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Journal of Physics: Condensed Matter - latest papers
Latest articles for Journal of Physics: Condensed Matter
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Non-solvents as physical blowing agents in printable silicone foams
Silicone foams were produced by dispersing an incompatible liquid phase (i.e. a non-solvent) into an uncured, liquid silicone. The formulation and processing parameters were varied to see the effect on porosity and pore morphology. Specifically, two fluorosilanes were added to stabilize the inclusion of fluorinated solvents as blowing agents. As the floruosilane content increased, the void content increased up to about 44 vol. % when the fluorosilane comprised 18 wt. % of the initial formulation. Fumed silica that was treated with a fluorinated silane was also used to try to stabilize the dispersed liquid. While the fumed silica content did not have a strong effect on the total void content, the morphology changed when silica content changed. Various fluorinated solvents with distinct chemical structures were used as the non-solvent and then removed after curing of the silicone. The interaction of the internal non-solvent phase and the silicone phase was expected to influence the porosity. These insights highlight how the manipulation of formulation and processing parameters, focusing on the inclusion of fluorosilanes and fluorinated solvents, contributes to the understanding of how incompatible liquid phases interact with silicone matrices to control porosity and pore morphology. Additionally, these interactions also influenced processability, leading to formulations that could be printed. Higher content of non-solvent inclusions could increase the yield stress and storage modulus of an uncured formulation, leading to the ability to tune intrastrand porosity while a 3D printed architecture could be modified to introduce porosity between the strands. In addition to fluorinated solvents, we also considered non-fluorinated solvents since these may be more industrially relevant in the future. Overall, this approach provides an alternative route to producing porous foams that can be 3D printed, which could be useful for applications like cushioning and protective gear.
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Study of the interplay between geometry and chemistry in nanoscale hydration
, a novel structural indicator developed to characterize water in hydration and nanoconfined environments, was recently introduced and initially applied to water in contact with self-assembled monolayers (SAMs), graphene-like systems, and proteins. In the present work, we employ this metric to characterize SAMs featuring cavities of varying sizes. We investigate the effects of geometry and chemical composition on surface hydration by incorporating hydroxyl groups ( ) as hydrophilic sites. When hydration implies the disruption of hydrogen bonds at the hydration layer, a defect interaction threshold (DIT) should be satisfied in order to give rise to hydrophilic behavior. This threshold is given by the amount of energy compensation a hydrogen bond defect receives in bulk water and is significantly lower than the hydrogen bond energy. Besides signaling the transition to hydrophobicity, we find that the DIT value is also relevant for nanoconfined environments. In this regard, our findings reveal that a water molecule cannot sustain more than one hydrogen-bonding site with an interaction weaker than the DIT; if a second site exceeds this threshold, the molecule desorbs. This finding quantifies the energy loss a water molecule can tolerate while maintaining wetting or hydration. Finally, by means of preliminary calculations on simplified model systems, we show that such knowledge may be instrumental in main contexts like protein binding, where removal of hydration water contributes as a relevant driving force.
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A first-principles approach to electromechanics in liquids
Electromechanics in fluids describes the response of the number density to electric fields, and thus provides a powerful means by which to control the behavior of liquids. While continuum approaches have proven successful in describing electromechanical phenomena in macroscopic bodies, their use is questionable when relevant length scales become comparable to a system’s natural correlation lengths, as commonly occurs in biological systems, nanopores, and microfluidics. Here, we present a first-principles theory for electromechanical phenomena in fluids. Our approach is based on the recently proposed hyperdensity functional theory (hyper-DFT) (Sammüller et al 2024 Phys. Rev. Lett.133 098201) in which we treat the charge density as an observable of the system, with the intrinsic Helmholtz free energy functional dependent upon both density and electrostatic potential. Expressions for the coupling between number and charge densities emerge naturally in this formalism, avoiding the need to construct density-dependent and spatially varying material parameters such as the dielectric constant. Furthermore, we make our theory practical by deriving a connection between hyper-DFT and local molecular field theory, which facilitates the use of machine learning to obtain explicit representations for the free energy functionals of systems with short-ranged electrostatic interactions, with long-ranged effects accounted for in a well-controlled mean-field fashion.
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Electronic structure of spin–orbit coupled vacancy ordered transition metal halides and formation of large magnetic anisotropy energy
Vacancy-ordered antifluorite materials (A2BX6) are garnering renewed attention as novel magnetic states driven by spin–orbit coupling (SOC) can be realized in them. In this work, by pursuing density functional theory calculations and model studies, we analyze the ground state electronic and magnetic structure of face-centered cubic antifluorites K2ReCl6 (KReC, 5d3), K2OsCl6 (KOsC, 5d4), and K2IrCl6 (KIrC, 5d5). We find that KReC stabilizes in the high-spin S = 3/2 state due to large exchange-splitting as compared to the SOC strength. The KOsC stabilizes in S = 1 simple Mott insulating state while KIrC stabilizes in = 1/2 spin–orbit-assisted Mott insulating state. The presence of an isolated metal-chloride octahedron makes these antifluorites weakly coupled magnetic systems with the nearest and next-nearest-neighbor spin-exchange parameters (J1 and J2) are of the order of 1 meV. For KReC and KOsC, the J1 and J2 are estimated to be antiferromagnetic and ferromagnetic, which leads to a Type-I antiferromagnetic ground state, whereas for KIrC, both J1 and J2 are antiferromagnetic, hence, it stabilizes with a Type-III antiferromagnetic state. Interestingly, in their equilibrium structure, these antifluorites possess large magnetic anisotropy energy (MAE) (0.6–4 meV/transition metal), which is at least one-to-two orders higher than traditional MAE materials like transition metals and multilayers formed out of them. Moreover, with epitaxial tensile/compressive strain, the MAE enhances by one order, becoming giant for KOsC (20–40 meV/Os).
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Optimization of p-type conductivity in lithium niobate by co-doping strategy
The development of lithium niobate (LiNbO3, LN)-based active devices has been limited by the absence of stable p-type conductivity. Overcoming this challenge is critical for unlocking the full potential of LN in optoelectronic applications. Through first-principles calculations, we have identified nitrogen (N) as the most effective p-type dopant among elements near oxygen (O) in the periodic table. However, mono-acceptor N doping creates defect levels 0.415 eV above the valence band maximum (VBM). We propose to passivate the N dopant by introducing a small amount of impurity magnesium (Mg). This doping process involves two steps: first, N–Mg co-doping with equal amounts creates fully occupied impurity bands; second, excess N doping the fully occupied impurity bands introduces shallow defect levels. Quantitative results show that in 2N+Mg co-doped LN the defect level is only 0.115 eV (about 0.3 eV lower than that of mono-acceptor N-doping) above the VBM. Correspondingly, the ɛ(0/−1) transition level is found at 0.067 eV (about 0.243 eV lower than that of mono-acceptor N-doping), indicating a typical shallow acceptor. Co-doping also lowers the defect formation energy by ∼1.9 eV, boosting dopant solubility. These key data demonstrate that the donor and excess acceptor co-doping strategy effectively converts the defect levels into shallow levels, reduces defect formation energy, and enhances local doping stability in p-type LN, thereby laying the foundation for the development of LN-based p–n junction and active optoelectronic devices.