Real-space Distribution Research Articles

Insight into the split and asymmetry of charge distribution in biased M-structure superlattice

The charge distribution in real space of an insertion variant based on an InAs/GaSb superlattice for an infrared detector is illustrated by in situ electron microscopy. The localization split of positive charge can be directly observed in the InAs/GaSb/AlSb/GaSb superlattice (M-structure) rather than in the InAs/GaSb superlattice. With the applied bias increasing from 0 to 4.5 V, the double peaks of positive charge density become asymmetrical gradually, with the peak integral ratio ranging from 1.13 to 2.54. Simultaneously, the negative charges move along the direction of the negative electric field. Without inserting the AlSb layer, the charge inversion occurs in both the hole wells and the electron wells of the InAs/GaSb superlattice under high bias. Such a discrepancy between the M-structure superlattice and the traditional superlattice suggests an effective reduction of tunneling probability of the M-structure design. Our result is of great help to understand the carrier immigration mechanism of the superlattice-based infrared detector.

Iterative initial condition reconstruction

Motivated by recent developments in perturbative calculations of the nonlinear evolution of large-scale structure, we present an iterative algorithm to reconstruct the initial conditions in a given volume starting from the dark matter distribution in real space. In our algorithm, objects are first moved back iteratively along estimated potential gradients, with a progressively reduced smoothing scale, until a nearly uniform catalog is obtained. The linear initial density is then estimated as the divergence of the cumulative displacement, with an optional second-order correction. This algorithm should undo nonlinear effects up to one-loop order, including the higher-order infrared resummation piece. We test the method using dark matter simulations in real space. At redshift $z=0$, we find that after eight iterations the reconstructed density is more than $95\%$ correlated with the initial density at $k\le 0.35\; h\mathrm{Mpc}^{-1}$. The reconstruction also reduces the power in the difference between reconstructed and initial fields by more than 2 orders of magnitude at $k\le 0.2\; h\mathrm{Mpc}^{-1}$, and it extends the range of scales where the full broadband shape of the power spectrum matches linear theory by a factor of 2-3. As a specific application, we consider measurements of the baryonic acoustic oscillation (BAO) scale that can be improved by reducing the degradation effects of large-scale flows. In our idealized dark matter simulations, the method improves the BAO signal-to-noise ratio by a factor of 2.7 at $z=0$ and by a factor of 2.5 at $z=0.6$, improving standard BAO reconstruction by $70\%$ at $z=0$ and $30\%$ at $z=0.6$, and matching the optimal BAO signal and signal-to-noise ratio of the linear density in the same volume. For BAO, the iterative nature of the reconstruction is the most important aspect.

Optical probing of the Coulomb interactions of an electrically pumped polariton condensate

We report on optical probing of the Coulomb interactions in an electrically driven exciton-polariton laser. By positioning a weak non-resonant Gaussian continuous wave-beam with a diameter of 2 μm inside an electrical condensate excited in a 20 μm diameter micropillar, we study a repulsion effect which is characteristic of the part-excitonic nature of the microcavity system in strong coupling. It manifests itself in a modified real space distribution of the emission pattern. Furthermore, polariton repulsion results in a continuous blueshift of the emission with the increased power of the probe beam. A Gross-Pitaevskii equation approach based on modeling the electrical and optical potentials explains our experimental data.

Scanning Tunneling Microscopy Observation of Phonon Condensate

Using quantum tunneling of electrons into vibrating surface atoms, phonon oscillations can be observed on the atomic scale. Phonon interference patterns with unusually large signal amplitudes have been revealed by scanning tunneling microscopy in intercalated van der Waals heterostructures. Our results show that the effective radius of these phonon quasi-bound states, the real-space distribution of phonon standing wave amplitudes, the scattering phase shifts, and the nonlinear intermode coupling strongly depend on the presence of defect-induced scattering resonance. The observed coherence of these quasi-bound states most likely arises from phase- and frequency-synchronized dynamics of all phonon modes, and indicates the formation of many-body condensate of optical phonons around resonant defects. We found that increasing the strength of the scattering resonance causes the increase of the condensate droplet radius without affecting the condensate fraction inside it. The condensate can be observed at room temperature.

Orientational order of liquids and glasses via fluctuation diffraction.

Liquids, glasses and other amorphous matter lack long-range order, which makes them notoriously difficult to study. Local atomic order is partially revealed by measuring the distribution of pairwise atomic distances, but this measurement is insensitive to orientational order and unable to provide a complete picture of diverse amorphous phenomena, such as supercooling and the glass transition. Fluctuation scattering with electrons and X-rays is able provide this orientational sensitivity, but it is difficult to obtain clear structural interpretations of fluctuation data. Here we show that the interpretation of fluctuation diffraction data can be simplified by converting it into a real-space angular distribution function. We calculate this function from simulated diffraction of amorphous nickel, generated with a classical molecular dynamics simulation of the quenching of a high temperature liquid state. We compare the results of the amorphous case to the initial liquid state and to the ideal f.c.c. lattice structure of nickel. We show that the extracted angular distributions are rich in information about orientational order and bond angles. The diffraction fluctuations are potentially measurable with electron sources and also with the brightest X-ray sources, like X-ray free-electron lasers.

MAPPING THE REAL-SPACE DISTRIBUTIONS OF GALAXIES IN SDSS DR7. I. TWO-POINT CORRELATION FUNCTIONS

ABSTRACT Using a method to correct redshift-space distortion (RSD) for individual galaxies, we mapped the real-space distributions of galaxies in the Sloan Digital Sky Survey (SDSS) Data Release 7 (DR7). We use an ensemble of mock catalogs to demonstrate the reliability of our method. Here, in the first paper in a series, we focus mainly on the two-point correlation function (2PCF) of galaxies. Overall the 2PCF measured in the reconstructed real space for galaxies brighter than agrees with the direct measurement to an accuracy better than the measurement error due to cosmic variance, if the reconstruction uses the correct cosmology. Applying the method to the SDSS DR7, we construct a real-space version of the main galaxy catalog, which contains 396,068 galaxies in the North Galactic Cap with redshifts in the range 0.01 ≤ z ≤ 0.12. The Sloan Great Wall, the largest known structure in the nearby universe, is not as dominant an overdense structure as it appears to be in redshift space. We measure the 2PCFs in reconstructed real space for galaxies of different luminosities and colors. All of them show clear deviations from single power-law forms, and reveal clear transitions from one-halo to two-halo terms. A comparison with the corresponding 2PCFs in redshift space nicely demonstrates how RSDs boost the clustering power on large scales (by about 40%–50% at scales ) and suppress it on small scales (by about 70%–80% on a scale of ).

Ultraconfined Plasmonic Hotspots Inside Graphene Nanobubbles.

We report on a nanoinfrared (IR) imaging study of ultraconfined plasmonic hotspots inside graphene nanobubbles formed in graphene/hexagonal boron nitride (hBN) heterostructures. The volume of these plasmonic hotspots is more than one-million-times smaller than what could be achieved by free-space IR photons, and their real-space distributions are controlled by the sizes and shapes of the nanobubbles. Theoretical analysis indicates that the observed plasmonic hotspots are formed due to a significant increase of the local plasmon wavelength in the nanobubble regions. Such an increase is attributed to the high sensitivity of graphene plasmons to its dielectric environment. Our work presents a novel scheme for plasmonic hotspot formation and sheds light on future applications of graphene nanobubbles for plasmon-enhanced IR spectroscopy.

Finite-temperature hydrodynamics for one-dimensional Bose gases: Breathing-mode oscillations as a case study

We develop a finite-temperature hydrodynamic approach for a harmonically trapped one-dimensional quasicondensate and apply it to describe the phenomenon of frequency doubling in the breathing-mode oscillations of its momentum distribution. The doubling here refers to the oscillation frequency relative to the oscillations of the real-space density distribution, invoked by a sudden confinement quench. We find that the frequency doubling is governed by the quench strength and the initial temperature, rather than by the crossover from the ideal Bose gas to the quasicondensate regime. The hydrodynamic predictions are supported by the results of numerical simulations based on a finite-temperature c-field approach, and extend the utility of the hydrodynamic theory for low-dimensional quantum gases to the description of finite-temperature systems and their dynamics in momentum space.

Novel trends in pair distribution function approaches on bulk systems with nanoscale heterogeneities

Novel materials for high performance applications increasingly exhibit structural order on the nanometer length scale; a domain where crystallography, the basis of Rietveld refinement, fails [1]. In such instances the total scattering approach, which treats Bragg and diffuse scattering on an equal basis, is a powerful approach. In recent years, the analysis of the total scattering data became an invaluable tool and the gold standard for studying nanocrystalline, nanoporous, and disordered crystalline materials. The data may be analyzed in reciprocal space directly, or Fourier transformed to the real-space atomic pair distribution function (PDF) and this intuitive function examined for local structural information. Here we give a number of illustrative examples, for convenience picked from our own work, of recent developments and applications of total scattering and PDF analysis to novel complex materials. There are many other wonderful examples from the work of others.

Coexistence and competition of spin-density-wave and superconducting order parameters in iron-based superconductors

We theoretically study the coexistence of spin density wave (SDW) and superconductivity (SC) in ironpnictide superconductors based on a three-orbital model, focusing on the momentum-space and real-space distributions of SDW and SC order parameters in the coexistence region. We show that a SDW–SC coexisting state lies in the T–n phase diagram, in qualitative agreement with those of NaFe1−xCoxAs and Ba(Fe1−xCox)2As2. In the SC state the pairing wavefunction has s± symmetry with sx2y2 and sx2+y2 components. In the coexisting state, the SDW and SC order parameters display strong orbital-selective competitions in momentum space, which also result in real-space modulation and spin singlet–triplet mixing in the Cooper pairing amplitude. We expect that the obtained features may be observed in future experiments.

Multiscale ab initio simulation of Ni-based alloys: Real-space distribution of atoms in γ + γ′ phase

Real-space partitioning of elements to γ and γ′ phases in Ni-based alloys were simulated at atomic scale using ab initio-based multiscale DFT+CEMC approach. The method uses density functional theory (DFT) calculations to parameterize cluster expansion (CE) of the configuration energy, followed by Monte Carlo (MC) simulations at finite temperatures (T). The set of clusters for the CE, which controls the accuracy of the expansion, was determined by the method of model selection by minimizing cross-validation score SCV using genetic algorithm (GA). The MC algorithm was accelerated through parallelization using spatial decomposition allowing simulation of system containing millions of atoms. Real space distribution of atoms was obtained for Ni–Al, Ni–Al–Ti and Ni–Al–Ti–Co alloys as a function of T and for select alloy compositions in γ+γ′ phase. For ternary Ni–Al–Ti alloys, simulation correctly captures preferential partitioning of Ti to γ′ whereas interplay of interactions determines the partitioning of Ti and Co in quaternary alloys. Simulation results were validated by comparing phase boundaries and the volume fraction of γ′ (obtained from real space) with thermodynamic calculations using experimental database. The approach opens up the route to calculate configuration-dependent alloy-specific parameters for higher length scale models for multi-component alloys.

The Electronic Properties of Single-Layer and Multilayer MoS2 under High Pressure

We analyze the changes in the electronic structures of single-layer and multilayer MoS2 under pressure using first-principles methods including van der Waals interactions. For single-layer MoS2, the bond angle is found to control the electronic structure around the band gap under the pressure. For multilayer and bulk MoS2, the changes in electronic structure under pressure are mainly controlled by the coupling of layers. Under pressure, the band gap of single-layer MoS2 changes from direct to indirect, while multilayer MoS2 becomes a band metal. Analysis of the real-space distribution of band-decomposed charge density shows that this behavior can be understood in terms of the different Mo d-electron orbitals making up the states near band gap including those at the top of valence band of Γ and K points and the bottom of conduction band along Λ and at the K point.

Quasiperiodicity and the nanoscopic morphology of some polyurethanes

When straining polyurethane elastomers (PUEs), it is often observed that the long-period peak of the small-angle X-ray scattering (SAXS) does not shift normally. An explanation is indicated for some PUEs in the real-space chord distribution. It exhibits a sequence of constant long-period bands. The band positions form a Fibonacci sequence. This relates to the underlying chemical synthesis by polyaddition of hard and soft modules, indicating a nearly quasiperiodic setup in sequences of stringed hard domains. These sequences appear to be the probes provided by SAXS for the study of morphology evolution in such PUEs. Should a regular-as-possible arrangement of physical crosslinks optimize a property of the material, then in the synthesis the mole fractionnHof hard modules should be chosen to benH= τ/(1 + τ) ≃ 0.62, where τ is the golden ratio.

Photorefractive writing and probing of anisotropic linear and nonlinear lattices

We study experimentally the writing of one- and two-dimensional photorefractive lattices, focusing on the often overlooked transient regime. Our measurements agree well with theory, in particular concerning the ratio of the drift to diffusion terms. We then study the transverse dynamics of coherent waves propagating in the lattices, in a few novel and simple configurations. For defocusing linear waves with broad transverse spectrum, we remark that both the intensity distributions in real space (‘discrete diffraction’) and Fourier space (‘Brillouin zone spectroscopy’) reflect the Bragg planes and band structure. For nonlinear waves, we observe modulational instability and discrete solitons formation in time domain. We discuss also the non-ideal effects inherent to the photo-induction technique: anisotropy, residual nonlinearity, diffusive term, non-stationarity.

Phase separation induced by density-spin interaction in one-dimensional extended t-J model

t-J model is one of important theoretical models in the study of high temperature superconductivity. Recent cold molecule experiments indicate that t-J model can be simulated by ultracold polar molecules. In the simulated t-J model, besides long-range dipolar interaction, density-spin interaction has also been introduced. In the present we study the effect of density-spin interaction in the one-dimensional extended t-J model by using the density matrix renormalization group method. We choose three sets of representative parameters, which correspond to three different phases in the ground state phase diagram of t-J model, to calculate the charge and spin density distribution in real space and the structure factor of density-density and spin-spin correlation functions. The results indicate that the nature of the system will not change if the intensity of the density-spin interaction is small, however if the intensity is large enough, the system enters the phase separation, in which the character is quiet different form that of the phase separation in the traditional t-J model.

Atomic scale study of corrugating and anticorrugating states on the bare Si(1 0 0) surface

In this article, we study the origin of the corrugating and anticorrugating states through the electronic properties of the Si(1 0 0) surface via a low-temperature (9 K) scanning tunneling microscope (STM). Our study is based on the analysis of the STM topographies corrugation variations when related to the shift of the local density of states (LDOS) maximum in the direction. Our experimental results are correlated with numerical simulations using the density-functional theory with hybrid Heyd–Scuseria–Ernzerhof (HSE06) functional to simulate the STM topographies, the projected density of states variations at different depths in the silicon surface as well as the three dimensional partial charge density distributions in real-space. This work reveals that the Si(1 0 0) surface exhibits two anticorrugating states at +0.8 and +2.8 V that are associated with a phase shift of the LDOS maximum in the unoccupied states STM topographies. By comparing the calculated data with our experimental results, we have been able to identify the link between the variations of the STM topographies corrugation and the shift of the LDOS maximum observed experimentally. Each surface voltage at which the STM topographies corrugation drops is defined as anticorrugating states. In addition, we have evidenced a sharp jump in the tunnel current when the second LDOS maximum shift is probed, whose origin is discussed and associated with the presence of Van Hove singularities.

Real-space observation of unbalanced charge distribution inside a perovskite-sensitized solar cell.

Perovskite-sensitized solar cells have reached power conversion efficiencies comparable to commercially available solar cells used for example in solar farms. In contrast to silicon solar cells, perovskite-sensitized solar cells can be made by solution processes from inexpensive materials. The power conversion efficiency of these cells depends substantially on the charge transfer at interfaces. Here we use Kelvin probe force microscopy to study the real-space cross-sectional distribution of the internal potential within high efficiency mesoscopic methylammonium lead tri-iodide solar cells. We show that the electric field is homogeneous through these devices, similar to that of a p-i-n type junction. On illumination under short-circuit conditions, holes accumulate in front of the hole-transport layer as a consequence of unbalanced charge transport in the device. After light illumination, we find that trapped charges remain inside the active device layers. Removing these traps and the unbalanced charge injection could enable further improvements in performance of perovskite-sensitized solar cells.

Ordered arrays of magnetic nanowires investigated by polarized small-angle neutron scattering

Polarized small-angle neutron scattering (PSANS) experimental results obtained on arrays of ferromagnetic Co nanowires ($\phi\approx13$ nm) embedded in self-organized alumina (Al$_{2}$O$_{3}$) porous matrices are reported. The triangular array of aligned nanowires is investigated as a function of the external magnetic field with a view to determine experimentally the real space magnetization $\vec{M}(\vec{r})$ distribution inside the material during the magnetic hysteresis cycle. The observation of field-dependentSANSintensities allows us to characterize the influence of magnetostatic fields. The PSANS experimental data are compared to magnetostatic simulations. These results evidence that PSANS is a technique able to address real-space magnetization distributions in nanostructured magnetic systems. We show that beyond structural information (shape of the objects, two-dimensional organization) already accessible with nonpolarized SANS, using polarized neutrons as the incident beam provides information on the magnetic form factor and stray fields \textgreek{m}0Hd distribution in between nanowires.

Radiographic and Tomographic Neutron Bragg Imaging for Quantitative Visualization of Wide Area Crystalline Structural Information

Recent status of the technical development of the Bragg-edge neutron transmission imaging and its application to material science is presented. The neutron Bragg imaging has the advantages in measuring large area with reasonable spatial resolution, and it is a non-destructive method capable of looking inside a bulk material. Therefore, various information that are quite different from EBSD, synchrotron microtomography and X-ray/neutron scattering can be obtained by this method. We carried out quantitative imaging to obtain crystalline microstructural information in ultralow-carbon steels that received the high pressure torsion (HPT). The real-space distributions of texture and grain/crystallite size of HPTed steels of four torsion numbers were quantitatively visualized at once. As a result, we could deduce unique distributions of microstructural information depending on each torsion number, and correlated them with real-space distributions of the Vickers hardness. We also successfully developed a versatile strain tomography technique that can obtain tensor values for strain although traditional CT techniques can deal with only scalar values. The new CT algorithm, the tensor CT method, is based on our original algorithm called FBP-EM. The strain tensor tomography using FBP-EM was successfully applied for the experimental measured result obtained with the VAMAS neutron strain analysis international standard sample.

Tuning topological edge states of Bi(111) bilayer film by edge adsorption.

Based on first-principles and tight-binding calculations, we report that the topological edge states of zigzag Bi(111) nanoribbon can be significantly tuned by H edge adsorption. The Fermi velocity is increased by 1 order of magnitude, as the Dirac point is moved from the Brillouin zone boundary to the Brillouin zone center, and the real-space distribution of Dirac states are made twice more delocalized. These intriguing changes are explained by an orbital filtering effect of edge H atoms, which pushes certain components of the p orbital of edge Bi atoms out of the band gap regime that reshapes the topological edge states. In addition, the spin texture of the Dirac states is also modified, which is described by introducing an effective Hamiltonian. Our findings not only are of fundamental interest but also have practical implications in potential applications of topological insulators.