1,648 research outputs found
Bi2Te_xSe_y series studied by resistivity and thermopower
We study the detailed temperature and composition dependence of the
resistivity, , and thermopower, , for a series of layered
bismuth chalcogenides BiTeSe, and report the stoichiometry
dependence of the optical band gap. In the resistivity of the most compensated
member, BiTeSe, we find a low-temperature plateau whose
onset temperature correlates with the high-temperature activation energy. For
the whole series can be described by a simple model for an extrinsic
semiconductor. By substituting Se for Te, the Fermi level is tuned from the
valence band into the conduction band. The maximum values of , bulk band
gap as well the activation energy in the resistivity are found for
Energy-dependent spatial texturing of the charge order in -CuTiSe
We report a detailed study of the microscopic effects of Cu intercalation on
the charge density wave (CDW) in 1\textit{T}-CuTiSe. Scanning tunneling
microscopy and spectroscopy (STM/STS) reveal a unique, Cu driven spatial
texturing of the charge ordered phase, with the appearance of energy dependent
CDW patches and sharp -phase shift domain walls (DWs). The energy and
doping dependencies of the patchwork are directly linked to the inhomogeneous
potential landscape due to the Cu intercalants. They imply a CDW gap with
unusual features, including a large amplitude, the opening below the Fermi
level and a shift to higher binding energy with electron doping. Unlike the
patchwork, the DWs occur independently of the intercalated Cu
distribution. They remain atomically sharp throughout the investigated phase
diagram and occur both in superconducting and non-superconducting specimen.
These results provide unique atomic-scale insight on the CDW ground state,
questioning the existence of incommensurate CDW domain walls and contributing
to understand its formation mechanism and interplay with superconductivity
Direct role of structural dynamics in electron-lattice coupling of superconducting cuprates
The mechanism of electron pairing in high-temperature superconductors is still the subject of intense debate. Here, we provide direct evidence of the role of structural dynamics, with selective atomic motions (buckling of copper–oxygen planes), in the anisotropic electron-lattice coupling. The transient structures were determined using time-resolved electron diffraction, following carrier excitation with polarized femtosecond heating pulses, and examined for different dopings and temperatures. The deformation amplitude reaches 0.5% of the c axis value of 30 Å when the light polarization is in the direction of the copper–oxygen bond, but its decay slows down at 45°. These findings suggest a selective dynamical lattice involvement with the anisotropic electron–phonon coupling being on a time scale (1–3.5 ps depending on direction) of the same order of magnitude as that of the spin exchange of electron pairing in the high-temperature superconducting phase
Dimensional cross-over of the charge density wave order parameter in thin exfoliated 1T-VSe
The capability to isolate one to few unit-cell thin layers from the bulk
matrix of layered compounds opens fascinating prospects to engineer novel
electronic phases. However, a comprehensive study of the thickness dependence
and of potential extrinsic effects are paramount to harness the electronic
properties of such atomic foils. One striking example is the charge density
wave (CDW) transition temperature in layered dichalcogenides whose thickness
dependence remains unclear in the ultrathin limit. Here we present a detailed
study of the thickness and temperature dependences of the CDW in VSe by
scanning tunnelling microscopy (STM). We show that mapping the real-space CDW
periodicity over a broad thickness range unique to STM provides essential
insight. We introduce a robust derivation of the local order parameter and
transition temperature based on the real space charge modulation amplitude.
Both quantities exhibit a striking non-monotonic thickness dependence that we
explain in terms of a 3D to 2D dimensional crossover in the FS topology. This
finding highlights thickness as a true tuning parameter of the electronic
ground state and reconciles seemingly contradicting thickness dependencies
determined in independent transport studies
MiNDSTEp differential photometry of the gravitationally lensed quasars WFI 2033-4723 and HE 0047-1756: microlensing and a new time delay
Aims. We present V and R photometry of the gravitationally lensed quasars WFI 2033-4723 and HE 0047-1756. The data were taken by the MiNDSTEp collaboration with the 1.54 m Danish telescope at the ESO La Silla observatory from 2008 to 2012.
Methods. Differential photometry has been carried out using the image subtraction method as implemented in the HOTPAnTS package, additionally using GALFIT for quasar photometry.
Results. The quasar WFI 2033-4723 showed brightness variations of order 0.5 mag in V and R during the campaign. The two lensed components of quasar HE 0047-1756 varied by 0.2–0.3 mag within five years. We provide, for the first time, an estimate of the time delay of component B with respect to A of Δt = (7.6 ± 1.8) days for this object. We also find evidence for a secular evolution of the magnitude difference between components A and B in both filters, which we explain as due to a long-duration microlensing event. Finally we find that both quasars WFI 2033-4723 and HE 0047-1756 become bluer when brighter, which is consistent with previous studies
Holographic imaging of the complex charge density wave order parameter
The charge density wave (CDW) in solids is a collective ground state
combining lattice distortions and charge ordering. It is defined by a complex
order parameter with an amplitude and a phase. The amplitude and wavelength of
the charge modulation are readily accessible to experiment. However, accurate
measurements of the corresponding phase are significantly more challenging.
Here we combine reciprocal and real space information to map the full complex
order parameter based on topographic scanning tunneling microscopy (STM)
images. Our technique overcomes limitations of earlier Fourier space based
techniques to achieve distinct amplitude and phase images with high spatial
resolution. Applying this analysis to transition metal dichalcogenides provides
striking evidence that their CDWs consist of three individual charge
modulations whose ordering vectors are connected by the fundamental rotational
symmetry of the crystalline lattice. Spatial variations in the relative phases
of these three modulations account for the different contrasts often observed
in STM topographic images. Phase images further reveal topological defects and
discommensurations, a singularity predicted by theory for a nearly commensurate
CDW. Such precise real space mapping of the complex order parameter provides a
powerful tool for a deeper understanding of the CDW ground state whose
formation mechanisms remain largely unclear
Preeminent role of the Van Hove singularity in the strong-coupling analysis of scanning tunneling spectroscopy for two-dimensional cuprates
In two dimensions the non-interacting density of states displays a Van Hove
singularity (VHS) which introduces an intrinsic electron-hole asymmetry, absent
in three dimensions. We show that due to this VHS the strong-coupling analysis
of tunneling spectra in high- superconductors must be reconsidered. Based
on a microscopic model which reproduces the experimental data with great
accuracy, we elucidate the peculiar role played by the VHS in shaping the
tunneling spectra, and show that more conventional analyses of strong-coupling
effects can lead to severe errors.Comment: 5 pages, 4 figure
A multi-adaptive framework for the crop choice in paludicultural cropping systems
The conventional cultivation of drained peatland causes peat oxidation, soil subsidence,
nutrient loss, increasing greenhouse gas emissions and biodiversity reduction. Paludiculture
has been identified as an alternative management strategy consisting in the cultivation of
biomass on wet and rewetted peatlands. This strategy can save these habitats and restore the
ecosystem services provided by the peatlands both on the local and global scale.
This paper illustrates the most important features to optimise the crop choice phase, which is
the crucial point for the success of paludiculture systems.
A multi-adaptive framework was proposed. It was based on four points that should be
checked to identify suitable crops for paludicultural cropping system: biological traits,
biomass production, attitude to cultivation and biomass quality. The main agronomic
implications were explored with the help of some results from a plurennial open-field
experimentation carried out in a paludicultural system set up in the Massaciuccoli Lake Basin
(Tuscany, Italy) and a complete example of the method application was provided. The tested
crops were Arundo donax L., Miscanthus × giganteus Greef et Deuter, Phragmites australis
L., Populus × canadensis Moench. and Salix alba L. The results showed a different level of
suitability ascribable to the different plant species proving that the proposed framework can
discriminate the behaviour of tested crops. Phragmites australis L.was the most suitable crop
whereas Populus × canadensis Moench and Miscanthus × giganteus Greef et Deuter (in the
case of biogas conversion) occupied the last positions in the ranking
Rewetting in Mediterranean reclaimed peaty soils and its potential for phyto-treatment use
A pilot experimental field combining rewetting of reclaimed peaty soils and water phyto-treatment was set up in the Massaciuccoli Lake basin (Tuscany, Italy) to reduce the water eutrophication and peat degradation caused by almost a century of drainage-based agricultural use.
In this paper, we investigated the restoration process occurring consequently to the conversion of a drained area in a natural wetland system (NWS) (the partial top soil removal, the realization of a perimeter levee to contain the waters, the rewetting with the drainage waters coming from the of surrounding cultivated areas) and the capability of the spontaneous vegetation to catch nutrients acting as a vegetation filter. To follow the restoration process over time (2012e2016), we used a mixed approach merging phytosociological surveys with ortophotos taken by an Unmanned Aerial Vehicle (UAV). During the last year of observation (2016), we performed destructive sampling on the most widespread plant communities
in the area (Phragmites australis and Myriophyllum aquaticum community) to quantify the biomass production and the uptake of nitrogen and phosphorus.
Stands of Phragmites australis (Cav.) Trin. ex Steud. yielded more than Myriophyllum aquaticum (Vell.) Verdc. (4.94 kg m-2 vs 1.08 kg m-2). M. aquaticum showed higher nutrient contents (2.04% of N and 0.35% of P), however P. australis was able to take up more nutrients within the NWS because of its larger cover
and productivity. In the perspective of maximizing the plant development and consequently the amount of nutrients extracted from treated waters, the authors suggest 4-5 year-long-harvesting turns, better occurring in spring-summer
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