63 research outputs found
Probing Magnetic Excitations and Correlations in Single and Coupled Spin Systems with Scanning Tunneling Spectroscopy
Spectroscopic measurements with low-temperature scanning tunneling
microscopes have been used very successfully for studying not only individual
atomic or molecular spins on surfaces but also complexly designed coupled
systems. The symmetry breaking of the supporting surface induces magnetic
anisotropy which lead to characteristic fingerprints in the spectrum of the
differential conductance and can be well understood with simple model
Hamiltonians. Furthermore, correlated many-particle states can emerge due to
the interaction with itinerant electrons of the electrodes, making these
systems ideal prototypical quantum systems. In this manuscript more complex
bipartite and spin-chains will be discussed additionally. Their spectra enable
to determine precisely the nature of the interactions between the spins which
can lead to the formation of new quantum states which emerge by interatomic
entanglement.Comment: 46 pages, 21 figure
Scanning tunneling spectroscopy at the single atom scale
This thesis reports measurements at the single atom scale by using low-temperature scanning tunneling microscopy (STM) and spectroscopy (STS). Different sample systems where analyzed with normal conducting and superconducting tips. Chapter 2 presents the theoretical aspects which have to be taken into account for a detailed analysis and a consistent interpretation of the STS measurements. In chapter 3 the creation of a hexagonally ordered superlattice of single Ce adatoms on Ag(111) is reported and understood within a scattering model of the surface state electrons with the adatoms. Furthermore, the change in the local density of states of the surface state in ordered and slightly disordered superlattices is measured and theoretically explained within a tight-binding model which allows to understand the creation and stability of the superlattice by an energy gain of the participating surface-state electrons. Because Ce atoms have a non-vanishing magnetic moment which is expected to interact with the continuous states of the supporting surface leading to a Kondo resonance, chapter 4 presents measurements on single Ce adatoms on different Ag surfaces. This chapter shows the difficulties to interpret the obtained data. For instance, bistable Ce adatoms are detected on Ag(100) which show drastical changes in their apparent height and spectral signature depending on the tunneling conditions. The possible physical processes behind these phenomena are discussed. While the results presented in the first chapters were obtained with a normal conducting tip, chapter 5 intensively discusses the opportunities superconducting tips offer in low-temperature STS measurements. Novel insight in and thorough understanding of Andreev reflection processes are obtained by using the unique possibility of having different superconducting gaps in the tip and the sample. Detailed analyses of the supercurrent at low tunneling resistances reveal tunneling currents which are not described within the standard resistivity shunted junction model, and are presumably due to self-induced tunneling or due to an additional quasiparticle tunneling channel which only exist in asymmetric junctions. Furthermore, the influence of single magnetic Co atoms inbetween the superconducting tunnel junction on the obtained spectrum is discussed
Structural and magnetic properties of FeMn (1...6) chains supported on CuN / Cu (100)
Heterogeneous atomic magnetic chains are built by atom manipulation on a
CuN/Cu (100) substrate. Their magnetic properties are studied and
rationalized by a combined scanning tunneling microscopy (STM) and density
functional theory (DFT) work completed by model Hamiltonian studies. The chains
are built using Fe and Mn atoms ontop of the Cu atoms along the N rows of the
CuN surface. Here, we present results for FeMn (=1...6) chains
emphasizing the evolution of the geometrical, electronic, and magnetic
properties with chain size. By fitting our results to a Heisenberg Hamiltonian
we have studied the exchange-coupling matrix elements for different chains.
For the shorter chains, , we have included spin-orbit effects in the
DFT calculations, extracting the magnetic anisotropy energy. Our results are
also fitted to a simple anisotropic spin Hamiltonian and we have extracted
values for the longitudinal-anisotropy and transversal-anisotropy
constants. These parameters together with the values for allow us to
compute the magnetic excitation energies of the system and to compare them with
the experimental data.Comment: 10 pages 8 figure
Symmetry mediated tunable molecular magnetism on a 2D material
The induction of unconventional superconductivity by twisting two layers of
graphene a small angle was groundbreaking1, and since then has attracted
widespread attention to novel phenomena caused by lattice or angle mismatch
between two-dimensional (2D) materials2. While many studies address the
influence of angle mismatch between layered 2D materials3-5 , the impact of the
absorption alignment on the physical properties of planar molecules on 2D
substrates has not been studied in detail. Using scanning probe microscopy
(SPM) we show that individual cobalt phthalocyanine (CoPc) molecules adsorbed
on the layered superconductor 2H-NbSe2 change drastically their charge and spin
state when the symmetry axes of the molecule and the substrate are twisted with
respect to each other. The CoPc changes from an effective spin-1/2 as found in
gas-phase6 to a molecule with non-magnetic ground-state. On the latter we
observe a singlet-triplet transition originating from an antiferromagnetic
interaction between the central-ion spin and a distributed magnetic moment on
the molecular ligands. Because the Ising superconductor 2H-NbSe2 lacks
inversion symmetry and has large spin-orbit coupling7 this intramolecular
magnetic exchange has significant non-collinear Dzyaloshinskii-Moriya (DM)8, 9
contribution.Comment: 4 figure
Potential Energy Driven Spin Manipulation via a Controllable Hydrogen Ligand
Spin-bearing molecules can be stabilized on surfaces and in junctions with
desirable properties such as a net spin that can be adjusted by external
stimuli. Using scanning probes, initial and final spin states can be deduced
from topographic or spectroscopic data, but how the system transitioned between
these states is largely unknown. Here we address this question by manipulating
the total spin of magnetic cobalt hydride complexes on a corrugated boron
nitride surface with a hydrogen- functionalized scanning probe tip by
simultaneously tracking force and conductance. When the additional hydrogen
ligand is brought close to the cobalt monohydride, switching between a corre-
lated S = 1 /2 Kondo state, where host electrons screen the magnetic moment,
and a S = 1 state with magnetocrystalline anisotropy is observed. We show that
the total spin changes when the system is transferred onto a new potential
energy surface defined by the position of the hydrogen in the junction. These
results show how and why chemically functionalized tips are an effective tool
to manipulate adatoms and molecules, and a promising new method to selectively
tune spin systems
Quantum Engineering of Spin and Anisotropy in Magnetic Molecular Junctions
Single molecule magnets and single spin centers can be individually addressed
when coupled to contacts forming an electrical junction. In order to control
and engineer the magnetism of quantum devices, it is necessary to quantify how
the structural and chemical environment of the junction affects the spin
center. Metrics such as coordination number or symmetry provide a simple method
to quantify the local environment, but neglect the many-body interactions of an
impurity spin when coupled to contacts. Here, we utilize a highly corrugated
hexagonal boron nitride (h-BN) monolayer to mediate the coupling between a
cobalt spin in CoHx (x=1,2) complexes and the metal contact. While the hydrogen
atoms control the total effective spin, the corrugation is found to smoothly
tune the Kondo exchange interaction between the spin and the underlying metal.
Using scanning tunneling microscopy and spectroscopy together with numerical
simulations, we quantitatively demonstrate how the Kondo exchange interaction
mimics chemical tailoring and changes the magnetic anisotropy
Electron spin secluded inside a bottom-up assembled standing metal-molecule nanostructure
Artificial nanostructures, fabricated by placing building blocks such as
atoms or molecules in well-defined positions, are a powerful platform in which
quantum effects can be studied and exploited on the atomic scale. Here, we
report a strategy to significantly reduce the electron-electron coupling
between a large planar aromatic molecule and the underlying metallic substrate.
To this end, we use the manipulation capabilities of a scanning tunneling
microscope (STM) and lift the molecule into a metastable upright geometry on a
pedestal of two metal atoms. Measurements at millikelvin temperatures and
magnetic fields reveal that the bottom-up assembled standing metal-molecule
nanostructure has an spin which is screened by the substrate
electrons, resulting in a Kondo temperature of only mK. We extract
the Land\'e -factor of the molecule and the exchange coupling to the
substrate by modeling the differential conductance spectra using a third-order
perturbation theory in the weak coupling and high-field regimes. Furthermore,
we show that the interaction between the STM tip and the molecule can tune the
exchange coupling to the substrate, which suggests that the bond between the
standing metal-molecule nanostructure and the surface is mechanically soft
Long spin relaxation times in a transition metal atom in direct contact to a metal substrate
Long spin relaxation times are a prerequisite for the use of spins in data
storage or nanospintronics technologies. An atomic-scale solid-state
realization of such a system is the spin of a transition metal atom adsorbed on
a suitable substrate. For the case of a metallic substrate, which enables
directly addressing the spin by conduction electrons, the experimentally
measured lifetimes reported to date are on the order of only hundreds of
femtoseconds. Here, we show that the spin states of iron atoms adsorbed
directly on a conductive platinum substrate have an astonishingly long spin
relaxation time in the nanosecond regime, which is comparable to that of a
transition metal atom decoupled from the substrate electrons by a thin
decoupling layer. The combination of long spin relaxation times and strong
coupling to conduction electrons implies the possibility to use flexible
coupling schemes in order to process the spin-information
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