Experimentelle Physik II

Research projects

Research interests

We are interested in the correlation of structural, electronic, and magnetic properties of surfaces and nanostructured materials. Our main experimental methods are (spin-polarized) scanning tunneling microscopy (STM). By varying, for example, the microscope temperature or the applied external field these techniques are applied in various environments.

Our research program is based on the three columns below:

Atomic scale transport properties

The Molecular Nanoprobe (MONA)

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.

We recently developed 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) (see here and the highlight in Nature Nanotechnology for further information).  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 future, serve as a me­thod to map the charge den­sity dis­tri­bu­tion around an arbi­trary quasi­par­ticle in­jec­tion point.

Imaging magnetic nanostructures

Spin-Polarized Scanning Tunneling Microscopy (SP-STM)

The under­stan­ding of mag­neti­cally ordered states in solid-state mate­rials is an ex­ci­ting field of re­search.  Nowadays, ad­van­ced sur­face analy­sis and mi­cros­copy tools allow for the de­tec­tion of mag­ne­tic sig­nals with un­pre­ce­den­ted sen­si­ti­vi­ty and spa­tial re­so­lu­tion.  In this con­text, spin-po­la­rized scan­ning tun­ne­ling mi­cros­copy (SP-STM) is of par­ti­cu­lar inter­est as it enables to image the sample's spin struc­ture with ato­mic re­so­lu­tion.  For exam­ple, SP-STM al­lowed for the first di­rect ima­ging of anti­ferro­mag­netic sur­fa­ces and do­main walls and of frus­tra­ted Nèel spin states.  Fur­ther­more, it turned out that the spin-orbit–induced Dzya­lo­shins­kii-Morija inter­action (DMI) can be very sig­ni­fi­cant at sur­fa­ces and inter­faces.  As a result, spin structures with increasing com­plexity were de­tec­ted.  Where­as col­linear ferro- or anti­ferro­magnetism gover­ned dominated the scientific debate in the past, we have wit­nes­sed a ple­thora of more com­plex non-col­li­near mag­ne­tic states since the advent of the current century, such as non-collinear anti­ferro­mag­nets and chi­ral spin cycloid.

Topological materials

Unraveling twisted electronic states

The discovery of topological insulators (TIs) about 10 years ago led to intense research efforts towards the utilization of these materials in spintronic, magneto-electric, or quantum computation devices.  We employ several scanning probe-based techniques, such as scanning tunneling spectroscopy (STS), quasiparticle interference (QPI), or Landau level spectroscopy (LLS), to investigate the correlation between dissipation-less quantized transport and the onset of ferromagnetism in three-dimensional (3D) TIs.  In the meantime, we studied a wide range of bulk- and surface-doped 3D TIs. Our results paint a consistent picture of how and under which conditions these dopands lead to long-range magnetic order which breaks time- reversal symmetry.  For example, we find that the fate of the topological surface states critically depends on the TM element: while V- and Fe-doped Sb2Te3 display resonant impurity states in the vicinity of the Dirac point, Cr and Mn impurities leave the energy gap unaffected.