Screening Of Coulomb Interaction Research Articles

Investigating the possible existence of hyper-heavy nuclei in a neutron-star environment

The synthesis of hyper-heavy elements is investigated under conditions simulating neutron star environment. The constrained molecular dynamics approach is used to simulate low energy collisions of extremely n-rich nuclei. A new type of the fusion barrier due to a “neutron wind” is observed when the effect of neutron star environment (screening of Coulomb interaction) is introduced implicitly. When introducing also a background of surrounding nuclei, the nuclear fusion becomes possible down to temperatures of 108 K and synthesis of extremely heavy and n-rich nuclei appears feasible. A possible existence of hyper-heavy nuclei in a neutron star environment could provide a mechanism of extra coherent neutrino scattering or an additional mechanism, resulting in x-ray burst or a gravitational wave signal and, thus, becoming another crucial process adding new information to the suggested models on neutron star evolution. These proceedings are part of a paper that has already been published and the relevant reference is: M. Veselský et al., PRC 106, L012802 (2022).

Tailoring the dielectric screening in WS2–graphene heterostructures

The environment contributes to the screening of Coulomb interactions in two-dimensional semiconductors. This can potentially be exploited to tailor material properties as well as for sensing applications. Here, we investigate the tuning of the band gap and the exciton binding energy in the two-dimensional semiconductor WS2 via the external dielectric screening. Embedding WS2 in van der Waals heterostructures with graphene and hBN spacers of thicknesses between one and 16 atomic layers, we experimentally determine both energies as a function of the WS2-to-graphene interlayer distance and the charge carrier density in graphene. We find that the modification to the band gap as well as the exciton binding energy are well described by a one-over-distance dependence, with a significant effect remaining at several nanometers distance, at which the two layers are electrically well isolated. This observation is explained by a screening arising from an image charge induced by the graphene layer. Furthermore, we find that the effectiveness of graphene in screening Coulomb interactions in nearby WS2 depends on its doping level and can therefore be controlled via the electric field effect. We determine that, at room temperature, it is modified by approximately 20% for charge carrier densities of 2 × 1012 cm−2.

Partial quenching of electronic Raman scattering in double-wall carbon nanotubes by interlayer coupling

Measuring electronic Raman scattering (ERS) has become an efficient method for structural characterization of metallic single-wall carbon nanotubes (SWCNT). However, applying this method to other types of SWCNT-based structures, e.g., those with strong van der Waals (VDW) coupling, is currently not well studied. In this work, we combine electron diffraction, Rayleigh and Raman spectroscopies to investigate the ERS process near 36 metallic transitions in 21 individual double-wall carbon nanotubes (DWCNTs) with all types of electronic configurations. We observe the partial suppression of ERS intensity in DWCNTs compared to SWCNTs and mainly attribute it to the effect of dielectric screening of Coulomb interactions. We probe ultra-pure macroscopic multi-chirality DWCNT solutions and identify the role of inhomogeneous broadening in observing ERS peaks in Raman spectra. Based on the experimental findings, we propose an adapted method for the structural identification of DWCNT samples from the ERS data. The obtained results can be generalized to the characterization of the emerging 1D VDW heterostructures based on metallic SWCNTs.

Screening of the Coulomb interaction in C3N: Reduced dimensionality and electronic structure effects

Coulomb interactions in two-dimensional semimetals like graphene are not screened in the usual way, and sizable long-range interaction has been found. With the aim of achieving new materials with nonconventional screening, as well as, in order to investigate various effects such as reduced dimensionality and electronic band dispersion, we calculate the strength of the effective Coulomb interaction $U$ between ${p}_{z}$ electrons in recently developed hexagonal ${\mathrm{C}}_{3}\mathrm{N}$ materials from the first principles using the constrained random-phase approximation. We find that the calculated parameters in the monolayer of the ${\mathrm{C}}_{3}\mathrm{N}$ are larger than the ones in graphene and remain sizable even in metallic bilayers. Nonlocal Coulomb interactions $U(r)$ in nonmetallic ${\mathrm{C}}_{3}\mathrm{N}$ nanoribbons are almost 1 eV smaller than the ones for ${\mathrm{C}}_{3}\mathrm{N}$ monolayer. Our results show that similar to graphene, screening in hexagonal ${\mathrm{C}}_{3}\mathrm{N}$ is also nonconventional. This controversial screening of Coulomb interaction stems from the electronic structure and massless Dirac dispersion below Fermi energy, which has to be preserved for symmetry reasons in hexagonal ${\mathrm{C}}_{3}\mathrm{N}$ layers.

Investigating the possible existence of hyper-heavy nuclei in a neutron-star environment

The synthesis of hyper-heavy elements is investigated under conditions simulating neutron star environment. The Constrained Molecular Dynamics (CoMD) approach is used to simulate low energy collisions of extremely n-rich nuclei. A new type of the fusion barrier due to a "neutron wind" is observed when the effect of neutron star environment (screening of Coulomb interaction) is introduced implicitly. When introducing also a background of surrounding nuclei, the nuclear fusion becomes possible down to temperatures of 10$^{8}$ K and synthesis of extremely heavy and n-rich nuclei appears feasible. A possible existence of hyper-heavy nuclei in a neutron star environment could provide a mechanism of extra coherent neutrino scattering or an additional mechanism, resulting in X-ray burst or a gravitational wave signal and, thus, becoming another crucial process adding new information to the suggested models on neutron star evolution.

Giant Photoresponse Enhancement in Mixed‐Dimensional Van der Waals Heterostructure through Dielectric Engineering

AbstractThe reduced dielectric screening in the out of plane direction, makes 2D materials sensitive to the surrounding environment, offering a unique platform with greatly tunable optoelectronic properties. Large exciton binding energy in 2D materials limits their photogeneration efficiency. The strong electric field generated at a p–n junction will help in separating these strongly bound electron hole pairs. Here, the present study demonstrates how engineering the surrounding dielectric environment would facilitate a mixed dimensional van der Waals p–n junction to improve the photoresponse to a great extent. A 3D silicon‐2D monolayer MoS2 heterostructure is fabricated as a model system. Nearly three orders of magnitude enhancement in photoresponse is observed by modulating the surrounding dielectric environment. This huge enhancement is attributed to the easy separation of photogenerated carriers due to the screening of Coulomb interaction. The dielectric also screens the impurity potential, reducing the charge carrier scattering. In addition, there is a change in the overall bandgap of the heterostructure producing a lower energy barrier for the charge carriers. The findings lay a general pathway for improving the efficiency of 2D material based photodetectors through dielectric engineering.

Generalized dynamical mean-field theory of two-sublattice systems with long-range interactions and its application to study charge and spin correlations in graphene

We investigate magnetic and charge correlations in graphene by using the formulation of extended dynamical mean-field theory (E-DMFT) for two-sublattice systems. First, we map the average non-local interaction onto the effective static interaction between different sublattices, which is treated together with the local interaction within an effective "two-orbital" local model. The remaining part of the non-local interaction is considered by introducing an effective retarded interaction within the E-DMFT approach. The non-local susceptibilities in charge and spin channel are further evaluated in the ladder approximation. We verify the applicability of the proposed method to describe the effect of uniformly screened long-range Coulomb potential $\propto 1/r$, as well as screened realistic long-range electron interaction [T. O. Wehling et al., Phys. Rev. Lett. 106, 236805 (2011)] in graphene. We show that the developed approach describes a competition of semimetal, spin density wave (SDW), and charge-density-wave (CDW) correlations. The obtained phase diagram is in a good agreement with recent results of functional renormalization group (fRG) for finite large graphene nanoflakes and scaling analysis of quantum Monte Carlo data on finite clusters. Similarly to the previously obtained results within the fRG approach, the realistic screening of Coulomb interaction by $\sigma$ bands causes moderate (strong) enhancement of critical long-range interaction strength, needed for the SDW (CDW) instability, compared to the results for the uniformly screened Coulomb potential.

Magnetic, charge, and transport properties of graphene nanoflakes

We investigate magnetic, charge and transport properties of hexagonal graphene nanoflakes (GNFs) connected to two metallic leads by using the functional renormalization group (fRG) method. The interplay between the on-site and long-range interactions leads to a competition of semimetal (SM), spin density wave (SDW), and charge-density-wave (CDW) phases. The ground-state phase diagrams are presented for the GNF systems with screened realistic long-range electron interaction [T. O. Wehling, et. al., Phys. Rev. Lett. 106, 236805 (2011)], as well as uniformly screened long-range Coulomb potential $\propto 1/r$. We demonstrate that the realistic screening of Coulomb interaction by $\sigma$ bands causes moderate (strong) enhancement of critical long-range interaction strength, needed for the SDW (CDW) instability, compared to the results for the uniformly screened Coulomb potential. This enhancement gives rise to a wide region of stability of the SM phase for realistic interaction, such that freely suspended GNFs are far from both SM-SDW and SM-CDW phase-transition boundaries and correspond to the SM phase. Close relation between the linear conductance and the magnetic or charge states of the systems is discussed. A comparison of the results with those of other studies on GNFs systems and infinite graphene sheet is presented.

Single-Atom Vacancy Doping in Two-Dimensional Transition Metal Dichalcogenides

ConspectusFaced with the growing quests of higher-performance chips, developing new channel semiconductors immune to short channel effects has become a realistic option for continuing Moore’s Law. With outstanding gate electrostatic capacitance, stable chemical properties, and suitable bandgap, two-dimensional (2D) transition metal dichalcogenides (TMDCs) are considered as potential candidates for next-generation channel materials. However, the practical applications of 2D TMDCs are severely limited by stable, precise, and controllable doping technologies, due to their ultrathin body and dangling bond-free surface. Compared to three-dimensional semiconductors, donors in 2D semiconductors need larger ionization energy which can be attributed to the reduced screening of Coulomb interaction and the larger bandgap induced by quantum confinement. Limited by the ultrathin body of 2D TMDCs and the strong film–substrate charge transfer, typical silicon-based substitutional doping technology encounters some headache difficulties in 2D TMDCs and hardly achieves high-concentration doping. The other two doping technologies also cannot take on this task either; local gate electrostatic doping cannot leave the aid of the external electric field. And surface charge transfer doping of molecule adsorbents behaves unstably (e.g., thermal desorption) or ineffectively modifies the original electronic structure. Fortunately, single-atom vacancies can effectively and precisely adjust the carrier concentration of 2D TMDCs and significantly enhance their conductivity. Therefore, clarifying the work rules and function mechanism of single-atom vacancy doping in 2D TMDCs is beneficial in creating a brand-new optimization strategy of electrical properties and overcoming the technical obstacles of the “lab-to-fab” transition for their practical applications in high-performance electronics and optoelectronics.In this Account, we summarize the state-of-the-art progress in single-atom vacancy doping in 2D TMDCs and highlight the applications in optoelectronic and electronic devices. First, the common defects and the density-largest-defect type in 2D TMDCs are demonstrated through experimental characterizations. Second, the healing and manufacturing strategies of chalcogen vacancies in 2D TMDCs are systematically summarized. Third, we clarify the doping mechanism of single-atom vacancies in 2D TMDCs and its regulation of the electrical properties including carrier concentration and carrier mobility. Fourth, the correlations between chalcogen vacancies in 2D TMDCs and the optical signals from Raman and photoluminescence spectroscopies are established, which will help to quickly and nondestructively evaluate the chalcogen vacancy concentration. Fifth, the current applications of single-atom vacancy doping of 2D TMDCs materials are reviewed, including complementary metal–oxide semiconductor (CMOS) logic inverters, homojunctions, Schottky diodes, and photovoltaic devices. Finally, the potential challenges and future development trends of single-atom vacancy doping for next-generation electronic and optoelectronic devices are pointed out. Overall, this Account guides on controllable and precise doping technologies for researchers in these fields from materials, electronics, and optoelectronics to promote the practical applications of 2D TMDCs.

The influence of Coulomb interaction screening on the excitons in disordered two-dimensional insulators

We study the joint effect of disorder and Coulomb interaction screening on the exciton spectra in two-dimensional (2D) structures. These can be van der Waals structures or heterostructures of organic (polymeric) semiconductors as well as inorganic substances like transition metal dichalcogenides. We consider 2D screened hydrogenic problem with Rytova–Keldysh interaction by means of so-called fractional Scrödinger equation. Our main finding is that above synergy between screening and disorder either destroys the exciton (strong screening) or promote the creation of a bound state, leading to its collapse in the extreme case. Our second finding is energy levels crossing, i.e. the degeneracy (with respect to index mu ) of the exciton eigenenergies at certain discrete value of screening radius. Latter effects may also be related to the quantum manifestations of chaotic exciton behavior in above 2D semiconductor structures. Hence, they should be considered in device applications, where the interplay between dielectric screening and disorder is important.

An electronic phase diagram of hole-doped BiCuSeO crystals determined by transport characterization under various growth conditions

Temperature-hole-concentration dependent electronic phase-diagram of BiCuSeO.

Skyrmionic order and magnetically induced polarization change in lacunar spinel compounds GaV4S8 and GaMo4S8 : Comparative theoretical study

We show how low-energy electronic models derived from the first-principles electronic structure calculations can help to rationalize the magnetic properties of two lacunar spinel compounds GaM4S8 with light (M=V) and heavy (M=Mo) transition-metal elements, which are responsible for different spin-orbit interaction strength. In the model, each magnetic lattice point was associated with the M4S4 molecule, and the model itself was formulated in the basis of molecular Wannier functions constructed for three magnetic t2 bands. The effects of rhombohedral distortion, spin-orbit interaction, band filling, and the screening of Coulomb interactions in the t2 bands are discussed in details. The electronic model is further treated in the superexchange approximation, which allows us to derive an effective spin model for the energy and electric polarization ($P$) depending on the relative orientation of spins in the bonds, and study the properties of this model by means of classical Monte Carlo simulations with the emphasis on the possible formation of the skyrmionic phase. While isotropic exchange interactions clearly dominate in GaV4S8, all types of interactions -- isotropic, antisymmetric, and symmetric anisotropic -- are comparable in the case of GaMo4S8. Particularly, large uniaxial exchange anisotropy has a profound effect on the properties of GaMo4S8. On the one hand, it raises the Curie temperature by opening a gap in the spectrum of magnon excitations. On the other hand, it strongly affects the skyrmionic phase by playing the role of a molecular field, which facilitates the formation of skyrmions, but makes them relatively insensitive to the external magnetic field in the large part of the phase diagram. We predict reversal of the magnetic dependence of $P$ in the case of GaMo4S8 caused by the reversal of direction of the rhombohedral distortion.

Annihilation mechanism of excitons in a MoS2 monolayer through direct Förster-type energy transfer and multistep diffusion

Atomically thin MoS2 layer is a direct bandgap semiconductor exhibiting strong electron-hole interaction due to the extreme quantum confinement and reduced screening of Coulomb interactions, which results in the formation of stable excitons at room temperature. Therefore, various excitonic properties of MoS2 monolayer are extremely important in determining the strength of light matter interactions including their radiative recombination lifetime and optoelectronic response. In this paper, we report a comprehensive study of the underlying annihilation mechanism of various types of exciton in MoS2 monolayer using the transient absorption spectroscopy. We rigorously demonstrate that the F\"orster-type resonance energy transfer is the main annihilation mechanism of A- and B-excitons while multistep diffusion process is responsible for C-exciton annihilation, which is supported by critical scientific evidence.

Changes in the Photoluminescence of Monolayer and Bilayer Molybdenum Disulfide during Laser Irradiation.

Various postsynthesis processes for transition metal dichalcogenides have been attempted to control the layer number and defect concentration, on which electrical and optical properties strongly depend. In this work, we monitored changes in the photoluminescence (PL) of molybdenum disulfide (MoS2) until laser irradiation generated defects on the sample flake and completely etched it away. Higher laser power was required to etch bilayer MoS2 compared to monolayer MoS2. When the laser power was 270 μW with a full width at half-maximum of 1.8 μm on bilayer MoS2, the change in PL intensity over time showed a double maximum during laser irradiation due to a layer-by-layer etching of the flake. When the laser power was increased to 405 μW, however, both layers of bilayer MoS2 were etched all at once, which resulted in a single maximum in the change of PL intensity over time, as in the case of monolayer MoS2. The dependence of the etching pattern for bilayer MoS2 on laser power was also reflected in position changes of both exciton and trion PL peaks. The subtle changes in the PL spectra of MoS2 as a result of laser irradiation found here are discussed in terms of PL quantum efficiency, conversion between trions and excitons, mean interatomic spacing, and the screening of Coulomb interaction.

Nonlinear Optical Absorption in GaSe upon Laser Excitation

The nonlinear absorption in GaSe crystals at high optical excitation levels is experimentally studied. A Nd:YAG laser (first and second harmonics, 1064 and 532 nm) and a liquid laser (tuning range 594–643 nm) were used as excitation sources. It is shown that the features observed in the exciton resonance region of the absorption, luminescence, and photoconductivity spectra of GaSe crystals at high optical excitation levels can be explained by exciton–exciton interaction and Coulomb interaction screening by free carriers.

Nonconventional screening of Coulomb interaction in hexagonal boron nitride nanoribbons

Strong excitonic effects is a very subtle issue in pristine hexagonal boron nitride (h-BN) and h-BN nanoribbons (h-BNNRs) due to large band gaps and reduced dimensionality. One of the reasons for such a large exciton binding energy (as large as 2.5 eV) is weak dielectric screening. Employing first-principles calculations in conjunction with the constrained random-phase approximation, we determine the strength of the Coulomb matrix elements for pristine h-BN and h-BNNRs with armchair and zigzag edges. Due to the nonconventional screening, the calculated off-site $U$ parameters for passivated h-BNNRs turn out to be rather sizable. Coulomb interaction is weakly screened at short distances and antiscreened at intermediate distances. Transition from screening to antiscreening takes place at a distance as low as 8 \AA{} in narrow passivated h-BNNR. The critical distance for the onset of antiscreening in hydrogen-terminated h-BNNRs is longer than in zero-dimensional molecules and clusters, but shorter than in graphene nanoribbons and carbon nanotubes. With increasing the width of the passivated h-BNNRs from the critical point about 12.6 \AA{}, the antiscreening effect is not observed. For completeness, on-site and long-range Coulomb interactions for metallic nonpassivated zigzag h-BNNRs are also reported.

Exciton routing in the heterostructure of a transition metal dichalcogenide monolayer on a paraelectric substrate

We propose a scheme for the spatial exciton energy control and exciton routing in a transition metal dichalcogenide (TMD) monolayer which lies on a quantum paraelectric substrate. It relies on the ultrasensitive response of the substrate dielectric permittivity to temperature changes, allowing for spatially inhomogeneous screening of Coulomb interaction in a monolayer. As an example, we consider the heterostructure of TMD and strontium titanate oxide SrTiO$_3$, where large dielectric screening can be attained. We study the impact of substrate temperature on the characteristic electronic features of TMD monolayers such as the particle bandgap and exciton binding energy, Bohr radius and nonlinearity (an exciton-exciton interaction). The combination of particle bandgap and exciton binding energy modulation results in the shift of the exciton resonance energy. Applying local heating, we create spatial patterns with varying exciton resonant energy and an exciton flow towards the energetically lower region of the sample.

Screening of Coulomb interactions in holography

We introduce Coulomb interactions in the holographic description of strongly interacting systems by performing a (current-current) double-trace deformation of the boundary theory. In the theory dual to a Reissner-Nordström background, this deformation leads to gapped plasmon modes in the density-density response, as expected from conventional RPA calculations. We further show that by introducing a (d + 1)-dimensional Coulomb interaction in a boundary theory in d spacetime dimensions, we recover plasmon modes whose dispersion is proportional to sqrt{left|mathbf{k}right|} , as observed for example in graphene layers. Moreover, motivated by recent experimental results in layered cuprate high-temperature superconductors, we present a toy model for a layered system consisting of an infinite stack of (spatially) two-dimensional layers that are coupled only by the long-range Coulomb interaction. This leads to low-energy ‘acoustic plasmons’. Finally, we compute the optical conductivity of the deformed theory in d = 3 + 1, where a logarithmic correction is present, and we show how this can be related to the conductivity measured in Dirac and Weyl semimetals.

Resonances in nonrelativistic free-free Gaunt factors with screened Coulomb interaction

The effect of Coulomb interaction screening on nonrelativistic free-free absorption is investigated by integrating the numerical continuum wave functions. The screened potential is taken to be in Debye-H\"uckel (Yukawa) form with a screening length $D$. It is found that the values of the free-free Gaunt factors for different Debye screening lengths $D$ for a given initial electron energy ${\ensuremath{\varepsilon}}_{i}$ and absorbing photon energy $\ensuremath{\omega}$ generally lie between those of the pure Coulomb field and field-free case. However, for initial electron energies below 0.1 Ry and fixed photon energy, the Gaunt factors show dramatic enhancements (broad and narrow resonances) in the vicinities of the critical screening lengths, ${D}_{nl}$, at which the energies of $nl$ bound states in the potential merge into the continuum. These enhancements of the Gaunt factors can be significantly higher than their values in the unscreened (Coulomb) case over a broad range of ${\ensuremath{\varepsilon}}_{i}$. The observed broad and narrow resonances in the Gaunt factors are related to the temporary formation of weakly bound (virtual) and resonant (quasibound) states of the low-energy initial electron on the Debye-H\"uckel potential when the screening length is in the vicinity of ${D}_{nl}$.

Screening of long-range Coulomb interaction in graphene nanoribbons: Armchair versus zigzag edges

We study the electronic screening of the long-range Coulomb interaction in graphene nanoribbons (GNRs) with armchair and zigzag edges as a function of the ribbon width by employing ab initio calculations in conjunction with the random-phase approximation. We find that in GNRs with armchair edges quantum confinement effects lead to oscillatory behavior of the on-site screened Coulomb interaction with the ribbon width. Furthermore, the reduced dimensionality and the existence of a band gap result in a nonconventional screening of the Coulomb interaction; that is, it is screened at short distances and antiscreened at intermediate distances, and finally, it approaches the bare (unscreened) interaction at large distances. In the case of GNRs with zigzag edges the presence of edge states strongly affects the screening, which leads to a strong reduction of the effective on-site Coulomb interaction (Hubbard $U)$ parameters at the edge. We find that the interactions turn out to be local; the nonlocal part is strongly screened due to edge states, making GNRs with zigzag edges correlated materials. On the basis of the calculated effective Coulomb interaction parameter $U$, we discuss the appearance of ferromagnetism at zigzag edges of GNRs within the Stoner model.