Kubo-Greenwood Formula Research Articles

Optical spectra in pure-silica-core single mode optical fibers after high-fluence reactor irradiation

The paper describes the study of radiation-induced attenuation (RIA) spectra in reactor-irradiated single mode at λ=1.55 μm optical fibers (OFs) with differente high-temperature coatings (polyimyde, aluminum and copper). The OFs were previously irradiated up to fast neutron fluence 1.8·1020 n/cm2 and gamma dose 2.32 GGy. After three years of storage at room temperature (RT), the RIA spectra were examined in the range of 650–1700 nm. All RIA mechanisms acting in this spectral range were identified: a tail of non-bridging oxygen hole center, a long-wavelength RIA (LWRIA) band with a maximum at λ > 1600 nm, unstructured “grey” loss due to coating microbendings or damage, and an absorption band peaking at 930 nm (E=1.33 eV). It was found out that the LWRIA can be significantly annealed at RT in polyimide-coated OFs. Grey loss were found to be more stable than those causing LWRIA.The LWRIA arising in reactor- and gamma-irradiated OFs have close thermal stability at RT and exhibit a similar shapes, which close to the absorption band of self-trapped holes (STH). This suggests a STH-like nature of the LWRIA.

Physical origin of enhanced electrical conduction in aluminum-graphene composites

The electronic and transport properties of aluminum-graphene composite materials were investigated using the ab initio plane wave density functional theory. The interfacial structure is reported for several configurations. In some cases, the face-centered aluminum (111) surface relaxes in a nearly ideal registry with graphene, resulting in a remarkably continuous interface structure. The Kubo–Greenwood formula and space-projected conductivity were employed to study electronic conduction in aluminum single- and double-layer graphene-aluminum composite models. The electronic density of states at the Fermi level is enhanced by the graphene for certain aluminum–graphene interfaces, thus improving electronic conductivity. In double-layer graphene composites, conductivity varies non-monotonically with temperature, showing an increase between 300 and 400 K at short aluminum-graphene distances, unlike the consistent decrease in single-layer composites.

Exploring finite-temperature electronic transport in CoSi alloys with transition metals (Cr, Mn, Fe, and Ni) using the KKR-CPA method

An effective combination of the KKR-CPA method and the Kubo–Greenwood formula allows quantitative reproduction of electrical resistivity of CoSi and its alloys with 3d transition metals.

Dangerous Bonds Individual of Hydrogenated Amorphous Silicon and Defect Absorption Spectra

In this work, defect absorption spectra for defects characteristic of hydrogenated amorphous silicon are theoretically studied. It is shown that in order to determine defect absorption spectra using the Kubo-Greenwood formula, the indefinite integral in this formula must be written in a certain form. It was discovered that electronic transitions involving defect states are divided into two parts depending on the energy of absorbed photons. The values of the partial defect absorption spectrum at low energies of absorbed photons have almost no effect on the overall defect absorption spectrum. It has been established that the main role in determining the defect absorption spectrum is played by partial spectra determined by optical transitions of electrons between allowed bands and defects. It is shown that with a power-law distribution of the density of electronic states in allowed bands, the spectra of optical transitions between them and defects do not depend on the value of this power.

Calculation of the Density of the Distribution of Electronic States in the Conduction Band from the Fundamental Absorption Spectra of Amorphous Semiconductors

The region of fundamental absorption in the optical spectra of amorphous semiconductors is theoretically studied using the Davis-Mott approximation according to the Kubo-Greenwood formula. As is known, three types of optical transitions of the electron can be observed in the fundamental absorption region; from the tail of the valence band to the conduction band, from the valence band to the conduction band and from the valence band to the tail of the conduction band. For all these electronic transitions, analytical expressions of the partial absorption spectra are obtained from two different types of the Kubo-Greenwood formula. The width of the optical mobility gap and the proportionality coefficient were determined in the analytical form of the interband absorption spectrum by fitting them to the experimental interband absorption spectrum. A new method is presented for calculating the density of distribution of electronic states in the conduction band of amorphous carbon based on the experimental interband absorption spectrum and the analytical expression of the Kubo-Greenwood formula written for the interband absorption spectrum.

How to read optical properties of matter via the Kubo-Greenwood approach.

Substances with a complex electronic structure exhibit non-Drude optical properties that are challenging to interpret experimentally and theoretically. In our recent paper [Phys. Rev. E 105, 035307 (2022)2470-004510.1103/PhysRevE.105.035307], we offered a computational method based on the continuous Kubo-Greenwood formula, which expresses dynamic conductivity as an integral over the electron spectrum. In this Letter, we propose a methodology to analyze the complex conductivity using liquid Zr as an example to explain its nontrivial behavior. To achieve this, we apply the continuous Kubo-Greenwood formula and extend it to include the imaginary part of the complex conductivity into the analysis. Our method is suitable for a wide range of substances, providing an opportunity to explain optical properties from ab initio calculations of any difficulty.

Tunability of electronic and thermoelectric properties of hexagonal boron nitride with carbon impurities under magnetic field: Tight binding investigation

Utilizing the Kubo-Greenwood formula, Tight Binding calculations were employed to examine the electronic and thermoelectric properties of hexagonal boron nitride (h-BN) with carbon impurity instead of boron, nitrogen and pairs boron-nitrogen. The electronic properties of the pristine monolayer BN are markedly impacted by the introduction of carbon dopants and its band gap reduction is directly correlated with the concentration of carbon impurities. The electronic properties of doped h-BN are influenced by the presence of a magnetic field, leading to subband separation and band gap narrowing, independent of the impurity types. The thermal conductivity and magnetic susceptibility of the CBN-doped monolayer BN structure are higher than those of the BC and NC doped h-BN structures and for all structures, their properties have a strong dependence on the magnetic field. The Lorenz Number for all structures has peak at the TM temperature which shifts to a lower temperature as the impurity concentration decreases.

Theoretical study on the origin of anomalous temperature-dependent electric resistivity of ferromagnetic semiconductor

Employing Korringa–Kohn–Rostoker Green’s function methodology, our investigation elucidates the previously obscure origins of the anomalous temperature-dependent electrical resistivity behavior of (Ga,Mn)As ferromagnetic semiconductors. Phonon and magnon excitations induced by temperature effects are addressed via the coherent potential approximation, while the Kubo–Greenwood formula is employed to compute transport properties. Consequently, the anomalous temperature-dependent electrical resistivity arising from the ferromagnetic–paramagnetic transition is successfully replicated. Our examination of electronic structures and magnetic interactions reveals pivotal roles played by antisite defects and interstitial Mn atoms in governing this behavior. As this approach enables both the estimation of temperature-dependent transport properties and the assessment of underlying mechanisms from a microscopic standpoint, it holds significant potential as a versatile tool across diverse fields.

Conductivity in flat bands from the Kubo-Greenwood formula

Conductivity in flat bands from the Kubo-Greenwood formula

Magneto-electronic and thermopower properties of B, N and Si doped monolayer graphene

Pure graphene is limited by its gapless energy band structure in catalytic reactions that are important for energy conversion applications. However, impurities can create a band gap and improve the electrical and photocatalytic properties of graphene, leading to a wide range of energy-related applications. In this study, the Kubo-Greenwood formula was used to investigate the thermopower properties of monolayer graphene doped with B, N, and Si atoms under various concentrations of impurities and magnetic fields. The impurity concentration was controlled by varying the number of unit cells. The electronic structure of the doped graphene are strongly influenced by the type and concentration of impurities, with a created energy gap that was sensitive to the impurity concentration. Furthermore, applying a magnetic field resulted in subband splitting and band gap reduction for all selected structures, regardless of the impurity type. The thermal properties of the doped structures are dependent on the impurity type, concentration, and magnetic field strength. It was observed that the thermal properties decreased with increasing impurity concentration and were affected by the magnetic field. The thermal properties of the nitrogen-doped structure is higher than those of the boron-doped and silicon-carbide-doped structures. Overall, these findings are significant for developing more efficient energy conversion devices.

Significant modification of the electronic structure of biased trilayer SiC and BN via magnetic field to achieve enhanced thermoelectric performance

The two-dimensional semiconducting materials with tunable electronic structure and high thermoelectric properties have a wide range of applications, especially in nanoscale devices. This study uses the Kubo-Greenwood formula in the framework of the Green function approach and the tight binding model to calculate the thermoelectric properties of trilayer SiC (3L-SiC). 3L-SiC is a semiconductor that can transform to metallic state with increasing applying the external fields with critical strength. The thermoelectric properties of the 3L-SiC and 3L-BN were analyzed under bias voltage, magnetic field and temperature up to 1500 K and compared with each other. The results show that thermal properties of the 3L-SiC are higher than that 3L-BN and both structures exhibit strong dependence on bias voltage and magnetic field with higher sensitivity to the magnetic field. The thermal properties of the selected structures increases with temperature and the external fields enhance the increasing rate. The behavior of the Lorenz Number in terms of temperature exhibits general pattern as increasing from zero to maximum value at TM and decreasing by further temperature increasing.

Modulating the electronic properties and thermal conductivity of trilayer BX (X=N,P) by bias voltage and magnetic field

Two-dimensional materials with controllable electronic properties have a significant effect on improving the performance of nanoelectronic devices. In this study, the electronic and thermal conductivity of the trilayer BP (3L-BP) and BN (3L-BN) in the presence of the bias voltage and magnetic field are investigated by tight binding model and Kubo-Greenwood formula. The different stacking types for all structures are selected. All selected configurations exhibit semiconductor properties and their electronic properties can be affected under the influence of an external electric field. Their band gap decrease to zero in critical field strength and the magnetic field shows a higher decreasing rate, compared to the bias voltage. The external fields increase the thermal conductivity of all cases and 3L-BP has higher thermal intensity than that 3L-BN. The results show that by applying stronger bias voltage or magnetic field, the thermal conductivity of 3L-BN structures can be enhanced in order to 3L-BP, independent of its wide band gap.

Electronic transport in copper–graphene composites

We investigate electronic transport properties of copper–graphene (Cu–G) composites using a density-functional theory (DFT) framework. Conduction in composites is studied by varying the interfacial distance of copper/graphene/copper (Cu/G/Cu) interface models. Electronic conductivity of the models computed using the Kubo–Greenwood formula shows that the conductivity increases with decreasing Cu–G distance and saturates below a threshold Cu–G distance. The DFT-based Bader charge analysis indicates increasing charge transfer between Cu atoms at the interfacial layers and the graphene with decreasing Cu–G distance. The electronic density of states reveals increasing contributions from both copper and carbon atoms near the Fermi level with decreasing Cu–G interfacial distance. By computing the space-projected conductivity of the Cu/G/Cu models, we show that the graphene forms a bridge to the electronic conduction at small Cu–G distances, thereby enhancing the conductivity.

Ab Initio Calculations of Transport and Optical Properties of Dense Zr Plasma Near Melting

The dynamic electrical conductivity of dense Zr plasma near melting is calculated using ab initio molecular dynamics and the Kubo–Greenwood formula. The antisymmetrization of the electronic wave function is considered with the determinant of one-electron wave functions; exchange and correlation effects are treated via an exchange–correlation functional. Optical properties are restored using the Kramers–Kronig transformation. The influence of computational parameters and inner shell electrons on the results is thoroughly investigated. We demonstrate the convergence of our computations and analyze comparison with experimental data.

Combined effect of stacking and magnetic field on the electrical conductivity and heat capacity of biased trilayer BP and BN

In this paper, the Kubo-Greenwood formula based on the tight-binding model is used to investigate the effects of the bias voltage and magnetic field on the electrical conductivity and heat capacity of the trilayer BP and BN with energy-stable stacking structures. The results show that electronic and thermal properties of the selected structures can be significantly modified by external fields. The position and intensity of DOS peaks and the band gap of selected structures are affected by the external fields. When external fields increases above critical value, the band gap decreases to zero and semiconductor-metallic transition occurs. The results show that the thermal properties of the BP and BN structures are zero in TZ temperature region and increase by temperature above TZ. The increasing rate for thermal properties depends on the stacking configuration and changes with the bias voltage and magnetic field. In the presence of the stronger field, the TZ region decreases below 100 K. Compared to the BP structures, the BN types with larger band gap has smaller electrical conductivity which can be increased in order to 3L-BP by applying the stronger magnetic field or bias voltage. These results are interesting for the future development of nanoelectronic devices.

Independent Dual-Channel Approach to Mesoscopic Graphene Transistors.

Graphene field-effect transistors (GFETs) exhibit unique switch and sensing features. In this article, GFETs are investigated within the tight-binding formalism, including quantum capacitance correction, where the graphene ribbons with reconstructed armchair edges are mapped into a set of independent dual channels through a unitary transformation. A new transfer matrix method is further developed to analyze the electron transport in each dual channel under a back gate voltage, while the electronic density of states of graphene ribbons with transversal dislocations are calculated using the retarded Green’s function and a novel real-space renormalization method. The Landauer electrical conductance obtained from these transfer matrices was confirmed by the Kubo–Greenwood formula, and the numerical results for the limiting cases were verified on the basis of analytical results. Finally, the size- and gate-voltage-dependent source-drain currents in GFETs are calculated, whose results are compared with the experimental data.

Engineering the thermodynamic properties of carbon doped boron nitride nanotubes by impurity concentration and electric field

The effects of the impurity concentration and electric field strength on the electronic and thermal properties of carbon doped boron nitride nanotubes are investigated by the tight binding model, Kubo-Greenwood formula and Green function approach. The position, intensity and number of the density of sate peaks are affected by electric field strength F and dopant concentration which leads to band gap reduction in the doped boron nitride nanotubes compared to the pure boron nitride nanotubes. The thermal functions of the doped boron nitride nanotubes are zero first in very low temperature due to existing the large band gap and become zero by increasing the temperature until reach their maximum values at TM and finally decrease significantly in higher temperature above the TM. The increasing (decreasing) pattern in low (high) temperature region occurs due to the band gap modifications (scattering and collision between charge carriers) and for all selected structures, they are related to the electric field strength and doping concentration. The Lorenz number of the doped boron nitride nanotubes with and without the electric field has a peak with LMax intensity at TM which in the presence of the external electric field, it moves to lower temperature with decreasing intensity. By increasing the nanotube radius, the thermal properties of the doped boron nitride nanotubes increase in terms of the temperature and this behavior becomes valid in the presence of the electric field.

Low lattice thermal conductance obtained by controllable quasiperiodic long-range disorder in ternary atomic ribbons with rock-salt structure

To achieve thermoelectric devices with high efficiency requires developing routes to modulate the vibrational properties of semiconductor materials that allow obtaining low lattice thermal conductance. Several efforts have been made to understand the thermal conduction of thermoelectric materials. In particular, semiconductors with rock-salt structure are promising for thermoelectric applications because of the possibility of modifying their thermal properties by composition and the huge quantity of compounds that can be built with this kind of structure. In this work, we investigate how the vibrational properties of ternary atomic ribbons with rock-salt structure can be controlled by Fibonacci long-range mass disorder structure. The study is performed through a real space analysis using the thermal Kubo–Greenwood formula and the Born interaction potential. The results show how the transmission of acoustical phonons with low frequency can be blocked by Fibonacci structures, leading to a progressive decrease of the lattice thermal conductance, which is favorable for thermoelectric applications.

Geometric origin of intrinsic spin hall effect in an inhomogeneous electric field

In recent years, the spin Hall effect has received great attention because of its potential application in spintronics and quantum information processing and storage. However, this effect is usually studied under the external homogeneous electric field. Understanding how the inhomogeneous electric field affects the spin Hall effect is still lacking. Here, we investigate a two-dimensional two-band time-reversal symmetric system and give an expression for the intrinsic spin Hall conductivity in the presence of the inhomogeneous electric field, which is shown to be expressed through the geometric quantities: quantum metric and interband Berry connection. We show that for Rashba and Dresselhaus systems, the inhomogeneous intrinsic spin Hall conductivity can be tuned with the Fermi energy. On the other hand, when people get physical intuition on transport phenomena from the wave packet, one issue appears. It is shown that the conductivity obtained from the conventional wave packet approach cannot be fully consistent with the one predicted by the Kubo-Greenwood formula. Here, we attempt to solve this problem.

First-principles study of the electronic and optical properties of Ho_{text{W}} impurities in single-layer tungsten disulfide

The electronic and optical properties of single-layer (SL) tungsten disulfide (WS_2) in the presence of substitutional Holmium impurities (Ho_{text{W}}) are studied. Although Ho is much larger than W, density functional theory (DFT) including spin-orbit coupling is used to show that Ho:SL WS_2 is stable. The magnetic moment of the Ho impurity is found to be 4.75mu _B using spin-dependent DFT. The optical selection rules identified in the optical spectrum match exactly the optical selection rules derived by means of group theory. The presence of neutral Ho_W impurities gives rise to localized impurity states (LIS) with f-orbital character in the band structure. Using the Kubo-Greenwood formula and Kohn-Sham orbitals we obtain atom-like sharp transitions in the in-plane and out-of-plane components of the susceptibility tensor, Imchi _{parallel } and Imchi _{perp }. The optical resonances are in good agreement with experimental data.