Diffusion Annihilation Research Articles

Photon Pair Condensation by Engineered Dissipation.

Dissipation can usually induce detrimental decoherence in a quantum system. However, engineered dissipation can be used to prepare and stabilize coherent quantum many-body states. Here, we show that, by engineering dissipators containing photon pair operators, one can stabilize an exotic dark state, which is a condensate of photon pairs with a phase-nematic order. In this system, the usual superfluid order parameter, i.e., single-photon correlation, is absent, while the photon pair correlation exhibits long-range order. Although the dark state is not unique due to multiple parity sectors, we devise an additional type of dissipators to stabilize the dark state in a particular parity sector via a diffusive annihilation process which obeys Glauber dynamics in an Ising model. Furthermore, we propose an implementation of these photon pair dissipators in circuit-QED architecture.

Trapped-Hole Diffusion in Photoexcited CdSe Nanorods

Surface charge-carrier traps are ubiquitous in colloidal semiconductor nanocrystals and fundamentally impact excited-state relaxation, making it critical to understand both their nature and their dynamics. Here, using photoluminescence upconversion and transient absorption spectroscopy, we study hole trapping and the dissociation between electrons and trapped holes in nonuniform CdSe nanorods and monitor their subsequent recombination dynamics. These recombination dynamics are described well with a diffusion–annihilation model wherein the trapped hole undergoes a random walk on the nanocrystal surface until it encounters the electron. This model fits the nonexponential excited-state decay over more than 7 orders of magnitude in time with a single adjustable parameter. The characterization of the spatial dynamics of trapped holes in CdSe nanostructures extends our fundamental understanding of excited-state dynamics in this important class of materials. The surface motion of trapped holes may have important...

Numerical Simulation on the Sintering Process of Nanocomposite Ceramic Tool Materials

A Monte Carlo Potts model for the sintering process of two-phase nanocomposite ceramic tool materials is proposed. The grain growth, pore diffusion and vacancy annihilation at grain boundaries are simulated in the model. Simulation results are presented and discussed. It is found that pore and nanophase can pin the ceramic matrix and decrease the growth rate of matrix grain during the microstructure evolution. The addition of nanoparticles is not only beneficial to the refinement of ceramic matrix grain and the formation of intra/intergranular-type microstructure but also advantageous to the densification of ceramic materials.

Diffusion–annihilation processes in weighted scale-free networks with an identical degreesequence

and diffusion–annihilation processes have so far been studied on weighted uncorrelatedscale-free networks and fractal scale-free networks. In the previous reports it was widelyaccepted that the segregation of particles in the processes is introduced by the fractalstructure. In this paper we study these processes on a family of weighted scale-freenetworks with an identical degree sequence. We find that the depletion zone andsegregation are essentially caused by disassortative mixing, namely, high-degree nodes tendto connect with low-degree nodes. Their influence on the processes is governed by thecorrelation between the weight and degree. Our finding suggests that the weight and degreedistribution do not suffice to characterize the diffusion–annihilation processes on weightedscale-free networks.

On the Formation of Filament Channels

From the Hα archive of the Big Bear Solar Observatory (BBSO) we have selected three examples showing fibril structures that change their orientation, over 1 or 2 days, from nearly perpendicular to nearly parallel to the polarity inversion line (PIL). In one case, the filament channel forms within a single decaying bipole; in the other two cases, it forms along the boundary between an active region and its surroundings. Comparing the Hα filtergrams with magnetograms from the Michelson Doppler Imager (MDI), we find that the fibrils become aligned with the PIL as supergranular convection brings opposite-polarity magnetic flux together; shearing motions along the PIL, when present, act mainly to accelerate the rate of diffusive annihilation. We conclude that the reorientation of the fibrils is due to the cancellation and submergence of the transverse field component (B⊥), leaving behind the preexisting axial field component (B∥). The latter may have been generated by photospheric differential rotation over longer timescales, or else was already present when the flux emerged. The filament channel forms slowly if B∥/B⊥ is initially small, as along the internal neutral line of a newly emerged bipole, but may appear within hours if this ratio is initially substantial, as where the dipole-like loops of an active region curve around its periphery. In all of our examples, filaments form within a day or so after the fibrils become aligned with the PIL, while barbs appear at a later stage, as flux elements continue to diffuse across the PIL and cancel with the majority-polarity flux on the other side.

Cubic boron nitride thin film growth by boron and nitrogen ion implantation

Cubic boron nitride $(c\text{\ensuremath{-}}\mathrm{B}\mathrm{N})$ thin films were deposited on silicon substrates using mass separated ion beam deposition (MSIBD). In order to investigate the influence of the ion energy on the growth of $c\text{\ensuremath{-}}\mathrm{B}\mathrm{N}$ films, $^{11}\mathrm{B}^{+}$ and $^{14}\mathrm{N}^{+}$ ions were implanted into $c\text{\ensuremath{-}}\mathrm{B}\mathrm{N}$ with ion energies ranging from $5\phantom{\rule{0.3em}{0ex}}\mathrm{keV}\phantom{\rule{0.3em}{0ex}}\text{to}\phantom{\rule{0.3em}{0ex}}43\phantom{\rule{0.3em}{0ex}}\mathrm{keV}$ and substrate temperatures $({T}_{S})$ from room temperature (RT) to $250\phantom{\rule{0.2em}{0ex}}\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}$. A systematic study on the interplay of ${E}_{\mathit{\text{ion}}}$ and ${T}_{S}$ has revealed a characteristic energy-dependent temperature threshold for $c\text{\ensuremath{-}}\mathrm{B}\mathrm{N}$ growth. This behavior is explained by dynamic annealing of defects caused by a penetrating ion in a collison cascade. In this picture, the suppression of defect accumulation that is crucial for maintaining cubic phase formation is attributed to temperature-driven back diffusion and subsequent annihilation of B and N interstitial recoils. The model is confirmed by analyzing the depth profile of implanted, isotopically pure $^{10}\mathrm{B}$, and its application for both $c\text{\ensuremath{-}}\mathrm{B}\mathrm{N}$ nucleation and growth is discussed.

Reaction–diffusion with a time-dependent reaction rate: the single-species diffusion–annihilation process

We study the single-species diffusion-annihilation process with a time-dependent reaction rate, lambda(t)=lambda_0 t^-omega. Scaling arguments show that there is a critical value of the decay exponent omega_c(d) separating a reaction-limited regime for omega > omega_c from a diffusion-limited regime for omega < omega_c. The particle density displays a mean-field, omega-dependent, decay when the process is reaction limited whereas it behaves as for a constant reaction rate when the process is diffusion limited. These results are confirmed by Monte Carlo simulations. They allow us to discuss the scaling behaviour of coupled diffusion-annihilation processes in terms of effective time-dependent reaction rates.

Diffusional rotation of crystals about a common interface

Abstract Polycrystalline materials can deform by a mechanism that involves grain shape changes occurring by the diffusive flow of material. Since grains maintain contact during flow, grain-boundary sliding is an inevitable consequence. This has confused the understanding of the overall mechanism to the extent that it has been claimed that deformation occurs entirely by sliding. The main evidence for diffusive shape changes comes from observations on precipitate-containing materials. Precipitates act as inert markers, accumulate on vacancy ‘sink’ boundaries and leave zones denuded of precipitate on those that act as sources. Zone size is related to the diffusive strain of each grain, although comparisons with theoretical predictions are confounded by the phenomenon of grain rotation. In the theory of diffusive flow, the creation and annihilation rate of vacancies is assumed constant over any boundary and thus the zone width should also be constant. If grain rotation occurs, however, the rate of vacancy cre...

Epithermal positron dynamics

Energetic positrons implanted into a solid rapidly lose energy, down to the order of an electron volt, through plasmon and single electron scattering. However, the later stages of thermalisation are slower, involving predominantly acoustic phonon scattering, with no discrete boundary between the epithermal and thermal equilibrium states. Both the initial stages of thermalisation (stopping) and the diffusion of thermal positrons have been extensively studied both theoretically and experimentally. Theoretically stopping and the thermalisation processes have been mainly studied using Monte Carlo techniques, whereas the thermal equilibrium state is usually described in terms of diffusion annihilation equations. Attempts to model epithermal positron transport in a similar manner have met with limited success. In this work the dynamics of near-thermal positrons are studied using a Fokker-Planck equation, with particle energy and depth below the surface as stochastic variables. The three components of the diffusion tensor are calculated from the stopping powers of positrons in the low energy limit, due to scattering with acoustic phonons and single electron scattering processes.

EXACT STEADY STATES OF ASYMMETRIC DIFFUSION AND TWO-SPECIES ANNIHILATION WITH BACK REACTION FROM THE GROUND STATE OF QUANTUM SPIN MODELS

We calculated the exact probability distribution of the equilibrium state of some dynamical models in d-dimensional hypercubic lattices (d≥1). In these models we have asymmetric diffusion or two-species annihilation with back reaction A+B↔Ø. Our results are derived by exploring the relationship between the master equation and the Hamiltonian of quantum spins. The models we study are related to the spin-S Heisenberg model.

Diffusion annihilation in one dimension and kinetics of the Ising model at zero temperature.

The relationship between the one-dimensional kinetic Ising model at zero temperature and diffusion annihilation in one dimension is studied. Explicit asymptotic results for the average domain size, average magnetization squared, and pair-correlation function are derived for the Ising model for arbitrary initial magnetization. These results are compared with known results for diffusion annihilation, and it is shown that there is only partial equivalence between the Ising model and diffusion annihilation. The results of Monte Carlo simulations for the domain-size distribution function for different initial magnetizations are also presented. In contrast to the case of diffusion annihilation, the domain-size distribution scaling function h(x) is found to depend nontrivially on the initial magnetization. The exponent \ensuremath{\tau} characterizing the small-x behavior of h(x) is determined exactly and is shown rigorously to be the same for both the Ising model and diffusion annihilation.

Origin of the annihilation radiation from the galactic center region

Abstract Observations show that there are two sources of positron annihilation radiation from the region of the Galactic Center: a variable, compact source at or near the Galactic Center and a steady, diffuse source in the galactic disk. We suggest that the annihilating positrons responsible for the variable, annihilation line radiation, observed from 1977 through 1979, result from photon-photon pair production, most likely around an accreting black hole, and that the annihilating, interstellar positrons, responsible for the steady, diffuse galactic annihilation radiation, result from the decay of radionuclei produced by thermonuclear burning in supernovae.

Aggregation in the presence of sources and sinks: A scaling theory.

A scaling generalization of the Smoluchowski equation is used to treat fluctuation effects in aggregation problems. In particular, we investigate the diffusion-limited cluster-cluster aggregation subject to the condition that single particles are fed into the system at a constant rate, h, while clusters larger than a fixed size are removed. Considering the zero-feed-rate limit as a critical point, we find that h plays the role of an external field conjugate to the order parameter which turns out to be the cluster density. The cluster density obeys dynamic scaling and, because of the finiteness of a kinetic coefficient, the dynamic critical exponents are expressible in terms of a static exponent. The exponents are determined by arguing that the zero-feed-rate process is in one universality class with the A+A\ensuremath{\rightarrow}0 diffusive annihilation problem. Our scaling theory is in agreement with available Monte Carlo simulation data.

Novel dimension-independent behaviour for diffusive annihilation on percolation fractals

The authors report the first studies of diffusive annihilation on fractal structures. They find super-universal (d-independent) behaviour for the time decay of the particle density; specifically, for the reaction A+A to 0 they find rho A approximately t-23/, while for the reaction A+B to 0 they find rho A approximately t-13/. A scaling theory for diffusive annihilation is developed that predicts that the first exponent is 1/2ds and the second is 1/4ds, where ds=2df/dw is the spectral dimension. Thus the findings support the Alexander-Orbach conjecture (1982) that ds is independent of d.

On the concentration of relic magnetic monopoles in the universe

The modern concentration of relic 't Hooft-Polyakov monopoles is shown to amount to ∼ 10 −19 cm −3, being determined by the diffusion annihilation rate at t<10 −5 s. To eliminate the contradiction with experimental upper limits some mechanism similar to quark confinement should forbid free monopole existence.