Cluster-based Superconducting Tunneling Networks

A 2D tunneling network consisting of nanoclusters placed on a surface is studied. It is shown that such a network is capable of transferring large supercurrent at high temperatures. For a realistic set of parameters the damping is quite small, and the smallness is due to strong renormalization of the capacitance of a cluster. The critical field also turns out to be larComment: 18 page

Shell Structure and Strengthening of Superconducting Pair Correlation in Nanoclusters

The existence of shell structure and the accompanying high degeneracy of electronic levels leads to the possibility of strong superconducting pairing in metallic nanoclusters with N~100-1000 delocalized electrons. The most favorable cases correspond to (a) "magic" clusters with strongly degenerate highest occupied and lowest unoccupied shells and a relatively small energy spacing between them as well as to (b) clusters with slightly incomplete shells and small Jahn-Teller splitting. It is shown that realistic sets of parameters lead to very high values of Tc as well as to a strong alteration of the energy spectrum. The impact of fluctuations is analyzed. Spectroscopic experiments aimed at detecting the presence of pair correlations are proposed. The pairing should also manifest itself via odd-even effects in cluster spectra, similar to the case of nuclei

Crystal Structures and Electronic Properties of Haloform-Intercalated C60

Using density functional methods we calculated structural and electronic properties of bulk chloroform and bromoform intercalated C60, C60 2CHX3 (X=Cl,Br). Both compounds are narrow band insulator materials with a gap between valence and conduction bands larger than 1 eV. The calculated widths of the valence and conduction bands are 0.4-0.6 eV and 0.3-0.4 eV, respectively. The orbitals of the haloform molecules overlap with the $\pi$ orbitals of the fullerene molecules and the p-type orbitals of halogen atoms significantly contribute to the valence and conduction bands of C60 2CHX3. Charging with electrons and holes turns the systems to metals. Contrary to expectation, 10 to 20 % of the charge is on the haloform molecules and is thus not completely localized on the fullerene molecules. Calculations on different crystal structures of C60 2CHCl3 and C60 2CHBr3 revealed that the density of states at the Fermi energy are sensitive to the orientation of the haloform and C60 molecules. At a charging of three holes, which corresponds to the superconducting phase of pure C60 and C60 2CHX3, the calculated density of states (DOS) at the Fermi energy increases in the sequence DOS(C60) < DOS(C60 2CHCl3) < DOS(C60 2CHBr3).Comment: 11 pages, 7 figures, 4 table

Hydrogen Pentagraphenelike Structure Stabilized by Hafnium: A High-Temperature Conventional Superconductor

The recent discovery of H3S and LaH10 superconductors with record high superconducting transition temperatures Tc at high pressure has fueled the search for room-temperature superconductivity in the compressed superhydrides. Here we introduce a new class of high Tc hydrides with a novel structure and unusual properties. We predict the existence of an unprecedented hexagonal HfH10, with remarkably high value of Tc (around 213–234 K) at 250 GPa. As concerns the novel structure, the H ions in HfH10 are arranged in clusters to form a planar “pentagraphenelike” sublattice. The layered arrangement of these planar units is entirely different from the covalent sixfold cubic structure in H3S and clathratelike structure in LaH10. The Hf atom acts as a precompressor and electron donor to the hydrogen sublattice. This pentagraphenelike H10 structure is also found in ZrH10, ScH10, and uH10 at high pressure, each material showing a high Tc ranging from 134 to 220 K. Our study of dense superhydrides with pentagraphenelike layered structures opens the door to the exploration of a new class of high Tc superconductors