6,188 research outputs found
A theoretical analysis of the chemical bonding and electronic structure of graphene interacting with Group IA and Group VIIA elements
We propose a new class of materials, which can be viewed as graphene
derivatives involving Group IA or Group VIIA elements, forming what we refer to
as graphXene. We show that in several cases large band gaps can be found to
open up, whereas in other cases a semimetallic behavior is found. Formation
energies indicate that under ambient conditions, sp and mixed sp/sp
systems will form. The results presented allow us to propose that by careful
tuning of the relative concentration of the adsorbed atoms, it should be
possible to tune the band gap of graphXene to take any value between 0 and 6.4
eV.Comment: 5 pages, 4 figures. Transferred to PR
Possible trace fossils of putative termite origin in the Lower Jurassic (Karoo Supergroup) of South Africa and Lesotho
Complex structures in the sandstones of the Lower Jurassic aeolian Clarens Formation (Karoo Supergroup) are found at numerous localities throughout southern Africa, and can be assigned to five distinct architectural groups: (1) up to 3.3-m high, free-standing, slab-shaped forms of bioturbated sandstones with elliptical bases, orientated buttresses and an interconnecting large burrow system; (2) up to 1.2-m high, free-standing, irregular forms of bioturbated sandstones with 2-cm to 4-cm thick, massive walls, empty chambers and vertical shafts; (3) about 0.15-m to 0.25-m high, mainly bulbous, multiple forms with thin walls (<2 cm), hollow chambers with internal pillars and bridges; (4) about 0.15-m to 0.2-m (maximum 1-m) high, free-standing forms of aggregated solitary spheres associated with massive horizontal, orientated capsules or tubes, and meniscate tubes; and (5) about 5 cmin diameter, ovoid forms with weak internal shelving in a close-fitting cavity. Based on size, wall thickness, orientation and the presence of internal chambers, these complex structures are tentatively interpreted as ichnofossils of an Early Jurassic social organism; the different architectures are reflective of the different behaviours of more than one species, the history of structural change in architectural forms (ontogenetic series) or an architectural adaptation to local palaeoclimatic variability. While exact modern equivalents are unknown, some of these ichnofossils are comparable to nests (or parts of nests) constructed by extant termites, and thus these Jurassic structures are very tentatively interpreted here as having been made by a soil-dwelling social organism, probably of termite origin. This southern African discovery, along with reported Triassic and Jurassic termite ichnofossils from North America, supports previous hypotheses that sociality in insects, particularity in termites, likely evolved prior to the Pangea breakup in the Early Mesozoic
Electronic structure and chemical bonding in Ti2AlC investigated by soft x-ray emission spectroscopy
The electronic structure of the nanolaminated transition metal carbide Ti2AlC
has been investigated by bulk-sensitive soft x-ray emission spectroscopy. The
measured Ti L, C K and Al L emission spectra are compared with calculated
spectra using ab initio density-functional theory including dipole matrix
elements. The detailed investigation of the electronic structure and chemical
bonding provides increased understanding of the physical properties of this
type of nanolaminates. Three different types of bond regions are identified;
the relatively weak Ti 3d - Al 3p hybridization 1 eV below the Fermi level, and
the Ti 3d - C 2p and Ti 3d - C 2s hybridizations which are stronger and deeper
in energy are observed around 2.5 eV and 10 eV below the Fermi level,
respectively. A strongly modified spectral shape of the 3s final states in
comparison to pure Al is detected for the buried Al monolayers indirectly
reflecting the Ti 3d - Al 3p hybridization. The differences between the
electronic and crystal structures of Ti2AlC, Ti3AlC2 and TiC are discussed in
relation to the number of Al layers per Ti layer in the two former systems and
the corresponding change of the unusual materials properties.Comment: 14 pages, 7 figures; PACS:78.70.En, 71.15.Mb, 71.20.-
Magnetic properties of Ruddlesden-Popper phases SrY(FeNi)O: A combined experimental and theoretical investigation
We present a comprehensive study of the magnetic properties of
SrY(FeNi)O ().
Experimentally, the magnetic properties are investigated using superconducting
quantum interference device (SQUID) magnetometry and neutron powder diffraction
(NPD). This is complemented by the theoretical study based on density
functional theory as well as the Heisenberg exchange parameters. Experimental
results show an increase in the N\'eel temperature () with the increase of
Y concentrations and O occupancy. The NPD data reveals all samples are
antiferromagnetically ordered at low temperatures, which has been confirmed by
our theoretical simulations for the selected samples. Our first-principles
calculations suggest that the 3D magnetic order is stabilized due to finite
inter-layer exchange couplings. The latter give rise to a finite inter-layer
spin correlations which disappear above the
Bonding mechanism in the nitrides Ti2AlN and TiN: an experimental and theoretical investigation
The electronic structure of nanolaminate Ti2AlN and TiN thin films has been
investigated by bulk-sensitive soft x-ray emission spectroscopy. The measured
Ti L, N K, Al L1 and Al L2,3 emission spectra are compared with calculated
spectra using ab initio density-functional theory including dipole transition
matrix elements. Three different types of bond regions are identified; a
relatively weak Ti 3d - Al 3p bonding between -1 and -2 eV below the Fermi
level, and Ti 3d - N 2p and Ti 3d - N 2s bonding which are deeper in energy
observed at -4.8 eV and -15 eV below the Fermi level, respectively. A strongly
modified spectral shape of 3s states of Al L2,3 emission from Ti2AlN in
comparison to pure Al metal is found, which reflects the Ti 3d - Al 3p
hybridization observed in the Al L1 emission. The differences between the
electronic and crystal structures of Ti2AlN and TiN are discussed in relation
to the intercalated Al layers of the former compound and the change of the
materials properties in comparison to the isostructural carbides.Comment: 18 pages, 7 figures;
http://link.aps.org/doi/10.1103/PhysRevB.76.19512
Thermo-mechanical behaviour of a compacted swelling clay
Compacted unsaturated swelling clay is often considered as a possible buffer
material for deep nuclear waste disposal. An isotropic cell permitting
simultaneous control of suction, temperature and pressure was used to study the
thermo-mechanical behaviour of this clay. Tests were performed at total
suctions ranging from 9 to 110 MPa, temperature from 25 to 80 degrees C,
isotropic pressure from 0.1 to 60 MPa. It was observed that heating at constant
suction and pressure induces either swelling or contraction. The results from
compression tests at constant suction and temperature evidenced that at lower
suction, the yield pressure was lower, the elastic compressibility parameter
and the plastic compressibility parameter were higher. On the other hand, at a
similar suction, the yield pressure was slightly influenced by the temperature;
and the compressibility parameters were insensitive to temperature changes. The
thermal hardening phenomenon was equally evidenced by following a
thermo-mechanical path of loading-heating-cooling-reloading
Accurate electronic band gap of pure and functionalized graphane from GW calculations
Using the GW approximation, we study the electronic structure of the recently
synthesized hydrogenated graphene, named graphane. For both conformations, the
minimum band gap is found to be direct at the point, and it has a
value of 5.4 eV in the stable chair conformation, where H atoms attach C atoms
alternatively on opposite sides of the two dimensional carbon network. In the
meta-stable boat conformation the energy gap is 4.9 eV. Then, using a supercell
approach, the electronic structure of graphane was modified by introducing
either an hydroxyl group or an H vacancy. In this last case, an impurity state
appears at about 2 eV above the valence band maximum.Comment: Submited to Physical Review Letter
Atom chip for BEC interferometry
We have fabricated and tested an atom chip that operates as a matter wave interferometer. In this communication we describe the fabrication of the chip by ion-beam milling of gold evaporated onto a silicon substrate. We present data on the quality of the wires, on the current density that can be reached in the wires and on the smoothness of the magnetic traps that are formed. We demonstrate the operation of the interferometer, showing that we can coherently split and recombine a Bose–Einstein condensate with good phase stability
The feasibility and outcome of a community-based primary prevention program for cardiovascular disease in the 21st century
Objective There is no evidence that systematic screening and risk factor modification in an unselected, asymptomatic population will reduce cardiovascular disease (CVD) mortality. This study aimed to evaluate the effectiveness of a primary care CVD prevention program on mortality during a 13-year follow-up. Design A risk factor survey was sent, followed by a nurse-led lifestyle counselling to respondents with at least one CVD risk factor, and a general practitioner's (GP) appointment for high-risk persons. Screening and interventions were performed during 2005-2006. Setting A public health care centre in the town of Harjavalta, Finland. Subjects All home-dwelling 45-70-year old inhabitants without manifested CVD or diabetes. Main outcome measures All-cause and CVD mortality. Results Altogether 74% (2121/2856) inhabitants responded to the invitation. The intervention was received by 1465 individuals (52% of the invited population): 398 risk persons had an appointment with a nurse, followed by an appointment with a GP for 1067 high-risk persons. During the follow-up, 370 persons died. Mortality among the non-respondents was twofold compared to the participants'. In subjects who received the intervention, the age- and gender-adjusted hazard ratio for all-cause mortality was 0.44 (95% CI: 0.36 to 0.54) compared to the subjects who did not receive the intervention. Conclusions Reducing mortality is possible in a primary care setting by raising health awareness in the community with screening, by targeted lifestyle counselling and evidence-based preventive medication for persons at high risk for CVD. Subjects not willing to participate in health surveys have the worst prognosis.Peer reviewe
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