239 research outputs found
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
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
Disentangling electron- and electric field-induced ring-closing reactions in a diarylethene derivative on Ag(111)
Using scanning tunneling microscopy and spectroscopy we investigate the
adsorption properties and ring-closing reaction of a diarylethene derivative
(C5F-4Py) on a Ag(111) surface. We identify an electron-induced reaction
mechanism, with a quantum yield varying from per electron
upon variation of the bias voltage from V. We ascribe the drastic
increase in switching efficiency to a resonant enhancement upon tunneling
through molecular orbitals. Additionally, we resolve the ring-closing reaction
even in the absence of a current passing through the molecule. In this case the
electric-field can modify the reaction barrier, leading to a finite switching
probability at 4.8 K. A detailed analysis of the switching events shows that a
simple plate-capacitor model for the tip-surface junction is insufficient to
explain the distance dependence of the switching voltage. Instead, describing
the tip as a sphere is in agreement with the findings. We resolve small
differences in the adsorption configuration of the closed isomer, when
comparing the electron- and field-induced switching product
Visualizing intramolecular distortions as the origin of transverse magnetic anisotropy
The magnetic properties of metalâorganic complexes are strongly influenced by conformational changes in the ligand. The flexibility of Fe-tetra-pyridyl-porphyrin molecules leads to different adsorption configurations on a Au(111) surface. By combining low-temperature scanning tunneling spectroscopy and atomic force microscopy, we resolve a correlation of the molecular configuration with different spin states and magnitudes of magnetic anisotropy. When the macrocycle exhibits a laterally undistorted saddle shape, the molecules lie in a S = 1 state with axial anisotropy arising from a square-planar ligand field. If the symmetry in the molecular ligand field is reduced by a lateral distortion of the molecule, we find a finite contribution of transverse anisotropy. Some of the distorted molecules lie in a S = 2 state, again exhibiting substantial transverse anisotropy
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
Electronic structure of an iron porphyrin derivative on Au(1â1â1)
Surface-bound porphyrins are promising candidates for molecular switches, electronics and spintronics. Here, we studied the structural and the electronic properties of Fe-tetra-pyridil-porphyrin adsorbed on Au(1â1â1) in the monolayer regime. We combined scanning tunneling microscopy/spectroscopy, ultraviolet photoemission, and two-photon photoemission to determine the energy levels of the frontier molecular orbitals. We also resolved an excitonic state with a binding energy of 420 meV, which allowed us to compare the electronic transport gap with the optical gap
Gating the charge state of a single molecule by local electric fields
The electron acceptor molecule TCNQ is found in either of two distinct
integer charge states when embedded into a monolayer of a charge
transfer-complex on a gold surface. Scanning tun- neling spectroscopy
measurements identify these states through the presence/absence of a zero-bias
Kondo resonance. Increasing the (tip-induced) electric field allows us to
reversibly induce the ox- idation/reduction of TCNQ species from their anionic
or neutral ground state, respectively. We show that the different ground states
arise from slight variations in the underlying surface potential, pictured here
as the gate of a three-terminal device.Comment: 5 pages, 4 figure
Direct imaging of the inducedâfit effect in molecular selfâassembly
Molecular recognition is a crucial driving force for molecular selfâassembly. In many cases molecules arrange in the lowest energy configuration following a lockâandâkey principle. When molecular flexibility comes into play, the inducedâfit effect may govern the selfâassembly. Here, the selfâassembly of dicyanovinylâhexathiophene (DCV6T) molecules, a prototype specie for highly efficient organic solar cells, on Au(111) by using lowâtemperature scanning tunneling microscopy and atomic force microscopy is investigated. DCV6T molecules assemble on the surface forming either islands or chains. In the islands the molecules are straightâthe lowest energy configuration in gas phaseâand expose the dicyano moieties to form hydrogen bonds with neighbor molecules. In contrast, the structure of DCV6T molecules in the chain assemblies deviates significantly from their gasâphase analogues. The seemingly energetically unfavorable bent geometry is enforced by hydrogenâbonding intermolecular interactions. Density functional theory calculations of molecular dimers quantitatively demonstrate that the deformation of individual molecules optimizes the intermolecular bonding structure. The intermolecular bonding energy thus drives the chain structure formation, which is an expression of the inducedâfit effect
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