Modulated Charge Distribution Research Articles

Influence of Hexagonal Boron Nitride on Electronic Structure of Graphene.

By performing first-principles calculations, we studied hexagonal-boron-nitride (hBN)-supported graphene, in which moiré structures are formed due to lattice mismatch or interlayer rotation. A series of graphene/hBN systems has been studied to reveal the evolution of properties with respect to different twisting angles (21.78°, 13.1°, 9.43°, 7.34°, 5.1°, and 3.48°). Although AA- and AB-stacked graphene/hBN are gapped at the Dirac point by about 50 meV, the energy gap of the moiré graphene/hBN, which is much more asymmetric, is only about several meV. Although the Dirac cone of graphene residing in the wide gap of hBN is not much affected, the calculated Fermi velocity is found to decrease with the increase in the moiré super lattice constant due to charge transfer. The periodic potential imposed by hBN modulated charge distributions in graphene, leading to the shift of graphene bands. In agreement with experiments, there are dips in the calculated density of states, which get closer and closer to the Fermi energy as the moiré lattice grows larger.

Patterns and driving forces of dimensionality-dependent charge density waves in 2H-type transition metal dichalcogenides

Charge density wave (CDW) is a startling quantum phenomenon, distorting a metallic lattice into an insulating state with a periodically modulated charge distribution. Astonishingly, such modulations appear in various patterns even within the same family of materials. Moreover, this phenomenon features a puzzling diversity in its dimensional evolution. Here, we propose a general framework, unifying distinct trends of CDW ordering in an isoelectronic group of materials, 2H-MX2 (M = Nb, Ta and X = S, Se). We show that while NbSe2 exhibits a strongly enhanced CDW order in two dimensions, TaSe2 and TaS2 behave oppositely, with CDW being absent in NbS2 entirely. Such a disparity is demonstrated to arise from a competition of ionic charge transfer, electron-phonon coupling, and electron correlation. Despite its simplicity, our approach can, in principle, explain dimensional dependence of CDW in any material, thereby shedding new light on this intriguing quantum phenomenon and its underlying mechanisms.

Local particle-hole pair excitations by SU(2) symmetry fluctuations

Understanding the pseudo-gap phase which opens in the under-doped regime of cuprate superconductors is one of the most enduring challenges of the physics of these compounds. A depletion in the electronic density of states is observed, which is gapping out part of the Fermi surface, leading to the formation of mysterious lines of massless excitations- the Fermi arcs. Here we give a new theoretical account of the physics of the pseudo-gap phase in terms of the emergence of local patches of particle-hole pairs generated by SU(2) symmetry fluctuations. The proliferation of these local patches accounts naturally for the robustness of the pseudo-gap phase to disturbances like disorder or magnetic field and is shown to gap out part of the Fermi surface, leading to the formation of the Fermi arcs. Most noticeably, we show that these patches induce a modulated charge distribution on the Oxygen atoms, in remarkable agreement with recent X-ray and STM observations.

Anisotropic electron transport through a rectangular antidot lattice

Rectangular antidot lattices are prepared by Ga focused ion beam implantation with lateral periodicities in the range of 0.1–2.0 microm. For current flow along the long periodicity of the antidot lattice the carrier density determined from the slope of the classical Hall resistance ϱ xy is 30% larger than the carrier density deduced from the minima of the magnetoresistance oscillations. Furthermore the Hall plateaus are shifted to the low field side of the extrapolation of the classical Hall resistance. For current flow along the short periodicity a dramatic negative magnetoresistance is observed similar to a square antidot lattice. We conclude that the transport properties of the rectangular antidot lattices are dominated by the strongly modulated charge distribution in these systems.

Microphotometric investigation of fixed holograms

Holographic gratings are written in copper-doped LiNbO3 crystals at temperatures of about 180 degrees C. At room temperature the gratings are fixed. The subsequent development process under homogeneous illumination is investigated by the microphotometric method. Measurements of the light intensity pattern at the exit face of the crystal reveal not only the change of the amplitude but also a phase shift of nearly pi . The dynamics of the development process can be fully understood by the assumption of a quasi-permanent modulated charge distribution and a second modulated charge distribution of optically excitable ions.