1,185 research outputs found
Temperature - pressure phase diagram of CeCoSi: Pressure induced high-temperature phase
We have studied the temperature-pressure phase diagram of CeCoSi by
electrical-resistivity experiments under pressure. Our measurements revealed a
very unusual phase diagram. While at low pressures no dramatic changes and only
a slight shift of the Ne\'{e}l temperature ( K) are observed,
at about 1.45 GPa a sharp and large anomaly, indicative of the opening of a
spin-density-wave (SDW) gap, appears at a comparatively high temperature K. With further increasing pressure shifts rapidly to low
temperatures and disappears at about 2.15 GPa, likely continuously in a quantum
critical point, but without evidence for superconductivity. Even more
surprisingly, we observed a clear shift of to higher temperatures upon
applying a magnetic field. We discuss two possible origins for , either
magnetic ordering of Co or a meta-orbital type of transition of Ce.Comment: 6 pages, 5 figure
Turing Instability in a Boundary-fed System
The formation of localized structures in the chlorine dioxide-idodine-malonic
acid (CDIMA) reaction-diffusion system is investigated numerically using a
realistic model of this system. We analyze the one-dimensional patterns formed
along the gradients imposed by boundary feeds, and study their linear stability
to symmetry-breaking perturbations (Turing instability) in the plane transverse
to these gradients. We establish that an often-invoked simple local linear
analysis which neglects longitudinal diffusion is inappropriate for predicting
the linear stability of these patterns. Using a fully nonuniform analysis, we
investigate the structure of the patterns formed along the gradients and their
stability to transverse Turing pattern formation as a function of the values of
two control parameters: the malonic acid feed concentration and the size of the
reactor in the dimension along the gradients. The results from this
investigation are compared with existing experiments.Comment: 41 pages, 18 figures, to be published in Physical Review
Nonlinearity arising from noncooperative transcription factor binding enhances negative feedback and promotes genetic oscillations
We study the effects of multiple binding sites in the promoter of a genetic
oscillator. We evaluate the regulatory function of a promoter with multiple
binding sites in the absence of cooperative binding, and consider different
hypotheses for how the number of bound repressors affects transcription rate.
Effective Hill exponents of the resulting regulatory functions reveal an
increase in the nonlinearity of the feedback with the number of binding sites.
We identify optimal configurations that maximize the nonlinearity of the
feedback. We use a generic model of a biochemical oscillator to show that this
increased nonlinearity is reflected in enhanced oscillations, with larger
amplitudes over wider oscillatory ranges. Although the study is motivated by
genetic oscillations in the zebrafish segmentation clock, our findings may
reveal a general principle for gene regulation.Comment: 11 pages, 8 figure
Avoided ferromagnetic quantum critical point in CeRuPO
CeRuPO is a rare example of a ferromagnetic (FM) Kondo-lattice system.
External pressure suppresses the ordering temperature to zero at about
GPa. Our ac-susceptibility and electrical-resistivity
investigations evidence that the type of magnetic ordering changes from FM to
antiferromagnetic (AFM) at about GPa. Studies in applied
magnetic fields suggest that ferromagnetic and antiferromagnetic correlations
compete for the ground state at , but finally the AFM correlations win.
The change in the magnetic ground-state properties is closely related to the
pressure evolution of the crystalline-electric-field level (CEF) scheme and the
magnetic Ruderman-Kittel-Kasuya-Yosida (RKKY) exchange interaction. The
N\'{e}el temperature disappears abruptly in a first-order-like fashion at
, hinting at the absence of a quantum critical point. This is consistent
with the low-temperature transport properties exhibiting Landau-Fermi-liquid
(LFL) behavior in the whole investigated pressure range up to 7.5 GPa.Comment: 12 figure
Temperature - pressure phase diagram of the superconducting iron pnictide LiFeP
Electrical-resistivity and magnetic-susceptibility measurements under
hydrostatic pressure up to p = 2.75 GPa have been performed on superconducting
LiFeP. A broad superconducting (SC) region exists in the temperature - pressure
(T-p) phase diagram. No indications for a spin-density-wave transition have
been found, but an enhanced resistivity coefficient at low pressures hints at
the presence of magnetic fluctuations. Our results show that the
superconducting state in LiFeP is more robust than in the isostructural and
isoelectronic LiFeAs. We suggest that this finding is related to the nearly
regular [FeP_4] tetrahedron in LiFeP.Comment: 4 pages, 4 figure
Pressure-induced change of the pairing symmetry in superconducting CeCu2Si2
Low-temperature (T) heat-capacity measurements under hydrostatic pressure of
up to p=2.1 GPa have been performed on single-crystalline CeCu2Si2. A broad
superconducting (SC) region exists in the T-p phase diagram. In the
low-pressure region antiferromagnetic spin fluctuations and in the
high-pressure region valence fluctuations had previously been proposed to
mediate Cooper pairing. We could identify these two distinct SC regions. We
found different thermodynamic properties of the SC phase in both regions,
supporting the proposal that different mechanisms might be implied in the
formation of superconductivity.Comment: 4 pages, 5 figure
The invasive colonial ascidian Didemnum vexillum on Georges Bank - Ecological effects and genetic identification
Since the discovery of the invasive colonial tunicate Didemnum vexillum Kott, 2002 on Georges Bank in 2002, research has focused on investigating the spread of the tunicate invasion, evaluating its potential impact on the benthic community, identifying it to species level, and determining its region of origin. The percent cover of Didemnum vexillum, measured from bottom photographs, ranges from 0-100% in individual photos and between 0-79% when averaged within photo transects. Individual photos represent an area of the seabed measuring ~ 0.39 m2 while photo transects range from ~ 700-1000 meters in length. Hydroids are the second most abundant epifaunal taxon. The macrofauna identified in bottom photo analysis comprises 21 different taxa, of which burrowing and non-burrowing anemones are the most numerous. Our detailed analysis of bottom photographs suggests that Didemnum vexillum is able to out-compete other epifaunal and macrofaunal taxa. An Analysis of Similarity (ANOSIM) test on macrofauna abundance data collected with a Naturalist dredge from 1994 to 2006, indicates that Didemnum vexillum has had a significant impact on the species composition of the benthic community. The abundance of two polychaete species, Nereis zonata Malmgren, 1867 and Harmothoe extenuata Grube, 1840, increased significantly in infested areas compared with uninfested areas, according to two-way Analysis of Variance (ANOVA). We found four distinct nucleotide sequences of the 18s rDNA gene among 17 samples of Didemnum species, three from Georges Bank and one from New Zealand. Two of the Georges Bank sequences were identified as Didemnum albidum Verrill, 1871, a species native to the northeast United States. The third sequence represents the invasive Didemnum vexillum from Georges Bank, and the fourth sequence an undescribed species from New Zealand (not D. vexillum)
Hydrostatic and chemical pressure tuning of CeFeAs1−xPxO single crystals
We carried out a combined P-substitution and hydrostatic pressure study on
CeFeAs_1-xP_xO single crystals in order to investigate the peculiar
relationship of the local moment magnetism of Ce, the ordering of itinerant Fe
moments, and their connection with the occurrence of superconductivity. Our
results evidence a close relationship between the weakening of Fe magnetism and
the change from antiferromagnetic to ferromagnetic ordering of Ce moments at
p*=1.95 GPa in CeFeAs_0.78P_0.22O. The absence of superconductivity in
CeFeAs_0.78P_0.22O and the presence of a narrow and strongly pressure sensitive
superconducting phase in CeFeAs_0.70P_0.30O and CeFeAs_0.65P_0.35O indicate the
detrimental effect of the Ce magnetism on superconductivity in P-substituted
CeFeAsO.Comment: 5 pages, 4 figure
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