1,248 research outputs found
Study of symmetry in F(R) theory of gravity
An action in which the Ricci scalar is nonminimally coupled with a scalar
field and contains higher order curvature invariant terms carries a conserved
current under certain conditions that decouples geometric part from the scalar
field. The conserved current relates the pair of arbitrary coupling parameters
and with the gravitational field variable, where
is the Brans-Dicke coupling parameter. The existence of such
conserved current may be helpful to sketch the cosmological evolution from its
early age till date in a single frame.Comment: 6 page
Complex edge effects in zigzag graphene nanoribbons due to hydrogen loading
We have performed density functional calculations as well as employed a
tight-binding theory, to study the effect of passivation of zigzag graphene
nanoribbons (ZGNR) by Hydrogen. We show that each edge C atom bonded with 2 H
atoms open up a gap and destroys magnetism for small widths of the nanoribbon.
However, a re-entrant magnetism accompanied by a metallic electronic structure
is observed from 8 rows and thicker nanoribbons. The electronic structure and
magnetic state are quite complex for this type of termination, with sp
bonded edge atoms being non-magnetic, whereas the nearest neighboring atoms are
metallic and magnetic. We have also evaluated the phase stability of several
thicknesses of ZGNR, and demonstrate that sp bonded edge atoms, with 2 H
atoms at the edge, should be stable at temperatures and pressures which are
reachable in a laboratory environment.Comment: 11 figure
First-principles calculations of exchange interactions, spin waves, and temperature dependence of magnetization in inverse-Heusler-based spin gapless semiconductors
Employing first principles electronic structure calculations in conjunction
with the frozen-magnon method we calculate exchange interactions, spin-wave
dispersion, and spin-wave stiffness constants in inverse-Heusler-based spin
gapless semiconductor (SGS) compounds MnCoAl, TiMnAl, CrZnSi,
TiCoSi and TiVAs. We find that their magnetic behavior is similar to
the half-metallic ferromagnetic full-Heusler alloys, i.e., the intersublattice
exchange interactions play an essential role in the formation of the magnetic
ground state and in determining the Curie temperature, . All
compounds, except TiCoSi possess a ferrimagnetic ground state. Due to the
finite energy gap in one spin channel, the exchange interactions decay sharply
with the distance, and hence magnetism of these SGSs can be described
considering only nearest and next-nearest neighbor exchange interactions. The
calculated spin-wave dispersion curves are typical for ferrimagnets and
ferromagnets. The spin-wave stiffness constants turn out to be larger than
those of the elementary 3-ferromagnets. Calculated exchange parameters are
used as input to determine the temperature dependence of the magnetization and
of the SGSs. We find that the of all compounds is
much above the room temperature. The calculated magnetization curve for
MnCoAl as well as the Curie temperature are in very good agreement with
available experimental data. The present study is expected to pave the way for
a deeper understanding of the magnetic properties of the inverse-Heusler-based
SGSs and enhance the interest in these materials for application in spintronic
and magnetoelectronic devices.Comment: Accepted for publ;ication in Physical Review
Vacancy-induced low-energy states in undoped graphene
We demonstrate that a nonzero concentration nv of static, randomly placed vacancies in graphene leads to a density w of zero-energy quasiparticle states at the band center ε=0 within a tight-binding description with nearest-neighbor hopping t on the honeycomb lattice. We show that wremains generically nonzero in the compensated case (exactly equal number of vacancies on the two sublattices) even in the presence of hopping disorder and depends sensitively on nv and correlations between vacancy positions. For low, but not-too-low, |ε|/t in this compensated case, we show that the density of states ρ(ε) exhibits a strong divergence of the form ρ_(Dyson)(ε)∼|ε|^(-1)/[log(t/|ε|)]^((y+1)), which crosses over to the universal low-energy asymptotic form (modified Gade-Wegner scaling) expected on symmetry grounds ρ_(GW)(ε)∼|ε|^(-1)e^(-b[log(t/|ε|)]2/3) below a crossover scale ε_c≪t. ε_c is found to decrease rapidly with decreasing nv, while y decreases much more slowly
Disorder induced metallicity in amorphous graphene
We predict a transition to metallicity when a sufficient amount of disorder
is induced in graphene. Calculations were performed by means of a first
principles stochastic quench method. The resulting amorphous graphene can be
seen as nanopatches of graphene that are connected by a network of disordered
small and large carbon rings. The buckling is minimal and we believe that it is
a result of averaging of counteracting random in-plane stress forces. The
linear response conductance is obtained by a model theory as function of
lattice distortions. Such metallic behaviour is a much desired property for
functionalisation of graphene to realize a transparent conductor, e.g. suitable
for touch-screen devices
Construction and Analysis of Projected Deformed Products
We introduce a deformed product construction for simple polytopes in terms of
lower-triangular block matrix representations. We further show how Gale duality
can be employed for the construction and for the analysis of deformed products
such that specified faces (e.g. all the k-faces) are ``strictly preserved''
under projection. Thus, starting from an arbitrary neighborly simplicial
(d-2)-polytope Q on n-1 vertices we construct a deformed n-cube, whose
projection to the last dcoordinates yields a neighborly cubical d-polytope. As
an extension of thecubical case, we construct matrix representations of
deformed products of(even) polygons (DPPs), which have a projection to d-space
that retains the complete (\lfloor \tfrac{d}{2} \rfloor - 1)-skeleton. In both
cases the combinatorial structure of the images under projection is completely
determined by the neighborly polytope Q: Our analysis provides explicit
combinatorial descriptions. This yields a multitude of combinatorially
different neighborly cubical polytopes and DPPs. As a special case, we obtain
simplified descriptions of the neighborly cubical polytopes of Joswig & Ziegler
(2000) as well as of the ``projected deformed products of polygons'' that were
announced by Ziegler (2004), a family of 4-polytopes whose ``fatness'' gets
arbitrarily close to 9.Comment: 20 pages, 5 figure
Vacancy-induced spin texture in a one dimensional Heisenberg antiferromagnet
We study the effect of a missing spin in a one dimensional
antiferromagnet with nearest neighbour Heisenberg exchange and six-spin
coupling using Quantum Monte-Carlo (QMC) and bosonization techniques.
For , the system is in a quasi-long range ordered
power-law antiferromagnetic phase, which gives way to a valence-bond solid
state that spontaneously breaks lattice translation symmetry for . We
study the ground state spin texture in the the ground state
of the system with a missing spin, focusing on the alternating
part . We find that our QMC results for at take on the
scaling form expected from bosonization considerations, but violate scaling for
. Within the bosonization approach, such violations of scaling arise
from the presence of a marginally irrelevant sine-Gordon interaction, whose
effects we calculate using renormalization group (RG) improved perturbation
theory. Our field-theoretical predictions are found to agree well with the QMC
data for .Comment: 9 pages, two-column PRB forma
Comprehending a Molecular Conundrum: Functional Studies of Ribosomal Protein Mutants from Salmonella enterica Serovar Typhimurium
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