24 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
Fluorescence and phosphorescence from individual C molecules excited by local electron tunneling
Using the highly localized current of electrons tunneling through a double
barrier Scanning Tunneling Microscope (STM) junction, we excite luminescence
from a selected C molecule in the surface layer of fullerene
nanocrystals grown on an ultrathin NaCl film on Au(111). In the observed
luminescence fluorescence and phosphorescence spectra, pure electronic as well
as vibronically induced transitions of an individual C molecule are
identified, leading to unambiguous chemical recognition on the single-molecular
scale
Dynamical Coulomb Blockade Observed in Nano-Sized Electrical Contacts
Electrical contacts between nano-engineered systems are expected to
constitute the basic building blocks of future nano-scale electronics. However,
the accurate characterization and understanding of electrical contacts at the
nano-scale is an experimentally challenging task. Here we employ
low-temperature scanning tunneling spectroscopy to investigate the conductance
of individual nano-contacts formed between flat Pb islands and their supporting
substrates. We observe a suppression of the differential tunnel conductance at
small bias voltages due to dynamical Coulomb blockade effects. The differential
conductance spectra allow us to determine the capacitances and resistances of
the electrical contacts which depend systematically on the island--substrate
contact area. Calculations based on the theory of environmentally assisted
tunneling agree well with the measurements.Comment: 5 pages, 3 figures, to appear in PR
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
A quantum pathway to overcome the trilemma of magnetic data storage
The three essential pillars of magnetic data storage devices are readability,
writeability, and stability. However, these requirements compete as magnetic
domain sizes reach the fundamental limit of single atoms and molecules. The
proven magnetic bistability of individual holmium atoms on magnesium oxide
appeared to operate within this magnetic trilemma, sacrificing writeability for
unprecedented stability. Using the magnetic stray field created by the tip of a
spin-polarized scanning tunneling microscope (SP-STM), we controllably move the
Ho state into the quantum regime, allowing us to write its state via the
quantum tunneling of magnetization (QTM). We find that the hyperfine
interaction causes both the excellent magnetic bistability, even at zero
applied magnetic field, and the avoided level crossings which we use to control
the magnetic state via QTM. We explore how to use such a system to realize a
high-fidelity single atom NOT gate (inverter). Our approach reveals the
prospect of combining the best traits of the classical and quantum worlds for
next generation data storage