Room temperature Peierls distortion in small radius nanotubes

By means of {\it ab initio} simulations, we investigate the phonon band structure and electron-phonon coupling in small 4-\AA diameter nanotubes. We show that both the C(5,0) and C(3,3) tubes undergo above room temperature a Peierls transition mediated by an acoustical long-wavelength and an optical $q=2k_F$ phonons respectively. In the armchair geometry, we verify that the electron-phonon coupling parameter $\lambda$ originates mainly from phonons at $q=2k_F$ and is strongly enhanced when the diameter decreases. These results question the origin of superconductivity in small diameter nanotubes.Comment: submitted 21oct2004 accepted 6jan2005 (Phys.Rev.Lett.

The role of the dopant in the superconductivity of diamond

We present an {\it ab initio} study of the recently discovered superconductivity of boron doped diamond within the framework of a phonon-mediated pairing mechanism. The role of the dopant, in substitutional position, is unconventional in that half of the coupling parameter $\lambda$ originates in strongly localized defect-related vibrational modes, yielding a very peaked Eliashberg $\alpha^2F(\omega)$ function. The electron-phonon coupling potential is found to be extremely large and T$_C$ is limited by the low value of the density of states at the Fermi level

Electrical transport properties of bulk MgB2 materials synthesized by the electrolysis on fused mixtures of MgCl2, NaCl, KCl and MgB2O4

Electrolysis was carried out on fused mixtures of MgCl2, NaCl, KCl and MgB2O4 under an Ar flow at 600C. Electrical resistivity measurements for the grown deposits show an onset of superconducting transition at 37 K in the absence of applied magnetic field. The resistivity decreases down to zero below 32 K. From an applied-field dependence of resistivity, an upper critical field and a coherence length were calculated to be 9.7 T and 5.9 nm at 0 K, respectively

Suppression of backward scattering of Dirac fermions in iron pnictides Ba(Fe1−x_{1-x}Rux_xAs)2_2

We report electronic transport of Dirac cones when Fe is replaced by Ru, which has an isoelectronic electron configuration to Fe, using single crystals of Ba(Fe$_{1-x}$Ru$_x$As)$_2$. The electronic transport of parabolic bands is shown to be suppressed by scattering due to the crystal lattice distortion and the impurity effect of Ru, while that of the Dirac cone is not significantly reduced due to the intrinsic character of Dirac cones. It is clearly shown from magnetoresistance and Hall coefficient measurements that the inverse of average mobility, proportional to cyclotron effective mass, develops as the square root of the carrier number (n) of the Dirac cones. This is the unique character of the Dirac cone linear dispersion relationship. Scattering of Ru on the Dirac cones is discussed in terms of the estimated mean free path using experimental parameters.Comment: 6 pages, 3 figures, To be published in Phys. Rev.

Electron and hole Dirac cone states in-pairs in Ba(FeAs)2_2 confirmed by magnetoresistance

The quantum transport of Dirac cone states in the iron pnictide Ba(FeAs)$_2$ with a d-\,multiband system is studied by using single crystal samples. The transverse magnetoresistance develops linearly against magnetic field at low temperatures. The transport phenomena are interpreted in terms of the 0$^{th}$ Landau level by applying the theory predicted by Abrikosov. The results of the semiclassical analyses of a two carrier system under low magnetic field limit show that both electron and hole reside as the high mobility states, being indicative to the fact that both electron- and hole Dirac cone states should be taken into account in pairs for having the real interpretation of low temperature electronic states in iron pnictides, being in contrast to the previous reports.Comment: 4 pages, 2 figure