We are dealing with the long-standing problem of the eigenstates and eigenvalues of 3-D rotators by off-center impurity ions in alkali halides. The ion runs along the brim of a sombrero-like vibronic potential. The quantum-mechanical motion depending on two angular coordinates, we consider the motion along theta at averaged phi, and vice versa. We find the off-center ion performing two rotations: one in (x,y) plane along the azimuth phi and another one along the position angle theta, respectively, which are both described as Mathieu's non-linear oscillators. The motion along z is complemented by an anharmonic vibrational mode.
Using first principles calculations we show that several alkali lead halides potentially present collective ferroelectric polarization. This should occur at least at the nanoscale; it could be detected macroscopically provided it is not concealed by lattice vibrations in the temperature range of stability of the cubic perovskite phase. For potassium lead halides and for alkali lead fluorides, remarkably, the ferroelectric behavior turns hyper-ferroelectric, suggesting a more robust ferroelectric polarization in spite of depolarization potentials induced by charge accumulation on surfaces or in…
Favorable optoelectronic properties and ease of fabrication make NiO a promising hole transport layer for perovskite solar cells. To achieve maximum efficiency, the electronic levels of NiO need to be optimally aligned with those of the perovskite absorber. Applying surface modifiers by adsorbing species on the NiO surface, is one of the most widespread strategies to tune its energy levels. Alkali halides are simple inorganic surface modifiers that have been extensively used in organic optoelectronics, however, rarely studied in perovskite solar cells. Using density functional theory (DFT) cal…
Electronic properties of metal induced gap states at insulator/metal interfaces -- dependence on the alkali halide and the possibility of excitonic mechanism of superconductivity
Kiguchi {\it et al.} [Phys. Rev. Lett. {\bf 90}，196803（2003）]中，我们用第一原理密度泛函方法理论上研究了在各种碱金属卤化物和由凝胶表示的金属之间的界面处的MIGS的电子性质。我们已经发现，在通常的倏逝态之上，MIGS通常在卤素位点上具有长尾巴，具有$p_z$样特征，其穿透深度（$λ$）大到本体碱金属卤化物的晶格常数的一半。这意味着，$λ$，虽然几乎不依赖于凝胶中的载流子密度，但由碱金属卤化物的晶格常数（因此由能隙）主导，其中$λ_{\rm LiF} < λ_{\rm LiCl} < λ_{\rm LiI}$。我们还提出了一种可能性的MIGS工作有利的激子介导的超导性。
The alkali halides, known as ionic crystals, have the NaCl-type or CsCl-type structure as the ground state. We study the structural, vibrational, and electronic properties of two-dimensional (2D) ionic crystals from first-principles. Two potential structures that are hexagonal and tetragonal are investigated as structural templates. Through phonon dispersion calculations, 8 and 16 out of 20 alkali halides in the hexagonal and tetragonal structures are dynamically stable, respectively. The electron energy gaps range from 6.8 eV for LiF to 3.9 eV for RbI and CsI in the tetragonal structure withi…
Off-center impurities in alkali halides: reorientation, electric polarization and binding to F center. V. Temperature-dependent electrostatic polarizabilities
Perovskite solar cells have shown remarkable efficiencies beyond 22%, through organic and inorganic cation alloying. However, the role of alkali-metal cations is not well-understood. By using synchrotron-based nano-X-ray fluorescence and complementary measurements, we show that when adding RbI and/or CsI the halide distribution becomes homogenous. This homogenization translates into long-lived charge carrier decays, spatially homogenous carrier dynamics visualized by ultrafast microscopy, as well as improved photovoltaic device performance. We find that Rb and K phase-segregate in highly conce…
