM |
H8.00005: Interplay of ballistic and diffusive superconductivity in the vortex core in the model two-band system MgB$_2$ K. Tanaka, M. Eschrig, D. F. Agterberg A revived interest in multi-band superconductivity has emerged due to the unexpected and interesting simultaneous presence of diffusive and ballistic bands in the superconductor MgB$_2$. Motivated by recent experimental data on the vortex state in MgB$_2$ obtained by scanning tunneling spectroscopy, we theoretically study the intriguing effects of superconductivity in a diffusive band (`$\pi$ band') induced by superconductivity in a ballistic band (`$\sigma $ band'). We apply a unique model that has been developed recently [1] for describing such a system, based on coupled Eilenberger and Usadel equations. Results are presented for the spatial variation of the order parameter, the current density, and the vortex core spectrum in the two bands. A particularly interesting result emerging from our studies is the possibility of additional bound states near the gap edge in the `strong' $\sigma$ band, which arise from hybridization with the `weak' $\pi$ band. The development of such gap-edge bound states is examined for various sets of physical parameters that are relevant for MgB$_2$. We will also discuss the induced Kramer-Pesch effect in the $\pi$ band and magnetic-field dependence of vortex core size. [1] K. Tanaka, D. F. Agterberg, J. Kopu, M. Eschrig, Phys. Rev. B {\bf 73}, 220501(R) (2006). [Preview Abstract] |
Tuesday, March 6, 2007 9:24AM - 9:36AM |
H8.00006: Breakdown of phase-locked states and limit of the upper critical field controlled by interband scattering in two-gap superconductors. Alexander Gurevich The effect of weak interband scattering on the maximum upper critical field $H_{c2}$, which can be achieved in a two-band superconductor by increasing intraband impurity scattering is considered. Using the two-gap Usadel equations, we show how weak interband scattering provides the crossover from the orbitally limited to the paramagnetically limited $H_{c2}$, both in bulk samples and thin films. The results are applied to describe high $H_{c2}$ values in carbon-doped MgB$_2$ films. It is shown that interband scattering produces mixed gradient terms $\propto Re(\Pi^{*}\Psi_\sigma^{*}\Pi\Psi_\pi)$ in the free energy, where $\Pi = \nabla +2\pi A/\phi_0$, and $\Psi_\sigma=\Delta_\sigma\exp(i\theta_\sigma)$ and $\Psi_\pi = \Delta_\pi\exp(i\theta_\pi)$ are intraband order parameters. The mixed gradient coupling can provide parametric excitation of the plasmon Legget mode or large-amplitude interband phase textures by ac currents. [Preview Abstract] |
Tuesday, March 6, 2007 9:36AM - 9:48AM |
H8.00007: Effect of oxygen alloying on scattering processes in MgB$_{2}$ Raghuram Gandikota, Rakesh Singh, Yi Shen, Nathan Newman, John Rowell The effect of oxygen alloying on T$_{c}$, resistivity, H$_{c2}$, and J$_{c}$ of MBE-grown MgB$_{2}$ films was studied. Oxygen was introduced either during growth or by ex-situ heating in oxygen atmosphere. While the concentration of oxygen increased from 1 to 9{\%}, \textit{dH}$_{c2}^{\vert \vert }$\textit{(0)/dT} of the films, at T$_{c}$, increases from 0.67T/K to 1T/K. J$_{c}$s greater than 400 kA/cm$^{2}$( at 8T, 4.2K) have been observed in these films and increased oxygen alloying changes J$_{c}$ very little. $H_{c2}^{\vert \vert }(0)$ and $H_{c2}^{-}$(0) values, obtained in these films with T$_{c}$s $\sim $ 31K, are as high as 43T and 32T respectively. These values are significantly higher than the maximum $H_{c2}^{\vert \vert }(0)$ value obtained for ion irradiated films, neutron irradiated MgB$_{2}$ bulk, and C-alloyed MgB$_{2}$ bulk. ~While the J$_{c}$ values of the oxygen alloyed films are higher than the C-alloyed Penn State films, the H$_{c2}$(0) values are, however, still significantly smaller than the record values found in the C-alloyed films. [Preview Abstract] |
Tuesday, March 6, 2007 9:48AM - 10:00AM |
H8.00008: MgB$_{2}$ Tunnel Junctions with Native or Thermal Oxide Barriers Raghuram Gandikota, Rakesh Singh, Yi Shen, Nathan Newman, John Rowell MgB$_{2 }$tunnel junctions (MgB$_{2}$/barrier/MgB$_{2})$ were fabricated using oxides of Mg and stoichiometric MgB$_{2}$, as the tunnel barrier. The sum of the superconducting gaps ($\pi $-gap) observed in conductance-voltage (G-V) measurements was as high as 4.3mV at 4.2 K and a finite value was found for temperatures above 30K. The G-V data exhibit smeared BCS densities of states, indicative of a degraded layer at the electrode/barrier interface. The presence of such an interface might also explain the lack of supercurrents above 20K in junctions exhibiting gap structures above 30K and even in shorted junctions. A subgap current was also observed and was not found to strongly depend on the oxide stoichiometry.~ [Preview Abstract] |
Tuesday, March 6, 2007 10:00AM - 10:12AM |
H8.00009: Superconductivity in M$_{n+1}$AX$_{n}$ compounds Michael Osofsky, R. J. Soulen, Jr., S. B. Qadri, M. W. Barsoum We present evidence for the presence of bulk superconductivity in several members of the M$_{n+1}$AX$_{n}$ (M=early transition metal, A=group A element, and X=carbon and/or nitrogen; n=1-3) family of compounds. Samples were synthesized by using standard ceramic techniques. We will present resistivity, susceptibility and specific heat data for these materials. We will also show detailed x-ray diffraction and microscopy data to demonstrate that impurity phases cannot account for the results. [Preview Abstract] |
Tuesday, March 6, 2007 10:12AM - 10:24AM |
H8.00010: Effects of Grain Size and Doping Level on the Critical Current Density of the Ti-sheathed MgB$_{2}$ Superconducting Wires with SiC Doping Gan Liang, Hui Fang, Cad Hoyt, Z. P. Luo, F. Yen, M. Hanna, A. Alessandrini, K. Salama The effects of the grain size and doping level on the critical current density ($J_{c})$ of the SiC-doped Ti-sheathed MgB$_{2}$ superconducting wires were studied. Two groups of samples were prepared: for the first group, the average size of the SiC grains was 20 nm and the doping levels were 5{\%}, 10{\%}, and 15{\%}; for the second group, the doping level of the SiC dopant was 10{\%} and the average sizes of the SiC particles were 20 nm, 43 nm, and 123 nm. All of the samples were sintered at 800 C for 30 minutes. Contrary to the $J_{c}$ results reported on the SiC-doped Fe-sheathed MgB$_{2}$ wires by some other groups, we found that the $J_{c}$ for the SiC-doped Ti-sheathed MgB$_{2}$ wires decreases with both the increase of SiC concentration and the decrease of the grain size. Only for the wires with average grain size of 123 nm, $J_{c}$ is greater than that of the un-doped MgB$_{2}$ wires. A simple model is proposed to explain the formation of the impurities in the cores of these doped MgB$_{2}$ wires. This unusual dependence of $J_{c}$ on the size and doping-level of the SiC dopant is discussed in association with the magnetization, resisitivity, XRD, TEM, and SEM results. [Preview Abstract] |
Tuesday, March 6, 2007 10:24AM - 10:36AM |
H8.00011: Superconducting and Normal State Properties of OsB$_2$* Yogesh Singh, A. Niazi, X. Zong, B.J. Suh$^{\dagger}$, M.W. Vannette, R. Prozorov, D.C. Johnston OsB$_{2}$ is a layered superhard metallic material that was found to superconduct below $T_{\rm c} = 2.1$~K.$^1$ We report the first detailed measurements of the static and dynamic magnetic susceptibilities $\chi$, electrical resistivity, heat capacity $C_{\rm p}$, penetration depth, and $^{11}$B NMR on OsB$_{2}$ to characterize its superconducting and normal state properties. The results confirm that OsB$_{2}$ is a bulk superconductor below $T_{\rm c} = 2.1$~K\@. Its properties can be described by a close to weak-coupling \emph{s}-wave BCS model with an electron-phonon coupling constant $\lambda = 0.4$--0.5, $\Delta(0)/(k_{\rm B}T_{\rm c}) \approx 1.9$, a small Ginzburg-Landau parameter $\kappa$ of order 5 or less, and a small zero-temperature critical magnetic field of roughly $500~$Oe. The $^{11}$B NMR measurements in the normal state show a nuclear spin-lattice relaxation time $T_1 = 2.1$~s at room temperature and a Korringa law with $T_1T = 610$~s$\cdot$K at lower $T$, and a correspondingly small $T$-independent Knight shift. These results indicate a small \emph{s} character of the conduction electron wave function at the B site at the Fermi level. Our results will be compared to corresponding data for MgB$_2$.\\ 1. J. K. Vandenberg et al., Mater. Res. Bull. {\bf 10}, 889 (1975).\\ $^*$Supported by the USDOE under Contract No.\ W-7405-Eng-82.\\ $\dagger$ Permanent address: Dept. Phys., The Catholic Univ. Korea. [Preview Abstract] |
Tuesday, March 6, 2007 10:36AM - 10:48AM |
H8.00012: Hot isostatic pressure synthesis of MgB2 from sonochemically modified precursors Brett A. McCarty, Amanda Toomey, Ruslan Prozorov Close to theoretical density bulk MgB2 was obtained by hot isostatic pressing (HIP) from precursors subject to high-intensity ultrasonic treatment. Comparative results obtained on unmodified and sonicated boron of various purities (from 80{\%} to 99{\%}) and in combination with various phases of magnesium are reported. Samples made from sonicated precursors showed improvements in superconducting properties without affecting Tc. Analysis of magnetization, SEM images, and XRD will be presented. [Preview Abstract] |
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