A remark on the twin “paradox”

The twin paradox is investigated using a method that emphasises the symmetry between the twins. All the calculations are performed using the Schwarzschild metric, making the result valid in general relativity as well in the special case of Minkowski space.     

Energy barriers associated with slip–twin interactions

The energetics of slip–coherent twin boundary (CTB) interactions are established under tensile deformation in face centered cubic (fcc) copper with molecular dynamics simulations, exploring the entire stereographic triangle. The CTBs serve as effective barriers in some crystal orientations more than others, consistent with experimental observations. The resulting dislocation structures upon slip–twin reactions are identified in terms of Burgers vector analysis. Visualization of the dislocation transmission, lock formation, dislocation incorporation to twin boundaries, dislocation multiplication at the matrix–twin interface and twin translation, growth, and contraction behaviors cover the most significant reactions that can physically occur providing a deeper understanding of the mechanical behavior of fcc alloys in the presence of twin boundaries. The results make a distinction between deformation and annealing twins interacting with incident dislocations and point to the considerable role both types of twins can play in strengthening of fcc metals.     

Top rare decays t→cV in mirror twin Higgs models

The decay t→cV(V=γ,Z,g)processes in mirror twin Higgs models with colorless top partners are studied in this paper.We report that the branchi ng ratios of these decays can strongly affect the standard model expectations in some parameter spaces and may be detectable according to the current precision electroweak measurements.Thus,constraints on the model parameters may be obtained from the branching fraction of the decay processes,which may serve as a robust detection tool for this new physics model.     

Interaction of point defects with twin boundaries in Au： A molecular dynamics study

The molecular dynamics simulation technique with many-body and semi-empirical potentials （based on the embedded atom method potentials） has been used to calculate the interactions of point defects with （1 1 1）, （1 1 3）, and （1 2 0） twin boundaries in Au at different temperatures. The interactions of single-, di-, and tri-vacancies （at on- and off-mirror sites） with the twin interfaces at 300 K are calculated. All vacancy clusters are favorable at the on-mirror arrangement near the （1 1 3） twin boundary. Single- and di-vacancies are more favorable at the on-mirror sites near the （1 1 l） twin boundary, while they are favorable at the oft-mirror sites near the （1 2 0） twin boundary. Almost all vacancy clusters energetically prefer to lie in planes closest to the interface rather than away from it, except for tri-vacancies near the （1 2 0） interface at the off-mirror site and for 3.3 and 3.4 vacancy clusters at both sites near the （1 1 1） interface, which are favorable away from the interface. The interaction energy is high at high temperatures.     

High repetition rate pulsed laser of twin wavelengths from KTiOPO4 optical parametric oscillation

In this paper, some key techniques of KTiOPO_4 optical parametric oscillation (OPO) laser with repetitive frequencies and twin wavelengths output have been analysed theoretically and studied experimentally in detail. An intracavity optical parametric oscillator (IOPO) has been applied. Operating under 20Hz, laser output energy of more than 38mJ with twin wavelengths 1.57μm and 1.06μm has been achieved. Results of the experiments agree well with the theoretical discussion.     

Pair production of neutral Higgs bosons from the left-right twin Higgs model via γγ collisions

The left-right twin Higgs (LRTH) model predicts the existence of the neutral Higgs bosons (h, φ<'0>), which can be produced in pairs (φ<'0>φ<'0>, hh, φ<'0>h) via γγ collisions at the next generation e<'+>e<'-> International Linear Collider (ILC). Our numerical results show that the production cross section of the neutral Higgs boson pair φ<'0>φ<'0> can reach 8.8 fb. The subprocess γγ→φ<'0>φ<'0> might be used to test the LRTH model in future ILC experiments.     

Pair production of neutral Higgs bosons from the left-right twin Higgs model via γγ collisions

The left-right twin Higgs (LRTH) model predicts the existence of the neutral Higgs bosons (h, Φ⁰), which can be produced in pairs (Φ⁰Φ⁰, hh, Φ⁰h) via γγ collisions at the next generation e⁺e^- International Linear Collider (ILC). Our numerical results show that the production cross section of the neutral Higgs boson pair Φ⁰Φ⁰ can reach 8.8 fb. The subprocess γγ→Φ⁰Φ⁰ might be used to test the LRTH model in future ILC experiments.     

Effect of O–O bonds on p-type conductivity in Ag-doped ZnO twin grain boundaries

Based on density functional theory, first-principles calculation is applied to study the electronic properties of undoped and Ag-doped Zn O-Σ7(12ˉ30) twin grain boundaries(GBs). The calculated result indicates that the twin GBs can facilitate the formation and aggregation of Ag substitution at Zn sites(AgZn) due to the strain release. Meanwhile, some twin GBs can also lower the ionization energy of AgZn. The density of state shows that the O–O bonds in GBs play a key role in the formation of a shallow acceptor energy level. When AgZnbonds with one O atom in the O–O bond, the antibonding state of the O–O bond becomes partially occupied. As a result, a weak spin splitting occurs in the antibonding state, which causes a shallow empty energy level above the valence band maximum. Further, the model can be applied to explain the origin of p-type conductivity in Ag-doped Zn O.     

Channeled substrate window twin stripe GaAs/GaAlAs visible laser: Fabrication and steady state analysis

GaAs/GaAlAs high power window stripe lasers are developed with straight ac-tive layer in region adjacent to facet and curved active layer in central part.Only one-step liq-uid-phase epitaxy(LPE)growth is used in the fabvrication to from two internal current patheslaterally,allowing of a very simple fabrication process.Optical coupling from two stimulatingregions makes less beam divergence.The steady state analysis of such laser structure agreewell with experimental results.     

Features of the twin structure of YBa2Cu3O7−x epitaxial films

X-ray diffraction is used to investigate YBa₂Cu₃O_7?x (YBCO) films on NdGaO₃(110) and a ${{(100)CeO_2 } \mathord{\left/ {\vphantom {{(100)CeO_2 } {(1\bar 102)}}} \right. \kern-0em} {(1\bar 102)}}Al_2 O_3 $ heterostructure. Symmetric, asymmetric, and axial geometries for θ and θ/2θ scans are used to obtain diffraction spectra from different crystallographic planes. The orientational and quantitative twinning characteristics of the films are determined. While the crystallographic parameters of these two types of film (the films are c-axis oriented with c=11.67 Å) are similar, there are differences in the twin structure. In particular, the features of the NdGaO₃ structure lead to the appearance of an angle differing from 90° (90.20°) between the possible (110) and θ/2θ twin planes in a YBCO film and a different number of twin components in each system of twins. It is concluded from an analysis of the broadening of reflections, which are sensitive to twinning, that there is not twinning in a 60% film of YBCO on Al₂O₃ with a CeO₂ buffer layer.     

Thermal disordering near a Σ=3{111} twin boundary in Cu3Au

Symmetrical and asymmetrical configurations of a =3 (111) twin boundary in Cu₃Au are studied by means of energy minimization and Monte Carlo simulations of a rigid lattice, using an adapted n-body potential. We find that relaxation occurring in the neighborhood of the boundary is larger for asymmetrical than for the symmetrical minimum energy configurations. From energy minimization the latter are also shown to correspond to the lowest excess energy. For the asymmetrical twin structure, Monte Carlo calculations of the long-range order parameter on a local basis show the existence of prewetting effects, i.e., the appearance well below the critical temperature of several partially disordered layers at each side of the twin boundary. In contrast, the order is preserved in the symmetrical twin in the same temperature range. These results are consistent with experimental observations of wetting in =3 (111) twins in Cu₃Au, and it is found to occur at lower temperature in asymmetrical than in symmetrical boundaries.     

Fifth-order attosecond polarization beats using twin color-locked noisy lights in cascade three-level system with Doppler-free approach

Fifth-order attosecond sum-frequency polarization beat (FASPB) is studied in a cascade three level system with the phase-conjugation fourth-order coherence function theory. An improved schematic diagram of geometry, which is different from that inducing fifth-order femtosecond different-frequency polarization beat (FFDPB), is used to obtain FASPB. By analyzing the cases that pump beams have either narrow or broad bandwidth, it is found that the temporal behavior of the sum-frequency polarization beat signal depends on the properties of the lasers and transverse relaxation rate of the atomic energy-level system. Finally, the cascaded four-wave mixing (FWM) processes and the difference between attosecond and femtosecond polarization beats have been discussed, it is found that cascaded or sequential lower processes can often obscure the direct fifth-order polarization beat processes.     

The effect of HCP on the formation of twin boundaries and dislocations in Ni–Co alloys

In this study, molecular dynamics (MD) was used to simulate the rapid solidification process of Ni₄₇Co₅₃ and Ni₄₈Co₅₂ alloys at a cooling rate of 10¹² K/s. The effects of HCP on the formation of twin boundaries and dislocations in two Ni–Co alloys are studied. It is found that the difference of HCP clusters is the main effect that producing discrepancies on microstructure of two alloys. The number of HCP clusters accounted for 9.23% in Ni₄₇Co₅₃ alloy. They are regularly arranged to form the number of single-layer twin boundaries, and each twin boundary ends in a dislocation. The FCC and HCP structures coexist in the same atomic layers, which is easy to create dislocations. The relatively standard FCC crystal and only 0.32% HCP clusters are formed in Ni₄₈Co₅₂ alloy at 300 K. That small amount of HCP clusters are dispersed on the surface, and cause the formation of dislocation in the border with FCC clusters.