bluuebunny/crossref_metadata_2025 huggingface dataset:
Machine learning-based approach for automatically tuned feedback-controlled electromigration
Authors: Iwata Y. , Sakurai T. , Shirakashi J.
Published: June 2020

Feedback-controlled electromigration (FCE) has been employed to control atomic junctions with quantized conductance. An FCE scheme is controlled by many parameters, such as the threshold differential conductance GTH, feedback voltage VFB, and voltage step VSTEP. It is considered possible to achieve a precise and stable control of the quantized conductance by automatically optimizing the FCE parameters. This motivated us to develop an approach based on machine learning (ML) to tune the feedback parameters of FCE. The ML system is composed of three kinds of engines, namely, learning, evaluation, and inference. The learning engine performs the FCE procedure with random parameters, collects various experimental data, and updates the database. Subsequently, four variables and a cost function are defined to evaluate the controllability of the quantized conductance. The evaluation engine scores the experimental data by using the defined cost function. Then, the control quality is evaluated in real time during the FCE procedure. The inference engine selects the new FCE parameter according to the evaluated data. These engines determine the optimal parameters without human intervention and according to the situation. Finally, we actually applied this system to the FCE procedure. The parameter is selected from sample data in the database according to the variation in controllability. As a result, the controllability gradually improves during the FCE procedure that uses the ML system. The results indicate that the proposed ML system can evaluate the controllability of the FCE procedure and change the VFB parameter in real time according to the situation.
Periodic orbits, superintegrability, and Bertrand’s theorem
Authors: Martínez-y-Romero R. P. , Núñez-Yépez H. N. , Salas-Brito A. L.
Published: June 2020

Periodic orbits are the key for understanding classical Hamiltonian systems. As we show here, they are the clue for understanding Bertrand’s result relating the boundedness, flatness, and periodicity of orbits with the functional form of the potentials producing them. This result, which is known as Bertrand’s theorem, was proved in 1883 using classical 19th century techniques. In this paper, we prove such a result using the relationship between the bounded plane and periodic orbits, constants of motion, and continuous symmetries in the Hamiltonian system.
Neutron yield enhancement and suppression by magnetization in laser-driven cylindrical implosions
Authors: Hansen E. C. , Davies J. R. , Barnak D. H. , Betti R. , Campbell E. M. , Glebov V. Yu. , Knauer J. P. , Leal L. S. , Peebles J. L. , Sefkow A. B. , Woo K. M.
Published: June 2020

In inertial confinement fusion, an externally applied magnetic field can reduce heat losses in the compressing fuel thereby increasing neutron-averaged ion temperatures and neutron yields. However, magnetization is only beneficial if the magnetic pressure remains negligible compared to the fuel pressure. Experiments and three-dimensional magneto-hydrodynamic simulations of cylindrical implosions on the OMEGA laser show ion temperature and neutron yield enhancements of up to 44% and 67%, respectively. As the applied axial magnetic field is increased to nearly 30 T, both experiments and simulations show yield degradation. For magnetized, cylindrical implosions, there exists an optimal magnetic field that maximizes the increase in yield. Limiting the fuel convergence ratio by preheating the fuel can further increase the benefit of magnetization. The results demonstrate that it is possible to create a plasma with a density of order 1 g/cm3 and an ion temperature greater than 1 keV with a magnetic pressure comparable to the thermal pressure, a new regime for laser-produced plasmas on OMEGA.
Inferring thermal ion temperature and residual kinetic energy from nuclear measurements in inertial confinement fusion implosions
Authors: Woo K. M. , Betti R. , Mannion O. M. , Forrest C. J. , Knauer J. P. , Goncharov V. N. , Radha P. B. , Patel D. , Gopalaswamy V. , Glebov V. Yu.
Published: June 2020

In inertial confinement fusion implosion experiments, the presence of residual anisotropic fluid motion within the stagnating hot spot leads to significant variations in ion-temperature measurements using neutron time-of-flight detectors along different lines of sight. The minimum ion-temperature measurement is typically used as representative of the thermal temperature. In the presence of isotropic flows, however, even the minimum Deuterium–Tritium (DT) neutron-inferred ion temperature can be well above the plasma thermal temperature. Using both Deuterium–Deuterium (DD) and DT neutron-inferred ion-temperature measurements, we show that it is possible to determine the contribution of isotropic flows and infer the DT burn-averaged thermal ion temperature. The contribution of large isotropic flows on driving the ratio of DD to DT neutron-inferred ion temperatures well below unity and approaching the lower bound of 0.8 is demonstrated in multimode simulations. The minimum DD neutron-inferred ion temperature is determined from the velocity variance analysis, accounting for the presence of isotropic flows. Being close to the DT burn-averaged thermal ion temperature, the inferred DD minimum ion temperatures demonstrate a strong correlation with the experimental yields in the OMEGA implosion database. An analytical expression is also derived to explain the effect of mode ℓ=1 ion-temperature measurement asymmetry on yield degradations caused by the anisotropic flows.
Chimera-like states induced by additional dynamic nonlocal wirings
Authors: Chandran P. , Gopal R. , Chandrasekar V. K. , Athavan N.
Published: June 2020

We investigate the existence of chimera-like states in a small-world network of chaotically oscillating identical Rössler systems with an addition of randomly switching nonlocal links. By varying the small-world coupling strength, we observe no chimera-like state either in the absence of nonlocal wirings or with static nonlocal wirings. When we give an additional nonlocal wiring to randomly selected nodes and if we allow the random selection of nodes to change with time, we observe the onset of chimera-like states. Upon increasing the number of randomly selected nodes gradually, we find that the incoherent window keeps on shrinking, whereas the chimera-like window widens up. Moreover, the system attains a completely synchronized state comparatively sooner for a lower coupling strength. Also, we show that one can induce chimera-like states by a suitable choice of switching times, coupling strengths, and a number of nonlocal links. We extend the above-mentioned randomized injection of nonlocal wirings for the cases of globally coupled Rössler oscillators and a small-world network of coupled FitzHugh–Nagumo oscillators and obtain similar results.
Hydrodynamic motion of guiding elements within a magnetic switchyard in fast ignition conditions
Authors: East J. L. H. , Hume E. J. , Lancaster K. L. , Robinson A. P. L. , Pasley J.
Published: June 2020

Magnetic collimation via resistivity gradients is an innovative approach to electron beam control for the cone-guided fast ignition variant of inertial confinement fusion. This technique uses a resistivity gradient induced magnetic field to collimate the electron beam produced by the high-intensity laser–plasma interaction within a cone-guided fast ignition cone-tip. A variant of the resistive guiding approach, known as the “magnetic switchyard,” has been proposed which uses shaped guiding elements to direct the electrons toward the compressed fuel. Here, the 1D radiation-hydrodynamics code HYADES is used to investigate and quantify the gross hydrodynamic motion of these magnetic switchyard guiding elements in conditions relevant to their use in fast ignition. Movement of the layers was assessed for a range of two-layer material combinations. Based upon the results of the simulations, a scaling law is found that enables the relative extent of hydrodynamic motion to be predicted based upon the material properties of the switchyard, thereby enabling optimization of material-combination choice on the basis of reducing hydrodynamic motion. A multi-layered configuration, more representative of an actual switchyard, was also simulated in which an outer Au layer is employed to tamp the motion of the outermost guiding element of the switchyard.
Sigmoidal synaptic learning produces mutual stabilization in chaotic FitzHugh–Nagumo model
Authors: Parker John E. , Short Kevin M.
Published: June 2020

This paper investigates the interaction between two coupled neurons at the terminal end of a long chain of neurons. Specifically, we examine a bidirectional, two-cell FitzHugh–Nagumo neural model capable of exhibiting chaotic dynamics. Analysis of this model shows how mutual stabilization of the chaotic dynamics can occur through sigmoidal synaptic learning. Initially, this paper begins with a bifurcation analysis of an adapted version of a previously studied FitzHugh–Nagumo model that indicates regions of periodic and chaotic behaviors. Through allowing the synaptic properties to change dynamically via neural learning, it is shown how the system can evolve from chaotic to stable periodic behavior. The driving factor between this transition is representative of a stimulus coming down a long neural pathway. The result that two chaotic neurons can mutually stabilize via a synaptic learning implies that this may be a mechanism whereby neurons can transition from a disordered, chaotic state to a stable, ordered periodic state that persists. This approach shows that even at the simplest level of two terminal neurons, chaotic behavior can become stable, sustained periodic behavior. This is achieved without the need for a large network of neurons.
Electron loss mechanisms in a miniature microwave discharge water neutralizer
Authors: Sato Yosuke , Koizumi Hiroyuki , Nakano Masakatsu , Takao Yoshinori
Published: June 2020

This study analyzes the mechanism of electron loss at the discharge chamber wall of a microwave discharge neutralizer via three-dimensional particle-in-cell simulations with Monte Carlo collisions (PIC–MCCs). The neutralizer employs electron cyclotron resonance discharges with two ring-shaped permanent magnets and 4.2-GHz microwaves, where the plasma is confined by a magnetic mirror. The PIC–MCC simulation results show that the electron extraction efficiency of a water neutralizer can be increased by two times in an optimized magnetic field configuration, which is a higher increased rate than that of a xenon neutralizer. However, the efficiency of 20% is still low (e.g., less than half of the xenon one) because many electrons are lost to the magnet surface. The loss is determined to be due to approximately 5-times higher ratio of electrons inside the loss cone in the water neutralizer than that in the xenon neutralizer. The electron velocity distributions of each neutralizer clearly show that the water neutralizer has a larger fraction of electrons parallel to the magnetic field than the xenon neutralizer. This result is attributed to the large number of electron collisions in the water neutralizer owing to the high neutral gas pressure.
A model of the current sheet in the Earth's magnetotail
Authors: Wang Ruoxi , Vasko Ivan Y. , Artemyev Anton V.
Published: June 2020

The thin current sheet of the Earth's magnetotail is the key element of the entire magnetosphere dynamics that significantly depends on a current sheet configuration. Spacecraft measurements have shown that the ion pressure tensor in thin current sheets is essentially isotropic, but widely used two-dimensional current sheet models with isotropic pressure tensor underestimate the current density in the current sheet. In this paper, we develop a set of one-dimensional kinetic models, which demonstrate that although the plasma can be on average isotropic, due to complex kinetic features of the ion distribution function, the current density can be larger than that predicted by isotropic models. These models demonstrate that the current density in thin current sheets crucially depends on the kinetic features of the ion distribution function.
Electronic trajectories in atomic physics: The chemical bond in the H2+ ion
Authors: Carati A. , Galgani L. , Gangemi F. , Gangemi R.
Published: June 2020

The H2+ ion is the simplest example in which a chemical bond exists, created by one electron between two protons. As all chemical bonds, it is usually considered inexplicable in a classical frame. Here, in view of the extremely large velocities attained by the electron near the protons, we consider a relativistic extension of the standard classical three-body model. This has a great impact since the reference unperturbed system (clamped protons) is no more integrable, and indeed by molecular dynamics simulations, we find that the modification entails the existence of a large region of strongly chaotic motions for the unperturbed system, which lead, for the full system, to a collapse of the molecule. For motions of generic type, with the electron bouncing between the protons, there exists an open region of motions regular enough for producing a bond. Such a region is characterized by the property that the electron’s trajectories have an angular momentum pφ along the inter-nuclear axis of the order of the reduced Planck’s constant ℏ. Moreover, special initial data exist for which the experimental bond length and oscillation frequency of the protons (but not the dissociation energy) are well reproduced. Also, well reproduced is the quantum potential, albeit only in an extended interval about the minimum.
3D modeling and simulation of high-current vacuum arc subjected to real external transverse magnetic field
Authors: Zhao Lihua , Gou Hangyuan , Yang Mengjie , Ren Junwen , Bai Xueyan , Wang Lijun , Huang Xiaolong
Published: June 2020

Vacuum circuit breakers (VCBs) are widely used in power distribution systems. When the VCB in a switchgear interrupts the short circuit current, the U type loop, which consists of a vacuum interrupter, an external busbar and a conductive rod, generates an external transverse magnetic field (ETMF). This field deflects the vacuum arc and affects the interruption performance of the VCB. In this paper, the real magnetic field data of the arc region are obtained by establishing a simulation model of an electrode system, taking into account the real ETMF and real axial magnetic field (AMF). Furthermore, the three dimensional steady magnetohydrodynamic model of the vacuum arc is established by using real magnetic field data as the boundary condition. The vacuum arc characteristics under the combined effect of AMF and the asymmetric transverse magnetic field (TMF) are studied. The simulation results show that the ETMF considerably changes the Bx of the arc column, which demonstrates asymmetry in the positive and negative directions, whereas By demonstrates a weaker asymmetry. Moreover, the asymmetric TMF deflects the distributions of the vacuum arc plasma and increases the partial heat flux density and axial current density. With the increase in the severity of the short circuit, the TMF asymmetry of the arc region is enhanced and the deflection of the vacuum arc plasma is also enhanced.
Measurement of local thermal contact resistance with a periodic heating method using microscale lock-in thermography
Authors: Ishizaki Takuya , Igami Taichi , Nagano Hosei
Published: June 2020

This study proposes a new method for measuring thermal contact resistance using lock-in thermography. By using lock-in thermography with an infrared microscope, the dynamic and spatially resolved temperature behavior of the contact interface was visualized on a microscale with one measurement. In addition, a new thermal contact resistance measurement principle was constructed after solving the three-dimensional thermal conduction equation in the cylindrical coordinates by considering a periodic heat source with a Gaussian intensity distribution and the relative position of the heating point to the sample edge, in the presence of thermal resistance at the contact interface. Consequently, the discontinuous behaviors of the temperature wave, amplitude, and phase lag at the contact interface were observed on a microscale. From that discontinuity, the local thermal contact resistance was analyzed as a fitting parameter by matching the theoretical curve to the measured amplitude and phase lag. Furthermore, the simultaneous analysis of the material thermal diffusivity was demonstrated and the validity of the measurements was confirmed.
Discrete light bullets in passively mode-locked semiconductor lasers
Authors: Seidel Thomas G. , Perego Auro M. , Javaloyes Julien , Gurevich Svetlana V.
Published: June 2020

In this paper, we analyze the formation and dynamical properties of discrete light bullets in an array of passively mode-locked lasers coupled via evanescent fields in a ring geometry. Using a generic model based upon a system of nearest-neighbor coupled Haus master equations, we show numerically the existence of discrete light bullets for different coupling strengths. In order to reduce the complexity of the analysis, we approximate the full problem by a reduced set of discrete equations governing the dynamics of the transverse profile of the discrete light bullets. This effective theory allows us to perform a detailed bifurcation analysis via path-continuation methods. In particular, we show the existence of multistable branches of discrete localized states, corresponding to different number of active elements in the array. These branches are either independent of each other or are organized into a snaking bifurcation diagram where the width of the discrete localized states grows via a process of successive increase and decrease of the gain. Mechanisms are revealed by which the snaking branches can be created and destroyed as a second parameter, e.g., the linewidth enhancement factor or the coupling strength is varied. For increasing couplings, the existence of moving bright and dark discrete localized states is also demonstrated.
Charging and discharging characteristics of a single gold nanoparticle embedded in Al2O3 thin films
Authors: Rezk Ayman , Abbas Yawar , Saadat Irfan , Nayfeh Ammar , Rezeq Moh'd
Published: June 2020

We demonstrate a metal-oxide-semiconductor based nonvolatile memory element structure with a single isolated gold nanoparticle (Au-NP) acting as the storage site. The Au-NPs are sandwiched between amorphous Al2O3 thin films, deposited using the atomic layer deposition method to form the blocking and tunneling layers. The current voltage characteristics of the fabricated structure during write/erase cycles are obtained using conductive mode atomic force microscopy (C-AFM) by probing a single isolated NP at a time. Consecutive voltage sweeps show a memory window in I–V characterization with an average of ΔV ∼0.9 V at a reading current of 1.5 nA, demonstrating the presence of stored charge in the Au-NP, which originates from the AFM metal-coated probe. The results show the writing and erasing operation of the device by charging and discharging a single Au-NP. Moreover, the physics is discussed in terms of the electric field enhancement due to the charge confinement and its effect on the conduction mechanisms.
Sedimentation of general shaped particles using a multigrid fictitious boundary method
Authors: Walayat Khuram , Talat Nazia , Jabeen Saqia , Usman Kamran , Liu Moubin
Published: June 2020

In this paper, a direct numerical simulation technique, the Finite Element Fictitious Boundary Method (FBM), is used to simulate fluid–solid two-phase flows of different general shaped particles. The momentum interactions between solid and fluid phases are handled by using the FBM. The continuity and momentum equations are solved on a fixed Eulerian grid that is independent of flow features by using a discrete projection scheme inside a multi-grid finite element approach. A detailed description is presented for the geometric representation and modeling of two-dimensional particles of different general shapes, i.e., circular, elliptical, square, rectangular, triangular, and pentagonal shapes inside the fluid. We discussed the effects of particle shapes and the influences on the settling behavior of the particles. A comparison of the settling trajectories of the particles of the same mass but with different shapes is presented. Moreover, depending upon the particle’s shape, some interesting facts are discovered, which have a great influence on the particles’ trajectory and settling velocity. Some very important correlations between the drag force coefficient and particle’s Reynolds numbers with different density ratios of particles are obtained. Furthermore, we also studied the settling behavior of elliptical and rectangular particles with different axis ratios and a boomerang particle with different concave angles.

The authors of the article agree to the retraction of the article effective AUGUST 20, 2021.
Parameter estimation of orbital angular momentum based continuous-variable quantum key distribution
Authors: Jin Di , Guo Ying , Wang Yijun , Huang Duan
Published: June 2020

In principle, the number of orbital angular momentum (OAM) eigenstates is unbounded; therefore, OAM multiplexing could greatly increase the transmission rate. Compared with optical fibers, free-space channels fluctuate randomly over time due to the effects of atmospheric turbulence. Free-space channel estimation is necessary. Here, we study the channel parameter estimation of continuous-variable quantum key distribution (CVQKD) based on OAM, especially the influence of atmospheric turbulence on transmittance and excess noise. By comparing the secret key rate in the case of single link and multiplexing, the impact of the estimated parameters on the performance of the CVQKD system is evaluated. Simulation results show that the value of the estimated parameter will affect the secret key rate.
Ion source perturbation and control in intense laser plasma interaction
Authors: Ramakrishna B. , Krishnamurthy S. , Tayyab M. , Bagchi S. , Makur K. , Trines Raoul , Scott Robbie , Robinson Alex , Chakera J. A.
Published: June 2020

We present here experimental results on the optimization of the mega-electronvolt ion source from the target front surface by using relativistic (1018 W/cm2) interactions with ultra-short laser pulses (50 fs). The source perturbation in the accelerated proton/ion beam was primarily controlled by the addition of a pre-pulse to main pulse contrast ratio. The 2D particle-in-cell simulations agreed well with the observed experimental results for the ion source perturbation and mitigation. This work provides insights into ion source perturbations (temporal and spatial) and the need to control them in intense laser–plasma interactions. Our results may assist in the efficient guiding of proton/ion beams to the core of fusion fuel or of ions in cancer therapy.
Fear induced explosive transitions in the dynamics of corruption
Authors: Bauzá F. , Soriano-Paños D. , Gómez-Gardeñes J. , Floría L. M.
Published: June 2020

In this article, we analyze a compartmental model aimed at mimicking the role of imitation and delation of corruption in social systems. In particular, the model relies on a compartmental dynamics in which individuals can transit between three states: honesty, corruption, and ostracism. We model the transitions from honesty to corruption and from corruption to ostracism as pairwise interactions. In particular, honest agents imitate corrupt peers while corrupt individuals pass to ostracism due to the delation of honest acquaintances. Under this framework, we explore the effects of introducing social intimidation in the delation of corrupt people. To this aim, we model the probability that an honest delates a corrupt agent as a decreasing function of the number of corrupt agents, thus mimicking the fear of honest individuals to reprisals by those corrupt ones. When this mechanism is absent or weak, the phase diagram of the model shows three equilibria [(i) full honesty, (ii) full corruption, and (iii) a mixed state] that are connected via smooth transitions. However, when social intimidation is strong, the transitions connecting these states turn explosive leading to a bistable phase in which a stable full corruption phase coexists with either mixed or full honesty stable equilibria. To shed light on the generality of these transitions, we analyze the model in different network substrates by means of Monte Carlo simulations and deterministic microscopic Markov chain equations. This latter formulation allows us to derive analytically the different bifurcation points that separate the different phases of the system.
Variable range hopping mechanism and modeling of isolation leakage current in GaN-based high-electron-mobility transistors
Authors: Zhu Jiejie , Zhang Yingcong , Uren Michael J. , Liu Siyu , Wang Pengfei , Mi Minhan , Hou Bin , Yang Ling , Kuball Martin , Ma Xiaohua , Hao Yue
Published: June 2020

Off-state leakage current of AlGaN/GaN high-electron-mobility transistors (HEMTs) with implant and mesa isolation was studied. Comparison of isolated pad structures with HEMTs shows that isolation leakage flowing through contact pads is the major leakage source in the studied GaN-based HEMTs whose gate finger is connected to an Ohmic contact pad. Then, circular metal-oxide-semiconductor field-effect-transistor devices were used to identify this isolation leakage path as surface isolation leakage at the SiN/nitride interface rather than bulk leakage in the buffer layer or implanted region. The temperature-dependent measurement shows that the two-dimensional variable range hopping mechanism dominates both the implant and mesa isolation leakage current. Mesa isolation results in a larger hopping probability and isolation leakage current than implant isolation. The isolation leakage current through gate and drain contact pads results in a non-zero switch of gate current in rectangular devices with either a Schottky gate or a metal-oxide-semiconductor gate, which is not observed in circular devices. Gate voltage for the switch of gate leakage current is linearly correlated with drain bias voltage, and the slope of the linear model represents the influence of drain bias on gate leakage current. This empirical model is independent of the fabrication process, provided that the same layout is used. With an increase in the source–drain distance from 4 μm to 10 μm, the ratio of gate–drain isolation resistance to gate–source isolation resistance increases from 1.38 to 2.33, leading to a decrease in the slope of empirical lines from 0.42 to 0.30.
Numerical analysis and semi-analytical prediction of the depth of holes drilled with combined ms and ns laser pulses
Authors: Qin Yuan , Förster Daniel J. , Weber Rudolf , Graf Thomas , Yang Sen , Zhu Dehua , Li Fengping
Published: June 2020

The evolution of the depth of holes drilled with a single ms laser pulse combined with a series of ns laser pulses for different processing parameters was studied numerically. The results show that the achievable hole depth can be increased by increasing the intensity of the ms pulses and by saving the absorbed energy that would be lost in overheating of the melt. The melt includes the one generated between subsequent ns pulses and the un-expelled melt after the effect of shock waves induced by the previous ns pulse. The overheating of the un-expelled melt is avoided by increasing the intensity of the ns pulses. Additionally, increasing the repetition rate of the ns pulses during the processing with a single ms pulse reduces the energy wasted for overheating the generated melt and the energy transferred radially to the rim of the hole. On the basis of the numerically obtained relations between the intensity of the ns pulse and the changed depth of the hole per pulse, a semi-analytical method was developed to predict the depth of the hole drilled with combined laser pulses. The applicability of the method is discussed and verified by comparing the semi-analytical results with the numerical and experimental results.
Mining of effective local order parameters for classifying crystal structures: A machine learning study
Authors: Doi Hideo , Takahashi Kazuaki Z. , Aoyagi Takeshi
Published: June 2020

Determining local structures of molecular systems helps the scientific and technological understanding of the function of materials. Molecular simulations provide microscopic information on molecular systems, but analyzing the resulting local structures is a non-trivial task. Many kinds of order parameters have been developed for detecting such local structures. Bond-orientational order parameters are promising for classifying local structures and have been used to analyze systems with such structures as body-centered cubic, face-centered cubic, hexagonal close-packed, and liquid. A specific set of order parameters derived from Lechner’s definitional equation are widely used to classify complex local structures. However, there has been no thorough investigation of the classification capability of other Lechner parameters, despite their potential to precisely distinguish local structures. In this work, we evaluate the classification capability of 112 species of bond-orientational order parameters including Lechner’s definitions. A total of 234 248 combinations of these parameters are also evaluated. The evaluation is systematically and automatically performed using machine learning techniques. To distinguish the four types of local structures, we determine the better set of two order parameters by comparing with a conventional set. A set of three order parameters is also suggested for better accuracy. Therefore, the machine learning scheme in the present study enables the systematic, accurate, and automatic mining of effective order parameters for classifying crystal structures.
Bright terahertz (THz) generation by frequency mixing of dichromatic lasers in inhomogeneous cold plasma: Scaling of THz field
Authors: Manendra  , Singh Kunwar Pal , Singh Beer Pal , Malik Anil K
Published: June 2020

We report a theoretical model for bright, radially polarized terahertz (THz) generation based on difference frequency generation in periodic density plasmas. An initial phase difference between two lasers is incorporated in our model. It is observed that the THz field significantly varies with the initial phase difference. It is also found that the THz field and efficiency depend on the periodic plasma density structure parameters (like amplitude nγ and wave vector γ). Our investigations reveal that close to the phase matching condition, and optimized values of laser and plasma parameters, peak THz fields ∼ 15 GV/m can be obtained for the laser field ∼ 5×1010 V/m. We also found that the THz field distribution can be controlled with laser field profile parameters. The conversion efficiency of ∼0.01 can be achieved by optimizing the laser field profile and plasma parameters. In our model, high field and radially polarized THz can be obtained to meet the demands of THz-matter interactions, nonlinear THz spectroscopy, imaging, etc. Radially polarized THz field is also useful to penetrate deeply into the layers inside the skin with less risk of collateral damage and thereby improved safety and efficacy of treatment.
An algorithm to find (and plug) “holes” in multi-dimensional surfaces
Authors: Pandey Ankit , Poirier Bill
Published: June 2020

We have developed an algorithm to detect holes in multi-dimensional real-valued surfaces—such as the potential energy surfaces (PESs) that describe the nuclear motion of molecules in the context of the Born–Oppenheimer approximation. For our purposes, a PES “hole” is defined as an unphysical saddle point, beyond which the potential energy drops (typically) without limit to negative infinity. PES holes are numerical artifacts that can arise when fitting PES functional forms to discrete ab initio data—even when the data is of high quality, and/or for comparatively few degrees of freedom (DOF). Often undetected, PES holes can have devastating effects on subsequent dynamical calculations, especially if they occur at low energies. In this paper, we present a highly efficient algorithm designed to systematically identify hole configurations and energies. The method is applied to a variety of molecular PESs ranging up to 30 DOF. A number of evidently previously undetected PES holes are reported here—surprisingly, even for PESs that have been available for decades. The code itself (Crystal) is presented together with a user manual. These tools may be of great benefit for PES developers, who can use the information they provide to fix holes, once identified. More generally, the methodology can be applied in any context involving multi-dimensional surfaces.
On applicability of absorbing rectilinear electron beams in high-frequency gyrotrons operating at cyclotron harmonics
Authors: Fokin Andrey P. , Glyavin Mikhail Yu. , Manuilov Vladimir N. , Osharin Ivan V. , Savilov Andrey V.
Published: June 2020

This paper presents the studies of electron–wave interaction in a high-frequency harmonic gyrotron with an additional absorbing rectilinear electron beam. The expression for the starting current of the gyrotron in the presence of an absorbing beam is obtained within the model of a fixed axial field structure. We investigate the effectiveness of mode selection by absorbing beam in gyrotrons with different resonator lengths to wavelength ratios on the basis of simple theory, as well as the results of numerical simulation within a self-consistent model.
Experimental characterization of ultrafast phenomena of secondary-ion emission induced by nanoscale energy deposition processes of energetic C60 impacts
Authors: Hirata K. , Yamada K. , Chiba A. , Hirano Y. , Narumi K. , Saitoh Y.
Published: June 2020

Energetic C60 ion impacts in the sub-MeV to MeV energy range, which can provide secondary ions (SIs) necessary for the identification of organic materials in high emission yields, are excellent ionization methods for highly sensitive SI mass spectrometry. Energetic C60 ion-impact-induced SI emission is an ultrafast phenomenon that occurs at the nanoscale around the impact point through complex processes triggered by energy deposition by impact; it is therefore difficult to observe in situ. Here, we experimentally characterized this phenomenon using information about the emission yields of various negative-SI (N-SI) species over a wide range of C60 impact energies. We found that the N-SI emission phenomena are influenced by variations in the volume and geometry of the regions, where SI and/or precursor particles are preferentially produced depending on the deposited energy density.
Nonlinear transport in nonequilibrium systems (with an application to Tokamak-plasmas)
Authors: Sonnino Giorgio , Nardone Pasquale , Peeters Philippe , Tirapegui Enrique
Published: June 2020

We show, for the first time, the explicit form of the nonlinear partial differential equations (PDEs) subject to the correct boundary conditions that have to be satisfied by transport coefficients having a vanishing skew-symmetric piece. We also report, for the first time, the nonlinear PDEs (with the appropriate boundary conditions) for transport coefficients when the thermodynamic system is subject to two thermodynamic forces. Since the proposed PDEs have been derived without neglecting any term present in the dynamical equations (i.e., the energy, mass, and momentum balance equations), we propose them as a good candidate for describing transport in thermodynamic systems also far from equilibrium (e.g., in the turbulent regime). The preliminary test was carried out by analyzing a concrete example where Onsager’s relationships manifestly disagree with experience: magnetically confined Tokamak-plasmas. More specifically, we focus our calculations to compute mass and energy transports in Frascati Tokamak Upgrade-plasmas subject to two thermodynamic forces. We show a good agreement between the theoretical predictions and the experimental data. The aim of this study is to apply our approach to the Divertor Tokamak Test Facility, to be built in Italy, and to the International Thermonuclear Experimental Reactor.
A comprehensive conductivity model for drift and micro-tearing modes
Authors: Larakers J. L. , Hazeltine R. D. , Mahajan S. M.
Published: June 2020

The parallel electrical conductivity is a crucial parameter in the study of the linear stability of drift-modes like the micro-tearing mode (MTM). The conductivity enters by closing the electromagnetic tearing layer equations. Recent progress in the understanding of the pedestal suggests that the MTM could play an important role in its structure and evolution. For this reason, we revisit and improve previous model conductivities. This parameter is generally derived from the linearized drift kinetic equation. In the past literature, it has been computed using either simplified collision operators or neglecting the spatial dependence away from the rational surface. A fully consistent expression for the conductivity that would accurately model the pedestal has not been available. By applying a novel variational procedure and with the full Fokker Plank collision operator, including electron–electron collisions, we compute a closed expression for the parallel electrical conductivity in the form of a rational function.
Fabrication of magnetocaloric La(Fe,Si)13 thick films
Authors: Dung N. H. , Doan N. B. , De Rango P. , Ranno L. , Sandeman K. G. , Dempsey N. M.
Published: June 2020

La(Fe,Si)13-based compounds are considered to be very promising magnetocaloric materials for magnetic refrigeration applications. Many studies have focused on this material family but only in bulk form. In this paper, we report the fabrication of thick films of La(Fe,Si)13, both with and without post-hydriding. These films exhibit magnetic and structural properties comparable to those of bulk materials. We also observe that the ferromagnetic phase transition has a negative thermal hysteresis, a phenomenon not previously found in this material but which may have its origins in the availability of a strain energy reservoir, as in the cases of other materials in which negative thermal hysteresis has been found. Here, it appears that the substrate acts to store strain energy. Our exploratory study demonstrates the viability of thick films of the La(Fe,Si)13 phase and motivates further work in the area, while showing that additional perspectives can be gained from reducing the dimensionality of magnetocaloric materials in which the magneto-volume effect is large.
Noise cancellation system for shaking optical lattice by controlling optical path
Authors: Yu Hoon , Kim Jung Bog
Published: June 2020

We present a simple way to control the phase of an optical lattice by detecting the interference signal of two beams. The optical lattice is intentionally shaken by varying the relative phase of the beams. However, the lattice may also be shaken by unwanted variations of the relative optical path length, e.g., due to mirror vibrations. The purpose of the servo is to attenuate these unwanted variations while the intended shaking remains. We demonstrate that the servo changes the relative phase between beams and follows the intended shaking function with 99% accuracy. The bandwidth for the acceptable attenuation of unwanted shaking, −13 dB, is measured to 1.2 kHz to control the atomic Bloch state. The servo will be implemented to attenuate the unknown system vibrations for a shaken lattice and engineer the momentum state of atoms trapped in the lattice. This idea can also be applied to any time varying experiment.
In situ cell for x-ray absorption spectroscopy of low volatility compound vapors
Authors: Levish Alexander , Winterer Markus
Published: June 2020

Technologically relevant gas phase processes rely on reactants in vapor form for the production of thin films and nanoparticles. An instrument is described which enables the investigation of such vapors by x-ray absorption spectroscopy. Corresponding in situ studies provide information about gas phase precursor chemistry and optimized synthesis processes. The setup consists of a sealed vapor container heated by a hot air bath. Inert gas filling and temperature monitoring are implemented. Fluid dynamic simulations reveal a homogeneous temperature distribution without hot or cold spots. Temperature stability better than 1 K for at least 190 min allows time-dependent measurements or improved signal to noise ratios by averaging of datasets. Iron acetylacetonate is studied as a model system. X-ray absorption spectra measured by fluorescence are of high quality, allowing a detailed analysis of X-ray Absorption Near Edge Structure (XANES) and extended x-ray absorption fine structure. A molecular structure transformation is observed in XANES spectra of iron acetylacetonate vapor above 480 K probably due to the loss of one ligand. The setup allows the investigation of low volatility compounds with vapor pressures above 2 kPa at temperatures up to 520 K.
Scintillation properties of organic–inorganic layered perovskite nanocrystals in glass
Authors: Kawano Naoki , Shinozaki Kenji , Nakauchi Daisuke , Kimura Hiromi , Yanagida Takayuki
Published: June 2020

We fabricated organic–inorganic perovskite nanocrystals in nanoporous glasses (OIPiG) where (C6H5C2H4NH3)2PbBr4 (Phe) was incorporated into about 4 nm diameter pores and evaluated their photoluminescence and scintillation properties, compared with the Phe single crystal. Both the samples showed emissions due to the recombination of free excitons in the inorganic layer under 280 nm excitation light. In scintillation, a weak and broad scintillation peak possibly due to the free excitons and bound excitons was observed in the OIPiG, while the Phe single crystal showed efficient scintillation due to free exciton emissions with a peak approximately at 435 nm. The scintillation decay time constant (4.4 ns) due to the free excitons for the OIPiG was found to be faster than that (6.6 ns) for the Phe single crystal. In addition, the afterglow levels were confirmed to be 340 ppm for the OIPiG and 20 ppm for the Phe single crystal. The afterglow level of the OIPiG was higher than that of the Phe single crystal but was almost equivalent to that for the commonly used inorganic scintillator CsI:Tl.
Resonant tunneling diodes strongly coupled to the cavity field
Authors: Limbacher B. , Kainz M. A. , Schoenhuber S. , Wenclawiak M. , Derntl C. , Andrews A. M. , Detz H. , Strasser G. , Schwaighofer A. , Lendl B. , Darmo J. , Unterrainer K.
Published: June 2020

We demonstrate resonant tunneling diodes, embedded in double metal cavities, strongly coupled to the cavity field, while maintaining their electronic properties. We measure the polariton dispersion and find a relative vacuum Rabi splitting of 11%, which explicitly qualifies for the strong-coupling regime. Additionally, we show that electronic transport has a significant influence on the polaritons by modulating the coupling strength. The merge between electronic transport and polaritonic physics in our devices opens up different perspectives of cavity quantum electro-dynamics and integrated photonics.
Oblique elastic plate impact on thin liquid layer
Authors: Khabakhpasheva T. I. , Korobkin A. A.
Published: June 2020

The present study is concerned with possible mechanisms of air entrainment in a thin liquid layer caused by oblique impact of a deformable body on the layer. The two-dimensional unsteady problem of oblique elastic plate impact is considered within the thin-layer approximation for the first time. The plate deflection is described by the Euler beam equation. The plate edges are free of stresses and shear forces. The plate deflections are comparable with the liquid layer thickness. It is revealed in this paper that, for a stiff plate, the initial impact by the trailing edge makes the plate rotate with the leading plate edge entering water before the wetted part of the plate arrives at this edge. The air cavity trapped in such cases can be as long as 40% of the plate length. For a flexible plate, the impact does not cause the plate rotation. However, the dry part of the plate in front of the advancing wetted region is deflected toward the liquid layer also trapping the air. The numerical results are presented for elastic and rigid motions of the plate, hydrodynamic pressure in the wetted part of the plate, position of this wetted part, and the flow beneath the plate.
The influence of Au substitution and hydrostatic pressure on the phase transitions and magnetocaloric properties of MnCoGe alloys
Authors: Chen Jing-Han , Trigg Aaron , Poudel Chhetri Tej , Young David P. , Dubenko Igor , Ali Naushad , Stadler Shane
Published: June 2020

In this work, the phase transitions and magnetocaloric properties of Mn1−xAuxCoGe (0≤x≤0.025) alloys were studied as a function of concentration x and applied hydrostatic pressure. The increasing substitution of Au for Mn results in the decrease of the first-order martensitic transition temperature, and this first-order martensitic transition was ultimately converted to a second-order magnetic transition when the Au substitution (x) reached 0.025. The magnitudes of the maximum magnetic entropy changes increased when the magnetic and structural transitions were coupled, which occurred for 0.005≤x≤0.020. The largest maximum magnetic entropy change for a field change of μ0ΔH=7 T was 33.1 J/kg K for the sample with x=0.020. Similar to the effect of Au substitution, the first-order martensitic transition temperature initially decreased, and then converted to second order, when the applied hydrostatic pressure reached a large enough value. Interestingly, both Au substitution and pressure application cause a volume reduction and, in both cases, the first-order martensitic transition temperature initially reduced and then converted to second-order. These results suggest two different methods of tuning the transition temperatures in these magnetocaloric materials. One can either apply hydrostatic pressure and temporarily adjust the transition temperatures or modify the composition chemically and permanently change the transition temperatures.
A weight-dependent local correlation density-functional approximation for ensembles
Authors: Loos Pierre-François , Fromager Emmanuel
Published: June 2020

We report a local, weight-dependent correlation density-functional approximation that incorporates information about both ground and excited states in the context of density functional theory for ensembles (eDFT). This density-functional approximation for ensembles is specially designed for the computation of single and double excitations within Gross–Oliveira–Kohn DFT (i.e., eDFT for neutral excitations) and can be seen as a natural extension of the ubiquitous local-density approximation in the context of ensembles. The resulting density-functional approximation, based on both finite and infinite uniform electron gas models, automatically incorporates the infamous derivative discontinuity contributions to the excitation energies through its explicit ensemble weight dependence. Its accuracy is illustrated by computing single and double excitations in one-dimensional (1D) many-electron systems in the weak, intermediate, and strong correlation regimes. Although the present weight-dependent functional has been specifically designed for 1D systems, the methodology proposed here is general, i.e., directly applicable to the construction of weight-dependent functionals for realistic three-dimensional systems, such as molecules and solids.