578 research outputs found
Closed-loop approach to thermodynamics
We present the closed loop approach to linear nonequilibrium thermodynamics
considering a generic heat engine dissipatively connected to two temperature
baths. The system is usually quite generally characterized by two parameters:
the output power and the conversion efficiency , to which we add a
third one, the working frequency . We establish that a detailed
understanding of the effects of the dissipative coupling on the energy
conversion process, necessitates the knowledge of only two quantities: the
system's feedback factor and its open-loop gain , the product of
which, , characterizes the interplay between the efficiency, the
output power and the operating rate of the system. By placing thermodynamics
analysis on a higher level of abstraction, the feedback loop approach provides
a versatile and economical, hence a very efficient, tool for the study of
\emph{any} conversion engine operation for which a feedback factor may be
defined
Time-resolved PhotoEmission Spectroscopy on a Metal/Ferroelectric Heterostructure
In thin film ferroelectric capacitor the chemical and electronic structure of
the electrode/FE interface can play a crucial role in determining the kinetics
of polarization switching. We investigate the electronic structure of a
Pt/BaTiO3/SrTiO3:Nb capacitor using time-resolved photoemission spectroscopy.
The chemical, electronic and depth sensitivity of core level photoemission is
used to probe the transient response of different parts of the upper
electrode/ferroelectric interface to voltage pulse induced polarization
reversal. The linear response of the electronic structure agrees quantitatively
with a simple RC circuit model. The non-linear response due to the polarization
switch is demonstrated by the time-resolved response of the characteristic core
levels of the electrode and the ferroelectric. Adjustment of the RC circuit
model allows a first estimation of the Pt/BTO interface capacitance. The
experiment shows the interface capacitance is at least 100 times higher than
the bulk capacitance of the BTO film, in qualitative agreement with theoretical
predictions from the literature.Comment: 7 pages, 10 figures. Submitted to Phys. Rev.
Interface Electronic Structure in a Metal/Ferroelectric Heterostructure under Applied Bias
The effective barrier height between an electrode and a ferroelectric (FE)
depends on both macroscopic electrical properties and microscopic chemical and
electronic structure. The behavior of a prototypical electrode/FE/electrode
structure, Pt/BaTiO3/Nb-doped SrTiO3, under in-situ bias voltage is
investigated using X-Ray Photoelectron Spectroscopy. The full band alignment is
measured and is supported by transport measurements. Barrier heights depend on
interface chemistry and on the FE polarization. A differential response of the
core levels to applied bias as a function of the polarization state is
observed, consistent with Callen charge variations near the interface.Comment: 9 pages, 8 figures. Submitted to Phys. Rev.
Tailoring strain in SrTiO3 compound by low energy He+ irradiation
The ability to generate a change of the lattice parameter in a near-surface
layer of a controllable thickness by ion implantation of strontium titanate is
reported here using low energy He+ ions. The induced strain follows a
distribution within a typical near-surface layer of 200 nm as obtained from
structural analysis. Due to clamping effect from the underlying layer, only
perpendicular expansion is observed. Maximum distortions up to 5-7% are
obtained with no evidence of amorphisation at fluences of 1E16 He+ ions/cm2 and
ion energies in the range 10-30 keV.Comment: 11 pages, 4 figures, Accepted for publication in Europhysics Letter
(http://iopscience.iop.org/0295-5075
Gaia eclipsing binary and multiple systems. Two-Gaussian models applied to OGLE-III eclipsing binary light curves in the Large Magellanic Cloud
The advent of large scale multi-epoch surveys raises the need for automated
light curve (LC) processing. This is particularly true for eclipsing binaries
(EBs), which form one of the most populated types of variable objects. The Gaia
mission, launched at the end of 2013, is expected to detect of the order of few
million EBs over a 5-year mission.
We present an automated procedure to characterize EBs based on the geometric
morphology of their LCs with two aims: first to study an ensemble of EBs on a
statistical ground without the need to model the binary system, and second to
enable the automated identification of EBs that display atypical LCs. We model
the folded LC geometry of EBs using up to two Gaussian functions for the
eclipses and a cosine function for any ellipsoidal-like variability that may be
present between the eclipses. The procedure is applied to the OGLE-III data set
of EBs in the Large Magellanic Cloud (LMC) as a proof of concept. The bayesian
information criterion is used to select the best model among models containing
various combinations of those components, as well as to estimate the
significance of the components.
Based on the two-Gaussian models, EBs with atypical LC geometries are
successfully identified in two diagrams, using the Abbe values of the original
and residual folded LCs, and the reduced . Cleaning the data set from
the atypical cases and further filtering out LCs that contain non-significant
eclipse candidates, the ensemble of EBs can be studied on a statistical ground
using the two-Gaussian model parameters. For illustration purposes, we present
the distribution of projected eccentricities as a function of orbital period
for the OGLE-III set of EBs in the LMC, as well as the distribution of their
primary versus secondary eclipse widths.Comment: 20 pages, 29 figures. Submitted to A&
A new method to identify subclasses among AGB stars using Gaia and 2MASS photometry
Aims: We explore the wealth of high quality photometric data provided by data
release 2 of the Gaia mission for long period variables (LPVs) in the Large
Magellanic Cloud. Our goal is to identify stars of various types and masses
along the Asymptotic Giant Branch.
Methods: For this endeavour, we developed a new multi-band approach combining
Wesenheit functions W_{RP,BP-RP} and W_{K_s,J-K_s} in the Gaia BP, RP and 2MASS
J, K_s spectral ranges, respectively, and use a new diagram
(W_{RP,BP-RP}-W_{K_s,J-K_s}) versus K_s to distinguish between different kinds
of stars in our sample of LPVs. We used stellar population synthesis models to
validate our approach.
Results:We demonstrate the ability of the new diagram to discriminate between
O-rich and C-rich objects, and to identify low-mass, intermediate-mass and
massive O-rich red giants, as well as extreme C-rich stars. Stellar evolution
and population synthesis models guide the interpretation of the results,
highlighting the diagnostic power of the new tool to discriminate between
stellar initial masses, chemical properties and evolutionary stages.Comment: accepted for publication in A&A Letters; 7 figures, 2 appendice
Field-effect control of superconductivity and Rashba spin-orbit coupling in top-gated LaAlO3/SrTiO3 devices
The recent development in the fabrication of artificial oxide
heterostructures opens new avenues in the field of quantum materials by
enabling the manipulation of the charge, spin and orbital degrees of freedom.
In this context, the discovery of two-dimensional electron gases (2-DEGs) at
LAlO3/SrTiO3 interfaces, which exhibit both superconductivity and strong Rashba
spin-orbit coupling (SOC), represents a major breakthrough. Here, we report on
the realisation of a field-effect LaAlO3/SrTiO3 device, whose physical
properties, including superconductivity and SOC, can be tuned over a wide range
by a top-gate voltage. We derive a phase diagram, which emphasises a
field-effect-induced superconductor-to-insulator quantum phase transition.
Magneto-transport measurements indicate that the Rashba coupling constant
increases linearly with electrostatic doping. Our results pave the way for the
realisation of mesoscopic devices, where these two properties can be
manipulated on a local scale by means of top-gates
Mesoscale magnetism at the grain boundaries in colossal magnetoresistive films
We report the discovery of mesoscale regions with distinctive magnetic
properties in epitaxial LaSrMnO films which exhibit
tunneling-like magnetoresistance across grain boundaries. By using
temperature-dependent magnetic force microscopy we observe that the mesoscale
regions are formed near the grain boundaries and have a different Curie
temperature (up to 20 K {\it higher}) than the grain interiors. Our images
provide direct evidence for previous speculations that the grain boundaries in
thin films are not magnetically and electronically sharp interfaces. The size
of the mesoscale regions varies with temperature and nature of the underlying
defect.Comment: 4 pages of text, 4 figure
TThermodynamics of the thermoelectric working fluid close to the superconducting phase transition
The bottleneck in state-of-the-art thermoelectric power generation and
cooling is the low performance of thermoelectric materials. While the adverse
effects of lattice phonons on performance can be mitigated, the main difficulty
remains to obtain a large thermoelectric power factor as the Seebeck
coefficient and the electrical conductivity cannot be increased independently.
Here, relating the thermoelastic properties of the electron gas that performs
the thermoelectric energy conversion, to its transport properties, we analyze
theoretically whether an electronic phase transition can enhance thermoelectric
conversion and at what cost. More precisely, we consider the
metal-to-superconductor phase transition in a model two-dimensional system, and
we seek to quantify the contribution of the 2D fluctuating Cooper pairs to the
power factor in the close vicinity of the critical temperature . In
addition, we provide experimental evidence of the rapid increase of the Seebeck
coefficient without decreasing the electrical conductivity near in
a 100-nm Ba(FeCo)As thin film with high structural quality
resulting in a power factor enhancement of approximately 300. This level of
performance cannot be achieved in a system with low structural quality as shown
experimentally with our sample degraded by ion bombardment as defects preclude
the strong enhancement of the Seebeck coefficient near the phase transition.
Finally, we theoretically discuss the ideal thermoelectric conversion
efficiency (i.e. disregarding adverse phonon effects) and show that driving the
electronic system to the vicinity of a phase transition may be an innovative
path towards a strong performance increase but at the cost of a narrow
temperature range of use of such materials.Comment: Submission to SciPos
Are short food supply chains more environmentally sustainable than long chains? a life cycle assessment (LCA) of the eco-efficiency of food chains in selected EU countries
Improving the eco-efficiency of food systems is one of the major global challenges faced by the modern world. Short food supply chains (SFSCs) are commonly regarded to be less harmful to the environment, among various reasons, due to their organizational distribution and thus the shortened physical distance between primary producers and final consumers. In this paper, we empirically test this hypothesis, by assessing and comparing the environmental impacts of short and long food supply chains. Based on the Life Cycle Assessment (LCA) approach, we calculate eco-efficiency indicators for nine types of food distribution chains. The analysis is performed on a sample of 428 short and long food supply chains from six European countries. Our results indicate that, on average, long food supply chains may generate less negative environmental impacts than short chains (in terms of fossil fuel energy consumption, pollution, and GHG emissions) per kg of a given product. The values of eco-efficiency indicators display a large variability across analyzed chains, and especially across different types of SFSCs. The analysis shows that the environmental impacts of the food distribution process are not only determined by the geographical distance between producer and consumer, but depend on numerous factors, including the supply chain infrastructure
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