2018

Nov. 29, 2018

Organic chem­istry allows for a basically infinite number of mo­di­fi­ca­tions of molecular struc­tures.  For example, substituents added to a molecule can lead to either a stronger or a weaker coupling to the substrate.  We performed a comparative study of tetra-tert-butyl phthalocyanine (ttbPc) isomers on a Ag(111) surface by means of low-temperature scanning tunneling microscopy to analyze the influence of substituents on the tautomerization, a proton transfer reaction, in single molecules.  By comparison with other well-studied molecules we find a decoupling of ttbPc from the surface.  We find significant differences between the four ttbPc isomers which naturally exist due to different bonding positions of the tert-butyl groups on the macrocycle.  The result has been published in the Journal of Physical Chemistry C.

Nov. 8, 2018

The control of the charge- and spin state of single molecules is key for the design of mo­le­cu­lar elec­tro­nic and spin­tro­nic de­vi­ces.  For swit­ching the spin or mag­ne­tic state of mo­le­cules two prin­ci­ples have been used in the past:  (1) atoms or small mo­le­cu­les were ad­ded to and re­mo­ved from the host mo­le­cule or (2) by mo­di­fying the shape of mo­le­cu­les which can exist in more than one con­fi­gu­ra­tion.  The re­a­li­za­tion of both me­thods is ra­ther dif­fi­cult.  In co­ope­ration with theo­rists around Prof. Giorgio Sangiovanni (TP I), in a re­cent pub­li­ca­tion our group presented an al­ter­na­tive way to achieve mul­tiple and re­ver­si­bly switchable spin states in a single mo­le­cu­les.   These re­sults show that a bi­stable mo­le­cu­lar state can be achie­ved by po­si­tio­ning single man­ga­nese phthalo­cyanine (MnPc) mo­le­cu­le on top of an anti­phase boun­da­ry of the (3×3)-recon­struc­ted sur­face alloy of bismuth on Ag(111).   In this environment we can switch single molecules bet­ween a high- and a low-spin state by using the elec­tric field of a sharp STM tip.  The result has been pub­lished in npj Quantum Materials. 

Aug. 18, 2018

Molecular machines rely on the directional motion of sub-molecular units, which is often achieved by built-in structural chiralities.  In a recent scanning tunneling microscopy study, Jens Kügel and Markus Leisegang discovered that even achiral H₂Pc and HPc molecules can perform chiral intra-molecular motion.  The deposition onto a Ag(100) surface leads to an adsorption-geometry-induced chirality which is caused by a rotation of the molecules with respect to the high-symmetry directions of the substrate.  As a result, tautomerization processes referentially occur in a clockwise or counterclockwise direction.  In contrast to built-in structural chiralities that are fixed by the molecular structure, the direction of proton motion in HPc on Ag(100) can be inverted by a rotation of the molecule on the substrate. The result has been published in ACS Nano.

May 14, 2018

By spin-po­la­rized STM, a mag­ne­ti­cal­ly sen­si­tive mi­cros­co­py tech­nique with ato­mic re­so­lu­tion ca­­pa­­bi­­li­­ty, we could ob­serve a no­vel theo­re­ti­cal­ly pre­dic­ted mag­ne­tic ground state, a so-called up-up-down-down (↑↑↓↓) spin struc­ture. As shown in the STM ima­ge above, this exo­tic mag­ne­tic struc­ture exists in three orien­ta­tio­nal do­mains in a mono-layer Fe on Rh(111). Cal­cu­la­tions per­for­med at the For­schungs­zentrum Jü­lich re­veal that this mag­ne­tic struc­ture is caused by a beyond-Heisen­berg inter­ac­tion. The result has been published in Phys. Rev. Lett.

Feb 27, 2018

The electrical resistance of materials or devices is often measured by the 4-point probe method to eliminate lead and contact resistance issues. In the recent decade, miniaturized versions, so-called nanoprobes, have been developed where four sharp tips are individually positioned by piezo-actuators and simultaneously imaged with a scanning electron microscope (see here for an example). However, the finite tip sharpness usually limits the minimal distance to about 100 nm.

In a new publication, which was also highlighted in Nature Nanotechnology, we describe the development of a novel method which enables to detect how charged quasiparticles propagate on length scales down to a few nm by remotely triggering the tautomerization of a single molecule with a scanning tunneling microscope (STM).  In analogy to the 4-point nanoprobe we coined it “molecular nanoprobe” (MONA).  In combination with atom-by-atom-engineered interferometers, MONA allows to unravel the quantum-mechanical wave nature of hot electrons. Two interferometers can even be combined to build an energy-dependent selector, which allows it to selectively switch one out of two molecules without changing the position of the STM tip. The MONA technique may, in the future, serve as a method to map the charge density distribution around an arbitrary quasiparticle injection point. more

Feb 13, 2018

In a recent highly collaborative publication in Phys. Rev. B with colleagues from Sweden, Italy, Russia, France, and Spain, we report on a syste­ma­tic in­ves­tiga­tion of the tran­sition metal-doped to­po­lo­gi­cal in­su­la­tor Sb₂Te₃. By com­bi­ning den­sity func­tio­nal theo­ry with com­ple­men­tary ex­pe­ri­men­tal tech­niques, i.e., STM, re­so­nant photo­emis­sion, and x-ray mag­ne­tic cir­cu­lar dichro­ism, we achie­ved a detailed charac­teri­zation of the elec­tro­nic and mag­netic pro­per­ties. Our results pro­vide ge­ne­ral guide­lines how to rea­lize a ro­bust QAHE by do­ping Sb₂Te₃ towards a ferromagnetic state.