Thermodynamic Properties of Molecular Cryocrystals of Nitrogen Type with FCC Structure: Contribution from Lattice Vibrations and Molecular Rotational Motion

Nguyen Quang Hoc(1), Nguyen Ngoc Anh(2), Nguyen The Hung(2), Nguyen Duc Hien(2) and Nguyen Duc Quyen(3)

The analytic expressions of thermodynamic quantities such as the Helmholtz free energy, the internal energy, the entropy, the molar specific heats under constant volume and under constant pressure, etc. of molecular cryocrystals of N2 type with fcc structure are obtained by the statistical moment method and the self-consistent field method taking account of the anharmonicity in lattice vibrations and molecular rotational motion. Numerical results for molecular cryocrystals of N2 type(N2, CO,CO2 and N2O) are compared with the experimental data.

Vo Thanh Lam, Nguyen Ngoc Hieu

The quinoid structure with two different bond lengths has been proposed for all types of carbon nanotubes (i.e., armchair, zigzag, and chiral). In this work we study electronic energy band structure of carbon nanotubes with Quinoid structure by using the tight-binding approximation. Effect of alternation bonds on band gap and the Fermi level is also investigated. The numerical calculations are performed with value of the difference between long and short bond lengths being up to 0.004 nm. The results are compared with well-know results for pristine carbon nanotubes.

Bui Duc Hung, Nguyen Vu Nhan, Luong Van Tung , and Nguyen Quang Bau

The quantum theory of the photostimulated effects in quantum wells has been studied based on the quantum kinetic equation for electrons with a parabolic potential . In this case, electrons system in quantum wells is placed in a constant electric field , a electromagnetic wave and in the presence of an intense laser field The effect of constant electric field , in quantum wells appears a constant current carrier along vector . But, in the presence of laser radiation and polarized electromagnetic wave can influence that current carrier, and do appear an electric field intensity vector with open circuit conditions. Hence, the analytic expressions of electric field intensity vector along the coordinate axes has been calculated. The dependence of the components on the frequency ï— of the laser radiation field, the frequency ï· of the electromagnetic wave field, the frequency of the parabolic potential are shown. From the analytic results, when , the result will turn back to the photostimulated kinetic effects in semiconductors.

Bui Thi Hanh(1) and Nguyen Van Lien (1,2)

Due to the Klein tunneling in graphene the electrostatic confinement potentials can not induce a bound state, but only the quasi-bound (resonant) states with a finite carrier life-time. We suggest a realistic model of disk-like graphene quantum dots which exhibit the quasi-bound states with carrier life-time long enough for practical applications. In the suggested model a quantum dot is characterized by the three parameters - the radius R of the circular top gate, the confinement potential magnitude , and the field in the dot boundary region. The last two parameters, and , play a critical role in determining the carrier life-time of quasi-bound states and both can be easily tuned by independently varying the voltages, applied to the top gate and to the back one. By solving the Dirac equation we calculated the resonant position as well as the level width of quasi-bound states for modeled quantum dots with different , , and and for various values of the electron angular momentum . Obtained results give an estimation for the dependence of the quasi-bound states life-time on the quantum number as well as the dot parameters and provide a range of parameter values when the quasi-bound states life-time could be long enough.

Dang Van Quan and Cao Huy Thien

The kinetics of the boson condensation for polaritons interacting in semiconductor quantum wire is studied within rate equations. The numerical results are performed of the quasi-stationary excitation. The resulting first-order spatial coherence of condensed polaritons are in qualitative agreement with experiment.

Nguyen Quoc Khanh, Nguyen Minh Quan and Nguyen Ngoc Thanh Nam

Chúng tôi đã tính độ linh động, điện trở và thời gian tán xạ của khí điện tử giả hai chiều trong giếng lượng tử SiGe/Si/SiGe ở nhiệt độ bất kỳ cho cả hai trường hợp có hoặc không có từ trường song song đối với hai cơ chế tán xạ tạp chất và nhám bề mặt. Chúng tôi xem xét hiệu ứng nhiều hạt, sự phụ thuộc vào bề dày lớp và chuyển pha kim loại-cách điện. Ở nhiệt độ thấp kết quả của chúng tôi phù hợp với kết quả của A. Gold (J. Appl. Phys. 108, 063710 (2010)) và với các kết quả thực nghiệm gần đây. Các kết quả ở nhiệt độ và từ trường khác không của chúng tôi kết hợp với các thí nghiệm sẽ cho phép xác dịnh các cơ chế tán xạ chủ yếu, các tham số tán xạ và hiệu ứng nhiều hạt.

Nguyen Quoc Khanh, Nguyen Ngoc Thanh Nam and Nguyen Minh Quan

Chúng tôi đã tính độ linh động, điện trở và thời gian tán xạ của khí điện tử giả hai chiều trong cấu trúc dị thể SiGe/Si ở nhiệt độ bất kỳ cho cả hai trường hợp có hoặc không có từ trường song song đối với hai cơ chế tán xạ tạp chất và nhám bề mặt. Chúng tôi xem xét hiệu ứng nhiều hạt, sự phụ thuộc vào vị trí pha tạp và chuyển pha kim loại-cách điện. Ở nhiệt độ thấp kết quả của chúng tôi phù hợp với kết quả của A. Gold ( Semicond. Sci. Technol. 26, 045017 (2011)) và với các kết quả thực nghiệm gần đây. Các kết quả ở nhiệt độ và từ trường khác không của chúng tôi kết hợp với các thí nghiệm sẽ cho phép xác dịnh các cơ chế tán xạ chủ yếu, các tham số tán xạ và hiệu ứng nhiều hạt.

C. Q. Ky and L. V. Vinh

Molecular dynamics simulation with Sutton-Chen type of embedded atom method is used to study the mechanical properties of amorphous Ni and Ni nanowires. An uniaxial tensile strain along the [001] axis with varying strain rates is applied to the sample to investigate the effects of strain rates and thermal conditions on the deformation characteristics and mechanical properties of the systems. The deformation mechanism of the systems is investigated in detail. The effect of structure on the observed deformation modes in models is also discussed.

Huynh Vinh Phuc(1), Luong Van Tung(1), Le Dinh(2) and Tran Cong Phong(2)

In this paper, the analytic expression for nonlinear absorption power (NAP) in doped semiconductor superlattice (DSSL) are obtained by using the operator projection technique in the case of electron-optical phonon scattering. We have obtained nonlinear optically detected electrophonon resonance (NODEPR) condition as a function of well width and concentration of the donor impurities. Anomalous behaviors of the NODEPR spectrum such as the splitting of NODEPR peaks for two photon absorption process are discussed. From the graphs of the NAP, we obtaine the NODEPR line-width as profiles of curves. Computational results show that the NODEPR line-width increases with temperature and decreases with well width. The contribution of two photons absorption process to absorption power is smaller than ones of one photon process.

Huynh Vinh Phuc(1), Luong Van Tung(1), Le Dinh(2) and Tran Cong Phong(2)

In this paper, we study the line-width of cyclotron resonance peak in graphene. Analytical expressions are derived for absorption power using projection operator methods. The dependence of absorption power on the photon energy is numerically calculated and graphically plotted. From the graphs of the absorption power, we obtained cyclotron resonance line-width (CRLW) as profiles of curves. The dependence of CRLW on the magnetic field B, and on the temperature T are considered.

Nguyen Quoc Thai

Khảo sát tiết diện sinh neutron trong phản ứng (p,n) với các mức năng lượng bắn phá khác nhau, ta nhận thấy rằng tiết diện sinh neutron trong phản ứng (p,n) phụ thuộc vào góc khối. Tiết diện sinh neutron tập trung nhiều nhất ở vùng góc khối bằng 0 đến 30độ tức là khu vực theo phương của proton tới và mở rộng xung quanh một góc khoảng 30độ.

Nguyen Dinh Nam, Do Tuan Long, Nguyen Duc Huy, Nguyen Quang Bau, Hoang Dinh Trien

The relative magnetoresistance in quantum wells with parabolic potential in the presence of magnetic field under the influences of electromagnetic wave is theoretically studied based on a set of quantum kinetic equations for the electron – phonon system. Analyzing the analytical expressions obtained, we see that the relative magnetoresistance depends on the intensity of electromagnetic waves , magnetic field B, the frequency of the radiation and the relaxation time of carrier. Comparing with the results obtained in case of bulk semiconductors, we see the differences and the influences of finite-size effects on the relative magnetoresistance.

Le Thanh Hai(1) and Nguyen Nhu Dat(2)

The dynamical response of an electron system confined in a semiconductor quantum wire is calculated within the random phase approximation. The dielectric constant mismatch and the effect of the metallic outer shell are taken into account.

Hoang Dinh Trien(1), Le Thai Hung(1), Vu Thi Hong Duyen(1), Nguyen Thu Huong(2), Nguyen Vu Nhan(2) and Nguyen Quang Bau(1)

Impact of the external magnetic field and the confinement of phonons on the nonlinear absorption coefficient (NAC) of a strong electromagnetic wave (EMW) by confined electrons in compositional superlattices is theoretically studied by using the quantum transport equation for electrons. The formula which shows the dependence of the NAC on the energy ($\hbar \Omega$), the amplitude (E0) of the strong EMW, the temperature (T) of the system, the energy ($\hbar \Omega_{B}$) of external magnetic field and quantum number m characterizing confined phonons is obtained. The analytic expressions are numerically evaluated, plotted and discussed for a specific of the GaAs-Al0.3Ga0.7As compositional superlattices. The results show clearly the difference in the spectrums and values of the NAC in this case from those in the case with out the impact of the external magnetic field and the confinement of phonons.

Nguyen Duong Bo and Tran Minh Tien

Quantum mixtures of ultracold atoms with multiple degeneracy are modeled by a multiple-degeneracy Falicov-Kimball model. The model is analyzed using the dynamical mean field theory. Multicolor charge-ordered or multicolor segregated phase are stabilized at low temperature depending on the model parameters, while the homogeneous mixture appears at high temperature. A phase diagram is also presented

Ha Dang Khoa(1) and Nguyen Thi Phuong Lan(2)

By the moment method we obtain the analytic expression of the Helmholtz free energy of the multi component alloy with face-centred cubic (fcc) and body-centred cubic (bcc) structure. Using the obtained expression of the Helmholtz free energy, we obtain the state equation, the equation for equilibrium long-order parameter and the analytic expressions for thermodynamic quantities of multi component alloy with cubic (fcc) and (bcc) structure such as the isothermal compressibility, the thermal expansion coefficient, the specific heats at constant volume CV and at constant pressure CP. etc. Our theoretical results are applied to investigate the multi component alloys 76Ni-15Cr-9Fe, 66Ni-29Cu-3Al, 35Ni-50Fe-15Cr, 60Ni-24Fe-16Cr. The numerical results for the order parameters, the specific heats, etc. are shown to agree well with the experimental data.

Do Chieu Ha(1) and Nguyen Nhat Khanh(2)

In this paper, the surrounded atom model is developed to study the order-disorder phase transition in binary alloys. We calculate the configurational free energy of the alloys, derive the equation of equilibrium and determine the critical temperature of the phase transition.

P.H. Kien(1) and P.K. Hung(2)

Tracer diffusion mechanism in amorphous Fe and Fe80B20 solids is studied using the statistic relaxation model containing 200000 atoms. It was found a large number of bubbles which could break up and leads to diffusion. A bubbles diffusion mechanism is proposed and the diffusion coefficient determined in term of this mechanism is in reasonable agreement with experimental data. The decrease in diffusion coefficient upon thermal annealing observed experimentally for most amorphous alloys is interpreted as a result of reduced number of bubbles in system. Keywords: Amorphous alloys; Tracer diffusion; Bubble; Simplex.

Le Thi Thao(2), Doan Thi Thanh Binh(3), Tran Trung Nguyen(1), Le The Vinh(1).

Microstructures, refractive index and anomalies in SiO2 have been investigated by means of a molecular dynamic method. The simulation is conducted for systems prepared at six different temperatures from 2100 to 3000 K. The microstructure properties is analyzed through the pair radial distribution function, coordination number, bond-angle distribution. We focus on the correlation between structural characteristics and refractive index under temperature. The simulation reveals The appearance of refractive index anomalies and density as temperature increases.

Doan Thi Thanh Binh(1), Tran Trung Nguyen(2), Bui Danh Hao(2), Le The Vinh(2)

Microstructures and refractive index of liquid TiO2 have been investigated by means of a molecular dynamic method. The simulation is conducted for systems prepared at six different temperatures from 1100 to 3100 K. The microstructure properties is analyzed through the pair radial distribution function, bond-angle distribution. We focus on the correlation between structural characteristics and refractive index under temperature. The simulation reveals the refractive index decreases linearly as temperature increases.

M T Lan(1), N T Thao(3), T V Mung(1), H V Hue(2), N V Hong(1) and P K Hung(1)

Microstructure, dynamics and diffusion in liquid MgO have been studied by Molecular dynamics Simulation. Model consists of 2000 atoms were constructed under a wide pressure range and at temperature of 3800 K. The local structure is analyzed through the coordination distribution, topology statistics of basic structural units MgOx (x=4, 5, 6). As regards to the structural dynamics, the processes bond-breaking/reconstruction, spatially heterogeneous dynamics, clustering, structural stability (life time of basic structural units) are investigated in detail. Structural dynamics assists us to better understand various important atomistic (molecular) properties and clarified the diffusion mechanism in liquid MgO.

Pham Duy Tan(1) and Pham Dinh Tam(2),

The dependence of lattice parameter and order parameter for Cu3Au alloy is represented by the equation of state and the equation of order parameter. These equations in simple analytic form are obtained by the statistical moment method. Numerical results of the lattice parameter and the order parameter from obtained equations in different temperatures and pressures are in good agreement with experiments.

Pham Dinh Tam(1), Pham Duy Tan(2), Nguyen Quang Hoc(3), Nguyen Hong Son(4).

The equation of state, the expressions of lattice parameter and thermal expansion coefficient in general form are obtained by the statistical moment method. Applying to Cu, Ag, Au, Al and Ni metals, we determine these properties in simple analytic form for each metal. Numerical results for the thermal expansion coefficient of these metals in different temperatures and pressures are in good agreement with experiments.

Vu Van Hung(1), Luu Thi Kim Thanh(2), Duong Dai Phuong(3)

Contribution of the free electrons to the paramagnetic susceptibility of metals at low temperature is investigated by using the q-deformed Fermi-Dirac statistics. Besides the general Pauli term, in our analytic expression of the paramagnetic susceptibility, the contribution of the q-deformed is also taken into account. Our numerical evaluation for some typical metals Na, K, Cs, Rb and Ba shows the adequation with one measured in experimental. In the low temperature limit, we also pointed out the weakly temperature dependence of the paramagnetic susceptibility of the metals.