5,645 research outputs found
Quantisation of 2D-gravity with Weyl and area-preserving diffeomorphism invariances
The constraint structure of 2D-gravity with the Weyl and area-preserving
diffeomorphism invariances is analysed in the ADM formulation. It is found that
when the area-preserving diffeomorphism constraints are kept, the usual
conformal gauge does not exist, whereas there is the possibility to choose the
so-called ``quasi-light-cone'' gauge, in which besides the area-preserving
diffeomorphism invariance, the reduced Lagrangian also possesses the SL(2,R)
residual symmetry. The string-like approach is applied to quantise this model,
but a fictitious non-zero central charge in the Virasoro algebra appears. When
a set of gauge-independent SL(2,R) current-like fields is introduced instead of
the string-like variables, a consistent quantum theory is obtained.Comment: 14 pages, Latex fil
A Weak Gravity Conjecture for Scalar Field Theories
We show that the recently proposed weak gravity conjecture\cite{AMNV0601} can
be extended to a class of scalar field theories. Taking gravity into account,
we find an upper bound on the gravity interaction strength, expressed in terms
of scalar coupling parameters. This conjecture is supported by some
two-dimensional models and noncommutative field theories.Comment: version published in JHE
Incommensurate phonon anomaly and the nature of charge density waves in cuprates
While charge density wave (CDW) instabilities are ubiquitous to
superconducting cuprates, the different ordering wavevectors in various cuprate
families have hampered a unified description of the CDW formation mechanism.
Here we investigate the temperature dependence of the low energy phonons in the
canonical CDW ordered cuprate LaBaCuO. We discover
that the phonon softening wavevector associated with CDW correlations becomes
temperature dependent in the high-temperature precursor phase and changes from
a wavevector of 0.238 reciprocal space units (r.l.u.) below the ordering
transition temperature up to 0.3~r.l.u. at 300~K. This high-temperature
behavior shows that "214"-type cuprates can host CDW correlations at a similar
wavevector to previously reported CDW correlations in non-"214"-type cuprates
such as YBaCuO. This indicates that cuprate CDWs may
arise from the same underlying instability despite their apparently different
low temperature ordering wavevectors.Comment: Accepted in Phys. Rev. X; 9 pages; 5 figures; 3 pages of
supplementary materia
Some Low Dimensional Evidence for the Weak Gravity Conjecture
We discuss a few examples in 2+1 dimensions and 1+1 dimensions supporting a
recent conjecture concerning the relation between the Planck scale and the
coupling strength of a non-gravitional interaction, unlike those examples in
3+1 dimensions, we do not have to resort to exotic physics such as small black
holes. However, the result concerning these low dimensional examples is a
direct consequence of the 3+1 dimensional conjecture.Comment: 7 pages, harvma
The Measure for the Multiverse and the Probability for Inflation
We investigate the measure problem in the framework of inflationary
cosmology. The measure of the history space is constructed and applied to
inflation models. Using this measure, it is shown that the probability for the
generalized single field slow roll inflation to last for e-folds is
suppressed by a factor , and the probability for the generalized
-field slow roll inflation is suppressed by a much larger factor
. Some non-inflationary models such as the cyclic model do not
suffer from this difficulty.Comment: 16 page
On geometric problems related to Brown-York and Liu-Yau quasilocal mass
We discuss some geometric problems related to the definitions of quasilocal
mass proposed by Brown-York \cite{BYmass1} \cite{BYmass2} and Liu-Yau
\cite{LY1} \cite{LY2}. Our discussion consists of three parts. In the first
part, we propose a new variational problem on compact manifolds with boundary,
which is motivated by the study of Brown-York mass. We prove that critical
points of this variation problem are exactly static metrics. In the second
part, we derive a derivative formula for the Brown-York mass of a smooth family
of closed 2 dimensional surfaces evolving in an ambient three dimensional
manifold. As an interesting by-product, we are able to write the ADM mass
\cite{ADM61} of an asymptotically flat 3-manifold as the sum of the Brown-York
mass of a coordinate sphere and an integral of the scalar curvature plus
a geometrically constructed function in the asymptotic region outside
. In the third part, we prove that for any closed, spacelike, 2-surface
in the Minkowski space for which the Liu-Yau mass is
defined, if bounds a compact spacelike hypersurface in ,
then the Liu-Yau mass of is strictly positive unless lies on
a hyperplane. We also show that the examples given by \'{O} Murchadha, Szabados
and Tod \cite{MST} are special cases of this result.Comment: 28 page
Quantum gates implementations in the separated ion-traps by fast laser pulses
An approach is proposed to implement the universal quantum gates between the
ions confined individually in the separated traps. Instead of the typical
adiabatic operations, performed for manipulating the ion-ion coupling, here the
switchable couplings between ions are implemented non-adiabatically by using
the fast laser pulses. Consequently, the desirable quantum gates between the
ions could be implemented by using only a series of laser pulses. The proposal
may be conveniently generalized to the quantum computation with the scalable
ion-traps.Comment: 10 pages, 3figure
- …