Atomic Velocity Research Articles

Resistivity as a Witness of Local Crystal Growth Conditions

A detailed knowledge of the growth front geometry during physical vapor transport (PVT) growth of SiC single crystals is beneficial to achieve high quality n+ SiC substrates for power device applications. In this report we show how mapping of resistivity in SiC wafers can shed light on local growth conditions, which are very difficult-to-study in situ. We consider both thermodynamic quantities (absolute temperature T and partial pressure pN₂) and geometric characteristics of the growth surface relevant to growth kinetic parameters, namely atomic terrace width and atomic step velocity. Specifically, we show how an elevation map of the growth surface can be reconstructed from a spatially-resolved measurement of resistivity in a SiC wafer by integrating the spatial derivative of elevation with respect to the basal plane, which is assumed to be related to local resistivity through dependence of non-equilibrium nitrogen incorporation on the atomic step velocity.

Molecular dynamics simulations and analyzation of Cu deposited on stainless steel substrate surfaces

Copper (Cu) is used in integrated circuits and microdevices and has the potential to replace aluminum alloys due to its low resistivity, strong electromigration properties, and affordability. However, a significant factor that influences the performance of devices at the micro and nano scales is the surface roughness of the deposits. LAMMPS software is employed to simulate the deposition Cu on an ideal state for a stainless-steel substrate. The deposition process and deformation behavior of Cu on the surface and the roughness of the deposition surface are analyzed. Taking the deposition process of Cu atoms as an object, the effects of different atomic numbers, different temperatures, different velocities, and different heights on the surface roughness of the deposits were investigated. The atomic structure composition of the deposition velocity is analyzed, and the radial distribution function is analyzed to reveal the microscopic mechanism of action. The results of the theoretical deposition and analysis show that the surface roughness increases with the number of atoms deposited and decreases with increasing substrate temperature. The surface roughness first decreases and then, after some fluctuation, stays constant at a particular level with increasing velocity. Additionally, as the deposition height increases, the surface roughness reduces. There is a nonlinear relationship between the various components and the deposited surface roughness. The surface quality of deposits can be improved during the deposition process by optimizing the deposition parameters of deposition atoms, substrate temperature, deposition velocity, and deposition height.

Wavelength modulation laser-induced fluorescence for plasma characterization.

Laser-Induced Fluorescence (LIF) spectroscopy is an essential tool for probing ion and atom velocity distribution functions (VDFs) in complex plasmas. VDFs carry information about the kinetic properties of species that is critical for plasma characterization. Accurate interpretation of these functions is challenging due to factors such as multicomponent distributions, broadening effects, and background emissions. Our research investigates the use of Wavelength Modulation (WM) LIF to enhance the sensitivity of VDF measurements. Unlike standard Amplitude Modulation (AM) methods, WM-LIF measures the derivative of the LIF signal. This approach makes variations in VDF shape more pronounced. VDF measurements with WM-LIF were investigated with both numerical modeling and experimental measurements. The developed model enables the generation of both WM and AM signals, facilitating comparative analysis of fitting outcomes. Experiments were conducted in a weakly collisional argon plasma with magnetized electrons and non-magnetized ions. Measurements of the argon ion VDFs employed a narrow-band tunable diode laser, which scanned the 4p4D7/2-3d4F9/2 transition centered at 664.553nm in vacuum. A lock-in amplifier detected the second harmonic WM signal, which was generated by modulating the laser wavelength with an externally controlled piezo-driven mirror of the diode laser. Our findings indicate that the WM-LIF signal is more sensitive to fitting parameters, allowing for better identification of VDF parameters such as the number of distribution components, their temperatures, and velocities. In addition, WM-LIF can serve as an independent method to verify AM measurements and is particularly beneficial in environments with substantial light noise or background emissions, such as those involving thermionic cathodes and reflective surfaces.

Influence of Sn grain orientation on mean-time-to-failure equation for microbumps in 3D IC technology

The mean-time-to-failure (MTTF) associated with microbumps is a pivotal factor influencing the reliability of cutting-edge consumer electronic devices. Within Sn-based solder joints, the orientation of Sn grains significantly impacts the diffusion velocity of Cu and Ni atoms, thereby impacting MTTF. While a large solder joint encompassing numerous Sn grains can be treated as isotropic, the scenario shifts in a small microbump with just one Sn grain, rendering it anisotropic. In such cases, the Sn grain orientation emerges as a dominant factor dictating the microbump's longevity. Herein, we investigated the influence of Sn grain orientation on MTTF of electromigration, thermomigration, and thermal aging. A new parameter, α, was introduced, which signifies the angle between the c-axis of the Sn grain and the direction of external stressing.

Self-calibrated atom-interferometer gyroscope by modulating atomic velocities.

Atom-interferometer gyroscopes have attracted much attention for their long-term stability and extremely low drift. For such high-precision instruments, self-calibration to achieve an absolute rotation measurement is critical. In this work, we propose and demonstrate the self-calibration of an atom-interferometer gyroscope. This calibration is realized by using the detuning of the laser frequency to control the atomic velocity, thus modulating the scale factor of the gyroscope. The modulation determines the order and the initial phase of the interference stripe, thus eliminating the ambiguity caused by the periodicity of the interferometric signal. This self-calibration method is validated through a measurement of the Earth's rotation rate, and a relative uncertainty of 162 ppm is achieved. Long-term stable and self-calibrated atom-interferometer gyroscopes have important applications in the fields of fundamental physics, geophysics, and long-time navigation.

Mechanism of enhanced thermal conductivity of hybrid nanofluids by adjusting mixing ratio of nanoparticles

At certain mixing ratios, hybrid nanofluids exhibit superior thermal conductivity performance compared with mono nanofluids, without significant enhancements in viscosity. Studying the microstructures formed by nanoparticles in the base fluid is key to revealing the mechanism of enhanced heat transfer in nanofluidic systems. Here, effects of varying the nanoparticle volume fraction (0.5–2.0 vol%) and Cu:Al volume mixing ratio (20:80, 40:60, 50:50, 60:40, and 80:20) of Cu-Al/Ar hybrid nanofluids were investigated using non-equilibrium molecular dynamics (NEMD) simulations at a temperature of 85 K. The mechanism of thermal conductivity enhancement was elucidated through quantitative analysis of the nanolayer structure and atom trajectory. The results showed that the thermal conductivities of Cu-Al/Ar hybrid nanofluids with a mixing ratio of 20:80 were lower than those of the corresponding Cu/Ar mono nanofluids with volume fraction below 1.3 vol% due to the low quantity of Cu nanoparticles. However, when the quantity of Cu nanoparticles was increased by using mixing ratios from 40:60 to 80:20, the thermal conductivities were higher at all volume fractions. Compared to Cu/Ar nanofluids, the thermal conductivity of Cu-Al/Ar hybrid nanofluids improved from 0.149 to 0.166 W/(m·K) which was higher 2.76 % to 14.48 % as the mixing ratio increased from 20:80 to 80:20 at 2.0 vol%. Radial distribution function (g(r)), the number density and diffusion rate calculations indicated that the density and velocity of Ar atoms around Cu atoms were higher than those of Al atoms due to stronger interactions between Cu-Ar atoms. The main reason for the enhanced thermal conductivity of hybrid nanofluids with increasing mixing ratio was the higher unit nanolayer density and Brownian motion velocity.

Modelling infrared spectra of the O-H stretches in liquid H2O based on a deep learning potential, the importance of nuclear quantum effects

ABSTRACT In this study, we have trained a deep learning (DL) potential for water using training datasets obtained from the DPLibrary (https://dplibrary.deepmd.net/). Subsequently, we conducted classical molecular dynamics simulation (DeePMD) and path-integral MD simulation (PI-DeePMD) of liquid water based on this deep potential (DP). Using the velocity-velocity auto-correlation function (VVAF) formula, we constructed infrared (IR) spectra for the O-H stretching mode based on the DeePMD simulation trajectories. Our results showed that the DeePMD/VVAF approach accurately captured the experimental result for O-H stretching vibration, with the O-H vibrational peak located at 3400 cm−1. Additionally, the PI-DeePMD approach successfully predicted the red-shift and broadening of the O-H stretching band of water, emphasising the importance of nuclear quantum effects (NQEs). In particular, the PI-DeePMD simulation correctly reproduced the shoulder located at 3250 cm−1, which was underestimated by classical DeePMD simulation. The success of the PI-DeePMD/VVAF approach can be attributed to the following reasons: (1) the DeePMD simulation improves the accuracy of calculating atomic velocities due to the DFT-level accuracy of the DP model; (2) the PI-DeePMD simulation takes into account the contribution from nuclear quantum effects; (3) the non-Condon effect is considered in the VVAF formalism.

Molecular dynamics investigation of the effect of nanostructured surfaces on flow boiling

As a prominent limiting factor in the performance of microelectronic devices, the improvement of heat dissipation is of utmost importance. The enhancement of heat and mass transfer during boiling by nanostructured surfaces has been demonstrated in many studies. In this study, a nanoscale flow-boiling model is developed by molecular dynamics method in order to deeply investigate the microscopic mechanism of nanostructure-enhanced heat transfer. Simple liquid argon is heated by grooved substrate to explore the effect of the depth of the cavities on the boiling heat transfer. The results indicate that the grooved surface improves heat transfer performance through two key mechanisms. Firstly, it induces bubble nucleation, and secondly, it delays the onset of film boiling. The presence of cavities allows the argon atoms within to absorb additional energy from the surrounding walls and be in closer proximity to the heating layer. This reduction in solid thermal resistance results in increased heat transfer efficiency, leading to earlier bubble nucleation and a greater propensity for nucleation within the cavities. The low horizontal velocity of argon atoms within the cavities suggests their ability to retain a significant number of atoms, thereby effectively mitigating the deterioration of heat transfer caused by the formation of a vapor film on the solid wall. The comprehensive performance is observed to improve with increasing depth of the cavities within the range investigated in this study.

Continuous cold rubidium atomic beam with enhanced flux and tunable velocity.

We present a cold atomic beam source based on a two-dimensional (2D)+ magneto-optical trap (MOT), capable of generating a continuous cold beam of 87Rb atoms with a flux up to 4.3 × 109 s-1, a mean velocity of 10.96(2.20) m/s, and a transverse temperature of 16.90(1.56) µK. Investigating the influence of high cooling laser intensity, we observe a significant population loss of atoms to hyperfine-level dark states. To account for this, we employ a multiple hyperfine level model to calculate the cooling efficiency associated with the population in dark states, subsequently modifying the scattering force. Simulations of beam flux at different cooling and repumping laser intensities using the modified scattering force are in agreement with experimental results. Optimizing repumping and cooling intensities enhances the flux by 50%. The influence of phase modulation on both the pushing and cooling lasers is experimentally studied, revealing that the mean velocity of cold atoms can be tuned from 9.5 m/s to 14.6 m/s with a phase-modulated pushing laser. The versatility of this continuous beam source, featuring high flux, controlled velocity, and narrow transverse temperature, renders it valuable for applications in atom interferometers and clocks, ultimately enhancing bandwidth, sensitivity, and signal contrast in these devices.

Production of warm ions in electron beam generated E × B plasma

Several recent experiments have demonstrated low-damage processing of 2D materials, such as graphene and single crystal diamond, using electron beam (e-beam) generated plasmas with applied crossed electric and magnetic (E × B) fields. The low damage of these sensitive materials is commonly attributed to the low energy of ions incident to the substrate surface and the ion confinement in E × B fields. In this work, measurements of atom and ion velocity distribution functions in an e-beam E × B plasma at sub-mTorr argon pressures using a laser-induced fluorescence diagnostic revealed the presence of a warm population of ions with temperatures of ∼ 1 eV that are sufficient to destroy the ion confinement in E × B fields and drive the ion flux by cross field diffusion in the direction opposite to the applied electric field, toward the plasma-bounded walls or substrate. Thus, it is this nearly ambipolar diffusion process that is responsible for the flux of charged particles impinging on the wall/substrate surface.

Analytical Solution of the Mixed Problem on a Segment for One-dimensional Ionization Equations in the Case of Constant Velocities of Atoms and Ions

Analytical Solution of the Mixed Problem on a Segment for One-dimensional Ionization Equations in the Case of Constant Velocities of Atoms and Ions

Controlled multiple spectral hole burning via a tripod-type atomic medium

In limit of saturation spectroscopy, we theoretically study the spectral hole burning (SHB) in the absorption spectrum of a probe field through a tripod atomic system. The response function for the probe field is calculated in a Doppler-broadened medium. Burning of spectral holes is observed only for the counter propagation of either one or both the coupling fields in the medium. The SHB is not observed below some critical temperature which is a condition for the electromagnetically induced transparency (EIT) in the medium. The most interesting and significant feature is that the Doppler broadening acts as a decoherence effect in case of EIT, however, the Doppler broadening acts inversely in case of SHB and consequently the burning effect enhances. The SHB is further enhanced and controlled by classes of the average velocity of atoms. The classes of high average atomic velocity in the medium increase the number of spectral hole burns (HBs). The widths of HBs can be controlled by the intensity of the driving fields. A single HB can be switched to multiple HBs in a well-controlled manner using different classes of high average atomic velocity. The various switchable holes can be burned in a desired position of the absorption spectrum which in turn simultaneously slow down multiple probe fields. The phenomenon of SHB may be useful in the construction of multichannel optical switching and storage devices.

Investigating the effect of external heat flux on the crack growth process in an aluminum nanoplate using molecular dynamics simulation

The effect of heat flux (HF) is crucial for understanding the behavior of materials, particularly in terms of crack propagation. HF can induce thermal stresses that accelerate crack growth, which can compromise the structural integrity and safety of materials. However, by studying the effect of HF, we can gain insights into the mechanisms that lead to crack initiation and growth. The importance of this study lies in its contribution to understanding the impact of external HF (EHF) on the cracking behavior of Aluminum nanoplates. For this purpose, EHF with 0.01, 0.02, 0.03, 0.05, and 0.1 W/m2 are added to the initial structure. By inserting EHF into the modeled samples, the atomic evolution of them is investigated. This study was performed using LAMMPS software and molecular dynamics (MD) simulation. Numerically, the maximum velocity of atoms increased from 4.81187 Å/ps to 4.81236 Å/ps as the EHF rose from 0.01 W/m2 to 0.1 W/m2. The maximum stress of samples increases from 151.689 GPa to 151.712 GPa by the EHF ratio enlarging. Finally, the crack length reaches to the maximum value (33.492 Å) by the EHF ratio set to 0.03 W/m2.By investigating the atomic evolution and crack growth process under different EHF values, the study provides valuable insights into the underlying mechanisms and thermal effects that influence crack propagation in these materials. This knowledge can be utilized to optimize the design and performance of aluminum nanoplates in practical applications, such as in the development of more robust and reliable structural components or devices.

Analytical Solution of Mixed Problems for the One-Dimensional Ionization Equations in the Case of Constant Velocities of Atoms and Ions

We consider the main initial–boundary value (mixed) problems for the nonlinear system of one-dimensional gas ionization equations in the case of constant velocities of gas atoms and ions resulting from ionization. The atom and ion concentrations are the unknowns in this system. We find a general formula for a sufficiently smooth solution of the system. It is shown that mixed problems for the system of one-dimensional ionization equations admit integration in closed-form analytical expressions. In the case of a mixed problem for a finite interval, the analytical solution is constructed using recurrence formulas each of which is defined in a triangle belonging to some triangulation, specified in the paper, of the domain where the unknown functions are defined.

Analysis of Velocity Autocorrelation Functions from Molecular Dynamics Simulations of a Small Peptide by the Generalized Langevin Equation with a Power-Law Kernel.

Internal motions play an essential role in the biological functions of proteins and have been the subject of numerous theoretical and spectroscopic studies. Such complex environments are associated with anomalous diffusion where, in contrast to the classical Brownian motion, the relevant correlation functions have power law decays with time. In this work, we investigate the presence of long memory stochastic processes through the analysis of atomic velocity autocorrelation functions. Analytical expressions of the velocity autocorrelation function spectrum obtained through a Mori-Zwanzig projection approach were shown to be compatible with molecular dynamics simulations of a small helical peptide (8-polyalanine).

Effect of atom diffusion on the efficiency of Bragg diffraction in atom interferometers.

The transition efficiency of atomic Bragg diffraction as mirrors and beam splitters in Bragg atom interferometers plays an essential role in impacting the fringe contrast and measurement sensitivity. This can be attributed to the properties of atomic sources, Bragg pulse shapes, the pulse duration, and the relative position deviation of the atoms and Bragg pulses. Here, we investigate the effect of the atomic source's diffusion and velocity width on the efficiency of Bragg diffraction of the moving cold atomic cloud. The transfer efficiency of Bragg mirrors and beam splitters are numerically simulated and experimentally measured, which are well consistent in comparison. We quantify these effects of atomic diffusion and velocity width and precisely compute how Bragg pulses' efficiencies vary as functions of these parameters. Our results and methodology allow us to optimize the Bragg pulses at different atomic sources and will help in the design of large momentum transfer mirrors and beam splitters in atom interferometry experiments.

Rapid thermal annealing effect on performance variations of solution processed indium–gallium–zinc-oxide thin-film transistors

In this paper, the 1 min annealing effect on the electrical characteristics of solution-processed amorphous indium–gallium–zinc-oxide thin-film transistors (a-IGZO TFTs) was analyzed in an ambient gas environment comprising He, Ar, N2, and O2 and an annealing temperature range from 400 to 600 °C for different active layer thicknesses. Even for the short annealing time of 1 min, the He-annealed TFTs show good performance with a threshold voltage of −1.27 V, a subthreshold slope of 605 mV/dec, and a field-effect mobility of 7.19 cm2/Vs under the thick active layer condition. The resulting good performance of the He-annealed TFT originates from the high thermal velocity of the He atom, which can be confirmed from the x-ray photoelectron spectroscopic measurement by the sharp definition of the active layer near the a-IGZO/gate insulator interface.

Hyperfine transition induced by atomic motion above a paraffin-coated magnetic film

We measured transitions between the hyperfine levels of the electronic ground state of potassium-39 atoms (transition frequency: 460 MHz) as the atoms moved through a periodic magneto-static field produced above the magnetic-stripe domains of a magnetic film. The period length of the magnetic field was 3.8 µm. The atoms were incident to the field as an impinging beam with the most probable velocity of 550 m s−1 and experienced a peak oscillating field of 20 mT. Unwanted spin relaxation caused by the collisions of the atoms with the film surface was suppressed by the paraffin coating on the film. We observed increasing hyperfine transition probabilities as the frequency of the field oscillations experienced by the atoms increased from 0 to 140 MHz for the atomic velocity of 550 m s−1, by changing the incident angle of the atomic beam with respect to the stripe domains. Numerical calculation of the time evolution of the hyperfine states revealed that the oscillating magnetic field experienced by the atoms induced the hyperfine transitions, and the main process was not a single-quantum transition but rather multi-quanta transitions.

Hydration structure and dynamics, ultraviolet–visible and fluorescence spectra of caffeine in ambient liquid water. A combined classical molecular dynamics and quantum chemical study

The hydration structure and related dynamics of caffeine diluted in ambient liquid water have been extensively studied by performing classical molecular dynamics simulations, using our previously developed potential model of caffeine and the TIP4P/2005 water model. The results obtained have revealed that the first hydration shell of caffeine contains on average 56 water molecules. Hydrated caffeine forms in total 3 hydrogen bonds with its neighbor water molecules, with the Ocaffeine … Hwater hydrogen bonds exhibiting similar lifetimes with the ones corresponding to Owater … Hwater hydrogen bonds in liquid water. The self-diffusion coefficient of caffeine has been found to be four times lower than the corresponding value for water, being also in agreement with recent experimental measurements. The presence of water molecules inside the solvation shell of caffeine changes significantly their low-frequency intermolecular vibrations, as reflected on the calculated atomic velocity time correlation functions and corresponding spectral densities. Using the estimated average intermolecular structure of the first hydration shell of caffeine, the molecular cluster caffeine@W56 was optimized via quantum chemical calculations and subsequently the time-dependent density functional theory was used in order to predict the ultraviolet–visible and fluorescence spectra of hydrated caffeine. The results obtained are in agreement with recent experimental studies, which have proposed that such spectroscopic measurements can be used for the direct determination of alkaloids in aqueous extracts of natural products. In this framework, multi-scale molecular modelling providing accurate predictions of experimental data could also be a very useful tool, linking theoretical physical chemistry with analytical chemistry applications.

Analytical Solution of Mixed Problems for the One-Dimensional Ionization Equations in the Case of Constant Velocities of Atoms and Ions

Analytical Solution of Mixed Problems for the One-Dimensional Ionization Equations in the Case of Constant Velocities of Atoms and Ions