Local Density Of States Research Articles

Highly Conductive Flexible Conductor Based on PEDOT:PSS/MWCNTs Nano Composite

Flexible textiles with strong electrical conductivities have enormous potential as active components in wearable electronics. In this study, we fabricated highly flexible electrical conductors based on cotton fabrics using multiwalled carbon nanotubes (MWCNTs) and poly(3,4-ethylenedioxythiophene)/poly(styrenesulfonate) (PEDOT:PSS) nanocomposites. We propose that mixing and drop-casting with different amounts of MWCNTs and a fixed amount of doped PEDOT:PSS using a cotton fabric provides a wide range of conductivities depending on the amount of MWCNTs in the mixture. Scanning electron microscopy (SEM) confirmed that the distribution of MWCNTs in the PEDOT:PSS films coated the surface of the cotton fabric, thereby increasing its electrical conductivity. We found that the amount of MWCNTs significantly affected the electrical properties of the nanocomposite cotton in two ways. First, the sheet resistance of the nanocomposite cotton decreased from 78.35 Ω/□ to 2.86 Ω/□ when the concentration of the nanocomposite was increased from 9.21 wt% to 60.27 wt%. This implies that the electrical properties of the nanocomposite cotton can be adjusted by controlling the amount of MWCNTs in the blend. Moreover, we found that the relationship between the sheet resistance and nanocomposite concentration obeys the power law with an exponent α ~ 1.676. Second, the study of the effect of temperature on the resistance indicates that the conductive nanocomposite exhibits semiconductor behavior in the temperature range 24–120 °C and obeys the variable range hopping model. The characteristic temperatures, resistance prefactor, and density of localized states and activation energies depend on the concentration of MWCNTs and can be described by power laws with exponents of 0.470, −1.292, −0.470 and 0.118, respectively. The novel nanocomposite cotton fabric developed in this study exhibits suitable electrical and thermal properties and good long-term electrical stability, which make the nanocomposite cotton fabric a potential flexible conductor with a wide range of electrical conductivities, making it suitable for various applications.

Imaging Friedel oscillations in rhombohedral trilayer graphene

Quasiparticle interference-induced spatial standing waves in local density of states, i.e., Friedel oscillations, are first visualized in rhombohedral trilayer graphene (rTG) by using scanning tunneling microscopy and spectroscopy. We show that the long-range standing-wave patterns of rTG can be created not only by the scattering off usual potential barriers including defects and step edges, but also by ABC-ABA stacking domain walls. For step edges as scatterers, both the surface step edge and the underlying step edge can effectively generate quasiparticle standing waves on the surface layer. For all observed types of scatterers in rTG, the Friedel oscillations always exhibit a $1/r$ spatial decay. This decay rate of Friedel oscillations is consistent with that in bilayer graphene, while slower than that in monolayer graphene, directly confirming previous theoretical predictions. Our results provide fundamental knowledge of the nature of scatterers in rTG, which would help us to better understand their microscopic scattering mechanisms.

One-dimensional transient crystals

A phase of solid state matter which exists in time dimension is investigated theoretically for a linear chain of quantum dots or atomic sites using the evolution operator technique and the Laplace transform technique.. The studies focus on the spectral density of states (DOS) function and its modifications after a sudden quench in the system. It is shown that this function reveals a very regular structure which oscillates in time and which corresponds to the stationary DOS of one-dimensional crystals. Thus, such a system stands for a transient crystal material which exists in time dimension but appears only for a short period and suddenly vanishes. Moreover, we observe different impulse propagation velocities due to the local DOS inertia, i.e., it is more difficult to rebuild a many-peaked structure of DOS related to longer chains. We also show that the transient crystal pattern in a linear chain can be observed for real electrodes characterized by the van Hove singularities or by a flat spectral density.

Quasi-BIC Modes in All-Dielectric Slotted Nanoantennas for Enhanced Er3+ Emission.

In the quest for new and increasingly efficient photon sources, the engineering of the photonic environment at the subwavelength scale is fundamental for controlling the properties of quantum emitters. A high refractive index particle can be exploited to enhance the optical properties of nearby emitters without decreasing their quantum efficiency, but the relatively modest Q-factors (Q ∼ 5-10) limit the local density of optical states (LDOS) amplification achievable. On the other hand, ultrahigh Q-factors (up to Q ∼ 109) have been reported for quasi-BIC modes in all-dielectric nanostructures. In the present work, we demonstrate that the combination of quasi-BIC modes with high spectral confinement and nanogaps with spacial confinement in silicon slotted nanoantennas lead to a significant boosting of the electromagnetic LDOS in the optically active region of the nanoantenna array. We observe an enhancement of up to 3 orders of magnitude in the photoluminescence intensity and 2 orders of magnitude in the decay rate of the Er3+ emission at room temperature and telecom wavelengths. Moreover, the nanoantenna directivity is increased, proving that strong beaming effects can be obtained when the emitted radiation couples to the high Q-factor modes. Finally, via tuning the nanoanntenna aspect ratio, a selective control of the Er3+ electric and magnetic radiative transitions can be obtained, keeping the quantum efficiency almost unitary.

Importance of accurately measuring LDOS maps using scanning tunneling spectroscopy in materials presenting atom-dependent charge order: The case of the correlated Pb/Si(111) single atomic layer

We address here general issues and show how to properly extract the local charge order in two-dimensional systems from scanning tunneling microscopy/spectroscopy (STM/STS) measurements. When the charge order presents spatial variations at the atomic scale inside the unit cell and is energy dependent, particular care should be taken. We show that the widely used lock-in technique performed while acquiring an STM topography in a closed feedback loop cannot be used to extract this local charge order from STS $dI/dV$ differential conductance maps. In such situations, the use of the lock-in technique leads to systematically incorrect $dI/dV$ measurements, giving a false local charge order. We show that a correct method is either to perform a constant height measurement or to perform a full grid of $dI/dV(V)$ spectroscopies, using a set point for the bias voltage outside the bandwidth of the correlated material where the local density of states (LDOS) is expected to be spatially homogeneous. We take as a paradigmatic example of two-dimensional material the 1/3 single-layer Pb/Si(111) phase. As large areas of this phase cannot be grown, charge ordering in this system is not accessible to angular resolved photoemission or grazing x-ray diffraction. Two previous investigations by STM/STS supplemented by ab initio density functional theory (DFT) calculations concluded that this material undergoes a phase transition to a low-temperature $3\ifmmode\times\else\texttimes\fi{}3$ reconstruction where one Pb atom moves up, the two remaining Pb atoms shifting down. A third STM/STS study by Adler et al. [F. Adler, S. Rachel, M. Laubach, J. Maklar, A. Fleszar, J. Sch\"afer, and R. Claessen, Phys. Rev. Lett. 123, 086401 (2019)] came to the opposite conclusion, i.e., that two Pb atoms move up, while one Pb atom shifts down. We show that this latter erroneous conclusion comes from an artifact induced by the lock-in technique. In contrast, using a full grid of $dI/dV$($V$) spectroscopy measurements, our results show that the energy-dependent LDOS maps agree very well with state-of-the-art DFT calculations, confirming the one-up two-down charge ordering. We show that this structural and charge reordering inside the $3\ifmmode\times\else\texttimes\fi{}3$ unit cell is equally driven by electron-electron interactions and the coupling to the substrate.

Normalization procedure for obtaining the local density of states from high-bias scanning tunneling spectroscopy

Differential conductance spectroscopy performed in the high bias regime---in which the applied voltage exceeds the sample work function---is a suboptimal measure of the local density of states due to the effects of the changing tunnel barrier. Additionally, the large applied voltage oftentimes makes constant-height measurement experimentally impractical, lending constant-current spectroscopy an advantageous edge; but the differential conductance in that case is even further removed from the local density of states due to the changing tip height. Here, we present a normalization scheme for extracting the local density of states from high bias scanning tunneling spectroscopy, obtained in either constant-current or constant-height mode. We extend this model to account for the effects of the in-plane momentum of the probed states to the overall current. We demonstrate the validity of the proposed scheme by applying it to laterally confined field-emission resonances, which appear as peak-shaped spectroscopic features with a well-defined in-plane momentum.

Non-Hermiticity induced exceptional points and skin effect in the Haldane model on a dice lattice

The interplay of topology and non-Hermiticity has led to diverse, exciting manifestations in a plethora of systems. In this work, we systematically investigate the role of non-Hermiticity in the Chern insulating Haldane model on a dice lattice. Due to the presence of a non-dispersive flat band, the dice-Haldane model hosts a topologically rich phase diagram with the non-trivial phases accommodating Chern numbers $\pm 2$. We introduce non-Hermiticity into this model in two ways -- through balanced non-Hermitian gain and loss, and by non-reciprocal hopping in one direction. Both these types of non-Hermiticity induce higher-order exceptional points of order three. We substantiate the presence and the order of these higher-order exceptional points using the phase rigidity and its scaling. Further, we construct a phase diagram to identify and locate the occurrence of these exceptional points in the parameter space. Non-Hermiticity has yet more interesting consequences on a finite-sized lattice. Unlike for balanced gain and loss, in the case of non-reciprocal hopping, the nearest-neighbour dice lattice system under periodic boundary conditions accommodates a finite, non-zero spectral area in the complex plane. This manifests as the non-Hermitian skin effect when open boundary conditions are invoked. In the more general case of the dice-Haldane lattice model, the non-Hermitian skin effect can be caused by both gain and loss or non-reciprocity. Fascinatingly, the direction of localization of the eigenstates depends on the nature and strength of the non-Hermiticity. We establish the occurrence of the skin effect using the local density of states, inverse participation ratio and the edge probability, and demonstrate its robustness to disorder. Our results place the dice-Haldane model as an exciting platform to explore non-Hermitian physics.

The magnetic field driven superconductor–metal transition in disordered hole-overdoped cuprates

By solving the Bogoliubov–de Gennes equations for a d-wave superconductor, we explore how the interplay between disorder and the orbital depairing of an external magnetic field influences the superconductor–metal transition of the hole-overdoped cuprates. For highly disordered systems, we find granular Cooper paring to persist above the critical field where the superfluid stiffness goes to zero. We also show that because the vortices are attracted to regions where the superconducting pairing is already weak, the Caroli–de Gennes–Matricon zero-bias peak in the local density of states at the vortex cores disappears already at moderate disorder.

Calculation of emission spectra of lithium compounds

AbstractWe studied the electronic structure of lithium metal, lithium fluoride, and spodumene using density functional theory. We performed calculations of the total, local and partial density of states, and x‐ray emission spectra. We observed changes in the Li K spectra shape and energy position due to the chemical structure and composition modification. We also outline possible interferences between emission bands.

Strong Er3+ radiative emission enhancement by quasi-BIC modes coupling in all-dielectric slot nanoantenna arrays

We have designed and realized all-dielectric lossless nanoantennas, in which a thin SiO2 layer doped with erbium ions is placed inside slotted silicon nanopillars arranged in a square array. The modal analysis has evi-denced that the slotted nanopillar array supports optical quasi-BIC resonances with ultra-high Q-factors (up to Q∼109), able to boost the electromagnetic local density of optical states in the optically active layer. Up to 3 orders of magnitude photoluminescence intensity increment and 2 orders of magnitude decay rate enhancement have been measured at room temperature when the Er3+ emission at about λ=1540 nm couples with the quasi-BIC resonances. Furthermore, by tailoring the nanopillar aspect ratio, the slot geometry has been exploited to obtain selective enhancements of the electric or magnetic dipole contribution to Er3+ radiative transitions in the NIR, keeping the emitter quantum efficiency almost unitary. Finally, by computing the angularly resolved elec-tromagnetic field enhancement inside the nanoslot, the nanoantenna directivity has been designed, proving that strong beaming effects can be obtained. Our findings have a direct impact on the development of bright and effi-cient photon sources operating at telecom wavelength that are of primary importance for quantum nanophotonic applications.

Surface lattice resonances enhanced directional amplified spontaneous emission on plasmonic honeycomb nanocone array.

Plasmonic arrays have emerged as a promising platform for investigating light-matter interactions enhanced by surface lattice resonance (SLR) at the nanoscale, which exhibit superior properties in localized field enhancement, narrow linewidth, and effective radiation loss suppression. In this study, an Al nanocone array in a honeycomb arrangement served as an optical cavity with a tip effect to realize the directional and polarized amplified spontaneous emission (ASE) of R6G. Based on the optical feedback between the degenerated SLR mode of high local density of states (LDOS) and the emission of gain media, 140-fold enhanced ASE was observed at an emission angle of 25° under TM polarization when the pump power density exceeded the threshold of 197.8 W cm-2. Moreover, polarization-resolved iso-frequency images indicated that a specific polarization dependence of ASE was modulated by the SLR mode. This study clarifies the interaction between the gain media and plasmonic system, which is beneficial for the generation of nanolasing with directional emission and lays a foundation for the plasmonic device.

Structural and vibrational properties of wurtzite ZnO with oxygen-deficient defects: ab initio and potential-based calculations.

The structural and vibrational properties of the ZnO wurtzite phase with oxygen vacancies in different charged states are studied using first-principles and potential-based methods. The calculations based on density-functional theory are performed to determine the atomic configurations around defects. The DFT results are discussed and compared with those obtained using the static lattice method in the traditional shell model. Both computational approaches predict the same character of crystal lattice relaxation around oxygen vacancies. The phonon local symmetrized densities of states are calculated using the Green function method. The frequencies of localized vibrations of various symmetry types induced by oxygen vacancies in neutral and positively charged states are determined. The calculation results allow estimating the effect of oxygen vacancies on the formation of the intense Raman peak.

Simulation of Supercell Defect Structure and Transfer Phenomena in TlInTe2

The local environment of atoms in a semiconductor compound TlInTe2 with tetragonal syngony is studied by the density functional theory (DFT). The introduction of a point defect (indium vacancies) into the TlInTe2 lattice is modeled using supercells. The DFT electronic properties (total and local partial densities of states (PDOS) of electrons) are modeled for the primitive TlInTe2 cell (16 atoms per unit cell) and for the defective TlInTe2 cell (where is the vacancy In) consisting of 32 atoms. The DFT-GGA calculations of the TlInTe2 band structure show that the band gap ( ) is = 1.21 eV. This value is significantly dif-ferent from the experimental value. The Hubbard model is used to correct the interaction of particles in the lattice. The DFT-GGA + U (U is the Hubbard potential) calculated by the TlInTe2 band gap is 0.97 eV. For the TlInTe2 supercell, the energies of the formation of a vacancy, the chemical potential of indium, and the standard enthalpy of the formation of TlInTe2 are calculated. When explaining the effect of various factors on the transport phenomena in TlInTe2, their thermal and electrical conductivity, both the DFT-calculated data and experimental data, are used. Taking into account the experimental data for the p-TlInTe2 crystals, the mechanism of conduction in the direction of structural chains (c axis of the crystal) is established. From the experimental data in the temperature range = 148–430 K, the band gap = 0.94 eV and the activation energy of impurity conduction = 0.1 eV (at 210–300 K) are estimated. At temperatures of ≤ 210 K, DC hopping conduction takes place in the p-TlInTe2 crystals. With this in mind, the following physical parameters are calculated for p-TlInTe2: the density of states localized near the Fermi level, their energy spread, and the average hopping distance.

Fission hindrances in transfermium nuclei

Very heavy nuclei owe their stability against spontaneous fission to quantum shell effects, which depend on the local density of single-particle states. The height but also the width and the structure of the barrier in multi-dimensional deformation space determine the fission half-lives. Other effects come into play, such as the conservation of quantum numbers and superfluidity or stiffness of the system in the fission process. This is why odd nuclei have longer fission partial half-lives with respect to their even neighbours and also why multi-quasi-particle states, such as high-K states, are thought to be more stable against fission than the ground state. We will report here on two different fission studies carried out with the GABRIELA detector array at the focal plane of the recoil separator SHELS. The first study concerns the fission properties of 253Rf, the most neutron deficient Rf isotope known to date. The second study focusses on a new measurement of the fission hindrance of the known 8- isomer in 254No.

Quasi-bound states and resonant skew scattering in two-dimensional materials with a Mexican-hat dispersion

Mexican-hat dispersion of band electrons in two-dimensional materials attracts a lot of interest, mainly due to the Van Hove singularity of the density of states near the band edge. In this paper, we show that there is one more feature of such a dispersion, which also leads to nontrivial effects. It consists in the fact that the sign of the effective mass in the momentum space near the central extremum is opposite to the sign of the mass outside this region. For this reason, any localized potential that repels quasiparticles in the outer region attracts quasiparticles in the central region and thereby creates quasi-bound states. We study these states in the case when the Mexican-hat dispersion is formed due to the hybridization of the inverted electron and hole bands, and the potential is created by a point defect. The energy and width of the resonance of the local density of states corresponding to a quasi-bound state are found, and it is shown that, under certain conditions, a quasi-bound state can transform into a bound state in a continuum of band states. The presence of quasi-bound states leads to nontrivial effects in the spin-dependent scattering of electrons. Due to the quasi-bound state, the skew scattering is strongly enhanced for electrons with energy near the resonance, and the skewness angle varies over a wide range depending on the energy. In addition, in a certain energy range, a nontrivial effect of scattering suppression appears in the direction opposite to the skewness angle.

Properties of (Cd, Zn)S films deposited by DC co-sputtering

CdS thin film is commonly used as a buffer layer in most thin film solar cells. However, the buffer layer with less absorption in the visible region of the solar spectrum as well as that which provides perfect lattice matching with the absorbers is required. The incorporation of Zn in the CdS films can affect the structural and optical properties of the buffer layer. In this work, films of CdS and (Cd,Zn)S films were deposited by DC sputtering utilizing CdS and Zn targets. The concentration of Zn in the CdS was varied by changing the sputtering power of the Zn target. The influence of Zn concentration on the structural and optical properties of CdS films was investigated using X-ray diffraction (XRD) and Ultraviolet-Visible-Near Infrared (UV-VIS-NIR) spectroscopy, respectively. The XRD patterns confirm the growth preferential orientation of all films along the (002) plane. UV-VIS spectroscopy results indicate absorption edge shifts toward lower wavelength with an increase of Zn concentration in the CdS. The band gap evaluated by the Tauc plot of these films varied from 2.40 eV to 2.50 eV. The increased bandgap induces the density of localized states on the edge of the bandgap which leads to the increase in the Urbach tail width.

Local density of optical states calculated by the mode spectrum in stratified media

The local density of optical states (LDOS) is an important physical concept, which can characterize the spontaneous emission of microcavities. In order to calculate the LDOS, the relationship between the mode spectrum and the LDOS is established. Then, based on the transfer matrix method and the effective resonator model, the leaky loss of the leaky mode and the mode spectrum in the one-dimensional photonic bandgap crystal waveguide are calculated, results of which indicate that the mode spectrum can characterize the leaky loss of the leaky mode. At last, the density of optical states (DOS), and the LDOS in each layer are calculated. The partial DOS and the partial LDOS in the quantum well, related to the fundamental leaky mode, can be used to find out the optimal location of the quantum well in the defect layer to couple more useful photons into the lasing mode for lasers.

Seeing moiré: Convolutional network learning applied to twistronics

Moir\'e patterns made of two-dimensional (2D) materials represent highly tunable electronic Hamiltonians, allowing a wide range of quantum phases to emerge in a single material. Current modeling techniques for moir\'e electrons require significant technical work specific to each material, impeding large-scale searches for useful moir\'e materials. In order to address this difficulty, we have developed a material-agnostic machine learning approach and test it here on prototypical one-dimensional (1D) moir\'e tight-binding models. We utilize the stacking dependence of the local density of states (SD-LDOS) to convert information about electronic band structure into physically relevant images. We then train a neural network that successfully predicts moir\'e electronic structure from the easily computed SD-LDOS of aligned bilayers. This network can satisfactorily predict moir\'e electronic structures, even for materials that are not included in its training data.

Charge Localization Induced by Pentagons on Ge(110).

The Ge(110) surface reconstructs into ordered and disordered phases, in which the basic unit is a five-membered ring of Ge atoms (pentagon). The variety of surface reconstructions leads to a rich electronic density of states with several surface states. Using scanning tunneling microscopy and spectroscopy, we have identified the exact origins of these surface states and linked them to either the Ge pentagons or the underlying Ge-Ge bonds. We show that even moderate fluctuations in the positions of the Ge pentagonal units induce large variations in the local density of states. The local density of states modulates in a precise manner, following the geometrical constraints on tiling Ge pentagons. These geometry-correlated electronic states offer a vast configurational landscape that could provide new opportunities in data storage and computing applications.

Surface state of the interorbital pairing state in the Sr2RuO4 superconductor

We study the (001) surface state of a recently proposed $E_g$ symmetry inter-orbital-odd spin-triplet s-wave superconducting (SC) state in Sr$_2$RuO$_4$ (SRO). We confirm that this pair potential is transformed into a chiral $d$-wave pair potential and a pseudo-Zeeman field in the band basis for a low-energy range. Due to the chiral $d$-wave pair potential, the surface states appear near zero energy in the momentum range enclosed by the nodal lines of the chiral d-wave pair potential for each band at the (001) surface. Nevertheless, the pseudo-Zeeman field gives band splitting of the surface states, and its splitting energy is much smaller than the SC energy gap. The local density of states (LDOS) at the (001) surface of the SC state has a pronounced peak structure at zero energy due to the surface states near zero energy when the order of the resolution is lower than the splitting energy. This peak structure is robust under perturbations, such as an orbital Rashba coupling or an $E_u$ SC pair potential at the surface.