Realization of low-energy type-Ⅱ Dirac fermions in(Ir_(1-x)Pt_x)Te_2 superconductors

Topological Dirac semimetals(DSMs) present a kind of topologically nontrivial quantum state of matter, which has massless Dirac fermions in the bulk and topologically protected states on certain surfaces. In superconducting DSMs, the effects of their nontrivial topology on superconducting pairing could realize topological superconductivity in the bulk or on the surface. As superconducting pairing takes place at the Fermi level E_F, to make the effects possible, the Dirac points should lie in the vicinity of E_F so that the topological electronic states can participate in the superconducting paring. Here,we show using angle-resolved photoelectron spectroscopy that in a series of(Ir_(1-x)Pt_x)Te_2 compounds, the type-Ⅱ Dirac points reside around E_F in the superconducting region, in which the bulk superconductivity has a maximum T_c of ～ 3 K.The realization of the coexistence of bulk superconductivity and low-energy Dirac fermions in(Ir_(1-x)Pt_x)Te_2 paves the way for studying the effects of the nontrivial topology in DSMs on the superconducting state.     

Intermediate Energy Pion-~(20)Ne Elastic Scattering in the α+~(16)O Model of (20)~Ne

We present an analysis of π-20Ne elastic scattering at intermediate energy basing on the α+16O model of the20Ne nucleus and in the framework of Glauber multiple scattering theory. Satisfactory agreement with the general features of the experimental data of pion elastic scattering on the neighboring 4N-type nuclei is obtained without any free parameters. Compared with the experimental angular distributions of pion elastic scattering on12C,16O,24Mg,and28Si nuclei, the difractive patterns and the positions of the dips and peaks in the angular distributions of π-20Ne elastic scattering are reasonably predicted by the calculations.     

A model with flavor-dependent gauged U(1)_(B-L_1)×U(1)_(B-L_2-L_3) symmetry

We propose a new model with flavor-dependent gauged U(1)_(B-L_1)×U(1)_(B-L_2-L_3) symmetry in addition to the flavor-blind symmetry in the Standard Model. The model contains three right-handed neutrinos to cancel gauge anomalies and several Higgs bosons to construct the measured fermion masses. We show the generic features of the model and explore its phenomenology. In particular, we discuss the current bounds on the extra gauge bosons from the K and B meson mixings as well as the LEP and LHC data, and focus on their contributions to the lepton flavor violating processes of ?_(i+1) →?_iγ(i=1,2).     

The effect of the rotational excitation of NO on the stereodynamics for the reaction C(~3P)+NO(X~2Π)→CN(X~2Σ~+)+O(~3P)

The stereodynamic properties of the reaction C(3P)+NO(X2Π)→CN(X2Σ+)+O(3P) in different rotational states of reactant NO are studied theoretically by using the quasiclassical trajectory method on 2A' and 2A' potential energy surfaces(PESs) at a collision energy of 0.06 eV.The vector properties in different rotational states on the two surfaces are discussed in detail.The results indicate that the rotational excitation of NO has considerable influence on the stereodynamic property of the reaction occurring on the two surfaces.At the same time,the calculated polarization-dependent differential cross sections(PDDCSs) in different initial rotational states manifest that products are strongly polarized at three scattering angles.     

Oxygen vacancy induced magnetism in BaTiO_(3-δ) and Nb:BaTiO_(3-δ) thin films

The unusual magnetic properties are observed in oxygen deficient BaTiO3-δ(BTO) and Nb:BaTiO3-δ(BNTO) thin films fabricated on SrTiO3 substrates by laser molecular beam epitaxy.The distinct magnetic hysteresis loops are observed in the oxygen deficient BTO and BNTO thin films in a temperature range of 5 to 300 K,whereas the diamagnetism is observed in both BTO and BNTO annealed at 1 atm of oxygen.The dopant Nb only enhances the magnetization in BNTO thin films,but has little effect on the coercivity.The magnetism of BTO and BNTO films is proposed to be the oxygen vacancies by origin.     

Properties of the πh_(9/2)  νi_(13/2) band in odd-odd ~(188)Au

A new rotational band has been identified and assigned to 188Au for the first time using the 173Yb(19F,4nγ) reaction at the beam energies of 86 and 90 MeV. This band is proposed to be built on the πh9/2  νi13/2 configuration by comparing the band properties with known bands in neighboring nuclei. The prolate-to-oblate shape transition through triaxial shape has been proposed to occur around 188Au for the πh9/2  νi13/2 bands in odd-odd Au isotopes on the basis of total Routhian surface (TRS) calculations.     

NMR AND DFT STUDY OF Co-C BOND ACTIVATION MECHANISM IN VITAMIN B_(12) COMPOUNDS

This report focuses on a theoretical study of the cobalt-cabon bond activation mechanism in Vitamin B12, its co enzyme complexes, and model compounds such as cobalamins (Cbl), cobaloximes and cobalt porphyrins. In particular, emphasis is placed on the nucleotide loop and its role in the base on and base off mechanism.     

1982年CUSPEA试题(二)(英文)

MODERN PHYSICS(4 hours)Solve 5 of the following 6 problemsB1Our universe is filled with black body radiation (photons) at a temperature T≈3.K. This isthought to be a relic of early developments following the "big bang".(i) Express the photon number density n analytically in terms of T and universal constants. Youranswer should explicitly show the dependence on T and on universal constants. However, a certainnumerical cofactor may be left in the form of a dimensionless integral which n…     

Simulation of surfactant effects on the growth of metal homoexpitaxial Sb-Ag/Ag(111)

A kinetic Monte Carlo simulation is performed in order to study the effect of Sb atoms as a surfactant on the growth of Ag on Ag(111).In our model the repulsive mechanism in which the surfactant Sb atoms repel diffusing Ag adatoms,and the exchange mechanism between Ag and Sb atoms,are considered.Our simulations show that the effects of Sb atoms for Ag/Ag(111) growth system are mainly to increase the chances for Ag atoms to overcome the Ehrlich-Schwoebel barrier both in the interlayer growth and along the edge diffusion.The influence of the coverage of Sb atoms and substrate temperature on the growth of Ag/Sb/Ag(111) is discussed.     

Au(111)表面钴酞菁分子自旋激发态的π电子辅助自旋弛豫

We present our investigation on the spin relaxation of cobalt phthalocyanine (CoPc) films on Au(111) (CoPc/Au(111)) surface using scanning tunneling microscopy and spectroscopy. The spin relaxation time derived from the linewidth of spin-flip inelastic electron tunneling spectroscopy is quantitatively analyzed according to the Korringa-like formula. We find that although this regime of the spin relaxation time calculation by just considering the exchange interaction between itinerant conduction electrons and localized d-shells (s-d exchange interaction) can successfully reproduce the experimental value of the adsorbed magnetic atom, it fails in our case of CoPc/Au(111). Instead, we can obtain the relaxation time that is in good agreement with the experimental result by considering the fact that the π electrons in CoPc molecules are spin polarized, where the spin polarized π electrons extended at the Pc macrocycle may also scatter the conduction electrons in addition to the localized d spins. Our analyses indicate that the scattering by the π electrons provides an efficient spin relaxation channel in addition to the s-d interaction and thus leads to much short relaxation time in such a kind of molecular system on a metal substrate.     

辉光放电等离子体辅助XeCl准分子激光溅射沉积碳氮薄膜

利用直流辉光放电等离子体辅助的脉冲激光沉积技术在Si衬底上生长了碳氮薄膜.通过扫描电子显微镜、X射线衍射、X射线光电子能谱、俄歇电子能谱等多种手段,对薄膜的形貌、成分、晶体结构、价键状态等特性进行了分析和确定.结果表明,沉积薄膜为含有非晶SiN和晶态氮化碳颗粒结构,晶态成分呈多晶态,主要为α-C₃N₄相、β-C₃N₄相,晶粒大小为40—60nm.碳氮之间主要以C-N非极性共价键形式相结合. 关键词： 脉冲激光沉积 直流辉光放电 碳氮薄膜     

Deposition of hexagonal boron nitride thin films on silver nanoparticle substrates and surface enhanced infrared absorption

Silver nanoparticle thin films with different average particle diameters are grown on silicon substrates. Boron nitride thin films are then deposited on the silver nanoparticle interlayers by radio frequency （RF） magnetron sputtering. The boron nitride thin films are characterized by Fourier transform infrared spectra. The average particle diameters of silver nanoparticle thin films are 126.6, 78.4, and 178.8 nm. The results show that the sizes of the silver nanoparticles have effects on the intensities of infrared spectra of boron nitride thin films. An enhanced infrared absorption is detected for boron nitride thin film grown on silver nanoparticle thin film. This result is helpful to study the growth mechanism of boron nitride thin film.     

Femtosecond laser interaction with pulsed-laser deposited carbon thin films of nanoscale thickness

The dependence of optical, electronic and thermal penetration zones on the thickness of nanoscale layers grown on silicon wafers is reported. Tetrahedral amorphous carbon (ta-C) and amorphous carbon nitride (a-C_xN_y) films were prepared by inverse pulsed laser deposition (IPLD). Single-pulse modification thresholds for femtosecond laser processing proved to be dependent on the actual film thickness below 60 nm for ta-C and 90 nm for a-C_xN_y. The modification behaviour was governed by multiphoton processes. An effective penetration depth of the laser radiation in a-C_xN_y was of ca. 110 nm in accordance with two-photon absorption. Both the emergence length of ballistic hot electrons and the heat diffusion length are negligible in these thin film materials. The lower bulk value of the threshold fluence of the a-C_xN_y films as compared to ta-C is mainly controlled by optical contributions due to nitrogen-related defects.     

Adjusting chemical bonding of hard amorphous CSixNy thin films by N*-plasma-assisted pulsed laser deposition

Hard amorphous carbon silicon nitride thin films have been grown by pulsed laser deposition (PLD) of various carbon silicon nitride targets by using an additional nitrogen RF plasma source on [100] oriented silicon substrates at room temperature. The influence of the number of laser shots per target site on the growth rate and film surface morphology was studied. Up to about 30 at. % nitrogen and up to 20 at. % silicon were found in the films by Rutherford backscattering spectroscopy (RBS) and X-ray photoelectron spectroscopy (XPS). The XPS of the films showed a clear correlation of binding energy to the variation of PLD parameters. The films show a universal hardness value up to 23 GPa (reference value for silicon substrate 14 GPa) in dependence on target composition and PLD parameters. The results emphasise the possibility of variation of chemical bonding and corresponding properties, such as nanohardness, of amorphous CSi_xN_y thin films by the plasma-assisted PLD process.     

Effects of experimental parameters on composition of boron carbon nitride thin films deposited by magnetron sputtering

We have synthesized boron carbon nitride thin films by radio frequency magnetron sputtering. The films structure and composition were characterized by X-ray diffraction, Fourier transform infrared spectroscopy and X-ray photoelectron spectroscopy. The results indicate that the three elements of B, C, N are chemically bonded with each other and atomic-level hybrids have been formed in the films. The boron carbon nitride films prepared in the present experiment possess a disordered structure. The influence of PN₂/PN₂+Ar, total pressure and substrate bias voltage on the composition of boron carbon nitride films is investigated. The atomic fraction of C atoms increases and the fractions of B, N decrease with the decrease of PN₂/PN₂+Ar from 75% to 0%. There is an optimum total pressure. That is to say, the atomic fractions of B, N reach a maximum and the fraction of C atoms reaches a minimum at the total pressure of 1.3 Pa. The boron carbon nitride films exhibit lower C content and higher B, N contents at lower bias voltages. And the boron carbon nitride films show higher C content and lower B, N contents at higher bias voltages.     

Surface properties of cubic boron nitride thin films

Studying the surface properties of cubic boron nitride (c-BN) thin films is very important to making it clear that its formation mechanism and application. In this paper, c-BN thin films were deposited on Si substrates by radio frequency sputter. The influence of working gas pressure on the formation of cBN thin film was studied. The surface of c-BN films was analyzed by X-ray photoelectron spectroscopy (XPS), and the results showed that the surface of c-BN thin films contained C and O elements besides B and N. Value of N/B of c-BN thin films that contained cubic phase of boron nitride was very close to 1. The calculation based on XPS showed that the thickness of hexagonal boron nitride (h-BN) on the surface of c-BN films is approximately 0.8 nm.     

120keV的N+注入SiC薄膜的光谱特性研究

文中主要研究了120keV的N⁺注入后SiC薄膜样品的光致发光谱(PL)和傅立叶红外光谱(FTIR)特性.从红外光谱可以看到有明显得碳氮单键、双键、三键等新结构生成.从PL光谱则发现365nm处的发光峰明显增强,这表明N⁺注入使得带隙中深的能级辐射中心复合的效率大幅度提高     

Study of N+ Implanted SiC Thin Films by FTIR and PL Spectroscopy

SiC films were deposited on Si substrates by RF co-sputtering of the Si and C compound target and implanted by 120keV N ions. The structure, optical property of thin films were studied by Fourier transform infrared spectrum(FTIR) and photoluminescence(PL) spectroscopy. The obtained results suggested that carbon nitride single, double and triple bonds are formed in the N ions implanted SiC films, and the luminescence intensity depends strongly on the quantity of N ions.     

An alternative procedure for the determination of the optical band gap and thickness of amorphous carbon nitride thin films

In this work, we propose an alternative procedure to obtain the optical band gap and the thickness of amorphous carbon nitride thin films that requires only the measurement of the absorbance spectrum of the samples. The method is based on an absorbance spectrum fitting (ASF) procedure using the Tauc model, which is widely applied to the study of amorphous semiconductors. With the aim of evaluating the proposed method two sets of carbon nitride samples deposited on glass substrates were analyzed; one prepared by pulsed laser ablation (PLA) and the second by magnetron sputtering. The obtained results using different conventional methods were compared with the results of the ASF method and a good agreement between the values and the tendencies with the experimental conditions used to prepare the films were observed.     

电子束蒸发制备氮化硼薄膜的红外光谱研究

用电子束蒸发法制备氮化硼薄膜, 分别研究束流大小和蒸发时间长短对薄膜质量的影响, 薄膜以红外吸收光谱标识。实验结果表明, 束流大小和蒸发时间长短对薄膜质量都有影响, 经过900℃氮气保护退火后, 都得到了高立方相含量的氮化硼薄膜。