Bulletin of the American Physical Society
2009 APS March Meeting
Volume 54, Number 1
Monday–Friday, March 16–20, 2009; Pittsburgh, Pennsylvania
Session X33: MgB2 and Multigap Superconductivity |
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Sponsoring Units: DCMP Chair: Mike Sumption, Ohio State University Room: 403 |
Thursday, March 19, 2009 2:30PM - 2:42PM |
X33.00001: Homogeneity and connectivity of doped MgB2 bulks and strands as probed by heat capacity and current transport Mike Sumption, Mike Susner, Z. Shi, E. Collings Homogeneity and current percolation have been investigated for MgB2 bulks and strands. Sintered bulks were compared to dense bulks produced by HIPping, infiltration, and spark plasma synthesis in terms of their homogeneity as measured by heat capacity and resistivity. Various levels of carbon based dopants were introduced in each case, with control samples for comparison. The influence of higher temperature processing on inhomogeneity was investigated. These results are compared to those of MgB2 based strands made both with and without C-based doping. In addition, comparisons of transport and magnetization measurements at higher magnetic fields showed the onset of a regime where the anisotropy between Jc parallel to the strand and Jc perpendicular to the strand grows rapidly. This leads to large differences between transport and magnetically measured values of not only critical current, but also the irreversibility fields, Birr. Such effects can be described in terms of three different regimes, defining the dimensionality of the system. These regimes are distinct from the various states of vortex matter present in the magnetic phase diagram of MgB2, but in coexisting with them they influence our estimates of these boundaries. [Preview Abstract] |
Thursday, March 19, 2009 2:42PM - 2:54PM |
X33.00002: Upper critical field study of CH$_{4}$ HPCVD carbon-doped MgB$_{2}$ F. Hunte, J. Jaroszynski, A. Gurevich, D.C. Larbalestier, Y. Zhu, P.M. Voyles, X.X. Xi, K. Chen, S. Baily, F. Balakirev, C.G. Zhuang, S. Meng , C. Y. Zhang, Q.R. Feng, Z.Z. Gan The $H_{c2}(T)$ of a set of four carbon-doped MgB$_{2}$ films grown on both SiC and Al$_{2}$O$_{3 }$substrates by HPCVD from methane CH$_{4}$ at flow rates from 7 to 10 sccm were measured in fields up to 65T. Compared to early metalorganic C sources which generated high $H_{c2}(0)$, these films have much lower resistivities and higher connectivities. The curvature of $H_{c2}(T)$ derived from low current four point magnetoresistance shows upturn at low temperatures, which is consistent with the dominance of \textit{$\pi $}-band scattering in the theory of dirty two-gap superconductivity. $H_{c2}^{\vert }$(0) $>$ 60T is close to the paramagnetic limit of $\sim $ 66T for the 10 sccm film on SiC, though still a little lower than for the previously used metalorganic (C$_{6}$H$_{7})_{2}$Mg. Differences in the $H_{c2}(T)$ behavior between films grown on the two substrates are attributed to variations in strain fields produced by the substrate coupled to the film at growth. [Preview Abstract] |
Thursday, March 19, 2009 2:54PM - 3:06PM |
X33.00003: Absolute Penetration Depth in MgB$_{2}$ Nicholai Salovich, Russell Giannetta, Matt Tillman, Paul Canfield Absolute penetration depth measurements were carried out on single crystals of MgB$_{2}$. $\lambda $(0) was determined by sputtering an Al film onto the sample crystal and measuring the change in Meissner screening as the Al film expels flux. The change in screening was measured with a tunnel diode oscillator [1]. Several samples were sputtered with films of different thicknesses and measured. Thickness dependent changes in Hc and Tc of the thin Al films provided a self-consistency check on properties of the films. Subsequent analysis using FIB/SEM and AFM independently measured the film thickness and roughness. Work at UIUC supported by NSF DMR-05-03882. Work at the Ames Laboratory was supported by the Department of Energy, Basic Energy Sciences under Contract No. DE-AC02-07CH11358. [1] R. Prozorov, et al, Appl. Phys. Lett. 77, 4202 (2000) [Preview Abstract] |
Thursday, March 19, 2009 3:06PM - 3:18PM |
X33.00004: High Upper Critical Field and Critical Current Density of Carbon-doped MgB$_{2}$ Films by HPCVD Using TMB Wenqing Dai, Ke Chen, Qi Li, Xiaoxing Xi Carbon-doping is effective to enhance upper critical field $H_{c2}$ and critical current density $J_{c}$ of MgB$_{2}$. Using Trimethylboron (TMB) as the doping source, we have successfully fabricated carbon-doped MgB$_{2}$ thin films by the Hybrid Physical-Chemical Vapor Deposition (HPCVD) method. Large temperature derivative $-dH_{c2}^{//ab} /dT$ values near $T_{c}$, as high as 8.3 T/K, have been achieved for heavily doped samples. These values are much higher than what have been reported before. With $T_{c}$ over 30 K, $H_{c2}^{//ab} (0)$ values over 100 T can be expected for these samples. For lightly doped films, $J_{c}$ values, larger than 10$^{5}$ A/cm$^{2}$ at 5 K under 9 T perpendicular field and 10$^{4}$ A/cm$^{2}$ at 20 K under 5 T perpendicular field, were obtained. The results demonstrate that carbon-doped MgB$_{2}$ films by HPCVD using TMB are promising for high field applications. [Preview Abstract] |
Thursday, March 19, 2009 3:18PM - 3:30PM |
X33.00005: Novel phase line in the mixed state of the multi-band MgB$_2$ single crystal Daichi Kubota, Takekazu Ishida High-quality MgB$_2$ single crystals of submillimeter size were synthesized successfully by the vapor transport method for carrying out magnetic torque measurements. The magnetic torque of MgB$_2$ has been analyzed by a single-band model as well as a multi-band model for an anisotropic superconductor. The anisotropy in the coherence length $\gamma_\xi = \xi_a/\xi_c$ and the anisotropy in the penetration depth $\gamma_\lambda = \lambda_c/\lambda_a$ with the multi-band Kogan theory for the magnetic torque were simultaneously determined by means of the least-squares fittings. A systematic variation of two-sorts of effective superconducting anisotropies, $\gamma_\xi$ and $\gamma_\lambda$, in the field-temperature plane was obtained as contours, and can be interpreted as a manifestation of the two-band nature of the MgB$_2$ superconductivity. It is not a gradual crossover but a drastic change in the electronic state that an MgB$_2$ superconductor transits from a $\pi-\sigma$ multiband superconductor in lower fields to a $\sigma$ single band superconductor in higher fields. [Preview Abstract] |
Thursday, March 19, 2009 3:30PM - 3:42PM |
X33.00006: Time Dependent Effects in Transport Measurements and Absence of Anisotropic Behavior in Polycrystalline MgB$_{2 }$ Murat Olutas, Atilla Kilic, Kivilcim Kilic, Atilgan Altinkok, Hakan Yetis The flux dynamics in polycrystalline sample of MgB$_{2}$ have been studied by current-voltage (I-V) measurements for different sweep rates (dI/dt) of transport current at zero magnetic field (H=0) and H$\ne $0, and also magnetovoltage measurements (V-H curves) for different sweep rates (dH/dt). It was observed that the time and hysteresis effects in I-V curves which appear upon cycling of transport current are not significant as compared to those of YBaCuO and BiSrCaCuO. The absence of hysteresis effects in I-V curves were attributed mainly to the absence of weak link structure in MgB$_{2}$. The hysteresis effects in V-H curves depend on field orientation of H with respect to I. Furthermore, it was observed that time dependent effects appear in V-H curves as the dH/dt varies and also the counterclockwise behavior which appears in forward region changes its character and becomes clockwise for reverse region. The magnetovoltage measurements were also carried out by varying the angle $\theta $ between H and I (V- $\theta $ curves). The V- $\theta $ curves show that there is no anisotropy in polycrystalline MgB$_{2}$. This behavior was discussed in terms of polycrystalline structure of MgB$_{2}$ and its band structure where the Fermi surface includes four sheets of one electron-like and three hole-like. The observations were interpreted mainly in terms of flux trapping in grains. [Preview Abstract] |
Thursday, March 19, 2009 3:42PM - 3:54PM |
X33.00007: Electrical Conductivity Percolation Effects in CrO$_{2}$-MgB$_{2}$ Nanocomposites Xiangdong Liu, Raghava Panguluri, Huang Zhi-Feng, Boris Nadgorny Cold-pressed half-metals/insulators are known to show an enhanced extrinsic powder magnetoresistance (MR) due to intergranular and intergrain spin-dependent tunneling. In this work we use a mixture of metal/superconductor (CrO$_{2})_{x}$(MgB$_{2})_{1-x}$ nanocomposites to study conduction percolation effects. The samples were cold pressed in the form of pellets from the mixture of pure CrO$_{2}$ and MgB$_{2}$ powders. Transport and magneto-transport properties of various composition compacts measured as a function of temperature. Magneto-transport measurements performed over a temperature range of 2-100K show a hysteretic behavior with the peak values of MR coinciding with the CrO$_{2}$ coercive fields, with a maximum MR value for the composition near the percolation threshold of $\sim $ 42{\%} at 2 K. The electrical resistivity displayed a sharp maximum near the percolation threshold, a feature that is likely to be unique for such type of systems. We will discuss the implications of our results for the analysis of conduction percolation within the framework of a simple percolation model and a possible connection to Andreev reflection effect in this system at low temperatures. [Preview Abstract] |
Thursday, March 19, 2009 3:54PM - 4:06PM |
X33.00008: Abrikosov flux-lines in a two-band superconductor with mixed dimensionality K. Tanaka, M. Eschrig We study electronic and thermodynamic properties of a two-band superconductor in the vortex state, in which one band is ballistic and quasi-two-dimensional (2D), and the other is diffusive and three-dimensional (3D). A circular cell approximation of the vortex lattice within the quasiclassical theory of superconductivity is applied to a recently developed model appropriate for such a two-band system [1,2]. Motivated by MgB$_2$, superconductivity in the 3D diffusive band is assumed to be ``weak'', i.e., mostly induced. We examine the intriguing effects of induced superconductivity, impurities, and Coulomb interactions in the 3D band on electronic structure in the 2D band. In particular, the Coulomb repulsion and the diffusivity in the ``weak'' band enhance suppression of the order parameter and enlargement of the vortex core by magnetic field in the ``strong'' band, resulting in reduced critical temperature and field. A particularly interesting feature found in our model is the appearance of additional bound states at the gap edge in the ``strong'' band. Furthermore, coupling with the ``weak'' band leads to reduced band gaps and van Hove singularities of energy bands of the vortex lattice in the ``strong'' band. [1] K. Tanaka, D. F. Agterberg, J. Kopu, and M. Eschrig, Phys. Rev. B {\bf 73} 220501(R) (2006). [2] K. Tanaka, M. Eschrig, and D. F. Agterberg, Phys. Rev. B {\bf 75} 214512 (2007). [Preview Abstract] |
Thursday, March 19, 2009 4:06PM - 4:18PM |
X33.00009: Flux penetration in mesoscale samples of multi-component unconventional superconductors David George Ferguson, Paul Goldbart Multi-component unconventional superconductors bring the possibility of unusual magnetic phenomena. Examples include spontaneous zero-field magnetization, and penetration by magnetic flux not only through one-dimensional vortices but also through two-dimensional domain walls in which superconductivity persists. How flux penetrates, both in and out of equilibrium, depends on the way in which the order parameter distorts---vortex versus domain wall---particularly in view of the fact that flux quantization is required for vortices but not for domain walls that traverse the sample. We study these issues in the setting of mesoscale samples, in which domain walls are more stable and any discreteness of flux penetration should be more readily observable, predicting an unusual variation of the magnetization with the applied magnetic field. The observation of such effects in ${\rm Sr}_{2}{\rm Ru}{\rm O}_{4}$, a proposed unconventional superconductor, via techniques such as cantilever torque magnetometry, should shed light on important issues such as the pairing symmetry and the prevalence of domain walls in bulk samples. [Preview Abstract] |
Thursday, March 19, 2009 4:18PM - 4:30PM |
X33.00010: Multi-Gap Superconductivity in Chevrel Phases Alexander Petrovic, Cedric Dubois, Gilles Santi, Christophe Berthod, Oystein Fischer, Rolf Lortz, Albin Demuer, Arlei Antunes, Antoine Pare, Diala Salloum, Patrick Gougeon, Michel Potel Sub-Kelvin scanning tunnelling spectroscopy (STS) in the quasi-3D Chevrel Phases PbMo$_6$S$_8$ and SnMo$_6$S$_8$ reveals two distinct superconducting gaps at $\sim$~3meV and $\sim$~1meV. The relative spectral contribution from each gap changes with the angle between tip and sample, implying a rather anisotropic Fermi surface. Complementary to our local probe studies, specific heat measurements confirm the strong coupling (2$\Delta$/$k_B$$T_c$~$\sim$~4.9) seen by STS in each material and provide further evidence for multi-gap superconductivity. Hexagonal vortex lattices have been imaged by STS for each compound, with vortex core spectroscopy indicating a pseudogap within the cores. No pseudogap is visible in normal-state spectra, suggesting that the gapped vortex cores may be a novel consequence of two-band superconductivity. [Preview Abstract] |
Thursday, March 19, 2009 4:30PM - 4:42PM |
X33.00011: Anomalous upper critical field of two-gap superconductor Lu$_2$Fe$_3$Si$_5$ Yasuyuki Nakajima, Hikaru Hidaka, Tsuyoshi Tamegai, Terukazu Nishizaki, Takahiko Sasaki, Norio Kobayashi Ternary-iron silicide superconductor Lu$_2$Fe$_3$Si$_5$ with $T_c$ = 6 K has attracted attention because of the anomalous superconducting properties, such as a large residual linear term in the superconducting specific heat and a reduced normalized specific heat jump at $T_c$ smaller than the BCS value. Our recent specific-heat study has revealed that these anomalies stem from the two distinct superconducting gaps. In order to clarify the details of the two-gap superconductivity in Lu$_2$Fe$_3$Si$_5$, we have prepared the high-quality single crystal and investigated the upper critical field $H_{c2}$ obtained by resistivity measurements. We find that $H_{c2}$ increases linearly with decreasing temperature down to $T_c/3$, and $H_{c2}(T=0)$ exceeds the orbital depairing field described by the simple WHH theory. We also find that the angular dependence of $H_{c2}$ is well described by anisotropic GL model unlike the case of typical two-gap superconductor MgB$_2$. We discuss the origin of these differences based on the nature of two gaps in the two superconductors. [Preview Abstract] |
Thursday, March 19, 2009 4:42PM - 4:54PM |
X33.00012: Concomitant enhancement of spin susceptibility and pairing interaction in the reduced carrier-density regime of Li$_x$ZrNCl superconductor Yuichi Kasahara, Tsukasa Kishiume, Takumi Takano, Katsuki Kobayashi, Yoshihiro Iwasa, Eiichi Matsuoka, Hideya Onodera, Yasujiro Taguchi Li-intercalated layered nitrides Li$_x$ZrNCl are novel superconductors, in which superconductivity emerges at relatively high transition temperature $T_c\sim12$ - 15~K with very low carrier density $\sim10^{21}$~cm$^{-3}$. The pristine $\beta$-ZrNCl is a simple band insulator, and electron doping is achieved by Li intercalation. Insulator-to-superconductor (IS) transition takes place at $x\sim0.05$ with maximum $T_c$ value of $\sim15$~K and $T_c$ decreases with further doping, which is opposite trend to the other superconductors in doped band insulators. Here we show the results of magnetic susceptibility measurements on Li$_x$ZrNCl with systematically controlled $x$. Estimated spin susceptibility $\chi_s$ is almost temperature-independent without substantial anisotropy. With decreasing $x$, $\chi_s$ evolves strongly, same as $T_c$. On the other hand, specific heat study revealed that the density of states is reduced but the pairing interaction is enhanced on the verge of IS transition. Therefore, our results may indicate that magnetic fluctuations are enhanced toward a band-insulator and that they are possibly responsible to superconductivity even in the present small carrier-density system. [Preview Abstract] |
Thursday, March 19, 2009 4:54PM - 5:06PM |
X33.00013: Absence of Superconductivity in the Hole Doped Li$_{0.38}$BC Engin Ozdas, Bora Kalkan, Ebru Gungor The existence of several borocarbides with crystal structures highly related to MgB$_{2}$, in which one of these, the layered LiBC has been predicted based on the electronic structure calculations that this compound should become superconducting on doping with holes. However, the superconducting features for Li off-stoichiometric borocarbide compounds have not been observed in any experimental studies, because of the difficulties in the sample preparation. In this work, the effects of synthesis conditions on the structure of Li$_{x}$BC samples with the different Li content and the phase stability were investigated. The structural studies showed that the intercalation process has a staging behavior as Li intercalated graphite and a novel Li vacancy ordered structure for off-stoichiometric stage-2 Li$_{0.38}$BC phase. The temperature dependence of the conductivity shows semiconducting behavior over the whole temperature range and the hopping type conduction improved by the hole doping. [Preview Abstract] |
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