Anisotropic Optical Conductivity Research Articles

Investigations of the electronic and optical properties of V2InC and V2InN based MAX phases in the framework of density functional theory

This work reports a new class of MAX phases based on V2InC and V2InN for advanced technological applications. We use the “density functional theory (DFT)-based full-potential linearized augmented-plane-wave (FP-LAPW) approach” to investigate the lattice constants, electronic, and optical properties of V2InC and V2InN. The lattice constants a (c) of the studied MAX phases have been recorded as 3.01 Å (12.60 Å) for V2InC and 3.03 Å (12.40 Å) for V2InN, which have been found well-matching to the available literature. Like other MAX phases, the studied compounds are metallic, as the valence band's energy states overlap with those in the conduction band over the Fermi level. The major contribution to the electronic structure is seen from the V-d, In-s, p, and X-p states. Similarly, the studied MAX phases show prominent light absorption, enhanced by order 10 in the visible range and 102 in the ultraviolet (UV) range of the electromagnetic spectrum. Additionally, the V2InC and V2InN demonstrated anisotropic optical conductivity, which shows interesting variations with the energy of incident light. The optical conductivity exhibits threshold behavior in the infrared, significant improvement in the visible range, and the highest conductivity in the ultraviolet range. The information provided in our study will likely be useful for future research on V2InC and V2InN MAX phases.

Anisotropic optical conductivity induced by magnetic atoms embedded in honeycomb lattices

We investigate the optical properties of a hexagonal lattice with spin-spin interaction. Transition metal atoms with 3d-orbitals, for example Cr, Fe, Co, Ni, can be embedded in the lattice, exhibiting a strong magnetic moment. Spin polarization controls the magnetic phases, which can be ferromagnetic (FM), ferrimagnetic (FR), or antiferromagnetic (AFM). The behavior of electrons can resemble both massless and massive particles when varying the angle between spin-spin interactions. This leads to the investigation of the electronic properties of alternative materials, such as MXenes, for optoelectronic devices with stable transmission across a wide range of light frequencies from near-infrared to the ultraviolet regime. Furthermore, anisotropic optical conductivity and transmittance could be used to identify the spin orientation and atomic arrangement in magnetic materials.

Strain-tuned optical conductivity of monolayer PbBiI

In this paper, we investigate the optical response of the PbBiI single-layer by developing a strain-induced Kane–Mele model from Peierls substitution and by employing the Kubo formula at low temperatures. We address three different regimes of uniform and non-uniform classes created by tuning the strength of the strain. From a detailed analysis of the electronic band structure, we find that the Rashba spin splitting gap is destroyed with strain, while the bulk gap slightly changes. We also find that interband optical transitions exhibit a blueshift spectrum with strain. Interestingly, all these findings are independent of the regime and class of strain. However, our simulations show that only the non-uniform class of strain leads to anisotropic optical conductivity. These results enhance optoelectronic applications of low-dimensional materials.

Effect of Uniform Strain on Graphene Surface Plasmon Excitations

This paper reports a study of the reflectance of the optical sensors based on graphene under uniform strain. Assuming the graphene layer is surrounded by two different semi-infinite dielectric media, the generalized Fresnel coefficients are derived as a function of usual quantities (e.g., dielectric constants, incident angles, and strain) and anisotropic optical conductivity. The strain not only changes the electronic band structure but also can be employed to tune the electronic collective excitations (plasmons) and thus the optical reflectance of graphene monolayers. One of the most common techniques for plasmon excitation is the Kretschmann configuration. It is based on the observation of a sharp minimum in the reflection coefficient versus the angle (or wavelength) curve. Because strain induces anisotropy in graphene optical conductivity the strain-dependent orientation plays an important role to manipulate the variation of graphene plasmon energy, which may be useful to synchronize graphene properties in plasmonic devices to enhance light-matter interactions.

Hyperbolic plasmons in massive tilted two-dimensional Dirac materials

We explore topological transitions in the type of propagation of surface electromagnetic modes in massive anisotropic tilted Dirac systems. The presence of tilting and mass gives rise to an indirect band gap that strongly modifies the joint density of states compared to the gapless system. New van Hove singularities appear, and the interplay between intra and interband transitions leads to an anisotropic optical conductivity with imaginary parts acquiring opposite signs in orthogonal directions, opening the possibility of having hyperbolic propagation of plasmons. Isofrequency contours and low plasmon losses, as obtained from the dispersion relation, show that transitions between purely anisotropic quasi-elliptical and well-defined, highly directional, hyperbolic modes are attainable only when tilt and mass coexist. Fine-tuning in the THz regime suggests that this might be useful as a dynamical probe of concurring tilt and mass in Dirac materials.

Anisotropic and tunable optical conductivity of a two-dimensional semi-Dirac system in the presence of elliptically polarized radiation

We investigate the effect of ellipticity ratio of the polarized radiation field on optoelectronic properties of a two-dimensional (2D) semi-Dirac (SD) system. The optical conductivity is calculated within the energy balance equation approach derived from the semiclassical Boltzmann equation. We find that there exists the anisotropic optical absorption induced via both the intra- and interband electronic transition channels in the perpendicular $xx$ and $yy$ directions. Furthermore, we examine the effects of the ellipticity ratio, the temperature, the carrier density, and the band-gap parameter on the optical conductivity of the 2D SD system placed in transverse and vertical directions, respectively. It is shown that the ellipticity ratio, temperature, carrier density, and band-gap parameter can play the important roles in tuning the strength, peak position, and shape of the optical conductivity spectrum. The results obtained from this study indicate that the 2D SD system can be a promising anisotropic and tunable optical and optoelectronic material for applications in innovative 2D optical and optoelectronic devices, which are active in the infrared and terahertz bandwidths.

Prediction of hyperbolic exciton-polaritons in monolayer black phosphorus

Hyperbolic polaritons exhibit large photonic density of states and can be collimated in certain propagation directions. The majority of hyperbolic polaritons are sustained in man-made metamaterials. However, natural-occurring hyperbolic materials also exist. Particularly, natural in-plane hyperbolic polaritons in layered materials have been demonstrated in MoO3 and WTe2, which are based on phonon and plasmon resonances respectively. Here, by determining the anisotropic optical conductivity (dielectric function) through optical spectroscopy, we predict that monolayer black phosphorus naturally hosts hyperbolic exciton-polaritons due to the pronounced in-plane anisotropy and strong exciton resonances. We simultaneously observe a strong and sharp ground state exciton peak and weaker excited states in high quality monolayer samples in the reflection spectrum, which enables us to determine the exciton binding energy of ~452 meV. Our work provides another appealing platform for the in-plane natural hyperbolic polaritons, which is based on excitons rather than phonons or plasmons.

Pressure-Induced Excitations in the Out-of-Plane Optical Response of the Nodal-Line Semimetal ZrSiS.

The anisotropic optical response of the layered, nodal-line semimetal ZrSiS at ambient and high pressure is investigated by frequency-dependent reflectivity measurements for the polarization along and perpendicular to the layers. The highly anisotropic optical conductivity is in very good agreement with results from density-functional theory calculations and confirms the anisotropic character of ZrSiS. Whereas the in-plane optical conductivity shows only modest pressure-induced changes, we found strong effects on the out-of-plane optical conductivity spectrum of ZrSiS, with the appearance of two prominent excitations. These pronounced pressure-induced effects can neither be attributed to a structural phase transition according to our single-crystal x-ray diffraction measurements, nor can they be explained by electronic correlation and electron-hole pairing effects, as revealed by theoretical calculations. Our findings are discussed in the context of the recently proposed excitonic insulator phase in ZrSiS.

Mechanically Tunable Near-Field Radiative Heat Transfer between Monolayer Black Phosphorus Sheets.

As a cutting-edge two-dimensional (2D) material, black phosphorous (BP) has been demonstrating a promising performance in the field of near-field radiative heat transfer (NFRHT) due to the excitation ability of its surface plasmon polaritons. Here, we have systematically demonstrated the effect of mechanical strain on the NFRHT between two separate BP sheets. First-principles calculations predict that a certain amount of mechanical strain (4% along biaxial or 6% along zigzag (ZZ)) can trigger an orthogonal switch of anisotropic optical conductivity by regulating the effective mass of electrons. The mismatched coupling of surface plasmon polaritons between the pristine and strained BP results in a 73% change in NFRHT. The studied mechanically tunable NFRHT unfolds a new degree of freedom for controlling the near-field heat transfer and sheds light on an invaluable approach to designing two-dimensional material-based thermal and electrical applications.

Unveiling optical in-plane anisotropy of 2D materials from oblique incidence of light

In this work, we present a theoretical study of the dispersion of linearly polarized light between two dielectric media separated by an anisotropic two-dimensional (2D) material under oblique incidence. Assuming that the 2D material is a conducting sheet of negligible thickness, generalized Fresnel coefficients are derived as a function of usual quantities (e.g. refraction indexes and scattering angles) and the anisotropic in-plane optical conductivity of the interstitial 2D material. In particular, we analyzed the modifications due to the 2D material of two classical optical phenomena: the Brewster effect and the total internal reflection. As a application, our general findings are particularized for uniaxially strained graphene. Effects of a uniaxial strain on the Brewster angle and the reflectance (under total internal reflection) are evaluated as a function of the magnitude and direction of strain.

Blue shift in the interband optical transitions of gated monolayer black phosphorus

The anisotropic frequency-dependent optical conductivity of single-layer puckered phosphorene is studied, and also its dependence on the electrostatic gating of the top and bottom sublayers. The effective Hamiltonian within the k→⋅p→ approximation is used to obtain the electronic band structure from which the interband optical transitions are evaluated using the Kubo formula. Inherent asymmetry in the dispersion energy of carriers along the armchair and zigzag edges gives rise to anisotropic optical features. The bandgap increases with gate voltage resulting in absorption at higher optical energies along the armchair edge, which is the so-called blue shift, while along the zigzag edge such a shift is not significant. The temperature and exciton effects on the interband response are concisely discussed. These results provide new insights which render phosphorene attractive for optoelectronic devices.

Combined temperature- and magnetic field-induced optical responses of phosphorene

The paper presents a theoretical investigation of the temperature and an out-of-plane Zeeman magnetic field effects on the interband optical conductivity of phosphorene. The electronic states are calculated with the tight-binding Hamiltonian and to compute the optical conductivity, the Kubo formalism is employed. The main result is that the multiple bands due to the perpendicular Zeeman splitting of the energy levels lead to the multiple resonance structures of the optical conductivity. It is found that the strong anisotropic interband optical conductivity of pristine phosphorene keeps the anisotropic feature when the temperature is increased. Importantly, our results show that the optical properties can be externally tuned via a magnetic field. In particular, when the magnetic field is applied, the unique anisotropic property breaks down and there is a little to no sign for temperature- and orientation-dependent transitions. The scientific soundness of findings is useful for practical aspects of phosphorene in spintronic.

Strong anisotropic optical conductivity in two-dimensional puckered structures: The role of the Rashba effect

We calculate the optical conductivity of an anisotropic two-dimensional system with Rashba spin-flip excitation within the Kubo formalism. We show that the anisotropic Rashba effect caused by an external field changes significantly the magnitude of the spin splitting. Furthermore, we obtain an analytical expression for the longitudinal optical conductivity associated with inter-band transitions as a function of the frequency for an arbitrary polarization angle. We find that the diagonal components of the optical conductivity tensor are direction-dependent and the spectrum of optical absorption is strongly anisotropic with an absorption window. The height and width of this absorption window are very sensitive to the system anisotropy. While the height of absorption peak increases with increasing effective mass anisotropy ratio, the peak intensity is larger when the light polarization is along the armchair direction. Moreover, the absorption peak width becomes broader as the density of state mass or Rashba interaction is enhanced. These features can be used to determine parameters relevant for spintronics through the optical absorption spectrum.

Influence of out-of-plane response on optical properties of two-dimensional materials: First principles approach

The ab initio calculation of optical spectra of sheet crystals usually arranges them in a three-dimensional superlattice with a sufficiently large interlayer distance. We show how the resulting frequency-dependent dielectric tensor is related to the anisotropic optical conductivity of an individual sheet or to the dielectric tensor of a corresponding film with thickness $d$. Their out-of-plane component is taken into account, in contrast to usual treatments. We demonstrate that the generalized transfer-matrix method to model the optical properties of a layer system containing a sheet crystal accounts for all tensor components. As long as $d\ensuremath{\ll}\ensuremath{\lambda}$ ($\ensuremath{\lambda}$-wavelength of light) this generalized formulation of the optical properties for anisotropic two-dimensional (2D) systems of arbitrary thickness reproduces the limits found in literature that are based either on electromagnetic boundary conditions for a conducting surface or on an isotropic dielectric tensor. For $s$-polarized light, the results are independent of the sheet description. For oblique incidence of $p$-polarized light, the tensor nature of the optical conductivity (or the dielectric function) of the sheet crystal strongly impacts on reflectance, transmittance, and absorbance due to the out-of-plane optical conductivity. The limit $d\ensuremath{\rightarrow}0$ should be taken in the final expressions. Example spectra are given for the group-IV honeycomb 2D crystals graphene and silicene.

Black phosphorus plasmonics: anisotropic elliptical propagation and nonlocality-induced canalization

We investigate unusual surface plasmons polariton (SPP) propagation and light–matter interactions in ultrathin black phosphorus (BP) films, a 2D material that exhibits exotic electrical and physical properties due to its extremely anisotropic crystal structure. Recently, it has been speculated that the ultra-confined surface plasmons supported by BP may present various topologies of wave propagation bands, ranging from anisotropic elliptic to hyperbolic, across the mid- and near-infrared regions of the electromagnetic spectrum. By carefully analyzing the natural nonlocal anisotropic optical conductivity of BP, derived using the Kubo formalism and an effective low-energy Hamiltonian, we demonstrate here that the SPP wavenumber cutoff imposed by nonlocality prohibits that they acquire an arbitrary hyperbolic topology, forcing operation in the canalization regime. The resulting nonlocality-induced canalization presents interesting properties, as it is inherently broadband, enables large light–matter interactions in the very near field, and allows extreme device miniaturization. We also determine fundamental bounds to the confinement of BP plasmons, which are significantly weaker than for graphene, thus allowing a larger local density of states. Our results confirm the potential of BP as a promising reconfigurable plasmonic platform, with exciting applications, such as planar hyperlenses, optoelectronic components, imaging, and communication systems.

Anisotropic optical conductivity and electron–hole asymmetry in doped monolayer graphene in the presence of the Rashba coupling

In this study, the optical conductivity of substitutionary doped graphene is investigated in the presence of the Rashba spin orbit coupling (RSOC). Calculations have been performed within the coherent potential approximation (CPA) beyond the Dirac cone approximation. Results of the current study demonstrate that the optical conductivity is increased by increasing the RSOC strength. Meanwhile it was observed that the anisotropy of the band energy results in a considerable anisotropic optical conductivity (AOC) in monolayer graphene. The sign and magnitude of this anisotropic conductivity was shown to be controlled by the external field frequency. It was also shown that the Rashba interaction results in electron–hole asymmetry in monolayer graphene.

Rediscovering black phosphorus as an anisotropic layered material for optoelectronics and electronics.

Graphene and transition metal dichalcogenides (TMDCs) are the two major types of layered materials under intensive investigation. However, the zero-bandgap nature of graphene and the relatively low mobility in TMDCs limit their applications. Here we reintroduce black phosphorus (BP), the most stable allotrope of phosphorus with strong intrinsic in-plane anisotropy, to the layered-material family. For 15-nm-thick BP, we measure a Hall mobility of 1,000 and 600 cm(2)V(-1)s(-1) for holes along the light (x) and heavy (y) effective mass directions at 120 K. BP thin films also exhibit large and anisotropic in-plane optical conductivity from 2 to 5 μm. Field-effect transistors using 5 nm BP along x direction exhibit an on-off current ratio exceeding 10(5), a field-effect mobility of 205 cm(2)V(-1)s(-1), and good current saturation characteristics all at room temperature. BP shows great potential for thin-film electronics, infrared optoelectronics and novel devices in which anisotropic properties are desirable.

Anisotropic in-plane optical conductivity in detwinned Ba(Fe1−xCox)2As2

We study the anisotropic in-plane optical conductivity of detwinned Ba(Fe1−xCox)2As2 single crystals for x = 0, 2.5 and 4.5% in a broad energy range (3 meV–5 eV) across their structural and magnetic transitions. For temperatures below the Neel transition, the topology of the reconstructed Fermi surface, combined with the distinct behavior of the scattering rates, determines the anisotropy of the low-frequency optical response. For the itinerant charge carriers, we are able to disentangle the evolution of the Drude weights and scattering rates and to observe their enhancement along the orthorhombic antiferromagnetic a-axis with respect to the ferromagnetic b-axis. For temperatures above the structural phase transition, uniaxial stress induces a finite in-plane anisotropy. The anisotropy of the optical conductivity, leading to significant dichroism, extends to high frequencies in the mid- and near-infrared regions. The temperature dependence of the dichroism at all dopings scales with the anisotropy ratio of dc conductivity, suggesting the electronic nature of the structural transition. Our findings bear testimony to a large nematic susceptibility that couples very effectively to the uniaxial lattice strain. In order to clarify the subtle interplay of magnetism and Fermi surface topology we compare our results with theoretical calculations obtained from density functional theory within the full-potential linear augmented plane-wave method.

Observing the anisotropic optical response of the heavy‐fermion compound UNi2Al3

AbstractThe optical conductivity of heavy fermions can reveal fundamental properties of the charge carrier dynamics in these strongly correlated electron systems. Here we extend the conventional techniques of infrared optics on heavy fermions by measuring the transmission and phase shift of THz radiation that passes through a thin film of UNi2Al3, a material with hexagonal crystal structure. We deduce the optical conductivity in a previously not accessible frequency range, and furthermore we resolve the anisotropy of the optical response (parallel and perpendicular to the hexagonal planes). At frequencies around 7 cm−1, we find a strongly temperature‐dependent and anisotropic optical conductivity that – surprisingly – roughly follows the dc behavior.

Anisotropic optical conductivity and two colors ofMgB2

We present the anisotropic optical conductivity of MgB$_{2}$ between 0.1 and 3.7 eV at room temperature obtained on single crystals of different purity by the spectroscopic ellipsometry and reflectance measurements. The bare (unscreened) plasma frequency $\omega_{p}$ is almost isotropic and equal to 6.3 eV, which contrasts some earlier reports of a very small value of $\omega_{p}$. The data suggests that the $\sigma$-bands are characterized by a stronger electron-phonon coupling $\lambda_ {tr}$ but smaller impurity scattering $\gamma_{imp}$, compared to the $\pi$-bands. The optical response along the boron planes is marked by an intense interband transition at 2.6 eV, due to which the reflectivity plasma edges along the a- and c-axes are shifted with respect to each other. As a result, the sample spectacularly changes color from a blueish-silver to the yellow as the polarization is rotated from the in-plane direction towards the c-axis. The optical spectra are in good agreement with the published {\it ab initio} calculations. The remaining discrepancies can be explained by the relative shift of $\sigma$-bands and $\pi$-bands by about 0.2 eV compared to the theoretical band structure, in agreement with the de Haas-van Alphen experiments. The widths of the Drude and the interband peaks are both very sensitive to the sample purity.