Abstract
In the present study, we report an intercomparison of various physical and electronic properties of MgB2 and AlB2. In particular, the results of phase formation, resistivity rho(T), thermoelectric power S(T), magnetization M(T), heat capacity (C (P) ), and electronic band structure are reported. The original stretched hexagonal lattice with a=3.083 , and c=3.524 of MgB2 shrinks in c-direction for AlB2 with a=3.006 , and c=3.254 . The resistivity rho(T), thermoelectric power S(T) and magnetization M(T) measurements exhibited superconductivity at 39 K for MgB2. Superconductivity is not observed for AlB2. Interestingly, the sign of S(T) is +ve for MgB2 the same is -ve for AlB2. This is consistent with our band structure plots. We fitted the experimental specific heat of MgB2 to Debye-Einstein model and estimated the value of Debye temperature (I similar to (D)) and Sommerfeld constant (gamma) for electronic specific heat. Further, from gamma, the electronic density of states (DOS) at Fermi level N(E (F)) is calculated. From the ratio of experimental N(E (F)) and the one being calculated from DFT, we obtained value of lambda to be 1.84, thus placing MgB2 in the strong coupling BCS category. The electronic specific heat of MgB2 is also fitted below T (c) using alpha-model and found that it is a two gap superconductor. The calculated values of two gaps are in good agreement with earlier reports. Our results clearly demonstrate that the superconductivity of MgB2 is due to very large phonon contribution from its stretched lattice. The same two effects are obviously missing in AlB2, and hence it is not superconducting. DFT calculations demonstrated that for MgB2, the majority of states come from sigma and pi 2p states of boron on the other hand sigma band at Fermi level for AlB2 is absent. This leads to a weak electron phonon coupling and also to hole deficiency as pi bands are known to be of electron type, and hence obviously the AlB2 is not superconducting. The DFT calculations are consistent with the measured physical properties of the studied borides, i.e., MgB2 and AlB2.