We report 51 V-NMR study on a high-quality powder sample of volborthite Cu3V2O7(OH)2·2H2O, a spin-1/2 Heisenberg antiferromagnet on a distorted kagomé lattice formed by isosceles triangles. In the magnetic fields below 4.5 T, a sharp peak in the nuclear spin-lattice relaxation rate 1/T1 accompanied with line broadening revealed a magnetic transition near 1 K. The low temperature phase shows anomalies such as a Lorentzian line shape, a 1/T1 ∝ T behavior indicating dense low energy excitations, and a large spin-echo decay rate 1/T2 pointing to unusually slow fluctuations.Another magnetic phase appears above 4.5 T with less anomalous spectral shape and dynamics.PACS numbers: 75.30. Kz, 75.40.Gb The search for exotic ground states in two-dimensional (2D) spin systems with frustrated interactions has been a challenge in condensed matter physics [1]. In particular, the ground state of the spin-1/2 Heisenberg model with a nearest neighbor interaction on a kagomé lattice, a 2D network of corner-sharing equilateral triangles, is believed to show no long-range magnetic order. Theories have proposed various ground states such as spin liquids with no broken symmetry with [2] or without [3] a spingap or symmetry breaking valence-bond-crystal states [4]. Candidate materials known to date, however, depart from the ideal kagomé model in one way or another such as disorder, structural distortion, anisotropy, or longer range interactions. Volborthite Cu 3 V 2 O 7 (OH) 2 ·2H 2 O is an example, which has distorted kagomé layers formed by isosceles triangles. Consequently, it has two Cu sites and two kinds of exchange interactions as shown in the inset of Fig. 1. The magnetic susceptibility χ obeys the CurieWeiss law χ = C/(T +θ W ) above 200 K with θ W = 115 K, exhibits a broad maximum at 20 K, and approaches a finite value at the lowest temperatures, indicating absence of a spin gap. The specific heat and χ data revealed no magnetic order down to 2 K, much lower than θ W [5]. This indicates strong effects of frustration common to the geometry of the kagomé lattice, even though the difference between J and J ′ should partially lift the massive degeneracy of low energy states of the ideal kagomé model. Dynamic measurements, however, suggest a magnetic transition at a lower temperature. The nuclear spinlattice relaxation rate 1/T 1 at the V sites shows a peak at 1.4 K, below which the nuclear magnetic resonance (NMR) spectrum begins to broaden [6]. The muon spin relaxation (µSR) experiments also detected slowing down of spin fluctuations with decreasing temperature towards 1 K [7]. These dynamic anomalies coincide with the appearance of hysteresis in χ, i.e. the difference between field-cooled and zero-field-cooled magnetization, suggesting a spin-glass like state [6]. However, impurity effects corresponding to the Curie term in χ of more than 0.5 %/Cu of spin-1/2 [5,8] have been impeding proper understanding of intrinsic properties of volborthite.Recently, H. Yoshida et al. have succeeded in reducing impurities down to 0.07 % by ...

We have synthesized high-quality single crystals of volborthite, a seemingly distorted kagome antiferromagnet, and carried out high-field magnetization measurements up to 74 T and ^{51}V NMR measurements up to 30 T. An extremely wide 1/3 magnetization plateau appears above 28 T and continues over 74 T at 1.4 K, which has not been observed in previous studies using polycrystalline samples. NMR spectra reveal an incommensurate order (most likely a spin-density wave order) below 22 T and a simple spin structure in the plateau phase. Moreover, a novel intermediate phase is found between 23 and 26 T, where the magnetization varies linearly with magnetic field and the NMR spectra indicate an inhomogeneous distribution of the internal magnetic field. This sequence of phases in volborthite bears a striking similarity to those of frustrated spin chains with a ferromagnetic nearest-neighbor coupling J_{1} competing with an antiferromagnetic next-nearest-neighbor coupling J_{2}.

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Reversible 2,6-dihydroxybenzoate decarboxylase from Rhizobium sp. strain MTP-10005 belongs to a nonoxidative decarboxylase family. We have determined the structures of the following three forms of the enzyme: the native form, the complex with the true substrate (2,6-dihydroxybenzoate), and the complex with 2,3-dihydroxybenzaldehyde at 1.7-, 1.9-, and 1.7-Å resolution, respectively. The enzyme exists as a tetramer, and the subunit consists of one (␣␤) 8 and is assumed to be the catalytic base. On the basis of the geometrical consideration, substrate specificity is uncovered, and the catalytic mechanism is proposed for the novel Zn 2؉ -dependent decarboxylation.The nonoxidative decarboxylation catalyzed by decarboxylases such as 2,3-dihydroxybenzoate (1-5), 2,5-dihydroxybenzoate (6), 3,4-dihydroxybenzoate (7), 4,5-dihydroxyphthalate (8 -10), and 4-hydroxybenzoate decarboxylase (11, 12) is a poorly understood reaction. These enzymes have been reported to require neither a cofactor such as NAD ϩ , pyridoxal 5Ј-phosphate, or thiamine monophosphate nor a pyruvoyl group for catalytic activity. In studies on these enzymes, the interest is focused on their substrate specificities and catalytic mechanisms.We isolated a thermophilic reversible 2,6-dihydroxybenzoate (␥-resorcylate) decarboxylase (GRDC) 2 from Rhizobium sp. strain MTP-10005 and characterized it (13). The GRDC catalyzes the decarboxylation of 2,6-and 2,3-dihydroxybenzoate to 1,3-dihydroxybenzene (resorcinol) and 1,2-dihydroxybenzene, respectively but does not act on 2,4-, 2,5-, 3,4-, 3,5-dihydroxybenzoate, 2-hydroxybenzoate, or 3-hydroxybenzoate (Scheme 1) . 2,6-Dihydroxybenzoate is an important intermediate of medicine and agricultural or industrial chemicals (14 -16). However, it is generated together with 2,4-dihydroxybenzoate as a by-product at a rate of about half and half by traditional chemical methods (17). 2,6-Dihydroxybenzoate is expected to be produced specifically from 2,6-dihydroxybenzene by the reverse carboxyl reaction of GRDC.Recently, Ishii et al. (18) reported the purification and characterization of GRDC from Rhizobium radiobacter WU-0108, Agrobacterium tumefaciens IAM12048 (19), and Pandoraea sp. 12B-2 (20). They reported that these enzymes also catalyze the reversible decarboxylation of 2,6-dihydroxybenzoate without cofactors and have a similar substrate specificity. Orotidine 5Ј-monophosphate decarboxylase catalyzes the cofactor-independent decarboxylation. On the basis of the x-ray structure of the enzyme, it is proposed that the decarboxylation of orotidine 5Ј-monophosphate proceeds by an electrophilic substitution mechanism in which decarboxylation and carbon-carbon bond protonation by Lys 62 occur in a concerted way (21,22). To elucidate the overall and active-site structure, the substrate recognition, and the reaction mechanism, we have determined the crystal structures of GRDC from the Rhizobium sp. strain MTP-10005 in the native form, GRDC complexed with the substrate 2,6-dihydroxybenzoate, and GRDC complexed with substrate anal...

We observe trion emission from suspended carbon nanotubes where carriers are introduced electrostatically using field-effect transistor structures. The trion peak emerges below the E11 emission energy at gate voltages that coincide with the onset of bright exciton quenching. By investigating nanotubes with various chiralities, we verify that the energy separation between the bright exciton peak and the trion peak becomes smaller for larger diameter tubes. Trion binding energies that are significantly larger compared to surfactant-wrapped carbon nanotubes are obtained, and the difference is attributed to the reduced dielectric screening in suspended tubes.