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First occurrence of mastixioid (Cornaceae) fossil in India and its biogeographic implications
Mastixioids in the family Cornaceae, are presently native only in limited areas of Asia, they have rich fossil fruit record in Cenozoic sediments of Europe and North America, but unfortunately none have been reported from Cenozoic sediments of India and Asia until now. Here, we report the occurrence of leaf remains (both impression and compression) along with carbonised fruits, resembling morphologically and anatomically those of the extant endemic species Mastixia arborea C.B. Clarke. Our materials were recovered from the middle Miocene to early Pleistocene Siwalik sediments exposed around West Kameng and Papumpare districts in Arunachal Pradesh, eastern Himalaya. These new fossil materials confirm the existence of Mastixia in the Miocene-Pleistocene Siwalik forests in India. At present the modern analogue does not grow in the eastern Himalaya and is endemic to the tropical evergreen forests of the Western Ghats, situated at the same palaeolatitude as the fossil locality. Extinction from the entire eastern Himalaya and probable movement of this taxon to the Western Ghats is likely due to climate change in the area, related to the Himalayan Orogeny during Miocene–Pleistocene times. The disappearance of Mastixia from this area may be related to the gradual intensification of rainfall seasonality since the late Miocene. The recovery of this species and our earlier-described evergreen taxa from the same Siwalik time (Mio-Pleistocene), suggest the existence of tropical, warm and humid climatic conditions during the period of deposition. The leaf and fruit remains are here described as new species, namely Mastixia asiatica Khan, Bera M et Bera S, sp. nov. and Mastixia siwalika Khan, Bera M et Bera S, sp. nov. respectively. This report documents the first fossil record of Mastixia leaf remains using both macro and micromorphological characters. We also review the historical phytogeography, and highlight the phytogeographic implication of, the mastixioids
Quantum Phase Transition from a Spin-liquid State to a Spin-glass State in the Quasi-1D Spin-1 System Sr1-xCaxNi2V2O8
We report a quantum phase transition from a spin-liquid state to a spin-glass
state in the quasi-one dimensional (1D) spin-1 system Sr1-xCaxNi2V2O8, induced
by a small amount of Ca-substitution at Sr site. The ground state of the parent
compound (x = 0) is found to be a spin-liquid type with a finite energy gap of
26.6 K between singlet ground state and triplet excited state. Both
dc-magnetization and ac-susceptibility studies on the highest Ca-substituted
compound (x = 0.05) indicate a spin-glass type magnetic ground state. With
increasing Ca-concentration, the spin-glass ordering temperature increases from
4.5 K (for the x = 0.015 compound) to 6.25 K (for the x = 0.05 compound). The
observed results are discussed in the light of the earlier experimental reports
and the theoretical predictions for a quasi-1D spin-1 system.Comment: 26 pages, 8 figures, 3 table
Enhanced Raman and photoluminescence response in monolayer MoS due to laser healing of defects
Bound quasiparticles, negatively charged trions and neutral excitons, are
associated with the direct optical transitions at the K-points of the Brillouin
zone for monolayer MoS. The change in the carrier concentration,
surrounding dielectric constant and defect concentration can modulate the
photoluminescence and Raman spectra. Here we show that exposing the monolayer
MoS in air to a modest laser intensity for a brief period of time enhances
simultaneously the photoluminescence (PL) intensity associated with both trions
and excitons, together with 3 to 5 times increase of the Raman intensity
of first and second order modes. The simultaneous increase of PL from trions
and excitons cannot be understood based only on known-scenario of depletion of
electron concentration in MoS by adsorption of O and HO molecules.
This is explained by laser induced healing of defect states resulting in
reduction of non-radiative Auger processes. This laser healing is corroborated
by an observed increase of intensity of both the first order and second order
2LA(M) Raman modes by a factor of 3 to 5. The A mode hardens by
1.4 cm whereas the E mode softens by 1 cm.
The second order 2LA(M) Raman mode at 440 cm shows an increase in
wavenumber by 8 cm with laser exposure. These changes are a
combined effect of change in electron concentrations and oxygen-induced lattice
displacements.Comment: 15 pages, 5 figures, Journal of Raman Spectroscopy, 201
Quantum Size Effects in the Atomistic Structure of Armchair-Nanoribbons
Quantum size effects in armchair graphene nano-ribbons (AGNR) with hydrogen
termination are investigated via density functional theory (DFT) in Kohn-Sham
formulation. "Selection rules" will be formulated, that allow to extract
(approximately) the electronic structure of the AGNR bands starting from the
four graphene dispersion sheets. In analogy with the case of carbon nanotubes,
a threefold periodicity of the excitation gap with the ribbon width (N, number
of carbon atoms per carbon slice) is predicted that is confirmed by ab initio
results. While traditionally such a periodicity would be observed in electronic
response experiments, the DFT analysis presented here shows that it can also be
seen in the ribbon geometry: the length of a ribbon with L slices approaches
the limiting value for a very large width 1 << N (keeping the aspect ratio
small N << L) with 1/N-oscillations that display the electronic selection
rules. The oscillation amplitude is so strong, that the asymptotic behavior is
non-monotonous, i.e., wider ribbons exhibit a stronger elongation than more
narrow ones.Comment: 5 pages, 6 figure
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