Positron Calculation Based On Superposed-Neutral-Atom Model

Positron calculation is an important and needful component in the research of materials with positron techniques.Comparing experiment data with the calculation results to analyze possible vacancy types and characters of positron annihilation is the requirment of a successful positron research.The Surperposed-Neutral-Atom model and Finite-Difference method(SNA-FD) is an important method of positron calculation.Compared with other methods,it needs little time and has the same precision.It can solve complex vacancy type and becomes a broad and general calculation method in the international positron research.In this article,a positron calculation system,based on the SNA-FD method,is finished after large amount of literatures have been studied.A full-scale test is done with the calculation of positron annihilaton in different types of vacancy of Al single-crystal.The result shows that this calculation system can make a quick and exact calculation about the positron annihilation in single crystal and varous types of vacancy.Several positron calculation works have been done at the base of this system.Positron annihilation has been studied for more than eighty elements of periodic table.Positron bulk lifetime and positron monovacancy lifetime are obtained.The calculated positron bulk lifetimes agree well with the experiment values,except for some special ones,which shows that the LDA method and the GGA mothod both can be used to the calculation of positron annihilation in single crystal.Lots of calculated positron monovacancy lifetimes agree with the experimet values,while some of calculated ones are a little different with the experiment values.It may be ascribed to the fix of neighbour atom around the monovacancy,which have ignored changes of atom position when the monovacancy is formed.This work offers a large number of data which can be used to analyze and compare with experiment values in the future.It bulids a theory platform for the later research of elements.Positron annihilation in several carbon allotropes has been studied.The results show that:Positron mainly annihilates in the interspace between layers in the single crystal of graphite.The calculated positron bulk lifetime in graphite is 208ps,which agrees with the experiment value of 215ps in literature;Positron mainly annihilate in the interspace of carbon atoms in the single crystal of diamond.The calculated positron bulk lifetime in diamond is 115ps,which agrees with the experiment value of about 110ps in literature.In the single crystal of C60 with a FCC structure,positron mainly appears outside the C60 molecule.The main annihilation space is the interspace between molecules.The calculated positron bulk lifetime in C60 is 352ps, which agrees with experiment value of 356ps in literature.In carbon nanotube bundles with different dimeters,as the diameter of carbon nanotubes increases,the main space where positron appears changes from the interspace of carbon tubes to the space inside carbon tubes,the radio between positron annihilation with valence eletrons and core eletrons becomes larger,the positron bulk lifetime in carbon nanotube increase rapidly first and come to be a constant at the end.The calculated positron lifetime of carbon nanotube with a dimeter of 0.8～1.6ns is 332～470ps,which agrees with the experiment value of 394ps.Positron annihilation has been studied in widly used compound semiconductors (ZnO GaN GaAs SiC InP).The positron bulk lifetime,positron monovacancy lifetime and positron divacancy lifetime have been calculated.What follows are samples of calculated results sorted by crystal type and calculated with GGA method(All calculated results except for metioned).Positron bulk lifetime in ZnO single crystal is 177ps,which agrees well with the experiment value of 170ps in litereture.Positron lifetime of Zn monovacancy is 237ps, which agrees well with the experiment value of 230ps in litereture.O monovacancy in ZnO single crystal can't capture positron.Positron around o monovacany can move to other place in single crystal and annihilate with electron there.It shows that positron technique can't obtain information of O monovacany in ZnO single crystal.Positron lifetime of Zn divacancy is 259ps.Positron lifetime of Zn-O complex vacancy is 266ps,which agrees with some confused experiment value of 260ps in literature and shows that Zn-O complex vacancy maybe exist in the sample.Calculated results show that O divacancy can capture positron.Positron lifetime of O divacancy is 188ps, which is near the value of positron bulk lifetime in ZnO single crystal.It shows that positron lifetime component of O divacancy can't be separated from ZnO bulk lifetime in the actual positron annihilation lifeime spectroscopy(PALS) with actual precision.Positron lifetime in GaN single crystal is 153ps,which agrees with the experiment value of 160ps in literature.Positron lifetime of Ga monovacancy is 214ps.N monovacancy can't capture positron.Positron around N monovacany can move to other place in single crystal and annihilate with electron there.It shows that positron technique can't obtain information of N monovacany in GaN single crystal.Positron lifetime of Ga divacancy is 227ps.Positron liftime of Ga-N complex vacancy is 238ps.N divacancy can capture positron.Positron lifetime of N divacancy is 161ps,which is near the value of positron bulk lifetime in GaN single crystal.It shows that Positron lifetime component of N divacancy can't be separated from GaN bulk lifetime in the actual positron annihilation lifeime spectroscopy(PALS) with actual precision.Positron bulk lifetime in GaAs single crystal is 221ps,which agrees with the experiment value of 229ps in literature.Both Ga monovacancy and As monovacancy can capture positron.Positron lifetime of Ga monovacancy is 263ps,which agrees well with the experiment value of 262ps in literature.Positron lifetime of As monovacancy is 255ps,which agrees well with the experiment value of 257ps in literature.Positron lifetime of Ga divacancy is 274ps.Positron lifetime of As divacancy is 271ps.Positron lifetime of Ga-As complex vacancy is 319ps.It shows that two positron monovacancy lifetime and two positron divacancy lifetime are near each other.The growth method and worked technics of samples and some other experimental techqiues must be considered to separate these components.Positron bulk lifetime in SiC single crystal is 131ps,which agrees with experiment value of 136ps in literature.Positron lifetime of Si monovacancy is 178ps, which agrees well with experiment value of 176ps in literature.C monovacancy in SiC single crystal can't capture positron.Positron around C monovacany can move to other place in single crystal and annihilate with electron there.It shows that positron technique can't obtain information of C monovacany in SiC single crystal.Positron lifetime of Si divacancy is 191ps,which is some different with experiment value of 210ps in literature.Calculated results show that C divacancy can capture positron.Positron lifetime in C divacancy is 144ps,which is near the value of positron bulk lifetime in SiC single crystal.It shows that Positron lifetime component of C divacancy can't be separated from SiC bulk lifetime in the actual positron annihilation lifeime spectroscopy(PALS) with actual precision.Positron lifetime of Si-C complex vacancy is 203ps,which agrees with the experiment value of 209ps in literature.Positron bulk lifetime in InP single crystal is 237ps,which agrees well with experiment value of 234ps in literature.Positron lifetime of In monovacancy is 294ps, which agrees with experiment value of 283ps.Calculated results show that P monovacancy can capture positron.Positron lifetime of P monovacancy is 255ps.Positron lifetime of In divacancy is 313ps.Positron lifetime of P divacancy is 264ps.Positron lifetime of In-P complex vacancy is 345ps,which agrees well with experiment value of 340ps in literature.Positron annihilaton in high temperature superconductors has been studied.Positron distribution and lifetime have been calculated in high temperature superconductors of YBaCuO and SmFeAsO.The results show that positron can't be captured by the O monovacancy in single crystal.