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Dimitar Pashov

Publications and source records attributed to Dimitar Pashov.

At least 19 recordsLinked to original sources
arXiv2026

Localized Excitons and Exciton-Phonon Coupling in Antiferromagnetic AgCrP2S6

AgCrP2S6 combines a low-symmetry thiophosphate framework with an antiferromagnetic Cr sublattice and nonmagnetic Ag sites, providing a setting in which covalency and magnetic localization compete in the low-energy optical response. We combine single-crystal x-ray diffraction, Raman spectroscopy, lattice-dynamical calculations, and self-consistent vertex corrected Feynman diagrammatic many-body approaches to determine the structural, electronic, and excitonic properties of bulk AgCrP2S6. The material remains monoclinic between 100 and 300 K, with additional Ag-site disorder resolved at low temperature, and the optimized structure is dynamically stable. The many-body calculations yield a reduced quasiparticle gap relative to Cr trihalides, but the lowest excitons remain predominantly local: the calculated low-energy (~1.4 eV) excitation cluster is best described as a weakly bright, strongly anisotropic Frenkel exciton with dominant onsite d-d character and substantial ligand-assisted d-p admixture. Symmetry analysis in the C2h setting identifies this state as predominantly Bu and constrains its leading exciton-phonon coupling channels to Ag diagonal renormalization and Bg-mediated bright-dark mixing. These results place AgCrP2S6 in a localized excitonic regime in which the stronger p-d hybridization, relative to the more ionic Cr trihalides, narrows the quasiparticle gap, while the antiferromagnetic exchange and Ag-site dilution disfavor the intersite coherence associated with strongly delocalized low-energy excitons.

cond-mat.mtrl-sci↗
arXiv2026

Manipulation of localized excitons in CrPS$_4$ by temperature and magnetic field

Layered van der Waals magnetic semiconductors provide a versatile platform for exploring excitonic phenomena intertwined with spin and lattice degrees of freedom, enabling excitons to act as sensitive probes of magnetic order. CrPS$_4$ is a layered antiferromagnetic semiconductor that hosts rich excitonic features whose microscopic origin and connection to magnetic ordering remain incompletely understood. Here, we investigate the electronic and excitonic properties of bulk CrPS$_4$ using a combination of many-body perturbation theory, dynamical mean-field theory, and photoluminescence-based experiments. Our calculations establish CrPS$_4$ as a direct-gap semiconductor with a bandgap of 2.48~eV in the antiferromagnetic phase. Several sub-bandgap excitonic transitions are predicted by theory, comprising multiple spin-allowed excitons and an additional spin-flip excitation, predominantly localized on the Cr$^{3+}$ ions. Temperature- and magnetic-field-dependent optical measurements reveal thermally driven exciton redistribution among localized states and identify characteristic energy shifts that provide clear optical signatures of magnetic phase transitions in CrPS$_4$. These results provide new insights into the excitonic transitions of antiferromagnets and suggest potential routes for all-optical sensing and light-driven control of their magnetic order.

cond-mat.str-el↗
arXiv2026

Tunable Magneto-Excitonic Coupling in Alloyed van der Waals Antiferromagnet

The unique coupling between magnetic order and photo-generated excitons, electron-hole pairs bound by Coulomb interaction, in layered magnetic semiconductors offers a powerful mechanism for controlling light-matter interactions. In the van der Waals antiferromagnet CrSBr, this coupling is exceptionally strong and manifests distinctly between two coexisting excitonic states: the localised, Frenkel-like XA exciton and the more delocalised, Wannier-Mott-like XB exciton, providing a unique playground for the optical control of magnetism. Here, we reveal how chlorine incorporation reshapes the magneto-optical interplay in CrSBr1-xClx by simultaneously modifying its electronic structure, excitonic properties, and magnetic interactions. Combining magneto-optical spectroscopy up to 85 T with state-of-the-art quasiparticle self-consistent GW (QSGW) calculations on alloy supercells, we show that Cl insertion progressively localises the excitonic wavefunctions and drives both states toward a more Frenkel-like regime. This evolution is accompanied by a systematic reduction of the magnetic-field-induced energy renormalisation, most prominently for the XB exciton. Our work connects exciton character directly to magneto-excitonic coupling. Furthermore, it establishes compositional alloying as an effective strategy for engineering the coupling between magnetic and optical properties in van der Waals magnetic semiconductors.

cond-mat.mtrl-sci↗
arXiv2026

Bond, orbital and spin order in d4/d6/d7 perovskite oxides: successes and limitations of foundation interatomic potentials

Foundation machine-learning interatomic potentials (MLIPs) are rapidly replacing density-functional theory (DFT) for modeling structure and nuclear dynamics, making their fidelity in strongly correlated systems an urgent question. We test three foundation potentials on the low-temperature order of three correlated, isostructural ABO3 perovskite oxides: LaMnO3 (d4), LaCoO3 (d6), and NdNiO3 (d7). We run molecular dynamics for 1 ns on 80- and 160-atom supercells from 50 to 300 K with no system-specific training. These oxides expose three distinct classes of low-temperature order that define a hierarchy of difficulty for the potentials. The scalar class, represented by NdNiO3, has a simple geometric fingerprint and is captured. The vector class, represented by LaMnO3, requires identifying which Cartesian axis carries the long bond at each site, and is captured in magnitude but not in symmetry. The on-site class, represented by the low-spin to high-spin crossover in LaCoO3, is a purely local multiplet population shift with no spatial order parameter and remains inaccessible to present-day MLIPs.

cond-mat.mtrl-sci↗
arXiv2026

Bright and Dark Excitons in CrSBr: Local Ligand-Field Character and Band-Coherent Optical Selection Rules

Magnetic van der Waals semiconductors such as CrSBr host an intricate exciton landscape whose physical interpretation has converged only recently. A many-body Feynman diagrammatic approach based on quasiparticle self-consistent GW with electron-hole ladder vertex corrections to the screened Coulomb interaction has established the electronic band gap, excitonic orbital character, real-space extent, binding energies, and bosonic-coupling signatures of the bright XA exciton near 1.34 eV and the higher XB manifold near 1.8 eV. These results agree well with ARPES and magneto-optical experiments and supersede the early Rydberg-like assignment of the excitons. What has remained unresolved is why these intense bright excitons coexist, within a few tens of meV, with companion states that are several orders of magnitude darker despite drawing from essentially the same single-particle transition manifold. Here we show that brightness is a band-coherent property of the excitonic eigenfunctions: bright and dark partners are sublattice-symmetric and sublattice-antisymmetric superpositions of the same ligand-field-like Bloch transitions across the two Cr atoms of the orthorhombic primitive cell. The commonly used Frenkel and Wannier-Mott labels describe what an exciton is made of, but brightness requires a symmetry-adapted interference rule between transition dipoles. Disentangling this bare excitonic structure is a prerequisite for interpreting the optical response of CrSBr once magnon, phonon, and photon couplings are included.

cond-mat.mtrl-sci↗
arXiv2026

Many-body description of two-dimensional van der Waals ferroelectric $\alpha-$In$_2$Se$_3$

Two-dimensional (2D) van der Waals ferroelectrics are recognized for enabling many applications, from memory and logic to neuromorphic computing, as well as transforming other materials to control electronic phase transitions and topological states. While these materials are typically weakly correlated and expected to have their ground-state properties well described with the commonly used density functional theory, by focusing on bilayers and trilayers of In$_2$Se$_3$ we show that this approach may not be reliable. The underlying electronic structure strongly depends on the polarization structure of the multilayer system and is surprisingly challenging to accurately calculate, requiring a high-fidelity many-body theory of the quasiparticle self-consistent \textit{GW} approximation. We develop this underlying description by extending the capabilities of Green function implementation within the open-source Questaal package. We show that even a sophisticated hybrid functional approach may fail to predict a nonvanishing gap in a bilayer In$_2$Se$_3$ and yields charge density, polarization, and band offsets that strongly deviate from the many-body picture. We discuss the implications of these computational advances for future opportunities in 2D ferroelectrics.

cond-mat.mtrl-sci↗
arXiv2026

Engineering photomagnetism in collinear van der Waals antiferromagnets

Achieving efficient ultrafast optical control of antiferromagnetic spin dynamics is a central goal for next-generation high-speed THz spintronic and magnonic devices. Resonant optical pumping of crystal-field-split d-d orbital multiplets in magnetic TM ions directly modulates exchange and spin-orbit interactions, inducing large-amplitude coherent spin precession. However, such effects are limited to a handful of systems and there is no general strategy to enhance d-d photomagnetism in antiferromagnets. Here, we demonstrate the engineering of photomagnetism via TM-ion doping in collinear van der Waals antiferromagnets. In Mn$_{1-x}$Ni$_x$PS$_3$, small amounts of Ni$^{2+}$ activate a strong photomagnetic response while largely preserving the N\'eel ground state. Even 10% Ni boosts the response by more than an order of magnitude compared to pure MnPS$_3$, with resonant pumping of Ni$^{2+}$ d-d transitions driving large-amplitude coherent spin precession and providing helicity-dependent phase control. Tuning the pump energy across the full Mn$_{1-x}$Ni$_x$PS$_3$ composition range shows that Ni excitations remain effective across competing N\'eel and zig-zag antiferromagnetic states while supporting tunable-frequency coherent spin precession. These results establish TM-ion doping as a versatile strategy to harness orbital multiplet excitations for ultrafast, low-dissipation spin control in van der Waals antiferromagnets.

cond-mat.mtrl-sci↗
arXiv2025

Origin of metal-insulator transition in rare-earth Nickelates

Rare-earth nickelates RNiO3 (R=rare-earth element) exhibit three kinds of phase transitions with decreasing temperature: a structural transition from a pseudo-cubic to a monoclinic phase, a metal- insulator transition (MIT), and a magnetic transition from a paramagnetic state to an ordered one. The first two occur at the same temperature, which has led to a consensus that the MIT is driven by lattice distortions. We show here that the primary driving force for the MIT is magnetic; however because of the unusual d7 configuration of Ni, additional flexibility in spin configurations are also needed which symmetry-lowing structural deformations make possible. The latter enable Ni to disproportionate into two kinds: a high-spin and a low-spin configuration, which allow the system to reduce its unfavorable orbital moment and also open a gap.

cond-mat.str-el↗
arXiv2025

Spin-quenching in molecule-transition-metal-dichalcogenide heterostructure through inverse proximity effect

A functional heterostructure is central to integrated circuitry in quantum photonics, optoelectronics, neuromorphic computing, spintronics, and straintronics. Recently, heterostructures combining 2D magnets and nonmagnetic transition metal dichalcogenides (TMDs) have been explored. In these, electron and hole wavefunctions are localized in 2D magnets but delocalized in TMDs. When combined, a proximity induced magnetic inter layer exciton can emerge, with energy differing by 20 to 30 meV from intra layer excitons and being two orders of magnitude darker, making it hard to detect and functionalize. Using a high fidelity ab initio many body diagrammatic approach, we show that functionality can be significantly enhanced in a transition metal molecule TMD interface. The molecular exciton exhibits charge transfer character and is extended, unlike the localized Frenkel excitation in 2D magnets. Moreover, the degree of localization and magnetic moment can be tuned by varying the molecular orientation relative to the TMD. This changes the proximity to the magnetic ion, altering screening and enabling a pathway to quench the ion's spin moment. This inverse proximity effect tunes the energies, spin states, and brightness of molecular and inter layer magnetic excitons, a mechanism absent in 2D magnet TMD systems. We also identify conditions under which the interlayer exciton becomes well separated and brighter than intra layer excitons, making it promising for protocols that probe and manipulate magnetic excitonic states.

cond-mat.str-el↗
arXiv2025

Quasiparticle Description of Angle-Resolved Photoemission Spectroscopy for SrCuO2

SrCuO2 has long been considered a near-archetypal realization of a quasi one dimensional (1D) system of interacting electrons with short-range interactions. Within this framework, experimental observations - interpreted through the lens of the 1D Hubbard model-suggest that electron and hole excitations decay into two types of (unphysical) collective bosonic modes: spinons, which carry the spin degree of freedom, and holons, which carry the charge degree of freedom. This model, known as spin-charge separation, is most directly evidenced by angle-resolved photoemission spectroscopy (ARPES), where a photo-induced hole decays into a continuum of these excitations. Here we present an alternative perspective grounded in first-principles, self-consistent, and parameter-free many-body perturbation theory. In this revised quasiparticle framework, ARPES can be understood as a one-body effect arising from mild disorder in a long-range antiferromagnetic ground state. the emergence of the so-called spinon branch arises naturally from spin disorder, the anomalous line widths are accurately captured, and we provide a compelling explanation for the spectral weight observed at the non-magnetic zone boundary. This reinterpretation provides a unified explanation for key experimental signatures previously attributed to spin-charge separation, including features observed in optical conductivity. Additionally, we show that SrCuO2 exhibits a nontrivial interchain coupling that significantly influences both its one-particle and two-particle spectral functions. By comparing the spectral features of SrCuO2 with those of La2CuO4, we argue that SrCuO2 shares notable similarities with the two-dimensional cuprates - both being rooted in a common CuO4 plaquette-based molecular orbital framework.

cond-mat.supr-con↗
arXiv2025

Deconstruction of the anisotropic magnetic interactions from spin-entangled optical excitations in van der Waals antiferromagnets

Magneto-optical excitations in antiferromagnetic d systems can originate from a multiplicity of light-spin and spin-spin interactions, as the light and spin degrees of freedom can be entangled. This is exemplified in van der Waals systems with attendant strong anisotropy between in-plane and out-of-plane directions, such as MnPS3 and NiPS3 films studied here. The rich interplay between the magnetic ordering and sub-bandgap optical transitions poses a challenge to resolve the mechanisms driving spin-entangled optical transitions, as well as the single-particle bandgap itself. Here we employ a high-fidelity ab initio theory to find a realistic estimation of the bandgap by elucidating the atom- and orbital-resolved contributions to the fundamental sub-bands. We further demonstrate that the spin-entangled excitations, observable as photoluminescence and absorption resonances, originate from an on-site spin-flip transition confined to a magnetic atom (Mn or Ni). The evolution of the spin-flip transition

cond-mat.mes-hall↗
arXiv2025

Challenges and opportunities in proximity-driven exciton-spin engineering in van der Waals heterostructures

van der Waals heterostructures consisting of transition metal dichalcogenides (TMDs) and two-dimensional (2D) magnets offer a versatile platform to study the coexistence and transformation of different excitons. By focusing on TMD WSe$_2$ and 2D magnetic CrI$_3$, as a bilayer WSe$_2$/CrI$_3$ and a trilayer CrI$_3$/WSe$_2$/CrI$_3$, we provide their description using a parameter-free, high-fidelity many-body perturbation theory. This ab initio approach allows us to elucidate the character of magnetic Frenkel excitons in CrI3 and how the nonmagnetic Wannier-Mott excitons in WSe2 are modified by the proximity of CrI3. We reveal novel proximity-induced interlayer excitons in these heterostructures. In contrast to the sensitivity of proximity-induced modifications of excitons in WSe$_2$, which depend on the interfacial details, the interlayer magnetic excitons are remarkably robust and are present across the different stacking configurations between WSe$_2$ and CrI$_3$, simplifying their experimental demonstration. These findings suggest unexplored opportunities for information transduction using magnetic excitons and integrating photonics, electronics, and spintronics in proximitized materials.

cond-mat.mtrl-sci↗
arXiv2025

Effective bands and band-like electron transport in amorphous solids

The localization of electrons caused by atomic disorder is a well-known phenomenon. However, what circumstances allow electrons to remain delocalized and retain band-like characteristics even when the crystal structure is completely absent, as found in certain amorphous solids, is less well understood. To probe this phenomenon, we developed a fully first-principles description of the electronic structure and charge transport in amorphous solids by combining a novel representation of the amorphous state with the state-of-the-art many-body (QSGW) electronic structure theory. Using amorphous In2O3 as an example, we demonstrate the accuracy of our approach in reproducing the band-like nature of the conduction electrons as well as their disorder-limited mobility. Our approach reveals the physical origins responsible for the electron delocalization and the survival of the band dispersions despite the absence of long-range order.

cond-mat.mtrl-sci↗
arXiv2024

Magnon-mediated exciton-exciton interaction in a van der Waals antiferromagnet

Excitons are fundamental excitations that govern the optical properties of semiconductors. Interacting excitons can lead to various emergent phases of matter and large nonlinear optical responses. In most semiconductors, excitons interact via exchange interaction or phase space filling. Correlated materials that host excitons coupled to other degrees of freedom offer hitherto unexplored pathways for controlling these interactions. Here, we demonstrate magnon-mediated excitonic interactions in CrSBr, an antiferromagnetic semiconductor. This interaction manifests as the dependence of exciton energy on exciton density via a magnonic adjustment of the spin canting angle. Our study demonstrates the emergence of quasiparticle-mediated interactions in correlated quantum materials, leading to large nonlinear optical responses and potential device concepts such as magnon-mediated quantum transducers.

cond-mat.mtrl-sci↗
arXiv2024

Good plasmons in a bad metal

Correlated materials may exhibit unusually high resistivity increasing linearly in temperature, breaking through the Mott-Ioffe-Regel bound, above which coherent quasiparticles are destroyed. The fate of collective charge excitations, or plasmons, in these systems is a subject of debate. Several studies suggest plasmons are overdamped while others detect unrenormalized plasmons. Here, we present direct optical images of low-loss hyperbolic plasmon polaritons (HPPs) in the correlated van der Waals metal MoOCl2. HPPs are plasmon-photon modes that waveguide through extremely anisotropic media and are remarkably long-lived in MoOCl2. Many-body theory supported by photoemission results reveals that MoOCl2 is in an orbital-selective and highly incoherent Peierls phase. Different orbitals acquire markedly different bonding-antibonding character, producing a highly-anisotropic, isolated Fermi surface. The Fermi surface is further reconstructed and made partly incoherent by electronic interactions, renormalizing the plasma frequency. HPPs remain long-lived in spite of this, allowing us to uncover previously unseen imprints of electronic correlations on plasmonic collective modes.

cond-mat.str-el↗
arXiv2024

Electron-Phonon Coupling in Many-Body Perturbation Theory: Developments within the Quasiparticle Self-Consistent GW approximation and LMTO Formalism

The calculation of electron-phonon (e-ph) coupling from first principles is a topic of great interest in materials science, offering a robust, non-empirical framework to understand and predict a wide range of physical phenomena. While significant progress has been made using the Kohn-Sham framework of density functional theory (KS-DFT), it is increasingly evident that standard approximations in KS-DFT often fall short of providing accurate results. These shortcomings are frequently linked to the non-local nature of the exchange-correlation potential, prompting the development of advanced methodologies within DFT and many-body perturbation theory. Despite these efforts, a highly reliable and efficient first-principles approach to accurately capture e-ph interactions remains elusive. To address this challenge, we introduce a novel field-theoretical methodology that integrates the foundational work of Baym and Hedin with the Quasiparticle Self-Consistent GW (QSGW) approximation, implemented within the Questaal electronic structure suite. Our approach, based on a response function framework, ensures that Pulay-like incomplete-basis-set corrections are not required to account for changes in basis functions, paving the way for a high-fidelity description of e-ph coupling.

cond-mat.mtrl-sci↗
arXiv2024

Giant exchange splitting in the electronic structure of A-type 2D antiferromagnet CrSBr

We present the evolution of the electronic structure of CrSBr from its antiferromagnetic ground state to the paramagnetic phase above T_N=132 K, in both experiment and theory. Low temperature angle-resolved photoemission spectroscopy (ARPES) results are obtained using a novel method to overcome sample charging issues, revealing quasi-2D valence bands in the ground state. The results are very well reproduced by our QSG\^W calculations, which further identify certain bands at the X points to be exchange-split pairs of states with mainly Br and S character. By tracing band positions as a function of temperature, we show the splitting disappears above T_N. The energy splitting is interpreted as an effective exchange splitting in individual layers in which the Cr moments all align, within the so-called A-type antiferromagnetic arrangement. Our results lay firm foundations for the interpretation of the many other intriguing physical and optical properties of CrSBr.

cond-mat.str-el↗