In the late 1950’s it was recognized that a current passing through a tunneling barrier contains direct information
about the electronicdensity of states (DOS) of the two electrodes enclosing such a tunnelingbarrier [1-3]. This fundamental insight was awarded
with the Nobel prize in1973. However, the challenge was in realizing such a tunneling barrier (typicallybetween a few Å up to a few nm in thickness).
At that time, the necessity for fabricating at least one of the electrodes as well as the tunneling barrier asthin films severely limited the
feasibility of tunneling experiments to providea general route for studying the DOS of a given material.
A major breakthrough was made when one of the electrodes was replaced by a sharply pointed tip the relative position of which (with respect to
the sample under investigation) could be steered by piezoelectric elements , again an achievement that was awarded with the Nobel prize (1986).
The tunneling barrier is defined by controlling the tunneling current, I, such that a finite distance between tip and sample is maintained. The
possibility of scanning the tip to obtain raster images led to the term Scanning Tunneling Microscope (STM). The DOS of the sample can be probed
by ramping the applied voltage, V, and register I(V) through the tunneling barrier at constant tip position with respect to the sample. This
Scanning Tunneling Spectroscopy (STS) can be conducted at predefined sample positions or in a grid-like fashion resulting in the unprecedented
possibility to obtain information on a material’s DOS with sub-atomic local, and sub-meV energy resolution.
It was soon realized that other physical properties (like atomic and magnetic forces) can also be probed by a scanning tip . Specifically the
invention of the Atomic Force Microscope (AFM)  sparked an enormous technological development into a field that is nowadays known as
Scanning Probe Microscopy (SPM). But also STM has profoundly matured, with applications ranging from the tracking of Brownian motions on surfaces
to inelastic tunneling spectroscopy, to just name a few examples.
In our research we focus on applications of STM/S in solid state physics of
bulk materials in which strong electronic correlations result in fascinating and exiting, yet often not fully understood new phenomena as, e.g.,
colossal magnetoresistance, quantum criticality or the fractional quantum Hall effect. For further reading, please,
 L. Esaki, Phys. Rev. 109 (1958) 603.
 J. C. Fisher and I. Giaever, J. Appl. Phys. 32 (1961) 172.
 J. Bardeen, Phys. Rev. Lett. 6 (1961) 57.
 G. Binnig, H. Rohrer, C. Gerber and E. Weibel, Appl. Phys. Lett. 40 (1982) 178.
 see, e.g., R. Wiesendanger, “Scanning Probe Microscopy and Spectroscopy” Cambridge University Press 1994.
 G. Binnig, C. F. Quate and C. Gerber, Phys. Rev. Lett. 56 (1986) 930.