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Sharma S.,Max Planck Institute of Microstructure Physics | Sharma S.,Indian Institute of Technology Roorkee | Dewhurst J.K.,Max Planck Institute of Microstructure Physics | Shallcross S.,Lehrstuhl fur Theoretische Festkorperphysik | And 3 more authors.
Journal of Chemical Theory and Computation | Year: 2015

We apply the bootstrap kernel within time-dependent density functional theory to study the one-dimensional chain of polymer polyphenylenevinylene and molecular crystals of picene and pentacene. The absorption spectra of poly(p-phenylenevinylene) has a bound excitonic peak that is well-reproduced. Pentacene and picene, electronically similar materials, have remarkably different excitonic physics, and this difference is also well captured. We show that the inclusion of local-field effects dramatically changes the spectra of both picene and pentacene but not for poly(p-phenylenevinylene). © 2015 American Chemical Society.


Sharma S.,Max Planck Institute of Microstructure Physics | Dewhurst J.K.,Max Planck Institute of Microstructure Physics | Shallcross S.,Lehrstuhl fur Theoretische Festkorperphysik | Gross E.K.U.,Max Planck Institute of Microstructure Physics
Physical Review Letters | Year: 2013

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, finding very good agreement in all cases. 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 eg and t2g symmetry projected states. © 2013 American Physical Society.


Shinohara Y.,Max Planck Institute of Microstructure Physics | Sharma S.,Max Planck Institute of Microstructure Physics | Sharma S.,Indian Institute of Technology Roorkee | Dewhurst J.K.,Max Planck Institute of Microstructure Physics | And 4 more authors.
New Journal of Physics | Year: 2015

The insulator to metal phase transition in NiO is studied within the framework of reduced density matrix functional theory (RDMFT) and density functional theory (DFT). We find that the spectral density obtained using RDMFT is in good agreement with experiments both for undoped as well as doped NiO. We find that the physical description of the hole-doping induced phase transition qualitatively differs depending on whether NiO is calculated within DFT or reduced density matrix functional. In the former case the underlying mechanism of the phase transition is identified to be a rigid shift of chemical potential, while in the latter case a redistribution of the spectral weight drives the transition. These latter results are found to be in good agreement with both experiments and previous many-body calculations. © 2015 IOP Publishing Ltd and Deutsche Physikalische Gesellschaft.


Shallcross S.,Lehrstuhl fur Theoretische Festkorperphysik | Sharma S.,Max Planck Institute of Microstructure Physics | Pankratov O.,Lehrstuhl fur Theoretische Festkorperphysik
Physical Review B - Condensed Matter and Materials Physics | Year: 2013

We identify an angle-dependent momentum scale as the fundamental property of a bilayer composed of mutually rotated graphene layers. The interlayer scattering processes at these characteristic momentum values define an effective Brillouin zone (a Jones zone) which, in general, differs from the Brillouin zone generated by the real-space lattice, which is physically irrelevant. From this we develop a numerical method that increases, for the twist bilayer, the efficiency of the standard tight-binding method by a factor of ≈103 at no loss of accuracy. The efficiency of the method is based on (i) the fact that the twist Hamiltonian is exceptionally sparse in a basis of single-layer graphene (SLG) states, (ii) a solution of trivial Diophantine problem (Bézout's identity) allows one to know in advance which matrix elements take nonzero values, and (iii) to access the electronic structure in a few electron volts about the Dirac point a truncated SLG basis consisting only of states in a somewhat larger energy window are required, leading to a much reduced size of the Hamiltonian. This allows a complete survey of the system which reveals (i) an angle-dependent series of van Hove singularities, (ii) an increasing mixing of SLG states as the twist angle is reduced leading to the appearance of localization of the twist bilayer wave functions at all energies in the small-angle limit, and (iii) a zero-energy peak in the density of states in an approximately self-similar small-angle regime. © 2013 American Physical Society.


Shallcross S.,Lehrstuhl fur Theoretische Festkorperphysik | Sharma S.,Max Planck Institute of Microstructure Physics | Kandelaki E.,Lehrstuhl fur Theoretische Festkorperphysik | Kandelaki E.,Ruhr University Bochum | Pankratov O.A.,Lehrstuhl fur Theoretische Festkorperphysik
Physical Review B - Condensed Matter and Materials Physics | Year: 2010

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 θ. 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 vF of the Dirac cones decreases as θ→0°; the form we derive for vF (θ) agrees with that found via a continuum approximation in. From tight-binding calculations for structures with 1.47°≤θ<30° we find agreement with this formula for θ5°. In contrast, for θ>5° this formula breaks down and the Dirac bands become strongly warped as the limit θ→0 is approached. For an ideal system of twisted layers the limit as θ→0° is singular as for θ>0 the Dirac point is fourfold degenerate, while at θ=0 one has the twofold degeneracy of the AB stacked bilayer. Interestingly, in this limit the electronic properties are in an essential way determined globally, in contrast to the " nearsightedness" of electronic structure generally found in condensed matter. © 2010 The American Physical Society.

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