1,547 research outputs found
Polar Dynamics at the Jahn-Teller Transition in Ferroelectric GaV4S8
We present a dielectric spectroscopy study of the polar dynamics linked to
the orbitally driven ferroelectric transition in the skyrmion host GaV4S8. By
combining THz and MHz-GHz spectroscopy techniques, we succeed in detecting the
relaxational dynamics arising from coupled orbital and polar fluctuations in
this material and traced its temperature dependence in the paraelectric as well
as in the ferroelectric phase. The relaxation time significantly increases when
approaching the critical temperature from both sides of the transition. It is
natural to assume that these polar fluctuations map the orbital dynamics at the
Jahn-Teller transition. Due to the first-order character of the
orbital-ordering transition, the relaxation time shows an enormous jump of
about five orders of magnitude at the polar and structural phase transition.Comment: 5 pages, 4 figure
Structural, magnetic, electric, dielectric, and thermodynamic properties of multiferroic GeV4S8
The lacunar spinel GeV4S8 undergoes orbital and ferroelectric ordering at the
Jahn-Teller transition around 30 K and exhibits antiferromagnetic order below
about 14 K. In addition to this orbitally driven ferroelectricity, lacunar
spinels are an interesting material class, as the vanadium ions form V4
clusters representing stable molecular entities with a common electron
distribution and a well-defined level scheme of molecular states resulting in a
unique spin state per V4 molecule. Here we report detailed x-ray, magnetic
susceptibility, electrical resistivity, heat capacity, thermal expansion, and
dielectric results to characterize the structural, electric, dielectric,
magnetic, and thermodynamic properties of this interesting material, which also
exhibits strong electronic correlations. From the magnetic susceptibility, we
determine a negative Curie-Weiss temperature, indicative for antiferromagnetic
exchange and a paramagnetic moment close to a spin S = 1 of the V4 molecular
clusters. The low-temperature heat capacity provides experimental evidence for
gapped magnon excitations. From the entropy release, we conclude about strong
correlations between magnetic order and lattice distortions. In addition, the
observed anomalies at the phase transitions also indicate strong coupling
between structural and electronic degrees of freedom. Utilizing dielectric
spectroscopy, we find the onset of significant dispersion effects at the polar
Jahn-Teller transition. The dispersion becomes fully suppressed again with the
onset of spin order. In addition, the temperature dependencies of dielectric
constant and specific heat possibly indicate a sequential appearance of orbital
and polar order.Comment: 15 pages, 9 figure
Polar and magnetic order in GaV4Se8
In the present work, we provide results from specific heat, magnetic
susceptibility, dielectric constant, ac conductivity, and electrical
polarization measurements performed on the lacunar spinel GaV4Se8. With
decreasing temperature, we observe a transition from the paraelectric and
paramagnetic cubic state into a polar, probably ferroelectric state at 42 K
followed by magnetic ordering at 18 K. The polar transition is likely driven by
the Jahn-Teller effect due to the degeneracy of the V4 cluster orbitals. The
excess polarization arising in the magnetic phase indicates considerable
magnetoelectric coupling. Overall, the behavior of GaV4Se8 in many respects is
similar to that of the skyrmion host GaV4S8, exhibiting a complex interplay of
orbital, spin, lattice, and polar degrees of freedom. However, its dielectric
behavior at the polar transition markedly differs from that of the Jahn-Teller
driven ferroelectric GeV4S8, which can be ascribed to the dissimilar electronic
structure of the Ge compound.Comment: 7 pages, 6 figures. Revised version according to suggestions of
referee
Information theoretic measures for quantifying sequence-ensemble relationships of intrinsically disordered proteins
Intrinsically disordered proteins (IDPs) contribute to a multitude of functions. De novo design of IDPs should open the door to modulating functions and phenotypes controlled by these systems. Recent design efforts have focused on compositional biases and specific sequence patterns as the design features. Analysis of the impact of these designs on sequence-function relationships indicates that individual sequence/compositional parameters are insufficient for describing sequence-function relationships in IDPs. To remedy this problem, we have developed information theoretic measures for sequence-ensemble relationships (SERs) of IDPs. These measures rely on prior availability of statistically robust conformational ensembles derived from all atom simulations. We show that the measures we have developed are useful for comparing sequence-ensemble relationships even when sequence is poorly conserved. Based on our results, we propose that de novo designs of IDPs, guided by knowledge of their SERs, should provide improved insights into their sequence-ensemble-function relationships
Pumps and water lifters for rural development
CER76-77ADW-JFR-EVR61.Includes bibliographical references (pages 167-178).June 1977
Metabolic capabilities of microorganisms involved in and associated with the anaerobic oxidation of methane
In marine sediments the anaerobic oxidation of methane with sulfate as electron acceptor (AOM) is responsible for the removal of a major part of the greenhouse gas methane. AOM is performed by consortia of anaerobic methane-oxidizing archaea (ANME) and their specific partner bacteria. The physiology of these organisms is poorly understood, which is due to their slow growth with doubling times in the order of months and the phylogenetic diversity in natural and in vitro AOM enrichments. Here we study sediment-free long-term AOM enrichments that were cultivated from seep sediments sampled off the Italian Island Elba (20◦C; hereon called E20) and from hot vents of the Guaymas Basin, Gulf of California, cultivated at 37◦C (G37) or at 50◦C (G50). These enrichments were dominated by consortia of ANME-2 archaea and Seep-SRB2 partner bacteria (E20) or by ANME-1, forming consortia with Seep-SRB2 bacteria (G37) or with bacteria of the HotSeep-1 cluster (G50). We investigate lipid membrane compositions as possible factors for the different temperature affinities of the different ANME clades and show autotrophy as characteristic feature for both ANME clades and their partner bacteria. Although in the absence of additional substrates methane formation was not observed, methanogenesis from methylated substrates (methanol and methylamine) could be quickly stimulated in the E20 and the G37 enrichment. Responsible for methanogenesis are archaea from the genus Methanohalophilus and Methanococcoides, which are minor community members during AOM (1–7h of archaeal 16S rRNA gene amplicons). In the same two cultures also sulfur disproportionation could be quickly stimulated by addition of zero-valent colloidal sulfur. The isolated partner bacteria are likewise minor community members (1–9h of bacterial 16S rRNA gene amplicons), whereas the dominant partner bacteria (Seep-SRB1a, Seep-SRB2, or HotSeep-1) did not grow on elemental sulfur. Our results support a functioning of AOM as syntrophic interaction of obligate methanotrophic archaea that transfer non-molecular reducing equivalents (i.e., via direct interspecies electron transfer) to obligate sulfate-reducing partner bacteria. Additional katabolic processes in these enrichments but also in sulfate methane interfaces are likely performed by minor community members
1985 riprap tests in flood control channels
CER85-86-JFR-AS-SRA-EVR-17.Includes bibliographical references (page 42).August, 1985.Prepared for U.S. Army Corps of Engineers, Waterways Experiment Station
Structural Fluctuations in the Spin Liquid State of Tb2Ti2O7
High resolution X-ray scattering measurements on single crystal Tb2Ti2O7
reveal finite structural correlations at low temperatures. This geometrically
frustrated pyrochlore is known to exhibit a spin liquid, or cooperative
paramagnetic state, at temperatures below ~ 20 K. Parametric studies of
structural Bragg peaks appropriate to the Fdm space group of Tb2Ti2O7
reveal substantial broadening and peak intensity reduction in the temperature
regime 20 K to 300 mK. We also observe a small, anomalous lattice expansion on
cooling below a density maximum at ~ 18 K. These measurements are consistent
with the development of fluctuations above a cooperative Jahn-Teller,
cubic-tetragonal phase transition at very low temperatures.Comment: 5 pages, 4 figures, submitted for publicatio
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