Comparative Experimental and Density Functional Theory (DFT) Study of the Physical Properties of MgB2 and AlB2

Abstract

In present study, we report an inter-comparison of various physical and electronic properties of MgB 2 and AlB 2 . In particular the results of phase formation, resistivity (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 Å, and c = 3.524 Å of MgB 2 shrinks in c-direction for AlB 2 with a = 3.006 Å, and c = 3.254 Å. The resistivity (T), thermoelectric power S(T) and magnetization M(T) measurements exhibited superconductivity at 39 K for MgB 2 . Superconductivity is not observed for AlB 2 . Interestingly, the sign of S(T) is +ve for MgB 2 the same is -ve for AlB 2 . This is consistent our band structure plots. We fitted the experimental specific heat of MgB 2 to Debye Einstein model and estimated the value of Debye temperature ( D ) and Sommerfeld constant ( ) for electronic specific heat. Further, from 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 λ to be 1.84, thus placing MgB 2 in the strong coupling BCS category. The electronic specific heat of MgB 2 is also fitted below T c using -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 MgB 2 is due to very large phonon contribution from its stretched lattice. The same two effects are obviously missing in AlB 2 and hence it is not superconducting. DFT calculations demonstrated that for MgB 2 the majority of states come from σ and π 2p states of boron on the other hand σ band at Fermi level for AlB 2 is absent. This leads to a weak electron phonon coupling and also to hole deficiency as π bands are known to be of electron type and hence obviously the AlB 2 is not superconducting. The DFT calculations are consistent with the measured physical properties of the studied borides, i.e., MgB 2 and AlB 2

Phonon anomalies predict superconducting Tcfor AlB2-type structures

“…For example, the PD anomaly is deep when the cusp size in the electronic band is large. If the cusp dips below the Fermi level, for example as with AlB 2 , the phonon anomaly does not occur; this is consistent with experimental data 49 that show no superconductivity for this composition. In addition, for superlattice constructs, multiple parallel cusps that intersect with the Fermi level occur.…”

Coexistence of four-band nodal rings and triply degenerate nodal points in centrosymmetric metal diborides

“…Especially, when spin-orbit coupling (SOC) is included, the TNP here transforms into a Dirac point that is close to the borderline between the type-I and type-II Dirac point categories. We clarify the symmetry protection of these novel band-crossing features and construct effective models for describing the [42][43][44] , and also excellent thermoelectricity (MgB 2 , and AlB 2 ) 45,46 . Thus our work reveals a promising platform for studying the interplay between these exotic properties and the topological band-crossings.…”

Effect of different-sized h-BN nano-particles on some properties of MgB2 superconductors

“…Addition of different sized h-BN did not affect the critical transition temperature of the superconducting MgB 2 sample significantly and the Table 1. An increment of DT may be attributed to the weak coupling between grains and insufficient crystallinity, accordingly critical current density values of the doped samples are lower in comparison to un-doped MgB 2 [33][34][35].…”