Collective interactions reveal atomic-scale non-adiabatic carrier–phonon coupling
Ursula Keller
Emeritus Physics Professor ETH Zurich, Switzerland, Link and Visiting Professor at Tampere University
Attosecond transient absorption spectroscopy (ATAS) show how ultrashort near-infrared laser pulses drive sub-femtosecond localization and collective interactions around transition metal centers in both bulk and low-dimensional systems. We have investigated four systems—Ti, MoSe₂, β-W, and MXene—revealing a unified picture of how localization-mediated screening and element- and orbital-specific local field effects (LFEs) can dominate ultrafast electronic behavior. We then used the LFEs to probe the carrier-phonon coupling which clearly showed a non-adiabatic delay of around 30 fs.
Let me first summarize the physical background of ultrafast localization observed and explained in our group for the first time: In Ti metal [Nature Physics 1999, Link] near-instantaneous localization of excited carriers onto 3d orbitals induces local screening that modifies absorption response, which cannot be explained by independent-particle models. We did not observe this for Al as a control experiment [PRX 2022, Link]. In β-tungsten [PRL 2023, Link], a transition is observed from initial Pauli blocking to delayed 5d-orbital localization and screening, marking a temporal crossover from free-electron and collective behavior. Compared to Ti, LFEs are less pronounced in β-tungsten due to the greater delocalization of 5d-orbitals relative to 3d orbitals, and because Ti has four times more excited states/atom. This explains why independent-electron dynamics initally dominate in β-tungsten with correlated-electron effects emerging only after carrier thermalization.
In layered MoSe₂ [PNAS 2023, Link], unexpectedly, we received contradictory answers when we investigated how the electrons of the compound semiconductor MoSe2 behave within a few femtoseconds after absorbing a short pulse of infrared light. Even though the electronic properties of MoSe2 are determined by the bonds of Mo and Se atoms, to which both elements contribute equally, the material shows opposite faces depending on through which of its atomic constituents it is probed. With the ATAS probe we used the fact that we were able to observe the energy region of the bonds in the compound semiconductor MoSe2 via low-lying energy levels of the Mo as well as the Se atoms, simultaneously. ATAS then reveals strikingly different responses from molybdenum and selenium atoms: Mo exhibits collective, localization-driven screening, while Se shows independent state-filling dynamics. These results demonstrate that d-orbital-specific many-body interactions persist even in compound semiconductors. As a result, the concept of an effective mass does not apply to these semiconductors.
The investigation with regards to functionalized MXene crystal Ti₃C₂Tₓ (Tₓ = O, OH, F) [1] we have used the ultrafast localization in the d-orbital as a probe to reveal orbital-specific modifications of LFEs induced by coherent phonons, probed via the shallow-core Ti 3p state. We demonstrate that phonon-induced changes in carrier localization modulate LFEs, resulting in carrier-, site-, and orbital-specific absorption signatures. These LFEs serve as sensitive fingerprints of electron-phonon coupling strength across the phonon spectrum, enabling direct access to the underlying relaxation dynamics with atomic-scale resolution. By tracking hot-carrier responses to coherent lattice motion, we observe a clear breakdown of the Born–Oppenheimer approximation: electrons lag behind the lattice oscillations by up to 31 ± 8 fs, while holes respond nearly instantaneously (12 ± 8 fs).
[1] S. Neb, D. Shin, F. Burri, M. Hollm, E. W. de Vos, D. A. Kuznetsov, C. R. Müller, A. Fedorov, S. A. Sato, A. Rubio, L. Gallmann, U. Keller, “Local fields reveal atomic-scale non-adiabatic heat dissipation, ”Science, submitted and under review
