1,084 research outputs found
Real-time effective-action approach to the Anderson quantum dot
The non-equilibrium time evolution of an Anderson quantum dot is
investigated. The quantum dot is coupled between two leads forming a
chemical-potential gradient. We use Kadanoff-Baym dynamic equations within a
non-perturbative resummation of the s-channel bubble chains. The effect of the
resummation leads to the introduction of a frequency-dependent 4-point vertex.
The tunneling to the leads is taken into account exactly. The method allows the
determination of the transient as well as stationary transport through the
quantum dot, and results are compared with different schemes discussed in the
literature (fRG, ISPI, tDMRG and QMC).Comment: 12 pages, 13 figure
Cosmological Symmetry Breaking, Pseudo-scale invariance, Dark Energy and the Standard Model
The energy density of the universe today may be dominated by the vacuum
energy of a slowly rolling scalar field. Making a quantum expansion around such
a time dependent solution is found to break fundamental symmetries of quantum
field theory. We call this mechanism cosmological symmetry breaking and argue
that it is different from the standard phenomenon of spontaneous symmetry
breaking. We illustrate this with a toy scalar field theory, whose action
displays a U(1) symmetry. We identify a symmetry, called pseudo-scale
invariance, which sets the cosmological constant exactly equal to zero, both in
classical and quantum theory. This symmetry is also broken cosmologically and
leads to a nonzero vacuum or dark energy. The slow roll condition along with
the observed value of dark energy leads to a value of the background scalar
field of the order of Planck mass. We also consider a U(1) gauge symmetry
model. Cosmological symmetry breaking, in this case, leads to a non zero mass
for the vector field. We also show that a cosmologically broken pseudo-scale
invariance can generate a wide range of masses.Comment: 18 pages, no figure
Reparametrization-Invariant Path Integral in GR and "Big Bang" of Quantum Universe
The reparametrization-invariant generating functional for the unitary and
causal perturbation theory in general relativity in a finite space-time is
obtained. The region of validity of the Faddeev-Popov-DeWitt functional is
studied. It is shown that the invariant content of general relativity as a
constrained system can be covered by two "equivalent" unconstrained systems:
the "dynamic" (with "dynamic" evolution parameter as the metric scale factor)
and "geometric" (given by the Levi-Civita type canonical transformation to the
action-angle variables where the energy constraint converts into a new
momentum). "Big Bang", the Hubble evolution, and creation of matter fields by
the "geometric" vacuum are described by the inverted Levi-Civita (LC)
transformation of the geomeric system into the dynamic one. The particular case
of the LC transformations are the Bogoliubov ones of the particle variables
(diagonalizing the dynamic Hamiltonian) to the quasiparticles (diagonalizing
the equations of motion). The choice of initial conditions for the "Big Bang"
in the form of the Bogoliubov (squeezed) vacuum reproduces all stages of the
evolution of the Friedmann-Robertson-Walker Universe in their conformal
(Hoyle-Narlikar) versions.Comment: 21 pages, latex, 4 figures in postscrip
Renormalisation Flow and Universality for Ultracold Fermionic Atoms
A functional renormalisation group study for the BEC-BCS crossover for
ultracold gases of fermionic atoms is presented. We discuss the fixed point
which is at the origin of universality for broad Feshbach resonances. All
macroscopic quantities depend only on one relevant parameter, the concentration
a k_F, besides their dependence on the temperature in units of the Fermi
energy. In particular, we compute the universal ratio between molecular and
atomic scattering length in vacuum. We also present an estimate to which level
of accuracy universality holds for gases of Li and K atoms.Comment: 19 pages, 3 figures, to be published in PR
Flow Equations for the BCS-BEC Crossover
The functional renormalisation group is used for the BCS-BEC crossover in
gases of ultracold fermionic atoms. In a simple truncation, we see how
universality and an effective theory with composite bosonic di-atom states
emerge. We obtain a unified picture of the whole phase diagram. The flow
reflects different effective physics at different scales. In the BEC limit as
well as near the critical temperature, it describes an interacting bosonic
theory.Comment: 4 pages, 4 figure
Hydrogen transport in superionic system Rb3H(SeO4)2: a revised cooperative migration mechanism
We performed density functional studies of electronic properties and
mechanisms of hydrogen transport in Rb3H(SeO4)2 crystal which represents
technologically promising class M3H(XO4)2 of proton conductors (M=Rb,Cs, NH4;
X=S,Se). The electronic structure calculations show a decisive role of lattice
dynamics in the process of proton migration. In the obtained revised mechanism
of proton transport, the strong displacements of the vertex oxygens play a key
role in the establishing the continuous hydrogen transport and in the achieving
low activation energies of proton conduction which is in contrast to the
standard two-stage Grotthuss mechanism of proton transport. Consequently, any
realistic model description of proton transport should inevitably involve the
interactions with the sublattice of the XO4 groups.Comment: 11 pages, 11 figures, to appear in Physical Review
Standard Model with Cosmologically Broken Quantum Scale Invariance
We argue that scale invariance is not anomalous in quantum field theory,
provided it is broken cosmologically. We consider a locally scale invariant
extension of the Standard Model of particle physics and argue that it fits both
the particle and cosmological observations. The model is scale invariant both
classically and quantum mechanically. The scale invariance is broken
cosmologically producing all the dimensionful parameters. The cosmological
constant or dark energy is a prediction of the theory and can be calculated
systematically order by order in perturbation theory. It is expected to be
finite at all orders. The model does not suffer from the hierarchy problem due
to absence of scalar particles, including the Higgs, from the physical
spectrum.Comment: 13 pages, no figures significant revisions, no change in results or
conclusion
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