Our Research

Our group researches the frontiers of theoretical condensed matter physics, focusing on quantum materials driven far from equilibrium, with a special emphasis on periodic driving (Floquet engineering). By harnessing time-periodic fields—particularly light—we study how electronic and magnetic properties in low-dimensional systems, such as graphene and topological insulators, can be dynamically controlled and reconfigured. This approach allows us to uncover and design novel quantum phases and topological states of matter that emerge uniquely under periodic driving. Ultimately, we aim to reveal new physical phenomena and functionalities with potential applications in spintronics, quantum information, and advanced quantum technologies.

Below, I highlight some of our most recent research directions:

Quantum Phenomena Induced by Vortex Light Beams

Light is commonly used to control materials by adjusting its brightness or polarization. However, light beams with a twisting wave front—called vortex light—carry orbital angular momentum, offering an entirely new way to interact with matter. In our recent research, we found that shining vortex light onto materials with Dirac-like electronic properties creates tiny, light-induced whirlpools that can trap special quantum states known as vortex states. This provides a novel approach to shaping and tuning the behavior of quantum materials. Our findings open up exciting opportunities to harness not just the intensity, but also the structure of light in designing the next generation of electronic devices.

Figure: (a) shows a schematic of a massive Dirac-like material driven by a circularly polarized vortex light beam (VLB). In this system, a rich variety of electronic states emerges, as depicted in (b). Panel (c) displays the quasi-energy spectrum for the configuration in (a), where the VLB carries an orbital angular momentum of l=3. The probability densities of two representative edge states are shown in (d) and (e), while (f) illustrates a bulk state, and (g) highlights a vortex state.

Floquet Engineering of 2D Magnetic Interactions

Light is much more than a probe—it can serve as a powerful tool to control how tiny magnetic moments, or spins, interact in two-dimensional systems. Our recent works demonstrate that by periodically driving both two- and three-dimensional electron systems, especially those with strong spin-orbit coupling or topological surface states, we can dramatically reshape how spins interact. Through Floquet engineering, we can finely tune the strength and nature of indirect magnetic exchange—also known as RKKY interaction—by selectively enhancing the Ising, Heisenberg, and Dzyaloshinskii–Moriya components of the magnetic exchange between localized spins. With this approach, simply adjusting the driving field allows us to switch between different magnetic regimes in real time. These advances open exciting possibilities for engineering magnetic order in materials, paving the way for the next generation of spintronic technologies.

Figure:  (a) Fermi surface of BiTeI prior to irradiation, showing spin-split contours with opposite helicities that are constant along kz​. (b) Fermi surface following illumination with circularly polarized light, highlighting how photon dressing alters the band structure. (c) Spin texture of the photo-dressed Fermi surface, now exhibiting variation with kz. Panels (d), (e), and (f) display the interlayer exchange interactions as a function of BiTeI thickness (layer separation): Jzz​ (collinear out-of-plane), Jxx (collinear in-plane), and Jxy (non-collinear or chiral). Dotted lines correspond to numerical results, while solid lines show analytical predictions. All calculations use a Fermi energy of 180 meV and a photon energy of 7.5 eV. Photon dressing in BiTeI enables tunable and non-collinear interlayer exchange couplings.

Magneto-Optical Probes of Topological Thin-Films

Thin films of topological insulators reveal a rich landscape of electronic behavior that can be tuned simply by changing their thickness. Our recent research shows that as the film becomes thinner or thicker, the coupling between its surfaces can cause protected edge states to emerge or disappear, signaling a transition between distinct topological phases. To probe these states, we employ magneto-optical techniques—specifically, the Faraday and Kerr effects—which provide a powerful and non-invasive method to distinguish electronic responses at the edge from those in the bulk of the film. Even in ultrathin or imperfect samples, this approach allows us to pinpoint and track the presence of topological edge states. These findings open new possibilities for detecting and controlling quantum phenomena in nano-engineered TI films, laying the groundwork for advanced electronic and optoelectronic devices that harness the unique properties of topological materials.

Figure: (a) Thickness dependence of the hybridization gap (Δ) in antimony telluride (Sb₂Te₃) thin films, shown both with and without particle-hole asymmetry. The inset in panel (a) shows Δ as a function of film thickness for bismuth selenide (Bi₂Se₃). Panels (b) and (c) display the low-energy electronic band structures for 8-quintuple-layer (QL) thin films of Sb₂Te₃ and Bi₂Se₃, respectively. Panel (d) presents the Landau level spectrum of an 8 QL Sb₂Te₃ thin film under a 25 tesla magnetic field. Solid horizontal lines indicate surface Landau levels, while dotted lines represent bulk Landau levels. Solid arrows mark allowed surface-to-surface optical transitions, and dotted arrows indicate bulk-to-surface transitions.