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Spontaneous light emission is known to be affected by the local density of states and enhanced when coupled to a resonant cavity. Here, we report on an experimental study of silicon-vacancy (SiV) color center fluorescence and spontaneous Raman scattering from subwavelength diamond particles supporting low-order Mie resonances in the visible range. For the first time to our knowledge, we have measured the size dependences of the SiV fluorescence emission rate and the Raman scattering intensity from individual diamond particles in the range from 200 to 450 nm. The obtained dependences reveal a sequence of peaks, which we explicitly associate with specific multipole resonances. The results are in agreement with our theoretical analysis and highlight the potential of intrinsic optical resonances for developing nanodiamond-based lasers and single-photon sources.

The properties of semiconductors can be crucially impacted by midgap states induced by dopants, which can be native or intentionally incorporated in the crystal lattice. For Bernal-stacked bilayer graphene (BLG), which has a tunable bandgap, the existence of midgap states induced by dopants has been conjectured, but never confirmed experimentally. Here, we report scanning tunneling microscopy and spectroscopy results, supported by tight-binding calculations, that demonstrate the existence of midgap states in BLG. We show that the midgap state in BLG -- for which we demonstrate gate-tunability -- appears when the dopant is hosted on the non-dimer sublattice sites. We further evidence the presence of narrow resonances at the onset of the high energy bands (valence or conduction, depending on the dopant type) when the dopants lie on the dimer sublattice sites. These results suggest that dopants/defects can play an important role in the transport and optical properties of multilayer graphene samples, especially at energies close to the band extrema., Includes supplementary material

We theoretically study the structural and electronic properties of a molecular conductor, α-(BEDT-TTF)2I3, using first-principles density-functional theory calculations, especially in its low-temperature charge-ordered state at ambient pressure. We apply a hybrid functional approach and compare the results with a conventional exchange-correlation functional within the generalized gradient approximation. By performing structural optimization, we found a stable charge-ordered solution for the former, in contrast to the latter approach where the magnitude of the charge imbalance becomes considerably small compared to that when the experimental structure is adopted. The electronic band structure near the Fermi level, with and without structural optimization, as well as the molecule-dependent local density of states of the charge-ordered state are discussed.

In the framework of the microscopic Bogoliubov–de Gennes theory, we investigate the vortex and skyrmionic states as well as phase transitions between them in mesoscopic superconducting squares with coexisted spin-triplet (p-wave) and spin-singlet (s-wave and d-wave) pairing symmetries. By choosing different chemical potential $$\mu $$ , the influence of next-nearest-neighbor hopping $$t'$$ on the evolution of mixed pairing orders is mainly discussed. For the case of small $$|\mu |$$ in which the triplet $$p_{x}\pm {i}p_{y}$$ -wave symmetry is dominant, the skyrmionic structures containing the vortex-antivortex pair become stable at appropriate external magnetic flux. The transitions between different coreless skyrmionic modes can take place with tuning $$t'$$ . For further enlarged $$|\mu |$$ , the singlet s-wave component is gradually enhanced in the present mescoscopic sample. We can obtain the transitions between different multivortex states and between the multivortex and skyrmionic states when $$t'$$ is varied. The corresponding zero-energy local density of states and the relative phase differences are provided to display different topological defects.

Local phenomena of materials generate strongly localized electromagnetic wave, called thermally excited evanescent waves, in the long-wavelength infrared spectra range. In order to directly detect the spectroscopic information of the thermally excited evanescent wave, we have developed passive near-field optical spectroscopy (SNOS) based on grating-based spectroscopy. Using the passive SNOS, we achieved detecting the thermally excited evanescent waves on an Au/SiC micropattern sample at different wavelengths. The detected near-field signals were well consistent with the electromagnetic local density of states of SiC and Au. Passive SNOS will be a powerful tool for numerous spectroscopic applications including a surface chemical analysis.

Using the tight-binding approach, we investigate the electronic and magneto-optical properties of bilayer phosphorene quantum dots (BLPQDs) in the presence of perpendicular electric and magnetic fields. The magneto-energy spectra of the BLPQDs exhibit Aharonov–Bohm oscillations. The period and the amplitude of the oscillation decrease with the size of the BLPQDs. An oscillatory behavior of the local density of states (LDOS) versus the magnetic field is observed, as well as the appearance of the spatial Aharonov–Bohm oscillations in the LDOS. In the absence of the electric field, there exists an s-fold degeneracy (s absolutely flat bands at exactly zero energy) arising from the edge-mode states, where s is the smaller value between M and N, where M and N are the number of phosphorus atoms along the x and y axis, respectively, in a rectangular BLBPQD. The absorption spectra of the BLPQDs are obtained for both in-plane and out-of-plane polarizations. Compared with the absorption spectra of graphene dots, the absorption of an out-of-plane polarization of the incident light is high compared to that of in-plane polarizations. On the other hand, the absorption spectra due to in-plane polarizations are almost the same in the case of graphene, whereas they are considerably different in BLBPQDs. Importantly, the appearance of several sharp and high absorption peaks in the near-infrared (NIR) range dictates the BLBPQDs for application and development of bioimaging, biomedicine and drug delivery technology. More importantly, both the location and intensity of these NIR peaks depend characteristically on the orientation of the polarization of the incident light, which can be desirably tuned by the simultaneous engineering of magnetic and electric fields. Such unique advantage of the anisotropic optical feature enables a new degree of freedom for achieving novel polarization-dependent photonic devices. The dual magnetic and electric field tunable optical and electrical features of the BLPQDs are expected to have important consequences for the development of multifunctional magneto-optoelectronic devices and provide insight into the applicability of quantum photopic technologies based on BLBPQDs.

International audience; Rock-salt lead selenide nanocrystals can be used as building blocks for large scale square superlattices via two-dimensional assembly of nanocrystals at a liquid-air interface followed by oriented attachment. Here we report Scanning Tunneling Spectroscopy (STS) measurements of the local density of states of an atomically coherent superlattice with square geometry made from PbSe nanocrystals. Controlled annealing of the sample permits the imaging of a clean structure and to reproducibly probe the band gap and the valence hole and conduction electron states. The measured band gap and peak positions are compared to the results of optical spectroscopy and atomistic tight-binding calculations of the square superlattice band Electronic properties of atomically coherent square PbSe nanocrystal superlattice resolved by Scanning Tunne structure. In spite of the crystalline connections between nanocrystals that induce significant electronic couplings, the electronic structure of the superlattices remains very strongly influenced by the effects of disorder and variability.

The Hamiltonian size reduction method or the mode space method applicable to general heterogeneous structures is developed in this work. The effectiveness and accuracy of the method are demonstrated for four example devices, such as GaSb/InAs tunnel field effect transistors (FETs), MoTe 2/SnS 2 bilayer vertical FETs, InAs nanowire FETs with a defect, and Si nanowire FETs with rough surfaces. The Hamiltonian size is reduced to around 5% of the original full-Hamiltonian size without losing the accuracy of the calculated transmission and local density of states in a practical sense. The method developed in this work can be used with any type of Hamiltonian and can be applied virtually to any hetero-structure, and so it has the potential to become an enabling technology for efficient simulations of hetero-structures.

Doped diamond electrodes have attracted significant attention for decades owing to their excellent physical and electrochemical properties. However, direct experimental observation of dopant effects on the diamond surface has not been available until now. Here, low-temperature scanning tunneling microscopy is utilized to investigate the atomic-scale morphology and electronic structures of (100)- and (111)-oriented boron-doped diamond (BDD) electrodes. Graphitized domains of a few nanometers are shown to manifest the effects of boron dopants on the BDD surface. Confirmed by first-principles calculations, local density of states measurements reveal that the electronic structure of these features is characterized by in-gap states induced by boron-related lattice deformation. The dopant-related graphitization is uniquely observed in BDD (111), which explains its electrochemical superiority over the (100) facet. These experimental observations provide atomic-scale information about the role of dopants in modulating the conductivity of diamond, as well as, possibly, other functional doped materials.

We use scanning gate microscopy to study electron transport through an open, gate-defined resonator in a Ga(Al)As heterostructure. Raster-scanning the voltage-biased metallic tip above the resonator, we observe distinct conductance modulations as a function of the tip position and voltage. Quantum-mechanical simulations reproduce these conductance modulations and reveal their relation to the partial local density of states in the resonator. Our measurements illustrate the current frontier between possibilities and limitations in imaging the local density of states in buried electron systems using scanning gate microscopy., Physical Review Research, 3 (3), ISSN:2643-1564

Shubnikov-de Haas oscillations are the most well-known magneto-oscillations in transport measurements. They are caused by Landau quantization of two-dimensional (2D) electron systems in the presence of a magnetic field. Here we demonstrate that a scanning tunneling microscope (STM) can locally measure similar magneto-oscillations in 2D systems. In Landau level spectroscopy measurements with fine magnetic-field increments, we observed fixed-energy magnetic-field-dependent oscillations of the local density of states. From the measured tunneling magneto-conductance oscillations acquired by STM, energy-momentum dispersions and Berry phases of a monolayer, Bernal-stacked bilayer, and ABC-stacked trilayer graphene were obtained. The reported method is applicable to a wide range of materials because it can obtain the magneto-oscillations of 2D systems larger than the magnetic length; importantly, it also requires no gate electrode.

The isostructural and isoelectronic transition-metal-dichalcogenides 1T-TaS2 and 1T-TaSe2 are layered materials with intricate electronic structures. Combining the molecular beam epitaxy growth, scanning tunneling microscopy measurements and first-principles calculations, we prepare monolayer 1T-TaS2 and TaSe2 and explore their electronic structures at the atomic scale. Both two-dimensional (2D) compounds exhibit commensurate charge density wave phase at low temperature. The conductance mapping identifies the contributions from different Ta atoms to the local density of states with spatial and energy resolution. Both 1T-TaS2 and 1T-TaSe2 monolayer are shown to be insulators, while the former has a Mott gap and the latter is a regular band insulator.

Fluorescent molecules are highly susceptible to their local environment. Thus, a fluorescent molecule near a plasmonic nanoparticle can experience changes in local electric field and local density of states that reshape its intrinsic emission spectrum. By avoiding ensemble averaging while simultaneously measuring the super-resolved position of the fluorophore and its emission spectrum, single-molecule hyperspectral imaging is uniquely suited to differentiate changes in the spectrum from heterogeneous ensemble effects. Thus, we uncover for the first time single-molecule fluorescence emission spectrum reshaping upon near-field coupling to individual gold nanoparticles using hyperspectral super-resolution fluorescence imaging, and we resolve this spectral reshaping as a function of the nanoparticle/dye spectral overlap and separation distance. We find that dyes bluer than the plasmon resonance maximum are red-shifted and redder dyes are blue-shifted. The primary vibronic peak transition probabilities shift to favor secondary vibronic peaks, leading to effective emission maxima shifts in excess of 50 nm, and we understand these light-matter interactions by combining super-resolution hyperspectral imaging and full-field electromagnetic simulations.

We studied surface corrosion effects on Zr and UN using first-principles density functional theory-based calculations. We focused on the energetics of Zr (1000), UN (100) and UN (110) surfaces, exposed to water and oxygen. Average distance between the terminating UN (100) surface and bulk increases due to the presence of additional oxygen content, as well as for the (110) surface. The average distance between the surface layer and bulk is greater in the (110) surface than the (100) surface after water adsorption. Oxygen concentration determines whether H2 or oxynitrde is formed on the (110) surface. Local density of states and partial charge density show the bonding between the UN surfaces and adsorbates. From an electronic energy of −2 eV to the Fermi level, the majority of electrons are found to be localized around U atoms. Electron localization function calculations further reveal the corrosion mechanism details.

Interface engineering of CdS/CZTS(Se) is an important aspect of improving the performance of buffer/absorber heterojunction combination. It has been demonstrated that the crossover phenomenon due to the interface recombination can be drastically eliminated by interface modification. Therefore, in-depth studies across the CdS/CZTS(Se) junction properties, as well as effective optimization processes, are very crucial for achieving high-efficiency CZTSSe solar cells. Here, we present a comprehensive study on the effects of soft-baking (SB) temperature on the junction properties and the corresponding optoelectronic and interface-structural properties. Based on in-depth photoemission studies corroborated with structural and composition analysis, we concluded that interdiffusion and intermixing of CZTSSe and CdS phases occurred on the Cu-poor surface of CZTSSe at elevated SB temperatures, and the interface dipole moments induced by electrostatic potential fluctuation were thus significantly eliminated. In contrast, with low SB temperature, the CdS/CZTSSe heterojunction revealed very sharp interface with very short interdiffusion, forming interface dipole moments and drastically deteriorating device performance. These post thermal treatments also significantly suppress defect energy level of interface measured by admittance spectroscopy from 294 to 109 meV due to CdS/CZTSSe interdiffusion. Meanwhile, the interdiffusion effects on the shift of valence band maximum, conduction band minimum and band offset across the heterojunction of thermally treated CdS/CZTSSe interface are spatially resolved at the atomic scale by measuring the local density of states with cross-sectional scanning tunneling microscopy and spectroscopy. A significant enhancement in the power conversion efficiency from 4.88% to 8.48% is achieved by a facile interface engineering process allowing a sufficient intermixing of CdS/Cd and CZTSSe/Se phases without detrimental recombination centers.

The photoluminescence quantum yield characterizes the performance of emitters for applications in optical devices, as reporters or probes in material and analytical sciences, and for sensing applications. Quantum yield measurements are challenging for luminescent molecules and nanocrystals immobilized in thin films for many sensor applications, particularly if spatially resolved quantitative luminescence information is desired. We show here that a Drexhage-type experiment, where a silver-coated millimeter-sized sphere is used to modify the local density of states, can provide an elegant approach to counter this challenge. As a representative example of the potential of this method, we measure the pH-dependent photoluminescence quantum yield of fluorescein isothiocyanate bound to a thin mesoporous silica film. The results were compared with those of the studies on the pH dependence of the same dye in solution. We found that our approach can link single fluorophore studies to ensemble measurements and pave ...

Nanoscale heterostructure engineering is the main target for the development of optoelectronic devices. In this sense, a precise knowledge of local electronic response after materials processing is...

Flower defect in graphene is observed in the experiment and can be generated by electron beam exposure. In this work, we investigate the spin-polarized transport through zigzag graphene nanoribbons containing a flower defect by using nonequilibrium Green’s functions in combination with the density functional theory. We found high spin polarization in Zigzag Graphene Nanoribbon (ZGNR) with a flower defect. The dependence of spin polarization on the width of ZGNR is also studied. The underlying mechanism is analyzed through the flower defect–induced different distributions of spin up and spin down local density of states.

In this paper, we study electrical properties of AB-stacked bilayer graphene-DNA hybrid nanostructure connected to two semi-infinite monolayer graphene nanoribbon leads. We investigated two types of devices with both zigzag and armchair edges and the results are evaluated by replacing the junction position. Our calculations are based on the tight-binding approximation and the Green's function method in which the electrical transmission, the density of states (DOS) and the local density of states (LDOS) are calculated numerically. In the metallic bilayer graphene device via creating a nanopore and translocating the DNA molecule, the electron transmission reduces and an energy gap opens, so that, the metal-semiconductor phase transition occurs. Also, our results indicate that the model AC leads to a more considerable energy gap than model AB. Our results have important applications in DNA sensing.

We studied the valley dependent transport in a T junction consisting of an armchair lead and two zigzag leads. Electrons transmitted from the armchair lead to the two outgoing zigzag leads can be valley polarized. When the two outgoing leads have different widths, electrons are pushed into the wider lead and as a result, the valley polarization of the current in the narrow lead is enhanced with an oscillatory dependence on energy. The oscillation pattern is determined by the widths of the two zigzag leads. We analyzed the total local density of states of the device region of the junction and cannot find features that attribute this enhancement to quasi-bound state formation.

By applying scanning tunneling microscope (STM), we directly observed the coexistence of superconductivity and Moire pattern on Se terminated surface of PbTaSe2. Two different Moire periodicities with 9.7 and 24 times of the crystal lattice were observed and the Moire patterns showed distinct characters at different sample biases. Scanning tunneling spectroscopy (STS) studies revealed that the electronic structures were greatly modified by the Moire patterns, and Moire patterns with different manifestations had distinct electronic structures outside the superconducting (SC) gap. The main difference of the electronic structures of the bright and dark area of the Moire pattern is a 1 meV energy shift of the peak positions on the local density of states (LDOS). We argued that the interlayer chemical bond and the multiple rotation conditions at the surface were responsible for the various manifestations of Moire patterns and the great modifications of electronic structures. On surfaces with different Moire patterns, the SC gaps are only slightly changed and remains homogeneous across the surfaces, which indicates that the proximity effect is the origin of superconductivity on Se surfaces.

Simulations based on solving the Kohn-Sham (KS) equation of density functional theory (DFT) have become a vital component of modern materials and chemical sciences research and development portfolios. Despite its versatility, routine DFT calculations are usually limited to a few hundred atoms due to the computational bottleneck posed by the KS equation. Here we introduce a machine-learning-based scheme to efficiently assimilate the function of the KS equation, and by-pass it to directly, rapidly, and accurately predict the electronic structure of a material or a molecule, given just its atomic configuration. A new rotationally invariant representation is utilized to map the atomic environment around a grid-point to the electron density and local density of states at that grid-point. This mapping is learned using a neural network trained on previously generated reference DFT results at millions of grid-points. The proposed paradigm allows for the high-fidelity emulation of KS DFT, but orders of magnitude faster than the direct solution. Moreover, the machine learning prediction scheme is strictly linear-scaling with system size.

Structural and electronic properties of bilayer graphene films and nanostructures obtained through the graphitization of SiC(0001) were investigated in this work using scanning tunneling microscopy and spectroscopy. We report on the observation of triangular nanostructures which result from extended stacking faults in the SiC substrate and their effects on graphene layers that are formed on top of them. Spectroscopic measurements revealed distinct electronic responses as a function of the local hydrogen intercalation. Spectroscopic signatures ranging from single- to double-layer graphene, as well as intermediate states were observed as a consequence of the (in)complete hydrogen intercalation process. High resolution topographic scanning tunneling microscopy images at resonant bias voltages inside triangular nanostructures reveal that the bottom layer of the bilayer graphene film is still bonded to the substrate. Therefore, the triangular nanostructures present edges and facets with the coexistence of carbon atoms in sp3 and sp2 hybridizations. Using atomistic calculations we have modeled the local density of states of these objects reproducing their electronic response. The generation of regions with distinct electronic responses is potentially interesting for high-density data storage with hidden bit capabilities., The authors acknowledge experimental support by the Brazilian Synchrotron Light Laboratory (LNLS). Financial support was provided by CNPq, CAPES, FAPEMIG and INCT-Nano-Carbono. This work was partially financed by the Project FIS2016-76617-P of the Spanish Ministry of Economy and Competitiveness MINECO, the Basque Government under the ELKARTEK project (SUPER), and the University of the Basque Country (Grant No. IT-756-13).

The Dirac-like cones underlie many unique properties of photonic crystals (PhCs). This paper aims to design fabrication-friendly PhCs with Dirac-like cones for transverse magnetic (TM) modes and transverse electric (TE) modes at different specific frequencies. By maximizing the minimum of a collection of the local density of states corresponding to different judiciously selected sources, this paper demonstrates that Dirac-like cones formed by the degeneracy of a doubly degenerate mode and a single mode at different desired frequencies are successfully obtained. The exotic wave manipulation properties associated with Dirac-like cones, such as cloaking, wavefront shaping and tunneling through bent channels, are exhibited based on the optimized structures. This paper also demonstrates that the proposed method could be used for the design of PhCs with one Dirac-like cone at ω , and one monopolar band at 2 ω at the Γ point, and PhCs with third order Dirac-like cones, which have potential applications in nonlinear optics. All topological patterns of the optimized PhCs are reported and have regular and smooth features, meaning they can be readily fabricated.

Synthesis of atomically pure carbon nanotubes (CNT) is difficult. Often, defects form in the structures consisting of carbon rings of non-uniform size and shape. Employing density functional theory combined with a Greens function scattering approach; we investigate the electronic properties of defective CNTs. We use the sculpturene method to form CNTs with a range of defect types, consisting of four-, five-, seven-, eight-, and ten-atom rings. We find that these defects have a non-trivial effect on the transport, often leading to decreased conductance, but also effecting the band-gap (Eg). We also plot the local density of states around the Fermi energy for a range of systems and generally find that higher levels of disorder cause a greater degree of localisation, which helps to explain the electronic properties.

Among the recent achievements of sodium-ion battery (SIB) electrode materials, hybridization of two-dimensional (2D) materials is one of the most interesting appointments. In this work, we propose to use the 2D hybrid composites of SnS2 with graphene or graphene oxide (GO) layers as the SIB anode, based on the first-principles calculations of their atomic structures, sodium intercalation energetics, and electronic properties. The calculations reveal that a graphene or GO film can effectively support not only the stable formation of a heterointerface with the SnS2 layer but also the easy intercalation of a sodium atom with low migration energy and acceptable low volume change. The electronic charge-density differences and the local density of states indicate that the electrons are transferred from the graphene or GO layer to the SnS2 layer, facilitating the formation of a heterointerface and improving the electronic conductance of the semiconducting SnS2 layer. These 2D hybrid composites of SnS2/G or GO ar...

We study the spin-polarized transport induced by photoirradiation in zigzag silicene nanosystem, based on tight-binding approach, Green's function method and Landauer–Buttiker formula. By applying strong circular polarized light, silicene nanosystem can be transformed into a quantum Hall insulator, where the spin-down subband is gapped while the spin-up subband persists gapless edge state. Therefore, the dc conductance is dominated by the spin-up electrons, and the spin polarization can reach almost 100% around the Fermi energy. The spatial-resolved local density of states confirm that the spin-up electrons transport at two edges of the nanosystem in opposite current directions. Furthermore, because of the topological origin of the edge state, the spin-polarized transport is very robust against the size change of the nanosystem.

Metallic nanostructures can strongly influence the fluorescence emission process as they profoundly alter the field and local density of states at the nanoscale. So far, a number of studies use nanostructures with metallic nanogaps to control and enhance the fluorescence emission. Metallic nanoparticles on mirror (NPoM) nanostructures have drawn intense research interest due to their ease of fabrication and ability to form metallic nanogaps down to subnanometers. However, the emitters are usually randomly located in the whole gap region which prohibits a precise control of the fluorescence emission and a quantitative analysis. This study reports an NPoM nanostructure composed of gold nanorods (NRs) and a gold film with double polymethyl methacrylate (PMMA) layers as the spacer that sandwiches the molecules. With a two-step spin coating method, the thickness of each PMMA layer can be precisely controlled by varying the spin coating parameters. Therefore, the vertical position of the emitters in the nanogap can be adjusted as expected. This double layer structure results in a tunable and enhanced fluorescence emission. The experimental results show that the enhancement of the fluorescence intensity and the spontaneous decay rate is sensitive to the vertical position of the emitters and the thickness of the PMMA spacer, and unveil the complete evolution from fluorescence enhancement to quenching as the emitter approaches either the gold NR or the gold film, which are well predicted by the theoretical simulation results. The proposed results may have promising applications associated with the modification of spontaneous emission in plasmonic nanogap structures.

The spectral functions are studied in conjunction with the dyadic Green’s functions for various media. The dyadic Green’s functions are found using the eigenfunction expansion method for homogeneous, inhomogeneous, periodic, lossless, lossy, and anisotropic media, guided by the BlochFloquet theorem. For the lossless media cases, the spectral functions can be directly related to the photon local density of states, and hence, to the electromagnetic energy density. For the lossy case, the spectral function can be related to the field correlation function. Because of these properties, one can derive properties for field correlations and the Langevin-source correlations without resorting to the fluctuation dissipation theorem. The results are corroborated by the fluctuation dissipation theorem. An expression for the local density of states for lossy, inhomogeneous, and dispersive media has also been suggested. PACS numbers: Valid PACS appear here

The spatial dependence of the local density of states at the Fermi level of a stepped Au(111) surface is studied by thermovoltage scanning tunneling microscopy. The periodicity of the standing waves is not given by ${\ensuremath{\lambda}}_{\text{F}}/2=1.8$ nm, as expected based on the band diagram of Au(111), but rather varies between 1.5 and 2.1 nm depending on the exact width of the terraces. This counterintuitive result can be understood by considering the superposition of incident and reflected electron waves, which have a periodicity of ${\ensuremath{\lambda}}_{\text{F}}/2$. The change in periodicity is a direct consequence of the decaying nature of the Friedel oscillations.

Iterative Green's function, based on cyclic reduction of block tridiagonal matrices, has been the ideal algorithm, through tight-binding models, to compute the surface density-of-states of semi-infinite topological electronic materials. In this paper, we apply this method to photonic and acoustic crystals, using finite-element discretizations and a generalized eigenvalue formulation, to calculate the local density-of-states on a single surface of semi-infinite lattices. The three-dimensional (3D) examples of gapless helicoidal surface states in Weyl and Dirac crystals are shown and the computational cost, convergence and accuracy are analyzed., 7 pages, 4 figures

We apply two of the most popular tools used in condensed matter physics to the topological characterization of dispersion bands in photonic crystals. We focus on a family of crystals formed by 2D cylinders placed as concentric hexagons in a triangular lattice. Applying the recently developed theory of "Topological Quantum Chemistry" [1] , we show that the symmetry of the Bloch’s wave functions can be used to determine the position of localized photonic Wannier functions — which can be moved in the unit cell by engineered band inversions. We can show this movement performing a discrete calculation of Wilson loops along one k -direction in the Brillouin zone [2] . Additionally, there is an isolated set of bands which does not admit a well-localized Wannier description, i.e topological bands. This represents a physical instance of "fragile" topology in photonic crystals [3] . Finally, we explore the local density of states (LDOS), and density of states (DOS) [4] . Through this analysis, we can analyze the distribution of the density of states for sets of isolated bands to conclude that the positions of their maximum concentration are related to the localization of maximally localized photonic Wannier functions. We expect that this approach will help the community to understand topological effects in photonic crystals in a more intuitive way.

In a Type II superconductor, the vortex core behaves like a normal metal. Consequently, the single-particle density of states in the vortex core of a conventional Type II superconductor remains either flat or (for very clean single crystals) exhibits a peak at zero bias due to the formation of Caroli-de Gennes-Matricon bound state inside the core. Here we report an unusual observation from scanning tunneling spectroscopy measurements in a weakly pinned thin film of the conventional s-wave superconductor a-MoGe, namely, that a soft gap in the local density of states continues to exist even at the center of the vortex core. We ascribe this observation to rapid fluctuation of vortices about their mean position that blurs the boundary between the gapless normal core and the gapped superconducting region outside. Analyzing the data as a function of magnetic field we show that the variation of fluctuation amplitude as a function of magnetic field is consistent with quantum zero-point motion of vortices., arXiv admin note: substantial text overlap with arXiv:1905.01045

Phase singularities appear ubiquitously in wavefields, regardless of the wave equation. Such topological defects can lead to wavefront dislocations, as observed in a humongous number of classical wave experiments. Phase singularities of wave functions are also at the heart of the topological classification of the gapped phases of matter. Despite identical singular features, topological insulators and topological defects in waves remain two distinct fields. Realising 1D microwave insulators, we experimentally observe a wavefront dislocation – a 2D phase singularity – in the local density of states when the systems undergo a topological phase transition. We show theoretically that the change in the number of interference fringes at the transition reveals the topological index that characterises the band topology in the insulator., Topological properties are defined from singularities of the wave functions delocalised in the bulk of the material, but most experimental manifestations remain indirect. Here, the authors observe a wavefront dislocation as a direct evidence of the phase singularity of the delocalised wave functions in 1D microwave photonic insulators.

Silicon (Si) donors in GaAs have been the topic of extensive studies since Si is the most common and well understood n-type dopant in III-V semiconductor devices and substrates. The indirect bandgap of AlAs compared to the direct one of GaAs leads to interesting effects when introducing Si dopants. Here we present a study of cross-sectional scanning tunneling microscopy (X-STM) and density functional theory (DFT) calculations to study Si donors in AlAs at the atomic scale. Based on their crystal symmetry and contrast strengths, we identify Si donors up to four layers below the (110) surface of AlAs. Interestingly, their short-range local density of states (LDOS) is very similar to Si atoms in the (110) surface of GaAs. Additionally we show high-resolution images of Si donors in all these layers. For empty state imaging, the experimental and simulated STM images based on DFT show excellent agreement for Si donor up to two layers below the surface.

We prove some local estimates on the trace of spectral projectors for random Schrodinger operators restricted to cubes \(\Lambda \subset \mathbb {R}^d\). We also present a new proof of the spectral averaging result based on analytic perturbation theory. Together, these provide another proof of the Wegner estimate with an explicit form of the constant and an alternate proof of the Birman-Solomyak formula. We also use these results to prove the Lipschitz continuity of the local density of states function for a restricted family of random Schrodinger operators on cubes \(\Lambda \subset \mathbb {R}^d\), for \(d \geqslant 1\). The result holds for low energies without a localization assumption but is not strong enough to extend to the infinite-volume limit.

In previous reports, experimental studies have shown that both thermal stability and strength can be controlled by grain boundary (GB) segregation. In this study, we investigate the segregation behavior of solute (Ti, Ag and Ta) atoms to low/high-angle symmetric tilt grain boundaries (STGBs) of W using density functional theory (DFT) calculations and supported by TEM experiments. We found no segregation preference for Ti or Ta at low-angle STGBs; however, they exhibit a slight segregation tendency to the core of high-angle STGBs. In contrast, Ag is more prone to segregate in and all around the GB plane. We estimated the mechanical and electronic contributions to solution energy and found that the electronic contribution is dominant. Furthermore, the role of d − valence electrons of solute and W atoms, was analyzed using the local density of states (PDOS). We found that substantial d − valence electrons hybridization in the case of Ta plays an important role in stabilizing W-Ta bonds, while the anisotropic nature of W-Ti bond contributes to stabilize surrounding W atoms. Charge transfer analysis revealed that Ti and Ta lose electrons to W atoms. Contrary to the electronegativity rule, Ag atoms gain charge from neighboring W atoms and excellent s − s hybridization may explain the increased GB segregation of Ag atoms.

Fermion bound states at the center of the core of a vortex in a two-dimensional superconductor are investigated by analytically solving the Bogoliubov–de Gennes equations in a magnetic field. The metallic surface states of a strong topological insulator become superconducting via proximity effect with an s-wave superconductor. Due to the magnetic field, the states undergo Landau quantization. A zero-energy Majorana state arises for the Landau level $$n=0$$ together with an equally spaced ( $$\Delta ^2_{\infty }/E_F$$ ) sequence of fermion excitations. The spin-momentum locking due to the Rashba spin–orbit coupling is key to the formation of the Majorana state. Extending previous results in zero magnetic field, we present analytical expressions for the energy spectrum and the wave functions of the bound states in a finite magnetic field. The solutions consist of harmonic oscillator wave functions (associated Laguerre polynomials) times a function that falls off exponentially with distance $$\rho $$ from the core of the vortex as $$\exp [-\int _0^{\rho } d\rho ' \Delta (\rho ')/v_F]$$ . An analytic expression for the local density of states (LDOS) for the bound states is obtained. It depends on two length scales, $$1/k_F$$ and the magnetic length, $$l_H$$ , and the angular momentum index $$\mu $$ . The particle-hole symmetry is broken in the LDOS as a consequence of the spin–orbit coupling and the chirality of the vortex. We also discuss the spin polarization of the bound states.

Spectromicroscopy techniques with fast electrons can quantitatively measure the optical response of excitations with unrivalled spatial resolution. However, owing to their inherently scalar nature, electron waves cannot access the polarization-related quantities. Despite promising attempts based on the conversion of concepts originating from singular optics (such as vortex beams), the definition of an optical polarization analogue for fast electrons has remained an open question. Here we establish such an analogue using the dipole transition vector of the electron between two well-chosen singular wave states. We show that electron energy loss spectroscopy allows the direct measurement of the polarized electromagnetic local density of states. In particular, in the case of circular polarization, it directly measures the local optical spin density. This work establishes electron energy loss spectroscopy as a quantitative technique to tackle fundamental issues in nano-optics, such as super-chirality, local polarization of dark excitations or polarization singularities at the nanoscale. The functionality of electron energy loss spectroscopy can be extended to include a polarization analogue constructed via the dipole transition vector between two electronic states, bringing it closer to its optical counterpart.

We theoretically express quantum transport at Dirac points via graphene quantum billiard as a non-magnetic material to connect metallic leads. Our results indicate that the quantum billiard of graphene is similar to a resonant tunnelling device. The centerpiece size and the Fermi energy of the graphene quantum billiard play an important role in the resonant tunnelling. In graphene, change of carrier density can affect plasmon polaritons. At the Dirac point, the conductivity of graphene depends on the geometry, so that the conduction of the evanescent modes is close to the theoretical value of 4e2/πh (where Planck's constant and the electron charge are denoted by h and e, respectively.). This transport property can be used to justify chaotic quantum systems and ballistic transistors. Our theoretical results demonstrate that the local density of state of the graphene sheet for EL = ER = 0 is larger than EL = ER = t (where EL (ER) is onsite energy of the left (right) metallic lead) unlike the current obtained from the calculations.

Based on a model Hamiltonian including the on-site repulsive interaction among the electrons, we investigate the spatial configurations of mixed spin-triplet (dominant p-wave) and spin-singlet (coexisted s-wave and d-wave) pairing symmetries in bulk superconducting systems by numerically solving the Bogoliubov–de Gennes equations. The influences of the Zeeman field and the next-nearest-neighbor hopping on the order-parameter modulations are mainly discussed. Besides the quantized vortex states with point-like cores, the unclosed vortex chains and enclosed skyrmionic modes can remain stable in the present bulk sample. Furthermore, nonuniform patterns of the vortex lattice and the elliptical-like vortices can be obtained at appropriate applied flux. The corresponding zero-energy local density of states as well as the relative phase differences useful to display the skyrmionic texture are provided

The discovery of interaction-driven insulating and superconducting phases in moir\'e van der Waals heterostructures has sparked considerable interest in understanding the novel correlated physics of these systems. While a significant number of studies have focused on twisted bilayer graphene, correlated insulating states and a superconductivity-like transition up to 12 K have been reported in recent transport measurements of twisted double bilayer graphene. Here we present a scanning tunneling microscopy and spectroscopy study of gate-tunable twisted double bilayer graphene devices. We observe splitting of the van Hove singularity peak by ~20 meV at half-filling of the conduction flat band, with a corresponding reduction of the local density of states at the Fermi level. By mapping the tunneling differential conductance we show that this correlated system exhibits energetically split states that are spatially delocalized throughout the different regions in the moir\'e unit cell, inconsistent with order originating solely from onsite Coulomb repulsion within strongly-localized orbitals. We have performed self-consistent Hartree-Fock calculations that suggest exchange-driven spontaneous symmetry breaking in the degenerate conduction flat band is the origin of the observed correlated state. Our results provide new insight into the nature of electron-electron interactions in twisted double bilayer graphene and related moir\'e systems., Comment: 24 pages, 5 figures

In this work we investigate the electromagnetic local density of states (LDOS) near a twisted bilayer graphene (TBG) deposited over a general isotropic substrate. The band structure of the TBG is calculated within a tight-binding framework, and then used to determine the TBG's conductivity. The latter presents a nontrivial dependence upon the angle of twist, which shows up in the LDOS, allowing for a moir\'e pattern-dependent quantum emission. For some specific twist angles we show that it is possible to either enhance or decrease the LDOS by an order of magnitude at selected frequencies when compared to the monolayer. This impressive variation is explained in terms of the presence/absence of well defined surface plasmon polaritons (SPPs). Altogether our findings demonstrate that TBG is an alternative, versatile material platform for controlling spontaneous emission and enhancing light-matter interactions, and pave the way for further studies and applications of quantum emission in the emerging class of two-dimensional moir\'e materials.

We test the valley-filtering capabilities of a quantum dot inscribed by locally straining an $\ensuremath{\alpha}\text{\ensuremath{-}}{\mathcal{T}}_{3}$ lattice. Specifically, we consider an out-of-plane Gaussian bump in the center of a four-terminal configuration and calculate the generated pseudomagnetic field having an opposite direction for electrons originating from different valleys, the resulting valley-polarized currents, and the conductance between the injector and collector situated opposite one another. Depending on the quantum dot's width and width-to-height ratio, we detect different transport regimes with and without valley filtering for both the $\ensuremath{\alpha}\text{\ensuremath{-}}{\mathcal{T}}_{3}$ and dice lattice structures. In addition, we analyze the essence of the conductance resonances with a high valley polarization in terms of related (pseudo-) Landau levels, the spatial distribution of the local density of states, and the local current densities. The observed local charge and current density patterns reflect the local inversion symmetry breaking by the strain, besides the global inversion symmetry breaking due to the scaling parameter $\ensuremath{\alpha}$. By this way we can also filter out different sublattices.

International audience; We show numerically how lanthanide-doped upconverter nanocrystals inserted at optimal positions in adjustable and smooth 2D plasmonic arrays may change and potentially control the whole UV-visible-NIR reflectance spectra displayed by these structures. The analysis and interplay between a custom-built simple 5-levels 2-electrons quantum model and the bare structure have been studied in depth and revealed very fundamental and interesting physics. Essentially, the largely and selectively enhanced white-light excitation field and optimized local density of states at the location of the emitters favor some energy transfer paths and a redistribution of light energy in a broad spectral range. Interestingly, the extent to which the spectra can be modulated owing to the emission properties of such very robust multilevel emitters may find interesting applications and notably allow increased efficiency of emission in Light Emitting Devices or solar light redistribution and collection in Solar cells, where conversions from one color to any other one play a major role.

Infinite quasiperiodic arrangements in space, such as quasicrystals, are typically described as projections of higher-dimensional periodic lattices onto the physical dimension. The concept of a reference higher-dimensional space, called a superspace, has proved useful in relation to quasiperiodic systems. Although some quantum-mechanical systems in quasiperiodic media have been shown to admit quasiperiodic states, any sort of general Hamiltonian formalism in superspace is lacking to this date. Here, we show how to extend generic quantum-mechanical Hamiltonians to higher dimensions in such a way that eigenstates of the original Hamiltonian are obtained as projections of the Hamiltonian in superspace, which we call the super Hamiltonian. We apply the super Hamiltonian formalism to a simple, yet realistic one-dimensional quantum particle in a quasiperiodic potential without the tight-binding approximation, and obtain continuously labelled eigenstates of the system corresponding to a continuous spectrum. All states corresponding to the continuum are quasiperiodic. We also obtain the Green's functions for continuum states in closed form and, from them, the density of states and local density of states, and scattering states off defects and impurities. The closed form of this one-dimensional Green's function is equally valid for any continuum state in any one-dimensional single-particle quantum system admitting continuous spectrum. With the basis set we use, which is periodic in superspace, and therefore quasiperiodic in physical space, we find that Anderson-localised states are also quasiperiodic if distributional solutions are admitted, but circumvent this difficulty by generalising the superspace method to open boundary conditions (cont'd)., 11 pages, 6 figures. See article for full abstract