Spatial Density Fluctuations Research Articles

Theory of spin center sensing of diffusion and other surface electric dynamics

Surface electric dynamics influences the quantum coherence of near-surface spin centers (i.e., T1 and T2 times) through spatial and temporal fluctuations of the surface charge density and the electrostatic potential at the crystal surface that are described by the spectral noise density S(ω). We find that the electric noise's S(ω) dependence on the depth d of a spin center below the surface follows a power law d−γ not solely determined by whether the surface charges move as localized pointlike charges (monopoles) or dipoles. The exponent γ instead reveals the structure of the surface charge and surface potential correlation functions. For surface dynamics originating from diffusion, the spin center's relaxation and decoherence times exhibit a temporal crossover near the correlation time of the fluctuators and thus provide a quantitative fingerprint of the diffusive behavior of charged particles at surfaces. Published by the American Physical Society 2024

Reaction-Limited Quantum Reaction-Diffusion Dynamics.

We consider the quantum nonequilibrium dynamics of systems where fermionic particles coherently hop on a one-dimensional lattice and are subject to dissipative processes analogous to those of classical reaction-diffusion models. Particles can either annihilate in pairs, A+A→0, or coagulate upon contact, A+A→A, and possibly also branch, A→A+A. In classical settings, the interplay between these processes and particle diffusion leads to critical dynamics as well as to absorbing-state phase transitions. Here, we analyze the impact of coherent hopping and of quantum superposition, focusing on the so-called reaction-limited regime. Here, spatial density fluctuations are quickly smoothed out due to fast hopping, which for classical systems is described by a mean-field approach. By exploiting the time-dependent generalized Gibbs ensemble method, we demonstrate that quantum coherence and destructive interference play a crucial role in these systems and are responsible for the emergence of locally protected dark states and collective behavior beyond mean field. This can manifest both at stationarity and during the relaxation dynamics. Our analytical results highlight fundamental differences between classical nonequilibrium dynamics and their quantum counterpart and show that quantum effects indeed change collective universal behavior.

Quantum Vacuum Excitation of a Quasinormal Mode in an Analog Model of Black Hole Spacetime.

Vacuum quantum fluctuations near horizons are known to yield correlated emission by the Hawking effect. We use a driven-dissipative quantum fluid of microcavity polaritons as an analog model of a quantum field theory on a black-hole spacetime and numerically calculate correlated emission. We show that, in addition to the Hawking effect at the sonic horizon, quantum fluctuations may result in a sizable stationary excitation of a quasinormal mode of the field theory. Observable signatures of the excitation of the quasinormal mode are found in the spatial density fluctuations as well as in the spectrum of Hawking emission. This suggests an intrinsic fluctuation-driven mechanism leading to the quantum excitation of quasinormal modes on black hole spacetimes.

Laser-Ablated SiO2:TiO2 Plasma Optical Emission Spectroscopy

Laser ablation of sintered SiO2:TiO2 targets using Nd:YAG lasers at fundamental (1064[Formula: see text]nm) harmonic generation in the air has been studied using optical emission spectroscopy. Exploring the spatial fluctuations in electron temperature ([Formula: see text]) and electron density ([Formula: see text]), there is a discussion of how laser energy affects electron temperature ([Formula: see text]) and electron density ([Formula: see text]). Laser energy has also been shown to affect the intensity and velocity of neutral and ion species. Using existing data and theory, the findings were confirmed following the local thermodynamic equilibrium (LTE) hypothesis. Plasma properties such as Debye length ([Formula: see text]), ([Formula: see text]) and plasma frequency ([Formula: see text]) were also studied in this study. Using a laser, we found that all plasma parameters were affected. In addition, the calculated inverse Bremsstrahlung absorption coefficients ([Formula: see text]) were altered.

Local capacitance-voltage profiling and deep level transient spectroscopy of SiO2/SiC interfaces by scanning nonlinear dielectric microscopy

We extend our local capacitance-voltage profiling method for the real-space nanoscale investigation of the SiO2/SiC interface, which is still an issue in the application of SiC to power electronics devices. Our technique is based on time-resolved scanning nonlinear dielectric microscopy and can be used for collecting local capacitance-voltage and its voltage-derivative profiles at each measurement point. This technique can also be combined with local deep level transient spectroscopy for simultaneously imaging the spatial fluctuations of interface defect density in real-space. Here we use the proposed technique to measure SiC wafers with a thermal oxide layer and investigate the effect of interface nitridation. From the data volume obtained by our technique, various images are extracted and their correlation is analyzed to characterize and discuss the spatial fluctuations and hysteresis of the local capacitance-voltage profiles. As expected, the nitridation of the SiO2/SiC interface after thermal oxidation is effective for the reduction of hysteresis and fluctuations in local capacitance-voltage profiles. However, significant spatial fluctuations remain even after the nitridation treatment, which suggests the SiO2/SiC interface has intrinsic non-uniformity generating high surface potential fluctuations.

Quantum vacuum excitation of a quasi-normal mode in an analog model of black hole spacetime

Vacuum quantum fluctuations near horizons are known to yield correlated emission by the Hawking effect. In this talk, I will explain how a 1 dimensional flow of microcavity polaritons may be engineered to produce an effective curved spacetime with a black hole horizon. I will present numerical computations of correlated emission on this spacetime and show that, in addition to the Hawking effect at the sonic horizon, quantum fluctuations may result in a sizeable stationary excitation of a quasi-normal mode of the field theory. Observable signatures of the excitation of the quasi-normal mode are found in the spatial density fluctuations as well as in the spectrum of Hawking emission. I will explain how the driven-dissipative dynamics of the polariton fluid are key to observing the quantum excitation of the quasi-normal mode. Nonetheless, this observation suggests a general and intrinsic fluctuation-driven mechanism leading to the quantum excitation of quasi-normal modes on black hole spacetimes.

C60Br24/SWCNT: A Highly Sensitive Medium to Detect H2S via Inhomogeneous Carrier Doping

H2S is a toxic and corrosive gas, whose accurate detection at sub-ppm concentrations is of high practical importance in environmental, industrial, and health safety applications. Herein, we propose a chemiresistive sensor device that applies a composite of single-walled carbon nanotubes (SWCNTs) and brominated fullerene (C60Br24) as a sensing component, which is capable of detecting 50 ppb H2S even at room temperature with an excellent response of 1.75% in a selective manner. In contrast, a poor gas response of pristine C60-based composites was found in control measurements. The experimental results are complemented by density functional theory calculations showing that C60Br24 in contact with SWCNTs induces localized hole doping in the nanotubes, which is increased further when H2S adsorbs on C60Br24 but decreases in the regions, where direct adsorption of H2S on the nanotubes takes place due to electron doping from the analyte. Accordingly, the heterogeneous chemical environment in the composite results in spatial fluctuations of hole density upon gas adsorption, hence influencing carrier transport and thus giving rise to chemiresistive sensing.

Photonic probing of structural alterations in DNA specific mass density fluctuations in nuclei due to total body irradiation (TBI) via confocal imaging

Abnormalities within cells result in intracellular structural alterations ranging from nano to submicron scales. Accidental or deliberate exposure to total body irradiation has adverse effects on the nuclear DNAs of cells. Here, we study the molecular specific DNA spatial mass density fluctuations of chromatin of mice gut cell nuclei caused by the exposure to standard doses of 4-Gy total body irradiation, using the light localization technique called inverse participation ratio via confocal imaging. Results show radiation suppresses DNA spatial mass density fluctuations. And hence, the reduction and saturation in DNA mass density fluctuations are observed on different durations of post-irradiation.

First-principles method for x-ray Thomson scattering including both elastic and inelastic features in warm dense matter

We show that the entire x-ray Thomson scattering (XRTS) spectrum, including both elastic and inelastic features, can be calculated from first principles in the framework of quantum perturbation theories. Our derivation shows that the elastic scattering feature in a warm dense regime is different from that of condensed matter at low temperature. In addition to the contribution from spatial fluctuations of electronic density, which dominates elastic scattering at low temperature, there is an extra contribution from partially occupied inner-shell states, which is important in the warm dense regime. Calculated XRTS of isochorically heated beryllium agrees well with previous experimental measurements, which may give this method a further edge on interpreting XRTS spectra compared to empirical modeling methods.

Inevitable imprints of patchy reionization on the cosmic microwave background anisotropy

ABSTRACT Reionization of the cosmic neutral hydrogen by the first stars in the Universe is an inhomogeneous process, which produces spatial fluctuations in free electron density. These fluctuations lead to observable signatures in cosmological probes like the cosmic microwave background (CMB). We explore the effect of the electron density fluctuations on CMB using photon-conserving seminumerical simulations of reionization named SCRIPT. We show that the amplitude of the kinematic Sunyaev–Zeldovich (kSZ) and the B-mode polarization signal depends on the patchiness in the spatial distribution of electrons along with the dependence on mid-point and extent of the reionization history. Motivated by this finding, we provide new scaling relations for the amplitude of kSZ and the B-mode polarization signal which can capture the effects arising from the mean optical depth, width of reionization, and spatial fluctuations in the electron density arising from patchy reionization. We show that the amplitude of the kSZ and the B-mode polarization signal exhibits different dependency on the width of reionization and the patchiness of reionization, and hence a joint study of these CMB probes will be able to break the degeneracy. By combining external data sets from 21-cm measurements, the degeneracy can be further lifted by directly exploring the sizes of the ionized regions.

Abstract P6-06-04: Exploratory computational longitudinal analysis of mammographic microenvironment disruption preceding breast tumorigenesis

Abstract Despite an aggressive emphasis on screening mammography, there has been no change in the number of initial metastatic breast cancer diagnosis since 1975. Even more alarming is the lack of effective screening to differentiate dormant cancerous pathology that may spontaneously ignite into an aggressive metastatic cancer. In this context, screening mammography is ineffective and biased towards a reactive diagnosis, when cancer is already present. We seek to develop a predictive algorithm that is sensitive to currently undetectable alterations of the microenvironment that precede tumorigenesis. Histological features such as tissue disruption and loss of tissue homeostasis, which are critical early indicators of breast cancer, unfortunately fall well outside the detection abilities of a trained radiologist or current computer-aided detection/diagnostic methods. In a recent publication using a powerful wavelet-based multifractal image analysis method, we extracted quantitative metrics for tissue disruption from standard mammography, and inferred a correlation with loss of tissue homeostasis. The 2D Wavelet-Transform Modulus Maxima (WTMM) method was used to quantify density fluctuations from mammographic breast tissue via a statistical roughness exponent called the Hurst exponent (H). The two mammographic tissue radiological states, i.e., fatty and dense, are quantitatively characterized with this Hurst exponent: H>1/2 (correlated spatial density fluctuations) for dense tissue; and H<1/2 (anti-correlated spatial density fluctuations) for fatty tissue. However, tissue regions that are neither dense nor fatty, with H~1/2 (uncorrelated spatial density fluctuations), were found preferably in patients with tumorous breasts vs. normal breasts (p-value = 0.0006) [Marin et al. Medical Physics, 2017]. The underlying physical processes associated with a H=1/2 signature are those of randomness and disorganization. Indeed, incoherent angular motion and the consequential adoption of randomized cellular motility are associated with malignant growth in breast tissue. We hypothesize that this H~1/2 signature accompanies or even preceeds tumorigenesis. In an exploratory analysis aimed at verifying this hypothesis, we studied, for 22 breast cancer patients, the progression of healthy breast tissue microenvironment towards a disrupted state by performing a computational analysis of longitudinal sequences of mammograms multiple years prior to, and leading to diagnostic. Preliminary results indicate a measurable change in mammographic density fluctuations (and therefore, potentially loss of tissue homeostasis), between 1 and 3 years prior to radiologic detection / diagnostic. An eventual larger-scale clinical validation of such an analysis could lead to the routine implementation of predictive algorithms based on the multifractal characterization of tissue disruption that precedes tumorigenesis. Such predictive diagnostics empowers both patient and clinician to collaboratively assess all ramification of the disease and select optimal therapeutics. Citation Format: Andre Khalil, Brian Toner. Exploratory computational longitudinal analysis of mammographic microenvironment disruption preceding breast tumorigenesis [abstract]. In: Proceedings of the 2019 San Antonio Breast Cancer Symposium; 2019 Dec 10-14; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2020;80(4 Suppl):Abstract nr P6-06-04.

Stochastic continuum model for mycelium-based bio-foam

Mycelium, the root structure of fungi, grows naturally as a biodegradable filamentous material. This unique material has highly heterogeneous microstructure with pronounced spatial variability in density and exhibits strongly non-linear mechanical behavior. In this work we explore the material response in compression, under cyclic deformation, and develop an experimentally-validated multiscale model for its mechanical behavior. The deformation localizes in stochastically distributed sub-domains which eventually percolate to form macroscopic bands of high density material. This is reflected in the stress-strain curve as strain softening. Cycling at fixed macroscopic strain leads to deformation history dependence similar to the Mullins effect. To capture this behavior, we use a two-scale model. At the micro-scale, a random fiber network is used, while at the macroscale the spatial density fluctuations are captured using a stochastic continuum model. The density-dependent local constitutive behavior is defined by the microscale model. An empirical damage model is incorporated to account for the experimentally observed cyclic softening behavior of mycelium. The model is further validated by comparison with a separate set of experimental results. The model can be used to explore the effect of mesoscale density fluctuations on the overall mechanical behavior and to design mycelium-based products with desired mechanical performance.

Theory of Coulomb drag in spatially inhomogeneous 2D materials

Coulomb drag is a favored experimental probe of Coulomb interactions between layers of 2D materials. In reality, these layers display spatial charge density fluctuations known as puddles due to various imperfections. A theoretical formalism for incorporating density inhomogeneity into calculations has however not been developed, making the understanding of experiments difficult. Here, we remedy this by formulating an effective medium theory of drag that applies in all 2D materials. We show that a number of striking features at zero magnetic field in graphene drag experiment which have not been explained by existing literature emerge naturally within this theory. Applying the theory to a phenomenological model of exciton condensation, we show that the expected divergence in drag resistivity is replaced by a peak that diminishes with increasing puddle strength. Given that puddles are ubiquitous in 2D materials, this work will be useful for a wide range of future studies.

Comparative Multifractal Analysis of Dynamic Infrared Thermograms and X-Ray Mammograms Enlightens Changes in the Environment of Malignant Tumors.

There is growing evidence that the microenvironment surrounding a tumor plays a special role in cancer development and cancer therapeutic resistance. Tumors arise from the dysregulation and alteration of both the malignant cells and their environment. By providing tumor-repressing signals, the microenvironment can impose and sustain normal tissue architecture. Once tissue homeostasis is lost, the altered microenvironment can create a niche favoring the tumorigenic transformation process. A major challenge in early breast cancer diagnosis is thus to show that these physiological and architectural alterations can be detected with currently used screening techniques. In a recent study, we used a 1D wavelet-based multi-scale method to analyze breast skin temperature temporal fluctuations collected with an IR thermography camera in patients with breast cancer. This study reveals that the multifractal complexity of temperature fluctuations superimposed on cardiogenic and vasomotor perfusion oscillations observed in healthy breasts is lost in malignant tumor foci in cancerous breasts. Here we use a 2D wavelet-based multifractal method to analyze the spatial fluctuations of breast density in the X-ray mammograms of the same panel of patients. As compared to the long-range correlations and anti-correlations in roughness fluctuations, respectively observed in dense and fatty breast areas, some significant change in the nature of breast density fluctuations with some clear loss of correlations is detected in the neighborhood of malignant tumors. This attests to some architectural disorganization that may deeply affect heat transfer and related thermomechanics in breast tissues, corroborating the change to homogeneous monofractal temperature fluctuations recorded in cancerous breasts with the IR camera. These results open new perspectives in computer-aided methods to assist in early breast cancer diagnosis.

On the dissipation of the rotation energy of dust grains in interstellar magnetic fields

A new mechanism is described, analysed and visualized, for the dissipation of suprathermal rotation energy of molecules in magnetic fields, a necessary condition for their alignment. It relies upon the Lorentz force perturbing the motion of every atom of the structure, as each is known to carry its own net electric charge because of spatial fluctuations in electron density. If the molecule is large enough that the frequency of its lowest frequency phonon lies near or below the rotation frequency, then the rotation couples with the molecular normal modes and energy flows from the former to the latter. The rate of this exchange is very fast, and the vibrational energy is radiated away in the IR at a still faster rate, which completes the removal of rotation energy. The energy decay rate scales like the field intensity, the initial angular velocity, the number of atoms in the grain and the inverse of the moment of inertia. It does not depend on the susceptibility. Here, the focus is on carbon-rich molecules which are diamagnetic. The same process must occur if the molecule is paramagnetic or bathes in an electric field instead. A semi-empirical method of chemical modelling was used extensively to illustrate and quantify these concepts as applied to a hydrocarbon molecule. The motion of a rotating molecule in a field was monitored in time so as to reveal the energy transfer and visualize the evolution of its orientation towards the stable configuration.

A modeling approach to scattering and reverberation in shallow water

A physics-based modeling approach is described that allows prediction of reverberation in complex shallow water environments. An integral expression is presented for the backscatter intensity with a factorized integrand comprised of two kernels, the two-way propagator and the scattering kernel. The propagator is defined by Green’s function, describes the local intensity, and can be calculated using available models, such as PE, normal modes, or ray approximations. An expression for the scattering kernel is obtained using a unified approach to volume and roughness scattering [Ivakin, JASA Feb. 1998], and represents a sum of volume and roughness scattering coefficients. They are specified for sea-water column and rough heterogeneous seabed with continuous spatial fluctuations of compressibility and density, and/or discrete randomly distributed targets, such as bubbles, fish, shells, and others. Results of numerical simulations for shallow water reverberation time/range series, based on a PE propagation model, are presented, and potential contributions of different mechanisms of scattering are compared and discussed. The approach is applied to consider reverberation in a complex shelly sand/mud environment, such as one at the Target and Reverberation Experiment 2013 (TREX), and results in model/data comparisons based on analysis of TREX acoustic scattering data and environmental ground truth measurements. [Work supported by ONR-OA.]

Type-dependent irreversible stochastic spin models for genetic regulatory networks at the level of promotion–inhibition circuitry

We describe an approach to model genetic regulatory networks at the level of promotion–inhibition circuitry through a class of stochastic spin models that includes spatial and temporal density fluctuations in a natural way. The formalism can be viewed as an agent-based model formalism with agent behaviour ruled by a classical spin-like pseudo-Hamiltonian playing the role of a local, individual objective function. A particular but otherwise generally applicable choice for the microscopic transition rates of the models also makes them of independent interest. To illustrate the formalism, we investigate (by Monte Carlo simulations) some stationary state properties of the repressilator, a synthetic three-gene network of transcriptional regulators that possesses oscillatory behaviour.

Spatial density fluctuation of supersonic flow over a backward-facing step measured by nano-tracer planar laser scattering

Spatial density fluctuation of supersonic flow over a backward-facing step is studied by nano-tracer planar laser scattering, aero-optic method and proper orthogonal decomposition analysis. Measurements were carried out in a Ma = 3.0 low-noise indraft wind tunnel. By varying the superficial roughness of the wall upstream from the step, supersonic laminar flow and supersonic turbulent flow were generated over a backward-facing step. The reattachment regions of these two flows were focused on. Flow structures of supersonic boundary layer, free shear layer, $$Kelvin - Helmholtz$$ vortices and reattachment shock were revealed. Based on the density-grayscale calibration of the flow field, the density distribution and optical path difference were calculated. Proper orthogonal decomposition was applied to 400 samples of optical path difference distributions which were generated by performing ray tracing method. According to the principle of proper orthogonal decomposition and the aero-optic basic equations, peaks in the first mode of proper orthogonal decomposition could reveal intensive spatial density fluctuations. The results showed that around the location of the reattachment point, density changed dramatically in both of two flows spatially. The location of reattachment point judged by the intensive density variation basically agreed with that judged by velocity field or flow structures. Thus, it was found that reattachment of supersonic backward-facing step occurred with intensive spatial density fluctuation which caused complex aero-optic aberration.

Spatial density fluctuations and selection effects in galaxy redshift surveys

One of the main problems of observational cosmology is to determine the range in which a reliable measurement of galaxy correlations is possible. This corresponds to determining the shape of the correlation function, its possible evolution with redshift and the size and amplitude of large scale structures. Different selection effects, inevitably entering in any observation, introduce important constraints in the measurement of correlations. In the context of galaxy redshift surveys selection effects can be caused by observational techniques and strategies and by implicit assumptions used in the data analysis. Generally all these effects are taken into account by using pair-counting algorithms to measure two-point correlations. We review these methods stressing that they are based on the a-priori assumption that galaxy distribution is spatially homogeneous inside a given sample. We show that, when this assumption is not satisfied by the data, results of the correlation analysis are affected by finite size effects. In order to quantify these effects, we introduce a new method based on the computation of the gradient of galaxy counts along tiny cylinders. We show, by using artificial homogeneous and inhomogeneous point distributions, that this method identifies redshift dependent selection effects and disentangles them from the presence of large scale density fluctuations. We then apply this new method to several redshift catalogs and we find evidence that galaxy distribution, in those samples where selection effects are small enough, is characterized by power-law correlations with exponent γ=0.9 up to 20 Mpc/h followed by a change of slope that, in the range 20–100 Mpc/h, corresponds to a power-law exponent γ=0.25. Whether a crossover to spatial uniformity occurs at ∼ 100 Mpc/h or larger scales cannot be clarified by the present data.

On the separate recovery of spatial fluctuations in compressibility and mass density in pulse-echo ultrasound imaging using linear inverse scattering

In pulse-echo ultrasound imaging (PEUI) of soft tissues, the scattered sound field is governed by spatial fluctuations of the two mechanical parameters compressibility and mass density. Spatial fluctuations in compressibility act as isotropic monopole radiators while spatial fluctuations in mass density act as anisotropic dipole radiators. Conventional strategies for linear image reconstruction in PEUI, e.g., delay-and-sum, minimum variance, and synthetic aperture focusing, exclusively account for monopole scattering. This neglect of the inhomogeneous mass density might be accompanied by a loss of diagnostically relevant information, e.g., the detection of tissue abnormalities. In this study, we formulate a linear inverse scattering problem to recover separate, space-resolved maps of the spatial fluctuations in both mechanical parameters from measurements of the scattered acoustic pressure. The physical model accounts for frequency-dependent absorption and dispersion in accordance with the time causal model. The computational costs are effectively reduced by the usage of the fast multipole algorithm. The concept is evaluated using simulated and experimentally obtained radio frequency data.