Vadym Apalkov

Properties of graphene: a theoretical perspective

The electronic properties of graphene, a two-dimensional crystal of carbon atoms, are exceptionally novel. For instance, the low-energy quasiparticles in graphene behave as massless chiral Dirac fermions which has led to the experimental observation of many interesting effects similar to those predicted in the relativistic regime. Graphene also has immense potential to be a key ingredient of new devices, such as single molecule gas sensors, ballistic transistors and spintronic devices. Bilayer graphene, which consists of two stacked monolayers and where the quasiparticles are massive chiral fermions, has a quadratic low-energy band structure which generates very different scattering properties from those of the monolayer. It also presents the unique property that a tunable band gap can be opened and controlled easily by a top gate. These properties have made bilayer graphene a subject of intense interest. In this review, we provide an in-depth description of the physics of monolayer and bilayer graphene from a theorists perspective. We discuss the physical properties of graphene in an external magnetic field, reflecting the chiral nature of the quasiparticles near the Dirac point with a Landau level at zero energy. We address the unique integer quantum Hall effects, the role of electron correlations, and the recent observation of the fractional quantum Hall effect in the monolayer graphene. The quantum Hall effect in bilayer graphene is fundamentally different from that of a monolayer, reflecting the unique band structure of this system. The theory of transport in the absence of an external magnetic field is discussed in detail, along with the role of disorder studied in various theoretical models. Recent experminental observations of a metal–insulator transition in hydrogenated graphene is discussed in terms of a self-consistent theory and compared with related numerical simulations. We highlight the differences and similarities between monolayer and bilayer graphene, and focus on thermodynamic properties such as the compressibility, the plasmon spectra, the weak localization correction, quantum Hall effect and optical properties. Confinement of electrons in graphene is non-trivial due to Klein tunnelling. We review various theoretical and experimental studies of quantum confined structures made from graphene. The band structure of graphene nanoribbons and the role of the sublattice symmetry, edge geometry and the size of the nanoribbon on the electronic and magnetic properties are very active areas of research, and a detailed review of these topics is presented. Also, the effects of substrate interactions, adsorbed atoms, lattice defects and doping on the band structure of finite-sized graphene systems are discussed. We also include a brief description of graphane–gapped material obtained from graphene by attaching hydrogen atoms to each carbon atom in the lattice.

Optical-field-induced current in dielectrics

The time it takes to switch on and off electric current determines the rate at which signals can be processed and sampled in modern information technology. Field-effect transistors are able to control currents at frequencies of the order of or higher than 100 gigahertz, but electric interconnects may hamper progress towards reaching the terahertz (1012 hertz) range. All-optical injection of currents through interfering photoexcitation pathways or photoconductive switching of terahertz transients has made it possible to control electric current on a subpicosecond timescale in semiconductors. Insulators have been deemed unsuitable for both methods, because of the need for either ultraviolet light or strong fields, which induce slow damage or ultrafast breakdown, respectively. Here we report the feasibility of electric signal manipulation in a dielectric. A few-cycle optical waveform reversibly increases—free from breakdown—the a.c. conductivity of amorphous silicon dioxide (fused silica) by more than 18 orders of magnitude within 1 femtosecond, allowing electric currents to be driven, directed and switched by the instantaneous light field. Our work opens the way to extending electronic signal processing and high-speed metrology into the petahertz (1015 hertz) domain.

Controlling dielectrics with the electric field of light

The control of the electric and optical properties of semiconductors with microwave fields forms the basis of modern electronics, information processing and optical communications. The extension of such control to optical frequencies calls for wideband materials such as dielectrics, which require strong electric fields to alter their physical properties. Few-cycle laser pulses permit damage-free exposure of dielectrics to electric fields of several volts per ångström and significant modifications in their electronic system. Fields of such strength and temporal confinement can turn a dielectric from an insulating state to a conducting state within the optical period. However, to extend electric signal control and processing to light frequencies depends on the feasibility of reversing these effects approximately as fast as they can be induced. Here we study the underlying electron processes with sub-femtosecond solid-state spectroscopy, which reveals the feasibility of manipulating the electronic structure and electric polarizability of a dielectric reversibly with the electric field of light. We irradiate a dielectric (fused silica) with a waveform-controlled near-infrared few-cycle light field of several volts per angström and probe changes in extreme-ultraviolet absorptivity and near-infrared reflectivity on a timescale of approximately a hundred attoseconds to a few femtoseconds. The field-induced changes follow, in a highly nonlinear fashion, the turn-on and turn-off behaviour of the driving field, in agreement with the predictions of a quantum mechanical model. The ultrafast reversibility of the effects implies that the physical properties of a dielectric can be controlled with the electric field of light, offering the potential for petahertz-bandwidth signal manipulation.

Semimetallization of dielectrics in strong optical fields

At the heart of ever growing demands for faster signal processing is ultrafast charge transport and control by electromagnetic fields in semiconductors. Intense optical fields have opened fascinating avenues for new phenomena and applications in solids. Because the period of optical fields is on the order of a femtosecond, the current switching and its control by an optical field may pave a way to petahertz optoelectronic devices. Lately, a reversible semimetallization in fused silica on a femtosecond time scale by using a few-cycle strong field (~1 V/Å) is manifested. The strong Wannier-Stark localization and Zener-type tunneling were expected to drive this ultrafast semimetallization. Wider spread of this technology demands better understanding of whether the strong field behavior is universally similar for different dielectrics. Here we employ a carrier-envelope-phase stabilized, few-cycle strong optical field to drive the semimetallization in sapphire, calcium fluoride and quartz and to compare this phenomenon and show its remarkable similarity between them. The similarity in response of these materials, despite the distinguishable differences in their physical properties, suggests the universality of the physical picture explained by the localization of Wannier-Stark states. Our results may blaze a trail to PHz-rate optoelectronics.

Ultrafast field control of symmetry, reciprocity, and reversibility in buckled graphene-like materials

Center for Nano-Optics (CeNO) and Department of Physics and Astronomy,Georgia State University, Atlanta, Georgia 30303, USA(Dated: May 4, 2015)We theoretically show that buckled two-dimensional graphene-like materials (silicene and ger-manene) subjected to a femtosecond strong optical pulse can be controlled by the optical eldcomponent normal to their plane. In such strong elds, these materials are predicted to exhibitnon-reciprocal reection, optical recti cation and generation of electric currents both parallel andnormal to the in-plane eld direction. Reversibility of the conduction band population is also eld-and carrier-envelope phase controllable. There is a net charge transfer along the material planethat is also dependent on the normal eld component. Thus a graphene-like buckled material be-haves analogously to a eld-e ect transistor controlled and driven by the electric eld of light withsubcycle (femtosecond) speed.I. INTRODUCTION

Electrical current through DNA containing mismatched base pairs

Mismatched base pairs, such as different conformations of the G.A mispair, cause only minor structural changes in the host DNA molecule, thereby making mispair recognition an arduous task. Electron transport in DNA that depends strongly on the hopping transfer integrals between the nearest base pairs, which in turn are affected by the presence of a mispair, might be an attractive approach in this regard. We report here on our investigations, via the I-V characteristics, of the effect of a mispair on the electrical properties of homogeneous and generic DNA molecules. The I-V characteristics of DNA were studied numerically within the double-stranded tight-binding model. The parameters of the tight-binding model, such as the transfer integrals and on-site energies, are determined from first-principles calculations. The changes in electrical current through the DNA chain due to the presence of a mispair depend on the conformation of the G.A mispair and are appreciable for DNA consisting of up to 90 base pairs. For homogeneous DNA sequences the current through DNA is suppressed and the strongest suppression is realized for the G(anti).A(syn) conformation of the G.A mispair. For inhomogeneous (generic) DNA molecules, the mispair result can be either a suppression or an enhancement of the current, depending on the type of mispairs and actual DNA sequence.

Buckled Dirac Materials in Ultrashort and Strong Optical Field: Coherent Control and Reversibility Modulation

In this paper, we broadly investigate the interaction of Dirac materials (silicene and germanene) with a few-femtosecond intense optical pulse. We show that electron dynamics in such a short optical pulse is coherent, and its reversibility can be controlled by the polarization of the optical pulse, as well as the direction of propagation, i.e., angle of incidence. By varying the incident angle of the pulse, one can change the electron dynamics from highly irreversible at small angle of incidence (with respect to normal of the plane) to almost fully reversible at large angles. The reversibility of electron dynamics is also sensitive to the polarization of the pulse relative to the orientation of crystallographic planes in silicene/germanene. Such control of electron dynamics in buckled graphene materials is due to the sensitivity of interband coupling in buckled materials with respect to the component of optical field perpendicular to the silicene/germanene monolayer.

Tunability of the fractional quantum Hall states in buckled Dirac materials

We report on the fractional quantum Hall states of germanene and silicene where one expects a strong spin-orbit interaction. This interaction causes an enhancement of the electron-electron interaction strength in one of the Landau levels corresponding to the valence band of the system. This enhancement manifests itself as an increase of the fractional quantum Hall effect gaps compared to that in graphene and is due to the spin-orbit induced coupling of the Landau levels of the conduction and valence bands, which modifies the corresponding wave functions and the interaction within a single level. Due to the buckled structure, a perpendicular electric field lifts the valley degeneracy and strongly modifies the interaction effects within a single Landau level: in one valley the perpendicular electric field enhances the interaction strength in the conduction band Landau level, while in another valley, the electric field strongly suppresses the interaction effects.

Addendum: Optical-field-induced current in dielectrics

This corrects the article DOI: 10.1038/nature11567

Enhanced mid-infrared transmission through a metallic diffraction grating

We study theoretically an enhancement of the intensity of mid-infrared light transmitted through a metallic diffraction grating. We show that for s-polarized light the enhancement of the transmitted light is much stronger than for p-polarized light. By tuning the parameters of the diffraction grating, the enhancement of the transmitted light can be increased by a few orders of magnitude. The spatial distribution of the transmitted light is highly nonuniform with very sharp peaks, which have spatial widths of about 10 nm.