Local Density Of States Distribution Research Articles

Bulk-local-density-of-state correspondence in topological insulators

In the quest to connect bulk topological quantum numbers to measurable parameters in real materials, current established approaches often necessitate specific conditions, limiting their applicability. Here we propose and demonstrate an approach to link the non-trivial hierarchical bulk topology to the multidimensional partition of local density of states (LDOS), denoted as the bulk-LDOS correspondence. In finite-size topologically nontrivial photonic crystals, we observe the LDOS partitioned into three distinct regions: a two-dimensional interior bulk area, a one-dimensional edge region, and zero-dimensional corner sites. Contrarily, topologically trivial cases exhibit uniform LDOS distribution across the entire two-dimensional bulk area. Our findings provide a general framework for distinguishing topological insulators and uncovering novel aspects of topological directional band-gap materials, even in the absence of in-gap states.

Spin‐Polarized Edge‐State Transport in L‐Shaped MoS2 Nanodevice Modulated by Exchange Field and Electric Field

Herein, the spin‐polarized edge‐state transport properties in L‐shaped zigzag nanodevice () are investigated based on tight‐binding model, Green's function method, and ballistic transport theory. The spin conductance and spin polarization are calculated by considering external exchange field and electric field. It is shown in the results that the spin conductance is closely related to the configuration of atomic non‐consistence on the edge of . Meanwhile, the spin conductance is determined by the spin channel opening on the edge, which can be modulated by external fields. The configuration characteristic and external fields give rise to effects of sublattice mismatch and spin mismatch on the edge channels, respectively. As a consequence, mismatch effects are actually employed to modulate the spin channel on the edge of , and 100% spin polarization is realized selectively. Moreover, the corresponding local density of states distributions are plotted to reveal the modulation of the spin‐resolved edge‐state and the spin‐filtering effects in the real space. These spin‐polarized edge‐state transport properties in can have potential application in various kinds of spin valves and spin filters in nanocircuits.

Imaging of Plasmonic Chiral Radiative Local Density of States with Cathodoluminescence Nanoscopy.

Chiral light-matter interactions as an emerging aspect of quantum optics enable exceptional physical phenomena and advanced applications in nanophotonics through the nanoscale exploitation of photon-emitter interactions. The chiral radiative properties of quantum emitters strongly depend on the photonic environment, which can be drastically altered by plasmonic nanostructures with a high local density of states (LDOS). Hence, precise knowledge of the chiral photonic environment is essential for manipulating the chirality of light-matter interactions, which requires high resolution chiral characterization techniques. In this work, chiral radiative LDOS distributions of single plasmonic nanostructures that directly govern the chiral radiative spontaneous decay of quantum emitters are imaged at the nanoscale by using cathodoluminescence nanoscopy, enabling precise and highly efficient control of chiral photon emission for chiroptical technologies. Radiative LDOS hot-spots with the chirality larger than 93% are obtained by properly designing chiral plasmonic modes of Au nanoantennas. After fabricating monolayered WSe2 nanodisks (NDs) at chiral radiative LDOS hot-spots and forming ND/Au hybrid nanostructures, the chiral radiative properties of WSe2 NDs are significantly modified, leading to chiral photoluminescence. Our experimental concept and method provide an effective way to characterize and manipulate chiral light-matter interactions at the nanoscale, facilitating future applications in chiral quantum nanophotonics such as single-photon sources and light emission devices.

Deep-Subwavelength Resolving and Manipulating of Hidden Chirality in Achiral Nanostructures.

The chiral state of light plays a vital role in light-matter interactions and the consequent revolution of nanophotonic devices and advanced modern chiroptics. As the light-matter interaction goes into the nano- and quantum world, numerous chiroptical technologies and quantum devices require precise knowledge of chiral electromagnetic modes and chiral radiative local density of states (LDOS) distributions in detail, which directly determine the chiral light-matter interaction for applications such as chiral light detection and emission. With classical optical techniques failing to directly measure the chiral radiative LDOS, deep-subwavelength imaging and control of circular polarization (CP) light associated phenomena are introduced into the agenda. Here, we simultaneously reveal the hidden chiral electromagnetic mode and acquire its chiral radiative LDOS distribution of a single symmetric nanostructure at the deep-subwavelength scale by using CP-resolved cathodoluminescence (CL) microscopy. The chirality of the symmetric nanostructure under normally incident light excitation, resulting from the interference between the symmetric and antisymmetric modes of the V-shaped nanoantenna, is hidden in the near field with a giant chiral distribution (∼99%) at the arm-ends, which enables the circularly polarized CL emission from the radiative LDOS hot-spot and the following active helicity control at the deep-subwavelength scale. The proposed V-shaped nanostructure as a functional unit is further applied to the helicity-dependent binary encoding and the two-dimensional display applications. The proposed physical principle and experimental configuration can promote the future chiral characterization and manipulation at the deep-subwavelength scale and provide direct guidelines for the optimization of chiral light-matter interactions for future quantum studies.

Effect of Donor Building Blocks on the Charge-Transfer Characteristics of Diketopyrrolopyrrole-Based Donor-Acceptor-Type Semiconducting Copolymers.

We investigate the effect of donor (D) building blocks on the charge transportation characteristics of donor (D)-acceptor (A)-type semiconducting copolymers with alternating electron-donating and electron-accepting units to provide a basis for the rational design of high-performance semiconducting polymers. For this purpose, we studied three different diketopyrrolopyrrole (DPP)-based semiconducting copolymers comprising a common dithienyl-DPP [3,6-dithienyl-2,5-diketopyrrolo(3,4-c)pyrrole] and variable donor moieties: phenylene (P)-PDPPTPT, thiophene (T)-PDPP3T, and thienothiophene (TT)-PDPP2T-TT. Structural analysis using grazing incidence X-ray diffraction indicates that all three DPP-based copolymer films have edge-on phases but poor crystallinity of the films, except the PDPP2T-TT copolymer with branched alkyl side chains that are relatively long. The electrical measurements show that the DPP-based copolymer with a TT donor unit has the highest field-effect mobility value of 0.30 cm2/V s. To understand the role of the donor units in DPP-based D-A copolymers, further insight into the charge transportation behavior is realized by analyzing the temperature-dependent transfer curves of the DPP semiconducting copolymer-based field-effect transistors using the Gaussian disorder model. Compared to the DPP-based D-A-type semiconducting copolymer with a P-moiety and shorter-branched alkyl side chains that exhibit a broad distribution in the density of localized states (DOS) and a higher thermal-activated energy for charge hopping, the DPP copolymers with a TT-moiety and longer branched side chains have the narrowest DOS, the lowest activation energy, and thus the highest hole mobility. These results suggest that the higher mobilities obtained from PDPP2T-TT with a TT donor unit can be attributed to the suppressed DOS distribution near the transport level, which mainly originates from the narrowest energy band gap tuned with the orbital couplings of the DPP acceptor and TT donor units.

Correlating electronic and magnetic coupling in large magnetic molecules via scanning tunneling microscopy

In an effort to improve the spin coupling in single-molecule magnets, we rationally designed a new building-block molecule with significantly enhanced spin coupling compared to a previously established molecule. We relate this to a stabilization of aromaticity in the central connecting carbon ring, promoting the spin-polarization mechanism. This correlation between magnetic and electronic properties is supported by bulk measurements as well as submolecularly resolved scanning tunneling microscopy and spectroscopy experiments, where we found distinct differences in the local density of states distribution of the two molecules, especially at the central carbon ring. While the established molecule exhibits localized, spatially decoupled and even switchable states, the improved building block exhibits symmetric local density of states delocalized over the entire molecule, also revealing that this main characteristic electronic property is preserved upon adsorption on a metal surface. Due to their planar geometry, these molecules can serve as model systems for scanning-probe based studies of molecular magnetism.

Self-consistent electronic structure of multiquantum vortices in superconductors at T ≪ Tc

We investigate the multiquantum vortex states in a type-II superconductor in both ‘clean’ and ‘dirty’ regimes defined by impurity scattering rate. Within a quasiclassical approach we calculate self-consistently the order parameter distributions and electronic local density of states (LDOS) profiles. In the clean case we find the low temperature vortex core anomaly predicted analytically by Volovik (1993 JETP Lett. 58 455) and obtain the patterns of LDOS distributions. In the dirty regime multiquantum vortices feature a peculiar plateau in the zero energy LDOS profile, which can be considered as an experimental hallmark of multiquantum vortex formation in mesoscopic superconductors.

Density of States and Its Local Fluctuations Determined by Capacitance of Strongly Disordered Graphene

We demonstrate that fluctuations of the local density of states (LDOS) in strongly disordered graphene play an important role in determining the quantum capacitance of the top-gate graphene devices. Depending on the strength of the disorder induced by metal-cluster decoration, the measured quantum capacitance of disordered graphene can dramatically decrease in comparison with pristine graphene. This is opposite to the common belief that quantum capacitance should increase with disorder. To explain this counterintuitive behavior, we present a two-parameter model which incorporates both the non-universal power law behavior for the ADOS and a lognormal distribution of LDOS. We find excellent quantitative agreements between the model and measured quantum capacitance for three disordered samples in a wide range of Fermi energies. Thus, by measuring the quantum capacitance, we can simultaneously determine the ADOS and its fluctuations. It is the LDOS fluctuations that cause the dramatic reduction of the quantum capacitance.

Metal-to-Insulator Transition and Electron-Hole Puddle Formation in Disordered Graphene Nanoribbons

The experimentally observed metal-to-insulator transition in hydrogenated graphene is numerically confirmed for actual sized graphene samples and realistic impurity concentrations. The eigenstates of our tight-binding model with substitutional disorder corroborate the formation of electron-holepuddles with characteristic length scales comparable to the ones found in experiments. The puddles cause charge inhomogeneities and tend to suppress Anderson localization. Even though, monitoring the charge carrier quantum dynamics and performing a finite-size scaling of the local density of states distribution, we find strong evidence for the existence of localized states in graphene nanoribbons with short-range but also correlated long-range disorder.

Distribution of the local density of states as a criterion for Anderson localization: Numerically exact results for various lattices in two and three dimensions

Numerical approaches to Anderson localization face the problem of having to treat large localization lengths while being restricted to finite system sizes. We show that by finite-size scaling of the probability distribution of the local density of states (LDOS) this long-standing problem can be overcome. To this end we reexamine this method, propose numerical refinements, and apply it to study the dependence of the distribution of the LDOS on the dimensionality and coordination number of the lattice. Particular attention is given to the graphene lattice. We show that the system-size dependence of the LDOS distribution is indeed an unambiguous sign of Anderson localization, irrespective of the dimension and lattice structure. The numerically exact LDOS data obtained by us agree with a log-normal distribution over up to ten orders of magnitude and thereby fulfill a nontrivial symmetry relation previously derived for the nonlinear $\ensuremath{\sigma}$ model.

Determination of the density of localized states in semiconductors from the pre-recombination transient photoconductivity

In this paper we present a new transient photoconductivity (TPC) inversion method for the determination of the density of localized states (DOS) energy distribution g( E) in thin film semiconductor materials with exact matrix solution for g( E). The method, derived from the multiple trapping model, is based on prior determination of the exact transient trap occupation function and applies to the pre-recombination time range of the TPC. It is demonstrated by application to simulated TPC data that high energy resolution can be achieved for the case of continuous DOS distribution, appropriate to amorphous semiconductors, as well as for discrete level DOS such as in crystalline semiconductors.

Modulated photoconductivity method for investigation of band gap states distribution in silicon-based thin films

It has been demonstrated that the well known modulated photocurrent technique can be modified to escape imperfect data in intrinsic parameters of amorphous and microcrystalline silicon films and to simplify measurements of the density of localized states distribution in the band gaps of these semiconductors. The information on the density-of-states distribution can be extracted from temperature dependence measurements of the constant and modulated components of the photoconductivity in the film illuminated by the light modulated with some selected frequencies. The modified method has been applied to microcrystalline hydrogenated silicon films with n- and p-type conduction. The study has demonstrated that the tail of the density-of-states distribution near the valence band of microcrystalline hydrogenated silicon is less steep than that near the conduction band.

Superlattice with two δ-type defects: electronic structure and density of states

The electronic-level structure and local density-of-states (LDOS) distributions are investigated for an infinite GaAs/AlxGa1−xAs superlattice (SL) with two strongly spatially confined defect layers (the so-called δ-defects). The energy spectrum is determined by using a multi-matching procedure within the envelope-function approximation, while the LDOS is computed via combined factorization and direct methods in the framework of the Green's function formalism. In general, insertion of two δ-defects leads to the appearance of two localized states inside the minigaps, whose energy positions and space-charge distributions can be tuned by adjusting the defect parameters. A substantial change in the LDOS distribution is also observed. In particular, the region consisting of n periods between the defected SL layers exhibits—for large enough δ-defect strengths—an energy spectrum of n quasi-discrete resonances within the miniband range, whereas outside this region the LDOS takes the form characteristic of semi-infinite SL's terminated at the defect sites.

Surface states in double-well superlattices: symmetric vs asymmetric interwell coupling

Electronic level structure and the density of states (DOS) of a semi-infinite superlattice (SL), whose period consists of two GaAs quantum-well layers and two AlxGa1−xAs quantum-barrier layers (the so-called double-well basis), are investigated within an envelope-function approximation. A special attention is paid to surface effects due to the SL termination by a substrate or a cladding layer. In particular, the existence and properties of surface-localized states, as well as a consequent modification of the extended states forming SL minibands, are studied for various terminating conditions, determined by the choice of substrate and the sequence of SL layers approaching the SL surface. The resulting surface electronic structure is shown to be critically dependent on the geometry of the SL basis: For symmetrically (asymmetrically) coupled wells, crossing (anticrossing) of SL minibands occurs, which dramatically influences the energy spectrum of SL surface states as well as the local DOS distributions at the SL end.

Density of states of a superlattice with a δ defect in the subsurface region

The effect of a spatially and strongly confined ( δ-like) defect layer on the electronic properties of a semi-infinite superlattice (SL) is investigated, with emphasis placed on local density-of-states (LDOS) modifications at the SL end. The SL energy spectrum and the LDOS distributions, calculated within a modified Kronig–Penney type of approach, are studied for variable δ-defect parameters (i.e. its strength and position with respect to the SL surface) and different SL terminating conditions (depending on the choice of substrate). The possibility of a dramatic redistribution of extended states forming the SL minibands as a result of a single δ-defect sheet insertion in the subsurface SL region is indicated and discussed in detail.

Density of states of superlattices with multiple layers per period

Density-of-states (DOS) investigations of polytype superlattices (SLs), whose period consists of more than two different layers, are presented. Local DOS (LDOS) is computed as a function of both the electron energy and the space coordinate, which illustrates — in a direct way — the spatial localization, over the SL period, of states forming the energy minibands. A few examples of AlGaAs-based polytype SLs with a coupled-well, step-well and δ-doped basis are analysed, indicating that various LDOS distributions within SL minibands can be arranged. By changing the basis geometry, the electronic DOS can either be selectively confined to a particular SL layer or extend over the whole SL period, which provides a means of modifying the spatial overlap of states from different SL minibands and thus, tuning the respective interband transitions. In particular, polytype SLs with similar miniband structures may exhibit essentially different LDOS features and, consequently, the different transport and optical characteristics. Therefore, it is emphasized that — for a particular device application of a complex-basis SL — not only the electronic level structure should be appropriately designed, but also the corresponding space-charge distributions should be carefully considered.

Improved Laplace transform method to determine trap densities from transients: application to ZnO and films

A simple method is proposed for analysis of non-exponential transients based on the Laplace transform of a transient. It is useful for analysis of any non-exponential transient to obtain information on the density of localized states distribution. The improvement involved a correction of the tail regions of the distribution to provide a close fit between the simulated and experimental transient. By testing it on a number of different types of simulated transients, it was possible to ascertain the scope of this method. In principle this analysis can be applied to a variety of physical processes. In the present case it was applied to experimental photoconductivity transients for polycrystalline ZnO and films deposited by spray pyrolysis.

Distribution of local density of states in disordered metallic samples: Logarithmically normal asymptotics.

Asymptotical behavior of the distribution function of local density of states (LDOS) in disordered metallic samples is studied with making use of the supersymmetric $\sigma$--model approach, in combination with the saddle--point method. The LDOS distribution is found to have the logarithmically normal asymptotics for quasi--1D and 2D sample geometry. In the case of a quasi--1D sample, the result is confirmed by the exact solution. In 2D case a perfect agreement with an earlier renormalization group calculation is found. In 3D the found asymptotics is of somewhat different type: $P(\rho)\sim \exp(-\mbox{const}\,|\ln^3\rho|)$.

Transient space-charge-limited currents: The time-of-flight and post-transit analysis in hydrogenated amorphous silicon

Through evaluation of the post-transit part of the transient space-charge-limited currents (SCLC), it becomes possible to determine the hydrogenated amorphous silicon (a-Si:H) density of localized states (DOS) over a wide range of energies. The use of SCLC conditions results in the enhancement of the occupation of the deep states and hence in the corresponding increase of the signal. In combination with standard SCLC time-of-flight, and since it can be used for both electrons and holes, the method is suitable for the elucidation of the complete DOS distribution, for the examination of the emission times of deep traps, and for the determination of carrier drift mobilities, all on the same p-i-n structure (which may be a standard solar cell).

Classical theory of impurity bands in delta -doped n-i-p-i superlattices.

We investigate the density of localized states (DOS) of the classical two-dimensional impurity band in lightly doped \ensuremath{\delta}-n-i-p-i structures. The donor DOS distribution of the ground state of the n-i-p-i superlattice is simulated for T=0 K as a function of the compensation level K and the interlayer distance d, which permits an artificial reduction of the donor-acceptor Coulomb interaction. We compare the position of the Fermi level with analytical results in the case of low and high compensation. In the limit of low compensation and large interlayer distance Wigner crystallization is observed. The analysis of the Coulomb gap in the vicinity of the Fermi level shows significant deviations from the universal slope postulated for two-dimensional systems in the large disorder limit. Furthermore, the impurity bandwidth's dependence on the interlayer distance and the influence of residual three-dimensional background doping is disussed. Finally, we investigate the donor and acceptor impurity band in excited \ensuremath{\delta}-n-i-p-i structures and thus take advantage of the possibility to tune the electron filling factor F in one system of given compensation. A static model is compared with a dynamic one in which recombination, capture, and intralayer hopping processes are considered. The excitation-induced heating of the impurity system is described by introducing effective temperatures.