Inelastic neutron scattering studies of the quantum frustrated magnet clinoatacamite, γ\gamma-Cu2(OD)3Cl, a proposed valence bond solid (VBS)

The frustrated magnet clinoatacamite, $\gamma$-Cu$_2$(OH)$_3$Cl, is attracting a lot of interest after suggestions that at low temperature it forms an exotic quantum state termed a Valence Bond Solid (VBS) made from dimerised Cu$^{2+}$ ($S=1/2$) spins.\cite{Lee_clinoatacamite} Key to the arguments surrounding this proposal were suggestions that the kagom\'e planes in the magnetic pyrochlore lattice of clinoatacamite are only weakly coupled, causing the system to behave as a quasi-2-dimensional magnet. This was reasoned from the near 95$^\circ$ angles made at the bridging oxygens that mediate exchange between the Cu ions that link the kagom\'e planes. Recent work pointed out that this exchange model is inappropriate for $\gamma$-Cu$_2$(OH)$_3$Cl, where the oxygen is present as a $\mu_3$-OH.\cite{Wills_JPC} Further, it used symmetry calculations and neutron powder diffraction to show that the low temperature magnetic structure ($T<6$ K) was canted and involved significant spin ordering on all the Cu$^{2+}$ spins, which is incompatible with the interpretation of simultaneous VBS and N\'eel ordering. Correspondingly, clinoatacamite is best considered a distorted pyrochlore magnet. In this report we show detailed inelastic neutron scattering spectra and revisit the responses of this frustrated quantum magnet.Comment: Proceedings of The International Conference on Highly Frustrated Magnetism 2008 (HFM2008

LIR1 expressing human Natural Killer cell subsets differentially recognize isolates of human cytomegalovirus through the viral MHC Class I homolog UL18

This is the author accepted manuscript. The final version is available from the American Society for Microbiology via https://doi.org/10.1128/JVI.02614-15.Immune responses of Natural Killer (NK) cell are controlled by the balance between activating and inhibitory receptors, but the expression of these receptors varies between cells within an individual. Although NK cells are a component of the innate immune system, particular NK cell subsets expressing Ly49H are positively selected and increase in frequency in response to cytomegalovirus infection in mice. Recent evidence suggests that in humans certain NK subsets also have an increased frequency in the blood of HCMV infected individuals. However whether these subsets differ in their capacity of direct control of HCMV infected cells remains unclear. In this study we developed a novel in vitro assay to assess whether human NK cells subsets have differential abilities to inhibit HCMV growth and dissemination. NK cells expressing or lacking NKG2C did not display any differences when controlling viral dissemination. However, when in vitro expanded NK cells were used, cells expressing or lacking the inhibitory receptor Leukocyte Immunoglobulin-like receptor 1 (LIR1) were differentially able to control dissemination. Surprisingly, the ability of LIR1+ NK cells to control virus spread differed between HCMV viral strains, and this phenomenon was dependent on amino acid sequences within the viral ligand UL18. Together, the results here outlined an in vitro technique to compare the long-term immune responses of different human NK cell subsets, and suggest, for the first time, phenotypically defined human NK cell subsets may differentially recognise HCMV infected. IMPORTANCE HCMV infection is ubiquitous in most populations, it is not cleared by the host after primary infection but persists for life. The innate and adaptive immune system controls the spread of virus, of which Natural Killer (NK) cells play a pivotal role. NK cells can respond to HCMV infection by rapid, short-term non-specific innate responses, but evidence from murine studies suggested NK cells may display a long-term, memory like responses to murine cytomegalovirus infection. In this study, we developed a new assay that examines human NK cell subsets that have been suggested to play a long-term memory-like response to HCMV infection. We show that changes in a HCMV viral protein that interacts with an NK cell receptor can change the ability of NK cell subsets to control HCMV while the acquisition odf another receptor has no effect on virus control.This work was funded by the Wellcome Trust Grant WT094107AIA, the UK Medical Research Council Grant G0701279, MR/L008734/1 and MR/K021087/1 and supported by the NIHR Cambridge BRC Cell Phenotyping hub. The funders had no role in study design, data collection and interpretation, or the decision to submit the work for publication

Magnetic Ordering in the Spin-Ice Candidate Ho2_2Ru2_2O7_7

Neutron scattering measurements on the spin-ice candidate material Ho$_2$Ru$_2$O$_7$ have revealed two magnetic transitions at T $\sim$ 95 K and T $\sim$ 1.4 K to long-range ordered states involving the Ru and Ho sublattices, respectively. Between these transitions, the Ho$^{3+}$ moments form short-ranged ordered spin clusters. The internal field provided by the ordered S=1 Ru$^{4+}$ moments disrupts the fragile spin-ice state and drives the Ho$^{3+}$ moments to order. We have directly measured a slight shift in the Ho$^{3+}$ crystal field levels at 95 K from the Ru ordering.Comment: 4 pages, 5 figures, submitted to Phys. Rev. Letter

Near-IR spectroscopy of PKS1549-79: a proto-quasar revealed?

We present a near-IR spectrum of the nearby radio galaxy PKS1549-79 (z=0153). These data were taken with the aim of testing the idea that this object contains a quasar nucleus that is moderately extinguished, despite evidence that its radio jet points close to our line-of-sight. We detect broad Paschen Alpha emission (FWHM ~1745 km/s), relatively bright continuum emission, and a continuum slope consistent with a reddened quasar spectrum (3.1 < Av < 7.3), all emitted by an unresolved point source. Therefore we conclude that we have, indeed, detected a hidden quasar nucleus in PKS1549-79. Combined with previous results, these observations are consistent with the idea that PKS1549-79 is a young radio source in which the cocoon of debric left over from the triggering events has not yet been swept aside by circumnuclear outflows.Comment: 6 pages, 4 figures, accepted for publication in MNRA

Forced and internal components of observed Arctic sea-ice changes

The Arctic sea-ice cover is strongly influenced by internal variability on decadal timescales, affecting both short-term trends and the timing of the first ice-free summer. Several mechanisms of variability have been proposed, but how these mechanisms manifest both spatially and temporally remains unclear. The relative contribution of internal variability to observed Arctic sea-ice changes also remains poorly quantified. Here, we use a novel technique called low-frequency component analysis to identify the dominant patterns of winter and summer decadal Arctic sea-ice variability in the satellite record. The identified patterns account for most of the observed regional sea-ice variability and trends, and they thus help to disentangle the role of forced and internal sea-ice changes over the satellite record. In particular, we identify a mode of decadal ocean–atmosphere–sea-ice variability, characterized by an anomalous atmospheric circulation over the central Arctic, that accounts for approximately 30 % of the accelerated decline in pan-Arctic summer sea-ice area between 2000 and 2012 but accounts for at most 10 % of the decline since 1979. For winter sea ice, we find that internal variability has dominated decadal trends in the Bering Sea but has contributed less to trends in the Barents and Kara seas. These results, which detail the first purely observation-based estimate of the contribution of internal variability to Arctic sea-ice trends, suggest a lower estimate of the contribution from internal variability than most model-based assessments.</p