6,920 research outputs found
A fast version of the k-means classification algorithm for astronomical applications
Context. K-means is a clustering algorithm that has been used to classify
large datasets in astronomical databases. It is an unsupervised method, able to
cope very different types of problems. Aims. We check whether a variant of the
algorithm called single-pass k-means can be used as a fast alternative to the
traditional k-means. Methods. The execution time of the two algorithms are
compared when classifying subsets drawn from the SDSS-DR7 catalog of galaxy
spectra. Results. Single-pass k-means turn out to be between 20 % and 40 %
faster than k-means and provide statistically equivalent classifications. This
conclusion can be scaled up to other larger databases because the execution
time of both algorithms increases linearly with the number of objects.
Conclusions. Single-pass k-means can be safely used as a fast alternative to
k-means
Single magnetic adsorbates on s-wave superconductors
In superconductors, magnetic impurities induce a pair-breaking potential for
Cooper pairs, which locally affects the Bogoliubov quasiparticles and gives
rise to Yu-Shiba-Rusinov (YSR or Shiba, in short) bound states in the density
of states (DoS). These states carry information on the magnetic coupling
strength of the impurity with the superconductor, which determines the
many-body ground state properties of the system. Recently, the interest in
Shiba physics was boosted by the prediction of topological superconductivity
and Majorana modes in magnetically coupled chains and arrays of Shiba
impurities. Here, we review the physical insights obtained by scanning
tunneling microscopy into single magnetic adsorbates on the -wave
superconductor lead (Pb). We explore the tunneling processes into Shiba states,
show how magnetic anisotropy affects many-body excitations, and determine the
crossing of the many-body groundstate through a quantum phase transition.
Finally, we discuss the coupling of impurities into dimers and chains and their
relation to Majorana physics.Comment: 18 pages, 17 figures, revie
Long-range repulsive interaction between TTF molecules on a metal surface induced by charge transfer
The low-coverage adsorption of a molecular electron donor,
tetrathiafulvalene, on Au(111) is characterized by the spontaneous formation of
superlattice of monomers, whose spacing exceeds the equilibrium distance of
non-covalent interactions and depends on coverage. The origin of this peculiar
growth mode is due to a long-range repulsive interaction between molecules. The
analysis of molecular-pair distributions obtained by scanning tunneling
microscopy measurements permits us to determine that the nature of TTF
intermolecular interactions on Au (111) is electrostatic. A repulsion between
molecules is caused by the accumulation of charge due to electron donation into
the metal surface, as pictured through density functional theory calculations
Tuning the magnetic anisotropy of single molecules
The magnetism of single atoms and molecules is governed by the atomic scale
environment. In general, the reduced symmetry of the surrounding splits the
states and aligns the magnetic moment along certain favorable directions. Here,
we show that we can reversibly modify the magnetocrystalline anisotropy by
manipulating the environment of single iron(II) porphyrin molecules adsorbed on
Pb(111) with the tip of a scanning tunneling microscope. When we decrease the
tip--molecule distance, we first observe a small increase followed by an
exponential decrease of the axial anisotropy on the molecules. This is in
contrast to the monotonous increase observed earlier for the same molecule with
an additional axial Cl ligand. We ascribe the changes in the anisotropy of both
species to a deformation of the molecules in the presence of the attractive
force of the tip, which leads to a change in the level alignment. These
experiments demonstrate the feasibility of a precise tuning of the magnetic
anisotropy of an individual molecule by mechanical control.Comment: 16 pages, 5 figures; online at Nano Letters (2015
Optimal generalized quantum measurements for arbitrary spin systems
Positive operator valued measurements on a finite number of N identically
prepared systems of arbitrary spin J are discussed. Pure states are
characterized in terms of Bloch-like vectors restricted by a SU(2 J+1)
covariant constraint. This representation allows for a simple description of
the equations to be fulfilled by optimal measurements. We explicitly find the
minimal POVM for the N=2 case, a rigorous bound for N=3 and set up the analysis
for arbitrary N.Comment: LateX, 12 page
Magnetic anisotropy in Shiba bound states across a quantum phase transition
The exchange coupling between magnetic adsorbates and a superconducting
substrate leads to Shiba states inside the superconducting energy gap and a
Kondo resonance outside the gap. The exchange coupling strength determines
whether the quantum many-body ground state is a Kondo singlet or a singlet of
the paired superconducting quasiparticles. Here, we use scanning tunneling
spectroscopy to identify the different quantum ground states of Manganese
phthalocyanine on Pb(111). We observe Shiba states, which are split into
triplets by magnetocrystalline anisotropy. Their characteristic spectral weight
yields an unambiguous proof of the nature of the quantum ground state.Comment: 6 pages, 4 figure
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