Magnetic structure and spin dynamics of quasi-one-dimensional spin-chain antiferromagnet BaCo2V2O8

We report a neutron diffraction and muon spin relaxation muSR study of static and dynamical magnetic properties of BaCo2V2O8, a quasi-one-dimensional spin-chain system. A proposed model for the antiferromagnetic structure includes: a propagation vector k_AF = (0, 0, 1), independent of external magnetic fields for fields below a critical value H_c(T). The ordered moments, of 2.18 \mu_B per Co ion, are aligned along the crystallographic c-axis. Within the screw chains, along the c axis, the moments are arranged antiferromagnetically. In the basal planes the spins are arranged ferromagnetically (forming zig-zags paths) along one of the axis and antiferromagnetically along the other. The temperature dependence of the sub-lattice magnetization is consistent with the expectations of the 3D Ising model. A similar behavior is observed for the internal static fields at different muon stopping sites. Muon time spectra measured at weak longitudinal fields and temperatures much higher than T_N can be well described using a single muon site with an exponential muon spin relaxation that gradually changes into an stretched exponential on approaching T_N. The temperature-induced changes of the relaxation suggest that the Co fluctuations dramatically slow down and the system becomes less homogeneous as it approaches the antiferromagnetic state.Comment: 7 pages, 9 figure

Zn-induced wipeout effect on Cu NQR spectra in La2−x_{2-x}Srx_xCu1−y_{1-y}Zny_yO4_4

We report a systematic study of Zn-substitution effect on Cu NQR spectrum for high $T_c$ superconductors La$_{2-x}$Sr$_x$Cu$_{1-y}$Zn$_y$O$_4$ from carrier-underdoped to -overdoped regimes (polycrystalline samples, $x$ =0.10, 0.15, and 0.20). We observed no appreciable wipeout effect for the overdoped samples, a gradual and partial wipeout effect below about 80 K for the optimally doped ones, and very abrupt and full wipeout effect below about 40 K for the underdoped ones. The wipeout effect indicates a highly enhanced spectral weight of Cu spin fluctuations at a low frequency. We associate the wipeout effect with a Zn-induced local magnetism far above 40 K and with a localization effect below 40 K.Comment: 2 pages, 3 figures, accepted for publication in Physica C (LT23, Hiroshima 2002

Antifungal Activities of The Extracts From Some Tropical and Temperate Woods

Empat flavonoids (Sciadopitysin) (1), Ginkgetin (2), Ishorhamnetin (3), dan Quercetin (4), dua Texanes (Taxinine) (5) dan Taxol (6), dan Phenylisoserine methyl ester (7) dapat diisolasi dari Taxus cuspadata var. nana. Senyawa (7) juga dapat diisolasi dari Taxus chinensis. Senyawa (7) dapat diisolasi untuk pertama kalinya masing-masing dari T. cuspidata var. nana dan T. chinensis.Aktivitas antifungus senyawa-senyawa yang terisolasi tersebut dan tujuh turunan taxinine terhadap tujuh fungi patogenik tanaman (Cochliobolus miyabeanus, Alternaria kikuchiana, Giberella fujikuroi, Cladosporium cucumenium, Fusarium oxysporum, Colletotrichum fragariae dan Corynespora cassiicola) telah diukur. Taxinine dan tujuh turunan taxinine menunjukkan adanya aktivitas antifungus terhadap lima jenis fungi. Diantara tiga jenis kayu tropis (Amboyna,Kempas,Angsana),kayu Amboyna menunjukkan aktivitas antifungus terhadap empat fungi dari basidiomycetes (Le,Pp,G3,dan PLI). Selanjutnya, ekstrak methanolic dan fraksi larutnya nhexane dan ethyl ether dari kayu Amboyna menunjukkan aktivitas antifungus terhadap fungus Pp.Kata kunci: Antifungi, flavonoid, Texanes, phenylisoserine methyl ester, kayu tropis dan temperate, fungi patogenik tanaman, basidiomycetes

Mechanism of strong quenching of photosystem II chlorophyll fluorescence under drought stress in a lichen, Physciella melanchla, studied by subpicosecond fluorescence spectroscopy

AbstractThe mechanism of the severe quenching of chlorophyll (Chl) fluorescence under drought stress was studied in a lichen Physciella melanchla, which contains a photobiont green alga, Trebouxia sp., using a streak camera and a reflection-mode fluorescence up-conversion system. We detected a large 0.31 ps rise of fluorescence at 715 and 740 nm in the dry lichen suggesting the rapid energy influx to the 715–740 nm bands from the shorter-wavelength Chls with a small contribution from the internal conversion from Soret bands. The fluorescence, then, decayed with time constants of 23 and 112 ps, suggesting the rapid dissipation into heat through the quencher. The result confirms the accelerated 40 ps decay of fluorescence reported in another lichen (Veerman et al., 2007 [36]) and gives a direct evidence for the rapid energy transfer from bulk Chls to the longer-wavelength quencher. We simulated the entire PS II fluorescence kinetics by a global analysis and estimated the 20.2 ns−1 or 55.0 ns−1 energy transfer rate to the quencher that is connected either to the LHC II or to the PS II core antenna. The strong quenching with the 3–12 times higher rate compared to the reported NPQ rate, suggests the operation of a new type of quenching, such as the extreme case of Chl-aggregation in LHCII or a new type of quenching in PS II core antenna in dry lichens

Antifungal Activities of The Extracts From Some Tropical and Temperate Woods

Empat flavonoids (Sciadopitysin) (1), Ginkgetin (2), Ishorhamnetin (3), dan Quercetin (4), dua Texanes (Taxinine) (5) dan Taxol (6), dan Phenylisoserine methyl ester (7) dapat diisolasi dari Taxus cuspadata var. nana. Senyawa (7) juga dapat diisolasi dari Taxus chinensis. Senyawa (7) dapat diisolasi untuk pertama kalinya masing-masing dari T. cuspidata var. nana dan T. chinensis.Aktivitas antifungus senyawa-senyawa yang terisolasi tersebut dan tujuh turunan taxinine terhadap tujuh fungi patogenik tanaman (Cochliobolus miyabeanus, Alternaria kikuchiana, Giberella fujikuroi, Cladosporium cucumenium, Fusarium oxysporum, Colletotrichum fragariae dan Corynespora cassiicola) telah diukur. Taxinine dan tujuh turunan taxinine menunjukkan adanya aktivitas antifungus terhadap lima jenis fungi. Diantara tiga jenis kayu tropis (Amboyna,Kempas,Angsana),kayu Amboyna menunjukkan aktivitas antifungus terhadap empat fungi dari basidiomycetes (Le,Pp,G3,dan PLI). Selanjutnya, ekstrak methanolic dan fraksi larutnya nhexane dan ethyl ether dari kayu Amboyna menunjukkan aktivitas antifungus terhadap fungus Pp.Kata kunci: Antifungi, flavonoid, Texanes, phenylisoserine methyl ester, kayu tropis dan temperate, fungi patogenik tanaman, basidiomycetes