Captures of Hot and Warm Sterile Antineutrino Dark Matter on EC-decaying Ho-163 Nuclei

Capturing low-energy electron antineutrinos on radioactive Ho-163 nuclei, which decay into Dy-163 via electron capture (EC), is a noteworthy opportunity to detect relic sterile antineutrinos. Such hypothetical particles are more or less implied by current experimental and cosmological data, and they might be a part of hot dark matter or a candidate for warm dark matter in the Universe. Using the isotope Ho-163 as a target and assuming reasonable active-sterile antineutrino mixing angles, we calculate the capture rate of relic electron antineutrinos against the corresponding EC-decay background in the presence of sterile antineutrinos at the sub-eV or keV mass scale. We show that the signature of hot or warm sterile antineutrino dark matter should in principle be observable, provided the target is big enough and the energy resolution is good enough.Comment: 16 pages, 6 figures, more discussions and references added. To appear in JCA

Fermi Variability Study of the Candidate Pulsar Binary 2FGL~J0523.3-2530

The Fermi source 2FGL~J0523.3$-$2530 has recently been identified as a candidate millisecond pulsar binary with an orbital period of 16.5 hrs. We have carried out detailed studies of the source's emission properties by analyzing data taken with the Fermi Large Area Telescope in the 0.2--300 GeV energy range. Long-term, yearly variability from the source has been found, with a factor of 4 flux variations in 1--300 GeV. From spectral analysis, we find an extra spectral component at 2--3 GeV that causes the source brightening. While no orbital modulations have been found from the Fermi data over the whole period of 2008--2014, orbital modulation in the source's $>$2 GeV emission is detected during the last 1.5 yrs of the Fermi observation. Our results support the millisecond pulsar binary nature of 2FGL~J0523.3$-$2530. Multi-wavelength observations of the source are warranted in order to find any correlated flux variations and thus help determine the origin of the long-term variability, which currently is not understood.Comment: 8 pages, 5 figures, comments from Referee were incorporated, accepted for publication in Ap

Quantum Hall Effect in Thin Films of Three-Dimensional Topological Insulators

We show that a thin film of a three-dimensional topological insulator (3DTI) with an exchange field is a realization of the famous Haldane model for quantum Hall effect (QHE) without Landau levels. The exchange field plays the role of staggered fluxes on the honeycomb lattice, and the hybridization gap of the surface states is equivalent to alternating on-site energies on the AB sublattices. A peculiar phase diagram for the QHE is predicted in 3DTI thin films under an applied magnetic field, which is quite different from that either in traditional QHE systems or in graphene.Comment: 4 pages, 4 figure

A topological look at the quantum spin Hall state

We propose a topological understanding of the quantum spin Hall state without considering any symmetries, and it follows from the gauge invariance that either the energy gap or the spin spectrum gap needs to close on the system edges, the former scenario generally resulting in counterpropagating gapless edge states. Based upon the Kane-Mele model with a uniform exchange field and a sublattice staggered confining potential near the sample boundaries, we demonstrate the existence of such gapless edge states and their robust properties in the presence of impurities. These gapless edge states are protected by the band topology alone, rather than any symmetries.Comment: 5 pages, 4 figure