Yu Song

Giant Goos-Hanchen shift in graphene double-barrier structures

We report giant Goos-Hanchen shifts [Goos and Hanchen, Ann. Phys. 436, 333 (1947)] for electron beams tunneling through graphene double barrier structures. We find that inside the transmission gap for the single barrier, the shift displays sharp peaks with magnitudes up to the order of electron beam width and rather small full-widths-at-half-maximum, which may be utilized to design valley and spin beam splitters with wide tunability and high energy resolution. We attribute the giant shifts to quasibound states in the structures. Moreover, an induced energy gap in the dispersion can increase the tunability and resolution of the splitters.

Transport properties through graphene-based fractal and periodic magnetic barriers

We investigate the transmission of electrons in a single layer graphene system subjected to nanoscale magnetic barriers and wells arranged in the Cantor pre-fractal and the finite periodic distribution. We find that the angular threshold and angular asymmetry of the transmission spectra are closely related to the ratio between the magnitude of the vector potential and the incident energy (|A|/E), which also determine the number and width of the resonant domains for the finite periodically magnetic modulation and the splitting features for the pre-fractal distribution. For the finite periodically magnetic modulation, the position, magnitude, and interval of the oscillatory domains in the conductance spectra are determined by the value |A|. However, due to the disorder of the pre-fractal distribution, the oscillation of the conductance spectrum is less regular compared to the corresponding one of the finite periodic distribution. We also find that the conductance approaches the classical limit in the high-Fermi-energy region but exceeds it in the low-Fermi-energy region, and the critical point of the two regions is negatively correlated with the magnitude of the vector potential.

Negative differential resistances in graphene double barrier resonant tunneling diodes

We theoretically investigate negative differential resistance (NDR) of massless and massive Dirac Fermions in double barrier resonant tunneling diodes based on sufficiently short and wide graphene strips. The current-voltage characteristics calculated in a rotated pseudospin space show that, the NDR feature only presents with appropriate structural parameters for the massless case and the peak-to-valley current ratio can be enhanced exponentially by a tunable band gap. Remarkably, the lowest NDR operation window is nearly structure-free and can be almost solely controlled by a back gate, which may have potential applications in NDR devices with the operation window as a crucial parameter.We theoretically investigate negative differential resistance (NDR) of massless and massive Dirac Fermions in double barrier resonant tunneling diodes based on sufficiently short and wide graphene strips. The current-voltage characteristics calculated in a rotated pseudospin space show that the NDR feature only presents with appropriate structural parameters for the massless case, and the peak-to-valley current ratio can be enhanced exponentially by a tunable band gap. Remarkably, the lowest NDR operation window is nearly structure-free and can be almost solely controlled by a back gate, which may have potential applications in NDR devices with the operation window as a crucial parameter.

Generation of a fully valley-polarized current in bulk graphene

The generation of a fully valley-polarized current (FVPC) in bulk graphene is a fundamental goal in valleytronics. To this end, we investigate valley-dependent transport through a strained graphene modulated by a finite magnetic superlattice. It is found that this device allows a coexistence of insulating transmission gap of one valley and metallic resonant band of the other. Accordingly, a substantial bulk FVPC appears in a wide range of edge orientation and temperature, which can be effectively tuned by structural parameters. A valley-resolved Hall configuration is designed to measure the valley polarization degree of the filtered current.

High-performance liquid chromatography on silica modified with temperature-responsive polymers

SummaryThe separation selectivity of temperature-responsive poly(N-isopropylacrylamide)-modified silica as a packing material for high performance liquid chromatography was investigated with steroids, alkaloids, and substituted anilines as solutes. The elution profiles of the solutes depended on the temperature of the column and the methanol content of the mobile phase, indicating that the separation selectivity could be controlled by the column temperature or the mobile phase composition.

Spin filter and spin valve in ferromagnetic graphene

We propose and demonstrate that a EuO-induced and top-gated graphene ferromagnetic junction can be simultaneously operated as a spin filter and a spin valve. We attribute such a remarkable result to a coexistence of a half-metal band and a common energy gap for opposite spins in ferromagnetic graphene. We show that both the spin filter and the spin valve can be effectively controlled by a back gate voltage, and they survive for practical metal contacts and finite temperature. Specifically, larger single spin currents and on-state currents can be reached with contacts with work functions similar to graphene, and the spin filter can operate at higher temperature than the spin valve.

Ballistic collective group delay and its Goos–Hänchen component in graphene

We theoretically construct an experimental observable for the ballistic collective group delay (CGD) of all the particles on the Fermi surface in graphene. First, we reveal that lateral Goos-Hänchen (GH) shifts along barrier interfaces contribute an inherent component in the individual group delay (IGD). Then, by linking the complete IGD to spin precession through a dwell time, we suggest that the CGD and its GH component can be electrostatically measured by the conductance difference in a spin precession experiment under weak magnetic fields. Such an approach is feasible for almost any Fermi energy. We also indicate that it is a generally nonzero self-interference delay that relates the IGD to the dwell time in graphene.

Electrically induced bound state switches and near-linearly tunable optical transitions in graphene under a magnetic field

We study the low-lying excited spectra and optical transitions of a single Dirac electron in a graphene sheet that is subjected to a homogeneous magnetic field and an electrostatic potential produced by an applied top gate of disk shape. Numerical results based on the Dirac equation and the transfer matrix method show that in the regime of a small circle-inner magnetic flux, the variable potential induces switches between extended Landau-type bound states and localized quantum-dot-type ones. We indicate that the frequency of emitted or absorbed photons can be tuned almost linearly by the potential within specific ranges, which may have potential applications as a near-linearly-controlled photon frequency filter. These properties are robust to the sharpness of the potential boundary.

Bound states in a hybrid magnetic-electric quantum dot

We propose a hybrid magnetic-electric quantum dot defined by a missing magnetic flux and an electrostatic dot potential in a same circular region, which can be realized through an electrode-controlled (Vg) superconducting disk deposited atop a two-dimensional electron gas in a homogeneous perpendicular magnetic field (Ba). We find that when Vg/Ba=eℏ/4m∗, all ground states with m the (angular momentum) ≤0 recover the degeneracy Landau levels (LLs), although for general cases m-dependent energy deviations from the LLs happen. We also find that the magnetic-field-dependent energy spectrum exhibits quite different features for dot potentials with different signs, e.g., angular momentum transitions occurring in the positive case and coexistence of quantum-dot-kind and LL-kind levels for a small Ba in the negative one. Moreover, as the dot potential varies in a middle range, the energy spectrum shows step-type profiles, which are related to the nonmonotonous change in the probability for the electron to stay in...

Electric-field-induced extremely large change in resistance in graphene ferromagnets

A colossal magnetoresistance (~) and an extremely large magnetoresistance (~) have been previously explored in manganite perovskites and Dirac materials, respectively. However, the requirement of an extremely strong magnetic field (and an extremely low temperature) makes them not applicable for realistic devices. In this work, we propose a device that can generate even larger changes in resistance in a zero-magnetic field and at a high temperature. The device is composed of graphene under two strips of yttrium iron garnet (YIG), where two gate voltages are applied to cancel the heavy charge doping in the YIG-induced half-metallic ferromagnets. By calculations using the Landauer–Buttiker formalism, we demonstrate that, when a proper gate voltage is applied on the free ferromagnet, changes in resistance up to () can be achieved at the liquid helium (nitrogen) temperature and in a zero magnetic field. We attribute such a remarkable effect to a gate-induced full-polarization reversal in the free ferromagnet, which results in a metal-state to insulator-state transition in the device. We also find that the proposed effect can be realized in devices using other magnetic insulators, such as EuO and EuS. Our work should be helpful for developing a realistic switching device that is energy saving and CMOS-technology compatible.