Lihuan Sun

Scanning tunneling spectroscopy (STS) is a unique technique to probe the local density of states (LDOS) at the atomic scale by measuring the tunneling conductance between a sharp tip and a sample surface. However, the technique suffers of well-known limitations, the so-called set-point effect, which can potentially introduce artifacts in the measurements. We compare several STS imaging schemes applied to the LDOS modulations of the charge density wave state on atomically flat surfaces, and we demonstrate that only constant-height STS is capable of mapping the intrinsic LDOS. In the constant-current STS, commonly used and easier to implement, the tip-sample distance variations imposed by the feedback loop result in set-point-dependent STS images and possibly mislead the identification of the CDW gap edges.

Inducing and controlling spin-orbit coupling (SOC) in graphene is key to create topological states of matter, and for the realization of spintronic devices. Placing graphene onto a transition metal dichalcogenide is currently the most successful strategy to achieve this goal, but there is no consensus as to the nature and the magnitude of the induced SOC. Here, we show that the presence of backscattering in graphene-on-WSe₂ heterostructures can be used to probe SOC and to determine its strength quantitatively, by imaging quasiparticle interference with a scanning tunneling microscope. A detailed theoretical analysis of the Fourier transform of quasiparticle interference images reveals that the induced SOC consists of a valley-Zeeman (λ_vZ ≈ 2 meV) and a Rashba (λ_R ≈ 15 meV) term, one order of magnitude larger than what theory predicts, but in excellent agreement with earlier transport experiments. The validity of our analysis is confirmed by measurements on a 30 degree twist angle heterostructure that exhibits no backscattering, as expected from symmetry considerations. Our results demonstrate a viable strategy to determine SOC quantitatively by imaging quasiparticle interference.

We investigate the orientation switching of individual azobenzene molecules adsorbed on a Au(111) surface using a laser-assisted scanning tunneling microscope (STM). It is found that the rotational motion of the molecule can be regulated by both sample bias and laser wavelength. By measuring the switching rate and state occupation as a function of both bias voltage and photon energy, the threshold in sample bias and the minimal photon energy are derived. It has been revealed that the tip-induced local electrostatic potential remarkably contributes to the reduction in hopping barrier. We also find that the tunneling electrons and photons play distinct roles in controlling rotational dynamics of single azobenzene molecules on the surface, which are useful for understanding dynamic behaviors in similar molecular systems.

Two low-energy excitations of a single water molecule are observed in inelastic electron tunneling spectroscopy (IETS), where a significant enhancement is achieved by attaching the molecule to the tip apex in a scanning tunneling microscope (STM). Density functional theory (DFT) simulations and quantum mechanical calculations of an asymmetric top are carried out to reveal the origin of both excitations. Variations in tunneling junction separation give rise to the quantum confinement effect on the quantum state of a water molecule in the tunneling junction. Our results demonstrate a potential method to measure the dynamic behavior of a single molecule confined in a tunneling junction, where the molecule-substrate interaction can be purposely tuned.

Revealing the nature of hydrogen bond network in water structures is one of the imperative objectives of science. Using a low-temperature scanning tunneling microscope，a gallery of water clusters on Au(111) surface were directly imaged with molecular resolution by functionalized tip. Their internal structures as well as the geometry variations with the increasing of size were identified. In contrast to a buckled water hexamer predicted by previous theoretical calculations, our results present deterministic evidences for a flat configuration of water hexamers on Au(111), corroborated by density functional theory calculations with properly implemented van der Waals corrections. The consistency between the experimental observations and improved theoretical calculations not only renders the internal structures of absorbed water clusters unambiguously, but also directly manifests the crucial role of van der Waals interactions in constructing water-solid interfaces.