Jiayu Dai

Probing the interfacial thermal transport under extremely large temperature difference (ELTD) is critical for thermal protection and heat dissipation in extreme environments yet challenging. However, conventional optical thermoreflectance methods work only over narrow temperature ranges and thus fail for ELTD conditions characterized by a broad temperature evolution range. Here, we circumvent this limitation by employing a table-top ultrafast X-ray diffraction (UXRD) system, which exploits the nearly linear lattice thermal expansion over a wide temperature range. We investigate interfacial thermal transport in a W thin film on a MgO substrate under ELTD conditions and determine that the interfacial thermal conductance (ITC) value increases from 6.43 × 10⁸ W m⁻² K⁻¹ to 8.71 × 10⁸ W m⁻² K⁻¹ as the maximum temperature of W rises from 670 K to 2334 K, accompanied by an increase in the maximum interface temperature difference from 302 K to 1451 K based on the finite volume method (FVM). The results reside at a relatively high level among reported ITC values for conventional metal-oxide interfaces, which may originate from the simultaneous increase in interface temperature difference and mean interface temperature. This work demonstrates UXRD as an effective tool for ELTD thermal measurements and fills a critical knowledge gap regarding interfacial thermal transport under extreme conditions.

The synthesis of high-purity bulk hexagonal diamond (HD) remains a formidable challenge, and a more detailed understanding of the transformation pathway from hexagonal graphite (HG) to HD remains elusive. Here, we elucidate the pathway selection mechanism and identify the favorable conditions for synthesizing high-purity bulk HD. Using deep potential molecular dynamics simulations, we systematically investigate the transformation under interlayer uniaxial compression across a broad temperature and pressure range. Our results demonstrate that interlayer stress governs the transformation pathway by regulating interlayer sliding of graphite layers. At low stress, the HG-to-HD transition proceeds through an intermediate orthorhombic graphite (OG) phase, whereas at high stress, it proceeds through a post-graphite phase; in both pathways, the final conversion to HD is completed via a buckling mechanism. Although temperature does not change the pathway type, it synergistically interacts with stress to regulate the formation of the AB′A-stacked OG phase, thereby determining the phase purity of HD. At 900 K and 55 GPa, an HD fraction of up to 86.7% is achieved. The interlayer stress-temperature map of the HD fraction exhibits an isolated island-like distribution, revealing an exceptionally narrow synthesis window. Moreover, the distribution of OG atom counts on the interlayer stress-temperature map provides a clear signature of the boundary separating the two transformation pathways. This work uncovers the pathway-selection mechanism for carbon phase transitions under extreme anisotropic stress conditions and establishes a theoretical framework for the controlled synthesis of high-purity bulk HD.

Interlayer coupling governs the band structure and ultrafast carrier dynamics of van der Waals (vdW) materials, serving as a fundamental degree of freedom for manipulating optoelectronic performance. However, energy-resolved characterization of its impacts on photoexcited carriers remains scarce. In this work, we use femtosecond transient reflectance spectroscopy with near-infrared supercontinuum probing to explore energy-resolved carrier relaxation in a Bernal-stacked epitaxial graphite film on nickel. An abrupt variation of carrier cooling rate is observed at a probe energy of 1.21 eV, which directly manifests the π-band splitting induced by interlayer coupling in graphite. Such band splitting opens additional scattering channels, thus leading to a pronounced cooling rate enhancement once the carrier energy exceeds the π-band splitting threshold. In addition, the increase in relaxation time under high pump fluences can be attributed to the hot phonon effect. This study presents direct experimental observation of anomalous energy-resolved carrier relaxation stemming from interlayer-coupling-induced band reconstruction in graphite, demonstrating the capability of broadband ultrafast spectroscopy to resolve subtle band-structure features in diverse layered vdW materials.

Under ramp compression, water can rapidly freeze into ice VII within nanoseconds, in stark contrast to its freezing behavior under ambient conditions, which typically takes several seconds. Understanding this rapid freezing process is essential for planetary science and high-pressure physics. However, the underlying mechanisms, particularly at the molecular scale, remain poorly understood. Here, we employ deep neural network-based molecular dynamics simulations to investigate the ramp compression of water. Two notable phenomena are observed: (1) extreme ramp compression delays nucleation to pressures exceeding those predicted by classical nucleation theory, indicating non-equilibrium nucleation; (2) the formation of a thermally elevated nucleus and an interfacial layer with high rotational mobility accelerates hydrogen bond rearrangement, thereby facilitating ice growth. Inspired by these findings, a scaling law is proposed to predict the transition pressure to ice VII under ramp compression, revealing that the metastable water can be overdriven far beyond its known limits. By bridging atomistic dynamics with macroscopic phase transitions, our work provides transformative insights into phase transitions under dynamic loading.

Lighter-element materials usually possess higher lattice thermal conductivity (LTC) due to both higher group velocities and longer mean free paths (MFPs) than their heavier-element counterparts. In this work, using broadband frequency-domain thermoreflectance, we measure the LTC and MFP spectra of corundum-structured α-Al2O3 and α-Ga2O3 crystals and report comparable maximum MFPs yet an approximately fourfold difference in LTC. For α-Al2O3, diffusive transport persists down to a thermal penetration depth of 135 nm, yielding an LTC of 34.9 ± 1.1 W m−1 K−1 and a maximum MFP smaller than 135 nm. For α-Ga2O3, ballistic transport appears below 120 nm, exhibiting an LTC of 9.2 ± 0.53 W m−1 K−1 and a maximum MFP of about 120 nm. This counterintuitive finding challenges the conventional expectation, providing experimental insights into the fundamental understanding and design of thermal transport in functional materials.

Electric-field control of spintronic terahertz (THz) emitters offers an attractive route toward compact and integrated THz devices. However, the quantitative relation between electric field and magnetic field orientation in dual manipulation of THz polarization remains unclear. Here, we investigate THz emission from Pt/Ni bilayers on ferroelectric Pb(Mg1/3Nb2/3)O3-PbTiO3 substrate. Rotated magnetic-field-dependent THz emission measurements, combined with a Stoner–Wohlfarth macrospin analysis, quantitatively determine the voltage-dependent anisotropy fields and their orientations. By experimentally comparing the vectorial THz response at representative magnetic-field orientations, we achieve an appreciable electric-field control of THz polarization rotation of up to 78° at a magnetic field angle of θH = 90°. Utilizing this polarization manipulation at the optimized magnetic field angle, we propose an electrically encoded THz far-field imaging scheme via delayed two-pulse excitation. These results provide a new approach for electrically designing structured THz emission.

Phonons, as the primary heat carriers, play an important role in the thermal management of AlN-based semiconductor devices. Acoustic phonon dispersion curves of a large single crystal of hexagonal AlN have been measured by inelastic neutron scattering experiments along different high symmetry directions. The corresponding longitudinal and transverse sound velocities and the associated elastic constants have been deduced. The results show good agreement with first principles calculations as well as the previous experimental data. The influence of pressure on the the lattice dynamics and mechanical properties has also been predicted.