Bulletin of the American Physical Society
2008 APS March Meeting
Volume 53, Number 2
Monday–Friday, March 10–14, 2008; New Orleans, Louisiana
Session P11: Focus Session: MgB2-like: Disorder in Novel Superconductors |
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Sponsoring Units: DMP Chair: Alex Gurevich, Florida State University Room: Morial Convention Center RO9 |
Wednesday, March 12, 2008 8:00AM - 8:12AM |
P11.00001: Penetration depth study of Li, C and Li$+$C doped MgB$_2$ single crystals Catalin Martin, Matthew Vannette, Ruslan Prozorov, J. Karpinski, N. Zhigadlo, R. Khasanov Magnetic penetration depth was studied in single crystals of MgB$_2$, pure and doped with Li, C and Li+C. At zero applied field London penetration depth is measured and superfluid density, n$_s$, can be evalutated. We analyze effect of the dopings on n$_s$, thus on the superconductiong gaps. In the vortex state, Campbell penetration depth is measured and it provides information about true critical current unaffected by the magnetic relaxation. In particular, penetration depth in vortex state becomes strongly irreversible, and we show that C doping enhances the irreversibility region, whereas the substitution with Li does not affect it. The results are discussed in terms of two-gap nature of MgB$_2$. [Preview Abstract] |
Wednesday, March 12, 2008 8:12AM - 8:24AM |
P11.00002: Nano-scale TiB$_{2}$ Precipitates in MgB$_{2}$ Superconducting Wire D.K. Finnemore, M.E. Tillman, P.C. Canfield, S.L. Bud'ko, Y.Q. Wu, M.J. Kramer , J.V. Marzik, M. Rindfleisch , S.T. Hannahs Superconducting MgB$_{2}$ wire containing uniformly dispersed, nano-scale TiB$_{2}$ precipitates has been produced using a plasma synthesis method to make the starting Ti doped B powder. A powder-in-tube method is used for making the wire. Sub-micrometer size MgB$_{2}$ grains are decorated with a random dispersal of 5 to 20 nm precipitates. The superconducting properties are reported for a family of powder-in-tube wires fabricated with pure boron, carbon doped boron, and titanium doped boron. The particle size of the doped boron powder, of 50 to 100 nm, permits low reaction temperatures and short reaction times. The best reaction temperatures are in the neighborhood of 700 C for reaction times of approximately 20 min. For the Ti doped, PIT wire, at T = 20 K, J$_{c}$ crosses 10,000 A/cm$^{2}$ at 1.8 T. [Preview Abstract] |
Wednesday, March 12, 2008 8:24AM - 8:36AM |
P11.00003: Properties of Carbon Doped MgB$_{2}$ Films by HPCVD Using TMB Wenqing Dai, R.H.T. Wilke, Ke Chen, Qi Li, Xiaoxing Xi Carbon-doping is an effective way to enhance the upper critical field of MgB$_{2}$. Our previous carbon-alloyed MgB$_{2}$ films using (MeCp)$_{2}$Mg as the carbon source show dramatically increased $H_{c2}^{\vert \vert}$ values to over 60 T at low temperatures. Structure analyses of these films indicate that only part of the carbon is doped into the MgB$_{2}$ lattice and the rest forms highly resistive foreign phases in the grain boundaries. To fabricate more homogeneously carbon doped thin films, gaseous trimethylboron (TMB) was used as the carbon source. The normal state resistivity of carbon doped films using TMB increases much more slowly with carbon concentration, demonstrating a better connection between the MgB$_{2}$ grains. However the relatively high growth temperature, required to decompose TMB, limits the film thickness in the original Hybrid Physical-Chemical Vapor Deposition (HPCVD) setup. A hot wire is then installed in the HPCVD system to help decompose TMB while the substrate and Mg bulk source are kept at relatively low temperature. Initial results of these films will be presented. [Preview Abstract] |
Wednesday, March 12, 2008 8:36AM - 9:12AM |
P11.00004: Inhomogeneous Nanoscale Disorder in Doped Magnesium Diboride Invited Speaker: Using TEM and STEM imaging and microanalysis, we have shown that doped MgB$_{2}$ has a variety of forms of disorder at a length scale of 5-50 nm. In [0001]-normal, carbon-doped HPCVD thin films, there is a thin, amorphous, C-rich layer which separates domains of lightly carbon-doped MgB$_{2}$. Carbon also causes significant spread in the local in-plane orientation and c-axis direction of the MgB$_{2}$. We also observed disorder in the orientation of small MgB$_{2}$ domains in oxygen-doped thin films grown by MBE. Both of types of films show dramatic enhancement in the upper critical magnetic field compared to pure MgB$_{2}$, with extrapolated $H_{c2}$(0 $K)$ reaching 65-70 T for some samples. The origin of this increase may be in the observed disorder, particularly in confinement of the superconducting MgB$_{2}$ domains by non-superconducting second phase layers. [Preview Abstract] |
Wednesday, March 12, 2008 9:12AM - 9:24AM |
P11.00005: The Ca and Yb Isotope Effect in Superconducting Ca and YbC$_{6}$ David Hinks, Dan Rosenmann, Helmut Claus The isotope effect ($\alpha $ = -$\partial $logT$_{c}$/$\partial $logM where M is the isotope mass and T$_{c}$ is the transition temperature) is a measure of the phonon contribution to the superconductivity. We have measured both the Ca and the Yb isotope effect in intercalated highly orientated pyrolytic graphic (HOPG) by vapor phase transport of the isotopes. We find a large and, within the experimental error, equal $\alpha $ for each element, 0.36(4) and 0.40(4) for Yb and Ca, respectively. The value for Ca is larger then theoretically predicted indicating a stronger electron-phonon coupling. [Preview Abstract] |
Wednesday, March 12, 2008 9:24AM - 9:36AM |
P11.00006: Crystal Structures and Physical Properties of One-Dimensional Borides $A_{1+\varepsilon }M_{4}$B$_{4}$ ($A$ = Ca, Y; La; $M$ = Fe, Co, Ru) Yukari Katsura, Hiraku Ogino, Yutaka Matsumura, Shigeru Horii, Jun-ichi Shimoyama, Kohji Kishio We investigated crystal structures and physical properties of La$_{1+\varepsilon }$Fe$_{4}$B$_{4}$ (\textit{$\varepsilon $ }$\sim $ 0.06) and three new borides Y$_{1+\varepsilon }$Fe$_{4}$B$_{4}$(\textit{$\varepsilon $ }$\sim $ 0.16), CaCo$_{4}$B$_{4}$ and Ca$_{1+\varepsilon }$Ru$_{4}$B$_{4}$ (\textit{$\varepsilon $ }$\sim $ 0.13). Polycrystalline bulk samples were obtained by reacting metal boride precursors (FeB, CoB, RuB) with Ca, Y and La metals. Microstructure were analyzed using a SEM with an EDX. Crystal structures were analyzed through TEM studies and Rietveld analysis of powder XRD patterns. Electrical resistivity and magnetization measurements were carried out from 1.8 to 300 K. We found that all these compounds belong to the same structural family as RE$_{1+\varepsilon }$Fe$_{4}$B$_{4}$ (RE = La-Tm): a tetragonal lattice composed of one-dimensional channels of FeB and single atomic chains of RE. Incommensurate structures along $c$-axis were observed in La$_{1+\varepsilon }$Fe$_{4}$B$_{4}$, Y$_{1+\varepsilon }$Fe$_{4}$B$_{4}$ and Ca$_{1+\varepsilon }$Ru$_{4}$B$_{4}$. The La$_{1+\varepsilon }$Fe$_{4}$B$_{4}$ bulks exhibited type-II superconductivity below 6.0 K, although this might be due to the superconductivity of dirty $\beta $-La remained in the bulks. The other new borides did not show superconductivity down to 1.8 K. [Preview Abstract] |
Wednesday, March 12, 2008 9:36AM - 9:48AM |
P11.00007: Observation of a new phase in the Li-B system. Eduard Galstyan, Yuyi Xue, Yanyi Sun, Irene Rusakova, Ning Wang, Karen Mkhoyan, Ching-Wu Chu The simple compounds of boron and light elements have attracted interest since the unexpected discovery of high temperature phonon-coupled superconductivity (SC) in MgB$_{2}$. The numerous theoretical works show that the Li-B system has electronic features similar to those in MgB$_{2}$ and compounds in the system are expected to be superconducting. In fact, there were early reports of Li-B compounds. Although the small Li-size suggests a rich phase-diagram, the previous investigations discovered only one stable compound over a broad initial stoichiometry. The extreme reactivity and hydrolyses of Li in air may be partially responsible for this. We were able to synthesize a new Li-B pure phase that belongs to hexagonal symmetry through a new synthesis procedure. We report on the structural, magnetic, and electron energy loss spectra (EELS) analyses of this compound. Despite the similarity with the electronic structure of MgB$_{2}$, the Li-B system has not yet shown the existence of SC. [Preview Abstract] |
Wednesday, March 12, 2008 9:48AM - 10:00AM |
P11.00008: Superconductivity in heavily boron-doped single crystalline and nanocrystalline diamond thin films P. Achatz, C. Marcenat, E. Bustarret, T. Klein, O.A. Williams, J.A. Garrido, M. Stutzmann Our results show that the critical boron concentration $n_{c}$ for single crystalline diamond (scd) and nanocrystalline diamond (ncd) thin films is the same for the normal to superconducting and for the non-metal to metal transitions, on the order of $5 \times 10^{20}\ cm^{-3}$, in agreement with estimates derived from various theoretical approaches. In scd material, a variable range hopping behaviour was clearly observed on the insulating side of the transition, and, as expected, the characteristic temperature $T_{0}$ tended toward zero at the transition. On the metallic side, the zero temperature conductivity $\sigma_{0}$ scaled with $(n_{B}/n_{c} - 1)^{\nu}$ with $\nu \approx 1$. The critical temperature $T_{c}$ remained high in the vicinity of the metal-non metal transition, and it was rather found to scale with $(n_{B}/n_{c} - 1)^{1/2}$. These results led us to propose that the electron-phonon coupling parameter $\lambda$ remains large down to $n_{B}/n_{c} \approx 1.1$, and to examine the metal-insulator transition and the parameter set ($\lambda$, $\mu$) in terms of scaling laws. Low temperature magnetotransport measurements and the possible occurence of a superconductor-insulator transition in heavily boron-doped ncd will be discussed. [Preview Abstract] |
Wednesday, March 12, 2008 10:00AM - 10:12AM |
P11.00009: Tracing the evolution of two energy gaps of MgB2 with increasing disorder Mauricio Escobar M., Yong-Jihn Kim Previously we have determined the phonon-mediated matrix elements for MgB2 using the two band model and the scattered states. We noticed that, in the dirty limit, where the resistance ratio is about 3, the impurity scattering does not reduce the matrix elements much due to the Cooper pair size effect, whereas, in the weak localization limit, where the resistance ratio is less than 3, all the matrix elements are decreasing significantly due to weak localization. Now we calculate numerically the matrix elements and solve the gap equations with increasing disorder. We determine when the two energy gaps merge into one common energy gap and compare our calculations with experiments. We also calculate the transition temperature, Tc, as a function of the resistance ratio. [Preview Abstract] |
Wednesday, March 12, 2008 10:12AM - 10:24AM |
P11.00010: Magnetic field-tuned superconductor-insulator transition in quenched-condensed ultrathin Be films Zuxin Ye, Wenhao Wu We quenched condensed ultrathin Be films onto glass slides inside a dilution refrigerator with the substrates held near 10 K. The films were first tuned from insulating to superconducting by increasing the thickness in fine steps. The thickness-tuned transition occurs at a normal state sheet resistance R$_{N} \quad \sim $ 13 k-Ohm measured at 10 K. For superconducting films of various thickness, the field-tuned superconductor-insulator transition was then investigated. Remarkably, the critical resistance of the field-tuned transition was found to be R$_{C}$ = h/4e$^{2}$, independent of the thickness for films of R$_{N}$ ranging from 11 to 6 k-Ohm, critical temperature T$_{C}$ ranging from 1.3 to 5.2 K, and critical field B$_{C}$ ranging from 1 to 8 T. This result is a strong evidence for a duality quantum phase transition from a vortex glass in the superconducting state to a Bose glass in the field-induced insulating state. For thicker films with R$_{N} \quad <$ 5 k-Ohm, the critical resistance no longer remained at h/4e$^{2}$ but was nearly equal to R$_{N}$. This observation suggested that these thick films were no longer in the vicinity of the quantum critical point. [Preview Abstract] |
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