Polymorphic crystalline wetting layers on crystal surfaces

Analogous to surface premelting, we propose that a crystal surface can undergo a pre-solid-solid transition, i.e. developing a thin polymorphic crystalline layer before reaching the solid-solid transition temperature if two crystals can form a low-energy coherent interface. We confirm this in simulations and colloid experiments at single-particle resolution. The power-law increase of surface layer thickness is analogous to premelting. Different kinetics and reversibilities of surface-crystal growth are observed in various systems. Surface crystals exist not only under thermal equilibrium, but also during melting, crystallization, and grain coarsening. Furthermore, the premelting and pre-solid-solid transition can coexist, resulting double surface wetting layers. We hypothesize that such surface phenomena exist in atomic and molecular crystals, which provide a novel way to tune material properties.

Effects of TiB2 Particles on the Microstructure Evolution and Mechanical Properties of B4C/TiB2 Ceramic Composite

B4C/TiB2 ceramic composites reinforced with three size scales (average particle size: 7 μm, 500 nm, and 50 nm) of TiB2 were prepared by using a pressureless sintering furnace at 2100 °C under Ar atmosphere for 60 min. The results demonstrated that during the sintering process, TiB2 located on the boundaries between different B4C grains could inhibit the grain growth which improved the mass transport mechanism and sintering driving force. A semi-coherent interface between B4C and SiC was found, which is supposed to help to reduce the interface energy and obtain good mechanical properties of the B4C/TiB2 ceramic composite. On sample cooling from sintering temperature to room temperature, the residual tensile stress fields formed at the TiB2 interfaces owning to the thermo-elastico properties mismatched, which might have contributed to increase the ability of the sample to resist crack propagation. The results showed that the relative density, Vickers hardness, and fracture toughness of the composite with 20 wt.% submicron and 10 wt.% nano-TiB2 were significantly improved, which were 98.6%, 30.2 GPa, and 5.47 MPa·m1/2, respectively.

Coupling light to a nuclear spin gas with a two-photon linewidth of five millihertz

Nuclear spins of noble gases feature extremely long coherence times but are inaccessible to optical photons. Here, we realize a coherent interface between light and noble-gas spins that is mediated by alkali atoms. We demonstrate the optical excitation of the noble-gas spins and observe the coherent back action on the light in the form of high-contrast two-photon spectra. We report on a record two-photon linewidth of 5 ± 0.7 mHz above room temperature, corresponding to a 1-min coherence time. This experiment provides a demonstration of coherent bidirectional coupling between light and noble-gas spins, rendering their long-lived spin coherence accessible for manipulations in the optical domain.