The excitonic insulator is an electronically driven phase of matter that emerges upon the spontaneous formation and Bose condensation of excitons. Detecting this exotic order in candidate materials is a subject of paramount importance, as the size of the excitonic gap in the band structure establishes the potential of this collective state for superfluid energy transport. However, the identification of this phase in real solids is hindered by the coexistence of a structural order parameter with the same symmetry as the excitonic order. Only a few materials are currently believed to host a dominant excitonic phase, Ta₂NiSe₅ being the most promising. In this talk, I will describe how advanced protocols based on time- and angle-resolved photoemission spectroscopy can shed light on primary order parameter of a candidate excitonic insulator [1]. Finally, I will discuss the opportunities offered by the development of novel momentum microscopy tools to extend these studies to the realm of two-dimensional material flakes that may host similar physics.

[1] E. Baldini et al., Proc. Natl. Acad. Sci. 120, e2221688120 (2023)

With the exception of the modified Auger parameter, X-ray induced Auger electron (X-AES) transitions aren’t exploited to their full potential. Indeed, they can provide as much information (oxidation degree, chemical environment, atomic composition) as the classic photopeaks used in XPS, but their shapes' complexities limit their decompositions.

[1] J.J. Moré, Numer. Anal. 630, 105 (1978).

[2]G.H. Golub and C. Reinsch, Linear Algebr. 420, 403 (1971).

[3] S. Béchu et al., Appl. Surf. Sci. 447, 528 (2018).

⁺Author for correspondence: solene.bechu@uvsq.fr

The photoelectric effect is sensitive to both the occupied electronic density of states and the electromagnetic field distribution. Thus, capturing the energy, yield, and spatial origin of photoelectrons from the sample enables us to examine local electronic properties and light-matter interactions concurrently. In this talk, we will describe two case studies using photoelectron emission microscopy (PEEM), revealing the spatial variations of Schottky barrier height between WS₂ and Au, and the local electromagnetic near-field profiles of Si metasurfaces. We will discuss the impact of crystallographic facets of Au grains as well as how the attractive interaction of Au with WS₂ can modify the crystallographic alignment among WS₂ layers. For near-field imaging, we will demonstrate the sensitivity of photoemission yield to the light absorptivity in visible to near infrared range, and evaluate the field profiles around Si meta atoms at the sub-(photon) wavelength scale on and off resonance excitation. Altogether we will discuss the potential of photoelectron imaging to examine the intertwined light-matter coupled phenomena abundant in two-dimensional and quantum materials.

The work was supported by Sandia’s LDRD program and in part by the US Department of Energy, Office of Basic Energy Sciences, Division of Materials Sciences and Engineering). The work performed at the U.S. Naval Research Laboratory (NRL) was supported through Base Programs funded by the Office of Naval Research and through the NeuroPipe ARAP funded by the Office of the Secretary of Defense. Samples were fabricated, in part, at the Center for Integrated Nanotechnologies, an Office of Science User Facility operated for the US Department of Energy, Office of Science. Sandia National Laboratories is a multimission laboratory managed and operated by National Technology and Engineering Solutions of Sandia, LLC., a wholly owned subsidiary of Honeywell International, Inc., for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-NA0003525. This paper describes objective technical results and analysis. Any subjective views or opinions that might be expressed in the paper do not necessarily represent the views of the U.S. Department of Energy or the United States Government.

The advent of intrinsic magnetic topological insulators enables us to envisage various low-dimensional topological orders, such as the quantum anomalous Hall insulators and the axion insulators, at realistic cryogenic temperatures. These materials are represented by MnBi₂Te₄ and its derived superlattices MnBi_2nTe_3n+1. However, it has been controversial whether these materials exhibit the key ingredient for magnetic topological phases: an energy gap due to the time-reversal symmetry breaking. Moreover, the construction of high-quality magnetic topological insulators at the ultrathin limit has met significant challenges. In this talk, I will present a new technique, layer-encoded frequency-domain photoemission spectroscopy, which allows us to decipher the layer origins of various electronic states. By encoding layer indices with intralayer phonon frequencies, we measure the strengths of coupling with layer-specific phonons. This experiment reveals that the topological surface states on antiferromagnetic MnBi₄Te₇ are partially relocated to the nonmagnetic layers, reconciling the mystery of vanishing broken-symmetry gaps [1]. Moreover, I will present our recent progress on the “carpet-growth” of Bi₂Te₃ ultrathin films and MnBi₂Te₄/Bi₂Te₃ heterostructures using molecular beam epitaxy. These thin films extend coherently across a millimeter spatial scale without disruptions by substrate step edges. Angle-resolved photoemission spectroscopy studies yield unprecedentedly sharp electronic structures in agreement with first-principles calculations layer-by-layer, and suggest opportunities to realize the quantum spin Hall effect and quantum anomalous Hall effect at near-ambient temperatures [2].

[1] W. Lee et al., Nature Physics 19, 950-955 (2023).