482 research outputs found
Electronic structure of turbostratic graphene
We explore the rotational degree of freedom between graphene layers via the
simple prototype of the graphene twist bilayer, i.e., two layers rotated by
some angle . It is shown that, due to the weak interaction between
graphene layers, many features of this system can be understood by interference
conditions between the quantum states of the two layers, mathematically
expressed as Diophantine problems. Based on this general analysis we
demonstrate that while the Dirac cones from each layer are always effectively
degenerate, the Fermi velocity of the Dirac cones decreases as ; the form we derive for agrees with that found via a
continuum approximation in Phys. Rev. Lett., 99:256802, 2007. From tight
binding calculations for structures with we
find agreement with this formula for . In contrast, for
this formula breaks down and the Dirac bands become
strongly warped as the limit is approached. For an ideal system
of twisted layers the limit as is singular as for the Dirac point is fourfold degenerate, while at one has the
twofold degeneracy of the stacked bilayer. Interestingly, in this limit
the electronic properties are in an essential way determined \emph{globally},
in contrast to the 'nearsightedness' [W. Kohn. Phys. Rev. Lett., 76:3168,
1996.] of electronic structure generally found in condensed matter.Comment: Article as to be published in Phys. Rev B. Main changes: K-point
mapping tables fixed, several changes to presentation
Spectral density and metal-insulator phase transition in Mott insulators within RDMFT
We present a method for calculating the spectrum of periodic solids within
reduced density matrix functional theory. This method is validated by a
detailed comparison of the angular momentum projected spectral density with
that of well established many-body techniques, in all cases finding an
excellent agreement. The physics behind the pressure induced insulator-metal
phase transition in MnO is investigated. The driving mechanism of this
transition is identified as increased crystal field splitting with pressure,
resulting in a charge redistribution between the Mn and symmetry
projected states.Comment: arXiv admin note: text overlap with arXiv:0912.111
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Valley control by linearly polarized laser pulses: example of WSe2
Electrons at the band edges of materials are endowed with a valley index, a quantum number locating the band edge within the Brillouin zone. An important question is then how this index may be controlled by laser pulses, with current understanding that it couples exclusively via circularly polarized light. Employing both tight-binding and state-of-the-art time dependent density function theory, we show that on femtosecond time scales valley coupling is a much more general effect. We find that two time separated linearly polarized pulses allow almost complete control over valley excitation, with the pulse time difference and polarization vectors emerging as key parameters for valley control. Our findings highlight the possibility of controlling coherent electronic excitation by successive femtosecond laser pulses, and offer a route towards valleytronics in two-dimensional materials
Magnetic phase diagrams from non-collinear canonical band theory
A canonical band theory of non-collinear magnetism is developed and applied to the close packed fcc and bcc crystal structures. This is a parameter-free theory where the crystal and magnetic symmetry and exchange splitting uniquely determine the electronic bands. In this way, we are able to construct phase diagrams of magnetic order for the fcc and bcc lattices. Several examples of non-collinear magnetism are seen to be canonical in origin, in particular, that of γ-Fe. In this approach, the determination of magnetic stability results solely from changes in kinetic energy due to spin hybridization, and on this basis we are able to analyze the microscopic reasons behind the occurrence of non-collinear magnetism in the elemental itinerant magnets
Quantum interference at the twist boundary in graphene
We explore the consequences of a rotation between graphene layers for the electronic spectrum. We derive the commensuration condition in real space and show that the interlayer electronic coupling is governed by an equivalent commensuration in reciprocal space. The larger the commensuration cell, the weaker the interlayer coupling, with exact decoupling for incommensurate rotations and in the θ → 0 limit. Furthermore, from first-principles calculations we determine that even for the smallest possible commensuration cell the decoupling is effectively perfect, and thus graphene layers will be seen to decouple for all rotation angles
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