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Charged water interfaces are ubiquitous and responsible for many important processes in nature and modern technologies, such as protein folding, electrochemistry, and photocatalysis. They appear in the form of an electric double layer that can be divided into two sub-layers. One is the so-called Stern layer composed of one to two hardly mobile, hydrogen-bonded, water monolayers next to the charged plane. The other is the diffuse layer, in which ions assuming the Poisson-Boltzmann distribution set up a dc field distribution. Being directly associated with the charged surface, the Stern layer directly controls the microscopic energy transfer and chemical reaction pathway at the interface. However, despite extensive studies on charged water interfaces, current knowledge on the microscopic structure of the Stern layer is still extremely limited. The difficulty lies in the inability of existing techniques to selectively extract structural information about the Stern layer in the presence of the diffuse layer. We now have developed a sum-frequency spectroscopy method that allows us to obtain the vibrational spectrum, and hence the microscopic structural information, of a Stern layer. Application of the method to a prototype lipid (fatty acid)-water interface reveals significant variation of its Stern layer structure upon deprotonation of the lipid headgroup. The measurement also yields a spectrum that characterizes the dc-field-induced sum-frequency generation from bulk water in general, and can help to deduce vibrational spectra of the Stern layer of other charged water interfaces. This unique ability of our method provides opportunities to gain better microscopic understanding of properties and functionality of charged water interfaces.

A phonon nanoscopy method, based on the picosecond ultrasonics technique, capable of studying the complex acoustic reflection coefficient at frequency up to 1 THz is proposed and demonstrated. By measuring the reflection coefficient at the same surface location at the interface between GaN and air, and between GaN and the material to characterize, we get access to the THz amplitude and phase spectra of the acoustic phonon reflection. The retrieval of both these pieces of information then allows the calculation of the attenuation in a wide range of frequency and gives new insight into the Kapitza anomaly. This method is then applied to cubic ice, and the measurements of the elastic properties, the phonon anharmonic decay spectrum up to 1 THz, as well as the measurements of the thermal phonon lifetime at 150 K are all achieved.

We utilize steady-state and transient optical spectroscopies to examine the responses of nonthermal quasiparticles with respect to orbital modifications in normal-state iron-chalcogenide superconductors. The dynamics shows the emergence of gaplike quasiparticles (associated with a $\ensuremath{\sim}36\text{ }\text{ }\mathrm{meV}$ energy gap) with a coincident transfer of the optical spectral weight in the visible range, at temperatures above the structural distortion. Our observations suggest that opening of the high-temperature gap and the lattice symmetry breaking are possibly driven by short-range orbital and/or charge orders, implicating a close correlation between electronic nematicity and precursor order in iron-based superconductors.

We report a direct determination of the specular scattering probability of acoustic phonons at a crystal boundary by observing the escape of incident coherent phonons from the coherent state during reflection. In the sub-THz frequency range where the phonon wavelength is much longer than the lattice constant, the acoustic phonon-interface interaction is found to agree well with the macroscopic theory on wave scattering from rough surfaces. This examination thus quantitatively verifies the dominant role of atomic-scale corrugations in the Kapitza anomaly observed at 1-10 K and further opens a new path to nondestructively estimate subnanoscale roughness of buried interfaces.