Multiple frequency radiofrequency-optical resonance in a four-level model of rubidium atoms with optical pumping

We consider a four-level model for alkali metal atoms with optical pumping by nonresonant light under conditions when magnetic dipole transitions are induced between energy sublevels of the hyperfine structure in the ground state. We present the dependences of the observed signal as a function of the frequency detuning of the applied rf fields relative to the resonant value, calculated in the density matrix formalism. We note the absence of a light shift in the radiofrequency-optical resonance signal, independent of the amplitude of the rf field and the optical and thermal relaxation rates. We show that when using a modulation technique for phase detection of the signal, its maximum discrimination ability is observed under conditions for simultaneous modulation of the pump light intensity and the frequency of the rf field, which in principle does not occur in the classical two-level model for optically oriented atoms in magnetic resonance. __________ Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 73, No. 3, pp. 326–329, May–June, 2006.     

On the Positivity of the Coefficients of a Certain Polynomial Defined by Two Positive Definite Matrices

It is shown that the polynomialp(t) = Tr[(A+tB)^m]has positive coefficients when m = 6 and A and B are any two 3-by-3 complex Hermitian positive definite matrices. This case is the first that is not covered by prior, general results. This problem arises from a conjecture raised by Bessis, Moussa, and Villani in connection with a long-standing problem in theoretical physics. The full conjecture, as shown recently by Lieb and Seiringer, is equivalent to p(t) having positive coefficients for any m and any two n-by-n positive definite matrices. We show that, generally, the question in the real case reduces to that of singular A and B, and this is a key part of our proof.     

Phase transition and optoelectronic properties of MgH2

In this article, structural and electronic properties of MgH₂ have been studied. The aim behind this study was to find out the ground state crystal structure of MgH₂. For the purpose, density functional theory (DFT)-based full-potential linearized augmented plane wave (FP-LAPW) calculations have been performed in three different space groups: P4₂/mnm (α-MgH₂), Pa3 (β-MgH₂) and Pbcn (γ-MgH₂). It has been found that the ground state structure of MgH₂ is α-MgH₂. The present study shows that α-MgH₂ transforms into γ-MgH₂ at a pressure of 0.41 GPa. After further increase in pressure, γ-MgH₂ transforms into β-MgH₂ at a pressure of 3.67 GPa. The obtained results are in good agreement with previously reported experimental data. In all the studied phases, the behavior of MgH₂ is insulating and its optical conductivity is around 6.0 eV. The α-MgH₂ and γ-MgH₂ are anisotropic materials while β-MgH₂ is isotropic in nature.     

Topological nature of in‐gap bound states in disordered large‐gap monolayer transition metal dichalcogenides

We propose a physical model based on disordered (a hole punched inside a material) monolayer transition metal dichalcogenides (TMDs) to demonstrate a large‐gap quantum valley Hall insulator. We find an emergence of bound states lying inside the bulk gap of the TMDs. They are strongly affected by spin–valley coupling, rest‐ and kinetic‐mass terms and the hole size. In addition, in the whole range of the hole size, at least two in‐gap bound states with opposite angular momentum, circulating around the edge of the hole, exist.Their topological insulator (TI) feature is analyzed by the Chern number, characterized by spacial distribution of their probabilities and confirmed by energy dispersion curves (energy vs. angular momentum). It not only sheds light on overcoming low‐temperature operating limitation of existing narrow‐gap TIs, but also opens an opportunity to realize valley‐ and spin‐qubits. (© 2016 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)     

Multi‐crystalline silicon solar cells with metal‐assisted nano‐texturing using HNO3 as hole injection agent

In this study, metal‐assisted etching (MAE) with nitric acid (HNO₃) as a hole injecting agent has been employed to texture multi‐crystalline silicon wafers. It was previously proven that addition of HNO₃ enabled control of surface texturing so as to form nano‐cone shaped structures rather than nanowires. The process parameters optimized for optically efficient texturing have been applied to multi‐crystalline wafers. Fabrication of p‐type Al:BSF cells have been carried out on textured samples with thermal SiO₂/PECVD‐SiN_x stack passivation and screen printed metallization. Firing process has been optimized in order to obtain the best contact formation. Finally, j_sc enhancement of 0.9 mA/cm² and 0.6% absolute increase in the efficiency have been achieved. This proves that the optimized MAE texture process can be successfully used in multi‐crystalline wafer texturing with standard passivation methods.

J –V curves and SEM images of the nano and iso‐textured samples. j_sc enhancement of 0.9 mA/cm² together with 0.6% absolute efficiency gain was observed on nano‐textured samples.     

亚波长金属线阵中一维时间反演电磁波的聚焦机理研究

基于周期金属线阵的等效电介质模型, 并使用传输线理论方法, 得到了金属线阵中时间反演电磁波的解析表达式. 接着, 在等效电介质模型的适用范围内, 讨论了金属线半径、金属线阵周期、金属线阵周期数对时间反演电磁波聚焦的影响. 从而在理论上证实, 这种结构可以实现远场时间反演电磁波聚焦, 同时, 也为时间反演技术提供了工程参考.     

Study on structure and properties of transition metal doped BiF3 by first-principles

Structure and physical properties of BiF₃ doped with M=Cr, Cu, Fe, Mn, Ni, Ti, V and Co are calculated by the DFT+U method. Effect of metal doping on the electronic structure and optical response of host materials BiF₃ is investigated systematically. New energy levels are formed and located within the band gap, which could decrease the recombination rate of e⁻/h⁺ pairs. Furthermore, transition metal doping extends the optical absorption of BiF₃ to the visible spectral region.     

3d过渡金属原子单层在Pd(001)表面磁性的第一性原理研究

用第一性原理基础上的超软赝势方法的总能计算，研究了3d过渡金属(Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn)在Pd(001)表面的单层p(1×1)和c(2×2)结构的表面磁性和总能. 所得结果表明：对于Sc, Ti, V和Cr只存在p(1×1)的铁磁性结构，而Mn只有c(2×2)的反铁磁结构存在. Fe, Co和Ni这三种元素上述两种结构都存在，但是总能上p(1×1)的铁磁结构要低些，因此是比较稳定的结构. 而Cu和Zn在该表面上的单层中不存在上述两种结构. 对于V的p(1×1)铁磁结构，计算得到的每个V原子磁矩为2.41μ_B，大于用全电子方法得到的0.51μ_B. 两种计算方法得到其他金属原子 (Cr，Mn，Fe，Co，Ni)的表面磁矩比较相近，都比孤立原子磁矩略小. 关键词： Pd(001)表面 过渡金属原子单层 表面磁性     

Measurements of NO2 mixing ratios with topographic target light scattering-differential optical absorption spectroscopy system and comparisons to point monitoring technique

A topographic target light scattering-differential optical absorption spectroscopy(ToTaL-DOAS) system is developed for measuring average concentrations along a known optical path and studying surface-near distributions of atmospheric trace gases.The telescope of the ToTaL-DOAS system points to targets which are located at known distances from the measurement device and illuminated by sunlight.Average concentrations with high spatial resolution can be retrieved by receiving sunlight reflected from the targets.A filed measurement of NO2 concentration is performed with the ToTaL-DOAS system in Shijiazhuang in the autumn of 2011.The measurement data are compared with concentrations measured by the point monitoring technique at the same site.The results show that the ToTaL-DOAS system is sensitive to the variation of NO2 concentrations along the optical path.     

Hydrogen adsorption and desorption on the Pt and Pd subnano clusters – a review

In this review, we present our recent first principles studies on the sequential H₂ dissociative chemisorption and H desorption on the Pt_n and Pd_n clusters (n=2-9, 13). Upon full saturation by H atoms, the calculated H₂ dissociative chemisorption energy and H desorption energy on Pt₁₃ and Pd₁₃ clusters are similar to the corresponding values on smaller close-packed clusters. Indeed, the catalytic performances of these subnano clusters do not vary significantly with the particle sizes or shapes. Instead, they are dependent on the surface metal atoms which can be accessed by H atoms. In addition to the coverage dependency of the H₂dissociative chemisorption and H sequential desorption energies, the phase transition of both Pt₁₃ and Pd₁₃from the icosahedral to fcc-like structures at certain H coverage was also investigated.