Bulletin of the American Physical Society
2007 APS March Meeting
Volume 52, Number 1
Monday–Friday, March 5–9, 2007; Denver, Colorado
Session A8: Focus Session: Novel Superconductors I: Doping and Impurities in MgB2 |
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Sponsoring Units: DMP Chair: Alexander Golubov, Twente University Room: Colorado Convention Center Korbel 1C |
Monday, March 5, 2007 8:00AM - 8:12AM |
A8.00001: $MgB_2$: doped or with pressure, four systems same behaviour Pablo de la Mora, Sabina Ruiz-Chavarria, Ulises Estevez, Gustavo Tavizon $MgB_2$, the intermediate $T_c$ superconductor, can be doped with carbon, aluminium and scandium and it has been also studied experimentally under pressure, in these four cases $T_c$ diminishes. In previous studies we have shown, with electronic structure calculations, that when $Mg$ is substituted with $Sc$ [$(Mg,Sc)B_2$] the drop of $T_c$ can be associated with the loss of electrical anisotropy of the $\sigma$-bands [1]. When $Mg$ is substituted with $Al$ $[(Mg,Al)B_2]$ or $B$ is substituted with $C$ [$Mg(B,C)_2$] then, with a change of doping scale, a common $T_c$ curve is obtained for both systems, comparison with the $\sigma$-$DOS$ shows that $T_c$ drop is due to $\sigma$-band-filling and to $\sigma$-band anisotropy loss [2]. In further studies we have found that both these features, $\sigma$-band anisotropy reduction and the loss of $\sigma$-band-carriers, can be associated to the drop of $T_c$ in these three doped systems [$Mg(B,C)_2$, $Mg,Al)B_2$ and $(Mg,Sc)B_2$] and in $MgB_2$ under pressure. All these studies show that: (a) with a change of doping scale then $T_c$ in both the C and Al doped systems follows the same curve which is very close to the $\sigma$-DOS; (b) for the four systems both the $\sigma$-band anisotropy and the number of $\sigma$-carriers are two fundamental physical properties of the relatively high $T_c$ in $MgB_2$. [1] J. Phys.: Condens. Matter 18 (2006) 1403-1412 [2] cond-mat/0606019 [Preview Abstract] |
Monday, March 5, 2007 8:12AM - 8:24AM |
A8.00002: First principles study of the electronic structure and phonon properties for Al and C-doped MgB$_2$ O. de la Pe\~{n}a-Seaman, R. de Coss, R. Heid, K.-P. Bohnen We have studied the structural, electronic and lattice dynamic properties of the superconducting alloys Al and C-doped MgB$_2$ within the framework of density functional perturbation theory, using a mixed-basis pseudopotential method and the virtual crystal approximation (VCA) for modeling the alloy. For both systems the structural parameters were determined on the following ranges, 0$\leq$x$\leq$1 for Mg$_{1-x}$Al$_x$B$_2$ and 0$\leq$x$\leq$0.4 for MgB$_{2(1-x)}$C$_{2x}$, finding a very good agreement between the calculated structural parameters and experimental data. The complete phonon dispersion curves were calculated for selected Al and C-concentrations. The calculated phonon bands for MgB$_2$ using the LDA and GGA approximations are compared in detail with the experimental data available in the literature. The evolution of the full-dispersion curves are analyzed as a function of Al and C-concentration, specially the E$_{2g}$-phonon mode frequency. In agreement with the experimental observed behavior, we find strong renormalization of the E$_{2g}$-mode for both Al and C-doped MgB$_2$. Additionally, we found a strong reduction of the E$_{2g}$-band dispersion with the filling of the $\sigma$-band. This research was supported by CONACYT, M\'{e}xico under Grant No. 43830-F. [Preview Abstract] |
Monday, March 5, 2007 8:24AM - 8:36AM |
A8.00003: Carbon and Aluminum Doping in MgB$_{2}$. Similarities and differences Peter Samuely, Pavol Szabo, Zuzana Holanova, Manuel Angst, Rudeger Wilke, Sergey Bud'ko, Paul Canfield Both carbon and aluminum dope the magnesium diboride by one extra electron which leads to filling of the most important $\sigma $ band and decreasing of the transition temperature. The point-contact spectroscopy in magnetic field is used to address the evolution of two superconducting energy gaps and density of states in the doped systems with $T_{c}$'s from 39 to 22 K. The similarities and differences in the inferred \textit{interband} and \textit{intraband} scatterings introduced by these two substitutions are discussed. It is shown that the two gap superconductivity is retained in all studied cases. The carbon doping is effective in increasing of the intraband scattering mainly in the $\pi $ band. This leads to important enhancement of the upper critical field. The approaching of two gaps is stronger in the Al-doped systems but the interband scattering is yet not large enough to merge two gaps. The full merging can be expected only for higher dopings, in the samples with $T_{c}$'s below 10 -- 15 K. Al substitution does not affect strongly the intraband scattering leaving the samples in the clean limit. [Preview Abstract] |
Monday, March 5, 2007 8:36AM - 8:48AM |
A8.00004: Carbon Doped MgB2 Thin Films using TMB R.H.T. Wilke, Qi Li, X.X. Xi, D.R. Lamborn, J. Redwing The most effective method to enhance the upper critical field in MgB2 is through carbon doping. In the case of thin films, ``alloying'' with carbon has resulted in enhanced Hc2 values estimated to be as high as 70 T for H parallel to ab and 40 T for H perpendicular ab [1]. ``Alloying'' refers to the in-situ Hybrid Physical-Chemical Vapor Deposition (HPCVD) of carbon containing MgB2 films using (C5H5)2Mg as the carbon source. While these films exhibit enhanced Hc2 values, there are amorphous boron- carbon phases in the grain boundaries that reduce the cross section area for superconducting current. We present here the results of our attempts to make more homogeneously carbon doped thin films using gaseuous trimethyl-boron (TMB) as the carbon source. Initial results indicate different behavior upon carbon doping using TMB from carbon-alloying. The microstructures and upper critical fields of the carbon doped films using TMB and carbon alloyed films will be compared. [1] V. Braccini et al., Phys. Rev. B 71 (2005) 012504. [2] A.V. Pogrebnyakov et al., Appl. Phys. Lett 85 (2004) 2017. [Preview Abstract] |
Monday, March 5, 2007 8:48AM - 9:00AM |
A8.00005: Disorder in carbon-doped HPCVD MgB$_{2}$ thin films Ye Zhu, P.M. Voyles, A.V. Pogrebnyakov, X.X. Xi Carbon-doped MgB$_{2}$ films prepared by hybrid physical-chemical vapor deposition have the highest $H_{c2}$ ($\sim $70 T at 0 K for H parallel to \textit{ab} plane) of all MgB$_{2}$ materials. We have characterized the nanoscale structure and chemistry of one such film by TEM and STEM. The C concentration in the Mg(B$_{1-x}$C$_{x})_{2}$ grains from EELS is not dramatically higher than that of C-doped bulk MgB$_{2}$, so doping does not explain the high $H_{c2}$. Instead, the doped film has a variety of forms of structural disorder at length scales down to 5 nm, which may be sufficient to explain the $H_{c2}$ of these films. These include MgB$_{2}$ domains with a 30 degree rotation about the $c$-axis, small angle rotations about $c$-axis, and a small tilt of the $c$-axis. There are also amorphous, C-rich regions between some MgB$_{2}$ domains. The amorphous phase comes from the oversupply of C during growth, which may also cause the other disorder by interrupting epitaxial film growth. This work is supported by the FRG on MgB$_{2}$, NSF DMR-0514592. [Preview Abstract] |
Monday, March 5, 2007 9:00AM - 9:12AM |
A8.00006: Correlated enhancement of H$_{c2}$ and J$_{c}$ in carbon nanotube-doped MgB$_{2}$. M. Jaime, A. Serquis, G. Serrano, S. Moreno, L. Civale, B. Maiorov, F. Balakirev We achieved simultaneous enhancement of upper critical magnetic field, H$_{c2}$, and critical current density, J$_{c}$, by doping polycrystalline samples of MgB$_{2}$ with double-wall carbon nanotubes (DWCNT), a source of atomic carbon. The optimum DWCNT content from the point of view of the J$_{c}$ is in the range 2.5-10{\%} at depending on field and temperature. Record values for H$_{c2}$ (4K) = 41.9 T (with extrapolated H$_{c2}$(0) $\approx $ 44.4 T) are reached in a bulk sample with 10{\%} at DWCNT content. The measured H$_{c2}$ vs T in all samples are successfully~described using a theoretical model for a two-gap superconductor in the dirty limit first proposed by Gurevich \textit{et al}. [Preview Abstract] |
Monday, March 5, 2007 9:12AM - 9:24AM |
A8.00007: Point-Contact Andreev-Reflection Spectroscopy in Neutron-Irradiated Mg$^{11}$B$_2$ R.S. Gonnelli, A. Calzolari, D. Daghero, M. Tortello, G.A. Ummarino, V. A. Stepanov, C. Tarantini, P. Manfrinetti We report recent results of point-contact spectroscopy (PCS) in Mg$^{11}$B$_2$ polycrystalline samples irradiated with neutrons at different fluences up to $\Phi$ = 1.4 $\cdot$ 10$^ {20}$ cm$^{-2}$. A strong depression of the bulk critical temperature $T_c$ down to about 8.7 K was observed after irradiation. The gaps $\Delta_{\pi}$ and $\Delta_{\sigma}$ were obtained from the experimental Andreev-reflection conductance curves through a two-band Blonder-Tinkham-Klapwijk fit and reported as a function of the Andreev critical temperature of the junctions, $T_c^A$. The resulting $\Delta_{\pi}$($T_c^A$) and $\Delta_{\sigma}$($T_c^A$) curves clearly show a merging of the gaps when $T_c^A <$ 9 K, which perfectly confirms the findings of recent specific-heat measurements in the same samples. ``Anomalous'' contacts with $T_c^A > T_c$ and a different dependence of the gaps on $T_c^A$ with respect to ``standard'' ones were obtained in samples irradiated at the highest fluences. The possible origin of these anomalies is discussed in terms of local current-induced annealing and/or nanoscale inhomogeneities - indeed observed by STM in the most irradiated samples. [Preview Abstract] |
Monday, March 5, 2007 9:24AM - 9:36AM |
A8.00008: Reflection of two band properties in the magnetic penetration depth of ion irradiated MgB$_{2}$ S.D. Kaushik, S. Patnaik Multiband superconductivity in MgB$_{2}$ has wide ranging ramifications for its transport characteristics. Using ion irradiation we have previously reported that by carefully choosing the type and density of defects, it is possible to control the inter and intra band scattering between the 2D $\sigma $ and isotropic $\pi $ bands of MgB$_{2}$. Here we report on the reflection of this defect induced modified scattering mechanism on the Meissner and mixed state penetration depth as a function of temperature, dc magnetic field, and defect density. The measurements are carried out using an ultrastable rf tunnel diode oscillator. The samples include unirradiated and those irradiated with 200 MeV Au$^{15+}$ and 100 MeV Si$^{8+}$. The fits to the superfluid density over the entire temperature range give information about the evolution of two gaps with progressive dirtying. From the mixed state measurements the bulk pinning force constant and flux flow resistivity are estimated. We also compare the superconducting properties of MgB$_{2}$ with 2H-NbSe$_{2}$ (T$_{c}$ = 7.3 K). [Preview Abstract] |
Monday, March 5, 2007 9:36AM - 9:48AM |
A8.00009: Disorder induced evolution of two energy gaps in MgB2 Yong-Jihn Kim We study disorder effect on MgB2 superconductivity using the two band model by Suhl, Matthias, and Walker. We stress the importance of the Cooper pair size effect in the response of the BCS superconductor to the perturbation: the bounded Cooper pairs see the impurities within the range of the coherence length. This effect will undermine the initial decrease of the Tc and the big energy gap due to disorder, until the resistance ratio reaches about $\sim $3. For the resistance ratio less than 3, weak localization starts to decouple electrons and phonons, leading to the significant decrease of both the Tc and the big gap. In particular, we trace the evolution of two energy gaps of MgB2 as a function of disorder. Estimating the inter-band scattering rate from the experimental data, we compare our calculations with experiments. We also calculate the transition temperature, Tc as a function of the resistance ratio. [Preview Abstract] |
Monday, March 5, 2007 9:48AM - 10:24AM |
A8.00010: Effects of Magnetic and Non-Magnetic Impurities in MgB$_2$: A Point-Contact Study of Single Crystals Invited Speaker: We studied the effects of chemical substitutions, either magnetic (Mn) or non-magnetic (Al, C), on the energy gaps of MgB$_2$ by means of directional point-contact spectroscopy (PCS) in state-of-the-art single crystals. Here we discuss two noticeable cases, i.e. Mg$_{1-x}$Mn$_{x}$B$_2$ crystals with $x$ up to 0.015, and Mg$_{1-x}$Al$_x$B$_2$ crystals with $x$ up to 0.32. In both cases, we used a pressure-less PCS technique in which a thin Au wire is put in contact with the side surface of the crystal by means of a small drop of Ag paint. The gaps $\Delta_{\sigma}$ and $\Delta_{\pi}$ were obtained through a two-band Blonder-Tinkham-Klapwijk (BTK) fit of the Andreev-reflection conductance curves of the resulting contacts. Both in Mn- and Al-doped MgB$_2$, the gaps decrease on decreasing the critical temperature of the contacts, $T_{c}^{A}$ (at which the Andreev-reflection structures disappear), but remain clearly distinct down to $T_{c}^{A}\simeq 10$ K. Once analysed within the two-band Eliashberg theory, the $\Delta_{\sigma}$ and $\Delta_{\pi}$ vs. $T_{c}^{A}$ curves give information about the effects of Mn and Al substitutions on the different scattering channels (interband and intraband, magnetic or non-magnetic). It turns out that the main effect of Mn is to increase the spin-flip scattering within the $\sigma$ band (with smaller contributions from either the $\pi-\pi$ or the $\sigma-\pi$ channels), as also confirmed by first-principle bandstructure calculations. In the case of Al, the band-filling effect is largely dominant. An increase in non-magnetic interband scattering is possible, but small enough not to give rise to gap merging. \newline \newline In collaboration with G.A. Ummarino, A. Calzolari, M. Tortello, D. Delaude, R.S. Gonnelli, Dipartimento di Fisica and CNISM, Politecnico di Torino, Italy; V.A. Stepanov, P.N. Lebedev Physical Institute, RAS, Moscow, Russia; N.D. Zhigadlo, J. Karpinski, Laboratory for Solid State Physics, ETHZ, Zurich, Switzerland; and S. Massidda, Dipartimento di Fisica, Universit\`{a} di Cagliari, Italy. [Preview Abstract] |
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