Theoretical Investigation On Electric And Magnetic Properties Of Gapless Semiconductor PbPdO₂

PbPdO₂ is considered as a new gapless semiconductor,in which the unique band structure makes it highly sensitive to external effects such as temperature,electric field,current and magnetic field.Meanwhile,the use of transition family metal elements(e.g.,Co?Mn?Ni)to replace metal lattice sites in PbPdO₂ or the use of oxygen vacancy regulation can make semiconductors produce novel physical properties,such as peculiar colossal electroresistance and giant magnetoresistance effects,which will strongly promotes the wide application of PbPdO₂ semiconductors in the field of spin electronic devices and new energy materials.Therefore,in recent years,PbPdO₂ gapless semiconductor materials have attracted extensive attention as far.In light of the novel electrical and magnetic properties of PbPdO₂-based semiconductors,the first-principle density functional theory(DFT)and Monte Carlo simulation were applied to investigate the electrical and magnetic behaviors of PbPdO₂-based semiconductors with different microstructure.The main contents of our work are as follows:(1)The band structure and atomic structure stability of bulk PbPdO₂ materials were investigated.The phonon spectrum of equilibrium structure for PbPdO₂was calculated.The obtained results indicate that equilibrium structure of PbPdO₂was phonon spectrum without imaginary-frequency component and also the lattice constant of PbPdO₂with stable structure was well consistent with that of experimental results.When there exists strain along a,b,and c axis(i.e.,lattice constant deviation),metal-insulator transition occurs in the electronic band structure of PbPdO₂.However,there was imaginary-frequency component in phonon spectra and the corresponding structure of PbPdO₂becomes unstable.The calculated results indicate that the physical properties of PbPdO₂were extremely sensitive to the variation in lattice constants.We can expect that the PbPdO₂-based semiconductors with excellent structural stability and performances could be synthesized by optimized preparation process including doping defect and calcination temperature.(2)The electronic band structure of PbPd_0.875Co_0.125O₂was investigated,in which Co atoms substitute different Pd-lattice in PbPdO₂.It was found that the insulating antiferromagnetic ground state of Co atoms was induced by the indirect exchange effect of Co-O-Co(180°).If the band crossover and p-d coupling occur under a certain molecular field,Co-doped PbPdO₂ semiconductors would exhibit metallic ferromagnetism.Additionally,Monte Carlo simulations were also employed to study the potential mechanism behind the ferromagnetism of PbPd_0.875Co_0.125O₂.Our work can well explain the high-T_c ferromagnetism of PbPd_0.875Co_0.125O₂ diluted magnetic semiconductors,which can provide a new route for designing high-T_c spin electronic devices.(3)The electronic structure and electrical properties of PbPdO₂ and PbPd_0.75Co_0.25O₂ultrathin slabs with(002)preferred orientations were systematically investigated using first-principle calculation method.The obtained results show that the change in the band structure of PbPdO₂was strongly dependent on the bond angle and bond length.It was found that the uniaxial strain along different direction in two dimensional planes induces the anisotropic variety of charge carrier concentrations.Interestingly,the carrier concentrations in the PbPd_0.75Co_0.25O₂slab could increase up to 5-6 orders of magnitude under the external strain.Our results could well explain the novel electrical behaviors of PbPdO₂-based semiconductors under large strain,which further confirms the promising candidate of for PbPdO₂-based semiconductors as piezoresistance material.(4)The modulation on the electronic structure of two-dimensional PbPdO₂ slab with(002)preferred orientation was systematically investigated using the first-principles calculation.The calculated results indicate that PbPdO₂ ultrathin slab possesses a small indirect gap while an indirect-direct band gap transition occurs when a moderate 2%compression or tensile strain was applied on the slab.Moreover,indirect-direct band gap transition induced by strain effect was also studied based on the charge density difference at different point of valence band,which could be explained by the charge transfer and charge polarization barrier resulting from the change in bond length and angle of Pd-O bonding under the strain.In the case of PbPdO₂ slab with(002)preferred orientation,the carrier mobilities of electrons and holes were calculated as 11645.31 and 694.60 cm²V^-1s^-1along the x-axis direction,935.05 and 16.05 cm²V^-1s^-1 along the y-axis direction,respectively.These calculated mobilities of electrons were larger than those for two dimensional Mo S₂(⁴00 cm²V^-1s^-1),and being comparable to those for In Se(¹0³ cm²V^-1s^-1)and black phosphorene(10³¹0⁴ cm²V^-1s^-1).(5)Based on the interfacial lattice mismatch model for 2-D PbPdO₂ slab with different orientations,the interfacial lattice mismatch between Mg O(100)film and PbPdO₂ slab with((211)and(002)preferred orientation was studied.The corresponding effect of interface strain on the band structure of PbPdO₂ slab was systematically investigated.It was found that the electronic structure of PbPdO₂slab could not change remarkably if the interface strain was limited within 5%.The good consistency between the calculated results and the experimental results also confirms the validity and feasibility of our built interfacial model,which provides solid foundation for theoretical investigation in the future.(6)The electronic structure of two-dimensional PbPdO₂ with(002)and(211)preferred orientations under strain were investigated using first-principles calculation.The calculated results indicate that PbPdO₂ with(002)and(211)preferred orientations under strain exhibit different electric field dependence on band-gap and carrier concentration.Compared to PbPdO₂ slabs with(211)preferred orientation,PbPdO₂ slabs with(002)preferred orientation has smaller band gap while exhibits more sensitive electric field dependence of band-gap.In addition,the electric field modulation of the resistivity might reach up to 3-4orders of magnitude in(002)oriented PbPdO₂ slab,which could well explain the potential mechanism behind giant magnetoresistance in PbPdO₂.