Bulletin of the American Physical Society
APS March Meeting 2013
Volume 58, Number 1
Monday–Friday, March 18–22, 2013; Baltimore, Maryland
Session R35: Novel Superconductors I |
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Sponsoring Units: DCMP Chair: James Eckstein, University of Illinois at Urbana-Champaign Room: 343 |
Wednesday, March 20, 2013 2:30PM - 2:42PM |
R35.00001: The Introduction of substitutional and non-substitutional dopants into MgB2 in high pressure/Temperature or non-equilibrium regimes Mike Sumption In an attempt to study the effect of doping of MgB2 under conditions leading to efficient doping, we used both an high temperature/high pressure induction furnace to dope into MgB2 bulks at temperatures up to 1600 C and 1500 Psi, and thin film, PLD multilayer and mixed layer film fabrication. The high temperature/high pressure formation was used to explore the solubility at high temperatures of various dopants, and the thin film formation was an attempt to use non-equilibrium conditions to inject dopants more effectively. The dopants used were C, Ti, and Zr. C was seen to reach a maximal level at 4 at{\%} C site substituted into MgB2, as evidenced by EPMA and XRD results. Zr, of interest as a possible Mg site substitution in MgB2 was not seen to enter into the MgB2 phase (instead segregating) in the bulk high temperature/high pressure experiments, but was seen to enter in during PLD, as evidenced by STEM and XRD results. Ti additions were attempted in the high pressures and temperature rig, with some evidence for dopant introduction. Critical field measurements on the Zr doped samples where seen to suppress Bc2 for all except very low levels of Ti addition, presumably associated with the much greater doping efficiency. [Preview Abstract] |
Wednesday, March 20, 2013 2:42PM - 2:54PM |
R35.00002: The Penetration Depth of MgB$_2$ as measured by DC SQUIDs Daniel Cunnane, Ke Chen, X.X. Xi High-speed superconducting circuits may benefit from the high $T_{c}$ and large superconducting gap of MgB$_{2}$. Nb remains the state of the art for superconducting electronics partly because of its small penetration depth and its isotropic nature. A microscopic theory on the penetration depth of multiband superconductors states that a clean MgB$_{2}$ sample is nearly isotropic while a sample in the dirty limit is anisotropic. We have made and measured DC SQUIDs using MgB$_{2}$ Josephson junctions to determine the inductance of an MgB$_{2}$ microstrip. The penetration depth along the c-axis, $\lambda_{c}$, was calculated using the inductance value and dimensions of the microstrip. We have previously reported the absolute value of the penetration depth of our MgB$_{2}$ films to be around 40 nm. Now we have made devices with film ranging from the clean limit to the dirty limit by adding defects during the deposition. The absolute value of $\lambda_{c}$ at low temperature is compared to the cleanliness of the film. The temperature dependence was also measured which is non-trivial due to the two-gap nature of MgB$_{2}$. These results are compared with theory that confirmed our previous results. [Preview Abstract] |
Wednesday, March 20, 2013 2:54PM - 3:06PM |
R35.00003: Superconducting properties of aligned flexible networks and yarns of MgB$_2$-CNT nanowires Julia Bykova, M\'{a}rcio Dias Lima, Derrick Tolly, Carter Haines, Austin Howard, Myron Salamon, Ray Baughman, Anvar Zakhidov Magnesium diboride (MgB$_2$) has attracted great interest due to its outstanding superconducting characteristics. Literature reports showed that addition of carbon nanotubes (CNT) to a MgB$_2$ matrix significantly improves its properties: CNTs can carry extremely high currents and also provide electrical and mechanical connection between MgB$_{2}$ grains. Here we present a new method to produce networks of aligned MgB$_{2}$-CNT nanowires which can be spinned into flexible yarns. Free-standing, aligned CNT sheets were used as a starting network. A conformal layer of boron was deposited on CNTs by Laser Assisted Chemical Vapor Deposition. The resultant boron-CNT nanowires (thickness of 70$\pm10$ nm) were exposed to magnesium vapor and were converted into MgB$_{2}$-CNT composites. The MgB$_{2}$-CNT arrays are flexible and can be easily bent and even twisted. Critical temperature reaches 37 K and depends on thickness and crystalline structure of nanowires. Critical current and critical fields were shown to be comparable or even better than standard MgB$_{2}$ wires. We discuss the correlation of observed two step behavior in electric transport curves with interconnects between MgB$_{2}$-CNT nanowires and Josephson junction network formation. [Preview Abstract] |
Wednesday, March 20, 2013 3:06PM - 3:18PM |
R35.00004: Enhancement of lower critical field in thin MgB$_{2}$ films and MgB$_{2}$/MgO multilayers Teng Tan, Evan Johnson, Narendra Acharya, Michael Hambe, Ke Chen, Alex Krick, Steven May, Xiaoxing Xi Magnesium diboride is a conventional superconductor with a high $T_{c}$ of 39 K, a low residual resistivity of \textless\ 0.1 $\mu \Omega $cm (at 42 K), and higher thermodynamic critical field $H_{c}$ values than Nb. These properties make MgB$_{2}$ a promising superconductor as an alternative to Nb for future SRF cavities. However, the lower critical field $H_{c1}$ of MgB$_{2}$ is low, and vortex dissipation above $H_{c1}$ can lead to degradation of the quality factor and low RF breakdown field. Here, we report an enhancement of $H_{c1}$ in thin MgB$_{2}$ films and MgB$_{2}$/MgO multilayers. The value of $H_{c1}$(5K) is increased from 40 mT in a 300 nm-thick MgB$_{2}$ film to 180 mT when the MgB$_{2}$ layer thickness is 100 nm either in a single-layer film or in a MgB$_{2}$/MgO multilayer with a total MgB$_{2}$ layer thickness of 300 nm. Superconducting MgB$_{2}$ thin films have been coated \textit{in-situ }on the inner wall of a SRF cavity using the hybrid physical chemical vapor deposition (HPCVD) technique. The characterization of the coating will be presented. [Preview Abstract] |
Wednesday, March 20, 2013 3:18PM - 3:30PM |
R35.00005: Thickness dependence of superconducting properties in magnesium diboride thin films Douglas Beringer, Cesar Clavero, Teng Tan, Xiaoxing Xi, Rosa Lukaszew Thin film MgB$_{2}$ is a promising material currently researched for improvements in superconducting radio frequency (SRF) technology and applications. At present, bulk niobium SRF accelerating cavities suffer from a fundamental upper limit in maximally sustained accelerating gradients; however, a scheme involving multi-layered superstructures consisting of superconducting-insulating-superconducting (SIS) layers has been proposed to overcome this fundamental material limit of 50 MV/m. The SIS multi-layer paradigm is reliant upon implementing a thin shielding material with a suitably high Hc1 which may prevent early field penetration in a bulk material layer and consequently delay the high field breakdown. It has been predicted that for thin superconducting films --- thickness less than the London penetration depth ($\sim$ 140 nm in the case of MgB$_{2})$ --- the lower critical field Hc1 will be enhanced with decreasing thickness. Thus, MgB$_{2}$, with a high bulk Hc1 value is a prime candidate for such SIS structures. Here we present our study on the structure, surface morphology and superconducting properties on a series of MgB$_{2}$ thin films and correlate the effects of film thickness and surface morphology on Hc1. [Preview Abstract] |
Wednesday, March 20, 2013 3:30PM - 3:42PM |
R35.00006: High resolution $^{11}$B NMR of MgB$_2$ using cryogenic magic-angle spinning Raivo Stern, Peter Beckett, Mark S. Denning, Ivo Heinmaa, Mukesh C. Dimri, Edward A. Young, Marina Carravetta Static and magic-angle spinning (MAS) $^{11}$B NMR data at 4.7 T and 8.5 T have been obtained under cryogenic conditions on a diluted sample of magnesium diboride powder in the normal and superconducting state. We demonstrate that MAS NMR is possible on type-II superconductors despite the sample rotation. The data provide accurate information on the magnetic shift variation and longitudinal relaxation data down to a temperature of 8 K, with a resolution improvement over the entire temperature range. The onset of superconductivity is unaffected by the sample rotation, as revealed by a steep variation of the magnetic shift just below the critical temperature. [Preview Abstract] |
Wednesday, March 20, 2013 3:42PM - 3:54PM |
R35.00007: The full 3D electronic band structure of MgB2 determined by soft x-ray ARPES Yasmine Sassa, Martin Mansson, Bastian M. Wojek, Masaki Kobayashi, Olof Gotberg, Vladimir Strocov, Nikolai Zhigadlo, Oscar Tjernberg, Bertram Batlogg MgB$_{2}$ is a prototypical multi-band multi-gap superconductor with electron-phonon coupling driving T$_{c}$ up to 40~K. Surprisingly, the experimental knowledge of the electronic band structure is rather limited. Here, we present the first results of angle-resolved photoelectron spectroscopy (ARPES) studies on high quality MgB$_{2}$ single crystals, employing photons in the soft x-ray range with variable energy. We have been able to measure the band dispersion not only in the $k_{\rm x}$-$k_{\rm y}$ plane, but also probe in detail the $k_{\rm z}$ dependence and thus, the 3D nature of the bands. Furthermore, we have found the ARPES intensities to be strongly polarization dependent and their analysis provides an excellent agreement with the orbital nature of the electronic states. The calculated electronic band structure captures very well all the features revealed in our experiment. [Preview Abstract] |
Wednesday, March 20, 2013 3:54PM - 4:06PM |
R35.00008: Fragile Structure Transition in Mo3Sb7 J.-Q. Yan, M.A. McGuire, A.F. May, D.G. Mandrus, B.C. Sales Despite a relatively low superconducting transition temperature T$_{\mathrm{c}} =$ 2.08 K, the Zintl compound Mo$_{3}$Sb$_{7}$ has attracted considerable interest due to the possible involvement of magnetism in superconducting pairing, and promising thermoelectric performance with proper doping. Mo$_{3}$Sb$_{7}$ crystallizes in a Ir$_{3}$Ge$_{7}$-type cubic structure with space group Im3m at room temperature. A structure transition from cubic to tetragonal (I4/mmm) was observed at 53 K and this symmetry lowering is accompanied by the opening of a 120 K spin gap. Here, we will present the growth of Mo$_{3}$Sb$_{7}$ single crystals and our work in exploring the correlation between the low-temperature superconductivity, the structure transition, and the spin gap. The low-temperature superconductivity was observed in both the cubic and tetragonal phases. The structure transition was found to be extremely sensitive to Te or Ru substitution which shifts the Fermi level toward the valence band edge. Work at ORNL was supported by the U.S. Department of Energy, Basic Energy Sciences, Materials Sciences and Engineering Division. [Preview Abstract] |
Wednesday, March 20, 2013 4:06PM - 4:18PM |
R35.00009: Crystal growth, complex phase diagram and high pressure studies of layer compound PdBi$_{2}$ Kui Zhao, Xiyu Zhu, Bing Lv, Yuyi Xue, Paul Chu Among the different Pd-Bi Alloys, $\beta $-PdBi$_{2}$, which is crystallized in a layered tetragonal (I4/mmm) structure, has been identified as a superconductor with transition temperature at $\sim$ 5.4K. Band structure calculation indicates that the interlayer Bi-Bi bonds are weak but not negligible, which implies the 3D bonding character of this compound. In order to enhance or weaken the interlayer bonding and ultimately increase the Tc in this system, high pressure measurement, isovalent chemical substitution of Bi with Sb, and chemical intercalation using transition metal Cu and alkali metal Na, are applied to the system. Meanwhile, aliovalent chemical substitution on the Bi site by Pb is also carried out. The magnetic, electrical, and calorimetric properties of these compounds are determined at ambient pressure and compared. The detailed high pressure results and the complete phase diagram of chemical substitution and intercalation will be presented and discussed. [Preview Abstract] |
Wednesday, March 20, 2013 4:18PM - 4:30PM |
R35.00010: Superconductivity with extremely large upper critical fields in Nb$_{2}$Pd$_{0.81}$S$_{5}$ Qiu Run Zhang, Gang Li, Daniel Rhodes, Andhika Kiswandhi, Tiglet Basara, J Sung, Theo Siegrist, Michelle Johannes, Luis Balicas Here, we report the discovery of superconductivity in a new transition metal-chalcogenide compound, i.e. Nb$_{2}$Pd$_{0.81}$S$_{5}$, with a transition temperature $T_{c} \cong $ 6.6 K. Despite its relatively low $T_{c}$, it displays remarkably high and anisotropic superconducting upper critical fields, e.g. $\mu_{0}H_{c2}$ ($T \to $ 0 K) \textgreater\ 37 T for fields applied along the crystallographic $b$-axis. This value is considerably larger than the value reported for the technologically relevant Nb$_{3}$Sn compound ($\mu_{0}H_{c2}$ $\sim$ 30 T, with $T_{c} =$ 18 K)$^{1,2}$. Its ratio of $\mu_{0}H_{c2}$ ($T$ $\to $ 0 K) to $T_{c}$, is also larger than those of the new Fe based superconductors, e.g. $\beta $-FeSe (20 T/8.7 K)$^{3}$, Ba$_{1-x}$K$_{x}$Fe$_{2}$As$_{2}$ ($\sim$ 70 T/28 K)$^{4}$, and even higher than the reported ratio for the Chevrel-phase PbMo$_{6}$S$_{8}$(60T/13.3 K)$^{5}$ compound. For a field applied perpendicularly to the $b$-axis, $\mu_{0}H_{c2}$ shows a linear dependence in temperature which coupled to a temperature-dependent anisotropy of the upper critical fields, suggests that Nb$_{2}$Pd$_{0.81}$S$_{5}$ is a multi-band superconductor. This is confirmed by band structure calculations which reveal nearly cylindrical and quasi-one-dimensional Fermi surface sheets having hole and electron character, respectively. [Preview Abstract] |
Wednesday, March 20, 2013 4:30PM - 4:42PM |
R35.00011: Synthesis, structure, chemical doping and high pressure studies of the SrPt$_{3}$ P with unique structure features BenMaan Jawdat, Bing Lv, Xiyu Zhu, Yuyi Xue, Ching Chu Superconductivity up to 8.4K was reported by Takayama et al.\footnote{T. Takayama, K. Kuwano, D. Hirai, Y. Katsura, A. Yamamoto, and H. Takagi, Phys. Rev. Lett., 108, 237001(2012).} in APt$_{3}$P (A$=$Sr, Ca and La) in 2012 with structural information based only on X-ray powder refinement. The compounds are suggested to crystallize in an antiperovskite-based structure closely related to that of the heavy fermion superconductor CePt$_{3}$Si but are nonpolar unlike CePt$_{3}$Si. Both small single crystals and polycrystalline samples of SrPt$_{3}$P, the compound with the highest T$_{c}$ of this class of materials, are synthesized through solid state reactions. In this presentation, full and detailed structural information will be revealed based on X-ray single crystal analysis. Different chemical doping on different sites and high pressure studies have been carried out on the compound of SrPt$_{3}$P. The results and its implication will be presented and discussed. [Preview Abstract] |
Wednesday, March 20, 2013 4:42PM - 4:54PM |
R35.00012: Revealing the superconducting state of CaC6 by angle-resolved photoelectron spectrocopy Shuolong Yang, Jonathan Sobota, Chris Howard, Chris Pickard, Makoto Hashimoto, Donghui Lu, Sung-Kwan Mo, Mark Ellerby, Zhi-Xun Shen We studied the electronic band structure of CaC$_{6}$ using angle-resolved photoelectron spectroscopy (ARPES). We were able to make direct connections to the DFT calculation and identify various electron- and hole-pockets both at the $\Gamma$- and K-points. Most importantly, we convincingly observed the interlayer band, which was predicted to be responsible for superconductivity and to display a near-free-electron-like dispersion. The near-circular Fermi surface of the interlayer band is clearly separate from the carbon-derived bands, which enables a pocket-dependent superconducting gap analysis near the $\Gamma$-point. Distinct electron-phonon coupling regimes were observed for the interlayer and the carbon-derived bands using self-energy analysis in agreement with previous studies. [Preview Abstract] |
Wednesday, March 20, 2013 4:54PM - 5:06PM |
R35.00013: Superconductivity in layered cobaltates: a functional RG treatment Christian Platt, Maximilian Kiesel, Werner Hanke, Ronny Thomale The superconducting state of water-intercalated cobaltates is still poorly understood. Starting with an effective three orbital model which fits the experimentally observed Fermi surface, we apply the functional renormalization group and study the phase diagram of Na$_x$CoO$_2$ as a function of doping. Here, we find ferromagnetic and triplet-pairing tendencies near van-Hove filling as well as $(d+id)$-superconductivity for larger dopings. The calculated gap function in this $(d+id)$-phase reveals a near-nodal behavior, and an increased CoO$_2$ layer distance promotes the ferromagnetic and triplet-pairing channels. Our findings are consistent with recent experimental observations. The cobaltates thus establish a chiral singlet superconductor based on transition metal oxides. [Preview Abstract] |
Wednesday, March 20, 2013 5:06PM - 5:18PM |
R35.00014: Evidence for phase separation between the co-existing Density Wave and Superconducting orders in (TMTSF)$_2$PF$_6$ Arjun Narayanan, Paul Chaikin Resistance, Thermopower and Angular Dependent Magnetoresistance(AMRO) measurements were used to study the organic conductor (TMTSF)$_{2}$PF$_{6}$ at pressures where co-existence between Superconducting and Spin Density Wave orders occurs. While in other material families such coexistence is poorly understood, in (TMTSF)$_{2}$PF$_{6}$ a clear picture is emerging. Various suggestions had been made regarding the coexistence phase, including homogenous phases showing microscopic coexistence, Soliton walls in Density Waves, and phase separation between normal metal and density wave regions. \textit{We provide strong evidence for the phase separation scenario in (TMTSF)$_2$PF$_6$.} The existence of domains and their pattern of distribution are unambiguously evidenced by thermopower and resistivity anisotropies. The metallic domains are identified as the regular high pressure metal by various unique signatures such as Field Induced Density Waves(FISDW), AMRO and the superconducting Tc. Some surprising details of the evolution of FISDW and AMRO with pressure in the coexistence phase will also be discussed. [Preview Abstract] |
Wednesday, March 20, 2013 5:18PM - 5:30PM |
R35.00015: Electron correlations in C$_{60}$ and aromatic superconductors Yusuke Nomura, Kazuma Nakamura, Ryotaro Arita Recent discovery of superconductivity in fcc/A15 Cs$_3$C$_{60}$ under pressure and in aromatic compounds (ex. alkali-doped picene) has stimulated a renewed interest in molecular superconductors such as K$_3$C$_{60}$ and Rb$_3$C$_{60}$. To clarify the mechanism of the superconductivity, it is essential to understand low-energy electronic structure of these systems. In the present study, we perform a systematic study for understanding the relation between electronic correlation and superconductivity in C$_{60}$ and aromatic compounds [1]. We derived, from first principles, extended Hubbard models for twelve compounds: fcc K$_3$C$_{60}$, Rb$_3$C$_{60}$, Cs$_3$C$_{60}$ (with three different lattice constants), A15 Cs$_3$C$_{60}$ (with four different lattice constants), doped solid picene, coronene, and phenanthrene. We show that these compounds are strongly correlated and have similar energy scales of their bandwidths and interaction parameters. However, they have a different trend in the relation between the strength of the electronic correlation and superconducting-transition temperature. While the C$_{60}$ compounds have a positive correlation, the aromatic compounds exhibit a negative correlation. [1] Y. Nomura, K. Nakamura, and R. Arita, Phys. Rev. B ${\bf 85}$, 155452 (2012). [Preview Abstract] |
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