Perfect and defective surface structures for (001) SrTiO3 are determined in considering a supercell with 11 atomic layers using the first-principles calculations. The amplitude of the surface rumpling for the SrO-terminated surface is much larger than that for the TiO2-terminated surface, although both SrO- and TiO2-terminated surfaces are stable for a comparable range of the TiO2 chemical potential. The distance between the first and second planes compresses while that of the second and third planes expands due to the relaxation of the slab. The top sites of the oxygen atoms of SrO-termined surface and the fourfold symmetry hollow sites of TiO2-termined surface are favorable for Ti or Sr adsorbate. The relative stability of the defect species or reactions varies with the equilibrium conditions. The dominant surface defect is Ti substitutional defect and a TiO–TiO2 double layers may form at the surface, which has been confirmed by experiments.

The electronic structures and thermoelectric properties of semiconducting transition-metal dichalcogenide monolayers MX2 (M=Zr, Hf, Mo, W and Pt; X=S, Se and Te) are investigated by combining first-principles and Boltzmann transport theory, including spin-orbital coupling (SOC). It is found that the gap decrease increases from S to Te in each cation group, when the SOC is opened. The spin-orbital splitting has the same trend with gap reducing. Calculated results show that SOC has noteworthy detrimental effect on p-type power factor, while has a negligible influence in n-type doping except W cation group, which can be understood by considering the effects of SOC on the valence and conduction bands. For WX2 (X=S, Se and Te), the SOC leads to observably enhanced power factor in n-type doping, which can be explained by SOC-induced band degeneracy, namely bands converge. Among all cation groups, Pt cation group shows the highest Seebeck coefficient, which leads to best power factor, if we assume scattering time is fixed. Calculated results show that MS2 (M=Zr, Hf, Mo, W and Pt) have best p-type power factor for all cation groups, and that MSe2 (M=Zr and Hf), WS2 and MTe2 (M=Mo and Pt) have more wonderful n-type power factor in respective cation group. Therefore, these results may be useful for further theoretical prediction or experimental search of excellent thermoelectric materials from semiconducting transition-metal dichalcogenide monolayers.

Ruthenium-based perovskite systems are attractive because their structural, electronic and mag-12 netic properties can be systematically engineered. SrRuO 3 /SrTiO 3 superlattice, with its period

Pressure dependence of electronic structures and thermoelectric properties of Mg2Sn 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 Γ 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 Mg2Sn can be improved in n-type doping by pressure. Thermoelectric material by using the Seebeck effect can convert waste heat directly to electricity to solve energy problems. The performance of thermoelectric material can be characterized by dimensionless figure of merit [1,2], ZT = S 2 σT /(κ e + κ L ), where S, σ, T, κ e and κ L are the Seebeck coefficient, electrical conductivity, absolute temperature, the electronic and lattice thermal conductivities, respectively. Bismuthtellurium systems [3,4], silicon-germanium alloys [5,6], lead chalcogenides [7,8] and skutterudites [9,10] have been identified as excellent thermoelectric material for thermoelectric devices. For thermoelectric research, the main objective is to search for high ZT materials, which has proven to be interesting and challenging.The thermoelectric material Mg 2 X(X = Si, Ge, Sn) composed of abundant, low-cost elements and their alloys have attracted much recent attention [11][12][13], and various doping strategies have been adopted to attain high ZT [14][15][16]. Pressure by tuning the electronic structures of materials can accomplish many interesting phenomenons like recent pressure-induced high-Tc superconductivity in (H 2 S) 2 H 2 [17,18]. Here, we use first-principle calculations and Boltzmann transport theory to address the pressure dependence of thermoelectric properties in the Mg 2 Sn. Calculated results show that the pressure dependence of energy band gap with mBJ+SOC is consistent with one with mBJ [19], and first increases, and then decreases. Pressure can significantly improve power factor in n-type doping below the critical pressure. It is found that pressure can reduce the lattice thermal conductivity in certain pressure range. These lead to enhanced ZT , and make Mg 2 Sn become more efficient for thermoelectric application in n-type doping by pressure. So, pressure tuning offers a very effective method to search for materials with enhanced thermoelectric properties.The rest of the paper is organized as follows. Firstly, we shall give our computational details. Secondly, weEnergy ( The energy band gap (Gap) and the value of spin-orbit splitting at Γ point (∆so) as a function of pressure...