Chin Shen Ong

Bismuth-based compounds, such as Bi1 _-x Sb _x or Bi ₂ Te ₃ , have outstanding electronic properties especially for advanced quantum devices. However, the potential of low-dimensional group V-Bi materials is largely undetermined. Here, we report the experimental realization of a two-dimensional (2D) BiAs layer with giant Rashba spin splitting, grown on an InAs(111)B substrate via molecular beam epitaxy. ARPES reveals the emergence of a prominent M-shaped band structure and a distinct electron pocket at the Fermi level. DFT, complemented by synchrotron-based XPS, LEED, and STM, confirms strong spin-orbit coupling and a giant Rashba coefficient of these electronic states. An As overlayer, remaining from the fabrication process, preserves a structural shift between substrate and BiAs layer, essential for the Rashba splitting, while preventing undesirable reconstruction. This indicates how well-known capping techniques can stabilize new 2D compounds with interesting electronic structures, especially BiAs, a promising candidate to open possibilities for next-generation spintronic devices and field-effect transistors.

High-temperature ceramic compound Al ₅ C ₃ N belongs to the scarcely studied Al–C–N system. It was originally reported as an ordered compound in noncentrosymmetric space group P 6 ₃ mc and described as a nanolaminate with an –Al ₂ C–AlN–Al ₂ C ₂ – stacking sequence. The recently reported structural disorder in the related compound Al ₄ SiC ₄ led us to question this proposed structure for Al ₅ C ₃ N and investigate the possibility of a disordered structure in the centrosymmetric space group P 6 ₃ / mmc . In the present work, we employed different synthesis routes to maximize the yield and quality of the desired phase, and applied a variety of techniques to probe the Al ₅ C ₃ N crystal structure. Our single-crystal X-ray diffraction analysis clearly indicates that noncentrosymmetric space group P 6 ₃ mc must be rejected. From a joint refinement of single-crystal X-ray and neutron powder diffraction data, the occupancies of C and N were refined at two sites in P 6 ₃ / mmc resulting in the stacking sequence –Al ₂ C–Al(C/N)–Al ₂ (C/N) ₂ –. Furthermore, DFT calculations show that a centrosymmetric disordered structure described in a supercell has the lowest energy, 0.2 eV per formula unit, relative to the previously reported P 6 ₃ mc structure. The calculated band structure shows both direct and indirect bandgaps, which leads to implications for the physical properties. Finally, STEM analysis provides additional evidence that the crystal structure of Al ₅ C ₃ N is better described in centrosymmetric space group P 6 ₃ / mmc .

A major challenge in condensed matter physics is integrating topological phenomena with correlated electron physics to leverage both types of states for next-generation quantum devices. Metal-insulator transitions are central to bridging these two domains while simultaneously serving as on-off switches for electronic states. Here, we demonstrate how the prototypical material of K ₂ Cr ₈ O ₁₆ undergoes a ferromagnetic metal-insulator transition accompanied by a change in band topology. Through inelastic x-ray and neutron scattering experiments combined with first-principles theoretical calculations, we show that this transition is not driven by a Peierls mechanism, given the lack of phonon softening. Instead, we establish the transition as a topological metal-insulator transition within the ferromagnetic phase with potential axionic properties, where electron correlations play a key role in stabilizing the insulating state. These results reveal how a metal-insulator transition provides a pathway through which magnetism, topology, and electronic correlations interact.

A major challenge in condensed matter physics is integrating topological phenomena with correlated electron physics to leverage both types of states for next-generation quantum devices. Metal-insulator transitions (MITs) are central to bridging these two domains while simultaneously serving as 'on-off' switches for electronic states. Here, we demonstrate how the prototypical material of K2Cr8O16 undergoes a ferromagnetic MIT accompanied by a change in band topology. Through inelastic x-ray and neutron scattering experiments combined with first-principles theoretical calculations, we demonstrate that this transition is not driven by a Peierls mechanism, given the lack of phonon softening. Instead, we establish the transition as a topological MIT within the ferromagnetic phase (topological-FM-MIT) with potential axionic properties, where electron correlations play a key role in stabilizing the insulating state. This work pioneers the discovery of a topological-FM-MIT and represents a fundamentally new class of topological phase transitions, revealing a unique pathway through which magnetism, topology, and electronic correlations interact.

Ultrafast magnetization dynamics driven by ultrashort pump lasers is typically explained by changes in the electronic populations and scattering pathways of excited conduction electrons. This conventional approach overlooks the fundamental role of quantum mechanical selection rules, governing transitions from the core states to the conduction band, that form the key method of the probing step in these experiments. By employing fully time-dependent density functional theory, we reveal that these selection rules profoundly influence the interpretation of ultrafast spin dynamics at specific probe energies. Our analysis for hcp Co and fcc Ni at the M edge demonstrates that the transient dynamics, as revealed in pump-probe experiments, arises from a complex interplay of optical excitations of the M shell. Taking into account the selection rules and conduction electron spin flips leads to highly energy-dependent dynamics. These findings address long-standing discrepancies in experimental transverse magneto-optical Kerr effect measurements and show that only through meticulous consideration of the matrix elements at the probe stage, can one ensure that the magnetization dynamics is revealed in its true nature, instead of being muddled by artifacts arising from the choice of probe energy.

Al4SiC4 is a ternary wide-band-gap semiconductor with a high strength-to-weight ratio and excellent oxidation resistance. It consists of slabs of Al4C3 separated by SiC layers with the space group of P6(3)mc. The space group allows Si to occupy two different 2a Wykoff sites, with previous studies reporting that Si occupies only one of the two sites, giving it an ordered structure. Another hitherto unexplored possibility is that Si can be randomly distributed on both 2a sites. In this work, we revisit the published ordered crystal structure using experimental methods and density functional theory (DFT). Al4SiC4 was synthesized by high-temperature sintering at 1800 degrees C from a powder mixture of Al4C3 and SiC. Neutron diffraction confirmed that Al4SiC4 crystallized with the space group of P6(3)mc, with diffraction patterns that could be fitted to both the ordered and the disordered structures. Scanning transmission electron microscopy, however, provided clear evidence supporting the latter, with DFT calculations further confirming that it is 0.16 eV lower in energy per Al4SiC4 formula unit than the former. TEM analysis revealed Al vacancies in some of the atomic layers that can introduce p-type doping and direct band gaps of 0.7 and 1.2 eV, agreeing with our optical measurements. Finally, we propose that although the calculated formation energy of the Al vacancies is high, the vacancies are stabilized by entropy effects at the high synthesis temperature. This indicates that the cooling procedure after high-temperature synthesis can be important in determining the vacancy content and the electronic properties of Al4SiC4.

Transition metal trichalcogenides (TMTCs) offer remarkable opportunities for tuning electronic states through modifications in chemical composition, temperature, and pressure. Despite considerable interest in TMTCs, there remain significant knowledge gaps concerning the evolution of their electronic properties under compression. In this study, we employ experimental and theoretical approaches to comprehensively explore the high-pressure behavior of the electronic properties of TiS₃, a quasi-one-dimensional (Q1D) semiconductor, across various temperature ranges. Through high-pressure electrical resistance and magnetic measurements at elevated pressures, we uncover a distinctive sequence of phase transitions within TiS₃, encompassing a transformation from an insulating state at ambient pressure to the emergence of an incipient superconducting state above 70 GPa. Our findings provide compelling evidence that superconductivity at low temperatures of ∼2.9 K is a fundamental characteristic of TiS₃, shedding new light on the intriguing high-pressure electronic properties of TiS₃ and underscoring the broader implications of our discoveries for TMTCs in general.