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1.
Using the coupled cluster method we investigatespin-s J 1-J′ 2 Heisenberg antiferromagnets (HAFs) on an infinite, anisotropic, two-dimensional triangular lattice for the two cases where the spin quantum number s = 1 and s = $\frac{3} {2}$\frac{3} {2}. With respect to an underlying square-lattice geometry the model has antiferromagnetic (J 1 > 0) bonds between nearest neighbours and competing (J′ 2 > 0) bonds between next-nearest neighbours across only one of the diagonals of each square plaquette, the same diagonal in each square. In a topologically equivalent triangular-lattice geometry, the model has two types of nearest-neighbour bonds: namely the J′ 2κJ 1 bonds along parallel chains and the J 1 bonds producing an interchain coupling. The model thus interpolates between an isotropic HAF on the square lattice at one limit (κ = 0) and a set of decoupled chains at the other limit (κ → ∞), with the isotropic HAF on the triangular lattice in between at κ = 1. For both the spin-1 model and the spin-$\frac{3} {2}$\frac{3} {2} model we find a second-order type of quantum phase transition at κ c = 0.615 ± 0.010 and κ c = 0.575 ± 0.005 respectively, between a Néel antiferromagnetic state and a helically ordered state. In both cases the ground-state energy E and its first derivative dE/ are continuous at κ = κ c , while the order parameter for the transition (viz., the average ground-state on-site magnetization) does not go to zero there on either side of the transition. The phase transition at κ = κ c between the Néel antiferromagnetic phase and the helical phase for both the s = 1 and s = $\frac{3} {2}$\frac{3} {2} cases is analogous to that also observed in our previous work for the s = $\frac{1} {2}$\frac{1} {2} case at a value κ c = 0.80 ± 0.01. However, for the higher spin values the transition appears to be of continuous (second-order) type, exactly as in the classical case, whereas for the s = $\frac{1} {2}$\frac{1} {2} case it appears to be weakly first-order in nature (although a second-order transition could not be ruled out entirely).  相似文献   

2.
The Raman spectra of the (GaN)129, (SiO2)86, and (GaN)54(SiO2)50 nanoparticles were calculated using the molecular dynamics method. The spectrum of (SiO2)86 had three broad bands only, whereas the Raman spectrum of (GaN)129 contained a large number of overlapping bands. The form of the Raman spectrum of (GaN)54(SiO2)50 was determined by the arrangement of the GaN and SiO2 components in it. The nanoparticle with a GaN nucleus had a continuous fairly smooth spectrum over the frequency range 0 ≤ ω ≤ 600 cm−1, whereas the spectrum of the nanoparticle with a SiO2 nucleus contained well-defined bands caused by vibrations of groups of atoms of different kinds and atoms of the same kind.  相似文献   

3.
The possibility of mechanical detection of Casimir friction with the use of a noncontact atomic force microscope is discussed. A SiO2 probe tip located above a graphene-coated SiO2 substrate is subjected to the frictional force caused by a fluctuating electromagnetic field produced by a current in graphene. This frictional force will create the bend of a cantilever, which can be measured by a modern noncontact atomic force microscope. Both the quantum and thermal contributions to the Casimir frictional force can be measured using this experimental setup. This result can also be used to mechanically detect Casimir friction in micro- and nanoelectromechanical systems.  相似文献   

4.
The discrete breathers in graphane in thermodynamic equilibrium in the temperature range 50–600 K are studied by molecular dynamics simulation. A discrete breather is a hydrogen atom vibrating along the normal to a sheet of graphane at a high amplitude. As was found earlier, the lifetime of a discrete breather at zero temperature corresponds to several tens of thousands of vibrations. The effect of temperature on the decay time of discrete breathers and the probability of their detachment from a sheet of graphane are studied in this work. It is shown that closely spaced breathers can exchange energy with each other at zero temperature. The data obtained suggest that thermally activated discrete breathers can be involved in the dehydrogenation of graphane, which is important for hydrogen energetics.  相似文献   

5.
Magnetic core/shell (CS) nanocomposites (MNCs) are synthesized using a simple method, in which a magnesium ferrite nanoparticle (MgFe2O4) is a core, and an amorphous silicon dioxide (silica SiO2) layer is a shell. The composition, morphology, and structure of synthesized particles are studied using X-ray diffraction, field emission electron microscopy, transmission electron microscopy (TEM), energy-dispersive spectroscopy (EDS), scattering electrophoretic photometer, thermogravimetric analysis (TGA), and Mössbauer spectroscopy. It is found that the MgFe2O4/SiO2 MNC has the core/shell structure formed by the Fe?O–Si chemical bond. After coating with silica, the MgFe2O4/SiO2 MNC saturation magnetization significantly decreases in comparison with MgFe2O4 particles without a SiO2 shell. Spherical particles agglomerated from MgFe2O4 nanocrystallites ~9.6 and ~11.5 nm in size function as cores coated with SiO2 shells ~30 and ~50 nm thick, respectively. The total size of obtained CS MNCs is ~200 and 300 nm, respectively. Synthesized CS MgFe2O4/SiO2 MNCs are very promising for biomedical applications, due to the biological compatibility of silicon dioxide, its sizes, and the fact that the Curie temperature is in the region required for hyperthermal therapy, 320 K.  相似文献   

6.
In this article, the lubrication properties of ZrO2/SiO2 composite nanoparticles modified with aluminum zirconium coupling agent as additives in lubricating oil under variable applied load and concentration fraction were reported. It was demonstrated that the modified nanoparticles as additives in lubrication can effectively improve the lubricating properties. Under an optimized concentration of 0.1 wt%, the average friction coefficient was reduced by 16.24%. This was because the nanoparticles go into the friction zone with the flow of lubricant, and then the sliding friction changed to rolling friction with a result of the reduction of the friction coefficient.  相似文献   

7.
Refractive-index tailoring and morphological evolutions in two different thin film composite systems of gadolinia–silica (Gd2O3:SiO2) and zirconia–silica (ZrO2:SiO2) deposited through reactive electron-beam codeposition processes are discussed in this research paper. For Gd2O3:SiO2 the refractive-index tuning has been achieved from 1.45 to 2.18, whereas in the case of ZrO2:SiO2 the achieved tunable range is from 1.45 to 2.45 in the ultraviolet region. Under certain compositional mixings with lower silica fractions both the systems demonstrated relative microstructural and morphological densifications. Such evolutions were very successfully derived through phase-modulated ellipsometry and atomic force microscopy. The composition-dependent refractive-index tailoring and microstructural densifications have been investigated by adopting Tauc–Lorentz and single-effective-oscillator models. The morphological correlation functions have also very aptly supported such evolutions in these composite films. These experimental results indicate their favourable properties and applicability down to the extreme ultraviolet wavelength region of the electromagnetic spectrum. PACS 42.79.Wc; 78.66.-w; 78.20.Ci; 61.16.Ch; 42.70.-a;68.55.-a; 68.35.Bs; 81.15.Ef  相似文献   

8.
Si/SiO2 Fabry–Pérot microcavities with a silicon nanocrystal (Si-nc) active spacer have been realized using a novel process based on a reactive magnetron sputtering of a pure silica target. Spectral, spatial and temporal behaviours of the quantum dots confined inside the resonator are detailed. Compared with a reference sample, the spectral and spatial emission distributions are significantly narrowed and the forward emission intensity is enhanced. Time resolved photoluminescence measurements also revealed an increase of the spontaneous emission rate. PACS 42.70.Qs; 78.55.-m; 78.66.-w  相似文献   

9.
A large variety of glass and glass ceramics may be obtained by sol-gel process from hydrolysis of tetraethoxysilane. The transformation involves hydrolysis and polycondensation reactions leading to the growth of clusters that eventually collide together to form a gel. The structure and properties of the final product have been found to be strongly dependent on the initial conditions of preparation. Silica nanocomposites based on Fe2O3/SiO2 were prepared with the help of ultrasonic activation and subsequent annealing in nitrogen atmosphere or air with concentrations of iron oxide of about 20 to 30wt.%.  相似文献   

10.
SiO2-TiO2 films [Si:Ti = 1:(0.06–2.3)] are obtained by the sol-gel method. The structural and photoluminescent properties of the films and powders heat-treated at different temperatures are studied. It is shown that after 700°C the composite consists of TiO2 crystallites that are structurally similar to anatase and distributed in an amorphous SiO2 matrix. The photoluminescence spectra have maxima at 450–500 nm. The photoluminescence intensity depends on the treatment temperature and TiO2 content. __________ Translated from Zhurnal Prikladnoi Spektroskopii Vol. 74, No. 3, pp. 357–361, May–June, 2007.  相似文献   

11.
The controllable synthesis and characterization of novel thermally stable silver-based particles are described. The experimental approach involves the design of thermally stable nanostructures by the deposition of an interfacial thick, active titania layer between the primary substrate (SiO2 particles) and the metal nanoparticles (Ag NPs), as well as the doping of Ag nanoparticles with an organic molecule (Congo Red, CR). The nanostructured particles were composed of a 330-nm silica core capped by a granular titania layer (10 to 13 nm in thickness), along with monodisperse 5 to 30 nm CR-Ag NPs deposited on top. The titania-coated support (SiO2/TiO2 particles) was shown to be chemically and thermally stable and promoted the nucleation and anchoring of CR-Ag NPs, which prevented the sintering of CR-Ag NPs when the structure was exposed to high temperatures. The thermal stability of the silver composites was examined by scanning electron microscopy (SEM) and high-resolution transmission electron microscopy (HRTEM). Larger than 10 nm CR-Ag NPs were thermally stable up to 300 °C. Such temperature was high enough to destabilize the CR-Ag NPs due to the melting point of the CR. On the other hand, smaller than 10 nm Ag NPs were stable at temperatures up to 500 °C because of the strong metal-metal oxide binding energy. Energy dispersion X-ray spectroscopy (EDS) was carried out to qualitatively analyze the chemical stability of the structure at different temperatures which confirmed the stability of the structure and the existence of silver NPs at temperatures up to 500 °C.  相似文献   

12.
Areas of single-layer MoS2 film can be prepared in a tube furnace without the need for temperature control. The films were characterized by means of Raman spectroscopy, photoluminescence, low-energy electron diffraction and microscopy, and X-ray photoelectron spectroscopy and mapping. Transport measurements show n-doped material with a mobility of 0.26 cm2 V-1 s-1.  相似文献   

13.
14.
The structure of polycrystalline Fe films grown on an oxidized Si(001) surface at room temperature has been studied by the technique of high-energy electron diffraction. It has been found that the grain orientation in the films depends of the amount of deposited iron. In Fe films less than 5 nm thick, grains have been found to be randomly oriented. Fe films more than 5 nm in thickness exhibit the (111) texture with an axis coinciding with the surface normal. The angular dispersion of the [111] direction in the Fe lattice from the surface normal is ±25°. It has been found that as the Fe films become thicker, the (111) texture changes to the (110) texture.  相似文献   

15.
The optical and magnetooptical properties of the new granular nanocomposites (CoFeB)/(SiO2) and (CoFeZr)/(Al2O3), which are grains of amorphous ferromagnetic alloys embedded in dielectric matrices, have been studied. The dependence of the optical, magnetooptical, and magnetic properties of the nanocomposites on their qualitative and quantitative composition, as well as on the conditions of their preparation, was investigated. Spectra of the dielectric functions ε = ε1 ? iε2 were obtained by the ellipsometric method in the range 0.6–5.4 eV. Above 4.2 eV, the absorption coefficient of the (CoFeB)/(SiO2) composites was found to be close to zero for all magnetic-grain concentrations. The polar Kerr effect measured at a photon energy of 1.96 eV in dc magnetic fields of up to 15 kOe reaches values as high as 0.25°–0.3° for these nanocomposites and depends only weakly on the conditions of preparation. On the other hand, the (CoFeZr)/(Al2O3) nanostructures reveal a considerable difference in the concentration dependences of the Kerr effect between samples prepared in a dc magnetic field and in zero field.  相似文献   

16.
This paper reports a systematic investigation of the growth and attachment of small gold nanoparticles to the functionalized surface of larger silica nanoparticles by three different methods. Nearly monodispersed silica particles and gold nanoparticles were prepared by sol-gel method. The size of the particle could be altered by changing the concentration of reactants, temperature and the time for which they react. The nanocoreshell particles prepared by three different methods were studied using scanning electron microscopy (SEM), UV-vis spectroscopy and Fourier transform infrared spectroscopy. We have found that the third method (c), a combination of the first two methods (a) and (b), has given better results.   相似文献   

17.
Heterogeneous photo-Fenton SiO2/Fe3O4/C@TiO2 (SFCT) catalyst with a core-multishell structure and a diameter of about 550 nm was successfully prepared and was characterized by scanning electron microscopy (SEM), TEM, XRD, Raman, and Fourier transform infrared (FT-IR). The results illustrated that anatase TiO2 coexisted with rutile TiO2, in which the anatase phase was the main crystal phase. In addition, the catalytic activity of SFCT catalyst had been evaluated in the catalytic degradation on p-nitrophenol (PNP). The influence factors on the PNP degradation, including SFCT component ratio (m SFC/ m TiO2), H2O2 dosage, solution pH, and PNP concentration, had been investigated. And the contrast experiments about the photo-Fenton catalytic mechanism revealed that the SFCT-2 catalyst possessed a superior activity in the neutral environment due to the optimal activity matching between Fe3O4 and TiO2, and it exhibited the stable catalytic performance after five successive recycles. Therefore, the SFCT-2 catalyst had a promising application for the photo-Fenton degradation of organic contaminant.  相似文献   

18.
The thermal effects produced by continuous-wave laser radiation on free-standing Si/SiO2 superlattices are studied. We compare two samples with different SiO2 layer thicknesses (2 and 6 nm) and the same Si layer thickness (2 nm). The as-prepared free-standing superlattices contain some amount of Si nanocrystals (Si-nc). Intense laser irradiation at 488 nm of the as-prepared samples enhances the Raman scattering of Si-nc by two orders of magnitude. This laser-induced crystallization originates from melting of Si nanostructures in silica, which makes Si-nc better ordered and better isolated from the oxide surrounding. Continuous-wave laser control of Si-nc stress was achieved in these samples. In the proposed model, intense laser radiation melts Si-nc, and Si crystallization upon cooling down from the liquid phase in a silica matrix leads to compressive stress. The Si-nc stress can be tuned in the ∼3 GPa range using laser annealing below the Si melting temperature. The high laser-induced temperatures were verified with Raman spectroscopy. The laser-induced heat leads to a strongly nonlinear rise of light emission. The light emission is also observed in the anti-Stokes region, and its temperature dependence is practically the same for the two studied samples. The laser-induced temperature is essentially controlled by the absorbed laser power. PACS 78.55.-m; 78.20.-e; 68.55.-a; 78.30.-j  相似文献   

19.
Dielectric properties of the composites based on nanodisperse silica hydrosol and ferroelectric triglycinesulfate (TGS) are investigated. The studies allow us to expect the presence of the ferroelectric state in the composites.  相似文献   

20.
The photoluminescence of quantum dots in Si/Ge/SiO2/Si and Si/Ge/Si structures is investigated as a function of temperature. The low activation energies for the temperature quenching of photoluminescence of germanium quantum dots in both structures are explained in terms of the thermally stimulated capture of holes from quantum dots to the energy levels of defects localized in their vicinity.  相似文献   

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