College of Arts and Sciences

Antiferromagnetic resonance and in-gap terahertz continuum in Kitaev Honeycone magnet α−RuCl3

Spin-1/2 moments in the antiferromagnetic Mott insulator α-RuCl3 are coupled by strongly anisotropic bond-dependent exchange interactions on a honeycomb lattice. Intense study of α- RuCl3 by inelastic scattering has been driven by the proposal that its low energy excitations may be adiabatically connected to the Majorana quasiparticles that emerge in the exact solution of the Kitaev spin liquid model. In my talk, I will present optical absorption measurements using time- domain terahertz spectroscopy in the range 0.3 to 10 meV that reveal several new features of the low-energy spectrum of α-RuCl3 [1].

Continue reading… Liang Wu, UC Berkeley, MPPL3, Antiferromagnetic resonance and in-gap terahertz continuum in Kitaev Honeycomb magnet α−RuCl3

Quantized electro-dynamical responses in topological materials

Although solid-state systems are usually considered “dirty” with impurities and imperfections, it is still the case that macroscopic, quantized phenomena can be observed in the form of the Josephson effect in superconductors and the quantum Hall effect in 2DEG. Combinations of these measurements allow you to determine Planck’s constant and the fundamental charge in a solid-state setting. In my talk, I will show you the observation of a new quantized response in units of the fine structure constant in a new class of material so called “topological insulators” (Tis). First,

Continue reading… Liang Wu (Berkeley); Michelson Postdoctoral Prize Lecture

Giant nonlinear optical responses in Weyl semimetals

Recently Weyl quasi-particles have been observed in transition metal monopnictides (TMMPs) such as TaAs, a class of noncentrosymmetric materials that heretofore received only limited attention. The question that arises now is whether these materials will exhibit novel, enhanced, or technologically applicable properties. The TMMPs are polar metals, a rare subset of inversion- breaking crystals that would allow spontaneous polarization, were it not screened by conduction electrons. Despite the absence of spontaneous polarization, polar metals can exhibit other signatures, most notably second-order nonlinear optical polarizability, leading to phenomena such as second-harmonic generation (SHG).

Continue reading… Liang Wu, University California Berkeley, MPPL2,Giant nonlinear optical responses in Weyl semimetals

Low-energy Electrodynamics of 3D Topological Insulators

Topological insulators (TIs) are a recently discovered state of matter characterized by an “inverted” band structure driven by strong spin-orbit coupling. One of their most touted properties is the existence of robust “topologically protected” surface states.  I will discuss what topological protection means for transport experiments and how it can be probed using the technique of time- domain THz spectroscopy applied to 3D TI thin films of Bi2Se3.  By measuring the low frequency optical response, we can follow their transport lifetimes as we drive these materials via chemical substitution through a quantum phase transition into a topologically trivial regime [1].

Continue reading… Liang Wu, University California Berkeley, MPPL1, Low-energy Electrodynamics of 3D Topological Insulators