LASSP & AEP Seminar: Mengkun Liu (Stony Brook)
Landau level Nanoscopy of charge and heat transport in Dirac heterostructures
In contemporary condensed matter physics and photonics, four key length scales play an essential role in shaping the behavior of quantum materials: (1) the polaritonic wavelength (λ) in the infrared (IR) and terahertz (THz) frequency range, which governs light confinement and light-matter interactions; 2) the magnetic lengths l_B =√(ℏ/eB)=257Å/√(B[T]) determined by the magnetic field B, which constrains electron motion; 3) the diffusion length D of the hot carriers at interfaces and the edges, which dictates energy relaxation, and 4) the periodicities of superlattices induced by moiré engineering, which defines the energy scale of emerging quantum phases. For instance, the commensurability of the magnetic lengths (e.g. ~10 nm for graphene at 7T) and superlattice constant (e.g. ~10 nm for twisted bilayer graphene at ‘magic angle’) would give rise to exotic fractal quantum states. In this talk, I will present: 1) A cutting-edge optical spectroscopy technique, Landau-level nanoscopy, capable of simultaneously probing all four critical length scales in a single experiment; 2) the discovery of a new class of infrared polaritons that can be tuned by magnetic fields, enhancing our ability to manipulate light-matter interactions and probe many-body physics at the nanoscale; 3) nanoscale mapping of thermoelectric properties in low-dimensional electron systems within the quantum Hall regime, revealing strong violations of the Wiedemann-Franz law. Our approach establishes Landau-level nanoscopy as a versatile platform for investigating magneto-optical effects and many-body interactions with unprecedented spatial resolutions. Our preliminary results also set the stage for future spectroscopic explorations of topological and chiral photonic phenomena in complex quantum materials using low-energy photons.
Bio:
Mengkun Liu earned his Ph.D. from Boston University, where he worked with Prof. Richard D. Averitt. He later joined Prof. D. N. Basov’s lab at UC San Diego and was appointed as an assistant professor in the Department of Physics and Astronomy at Stony Brook University in 2015. He currently holds a joint appointment at Brookhaven National Laboratory and was promoted to associate professor at Stony Brook University in 2020.
Prof. Liu’s Ultrafast & Near-field Infrared Laboratory explores the future of materials science using advanced infrared and THz near-field microscopy and nano-Fourier transform IR spectroscopy. Their nanoscale imaging and manipulation of light-matter interactions boasts a spatial resolution over 1,000 times higher than conventional optics. The lab studies complex materials like transition metal oxides, low-dimensional Dirac/Weyl microstructures, superconductors, and multiferroics by accessing the native length and time scales of electron and lattice motion and monitoring energy excitations, unlocking the mysteries of Mott physics and other many body effects and bringing closer to practical applications.
