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 ($h$-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 $\Delta J=2$ transitions of freely spinning para-H$_2$ and ortho-D$_2$ molecules. The line shape variations in RES for H$_2$ among the different surfaces can be traced back and naturally explained by a resonance mediated tunneling mechanism. RES data for H$_2$/$h$-BN/Rh(111) suggests a local intrinsic gating on this surface due to lateral variations in the surface potential. An RES inspection of H$_2$, HD, and D$_2$ mixtures finally points to a multi molecule excitation, since either of the three $J=0\rightarrow2$ 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 C60_{60} 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$_{60}$ 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$_{60}$ 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