A structural search leads to the prediction of a novel alkaline earth nitride BeN4 containing a square planar N42- ring. This compound has a particular chemical bonding pattern giving it potential as a high-energy-density material. The P4/nmm phase of BeN4 may be stable under ambient conditions, with a bandgap of 3.72 eV. It is predicted to have high thermodynamic and kinetic stability due to transfer of the outer-shell s electrons of the Be atom to the N4 cluster, with the outer-shell 2p orbital accommodating the lone-pair electrons of N42-. The total of six π electrons is the most striking feature, indicating that the square planar N42- exhibits aromaticity. Under ambient conditions, BeN4 has a high energy density (3.924 kJ/g relative to Be3N2 and N2 gas), and its synthesis might be possible at pressures above 31.6 GPa.

Polymeric nitrogen has attracted much attention owing to its possible application as an environmentally safe high-energy-density material. Based on a crystal structure search method accelerated by the use of machine learning and graph theory and on first-principles calculations, we predict a series of metal nitrides with chain-like polynitrogen (P21-AlN6, P21-GaN6, P-1-YN6, and P4/mnc-TiN8), all of which are estimated to be energetically stable below 40.8 GPa. Phonon calculations and ab initio molecular dynamics simulations at finite temperature suggest that these nitrides are dynamically stable. We find that the nitrogen in these metal nitrides can polymerize into two types of poly-N42- chains, in which the π electrons are either extended or localized. Owing to the presence of the polymerized N4 chains, these metal nitrides can store a large amount of chemical energy, which is estimated to range from 4.50 to 2.71 kJ/g. Moreover, these compounds have high detonation pressures and detonation velocities, exceeding those of conventional explosives such as TNT and HMX.

Synchrotron radiation x-ray diffraction investigations of iron (Fe) and nickel (Ni) are conducted at pressures up to 354 and 368 GPa, respectively, and the equations of state (EOSs) at 298 K for the two elements are obtained for data extending to pressures as high as those at the center of the Earth, using the latest Pt-EOS pressure scale. From a least-squares fit to the Vinet equation using the observed pressure–volume data, the isothermal bulk modulus K0 and its pressure derivative K0′ are estimated to be 159.27(99) GPa and 5.86(4) for hcp-Fe, and 173.5(1.4) GPa and 5.55(5) for Ni. By comparing the present EOSs and extrapolated EOSs reported in the literature for Fe and Ni, the volumes of Fe and Ni at 365 GPa are found to be 2.3% and 1.5% larger than those estimated from extrapolated EOSs in previous studies, respectively. It is concluded that these discrepancies are due to the pressure scale. The present results suggest that the densities of Fe and Ni at a pressure of 365 GPa corresponding to the center of the Earth are 2.3% and 1.5%, respectively, lower than previously thought.

Clathrate-like or caged compounds have attracted great interest owing to their structural flexibility, as well as their fertile physical properties. Here, we report the pressure-induced reemergence of superconductivity in BaIr2Ge7 and Ba3Ir4Ge16, two new caged superconductors with two-dimensional building blocks of cage structures. After suppression of the ambient-pressure superconducting (SC-I) states, new superconducting (SC-II) states emerge unexpectedly, with Tc increased to a maximum of 4.4 and 4.0 K for BaIr2Ge7 and Ba3Ir4Ge16, respectively. Combined with high-pressure synchrotron x-ray diffraction and Raman measurements, we propose that the reemergence of superconductivity in these caged superconductors can be ascribed to a pressure-induced phonon softening linked to cage shrinkage.

Aneutronic fusion reactions such as proton–boron fusion could efficiently produce clean energy with quite low neutron doses. However, as a consequence, conventional neutron spectral methods for diagnosing plasma ion temperature would no longer work. Therefore, finding a way to probe the ion temperature in aneutronic fusion plasmas is a crucial task. Here, we present a method to realize ultrafast in situ probing of 11B ion temperature for proton–boron fusion by Doppler broadening of the nuclear resonance fluorescence (NRF) emission spectrum. The NRF emission is excited by a collimated, intense γ-ray beam generated from submicrometer wires irradiated by a recently available petawatt (PW) laser pulse, where the γ-ray beam generation is calculated by three-dimensional particle-in-cell simulation. When the laser power is higher than 1 PW, five NRF signatures of a 11B plasma can be clearly identified with high-resolution γ-ray detectors, as shown by our Geant4 simulations. The correlation between the NRF peak width and 11B ion temperature is discussed, and it is found that NRF emission spectroscopy should be sensitive to 11B ion temperatures Ti > 2.4 keV. This probing method can also be extended to other neutron-free-fusion isotopes, such as 6Li and 15N.

A Z-pinch dynamic hohlraum can create the high-temperature radiation field required by indirect-drive inertial confinement fusion. A dynamic hohlraum with peak radiation temperature over 300 eV can be obtained with a >50 MA Z-pinch driver according to the scaling law of dynamic hohlraum radiation temperature vs drive current. Based on a uniform 300 eV radiation temperature profile with a width of 10 ns, three double-shell capsules with radii of 2, 2.5, and 3 mm are proposed, and the corresponding fusion yields from a one-dimensional calculation are 28.8, 56.1, and 101.6 MJ. The implosion dynamics of the 2.5 mm-radius capsule is investigated in detail. At ignition, the areal density of the fuel is about 0.53 g/cm2, the fuel pressure is about 80 Gbar, and the central ion temperature is about 4.5 keV, according to the one-dimensional simulation. A two-dimensional simulation indicates that the double-shell capsule can implode nearly spherically when driven by the radiation field of a Z-pinch dynamic hohlraum. The sensitivities of the fusion performance to the radiation temperature profiles and to deviations in the capsule parameter are investigated through one-dimensional simulation, and it is found that the capsule fusion yields are rather stable in a quite large parameter space. A one-dimensional simulation of a capsule embedded in 50 mg/cm3 CH foam indicates that the capsule performance does not change greatly in the mimicked environment of a Z-pinch dynamic hohlraum. The double-shell capsules designed here are also applicable to laser indirect-drive inertial fusion, if a laser facility can produce a uniform 300 eV radiation field and sustain it for about 10 ns.

The structural evolution of laser-excited systems of gold has previously been measured through ultrafast MeV electron diffraction. However, there has been a long-standing inability of atomistic simulations to provide a consistent picture of the melting process, leading to large discrepancies between the predicted threshold energy density for complete melting, as well as the transition between heterogeneous and homogeneous melting. We make use of two-temperature classical molecular dynamics simulations utilizing three highly successful interatomic potentials and reproduce electron diffraction data presented by Mo et al. [Science 360, 1451–1455 (2018)]. We recreate the experimental electron diffraction data, employing both a constant and temperature-dependent electron–ion equilibration rate. In all cases, we are able to match time-resolved electron diffraction data, and find consistency between atomistic simulations and experiments, only by allowing laser energy to be transported away from the interaction region. This additional energy-loss pathway, which scales strongly with laser fluence, we attribute to hot electrons leaving the target on a timescale commensurate with melting.

The interaction between a molecular cloud and an external agent (e.g., a supernova remnant, plasma jet, radiation, or another cloud) is a common phenomenon throughout the Universe and can significantly change the star formation rate within a galaxy. This process leads to fragmentation of the cloud and to its subsequent compression and can, eventually, initiate the gravitational collapse of a stable molecular cloud. It is, however, difficult to study such systems in detail using conventional techniques (numerical simulations and astronomical observations), since complex interactions of flows occur. In this paper, we experimentally investigate the compression of a foam ball by Taylor–Sedov blast waves, as an analog of supernova remnants interacting with a molecular cloud. The formation of a compression wave is observed in the foam ball, indicating the importance of such experiments for understanding how star formation is triggered by external agents.