First-principle Study On The Optical Properties Of New Diluted Magnetic Semiconductor (La_1-xBa_x)(Ag_1-yMn_y)SO And Some Experimental Results

With the rapid development of the science and technology in our information society,it puts forward higher requirements for the information transmission,processing and storage capacity.Diluted magnetic semiconductors have the dual properties of spin and charge,which are of great significance for the preparation for spin optoelectronic devices.The new types of dilute magnetic semiconductors based on iron-based superconductors own decoupled charge and spin doping.Recently,with the development of iron-based superconductors,several types of diluted magnetic semiconductors are discovered and synthesized,such as"111","122"and"1111"phases.In this dissertation,the electronic structures and optical properties of(La_1-xBa_x)(Ag_1-yMn_y)SO systems are discussed mainly by CASTEP module in Material Studio based on the first-principle calculation of density functional theory.The structural model,the electronic structure and optical properties?absorbance index,reflection coefficient and dielectric constant?are studied,especially for the optical properties.In addition,(La_1-xBa_x)(Ag_0.925Mn_0.075)SO?x=0,0.025,0.05,0.075?polycrystalline samples are synthetized by solid state method.The samples are investigated by XRD,low-temperature electric resistivity,magnetic properties and PL spectrum measurements.It is a new diluted magnetic semiconductor when x=0.075.The detail is as follows.1.The parent La AgSO is calculated by first-principle method and compared with the experimental parameters and results reported,conforming the reliability of our calculation method.There are two obvious peaks in the curve of absorbance index and reflection coefficient,sited at 65 nm and 150 nm,respectively.The static dielectric constant of parent LaAgSO is 6.5.2.As for the two-site doping systems(La_0.75Ba_0.25)(Ag_0.75Mn_0.25)SO and(La_0.875Ba_0.125)(Ag_0.875Mn_0.125)SO,Ba ion substituted for La ion produces charge and Mn ion for Ag ion produces spin.Thus we construct 2×1×1 and 2×2×1supercells,find out the optimum structure,and then calculate the energy gap and density of states,taking account of spin.At last we discuss the optical properties.The reflection peaks of(La_0.75Ba_0.25)(Ag_0.75Mn_0.25)SO locate at 67 nm?about 6%?and150 nm?95%?.The absorption peaks are at 67 nm?1.1×10⁵/cm?and 175nm?2.25×10⁵/cm?.The static dielectric constant is 5.8.Corresponding to(La_0.875Ba_0.125)(Ag_0.875Mn_0.125),they are 70 nm?about 2%?and 175 nm?85%?,70nm?4×10⁴/cm?and 180 nm?1.6×10⁵/cm?and 4.6 respectively.Obviously,the positions of the reflection and absorption peaks show?red shift?.3.Compare the absorbance index,reflection coefficient and dielectric constant of(La_0.75Ba_0.25)(Ag_0.75Mn_0.25)SO with(La_0.875Ba_0.125)(Ag_0.875Mn_0.125),it is interesting that all the values of three parameters of parent LaAgSO are biggest,which of the co-dopant 0.125 sample are smallest,while which of co-dopant 0.25sample is in the middle of them.The reason it is probably that Ba ion substituted for La ion and Mn ion for Ag ion have reverse effect on the optical properties of new diluted magnetic semiconductors.Considering of the two-site dopants influences one another,i.e.It will present ferromagnetism only if both spin and charge doping synchronously and reaching a certain degree,not just Ba in place of La or Mn of Ag.Therefore we design and calculate the two other systems-(La_0.75Ba_0.25)(Ag_0.875Mn_0.125)SO and(La_0.875Ba_0.125)(Ag_0.75Mn_0.25).The results are as follows:compare(La_0.875Ba_0.125)(Ag_0.75Mn_0.25)SO with(La_0.75Ba_0.25)(Ag_0.75Mn_0.25)SO where Mn ionic concentration is 0.25,the value of the absorption peak increases dramatically with the Ba dopant increasing from 0.125 to 0.25.However,it decreases obviously with the Mn dopant increasing from 0.125 to 0.25 when Ba ionic concentration equals to 0.125,compared(La_0.875Ba_0.125)(Ag_0.75Mn_0.25)SO with(La_0.875Ba_0.125)(Ag_0.875Mn_0.125)SO.It conforms to our proposal and illustrates further that charge and spin decoupled doping influences one another even in the optical properties of new diluted magnetic semiconductors.4.By experiment,we synthesize(La_1-xBa_x)(Ag_0.925Mn_0.075)SO polycrystal system?x=0,0.025,0.05 and 0.075?by solid state method.All the samples are pure phase within the XRD measurement accuracy scale.They are semiconductors by the low–temperature-electric resistivity measurement.(La_0.925Ba_0.075)(Ag_0.925Mn_0.075)SO is conformed to be a new diluted magnetic semiconductor with the Curie temperature Tc¹40 K by magnetization curve and hysteresis loop.It shows that the FWHM increases and the intensity decreases with the Ba dopant addition by photoluminescence spectra.