Magnetic properties of low-dimensional quantum spin systems made of stable organic biradicals PNNNO, F2PNNNO, and PIMNO

Stable organic biradical crystals PNNNO, F2PNNNO, and PIMNO of the PNNNO family were synthesized. {PNNNO=2-[4′- (N-tert-butyl-N-oxyamino)phenyl]-4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazol-1- oxyl 3-oxide, F2PNNNO= 2-[2′,6′,-difluoro-4′-(N-tert-butyl-7V-oxyamino)phenyl]-4,4,5, 5-tetramethyl-4, 5-dihydro-1H-imidazol-1-oxyl 3-oxide, PIMNO=2-[4′-(N-tert-butyl-N-oxyamino)-phenyl]-4,4,5, 5-tetramethyl-4,5-dihydro-1H-imidazol-1-oxyl.} PNNNO and PIMNO crystallize to form quasi-one-dimensional lattices, but F2PNNNO to form a quasi-two-dimensional lattice. The temperature dependences of the susceptibility and the high-field magnetization process up to 34 T were measured down to 0.5 K. The results are analyzed by comparing with the theoretical calculations based on the crystal structures. PNNNO and PIMNO are considered to be antiferromagnetic Heisenberg spin chains consisting of S=1/2 spin pairs (dimers) in which the two spins are coupled ferromagnetically. At low temperatures, an antiferromagnetic ordering occurs in these crystals, which is confirmed by the thermodynamic discussion through specific heat measurements. On the other hand, F2PNNNO is thought to be a two-dimensional Heisenberg system, in which the spin pairs are connected by two types of antiferromagnetic interactions. The ground state is singlet. The high-field magnetization process shows a two-step saturation with a plateau of the half value of the saturation magnetization

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First Emulsion γ-Ray Telescope Imaging of the Vela Pulsar by the GRAINE 2018 Balloon-borne Experiment

We are developing the Gamma-Ray Astro-Imager with Nuclear Emulsion project, designed for 10 MeV–100 GeV cosmic γ-ray observations with a high angular resolution (5'/0:[fdg]08 at 1–2 GeV) and a polarization-sensitive large-aperture (∼10 ㎡) emulsion telescope for repeated long-duration balloon flights. In 2018, a balloon-borne experiment was carried out in Australia with a 0.38 ㎡ sensitive area and a flight duration of 17.4 hr, including 6.7 hr of Vela observations. Significant improvements compared with the 2015 balloon-borne experiment were achieved by a factor of 5, including both an increase in effective area × time and a reduction in the background contribution. We aimed to demonstrate the telescope's overall performance based on detection and imaging of a known γ-ray source, the Vela pulsar. A robust detection of the Vela pulsar was achieved with a 68% containment radius of 0:[fdg]42, at a significance of 6σ, at energies above 80 MeV. The resulting angular profile is consistent with that of a pointlike source. We achieved the current best imaging performance of the Vela pulsar using an emulsion γ-ray telescope with the highest angular resolution of any γ-ray telescope to date