The DFT and molecular dynamics multiscale study of the corrugation of graphene on Ru(0001): the unexpected stability of the moire-buckled structure

Results from first principles density functional theory (DFT) calculations and classical molecular dynamics (CMD) simulations are presented on moire-corrugation of graphene (gr). We find that the moire-corrugated graphene could be surprisingly stable against the perfectly flat gr-sheet as pointed out by CMD simulations and DFT calculations. We also show that using the cost-effective CMD approach one can simulate graphene on e.g. Ru(0001) with a correct binding registry and reasonable corrugation and adhesion energy. A new force field has been parameterized for the interface using an angular-dependent Abell-Tersoff potential. The newly parameterized Abell-Tersoff interface potential provides correct moire superstructures in accordance with scanning tunnelling microscopy images and with DFT results. Based on ab initio DFT calculations, we also find that the CMD moire superstructure can be used as a preoptimized structure for DFT calculations and for further geometry optimization. The nearly flat gr (the corrugation $\xi \approx 0.2$ $\hbox{\AA}$) on Ru(0001) is slightly energetically unfavorable vs. the moire-corrugated gr-system ($\xi \approx 2.0$ $\hbox{\AA}$) as revealed by van der Waals DFT structural relaxation.Comment: 13 pages, 6 figures, 3 tables. arXiv admin note: substantial text overlap with arXiv:1312.478

The checkerboard modulation and the inter-layer asymmetry of the hole density in cuprates

The 2D pair-condensate is characterized by a charge ordered state with a "checkerboard" pattern in the planes and with an alternating superstructure along the c-axis.We find that Coulomb energy gain occurs along the c-axis, which is proportional to the measured condensation energy and is due to inter-layer charge complementarity (charge asymmetry of the boson condensate. The static c-axis dielectric constant and the in-plane coherence length are also calculated for various cuprates and compared with the available experimental data. We find excellent agreement .Comment: 4 pages, 3 figures, 2 tables, poster presentation at the International Conference on 'Dynamic Inhomogenities in Complex Oxides' (June 2003, Bled, Slovenia

Fe impurity induced ion-nanopatterning: atomistic simulations using a new force field for FeSi

The ion-bombardment induced nanopatterning of Si(001) has been simulated by atomistic simulations with and without Fe impurity. The surface contamination has been simulated by using a new force field developed for FeSi. This is a fitted bond order potential (BOP) given for Si and for Fe by Albe et al. This BOP formula has been optimized simultaneously for FeSi, Si and for Fe. Using this new force field we are able to follow the ion-beam assisted deposition for Si in the presence of Fe contamination in the surface region. As an overall result, we get an unexpectedly rich variety of nanopatterns formed by the reorganization of the crater rims of the individual ion impacts. The previously thought simple atomistically roughened surfaces show unprecedented landscapes and topography with nanoscale features. The characteristic size of the units of the pattern is in the range of a few nms. Typical of the occurred pattern is the network of interconnected elongated adatom islands. We also see the self-organization of this pattern upon ion-bombardments. At $50^{\circ}$ impact angle we get a nanoporous surface (sponge-like) both for Fe-contaminated and Fe-free simulations. At $70^{\circ}$ of impact (grazing angle of incidence) the pattern resembles to that of elongated atomic chains (adatom islands) along the ion impact direction. This latter pattern could be understood as a prepattern state towards rippling. At lower angles ($30^{\circ}$) nanoholes rule the landsdcape. The obtained pattern corresponds to low fluence experiments which are used to consider as simple roughening without showing any sing of patterning.Comment: 7 pages, 12 figures, 3 table

Charge ordering and inter-layer coupling in cuprates

We analyze the superconducting state and c-axis charge dynamics of cuprates at optimal doping using a charge ordered bilayer superlattice model in which pairing is supported by inter-layer Coulomb energy gain (potential energy driven superconductivity).The 2D pair-condensate can be characterized by a charge ordered state with a "checkerboard" like pattern seen by scanning tunneling microscopy.The pair condensation might lead to the sharp decrease of the normal state c-axis anisotropy of the hole content and hence to the decrease of inter-layer dielectric screening.The decrease of the c-axis dielectric screening can be the primary source of the condensation energy below $T_c$. We find that a net gain in the electrostatic energy occurs along the c-axis, which is proportional to the measured condensation energy ($U_0$) and with $T_c$:$E_c^{3D} \approx 2 (\xi_{ab}/a_0+1)^2 U_0 \approx k_B T_c$ and is due to inter-layer charge complementarity (charge asymmetry of the boson condensate) where $\xi_{ab}$ is the coherence length of the condensate and $a_0 \approx 3.9 \AA$ is the in-planelattice constant.Comment: 13 pages, 7 figure

Condensation energy, charge ordering and inter-layer coupling in cuprates

The correlation between the condensation energy and the critical temperature is studied within a charge ordered superlattice bilayer model in which pairing is supported by inter-layer Coulomb energy gain (potential energy driven superconductivity).The 2D pair-condensate can be characterized by a charge ordered state with a "checkerboard" like pattern seen by scanning tunneling microscopy.The drop of the c-axis dielectric screening can be the primary source of the condensation energy at optimal doping.We find that Coulomb energy gain occurs along the c-axis, which is proportional to the measured condensation energy ($U_0$) and to $T_c$: $E_c^{3D} \approx 2 (\xi_{ab}/a_0+1)^2 U_0 \approx k_B T_c$ and is due to inter-layer charge complementarity (charge asymmetry of the boson condensate)where $\xi_{ab}$ is the coherence length of the condensate and $a_0 \approx 3.9 \AA$ is the in-plane lattice c onstant.The static c-axis dielectric constant $\epsilon_c$ is calculated for various cuprates and compared with the available experimental data.Comment: 4 pages, 2 figure

Possible three-dimensional chiral charge ordered superconducting state in cuprates

The 2D pair-condensate is characterized by a fluctuating chiral charge ordered state with a "checkerboard" pattern in the CuO_2 planes and with an alternating supermodulation along the c-axis in such a way that the adjacent layers are mirror images of one anothers electronic state. Planar chiral order is revealed with a recent circular dichroism (CD) ARPES experiment (A. Kaminski, et al., Nature, 416, 611. (2002)). We propose further CD experiments on ultrathin films with varying thickness and argue that the odd number of unit cells along the c-axis might provide dichroism hence might support the picture of 3D chirality in cuprates. We find that Coulomb energy gain occurs along the c-axis within a multilayer chiral charge ordered state, which is proportional to the measured bilayer condensation energy and to $T_c$ at optimal doping. Within our approach the superconducting (SC) pair is composed of the hole content of the coherence area and the self-repulsion of the condensate is compensated by the gain in the inter-layer Coulomb energy below $T_c$. The SC condensate and the fluctuating charge order can also be described by a dynamical inter-layer electrostatic complementarity.Comment: 10 pages, 3 figure

Pair condensation and inter-layer coupling in cuprates: pairing on a superlattice

We analyze the superconducting state and the c-axis charge dynamics of cuprates using a charged ordered bilayer superlattice model in which pairing is supported by inter-layer Coulomb energy gain. The superlattice nature of high temperature superconductivity is experimentally suggested by the smallness of the coherent length \xi = 10 to 30 A which is comparable with a width of a 4X4 to 8X8 square supercell lattice layer. The temperature induced 2D-3D quantum phase transition of the hole-content is also studied. Pair condensation leads to the sharp decrease of the normal state c-axis anisotropy of the hole-content and reduces inter-layer dielectric screening. The decrease of the c-axis dielectric screening can be the primary source of the condensation energy. The 2D pair condensate can be characterized by a charge ordered state with a "checkboard" pattern seen by scanning tunneling microscopy.Comment: 11 pages, 4 figure

Intermixing in Cu/Co: molecular dynamics simulations and Auger electron spectroscopy depth profiling

The ion-bombardment induced evolution of intermixing is studied by molecular dynamics simulations and by Auger electron spectroscopy depth profiling analysis (AESD) in Cu/Co multilayer. It has been shown that from AESD we can derive the low-energy mixing rate and which can be compared with the simulated values obtained by molecular dynamics (MD) simulations. The overall agreement is reasonably good hence MD can hopefuly be used to estimate the rate of intermixing in various interface systems.Comment: 3 pages, 2 figure

Simulated ion-sputtering and Auger electron spectroscopy depth profiling study of intermixing in Cu/Co

The ion-bombardment induced evolution of intermixing is studied by molecular dynamics simulations and by Auger electron spectroscopy depth profiling analysis (AESD) in Cu/Co multilayer. It has been shown that from AESD we can derive the low-energy mixing rate and which can be compared with the simulated values obtained by molecular dynamics (MD) simulations.The overall agreement is reasonably good hence MD can hopefully be used to estimate the rate of intermixing in various interface systems.Comment: 3 pages, 3 figure

Fingerprint of super-interdiffusion: anomalous intermixing in Pt/Ti

The ion-sputtering induced transient enhanced intermixing has been studied by molecular dynamics (MD) simulations in Pt/Ti and its anomalous nature has been explained as a superdiffusive transient enhanced interdiffusion. We find ballistic mixing and a robust mass effect in Pt/Ti. The sum of the square of atomic displacements ($$) asymptotically grows nonlinearily and scales as $N^2$ and $\sim t^2$, where $N$ and $t$ are the ion-number fluence and the time of ion-sputtering, respectively. This anomalous behavior explains the high diffusity tail in the concentration profile obtained by Auger electron spectroscopy depth profiling (AES-DP) analysis in Pt/Ti/Si substrate (Pt/Ti) multilayer. In Ta/Ti/Pt/Si multilayer we find a linear time scaling of $\propto t$ at the Ti/Pt interface indicating the suppression of superdiffusive features. We propose a qualitative explanation based on the accelerative effect of nonlinear forces provbided by the anharmonic host lattice.Comment: 8 pages, 4 figures, RevTex forma