包含倾斜晶界的双晶ZnO的热输运行为

用嵌入位错线法和重合位置点阵法构建含有小角度和大角度倾斜角的双晶氧化锌纳米结构.用非平衡分子动力学方法模拟双晶氧化锌在不同倾斜角度下的晶界能、卡皮查热阻,并研究了样本长度和温度对卡皮查热阻和热导率的影响.模拟结果表明,晶界能在小角度区域随倾斜角线性增加,而在大角度区域达到稳定,与卡皮查热阻的变化趋势一致.热导率随样本长度的增加而增加,卡皮查热阻表现出相反的趋势.然而随着温度的增加,热导率和卡皮查热阻都减小.通过比较含5.45°和38.94°晶界样本的声子态密度,发现声子光学支对热传导的影响不大,主要由声子声学支贡献,大角度晶界对声子散射作用更强,声学支波峰向低频率移动.     

Bulk and boundary critical behavior at Lifshitz points

Lifshitz points are multicritical points at which a disordered phase, a homogeneous ordered phase, and a modulated ordered phase meet. Their bulk universality classes are described by natural generalizations of the standard φ⁴ model. Analyzing these models systematically via modern field-theoretic renormalization group methods has been a long-standing challenge ever since their introduction in the middle of 1970s. We survey the recent progress made in this direction, discussing results obtained via dimensionality expansions, how they compare with Monte Carlo results, and open problems. These advances opened the way towards systematic studies of boundary critical behavior atm-axial Lifshitz points. The possible boundary critical behavior depends on whether the surface plane is perpendicular to one of them modulation axes or parallel to all of them. We show that the semi-infinite field theories representing the corresponding surface universality classes in these two cases of perpendicular and parallel surface orientation differ crucially in their Hamiltonian’s boundary terms and the implied boundary conditions, and explain recent results along with our current understanding of this matter.     

Euclidean pseudoduality and boundary conditions in sigma models

We discuss pseudoduality transformations in two-dimensional conformally invariant classical sigma models, and extend our analysis to a given boundaries of world-sheet, which gives rise to an appropriate framework for the discussion of the pseudoduality between D-branes. We perform analysis using the Euclidean spacetime and show that structures on the target space can be transformed into pseudodual manifold identically. This map requires that torsions and curvatures related to individual spaces are the same when connections are Riemannian. Boundary pseudoduality imposes locality condition.     

Diagrammatic perturbation approach to finite-size and surface critical behavior for Dirichlet boundary conditions

A recent field-theoretic treatment of finite-size effects in the presence of Dirichlet boundary conditions is developed further. A simplified perturbation approach is introduced that makes higher orders amenable to diagrammatic evaluation. As an illustration the surface energy and surface specific heat are calculated up to two-loop order. Further suggestions of future research are discussed.     

Asymmetric tilt grain boundary structure and energy in copper and aluminium

Atomistic simulations were employed to investigate the structure and energy of asymmetric tilt grain boundaries in Cu and Al. In this work, we examine the Σ5 and Σ13 systems with a boundary plane rotated about the ? 100 ? misorientation axis, and the Σ9 and Σ11 systems rotated about the ? 110 ? misorientation axis. Asymmetric tilt grain boundary energies are calculated as a function of inclination angle and compared with an energy relationship based on faceting into the two symmetric tilt grain boundaries in each system. We find that asymmetric tilt boundaries with low index normals do not necessarily have lower energies than boundaries with similar inclination angles, contrary to previous studies. Further analysis of grain boundary structures provides insight into the asymmetric tilt grain boundary energy. The Σ5 and Σ13 systems in the ? 100 ? system agree with the aforementioned energy relationship; structures confirm that these asymmetric boundaries facet into the symmetric tilt boundaries. The Σ9 and Σ11 systems in the ? 110 ? system deviate from the idealized energy relationship. As the boundary inclination angle increases towards the Σ9 (221) and Σ11 (332) symmetric tilt boundaries, the minimum energy asymmetric boundary structures contain low index {111} and {110} planes bounding the interface region.     

Computer simulation of vacancy migration in a fcc tilt boundary

Vacancy jumps in a bicrystal model of Σ = 5 (36.9°) [001] tilt boundary in aluminum have been observed at temperatures between 700 and 800 K by means of molecular dynamics simulation. Preliminary results indicated considerable structure dependence and yielded an activation energy for vacancy migration of 0.38 eV. In contrast to a previous study of vacancy migration in a bcc tilt boundary, thermal activation of a boundary interstitial by Frenkel pair production was not observed.     

Non-equilibrium grain boundary cosegregation of Mo and P

The segregation of P at grain boundaries is believed to be an important cause of temper embrittlement in steels. As an alloy element, Mo may reduce the embrittlement. However, the concentration measured by Auger electron spectroscopy at the grain boundary in 2.25Cr1MoV and 12Cr1MoV showed that the concentration of P increased with that of Mo, which indicates that Mo and P cosegregated to the grain boundary in Cr-Mo steels.     

Pressure induced quantum critical point and non-fermi-liquid behavior in BaVS3

We report on experiments giving evidence for quantum effects of electromagnetic flux in barium alumosilicate glass. In contrast to expectation, below 100 mK the dielectric response becomes sensitive to magnetic fields. The experimental findings include both lifting of the dielectric saturation by weak magnetic fields and oscillations of the dielectric response in the low temperature resonant regime. As the origin of these effects we suggest that the magnetic induction field violates the time reversal invariance leading to a flux periodicity in the energy levels of tunneling systems. At low temperatures, this effect is strongly enhanced by the interaction between tunneling systems and thus becomes measurable.     

Quantum critical scaling and the origin of non-fermi-liquid behavior in Sc1-xUxPd3

We used inelastic neutron scattering to study magnetic excitations of Sc1-xUxPd3 for U concentrations (x=0.25, 0.35) near the spin glass quantum critical point (QCP). The excitations are spatially incoherent, broad in energy (E=variant Planck's over 2piomega), and follow omega/T scaling at all wave vectors investigated. Since similar omega/T scaling has been observed for UCu5-xPdx and CeCu6-xAux near the antiferromagnetic QCP, we argue that the observed non-Fermi-liquid behavior in these f-electron materials arises from the critical phenomena near a T=0 K phase transition, irrespective of the nature of the transition.     

A geometrical construction of rational boundary states in linear sigma models

Starting from the geometrical construction of special Lagrangian submanifolds of a toric variety, we identify a certain subclass of A-type D-branes in the linear sigma model for a Calabi–Yau manifold and its mirror with the A- and B-type Recknagel–Schomerus boundary states of the Gepner model, by reproducing topological properties such as their labeling, intersection, and the relationships that exist in the homology lattice of the D-branes. In the non-linear sigma model phase these special Lagrangians reproduce an old construction of 3-cycles relevant for computing periods of the Calabi–Yau, and provide insight into other results in the literature on special Lagrangian submanifolds on compact Calabi–Yau manifolds. The geometrical construction of rational boundary states suggests several ways in which new Gepner model boundary states may be constructed.     

Variation of the critical behavior in metamagnets

From a study of a simple mean field model for metamagnets we predict that new types of critical behavior may be observed in FeBr₂ and other similar systems.     

Computer simulation of the critical behavior of 3D disordered ising model

The critical behavior of the disordered ferromagnetic Ising model is studied numerically by the Monte Carlo method in a wide range of variation of concentration of nonmagnetic impurity atoms. The temperature dependences of correlation length and magnetic susceptibility are determined for samples with various spin concentrations and various linear sizes. The finite-size scaling technique is used for obtaining scaling functions for these quantities, which exhibit a universal behavior in the critical region; the critical temperatures and static critical exponents are also determined using scaling corrections. On the basis of variation of the scaling functions and values of critical exponents upon a change in the concentration, the conclusion is drawn concerning the existence of two universal classes of the critical behavior of the diluted Ising model with different characteristics for weakly and strongly disordered systems.     

Nonasymptotic critical 3D-XY behavior for the nematic-smectic-A 1 liquid crystal transition

High-resolution X-ray data near the nematic (N)-smectic-A ₁ (SmA ₁) transition are reported for two polar liquid-crystal systems with large nematic ranges. The correlation volume ξ_∥ ξ _⊥ ² , the smectic order-parameter susceptibility σ, and the heat capacityC _p of these systems and two other N-SmA ₁ systems are in excellent agreement with exact theoreticalpreasymptotic 3D-XY predictions. The importance of correction terms for ξ_∥ ξ _⊥ ² and σ as well asC _p is demonstrated. Universal features of the 3D-XY model are obeyed except for anisotropy in the correlation lengths ξ_∥ and ξ_⊥.     

Increase of YBa2Cu3O7 critical currents by Mo substitution and high-pressure oxygen annealing

We have previously shown for powders that Mo substitution into the CuO chains of YBa₂Cu₃O₇ can create effective pinning centres which significantly increase the critical current density (j_c) in 7 T field by a factor of 4 and 10 at 50 and 60 K, respectively (Rogacki et al., 2006) [1]. The present work reports on the influence of the Mo substitution and high-pressure oxygen annealing on the pinning properties and critical currents of YBa₂Cu_3-xMo_xO_7-d by comparing pure (x = 0, d > 0) and substituted (x = 0.03, d < 0) single crystals. Pinning properties have been investigated by measurements of magnetization loops and calculations of j_c in the ab-plane, in the temperature range from 2 to 90 K and in fields up to 14 T. Depending on the Mo substitution and the oxygen treatment, several types of pinning centres increasing j_c have been revealed and analysed in the frame of Dew-Hughes’ and Kramer's models.     

The role of misfit dislocations in tilt boundary formation in heterosystems with nonsingular orientations

The structural state of Ge _x Si_{1 ? x} films on miscut Si(1 1 13) substrates is studied by X-ray diffraction. Triclinic distortions that arise in the Ge _x Si_{1 ? x} film are analyzed. The coincidence of the tilt axis and the miscut axis is established. Different misfit-dislocation networks that generate tilt boundaries are considered.