Cem Sevik

Charge density waves (CDWs) are ordered quantum states of conduction electrons accompanied by periodic lattice distortions. Raman scattering (RS) spectroscopy is, therefore, well suited for probing CDW-induced structural modulations. The CDW state in quasi-one-dimensional NbTe4 is investigated using RS spectroscopy. At T = 5 K, the resonantly enhanced Raman spectrum exhibits 25 phonon modes. Polarization-dependent measurements reveal a strong coupling between phonon-mode symmetry and crystallographic symmetry, with modes polarized parallel or perpendicular to the crystallographic c axis, along which the one-dimensional structure is elongated. Temperature-dependent RS measurements identify a transition between commensurate and incommensurate CDW phases, accompanied by pronounced thermal hysteresis, with transition temperatures of approximately 45 K upon cooling and 90 K upon warming. The hysteresis width depends on the warming rate, indicating a finite nucleation rate of CDW domains and suggesting potential relevance for memory-device applications.

Describing the electronic and excitonic properties of two-dimensional metallic materials is challenging due to the reduced dielectric screening, which enhances many-body interactions and influences the optical response. In this work, we present a comprehensive study of many-body effects on the optical properties of two-dimensional (2D) metallic MXenes—a large family of emerging layered materials with significant potential for optoelectronic, sensing, and energy-harvesting applications. Using state-of-the-art methods, we explicitly treat intraband transitions and make use of a full frequency description of the screened Coulomb interaction, two aspects that are particularly important when treating many-body effects in metals. Our results reveal that many-body effects substantially modify the band structures of these metallic monolayers, reflecting the limited screening characteristic of atomically thin systems. The GW corrections lead to pronounced changes in the absorption spectra already at the independent-particle level. In contrast, the inclusion of electron-hole interactions through the Bethe-Salpeter equation (BSE) produces comparatively smaller modifications, which we attribute to the finite density of states at the Fermi level in these metallic systems. Overall, our findings highlight the necessity of explicitly accounting for many-body interactions to achieve reliable predictions of the optical properties of 2D metallic materials, and they establish key design principles for MXene-based optoelectronic applications.

Naturally stacked transition metal dichalcogenide (TMD) bilayers offer clean interfaces and simplified fabrication for exciton studies, yet they are dominated by momentum-dark-layer-hybridized excitons, limiting their optical applications. Unhybridized interlayer excitons (IXs) with optical activity and large dipole moments of layer thickness present superior prospects for exploring many-body physics and optoelectronics, but remain unexplored in natural bilayers. Here, we demonstrate efficient IX emission in fully encapsulated WSe₂ natural bilayers, with a threshold electric field 20 times below theoretical predictions. We confirm their unhybridized IX nature by a dipole length of 0.62 nm, characteristic of K-K' interlayer transitions. The heterostructure-like asymmetric linear Stark shift indicates an origin in heterostrain-induced layer degeneracy breaking, which is caused by local stacking disruption. This mechanism explains the notably reduced threshold. These heterostrain-localized unhybridized IXs exhibit superior properties compared to the layer-hybridized excitons, including strong dipolar repulsion (0.47 meV/μW), long lifetime (36 ns), high linear polarization (0.39), and directional long-range propagation (5 μm) with preserved in situ linear polarization and narrow line width. Our findings demonstrate natural bilayers as a field-tunable platform for engineering excitonic states and their interaction with light, which are promising for exploring quantum many-body effects and developing future optoelectronic applications.

Excitons in monolayer transition-metal dichalcogenides (TMDs) have garnered significant attention because of their large binding energies due to weakly screened Coulomb interaction, and direct bandgap at the K/K$^\prime$ point in the hexagonal Brillouin zone featuring spin-polarised bands due to spin-orbit coupling and lack of inversion symmetry. This makes them prospective for next-generation optoelectronic and quantum devices. However, despite the intense research activity, the reported values for exciton binding energies, quasiparticle gaps, and spectral features exhibit substantial variation across both experimental and theoretical studies. In this article, we present a comprehensive and critical assessment of the current understanding of excitonic properties in single-layer TMDs, integrating results from the angle-resolved photoemission spectroscopy (ARPES), photoluminescence (PL) measurements, and other experimental techniques with first-principles theoretical insights. Special emphasis is placed on the comparison and reconciliation of discrepancies observed across different experimental setups and sample qualities. Furthermore, we highlight our state-of-the-art GW-BSE calculations, which include both equilibrium and laterally strained systems, to systematically analyse the behaviour of direct and indirect excitons. By evaluating the effect of strain as a tunable control variable, we demonstrate its potential to engineer excitonic properties, supported by cross-validation against prior theoretical predictions and experimental findings. In doing so, we clarify the sources of discrepancies in the literature and offer a unified perspective on excited-state engineering strategies in two-dimensional TMDs.

Group-VI transition metal dichalcogenides (TMDs), MoS2 and MoSe2, have emerged as prototypical low-dimensional systems with distinctive phononic and electronic properties, making them attractive for applications in nanoelectronics, optoelectronics, and thermoelectrics. However, their reported lattice thermal conductivities (κ) remain highly inconsistent, with experimental values and theoretical predictions differing by more than an order of magnitude. These discrepancies stem from uncertainties in measurement techniques, variations in computational protocols, and ambiguities in the treatment of higher-order anharmonic processes. In this study, we critically review these inconsistencies, first by mapping the spread of experimental and modeling results, and then by identifying the methodological origins of divergence. To this end, we bridge first-principles calculations, molecular dynamics simulations, and state-of-the-art machine learning force fields (MLFFs), including recently developed foundation models. We train and benchmark GAP, MACE, NEP, and HIPHIVE against density functional theory and rigorously evaluate the impact of third- and fourth-order phonon scattering processes on κ. The computational efficiency of MLFFs enables us to extend convergence tests beyond conventional limits and to validate predictions through homogeneous nonequilibrium molecular dynamics as well. Our analysis demonstrates that, contrary to some recent claims, fully converged four-phonon processes contribute negligibly to the intrinsic thermal conductivity of both MoS2 and MoSe2. These findings not only refine the intrinsic transport limits of 2D TMDs but also establish MLFF-based approaches as a robust and scalable framework for predictive modeling of phonon-mediated thermal transport in low-dimensional materials.