Change In Atomic Density Research Articles

Strong laser fields and their power to generate controllable high-photon-number coherent-state superpositions

Recently, intensely driven laser-matter interactions have been used to connect the fields of strong laser field physics with quantum optics by generating non-classical states of light. Here, we make a further key step and show the potential of strong laser fields for generating controllable high-photon-number coherent-state superpositions. This has been achieved by using two of the most prominent strong-laser induced processes: high-harmonic generation and above-threshold ionization. We show how the obtained coherent-state superpositions change from an optical Schr\"odinger "cat" state to a "kitten" state by changing the atomic density in the laser-atom interaction region, and we demonstrate the generation of a 9-photon shifted optical "cat" state which, to our knowledge, is the highest photon number optical "cat" state experimentally reported. Our findings anticipate the development of new methods that naturally lead to the creation of high-photon-number controllable coherent-state superpositions, advancing investigations in quantum technology.

(Invited) Sub-Nm Quantification of Point Defects in Ceramics and Semiconductors Using Atom Probe Tomography

Many material properties are dependent upon local defect chemistry and associated charge compensations. Properties such as electronic and ionic transport, intergranular fracture, dielectric constant, and electrical breakdown are dependent upon the accumulation of point defects at internal interfaces such as grain boundaries and heterojunctions. Quantification of these point defect accumulations has been difficult, either due to light element sensitivity, detectability limits, or the 3-D nature of the internal interfaces. Recent work has shown that Atom Probe Tomography (APT) has the requisite counting statistics, 3-dimensionality, detectability limits, and spatial resolution to quantify point defect accumulations at grain boundaries and heterojunctions. Utilizing the 3-D atom by atom nature of APT data has enabled the conversion of analytical characterization data directly into space charge voltages and band alignments, ultimately enabling the direct relationship between materials characterization and materials or device properties. The ability of APT to quantify point defect distributions in 3-D has limitations that are dependent upon the type of defect. Individual vacancies on either oxygen or metal cation sites (Vo ** or Vm ’) have been quantified with APT at grain boundaries and heterojunctions. Quantification of vacancies are heavily dependent upon the detection efficiency of the APT experiment and upon the volume being sampled. Interstitial oxygen or metal cations (Oi ” or Mi *) can be observed but again is limited to the counting statistics of the volume being analyzed. However, their exact location can not be determined at this point as APT is limited in the necessary spatial resolution. Substitutional cations and defect pairs (MN ’) can be quantified as long as the cations are distinguishable in the mass spectrum. Substitutional defects coupled with vacancies (common ordered defect pairs) have been quantified at individual grain boundaries and, in some cases, allow for the calculation of electronic energy levels at interfaces. Schottky defects (Vo ** + Vm ’’) may be quantified using APT, as these defect pairs result in a change in atomic density, however, their quantification is limited by the detection efficiency of the APT instrument. Frenkel pairs on either the oxygen or metal cation sites (Vo ** + Oi ”) do not result in a change in atomic density and are only dependent upon site occupancies. Similar to interstitials, APT does not have the spatial resolution or detection efficiency necessary to identify individual Frenkel pairs. Such analyses will have to wait for Atomic Scale Tomography, of which APT may play a part. Ultimately, the combination of APT’s 3-D spatial and chemical resolution combined with analytical techniques such as EELS that allows for cation valence and free carrier energy levels to be determined will be the most complete solution to defect chemistry quantification.

Density dependence of the room temperature thermal conductivity of atomic layer deposition-grown amorphous alumina (Al2O3)

We report on the thermal conductivity of atomic layer deposition-grown amorphous alumina thin films as a function of atomic density. Using time domain thermoreflectance, we measure the thermal conductivity of the thin alumina films at room temperature. The thermal conductivities vary ∼35% for a nearly 15% change in atomic density and are substrate independent. No density dependence of the longitudinal sound speeds is observed with picosecond acoustics. The density dependence of the thermal conductivity agrees well with a minimum limit to thermal conductivity model that is modified with a differential effective-medium approximation.

TRI3DYN – Collisional computer simulation of the dynamic evolution of 3-dimensional nanostructures under ion irradiation

A computer program is described which allows fully three-dimensional dynamic collisional simulations of ion irradiation effects in particular in nanosystems. The program is based on the well-known TRIDYN code which describes dynamic modifications along one dimension. In a computational volume composed of fixed and equisized voxels, local atomic density changes due to ion implantation or recoil relocation are relaxed to constant predefined atomic volumes. This is accomplished by interaction with neighboring voxels and by material transport towards and away from the surface. Arbitrary initial system shapes and elemental distributions can be defined as well as a wide range of irradiation conditions including non-uniform beam profiles. The paper addresses details of the involved algorithms and discusses potential artifacts with respect to surface sputtering, broadening of compositional profiles, and surface contours. It demonstrates the capabilities of the program by examples of focused ion beam erosion, self-organized surface pattering, and ion implantation and sputter-shaping of nanostructures.

A study of laser action on a free-molecular flow of atoms in a capillary

We found experimentally that resonance laser irradiation of a rarefied flow of atoms in a capillary causes their density at the exit to decrease. We established a dependence of the change in atomic density on the excitation intensity. The observed effect is shown to be related to the flow heating caused by hyperelastic collisions of excited atoms with the capillary wall. We describe the gas heating in a capillary taking into account the energy relaxation of hot atoms as they collide with the wall. We show that our experimental results can be explained by assuming that the energy accommodation coefficient for hot atoms during their collisions with the wall is close to unity.

Determination of interface growth with atomic resolution in FeCo–Si multilayers

In situ fast kinetic ellipsometry, X-ray and polarized neutron scattering are used to investigate the growth of sputtered Fe 89Co 11–Si multilayers with atomic resolution and to determine interface thickness and composition. The values of these properties depend on the sputtering parameters. Under the conditions of minimal thickness of the interface layers we find on top of the silicon layer 15 Å (Fe 89Co 11) 0.5Si 0.5 and on top of the iron cobalt layer 19 Å (Fe 89Co 11) 0.67Si 0.33. During the growth of the iron cobalt layer a phase transition occurs. After the formation of the interface layer on top of the silicon layer the iron cobalt alloy starts to grow in an amorphous phase until a thickness of 25 Å is reached. Then it crystallizes down to the interface layer whereby it shrinks to 18.5 Å, the atomic density changes nearly proportionally to the imaginary part of the index of refraction. As will be shown, the growth of the layers and the formation of the interface layers can be followed on an atomic scale.

TEACHING NOTES

The text of the article is available in the PDF.

Change in atomic density of glassy carbon by Na ion implantation

A study has been made of the change in atomic density of Na-implanted glassy carbon (GC) with a density of 1.5 g/cm 3, comparing the depth profiles measured by secondary ion mass spectrometry (SIMS) and Rutherford backscattering spectrometry (RBS). Na ions of 50 keV were implanted in GC with doses ranging from 5×10 14 to 1×10 17 ions/cm 2 at room temperature. SIMS was carried out, using Cs + ions of 8.5 keV as primary ions. Secondary ions detected were 12C + and 23Na + ions. RBS was carried out using 1.5 MeV He ions with a fluence of 10 μC. SIMS results show that the Na depth profile is Gaussian for a low dose and a Gaussian-like distribution with a hump for intermediate doses. The profile for the highest dose shows Na enrichment at the surface and wide spreading. The RBS results were similar to those of SIMS. The correspondence between SIMS and RBS profiles suggests that the atomic density of implanted layers increases from 1.5 to 2.2 g/cm 3 at intermediate doses and returns to 1.7 g/cm 3 at a high dose. It was concluded that Na implantation in GC causes the increase in atomic density of GC surface layers.

ガラス状炭素へイオン注入したＮａの深さ分布と注入層の密度変化

ガラス状炭素へイオン注入したＮａの深さ分布と注入層の密度変化

Study on atomic density changes in the liquid Al-Si alloys by X-ray diffraction method

Study on atomic density changes in the liquid Al-Si alloys by X-ray diffraction method

Molecular Dynamics Analysis of Nucleation in Structural Transformation

The characteristics of the microstructure produced by a solid/solid transformation greatly depend on the nucleation process of the product phase. The objective of this study is to obtain a definite image of the atomistic nucleation process of structural transformation, which accompanies a change in atomic density, by clarifying (a) the spatial change with time, and (b) the role of dislocations in the nucleation process in computer-modeled metallic crystals. We employed a NPT ensemble attained by the constraint method proposed by Evans and Morriss. To model a fcc/bcc transformation: (a) relaxation of the initial configuration of perfect and defected fcc lattice is done by using the Finnis-Sinclare potential that stabilizes the fcc structure, and then (b) the system is set to be transformed by applying a bcc potential to all atoms at once. Using Voronoi polyhedra analysis, the crystal structure is distinguished on an atomic scale. Changes in crystal structure are found to occur in the system modeled by the NTP ensemble and the potential-change procedure employed in the present study. In a perfect crystal, the change in the number of bcc constituent atoms saturates at about 40% in all atoms in the system and consequently the structural transformation does not proceed further. It is thought that this is the state of fluctuation. In a crystal containing a dislocation, structural transformation starts around the dislocation and it is observed that the crystallographic orientation of formed crystals is restricted. The number of bcc constituent atoms increases steadily. Whether nucleation of structural transformation occurs or not is clearly distinguished by the initial state of the crystal.

Direct imaging of spatially varying potential and charge across internal interfaces in solids

The importance of spatially varying potential (and thus charge) across lattice discontinuities in solids has been recognized in many technologically important systems, especially those containing electrically active interfaces, e.g. electroceramics. The presence of spatially varying potential across electroceramic interfaces has been indirectly deduced and analyzed using predominantly bulk measurement techniques like I-V, C-V curves and impedance spectroscopy. Direct imaging of spatially varying electrostatic potential profile and determination of the sign, magnitude and spatial distribution of the associated interface- and space- charge (and therefore defect density) in electroceramics have remained elusive.We have utilized the technique of transmission high energy electron holography to directly image and quantify the electrostatic potential across grain boundaries (GBs) in SrTiO3, a functional electroceramic. The phase of the exit wave function at the GB region can be altered by : (i) variation in local mean inner potential (related to the change in density of atoms at the GB), (ii) differential diffraction conditions across the interface, (iii) change in the local specimen thickness and, (iv) presence of local electrostatic (electrical charge) and magnetic potential.

Atomic density changes in carbon due to carbon bombardment

A discrepancy occurred between two measured results in the literature concerning the peak position of the sp 3 content of deposited—mainly sp 2 bonding—carbon layers. The peak position was found at about 40 eV projectile energy in the case of MSIB, while at about 90 eV in MWCVD and RFCVD deposition. A Monte Carlo simulation is used to demonstrate a possible reason for the difference. It can be understood via simulation that the masses of the bombarding species are responsible for the different density peak positions. In the first experimental case carbon atoms, in the second cases methyl radicals were used in the nucleation technique.

High resolution study of the structure of the Σ=5 [001] twist grain boundary in NiO

The existence of grain boundaries (GBs) in many advanced ceramics can significantly alter their performance, rendering the study of GB structure an essential part of understanding ceramic properties on a fundamental level. Furthermore, the debate regarding the stability of [001] twist GBs in rocksalt-structured oxides provides the impetus for comparing experimental observations of the Σ=5 [001] twist GB structure in NiO with the stable GB structure calculated by Tasker and Duffy (Fig. 1). The Tasker and Duffy structure was obtained by introducing a Schottky defect in each of the GB unit cells, resulting in a 20% decrease in atomic density at the GB. High resolution TEM (HRTEM) employing Fresnel fringe and lattice imaging techniques (Fig. 2) supplemented with image simulations of six possible configurations for the NiO Σ=5 [001] twist GB structure using the TEMPAS program (Fig. 3) were the primary experimental methods used in an attempt to detect this reduced atomic density in NiO Σ=5 [001] twist GBs manufactured by a modified Schober-Balluffi method. Since the Fresnel fringe technique consists of a change in Fresnel fringe contrast with defocus due to a change in mean inner potential at the GB, it becomes necessary to consider the contribution of the various causes of a change in mean inner potential to the resulting contrast. These causes include: (i) a change in atomic density or a change in atomic structure at the GB, (ii) the existence of a space charge at the GB, (iii) the existence of a thin amorphous film at the GB, (iv) the presence of impurities at the GB or a change in composition across the GB, and (v) GB grooving. The contribution of a GB space charge to the change in mean inner potential at the GB is assumed to be small, given that the space charge associated with with a GB in an ionic crystal is on the order of a few tenths of a volt, whereas the mean inner potential at the GB is on the order of 15 - 20 V. An attempt to address the impurity and grooving issues was made by performing EDS and EELS across the GB in an ultra-high vacuum, high resolution STEM.

The effect of surface reconstruction on the capacitance of Au(100) surfaces

The effect of reconstruction of the Au(100) surface on the double-layer capacity was measured using standard ac methods combined with UHV surface preparation and ex situ structure analysis. The (5 × 20) reconstruction present on the UHV-prepared surface was found to be stable in dilute HClO 4 over a wide range of electrode potential. In dilute H 2SO 4, the (5 × 20) reconstruction was lifted at potentials near the pzc by specific adsorption of HSO − 4 even when the anion was present at very low concentrations, eg 10 −5 M. In 0.01 M HClO 4, the reconstruction was lifted only at potentials 0.2 V positive of the pzc, and was restored when the potential was made ca 0.1 V negative of the pzc for the (1 × 1) surface, ie the (5 × 20) ⇋ (1 × 1) transformation was quasi-reversible in dilute HClO 4. Atomic scale roughness, eg steps, appeared to lower the stability of the (5 × 20) structure. The capacity curve for the (5 × 20) reconstructed surface was dramatically different from that of the (1 × 1) surface, with a negative shift in the pzc of ca 0.2 V following the transformation (5 × 20) → (1 × 1), and a decrease in the capacitance of 20–25%, approximately equal to the change in atomic density of the surface. The vacuum work functions (Φ) of the emersed (5 × 20) and (1 × 1) suraces were measured using photoemission spectroscopy, and found to be 5.30 ± 0.05, and 5.05 ± 0.05 eV, respectively, which gives a ΔΦ consistent with the relative values of the pzcs. Our results were consistent in every respect with the previous findings of Kolb and co-workers on the effect of reconstruction on the double-layer capacity of Au(100) surfaces.

Stability of magnetic and magneto-optic properties in TbCo sputtered films

Stability of magnetic and magnetooptic properties in TbCo sputtered films is investigated in air at room temperature. Various kinds of substrate bias voltage is applied during sputtering. Saturation magnetization changes fastest in the film with -200 V bias due to preferential oxidation of Tb. The rate limiting step of this change is diffusion. Faraday rotation angle for saturation magnetization does not change with time, but Kerr rotation angle increases with oxidation. Change in atomic density of the TbCo film with substrate bias is investigated by Rutherford backscattering experiment. Stability of the film is related to the film density.

Diffusion and Heterogeneous Reaction. X. Diffusion Coefficient Measurements of Atomic Oxygen through Various Gases

Steady-state and time-transient measurements of the decay of oxygen atoms in a cylinder with catalytic walls have been used to determine the diffusion coefficient of atomic oxygen in O2, He, Ar, and Kr. The relative change in atom density as a function of distance (under steady-state conditions) and time (after the atom source is shut off) was followed by observing the luminescence associated with the reaction of oxygen atoms with nitric oxide. The diffusion coefficients at 298°K and 1 atm were found to be: Do−o2 = 0.27; Do−He = 0.73; Do−Ar = 0.39; Do−Kr = 0.16 cm2sec−1.