55 Co is not only present in abundance in presupernova phase but is also advocated to play a decisive role in the core collapse of massive stars. The spectroscopy of electron capture and emitted neutrinos yields useful information on the physical conditions and stellar core composition. B(GT) values to low-lying states are calculated microscopically using the pn-QRPA theory. Our rates are enhanced compared to the reported shell model rates. The enhancement is attributed partly to the liberty of selecting a huge model space, allowing consideration of many more excited states in our rate calculations. Unlike previous calculations the so-called Brinks hypothesis is not assumed leading to a more realistic estimate of the rates. The electron and positron capture rates are calculated over a wide temperature (0.01 × 10 9 − 30 × 10 9 K) and density (10 − 10 11 gcm −3 ) grid.
We report magnetic susceptibility (χ) and heat capacity (C p ) measurements along with ab-initio electronic structure calculations on PbCuTe 2 O 6 , a compound made up of a three dimensional 3D network of corner-shared triangular units. The presence of antiferromagnetic interactions is inferred from a Curie-Weiss temperature (θ CW ) of about −22 K from the χ(T ) data. The magnetic heat capacity C m data show a broad maximum at T max ≃ 1.15 K (i.e.T max /θ CW ≃ 0.05), which is analogous to the the observed broad maximum in the C m /T data of a hyper-Kagome system, Na 4 Ir 3 O 8 . In addition, C m data exhibit a weak kink at T * ≃ 0.87 K. While the T max is nearly unchanged, the T * is systematically suppressed in an increasing magnetic field (H) up to 80 kOe. For H ≥ 80 kOe, the C m data at low temperatures exhibit a characteristic power-law (T α ) behavior with an exponent α slightly less than 2. Hopping integrals obtained from the electronic structure calculations show the presence of strongly frustrated 3D spin interactions along with non-negligible unfrustrated couplings. Our results suggest that PbCuTe 2 O 6 is a candidate material for realizing a 3D quantum spin liquid state at high magnetic fields.
Electron captures on nuclei play an important role in the collapse of stellar core in the stages leading to a type-II supernova. Recent observations of subluminous Type II-P supernovae (e.g., 2005cs, 2003gd, 1999br) were able to rekindle the interest in 8-10 M which develop O+Ne+Mg cores. We used the proton-neutron quasiparticle random phase approximation (pn-QRPA) theory to calculate the B(GT) strength for 24 Mg → 24 Na and its associated electron capture rates for incorporation in simulation calculations. The calculated rates, in this article, have differences with the earlier reported shell model and Fuller, Fowler, and Newman [2] (hereafter F 2 N) rates. We compared Gamow-Teller (GT) strength distribution functions and found fairly good agreement with experiment and shell model. However, the GT centroid and the total GT strength, which are useful in the calculation of electron capture rates in the core of massive presupernova stars, lead to the enhancement of our rate up to a factor of 4 compared to the shell model rates at high temperatures and densities.
The Gamow-Teller strength (GT) distributions and electron capture rates on 55 Co and 56 Ni have been calculated using the proton-neutron quasiparticle random phase approximation theory. We calculate these weak interaction mediated rates over a wide temperature (0.01x10 9 -30x10 9 K) and density (10 -10 11 g cm -3 ) domain. Electron capture process is one of the essential ingredients involved in the complex dynamics of supernova explosion. Our calculations of electron capture rates show differences with the reported shell model diagonalization approach calculations and are comparatively enhanced at presupernova temperatures. We note that the GT strength is fragmented over many final states.
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