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Atomic-Scale Dynamics of Quantum Materials

Periodic Reporting for period 4 - dasQ (Atomic-Scale Dynamics of Quantum Materials)

Reporting period: 2019-12-01 to 2021-07-31

Interaction between electrons is at the heart of correlated-electron effects and marks one of the frontiers of modern condensed matter physics: a new class of solids where many-body physics and quantum effects determine the macroscopically observable properties. Key examples are high-temperature superconductors, charge- and spin-density wave materials and electronic insulators such as Mott insulators. By understanding these quantum materials, a new generation of devices may become available, greatly boosting our ability to handle information or harvest energy.

The goal of the dasQ project is to resolve the microscopic dynamics of electronic phases at the intrinsic length, time and energy scales of the underlying electron-electron interaction.

A key difficulty is that correlated-electron materials present static and dynamic inhomogeneity on multiple time and length scales that are profoundly linked to the emergence of the correlated behavior. To unravel this complexity, the dasQ project combines ultrafast pump probe spectroscopy at THz wavelength with atomic-resolution scanning tunneling microscopy into a tool that offers simultaneous atomic spatial and femtosecond time resolution. Strong enhancement of THz radiation in the STM’s tunnel junction enables highly local interaction with charge-ordered electronic phases and quasiparticle excitations. The extreme sensitivity of this new microscope to femtosecond-fast fluctuations of a material’s electronic structure enables it to resolve collective electronic excitations and their variation across inhomogeneous phases.
The dasQ project focusses this new capability on superconductors and materials with spatial charge order ranging from charge-density wave materials and Mott insulators to excitonic insulators. The tip-enhanced electric field of the THz excitation provides a strong local perturbation that can launch phase excitation wave packets in charge-density wave materials, electrically induce phase transitions to metastable electronic phases and even launch coherent acoustic phonon wave packets that locally alter material properties.

The findings of dasQ have proven that ultrafast dynamics of elementary excitations can indeed be observed with simultaneous atomic spatial and femtosecond temporal resolution. This provides a new experimental approach to the exploration of many-body phases in matter at the intrinsic length scale of charge correlation. The ability to transiently induce phase transitions by THz driving highlights new methods of controlling materials electrically at ultrafast timescales, thus providing insights for scaling of electronic devices using quantum materials.
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.
The dasQ project pushes the boundaries of high-precision microscopy and creates microscopes with a hitherto inaccessible combination of ultrafast time resolution and atomic spatial resolution. The first successful measurements of the electron motion in the charge-density wave compound NbSe2 showed intriguing results. We found that the dynamical behavior is inhomogeneous at a scale of just one nanometer and even collective modes of the charge-density wave state feature highly localized excitations at atomic pinning sites. We expect that other correlated-electron phases, in particular high-temperature superconductivity exhibit similar spatial fluctuations, which we plan to verify and quantify with the techniques developed in dasQ. Once we understand the interplay between spatial inhomogeneity and ultrafast fluctuations of collective modes we will continue to explore methods to control charge order locally by tip interaction, atom manipulation and coherent driving with THz fields. These experiments will provide a new view into the microscopic processes governing correlated-electron behavior.
Ultrafast scanning tunneling microscope. Image: Sebastian Loth