Observation of an Excitonic Mott Transition through Ultrafast Core-cum-Conduction Photoemission Spectroscopy

Editors' Suggestion
Open Access

Observation of an Excitonic Mott Transition through Ultrafast Core-cum-Conduction Photoemission Spectroscopy

Maciej Dendzik, R. Patrick Xian, Enrico Perfetto, Davide Sangalli, Dmytro Kutnyakhov, Shuo Dong, Samuel Beaulieu, Tommaso Pincelli, Federico Pressacco, Davide Curcio, Steinn Ymir Agustsson, Michael Heber, Jasper Hauer, Wilfried Wurth, Günter Brenner, Yves Acremann, Philip Hofmann, Martin Wolf, Andrea Marini, Gianluca Stefanucci, Laurenz Rettig, and Ralph Ernstorfer

Phys. Rev. Lett. 125, 096401 – Published 24 August 2020

Abstract

Time-resolved soft-x-ray photoemission spectroscopy is used to simultaneously measure the ultrafast dynamics of core-level spectral functions and excited states upon excitation of excitons in ${WSe}_{2}$ . We present a many-body approximation for the Green’s function, which excellently describes the transient core-hole spectral function. The relative dynamics of excited-state signal and core levels clearly show a delayed core-hole renormalization due to screening by excited quasifree carriers resulting from an excitonic Mott transition. These findings establish time-resolved core-level photoelectron spectroscopy as a sensitive probe of subtle electronic many-body interactions and ultrafast electronic phase transitions.

Received 19 March 2020
Revised 7 June 2020
Accepted 24 July 2020

DOI:https://doi.org/10.1103/PhysRevLett.125.096401

Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI. Funded by Bibsam.

Published by the American Physical Society

Physics Subject Headings (PhySH)

Electronic structure Excitons

Transition metal dichalcogenides

Green's function methods Time & angle resolved photoemission spectroscopy X-ray photoelectron spectroscopy

Condensed Matter, Materials & Applied Physics

Authors & Affiliations

Maciej Dendzik ^1,2,†, R. Patrick Xian ¹, Enrico Perfetto^3,4, Davide Sangalli ^3,5, Dmytro Kutnyakhov⁶, Shuo Dong ¹, Samuel Beaulieu ¹, Tommaso Pincelli¹, Federico Pressacco ⁷, Davide Curcio⁸, Steinn Ymir Agustsson ⁹, Michael Heber⁶, Jasper Hauer¹, Wilfried Wurth^6,7,*, Günter Brenner⁶, Yves Acremann¹⁰, Philip Hofmann⁸, Martin Wolf¹, Andrea Marini ³, Gianluca Stefanucci ^4,11, Laurenz Rettig ¹, and Ralph Ernstorfer ^1,‡

¹Fritz Haber Institute of the Max Planck Society, Faradayweg 4-6, 14915 Berlin, Germany
²Department of Applied Physics, KTH Royal Institute of Technology, Hannes Alfvéns väg 12, 114 19 Stockholm, Sweden
³CNR-ISM, Division of Ultrafast Processes in Materials (FLASHit), Area della Ricerca di Roma 1, Via Salaria Km 29.3, I-00016 Monterotondo Scalo, Italy
⁴Department of Physics, Tor Vergata University of Rome, Via della Ricerca Scientifica 1, 00133 Rome, Italy
⁵Department of Physics, University of Milan, via Celoria 16, I-20133 Milan, Italy
⁶DESY Photon Science, Notkestr. 85, 22607 Hamburg, Germany
⁷Center for Free-Electron Laser Science CFEL, Hamburg University, Luruper Chausee 149, 22761 Hamburg, Germany
⁸Department of Physics and Astronomy, Interdisciplinary Nanoscience Center (iNANO), Aarhus University, Ny Munkegade 120, 8000 Aarhus C, Denmark
⁹Institute of Physics, Johannes Gutenberg University Mainz, D-55128 Mainz, Germany
¹⁰Department of Physics, Laboratory for Solid State Physics, ETH Zurich, Otto-Stern-Weg 1, 8093 Zurich, Switzerland
¹¹INFN, Sezione di Roma Tor Vergata, Via della Ricerca Scientifica 1, 00133 Rome, Italy

^*Deceased.
^†dendzik@kth.se
^‡ernstorfer@fhi-berlin.mpg.de

Article Text

Click to Expand

Supplemental Material

Click to Expand

References

Click to Expand

Issue

Vol. 125, Iss. 9 — 28 August 2020

Reuse & Permissions

Author publication services for translation and copyediting assistance advertisement

Images

Figure 1
(a) Sketch of the experimental setup for pump-probe trXPS on ${WSe}_{2}$ using a momentum microscope, which enables the simultaneous analysis of core, valence, and conduction electrons. A sample is excited with a near-infrared (NIR) pulse and probed with an extreme ultraviolet (XUV) pulse. (b) Energy band diagram of the photoexcited ${WSe}_{2}$ near the $\bar{K}$ valley: the NIR pulse (red) resonant with the exciton energy generates excited carrier populations in the valence ( $h^{+}$ ) and conduction ( $e^{-}$ ) bands. Transient conduction band population as well as the induced renormalization of the screened core-hole spectral function are detected by the XUV pulse (violet). The core linewidth is affected by the screening (polarization bubbles) generated by particle-hole excitations both in the valence and conduction bands, here included through the self-energy $Σ$ . (c) Core-cum-conduction trXPS spectra showing W $4 f_{5 / 2}$ (left) and momentum-integrated conduction band (right) regions, before (blue) and after the excitation (red). Colored arrows indicate corresponding full width at half maximum of the spectra and the shaded area is the symmetric part of the line shape, illustrating its asymmetry.
Reuse & Permissions
Figure 2
(a) Representative energy spectra at four delay times marked in (e). Red points, black lines, and blue lines mark experimental data, fit result, and normalized residuals, respectively. Normalized residuals are presented in multiples of the data’s Poisson distribution standard error. (e) trXPS spectrum of W $4 f_{5 / 2}$ as a function of pump-probe delay time. Photoemission intensity is encoded in the false-color scale. (f) Corresponding modeled spectral function, defined by Eqs. (1)–(4).
Reuse & Permissions
Figure 3
(a) Comparison of the time-dependent effective screening $n_{QFC} ν^{2}$ (black circles) and broadening $γ$ (red triangles), obtained from fitting the theoretical model to the experimental data presented in Fig. 2. (b) Comparison of the photoemission signal above the Fermi level, corresponding to the CB population, (blue squares) and $γ$ (red triangles). (c) Decomposition of the dynamics of excitons (red squares), QFC (blue circles), and total excited carriers (green triangles). Black lines present fitted double exponential and single exponential decay for excitons and QFC, correspondingly. (d) The degree of ionization dynamics, as defined in the main text. Three different temporal regions are marked by the background color: before the excitation (white), Mott transition (red), and QFC regions (blue).
Reuse & Permissions
Figure 4
Illustration of the effect of excitons and QFCs on the core-hole line shape. Charge-neutral electron-hole pairs only weakly screen the core holes and have a negligible effect on its spectral function (left). In contrast, single-particle-like QFCs more effectively screen the localized charge residing in the core hole, resulting in a renormalization of the photoemission line shape (right).
Reuse & Permissions

Physical Review Letters