Khoa Vật lý, Trường Đại học Khoa học Tự nhiên - Đại học Quốc gia Hà Nội

Bài báo này nghiên cứu ảnh hưởng của nhiệt độ nung đến cấu trúc tinh thể, nhiệt độ tới hạn (Tc) và độ dẫn điện thừa của hệ thống Bi-Pb-Sr-Ca-Cu-O (BPSCCO). Các mẫu BPSCCO khối được chế tạo bằng phương pháp phản ứng trạng thái rắn. Bốn nhiệt độ khác nhau 835 °C, 840 °C, 845 °C, và 850 °C được áp dụng để nung bốn mẫu khác nhau. Cấu trúc tinh thể của các mẫu đã được điều tra thông qua các phép đo nhiễu xạ tia X (XRD) và kính hiển vi điện tử quét (SEM). Tính siêu dẫn của các mẫu được phân tích dựa trên phép đo điện trở phụ thuộc nhiệt độ. Kết quả thực nghiệm cho thấy sự tồn tại của cả hai pha Bi-2223 và Bi-2212 trong tất cả các mẫu. Về mặt định lượng, tỷ lệ thể tích của pha Bi-2223 được phát hiện là tăng từ 53.56% lên 75.97%, và kích thước hạt trung bình của pha Bi-2223 được quan sát là mở rộng từ 57.95 nm lên 86.50 nm khi nhiệt độ nung tăng từ 835 °C lên 850 °C. Ngoài ra, các phân tích độ dẫn điện thừa dựa trên lý thuyết Aslamazov - Larkin (AL) và Lawrence - Doniach (LD) cho thấy sự giảm đi của chiều dài hợp nhất (ξc(0)) từ 1.957 Å xuống 1.565 Å và khoảng cách giữa các lớp hiệu dụng (d) từ 79.7 Å xuống 64.5 Å. Trong khi đó, sức mạnh liên kết giữa hai mặt phẳng CuO2 (J) được ước lượng là tăng từ 0.00083 lên 0.00137. Những kết quả này có thể là bằng chứng để kết luận rằng việc tăng nhiệt độ nung rõ ràng cải thiện tính siêu dẫn trong hệ thống BPSCCO.

Abstract: This work studies on the thermodynamic properties of the ultra-thin magnetic films within the framework of a transverse Ising model. Equations for free energy, entropy and specific heat of spin system are obtained by using the mean field approximation. We also analyze the effect of the next nearest neighbor interaction on critical temperature, layer magnetization and specific heat of the thin films. Keywords: Magnetic ultra-thin films, transverse Ising model, next nearest neighbor interaction, critical temperature.

This report presented an effective way to synthesize MnO2/graphene nanocomposites using the plasma electrolysis method for photocatalytic degradation of methyl orange dye in water. The morphology, structure, and chemical composition of MnO2/graphene nanocomposites materials were investigated through scanning electron microscopy (SEM), Raman spectra, and Fourier-transform infrared (FTIR) spectroscopy, respectively. SEM results showed that MnO2 nanoparticles with particle sizes of 30-50 nm were attached uniformly on the surface of graphene nanosheets. The photodegradation activity was performed under UV-visible irradiation to evaluate the potential application of MnO2/graphene nanocomposites.

Noncommutative Quantum Field (NCQF) is a field defined over a space endowed with a noncommutative structure. In the last decade, the theory of NCQF has been studied intensively, and many qualitatively new phenomena have been discovered. In this article we study one of these phenomena known as UV/IR mixing.Keywords: Noncommutative quantum field theory.

Mn-doped ZnS (ZnS/Mn) nanoparticles synthesized by sono-chemical co-precipitation are combined with ZnO nanoparticles synthesized by sol-gel method for application in two-barcode encryption. Nanoparticles are mixed together in different mass proportions of ZnO: ZnS/Mn respectively being 5: 1, 10: 1, 15: 1, 20: 1, 25: 1.30 : 1 and 35: 1, before being pressed in 1 cm diameter, 1 mm thick KBr pellets. The photoluminescence (PL) of the tablet shows that the mass proportion of two materials could be distinghuished by the proportion of the PL intensities at 515 nm and 600 nm, which promises potential manufacturing ability two-substant/two-barcode information storage systems for civil security applications.Keywords: Photoluminescent encryption, Zn-based semiconductor, civil application.

The TiO2/Au nanocomposite thin films play a very important role in the plasmonic solar cell. In such as composite systems, the localized surface plasmon resonance (LSPR) of embedded metal nanoparticles determines the light absorption enhancement of the semiconductive part (TiO2); hence, enhances the efficiency of the plasmatic solar cell. The characterizations of the TiO2/Au nanocomposite thin films – such as light absorption, scattering and LSPR capacities - depend on many parameters: sizes, weight ratio of metal nanoparticles, thickness of the thin film layers and the arrangements of TiO2/Au composite in integrated-matrix system. In this work, we outline the preparation method of the TiO2/Au nanocomposite thin films, then we investigate the effect of technological conditions – such as TiO2/Au mixture solution concentration, TiO2/Au weight ratio and number of spin-coating layers – to the surface morphology of the nanocomposite to find out the optimum fabrication conditions. Some experimental results of thin film such as structural property (X ray diffraction, thin film morforlogy…) and optical property (absorption coefficience concerning LSPR…) will also be shown and discussed.Keywords: TiO2/Au thin film, LSPR, plasmonic solar cell.

In recent years, glucose oxidase (GOx) based sugar level detecting techniques have been intensively developed. In order to improve the diagnosis and desease treatment in low- and middle-income countries, the low cost, easily processing, but still high sensitive sensing systems/equipments play a very important rules in biomedicine and life science. In this work, lead sulfide (PbS) nanocolloids were used as electron receptor. The results showed that the sensitivity of the glucose sensor reached 546.2 μA cm-2 mM-1. It is note that, some early works on GOx based glucose sensor only reached sensitivity less than 100 μA cm-2 mM-1.Keywords: Lead sulfide nanoparticles, glucose sensor, glucose oxidase.

We apply classical Monte Carlo simulation to examine the thermodynamic properties of perovskites described by the Ising model with competitive magnetic interactions. By correspondingly adjusting the ferromagnetic-interaction and antiferromagnetic-interaction probabilities, \(p\) and \((1-p)\), in the regime \(p \ge 0.5\), the temperature dependence of magnetization, total energy, spin susceptibility, and specific heat consistently show a ferromagnetic to paramagnetic (FM-PM) phase transition at a critical temperature \(T_c\). Besides, the inverse susceptibility is confirmed to follow Curie-Weiss's law above another critical temperature \(T_{CW}\). By increasing the FM interaction probability, we have observed the FM-PM critical temperature \(T_c\) shifted to the higher value while the Curie-Weiss critical temperature \(T_{CW}\) moves to the lower. The different values between these two critical temperatures imply the inhomogeneity of the systems having phase separation, thus in agreement with the increased homogeneity with increasing \(p\).

CeF3nanoparticles doped with 0; 1.0; 1.5; 2.0; 2.5; 3.0 and 4.0 mol% Sm3+ were prepared by co-precipitation technique. These nanoparticles were studied by X-ray diffraction (XRD), transmission electron microscopy (TEM), photoluminescence (PL), photoluminescence excitation (PLE) spectra, energy-dispersive X-ray (EDS) and absorption spectra. The PL spectra exhibit a group of four emission lines, which are assigned to the transitions from the excited state 4G5/2 to the ground states 6HJ with J = 5/2; 7/2; 9/2 and 11/2 of Sm3+ ion. The intensity of PL related to Sm3+ ion reached to a maximum when the Sm dopant content was 2 mol%. The PLE spectra show 8 lines, which are attributed to the absorption transitions from the 6H5/2 ground state to the 4H(1)9/2, 4D(2)3/2, 6P7/2, 4F(3)7/2, 6P5/2, 4M17/2, 4I(3)13/2 and 4M15/2 excited states. Six lines among eight excitation lines were observed in the diffuse reflection spectra.Keywords: Co-precipitation, samarium doped cerium fluoride, nanopaticles, absorption, photoluminescence.

The Gaussian spin fluctuation theory is applied to study magnetic properties (Curie temperature, magnetization) of EuO  ultra-thin films within nearest neighbor and next nearest neighbor exchange approximation. The dependence of Curie temperature on the thickness of the free EuO ultrathin film is calculated and compared with the results of the other mean field method- constant coupling approximation. The effect of nearly independence of saturation magnetization on the EuO film’s thickness measured  in [8] can be explained by controlling the next nearest neighbor exchange in presence of substrates.