arXiv subjects

Ittai Sidilkover

Publications and source records attributed to Ittai Sidilkover.

5 recordsLinked to original sources

arXiv2026

Light-induced trion-exciton competition revealed by ultrafast photoemission

Strong Coulomb interactions in low-dimensional quantum materials give rise to emergent bound states such as excitons and trions. Trions are conventionally secondary excitations, requiring both optical excitation and charge doping. In quasi-one-dimensional Ta$_2$NiS$_5$, however, an exceptionally large binding energy exceeding the single-particle band gap stabilizes an equilibrium trion gas under surface doping alone. Here, using time- and angle-resolved photoemission spectroscopy, we show that trions can also be generated purely optically, without external charge. Following photoexcitation of pristine Ta$_2$NiS$_5$ we observe a bright, momentum-localized, in-gap feature with a slow, fluence-dependent relaxation. Rate-equation modeling identifies it as a mixed population of trions and excitons, with the trions formed via an unconventional single-particle pathway that requires no photoexcited hole. The trion-exciton composition is controlled by pump fluence, with trions dominating the late-time relaxation. In surface doped samples, the same model with unchanged rates reproduces the full dynamics, linking the light-induced trions to their equilibrium counterparts through pump-induced dissociation and recapture. These results establish trARPES as a direct probe of charged quasiparticles far from equilibrium and open routes to optical control of neutral and charged excitations in correlated materials.

cond-mat.str-el↗

arXiv2026

Resolving the phase of a Dirac topological state via interferometric photoemission

The electronic wavefunction is at the heart of physical phenomena, defining the frontiers of quantum materials research. While the amplitude of the electron wavefunction in crystals can be measured with state-of-the-art probes in unprecedented resolution, its phase has remained largely inaccessible, obscuring rich electronic information. Here we develop a quantum-path electron interferometer based on time- and angle-resolved photoemission spectroscopy, that enables the reconstruction of phase information associated with electronic states, as encoded in the photoemission transition amplitudes - with energy and momentum resolution. We demonstrate the scheme by resolving the phase along the Dirac electronic band of a prototypical topological insulator and observe a resonance-associated phase jump as well as a momentum and phase synchronized inversion revealing the helicity of the Dirac cone. We show the interferometer can be optically controlled by the polarization of the absorbed light, allowing a differential measurement of the phase - a crucial component for extracting phase information from an interferogram. This photo-electron-interferometer provides direct experimental access to the phase of electronic transition amplitudes. Its implementation relies on experimentally accessible conditions - such as the presence of a suitable intermediate state and polarization-selective coupling - and can therefore be extended to a wide class of materials.

cond-mat.mes-hall↗

arXiv2026

Trion gas on the surface of a failed excitonic insulator

Trions, three-body bound states composed of an exciton and an additional charge, are typically fragile and require external excitation to form. Here, we report the spontaneous emergence of a stable trion gas at the surface of the layered semiconductor Ta2NiS5, revealed through angle-resolved photoemission spectroscopy. We observe a sharp, highly localized in-gap feature that cannot be explained by conventional band-theory. Instead, we argue that it arises from the formation of negative trions, stabilized by surface-induced band bending and the material's quasi-one-dimensional geometry. Unlike excitons, these trions form without optical pumping and persist at equilibrium, marking a rare example of an interaction-driven surface state in a nominally conventional semiconductor. Our findings establish Ta2NiS5 as a unique platform for exploring many-body physics at surfaces and open new avenues for studying and controlling collective excitations in low-dimensional systems.

cond-mat.str-el↗

arXiv2025

Mesoscale variations of chemical and electronic landscape on the surface of Weyl semimetal Co$_3$Sn$_2$S$_2$ visualized by ARPES and XPS

The multiple crystalline terminations in magnetic Weyl semimetal Co$_3$Sn$_2$S$_2$ display distinct topological and trivial surface states, which have successfully been distinguished experimentally. However, a model of pure terminations is known to be inadequate because these surfaces exhibit a high degree of spatial heterogeneity and point disorder. Here we perform a spectromicroscopy study of the surface chemistry and surface electronic structure using photoemission measurements in combination with first-principles calculations of core levels. We identify an intermediate region with properties distinct from both the sulfur and tin terminations, and demonstrate that the spectral features in this region can be associated with a disordered termination with a varying density of surface tin vacancies. This work establishes heuristics for identifying variable surface disorder using photoemission, an important prerequisite to experimentally establishing the behavior of momentum-space topological surface features subject to variable surface disorder on a single cleave.

cond-mat.mtrl-sci↗

arXiv2025

Reexamining Circular Dichroism in Photoemission From a Topological Insulator

The orbital angular momentum (OAM) of electron states is an essential ingredient for topological and quantum geometric quantities in solids. For example, Dirac surface states with helical spin- and orbital-angular momenta are a hallmark of a 3D topological insulator. Angle-resolved photoemission spectroscopy (ARPES) with variable circular light polarization, known as circular dichroism (CD), has been assumed to be a direct probe of OAM and, by proxy, of the Berry curvature of electronic bands in energy- and momentum-space. Indeed, topological surface states have been shown to exhibit angle-dependent CD (CDAD), and more broadly, CD is often interpreted as evidence of spin-orbit coupling. Meanwhile, it is well-established that CD originates from the photoemission matrix elements, which can have extrinsic contributions related to the experimental geometry and the inherently broken inversion symmetry at the sample surface. Therefore, it is important to broadly examine CD-ARPES to determine the scenarios in which it provides a robust probe of intrinsic material physics. We performed CD-ARPES on the canonical topological insulator $\mathrm{Bi}_2\mathrm{Se}_3$ over a wide range of incident photon energies. Not only do we observe angle-dependent CD in the surface states, as expected, but we also find CD of a similar magnitude in virtually all bulk bands. Since OAM is forbidden by inversion symmetry in the bulk, we conclude this originates from symmetry-breaking in the photoemission process. Comparison with theoretical calculations supports this view and suggests that $\textit{hidden}$ OAM - localized to atomic sites within each unit cell - contributes significantly. Additional effects, including inter-atomic interference and final-state resonances, are responsible for the rapid variation of the CDAD signal with photon energy.

cond-mat.mtrl-sci↗