Tunable and low-loss correlated plasmons in Mott-like insulating oxides

Plasmon Excitations across the Charge-Density-Wave Transition in Single-Layer TiSe2

1T-TiSe2 is believed to possess a soft electronic mode, i.e., plasmon or exciton, that might be responsible for the exciton condensation and charge-density-wave (CDW) transition. Here, we explore collective electronic excitations in single-layer 1T-TiSe2 by using the ab initio electromagnetic linear response and unveil intricate scattering pathways of the two-dimensional (2D) plasmon mode near the CDW phase. We found the dominant role of plasmon-phonon scattering, which in combination with the CDW gap excitations leads to the anomalous temperature dependence of the plasmon line width across the CDW transition. Below the transition temperature TCDW a strong hybridization between the 2D plasmon and CDW excitations is obtained. These optical features are highly tunable due to temperature-dependent CDW-related modifications of electronic structure and electron-phonon coupling and make CDW-bearing systems potentially interesting for applications in optoelectronics and low-loss plasmonics.

Unrevealing tunable resonant excitons and correlated plasmons and their coupling in new amorphous carbon-like for highly efficient photovoltaic devices

An understanding on roles of excitons and plasmons is important in excitonic solar cells and photovoltaic (PV) technologies. Here, we produce new amorphous carbon (a-C) like films on Indium Tin Oxide (ITO) generating PV cells with efficiency three order of magnitude higher than the existing biomass-derived a-C. The amorphous carbon films are prepared from the bioproduct of palmyra sap with a simple, environmentally friendly, and highly reproducible method. Using spectroscopic ellipsometry, we measure simultaneously complex dielectric function, loss function as well as reflectivity and reveal coexistence of many-body resonant excitons and correlated-plasmons occurring due to strong electronic correlations. X-ray absorption and photoemission spectroscopies show the nature of electron and hole in defining the energy of the excitons and plasmons as a function of N or B doping. Our result shows new a-C like films and the importance of the coupling of resonant excitons and correlated plasmons in determining efficiency of photovoltaic devices.

Tailoring Optical Properties in Transparent Highly Conducting Perovskites by Cationic Substitution

SrMoO₃ , SrNbO₃ , and SrVO₃ are remarkable highly conducting d¹ (V, Nb) or d² (Mo) perovskite metals with an intrinsically high transparency in the visible. A key scientific question is how the optical properties of these materials can be manipulated to make them suitable for applications as transparent electrodes and in plasmonics. Here, it is shown how 3d/4d cationic substitution in perovskites tailors the relevant materials parameters, i.e., optical transition energy and plasma frequency. With the example of the solid-state solution SrV_{1- x} Mo_x O₃ , it is shown that the absorption and reflection edges can be shifted to the edges of the visible light spectrum, resulting in a material that has the potential to outperform indium tin oxide (ITO) due to its extremely low sheet resistance. An optimum for x = 0.5, where a resistivity of 32 µΩ cm (≈12 Ω sq^-1 ) is paired with a transmittance above 84% in the whole visible spectrum is found. Quantitative comparison between experiments and electronic structure calculations show that the shift of the plasma frequency is governed by the interplay of d-band filling and electronic correlations. This study advances the knowledge about the peculiar class of highly conducting perovskites toward sustainable transparent conductors and emergent plasmonics.

Observation of resonant exciton and correlated plasmon yielding correlated plexciton in amorphous silicon with various hydrogen content

Hydrogenated amorphous silicon (a-Si: H) has received great attention for rich fundamental physics and potentially inexpensive solar cells. Here, we observe new resonant excitons and correlated plasmons tunable via hydrogen content in a-Si: H films on Indium Tin Oxide (ITO) substrate. Spectroscopic ellipsometry supported with High Resolution-Transmission Electron Microscopy (HR-TEM) is used to probe optical properties and the density of electronic states in the various crystallinity from nano-size crystals to amorphous a-Si: H films. The observed optical and electronic structures are analyzed by the second derivative with analytic critical-point line shapes. The complex dielectric function shows good agreement with microscopic calculations for the energy shift and the broadening inter-band transitions based on the electron-hole interaction. Interestingly, we observe an unusual spectral weight transfer over a broad energy range revealing electronic correlations that cause a drastic change in the charge carrier density and determine the photovoltaic performance. Furthermore, the interplay of resonant excitons and correlated plasmons is discussed in term of a correlated plexciton. Our result shows the important role of hydrogen in determining the coupling of excitons and plasmons in a-Si: H film for photovoltaic devices.

Ultra-flat and long-lived plasmons in a strongly correlated oxide

Plasmons in strongly correlated systems are attracting considerable attention due to their unconventional behavior caused by electronic correlation effects. Recently, flat plasmons with nearly dispersionless frequency-wave vector relations have drawn significant interest because of their intriguing physical origin and promising applications. However, these flat plasmons exist primarily in low-dimensional materials with limited wave vector magnitudes (q < ~0.7 Å⁻¹). Here, we show that long-lived flat plasmons can propagate up to ~1.2 Å⁻¹ in α-Ti₂O₃, a strongly correlated three-dimensional Mott-insulator, with an ultra-small energy fluctuation (<40 meV). The strong correlation effect renormalizes the electronic bands near Fermi level with a small bandwidth, which is responsible for the flat plasmons in α-Ti₂O₃. Moreover, these flat plasmons are not affected by Landau damping over a wide range of wave vectors (q < ~1.2 Å⁻¹) due to symmetry constrains on the electron wavefunctions. Our work provides a strategy for exploring flat plasmons in strongly correlated systems, which in turn may give rise to novel plasmonic devices in which flat and long-lived plasmons are desirable.

Dispersionless plasmons could find important practical applications, but previous demonstrations have been limited to 2D materials and small momentum range. Here the authors report ultra-flat plasmons propagating over a wide range of momenta in a 3D strongly correlated oxide α-Ti₂O₃.

Low-Loss Tunable Infrared Plasmons in the High-Mobility Perovskite (Ba,La)SnO3

BaSnO₃ exhibits the highest carrier mobility among perovskite oxides, making it ideal for oxide electronics. Collective charge carrier oscillations known as plasmons are expected to arise in this material, thus providing a tool to control the nanoscale optical field for optoelectronics applications. Here, the existence of relatively long-lived plasmons supported by high-mobility charge carriers in La-doped BaSnO₃ (BLSO) is demonstrated. By exploiting the high spatial and energy resolution of electron energy-loss spectroscopy with a focused beam in a scanning transmission electron microscope, the dispersion, confinement ratio, and damping of infrared localized surface plasmons (LSPs) in BLSO nanoparticles are systematically investigated. It is found that LSPs in BLSO exhibit a high degree of spatial confinement compared to those sustained by noble metals and have relatively low losses and high quality factors with respect to other doped oxides. Further analysis clarifies the relation between plasmon damping and carrier mobility in BLSO. The results support the use of nanostructured degenerate semiconductors for plasmonic applications in the infrared region and establish a solid alternative to more traditional plasmonic materials.

Unravelling strong electronic interlayer and intralayer correlations in a transition metal dichalcogenide

Electronic correlations play important roles in driving exotic phenomena in condensed matter physics. They determine low-energy properties through high-energy bands well-beyond optics. Great effort has been made to understand low-energy excitations such as low-energy excitons in transition metal dichalcogenides (TMDCs), however their high-energy bands and interlayer correlation remain mysteries. Herewith, by measuring temperature- and polarization-dependent complex dielectric and loss functions of bulk molybdenum disulphide from near-infrared to soft X-ray, supported with theoretical calculations, we discover unconventional soft X-ray correlated-plasmons with low-loss, and electronic transitions that reduce dimensionality and increase correlations, accompanied with significantly modified low-energy excitons. At room temperature, interlayer electronic correlations, together with the intralayer correlations in the c-axis, are surprisingly strong, yielding a three-dimensional-like system. Upon cooling, wide-range spectral-weight transfer occurs across a few tens of eV and in-plane p-d hybridizations become enhanced, revealing strong Coulomb correlations and electronic anisotropy, yielding a two-dimensional-like system. Our result shows the importance of strong electronic, interlayer and intralayer correlations in determining electronic structure and opens up applications of utilizing TMDCs on plasmonic nanolithrography.

Coupled harmonic oscillator models for correlated plasmons in one-dimensional and quasi-one-dimensional systems

A new phenomenon of correlated plasmons was first observed in the insulating phase of the Sr_1-xNb_1-yO_3+δfamily (Asamaraet al2017Nat. Commun.815271). The correlated plasmons are tunable, have multiple plasmonic frequencies, and exhibit low loss-making them desirable in numerous plasmonic applications. However, their fundamental mechanism is yet to be explored. While conventional plasmons can be understood solely by considering long-range interactions, unconventional correlated plasmons arise in correlated electron systems and require consideration of the short-range interactions. Here, we report how the interplay of short-range and long-range interactions determines the correlated plasmon phenomena through a coupled harmonic oscillator model of both 1D and quasi-1D systems. In each system, the impact of various physical parameters like the number of oscillators, energy scale, free electron scattering parameter, quasi-particle concentration, charges, effective masses, and Coulomb interaction strengths are explored to gain an understanding of their impact on the complex dielectric function and loss function. We study both cases where the parameters are the same for all quasi-particles and where effective mass, Coulomb interaction strength, and charge are varied for individual quasi-particles. In an extended model of the quasi-1D system, we study both cases where the rung symmetry of all parameters is conserved and where it is broken. When rung symmetry is conserved, the overall trends in optical and plasmonic peaks are the same as the 1D model, though the peaks tend to shift to higher energies and amplitudes. When rung symmetry is broken, the quasi-1D behavior deviates significantly from the 1D model, including an increase in the maximum possible number of optical and plasmonic peaks. Overall, our results demonstrate the significance of the interplay of short-range and long-range interactions in determining the correlated plasmons and identifying how various parameters can be used to tune the resulting plasmons.

A New Spin-Correlated Plasmon in Novel Highly Oriented Single-Crystalline Gold Quantum Dots

A concept of spin plasmon, a collective mode of spin-density, in strongly correlated electron systems has been proposed since the 1930s. It is expected to bridge between spintronics and plasmonics by strongly confining the photon energy in the subwavelength scale within single magnetic-domain to enable further miniaturizing devices. However, spin plasmon in strongly correlated electron systems is yet to be realized. Herein, we present a new spin correlated-plasmon at room temperature in novel Mott-like insulating highly oriented single-crystalline gold quantum-dots (HOSG-QDs). Interestingly, the spin correlated-plasmon is tunable from the infrared to visible, accompanied by spectral weight transfer yielding a large quantum absorption midgap state, disappearance of low-energy Drude response, and transparency. Supported with theoretical calculations, it occurs due to an interplay of surprisingly strong electron-electron correlations, s-p hybridization and quantum confinement in the s band. The first demonstration of the high sensitivity of spin correlated-plasmon in surface-enhanced Raman spectroscopy is also presented.

Correlated oxide Dirac semimetal in the extreme quantum limit

Quantum materials (QMs) with strong correlation and nontrivial topology are indispensable to next-generation information and computing technologies. Exploitation of topological band structure is an ideal starting point to realize correlated topological QMs. Here, we report that strain-induced symmetry modification in correlated oxide SrNbO₃ thin films creates an emerging topological band structure. Dirac electrons in strained SrNbO₃ films reveal ultrahigh mobility (μ_max ≈ 100,000 cm²/Vs), exceptionally small effective mass (m* ~ 0.04m_e), and nonzero Berry phase. Strained SrNbO₃ films reach the extreme quantum limit, exhibiting a sign of fractional occupation of Landau levels and giant mass enhancement. Our results suggest that symmetry-modified SrNbO₃ is a rare example of correlated oxide Dirac semimetals, in which strong correlation of Dirac electrons leads to the realization of a novel correlated topological QM.

Nanoscale dielectric grating polarizers tuned to 4.43 eV for ultraviolet polarimetry

Transmissive dielectric wire grid polarizers tuned to 4.43 eV (Mg II line, 280 nm), an important diagnostic line for solar physics, are presented in this communication. The polarizers are based on TiO₂ gratings and designed with a period of ∼140 nm (7143 lines/mm), 40 nm line width (duty cycle of 0.286), and 100 nm line height. Several gratings are fabricated through electron beam lithography combined with reactive ion etching, whereby two parameters in the nanofabrication process are explored: e-beam dosage on the photoresist and TiO₂ etching time. Polarization of samples is optically characterized using a spectroscopic ellipsometer in transmission mode, achieving the best result with an extinction ratio of ∼109 and a transmittance of 16.4% at the target energy of 4.43 eV. The shape of the gratings is characterized through atomic force microscopy (AFM) and scanning electron microscopy (SEM); the measured AFM profiles are distorted by the tip geometry, hence a simple deconvolution procedure is implemented to retrieve the real profile. By analysing the AFM and SEM profiles, we find that the real shapes of the different gratings are close to the design, but with a larger duty cycle than the intended value. With the real grating geometry, an improved model of the best sample was built with a finite-difference time-domain (FDTD) method that matches the result obtained through optical characterization.

Electronic and plasmonic phenomena at nonstoichiometric grain boundaries in metallic SrNbO3

Grain boundaries could exhibit exceptional electronic structure and exotic properties, which are determined by a local atomic configuration and stoichiometry that differs from the bulk. However, optical and plasmonic properties at the grain boundaries in metallic oxides have rarely been discussed before. Here, we show that non-stoichiometric grain boundaries in the newly discovered metallic SrNbO₃ photocatalyst show exotic electronic, optical and plasmonic phenomena in comparison to bulk. Aberration-corrected scanning transmission electron microscopy and first-principles calculations reveal that a Nb-rich grain boundary exhibits an increased carrier concentration with quasi-1D metallic conductivity, and newly induced electronic states contributing to the broad energy range of optical absorption. More importantly, dielectric function calculations reveal extended and enhanced plasmonic excitations compared with bulk SrNbO₃. Our results show that non-stoichiometric grain boundaries might be utilized to control the electronic and plasmonic properties in oxide photocatalysis.

Atomic scale characterization of point and extended defects in niobate thin films

Niobium-based oxides have a wide range of applications owing to their rich crystal and electronic structures. Defects at the atomic scale are always unavoidable and will affect their functionalities, especially when in the form of thin films. Here, atomic resolution scanning transmission electron microscopy and electron energy loss spectroscopy have been performed on various defects (point, line, planar defects and segregated phases) in alkaline and alkaline-earth niobate thin films: CaZrO₃ modified (K, Na)NbO₃ and strontium niobate (SNO), respectively. In CaZrO₃ modified (K,Na)NbO₃ thin films, a tetragonal tungsten bronze phase was found, with a sharp boundary with the perovskite phase. In SNO thin films, several kinds of point defects and antiphase boundaries are commonly observed. In addition, a strongly Sr deficient phase, SrNb₂O₆, precipitates inside the SrNbO₃ phase with a coherent interface. The different oxidation states of Nb in SrNbO₃ and SrNb₂O₆ were revealed from the O K edge. Our characterization of the point defects and extended defects in niobate thin films offers practical guidelines for thin film deposition or discovery of defect-based novel functionalities.

A soft x-ray-ultraviolet (SUV) beamline and diffractometer for resonant elastic scattering and ultraviolet-vacuum ultraviolet reflectance at the Singapore synchrotron light source

A new beamline and a six-circle UHV diffractometer have been constructed at the Singapore Synchrotron Light Source with a broad energy coverage from 3.5 to 1500 eV. The beamline is optimized for ultraviolet-vacuum-ultraviolet optical reflectivity and resonant soft X-ray scattering with medium energy resolution over a broad energy range, achieved by using a self-focusing monochromator consisting of a plane mirror and three variable line spacing gratings. The unique character of the diffractometer comprises 4-circles in the vertical plane and 2-circles in the horizontal plane. Thirteen motions are available inside the UHV chamber with a base pressure of 1 × 10^-9 mbar. Two sample holders working independently over a temperature range of 37 K-400 K are controlled by a closed-cycle cryostat, while the bottom holder inside a high field compact pulsed magnet is available for measurements requiring a magnetic field.