Metal to Insulator Transition in Anisotropic 2D Semiconductor
The metal-to-insulator transition (MIT) in low-dimensional materials provides a platform for studying correlated electron phenomena. In this work, we investigate the highly anisotropic MIT in the van der Waals semiconductor ReSe₂, where the transition is driven by carrier density tuning via electrostatic gating. Our transport measurements show that along the hard-axis direction, ReSe₂ remains insulating, while in the perpendicular (easy-axis) direction, it becomes metallic. This anisotropic electronic melting suggests a novel smectic-like phase, where electronic transport is strongly direction-dependent. Our findings provide new insights into the interplay between anisotropy, electron localization, and phase transitions in low-dimensional materials, offering potential implications for anisotropic electronic applications and fundamental condensed matter physics.
Haleem Kim is a 4th-year Physics Ph.D. student with the Ultrafast Nano-Optics group of Professor Feng Wang at the University of California, Berkeley and Lawrence Berkeley National Laboratory and a Kavli Energy NanoScience Institute graduate student fellow. Haleem received his B.A. from Seoul National University in 2019. Haleem’s graduate research at Berkeley focuses broadly on the probing emergent phenomenon in 2D materials using transport measurement and optical spectroscopy.
The Role of Entropy in Scaffolding the PSII Energy Transfer Network
Photosystem II (PSII) can achieve near-unity quantum efficiency of light harvesting in ideal conditions and can dissipate excess light energy as heat to prevent the formation of reactive oxygen species under light stress. Understanding how this pigment-protein complex accomplishes these opposing goals is a topic of great interest that has so far been explored primarily through the lens of the system energetics. Despite PSII’s known flat energy landscape, a thorough consideration of the entropic effects on energy transfer in PSII is lacking. In this work, we aim to discern the free energetic design principles underlying the PSII energy transfer network. To accomplish this goal, we employ a structure-based rate matrix and compute the free energy terms in time following a specific initial excitation to discern how entropy and enthalpy drive ensemble system dynamics. We find that the interplay between the entropy and enthalpy components differ among each protein subunit, which allows each subunit to fulfill a unique role in the energy transfer network. This individuality ensures PSII can accomplish efficient energy trapping in the reaction center (RC), effective nonphotochemical quenching (NPQ) in the periphery, and robust energy trapping in the other-monomer RC if the same-monomer RC is closed. We also show that entropy, in particular, is a dynamically tunable feature of the PSII free energy landscape accomplished through regulation of LHCII binding. These findings help rationalize natural photosynthesis and provide design principles for novel, more efficient solar energy harvesting technologies.
Johanna Hall is a 3rd-year Chemistry Ph.D. candidate in Graham Fleming’s group at UC Berkeley. In her research, she uses a combination of experimental and computational techniques to study the energy transfer networks in photosynthetic light harvesting systems. Findings from this research could be used to improve novel solar energy technologies. Johanna obtained a B.S. in Environmental Engineering with minors in Biology and Global Development from the Georgia Institute of Technology in 2022.