Optical properties and plasmons in moiré structures.

The discoveries of numerous exciting phenomena in twisted bilayer graphene (TBG) are stimulating significant investigations on moiré structures that possess a tunable moiré potential. Optical response can provide insights into the electronic structures and transport phenomena of non-twisted and twisted moiré structures. In this article, we review both experimental and theoretical studies of optical properties such as optical conductivity, dielectric function, non-linear optical response, and plasmons in moiré structures composed of graphene, hexagonal boron nitride (hBN), and/or transition metal dichalcogenides. Firstly, a comprehensive introduction to the widely employed methodology on optical properties is presented. After, moiré potential induced optical conductivity and plasmons in non-twisted structures are reviewed, such as single layer graphene-hBN, bilayer graphene-hBN and graphene-metal moiré heterostructures. Next, recent investigations of twist-angle dependent optical response and plasmons are addressed in twisted moiré structures. Additionally, we discuss how optical properties and plasmons could contribute to the understanding of the many-body effects and superconductivity observed in moiré structures.

Tunable electronic properties and optoelectronic characteristics of MoGe2N4/SiC van der Waals heterostructure.

The assembly of van der Waals (vdW) heterostructure with easily regulated electronic properties provides a new way for the expansion of two-dimensional materials and promotes the development of optoelectronics, sensors, switching devices and other fields. In this work, a systematic investigation of the electronic properties of MoGe2N4/SiC heterostructures using density functional theory has been conducted, along with the modulation of electronic properties by vertical strain and the potential application prospects in optoelectronic devices. The results show that MoGe2N4/SiC heterostructure has excellent dynamic and thermal stability and belongs to type-II band alignment semiconductors. This is extremely beneficial for the separation of photo-generating electron-hole pairs, so it has important significance for the development of photovoltaic materials. In addition, under the control of vertical strain, the semiconductor-metal transition occurs in the MoGe2N4/SiC heterostructure when the compressive strain reaches 6%. In the case of compressive strain less than 6% and tensile strain, the MoGe2N4/SiC heterostructure maintains the type-II band alignment semiconductor characteristics. Meanwhile, we find that the MoGe2N4/SiC heterostructure has optical absorption coefficients of up to 105in the visible and ultraviolet light ranges, which can improve the absorption coefficients of the MoGe2N4and SiC monolayer in some visible light regions. Finally, the optical conductivity of the MoGe2N4/SiC heterostructure exhibits significant anisotropy, with the armchair direction displaying higher conductivity within the orange light range. In conclusion, the formation of vdW heterostructure by vertically stacking MoGe2N4and SiC monolayers can effectively improve their electronic and optical properties, which provides a valuable reference for the future development of electronic devices and photovoltaic materials.

Mid-Infrared Mapping of Four-Layer Graphene Polytypes Using Near-Field Microscopy.

The mid-infrared (MIR) spectral region attracts attention for accurate chemical analysis using photonic devices. Few-layer graphene (FLG) polytypes are promising platforms, due to their broad absorption in this range and gate-tunable optical properties. Among these polytypes, the noncentrosymmetric ABCB/ACAB structure is particularly interesting, due to its intrinsic bandgap (8.8 meV) and internal polarization. In this study, we utilize scattering-scanning near-field microscopy to measure the optical response of all three tetralayer graphene polytypes in the 8.5-11.5 µm range. We employ a finite dipole model to compare these results to the calculated optical conductivity for each polytype obtained from a tight-binding model. Our findings reveal a significant discrepancy in the MIR optical conductivity response of graphene between the different polytypes than what the tight-binding model suggests. This observation implies an increased potential for utilizing the distinct tetralayer polytypes in photonic devices operating within the MIR range for chemical sensing and infrared imaging.

Investigation of structural, electronic and optical properties of two-dimensional MoS2-doped-V2O5 composites for photocatalytic application: a density functional theory study.

In the present research, the structural, electronic and optical properties of transition metal dichalcogenide-doped transition metal oxides MoS2-doped-V2O5 with various doping concentrations (x = 1-3%) of MoS2 atoms are studied by using first principles calculation. The generalized gradient approximation Perdew-Burke-Ernzerhof simulation approach is used to investigate the energy bandgap (Eg) of orthorhombic structures. We examined the energy bandgap (Eg) decrement from 2.76 to 1.30 eV with various doping (x = 1-3%) of molybdenum disulfide (MoS2) atoms. The bandgap nature shows that the material is a well-known direct bandgap semiconductor. MoS2 doping (x = 1-3%) atoms in pentoxide (V2O5) creates the extra gamma active states which contribute to the formation of conduction and valance bands. MoS2-doped-V2O5 composite is a proficient photocatalyst, has a large surface area for absorption of light, decreases the electron-hole pairs recombination rate and increases the charge transport. A comprehensive study of optical conductivity reveals that strong peaks of MoS2-doped-V2O5 increase in ultraviolet spectrum region with small shifts at larger energy bands through increment doping x = 1-3% atoms of MoS2. A significant decrement was found in the reflectivity due to the decrement in the bandgap with doping. The optical properties significantly increased by the decrement of bandgap (Eg). Two-dimensional MoS2-doped-V2O5 composite has high energy absorption, optical conductivity and refractive index, and is an appropriate material for photocatalytic applications.

The Evolution of MXenes Conductivity and Optical Properties Upon Heating in Air.

MXenes, a family of 2D transition-metal carbides and nitrides, have excellent electrical conductivity and unique optical properties. However, MXenes oxidize in ambient conditions, which is accelerated upon heating. Intercalation of water also causes hydrolysis accelerating oxidation. Developing new tools to readily characterize MXenes' thermal stability can enable deeper insights into their structure-property relationships. Here, in situ spectroscopic ellipsometry (SE) is employed to characterize the optical properties of three types of MXenes (Ti3 C2 Tx , Mo2 TiC2 Tx , and Ti2 CTx ) with varied composition and atomistic structures to investigate their thermal degradation upon heating under ambient environment. It is demonstrated that changes in MXene extinction and optical conductivity in the visible and near-IR regions correlate well with the amount of intercalated water and hydroxyl termination groups and the degree of oxidation, measured using thermogravimetric analysis. Among the three MXenes, Ti3 C2 Tx and Ti2 CTx , respectively, have the highest and lowest thermal stability, indicating the role of transition-metal type, synthesis route, and the number of atomic layers in MXene flakes. These findings demonstrate the utility of SE as a powerful in situ technique for rapid structure-property relationship studies paving the way for the further design, fabrication, and property optimization of novel MXene materials.

Theoretical studies on optical properties of Beltrami-shaped curved graphene.

We compute the optical conductivity and polarization for an out-of-plane deformation in graphene nanostructure using a theoretical approach based on Dirac equation solutions on curved2+1dimensional space-time, where the space part is considered to correspond to the Beltrami pseudosphere which belongs to the family of surfaces with negative constant Gaussian curvature. We found that different parameters of deformation along one direction translate into an enhancement of the optical conductivity peaks and polarization magnitude in the far-infrared frequencies. This allows for a very high degree of polarization with a single layer graphene and opens up a potential prospect of employing graphene layer as efficient polarizers. Therefore the experimental predictions related to the electronic configuration of the corresponding graphene-like sample may be explicitly worked out.

Abrupt change reflected by the van Hove singularity in the optical spectra of CaMn2P2.

The trigonal CaAl2Si2-type structure CaMn2P2has been reported undergoing an exotic first order phase transition at the critical temperatureTN=69.5K. In this paper, we present the optical spectra for theab-plane of CaMn2P2single crystal from 300 K to 10 K for the first time. In the real part of the optical conductivity spectraσ1(ω), a direct gap could be determined at all temperatures without any Drude term visible, i.e., the sample goes through the first order phase transition from one insulator state to the other insulator state. At higher energy, an asymmetric sharp interband transition peak appears in allσ1(ω) spectra, which indicates a divergence of the joint density of states. This sharp peak could be well described by the two dimensional van Hove singularity function. In particular, this peak is very sensitive to the first order phase transition, especially the peak positionEtwhose the most prominent blue shift occurs only when the first order transition happens. Our data and analysis reveal that the first order phase transition leads to a weak partial re-normalization of the band structure. Our study will be useful in further investigations about the mechanism of the first order phase transition in the insulator.

Electronic, Optical, Thermoelectric and Elastic Properties of RbxCs1-xPbBr3 Perovskite.

Inorganic halide perovskites of the type AMX3, where A is an inorganic cation, M is a metal cation, and X is a halide anion, have attracted attention for optoelectronics applications due to their better optical and electronic properties, and stability, under a moist and elevated temperature environment. In this contribution, the electronic, optical, thermoelectric, and elastic properties of cesium lead bromide, CsPbBr3, and Rb-doped CsPbBr3, were evaluated using the density functional theory (DFT). The generalized gradient approximation (GGA) in the scheme of Perdew, Burke, and Ernzerhof (PBE) was employed for the exchange-correlation potential. The calculated value of the lattice parameter is in agreement with the available experimental and theoretical results. According to the electronic property results, as the doping content increases, so does the energy bandgap, which decreases after doping 0.75. These compounds undergo a direct band gap and present an energies gap values of about 1.70 eV (x = 0), 3.76 eV (x = 0.75), and 1.71 eV (x = 1). The optical properties, such as the real and imaginary parts of the dielectric function, the absorption coefficient, optical conductivity, refractive index, and extinction coefficient, were studied. The thermoelectric results show that after raising the temperature to 800 K, the thermal and electrical conductivities of the compound RbxCs1-xPbBr3 increases (x = 0, 0.25, 0.50 and 1). Rb0.75Cs0.25PbBr3 (x = 0.75), which has a large band gap, can work well for applications in the ultraviolet region of the spectrum, such as UV detectors, are potential candidates for solar cells; whereas, CsPbBr3 (x = 0) and RbPbBr3 (x = 1), have a narrow and direct band gap and outstanding absorption power in the visible ultraviolet energy range.

Optical Conductivity as a Probe of the Interaction-Driven Metal in Rhombohedral Trilayer Graphene.

Study of the strongly correlated states in van der Waals heterostructures is one of the central topics in modern condensed matter physics. Among these, the rhombohedral trilayer graphene (RTG) occupies a prominent place since it hosts a variety of interaction-driven phases, with the metallic ones yielding exotic superconducting orders upon doping. Motivated by these experimental findings, we show within the framework of the low-energy Dirac theory that the optical conductivity can distinguish different candidates for a paramagnetic metallic ground state in this system. In particular, this observable shows a single peak in the fully gapped valence-bond state. On the other hand, the bond-current state features two pronounced peaks in the optical conductivity as the probing frequency increases. Finally, the rotational symmetry breaking charge-density wave exhibits a minimal conductivity with the value independent of the amplitude of the order parameter, which corresponds precisely to the splitting of the two cubic nodal points at the two valleys into two triplets of the band touching points featuring linearly dispersing quasiparticles. These features represent the smoking gun signatures of different candidate order parameters for the paramagnetic metallic ground state, which should motivate further experimental studies of the RTG.

Fluorite-related iridate Pr3IrO7: crystal growth, structure, magnetism, thermodynamic, and optical properties.

Spin-orbit coupling in heavy 5dmetal oxides, in particular, iridates have received tremendous interest in recent years due to the realization of exotic electronic and magnetic phases. Here, we report the synthesis, structural, magnetic, thermodynamic, and optical properties of the ternary iridate Pr3IrO7. Single crystals of Pr3IrO7have been grown by the KF flux method. Structural analysis shows that Pr3IrO7crystallizes in an orthorhombic phase withCmcmsymmetry. The electron energy loss spectroscopy study indicates that Pr is in a 3+ valence state, which implies a 5+ oxidation state of Ir. Magnetization data measured at high and low magnetic fields do not exhibit any bifurcation betweenMZFCandMFC, however, a weak hump inM(T) is observed atT∗∼10.4 K. The specific heat data reveal two maxima at ∼253 and ∼4.8 K. The optical conductivityσ1(ω)spectrum shows 24 infrared-active phonon modes and reveals an insulating behavior with an optical gapΔOPof size ∼500 meV. During cooling down, the temperature-dependent reflectivity spectrum reveals eight extra phonon modes below the structural phase transition (∼253 K). An anomaly is observed at aroundT∗in the temperature evolution of infrared-active mode frequencies suggesting the presence of significant spin-phonon coupling in the system.

Experimental Observation of ABCB Stacked Tetralayer Graphene.

In tetralayer graphene, three inequivalent layer stackings should exist; however, only rhombohedral (ABCA) and Bernal (ABAB) stacking have so far been observed. The three stacking sequences differ in their electronic structure, with the elusive third stacking (ABCB) being unique as it is predicted to exhibit an intrinsic bandgap as well as locally flat bands around the K points. Here, we use scattering-type scanning near-field optical microscopy and confocal Raman microscopy to identify and characterize domains of ABCB stacked tetralayer graphene. We differentiate between the three stacking sequences by addressing characteristic interband contributions in the optical conductivity between 0.28 and 0.56 eV with amplitude and phase-resolved near-field nanospectroscopy. By normalizing adjacent flakes to each other, we achieve good agreement between theory and experiment, allowing for the unambiguous assignment of ABCB domains in tetralayer graphene. These results establish near-field spectroscopy at the interband transitions as a semiquantitative tool, enabling the recognition of ABCB domains in tetralayer graphene flakes and, therefore, providing a basis to study correlation physics of this exciting phase.

Structural, nonlinear optical and antimicrobial properties of sol-gel derived, Fe-doped CuO thin films.

Undoped and Fe-doped CuO thin films with different weight ratios (3, 6, and 9 wt.% of Fe) were deposited onto glass substrates using the sol-gel spin coating technique. X-ray diffraction analysis of these samples indicated that all the films were polycrystalline, and crystallite size decreased with doping concentration. As revealed by scanning electron microscopy, Fe doping increased average particle size and improved size distribution in films. The bandgap of undoped CuO thin film was tuned from 3.48 to 2.79 by the addition of 9 wt.% Fe, and reasonable explanations have been presented. Optical parameters, such as refractive index, extinction coefficient, dielectric constant, and optical conductivity, were calculated for optoelectronic applications. Finally, antimicrobial properties were measured for their possibility to be used as disinfectants, and the antifungal activity of Fe-doped CuO thin films was shown to be more effective.

Far-field thermal radiation properties of graphene under uniaxial strain.

Electronic band structure and optical conductivity of single-layer graphene could be altered by applied uniaxial strain. Valley and space inversion symmetries are broken. Dirac cones are deformed. We investigate the effect of uniaxial strain on the radiative properties of graphene from the perspective of direction and modulus. Optical conductivity exhibits wealthy phenomenon due to the degeneracy of the energy band broken by strain. The total energy radiation exhibits a novel behavior of periodicity inθ, in accordance with the symmetry of the hexagonal honeycomb lattice.

Prediction of Strong Transversal s(TE) Exciton-Polaritons in C60 Thin Crystalline Films.

If an exciton and a photon can change each other's properties, indicating that the regime of their strong bond is achieved, it usually happens in standard microcavity devices, where the large overlap between the 'confined' cavity photons and the 2D excitons enable the hybridization and the band gap opening in the parabolic photonic branch (as clear evidence of the strong exciton-photon coupling). Here, we show that the strong light-matter coupling can occur beyond the microcavity device setup, i.e., between the 'free' s(TE) photons and excitons. The s(TE) exciton-polariton is a polarization mode, which (contrary to the p(TM) mode) appears only as a coexistence of a photon and an exciton, i.e., it vanishes in the non-retarded limit (c→∞). We show that a thin fullerene C60 crystalline film (consisting of N C60 single layers) deposited on an Al2O3 dielectric surface supports strong evanescent s(TE)-polarized exciton-polariton. The calculated Rabi splitting is more than Ω=500 meV for N=10, with a tendency to increase with N, indicating a very strong photonic character of the exciton-polariton.

Dynamical polarization, optical conductivity and plasmon mode of a linear triple component fermionic system.

We investigate the density and optical responses of a linear triple component fermionic system in both non-interacting and interacting regimes by computing its dynamical polarization function, random phase approximation dielectric function, plasmon mode and long wavelength optical conductivity and compare the results with those of Weyl fermions and three-dimensional free electron gas. Linear triple component fermions are pseudospin-1 generalization of Weyl fermions, consisting of two linearly dispersive bands and a flat band. The presence of flat band brings about notable modifications in the response properties with respect to Weyl fermions such as induction of a new region in the particle-hole continuum, increased static polarization, reduced plasmon gap, shift in absorption edge, enhanced rate of increase in energy absorption with frequency and highly suppressed intercone transitions in the long wavelength limit. The plasmon dispersion follows the usualω∼ω0+ω1q2nature as observed in other three-dimensional systems.

Optical conductivities in triple fermions with different monopole charges.

We investigate the linear optical conductivities of the newly-discovered triple-component semimetals. Due to the exactly flat band, the optical conductivity relates to the transition between the zero band and the conduction band directly reflecting the band structure of the conduction electrons in contrast to the other materials. For the low-energy models with various monopole charges, the diagonal conductivities show strong anisotropy. Theω-dependence of interband conductivities for a general low-energy model is deduced. The real part of the interbandσxxalways linearly depends on the optical frequency, while the one ofσzzis proportional toω2/n-1. This can be a unique fingerprint of the monopole charge. For the lattice models, there also exists the optical anomalous Hall conductivity, where a sign change may appear. The characteristic frequencies of the kink structures are calculated, strictly. Our work will help us to establish the basic picture of linear optical response in topological triple-component semimetals and identify them from other materials.

Optical conductivity and superconductivity in highly overdoped La2-x Ca x CuO4 thin films.

We have used atomic layer-by-layer oxide molecular beam epitaxy to grow epitaxial thin films of [Formula: see text] with x up to 0.5, greatly exceeding the solubility limit of Ca in bulk systems ([Formula: see text]). A comparison of the optical conductivity measured by spectroscopic ellipsometry to prior predictions from dynamical mean-field theory demonstrates that the hole concentration p is approximately equal to x. We find superconductivity with [Formula: see text] of 15 to 20 K up to the highest doping levels and attribute the unusual stability of superconductivity in [Formula: see text] to the nearly identical radii of La and Ca ions, which minimizes the impact of structural disorder. We conclude that careful disorder management can greatly extend the "superconducting dome" in the phase diagram of the cuprates.

Blueshifted dielectric properties and optical conductivity of new nanoscale nickel-(II)-tetraphenyl-21H,23H-porphyrin films as a function of UV illumination for energy storage applications.

Pristine thermally evaporated nickel-(II)-tetraphenyl-21H,23H-porphyrin (NiTPP) thin films are amorphous, but after 4 and 8 h of UV illumination, the films become crystalline with preferred orientations of (112), (103) and (004) and crystallite sizes of (13, 18, 16) and (42, 31, 38) nm after 4 and 8 h, respectively. After UV illumination for 4 and 8 h, the NiTPP thin films are characterized by blueshifted absorption coefficients, increasing the optical and fundamental gap values and decreasing the dispersion parameter values. The dielectric properties display energy storage regions corresponding to the peak values of optical conductivity, which provides an elegant confirmation of the tailoring and tuning of band gaps, energy storage properties and optical conductivity by UV illumination time. Therefore, NiTPP films may be good candidates for environmental and energy storage applications.

Effective impedance of two-dimensional metal with retardation effect.

Optical absorption with retardation effect is discussed for two-dimensional (2D) metal. The absorption is given by the induced Joule heat in the metal and it is proportional to Re(σ)/|ɛ|2in whichσandɛdenote conductivity and dielectric function, respectively. Here, we investigate the effective impedance in both retarded and non-retarded regions of surface plasmon by discussing the response of the current density to the electric fields. The absorption formula Re(σ)/|ɛ|2is compared with the formula Re(σ/ɛ) that is commonly used for the absorption in carbon nanotube. We show that Re(σ/ɛ) is equal to Re(σ)/|ɛ|2only in the non-retarded region. The physical reason for Re(σ/ɛ) ≠ Re(σ)/|ɛ|2in the retarded region is that the induced current density is not out-of-phase with the induced electric field, which is explained by the effective impedance for both regions. The opposite response of the current to the induced electric field distinguishes the retarded and non-retarded regions. The calculated optical absorption spectra by Re(σ)/|ɛ|2reproduce the absorption spectra by solving the Maxwell equation as a function of the angular frequency of light or incident angle relative to the 2D surface, which makes Re(σ)/|ɛ|2a general representation of absorption.

Optical conductivity of triple point fermions.

As a low-energy effective theory on non-symmorphic lattices, we consider a generic triple point fermion Hamiltonian, which is parameterized by an angular parameterλ. We find strongλdependence in both Drude and interband optical absorption of these systems. The deviation of theT2coefficient of the Drude weight from Dirac/Weyl fermions can be used as a quick way to optically distinguish the triple point degeneracies from the Dirac/Weyl degeneracies. At the particularλ=π/6 point, we find that the 'helicity' reversal optical transition matrix element is identically zero. Nevertheless, deviating from this point, the helicity reversal emerges as an absorption channel.