The European Physical Journal B最新文献

Physical equations provide good access to explore correlation between physical variables in some realistic systems such as nonlinear circuits and electromechanical devices, and further dynamical analysis requires consistence of physical units on the involved variables and parameters. Therefore, proper nondimensionalization is essential for revealing the intrinsic dynamics of the system by converting the physical equations into equivalent dimensionless theoretical models. For example, for a RLC (C: capacitor, L: inductor, R: resistor) circuit, both [RC] and [LC]^1/2 can be used to time scale for obtaining dimensionless time variable as τ = t/RC, or τ = t/(LC)^1/2, while a mechanical oscillator requires τ = t/[m/K]^1/2, in which K and m denote elasticity coefficient for the spring and mss of the oscillator, respectively. This paper investigates an FitzHugh-Nagumo (FHN) neuron coupled with an electromechanical arm device, which involves two inherent time scales associated with the [RC] and the [m/K]^1/2. By introducing different time bases, two nondimensional formulations are derived and compared. The results show that the two theoretical models are related through a linear time-scaling transformation, indicating their dynamical equivalence and consistence. Numerical simulations based on bifurcation diagrams and the largest Lyapunov exponent confirm that the two formulations exhibit identical qualitative dynamical behaviors. Furthermore, energy analysis reveals distinct distribution patterns under different dynamical regimes. Finally, simulation on Multisim is verified for clarifying the influence of time scale on the theoretical models and the dynamical characteristic. This study provides a systematic understanding of time-scale effects in electromechanical nonlinear systems and offers guidance for interpreting models under different nondimensional frameworks.

Graphical abstract

Schematic diagram for electromechanical device coupled by a neural circuit. The red box is the electromechanical arm/leg branch, see Ref.[13]. Time scale can be selected with τ = t/RC, or τ = t/(LC)^1/2, τ = t/[m/K]^1/2, in which K and m denote elasticity coefficient for the spring and mss of the oscillator, respectively, the theoretical model will be presented in different forms.

Photovoltaic devices represent one of the most promising pathways toward sustainable energy conversion. In this work, we investigate the electronic structure of orthorhombic Cu₂ZnSnS₄ and Cu₂CdSnS₄ chalcogenides as potential high-efficiency solar absorbers, using density functional theory combined with evolutionary algorithms for crystal structure prediction. Beyond the conventional tetragonal kesterite ((Ioverline{4 })) and stannite ((I4overline{2 }m)) ground-state phases, our structure search reveals a previously unreported orthorhombic polymorph adopting the (Pmn{2}_{1}) space group. The orthorhombic phase is found to be nearly degenerate in energy with the tetragonal phases. For Cu₂ZnSnS₄, the energy difference between kesterite and the orthorhombic phase is only 5.5 meV/atom. For Cu₂CdSnS₄, the energy difference between stannite and the orthorhombic phase is merely 0.7 meV/atom. These remarkably small energy differences, well within typical experimental thermal fluctuations, indicate that the Pmn2₁ structure is both thermodynamically competitive and synthetically accessible. Mechanical stability is confirmed by elastic constants satisfying the Born criteria, while phonon calculations reveal no imaginary frequencies across the entire Brillouin zone, confirming dynamical stability. The calculated formation energies of − 0.42 eV/atom for Cu₂ZnSnS₄ and − 0.43 eV/atom for Cu₂CdSnS₄ further attest to the thermodynamic viability of both compounds. Using the quasiparticle TB-mBJ + U approach, we find direct bandgaps of 1.374 eV for Cu₂ZnSnS₄ and 1.28 eV for Cu₂CdSnS₄. These values are ideally positioned within the optimal 1.0–1.4 eV window for single-junction photovoltaics. The electron effective masses are exceptionally low, with density-of-states averages of 0.139 m₀ for Cu₂ZnSnS₄ and 0.122 m₀ for Cu₂CdSnS₄. These low masses yield room-temperature electron mobilities of approximately 9000 cm² V⁻¹ s⁻¹ for Cu₂ZnSnS₄ and 13 000 cm² V⁻¹ s⁻¹ for Cu₂CdSnS₄. Optical absorption spectra exhibit sharp direct-gap onsets exceeding 10⁴ cm⁻¹. Cu₂CdSnS₄ displays a steeper absorption edge and a higher peak intensity of 1.11 × 10⁵ cm⁻¹ at 2.54 eV, compared with 1.06 × 10⁵ cm⁻¹ at 2.76 eV for Cu₂ZnSnS₄. When evaluated using the spectroscopic limited maximum efficiency (SLME) metric, both materials demonstrate outstanding photovoltaic performance. Cu₂ZnSnS₄ achieves a maximum efficiency of 32% at a thickness of 30 μm, while Cu₂CdSnS₄ reaches 32.2% at a substantially reduced thickness of 20 μm.

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Volleyballenes have emerged as materials with potential applications in hydrogen adsorption and pollution remediation. In this study, a new set of compounds is predicted, where Sc atoms are substituted with Ca, Ti, and V atoms. Additionally, the stability of the parent volleyballene is explored when its carbon framework displays hexagonal–hexagonal rings (Stone–Wales defects) instead of pentagonal–pentagonal rings, and their number yielding stable structures is determined. New structures exhibit enhanced stability, with cohesion energies ranging from approximately − 9.0 to − 7.0 eV (− 7.36 eV for [Ca₂₀C₆₀]¹⁻, − 8.67 eV for [Ti₂₀C₆₀]¹⁻ and − 9.07 eV for [V₂₀C₆₀]¹⁺). Moreover, their calculated IR spectra are reported to aid in their identification and characterization.

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Ab initio calculations based on the FP-LAPW technique, employing the PBE-GGA approximation within the WIEN2k computational code, were performed to investigate the structural, electronic, and optical properties of gallium phosphide (GaP) in a graphene-like structure. The analysis demonstrates that graphene-like GaP is a wide-bandgap semiconductor characterized by a direct bandgap of 2.14 eV. Additionally, various optical parameters, including the dielectric function, energy loss function, refractive index, extinction coefficient, optical conductivity, absorption coefficient, and reflectivity, were determined for the graphene-like GaP structure. Based on the evaluation of these optical properties, we propose that graphene-like GaP represents a promising candidate for integration into optoelectronic applications.

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Investigation of structural, electronic, and optical properties of graphene-like GaP. The color purple represents the atomic element Ga, whereas the color yellow represents the atomic element P.

Two-dimensional (2D) piezoelectric materials have received significant attention due to their anisotropic structures and unique properties. However, the development of high-performance 2D materials with a strong out-of-plane piezoelectric response remains a challenge. In this work, we used first-principles calculations to systematically investigate the structural, electronic, elastic, and piezoelectric properties of Cu-N and Al-N co-doped monolayer (ML) β-Ga₂O₃. The results indicate that Cu-N β-Ga₂O₃ is thermodynamically stable, while Al-N β-Ga₂O₃ exhibits mechanical stability. Cu-N co-doping induces metallicity and magnetism in β-Ga₂O₃, making it unsuitable for piezoelectric applications. In contrast, Al-N co-doping preserves the semiconducting behavior of β-Ga₂O₃, induces a significant out-of-plane piezoelectric response (e₃₁ = 3.28 × 10^–10 C/m, e₃₂ = 3.07 × 10^–10 C/m), retains good flexibility, as evidenced by a 10.70% reduction in Young’s modulus compared to Al-doped β-Ga₂O₃.This work demonstrates that Al-N co-doping is a promising strategy for achieving multifunctional β-Ga₂O₃ with integrated piezoelectricity and flexibility, offering great promise for applications in wearable electronics and flexible sensors.

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In this work, n-type Czochralski silicon irradiated with 2 MeV electrons at a fluence of 2 × 10¹⁷ e/cm² was investigated using a combination of Raman spectroscopy, infrared (IR) spectroscopy, and Hall effect measurements. The degree of crystallinity (K_cryst), determined from the intensity ratio of the 520.6 cm⁻¹ and 470 cm⁻¹ Raman bands, decreases from 93.3% in the initial sample to 76.1% after irradiation, indicating increased structural disorder. IR spectra reveal the formation of oxygen-related bonding configurations, including Si–O–Si and P–O bonds, as well as an enhancement of the band near 606 cm⁻¹, which is associated with oxygen-related vacancy complexes. These spectroscopic changes are consistent with Hall measurements, which show a significant increase in resistivity (up to 10⁴ Ω·cm) accompanied by a strong reduction in carrier concentration. The results indicate that radiation-induced defect centers, most likely related to vacancy–oxygen complexes, play a key role in carrier removal. Compared to previous studies, the present work demonstrates a systematic correlation between spectroscopic signatures and electrical degradation in electron-irradiated silicon.

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Organic single crystals of piperazinium bis (trifluoroacetic acid) (PTFA) was synthesized by the slow evaporation method. The centrosymmetric nature of the PTFA crystal was confirmed through single crystal XRD analysis with the P-1 space group. The molecular structure of PTFA was optimized by Density functional theory (DFT) with the B3LYP 6–311 +  + G (d, p) basis set to calculated the structural parameters of the compound were compared with the experimental XRD data. The presence of various functional groups and their frequencies were confirmed through FT-IR spectral analysis, based on the potential energy distribution (PED) used for assignments and correlations of the compound’s fundamental modes. The electronic transition spectrum of the PTFA molecule was obtained via UV–Vis spectroscopy in various solvents. The analysis of HOMO & LUMO molecular orbitals indicates a significant charge transfer, while Natural Bond Orbital (NBO) analysis revealed donor–acceptor interactions. The molecular electrostatic potential (MEP) and Fukui functions, derived from Mulliken charges, identified a reactive region of the molecule. The chemical bonding was explored through Electron Localization Function (ELF) & Localized Orbital Locator (LOL) analyses and the weak interactions was identified in RDG analysis with inter & intra-fragment interactions illustrated by scatter plots. The first-order hyperpolarization calculation confirms the nonlinear optical activity of the PTFA crystal. Besides, the Hirshfeld surface analysis and 2D fingerprint maps were used for quantitative mapping of intermolecular interactions.

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