6,820 research outputs found
Quantum suppression of shot noise in atom-size metallic contacts
The transmission of conductance modes in atom-size gold contacts is
investigated by simultaneously measuring conductance and shot noise. The
results give unambiguous evidence that the current in the smallest gold
contacts is mostly carried by nearly fully transmitted modes. In particular,
for a single-atom contact the contribution of additional modes is only a few
percent. In contrast, the trivalent metal aluminum does not show this property.Comment: Fig. 2 replaced, small errors correcte
Wide energy-window view on the density of states and hole mobility of poly(p-phenylene vinylene)
Using an electrochemically gated transistor, we achieved controlled and
reversible doping of poly(p-phenylene vinylene) in a large concentration range.
Our data open a wide energy-window view on the density of states (DOS) and
show, for the first time, that the core of the DOS function is Gaussian, while
the low-energy tail has a more complex structure. The hole mobility increases
by more than four orders of magnitude when the electrochemical potential is
scanned through the DOS.Comment: 4 pages, 4 figure
Ageing effects around the glass and melting transitions in poly(dimethylsiloxane) visualized by resistance measurements
The process of ageing in rubbers requires monitoring over long periods (days
to years). To do so in non-conducting rubbers, small amounts of carbon-black
particles were dispersed in a fractal network through the rubber matrix, to
make the rubber conducting without modifying its properties. Continuous
monitoring of the resistance reveals the structural changes around the glass
and melting transitions and especially details about the hysteresis and ageing
processes. We illustrate the method for the semicrystalline polymer
poly(dimethylsiloxane) (PDMS).Comment: 4 pages, 4 figure
De betekenis van Johan Buitendags stellingname in theologie der natuur
This article presents the importance of Buitendag’s stance in the so-called ‘theology of nature’. His theological statements endeavour to understand reality in conversation with other academic disciplines to see things in a wider and holistic perspective. Following a suggestion of Moltmann, theology must not restrict itself to internal ecclesiastical and personal faith topics but search for ‘the truth of the whole’. It is argued that Buitendag’s concept of holism is different from Moltmann’s ‘the truth of the whole’. Moltmann’s holism is eschatologically directed after history, but is meaningless in a contemporary debate. His concept of history seems to be problematic too. Buitendag’s holism is more Quinean as a comprehensive relative approach, bottom-up from contemporary insights within different academic disciplines. His theological approach looks like an ellipsis, involving both an ontological and epistemological focus. He defends (Trinitarian) communion as the primary concept, ontologically, which biologists may recognise in their observations of animal communities too. His theology shows a panentheistic perspective for the discourse on divine immanent agency by using as analogy the mind-body relationship in a sophisticated way. Buitendag shows the importance of this perspective for theological hermeneutics. This article presents some logical and theological problems in a panentheistic view which some prominent supporters defend as ‘reality depicting’. Buitendag avoids this because of a relational ontology
Magnetic dipolar ordering and relaxation in the high-spin molecular cluster compound Mn6
Few examples of magnetic systems displaying a transition to pure dipolar
magnetic order are known to date, and single-molecule magnets can provide an
interesting example. The molecular cluster spins and thus their dipolar
interaction energy can be quite high, leading to reasonably accessible ordering
temperatures, provided the crystal field anisotropy is sufficiently small. This
condition can be met for molecular clusters of sufficiently high symmetry, as
for the Mn6 compound studied here. Magnetic specific heat and susceptibility
experiments show a transition to ferromagnetic dipolar order at T_{c} = 0.16 K.
Classical Monte-Carlo calculations indeed predict ferromagnetic ordering and
account for the correct value of T_{c}. In high magnetic fields we detected the
contribution of the ^{55}Mn nuclei to the specific heat, and the characteristic
timescale of nuclear relaxation. This was compared with results obtained
directly from pulse-NMR experiments. The data are in good mutual agreement and
can be well described by the theory for magnetic relaxation in highly polarized
paramagnetic crystals and for dynamic nuclear polarization, which we
extensively review. The experiments provide an interesting comparison with the
recently investigated nuclear spin dynamics in the anisotropic single molecule
magnet Mn12-ac.Comment: 19 pages, 11 eps figures. Contains extensive discussions on dipolar
ordering, specific heat and nuclear relaxation in molecular magnet
NMR evidence for two-step phase-separation in Nd_{1.85}Ce_{0.15}CuO_{4-delta}
By Cu NMR we studied the spin and charge structure in
Nd_{2-x}Ce_{x}CuO_{4-delta}. For x=0.15, starting from a superconducting
sample, the low temperature magnetic order in the sample reoxygenated under 1
bar oxygen at 900^0 C, reveals a peculiar modulation of the internal field,
indicative for a phase characterized by large charge droplets ('Blob'-phase).
By prolonged reoxygenation at 4 bar the blobs brake up and the spin structure
changes to that of an ordered antiferromagnet (AF). We conclude that the
superconductivity in the n-type systems competes with a genuine type I
Mott-insulating state
Superconductivity in a Molecular Metal Cluster Compound
Compelling evidence for band-type conductivity and even bulk
superconductivity below K has been found in
Ga-NMR experiments in crystalline ordered, giant Ga
cluster-compounds. This material appears to represent the first realization of
a theoretical model proposed by Friedel in 1992 for superconductivity in
ordered arrays of weakly coupled, identical metal nanoparticles.Comment: 5 pages, 4 figure
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