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首页> 外文期刊>Journal of Superconductivity and Novel Magnetism >Comparative Experimental and Density Functional Theory (DFT) Study of the Physical Properties of MgB_2 and AlB_2
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Comparative Experimental and Density Functional Theory (DFT) Study of the Physical Properties of MgB_2 and AlB_2

机译：MgB_2和AlB_2物理性质的比较实验和密度泛函理论（DFT）研究

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In the present study, we report an intercompari-son of various physical and electronic properties of MgB_2 and AlB_2. In particular, the results of phase formation, resistivity ρ(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 A, and c = 3.524 A of MgB_2 shrinks in indirection for AlB_2 with a = 3.006 A, and c = 3.254 A. The resistivity ρ(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 AIB2. This is consistent with our band structure plots. We fitted the experimental specific heat of MgB_2 to Debye-Einstein model and estimated the value of Debye temperature (ΘD) and Sommerfeld constant (γ) for electronic specific heat. Further, from γ, 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 α-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 α 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.
机译：在本研究中，我们报告了MgB_2和AlB_2的各种物理和电子性质的相互比较。特别是，报告了相形成，电阻率ρ（T），热电功率S（T），磁化强度M（T），热容（C_p）和电子能带结构的结果。对于a = 3.083 A和c = 3.524 A的MgB_2的原始拉伸六角形晶格，对于AlB_2在a = 3.006 A和c = 3.254 A的方向上间接收缩。电阻率ρ（T），热电功率S（T）和磁化强度M（T）测量显示MgB_2在39 K下具有超导性。没有观察到AlB_2的超导性。有趣的是，对于MgB_2，S（T）的符号为+ ve，对于AIB2，其符号为-ve。这与我们的能带结构图一致。我们将MgB_2的实验比热拟合到Debye-Einstein模型，并估计了电子比热的Debye温度（ΘD）和Sommerfeld常数（γ）的值。此外，根据γ，计算出费米能级N（E_F）下的状态的电子密度（DOS）。从实验N（E_F）的比率和DFT计算得出的比率，我们得出λ值为1.84，从而将MgB_2置于强耦合BCS类别中。 MgB_2的电子比热也使用α模型拟合到T_c之下，发现它是一个两间隙超导体。两个缺口的计算值与先前的报告非常吻合。我们的结果清楚地表明，MgB_2的超导性是由于其拉伸晶格中很大的声子贡献。 AlB_2中显然缺少相同的两个效应，因此它不是超导的。 DFT计算表明，对于MgB_2，大多数状态来自硼的α和π2p状态，而AlB_2的费米能级不存在σ带。这导致弱的电子声子耦合，并且还导致空穴不足，因为已知π带是电子类型的，因此很明显，AlB_2不是超导的。 DFT计算与所研究的硼化物，即MgB_2和AlB_2的测得的物理性质一致。

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