We have studied the variation of superconducting critical temperature Tc as a function of charge density and lattice parameters in Mg1-xAlxB2 superconducting samples. The AB2 heterostucture of metallic boron layers (intercalated by A = magnesium, aluminum layers, playing the role of spacers) is made by direct chemical reaction. The spacing between boron layers and their charge density are controlled by chemical substitution of Mg by Al atoms. We show that high Tc superconductivity is realized by tuning the chemical potential at a `shape resonance' according with the patent for `high-temperature superconductors made by metal heterostructures at the atomic limit'. The energy width of the superconducting shape resonance is found to be about 400 meV.

Here we report synthesis and characterization of Mg1−xScxB2 (0.12Tc>6 K. We find that the Sc doping moves the chemical potential through the 2D/3D electronic topological transition (ETT) in the σ band where the "shape resonance" of interband pairing occurs. In the 3D regime beyond the ETT we observe a hardening of the E2g Raman mode with a significant line-width narrowing due to suppression of the Kohn anomaly over the range 0

The experimental determination of the scaling of the superconducting critical temperature (T-c) vs the Fermi temperature (T-f) of the holes in the boron sigma subband is presented. The Fermi level has been tuned near the "shape resonance," i.e., the two- to three-dimensional crossover of the Fermi surface of the boron sigma subband by changing the Al/Mg content in Al1-xMgxB2. The product k(f)xi(0) of the Fermi wave vector (k(f)) times the superconducting Pippard coherence length (xi(0)), that is a measure of the pairing strength, remains constant, k(f)xi(0)=90 for x>0.66. This high-T-c phase occurs in the boron superlattice under a tensile microstrain in the range 3%

Thermal conductivity of superconducting MgB 2 was studied in both the superconducting and the normal state region. The latter is almost equally determined by the electronic -and the lattice contribution to the total thermal conductivity. In the superconducting state, however, the lattice contribution is larger. The electronic thermal conductivity below T c was derived from the experimental data considering the Bardeen-Rickayzen-Tewordt theory together with the model of Geilikman. The analysis shows that electron scattering on static imperfections dominates.

Various concentrations of the solid solution Yb2Pd2In1−xSnx ranging from x = 0 to 1 with the tetragonal Mo2FeB2-type structure were prepared and their physical properties were studied by means of bulk and spectroscopic measurements as a function of temperature, pressure and magnetic fields. Results deduced from these studies indicate a slight variation of the valency of the Yb ions, from for both border compounds to ν = 3 at x = 0.6. This variation gives rise to the appearance of long range magnetic order in a narrow concentration region around x = 0.6, while both border compounds remain non-magnetic. The outstanding concentration dependence for the ordered regime may be indicative of two quantum critical points, a novel feature not yet observed in Yb compounds.