Giovanni Vignale

Highly confined low-loss plasmons in graphene–boron nitride heterostructures

Graphene plasmons were predicted to possess simultaneous ultrastrong field confinement and very low damping, enabling new classes of devices for deep-subwavelength metamaterials, single-photon nonlinearities, extraordinarily strong light-matter interactions and nano-optoelectronic switches. Although all of these great prospects require low damping, thus far strong plasmon damping has been observed, with both impurity scattering and many-body effects in graphene proposed as possible explanations. With the advent of van der Waals heterostructures, new methods have been developed to integrate graphene with other atomically flat materials. In this Article we exploit near-field microscopy to image propagating plasmons in high-quality graphene encapsulated between two films of hexagonal boron nitride (h-BN). We determine the dispersion and plasmon damping in real space. We find unprecedentedly low plasmon damping combined with strong field confinement and confirm the high uniformity of this plasmonic medium. The main damping channels are attributed to intrinsic thermal phonons in the graphene and dielectric losses in the h-BN. The observation and in-depth understanding of low plasmon damping is the key to the development of graphene nanophotonic and nano-optoelectronic devices.

Unipolar spin diodes and transistors

Unipolar devices constructed from ferromagnetic semiconducting materials with variable magnetization direction are shown theoretically to behave very similarly to nonmagnetic bipolar devices such as the p-n diode and the bipolar (junction) transistor. Such devices may be applicable for magnetic sensing, nonvolatile memory, and reprogrammable logic.

TIME-DEPENDENT DENSITY FUNCTIONAL THEORY BEYOND THE ADIABATIC LOCAL DENSITY APPROXIMATION

In the current density functional theory of linear and nonlinear time-dependent phenomena, the treatment of exchange and correlation beyond the level of the adiabatic local density approximation is shown to lead to the appearance of viscoelastic stresses in the electron fluid. Complex and frequency-dependent viscosity/elasticity coefficients are microscopically derived and expressed in terms of properties of the homogeneous electron gas. As a first consequence of this formalism, we provide an explicit formula for the linewidths of collective excitations in electronic systems.

Dynamical Corrections to the DFT-LDA Electron Conductance in Nanoscale Systems

Using time-dependent current-density functional theory, we derive analytically the dynamical exchange-correlation correction to the dc conductance of nanoscale junctions. The correction pertains to the conductance calculated in the zero-frequency limit of time-dependent density functional theory within the adiabatic local-density approximation. In particular, we show that in linear response, the correction depends nonlinearly on the gradient of the electron density; thus, it is more pronounced for molecular junctions than for quantum point contacts. We provide specific numerical examples to illustrate these findings.

Engineering artificial graphene in a two-dimensional electron gas

At low energy, electrons in doped graphene sheets behave like massless Dirac fermions with a Fermi velocity, which does not depend on carrier density. Here we show that modulating a two-dimensional electron gas with a long-wavelength periodic potential with honeycomb symmetry can lead to the creation of isolated massless Dirac points with tunable Fermi velocity. We provide detailed theoretical estimates to realize such artificial graphenelike system and discuss an experimental realization in a modulation-doped GaAs quantum well. Ultrahigh-mobility electrons with linearly dispersing bands might open new venues for the studies of Dirac-fermion physics in semiconductors.

Microscopic Theory of the Inverse Edelstein Effect

We provide a precise microscopic definition of the recently observed inverse Edelstein effect in which a nonequilibrium spin accumulation in the plane of a two-dimensional (interfacial) electron gas drives an electric current perpendicular to its own direction. The drift-diffusion equations that govern the effect are presented and applied to the interpretation of the experiments.

Center of Mass and Relative Motion in Time Dependent Density Functional Theory

I acknowledge support from NSF Grant No. DMR-9403908 and the hospitality of the ITP and of the Physics Department at UCSB where part of the work was done under NSF Grants No. PHY89-04035 and No. DMR-9308011.

Nonuniqueness of the Potentials of Spin-Density-Functional Theory

It is shown that, contrary to widely held beliefs, the potentials of spin-density-functional theory (SDFT) are not unique functionals of the spin densities. Explicit examples of distinct sets of potentials with the same ground-state densities are constructed. These findings imply that the zero-temperature exchange-correlation energy is not always a differentiable functional of the spin density. As a consequence, various types of applications of SDFT must be critically reexamined.

Drude weight, plasmon dispersion, and ac conductivity in doped graphene sheets

We demonstrate that the plasmon frequency and Drude weight of the electron liquid in a doped graphene sheet are strongly renormalized by electron-electron interactions even in the long-wavelength limit. This effect is not captured by the random-phase approximation (RPA), commonly used to describe electron fluids, and is due to coupling between the center-of-mass motion and the pseudospin degree of freedom of the graphene’s masslessDiracfermions.Bymakinguseofdiagrammaticperturbationtheorytofirstorderintheelectron-electron interaction, we show that this coupling enhances both the plasmon frequency and the Drude weight relative to the RPA value. We also show that interactions are responsible for a significant enhancement of the optical conductivity at frequencies just above the absorption threshold. Our predictions can be checked by far-infrared spectroscopy or inelastic light scattering.

Magnetic Fields and Density Functional Theory

Publisher Summary The current-density-functional theory ( CSDFT) reviewed in this chapter is a rigorous formulation of a many-body problem of nonrelativistic interacting fermions in gauge fields. In addition to spin polarization, it includes for the first time the effect of orbital currents. A central result is the self-consistent Schroedinger-type equation showing that the replacement of the external vector potential by an effective one (including exchange-correlation effects) must be done in the linear term but not in the quadratic one. Although, at first sight, this appears to violate gauge invariance and hence the continuity equation a careful consideration of the transformation of the effective potentials based on the symmetry of exchange-correlation energy reveals that there is no such violation. The emphasis is on the appearance of an exchange-correlation contribution to the vector potential has nothing to do with the fact circulating currents generate, according to Maxwells equations, a “classical” contribution to the magnetic field. This effect is extremely small and could be accounted for by replacing the external vector potential by the self-consistent potential.