1,410 research outputs found
Energy transmutation in nonequilibrium quantum systems
We investigate the particle and heat transport in quantum junctions with the
geometry of star graphs. The system is in a nonequilibrium steady state,
characterized by the different temperatures and chemical potentials of the heat
reservoirs connected to the edges of the graph. We explore the
Landauer-Buettiker state and its orbit under parity and time reversal
transformations. Both particle number and total energy are conserved in these
states. However the heat and chemical potential energy are in general not
separately conserved, which gives origin to a basic process of energy
transmutation among them. We study both directions of this process in detail,
introducing appropriate efficiency coefficients. For scale invariant
interactions in the junction our results are exact and explicit. They cover the
whole parameter space and take into account all nonlinear effects. The energy
transmutation depends on the particle statistics.Comment: Version to appear in Journal of Physics A: Mathematical and Genera
Non-linear quantum noise effects in scale invariant junctions
We study non-equilibrium steady state transport in scale invariant quantum
junctions with focus on the particle and heat fluctuations captured by the
two-point current correlation functions. We show that the non-linear behavior
of the particle current affects both the particle and heat noise. The existence
of domains of enhancement and reduction of the noise power with respect to the
linear regime are observed. The impact of the statistics is explored. We
demonstrate that in the scale invariant case the bosonic particle noise exceeds
the fermionic one in the common domain of heat bath parameters. Multi-lead
configurations are also investigated and the effect of probe terminals on the
noise is discussed.Comment: LaTex, 1+20 pages, 10 figure
Constraints on Single-Field Inflation
Many alternatives to canonical slow-roll inflation have been proposed over
the years, one of the main motivations being to have a model, capable of
generating observable values of non-Gaussianity. In this work, we (re-)explore
the physical implications of a great majority of such models within a single,
effective field theory framework (including novel models with large
non-Gaussianity discussed for the first time below.) The constraints we
apply---both theoretical and experimental---are found to be rather robust,
determined to a great extent by just three parameters: the coefficients of the
quadratic EFT operators and , and the
slow-roll parameter . This allows to significantly limit the
majority of single-field alternatives to canonical slow-roll inflation. While
the existing data still leaves some room for most of the considered models, the
situation would change dramatically if the current upper limit on the
tensor-to-scalar ratio decreased down to . Apart from inflationary
models driven by plateau-like potentials, the single-field model that would
have a chance of surviving this bound is the recently proposed slow-roll
inflation with weakly-broken galileon symmetry. In contrast to
\textit{canonical} slow-roll inflation, the latter model can support even if driven by a convex potential, as well as generate observable
values for the amplitude of non-Gaussianity.Comment: 19+10 pages, 6 figure
Thermoelectric efficiency of critical quantum junctions
We derive the efficiency at maximal power of a scale-invariant (critical)
quantum junction in exact form. Both Fermi and Bose statistics are considered.
We show that time-reversal invariance is spontaneously broken. For fermions we
implement a new mechanism for efficiency enhancement above the Curzon-Ahlborn
bound, based on a shift of the particle energy in each heat reservoir,
proportional to its temperature. In this setting fermionic junctions can even
reach at maximal power the Carnot efficiency. The bosonic junctions at maximal
power turn out to be less efficient then the fermionic ones.Comment: 4.5 pages, 4 figures, 2 references and some clarifying remarks adde
Weakly Broken Galileon Symmetry
Effective theories of a scalar invariant under the internal
\textit{galileon symmetry} have been extensively
studied due to their special theoretical and phenomenological properties. In
this paper, we introduce the notion of \textit{weakly broken galileon
invariance}, which characterizes the unique class of couplings of such theories
to gravity that maximally retain their defining symmetry. The curved-space
remnant of the galileon's quantum properties allows to construct (quasi) de
Sitter backgrounds largely insensitive to loop corrections. We exploit this
fact to build novel cosmological models with interesting phenomenology,
relevant for both inflation and late-time acceleration of the universe.Comment: 26+8 pages, 2 figures, 2 table
Soft Theorems For Shift-Symmetric Cosmologies
We derive soft theorems for single-clock cosmologies that enjoy a shift
symmetry. These so-called consistency conditions arise from a combination of a
large diffeomorphism and the internal shift-symmetry and fix the squeezed limit
of all correlators with a soft scalar mode. As an application, we show that our
results reproduce the squeezed bispectrum for Ultra-slow-roll inflation, a
particular shift-symmetric, non-attractor model which is known to violate
Maldacena's consistency relation. Similar results have been previously obtained
by Mooij and Palma using background-wave methods. Our results shed new light on
the infrared structure of single-clock cosmological spacetimes.Comment: 4 pages, v2: citation added, v3: citations added and edited in
accordance with published versio
Large Non-Gaussianity in Slow-Roll Inflation
Canonical models of single-field, slow-roll inflation do not lead to
appreciable non-Gaussianity, unless derivative interactions of the inflaton
become uncontrollably large. We propose a novel slow-roll scenario where scalar
perturbations propagate at a subluminal speed, leading to sizeable equilateral
non-Gaussianity, , largely insensitive
to the ultraviolet physics. The model is based on a low-energy effective theory
characterized by weakly broken invariance under internal galileon
transformations, , which protects the properties of
perturbations from large quantum corrections. This provides the unique
alternative to models such as DBI inflation in generating strongly
subluminal/non-Gaussian scalar perturbations.Comment: 5 page
Stability of Geodesically Complete Cosmologies
We study the stability of spatially flat FRW solutions which are geodesically
complete, i.e. for which one can follow null (graviton) geodesics both in the
past and in the future without ever encountering singularities. This is the
case of NEC-violating cosmologies such as smooth bounces or solutions which
approach Minkowski in the past. We study the EFT of linear perturbations around
a solution of this kind, including the possibility of multiple fields and
fluids. One generally faces a gradient instability which can be avoided only if
the operator is present and its coefficient changes sign
along the evolution. This operator (typical of beyond-Horndeski theories) does
not lead to extra degrees of freedom, but cannot arise starting from any theory
with second-order equations of motion. The change of sign of this operator
prevents to set it to zero with a generalised disformal transformation.Comment: 18 pages, 2 figures. v2: minor changes; references added; version
published in JCA
Fermions at finite density in the path integral approach
We study relativistic fermionic systems in spacetime dimensions at
finite chemical potential and zero temperature, from a path-integral point of
view. We show how to properly account for the term that projects
on the finite density ground state, and compute the path integral analytically
for free fermions in homogeneous external backgrounds, using complex analysis
techniques. As an application, we show that the symmetry is always
linearly realized for free fermions at finite charge density, differently from
scalars. We study various aspects of finite density QED in a homogeneous
magnetic background. We compute the free energy density, non-perturbatively in
the electromagnetic coupling and the external magnetic field, obtaining the
finite density generalization of classic results of Euler--Heisenberg and
Schwinger. We also obtain analytically the magnetic susceptibility of a
relativistic Fermi gas at finite density, reproducing the de Haas--van Alphen
effect. Finally, we consider a (generalized) Gross--Neveu model for
interacting fermions at finite density. We compute its non-perturbative
effective potential in the large- limit, and discuss the fate of the vector and axial symmetries.Comment: 34 pages, 6 figures. v2: one reference added, matches published
versio
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