Experiments capable of resolving ultrafast electron dynamics, e.g. time-resolved photoemission or ultrafast optical spectroscopy, typically lack spatial resolution. Even new techniques using x-ray free electron lasers are often insensitive to spatial inhomogeneity because they are momentum-space probes and average over large areas in real space. Scanning tunneling microscopy, on the other hand, offers atomic resolution in real space. But until recently it had been blind to ultrafast dynamics.
The first key achievement of the dasQ project is the successful development of a new scanning tunneling microscope that uses THz pulses to capture ultrafast electron dynamics stroboscopically. The new microscope reaches 190 fs time resolution while maintaining atomic spatial resolution and high detection sensitivity of a fraction of an electron per pulse (0.002 e-/pulse).
This new microscope can record movies of the motion of electrons on surfaces and was first applied to explore the interplay between collective modes of a charge-density wave material and atomic defects. For the first time, it was possible to observe how a single defect captures the charge density wave and prevents its free motion through the material that would otherwise carry electric current without loss of energy.
Beyond imaging surface dynamics, the extreme enhancement of the THz pulses’ electric field in the tunnel junction was found to be of sufficient strength to modify materials. Ultrafast displacement currents and ultrafast Coulomb forces that are induced in the materials’ surfaces by the THz pulses can drive strong excitations at a timescale of less than one picosecond. With this unconventional interaction method, it became possible to induce nanoscale phase transitions to metastable electronic states. This provided a new avenue to measure the elusive atomic-scale fluctuations in superconductors: at defects that modify the superconducting ground state, picosecond-fast electric driving found long-lived excitations with lifetimes in the nanosecond range that strongly modify the material’s conductivity.
These findings have resulted in several peer-reviewed publications and presentations at international conferences, workshops and colloquia. The dasQ project demonstrates that elementary excitations of electrons, phonons and spins can be observed with simultaneous atomic spatial and femtosecond temporal resolution. This new capability sheds light on the intricate links between spatial heterogeneity and ultrafast electronic dynamics in matter and highlights possibilities how electronic phases can be controlled electrically at nanometer length scales and ultrafast speeds.
