9 research outputs found
Microwave dielectric study of spin-Peierls and charge ordering transitions in (TMTTF)PF salts
We report a study of the 16.5 GHz dielectric function of hydrogenated and
deuterated organic salts (TMTTF)PF. The temperature behavior of the
dielectric function is consistent with short-range polar order whose relaxation
time decreases rapidly below the charge ordering temperature. If this
transition has more a relaxor character in the hydrogenated salt, charge
ordering is strengthened in the deuterated one where the transition temperature
has increased by more than thirty percent. Anomalies in the dielectric function
are also observed in the spin-Peierls ground state revealing some intricate
lattice effects in a temperature range where both phases coexist. The variation
of the spin-Peierls ordering temperature under magnetic field appears to follow
a mean-field prediction despite the presence of spin-Peierls fluctuations over
a very wide temperature range in the charge ordered state of these salts.Comment: 7 pages, 6 figure
A Comparison of rpoB and 16S rRNA as Markers in Pyrosequencing Studies of Bacterial Diversity
Background: The 16S rRNA gene is the gold standard in molecular surveys of bacterial and archaeal diversity, but it has the disadvantages that it is often multiple-copy, has little resolution below the species level and cannot be readily interpreted in an evolutionary framework. We compared the 16S rRNA marker with the single-copy, protein-coding rpoB marker by amplifying and sequencing both from a single soil sample. Because the higher genetic resolution of the rpoB gene prohibits its use as a universal marker, we employed consensus-degenerate primers targeting the Proteobacteria.
<p/>Methodology/Principal Findings: Pyrosequencing can be problematic because of the poor resolution of homopolymer runs. As these erroneous runs disrupt the reading frame of protein-coding sequences, removal of sequences containing nonsense mutations was found to be a valuable filter in addition to flowgram-based denoising. Although both markers gave similar estimates of total diversity, the rpoB marker revealed more species, requiring an order of magnitude fewer reads to obtain 90% of the true diversity. The application of population genetic methods was demonstrated on a particularly abundant sequence cluster.
<p/>Conclusions/Significance: The rpoB marker can be a complement to the 16S rRNA marker for high throughput microbial diversity studies focusing on specific taxonomic groups. Additional error filtering is possible and tests for recombination or selection can be employed
High magnetic field NMR investigation of the spin density wave phase of TMTSFPF
We report proton NMR measurements of the effect of very high magnetic fields up to 44.7 T
(1.9 GHz) on the spin density wave (SDW) transition of the organic conductor TMTSFPF.
Up to 1.8 GHz, no effect of critical slowing close to the transition is seen on the proton
relaxation rate (1/T), which is determined by the SDW fluctuations associated with the phase
transition at the NMR frequency. Thus, the correlation time for such fluctuations is less than
s. A possible explanation for the absence of longer correlation times is that the transition
is weakly first order, so that the full critical divergence is never achieved. The measurements
also show a dependence of the transition temperature on the orientation of the magnetic field
and a quadratic dependence on its magnitude that agrees with earlier transport measurements
at lower fields. The UCLA part of this work was supported by NSF Grant DMR-0072524