Federico Cilento

Altermagnetism defies conventional classifications of collinear magnetic phases, combining compensated magnetization with momentum‐dependent, nonrelativistic spin splitting. While altermagnetic band splitting has been established in three‐dimensional compounds such as MnTe and CrSb, layered altermagnets remain far less explored spectroscopically. Here, combining termination‐resolved micro‐ARPES, fine photon‐energy mapping, spin‐ARPES, and density functional theory, we map the three‐dimensional fermiology of the layered g ‐wave altermagnet , obtained by ordered Co intercalation of 2H‐. We resolve the spin–split branches of the altermagnetic pairs directly in the raw photoemission spectra and, with spin‐ARPES at these experimentally identified momenta, show the reversal of the spin polarization at opposite momenta and between two regions of the sample, consistent with opposite time‐reversal domains, and its absence along the symmetry‐protected nodal –K direction. We further report a weak low‐temperature anomaly near T ₀ ≈ 50–60 K, detected by magnetometry, temperature‐dependent micro‐ARPES, and ultrafast reflectivity, whose microscopic origin remains open. These results provide a momentum‐resolved benchmark of the electronic structure of a layered altermagnet.

With this Comment, we bring to the attention of the scientific community that, to the best of our knowledge, no circularly polarized third harmonics in gases can be generated by starting from a circularly polarized fundamental beam. Hence, the evidences reported in the paper by Gao et al. [Rev. Sci. Instrum. 96, 093004 (2025)] should arise from complementary effects, or, if proved, they definitely need extensive explanation and discussion.

The dispersion of topological surface states in three-dimensional topological insulators (TIs) is commonly attributed to the atomic spin-orbit coupling (SOC) of the constituent elements. Whether the observable surface-state dispersion reflects this bare atomic value, or instead an effective SOC renormalized by surface–bulk hybridization, remains an open question. Here, we investigate this issue for the occupied and unoccupied surface-state dispersion in Bi 2 Se 3 and Bi 2 Te 3 by combining angle-resolved photoemission (ARPES) and time-resolved (tr-ARPES) at a fixed 1 ps delay time with one-step photoemission calculations. Comparison between experimental and calculated spectra indicates that the SOC governing the surface-state dispersion deviates from the atomic expectation, with this effect more clearly resolved in Bi 2 Se 3 than in Bi 2 Te 3 . The calculations, which explicitly include hybridization between topological surface states and bulk-derived surface resonance states, provide a microscopic framework for this behavior. These results indicate that the spin-orbit interaction governing topological surface states is effectively shaped by surface–bulk hybridization and can vary significantly across materials.

Halogenated diradicaloids display long-lived charge-separated states in solution but short-lived singlets in films, where packing restricts twisting dynamics.

We report on a setup for photoluminescence (PL) spectroscopy, allowing to perform time resolved, steady-state, and spatially resolved PL and PL imaging. Time domain information (with ≈50 ps resolution) is achieved via time-correlated single photon counting (TCSPC), while the spectral information (in the ≈1–4 eV NIR-UV energy range) is accessed through a common-path interferometer based on birefringence, with the PL spectrum being retrieved via a Fourier transform approach. The micro-PL approach allows us to measure the PL signal of micron-sized samples, such as exfoliated 2D semiconductors. Moreover, the high efficiency of signal collection and analysis allows us to resolve weak PL signals from indirect-bandgap semiconductors. We measure the time-, frequency-, and spatially resolved PL on exfoliated flakes of WSe2, MoS2, and WS2 as examples to display the potential of our new apparatus.

We present that halogen‐substituted and air‐stable Thiele‐type diradicaloids with folded ( 1 ) and planar ( 2 , 3 ) structures can be obtained by switching the positions of fluorine and chlorine substituents. The strongly distorted p ‐quinodimethane 1 demonstrates emission in the UV–visible/near‐infrared (UV–vis/NIR) range with a photoluminescence quantum yield (PLQY) of 100%, an exceedingly large Stokes shift of up to 2.0 eV, an excited state lifetime of 81 ns, and pronounced solvatochromic emission (from 1.81 eV in n ‐pentane to approximately 2.0 eV in dichloromethane). Dual emission in pentane, transient absorption spectroscopy, quantum chemical calculations, as well as the comparison with compounds 2 and 3 reveal that the exceptional photophysical properties of closed‐shell 1 are thanks to interconversion with its planar conformer 1 ^flat . The key here is that the conformer 1 ^flat , which is generated upon photoexcitation, excels with a pronounced singlet diradical character ( y ₀ = 0.78) and a dark doubly excited (DE) S1 state. Our findings delineate how to leverage conformational equilibria to design bright luminescent materials based on hidden and thus air‐stable organic diradicals.

We present that halogen‐substituted and air‐stable Thiele‐type diradicaloids with folded ( 1 ) and planar ( 2 , 3 ) structures can be obtained by switching the positions of fluorine and chlorine substituents. The strongly distorted p ‐quinodimethane 1 demonstrates emission in the UV–visible/near‐infrared (UV–vis/NIR) range with a photoluminescence quantum yield (PLQY) of 100%, an exceedingly large Stokes shift of up to 2.0 eV, an excited state lifetime of 81 ns, and pronounced solvatochromic emission (from 1.81 eV in n ‐pentane to approximately 2.0 eV in dichloromethane). Dual emission in pentane, transient absorption spectroscopy, quantum chemical calculations, as well as the comparison with compounds 2 and 3 reveal that the exceptional photophysical properties of closed‐shell 1 are thanks to interconversion with its planar conformer 1 ^flat . The key here is that the conformer 1 ^flat , which is generated upon photoexcitation, excels with a pronounced singlet diradical character ( y ₀ = 0.78) and a dark doubly excited (DE) S1 state. Our findings delineate how to leverage conformational equilibria to design bright luminescent materials based on hidden and thus air‐stable organic diradicals.

We use time- and angle-resolved photoemission spectroscopy (TR-ARPES) combined with density functional theory to investigate ultrafast carrier dynamics in low-symmetry layered semiconducting PdSe ₂ . The indirect bandgap is determined to be 0.55 eV. Following photoexcitation above this gap, we resolve a valence band shift and broadening, both lasting less than a picosecond, consistent with bandgap renormalization and carrier scattering, indicative of strong many-body interactions. Subsequently, hot carriers populate the conduction band minimum and are captured by defect states. A surface photovoltage (SPV) of ~67 meV emerges, persisting for over 50 ps, driven by defect-assisted charge separation. The formation of native vacancies, promoted by the low-symmetry lattice, likely gives rise to the mid-gap states responsible for this long-lived SPV response. Detailed analysis of TR-ARPES spectra disentangles the contributions of bandgap renormalization, carrier scattering, defect states, and SPV. These findings establish PdSe ₂ as a prototypical layered quantum material exhibiting exotic photoresponses on ultrafast timescales.

Charge-density-wave (CDW) materials having a strong electron-phonon coupling provide a powerful platform for investigating the intricate interplay between lattice fluctuations and a macroscopic quantum order. Here, using time- and angle-resolved photoemission spectroscopy (TR-ARPES), we reveal that the CDW gap closure in VTe ₂ is dominated by an incoherent process evolving on a sub-picosecond timescale, challenging the conventional view that the gap dynamics is primarily governed by the excitation of the CDW amplitude modes. Our findings, supported by a three-temperature model, show that the CDW gap evolution can be described by considering the population of a subset of strongly-coupled optical phonon modes, which leads to an increase in the lattice fluctuations. This microscopic framework extends beyond VTe ₂ , offering a universal perspective for understanding the light-induced phase transition in strongly-coupled CDW systems.