Search

Energy transfer of a van der Waals heterostructure formed by monolayers of WSe2 and PtSe2 is studied by steady-state photoluminescence (PL) and time-resolved transient absorption spectroscopy. The heterostructure sample is fabricated by transferring a mechanically exfoliated WSe2 monolayer onto a PtSe2 monolayer film obtained by chemical vapor deposition. The sample is thermally annealed to improve the interface quality. PL of the heterostructure is quenched by four times compared to the individual WSe2 monolayer, indicating excitation transfer from WSe2 to PtSe2. Femtosecond transient absorption measurements with two configurations show that both the electrons and the holes can transfer from WSe2 to PtSe2 on a sub-picosecond time scale, while neither can transfer from PtSe2 to WSe2. These results indicate that WSe2 and PtSe2 monolayers form a type-I band alignment with both the conduction band minimum and the valence band maximum in the PtSe2 layer.

The service performance of the turbine blade root of an aero-engine depends on the microstructures in its superficial layer. This work investigated the surface deformation structures of turbine blade root of single crystal nickel-based superalloy produced under different creep feed grinding conditions. Gradient microstructures in the superficial layer were clarified and composed of a severely deformed layer (DFL) with nano-sized grains (48–67 nm) at the topmost surface, a DFL with submicron-sized grains (66–158 nm) and micron-sized laminated structures at the subsurface, and a dislocation accumulated layer extending to the bulk material. The formation of such gradient microstructures was found to be related to the graded variations in the plastic strain and strain rate induced in the creep feed grinding process, which were as high as 6.67 and 8.17 × 107 s−1, respectively. In the current study, the evolution of surface gradient microstructures was essentially a transition process from a coarse single crystal to nano-sized grains and, simultaneously, from one orientation of a single crystal to random orientations of polycrystals, during which the dislocation slips dominated the creep feed grinding induced microstructure deformation of single crystal nickel-based superalloy.

Using entropy density of Dirac field near the event horizon of a rectilinear non-uniformly accelerating Kinnersley black hole, the law for the thermal radiation of black hole is studied and the instantaneous radiation energy density is obtained. It is found that the instantaneous radiation energy density of a black hole is always proportional to the quartic of the temperature on event horizon in the same direction. That is to say, the thermal radiation of a black hole always satisfies the generalized Stefan–Boltzmann law. In addition, the derived generalized Stefan–Boltzmann coefficient is no longer a constant, but a dynamic coefficient related to the space–time metric near the event horizon and the changing rate of the event horizon in black holes.

Using the related formula of dynamic black hole, we have calculated the instantaneous radiation energy density of the slowly changing dynamic Kerr–Newman black hole. It is found that the instantaneous radiation energy density of a black hole is always proportional to the quartic of the temperature of the event horizon in the same direction. By using the Hamilton–Jacobin equation of scalar particles in the curved spacetime, the spontaneous radiation of the slowly changing dynamic Kerr–Newman black hole is studied. The energy condition for the occurrence of the spontaneous radiation is obtained.

First principles calculations were carried out to investigate the energetics of point defects, including solute atoms, vacancies, and antisite defects, in titanium solid solution and intermetallics. Their influence on the mechanical behavior of titanium alloys and intermetallic compounds were discussed. The self consistent procedure proposed by the authors was applied to TiAl intermetallic compounds. The ordering parameter was redefined to include the contribution of vacancies to the disordering of intermetallic compounds, so that the new approach can be applied to both strongly and weakly ordered compounds. Concentrations of point defects of various kinds can be estimated for intermetallics of different composition at different temperature. The solid solution strengthening of alloying elements through short range ordering was studied for titanium alloys and the solid solution hardening rate was estimated for various alloying elements. The solute-vacancy interaction in titanium alloys was calculated and its influence on the diffusion and creep behavior was discussed. These calculations provided some useful information for the selection of alloying elements in designing new titanium alloys.

The melting transition of vortex lattice in type-II superconductors with magnetic field perpendicular to the anisotropy axis is studied in the framework of the elastic theory. It is found that the thermal fluctuations in the two directions transverse to the magnetic field are approximately proportional to each other with the constant, different from the anisotropy parameter, in a wide portion of phase diagram. One can introduce two Lindemann numbers with the ratio determined by the thermal fluctuations to draw a single melting line. The melting line thus obtained fits the experimental phase boundary quite well.

In this work, self-assembled ferroelectric BiFeO3 (BFO) and Pb(Zr,Ti)O3 (PZT) nanocapacitors were fabricated by a one-step pulsed-laser deposition process. Each individual nanocapacitor consists of a SrRuO3 or LaSrMnO3 bottom electrode layer, an epitaxial ferroelectric middle layer and a self-assembled nanoisland of conductive Bi2O3 or PbO2 as the top nanoelectrode. The nanoelectrodes have a lateral size of 10-100 nm depending on various deposition equivalent thickness. The as-fabricated nanocapacitors exhibit unique so-called anti-domain structures, with opposite polarization orientation to that of the naked ferroelectric films, which can be understood by the different interface built-in-voltages between their neighboring layers. They also show apparent reduced coercive fields and enhanced piezoelectricity compared to the naked films, as revealed by the switching spectroscopy piezoresponse force microscopy (SSPFM) and band-excitation mapping. Besides that, individual addressable polarization writing and erasing properties were also observed in these nanocapacitors and the written domain can maintain stability up to 12 h, which is promising for data storage devices.

Special quasi-random structure (SQS) and coherent potential approximation (CPA) are techniques widely employed in the first-principles calculations of random alloys. Here we scrutinize these approaches by focusing on the local lattice distortion (LLD) and the crystal symmetry effects. We compare the elastic parameters obtained from SQS and CPA calculations, taking the random face-centered cubic (fcc) Ti1-xAlx (0

By applying the on-site Coulomb interaction (Hubbard term U) to the Ti d orbital, the influence of electron localization on the electronic structure as well as the transport of oxygen vacancies (VO) in rutile was investigated. With U = 4.5 eV, the positions of defect states in the bandgap were correctly reproduced. The unbonded electrons generated by taking out one neutral oxygen atom are spin parallel and mainly localized on the Ti atoms near VO, giving rise to a magnetic moment of 2 μB, in agreement with the experimental finding. With regard to the migration barrier of VO, surprisingly, we found that U = 4.5 eV only changed the value of the energy barrier by ±0.15 eV, depending on the diffusion path. The most probable diffusion path (along [110]) is the same as that calculated by using the traditional GGA functional. To validate the GGA + U method itself, a hybrid functional with a smaller supercell was used, and the trend of the more probable diffusion path was not changed. In this regard, the traditional GGA functional might still be reliable in the study of intrinsic-defect transportation in rutile. Analyzing the atomic distortion and density of states of the transition states for different diffusion paths, we found that the anisotropy of the diffusion could be rationalized according to the various atomic relaxations and the different positions of the valence bands relative to the Fermi level of the transition states.

Patterns such as periodic subharmonic waves and kink-pairs are observed experimentally in a two-dimensional vertically vibrated granular layer located in an annular channel which has a sawtooth-shaped base. Using the thickness of the granular layer and external excitation as control parameters, we find the transitions between different patterns in the layer. The dependence of the kink-pair's velocity on the external excitation is revealed. The local velocity fields of the granular layer indicate that, in addition to horizontal transport in the layer, there is convection in the node area of a kink.

Ab initio calculations, based on the exact muffin-tin orbitals method are used to determine the elastic properties of Cu-Au alloys with Au/Cu ratio 1/3. The compositional disorder is treated within the coherent potential approximation. The lattice parameters and single-crystal elastic constants are calculated for different partially ordered structures ranging from the fully ordered L1(2) to the random face centered cubic lattice. It is shown that the theoretical elastic constants follow a clear trend with the degree of chemical order: namely, C(11) and C(12) decrease, whereas C(44) remains nearly constant with increasing disorder. The present results are in line with the experimental findings that the impact of the chemical ordering on the fundamental elastic parameters is close to the resolution of the available experimental and theoretical tools.

The optical property and spectroscopy of selected kinds of alcohols are studied based on the terahertz time-domain spectroscopy (THz-TDS) in the spectral range of 0.5-2.5 THz. The B3LYP method in conjunction with the 6-31G (d) basis set was used to optimize the geometric structure of samples and simulate the alcohols spectrum. No absorption peak occurs in THz wave band based on our simulation, suggesting that the present experimental absorption peaks resulted from the intermolecular interaction.

The fact that trapezoid clusters exist in 2D vertically vibrated granular systems leads us to construct a cluster model, in which wave-like motions are explained as the result of cluster-plate and cluster-cluster collisions. By analyzing the collision of one cluster with the plate in detail, we deduce a basic equation from velocity relationship, which could be separated into two correlative equations: one relates wave-like motion with exciting acceleration, and we call it the excitation condition; the other relates wavelength with exciting frequency, viz., the dispersion relation. The theoretical results are in agreement with the experimental ones, which supports the idea of the cluster model. Moreover, from the cluster model, we also predict a possibility of abnormal dispersion relation of a 2D granular system.

By the statistical entropy of the Dirac field of the static spherically symmetric black hole, the result is obtained that the radiation energy flux of the black hole is proportional to the quartic of the temperature of its event horizon. That is, the thermal radiation of the black hole always satisfies the generalised Stenfan–Boltzmann law. The derived generalised Stenfan–Boltzmann coefficient is no longer a constant. When the cut-off distance and the thin film thickness are both fixed, it is a proportional coefficient related to the space-time metric near the event horizon and the average radial effusion velocity of the radiation particles from the thin film. Finally, the radiation energy fluxes and the radiation powers of the Schwarzschild black hole and the Reissner–Nordstrom black hole are derived, separately.

The heat conduction in a one-dimensional (1D) hard-point model with mass gradient is studied. Using numerical simulation, we find an asymmetric heat conduction in this model with greater heat current in the direction of mass increase. The increase of temperature gradient, mass gradient and system size are found to enhance the asymmetric heat conduction. Based on the collision dynamic of a hard-point particle, we give a qualitative explanation for the underlying mechanism of asymmetric effect.