Inversion of two-band superconductivity at the critical electron doping of (Mg,Al)B-2

Electron energy-loss spectroscopy (EELS) was combined with heat capacity measurements to probe changes of electronic structure and superconductivity in Mg(1-x)AlxB2. A simultaneous decrease of EELS intensity from sigma-band hole states and the magnitude of the sigma gap was observed with increasing x, thus verifying that band filling results in the loss of strong superconductivity. These quantities extrapolated to zero at x approximate to 0.33 as inferred from the unit cell volume. However, superconductivity was not quenched completely, but persisted with T-c< 7 K up to about x approximate to 55. Only the pi band had detectable density of states for 0.33 less than or similar to x less than or similar to 0.55, implying an inversion of the two-band hierarchy of MgB2 in that regime. Since pi-band superconductivity is active in other materials such as intercalated graphite, implications for new materials with high T-c are discussed

Scattering theory of interface resistance in magnetic multilayers

The scattering theory of transport has to be applied with care in a diffuse environment. Here we discuss how the scattering matrices of heterointerfaces can be used to compute interface resistances of dirty magnetic multilayers. First principles calculations of these interface resistances agree well with experiments in the CPP (current perpendicular to the interface plane) configuration.Comment: submitted to J. Phys. D (special issue at the occasion of Prof. T. Shinjo's 60th birthday

Phonons and specific heat of linear dense phases of atoms physisorbed in the grooves of carbon nanotube bundles

The vibrational properties (phonons) of a one-dimensional periodic phase of atoms physisorbed in the external groove of the carbon nanotube bundle are studied. Analytical expressions for the phonon dispersion relations are derived. The derived expressions are applied to Xe, Kr and Ar adsorbates. The specific heat pertaining to dense phases of these adsorbates is calculated.Comment: 4 PS figure

Electron doping in MgB2 studied by electron energy-loss spectroscopy

The electronic structure of electron-doped polycrystalline Mg1-xAlx(B1-yCy)(2) was examined by electron energy-loss spectroscopy (EELS) in a scanning transmission electron microscope (STEM) and first-principle electronic structure calculations. We found significant changes in the boron K edge fine structure, suggesting the two bands of the B K edge, the sigma and the pi band, are being simultaneously filled as the electron doping concentration of Mg1-xAlx(B1-yCy)(2) was increased. Our density-functional theory calculations confirm the filling of the sigma band states close to the Fermi level, which is believed to cause the loss of superconductivity in highly doped MgB2, since the electron-phonon coupling of these states is thought to be responsible for the high superconducting transition temperature. Our results do not show significant differences in the electronic structure for electron doping on either the Mg or the B site, although many superconducting properties, such as T-c or H-c2 differ considerably for C- and Al-doped MgB2. This behavior cannot be satisfactorily explained by band filling alone, and effects such as interband scattering are considered

Specific heats of dilute neon inside long single-walled carbon nanotube and related problems

An elegant formula for coordinates of carbon atoms in a unit cell of a single-walled nanotube (SWNT) is presented and the potential of neon (Ne) inside an infinitely long SWNT is analytically derived out under the condition of the Lennard-Jones potential between Ne and carbon atoms. Specific heats of dilute Ne inside long (20, 20) SWNT are calculated at different temperatures. It is found that Ne exhibits 3-dimensional (3D) gas behavior at high temperature but behaves as 2D gas at low temperature. Especially, at ultra low temperature, Ne inside (20, 20) nanotubes behaves as lattice gas. A coarse method to determine the characteristic temperature $\mathcal{T}_c$ for low density gas in a potential is put forward. If $\mathcal{T}\gg \mathcal{T}_c$, we just need to use the classical statistical mechanics without solving the Shr\"{o}dinger equation to consider the thermal behavior of gas in the potential. But if $\mathcal{T}\sim \mathcal{T}_c$, we must solve the Shr\"{o}dinger equation. For Ne in (20,20) nanotube, we obtain $\mathcal{T}_c\approx 60$ K.Comment: 14 pages, 7 figure

The Effect of CoPt Crystallinity and Grain Texturing on Properties of Exchange-Coupled Fe/CoPt Systems

The effect of the crystallinity and the grain texturing of CoPt hard layers on exchange coupled Fe/CoPt soft/hard magnetic systems was studied using gradient thickness multilayer thin films. We have studied the hard layer structures by transmission electron microscopy and x-ray diffraction, and characterized the exchange coupling interaction through magnetization loops obtained by the magneto-optical Kerr effect measurement. We found that exchange coupling strongly depends on the crystalline characteristics of the CoPt hard layer. There is correlation between the mixture of different grain orientations of the CoPt hard layer and coupling efficiency. In particular, interlayer coupling is enhanced when there is only one orientation, namely, the L10 CoPt structure with its c-axis inclined at 45° with respect to the substrate plane. An increased degree of mixture of the latter with the in-plane-c-axis L10 CoPt structure is detrimental to obtaining one-phase-like magnetization loops. The present work points to the importance of controlling the crystalline properties of the hard layer in order to enhance the maximum energy product (BH)max in hard/soft nanocomposite magnets

nanocomposite magnets

Experimental evidence of dipolar interaction in bilaye