181 research outputs found
Distinction of Nuclear Spin States with the Scanning Tunneling Microscope
We demonstrate rotational excitation spectroscopy with the scanning tunneling
microscope for physisorbed hydrogen and its isotopes hydrogen-deuterid and
deuterium. The observed excitation energies are very close to the gas phase
values and show the expected scaling with moment of inertia. Since these
energies are characteristic for the molecular nuclear spin states we are able
to identify the para and ortho species of hydrogen and deuterium, respectively.
We thereby demonstrate nuclear spin sensitivity with unprecedented spatial
resolution
Rotational Excitation Spectroscopy with the STM through Molecular Resonances
We investigate the rotational properties of molecular hydrogen and its
isotopes physisorbed on the surfaces of graphene and hexagonal boron nitride
(-BN), grown on Ni(111), Ru(0001), and Rh(111), using rotational excitation
spectroscopy (RES) with the scanning tunneling microscope. The rotational
thresholds are in good agreement with transitions of freely
spinning para-H and ortho-D molecules. The line shape variations in RES
for H among the different surfaces can be traced back and naturally
explained by a resonance mediated tunneling mechanism. RES data for
H/-BN/Rh(111) suggests a local intrinsic gating on this surface due to
lateral variations in the surface potential. An RES inspection of H, HD,
and D mixtures finally points to a multi molecule excitation, since either
of the three rotational transitions are simultaneously
present, irrespective of where the spectra were recorded in the mixed
monolayer
Thermal and magnetic field stability of holmium single atom magnets
We use spin-polarized scanning tunneling microscopy to demonstrate that Ho
atoms on magnesium oxide exhibit a coercive field of more than 8 T and magnetic
bistability for many minutes, both at 35 K. The first spontaneous magnetization
reversal events are recorded at 45 K for which the metastable state relaxes in
an external field of 8 T. The transverse magnetic anisotropy energy is
estimated from magnetic field and bias voltage dependent switching rates at 4.3
K. Our measurements constrain the possible ground state of Ho single atom
magnets to either Jz = 7 or 8, both compatible with magnetic bistability at
fields larger than 10 mT.Comment: 4 pages and supplemental informatio
Adsorption Sites of Individual Metal Atoms on Ultrathin MgO(100) Films
We use Ca doping during growth of one and two monolayer thick MgO films on
Ag(100) to identify the adsorption sites of individual adatoms with scanning
tunneling microscopy. For this we combine atomic resolution images of the bare
MgO layer with images of the adsorbates and the substitutional Ca atoms taken
at larger tip-sample distance. For Ho atoms, the adsorption sites depend on MgO
thickness. On the monolayer, they are distributed on the O and bridge sites
according to the abundance of those sites, 1/3 and 2/3 respectively. On the MgO
bilayer, Ho atoms populate almost exclusively the O site. A third species
adsorbed on Mg is predicted by density functional theory and can be created by
atomic manipulation. Au atoms adsorb on the bridge sites for both MgO
thicknesses, while Co and Fe atoms prefer the O sites, again for both
thickness.Comment: 8 pages, 9 figures, part of the work presented at the DPG Spring
meeting in Dresden, 201
Magnetism of individual atoms adsorbed on surfaces
Magnetic impurities in solids cause manifold changes in their macroscopic properties, such as anomalous low temperature resistance due to Kondo screening, reduction of the superconducting transition temperature due to local suppression of the order parameter, they create magnetic signatures in semiconductors, and lead to inelastic spin excitations in tunnel junctions. In the present paper we review what has been learnt about these effects from a surface science approach. Placing the magnetic impurities at well defined adsorption sites on single crystal surfaces makes their effect on the host, as well as their own magnetic properties better accessible to experiments, and also better understandable since the atomic environment of the impurity is exactly known lending comparison with theory more direct. After an introduction we discuss X-ray magnetic circular dichroism measurements which are spatially averaging and therefore report on ensemble properties. One of the recent progresses achieved in surface science is the preparation of well defined ensembles, such as surfaces with only single adatoms, each of them in an identical atomic environment and with sufficient mutual distance to exclude interactions. Due to this approach we can now determine the electronic configuration of individual adatoms, their hybridization with the host, and quantify their spin and orbital moments, as well as the spin–orbit induced magneto-crystalline anisotropy, which can be orders of magnitude larger than thin film and bulk values. In the second part we review recent progress in revealing the magnetic properties of individual atoms with the scanning tunneling microscope (STM). With this technique the spatial extent of the Kondo screening cloud and of subgap excitations in the superconductor quasiparticle density of states became apparent. We outline the first pioneering experiments measuring transport through reversible atomic point contacts containing magnetic atoms and measurements using the subgap features caused in superconducting STM tips to detect the magnetism of individual atoms. We then describe experiments using inelastic spin excitation spectroscopy to pin down the magnetic ground state and anisotropy energy of magnetic impurities. We continue with spin-polarized STM experiments reporting magnetization curves of individual magnetic adatoms and finish by a description of the most recent spin-excitation experiments revealing the necessary anisotropy environment for a high spin impurity to display the Kondo effect
Ring State for Single Transition Metal Atoms on Boron Nitride on Rh(111)
The low-temperature adsorption of isolated transition metal adatoms (Mn, Co, and Fe) onto hexagonal boron nitride monolayers on Rh(111) creates a bistable adsorption complex. The first state considerably weakens the hexagonal boron nitride- (h-BN-) substrate bond for 60 BN unit cells, leading to a highly symmetric ring in STM images, while the second state is imaged as a conventional adatom and leaves the BN-substrate interaction intact. We demonstrate reversible switching between the two states and, thus, controlled pinning and unpinning of the h-BN layer from the metal substrate. I(z) and d ln(l)/dz curves are used to reveal the BN deformation in the ring state
Competing Interactions in the Self-Assembly of NC-Ph-3-CN Molecules on Cu(111)
We report on low-temperature scanning tunneling microscopy and spectroscopy measurements on NC-Ph-3-CN molecules adsorbed at 300 K on a Cu(111) surface. Upon cooling, the molecules form chain and honeycomb structures, incorporating Cu adatoms supplied by the substrate as metal linkers. In these assemblies, the molecules align along two main directions, with a relative abundance that depends on the coordination number and on the bond length. We show spectroscopic data about the unoccupied molecular orbitals and investigate the patterns obtained by depositing different amounts of molecules. Comparison of these results with the ones obtained for NC-Ph-5-CN molecules on the same substrate enables us to establish a hierarchy of the different interaction forces at work in the system
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