arXiv subjects

Abigail Dishi

Publications and source records attributed to Abigail Dishi.

4 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

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

Chiral charge density wave in 4Hb- and 1T-TaS$_2$: The Role of interlayer coupling

We use micro-angle-resolved photoemission spectroscopy (micro-ARPES) to investigate chiral charge density waves (CDWs) in 4Hb-TaS$_2$ with micron-scale spatial resolution. In the 1T layers of 4Hb-TaS$_2$, we uncover coexisting left- and right-handed CDW domains and resolve four distinct spectral patterns arising from the interplay of chirality and rotational stacking. In contrast, bulk 1T-TaS$_2$ exhibits a uniform chirality. In addition, 4Hb-TaS$_2$ shows negligible out-of-plane dispersion of the 1T-derived bands, in contrast to the pronounced interlayer coupling observed in bulk 1T-TaS$_2$. Density functional theory (DFT) calculations corroborate this picture, revealing that the interlayer interaction of the chiral order in 4Hb-TaS$_2$ is nearly two orders of magnitude weaker than in the 1T polytype. Our findings establish 4Hb-TaS$_2$ as a quasi-two-dimensional platform for exploring tunable chiral CDW phenomena.

cond-mat.str-el↗

arXiv2025

Emergence of a Fermi-surface in the current-driven Hidden state of 1T-TaS$_2$

The origin of the insulating state in 1T-TaS$_2$ has long been a subject of debate. A short current pulse transforms this insulating state into a metastable metallic phase. Using micro-ARPES, we investigate the electronic structure of this phase and uncover spatially dependent modifications caused by the current pulse. In some regions of the sample, a Fermi surface emerges, while others remain gapped. Detailed band structure analysis reveals that the metallic regions exhibit an electronic structure similar to that observed in the high-temperature phase of 1T-TaS$_2$, characterized by suppressed energy gaps and bands crossing the Fermi level. Furthermore, the metallic and insulating regions display distinct dispersions along the out-of-plane direction. These observations suggest a scenario in which the current pulse breaks the star-of-David dimers characteristic of the insulating phase, implicating these dimers as the likely origin of the insulating behavior in 1T-TaS$_2$.

cond-mat.str-el↗