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1. Effects of charge fluctuation and charge regulation on the phase transitions in stoichiometric VO2

   https://doi.org/10.1038/s41598-020-73447-9  

   Joshi, Siddharth; Smieszek, Nicholas; Chakrapani, Vidhya (October 2020, Scientific Reports)

   Abstract Detailed electrical and photoemission studies were carried out to probe the chemical nature of the insulating ground state of VO₂, whose properties have been an issue for accurate prediction by common theoretical probes. The effects of a systematic modulation of oxygen over-stoichiometry of VO₂from 1.86 to 2.44 on the band structure and insulator–metal transitions are presented for the first time. Results offer a different perspective on the temperature- and doping-induced IMT process. They suggest that charge fluctuation in the metallic phase of intrinsic VO₂results in the formation of e⁻and h⁺pairs that lead to delocalized polaronic V³⁺and V⁵⁺cation states. The metal-to-insulator transition is linked to the cooperative effects of changes in the V–O bond length, localization of V³⁺electrons at V⁵⁺sites, which results in the formation of V⁴⁺–V⁴⁺dimers, and removal of$$\pi^{*}$$ ${\pi }^{\ast }$screening electrons. It is shown that the nature of phase transitions is linked to the lattice V³⁺/V⁵⁺concentrations of stoichiometric VO₂and that electronic transitions are regulated by the interplay between charge fluctuation, charge redistribution, and structural transition. 

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2. Universal Nonthermal Power-law Distribution Functions from the Self-consistent Evolution of Collisionless Electrostatic Plasmas

   https://doi.org/10.3847/1538-4357/ad91a1  

   Banik, Uddipan; Bhattacharjee, Amitava; Sengupta, Wrick (December 2024, The Astrophysical Journal)

   Abstract Collisionless systems often exhibit nonthermal power-law tails in their distribution functions. Interestingly, collisionless plasmas in various physical scenarios (e.g., the ion population of the solar wind) feature av⁻⁵tail in their velocity (v) distribution, whose origin has been a long-standing puzzle. We show this power-law tail to be a natural outcome of the collisionless relaxation of driven electrostatic plasmas. Using a quasi-linear analysis of the perturbed Vlasov–Poisson equations, we show that the coarse-grained mean distribution function (DF),f₀, follows a quasi-linear diffusion equation with a diffusion coefficientD(v) that depends onvthrough the plasma dielectric constant. If the plasma is isotropically forced on scales larger than the Debye length with a white-noise-like electric field,D(v) ∼v⁴forσ<v<ω_P/k, withσthe thermal velocity,ω_Pthe plasma frequency, andkthe characteristic wavenumber of the perturbation; the corresponding quasi-steady-statef₀develops av^{−(d+ 2)}tail inddimensions (v⁻⁵tail in 3D), while the energy (E) distribution develops anE⁻²tail independent of dimensionality. Any redness of the noise only alters the scaling in the highvend. Nonresonant particles moving slower than the phase velocity of the plasma waves (ω_P/k) experience a Debye-screened electric field, and significantly less (power-law suppressed) acceleration than the near-resonant particles. Thus, a Maxwellian DF develops a power-law tail, while its core (v<σ) eventually also heats up but over a much longer timescale. We definitively show that self-consistency (ignored in test-particle treatments) is crucial for the emergence of the universalv⁻⁵tail. 

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3. Improved thermonuclear rate of ⁴² Ti( p , γ ) ⁴³ V and its astrophysical implication in the rp process

   https://doi.org/10.1051/0004-6361/202347054  

   Hou, S Q; Iliadis, C; Pignatari, M; Liu, J B; Trueman, T_C L; Li, J G; Xu, X X (September 2023, Astronomy & Astrophysics)

   Context.Accurate⁴²Ti(p,γ)⁴³V reaction rates are crucial for understanding the nucleosynthesis path of the rapid capture process (rpprocess) that occurs in X-ray bursts. Aims.We aim to improve the thermonuclear rates of⁴²Ti(p,γ)⁴³V based on more complete resonance information and a more accurate direct component, together with the recently released nuclear masses data. We also explore the impact of the newly obtained rates on therpprocess. Methods.We reevaluated the reaction rate of⁴²Ti(p,γ)⁴³V by the sum of the isolated resonance contribution instead of the Hauser-Feshbach statistical model. We used a Monte Carlo method to derive the associated uncertainties of new rates. The nucleosynthesis simulations were performed via the NuGrid post-processing code ppn. Results.The new rates differ from previous estimations due to the use of a series of updated resonance parameters and a direct S factor. Compared with the previous results from the Hauser-Feshbach statistical model, which assumes compound nucleus⁴³V with a sufficiently high-level density in the energy region of astrophysical interest, large differences exist over the entire temperature region ofrp-process interest, up to two orders of magnitude. We consistently calculated the photodisintegration rate using our new nuclear masses via the detailed balance principle, and found the discrepancies among the different reverse rates are much larger than those for the forward rate, up to ten orders of magnitude at the temperature of 10⁸K. Using a trajectory with a peak temperature of 1.95×10⁹K, we performed therp-process nucleosynthesis simulations to investigate the impact of the new rates. Our calculations show that the adoption of the new forward and reverse rates result in abundance variations for Sc and Ca of 128% and 49%, respectively, compared to the variations for the statistical model rates. On the other hand, the overall abundance pattern is not significantly affected. The results of using new rates also confirm that therp-process path does not bypass the isotope⁴³V. Conclusions.Our study found that the Hauser-Feshbach statistical model is inappropriate to the reaction rate evaluation for⁴²Ti(p,γ)⁴³V. The adoption of the new rates confirms that the reaction path of⁴²Ti(p,γ)⁴³V(p,γ)⁴⁴Cr(β⁺)⁴⁴V is a key branch of therpprocess in X-ray bursts. 

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4. Threshold voltage control with high-temperature gate-oxide annealing in ultrawide bandgap AlGaN-channel MOSHFETs

   https://doi.org/10.35848/1882-0786/ac8bc4  

   Mollah, Shahab; Hussain, Kamal; Mamun, Abdullah; Alam, Md Didarul; Chandrashekhar, MVS; Simin, Grigory; Khan, Asif (September 2022, Applied Physics Express)

   Abstract We report threshold voltage (V_TH) control in ultrawide bandgap Al_0.4Ga_0.6N-channel metal oxide semiconductor heterostructure field-effect transistors using a high-temperature (300 °C) anneal of the high-kZrO₂gate-insulator. Annealing switched the polarity of the fixed charges at the ZrO₂/AlGaN interface from +5.5 × 10¹³cm⁻²to −4.2 × 10¹³cm⁻², pinningV_THat ∼ (−12 V), reducing gate leakage by ∼10³, and improving subthreshold swing 2× (116 mV decade⁻¹). It also enabled the gate to repeatedly withstand voltages from −40 to +18 V, allowing the channel to be overdriven doubling the peak currents to ∼0.5 A mm⁻¹. 

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5. Ynamide and Azaalleneyl. Acid‐Base Promoted Chelotropic and Spin‐State Rearrangements in a Versatile Heterocumulene [(Ad)NCC( ^t Bu)] ⁻

   https://doi.org/10.1002/anie.202401433  

   Russell, John_B; Jafari, Mehrafshan_G; Kim, Jun‐Hyeong; Pudasaini, Bimal; Ozarowski, Andrew; Telser, Joshua; Baik, Mu‐Hyun; Mindiola, Daniel_J (April 2024, Angewandte Chemie International Edition)

   Abstract We introduce the heterocumulene ligand [(Ad)NCC(^tBu)]⁻(Ad=1‐adamantyl (C₁₀H₁₅),^tBu=tert‐butyl, (C₄H₉)), which can adopt two forms, the azaalleneyl and ynamide. This ligand platform can undergo a reversible chelotropic shift using Brønsted acid‐base chemistry, which promotes an unprecedented spin‐state change of the [V^III] ion. These unique scaffolds are prepared via addition of 1‐adamantyl isonitrile (C≡NAd) across the alkylidyne in complexes [(BDI)V≡C^tBu(OTf)] (A) (BDI⁻=ArNC(CH₃)CHC(CH₃)NAr), Ar=2,6‐ⁱPr₂C₆H₃) and [(dBDI)V≡C^tBu(OEt₂)] (B) (dBDI²⁻=ArNC(CH₃)CHC(CH₂)NAr). ComplexAreacts with C≡NAd, to generate the high‐spin [V^III] complex with a κ¹‐N‐ynamide ligand, [(BDI)V{κ¹‐N‐(Ad)NCC(^tBu)}(OTf)] (1). Conversely,Breacts with C≡NAd to generate a low‐spin [V^III] diamagnetic complex having a chelated κ²‐C,N‐azaalleneyl ligand, [(dBDI)V{κ²‐N,C‐(Ad)NCC(^tBu)}] (2). Theoretical studies have been applied to better understand the mechanism of formation of2and the electronic reconfiguration upon structural rearrangement by the alteration of ligand denticity between1and2. 

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