【作者】 唐晟；

【导师】 郑文琛；

【作者基本信息】 四川大学 ， 凝聚态物理， 2004， 硕士

【摘要】 研究固体材料在极端条件下(如强电场、强磁场、高温、高压、低温、超低压等)的物性是凝聚态物理中很受关注的问题。它有助于探索材料各种性质的微观机制,揭示材料新的性能并拓宽其应用领域。本文拟就强电场下晶体中过渡金属离子的电子顺磁共振(EPR)谱进行研究。对于在有对称中心的晶体中的过渡金属离子,其EPR谱无一级电场效应,二级电场效应非常弱而难以观测。但对于某些材料,却可以观察到有较强的电场效应。人们发现,这种强的EPR电场效应发生在(i)过渡金属离子与晶格的结合比较松散和(ii)过渡金属离子所处位置局部电场比较强两种情况中。因而推测:处于对称中心的过渡金属离子在电场作用下发生了偏离中心的位移,引起了大的晶格畸变,造成EPR谱大的变化。但时至今日,很少有理论工作定量地研究这种位移。我们用微观机制理论和经验的叠加模型研究了Mn2+在SrCl2、Fe3+在KTaO3和SrTiO3晶体中的零场分裂在强电场下的变化。微观机制采用对6S态离子EPR谱有重要贡献的旋-轨耦合机制、相对论机制和重叠及共价机制。计算中所用参数通过研究晶体光谱和结构参数来得到。由此获得的电场感应的杂质位移和由叠加模型所得的杂质位移将和通过力平衡方程估算的结果进行对照,具体的工作和结果有:1、在对SrCl2: Mn2+离子掺杂的研究过程中,我们发现,在各种电场强度下发生的零场分裂D值,主要是由于在电场作用下,Mn2+离子沿着电场方向(即C3轴方向)发生了一定的位移S引起,同时我们也获得了不同电场强度<WP=5>下的位移大小。这些结果与力平衡方程所得结果比较接近。2、KTaO3: Fe3+中Fe3+离子由于受到电场作用,沿[001]方向发生了位移。我们计算获得了在不同的电场强度和温度下Fe3+离子的位移。零场分裂与温度强烈的相关性是由于KTaO3介电常数与温度强烈有关引起。这一点与力平衡方程所得的结果类似。3、对于四角相SrTiO3中Fe3+离子的场致零场分裂,同样是由于Fe3+离子沿四角(FeO6)9(团簇的电场方向发生了一定的位移S引起,正是该位移导致了零场分裂变化。而且,该场致零场分裂和场致Fe3+位移对温度很敏感, 这是由于SrTiO3晶体的介电常数和局部电场(内电场)对温度很敏感引起。上述结果也与力平衡方程所得结果类似。利用本文的方法可以合理地计算和解释6S(3d5)离子在上述三种材料中的场致零场分裂变化,这说明了本方法的有效性和合理性,可适用于其它类似掺杂晶体的场致EPR研究。更多还原

【Abstract】 The studies of the physical properties of solids in the extreme conditions (e.g., strength electric field, strength magnetic field, high temperature, low pressure, high pressure and low temperature, etc.) have received much attention because they can help to understand the microscopic mechanisms of properties in solids, to discover the new functions of materials and to improve the applications of materials. In this paper, we study the effects of applied electric fields on the electron paramagnetic resonance (EPR) spectra for 3dn ions in crystals with inversion symmetry.The electric-field-induced changes of EPR spectra can often be found in the systems of 3dn ions without inversion symmetry (first-order electric-field effect). When a system possesses inversion symmetry, the induced EPR changes, in principle, cannot be observed because the second-order electric-field effect is very weak. However, for 3dn ions in some systems with inversion symmetry, the strong electric-field effects on the EPR spectra were observed. It is found that these strong electric-field effects occur in the cases of (i) the loose binding between transition metal ion and lattice and of (ii) the strong local electric field at transition metal ion site. So, some people suggested that these effects come from the off-center displacements of 3dn ions induced by the electric field. However, until now, few quantitative and theoretical studies have been made for them.We apply the microscopic mechanism theory and the empirical superposition <WP=7>model to study the electric-field-induced zero-field splittings for Mn2+ in cubic SrCl2, Fe3+ in cubic KTaO3 and tetragonal SrTiO3 crystals. In the microscopic theory, three important mechanisms (i.e., spin-orbit (SO) coupling mechanism, relativistic (RE) mechanism, covalency and overlap (CO) mechanism) are used and the parameters used in the calculations are obtained from the optical spectra and the structural data. The electric-field-induced displacements of 3d5 ions obtained from the above theory and model are comparable with those estimated from the force-balance equations. The results are as follows:1. For the cubic SrCl2: Mn2+, we found that zero-field splitting D can be attributed primarily to the electric-field-induced displacement of Mn2+ ion along the electric-field direction (i.e., C3 axis) and the displacements at various strengths of electric-field are estimated. They are close to those obtained from the force-balance equation.2. The displacements of Fe3+ along [001] direction in KTaO3 crystal are caused by the electric field. We obtain the displacements of Fe3+ in different electric field strengths and temperatures. It is suggested that the changes of electric-field-induced zero-field splitting can be attributed primarily to the above displacements and the temperature dependences of the electric-field-induced zero-field splitting are due mainly to the change in the dielectric constant of KTaO3 with temperature.3. From the studies of the electric-field-induced zero-field splittings for Fe3+ in tetragonal phase of SrTiO3, it is found that the induced zero-field splittings can be reasonably explained by considering suitable Fe3+ displacements along the electric field direction in the tetragonal (FeO6)9( groups. The strong temperature dependences of electric-field-induced Fe3+ displacement and hence of zero-field splitting are due to the strong temperature dependences of dielectric constant and local electric field (internal electric field) in SrTiO3.So, the above methods can calculate and explain the changes of electric-field-induced zero-field splittings for 6S-state(3d5)ions in crystals with inversion symmetry. The methods can also be applied in the investigations of other <WP=8>similar cases.更多还原