Van Thanh Ngo

In this review, we show our results with new interpretation on the critical exponents of thin films obtained by high-performance multi-histogram Monte Carlo simulations. The film thickness Nz consists of a few layers up to a dozen of layers in the z direction. The free boundary condition is applied in this direction while in the xy plane periodic boundary conditions are used. The large xy plane size, from 702 to 1602, is used for finite-size scaling. The Ising model is used with nearest-neighbor (NN) interaction. When Nz=1, namely the two-dimensional (2D) system, we find strictly the critical exponents given by the renormalization group. While, for Nz>1, the critical exponents calculated with the high-precision multi-histogram technique show that they deviate systematically from the 2D values and get closer to the 3D critical exponents for Nz>13 . If we argue that as long as the thickness Nz is small enough, the correlation length in the z direction is finite, it does not affect the nature of the phase transition, namely it remains in the 2D universality class. This argument is in contradiction with our numerical results which show a systematic deviation from 2D values: If we use these values of critical exponents in the hyperscaling relation with d=2, then the hyperscaling relation is violated. However, if we use the hyperscaling relation and the critical exponents obtained for Nz>1 to calculate the dimension of the system, we find the system dimension between 2 and 3. This can be viewed as an "effective" dimension. More discussion is given in the paper. We also show the cross-over between the first- and second-order transition while varying the film thickness. In addition, we will show evidence that when a 2D system has two order-parameters of different symmetries with a single transition, the critical exponents are new, suggesting a universality class of coupled two-symmetry breakings. In this case, the 2D hyperscaling does not hold. Another case is the 3D Ising model coupled to the lattice vibration: the critical exponents deviate from the 3D Ising ones, the results suggest the violation of the hyperscaling.

Rối loạn nhịp thất (RLNT) là tình trạng lâm sàng về nhịp hay gặp trước và sau phẫu thuật cầu nối chủ vành (CNCV). Các nghiên cứu trước đây chủ yếu tập trung vào RLNT giai đoạn sớm sau phẫu thuật (giai đoạn hồi sức), không có nhiềunghiên cứu về đặc điểm (tỉ lệ mắc, yếu tố nguy cơ) của RLNTở bệnh mạch vành (BMV) ổn định, nhồi máu cơ tim(NMCT) giai đoạn trước và giai đoạn ổn định sau phẫu thuật CNCV. Vì lý do đó, chúng tôi nghiên cứu đề tài này nhằm mục tiêu xác định đặc điểm RLNTvà tìm hiểu một số yếu tố ảnh hưởng RLNTtrước và sau phẫu thuật CNCV giai đoạn ổn định. Đối tượng nghiên cứu:171 bệnh nhân trải qua phẫu thuật CNCVtrong đó NMCT (n=52), BMV ổn định (n=119) tại Bệnh viện Tim Hà nội từ 6/2016 đến 8/2018. Phương pháp nghiên cứu: mô tả cắt ngang, tiến cứu. Kết quả:Tuổi trung bình 65,05 ± 7,41 năm, chủ yếu là nam giới (78,4%). Số lượng, tỉ lệ %ngoại tâm thu (NTT) thất trước phẫu thuật ở nhóm NMCT (n=52) cao hơn so với nhóm BMV ổn định (n=119) với p≥1000 pg/ml thì NTT thất với lown ≥ 3 tăng gấp 3,04 lần (p=0,05).Cùng các YTNC, nữ giới, EF < 50% và proBNP ≥1000 pg/ml có nguy cơ RLNT với Lown ≥ 3 cao gấp 2,9 đến 3,4 lần giai đoạn ổn định sau phẫu thuật 7 ngày (pKết luận:Bệnh nhân NMCT có đặc điểm RLNT trước mổ cao hơn bệnh nhân BMV ổn định, sau mổ 7 ngày không còn sự khác biệt.ProBNP ≥ 1000pg / ml là yếu tố nguy cơ độc lập vớiNTT thất mức độ nặng trước và sau phẫu thuật CNCV ở cả 2 nhóm NMCT và BMV ổn định.

We study the nature of the smectic–isotropic phase transition using a mobile 6-state Potts model. Each Potts state represents a molecular orientation. We show that with the choice of an appropriate microscopic Hamiltonian describing the interaction between individual molecules modeled by a mobile 6-state Potts spins, we observe the smectic phase dynamically formed when we cool the molecules from the isotropic phase to low temperatures (T). In order to elucidate the order of the transition and the low-T properties, we use the high-performance Wang–Landau flat energy-histogram technique. We show that the smectic phase goes to the liquid (isotropic) phase by melting/evaporating layer by layer starting from the film surface with increasing T. At a higher T, the whole remaining layers become orientationally disordered. The melting of each layer is characterized by a peak of the specific heat. Such a succession of partial transitions cannot be seen by the Metropolis algorithm. The successive layer meltings/evaporations at low T are found to have a first-order character by examining the energy histogram. These results are in agreement with experiments performed on some smectic liquid crystals.

We study the critical behaviour of very thin magnetic films. This system can be described by the q-state clock model. In order to determine the critical exponents of the system when there exists the Berezinskii–Kosterlitz–Thouless phase between the two phase transitions, we introduce a new technique for calculating the order parameter. The simulation is performed by very high-resolution Monte Carlo method with including the Wang–Landau algorithm. The results showed that the Berezinskii–Kosterlitz–Thouless phase starts to occur when with a small symmetry breaking field. We obtained not only the critical exponents of a common transition at high temperature but also the ones of unclear transition at low temperature.

Green function technique is a very efficient theoretical tool for the study of dynamical quantum processes in many-body systems. For the study of dynamical quantum processes in graphene ribbons it is necessary to know two-point Green functions of free Dirac fermions in these materials. The purpose of present work is to establish explicit expressions of two-point Green functions of free Dirac fermions in single-layer graphene ribbons with zigzag and armchair edges. By exactly solving the system of Dirac equations with appropriate boundary conditions on the edges of graphene ribbons we derive formulae determining wave functions of free Dirac fermions in above-mentioned materials. Then the quantum fields of free Dirac fermions are introduced, and explicit expressions of two-point Green functions of free Dirac fermions in single-layer graphene ribbons with zigzag and armchair edges are established.

In this paper, we investigated the role of the disulfide bridge in the structural stability of wild-type human neuroglobin. The classical simulation of the neuroglobin without the disulfide bridge was performed for a long simulation run of 240~ns using a new parameter set of Gromos96 force field and the latest data entry as the initial topologies. We used the analyzed data of original neuroglobin with the remained disulfide bridge to compare to the ones from this simulation. Our results showed that, the structure of neuroglobin was still very stable although the disulfide bridge was absent. There was only a few residues in B and C helices having a higher mobility. The most interesting result we obtained was that the increasing distance between the distal histidine and heme group could allow oxygen to bind more easily.

We propose a new model in order to study behaviors of self-organized system such as a group of animals. We assume that the individuals have two degrees of freedom corresponding one to their internal state and the other to their external state. The external state is characterized by its moving orientation. The rule of the interaction between the individuals is determined by the internal state which can be either in the non-excited state or in the excited state. The system is put under a source of external perturbation called "noise". To study the behavior of the model with varying noise, we use the Monte-Carlo simulation technique. The result clearly shows two first-order transitions separating the system into three phases: with increasing noise, the system undergoes a phase transition from a dilute disordered phase to an ordered compact phase and then to the disordered dispersed phase. These phases correspond to behaviors of animals: uncollected state at low noise, flocking at medium noise and runaway at high noise, respectively.