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Shiv Nadar University

(SoNS) School of Natural Sciences

Detailed magnetization, specific heat and Li7 nuclear magnetic resonance (NMR) measurements on single crystals of the hyperhoneycomb Kitaev magnet $\beta$-Li2IrO3 are reported. At high temperatures, anisotropy of the magnetization is reflected by the different Curie-Weiss temperatures for different field directions, in agreement with the combination of a ferromagnetic Kitaev interaction (K) and a negative off-diagonal anisotropy ($\Gamma$) as two leading terms in the spin Hamiltonian. At low temperatures, magnetic fields applied along a or c have only a weak effect on the system and reduce the N{\'{e}}el temperature from 38 K at 0 T to about 35.5 K at 14 T, with no field-induced transitions observed up to 58 T on a powder sample. In contrast, the field applied along b causes a drastic reduction in the TN that vanishes around Hc=2.8T, giving way to a crossover toward a quantum paramagnetic state. Li7 NMR measurements in this field-induced state reveal a gradual line broadening and a continuous evolution of the line shift with temperature, suggesting the development of local magnetic fields. The spin-lattice relaxation rate shows a peak around the crossover temperature 40 K and follows power-law behavior below this temperature.

Physics

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Fulltext

Physical Review Materials

Anisotropic temperature-field phase diagram of single crystalline $\beta$-Li2IrO3: Magnetization, specific heat and Li 7 NMR study

In this work we have considered a minimal extension of Standard Model by a local U(1) gauge group in order to accommodate a stable (fermionic) Dark Matter (DM) candidate. We have focussed on parameter regions where DM possesses adequate self-interaction, owing to the presence of a light scalar mediator (the dark Higgs), alleviating some of the tensions in the small-scale structures. We have studied the scenario in the light of a variety of data, mostly from dark matter direct searches, collider searches and flavor physics experiments, with an attempt to constrain the interactions of the standard model (SM) particles with the ones in the Dark Sector (DS). Assuming a small gauge kinetic mixing parameter, we find that for rather heavy DM the most stringent bound on the mixing angle of the Dark Higgs with the SM Higgs boson comes from dark matter direct detection experiments, while for lighter DM, LHC constraints become more relevant. Note that, due to the presence of very light mediators, the effective operator approach to the direct detection is inapplicable here and these constraints have been re-evaluated for our scenario. In addition, we find that the smallness of the relevant portal couplings, as dictated by data, critically suppress the viability of DM production by the standard “freeze-out” mechanism in such simplified scenarios. In particular, the viable DM masses are ≲ O(2) GeV i.e. in the regions where direct detection limits tend to become weak. For heavier DM with large self-interactions, we hence conclude that non-thermal production mechanisms are favored. Lastly, future collider reach of such a simplified scenario has also been studied in detail.

Journal of High Energy Physics

Scope of self-interacting thermal WIMPs in a minimal U(1)D extension and its future prospects

Pure organic white light emitters with an afterglow (AG) feature have unique advantages and various potential applications. However, the studies of single-component organic white light emitters with AG and visible light excitation (VLE)-dependent efficient room-temperature phosphorescence (RTP) features remain a challenging area of research in photophysics. Herein, we synthesize three terephthalonitriles in which 2,3,5,6-positions are covalently attached to 2-fluoro-phenoxy (TOF), 2-chloro-phenoxy (TOC), and 2-methoxy-phenoxy groups (TOM) to give twisted geometries. We observed that powder samples of both TOC and TOM show white light emission with CIE coordinates of (0.32, 0.38) and (0.26, 0.33), respectively, while TOM gives VLE-dependent efficient RTP under ambient conditions. In addition, both TOC and TOF exhibit a dim AG feature. Spectroscopic studies reveal that dual emission of these chromophores originates via radiative decay of monomeric excited states (singlet, triplet), dimer-like excited states (DLES) (singlet, triplet), and aggregated triplet states. Detailed spectroscopic and X-ray analyses reveal the signature of DLES that is formed via conformational reorganization of the phenoxy donors in the excited states. Single-crystal X-ray diffraction analysis shows that the multiple lp(O)···π(CN)/C=C, Cl/F···π, and hydrogen-bonding interactions in the X-ray structures play a significant role in facilitating intersystem crossing, stabilizing multiple triplet states, and suppressing nonradiative decay, thereby triggering dim afterglow under ambient conditions. We found that TOC and TOF exhibit persistent RTP (PRTP) with lifetimes of 139 and 736 ms, respectively, when they are embedded in the polyvinyl alcohol matrix. Given the PRTP feature, invisible security ink application is developed. These results provide a guidance to design white light-emitting materials with afterglow and visible light-activated efficient RTP features. Copyright © 2019 American Chemical Society.

Chemistry

Journal of Physical Chemistry C

Use of Dimeric Excited States of the Donors in D4-A Systems for Accessing White Light Emission, Afterglow, and Invisible Security Ink

Polybenzoxazines (PBzs) are emerging as a highly promising and superior class of thermoset polymers for a variety of applications. However, it remains a significant challenge to substantially lower the ring-opening polymerization (ROP) temperature with an ease in processability. On the other hand, biomacromolecule chitosan (CS) is explored extensively, but its practical applications have been precluded by poor thermal and mechanical properties. Here, we developed a fully biobased copolymer of vanillin benzoxazine (V-fa) monomer with CS, which is effective in providing mutual benefits, effective lowering in ROP temperature of benzoxazine (70 °C), and an enhanced thermal stability of CS (by 85 °C, and with char yield of â32\%). To understand this unusual lowering in ROP temperature, we investigated the structural interaction mechanism between solvated CS and V-fa using in situ NMR studies. The analysis of fully intercalated co-structure demonstrated that there is a strong preference for ROP over Schiff base reaction. It is anticipated that benzoxazine molecules move within the interplanar distance of CS as supported by powder X-ray diffraction studies. An increase in V-fa content in feed ratio led to a placement of V-fa units from random to a systematic and hierarchical arrangement within the CS framework followed by its subsequent polymerization. The synergistic interactions were further supported by Fourier transform infrared, differential scanning calorimetry, scanning electron micrsocopy, thermogravimetric analysis, and tensile studies. Current work represents preparation of CS benzoxazine copolymers using a low-cost, efficient, and sustainable approach to assist metal-free ROP reaction of Bz to afford low curable temperature processable films. A new strategy is devised for the utility of CS-PBzs copolymers, enabling their extension to innovative applications in cross-domains. © 2019 American Chemical Society.

ACS Sustainable Chemistry and Engineering

Sustainable Framework of Chitosan-Benzoxazine with Mutual Benefits: Low Curing Temperature and Improved Thermal and Mechanical Properties

A nonnegative matrix A is primitive if for some positive integer m all entries in Am are positive. The smallest such m is called the exponent of A and written exp(A). For the class of primitive companion matrices X, we find exp(A) for certain A ∈ X. We find certain values of m for which there is an n×n primitive companion matrix (for given n) with exponent m. We also propose open problems for further research. © 2019 Rocky Mountain Mathematics Consortium.

Mathematics

Rocky Mountain Journal of Mathematics

Exponents of primitive companion matrices

The first manganese-catalyzed cyclopropanation of indoles is reported in moderate to excellent yield with methyl-2-diazo-2-arylacetates. This new strategy involved acetyl (COCH 3 ) as the directing group and exhibited exceptional functional group tolerance. In the absence of stereodirecting groups the desired products were obtained as a mixture of diastereomers (7:3 → 8:2). Control experiments and DFT studies elucidated the probable pathway for the formation of cyclopropane-fused indole product. Deacetylation of the final products afforded both C3-substituted NH-indoles. © 2019 American Chemical Society.

Organic Letters

Directing-group-assisted manganese-catalyzed cyclopropanation of indoles

The C-branched pentose sugar d-apiose is important for plant cell wall development. Its biosynthesis as urdine diphosphate-d-apiose (UDP-d-apiose) involves decarboxylation of the UDP-d-glucuronic acid precursor coupled with pyranosyl-to-furanosyl sugar ring contraction. This unusual multistep reaction is catalysed in a single active site by UDP-d-apiose/UDP-d-xylose synthase (UAXS). Here we decipher the UAXS catalytic mechanism on the basis of crystal structures of the enzyme (which is from Arabidopsis thaliana), molecular dynamics simulations that are expanded by hybrid quantum mechanics/molecular mechanics calculations and mutational mechanistic analyses. Our studies show how UAXS uniquely integrates a classical catalytic cycle of oxidation and reduction by a tightly bound nicotinamide co-enzyme with retroaldol/aldol chemistry for the sugar ring contraction. We further demonstrate that decarboxylation occurs only after the sugar ring opening and identify that the thiol group of Cys100 steers the sugar skeleton rearrangement by proton transfer to and from C3′. The mechanistic features of UAXS highlight the evolutionary expansion of the basic catalytic apparatus of short-chain dehydrogenases/reductases for functional versatility in sugar biosynthesis. © 2019, The Author(s), under exclusive licence to Springer Nature Limited.

Nature Catalysis

Deciphering the enzymatic mechanism of sugar ring contraction in UDP-apiose biosynthesis

In this chapter, we provide a brief overview of the current status, challenges and scope for charge density analysis in proteins. The approach, basic requirements, available tools for performing protein charge density analysis are discussed here. A brief review of reported charge density studies on six protein systems ranging from 4.7 to 55 kDa and with resolutions ranging from 0.78 to 0.48 Å, and the key outcomes, which were inaccessible from the routine structural studies, are highlighted. The use of neutron diffraction data and its joint refinement with X-ray diffraction data for quantifying interactions involving water molecules themselves and with the protein residues is also discussed for a lone reported protein complex. Further, we perform a topological analysis of main-chain N-HO=C hydrogen bonds both in α-helices and β-sheets of a high-resolution (0.66 Å) protein structure of hAR based on electron densities from the ELMAM2 database. The quantitative characterizations reveal that the N-HO=C type hydrogen bonds in proteins follow the same trend as those in small molecules. Furthermore, both the electrostatic and dissociation energies of H-bonds are estimated based on the electron densities and electrostatic potentials. The electrostatic energies are comparable with the experimental values. The energies were also found to correlate well with the topological properties of N-HO=C hydrogen bonds. © The Royal Society of Chemistry 2019.

Monographs in Supramolecular Chemistry

CHAPTER 6: Charge Density Studies and Topological Analysis of Hydrogen Bonds in Proteins

The possibilities of precise control over the wetting characteristics of carbon-based hybrid nanostructures consisting of both sp2 and sp3 hybridized carbons using the electrowetting technique were demonstrated. An excellent polarity-dependent electrowetting behavior in the presence of an electrolyte followed an abrupt transition from the highly hydrophobic (contact angle ∼ 142°) Cassie-Baxter states to a hydrophilic (∼30°) Wenzel state, where diamond films acted as the anode. In addition, we also reported a remarkable transition from weakly hydrophobic to nearly superhydrophobic diamond nanostructures by chemical and morphological manipulations. The unique structural properties with precisely tailored morphology and surface roughness enabled such transitions on the nanostructured surface. This approach of preparing environmental stable hydrophobic surfaces with polarity-dependent wetting and precise control of the wetting mode transition could be used in numerous applications such as the electrochemical transport of liquids, supercapacitors, and low-friction microfluidics. © 2019 The Royal Society of Chemistry.

Journal of Materials Chemistry A

Direct synthesis of electrowettable nanostructured hybrid diamond

Tumor initiating cells (TIC) are resistant to conventional anticancer therapy and associated with metastasis and relapse in cancer. Although various TIC markers and their antibodies have been proposed, it is limited to the use of antibodies for in vivo imaging or treatment of TIC. In this study, we discovered heme oxygenase 2 (HMOX2) as a novel biomarker for TIC and developed a selective small molecule probe TiNIR (tumor initiating cell probe with near infrared). TiNIR detects and enriches the functionally active TIC in human lung tumors, and through the photoacoustic property, TiNIR also visualizes lung TIC in the patient-derived xenograft (PDX) model. Furthermore, we demonstrate that TiNIR inhibits tumor growth by blocking the function of HMOX2, resulting in significantly increased survival rates of the cancer model mice. The novel therapeutic target HMOX2 and its fluorescent ligand TiNIR will open a new path for the molecular level of lung TIC diagnosis and treatment. Copyright © 2019 American Chemical Society.