We have examined the magnetic structure of the kagomé lattice antiferromagnet potassium jarosite "K jarosite: KFe 3 ͑OH͒ 6 ͑SO 4 ͒ 2 … by means of powder neutron diffraction. Extremely high degeneracy of the ground states prevents the long-range magnetic ordering at any temperature and the )ϫ) structure is predicted theoretically to be favored rather than the qϭ0 structure at Tϭ0 in a kagomé lattice Heisenberg antiferromagnet. Nevertheless, K jarosite shows long-range magnetic ordering at 65 K and the ordered magnetic structure was found to be the qϭ0 structure. In addition, although the qϭ0 structure has two degenerated states of ''positive'' and ''negative'' chirality, the observed magnetic structure contains only elemental triangles of positive chirality. We found that a weak single-ion-type anisotropy is crucial for selecting the observed magnetic structure. The long-range magnetic ordering at finite temperature in the jarosite family of compounds can be ascribed to this anisotropy. PHYSICAL

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The spin dynamics of the Heisenberg kagomé lattice antiferromagnet, potassium jarosite KFe 3 (OH) 6 (SO 4 ) 2 , have been investigated by means of NMR. The NMR spectra confirm the long-range magnetic ordering below 65 K and the qϭ0 type 120°spin structure with positive chirality in the ordered phase. Though the Heisenberg kagomé lattice antiferromagnet is considered theoretically to remain disordered down to zero temperature due to the continuous degeneracy of the ground state, the long-range magnetic ordering at the finite temperature is realized in this compound due to the weak anisotropy. The spin-lattice relaxation rate, 1/T 1 , in the ordered phase decreases sharply with lowering temperature. The experimental rate is well explained by the twomagnon process of the spin waves having an energy gap of 15 K. The temperature dependence of the sublattice magnetization also supports the existence of the spin wave. These are the first experimental evidence that the low-energy excitation in the frustrated classical kagomé lattice antiferromagnet is described by the spin wave. We calculate the spin wave in the qϭ0 type 120°spin structure with weak single-ion-type anisotropy, and discuss the characteristics of the spin fluctuations in the Heisenberg kagomé lattice antiferromagnet.

We have studied the process by which purified Oxytricha macronuclear DNA associates with itself to form large aggregates. The various macronuclear DNA molecules all have the same terminal or telomeric DNA sequences that are shown below. 5' C4A4C4A4C4--mean length----G4T4G4T4G4T4G4T4G4 G4T4G4T4G4T4G4T4G4-----2.4 kb------C4A4C4A4C4. When incubated at high concentrations, these telomeric sequences cohere with one another to form an unusual structure--one that is quite different from any DNA structure so far described. The evidence for this is the following: 1) These sequences cohere albeit slowly, in the presence of relatively high concentrations of Na+, and no other cation tested. This contrasts with the rapid coherence of complementary single-chain terminals of normal DNA (sticky ends) which occurs in the presence of any cation tested. 2) If the cohered form is transferred into buffers containing a special cation, K+, it becomes much more resistant to dissociation by heating. We estimate that K+ increases the thermal stability by 25 degrees or more. The only precedent known (to us) for a cation-specific stabilization is that seen in the quadruplex structure formed by poly I. The thermal stability of double helical macronuclear DNA depends on the cation concentration, but not the cation type. Limited treatment with specific nucleases show that the 3' and 5'-ended strands are essential for the formation of the cohering structure. Once in the cohered form, the telomeric sequences are protected from the action of nucleases. Coherence is inhibited by specific, but not by non-specific, synthetic oligomers, and by short telomeric fragments with or without their terminal single chains. We conclude that the coherence occurs by the formation of a novel condensed structure that involves the terminal nucleotides in three or four chains.

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