arXiv subjects

Cem Sevik

Publications and source records attributed to Cem Sevik.

At least 19 recordsLinked to original sources

arXiv2026

Raman scattering fingerprints of the charge density wave state in one-dimensional NbTe$_4$

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. We investigate the CDW state in quasi-one-dimensional NbTe$_4$ 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.

cond-mat.mtrl-sci↗

arXiv2026

Excitonic Landscape of Monolayer Transition-Metal Dichalcogenides: Experimental Discrepancies, Theoretical Advances, and Strain Dependence

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.

physics.optics↗

arXiv2025

Thermal Conductivity Limits of MoS$_2$ and MoSe$_2$: Revisiting High-Order Anharmonic Lattice Dynamics with Machine Learning Potentials

Group-VI transition metal dichalcogenides (TMDs), MoS$_2$ and MoSe$_2$, have emerged as prototypical low-dimensional systems with distinctive phononic and electronic properties, making them attractive for applications in nanoelectronics, optoelectronics, and thermoelectrics. Yet, their reported lattice thermal conductivities ($\kappa$) 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. %MACE-OMAT-0, UMA, and NEP89. We train and benchmark GAP, MACE, NEP, and \textsc{HIPHIVE} against density functional theory (DFT) and rigorously evaluate the impact of third- and fourth-order phonon scattering processes on $\kappa$. 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 MoS$_2$ and MoSe$_2$. 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.

cond-mat.mtrl-sci↗

arXiv2025

Strain-tuned magnetoelectric properties of monolayer NiX$_2$ (X = I, Br): a first-principles analysis

Using \textit{ab initio} methodology, we reveal a strain-mediated approach to precisely tune the magnetoelectric coupling and spin-driven emergent polarization of NiX$_2$ (X = I, Br) monolayers. In the absence of strain, these systems spontaneously stabilize non-collinear spin states that break the inversion symmetry, inducing a ferroelectric polarization in the plane of the material. We show that biaxial and uniaxial strains broadly modulate the magnetoelectric response in these materials through two distinct mechanisms: (i) direct modification of the magnetoelectric tensor components, and (ii) tuning of the characteristic propagation vectors of a spin texture. This dual mechanism enables precise control over the magnitude of the spin-induced electric polarization of these materials. With respect to the achievable magnitude of the electric polarization, we demonstrate the critical role of third-nearest-neighbor spin-pair contributions, which can increase under strain to levels that compete with or even exceed the polarization driven by first-nearest-neighbor effects. These findings offer important insights into low-dimensional piezo-magnetoelectricity and expand the possibilities for designing multifunctional two-dimensional straintronic devices.

cond-mat.mtrl-sci↗

arXiv2025

Machine-Learning Interatomic Potential for Twisted Hexagonal Boron Nitride: Accurate Structural Relaxation and Emergent Polarization

The emerging ferroelectric properties of two-dimensional (2D) heterostructures are at the forefront of science and prospective technology. In moir\'e bilayers, twisting or heterostructuring causes local atomic reconstruction, which even at picometer scale, can lead to pronounced ferroelectric polarization. Accurately determining this reconstruction utilizing ab initio methods is unfeasible for the relevant system sizes, but modern machine-learning interatomic potentials offer a viable solution. Here, we present the Gaussian Approximation Potential for twisted hexagonal boron nitride (hBN) layers validated against ab initio datasets. This approach enables the precise analysis of their structural properties, which is particularly relevant at small twist angles. We couple the structural information to a tight-binding model based on accurate interatomic positioning, and determine the twist-dependent polarization, yielding results that closely align with previous experimental findings - even at room temperature. This methodology enables further studies that are unattainable otherwise and is transferable to other 2D materials of interest.

cond-mat.mtrl-sci↗

arXiv2025

State- and momentum-dependent nonlinear Stark effect of interlayer excitons in bilayer WSe$_2$

Interlayer excitons in van der Waals heterostructures offer rich collective phases, prospective optoelectronic applications, and versatile tunability, where control by electronic means is particularly relevant and practical. Here, in the case of bilayer WSe$_2$, we reveal how layer localization of excitons governs their response to an external electric field. Using Many-Body Perturbation Theory, we calculate the exciton dispersion for different stacking symmetries under applied electric field and/or strain, in order to map the landscape of competing low-energy excitons in four distinct finite-momentum valleys. While intralayer excitons are not affected by the electric field, some interlayer ones exhibit a nonlinear Stark shift that becomes linear after a critical threshold. The degree of nonlinearity is a direct measure of the layer hybridization of the electronic subcomponents of the exciton. Our findings explain the peculiar Stark-shift regimes observed in recent experiments, the nature of (anti)symmetric spectral shifts around zero field, and the sensitivity of dipolar excitons to external perturbations, all highly relevant to their further applications in excitonic condensates, optoelectronics devices and quantum emitters.

cond-mat.mes-hall↗

arXiv2024

Theory of a two-dimensional anharmonic piezoelectric crystal resonator

We developed a lattice dynamical theory of an atomically-thin compressional piezoelectric resonator. Acoustic and optical dynamic displacement response functions are derived and account for frequency-dependent electromechanical coupling. The dynamic susceptibilities for the direct and the converse piezoelectric effects are found equal. The mechanical resonant behavior of longitudinal in-plane displacement waves is investigated as a function of the lateral crystal size and of temperature in the classical and in the quantum regime. In the former case the quality factor of the resonator is inversely proportional to temperature and to crystal size. Below a cross-over temperature the quantum zero-point fluctuations become dominant and put an upper limit on the quality factor which is size independent. As experimentally relevant examples, the theory is applied on two-dimensional hexagonal boron nitride and molybdenum disulfide.

cond-mat.mes-hall↗

arXiv2024

Enhanced superconductivity of hydrogenated $\beta_{12}$ borophene

Borophene stands out among elemental two-dimensional materials due to its extraordinary physical properties, including structural polymorphism, strong anisotropy, metallicity, and the potential for phonon-mediated superconductivity. However, confirming superconductivity in borophene experimentally has been evasive to date, mainly due to the detrimental effects of metallic substrates and its susceptibility to oxidation. In this study, we present an \textit{ab initio} analysis of superconductivity in the experimentally synthesized hydrogenated $\beta_{12}$ borophene, which has been proven to be less prone to oxidation. Our findings demonstrate that hydrogenation significantly enhances both the stability and superconducting properties of $\beta_{12}$ borophene. Furthermore, we reveal that tensile strain and hole doping, achievable through various experimental methods, significantly enhance the critical temperature, reaching up to 29 K. These findings not only promote further fundamental research on superconducting borophene and its heterostructures, but also position hydrogenated borophene as a versatile platform for low-dimensional superconducting electronics.

cond-mat.supr-con↗

arXiv2024

Strain-tunable magnetic and magnonic states in Ni-dihalide monolayers

Monolayer NiI$_2$ garners large research interest due to its multiferroic behavior stemming from the interplay between its non-collinear magnetic order and the spin-orbit coupling. This prompts an investigation into the stability of the magnetic order in NiI$_2$ and similar materials under external stimuli. In this work, we report the effect of biaxial and uniaxial strain on the magnetic ground state, the critical temperature, and the magnonic properties of the NiX$_2$ (X = I, Br, Cl) monolayers. For all three materials, we reveal intricate strain-dependent phase diagrams, including ferromagnetic, helimagnetic, and skyrmionic phases. Moreover, we discuss the necessity of considering the biquadratic exchange interaction in the latter analysis. We reveal that the biquadratic exchange significantly alters both the magnetic ground state and the critical temperature of the magnetic order, and we demonstrate that its importance becomes even more explicit when monolayer Ni-dihalides are strained. Finally, we calculate the magnonic dispersion for the predicted magnetic states, showing that the skyrmionic phase functions as a magnonic crystal, and demonstrate the presence of strain-tunable soft magnon modes at finite wavevectors in the helimagnetic phase.

cond-mat.mes-hall↗

arXiv2024

Photoluminescence enhancement at the vertical van der Waals semiconductor-metal heterostructures

Excitons in monolayer transition metal dichalcogenides (TMDCs) offer intriguing new possibilities for optoelectronics with no analogues in bulk semiconductors. Yet, intrinsic defects in TMDCs limit the radiative exciton recombination pathways. As a result, the photoluminescence (PL) quantum yield (QY) is limited. Methods like superacid treatment, electrical doping, and plasmonic engineering can inhibit nonradiative decay channels and enhance PL. Here, we show a more straightforward approach that allows PL enhancement. An engineered vertical van der Waals (vdW) metal-monolayer semiconductor junction (MSJ) results in PL enhancement of more than an order of magnitude at technologically relevant excitation powers. Such MSJ can be constructed by vertically stacking metals with suitable work function either above or below a monolayer semiconducting TMDC. Our experiments reveal that the underlying PL enhancement mechanism is to be the suppressed exciton quenching due to the absence of metal-induced gap states and weak Fermi level pinning, thanks to the vdW gapped interface between the metal and the TMDC. Our time-resolved PL measurements further indicate that reduced exciton-exciton annihilation, even at high generation rates, contributes to the observed PL enhancement. The PL intensity is further increased by the proximity of surface plasmons in the metal with the TMDC layer. Our findings shed light on the interaction at vdW metal-semiconductor interfaces and offer a path to improving the optoelectronic performance of semiconducting TMDCs.

cond-mat.mes-hall↗

arXiv2023

First-principles exploration of superconductivity in intercalated bilayer borophene phases

We explore the emergence of phonon-mediated superconductivity in bilayer borophenes by controlled intercalation with elements from the groups of alkali, alkaline-earth, and transition metals, using systematic first-principles and Eliashberg calculations. We show that the superconducting properties are primarily governed by the interplay between the out-of-plane ($p_{z}$) boron states and the partially occupied in-plane ($s+p_{x,y}$) bonding states at the Fermi level. Our Eliashberg calculations indicate that intercalation with alkaline-earth elements leads to the highest superconducting critical temperatures ($T_{c}$). Specifically, Be in $\delta_{4}$, Mg in $\chi_{3}$, and Ca in the kagome bilayer borophene demonstrate superior performance with $T_{c}$ reaching up to 58~K. Our study therefore reveals that intercalated bilayer borophene phases are not only more resilient to chemical deterioration, but also harbor enhanced $T_{c}$ values compared to their monolayer counterparts, underscoring their substantial potential for the development of boron-based two-dimensional superconductors.

cond-mat.supr-con↗

arXiv2022

Intrinsic control of interlayer exciton generation rate in van der Waals materials via Janus layers

We demonstrate the possibility of engineering the optical properties of transition metal dichalcogenide heterobilayers when one of the constitutive layers has a Janus structure. This has important consequences for the charge separation efficiency. We investigate different MoS$_2$@Janus layer combinations using first-principles methods including electron-hole interactions (excitons) and exciton-phonon coupling. The direction of the intrinsic electric field from the Janus layer modifies the electronic band alignments and, consequently, the energy separation between interlayer exciton states -- which usually have a very low oscillator strength and hence are almost dark in absorption -- and bright in-plane excitons. We find that in-plane lattice vibrations strongly couple the two states, so that exciton-phonon scattering may be a viable generation mechanism for interlayer excitons upon light absorption. In particular, in the case of MoS$_2$@WSSe, the energy separation of the low-lying interlayer exciton from the in-plane exciton is resonant with the transverse optical phonon modes (40 meV). We thus identify this heterobilayer as a prime candidate for efficient electron-hole pair generation with efficient charge carrier separation.

cond-mat.mes-hall↗

arXiv2022

Surface termination dependence of electronic and optical properties in Ti$_2$CO$_2$ MXene monolayers

Two-dimensional (2D) MXenes are a rapid growing family of 2D materials with rich physical and chemical properties where their surface termination plays an essential role. Among the various 2D MXenes, functionalization of the Ti$_{n}$C$_{n-1}$ phase with oxygen (O) atoms makes them attractive for optoelectronic applications due to their optical gap residing in the infrared or visible region. In this manuscript, we theoretically investigate the electronic and optical properties of four different O-atom-functionalized Ti$_{n}$C$_{n-1}$ MXene monolayers using state-of-the-art, first-principles techniques. In particular, we calculate the quasiparticle corrections on top of density functional theory (DFT) at the GW level and the exciton-dominated optical spectra by solving the Bethe-Salpeter equation (BSE) also at finite momentum. We find that all but one of the monolayer models are indirect band gap semiconductors where quasiparticle corrections are very important ($\sim 1$ eV). The optical spectra are instead dominated by direct and indirect excitons with large binding energies (between $0.5$ and $1$ eV). Most direct excitons lie above $1.5$ eV, while the indirect ones are below: therefore, we conclude that Ti$_{n}$C$_{n-1}$ should display strong absorption in the visible region, but phonon-assisted emission in the infrared. Our work thus reveals the potential usage of surface terminations to tune the optical and electronic properties of Ti$_{n}$C$_{n-1}$ MXene monolayers, while emphasizing the pivotal role of many-body effects beyond DFT to obtain accurate prediction for these systems.

cond-mat.mtrl-sci↗

arXiv2022

Superconductivity in functionalized niobium-carbide MXenes

We show the effect of Cl and S functionalization on the superconducting properties of layered (bulk) and monolayer niobium carbide (Nb$_2$C) MXene crystals, based on first-principles calculations combined with Eliashberg theory. For the bulk layered Nb$_2$CCl$_2$, the calculated superconducting transition temperature ($T_c$) is in very good agreement with the recently measured value of 6 K. We show that $T_c$ is enhanced to 10 K for monolayer Nb$_2$CCl$_2$, due to an increase in the density of states at the Fermi level, and the corresponding electron-phonon coupling. We further demonstrate a feasible gate-induced enhancement of $T_c$ up to 40 K for both bulk-layered and monolayer Nb$_2$CCl$_2$ crystals. For the S-functionalized cases our calculations reveal the importance of phonon softening in understanding their superconducting properties. Finally, we predict that Nb$_3$C$_2$S$_2$ in bulk-layered and monolayer form is potentially superconducting, with a $T_c$ around 30 K. Considering that Nb$_2$C is not superconducting in pristine form, our findings promote functionalization as a pathway towards robust superconductivity in MXenes.

cond-mat.supr-con↗

arXiv2022

Enhancing superconductivity in MXenes through hydrogenation

Two-dimensional transition metal carbides and nitrides (MXenes) are an emerging class of atomically-thin superconductors, whose characteristics are highly prone to tailoring by surface functionalization. Here we explore the use of hydrogen adatoms to enhance phonon-mediated superconductivity in MXenes, based on first-principles calculations combined with Eliashberg theory. We first demonstrate the stability of three different structural models of hydrogenated Mo- and W-based MXenes. Particularly high critical temperatures of over 30 K are obtained for hydrogenated Mo$_2$N and W$_2$N. Several mechanisms responsible for the enhanced electron-phonon coupling are uncovered, namely (i) hydrogen-induced changes in the phonon spectrum of the host MXene, (ii) emerging hydrogen-based phonon modes, and (iii) charge transfer from hydrogen to the MXene layer, boosting the density of states at the Fermi level. Finally, we demonstrate that hydrogen adatoms are moreover able to induce superconductivity in MXenes that are not superconducting in pristine form, such as Nb$_2$C.

cond-mat.supr-con↗

arXiv2021

High-temperature multigap superconductivity in two-dimensional metal-borides

Using first-principles calculations in combination with the Eliashberg formalism, we systematically investigated phonon-mediated superconductivity in two-dimensional (2D) metal-boride crystals, consisting of a boron honeycomb network doped by diverse metal elements. Such 2D metal-boride compounds, named MBenes, are chemically exfoliable from single-crystalline layered ternary borides (MAB phases). First, we identified the MBene layers with potential for superconductivity via isotropic Eliashberg calculations, considering a wide range of metal elements, with focus on alkaline earth and transition metals. Subsequently, we performed a detailed analysis of the prominent superconducting MBenes by solving the anisotropic Eliashberg equations. The obtained high critical temperatures (up to 72 K), as well as the rich multigap superconducting behavior, recommend these crystals for further use in multifunctional 2D heterostructures and superconducting device applications.

cond-mat.supr-con↗

arXiv2020

Hot electron relaxation in normal state of iron pnictides: memory function approach

This study leads to the investigation of the non-equilibrium electron relaxation in the normal state of iron pnictides. Here we consider the relaxation of electrons due to their coupling with magnons and phonons in the metallic state of iron pnictides using the memory function approach. In the present model, electrons live at a higher temperature than that of the phonon and magnon baths, mimicking a non-equilibrium steady state situation. Further we analyze theoretically the generalized Drude scattering rate within the framework of Two Temperature Model and study the full frequency and temperature behavior for it. In zero frequency regime, the rate of electron-magnon scattering and electron-phonon scattering shows a linear temperature dependence at higher temperature values greater than Bloch-Gr\"{u}neisen temperature. Whereas at lower temperature values, $T\ll\Theta_{BG}$, corresponding scattering rates follow the temperature behavior as ($1/\tau_{e-p} \varpropto T^3$) and ($1/\tau_{e-m} \varpropto T^{3/2}$), respectively. In the AC regime, we compute that $1/\tau \propto \omega^2$ for $\omega\ll\omega_{BG}$ and for the values greater than the Bloch-Gr\"{u}neisen frequency, it is $\omega$-independent. Also, in lower frequency and zero temperature limit, we have observed the different frequency scale of electron-magnon and electron-phonon scattering i.e ($1/\tau \propto \omega^{3/2}$) and ($1/\tau\propto \omega^{3}$). These results can be viewed with the pump-probe experimental setting for the normal state of iron pnictides.

cond-mat.supr-con↗

arXiv2020

First-principles discovery of stable two-dimensional materials with high-level piezoelectric response

The rational design of two-dimensional piezoelectric materials has recently garnered great interest due to their increasing use in technological applications, including sensor technology, actuating devices, energy harvesting, and medical applications. Several materials possessing high piezoelectric response have been reported so far, but a high-throughput first-principles approach to estimate the piezoelectric potential of layered materials has not been performed yet. In this study, we systematically investigated the piezoelectric ($e_{11}$, $d_{11}$) and elastic (C$_{11}$ and C$_{12}$) properties of 128 thermodynamically stable two-dimensional (2D) semiconductor materials by employing first-principle methods. Our high-throughput approach demonstrates that the materials containing Group-\textrm{V} elements produce significantly high piezoelectric strain constants, $d_{11}$ $>$ 40 pmV$^{-1}$, and 49 of the materials considered have the $e_{11}$ coefficient higher than MoS$_{2}$ insomuch as BrSSb has one of the largest $d_{11}$ with a value of 373.0 pmV$^{-1}$. Moreover, we established a simple empirical model in order to estimate the $d_{11}$ coefficients by utilizing the relative ionic motion in the unit cell and the polarizability of the individual elements in the compounds.

cond-mat.mes-hall↗