Dongsung Choi

The advent of two-dimensional moiré systems has revolutionized the exploration of phenomena arising from strong correlations and nontrivial band topology. Recently, a moiré superstructure formed by two coexisting charge density waves with slightly mismatched wavevectors has been realized. These incommensurate charge density waves can collectively exhibit commensurability, forming a jointly commensurate charge density wave that provides a paradigm for controlling moiré potential and periodicity. Here, we use time-resolved diffraction to probe the light-induced dynamics of the jointly commensurate charge density wave in EuTe ₄ . Our measurements distinguish the instantaneous quenching of its amplitude, as verified by time-resolved photoemission spectroscopy, from the slower evolution of phase fluctuations. Moreover, while the wavevector remains locked upon photoexcitation, the correlation length decreases exclusively perpendicular to the wavevector, revealing the formation of shear-type defects. Together, our multimodal approach reconstructs the temporal evolution of the jointly commensurate charge density wave, highlighting its robustness out of equilibrium and providing insights into the optical manipulation of moiré materials through defect control.

Ultrafast photoexcitation offers a novel approach to manipulating quantum materials. One of the long-standing goals in this field is to achieve optical control over topological properties. However, the impact on their electronic structures, which host gapless surface states, has yet to be directly observed. Here, using time- and angle-resolved photoemission spectroscopy, we visualize the photoinduced evolution of the band structure in Bi_{y}(Pb_{1-x}Sn_{x})_{1-y}Se(111) films from topological to trivial insulators. Following near-infrared ultrafast laser excitation, we observe that the topological surface state opens a substantial gap of up to 0.1 eV. Considering the topological phase diagram associated with lattice distortion and atomic displacement, we show that a uniaxial strain generated by the ultrafast optical pulse is sufficiently effective and strong for the observed topological phase transition. Our Letter highlights the potential of optical tuning of materials through laser excitation to control topological properties on ultrafast timescales.

Sr ₂ IrO ₄ has attracted considerable attention due to its structural and electronic similarities to La ₂ CuO ₄ , the parent compound of high- T _c superconducting cuprates. It was proposed as a strong spin–orbit-coupled J _eff = 1/2 Mott insulator, but the Mott nature of its insulating ground state has not been conclusively established. Here, we use ultrafast laser pulses to realize an insulator–metal transition in Sr ₂ IrO ₄ and probe the resulting dynamics using time- and angle-resolved photoemission spectroscopy. We observe a gap closure and the formation of weakly renormalized electronic bands in the gap region. Comparing these observations to the expected temperature and doping evolution of Mott gaps and Hubbard bands provides clear evidence that the insulating state does not originate from Mott correlations. We instead propose a correlated band insulator picture, where antiferromagnetic correlations play a key role in the gap opening. More broadly, our results demonstrate that energy–momentum-resolved nonequilibrium dynamics can be used to clarify the nature of equilibrium states in correlated materials.

We present a novel test structure for the study of sidewall ohmic contacts to III–V fins for FinFETs. We apply it to the characterization of the impact of digital etch (DE), used to trim the fin width, and thermal annealing on the contact resistivity of Mo/ $\text{n}^{+}$ -InGaAs fin sidewall contacts. To obtain sidewall contacts, we leave in place the etch mask that is used to define the fins in a reactive ion etching process and deposit conformal Mo around them. We present a model that describes well the electrical characteristics of the test structure. In our results, we find that the specific acid that is used for DE or the number of DE cycles that are performed have a minor impact on contact resistivity. Thermal annealing is found to significantly improve the sidewall contact resistivity, with the best value of $3.7~ \pm ~0.2~ {\Omega } \cdot {\mu } \text{m}^{{2}}$ obtained after annealing at 400 °C. This is about three times higher than the reported contact resistivity for this contact technology with the contact wrapping around the top and the sidewall surfaces of fins, about $1.3~ {\Omega } \cdot {\mu }\text{m}^{{2}}$ . Also, we find that the width of the non-conductive region under the sidewall surface of Mo contact, which we term “deadzone,” can be significantly mitigated by thermal annealing. Our work highlights the importance of understanding sidewall ohmic contact formation for future high-performance InGaAs FinFETs.