Ensemble Of Defects Research Articles

Optical properties of ZnSe(Te) with ytterbium impurity.

We report the results on infrared transmission measurements of non-doped and tellurium-doped crystals of zinc selenide grown from the melt. It was found that non-doped samples feature high transmission (50%-60%) for the wavelengths of 1-22μm. The efficient scintillating crystals of ZnSe(Te) are almost opaque for λ>7 μm. Doping these samples with ytterbium from the gas phase does not achieve any significant transmission increase for non-doped ZnSe samples in the spectral range of 1-22μm. However, it considerably increases (up to 50%) transmission for doped ZnSe(Te) at the wavelengths λ>10 μm. These optical peculiarities were analyzed taking into account restructurization of point defect ensembles created by Te and Yb impurities.

Detection of a weak magnetic field via cavity-enhanced Faraday rotation

We study the sensitive detection of a weak static magnetic field via Faraday rotation induced by an ensemble of spins in a bimodal degenerate microwave cavity. We determine the limit of the resolution for the sensitivity of the magnetometry achieved using either single-photon or multiphoton inputs. For the case of a microwave cavity containing an ensemble of Nitrogen-vacancy defects in diamond, we obtain a magnetometry sensitivity exceeding $0.5~\text{\nano\tesla}/\sqrt{\text{\hertz}}$, utilizing a single photon probe field, while for a multiphoton input we achieve a sensitivity about $1 \text{\femto\tesla}/\sqrt{\text{\hertz}}$, using a coherent probe microwave field with power of $P_\text{in}=1~\text{\nano\watt}$.

Direct comparison of defect ensembles extracted from damage probability and raster scan measurements

The presented study addresses the characterization of nanometer sized defects acting as damage precursors in nanosecond laser pulse duration regime. Two approaches are used to extract distributions of localized damage precursors, namely, damage probability and damage density measurements. Testing is performed on uncoated and SiO2 monolayer film deposited fused silica substrate exposed with pulsed UV irradiation (355 nm, 4.8 ns). Then, a direct comparison of damage precursor ensembles obtained from both methods is carried out. Our analysis indicates apparent differences between both methods that are discussed in detail. Contamination by ablation products is identified as one of the key factors that influence damage density measurements.

<i>In Situ</i> Dynamic Experiments and Modeling in Multiscale Kinetics of Damage-Failure Transition

Scaling regularities that reveal the power law statistics of fragmentation and self-similarity of damage-failure transitions are linked to specific type of critical phenomena in ensembles of typical mesoscopic defects – structural-scaling transition. Taking into account nonlinearity of damage-failure transition the scaling effects were explained as the consequence of subjection of damage kinetics to the intermediate asymptotical (self-similar) solution. This solution has the nature of multiscale blow-up dissipative structures, represents the set of collective modes of defects responsible for the damage localization stage. Original in-situ experiments supported the assumption concerning the role of multiscale blow-up collective modes of defects in qualitative different scenario of dynamic crack propagation, failure of shocked materials, fragmentation statistics.

Non-thermal fixed point in a holographic superfluid

We study the far-from-equilibrium dynamics of a (2+1)-dimensional superfluid at finite temperature and chemical potential using its holographic description in terms of a gravitational system in 3+1 dimensions. Starting from various initial conditions corresponding to ensembles of vortex defects we numerically evolve the system to long times. At intermediate times the system exhibits Kolmogorov scaling the emergence of which depends on the choice of initial conditions. We further observe a universal late-time regime in which the occupation spectrum and different length scales of the superfluid exhibit scaling behaviour. We study these scaling laws in view of superfluid turbulence and interpret the universal late-time regime as a non-thermal fixed point of the dynamical evolution. In the holographic superfluid the non-thermal fixed point can be understood as a stationary point of the classical equations of motion of the dual gravitational description.

Partially ordered vortex lattices in the high-field low-temperature mixed state of quasi-two-dimensional organic superconductors

We report the results of high-field, low-temperature $\ensuremath{\mu}\mathrm{SR}$ measurements of the quasi-two-dimensional organic superconductors $\ensuremath{\kappa}\text{\ensuremath{-}}{(\mathrm{ET})}_{2}\mathrm{Cu}{(\mathrm{NCS})}_{2}$, and $\ensuremath{\kappa}\text{\ensuremath{-}}{(\mathrm{ET})}_{2}\mathrm{Cu}[\text{N}{(\mathrm{CN})}_{2}]\mathrm{Br}$. The $\ensuremath{\mu}\mathrm{SR}$ lineshapes for these compounds indicate the existence of partially ordered vortex lattice phases in the high-magnetic-field regime, up to 2.5 T for the former compound and 4 T for the latter compound. The observed sharp loss of order is found to be consistent with a vortex-lattice melting transition that is predicted by numerical simulations of weakly coupled layers of pancake vortices. It is argued that the robustness of the partially ordered vortex lattice phases could be due to strong flux-line pinning by a dilute ensemble of defects.

Nonequilibrium transitions in ensembles of defects attributed to dynamic localization of plastic deformation

Mechanisms of instability and localization of plastic shear during dynamic deformation of metals has been studied theoretically and experimentally. Instability mechanisms are attributed to collective effects in ensembles of microshears in spatially localized areas. Infrared scanning in situ of the instability zone and consequent study of dislocation substructure confirm the crucial role of nonequilibrium transitions in ensembles of defects during development of localized plastic flow. A model of plastic shear instability was developed based on equations that express the correlation between nonequilibrium transitions and mechanisms of structural relaxation and plastic flow.

On the possibility of increasing lifetime of a neutron generator target through laser-induced nanorelief generation at the film – substrate interface

The evolution of an ensemble of point defects (vacancies and interstitials) produced by irradiating a tritium-saturated titanium target (sorbent film on substrate) with deuterium is considered. It is shown that when the concentration of defects exceeds a certain critical value, the film undergoes transition into a quasi-periodic bent state with simultaneous clustering of defect aggregates at the relief extrema, spontaneously created at the film – substrate interface and at the film free surface. The aggregation of vacancies leads to the formation of pores. The subsequent deadherence or rapture of the film in the regions of large pores at the film – substrate interface leads to irreversible degradation of the target. We discuss the possibility of suppressing this effect by nanostructuring a laser-induced nanorelief at the substrate surface before the sorbent film deposition. Different types of such a relief are experimentally demonstrated.

Damage accumulation in quasibrittle fracture.

The strength of quasibrittle materials depends on the ensemble of defects inside the sample and on the way damage accumulates before failure. Using large-scale numerical simulations of the random fuse model, we investigate the evolution of the microcrack distribution as the applied load approaches the fracture point. We find that the distribution broadens mostly due to a tendency of cracks to coalesce in a way that increases with system size. We study how the observed behavior depends on the disorder present in the sample and relate the results with fracture size effects.

Starobinsky-like inflation in dilaton-brane cosmology

We discuss how Starobinsky-like inflation may emerge from dilaton dynamics in brane cosmology scenarios based on string theory, in which our universe is represented as a three-brane. The effective potential may acquire a constant term from a density of effectively point-like non-pertubative defects on the brane. Higher-genus corrections generate corrections to the effective potential that are exponentially damped at large field values, as in the Starobinsky model, but at a faster rate, leading to a smaller prediction for the tensor-to-scalar perturbation ratio r. This may be compensated partially by logarithmic deformations on the world-sheet due to recoil of the defects due to scattering by string matter on the brane, which tend to enhance the tensor-to-scalar ratio. Quantum fluctuations of the ensemble of D-brane defects during the inflationary period may also enhance the tensor-to-scalar ratio.

Kinetics of defect accumulation and duality of the weller curve in gigacycle fatigue of metals

We propose a model describing the kinetics of accumulation of defects under cyclic loading in metals. Analysis of experimental data on the initial defect distribution and of the role of the free surface of the sample in straining makes it possible to explain the features of generation of fatigue cracks in the bulk, which is typical of the gigacycle fatigue regime at a low stress level. The duality of the Weller curve in the gigacycle fatigue regime is attributed to the emergence of a fine-grain region in the form of a dissipative peaking structure in the defect ensemble.

Modulated optical properties of nonpolar gallium nitride via surface in-situ functionalization with cysteamine assisted phosphoric acid

In-situ functionalization of nonpolar a-plane gallium nitride (GaN) surface was achieved by adding cysteamine to phosphoric acid, aiming to modulate its optical properties. The emission properties and oxide formation were explored through surface characterization with atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS), photoluminescence (PL), and water contact angle. Nonpolar a-plane bulk GaN sample sliced from a GaN boule and nonpolar a-plane GaN thin layer heteroepitaxially grown on r-plane sapphire were used to elucidate the effects of in-situ functionalization of identical surface orientation of GaN crystals with different defect ensembles. The addition of cysteamine to the phosphoric acid solution was found to result in: (i) increased surface roughness, (ii) no change to hydrophobicity, (iii) decreased oxygen content at high solution temperatures and increased gallium and nitrogen content versus phosphoric acid solutions at similar temperatures without cysteamine. The in-situ functionalization resulted in enhanced PL intensity from the nonpolar bulk GaN, while the PL intensity from the nonpolar heteroepitaxially grown GaN layer on sapphire was significantly reduced. The opposite PL modulation was explained by the effects of different defects present in the two samples on the nonradiative recombination.

Scaling Invariance of Fatigue Crack Growth in Aluminum Alloy

The role of the collective behavior of defect ensembles at the crack tip and the laws of fatigue crack propagation in aluminum- magnesium alloy have been studied under conditions of symmetric tension–compression gigacycle loading at 20kHz using ultrasonic fatigue testing machine (Shimadzu USF2000). 3D New View 5010 interferometer profiler high resolution data of defect induced roughness in the crack process zone under fatigue crack path revealed the existence of two characteristic scales: the scale of the process zone and the correlation length that is the scale when the correlated behavior of defect induced roughness has started.

A non Abelian effective model for ensembles of magnetic defects in 3D Yang–Mills theory

We construct a non Abelian model for SU(2) Yang–Mills theory in an Euclidean three-dimensional spacetime and study its different phases. The model contains a center vortex sector coupled to a dual effective field encoding information about how the vortices are paired in the ensemble. The possible phases in the parameter space are interpreted in terms of the proliferation of either closed center vortices or closed chains, where the endpoints of open vortices are attached in pairs to monopole-like defects.

Charge noise and spin noise in a semiconductor quantum device

Solid-state systems which mimic two-level atoms are being actively developed. Improving the quantum coherence of these systems, for instance spin qubits or single photon emitters using semiconductor quantum dots, involves dealing with noise. The sources of noise are inherent to the semiconductor and are complex. Charge noise results in a fluctuating electric field, spin noise in a fluctuating magnetic field at the location of the qubit, and both can lead to dephasing and decoherence of optical and spin states. We investigate noise in an ultra-pure semiconductor using a minimally-invasive, ultra-sensitive, local probe: resonance fluorescence from a single quantum dot. We distinguish between charge noise and spin noise via a crucial difference in their optical signatures. Noise spectra for both electric and magnetic fields are derived. The noise spectrum of the charge noise can be fully described by the fluctuations in an ensemble of localized charge defects in the semiconductor. We demonstrate the "semiconductor vacuum" for the optical transition at frequencies above 50 kHz: by operating the device at high enough frequencies, we demonstrate transform-limited quantum dot optical linewidths.

Current Models of Dynamic Deformation and Fracture of Condensed Matter

The main types (groups) of models of deformation and the fracture, used for the solution of problems of dynamic and shock wave deformation of materials, are considered. All models are divided into three main groups: macroscopic (models of mechanics of continuous medium), microstructural (based on the description of evolutions of ensemble of defects ) and atomistic (are used in calculations by methods of molecular dynamics and quantum mechanics). The short characteristic of models of the listed groups is given. Some approaches to development of the most perspective multilevel models are described. The simple test for applicability of models for the description of shock and wave processes are offered. Approaches to the description of destruction of materials and used at this criterion are considered. The perspective directions of development of models of dynamic deformation and fracture are suggested.

Collective effects in a defect system of the structural element scale under deformation of a heterogeneous material with a regular structure

The accumulation of cracks (fibers break) under tensile deformation in a conventional model composite with a “small-scale” (on the order of 10 μm) structural element is studied by detecting acoustic emission signals in the linear location mode. The statistical analysis of the acoustic emission signal flow shows that an increase in correlation between breaks of fibers in one area is not fatal: the process is suddenly interrupted in this area and simultaneously initiated in the adjacent one. This process is governed solely by the relaxation mechanism of local stress redistribution. An ensemble of defects arising in one area produces a heterogeneity of a new scale. Because of the compatibility of strains, this qualitatively changes the structure of the object on a higher scale level. In the case of multiple micro- and mesocracking, the initiation of a main crack (macrofracture) depends on the stress field redistribution among higher scale areas compared with a typical structural element.

Approach to Dark Spin Cooling in a Diamond Nanocrystal

Using a Hartman-Hahn protocol, we demonstrate spin polarization transfer from a single, optically polarized nitrogen-vacancy (NV) center to the ensemble of paramagnetic defects hosted by an individual diamond nanocrystal. Owing to the strong NV-bath coupling, the transfer takes place on a short, microsecond time scale. Upon fast repetition of the pulse sequence, we observe strong polarization transfer blockade, which we interpret as an indication of spin bath cooling. Numerical simulations indicate that the spin bath polarization is nonuniform throughout the nanoparticle, averaging approximately 5% over the crystal volume, but reaching up to 25% in the immediate vicinity of the NV. These observations may prove relevant to the planning of future bath-assisted magnetometry tests.

Microstructural defect evolution in neutron – Irradiated 12Cr18Ni9Ti stainless steel during subsequent isochronous annealing

Transmission electron microscopy and microhardness measurements were used to examine changes in microstructure and associated strengthening induced in austenitic stainless steel 12Cr18Ni9Ti irradiated to ∼0.001 and ∼5dpa in the WWR-K reactor before and after being subjected to post-irradiation isochronal annealing. The relatively low values of irradiation temperature and dpa rate (∼80°C and ∼1.2×10−8dpa/s) experienced by this steel allowed characterization of defect microstructures over a wide range of defect ensembles, all at constant composition, produced first by irradiation and then by annealing at temperatures between 450 and 1050°C. It was shown that the dispersed barrier hardening model with commonly accepted physical properties successfully predicted the observed hardening. It was also observed that when TiC precipitates form at higher annealing temperatures, the alloy does not change in hardness, reflecting a balance between precipitate-hardening and matrix-softening due to removal of solute-strengthening elements titanium and carbon. Such matrix-softening is not often considered in other studies, especially where the contribution of precipitates to hardening is a second-order effect.

Point Defects in SiC as a Promising Basis for Single-Defect, Single-Photon Spectroscopy with Room Temperature Controllable Quantum States

The unique quantum properties of the nitrogen–vacancy (NV) center in diamond have motivated efforts to find defects with similar properties in silicon carbide (SiC), which can extend the functionality of such systems not available to the diamond. As an example, results of experiments on electron paramagnetic resonance (EPR) and optically detected magnetic resonance (ODMR) are presented suggests that silicon vacancy (VSi) related point defects in SiC possess properties the similar to those of the NV center in diamond, which in turn make them a promising quantum system for single-defect and single-photon spectroscopy in the infrared region. Depending on the defect type, temperature, SiC polytype, and crystalline position, two opposite schemes have been observed for the optical alignment of the high-spin ground state spin sublevels population of the VSi-related defects upon irradiation with unpolorized light. Spin ensemble of VSi-related defects are shown to be prepared in a coherent superposition of the spin states even at room temperature. Zero-field (ZF) ODMR shows the possibility to manipulate of the ground state spin population by applying radiofrequency field. These altogether make VSi-related defects in SiC very favorable candidate for spintronics, quantum information processing, and magnetometry.