Disorder from order among anisotropic next-nearest-neighbor Ising spin chains in SrHo2_2O4_4

We describe why Ising spin chains with competing interactions in $\rm SrHo_2O_4$ segregate into ordered and disordered ensembles at low temperatures ($T$). Using elastic neutron scattering, magnetization, and specific heat measurements, the two distinct spin chains are inferred to have N\'eel ($\uparrow\downarrow\uparrow\downarrow$) and double-N\'eel ($\uparrow\uparrow\downarrow\downarrow$) ground states respectively. Below $T_\mathrm{N}=0.68(2)$~K, the N\'eel chains develop three dimensional (3D) long range order (LRO), which arrests further thermal equilibration of the double-N\'eel chains so they remain in a disordered incommensurate state for $T$ below $T_\mathrm{S}= 0.52(2)$~K. $\rm SrHo_2O_4$ distills an important feature of incommensurate low dimensional magnetism: kinetically trapped topological defects in a quasi$-d-$dimensional spin system can preclude order in $d+1$ dimensions.Comment: 10 pages, 10 figure

Population pulsation resonances of excitons in monolayer MoSe2 with sub 1 {\mu}eV linewidth

Monolayer transition metal dichalcogenides, a new class of atomically thin semiconductors, possess optically coupled 2D valley excitons. The nature of exciton relaxation in these systems is currently poorly understood. Here, we investigate exciton relaxation in monolayer MoSe2 using polarization-resolved coherent nonlinear optical spectroscopy with high spectral resolution. We report strikingly narrow population pulsation resonances with two different characteristic linewidths of 1 {\mu}eV and <0.2 {\mu}eV at low-temperature. These linewidths are more than three orders of magnitude narrower than the photoluminescence and absorption linewidth, and indicate that a component of the exciton relaxation dynamics occurs on timescales longer than 1 ns. The ultra-narrow resonance (<0.2 {\mu}eV) emerges with increasing excitation intensity, and implies the existence of a long-lived state whose lifetime exceeds 6 ns.Comment: (PRL, in press

The structure of the BfrB-Bfd complex reveals protein-protein interactions enabling iron release from bacterioferritin

This document is the Accepted Manuscript version of a Published Work that appeared in final form in the Journal of the American Chemical Society, copyright © American Chemical Society after peer review and technical editing by the publisher. To access the final edited and published work see http://doi.org/10.1021/ja305180n.Ferritin-like molecules are unique to cellular iron homeostasis because they can store iron at concentrations much higher than those dictated by the solubility of Fe3+. Very little is known about the protein interactions that deliver iron for storage, or promote the mobilization of stored iron from ferritin-like molecules. Here, we report the X-ray crystal structure of Pseudomonas aeruginosa bacterioferritin (Pa-BfrB) in complex with bacterioferritin-associated ferredoxin (Pa-Bfd) at 2.0 Å resolution. As the first example of a ferritin-like molecule in complex with a cognate partner, the structure provides unprecedented insight into the complementary interface that enables the [2Fe-2S] cluster of Pa-Bfd to promote heme-mediated electron transfer through the BfrB protein dielectric (~18 Å), a process that is necessary to reduce the core ferric mineral and facilitate mobilization of Fe2+. The Pa-BfrB-Bfd complex also revealed the first structure of a Bfd, thus providing a first view to what appears to be a versatile metal binding domain ubiquitous to the large Fer2_BFD family of proteins and enzymes with diverse functions. Residues at the Pa-BfrB-Bfd interface are highly conserved in Bfr and Bfd sequences from a number of pathogenic bacteria, suggesting that the specific recognition between Pa-BfrB and Pa-Bfd is of widespread significance to the understanding of bacterial iron homeostasis

Electrical Control of Second-Harmonic Generation in a WSe2 Monolayer Transistor

Nonlinear optical frequency conversion, in which optical fields interact with a nonlinear medium to produce new field frequencies, is ubiquitous in modern photonic systems. However, the nonlinear electric susceptibilities that give rise to such phenomena are often challenging to tune in a given material, and so far, dynamical control of optical nonlinearities remains confined to research labs as a spectroscopic tool. Here, we report a mechanism to electrically control second-order optical nonlinearities in monolayer WSe2, an atomically thin semiconductor. We show that the intensity of second-harmonic generation at the A-exciton resonance is tunable by over an order of magnitude at low temperature and nearly a factor of 4 at room temperature through electrostatic doping in a field-effect transistor. Such tunability arises from the strong exciton charging effects in monolayer semiconductors, which allow for exceptional control over the oscillator strengths at the exciton and trion resonances. The exciton-enhanced second-harmonic generation is counter-circularly polarized to the excitation laser, arising from the combination of the two-photon and one-photon valley selection rules that have opposite helicity in the monolayer. Our study paves the way towards a new platform for chip-scale, electrically tunable nonlinear optical devices based on two-dimensional semiconductors.Comment: Published in Nature Nanotechnolog

Strongly anisotropic spin relaxation in graphene/transition metal dichalcogenide heterostructures at room temperature

Graphene has emerged as the foremost material for future two-dimensional spintronics due to its tuneable electronic properties. In graphene, spin information can be transported over long distances and, in principle, be manipulated by using magnetic correlations or large spin-orbit coupling (SOC) induced by proximity effects. In particular, a dramatic SOC enhancement has been predicted when interfacing graphene with a semiconducting transition metal dechalcogenide, such as tungsten disulphide (WS$_2$). Signatures of such an enhancement have recently been reported but the nature of the spin relaxation in these systems remains unknown. Here, we unambiguously demonstrate anisotropic spin dynamics in bilayer heterostructures comprising graphene and WS$_2$. By using out-of-plane spin precession, we show that the spin lifetime is largest when the spins point out of the graphene plane. Moreover, we observe that the spin lifetime varies over one order of magnitude depending on the spin orientation, indicating that the strong spin-valley coupling in WS$_2$ is imprinted in the bilayer and felt by the propagating spins. These findings provide a rich platform to explore coupled spin-valley phenomena and offer novel spin manipulation strategies based on spin relaxation anisotropy in two-dimensional materials

Assessment of cataract surgical outcomes in settings where follow-up is poor: PRECOG, a multicentre observational study

Background Poor follow-up after cataract surgery in developing countries makes assessment of operative quality uncertain. We aimed to assess two strategies to measure visual outcome: recording the visual acuity of all patients 3 or fewer days postoperatively (early postoperative assessment), and recording that of only those patients who returned for the fi nal follow-up examination after 40 or more days without additional prompting. Methods Each of 40 centres in ten countries in Asia, Africa, and Latin America recruited 40–120 consecutive surgical cataract patients. Operative-eye best-corrected visual acuity and uncorrected visual acuity were recorded before surgery, 3 or fewer days postoperatively, and 40 or more days postoperatively. Clinics logged whether each patient had returned for the fi nal follow-up examination without additional prompting, had to be actively encouraged to return, or had to be examined at home. Visual outcome for each centre was defi ned as the proportion of patients with uncorrected visual acuity of 6/18 or better minus the proportion with uncorrected visual acuity of 6/60 or worse, and was calculated for each participating hospital with results from the early assessment of all patients and the late assessment of only those returning unprompted, with results from the fi nal follow-up assessment for all patients used as the standard. Findings Of 3708 participants, 3441 (93%) had fi nal follow-up vision data recorded 40 or more days after surgery, 1831 of whom (51% of the 3581 total participants for whom mode of follow-up was recorded) had returned to the clinic without additional prompting. Visual outcome by hospital from early postoperative and fi nal follow-up assessment for all patients were highly correlated (Spearman’s rs=0·74, p<0·0001). Visual outcome from fi nal followup assessment for all patients and for only those who returned without additional prompting were also highly correlated (rs=0·86, p<0·0001), even for the 17 hospitals with unprompted return rates of less than 50% (rs=0·71, p=0·002). When we divided hospitals into top 25%, middle 50%, and bottom 25% by visual outcome, classifi cation based on fi nal follow-up assessment for all patients was the same as that based on early postoperative assessment for 27 (68%) of 40 centres, and the same as that based on data from patients who returned without additional prompting in 31 (84%) of 37 centres. Use of glasses to optimise vision at the time of the early and late examinations did not further improve the correlations. Interpretation Early vision assessment for all patients and follow-up assessment only for patients who return to the clinic without prompting are valid measures of operative quality in settings where follow-up is poor

Rac1 as a therapeutic target in ovarian cancer

Rac1 is a high value therapeutic target for cancer based on its tumor promoting activities, yet clinical applications targeting Rac1 are in their infancy. High expression and hyperactivation of Rac1 in ovarian cancer, along with our identification of R-ketorolac as a novel Rac1 and Cdc42 selective inhibitor with translational potential, prompt us to test the hypothesis that targeting Rac1 has therapeutic utility for ovarian cancer. Ascites tumor cell samples from ovarian cancer patients in a prospective study receiving racemic ketorolac for clinically indicated use in pain relief were previously reported to show time dependent reduction of Rac1 and Cdc42 activities post-treatment. New RNA seq data of these patient samples reveals significant changes of genes involved in cell adhesion, cytokine-mediated signaling and cytokine production pathways. Conversely, the identified downregulated genes were overexpressed and associated with worse survival in ovarian cancer patients analyzed through The Cancer Genome Atlas (TCGA). Among the downregulated genes in the NOD pathway are chemokines and pro-inflammatory cytokines. Follow-up cytokine panels from patients confirm that racemic ketorolac treatment reduces the levels of immunosuppressive cytokines IL-6, IL-10 and RANTES in ascites fluids. Together, these data indicate there may be a benefit to the anti-inflammatory activity of the S- enantiomer, as well as the GTPase inhibitory activity of the R- enantiomer of ketorolac for ovarian cancer treatment