Pressure enhanced thermoelectric properties in Mg2Sn

Pressure dependence of electronic structures and thermoelectric properties of $\mathrm{Mg_2Sn}$ are investigated by using a modified Becke and Johnson (mBJ) exchange potential, including spin-orbit coupling (SOC). The corresponding value of spin-orbit splitting at $\Gamma$ point is 0.47 eV, which is in good agreement with the experimental value 0.48 eV. With the pressure increasing, the energy band gap first increases, and then decreases. In certain doping range, the power factor for n-type has the same trend with energy band gap, when the pressure increases. Calculated results show that the pressure can lead to significantly enhanced power factor in n-type doping below the critical pressure, and the corresponding lattice thermal conductivity near the critical pressure shows the relatively small value. These results make us believe that thermoelectric properties of $\mathrm{Mg_2Sn}$ can be improved in n-type doping by pressure.Comment: 4 pages, 6 figure

Nonequilibrium current driven by a step voltage pulse: an exact solution

One of the most important problems in nanoelectronic device theory is to estimate how fast or how slow a quantum device can turn on/off a current. For an arbitrary noninteracting phase-coherent device scattering region connected to the outside world by leads, we have derived an exact solution for the nonequilibrium, nonlinear, and time-dependent current driven by both up- and down-step pulsed voltages. Our analysis is based on the Keldysh nonequilibrium Green's functions formalism where the electronic structure of the leads as well as the scattering region are treated on an equal footing. A model calculation for a quantum dot with a Lorentzian linewidth function shows that the time-dependent current dynamics display interesting finite-bandwidth effects not captured by the commonly used wideband approximation

Structure and Dielectric Properties of Amorphous High-kappa Oxides: HfO2, ZrO2 and their alloys

High-$\kappa$ metal oxides are a class of materials playing an increasingly important role in modern device physics and technology. Here we report theoretical investigations of the properties of structural and lattice dielectric constants of bulk amorphous metal oxides by a combined approach of classical molecular dynamics (MD) - for structure evolution, and quantum mechanical first principles density function theory (DFT) - for electronic structure analysis. Using classical MD based on the Born-Mayer-Buckingham potential function within a melt and quench scheme, amorphous structures of high-$\kappa$ metal oxides Hf$_{1-x}$Zr$_x$O$_2$ with different values of the concentration $x$, are generated. The coordination numbers and the radial distribution functions of the structures are in good agreement with the corresponding experimental data. We then calculate the lattice dielectric constants of the materials from quantum mechanical first principles, and the values averaged over an ensemble of samples agree well with the available experimental data, and are very close to the dielectric constants of their cubic form.Comment: 5 pages, 4 figure

Enhancement of parametric pumping due to Andreev reflection

We report properties of parametric electron pumping in the presence of a superconducting lead. Due to a constructive interference between the direct reflection and the multiple Andreev reflection, the pumped current is greatly enhanced. For both quantum point contacts and double barrier structures at resonance, we obtain exact solutions in the weak pumping regime showing that $I_p^{NS} = 4 I_p^N$, which should be compared with the result of conductance $G_{NS} = 2G_N$. Numerical results are also provided for the strong pumping regime showing interesting Andreev assisted pumping behaviour