811 research outputs found
Charge order from structured coupling in VSe<sub>2</sub>
Charge order -- ubiquitous among correlated materials -- is customarily described purely as an instability of the electronic structure. However, the resulting theoretical predictions often do not match high-resolution experimental data. A pertinent case is 1T-VSe2, whose single-band Fermi surface and weak-coupling nature make it qualitatively similar to the Peierls model underlying the traditional approach. Despite this, its Fermi surface is poorly nested, the thermal evolution of its charge density wave (CDW) ordering vectors displays an unexpected jump, and the CDW gap itself evades detection in direct probes of the electronic structure. We demonstrate that the thermal variation of the CDW vectors is naturally reproduced by the electronic susceptibility when incorporating a structured, momentum-dependent electron-phonon coupling, while the evasive CDW gap presents itself as a localized suppression of spectral weight centered above the Fermi level. Our results showcase the general utility of incorporating a structured coupling in the description of charge ordered materials, including those that appear unconventional
Superconductivity and hybrid soft modes in TiSe
The competition between superconductivity and other ground states of solids
is one of the challenging topics in condensed matter physics. Apart from
high-temperature superconductors [1,2] this interplay also plays a central role
in the layered transition-metal dichalcogenides, where superconductivity is
stabilized by suppressing charge-density-wave order to zero temperature by
intercalation [3] or applied pressure [4-7]. 1T-TiSe forms a prime example,
featuring superconducting domes on intercalation as well as under applied
pressure. Here, we present high energy-resolution inelastic x-ray scattering
measurements of the CDW soft phonon mode in intercalated CuTiSe and
pressurized 1T-TiSe along with detailed ab-initio calculations for the
lattice dynamical properties and phonon-mediated superconductivity. We find
that the intercalation-induced superconductivity can be explained by a solely
phonon-mediated pairing mechanism, while this is not possible for the
superconducting phase under pressure. We argue that a hybridization of phonon
and exciton modes in the pairing mechanism is necessary to explain the full
observed temperature-pressure-intercalation phase diagram. These results
indicate that 1T-TiSe under pressure is close to the elusive state of the
excitonic insulator
Chemical ecology of antibiotic production by actinomycetes
Actinomycetes are a diverse family of filamentous bacteria that produce a plethora of natural products relevant for agriculture, biotechnology and medicine, including the majority of the antibiotics we use in the clinic. Rather than as free-living bacteria, many actinomycetes have evolved to live in symbiosis with among others plants, fungi, insects and sponges. As a common theme, these organisms profit from the natural products and enzymes produced by the actinomycetes, for example, for protection against pathogenic microbes, for growth promotion or for the degradation of complex natural polymers such as lignocellulose. At the same time, the actinomycetes benefit from the resources of the hosts they interact with. Evidence is accumulating that these interactions control the expression of biosynthetic gene clusters and have played a major role in the evolution of the high chemical diversity of actinomycete-produced secondary metabolites. Many of the biosynthetic gene clusters for antibiotics are poorly expressed under laboratory conditions, but they are likely expressed in response to host-specific demands. Here, we review the environmental triggers and cues that control natural product formation by actinomycetes and provide pointers as to how these insights may be harnessed for drug discovery
An Alternative Interpretation of Recent ARPES Measurements on TiSe2
Recently there has been a renewed interest in the charge density wave
transition of TiSe2, fuelled by the possibility that this transition may be
driven by the formation of an excitonic insulator or even an excitonic
condensate. We show here that the recent ARPES measurements on TiSe2 can also
be interpreted in terms of an alternative scenario, in which the transition is
due to a combination of Jahn-Teller effects and exciton formation. The hybrid
exciton-phonons which cause the CDW formation interpolate between a purely
structural and a purely electronic type of transition. Above the transition
temperature, the electron-phonon coupling becomes ineffective but a finite
mean-field density of excitons remains and gives rise to the observed diffuse
ARPES signals.Comment: 4 pages, 2 figure
Pressure tuning of competing magnetic interactions in intermetallic CeFe_2
We use high-pressure magnetic x-ray diffraction and numerical simulation to determine the low-temperature magnetic phase diagram of stoichiometric CeFe_2. Near 1.5 GPa we find a transition from ferromagnetism to antiferromagnetism, accompanied by a rhombohedral distortion of the cubic Laves crystal lattice. By comparing pressure and chemical substitution we find that the phase transition is controlled by a shift of magnetic frustration from the Ce-Ce to the Fe-Fe sublattice. Notably the dominant Ce-Fe magnetic interaction, which sets the temperature scale for the onset of long-range order, remains satisfied throughout the phase diagram but does not determine the magnetic ground state. Our results illustrate the complexity of a system with multiple competing magnetic energy scales and lead to a general model for magnetism in cubic Laves phase intermetallic compounds
Resolving magnetic frustration in a pyrochlore lattice
CeFe is a geometrically frustrated ferromagnet that lies close to an
instability at which a subtle change in the lattice symmetry couples to a
transition to antiferromagnetism. We use x-ray diffraction, diamond-anvil-cell
techniques, and numerical simulation to identify the ground states and to
quantitatively illustrate effects of competing magnetic energy scales and
geometrical frustration on the magnetic phase diagram. Comparison of phase
transitions under both chemical substitution and applied pressure suggests a
general solution to the physics of pyrochlore rare earth inter-metallic
magnets.Comment: 5 pages, 4 figure
Occurrence, Fate, and Related Health Risks of PFAS in Raw and Produced Drinking Water
This study investigates
human exposure to per- and polyfluoroalkyl
substances (PFAS) via drinking water and evaluates human health risks.
An analytical method for 56 target PFAS, including ultrashort-chain
(C2–C3) and branched isomers, was developed. The limit of detection
(LOD) ranged from 0.009 to 0.1 ng/L, except for trifluoroacetic-acid
and perfluoropropanoic-acid with higher LODs of 35 and 0.24 ng/L,
respectively. The method was applied to raw and produced drinking
water from 18 Dutch locations, including groundwater or surface water
as source, and applied various treatment processes. Ultrashort-chain
(300 to 1100 ng/L) followed by the group of perfluoroalkyl-carboxylic-acids
(PFCA, ≥C4) (0.4 to 95.1 ng/L) were dominant. PFCA and perfluoroalkyl-sulfonic-acid
(≥C4), including precursors, showed significantly higher levels
in drinking water produced from surface water. However, no significant
difference was found for ultrashort PFAS, indicating the need for
groundwater protection. Negative removal of PFAS occasionally observed
for advanced treatment indicates desorption and/or degradation of
precursors. The proportion of branched isomers was higher in raw and
produced drinking water as compared to industrial production. Drinking
water produced from surface water, except for a few locations, exceed
non-binding provisional guideline values proposed; however, all produced
drinking waters met the recent soon-to-be binding drinking-water-directive
requirements
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