1,027 research outputs found

### Dislocations in stacking and commensurate-incommensurate phase transition in bilayer graphene and hexagonal boron nitride

Dislocations corresponding to a change of stacking in two-dimensional
hexagonal bilayers, graphene and boron nitride, and associated with boundaries
between commensurate domains are investigated using the two-chain
Frenkel-Kontorova model on top of ab initio calculations. Structural
transformations of bilayers in which the bottom layer is stretched and the
upper one is left to relax freely are considered for gradually increased
elongation of the bottom layer. Formation energies of dislocations, dislocation
width and orientation of the boundary between commensurate domains are analyzed
depending on the magnitude and direction of elongation. The second-order phase
transition from the commensurate phase to the incommensurate one with multiple
dislocations is predicted to take place at some critical elongation. The order
parameter for this transition corresponds to the density of dislocations, which
grows continuously upon increasing the elongation of the bottom layer above the
critical value. In graphene and metastable boron nitride with the layers
aligned in the same direction, where elementary dislocations are partial, this
transition, however, is preceded by formation of the first dislocation at the
elongation smaller than the critical one. The phase diagrams including this
intermediate state are plotted in coordinates of the magnitude and direction of
elongation of the bottom layer.Comment: 15 pages, 9 figure

### Comparison of performance of van der Waals-corrected exchange-correlation functionals for interlayer interaction in graphene and hexagonal boron nitride

Exchange-correlation functionals with corrections for van der Waals
interactions (PBE-D2, PBE-D3, PBE-D3(BJ), PBE-TS, optPBE-vdW and vdW-DF2) are
tested for graphene and hexagonal boron nitride, both in the form of bulk and
bilayer. The characteristics of the potential energy surface, such as the
barrier to relative sliding of the layers and magnitude of corrugation, and
physically measurable properties associated with relative in-plane and
out-of-plane motion of the layers including the shear modulus and modulus for
axial compression, shear mode frequency and frequency of out-of-plane
vibrations are considered. The PBE-D3(BJ) functional gives the best results for
the stackings of hexagonal boron nitride and graphite that are known to be
ground-state from the experimental studies. However, it fails to describe the
order of metastable states of boron nitride in energy. The PBE-D3 and vdW-DF2
functionals, which reproduce this order correctly, are identified as the
optimal choice for general studies. The vdW-DF2 functional is preferred for
evaluation of the modulus for axial compression and frequency of out-of-plane
vibrations, while the PBE-D3 functional is somewhat more accurate in
calculations of the shear modulus and shear mode frequency. The best
description of the latter properties, however, is achieved also using the
vdW-DF2 functional combined with consideration of the experimental interlayer
distance. In the specific case of graphene, the PBE-D2 functional works very
well and can be further improved by adjustment of the parameters.Comment: 22 pages, 4 figue

### Ambient betatron motion and its excitation by ghost lines in Tevatron

Transverse betatron motion of the Tevatron proton beam is measured and
analyzed. It is shown that the motion is coherent and excited by external
sources of unknown origins. Observations of the time varying ghost lines in the
betatron spectra are reported.Comment: 9 p

### Structure and energetics of carbon, hexagonal boron nitride and carbon/hexagonal boron nitride single-layer and bilayer nanoscrolls

Single-layer and bilayer carbon and hexagonal boron nitride nanoscrolls as
well as nanoscrolls made of bilayer graphene/hexagonal boron nitride
heterostructure are considered. Structures of stable states of the
corresponding nanoscrolls prepared by rolling single-layer and bilayer
rectangular nanoribbons are obtained based on the analytical model and
numerical calculations. The lengths of nanoribbons for which stable and
energetically favorable nanoscrolls are possible are determined. Barriers to
rolling of single-layer and bilayer nanoribbons into nanoscrolls and barriers
to nanoscroll unrolling are calculated. Based on the calculated barriers
nanoscroll lifetimes in the stable state are estimated. Elastic constants for
bending of graphene and hexagonal boron nitride layers used in the model are
found by density functional theory calculations.Comment: 9 pages, 6 figure

### Electrostatic Patch Effect in Cylindrical Geometry. III. Torques

We continue to study the effect of uneven voltage distribution on two close
cylindrical conductors with parallel axes started in our papers [1] and [2],
now to find the electrostatic torques. We calculate the electrostatic potential
and energy to lowest order in the gap to cylinder radius ratio for an arbitrary
relative rotation of the cylinders about their symmetry axis. By energy
conservation, the axial torque, independent of the uniform voltage difference,
is found as a derivative of the energy in the rotation angle. We also derive
both the axial and slanting torques by the surface integration method: the
torque vector is the integral over the cylinder surface of the cross product of
the electrostatic force on a surface element and its position vector. The
slanting torque consists of two parts: one coming from the interaction between
the patch and the uniform voltages, and the other due to the patch interaction.
General properties of the torques are described. A convenient model of a
localized patch suggested in [2] is used to calculate the torques explicitly in
terms of elementary functions. Based on this, we analyze in detail patch
interaction for one pair of patches, namely, the torque dependence on the patch
parameters (width and strength) and their mutual positions. The effect of the
axial torque is then studied for the experimental conditions of the STEP
mission.Comment: 28 pages, 6 Figures. Submitted to Classical Quantum Gravit

### Electrostatic Patch Effect in Cylindrical Geometry. I. Potential and Energy between Slightly Non-Coaxial Cylinders

We study the effect of any uneven voltage distribution on two close
cylindrical conductors with parallel axes that are slightly shifted in the
radial and by any length in the axial direction. The investigation is
especially motivated by certain precision measurements, such as the Satellite
Test of the Equivalence Principle (STEP). By energy conservation, the force can
be found as the energy gradient in the vector of the shift, which requires
determining potential distribution and energy in the gap. The boundary value
problem for the potential is solved, and energy is thus found to the second
order in the small transverse shift, and to lowest order in the gap to cylinder
radius ratio. The energy consists of three parts: the usual capacitor part due
to the uniform potential difference, the one coming from the interaction
between the voltage patches and the uniform voltage difference, and the energy
of patch interaction, entirely independent of the uniform voltage. Patch effect
forces and torques in the cylindrical configuration are derived and analyzed in
the next two parts of this work.Comment: 26 pages, 1 Figure. Submitted to Classical and Quantum Gravit

### Universal Torsion-Induced Interaction from Large Extra Dimensions

We consider the Kaluza-Klein (KK) scenario in which only gravity exists in
the bulk. Without the assumption of symmetric connection, the presence of brane
fermions induces torsion. The result is a universal axial contact interaction
that dominates those induced by KK gravitons. This enhancement arises from a
large spin density on the brane. Using a global fit to Z-pole observables, we
find the 3 sigma bound on the scale of quantum gravity to be 28 TeV for n=2. If
Dirac or light sterile neutrinos are present, the data from SN1987A increase
the bound to \sqrt{n}M_S >= 210 TeV.Comment: 9 pages REVTeX, 1 postscript figure, uses axodraw.st

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