Gunnar Schulze

Competition of Superconducting Phenomena and Kondo Screening at the Nanoscale

A manganese complex adsorbed on a superconducting lead surface creates a mosaic of two magnetic ground states. Magnetic and superconducting interactions couple electrons together to form complex states of matter. We show that, at the atomic scale, both types of interactions can coexist and compete to influence the ground state of a localized magnetic moment. Local spectroscopy at 4.5 kelvin shows that the spin-1 system formed by manganese-phthalocyanine (MnPc) adsorbed on Pb(111) can lie in two different magnetic ground states. These are determined by the balance between Kondo screening and superconducting pair-breaking interactions. Both ground states alternate at nanometer length scales to form a Moiré-like superstructure. The quantum phase transition connecting the two (singlet and doublet) ground states is thus tuned by small changes in the molecule-lead interaction.

Resonant electron heating and molecular phonon cooling in single C60 junctions

We study heating and heat dissipation of a single C(60) molecule in the junction of a scanning tunneling microscope by measuring the electron current required to thermally decompose the fullerene cage. The power for decomposition varies with electron energy and reflects the molecular resonance structure. When the scanning tunneling microscope tip contacts the fullerene the molecule can sustain much larger currents. Transport simulations explain these effects by molecular heating due to resonant electron-phonon coupling and molecular cooling by vibrational decay into the tip upon contact formation.

Reversing the Thermal Stability of a Molecular Switch on a Gold Surface: Ring-Opening Reaction of Nitrospiropyran

The ring-opening/closing reaction between spiropyran (SP) and merocyanine (MC) is a prototypical thermally and optically induced reversible reaction. However, MC molecules in solution are thermodynamically unstable at room temperature and thus return to the parent closed form on short time scales. Here we report contrary behavior of a submonolayer of these molecules adsorbed on a Au(111) surface. At 300 K, a thermally induced ring-opening reaction takes place on the gold surface, converting the initial highly ordered SP islands into MC dimer chains. We have found that the thermally induced ring-opening reaction has an activation barrier similar to that in solution. However, on the metal surface, the MC structures turn out to be the most stable phase. On the basis of the experimentally determined molecular structure of each molecular phase, we ascribe the suppression of the back reaction to a stabilization of the planar MC form on the metal surface as a consequence of its conjugated structure and large electric dipole moment. The metal surface thus plays a crucial role in the ring-opening reaction and can be used to alter the stability of the two isomers.

Switching ability of nitro-spiropyran on Au(111): electronic structure changes as a sensitive probe during a ring-opening reaction

Spiropyran is a prototype molecular switch which undergoes a reversible ring-opening reaction by photoinduced cleavage of a C-O bond in the spiropyran (SP) to the merocyanine (MC) isomer. While the electronic states and switching behavior are well characterized in solution, adsorption on metal surfaces crucially affects these properties. Using two-photon photoemission and scanning tunneling spectroscopy, we resolve the molecular energy levels on a Au(111) surface of both isomeric forms. Illumination at various wavelengths does not yield any observable switching rate, thus evidencing a very small upper limit of the quantum efficiency. Electron-induced switching from the SP to the MC isomer via generation of a negative ion resonance can be detected with a quantum yield of (2.2 ± 0.2) × 10(-10) events/electron in tunneling spectroscopy. In contrast, the back reaction could not be observed. This study reveals that the switching properties of surface-bound molecular switches can be very different compared with free molecules, reflecting the strong influence of the interaction with the metal substrate.

Dynamic Jahn-Teller effect in electronic transport through single C 60 molecules

Low-temperature scanning tunneling spectroscopy and first-principles calculations are used to characterize electron transport through vibronic states of

Resonant heating and substrate-mediated cooling of a single C60 molecule in a tunnel junction

{\text{C}}_{60}

Induction of a Photostationary Ring-Opening—Ring-Closing State of Spiropyran Monolayers on the Semimetallic Bi(110) Surface

molecules. This is achieved by positioning a

Inducing the Rotation of a Single Phenyl Ring with Tunneling Electrons

{\text{C}}_{60}

Reducing the Molecule-Substrate Coupling in C60-Based Nanostructures by Molecular Interactions

molecule on top of a molecular self-assembled template on Au(111). In these conditions, conductance spectra are shown to reveal the dynamic Jahn-Teller effect of the

Excitation of Jahn−Teller Active Modes during Electron Transport through Single C60 Molecules on Metal Surfaces

{\text{C}}_{60}