Session T41: Focus Session: Search for New Superconductors - Nanotubes and Fullerides

Search for Superconductivity in Charge Doped Composites of Carbon Nanotubes and Fullerides

The motivation of this study is to develop a highly sensitive method of microwave absorption in low magnetic fields (LFMA), combined with SQUID magnetometry, for searching for superconducting phases in nanocomposites in which carrier concentration is changed by in-situ doping. The unzipped ribbons of multiwalled carbon nanotubes (MWCNTs) (which are stacks of graphene ribbons) are infiltrated by $\mbox{C}_{60}$ fullerenes and doped chemically (by alkali metals) and electrochemically to modulate the carrier concentration. It is demonstrated that hysteretic LFMA appearing below $T_c$ is indeed a highly sensitive test for negligibly small superconducting (SC) phases (\mbox{$\sim10^{-11}\ \mbox{g}$}), and multi-phase SC composites with several $T_c$ can be detected and separated. At the same time, non-SC signals (which are quite similar to SC LFMA) have been found from samples containing magnetic nanoclusters (Fe, Ni/Co, etc.) which are catalyst residuals in CNT synthesis. The strategies of differentiating SC LFMA from magnetic LFMA by complementary SQUID/transport studies are suggested.   

1D goes 2D: A Kosterlitz Thouless transition in superconducting arrays of 4-Angstrom carbon nanotubes

We report superconducting resistive transition characteristics for array(s) of coupled 4-Angstrom single wall carbon nanotubes embedded in aluminophosphate-five (AFI) zeolite. The transition was observed to initiate at 15K with a slow resistance decrease switching to a sharp, order of magnitude drop between 7.5-6.0K with strong (anisotropic) magnetic field dependence. Both the sharp resistance drop and its attendant nonlinear IV characteristics are consistent with the manifestations of a Kosterlitz-Thouless (KT) transition that establishes quasi long range order in the plane transverse to the c-axis of the nanotubes, leading to an inhomogeneous system comprising 3D superconducting regions connected by weak links. Global coherence is established at below 5K with the appearance of a well-defined supercurrent gap at 2K.   

Raman signal analysis of SWCNT grown inside AlPO4-5 template with ethylene carbon source

We report on a new approach of growing carbon nanotubes inside the AFI zeolite template with ethylene as the carbon source, and the characterization of the resulting nanotube@AFI samples with Raman spectrum analysis and density functional theory simulations. Raman spectrum data of the ethylene-grown samples show many new features, and indicate that the new growth method enhances greatly the content of carbon nanotubes, with a much stronger RBM peak. Raman intensity can be simulated with density functional theory based method, for a direct comparison. These experimental and theoretical research methods, i.e. Raman spectrum, group theory analysis and density functional theory simulation, provide us with insight into the system, and help us to understand the novel superconducting behavior of the samples.   

Non-BCS superconductivity in fulleride superconductors

C$_{60}$-based solids are archetypal examples of molecular superconductors with T$_{c}$ as high as 33 K. T$_{c}$ of the face-centered cubic (fcc) A$_{3}$C$_{60}$ (A = alkali metal) fullerides increases monotonically with the interC$_{60}$ separation, which is in turn controlled by the sizes of the A$^{+}$ cations -- this physical picture has remained unaltered since 1992. Pressure-induced trace superconductivity (s/c fraction$<<$1{\%}) at $\sim $40 K was reported in 1995 in multiphase samples in the Cs$_{x}$C$_{60}$ phase field. Despite numerous attempts by many groups worldwide, this remained unverified and the structure and composition of the material responsible for superconductivity unidentified. This has hindered any attempt to push T$_{c}$ even higher and make contact with theory which predicts correlation-enhanced superconductivity for expanded fullerides near the metal-insulator transition. Here I will present our recent work in this field that led to the discovery of pressure-induced bulk superconductivity emerging out of a parent antiferromagnetic insulating state at the highest T$_{c}$ currently known for any molecular material [1,2].\\[4pt] [1] A.Y. Ganin et al. Nature Mater. 7, 367 (2008). \\[0pt] [2] Y. Takabayashi et al. Science 323, 1585 (2009).   

Electron-phonon coupling in A15 Cs$_3$C$_{60}$ and fcc alkali fullerides

We study the electron-phonon couplings of alkali-doped fullerides, fcc A$_3$C$_{60}$ (A=K,Rb) and A15 Cs$_3$C$_{60}$, using first-principles method based on the density functional theory. To improve the previous studies, we perform accurate calculations of phonon dispersion and electron-phonon coupling including momentum dependence and lattice symmetry. Furthermore, to estimate the superconducting transition temperature, we use Eliashberg equation instead of McMillan's formula. It is found that Eliashberg equation gives a quantitative improvement and that the obtained transition temperatures are reasonable compared to the experiments.   

Superconducting state of metallic clusters: Potential for room temperature superconductivity, nano-based tunneling networks

Nanoclusters form a new family of high temperature superconductors. We focus on small metallic nanoclusters M$_{n}$ (n is the number of atoms) which contain delocalized electrons. These electrons form energy shells similar to those in atoms or nuclei (e.g., s,p,d,..). The presence of the shell structure and the corresponding orbital degeneracy 2(2L+1) leads to an increase in the effective density of states and to a great strengthening of the pairing interaction. It turns out that under special, but perfectly realistic conditions, the superconducting pairing is very strong and leads to high T$_{c}$. For some specific clusters (e.g., Al$_{56}$, Zn$_{83}$) T$_{c}$ reaches $\sim$150 K and the energy spectrum becomes strongly modified. With a realistic sets of parameters, it should be possible to raise T$_{c}$ up to room temperature. Specific experiments capable of detecting this phenomenon can be identified (spectroscopic, magnetic and thermodynamic measurements). The observation of a heat capacity jump for Al$_{45}^{-}$ and Al$_{47}^{-}$ clusters at T$_{c}\sim $200K (Cao et al.,2008) yielded first experimental support for the phenomenon: the amplitude, width and position of the jumps are in good agreement with the theory. Pairing also raises the possibility of observing intercluster Josephson tunneling. The discrete nature of the spectrum makes the analysis very different from that for conventional superconductors. Especially interesting is the case of resonant tunneling. The effect is promising for the creation of superconducting tunneling networks at high T$_{c}$ (potentially at room temperatures), and with current densities greatly exceeding those of usual superconductors.   

Superconducting fluctuations in one-dimensional quasi-periodic ``metallic'' chains.

It is well known that a purely periodic chain of odd-electron atoms, nominally expected to exhibit metallic behavior, is unstable to charge/spin spatial displacement which lowers its ground state energy by gapping its multi-degenerate Fermi surface, in this case consisting of nesting parallel sheets. It is largely for these reasons that superconductivity is not observed in highly one-dimensional metals -- it is simply energetically more favorable for CDW/SDW gaps to form, rather than a BCS state, at least one mediated by electron-phonon coupling. In this talk, we explore the hypothetical electronic properties of a nominally ``metallic'' quasi-periodic chain using both an analytical approach and computationally with density functional theory, searching for configurations which yield ``gap-lets'' sufficiently small to permit the formation of BCS pairs as the new energetically favored ground state. The particular embodiment we examine is a string of aluminum atoms with interatomic spacings determined by a Fibonacci sequence. We propose a path to attempt synthesis of such a structure for experimental examination{\ldots}and perhaps leading to an entirely new class of higher temperature superconductors.   

Tuning Superconductivity in Two Dimensions with a Novel Metal-Graphene Hybrid Material

Using typical experimental techniques, such as chemical doping, it is difficult to isolate the effects of carrier density from disorder on a two-dimensional superconducting transition. To circumvent this problem, we have produced graphene sheets covered with a non-percolating network of nanoscale tin clusters. This network of disordered metal clusters efficiently dopes the graphene substrate and induces long-range superconducting correlations by injecting Cooper pairs into the graphene sheet. This allows us to study the superconducting transition at fixed disorder and variable carrier concentration by tuning the density of carriers via the field effect. We find that despite structural inhomogeneity on mesoscopic length scales (10-100 nm), this hybrid material behaves electronically as a homogenous dirty superconductor exhibiting separate amplitude and phase-ordering transitions. We identify a gate-voltage dependent transition of the Berezinskii-Kosterlitz-Thouless type and extract a vortex-antivortex dielectric constant higher than observed in similar systems. arXiv:0907.3661   

Pressure-induced superconductivity in thin films of boron-doped carbon nanotubes

We have reported that thin films of slightly boron-doped single-walled carbon nanotubes (B-SWNTs) can be superconductor at Tc of 12K [1]. Here, based on this, we show creation of paperlike thin film (Buckypaper) consisting of pseudo-two-dimensional network of B-SWNTs within weakly intertube van der Waals coupling (IVDWC) state. It was formed by sufficiently dissolving as-grown ropes of B-SWNTs and densely assembling them on silicon substrate. We find that superconducting transition temperature Tc of 8 K under absent pressure can be induced up to 19 K by applying a small pressure to the film and that a frequency in the radial breathing phonon drastically increases with applying pressure [2]. Discussion about IVDWC and distribution of B-SWNTs diameter imply the strong correlation. References [1] N. Murata, J. Haruyama, J. Reppert, A. M. Rao, T. Koretsune, S. Saito, Phys. Rev. Lett. 101, 027002 (2008) [2] J. Nakamura, J. Haruyama, M. Tachibana, J. Reppert,A. Rao, H. Sano, Y. Iye et al., Appl.Phys.Lett. 95, 142503 (2009)