Origin of magnetic moments and presence of a resonating valence bond state in Ba2_2YIrO6_6

While it was speculated that 5$d^4$ systems would possess non-magnetic $J$~=~0 ground state due to strong Spin-Orbit Coupling (SOC), all such systems have invariably shown presence of magnetic moments so far. A puzzling case is that of Ba$_2$YIrO$_6$, which in spite of having a perfectly cubic structure with largely separated Ir$^{5+}$ ($d^4$) ions, has consistently shown presence of weak magnetic moments. Moreover, we clearly show from Muon Spin Relaxation ($\mu$SR) measurements that a change in the magnetic environment of the implanted muons in Ba$_2$YIrO$_6$ occurs as temperature is lowered below 10~K. This observation becomes counterintuitive, as the estimated value of SOC obtained by fitting the RIXS spectrum of Ba$_2$YIrO$_6$ with an atomic $j-j$ model is found to be as high as 0.39~eV, meaning that the system within this model is neither expected to possess moments nor exhibit temperature dependent magnetic response. Therefore we argue that the atomic $j-j$ coupling description is not sufficient to explain the ground state of such systems, where despite having strong SOC, presence of hopping triggers delocalisation of holes, resulting in spontaneous generation of magnetic moments. Our theoretical calculations further indicate that these moments favour formation of spin-orbital singlets in the case of Ba$_2$YIrO$_6$, which is manifested in $\mu$SR experiments measured down to 60~mK.Comment: 20 Pages, 7 Figure

Redescription of poorly known neustonic calanoid copepod Pontella andersoni Sewell, 1912 (Family: Pontellidae) from the Bay of Bengal, Indian Ocean

The neustonic calanoid copepod of family Pontellidae Pontella andersoni Sewell, 1912 was originally described from the Bay of Bengal based on a single male and a damaged female with a description of copepodid stage III. Later, this species was described from Gulf of Mannar (based on a male specimen) and from West Java (based on immature specimens). In the present study, adult female and male specimens were collected from the coastal waters of northern Bay of Bengal. The species was easily distinguished from its congeners by a barrel shaped first urosomal somite in female, and two subequal spines with two knot-like projections in the first exopodal segment of right fifth leg, serrated spine and three processes in the second exopodal segment of left leg in male. The present note is an updated illustrative description of P. andersoni, since the previous descriptions were lacking the taxonomic resolution

Redescription of poorly known neustonic calanoid copepod Pontella andersoni Sewell, 1912 (Family: Pontellidae) from the Bay of Bengal, Indian Ocean

635-640The neustonic calanoid copepod of family Pontellidae Pontella andersoni Sewell, 1912 was originally described from the Bay of Bengal based on a single male and a damaged female with a description of copepodid stage III. Later, this species was described from Gulf of Mannar (based on a male specimen) and from West Java (based on immature specimens). In the present study, adult female and male specimens were collected from the coastal waters of northern Bay of Bengal. The species was easily distinguished from its congeners by a barrel shaped first urosomal somite in female, and two subequal spines with two knot-like projections in the first exopodal segment of right fifth leg, serrated spine and three processes in the second exopodal segment of left leg in male. The present note is an updated illustrative description of P. andersoni, since the previous descriptions were lacking the taxonomic resolution

Swarms of ctenophore Pleurobrachia pileus (O. F. Müller, 1776) in the waters of Sundarban: A menace to the fisheries?

1089-1092An unusual swarm of Pleurobrachia pileus (O. F. Müller, 1776) was recorded in the Sundarban coastal waters of Bay of Bengal, Northern Indian Ocean during the winter monsoon season of 2018. The species occurred in the Sundarban waters with an average abundance of 6,766 individual m-3 which in turn diminished the population of other zooplankton and fish. The hydrographical characters observed in the swarm stations favoured the proliferation of Pleurobrachia pileus and the unique factors were low temperature (18.9 – 20.9 °C), high salinity (~24 psu) and associated high biological production. The phenomenon of P. pileus swarm has not been reported earlier from the coastal waters of Sundarban, therefore, our incidental observation provides further insight to explore the study area

Anti-symmetric Compton scattering in LiNiPO4 : towards a direct probe of the magneto-electric multipole moment [version 2; peer review: 3 approved]

Background: Magnetoelectric multipoles, which break both space-inversion and time-reversal symmetries, play an important role in the magnetoelectric response of a material. Motivated by uncovering the underlying fundamental physics of the magnetoelectric multipoles and the possible technological applications of magnetoelectric materials, understanding as well as detecting such magnetoelectric multipoles has become an active area of research in condensed matter physics. Here we employ the well-established Compton scattering effect as a possible probe for the magnetoelectric toroidal moments in LiNiPO4. Methods: We employ combined theoretical and experimental techniques to compute as well as detect the antisymmetric Compton profile in LiNiPO4. For the theoretical investigation we use density functional theory to compute the anti-symmetric part of the Compton profile for the magnetic and structural ground state of LiNiPO4. For the experimental verification, we measure the Compton signals for a single magnetoelectric domain sample of LiNiPO4, and then again for the same sample with its magnetoelectric domain reversed. We then take the difference between these two measured signals to extract the antisymmetric Compton profile in LiNiPO4. Results: Our theoretical calculations indicate an antisymmetric Compton profile in the direction of the ty toroidal moment in momentum space, with the computed antisymmetric profile around four orders of magnitude smaller than the total profile. The difference signal that we measure is consistent with the computed profile, but of the same order of magnitude as the statistical errors and systematic uncertainties of the experiment. Conclusions: While the weak difference signal in the measurements prevents an unambiguous determination of the antisymmetric Compton profile in LiNiPO4, our results motivate further theoretical work to understand the factors that influence the size of the antisymmetric Compton profile, and to identify materials exhibiting larger effects