Dependence of the polytropic index of plasma on magnetic field

Determination of the polytropic index of plasma in a magnetic field provides description of plasma physics ranging from laboratory plasma in the Earth to black hole in the Universe. Accordingly, a lot of efforts have been devoted to revealing the dependence of polytropic index of plasma on magnetic field. A recent experiment performed in a magnetic nozzle reported the dependency of the polytropic index on the magnetic field strength in that the polytropic index changes from 5/3 to unity with decreasing the magnetic field strength. In this letter, we show that the polytropic index of plasma does not depend on magnetic field if the radial electric field is sustained. The polytropic index is measured to be close to 2, higher than the previously reported value, regardless of the change in the strength and configuration of magnetic field.Y

Exploring the nonextensive thermodynamics of partially ionized gas in magnetic field

Contrary to classical thermodynamics, which deals with systems in thermal equilibrium, partially ionized gases generally do not reach thermal equilibrium. Nonextensive statistical mechanics has helped extend classical thermodynamics to nonequilibrium ionized gas. However, the fundamental question on whether the statistics of non-Maxwellian electrons satisfy the laws of thermodynamics has not been resolved. Here, we verify the thermodynamic laws of reversible and adiabatic processes for a magnetically expanding ionized gas. Together with the experimental evidence of the non-Maxwellian electron distribution, the K distribution, which measures the thermal equilibrium states, shows the Tsallis entropy to be nearly constant and the polytropic index to be close to adiabatic values along a divergent magnetic field. These results verify that the collisionless magnetic expansion of a nonequilibrium plasma is reversible and adiabatic, and an isentropic process is the origin of the high-energy tail of the energy distribution far downstream.N

Non-maxwellian electron energy probability functions in an indirectly heated cathode bernas source

The indirectly heated cathode (IHC) Bernas source is used in plasma process devices as it can produce a high current of desired ion species. The formation of various ion species is associated with the complex electron production mechanism inside the source, including beam-plasma interaction and energy relaxation. However, plasma diagnostics inside IHC sources has been limited by the assumption of a Maxwellian distribution. In this work, the discharge characteristics of IHC plasmas were investigated by analyzing the electron energy probability functions (EEPFs) under various conditions. The shape of the EEPF is dominantly affected by the voltage applied to the hot-cathode and repeller (i.e., cathode voltage), whereby the kinetic energy of the beam electrons is determined. With increasing cathode voltage, the EEPFs show a bi-Maxwellian-like distribution with the bulk and energetic electron groups. The thermalization among the two electron groups with increasing magnetic field makes the EEPFs to follow a Maxwellian distribution. The absolute value of the density and effective temperature of the electron are mainly governed by the bulk electron group with a relatively higher density, and the changes in the electric potentials are explained in the context of the confinement of the energetic electrons.Y

Revealing the three-dimensional arrangement of polar topology in nanoparticles.

In the early 2000s, low dimensional ferroelectric systems were predicted to have topologically nontrivial polar structures, such as vortices or skyrmions, depending on mechanical or electrical boundary conditions. A few variants of these structures have been experimentally observed in thin film model systems, where they are engineered by balancing electrostatic charge and elastic distortion energies. However, the measurement and classification of topological textures for general ferroelectric nanostructures have remained elusive, as it requires mapping the local polarization at the atomic scale in three dimensions. Here we unveil topological polar structures in ferroelectric BaTiO3 nanoparticles via atomic electron tomography, which enables us to reconstruct the full three-dimensional arrangement of cation atoms at an individual atom level. Our three-dimensional polarization maps reveal clear topological orderings, along with evidence of size-dependent topological transitions from a single vortex structure to multiple vortices, consistent with theoretical predictions. The discovery of the predicted topological polar ordering in nanoscale ferroelectrics, independent of epitaxial strain, widens the research perspective and offers potential for practical applications utilizing contact-free switchable toroidal moments

Magnetic confinement and instability in partially magnetized plasma

Discharge with an external magnetic field is promising for various applications of low-temperature plasmas from electric propulsion to semiconductor processes owing to high plasma density. It is essential to understand plasma transport across the magnetic field because plasma confinement under the field is based on strong magnetization of light electrons, maintaining quasi-neutrality through the inertial response of unmagnetized ions. In such a partially magnetized plasma, different degrees of magnetization between electrons and ions can create instability and make the confinement and transport mechanisms more complex. Theoretical studies have suggested a link between the instability of various frequency ranges and plasma confinement, whereas experimental work has not been done so far. Here, we experimentally study the magnetic confinement properties of a partially magnetized plasma considering instability. The plasma properties show non-uniform characteristics as the magnetic field increases, indicating enhanced magnetic confinement. However, the strengthened electric field at the edge of the plasma column gives rise to the Simon-Hoh instability, limiting the plasma confinement. The variation of the edge-to-center plasma density ratio (h-factor) with the magnetic field clearly reveals the transition of the transport regime through triggering of the instability. Eventually, the h-factor reaches an asymptotic value, indicating saturation of magnetic confinement.N

Site selectivity of single dopant in high-nickel cathodes for lithium-ion batteries

Improving the structural stability of high-capacity high-Ni cathodes through doping has been investigated, but the structural stabilization mechanisms of dopants remain unclear. This study focused on unraveling the influence of individual dopants, Aluminium, Titanium, or Zirconium, on the structural stabilization of high-Ni cathodes. X-ray Diffraction and High-Angle Annular Dark-Field Scanning Transmission Electron Microscopy (HAADF-STEM) were employed for quantitative analysis of cation mixing, and for the first time, HAADF-STEM and deep learning were combined to improve the accuracy and efficiency of the analysis. The atomic-scale energy dispersive spectroscopy analysis identified transition metal sites as the primary doping sites in doped high-Ni cathodes. Density funtional theory calculations revealed that dopants enhance the interatomic bonds between Ni and O, thereby inhibiting cation mixing. Among the studied dopants, Ti was found to have the most substantial influence in enhancing structural stability. This study contributes to an understanding of single dopant on the structural stability of high-Ni cathodes, aiding the design of next-generation lithium-ion batteries. © 2024 The Authors11Nsciescopu

Stabilizing hidden room-temperature ferroelectricity via a metastable atomic distortion pattern

© 2020, The Author(s). Nonequilibrium atomic structures can host exotic and technologically relevant properties in otherwise conventional materials. Oxygen octahedral rotation forms a fundamental atomic distortion in perovskite oxides, but only a few patterns are predominantly present at equilibrium. This has restricted the range of possible properties and functions of perovskite oxides, necessitating the utilization of nonequilibrium patterns of octahedral rotation. Here, we report that a designed metastable pattern of octahedral rotation leads to robust room-temperature ferroelectricity in CaTiO3, which is otherwise nonpolar down to 0 K. Guided by density-functional theory, we selectively stabilize the metastable pattern, distinct from the equilibrium pattern and cooperative with ferroelectricity, in heteroepitaxial films of CaTiO3. Atomic-scale imaging combined with deep neural network analysis confirms a close correlation between the metastable pattern and ferroelectricity. This work reveals a hidden but functional pattern of oxygen octahedral rotation and opens avenues for designing multifunctional materials11sciescopu

Polar Metal Phase Induced by Oxygen Octahedral Network Relaxation in Oxide Thin Films

ABO(3)perovskite materials and their derivatives have inherent structural flexibility due to the corner sharing network of the BO(6)octahedron, and the large variety of possible structural distortions and strong coupling between lattice and charge/spin degrees of freedom have led to the emergence of intriguing properties, such as high-temperature superconductivity, colossal magnetoresistance, and improper ferroelectricity. Here, an unprecedented polar ferromagnetic metal phase in SrRuO3(SRO) thin films is presented, arising from the strain-controlled oxygen octahedral rotation (OOR) pattern. For compressively strained SRO films grown on SrTiO(3)substrate, oxygen octahedral network relaxation is accompanied by structural phase separation into strained tetragonal and bulk-like orthorhombic phases, and the asymmetric OOR evolution across the phase boundary allows formation of the polar phase, while bulk metallic and ferromagnetic properties are maintained. From the results, it is expected that other oxide perovskite thin films will also yield similar structural environments with variation of OOR patterns, and thereby provide promising opportunities for atomic scale control of material properties through strain engineering11sciescopu

Highly enhanced ferroelectricity in HfO2-based ferroelectric thin film by light ion bombardment

Continuous advancement in nonvolatile and morphotropic beyond-Moore electronic devices requires integration of ferroelectric and semiconductor materials. The emergence of hafnium oxide (HfO2)-based ferroelectrics that are compatible with atomic-layer deposition has opened interesting and promising avenues of research. However, the origins of ferroelectricity and pathways to controlling it in HfO2 are still mysterious. We demonstrate that local helium (He) implantation can activate ferroelectricity in these materials. The possible competing mechanisms, including He ion-induced molar volume changes, vacancy redistribution, vacancy generation, and activation of vacancy mobility, are analyzed. These findings both reveal the origins of ferroelectricity in this system and open pathways for nanoengineered binary ferroelectrics