Yunhao Lu

The pressure-induced phase transformation of semiconductors, particularly silicon (Si), has long been a fundamental problem in condensed matter physics. Decades of research have predominantly focused on external factors such as temperature, load, and unloading rate, leaving the role of inherent electronic properties fundamentally unresolved. This Letter addresses this gap by demonstrating that hole doping (i.e., hole concentration) is a critical governing factor in the pressure-induced phase transformation. We experimentally establish, for the first time, that high hole-doping redirects the phase transformation by facilitating the amorphization of Si. Our simulations powered by carrier-dependent interatomic potentials reveal that the three-coordinated Si atoms are dynamically stabilized by the localized holes during the decompression from $\ensuremath{\beta}$-tin (Si-II) phase, irreversibly driving the collapse of long-range order in Si to facilitate amorphization. Furthermore, we experimentally observe the hole-facilitated amorphization in Ge and SiC, while GaAs shows a somewhat weaker yet consistent trend. Our Letter unveils the hidden role of holes from mere charge carriers to fundamental mediators of structural stability under high pressure, establishing a new perspective on the interplay between electronic and structural properties in semiconductors.

Two-dimensional (2D) altermagnets have been attracting enormous research interest; however, the general guidance of global symmetry breaking poses a significant challenge of rather limited material candidates. Here, we propose a simple and widely applicable design strategy for generating both hidden and visible 2D altermagnetism in van der Waals antiferromagnetic multilayers. Based on conventional antiferromagnetic monolayers with spacetime-inversion symmetry, different types of altermagnetism can be realized, depending on interlayer magnetic interaction and local symmetry breaking. Notably, the approach has the advantage that it does not require a specific twisting/stacking order or global inversion symmetry breaking. Using a symmetry-guided framework and high-throughput computational screening, we identify diverse hidden and visible altermagnetic candidates. Beyond bilayer thicknesses, this unconventional layer-dependent nonrelativistic spin splitting mechanism exhibits a distinct odd-even effect, differing from that in conventional A-type antiferromagnets. Leveraging hidden altermagnetic order, we propose a universal multiferroic scheme for ferroelectric reversal of spin polarization. Our study greatly expands the family of 2D altermagnetic materials, circumvents stringent constraints associated with previous methods, and opens new pathways toward altermagnetic spintronics.

Ferroelectric vortices, with their unique topological polarization patterns, show great potential for next-generation electronics. Although they have been observed in compounds, their realization in single-element materials has remained elusive. Here, we experimentally prepare three-bilayer Bi(110) square-shaped islands on high-T_C superconductor (Bi₂Sr₂CaCu₂O_8+δ) substrates by molecular beam epitaxy. Scanning tunneling microscopy/spectroscopy combined with first-principles calculations suggests the possible existence of in-plane polarization and a ferroelectric vortex state in our islands, as well as reveals exceptional vortex stability against electric-field-driven switching. This work could pioneer single-element ferroelectric vortices, potentially advancing topological ferroelectrics design and expanding the understanding of related fundamental physics.

Multiferroic materials, which simultaneously exhibit ferroelectric and magnetic orders, offer tremendous potential for next-generation electronic and spintronic devices. Here, we propose a novel design strategy toward a new type of multiferroics: the interlayer self-doping multiferroics. We show that, due to the different band filling preferences of antiferromagnetic and ferromagnetic orderings, homobilayer systems with intermediate band filling exhibit an intrinsic instability toward interlayer self-doping. This results in an antiferromagnetic order in one layer and ferromagnetic order in the other, accompanied by out-of-plane ferroelectricity. Distinct from conventional type-I and type-II multiferroics, the ferroelectric and magnetic orders in interlayer self-doping multiferroics are intrinsically coupled yet not relying on spin-orbit coupling, enabling potential persistence at elevated temperatures. Using first-principles calculations, we validate this mechanism in two concrete systems: bilayer CrTe_{2} and bilayer FeTe. Notably, the multiferroicity in bilayer CrTe_{2} is predicted to operate robustly at room temperature. Our Letter unveils a new type of multiferroics, and it opens a new route for designing 2D ultrathin multiferroics with high transition temperature and robust magnetoelectric response.

Nanocrystal phase thermostability is critical for their applications, yet fundamentally governed by complex thermodynamic and kinetic variables. Understanding the stabilizing mechanisms and dominant factors requires atomic-level insights into dynamic evolution across surface and bulk regions under extreme conditions. Herein, we present a comprehensive in-situ investigation of individual single-crystalline anatase TiO ₂ nanorods using spherical aberration-corrected scanning transmission electron microscopy. By simultaneously acquiring environmental secondary electron images for surface topography and high-angle annular dark-field images for bulk atomic structures, we reveal the extraordinary phase stability of individual anatase nanorods governed by surface effects, distinct from aggregated nanorods. Anatase TiO ₂ nanorods undergo morphology reshaping and surface atomic reconstruction above 600 °C, involving transformation from high-index surfaces to (101) facets and the formation of (1 × 4)-reconstructed (001) surfaces. Remarkably, individual anatase TiO ₂ nanorods maintain the anatase structure even up to 1250 °C without transforming into the rutile phase. The restructuring lowers the total energy of the system, and acts as a kinetic “surface-locking” effect preventing rutile nucleation. Beyond elucidating the restructuring mechanisms and intrinsic thermostability of TiO ₂ nanocrystals, this work also establishes an effective pathway for simultaneously probing the complex structural evolution of nanomaterials across both surface and bulk regions.

Lithium borohydride (LiBH₄) is a promising hydrogen carrier owing to its high hydrogen storage capacity. However, the low reactivity of its dehydrogenation products, boron and LiH, towards dihydrogen molecules makes the re-generation of borohydrides extremely challenging. Here we theoretically unravel that the dissociation of H₂ into H atoms and its adsorption by the active B_spike atoms (surface-protruding boron atoms with low coordination and high reactivity) is a prerequisite for the formation of B-H bond, rather than the direct reaction between H₂ and B. Moreover, the proportion of B_spike atoms increases exponentially as the size of B clusters decreases, indicating that reducing B particle size to the ultrasmall scale is critical for enhancing hydrogenation reactivity. Thereby, we experimentally synthesize nanocomposites consisting of ultrafine LiBH₄ nanoparticles decorated with 3 nm Ni catalytic clusters for hydrogen storage. Upon dehydrogenation, these nanocomposites form B and LiH clusters in close proximity at 5-10 nm scale, while the Ni clusters remain intact. The Ni clusters not only facilitate the dissociation of H₂ into H atoms but also strongly interact with the B clusters, weakening B-B bond, which enables the hydrogenation of B/LiH back to LiBH₄ at temperatures as low as 30 °C under 100 bar H₂.

Two-dimensional (2D) multiferroics have attracted tremendous interest recently, yet their experimental realizations are quite rare. Here, we demonstrate strongly coupled polarization and magnetism in Li-encapsulated 2D silica (Li@SiO₂) through first-principles calculations. In Li@SiO₂, the antiferroelectric (AFE) phase is energetically more favorable than the ferroelectric (FE) phase, and both AFE and FE phases hold a zigzag-like antiferromagnetic (AFM) configuration. It is revealed that magnetism originates from polarization, suggesting strong magnetoelectric coupling. Intriguingly, by tuning of the electric field and working temperature, multiple distinct phases could be achieved, including a metallic paraelectric state, a semiconducting FE state with frustrated spins, and semiconducting FE and AFE states with AFM ordering. Effects of graphene and SiC substrates on multiferroics are also discussed for practical experiments. Our work not only provides a promising avenue for realizing 2D multiferroics in silicon-based systems but also sheds light on the feasibility of nonvolatile nanodevices integrated with the mature semiconductor technology.