13,220 research outputs found
Island formation without attractive interactions
We show that adsorbates on surfaces can form islands even if there are no
attractive interactions. Instead strong repulsion between adsorbates at short
distances can lead to islands, because such islands increase the entropy of the
adsorbates that are not part of the islands. We suggest that this mechanism
cause the observed island formation in O/Pt(111), but it may be important for
many other systems as well.Comment: 11 pages, 4 figure
Upper critical field in {BaKBiO}: magnetotransport versus magnetotunneling
Elastic tunneling is used as a powerful direct tool to determine the upper
critical field in the high- oxide BaKBiO. The
temperature dependence of inferred from the tunneling follows the
Werthamer-Helfand-Hohenberg prediction for type-II superconductors. A
comparison will be made with resistively determined critical field data.Comment: 4 pages incl. 5 figure
3D-2D crossover in the naturally layered superconductor (LaSe)1.14(NbSe2)
The temperature and angular dependencies of the resistive upper critical
magnetic field reveal a dimensional crossover of the superconducting
state in the highly anisotropic misfit-layer single crystal of
(LaSe)(NbSe) with the critical temperature of 1.23 K. The
temperature dependence of the upper critical field for
a field orientation along the conducting -planes displays a
characteristic upturn at 1.1 K and below this temperature the angular
dependence of has a cusp around the parallel field orientation. Both
these typical features are observed for the first time in a naturally
crystalline layered system.Comment: 7 pages incl. 3 figure
Group projector generalization of dirac-heisenberg model
The general form of the operators commuting with the ground representation
(appearing in many physical problems within single particle approximation) of
the group is found. With help of the modified group projector technique, this
result is applied to the system of identical particles with spin independent
interaction, to derive the Dirac-Heisenberg hamiltonian and its effective space
for arbitrary orbital occupation numbers and arbitrary spin. This gives
transparent insight into the physical contents of this hamiltonian, showing
that formal generalizations with spin greater than 1/2 involve nontrivial
additional physical assumptions.Comment: 10 page
Direct observation of charge order in triangular metallic AgNiO2 by single-crystal resonant X-ray scattering
We report resonant X-ray scattering measurements on the orbitally-degenerate
triangular metallic antiferromagnet 2H-AgNiO2 to probe the spontaneous
transition to a triple-cell superstructure at temperatures below 365 K. We
observe a strong resonant enhancement of the supercell reflections through the
Ni K-edge. The empirically extracted K-edge shift between the
crystallographically-distinct Ni sites of 2.5(3) eV is much larger than the
value expected from the shift in final states, and implies a core-level shift
of ~1 eV, thus providing direct evidence for the onset of spontaneous honeycomb
charge order in the triangular Ni layers. We also provide band-structure
calculations that explain quantitatively the observed edge shifts in terms of
changes in the Ni electronic energy levels due to charge order and
hybridization with the surrounding oxygens.Comment: 5 pages, 4 figure
Interaction effects in the spectrum of the three-dimensional Ising model
The two-point correlation functions of statistical models show in general
both poles and cuts in momentum space. The former correspond to the spectrum of
massive excitations of the model, while the latter originate from interaction
effects, namely creation and annihilation of virtual pairs of excitations. We
discuss the effect of such interactions on the long distance behavior of
correlation functions in configuration space, focusing on certain time-slice
operators which are commonly used to extract the spectrum. For the 3D Ising
model in the scaling region of the broken-symmetry phase, a one-loop
calculation shows that the interaction effects on time-slice correlations is
non negligible for distances up to a few times the correlation length, and
should therefore be taken into account when analysing Monte Carlo data.Comment: 10 pages, LaTeX file + 1 ps figure, uses axodraw.st
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