arXiv subjects

František Karlický

Publications and source records attributed to František Karlický.

15 recordsLinked to original sources

arXiv2026

Ab Initio Investigation of Excited States and Absorption Spectra of Small Mercury Clusters (Hg$_{2-5}$)

We report a systematic \textit{ab initio} investigation of the excited states and absorption spectra of small mercury clusters (Hg$_{\mathrm{2-5}}$). Benchmarking multi-reference configuration interaction (MR-CISD) and equation-of-motion coupled-cluster (EOM-CCSD) methods shows that EOM-CCSD/aug-cc-PVTZ-PP reproduces atomic excitation energies within $\pm$500 cm$^{-1}$ of experiment, whereas MR-CISD deviates by about $\pm$1100 cm$^{-1}$. For the mercury dimer, EOM-CCSD potential energy curves yield absorption bands in excellent agreement with experimental transitions, while asymptotic excitation energies closely match atomic references. Extending this methodology, we present the first systematic \textit{ab initio} study of absorption spectra of Hg$_{\mathrm{3}}$, Hg$_{\mathrm{4}}$, Hg$_{\mathrm{5}}$, which display broad bands in the ranges of 37000-60000 cm$^{-1}$. The consistent agreement with experimental benchmarks and diagnostics confirming single-reference character validates EOM-CCSD as a robust approach for describing excited states and optical spectra of mercury clusters.

physics.chem-ph↗

arXiv2026

DFTB coupled with NEGF study of the structural, electronic and transport properties of goldene 2D material

We report the structural, electronic, and transport properties of the goldene 2D material using the density functional tight-binding (DFTB) method. Electronic transport calculations were conducted in conjunction with the non-equilibrium Green's functions (NEGF) technique. Our study reveals that the Au 2D material is dynamically and thermally stable, and it possesses good elastic properties. On the other hand, goldene has a linear relationship between current and voltage at low potentials, indicating its metallic character. The calculated current-potential curve correlates well with transmission functions and the electronic density of states around the Fermi level. We also investigated the electronic structure and magnetic properties of silicon (Si)-doped Au 2D material. Our results show that the Si atom can induce a local magnetic state in the goldene monolayer. The resulting magnetic moment is 0.63 $μ_B$.

cond-mat.mtrl-sci↗

arXiv2025

Unveiling the magnetic behavior of C$_{\rm3}$N$_{\rm4}$ 2D material by defect creation, defect passivation, and transition metal adsorption

Using the density functional tight binding method (DFTB) and the GFN1-xTB (Geometries, Frequencies, and Noncovalent interactions Tight Binding) Hamiltonian, we have investigated the structural, electronic and magnetic properties of vacancy defects, hydrogen and oxygen passivated defects, and Fe adsorption in two-dimensional (2D) graphitic carbon nitride (gt-C$_3$N$_4$) 2D material. The ring shape is the most preferred vacancy evolution path, with significant stability of the semicircle fourfold C-N-C-N vacancy. We found that bare gt-C$_3$N$_4$ which is non-magnetic becomes magnetic by 2-, and 5-defects creation, hydrogen/oxygen passivation of the defects, and upon Fe adsorption. Interestingly, Fe atoms interact with the gt-C$_3$N$_4$ sheet and result in a ground ferromagnetic (FM) state. In addition, we investigate the effects of passivating the vacancies by hydrogen in gt-C$_3$N$_4$ on its structural, electrical, and magnetic properties. We found that substituting the 1, 2, and 3 vacancies with hydrogen and passivating the 6-defect with oxygen turns on magnetism in the system. Due to structural distortion, the passivated defects do not have a well-ordered magnetic orientation. However, passivating the remaining defected structures maintains the nonmagnetic state. It is also shown that passivation leads to a semiconductor with a band gap value higher than that of the bare material. We also calculate the electronic and magnetic properties of transition metal (TM) atoms, including V, Cr, Mn, Fe, Co, Ni-adsorbed gt-C$_3$N$_4$ monolayer. All TM atoms show slight lattice distortion, and the adsorbed system almost maintains the original structure type. Moreover, a FM alignment was observed with a total magnetic moments of 2.89 $μ_B$, 2 $μ_B$, and 1 $μ_B$ for V, Fe, and Co atoms, respectively. The Cr, Mn, and Ni atoms induce no magnetism to the non magnetic gt-C$_3$N$_4$ system.

cond-mat.mtrl-sci↗

arXiv2025

Predicting Fundamental Gaps of Chromium-Based 2D Materials Using GW Methods

Precise and accurate predictions of the two-dimensional (2D) material's fundamental gap are crucial for next-generation flexible electronic and photonic devices. We, therefore, evaluated the predictivity of the GW approach in its several variants built on various density functional theory (DFT) inputs. We identified reasons for significant discrepancies between generalized gradient approximation and hybrid DFT results for intricate cases of 2D materials containing chromium and evaluated the diverse behavior of subsequent quasiparticle corrections. We examined the impact of omitted vertex corrections using the more computationally intensive quasiparticle self-consistent QPGW and partially self-consistent QPGW$_0$ methodologies. We observed consistent trends across Cr-based and other 2D materials compared by advanced GW calculations, suggesting that single-shot G$_0$W$_0$@PBE can provide reasonable estimates of fundamental gaps when applied with caution. While this approach shows promise for a variety of 2D materials, including complicated antiferromagnetic chromium-based transition metal carbides (MXenes), further research is required to validate its reliability for strongly correlated systems. In contrast, the G$_0$W$_0$@HSE06 approach may strongly overestimate gaps in some cases.

cond-mat.mtrl-sci↗

arXiv2024

The effect of mixed termination composition in Sc, Ti, and V-based MXenes

In this study, we investigate the effect of mixed surface terminations (F, O, OH) on the properties of M$_{2}$C MXenes (M = Sc, Ti, V). We explore how different compositions and patterns of terminal groups affect the stability and electronic properties of these 2D materials. The bond dissociation energies and cohesion energies show a clear preference for F-terminations in Sc$_{2}$C, while Ti- and V-based MXenes prefer O-terminations. The data indicates that terminal groups on opposite sides of the MXene have little to no influence on each other's electronic structure, allowing for independent chemical environments on each side. Semiconducting forms of studied MXenes (Sc$_{2}$CF$_{2}$, Sc$_{2}$C(OH)$_{2}$ and Ti$_{2}$CO$_{2}$) showed very high sensitivity to conduction-inducing terminations (O, O, and F, respectively) with even minuscule amounts ($\approx$1%) causing the materials to become conductive. This high sensitivity of the band gap to surface terminations may offer an explanation for the challenges in synthesizing semiconducting forms of MXenes.

cond-mat.mtrl-sci↗

arXiv2024

Mn$_2$C MXene Functionalized by Oxygen is a Semiconducting Antiferromagnet and Efficient Visible Light Absorber

Manganese-based MXenes are promising two-dimensional materials due to the broad palette of their magnetic phases and the possibility of experimental preparation because the corresponding MAX phase was already prepared. Here, we systematically investigated geometrical conformers and spin solutions of oxygen-terminated Mn$_2$C MXene and performed subsequent many-body calculations to obtain reliable electronic and optical properties. Allowing energy-lowering using the correct spin ordering via supercell magnetic motifs is essential for the Mn$_2$CO$_2$ system. The stable ground-state Mn$_2$CO$_2$ conformation is antiferromagnetic (AFM) one with zigzag lines of up and down spins on Mn atoms. The AFM nature is consistent with the parent MAX phase and even the clean depleted Mn$_2$C sheet. Other magnetic states and geometrical conformations are energetically very close, providing state-switching possibilities in the material. Subsequent many-body GW and Bethe-Salpeter equation (BSE) calculations provide indirect semiconductor characteristics of AFM Mn$_2$CO$_2$ with a fundamental gap of 2.1 eV (and a direct gap of 2.4 eV), the first bright optical transition at 1.3 eV and extremely strongly bounded (1.1 eV) first bright exciton. Mn$_2$CO$_2$ absorbs efficiently the whole visible light range and near ultraviolet range (between 10 - 20%).

cond-mat.mtrl-sci↗

arXiv2024

Strain-Induced Changes of Electronic and Optical Properties of Zr-based MXenes

Zr-based MXenes recently attracted attention because of its experimental preparation showing temperature stability, mechanical strength, and promising energy, sensoric, and electrochemistry applications. However, necessary theoretical predictions at a precise/predictive level are complicated due to essential excitonic features and strong electron correlation (i.e., a necessity to go beyond standard density functional theory, DFT). Contrary to the prevailing focus on oxygen-terminated MXenes and standard predictions of other Zr-based MXenes as conductors, based on the hybrid DFT and GW many-body perturbational theory, we were able to find seven different semiconductors (five of them for their equilibrium geometry and two others under slight tensile biaxial strain) in the case of two- and three-layered Zr$_2$CT$_2$ and Zr$_3$C$_2$T$_2$ configurations with various terminations (T = O, F, S, Cl). We observed semiconductor-to-conductor transition induced by strain in the majority of such Zr-based MXenes at experimentally achievable strain range. Furthermore, using the Bethe-Salpeter equation (BSE), we demonstrated that selected semiconducting Zr-based MXenes possess high optical absorption efficiency (20-30%) in the visible light range, underscoring their potential in photonic applications. The high sensitivity of Zr-based MXenes to external conditions and functionalization combined with the thermal stability make the materials promising for applications at operational temperatures in electronic and optical technologies.

cond-mat.mtrl-sci↗

arXiv2024

MXene's Surface Functionalization Patterns and Their Impacts on Magnetism

Two-dimensional transition metal carbides and nitrides (MXenes) are a perspective group of materials with a broad palette of applications. Surface terminations are a product of the MXene preparation, and post-processing can also lead to partial coverage. Despite applicability and fundamental properties being driven by termination patterns, it is not fully clear, how they behave on MXene surfaces with various degrees of surface coverage. Here, as the first step, we used density functional theory to predict possible patterns in prototypic Ti$_2$C MXene, demonstrating the different behavior of the two most frequent terminal atoms, oxygen, and fluorine. Oxygen (with formal charge -2$e$) prefers a zigzag line both-side adsorption pattern on bare Ti$_2$C, attracting the next adsorbent to a minimal distance. Oxygen defects in fully O-terminated MXene tend to form similar zigzag line vacancy patterns. On the other hand, fluorine (with a formal charge of -1$e$) prefers one-side flake (island) adsorption on bare Ti$_2$C and a similar desorption style from fully fluorinated Ti$_2$C. MXene magnetic behavior is subsequently driven by the patterns, either compensating locally and holding the global magnetic state of the MXene until some limit (oxygen case) or gradually increasing total magnetism by summing of local effects (fluorine case). The systematic combinatoric study of Ti$_2$CT$_x$ with various coverages ($0 \leq x \leq 2$) of distinct terminal atoms T = O or F brings encouraging possibilities of tunable behavior of MXenes and provides useful guidance for its modeling towards electronic nanodevices.

cond-mat.mtrl-sci↗

arXiv2024

Prediction of induced magnetism in 2D Ti$_2$C based MXene by manipulating the mixed surface functionalization and metal substitution computed by xTB model Hamiltonian of the DFTB method

We employed the recently developed density functional tight binding (DFTB) method's Hamiltonian, GFN1-xTB, for modeling the mixed termination in Ti$_{\rm2}$C MXene, namely three types of termination by combining -O and -OH, -O and -F, as well as -F and -OH. We demonstrated that the approach yields reliable predictions for electronic and magnetic properties of such MXenes. The first highlighted result is that the mixed surface functionalization in Ti$_{\rm2}$CA$_{\rm x}$B$_{\rm y}$ MXene induces spin polarization with diverse magnetic alignments, including ferromagnetism and two types of antiferromagnetism. We further identified the magnetic alignment for the investigated MXene in terms of the compositions of the terminal groups. Moreover, the effect of the transition metal (Ti) substituted by the Sc atom on the electronic and magnetic properties were also investigated. We found that the studied systems maintain the magnetism and the metallic characteristics. A magnetic transition from antiferromagnetic (AFM) to ferrimagnetic (FiM) ordering was found for ScTi$_{\rm15}$C$_{\rm8}$F$_{\rm8}$(OH)$_{\rm8}$ and ScTi$_{\rm15}$C$_{\rm8}$F$_{\rm12}$(OH)$_{\rm4}$ compounds. Finally, we proved that incorporating the Sc atom into the lattice of Ti$_{\rm2}$CO$_{\rm2}$ and mixed surface termination Ti$_{\rm2}$CA$_{\rm x}$B$_{\rm y}$ is an effective strategy to induce magnetism. Our study may provide a new potential application for designing MXene-based spintronics.

cond-mat.mtrl-sci↗

arXiv2024

Phonon Assisted Exciton Processes in Two-Dimensional Tungsten Monocarbide

n this study, we utilize a rigorous ab initio-based finite momentum Bethe-Salpeter equation to investigate the photoluminescence emission in two-dimensional hexagonal tungsten carbide (h-WC). This thermodynamically stable monolayer exhibits an indirect optical gap, resulting in phonon-assisted emission. We observe that light absorption is a direct process centered around the direct quasiparticle gap, while light emission is indirect and requires modes between $Γ$-$M$ in the phonon dispersion. The emission lines feature prominent phonon replicas at cryogenic temperatures, particularly near-infrared wavelengths (1.09 and 1.17 eV), and we observe exciton thermalization with the crystal beyond 25 K. Additionally, non-radiative recombination is a remarkably fast process, occurring at order of a few femtoseconds (4.8 fs at 0 K and 2.8 fs at 300 K) compared to radiative recombination (2.3 ps at 0 K and 214 ns at 300 K). These optical characteristics of 2D h-WC may facilitate the promise of photon-emitter devices for near-infrared signal communication.

cond-mat.mtrl-sci↗

arXiv2024

Excitons, Optical Spectra, and Electronic Properties of Semiconducting Hf-based MXenes

Semiconducting MXenes are an intriguing two-dimensional (2D) material class with promising electronic and optoelectronic properties. Here, we focused on recently prepared Hf-based MXenes, namely Hf$_3$C$_2$O$_2$ and Hf$_2$CO$_2$. Using the first-principles calculation and excited state corrections, we proved its dynamical stability, reconciled its semiconducting behavior, and obtained fundamental gaps by the many-body GW method (indirect 1.1 eV and 2.2 eV, respectively, direct 1.4 eV and 3.5 eV, respectively). Using the Bethe-Salpeter equation (BSE) we subsequently provided optical gaps (0.9 eV and 2.7eV, respectively), exciton binding energies, absorption spectra, and other properties of excitons in both Hf-based MXenes. The indirect character of both 2D materials further allowed a significant decrease of excitation energies by considering indirect excitons with exciton momentum along the $Γ$-M path in the Brillouin zone. The first bright excitons are strongly delocalized in real space while contributed by only a limited number of electron-hole pairs around the M point in the k-space from the valence and conduction band. A diverse range of excitonic states in Hf$_3$C$_2$O$_2$ MXene lead to a 4\% and 13\% absorptance for the first and second peaks in the infrared region of absorption spectra, respectively. In contrast, a prominent 28\% absorptance peak in the visible region appears in Hf$_2$CO$_2$ MXene. Results from radiative lifetime calculations indicate the promising potential of these materials in optoelectric devices requiring sustained and efficient exciton behavior.

cond-mat.mtrl-sci↗

arXiv2021

Lattice dynamics in the conformational environment of multilayered hexagonal boron nitride (h-BN) conveys to peculiar infrared optical responses

Stacking mismatches in hexagonal boron nitride (h-BN) nanostructures affect their photonic, mechanical, and thermal properties. To access information about the stacked configuration of layered ensembles, highly sophisticated techniques like X-ray photoemission spectroscopy or electron microscopy are necessary. Here, instead, by taking advantage of the geometrical and chemical nature of h-BN, we show how simple structural models, based on shortened interplanar distances, can produce effective charge densities. Accounting these in the non-analytical part of the lattice dynamical description makes it possible to access information about the composition of differently stacked variants in experimental samples characterized by infrared spectroscopy. The results are obtained by density functional theory and confirmed by various functionals and pseudopotential approximations. Even though the method is shown on h-BN, the conclusions are more general and show how effective dielectric models can be considered as valuable theoretical pathways for the vibrational structure of any layered material.

cond-mat.mtrl-sci↗

arXiv2014

Challenges in the Theoretical Description of Nanoparticle Reactivity: Nano Zero-Valent Iron

The reactivity of iron atoms, clusters and nanoparticles (nZVI) is of increasing interest owing to their important practical applications, ranging from the steel industry to water remediation technologies. Here, we provide an overview of computational methods and models that can be applied to study nZVI reactions and discuss their benefits and limitations. We also report current progress in calculations through recent examples treating the reactivity of nZVI particles. Finally, we consider the potential use of highly accurate methods with favorable scaling (such as quantum Monte Carlo or random phase approximation), which are currently considered too computationally expensive but are expected to become more amenable in the future as computer power increases.

physics.chem-ph↗

arXiv2014

Band Gaps and Optical Spectra from Single- and Double-Layer Fluorographene to Graphite Fluoride: Many-Body Effects and Excitonic States

We compare first-principle band gaps and optical absorption spectra of single- and double-layer fluorographene with bulk graphite fluoride. The electronic properties are calculated using the many-body GW approximation and the optical spectra using the Bethe-Salpeter equation (BSE). The inclusion of electron-hole interactions is crucial for predicting low energy excitonic absorption peaks. The position of the first exciton peak is identical in single-, double- and multilayer fluorographene, which may indicate that the onset of the absorption spectra does not differ in these materials.

cond-mat.mtrl-sci↗

arXiv2012

Band gaps and structural properties of graphene halides and their derivates: A hybrid functional study with localized orbital basis sets

DFT calculations of the electronic structure of graphane and stoichiometrically halogenated graphene derivatives (fluorographene and other analogous graphene halides) show (i) localized orbital basis sets can be successfully and effectively used for such 2D materials; (ii) several functionals predict that the band gap of graphane is greater than that of fluorographene, whereas HSE06 gives the opposite trend; (iii) HSE06 functional predicts quite good values of band gaps w.r.t benchmark theoretical and experimental data; (iv) the zero band gap of graphene is opened by hydrogenation and halogenation and strongly depends on the chemical composition of mixed graphene halides; (v) the stability of graphene halides decreases sharply with increasing size of the halogen atom - fluorographene is stable, whereas graphene iodide spontaneously decomposes. In terms of band gap and stability, the C2FBr, and C2HBr derivatives seem to be promising materials, e.g., for (opto)electronics applications, because their band gaps are similar to those of conventional semiconductors, and they are expected to be stable under ambient conditions. The results indicate that other fluorinated compounds (CaHbFc and CaFbYc, Y = Cl, Br, I) are stable insulators.

cond-mat.mtrl-sci↗