12 research outputs found
Dirac fermion time-Floquet crystal: manipulating Dirac points
We demonstrate how to control the spectra and current flow of Dirac electrons
in both a graphene sheet and a topological insulator by applying either two
linearly polarized laser fields with frequencies and or a
monochromatic (one-frequency) laser field together with a spatially periodic
static potential(graphene/TI superlattice). Using the Floquet theory and the
resonance approximation, we show that a Dirac point in the electron spectrum
can be split into several Dirac points whose relative location in momentum
space can be efficiently manipulated by changing the characteristics of the
laser fields. In addition, the laser-field controlled Dirac fermion band
structure -- Dirac fermion time-Floquet crystal -- allows the manipulation of
the electron currents in graphene and topological insulators. Furthermore, the
generation of dc currents of desirable intensity in a chosen direction occurs
when applying the bi-harmonic laser field which can provide a straightforward
experimental test of the predicted phenomena.Comment: 9 pages, 7 figures, version that will appear in Phys. Rev.
Josephson-like currents in graphene for arbitrary time-dependent potential barriers
From the exact solution of the Dirac-Weyl equation we find unusual currents
j_y running in y-direction parallel to a time-dependent scalar potential
barrier W(x,t) placed upon a monolayer of graphene, even for vanishing momentum
component p_y. In their sine-like dependence on the phase difference of wave
functions, describing left and right moving Dirac fermions, these currents
resemble Josephson currents in superconductors, including the occurance of
Shapiro steps at certain frequencies of potential oscillations. The
Josephson-like currents are calculated for several specific time-dependent
barriers. A novel type of resonance is discovered when, accounting for the
Fermi velocity, temporal and spatial frequencies match.Comment: 8 pages, 4 figur
Terahertz Josephson plasma waves in layered superconductors: spectrum, generation, nonlinear, and quantum phenomena
The recent growing interest in terahertz (THz) and sub-THz science and
technology is due to its many important applications in physics, astronomy,
chemistry, biology, and medicine. We review the problem of linear and
non-linear THz and sub-THz Josephson plasma waves in layered superconductors
and their excitations produced by moving Josephson vortices. We start by
discussing the coupled sine-Gordon equations for the gauge-invariant phase
difference of the order parameter in the junctions, taking into account the
effect of breaking the charge neutrality, and deriving the spectrum of
Josephson plasma waves. We also review surface and waveguide Josephson plasma
waves. We review the propagation of weakly nonlinear Josephson plasma waves
below the plasma frequency, which is very unusual for plasma-like excitations.
In close analogy to nonlinear optics, these waves exhibit numerous remarkable
features, including a self-focusing effect, and the pumping of weaker waves by
a stronger one. We also present quantum effects in layered superconductors,
specifically, the problem of quantum tunnelling of fluxons through stacks of
Josephson junctions. We discuss the Cherenkov and transition radiations of the
Josephson plasma waves produced by moving Josephson vortices. We also discuss
the problem of coherent radiation (superradiance) of the THz waves by exciting
uniform Josephson oscillations. The effects reviewed here could be potentially
useful for sub-THz and THz emitters, filters, and detectors
Dimer currents on one dimensional asymmetric substrates
Transport on a one dimensional asymmetric substrate is studied for the case of short, soft charged dimers, formed by two equal masses carrying charges of opposite sign. The stationary dimer currents are computed as functions of the drive orientation and the dimer parameters. Optimal transport conditions are predicted to depend on the dimer elastic constant, in close agreement with the simulation data. Correspondingly, the rectification dimer currents sustained by low frequency ac drives reveal a rich phenomenology, also investigated in detail
Ratcheting of driven attracting colloidal particles: temporal density oscillations and current multiplicity
We consider the unidirectional particle transport in a suspension of colloidal particles which interact with each other via a pair potential having a hard-core repulsion plus an attractive tail. The colloids are confined within a long narrow channel and are driven along by a dc or an ac external potential. In addition, the walls of the channel interact with the particles via a ratchetlike periodic potential. We use dynamical density functional theory to compute the average particle current. In the case of dc drive, we show that as the attraction strength between the colloids is increased beyond a critical value, the stationary density distribution of the particles loses its stability leading to depinning and a time-dependent density profile. Attraction induced symmetry breaking gives rise to the coexistence of stable stationary density profiles with different spatial periods and time-periodic density profiles, each characterized by different values for the particle current