Study Of Rutile Superlattices From First Principles Calculations

Recently, first-principle calculation based on density functional theory becomes a popular means of scientific research now. Particularly, first-principle calculation became important method of researching and designing new type materials with the application of high-speed computers.Transition metal oxides have momentous application value because of the special physical properties, scientists investigated chromium dioxide(CrO2), titanium dioxide(TiO2), vanadium dioxide(VO2), rubidium dioxide(RuO2), iridium dioxide(IrO2), zirconium dioxide(ZrO2), stannic dioxide(SnO2), manganese dioxide (MnO2)both in theory and experiment which have the same rutile structure from the last century. There are a number of relevant reports. This kind of oxides are widely used in industry. In recent years, with the increase of technology, it can be realize to grow layer by layer by means of molecular beam epitaxy in lab. The lattice mismatch of CrO2, TiO2 and VO2 which have the same structure is in 4%. It means that we can obtain heterostructure of CrO2/TiO2 and CrO2/VO2 in certain conditions. Considering that the complexity of nano-structures, it is significance for applying first-principle calculation to explore and research nature of nano-structures.In this work, there're four cases are discussed.1. Build superlattices along (001) direction with lattice constant of CrO2. Cr layers and Ti layers are arranged along (001) direction with certain ratio in this structure which is represented as (CrO2)m/(TiO2)n.2. Build superlattices along (001) direction with lattice constant of CrO2. Cr layers and V layers are arranged along (001) direction with certain ratio in this structure which is represented as (CrO2)m/(VO2)n.3. Build superlattices along (001) direction with lattice constant of TiO2. Ti layers and Cr layers are arranged along (001) direction with certain ratio in this structure which is represented as (TiO2)m/(CrO2)n.4. Build superlattices along (001) direction with lattice constant of VO2. V layers and Cr layers are arranged along (001) direction with certain ratio in this structure which is represented as (VO2)m/(CrO2)n, m,n=1,2,3 in the cases. The result of calculation reveal that after optimization for (CrO2)m/(TiO2)n and (TiO2)m/(CrO2)n, the band length of Ti-O become longer while Cr-O become shorter. After optimization for (CrO2)m/(VO2)n and (VO2)m/(CrO2)n, The change of band length of Cr-O and V-0 are uncertain. Except (VO2)2/(CrO2)2,all superlattices show ferromagnetic and half-metal compound in these four kinds of superlattices. We make structural relaxation which fixed a,b axis and changed c axis, found that c-axis in most of superlattices have not evidence of contracts and stretched. It shows that magnetic moment is 2μB per Cr atom, 0μB per Ti atom, 1μB per V atom in most of superlattices and it doesn't change with the ratio of Cr, Ti, V layers. The band gap for spin down of (CrO2)m/(TiO2)n and (TiO2)m/(CrO2)n is little difference with CrO2. When the number of Ti layers is fixed, the band gaps for spin down become small with increasing Cr layers. To (CrO2)m/(VO2)n and (VO2)m/(CrO2)n superlattices, the band gaps for spin down are smaller than the gap of CrO2. Compare t2g states of Cr in CrO2 cell, these t2g states of (CrO2)m/(TiO2)n and (TiO2)m/(CrO2)n move to high energy because of the effect of Ti. Otherwise, the t2g states of (CrO2)m/(VO2)n and (VO2)m/(CrO2)n move to low energy affected by V.