Fine structure and isotope shift of the 3s4d 3 D-3s3p 3 P, 3s5d 3 D-3s3p 3 P and 3p 2 3 P-3s3p 3 P Transitions of MgI

Summary We report the measurements of the³ D(3s4d)-³ P(3s3p)³ D(3s5d)-³ P(3s3p), and³ P(3p ²)-³ P(3s3p) transition frequency of MgI, the fine-structure separation and isotope shift between²⁴Mg and²⁶Mg. The measurements have been performed in a metastable atomic beam; a good agreement is found for data already existing in the literature. The accuracy of the measurements reported in this paper is mainly limited by the Doppler broadening of theI ₂ transitions used as a reference and by the precision in the knowledge of the related wavelengths.     

Description of the symmetry groupSU 3/Z 3 of the octet model

A study is made of the relationship ofS U 3 and its adjoint groupS U3/Z3 to the subgroup ofR8 which leaves invariant not only the lengtha _i a _i of a real eight component vectora _i but also the cubic invariantd _{i j k} a _i a _j a _k ,d _{i j k} being the totally symmetric isotropicS U3 tensor introduced byGell-Mann. A formula for the rotationR ε R8 corresponding toU ε S U3, and a formula inverse to this, which provides a way of parametrizingU ε S U3, are derived.     

Intrinsic Instability of the Hybrid Halide Perovskite Semiconductor CH_3NH_3PbI_3

The organic-inorganic hybrid perovskite CH_3NH_3PbI_3 has attracted significant interest for its high performance in converting solar light into electrical power with an efficiency exceeding 20%. Unfortunately, chemical stability is one major challenge in the development of CH_3NH_3PbI_3 solar cells. It was commonly assumed that moisture or oxygen in the environment causes the poor stability of hybrid halide perovskites, however, here we show from the first-principles calculations that the room-temperature tetragonal phase of CH_3NH_3PbI_3 is thermodynamically unstable with respect to the phase separation into CH_3NH_3I + PbI_2, i.e., the disproportionation is exothermic,independent of the humidity or oxygen in the atmosphere. When the structure is distorted to the low-temperature orthorhombic phase, the energetic cost of separation increases, but remains small. Contributions from vibrational and configurational entropy at room temperature have been considered, but the instability of CH_3NH_3PbI_3 is unchanged. When I is replaced by Br or Cl, Pb by Sn, or the organic cation CH_3NH_3 by inorganic Cs, the perovskites become more stable and do not phase-separate spontaneously. Our study highlights that the poor chemical stability is intrinsic to CH_3NH_3PbI_3 and suggests that element-substitution may solve the chemical stability problem in hybrid halide perovskite solar cells.     

Effects of the progressive substitution of La3+ by Gd3+ in the magnetic and transport properties of La2/3Ca1/3MnO3

A systematic study of the magnetic and transport properties of the system (La_1−xGd_x)_0.67Ca_0.33MnO₃ (0⩽x⩽1) is presented in this article. Through this series, a great crystallographic distortion is achieved but keeping constant the electronic density. At low Gd³⁺ content, these ions behaves in a paramagnetic state following a Brillouin function. Low-temperature ferromagnetism remains unchanged. However, for the samples with large Gd³⁺ content a complex magnetic behavior arises from several competing interaction as super-exchange, double-exchange and also the interaction between Gd and Mn ions. Resistivity and thermopower follow a progressive increase as Gd³⁺ content does, but the polaron energy in the paramagnetic range remains independent of all these changes. Thermopower infinite value is also determined and analyzed.     

Evolution of the interfacial structure of LaAlO3 on SrTiO3

The evolution of the atomic structure of LaAlO_{3} grown on SrTiO_{3} was investigated using surface x-ray diffraction in conjunction with model-independent, phase-retrieval algorithms between two and five monolayers film thickness. A depolarizing buckling is observed between cation and oxygen positions in response to the electric field of polar LaAlO_{3}, which decreases with increasing film thickness. We explain this in terms of competition between elastic strain energy, electrostatic energy, and electronic reconstructions. Based on these structures, the threshold for formation of a two-dimensional electron system at a film thickness of 4 monolayers is quantitatively explained. The findings are also qualitatively reproduced by density-functional-theory calculations.     

E-T phase diagrams of a solid solution of the multicomponent PbZn1/3Nb2/3O3-PbMg1/3Nb2/3O3-PbNi1/3Nb2/3O3-PbTiO3 system near the morphotropic phase boundary

The specific features of the dielectric and pyroelectric responses of a solid solution in the barium-doped multicomponent yPbZn_1/3Nb_2/3O₃-mPbMg_1/3Nb_2/3O₃-nPbNi_1/3Nb_2/3O₃-xPbTiO₃ system of the composition y = 0.0982, m = 0.4541, n = 0.1477, and x = 0.3 near the morphotropic phase boundary have been investigated. It has been assumed that the maxima revealed in the dependences of the reversible permittivity on the electric field strength with both the forward and backward changes in the field (E ₌) are associated with the induced phase transition. Based on the experimental results, the E-T phase diagrams are constructed in the temperature range from ?100 to 150°C for different temperature-field regimes: (i) variation in the electric field E ₌ at a fixed temperature of the sample and (ii) variation in the temperature of the sample at a constant value of E ₌. It has been found that there is a singular point in the E-T phase diagram and that, in the vicinity of this point, the dielectric and pyroelectric responses of the studied ceramics exhibit specific features.     

Structural,spectroscopic, and thermophysical investigations of the oxyfluorides CsZnMoO3F3 and CsMnMoO3F3 with the pyrochlore structure

The structure, infrared and Raman spectra, heat capacity, and thermal expansion have been investigated. It has been shown that, down to liquid-helium temperatures, the cubic pyrochlore structure of the CsMnMoO₃F₃ and CsZnMoO₃F₃ oxyfluorides remains stable. The influence of cation substitution on individual features of the properties of the oxyfluorides has been analyzed.     

Numerical density of3He in coexisting phases of the system CO3-3He

The method of slow-neutron transmission is used to investigate the behavior of the numerical density of³He in the binary solution 2.46%³He + 97.54% CO₂₀ on the liquid-vapor boundary curve. Data on the numerical density are analyzed within the framework of the expanded theory of scale transformations.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 9, pp. 63–67, September, 1983     

Dynamics of the Yb3+ to Er3+ energy transfer in LiNbO3

The energy transfer dynamics between Yb³⁺ and Er³⁺ ions in lithium niobate is investigated after ytterbium-pulsed excitation at 920 nm. The sensitisation of the LiNbO₃:Er³⁺ system with Yb³⁺ ions does not modify the lifetime of the ⁴I_13/2 erbium level (1.5-μm emission), whereas it induces a marked, concentration-dependent change in the lifetime of the ²F_5/2 (Yb³⁺) and ⁴S_3/2 (Er³⁺) multiplets (1060-nm and 550-nm emissions, respectively). The results are analysed by using the rate-equation formalism and cross-relaxation model for the energy transfer. Received: 15 October 1998 / Revised version: 24 November 1998 / Published online: 24 February 1999     

Determination of the Position of the Impurity Yb3+ ion in the CsCaF3 crystals

CsCaF₃ crystals doped with Yb³⁺ ions have been studied using the electron paramagnetic resonance and optical spectroscopy methods. Several types of paramagnetic Yb³⁺ centers were found, among which a paramagnetic center in the unusual position was established. The parameters of the corresponding spin Hamiltonians and schemes of the energy levels of the observed centers were determined.     

Enhancing the photo-luminescence stability of CH3NH3PbI3 film with ionic liquids

The methylammonium lead triiodide (CH₃NH₃PbI₃)-based perovskite shows a great alluring prospect in areas of solar cells, lasers, photodetectors, and light emitting diodes owing to their excellent optical and electrical advantages. However, it is very sensitive to the surrounding oxygen and moisture, which limits its development seriously. It is urgent to spare no effort to enhance its optical and electrical stability for further application. In this paper, we synthesize the MAPbI₃ perovskite film on the glass substrate with/without the ionic liquid (IL) of 1-Butyl-3-methylimidazolium tetrafluoroborate (BMIMBF₄) by a simple two-step sequential solution method. The additive of BMIMBF₄ can improve the quality of crystal structure. Moreover, the photo-luminescence (PL) intensity of MAPbI₃ film with BMIMBF₄ is much stronger than the pure MAPbI₃ film after a week in the air, which is almost ten-fold of the pure one. Meanwhile, under the illumination of 405-nm continuous wave (CW) laser, the fluorescent duration of the MAPbI₃ film with BMIMBF₄ is approximately 2.75 min, while the pure MAPbI₃ film is only about 6 s. In fact, ionic liquid of BMIMBF₄ in the perovskite film plays a role of passivation, which prevents the dissolution of MAPbI₃ into CH₃NH₃ and PbI₂ and thus enhances the stability of environment. In addition, the ionic liquid of BMIMBF₄ possesses high ionic conductivity, which accelerates the electron transport, so it is beneficial for the perovskite film in the areas of solar cells, photodetectors, and lasers. This interesting experiment provides a promising way to develop the perovskite's further application.     

Influence of the Multiplet Levels of Sm3+ and Eu3+ on the Heat Capacity of Multiferroic BiFeO3

Physics of the Solid State - The temperature dependence of the heat capacity of multiferroics BiFeO3, Bi0.90Sm0.10FeO3, and Bi0.90Eu0.10FeO3 has been investigated. It is found that the substitution...     

Raman spectroscopic study of the phase transitions in the (NH4)3WO3F3 oxyfluoride

The Raman spectra of (NH₄)₃WO₃F₃ perovskite-like oxyfluorides are measured in the frequency range 70–3600 cm^?1. The anomalies observed in the spectral parameters upon phase transitions at a temperature of 200 K under atmospheric pressure and at room temperature under a pressure of 1.4 GPa are interpreted.     

Determination of the Sn 4d line shape of the Sn/Ge(111) radical3 x radical3 and 3x3 surfaces

The Sn 4d line shapes of the Sn/Ge(111) sqrt[3]xsqrt[3] and 3x3 surfaces are currently under debate. By employing LEED, core-level, and valence band spectroscopy we have been able to determine the correct Sn 4d line shapes for these surfaces. Contrary to a recent study we conclude that the majority of the earlier reports present line shapes close to the correct ones. At 70 K we identify three 4d components in the 3x3 spectrum, two of which are identified with the two types of Sn atoms in the 3x3 cell. The third component is attributed to Sn atoms surrounding Ge substitutional defects.     

MC-simulation of the 3D,q=3 Potts model

We consider theq=3 Potts model in three dimensions by Monte Carlo simulations. The microcanonical density of states is calculated as a function of the internal energy of the system. We extrapolate the data for the simulated finite systems to the thermo-dynamic limit and find a discontinuous phase transition. This method is checked in the two-dimensional case, where exact results are known.     

One-loop-dimensional reduction of the linear σ model

We perform the dimensional reduction of the linear σ model at one-loop level. The effective potential of the reduced theory obtained from the integration over the nonzero Matsubara frequencies is exhibited. Thermal mass and coupling constant renormalization constants are given, as well as the thermal renormalization group equation which controls the dependence of the counterterms on the temperature. We also recover, for the reduced theory, the vacuum unstability of the model for large N.