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Atomic-Scale Dynamics of Quantum Materials

Periodic Reporting for period 3 - dasQ (Atomic-Scale Dynamics of Quantum Materials)

Reporting period: 2018-06-01 to 2019-11-30

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. 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. Strong enhancement of THz radiation in the STM’s tunnel junction enables highly local interaction with charge-ordered electronic phases and quasiparticle excitations.

We aim to unravel the microscopic mechanism of charge density wave capture at singular pinning sites and image the spatial inhomogeneity of quasiparticle fluctuations in superconductors. We will explore methods to control charge order locally by tip interaction, atom manipulation and coherent driving with THz fields. These experiments will impact many aspects of correlated-electron materials.

The success of the dasQ project will create new experiments that interact with many-body phases at the intrinsic length scale of charge correlation and will identify opportunities 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.

In parallel, work is in progress to encroach on the elusive quasiparticle fluctuations in a superconductor and to map their dynamics in real space. This will make it possible to determine where superconductivity is created most efficiently and may highlight pathways to tailor its emergence.
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