The acene series represents a unique model system to investigate the intriguing electronic properties of extended π-electron structures in the one-dimensional limit, which are important for applications in electronics and spintronics and for the fundamental understanding of electronic transport. Here we present the on-surface generation of the longest acene obtained so far:Dodecacene. Scanning tunneling spectroscopy gives access to the energy position and spatial distribution of its electronic states on the Au(111) surface. We observe that, after a progressive closing of the gap and a stabilization to about 1 eV at the length of decacene and undecacene, the energy gap of dodecacene unexpectedly increases to 1.4 eV. Considering the acene series as an exemplary general case, we discuss the evolution with length of the single tunneling resonances in comparison with ionization energy, electronic affinity, and optical gap.

The evolution of low-energy spin dynamics in the iron-based superconductor LaFeAsO 1−x F x was studied over a broad doping, temperature, and magnetic field range (x = 0-0.15, T 480 K, μ 0 H 30 T) by means of 75 As nuclear magnetic resonance. An enhanced spin-lattice relaxation rate divided by temperature (T 1 T ) −1 in underdoped superconducting samples (x = 0.045, 0.05, and 0.075) suggests the presence of antiferromagnetic spin fluctuations, which are strongly reduced in optimally doped (x = 0.10) and completely absent in overdoped (x = 0.15) samples. In contrast to previous analysis, Curie-Weiss fits are shown to be insufficient to describe the data over the whole temperature range. Instead, a Bloembergen-Purcell-Pound (BPP) model is used to describe the occurrence of a peak in (T 1 T ) −1 clearly above the superconducting transition, reflecting a progressive slowing down of the spin fluctuations down to the superconducting phase transition.

The realization of a train of molecule-gears working under the tip of a scanning tunneling microscope (STM) requires a stable anchor of each molecule to the metal surface. Such anchor can be promoted by a radical state of the molecule induced by a dissociation reaction. Our results, rationalized by density functional theory calculations, reveal that such open radical state at the cyclopentadiene core of star-shaped pentaphenylcyclopentadiene (PPCP) favors the anchoring. Furthermore, to allow the transmission of motion by STM manipulation, the molecule-gear should be equipped with specific groups facilitating the tip-molecule interactions. In our case, a tert-butyl group positioned at one tooth end of the gear benefits both the tipinduced manipulation and the monitoring of rotation. With this optimized molecular system we achieve reproducible and stepwise rotations of the single gears and transmit rotations up to three interlocked units.

Superimposed STM images of the six stable rotation stations of a DMBI-P molecule during a step by step clockwise rotation induced by voltage pulses (V = 0.5 V, I = 0.5 nA, t = 10 s).

The voltage-pulse manipulation of azulene-based single molecules and dimers adsorbed on Au(111) is studied with respect to dipole moment and charge distribution of the structures on the surface.