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Both the static and kinetic properties of defects play fundamental roles in determining the physical properties of semiconductors. Compared to the static properties of defects, a comprehensive understanding of the different defects diffusing in different types of semiconductors is still lacking. In this article, based on extensive first-principles calculations, we have done a comparative study on the diffusion mechanisms of point defects in Si (a typical elemental semiconductor) and GaN (a typical compound semiconductor). The significantly different diffusion mechanisms of vacancies and interstitials in Si and GaN result in significantly different recombination mechanisms, i.e., a novel synergistic effect to accelerate the annihilation of defects is observed in Si but not in GaN, indicating that an efficient self-recovery mechanism can exist in Si but not in GaN. Our results not only explain some experimental observations in Si and GaN under nonequilibrium irradiation conditions but also provide a good example to understand the different kinetic properties of defects in elemental and compound semiconductors.

Femtosecond lasers (fs-lasers) offer a powerful and advantageous tool for fabricating a very large variety of materials. When processing transparent dielectrics, structural defects, such as cracks and broken edges, are always present, thus restricting the precision of fs-laser processing. In this paper, the formation and suppression mechanism of fs-laser induced structural defects are systematically studied. We demonstrate a novel method to improve the processing precision of lithium niobate (LiNbO 3) by elevating the substrate temperature. A crack-free ablation hole with a smooth edge was fabricated at the substrate temperature of 1000 °C. Our results show that the increase of absorptivity and the suppression of incubation effects are responsible for the high precision processing at a high substrate temperature, which not only inhibits the formation of defects, but also additionally increases the efficiency of fs-laser processing. This work provides a simple method to efficiently suppress the defect formation induced by fs-laser in LiNbO 3 samples, paving the way for a new technique for high precision fs-laser processing.

A dislocation-based constitutive model is applied to revisit the anomalous thermal hardening behavior of fcc metals under shock loading. Two scale-dependent dislocation motion mechanisms are found to dominate the thermal hardening behavior of fcc metals under shock loading. In particular, because of the shear stress decay with the propagation of the shock wave, the relativistic effect of dislocation motion is only significant near the impact surface, whereas the phonon drag mechanism dominates dislocation motion in a macroscopic fcc metal. Furthermore, we provide a detailed picture of the thermal hardening behavior on the continuum scale, in which the role of the newly generated stress wave from plastic deformation in the dynamic deformation process is highlighted. We show that the mechanical response at the elastic precursor is mainly controlled by the stress wave emanating from the plastic front.

Gd 3 Ga 5 O 12 single crystals [gadolinium gallium garnet (GGG)] were shock compressed to elastic deformation, elastoplastic transformation, and structural phase-transition regimes with the corresponding single, two-wave, and three-wave structures. Velocity profiles at the front interface and the rear free surface of the sample are measured by a Doppler pin system. Results of these measurements are analyzed in detail, and the refractive index, n, at 1550 nm is obtained as a function of shock pressure or density. Incorporating previously published single-wave data above 100 GPa, characteristic variations in the refractive index are presented and discussed within the context of shock-induced polymorphism of the GGG. Gd 3 Ga 5 O 12 single crystals [gadolinium gallium garnet (GGG)] were shock compressed to elastic deformation, elastoplastic transformation, and structural phase-transition regimes with the corresponding single, two-wave, and three-wave structures. Velocity profiles at the front interface and the rear free surface of the sample are measured by a Doppler pin system. Results of these measurements are analyzed in detail, and the refractive index, n, at 1550 nm is obtained as a function of shock pressure or density. Incorporating previously published single-wave data above 100 GPa, characteristic variations in the refractive index are presented and discussed within the context of shock-induced polymorphism of the GGG.

The waveguide-cavity system has significant applications for sensing as well as for constructing highly integrated optical circuits. In this paper, we analyzed the mode coupling of resonant cavities integrated into a LiNbO 3 rectangular subwavelength waveguide, which is able to serve as an on-chip integrated platform at terahertz frequencies. Using a time-resolved imaging method, we visualized terahertz field evolution in the waveguide-cavity structure and determined the resonant characteristics in transmission and reflection spectra. Moreover, a cavity-cavity coupling was realized, which was demonstrated by the transmission window in the transmission spectrum. We also showed that the localized fields in the cavity-cavity structure were enhanced at the transmission window. These findings would be helpful for the development of terahertz on-chip waveguide-based information processing and sensing systems.

The interaction of shock-induced ejecta with gas beyond the free surface is a critical unsolved issue and being investigated broadly. Using models containing micrometer-sized gas environments, we perform molecular dynamics simulations to investigate the coupling interactions of surrounding gases with ejecta from shock-loaded tin surface. Ejected microjets experience progressively aggravated deceleration with increasing gas density, and particle flows ahead of jet tips are suppressed. Despite the drag effect, the primary fragmentation process is yet intrinsically dominated by a velocity gradient. The continuous interaction between ejecta and gas leads to the progressive formation of transmitted shock waves in background gases, which is jointly determined by ejecta velocity and thermophysical properties of gas. Meanwhile, a mixing layer between ejecta and gas is directly observed, leading to discrepant mass distributions of ejecta along shock direction. With increasing gas density, the volume density tends to rise in the mixing zone while the zone thickness decreases. Further, with the presence of gases, the size distribution of ejected particles is altered with an outstanding feature of enhanced formation of atomic particles. It is found that the stripping effect of gas dominates the growth of ejecta clusters in the transport process. The stripped particles strongly couple and flow with compressed gas, accompanied by recombination into subsequent clusters. As the gas density increases, both formation and annihilation of atomic particles are promoted. The revealed peculiarities provide microscopic views of ejecta interaction with ambient gas, which would further the understanding of gas effects on the breakup and transport of ejected particles.

Abnormal expansion phenomena occur in various types of shocked porous materials with relatively high porosity. In this work, we propose a way to treat the shock velocity D and particle velocity u data of shocked porous materials and then deduce a new equation of state (EOS). Validations show that this new EOS can well describe the pressure-volume data of porous materials over a wide range of porosity and pressure. In particular, the abnormal expansion is satisfactorily reproduced. Furthermore, based on the new EOS, a unified functional form of the EOS is deduced to describe the shock, isentropic, and isothermal compression of solid materials, which facilitates the mutual calculation among different kinds of compression states.Abnormal expansion phenomena occur in various types of shocked porous materials with relatively high porosity. In this work, we propose a way to treat the shock velocity D and particle velocity u data of shocked porous materials and then deduce a new equation of state (EOS). Validations show that this new EOS can well describe the pressure-volume data of porous materials over a wide range of porosity and pressure. In particular, the abnormal expansion is satisfactorily reproduced. Furthermore, based on the new EOS, a unified functional form of the EOS is deduced to describe the shock, isentropic, and isothermal compression of solid materials, which facilitates the mutual calculation among different kinds of compression states.

Luminescence emission spectra and decay kinetics of Ce3+ in YAG were investigated as a function of temperature (300–800 K) under excitation in the long wavelength tail of the lowest 4f–5d absorption spectrum. Both the anti-Stokes and Stokes emission intensity were found to exhibit an unusual non-monotonic temperature dependence compared to those commonly excited in the absorption band, where luminescence intensity decreases monotonically with increasing temperature. This phenomenon is well explained by a phonon-assisted anti-Stokes excitation model with strong vibronic interaction. It reveals that the intensity enhancement of Ce3+ anti-Stokes and Stokes emission is caused by the anti-Stokes excitation efficiency. Excitation efficiency is enhanced with increasing temperature, and thermal ionization of the 5d electron into the conduction band remains as the quenching mechanism of Ce3+ anti-Stokes and Stokes luminescence at high temperatures.Luminescence emission spectra and decay kinetics of Ce3+ in YAG were investigated as a function of temperature (300–800 K) under excitation in the long wavelength tail of the lowest 4f–5d absorption spectrum. Both the anti-Stokes and Stokes emission intensity were found to exhibit an unusual non-monotonic temperature dependence compared to those commonly excited in the absorption band, where luminescence intensity decreases monotonically with increasing temperature. This phenomenon is well explained by a phonon-assisted anti-Stokes excitation model with strong vibronic interaction. It reveals that the intensity enhancement of Ce3+ anti-Stokes and Stokes emission is caused by the anti-Stokes excitation efficiency. Excitation efficiency is enhanced with increasing temperature, and thermal ionization of the 5d electron into the conduction band remains as the quenching mechanism of Ce3+ anti-Stokes and Stokes luminescence at high temperatures.

Rectangular subwavelength waveguides are necessary for the development of micro/nanophotonic devices and on-chip platforms. Using a time-resolved imaging system, we studied the transient properties and the propagation modes of THz pulses in rectangular subwavelength dielectric waveguides. The dynamic process of THz pulses was systematically recorded to a movie. In addition, an anomalous group velocity dispersion was demonstrated in rectangular subwavelength waveguides. By using the effective index method, we theoretically calculated the modes in rectangular subwavelength waveguides, which agree well with the experiments and simulations. This work provides the opportunity to improve the analysis and design of the integrated platforms and photonic devices.

The phase transition of iron under shock compression has attracted much attention in recent decades because of its importance in fields such as condensed matter physics, geophysics, and metallurgy. At room temperature, the transition of iron from the α-phase (bcc) to the e-phase (hpc) occurs at a stress of 13 GPa. At high temperature, a triple point followed by transformation to the γ-phase (fcc) is expected. However, the details of the high-temperature phase transitions of iron are still under debate. Here, we investigate the phase-transition behavior of polycrystalline iron under compression from room temperature to 820 K. The results show that the shock-induced phase transition is determined unequivocally from the measured three-wave-structure profiles, which clearly consist of an elastic wave, a plastic wave, and a phase-transition wave. The phase transition is temperature-dependent, with an average rate Δσtr/ΔT of −6.91 MPa/K below 700 K and −34.7 MPa/K at higher temperatures. The shock α-e and α-γ p...

A novel explanation of the quasielastic release phenomenon in shock compressed aluminum is presented. A dislocation-based model, taking into account dislocation substructures and evolution, is applied to simulate the elastic plastic response of both single crystal and poly crystalline aluminum. The calculated results are in good agreement with experimental results from not only the velocity profiles but also the shear strength and dislocation density, which demonstrates the accuracy of our simulations. Simulated results indicate that dislocation immobilization during dynamic deformation results in a smooth increase of yield stress, which leads to the quasi-elastic release, while the generation of dislocations caused by the plastic release wave results in the appearance of a transition point between the quasielastic release and the plastic release in the profile., 13 pages and 12 figures

The quantum size and composition effects on the optical properties of FexZr1−xO2 (x=0, 7.5, 15, and 35 wt %, respectively) clusters were studied by measuring their photoacoustic spectra at room temperature. We observed a concomitant smearing of the band gap, narrowing the effective gap of the grains with increasing iron content. These phenomena are attributed partly to the formation of a separate Fe2O3 phase, likely residing at the grain boundaries. The appearance of five spectral bands may be ascribed to the electronic transition of the Fe3+ centers in the samples from the ground level to five excitation states. The exciton peaks occur and shift towards blue, which can be explained by the quantum size effect. The absorption edges exhibit a red shift with increasing Fe content due to the Burstein’s effect.

Strong oxides at high shock pressures have broad crossovers from elastic solids at ambient to failure by plastic deformation, to heterogeneous deformation to weak solids, to fluid-like solids that equilibrate thermally in a few ns, to melting and, at sufficiently high shock pressures and temperatures, to metallic fluid oxides. This sequence of crossovers in single-crystal cubic Gd3Ga5O12 (Gd-Ga Garnet-GGG) has been diagnosed by fast emission spectroscopy using a 16-channel optical pyrometer in the spectral range 400–800 nm with bandwidths per channel of 10 nm, a writing time of ∼1000 ns and time resolution of 3 ns. Spectra were measured at shock pressures from 40 to 290 GPa (100 GPa = 1 Mbar) with corresponding gray-body temperatures from 3000 to 8000 K. Experimental lifetimes were a few 100 ns. Below 130 GPa, emission is heterogeneous and measured temperatures are indicative of melting temperatures in grain boundary regions rather than bulk temperatures. At 130 GPa and 2200 K, GGG equilibrates thermally ...

The pressure dependence of the yield strength of tantalum was investigated experimentally up to 101 GPa at room temperature using a diamond anvil cell. A yield strength softening is observed between 52 and 84 GPa, whereas a normal trend is observed below 52 GPa and above 84 GPa. The onset pressure of the softening is in agreement with previous results obtained by the pressure gradient method and shock wave experiments. This unusual strength softening in tantalum is not related with structural transformation, preferred orientation, or material damage. Our measurements indicate that microscopic deviatoric strain is the major reason for the observed strength softening in tantalum.

A pressure sensor of SrB4O7:Sm2+ has been synthesized and the pressure shift of its 7D0-5F0 fluorescence line has been recalibrated at room temperature up to 48 GPa and 127 GPa hydrostatically and non-hydrostatically, respectively. Different from previous study, our results show that the calibrated relation in the quasi-hydrostatic pressure environment is quite different from that in the non-hydrostatic pressure environment. The yield strength of SrB4O7:Sm2+ as a function of the pressure has been determined by the pressure gradient method in a diamond anvil cell. The results show that the yield strength of SrB4O7:Sm2+ increases from 2.85 GPa at a pressure of 7.9 GPa to 4.22 GPa at 25.4 GPa and is much smaller than that of ruby. The relatively small high-pressure yield strength of SrB4O7:Sm2+ is at the same level of the most sample materials. This would result in a small pressure difference with the coexisting sample, thus lead to a small error in the pressure measurement. The smaller yield strength and excellent fluorescent spectral characters of SrB4O7:Sm2+ make it a good substitute for ruby as a pressure scale in high-pressure experiments, especially under non-hydrostatic pressure environments.

Rapidly solidified Nd9Fe73B12.6C1.4Ti4-xGax (x = 0, 0.5, 1) thick ribbons were prepared by melt spinning technique at a surface speed (vs) of 5 m/s. The influences of Gallium doping on the glass-forming ability (GFA), microstructure and magnetic properties of alloys were investigated. The coercivity of Nd-Fe-B-Ti-C nanocomposite alloy thick ribbons can be dramatically enhanced by 0.5 at. % Ga addition in a low cooling-rate regime, attributing to the weakened exchange coupling interaction and grain refinement. The results of XRD showed that the GFA of Nd-Fe-B-Ti-C alloy was enhanced by the Ga addition. Therefore, Gallium addition is effective to refine grain and improve the glass-forming ability, which provides the possibility to obtain the high performance of magnetic properties by using strip casting (SC) method.

The high-pressure melting curve of tantalum (Ta) has been the center of a long-standing controversy. Sound velocities along the Hugoniot curve are expected to help in understanding this issue. To that end, we employed a direct-reverse impact technique and velocity interferometry to determine sound velocities of Ta under shock compression in the 10-110 GPa pressure range. The measured longitudinal sound velocities show an obvious kink at ~60 GPa as a function of shock pressure, while the bulk sound velocities show no discontinuity. Such observation could result from a structural transformation associated with a negligible volume change or an electronic topological transition.

Single-crystal Gd3Ga5O12 has been studied at high dynamic pressures generated with plate impacts. Shock-wave profiles and Hugoniot points were measured with a picosecond time-resolved Doppler Pin System. For final shock pressures in the range 8.52-113 GPa, a two-wave structure is observed below 59.3 GPa, a three-wave structure at ∼88.5 GPa, and a single shock wave is observed at ∼113 GPa. Our data show that the Hugoniot elastic limit (HEL) of single-crystal Gd3Ga5O12 is strongly dependent on final shock pressure. The HEL increases from 7.65 to 24.2 GPa as final pressure increases from 8.52 to 88.5 GPa. A shock-induced phase transformation is observed at a pressure of ∼75.9 GPa, which is a little higher than the value reported previously (Mashimo et al., Phys. Rev. Lett. 96, 105504, 2006), but is consistent with previous DAC work (Mao et al., Phys. Rev. B 83, 054114, 2011).

NdxY6−xFe68Mo4B22 (x = 1–5) nanocomposites were prepared directly by the devitrification of amorphous rods. The effects of Y doping on the glass-forming ability, microstructure, and magnetic properties of the alloys were also investigated. GFA of the alloys was found to get enhanced through substituting Nd with Y and to increase with the growth of Y contents. Results also showed that the best glass former was Nd1Y5Fe68Mo4B22 with a critical diameter of 4 mm. The coercivity first increased after subsequent crystallization and then decreased with the reduction of Y content, which was closely related to the phase transition during the crystallization. The devitrified Nd3Y3Fe68Mo4B22 exhibited a maximal coercivity of 364.1 kA/m, resulting from strong exchange coupling between the hard and the soft phases.

The pressure-volume-temperature (P-V-T) equation of state (EOS) of MgO is widely used as a pressure scale in static compression experiments. However, there are remarkable inconsistencies among different previously proposed MgO pressure scales. We calculated the P-V-T EOS of MgO up to 300 GPa and 3000 K based on experimental shock Hugoniot data and a simple thermal pressure model within the Mie-Gruneisen-type analysis framework. All of the parameters used can be measured experimentally with high accuracies. We found that, in overall, the calculated P-V-T EOS of MgO has excellent agreement with the available volume compression data over a wide range of pressure and temperature. A comparison of our results with the previous theoretical investigations has also been performed and confirms that our calculated P-V-T EOS of MgO can be used as a reliable pressure scale for static experiments at high pressures and high temperatures.

A thermodynamic formulation is proposed for deducing 0 K isotherm from shock Hugoniot data. In comparison to previous published thermodynamic approaches, the characteristic of this one is not requiring heat capacity as input and the used values of Gruneisen parameter merely confined to around ambient condition. Therefore, it keeps away from the difficulties in determining reliable heat capacity and Gruneisen parameter at high temperatures by experiments and theories. The predicted 0 K isotherms for seven selected metals and their related parameters of initial densities, initial bulk moduli, and their first pressure derivatives are all in well agreement with available experiments and theoretical estimations.