Motivated by recent observation of magnetic field induced transition in LaCoO3 we study the effect of external field in systems close to instabilities towards spin-state ordering and exciton ...condensation. We show that, while in both cases the transition can be induced by an external field, temperature dependencies of the critical field have opposite slopes. Based on this result we argue that the experimental observations select the exciton condensation scenario. We show that such condensation is possible due to high mobility of the intermediate spin excitations. The estimated width of the corresponding dispersion is large enough to overrule the order of atomic multiplets and to make the intermediate spin excitation propagating with a specific wave vector the lowest excitation of the system.
For most of their existence, stars are fuelled by the fusion of hydrogen into helium. Fusion proceeds via two processes that are well understood theoretically: the proton-proton (pp) chain and the ...carbon-nitrogen-oxygen (CNO) cycle
. Neutrinos that are emitted along such fusion processes in the solar core are the only direct probe of the deep interior of the Sun. A complete spectroscopic study of neutrinos from the pp chain, which produces about 99 per cent of the solar energy, has been performed previously
; however, there has been no reported experimental evidence of the CNO cycle. Here we report the direct observation, with a high statistical significance, of neutrinos produced in the CNO cycle in the Sun. This experimental evidence was obtained using the highly radiopure, large-volume, liquid-scintillator detector of Borexino, an experiment located at the underground Laboratori Nazionali del Gran Sasso in Italy. The main experimental challenge was to identify the excess signal-only a few counts per day above the background per 100 tonnes of target-that is attributed to interactions of the CNO neutrinos. Advances in the thermal stabilization of the detector over the last five years enabled us to develop a method to constrain the rate of bismuth-210 contaminating the scintillator. In the CNO cycle, the fusion of hydrogen is catalysed by carbon, nitrogen and oxygen, and so its rate-as well as the flux of emitted CNO neutrinos-depends directly on the abundance of these elements in the solar core. This result therefore paves the way towards a direct measurement of the solar metallicity using CNO neutrinos. Our findings quantify the relative contribution of CNO fusion in the Sun to be of the order of 1 per cent; however, in massive stars, this is the dominant process of energy production. This work provides experimental evidence of the primary mechanism for the stellar conversion of hydrogen into helium in the Universe.
We use the LDA+U approach to search for possible ordered ground states of LaSrCoO4. We find a staggered arrangement of magnetic multipoles to be stable over a broad range of Co 3d interaction ...parameters. This ordered state can be described as a spin-density-wave-type condensate of dxy⊗dx2−y2 excitons carrying spin S = 1. Further, we construct an effective strong-coupling model, calculate the exciton dispersion and investigate closing of the exciton gap, which marks the exciton condensation instability. Comparing the layered LaSrCoO4 with its pseudo cubic analog LaCoO3, we find that for the same interaction parameters the excitonic gap is smaller (possibly vanishing) in the layered cobaltite.
Decrease in operating temperature of a silicon photomultiplier (SiPM) reduces its dark noise. Some experiments consider low temperatures to take advantage of this possibility. And one of them is the ...TAO detector. It requires operation at T≈−50°C. In this paper, the studies of operation of both Hamamatsu SiPMs and AdvanSID SiPMs in a temperature range of −100 °C to ＋23 °C are presented.
We present an ultrasonic study of relaxor-based solid solution (1−
x
) PbFe
2/3
W
1/3
O
3
–
x
PbTiO
3
(0.25 ≤
x
≤ 0.35) located within the morphotropic phase boundary (MPB). The longitudinal ...ultrasonic wave velocity and attenuation have been measured by the ultrasonic pulse-echo technique at a frequency of 10 MHz in a wide temperature range from 200 to 800 K. Attenuation peaks have been observed at characteristic temperatures, namely, at Burns temperature
T
B
and the additional distinctive temperature
T
*
. To explain the experimental results, we consider a modified polar nanoregions (PNRs) model taking into account fluctuation corrections of ultrasonic wave velocity and attenuation in the vicinity of both
T
B
and
T
*
. The obtained results confirm the dynamic nature of MPB effects which determine the physical properties of relaxor-based solid solutions.
Electrocaloric effect (ECE) as well as field induced pyroelectric and piezoelectric effects were investigated in relaxor ceramics 0.84 PbMg
1/3
Nb
2/3
O
3
–0.14 PbTiO
3
–0.02 SrTiO
3
. Dielectric and ...elastic properties of the solid solution were also studied. To improve the accuracy and reliability of the results obtained, the electrocaloric temperature change δ
T
was measured both by the well-known quasi-adiabatic technique with a contact temperature sensor and by the radiometric mid Infrared technique developed by the authors. The contribution of the secondary pyroelectric effect to the total pyroelectric and ECE was studied in detail. The effect of the induced piezoelectricity on the range of the temperature stability of pyroelectric and electrocaloric responses was shown. The resulting combination of material parameters at reasonable operating temperatures and electric fields, including both the possibility of increasing
δT
and the existence of its temperature-independent range from 10 to 80 °C, predestines the solid solution as a promising electrocaloric material.
Dark matter elastic scattering off nuclei can result in the excitation and ionization of the recoiling atom through the so-called Migdal effect. The energy deposition from the ionization electron ...adds to the energy deposited by the recoiling nuclear system and allows for the detection of interactions of sub-GeV/c^{2} mass dark matter. We present new constraints for sub-GeV/c^{2} dark matter using the dual-phase liquid argon time projection chamber of the DarkSide-50 experiment with an exposure of (12 306±184) kg d. The analysis is based on the ionization signal alone and significantly enhances the sensitivity of DarkSide-50, enabling sensitivity to dark matter with masses down to 40 MeV/c^{2}. Furthermore, it sets the most stringent upper limit on the spin independent dark matter nucleon cross section for masses below 3.6 GeV/c^{2}.
In the core of the Sun, energy is released through sequences of nuclear reactions that convert hydrogen into helium. The primary reaction is thought to be the fusion of two protons with the emission ...of a low-energy neutrino. These so-called pp neutrinos constitute nearly the entirety of the solar neutrino flux, vastly outnumbering those emitted in the reactions that follow. Although solar neutrinos from secondary processes have been observed, proving the nuclear origin of the Sun's energy and contributing to the discovery of neutrino oscillations, those from proton-proton fusion have hitherto eluded direct detection. Here we report spectral observations of pp neutrinos, demonstrating that about 99 per cent of the power of the Sun, 3.84 × 10(33) ergs per second, is generated by the proton-proton fusion process.
Celotno besedilo
Dostopno za:
DOBA, IJS, IZUM, KILJ, KISLJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
We present a search for dark matter particles with sub-GeV/c^{2} masses whose interactions have final state electrons using the DarkSide-50 experiment's (12 306±184) kg d low-radioactivity liquid ...argon exposure. By analyzing the ionization signals, we exclude new parameter space for the dark matter-electron cross section σover ¯_{e}, the axioelectric coupling constant g_{Ae}, and the dark photon kinetic mixing parameter κ. We also set the first dark matter direct-detection constraints on the mixing angle |U_{e4}|^{2} for keV/c^{2} sterile neutrinos.
The goals, objectives, and structure of the ontology of the Common Digital Space of Scientific Knowledge (CDSSK) are considered. The CDSSK is an integrated information structure that combines state ...scientific information systems presented on the Internet (the Great Russian Encyclopedia, the National Electronic Library, the State Catalog of Geographical Names, etc.) with industry information systems, databases, and electronic libraries (MathNet, Socionet, Scientific Heritage of Russia, etc.). CDSSK can be considered as an information basis for solving artificial intelligence problems. The article presents the unified structure of the CDSSK ontology developed at the Joint Supercomputer Center of the Russian Academy of Sciences and its modeling on an example of ten subject classes and eight auxiliary classes of objects of the CDSSK universal subspace.
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