Drude-like Behavior Research Articles

Ab-initio study on structural, magnetic, electronic and optical properties of SrCo1−xAxO3 (A = Fe or Cr, x = 0.125 and 0.25)

Herein, a first-principles study on the structural, electronic, optical and magnetic properties of SrCo[Formula: see text]AxO3 ([Formula: see text] or Cr, [Formula: see text] and 0.25) was conducted. For all calculations, ultrasoft pseudo-potentials plane wave (USPPW) and the generalized gradient approximation (GGA) with the default exchange-correlation parameterization of Perdew–Burke–Ernzerhof (PBE) were employed and with Hubbard correction. The crystal structures of SrCo[Formula: see text]A[Formula: see text]O3 and SrCo[Formula: see text]A[Formula: see text]O3 ([Formula: see text] or Cr) are determined as a supercell of [Formula: see text] cell cubic perovskite structure (Pm-3m) with symmetry space group No. 221. No energy gap was observed at the Fermi level for both SrCo[Formula: see text]Cr[Formula: see text]O3 and SrCo[Formula: see text]Cr[Formula: see text]O3 compounds, indicating their metallic properties. The magnetic moment calculations showed that the doping of Fe into SrCoO3 resulted in a higher net magnetic moment and all materials studied here was ferromagnetic ground states. Dielectric functions indicate the metallic conductivity for all materials studied here. The different values of the real part for materials present that this material has a Drude-like dielectric function behavior.

The new solid solution of double transition metal MXenes: Atomistic modeling of two-dimensional YScX (X= C and N)

In this paper, the structural and dynamical stability, electronic, optical, and thermodynamic properties of a new solid solution of two transition metals YScX (X = C and N) MXenes have been studied. The calculation is carried out based on density functional theory (DFT) and the pseudopotential technique. The results obtained from the structural, thermodynamical, and dynamical stability have shown that 2D YScC and YScN MXenes are structurally stable and their experimental synthesis is possible. Total density of states and band structure calculations show that YScX (X = C and N) MXene exhibits metallic behavior using LDA, GGA, and GGA + HSE06 approximations. According to the total density of states results, it can be concluded that the YScX MXenes have a pseudogap close to the Fermi level and at the left side, which justifies the structural stability of the studied monolayers. The plots of electron charge density distribution and electron localization function justify the metallic nature of the mentioned structures. The large negative values of the real part of the dielectric function indicate that 2D YScX (X = C and N) MXenes have a metallic Drude-like behavior. The saturation of the surface of 2D monolayers with various F, O, OH, and H functional groups does not change the metallic properties, the only exception that reveals the semiconductor property in the HSE06 method is YScCH2 Mxene where no atomic orbital intersects the Fermi level. In addition, the onset of the absorption spectrum of YScX (X = C and N) MXenes corresponds to zero photon energy, which is a confirmation of the metallic properties of the investigated structures. The GGA + HSE06 method enables faster electromagnetic beam propagation through the 2D materials, particularly in the x-direction of the hexagonal structure, resulting in a lower absorption coefficient and optical conductivity of 2D monolayers compared to the GGA method. The rising trend of entropy with temperature provides evidence for the endothermic behavior of YScC and YScN monolayers. Unlike heat capacity, the Debye temperature of YScX (X = C and N) MXenes decreases with respect to temperature T, according to the inverse relationship between the Debye temperature and the heat capacity. Due to structural and dynamic stability and layered structure with the nature of covalent and metallic bonds, YScX MXenes are a suitable choice for use in electrical connections and as a protective coating with a low friction level.

Ab-initio study of the optical properties of 2D Yn+1Cn (n = 1, 2, and 3) MXenes and bulk of YC

Calculations are accomplished in the framework of density functional theory and pseudopotential method with local density approximation (LDA) and QUANTUM-ESPRESSO/PWSCF computational code. Large negative values of ε1ω represent that materials exhibit Drude-like behavior while at higher energies, interband transitions occur. According to the results of the imaginary part of the dielectric functionε2ω, it can be stated that the absorption starts from small energies, which indicates that the bulk of YC and Yn+1Cn (n = 1, 2, and 3) monolayers have no energy bandgap and show a metallic nature. The sharp plasmonic energy peak for the YC structure in the x-direction can be seen at 18.61 eV; also the values of sharp plasmonic energy peaks for the Y2C, Y3C2, and Y4C3 in the x-direction are 13.40 eV, 11.98 eV, and 10.14 eV respectively. The results of the reflection spectrum show that the bulk of YC and 2D Yn+1Cn (n = 1, 2, and 3) MXenes have large values due to the presence of free electrons and plasma fluctuations, and confirm the metallic nature of the mentioned compounds. For the bulk of YC, the refractive index prominent peaks approach zero within the photon energy range of 13 eV up to 16 eV, while the corresponding range for the 2D Yn+1Cn MXenes is narrower and equal to 6 eV up to 7 eV. Also, the absorption spectrum in the bulk structure of YC and Yn+1Cn MXenes starts from zero photon energy which verifies the metallic behavior of the studied structures.

Electronic transport coefficients from density functional theory across the plasma plane

We investigate the thermopower and Lorenz number of hydrogen with Kohn-Sham density functional theory (DFT) across the plasma plane toward the near-classical limit, i.e., weakly degenerate and weakly coupled states. Our results are in concordance with certain limiting values for the Lorentz plasma, a model system which only considers electron-ion scattering. Thereby, we clearly show that the widely used method of calculating transport properties via the Kubo-Greenwood (KG) formalism does not capture electron-electron scattering processes. Our discussion also addresses the inadequateness of assuming a Drude-like frequency behavior for the conductivity of nondegenerate plasmas by revisiting the relaxation time approximation within kinetic theory.

Using Disorder to Identify Bogoliubov Fermi-Surface States.

We argue that a superconducting state with a Fermi surface of Bogoliubov quasiparticles, a Bogoliubov Fermi surface (BG-FS), can be identified by the dependence of physical quantities on disorder. Inparticular, we show that a linear dependence of the residual density of states at weak disorder distinguishes a BG-FS state from other nodal superconducting states. We further demonstrate the stability of supercurrent against impurities and a characteristic Drude-like behavior of the optical conductivity. Our results can be directly applied to electron irradiation experiments on candidate materials of BG-FSs, including Sr_{2}RuO_{4}, FeSe_{1-x}S_{x}, and UBe_{13}.

Observation of interacting polaronic gas behavior in Ta-doped TiO2 thin films via terahertz time-domain spectroscopy

Transparent conductive oxides (TCOs)—materials that have the twin desirable features of high optical transmission and electrical conductivity—play an increasingly significant role in the fields of photovoltaics and information technology. As an excellent TCO, Ta-doped anatase TiO2 shows great promise for a wide range of applications. Here, terahertz time-domain spectroscopy is used to study the complex optical conductivity σ̃ω of the TCO—heavily Ta-doped TiO2 thin films with different Ta-doping concentrations, in the frequency range of 0.3–2.7 THz and the temperature range of 10–300 K. Fitting the complex optical conductivity to a Drude-like behavior allows us to extract the temperature dependence of the effective mass, which suggests the existence of many-body large polarons. Moreover, the carrier scattering rate of Ta-doped TiO2 with different carrier concentrations agrees with the interacting polaron gas theory. Our results suggest that with increasing electron density in TiO2, the interaction between polarons is larger and electron–phonon coupling is smaller, which is beneficial for achieving high mobility and conductivity in TiO2.

Influence of annealing on the optical properties of monoclinic vanadium oxide VO2 prepared in nanoscale by hydrothermal technique

Terahertz and infrared reflectivity measurements were carried out on monoclinic vanadium oxide VO2 prepared directly in nanoscale by hydrothermal technique. The product was annealed in argon atmosphere at pressure 100 Pa for 6 h at temperatures 350 °C, 450 °C and 550 °C. The as-grown and annealed samples were investigated by XRD, HRTEM, VSM, DSC, THz, and FTIR techniques to explore the effect of annealing on the optical and electronic properties. The obtained results reveal the existence of monoclinic M-B-VO2 phase for the as-grown sample, while the annealed samples include two other oxidation phases V6O13 and V2O5 together with the monoclinic VO2 phase. This observation is supported by the data obtained from XRD, VSM, and DSC measurements. Both THz transmittance and infrared reflectivity measurements performed at room temperature show insulating behavior for the as-grown monoclinic VO2 sample whereas subtle modifications of monoclinic VO2 phase by annealing induces a metallic behavior via enhancement of surface area to volume ratio and increasing the grain boundaries as a result of embedded and distributed of two other phases V6O13 and V2O5 within the monoclinic VO2 matrix. Fitting of the optical conductivity deduced from both THz and infrared measurements clearly shows Drude-like behavior for the annealed sample while this behavior was not observed in case of the as-grown sample.

High figure of merit magneto-optics from interfacial skyrmions on topological insulators

In the Kerr rotation geometry, magneto-optic memory devices typically suffer from low figure-of-merit (FOM) and long write times. We show that skyrmions formed at the interface of a thin-film multiferroic and a topological insulator can give rise to high FOM magneto-optic Kerr effects (MOKEs). Huge differential MOKE can arise in parts of the phase diagram. Resonancelike features in the MOKE spectra arising from the induced low energy TI band gap, the multiferroic-film thickness, and the high energy Drude-like behavior are resolved and explained. The Fermi level dependence of the MOKE signatures is distinct for the different magnetic textures. This has broad implications for magnetic texture characterization, electro-optic modulators and isolators, and high density magnetic optic memory.

Coupled electron-ion Monte Carlo simulation of hydrogen molecular crystals.

We performed simulations for solid molecular hydrogen at high pressures (250 GPa ≤ P ≤ 500 GPa) along two isotherms at T = 200 K (phase III) and at T = 414 K (phase IV). At T = 200 K, we considered likely candidates for phase III, the C2c and Cmca12 structures, while at T = 414 K in phase IV, we studied the Pc48 structure. We employed both Coupled Electron-Ion Monte Carlo (CEIMC) and Path Integral Molecular Dynamics (PIMD). The latter is based on Density Functional Theory (DFT) with the van der Waals approximation (vdW-DF). The comparison between the two methods allows us to address the question of the accuracy of the exchange-correlation approximation of DFT for thermal and quantum protons without recurring to perturbation theories. In general, we find that atomic and molecular fluctuations in PIMD are larger than in CEIMC which suggests that the potential energy surface from vdW-DF is less structured than the one from quantum Monte Carlo. We find qualitatively different behaviors for systems prepared in the C2c structure for increasing pressure. Within PIMD, the C2c structure is dynamically partially stable for P ≤ 250 GPa only: it retains the symmetry of the molecular centers but not the molecular orientation; at intermediate pressures, it develops layered structures like Pbcn or Ibam and transforms to the metallic Cmca-4 structure at P ≥ 450 GPa. Instead, within CEIMC, the C2c structure is found to be dynamically stable at least up to 450 GPa; at increasing pressure, the molecular bond length increases and the nuclear correlation decreases. For the other two structures, the two methods are in qualitative agreement although quantitative differences remain. We discuss various structural properties and the electrical conductivity. We find that these structures become conducting around 350 GPa but the metallic Drude-like behavior is reached only at around 500 GPa, consistent with recent experimental claims.

Thermal transport and quasi-normal modes in Gauss–Bonnet-axions theory

We obtain the black brane solution in arbitrary dimensional Gauss–Bonnet-axions (GBA) gravity theory. And then the thermal conductivity of the boundary theory dual to this neutral black brane is explored. We find that the momentum dissipation suppresses the DC thermal conductivity while it is enhanced by larger GB parameter. The analytical and numerical results of DC thermal conductivity match very well. Also we study the effect of the momentum dissipation and the GB coupling on the AC thermal conductivity and fit the results by Drude-like behavior for low frequency. Finally, we analytical compute the quasi-normal modes (QNM) frequency of the perturbative master field in large dimensions limit. Our analytical QNM frequencies agree well with the numerical results in large enough finite dimensions.

Comparative study of Mo2Ga2C with superconducting MAX phase Mo2GaC: First-principles calculations

The structural, electronic, optical and thermodynamic properties of Mo2Ga2C are investigated using density functional theory (DFT) within the generalized gradient approximation (GGA). The optimized crystal structure is obtained and the lattice parameters are compared with available experimental data. The electronic density of states (DOS) is calculated and analyzed. The metallic behavior for the compound is confirmed and the value of DOS at Fermi level is 4.2 states per unit cell per eV. Technologically important optical parameters (e.g., dielectric function, refractive index, absorption coefficient, photo conductivity, reflectivity, and loss function) are calculated for the first time. The study of dielectric constant (ɛ1) indicates the Drude-like behavior. The absorption and conductivity spectra suggest that the compound is metallic. The reflectance spectrum shows that this compound has the potential to be used as a solar reflector. The thermodynamic properties such as the temperature and pressure dependent bulk modulus, Debye temperature, specific heats, and thermal expansion coefficient of Mo2Ga2C MAX phase are derived from the quasi-harmonic Debye model with phononic effect also for the first time. Analysis of Tc expression using available parameter values (DOS, Debye temperature, atomic mass, etc.) suggests that the compound is less likely to be superconductor.

Study of surface-enhanced Raman scattering of InAs particles of subwavelength apertures at terahertz frequencies

We present a theoretical mechanism for electric field enhancement with SERS of InAs particles of subwavelength apertures under THz excitation. The distribution of electric field confirms that there is a strong enhancement in the InAs particles at THz frequencies. The InAs with a Drude-like behavior in THz range, which is similar to metals at optical frequencies, leads to different SERS when the parameters of these two particles change. The SERS enhancement factor can reach [Formula: see text] under the certain conditions.

Absorption enhancement of graphene Salisbury screen in the mid-infrared regime

As an emerging optoelectronic material, graphene has exhibited negligible absorption in the mid-infrared due to its Drude-like behavior of free electrons. For this reason, existing graphene-based Salisbury screens with enhanced graphene absorption have been limited to terahertz frequencies, and are experiencing difficulty to extend to higher frequency region. We propose to utilize graphene interband conductivity instead of the commonly used intraband conductivity to realize graphene-based Salisbury screens in the mid-infrared. Distinct mid-infrared absorption features in graphene-based Salisbury screens are investigated numerically and analytically by means of transfer matrix method. In contrast to terahertz absorption arising from intraband transition, the enhanced absorption of graphene in the mid-infrared is dominated by the interband transition. For a single layer of graphene on top of a metallic plane, peak absorptions of 10 % are obtained at normal incidence, and nearly perfect absorptions close to 1 are achieved at near grazing angle for incident s-polarization. To further enhance the perfect absorption over wider incident angles, a graphene-dielectric multilayer stack is proposed and analyzed. These results are relevant to graphene optoelectronics for mid-infrared applications.

Electrical and thermal conductivities in dense plasmas

Expressions for the electrical and thermal conductivities in dense plasmas are derived combining the Chester-Thellung-Kubo-Greenwood approach and the Kramers approximation. The infrared divergence is removed assuming a Drude-like behaviour. An analytical expression is obtained for the Lorenz number that interpolates between the cold solid-state and the hot plasma phases. An expression for the electrical resistivity is proposed using the Ziman-Evans formula, from which the thermal conductivity can be deduced using the analytical expression for the Lorenz number. The present method can be used to estimate electrical and thermal conductivities of mixtures. Comparisons with experiment and quantum molecular dynamics simulations are done.

Bandgap closure and reopening inCsAuI3at high pressure

Results of high-pressure infrared (IR) and Raman spectroscopy measurements are presented for the mixed valence compound ${\mathrm{CsAuI}}_{3}$, where Au adopts ${\mathrm{Au}}^{\mathrm{I}}$ and ${\mathrm{Au}}^{\mathrm{III}}$ valency. Raman spectroscopy shows softening with pressure of the vibration modes in the ${\mathrm{Au}}^{\mathrm{III}}$-${\mathrm{I}}_{4}$ square planar units in the tetragonal phase, indicating a similar pressure-induced lattice distortion as found for the closely related compounds ${\mathrm{CsAuCl}}_{3}$ and ${\mathrm{CsAuBr}}_{3}$. Multiple features in the higher pressure spectra confirm that the high-pressure phase has a lower symmetry than the ambient pressure tetragonal structure, consistent with an orthorhombic structure discovered recently by x-ray diffraction measurements. From IR spectroscopy, we observed rapid bandgap closure at a rate of 0.2 eV/GPa in the tetragonal phase of ${\mathrm{CsAuI}}_{3}$, close to the tetragonal-orthorhombic phase transition. The IR reflectivity shows a Drude-like behavior implying metallic conductivity. However, as the compound fully transforms to the orthorhombic phase, the bandgap reopens and the Drude behavior in the reflectivity disappears.

A Drude model analysis of conductivity and free carriers in boron-doped diamond films and investigations of their internal stress and strain

Boron-doped diamond (BDD) has seen a substantial increase in interest for use as electrode coating material for electrochemistry and studies of deep brain stimulation mechanism. In this study, we present an alternative method for determining important characteristics, including conductivity, carrier concentration, and time constant, of such material by the signature of Drude-like metallic behavior in the far-infrared (IR) spectral range. Unlike the direct determination of conductivity from the four-point probe method, using far-IR transmittance provides additional information, such as whether the incorporation of boron results in a large concentration of carriers or in inducing defects in the diamond lattice. The slightly doped to medium-doped BDD samples that were produced using chemical vapor deposition and analyzed in this work show conductivities ranging between 5.5 and 11 (Ω cm)-1. Different growth conditions demonstrate that increasing boron concentration results in an increase in the carrier concentration, with values between 7.2 × 1016 and 2.5 × 1017 carriers/cm3. Addition of boron, besides leading to a decrease in the resistivity, also resulted in a decrease in the time constant, limiting BDD conductivity. Investigations, by confocal Raman mapping, of the induced stress in the material due to interaction with the substrate or to the amount of doping are also presented and discussed. The induced tensile stress, which was distributed closer to the film-substrate interface decreased slightly with doping.

Tight-Binding Calculations of the Optical Response of Optimally P-Doped Si Nanocrystals: A Model for Localized Surface Plasmon Resonance

We present tight-binding calculations in the random-phase approximation of the optical response of Silicon nanocrystals (Si NCs) ideally doped with large concentrations of phosphorus (P) atoms. A collective response of P-induced electrons is demonstrated, leading to localized surface plasmon resonance (LSPR) when a Si NC contains more than ≈10 P atoms. The LSPR energy varies not only with doping concentration but also with NC size due to size-dependent screening by valence electrons. The simple Drude-like behavior is recovered for NC size above 4 nm. Si NCs containing a large number of deep defects in place of hydrogenic impurities do not give rise to LSPR.

Ultralow-Dissipative Conductivity by Dirac Fermions in BaFe2As2

We report on the anomalous behavior of the complex conductivity of BaFe$_{2}$As$_{2}$, which is related to the Dirac cone, in the terahertz (THz)-frequency region. Above the spin-density-wave (SDW) transition temperature, the conductivity spectra follow the Drude model. In the SDW state, the imaginary part of the complex conductivity, $\sigma_2$, is suppressed in comparison to that expected according to the Drude model. The real part, $\sigma_1$, exhibits nearly Drude-like behavior. This behavior (i.e., almost no changes in $\sigma_1$ and the depression of $\sigma_2$) can be regarded as the addition of extra conductivity without any dissipations in the Drude-type conductivity. The origin of this ultralow-dissipative conductivity is found to be due to conductivity contribution from quasiparticles within the Dirac cone. In other words, we are able to observe the dynamics of Dirac fermions through the conductivity spectra of BaFe$_2$As$_2$, clearly and directly.

Charge density wave excitations in stripe-type charge ordered Pr0.5Sr0.5MnO3 manganite

We show that a charge-density wave mode submerged in large conductivity of the stripe-type charge-ordered (CO) crystal of Pr0.5Sr0.5MnO3 (PSMO) can be extracted by exciting selective crystallographic planes using polarized terahertz field. Low energy (1–7 meV) dynamics have been explored in the (100), (110), and (111) epitaxial films of PSMO manganite. Of (110) and (111) films representing bulk-like CO, the former exhibits a peak in the conductivity-energy spectrum with attributes of charge-density wave mode along a specific in-plane axis. On the contrary, the CO (111) and the phase-separated (100) films lack this low energy mode but exhibit a Drude-like spectral behavior in the metallic regime. These studies reveal a variety in isothermal charge dynamics originating from the anisotropic nature of the stripe charge-order.

Switching terahertz waves with gate-controlled active graphene metamaterials

The extraordinary electronic properties of graphene provided the main thrusts for the rapid advance of graphene electronics. In photonics, the gate-controllable electronic properties of graphene provide a route to efficiently manipulate the interaction of photons with graphene, which has recently sparked keen interest in graphene plasmonics. However, the electro-optic tuning capability of unpatterned graphene alone is still not strong enough for practical optoelectronic applications owing to its non-resonant Drude-like behaviour. Here, we demonstrate that substantial gate-induced persistent switching and linear modulation of terahertz waves can be achieved in a two-dimensional metamaterial, into which an atomically thin, gated two-dimensional graphene layer is integrated. The gate-controllable light-matter interaction in the graphene layer can be greatly enhanced by the strong resonances of the metamaterial. Although the thickness of the embedded single-layer graphene is more than six orders of magnitude smaller than the wavelength (<λ/1,000,000), the one-atom-thick layer, in conjunction with the metamaterial, can modulate both the amplitude of the transmitted wave by up to 47% and its phase by 32.2° at room temperature. More interestingly, the gate-controlled active graphene metamaterials show hysteretic behaviour in the transmission of terahertz waves, which is indicative of persistent photonic memory effects.