Hai-Yao Deng

Formation mechanism of bound states in graphene point contacts

Electronic localization in narrow graphene constrictions is theoretically studied, and it is found that long-lived quasibound states (QBSs) can exist in a class of ultrashort graphene quantum point contacts (QPCs). These QBSs are shown to originate from the dispersionless edge states that are characteristic of the electronic structure of generically terminated graphene, in which pseudo-time-reversal symmetry is broken. The QBSs can be regarded as interface states confined between two graphene samples, and their properties can be modified by changing the sizes of the QPC and the interface geometry. In the presence of bearded sites, these QBSs can be converted into bound states. Experimental consequences and potential applications are discussed.

Decomposition into propagating and evanescent modes of graphene ribbons

odinger equation, and they are useful in a number of physical problems. In the present paper, we establish a complete set of BMs for graphene ribbons at arbitrary energy. We derive analytical expressions for these modes and systematically classify them into propagating or evanescent mode. We also demonstrate their uses in efficient electronic transport simulations of graphene-based electronic devices within both the mode-matching method and the Green’s function framework. Explicit constructions of Green’s functions for infinite and semi-infinite graphene ribbons are presented.

Mode-Matching Approach to Current Blocking Effect in Graphene Nanoribbons

Zigzag graphene nanoribbon (ZGNR) p–n junctions display parity-dependent transport on the number of zigzag chains. We revisit this phenomenon using the mode-matching method and derive analytical solutions for the transmission probability. It is pointed out that the randomness of the interface tilting destroys the parity effect if the spread of the tilting parameter is larger than the lattice constant. We discuss the origin of the parity effect in connection with the lattice symmetry. Junctions with bearded ZGNRs are shown to strongly suppress electron transmission.

Strong mechanically induced effects in DC current-biased suspended Josephson junctions

Superconductivity is a result of quantum coherence at macroscopic scales. Two superconductors separated by a metallic or insulating weak link exhibit the AC Josephson effect: the conversion of a DC voltage bias into an AC supercurrent. This current may be used to activate mechanical oscillations in a suspended weak link. As the DC-voltage bias condition is remarkably difficult to achieve in experiments, here we analyze theoretically how the Josephson effect can be exploited to activate and detect mechanical oscillations in the experimentally relevant condition with purely DC current bias. We unveil how changing the strength of the electromechanical coupling results in two qualitatively different regimes showing dramatic effects of the oscillations on the DC-voltage characteristic of the device. These include the appearance of Shapiro-type plateaus for weak coupling and a sudden mechanically induced retrapping for strong coupling. Our predictions, measurable in state-of-the-art experimental setups, allow the determination of the frequency and quality factor of the resonator using DC only techniques.

A Universal Self-Amplification Channel for Surface Plasma Waves

Surface plasma waves (SPWs) have been extensively studied in the past two decades with a promise for many applications. However, the effort has so far been met with limited success. It is widely believed that a major caveat lies with the energy losses experienced by SPWs during their propagation. To compensate for the losses, amplifiers have been designed, which are all extrinsic and need an external agent to supply the energy. Here we theoretically show that there exists an intrinsic amplification channel for SPWs in the collision-less limit. We pin down the origin of this channel and analytically calculate the amplification rate. Our finding unveils a hitherto unchartered yet fundamental property of SPWs and may bear far-reaching practical consequences.

Possible instability of the Fermi sea against surface plasma oscillations

We derive a generic formalism for studying the energy conversion processes in bounded metals. Using this formalism we show that in the collision-less limit the Fermi sea of metals should experience an instability against surface plasma oscillations, which opens for the latter an intrinsic self-amplification channel. The origin of the instability is clarified as arising from novel effects resulting from the translation symetry breaking due to the very presence of surface. The amplification rate of this channel is analytically evaluated on the basis of energy conservation and the effects of losses are discussed. In particular, the unique role played by the surface in energy conversion is unveiled. In contrast with common wisdom and in line with observations, Landau damping is shown as always overcompensated and therefore poses no serious issues in sub-wavelength plasmonics.

Possible self-amplification channel for surface plasma waves

Surface plasma waves (SPWs) have been extensively studied in the past two decades with a promise for many applications. However, the effort has so far been met with limited success. It is widely believed that a major caveat lies with the energy losses experienced by SPWs during their propagation. To compensate for the losses, amplifiers have been designed, which are all extrinsic and need an external agent to supply the energy. Here we theoretically show that there exists an intrinsic amplification channel for SPWs in the collision-less limit. We pin down the origin of this channel and analytically calculate the amplification rate. Our finding unveils a hitherto unchartered yet fundamental property of SPWs and may bear far-reaching practical consequences.

Power Spectral Density of Free-Standing Viscoelastic Films by Adiabatic Approximation

In a previous study, we calculated the surface dynamics of noisy viscoelastic supported films by using an adiabatic approximation. An expression was derived for the time-dependent power spectral density (PSD), which was found to produce good agreement with experiment. In this study, we extend the treatment to viscoelastic free-standing films. Two sets of surface capillary normal modes, namely, the squeezing and bending modes, were found. The frequency dispersion relation of the former resembles that of supported films. The latter is distinctively different and diverges at long wavelengths. By incorporating the experimental conditions, we obtained satisfactory agreement between theory and experiment.