452 research outputs found
Enhanced electrocaloric efficiency via energy recovery.
Materials that show large and reversible electrically driven thermal changes near phase transitions have been proposed for cooling applications, but energy efficiency has barely been explored. Here we reveal that most of the work done to drive representative electrocaloric cycles does not pump heat and may therefore be recovered. Initially, we recover 75-80% of the work done each time BaTiO3-based multilayer capacitors drive electrocaloric effects in each other via an inductor (diodes prevent electrical resonance while heat flows after each charge transfer). For a prototype refrigerator with 24 such capacitors, recovering 65% of the work done to drive electrocaloric effects increases the coefficient of performance by a factor of 2.9. The coefficient of performance is subsequently increased by reducing the pumped heat and recovering more work. Our strategy mitigates the advantage held by magnetocaloric prototypes that exploit automatic energy recovery, and should be mandatory in future electrocaloric cooling devices
Molecular dynamics simulations of lead clusters
Molecular dynamics simulations of nanometer-sized lead clusters have been
performed using the Lim, Ong and Ercolessi glue potential (Surf. Sci. {\bf
269/270}, 1109 (1992)). The binding energies of clusters forming crystalline
(fcc), decahedron and icosahedron structures are compared, showing that fcc
cuboctahedra are the most energetically favoured of these polyhedral model
structures. However, simulations of the freezing of liquid droplets produced a
characteristic form of ``shaved'' icosahedron, in which atoms are absent at the
edges and apexes of the polyhedron. This arrangement is energetically favoured
for 600-4000 atom clusters. Larger clusters favour crystalline structures.
Indeed, simulated freezing of a 6525-atom liquid droplet produced an imperfect
fcc Wulff particle, containing a number of parallel stacking faults. The
effects of temperature on the preferred structure of crystalline clusters below
the melting point have been considered. The implications of these results for
the interpretation of experimental data is discussed.Comment: 11 pages, 18 figues, new section added and one figure added, other
minor changes for publicatio
Dark-Photon Search using Data from CRESST-II Phase 2
Identifying the nature and origin of dark matter is one of the major
challenges for modern astro and particle physics. Direct dark-matter searches
aim at an observation of dark-matter particles interacting within detectors.
The focus of several such searches is on interactions with nuclei as provided
e.g. by Weakly Interacting Massive Particles. However, there is a variety of
dark-matter candidates favoring interactions with electrons rather than with
nuclei. One example are dark photons, i.e., long-lived vector particles with a
kinetic mixing to standard-model photons. In this work we present constraints
on this kinetic mixing based on data from CRESST-II Phase 2 corresponding to an
exposure before cuts of 52\,kg-days. These constraints improve the existing
ones for dark-photon masses between 0.3 and 0.7\,keV/c.Comment: submitted EPJ
First results from the CRESST-III low-mass dark matter program
The CRESST experiment is a direct dark matter search which aims to measure
interactions of potential dark matter particles in an earth-bound detector.
With the current stage, CRESST-III, we focus on a low energy threshold for
increased sensitivity towards light dark matter particles. In this manuscript
we describe the analysis of one detector operated in the first run of
CRESST-III (05/2016-02/2018) achieving a nuclear recoil threshold of 30.1eV.
This result was obtained with a 23.6g CaWO crystal operated as a cryogenic
scintillating calorimeter in the CRESST setup at the Laboratori Nazionali del
Gran Sasso (LNGS). Both the primary phonon/heat signal and the simultaneously
emitted scintillation light, which is absorbed in a separate
silicon-on-sapphire light absorber, are measured with highly sensitive
transition edge sensors operated at ~15mK. The unique combination of these
sensors with the light element oxygen present in our target yields sensitivity
to dark matter particle masses as low as 160MeV/c.Comment: 9 pages, 9 figure
Results on light dark matter particles with a low-threshold CRESST-II detector
The CRESST-II experiment uses cryogenic detectors to search for nuclear
recoil events induced by the elastic scattering of dark matter particles in
CaWO crystals. Given the low energy threshold of our detectors in
combination with light target nuclei, low mass dark matter particles can be
probed with high sensitivity. In this letter we present the results from data
of a single detector module corresponding to 52 kg live days. A blind analysis
is carried out. With an energy threshold for nuclear recoils of 307 eV we
substantially enhance the sensitivity for light dark matter. Thereby, we extend
the reach of direct dark matter experiments to the sub-region and demonstrate
that the energy threshold is the key parameter in the search for low mass dark
matter particles.Comment: 8 pages, 8 figure
Results on MeV-scale dark matter from a gram-scale cryogenic calorimeter operated above ground
Models for light dark matter particles with masses below 1 GeV/c are a
natural and well-motivated alternative to so-far unobserved weakly interacting
massive particles. Gram-scale cryogenic calorimeters provide the required
detector performance to detect these particles and extend the direct dark
matter search program of CRESST. A prototype 0.5 g sapphire detector developed
for the -cleus experiment has achieved an energy threshold of
eV, which is one order of magnitude lower than previous
results and independent of the type of particle interaction. The result
presented here is obtained in a setup above ground without significant
shielding against ambient and cosmogenic radiation. Although operated in a
high-background environment, the detector probes a new range of light-mass dark
matter particles previously not accessible by direct searches. We report the
first limit on the spin-independent dark matter particle-nucleon cross section
for masses between 140 MeV/c and 500 MeV/c.Comment: 6 pages, 6 figures, v3: ancillary files added, v4: high energy
spectrum (0.6-12keV) added to ancillary file
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