arXiv subjects

Iulia Cojocariu

Publications and source records attributed to Iulia Cojocariu.

12 recordsLinked to original sources

arXiv2026

Evidence for Many-Body States in NiPS$_3$ Revealed by Angle-Resolved Photoelectron Spectroscopy

We present $μ$-ARPES spectra of the Mott-insulating van der Waals antiferromagnet NiPS$_3$. Signatures of strong correlations -- such as the onset of atomic or atomic-ligand multiplets and spin-orbit-entangled exciton have been observed in this material by various two-particle spectroscopies, but not previously in photoemission. Our measurements reveal a weakly dispersive feature at the valence-band edge that is absent in DFT+$U$ calculations and remains unchanged across the Néel transition. After critically examining and ruling out alternative interpretations, we show that an exact diagonalization of a NiS$_6$ cluster yields low-energy final-state configurations of mixed multiplet $d^7$ and $d^8\underline{L}$ character, whose energy differences are consistent with the observed additional feature. This implies that ARPES directly accesses local Ni-S multiplet physics in NiPS$_3$, revealing a many-body structure beyond mean-field theory. Our results confirm that NiPS$_3$ is an excellent model platform in which strong correlations, reduced dimensionality, and covalent metal-ligand bonding jointly shape both two- and single-particle spectroscopies, underscoring the need for a genuinely quantum many-body description of two-dimensional quantum materials.

cond-mat.str-el↗

arXiv2026

Robust spin splitting and fermiology in a layered altermagnet

Altermagnetism defies conventional classifications of collinear magnetic phases, standing apart from ferromagnetism and antiferromagnetism with its unique combination of spin-dependent symmetries, net-zero magnetization, and anomalous Hall transport. Although altermagnetic states have been realized experimentally, their integration into functional devices has been hindered by the structural rigidity and poor tunability of existing materials. First, through cobalt intercalation of the superconducting 2H-NbSe$_2$ polymorph, we induce and stabilize a robust altermagnetic phase and using both theory and experiment, we directly observe the lifting of Kramers degeneracy. Additionally, we present spectroscopic insight into a previously hinted low-temperature phase, and provide evidence of its electronic origin. While shedding light on overlooked aspects of altermagnetism, these findings open pathways to spin-based technologies and lay a foundation for advancing the emerging field of altertronics.

cond-mat.str-el↗

arXiv2026

Probing the band structure of the strongly correlated antiferromagnet NiPS3 across its phase transition

NiPS3 is an exfoliable van-der-Waals intralayer antiferromagnet with zigzag-type spin arrangement. It is distinct from other TMPS3 (TM: transition metal) materials by optical excitations into a strongly correlated state that is tied to the magnetic properties. However, the related, fundamental band structure across the antiferromagnetic phase transition has not been probed yet. Here, we use angular-resolved photoelectron spectroscopy with μm resolution in combination with DFT+U calculations for that purpose. We identify a characteristic band shift across TN. It is attributed to bands of mixed Ni and S character related to the superexchange interaction of Ni 3t2g orbitals. Moreover, we find a structure above the valence band maximum with little angular dispersion that could not be reproduced by the calculations. The discrepancy suggests the influence of many-body interactions beyond the DFT+U approximations in striking contrast to the results on MnPS3 and FePS3, where these calculations were sufficient for an adequate description.

cond-mat.str-el↗

arXiv2024

Orbital Topology of Chiral Crystals for Orbitronics

Chirality is ubiquitous in nature and manifests in a wide range of phenomena including chemical reactions, biological processes, and quantum transport of electrons. In quantum materials, the chirality of fermions, given by the relative directions between the electron spin and momentum, is connected to the band topology of electronic states. Here, we show that in structurally chiral materials like CoSi, the orbital angular momentum (OAM) serves as the main driver of a nontrivial band topology in this new class of unconventional topological semimetals, even when spin-orbit coupling is negligible. A nontrivial orbital-momentum locking of multifold chiral fermions in the bulk leads to a pronounced OAM texture of the helicoid Fermi arcs at the surface. Our findings highlight the pivotal role of the orbital degree of freedom for the chirality and topology of electron states, in general, and pave the way towards the application of topological chiral semimetals in orbitronic devices.

cond-mat.mes-hall↗

arXiv2024

Identifying band structure changes of FePS3 across the antiferromagnetic phase transition

Magnetic 2D materials enable novel tuning options of magnetism. As an example, the van der Waals material FePS3, a zigzag-type intralayer antiferromagnet, exhibits very strong magnetoelastic coupling due to the different bond lengths along different ferromagnetic and antiferromagnetic coupling directions enabling elastic tuning of magnetic properties. The likely cause of the length change is the intricate competition between direct exchange of the Fe atoms and superexchange via the S and P atoms. To elucidate this interplay, we study the band structure of exfoliated FePS3 by mu m scale ARPES (Angular Resolved Photoelectron Spectroscopy), both, above and, for the first time, below the Neel temperature TN. We find three characteristic changes across TN. They involve S 3p-type bands, Fe 3d-type bands and P 3p-type bands, respectively, as attributed by comparison with density functional theory calculations (DFT+U). This highlights the involvement of all the atoms in the magnetic phase transition providing independent evidence for the intricate exchange paths.

cond-mat.mtrl-sci↗

arXiv2023

Observation of termination-dependent topological connectivity in a magnetic Weyl kagome-lattice

Engineering surfaces and interfaces of materials promises great potential in the field of heterostructures and quantum matter designer, with the opportunity of driving new many-body phases that are absent in the bulk compounds. Here, we focus on the magnetic Weyl kagome system Co$_3$Sn$_2$S$_2$ and show how for different sample's terminations the Weyl-points connect also differently, still preserving the bulk-boundary correspondence. Scanning-tunnelling microscopy has suggested such a scenario indirectly. Here, we demonstrate this directly for the fermiology of Co$_3$Sn$_2$S$_2$, by linking it to the system real space surfaces distribution. By a combination of micro-ARPES and first-principles calculations, we measure the energy-momentum spectra and the Fermi surfaces of Co$_3$Sn$_2$S$_2$ for different surface terminations and show the existence of topological features directly depending on the top-layer electronic environment. Our work helps to define a route to control bulk-derived topological properties by means of surface electrostatic potentials, creating a realistic and reliable methodology to use Weyl kagome metals in responsive magnetic spintronics.

cond-mat.str-el↗

arXiv2023

Metalloporphyrins on Oxygen-Passivated Iron: Conformation and Order Beyond the First Layer

On-surface metal porphyrins can undergo electronic and conformational changes that play a crucial role in determining the chemical reactivity of the molecular layer. Therefore, understanding those properties is pivotal for the design and implementation of organic-based devices. Here, by means of photoemission orbital tomography supported by density functional theory calculations, we investigate the electronic and geometrical structure of two metallated tetraphenyl porphyrins (MTPPs), namely ZnTPP and NiTPP, adsorbed on the oxygen-passivated Fe(100)-p(1x1)O surface. Both molecules weakly interact with the surface as no charge transfer is observed. In the case of ZnTPP our data correspond to those of moderately distorted molecules, while NiTPP exhibits a severe saddle-shape deformation. From additional experiments on NiTPP multilayer films, we conclude that this distortion is a consequence of the interaction with the substrate, as the NiTPP macrocycle of the second layer turns out to be flat. We further find that distortions in the MTPP macrocycle are accompanied by an increasing energy gap between the highest occupied molecular orbitals (HOMO and HOMO-1). Our results demonstrate that photoemission orbital tomography can simultaneously probe the energy level alignment, the azimuthal orientation, and the adsorption geometry of complex aromatic molecules even in the multilayer regime.

cond-mat.mtrl-sci↗

arXiv2023

Electronic band structure changes across the antiferromagnetic phase transition of exfoliated MnPS$_3$ probed by $μ$-ARPES

Exfoliated magnetic 2D materials enable versatile tuning of magnetization, e.g., by gating or providing proximity-induced exchange interaction. However, their electronic band structure after exfoliation has not been probed, most likely due to their photochemical sensitivity. Here, we provide micron-scale angle-resolved photoelectron spectroscopy of the exfoliated intralayer antiferromagnet MnPS$_3$ above and below the Néel temperature down to one monolayer. The favorable comparison with density functional theory calculations enables to identify the orbital character of the observed bands. Consistently, we find pronounced changes across the Néel temperature for bands that consist of Mn 3d and 3p levels of adjacent S atoms. The deduced orbital mixture indicates that the superexchange is relevant for the magnetic interaction. There are only minor changes between monolayer and thicker films demonstrating the predominant 2D character of MnPS$_3$. The novel access is transferable to other MPX$_3$ materials (M: transition metal, P: phosphorus, X: chalcogenide) providing a multitude of antiferromagnetic arrangements.

cond-mat.mtrl-sci↗

arXiv2023

Twist angle dependent interlayer transfer of valley polarization from excitons to free charge carriers in WSe$_2$/MoSe$_2$ heterobilayers

We identify an optical excitation mechanism that transfers a valley polarization from photo-excited electron-hole pairs to free charge carriers in twisted WSe$_2$/MoSe$_2$ heterobilayers. For small twist angles, the valley lifetimes of the charge carriers are surprisingly short, despite the occurrence of interlayer excitons with their presumably long recombination and polarization lifetimes. For large twist angles, we measure an increase in both the valley polarization and its respective lifetime by more than two orders of magnitude. Interestingly, in such heterobilayers we observe an interlayer transfer of valley polarization from the WSe$_2$ layer into the MoSe$_2$ layer. This mechanism enables the creation of a photo-induced valley polarization of free charge carriers in MoSe$_2$, which amplitude scales with the gate-induced charge carrier density. This is in contrast to monolayer MoSe$_2$, where such a gate-tunable valley polarization cannot be achieved. By combining time-resolved Kerr rotation, photoluminesence and angle-resolved photoemission spectroscopy measurements with first principles calculations, we show that these findings can be explained by twist angle dependent interlayer scattering mechanisms involving the Q- and $Γ$-valleys.

cond-mat.mtrl-sci↗

arXiv2022

Can the "shadow" of graphene band clarify its flatness?

Graphene band renormalization at the proximity of the van Hove singularity (VHS) has been investigated by angle-resolved photoemission spectroscopy (ARPES) on the Li-doped quasi-freestanding graphene on the cobalt (0001) surface. The absence of graphene band hybridization with the substrate, the doping contribution well represented by a rigid energy shift and the excellent electron-electron interaction screening ensured by the metallic substrate offer a privileged point of view for such investigation. A clear ARPES signal is detected along the M point of the graphene Brillouin zone, giving rise to an apparent flattened band. By simulating the graphene spectral function from the density functional theory calculated bands, we demonstrate that the photoemission signal along the M point originates from the "shadow" of the spectral function of the unoccupied band above the Fermi level. Such interpretation put forward the absence of any additional strong correlation effects at the VHS proximity, reconciling the mean field description of the graphene band structure even in the highly doped scenario.

cond-mat.str-el↗

arXiv2020

Evaluation of molecular orbital symmetry via oxygen-induced charge transfer quenching at a metal-organic interface

Thin molecular films under model conditions are often exploited as benchmarks and case studies to investigate the electronic and structural changes occurring on the surface of metallic electrodes. Here we show that the modification of a metallic surface induced by oxygen adsorption allows the preservation of the geometry of a molecular adlayer, giving access to the determination of molecular orbital symmetries by means of near-edge x-ray absorption fine structure spectroscopy, NEXAFS. As a prototypical example, we exploited Nickel Tetraphenyl Porphyrin molecules deposited on a bare and on an oxygen pre-covered Cu(100) surface. We find that adsorbed atomic oxygen quenches the charge transfer at the metal-organic interface but, in contrast to a thin film sample, maintains the ordered adsorption geometry of the organic molecules. In this way, it is possible to disentangle π* and σ* symmetry orbitals, hence estimating the relative oscillator strength of core level transitions directly from the experimental data, as well as to evaluate and localize the degree of charge transfer in a coupled system. In particular, we neatly single out the σ* contribution associated with the N 1s transition to the mixed N 2px,y-Ni 3dx2-y2 orbital, which falls close to the leading π*-symmetry LUMO resonance.

cond-mat.mtrl-sci↗

arXiv2020

Molecular anchoring stabilizes low valence Ni(I)TPP on copper against thermally induced chemical changes

Many applications of molecular layers deposited on metal surfaces, ranging from single-atom catalysis to on-surface magnetochemistry and biosensing, rely on the use of thermal cycles to regenerate the pristine properties of the system. Thus, understanding the microscopic origin behind the thermal stability of organic/metal interfaces is fundamental for engineering reliable organic-based devices. Here, we study nickel porphyrin molecules on a copper surface as an archetypal system containing a metal center whose oxidation state can be controlled through the interaction with the metal substrate. We demonstrate that the strong molecule-surface interaction, followed by charge transfer at the interface, plays a fundamental role in the thermal stability of the layer by rigidly anchoring the porphyrin to the substrate. Upon thermal treatment, the molecules undergo an irreversible transition at 420 K, which is associated with an increase of the charge transfer from the substrate, mostly localized on the phenyl substituents, and a downward tilting of the latters without any chemical modification

cond-mat.mtrl-sci↗