We performed first-principle calculations based on the supercell and cluster approaches to investigate the neutral Al impurity in smoky quartz. Electron paramagnetic resonance measurements suggest that the oxygens around the Al center undergo a polaronic distortion which localizes the hole being on one of the four oxygen atoms. We find that the screened exchange hybrid functional successfully describes this localization and improves on standard local density approaches or on hybrid functionals that do not include enough exact exchange such as B3LYP. We find a defect level at about 2.5 eV above the valence band maximum, corresponding to a localized hole in a O 2p orbital. The calculated values of the g tensor and the hyperfine splittings are in excellent agreement with experiment.
Uncovering the mechanism of the impurity-selective Mott transition in paramagnetic V$_{2}$O$_{3}$
虽然单轨道和多轨道哈伯德模型的相图已经得到了很好的研究，但真正的莫特绝缘体的物理学往往要丰富得多，依赖于材料，而且知之甚少。在原型莫特绝缘体V $_{2}$ O $_{3}$中，化学压力最初被认为是解释为什么顺磁金属到反铁磁绝缘体的转变温度被Ti掺杂降低，而Cr掺杂增强相关性，最终使高温相顺磁绝缘。然而，最近的实验和理论都表明这种情况是站不住脚的。基于完整的结构优化，我们证明通过电荷自洽密度泛函理论和动力学平均场论的组合，在V $_{2}$ O $_{3}$相图中的变化是由缺陷引起的局部对称性破缺驱动的，所述局部对称性破缺由Cr和Ti掺杂剂与主体系统的显著不同的耦合引起。这一发现强调了莫特金属-绝缘体跃迁对局部环境的高灵敏度，以及准确计算单电子哈密顿量的重要性，因为相关性对它有重要的响应。
For erbium-doped amorphous oxides, such as those that are used in compact lightwave devices interfaced with silicon, values of the refractive indices are commonly obtained empirically. This work, combining experimental and theoretical studies, examines silica as the exemplary host and the influence of erbium doping on the refractive index. Analysis of data is presented for the spectral refractive index in the ultraviolet to near infrared wavelength range of heavily erbium-doped silica thin films prepared by spin coating a sol-gel precursor on silicon and subsequent vacuum annealing. Effective …
While several 2D thermometry techniques exist, there is a lack of 3D thermometry techniques that work for wide range of materials and offer good resolution in time, space and temperature. X-ray diffraction (XRD) and nuclear magnetic resonance (NMR) imaging can provide 3D temperature information. However, XRD is typically limited to crystalline materials while NMR is largely limited to liquids where the resonance lines are narrow. We investigate electron paramagnetic resonance (EPR) of n-type silicon and germanium for 3D thermometry. While in germanium the EPR linewidths are too broad, EPR line…
Electron paramagnetic resonance studies of the impurity Fe$^{3+}$ and Mn$^{4+}$ centers indicate that the symmetry of the crystal structure of millimeter-scale strontium titanate plates above 105 K lowers in the bulk from cubic to tetragonal, different from that of the conventional low-temperature antiferrodistortive phase. It is shown that the effect does not originate from the residual stress; its magnitude (observed tetragonal distortion) and particular manifestations depend on plate orientation, surface quality and the geometry of a sample. Peculiarities of the observed phenomenon are pres…
Structural relaxations around Ti, Cr and Fe impurities in alpha-Al2O3 probed by x-ray absorption near edge structure combined with first-principles calculations
