Search Results (41)

A novel ternary transition metal chalcogenide Ba₂Re₆Se₁₁, which crystallizes in the R−3c space group, was synthesized using a high-pressure and high-temperature technique. The lattice is constituted by Re₆Se₈ cube-octahedral clusters connected by additional apical Se anions via the Re-Se-Re pathway, while the Ba atoms reside in the cavities among the Re₆Se₈ units. High-pressure synchrotron X-ray diffraction measurements showed that Ba₂Re₆Se₁₁ maintains a trigonal structure up to a pressure of 60 GPa, with a bulk modulus of 193 GPa. The lattice stability is ascribed to the fully occupied valence bands of the molecular orbital of the Re₆Se₈ cluster with trivalent Re. This fully occupied orbital configuration also gives rise to the diamagnetic state of Ba₂Re₆Se₁₁, which was validated through magnetic measurements. The resistivity of Ba₂Re₆Se₁₁ is as low as several milliohm centimeters, and it follows the thermal activation mechanism at elevated temperatures and the three-dimensional variable-range hopping model at low temperatures, indicating that Ba₂Re₆Se₁₁ is a semiconductor or insulator in close vicinity to a metal–insulator transition. Full article

Layered compounds containing the T₂O plane (T = transition metal), which is the anti-type of the CuO₂ plane in cuprate superconductors, have been explored widely because of their diverse physical properties. Among them, KV₂Se₂O has attracted much attention due to its interesting physical properties, especially the magnetic order. In this work, we report a new isostructural chromium oxyselenide, RbCr₂Se₂O. It was synthesized using a solid-state method using Rb₂CO₃ as the source of Rb and O for the title compound, with the assistance of Ba. The compound crystallizes in the space group P4/mmm with lattice parameters a = 4.01123(8) Å and c = 7.49357(18) Å. Magnetic susceptibility measurements indicate an antiferromagnetic transition at 345 K for RbCr₂Se₂O and also above room temperature, as the Néel temperature is T_N ≈ 400 K for KV₂Se₂O. The analysis of variable temperature XRD data reveals the anisotropic thermal expansion of the RbCr₂Se₂O lattice. The almost unchanged lattice parameter a near the transition temperature and the broad peak with an onset temperature of ~360 K in the differential scanning calorimetry data may have a relationship with the magnetic ordering. The measurement of electrical resistivity demonstrates the semiconducting behavior of RbCr₂Se₂O. The thermal activation model and variable-range hopping model are proposed to describe the conduction mechanism in the high- and low-temperature ranges, respectively. Full article

Background and Objectives: Lower-extremity injuries are common among female athletes; however, their multifactorial predictors remain insufficiently understood. Given the interplay between physiological and psychological readiness in athletic performance, identifying the factors that influence lower limb performance is crucial for effective injury prevention. This study aimed to evaluate the predictive effects of physiological (VO₂peak, anaerobic power, agility, and isokinetic strength) and psychological (resilience and self-efficacy) variables on functional performance related to risk of injury. Materials and Methods: This cross-sectional study included 60 athletes with a mean age of 24.5 ± 6.90 years and mean body mass index of 23.12 ± 3.6 kg/m² (range: 16–30 kg/m²). The testing protocol included anthropometric measurements, the Lower Extremity Functional Test (LEFT), Wingate anaerobic cycling test, assessments of aerobic capacity, isokinetic muscle strength, and jumping performance (Single-Leg Hop [SLH] and Standing Long Jump [SLJ] tests). Psychological assessments included the General Self-Efficacy Scale (GSES) and a resilience questionnaire. A hierarchical regression analysis was performed. Results: The participants trained 5 ± 2 days per week and had 42 ± 39 months of sports experience. The mean VO_2peak was 40.82 ± 5.8 mL·kg⁻¹·min⁻¹, relative anaerobic peak power was 7.53 ± 1.92 W/kg, and fatigue index was 60.63 ± 15.41%. The mean isokinetic knee extension and flexion torque were 184.55 ± 44.60 N·m and 95.08 ± 21.44 N·m, respectively, with a flexion-to-extension ratio of 53.5%. The mean LEFT completion time was 160 ± 22 s. The participants demonstrated moderate resilience (BRS = 21 ± 4) and good self-efficacy (GSES = 33 ± 7.5). Among the psychological variables, GSES exhibited a modest negative correlation with LEFT (r = −0.28, p = 0.02). No significant associations were found between LEFT and psychological resilience. Longer LEFT completion times were associated with lower VO_2peak, mean power, and jump distance (p < 0.01). In the final model (R² = 0.58, p = 0.02), SLH (β = −0.54), VO_2peak (β = −10.32), and GSES (β = −0.70) were the strongest independent predictors of LEFT performance. Conclusions: SLH distance, VO_2peak, and general self-efficacy are key predictors of functional performance on the LEFT among female athletes. These factors may serve as practical indicators for identifying athletes who could benefit from targeted injury prevention programs. Full article

Accurate ultra-short-term wind power forecasting is vital for the secure and economic operation of power systems with high renewable penetration. Conventional models, however, struggle with multi-scale frequency feature extraction, dynamic cross-variable dependencies, and simultaneously capturing local fluctuations and global trends. This study proposes a novel hybrid framework termed VMD–ALIF–GraphBlock–MLLA–Transformer. A dual-decomposition strategy combining variational mode decomposition and adaptive local iterative filtering first extracts dominant periodic components while suppressing high-frequency noise. An adaptive GraphBlock with MixHop convolution then models structured and time-varying inter-variable dependencies. Finally, a multi-scale linear attention-enhanced Mamba-like module and Transformer encoder jointly capture short- and long-range temporal dynamics. Experiments on a real wind farm dataset with 10-min resolution demonstrate substantial superiority over State-of-the-Art baselines across 1-, 4-, and 8-step forecasting horizons. SHAP analysis further confirms excellent consistency with underlying physical mechanisms. The proposed framework provides a robust, accurate, and highly interpretable solution for intelligent wind power forecasting. Full article

Two-legged hopping is a well-established model for assessing leg stiffness; however, in existing studies, it is unclear whether the trial segment selection affects the results. This study aimed to assess if the selected hopping segment alters the value and individual variability (%CVind) of leg stiffness and kinetic performance metrics. Elite women athletes (42, volleyball, basketball, handball) and 14 non-athletic women performed barefoot two-legged hopping (130 bpm) on a force-plate (Kistler, 9286AA, sampling at 1000 Hz). Leg stiffness was estimated from the Fz registration (resonant frequency method). Four cumulative range segments (1–10, 1–20, 1–30, and 1–40 hops) and three segments of 10-hop subranges (11–20, 21–30, and 31–40) were analyzed (repeated measures one-way Anova, p ≤ 0.05, SPSS v30.0). The hopping segment did not significantly alter the leg stiffness value (segment average 30.6 to 31.2 kN/m) or its %CVind (segment average ≈ 3%). The kinetic performance metrics depicted a solid foundation for the extracted leg stiffness value, with %CVind not exceeding 6.2%. The results indicate a data collection of just 15 hops, in continuance reduced to a 10 hops segment (after excluding the first five to avoid neuromuscular adaptation) as a robust reference choice. Full article

In this work, nickel ferrite/carbon nanotubes–polyaniline (NiFe₂O₄@CNTs-PANI) nanocomposites with positive magnetoresistance (MR) phenomenon was obtained by the hydrothermal method combined with the surface-initiated polymerization (SIP) method. The NiFe₂O₄@CNTs-PANI nanocomposites’ resistivity decreased as the temperature increased, exhibiting typical semiconductor behavior. This temperature-dependent resistivity revealed a quasi-three-dimensional (3D) variable range hopping (VRH) charge carrier transport mechanism. Based on the wave function shrinkage model, its positive MR was studied. It was found that the localization length (a₀) and average hopping distance (R_hop) were decreased with increasing magnetic field strength, indicating the reduction in the hopping probability of the charge carrier, which was beneficial for the positive MR. Meanwhile, the values of a₀ and R_hop were decreased with increasing NiFe₂O₄@CNTs loading, expressing a nanofiller loading-dependent behavior. Full article

Balance control in pirouettes has previously been characterized by constraint of the topple angle. However, there is a paucity of research using the margin of stability (MoS) as a dynamic measure of balance related to pirouettes. Therefore, this study aimed primarily to examine the MoS as a metric of balance during a single-turn en dehors pirouette in healthy female amateur ballet dancers. Four participants performed pirouettes until five successful pirouettes were achieved without hopping or loss of balance. Three-dimensional motion capture was used to record the motion trajectories of anatomical markers based on the Plug-in-Gait and Oxford Foot models. Motion synchronized with ground reaction forces was used to calculate the center of pressure (CoP), base of support (BoS), center of the pivot foot, center of mass (CoM), and extrapolated center of mass (XCoM) throughout the turn phase, using laboratory (LCS) and virtual left foot (LFT) coordinate systems. In the LCS and LFT coordinate system, the excursions and patterns of motion of both the CoM and XCoM relative to the CoP were similar, suggesting a neurological relationship. Two different measures of the margin of stability (MoS) in the LFT coordinate system were tabulated: the distance between the (1) XCoM and CoP and (2) XCoM and BoS center. The magnitude of both versions of the MoS was greatest at turn initiation and toe-touch, which was associated with two foot contacts. The MoS values were at a minimum approximately 50% of the stance during the turn phase: close to zero along the anteroposterior (A/P) axis and approximately 50 mm along the mediolateral (M/L) axis. On average, MoS magnitudes were reduced (mean across participants: approximately 20 mm) along the A/P axis, and larger MoS magnitudes (mean across participants: approximately 50 mm) along the M/L axis throughout the turn phase. Although all turns analyzed were completed successfully, the larger MoS values along the M/L axis suggest a fall potential. The variability between trials within a dancer and across participants and trials was documented and showed moderate inter-trial (16% to 51%) and across-participant CV% (range: 10% to 28%), with generally larger variations along the A/P axis. Although our results are preliminary, they suggest that the MoS may be useful for detecting faults in the control of dynamic balance in dehors pirouette performance, as a part of training and rehabilitation following injury. Full article

The electrical conductivity of nanocrystalline CuMnO₂ samples, obtained by microwave-assisted hydrothermal synthesis (MWH), is studied by impedance spectroscopy over a frequency range of 30 Hz to 2 MHz and a temperature range from 30 to 120 °C. Three samples are prepared to start from a mixture of sulphate reactants, at two synthesis temperatures and different reaction times (of applying microwaves): sample S1 at 80 °C for 5 min; sample S2 at 120 °C for 5 min and sample S3 at 120 °C for one hour. The static conductivity values, σ_DC of samples S2 and S3, are approximately equal but larger than those of sample S1. This result suggests that using MWH synthesis at 120 °C, with different reaction times (samples S2 and S3), is sufficient for microwaves to be applied for at least 5 min to obtain samples with similar electrical properties. The experimental data were analysed based on three theoretical models, demonstrating that the most appropriate theoretical model to explain the electrical conduction mechanism in the samples is Mott’s variable range hopping (VRH) model. Using this model, the activation energy of conduction, (E_A,cond), the density of localized states near the Fermi level, N(E_F), the hopping distance, R_h(T), the hopping energy, W_h(T) and the charge carrier mobility (μ) were determined for the first time, for microwave-assisted hydrothermally synthesized crednerite. Additionally, the band gap energy (W_m) and hopping frequency (ω_h) were evaluated at various temperatures T. Understanding the electrical conduction mechanism in the polycrystalline CuMnO₂ materials is important for their use in photo-electrochemical and photocatalytic applications, photovoltaic devices, and, more recently, in environmental protection. Full article

Polypyrrole (PPy)-doped bismuth calcium manganite (BCM) nanocomposites were synthesized by chemical polymerization. The amorphous nature of the polypyrrole and the monoclinic crystal structure of the BCM particles (35–65 nm) were confirmed by various microstructural, X-ray powder, and spectroscopy techniques. The DC conductivity analysis via the correlated barrier-hopping (CBH) model and Mott’s variable-range hopping (MVRH) model showed that the nanocomposites exhibited ionic conduction. Activation energies, evaluated from the Arrhenius plots, showed that PPy/BCM-30 (30 wt.% of BCM) had the minimum value of 0.09 eV, indicating maximum conductivity and normal NTCR behavior, with resistance decreasing with temperature. The CBH model described the conduction process, and the AC conductivity measurements indicated that the conductivity was frequency-independent at lower frequencies but became dispersive and frequency-dependent at higher frequencies, conforming to Jonscher’s power law. The study revealed that the transport of electrical charge in the material followed the correlated barrier-hopping (CBH) model. These results demonstrate how promising PPy/BCM nanocomposites are for energy storage, sensors, and electronic materials. Full article

Hop (Humulus lupulus L.) is a climbing plant that contains essential components for beer production. Although Brazil is the third-largest beer producer in the world, it still relies on imports to meet demand. Some hop varieties have already adapted to the tropical climate, but nitrogen fertilization is essential for the proper development of plants. Digital image processing (DIP) and modeling technologies are emerging as fast and economical alternatives for monitoring the nutritional status of plants. This study evaluated the impact of image quality and the performance of models in classifying hop plants in terms of nitrogen concentration, using predictors extracted from leaf images. A total of 24 plants subjected to six levels of fertilization, ranging from 0 to 200% of the optimal level, were analyzed. The leaves were classified into two nitrogen concentration groups and the data organized into two sets: one containing only significant variables and another including all the variables in the model. Linear discriminant analysis (LDA) and quadratic discriminant analysis (QDA) models were estimated. The QDA models demonstrated great efficacy in classifying plants with a high nitrogen concentration, achieving over 80% accuracy, although performance was lower for plants with a lower nitrogen concentration. Full article

CuMn_1−xMg_xO₂ (x = 0–0.06) polycrystalline samples were prepared using the hydrothermal method at T = 100 °C for 24 h in Teflon-line stainless steel autoclaves. The samples were crystallized, forming crednerite structures (C2/m space group), and the Mg²⁺ substitution onto the Mn³⁺ site induced small changes in the unit cell parameters and volume. Based on complex impedance measurements made between 20 Hz and 2 MHz, at different concentrations of Mg ions (x), the electrical conductivity (σ), the electric modulus (M), and the complex dielectric permittivity (ε) were determined. The conductivity spectrum, σ(f, x), follows the Jonscher universal law and enables the determination of the static conductivity (σ_DC) of the samples. The results showed that, when increasing the concentration x from 0 to 6%, σ_DC varied from 15.36 × 10⁻⁵ S/m to 16.42 × 10⁻⁵ S/m, with a minimum of 4.85 × 10⁻⁵ S/m found at a concentration of x = 4%. Using variable range hopping (VRH) and correlated barrier hopping (CBH) theoretical models, the electrical mechanism in the samples was explained. The band gap energy (W_m), charge carrier mobility (μ), number density (N_C) of effective charge carriers, and hopping frequency (ω_h) were evaluated at different concentrations (x) of substitution with Mg. In addition, using measurements of the temperature dependence of σ_DC(T) between 300 and 400 K, the thermal activation energy (E_A) of the samples was evaluated. Additionally, the dielectric behavior of the samples was explained by the interfacial relaxation process. This knowledge of the electrical properties of the CuMn_1−xMgxO₂ (x = 0–0.06) polycrystalline crednerite is of interest for their use in photocatalytic, electronic, or other applications. Full article

The complex dielectric permittivity, ε (f, H) = ε′ (f, H) − i ε″ (f, H), in the microwave frequency range f, of (0.1–3) GHz and polarizing field values H, in the range of (0–135) kA/m, was measured for a kerosene-based ferrofluid with magnetite particles. A relaxation process attributed to interfacial type relaxation was highlighted, determining for the first time in the microwave field, the activation energy of the dielectric relaxation process in the presence of the magnetic field, E_A(H), in relation to the activation energy in zero field, E_A(H = 0). Based on the complex permittivity measurements and the Claussius–Mossotti equation, the dependencies on frequency (f), and magnetic field (H), of the polarizability (α) and electrical conductivity (σ), were determined. From the dependence of α(f,H), the electric dipolar moment, p, of the particles in the ferrofluid, was determined. The conductivity spectrum, σ(f,H), was found to be in agreement with Jonscher’s universal law and the electrical conduction mechanism in the ferrofluid was explained using both Mott’s VRH (variable range hopping) model and CBH (correlated barrier hopping) model. Based on these models and conductivity measurements, the hopping distance, R_h, of the charge carriers and the maximum barrier height, W_m, for the investigated ferrofluid was determined for the first time in the microwave field. Knowledge of these electrical properties of the ferrofluid in the microwave field is useful for explaining the mechanisms of polarization and control of electrical conductivity with an external magnetic field, in order to use ferrofluids in various technological applications in microwave field. Full article

Two ceramic samples of sodium tantalate (NaTaO₃), doped with metal ions of copper (Cu; sample S1) or aluminum (Al; sample S2), were obtained by the sol-gel method. Complex impedance measurements in the frequency range (200 Hz–2 MHz) and at temperatures between 30 °C and 90 °C allowed identification of a transition temperature from semiconductor-type behavior to conductor-type behavior for each sample (52 °C for sample S1 and 54 °C for sample S2). In the temperature range with semiconductor behavior, the activation energy of each sample was determined. Based on the Mott’s variable-range hopping (VRH) model, the density of localized states at the Fermi level, N(E_F), the hopping distance (R) and the hopping energy (W) were determined, for the first time, on NaTaO₃ samples doped with Cu or Al metal ions. The increase in N(E_F) of sample S2 compared to N(E_F) of sample S1 was explained by the decrease in the hopping distance of charge carriers in sample S2 compared to that in sample S1. Additionally, using the correlated barrier hopping (CBH) model, the energy band gap (W_m) and the hopping (crossover) frequency (ω_h) at various temperatures were determined. Knowledge of these electrical properties is very important for explaining the electrical conduction mechanisms in metal ion-doped compounds, with perovskite structure being of interest for the use of these materials in the conversion of thermoelectric energy, photocatalytic applications, electronics or other applications. Full article

Nowadays, the search for the coupled polymer nanocomposite thermoelectrics that exhibit a high value of thermoelectric figure of merit (ZT) and similar behaviour of physical properties for the use as legs of thermoelectric cells is a current challenge. The direct current (DC) conductivity is one of the three important components of thermoelectric figure of merit. The aim of this study was to obtain PANI-based nanothermoelectrics with Te⁰ and Bi₂Te₃ nanoparticles and MWCNT by mechanochemical methodology and to investigate the dependency of their DC electrical conductivity on temperature in the 298–353 K range using the Arrhenius and Mott’s variable range hopping (VRH) models. Inorganic Te⁰ and Bi₂Te₃ nanoparticles were pre-synthesized by the available and environmentally friendly method using a commercial tellurium powder. The samples obtained were characterized by X-ray diffractometry (XRD), IR and UV-Vis spectroscopy. The XRD study of ES-PANI/Te⁰ (4.4 wt% Te⁰) and ES-PANI/Bi₂Te₃ (2.9 wt% Bi₂Te₃) nanocomposites found that the nanoparticle average size was 32 nm and 17 nm, respectively. The DC conductivity study of the samples with different nanophase content (2.1, 4.4, 10.2 wt% Te⁰, 1.5, 2.9, 7.3 wt% Bi₂Te₃, 1.5 wt% MWCNT) by the two points measurement method reveals the following: (a) the presence of inorganic nanophase reduces the conductivity compared to the matrix, (b) the addition of MWCNT in ES-PANI increases its electrical conductivity, (c) the conductivity of ES-PANI/Te⁰ as well as ES-PANI/Bi₂Te₃ nanocomposite rises with the increasing inorganic nanophase content, (d) the observed increase in the electrical conductivity of MWCNT-based nanocomposites with increasing inorganic nanophase content is interrupted by a characteristic area of decrease in its value at average values of inorganic nanoparticles content (at Te⁰ content of 4.4 wt%, at Bi₂Te₃ content of 2.9 wt%), (e) a similar DC conductivity behaviour in ES-PANI/Te⁰—ES-PANI/Bi₂Te₃ and ES-PANI/Te⁰-MWCNT—ES-PANI/Bi₂Te₃-MWCNT nanocomposite pairs is observed. Full article

Anti-site disorder, arising due to the similar size of Fe and Mo ions in Sr₂FeMoO₆ (SFMO) double perovskites, hampers spintronic applicability by deteriorating the magnetic response of this double perovskite system. A higher degree of anti-site disorder can also completely destroy the half-metallicity of the SFMO system. To study the effects of different process gas conditions on the anti-site disorder, we have prepared a series of SFMO thin films on SrTiO₃ (001) single-crystal substrate using a pulsed laser deposition (PLD) technique. The films are grown either under vacuum or under N₂/O₂ partial gas pressures. The vacuum-grown SFMO film shows the maximum value of saturation magnetization (M_S) and Curie temperature (T_C), signaling the lowest anti-site disorder in this series. In other words, there is a long-range Fe/Mo-O-Mo/Fe ferrimagnetic exchange in the vacuum-grown thin film, thereby enhancing the magnetization. Further, all the SFMO films show a semiconducting state with a systematic increase in overall resistivity with the increased anti-site disorder. The electrical conduction mechanism is defined by the variable-range hopping model at low temperatures. Raman spectra show a weak Fano peak, suggesting the presence of electron–phonon coupling in SFMO thin films. These results show the significance of the process gas in causing anti-site disorder, tuning the degree of this disorder and therefore its influence on the structural, magnetic, electrical, and vibrational properties of SFMO thin films. Full article