Indian Journal of Physics最新文献

In the present article, we employ the geometry of absolute parallelism and parameterized absolute parallelism to develop a new theory, denoted as f(X, Y, E) gravity, where X = X (R, T) represents a parameterized curvature scalar, Y = Y(T) a parameterized torsion scalar, and E refers to the trace of the stress-energy tensor. From this new theory, we derive the modified theories f(R) gravity, f(R, E) gravity and f(T) gravity as special cases, where R represents a curvature scalar, T a torsion scalar. We introduce the f(X, Y, E) = λ X + γ Y + 2η E, where λ, γ and η are constants, and apply a quadratic deceleration parameter to describe the cosmological models of the Big Bang, Big Rip, and Big Crunch. Our study demonstrates that the evolution of the universe exhibits remarkably similar behavior across all proposed modified theories. Additionally, we discuss the energy conditions, revealing that the Dominant Energy Condition, Strong Energy Condition, and Weak Energy Condition are violated for these models.

Ortho-manganites are considered as promising candidates in ferro-electro-magnetism. The colossal magneto-resistive material demonstrates a first-order phase transition from a paramagnetic to a ferromagnetic state (TC), along with a transition from insulator to metallic behavior, meaning a change in resistivity. Here, we report synthesis and characterization of colossal magneto-resistive material 'pure LCMO' and 'LCMO-30% Au doped composite'. The results indicate that metal–insulator transition temperature was approximately 268 K for the LCMO-30% Au composite along with highest magneto-resistance (MR) of ~ − 63.8% at 268 K under 7 T applied magnetic field. Meanwhile, for the LCMO-30% Au composite, the maximum temperature coefficient of resistance (TCR) of ~ 11.02%/K was observed at 263 K and 1.77% at 275 K; in contrast, the pure LCMO exhibited a TCR of 2.36%/K at 260 K. The results indicate that the magneto-resistive properties of the LCMO-30% Au composite are significantly superior compared to the pure LCMO which is due to the fact that gold either accumulates at the grain boundaries (GBs) or on the surfaces of the LCMO grains, acting as an additive at the GBs without integrating into the primary LCMO lattice. Ultimately, results obtained were compared with the existing ones and demonstrated improvements over previously reported findings.

Novel crystals of strontium incorporated nickel cadmium oxalate (SNCO) trihydrate were grown by single diffusion method in silica hydro gel by optimizing various growth parameters. Field emission scanning electron microscopic (FESEM) images show the morphology of the crystal surface and the existence of Sr, Ni, Cd, C and O elements in the crystal lattice were identified using energy dispersive X-ray (EDX) analysis. The occurrences of C–O, C=O, C–C, O–H and M–O bonds were observed using Fourier transform infrared (FTIR) spectrophotometer. Powder X-ray diffractogram reveals the high crystallinity of the SNCO crystals, and exhibit triclinic crystal system with P-1 space group. Thermal studies (TG/DTG/DTA/DSC) show the high thermal stability (T_S > 600º C) of grown crystals. The high energy gap (E_g = 5.420 eV) from the UV–Visible absorption spectroscopy confirms the insulating behaviour of the crystals. Low electrical conductivity (μA), high leakage resistance (MΩ) and the variation of dielectric constant, dielectric loss, AC conductivity with frequency were studied and reported.

We investigate the Einstein-Hilbert black brane solution in four-dimensional Anti-de Sitter (AdS) spacetime supplemented by a quadratic Ricci scalar term (q L^2 R^2), where q is a dimensionless coupling constant and L is the AdS radius. The shear viscosity to entropy density ratio, (frac{eta }{s}), is calculated holographically, and deviations from the universal Kovtun-Son-Starinets (KSS) bound are analyzed. Our results indicate that (frac{eta }{s} = frac{1}{{4pi }}left( {1 - 24q} right)), demonstrating that the ratio falls below the conjectured lower limit for positive q, while it respects the bound for negative q. We confirm that our solutions smoothly reduce to the standard Einstein-Hilbert case when (q rightarrow 0), consistent with expectations. The physical implications of violating the KSS bound are discussed in depth, particularly regarding stability, causality, and the strongly coupled nature of the dual field theory. These findings provide valuable insights into the influence of higher curvature terms on holographic transport properties.

In this study, a simulation of the structural phase transition mechanism of SnO₂ (Cassiterite), selected among the rutile-type stable metal dioxides, has been carried out. Calculations were performed in the frame of density functional theory (DFT). Using the ab-initio technique, the structural phase transition behaviors and intermediate phases of SnO₂ under high pressure were investigated with the Siesta code. Generalized gradient approximation (GGA) was selected to find the exchange–correlation energy. Initially, the rutile-type SnO₂ structure was optimized at zero pressure, and then the pressure was increased up to 170 GPa with an increment of 10 GPa. The initial rutile-type structure of SnO₂ with space group P4₂/mnm (No. 136) transformed into the cotunnite type structure with space group Pnma (No. 62) at 100 GPa. During the phase transformation of SnO₂ from rutile type to cotunnite type at 100 GPa pressure, intermediate phases Pnnm (Orthorhombic) → P2₁ (Monoclinic) → P₁ (Triclinic) → Pc (Monoclinic) → P2₁/c (Monoclinic) were determined for the first time in this study. Energy and enthalpy calculations were performed to investigate the compatibility of the determined stable phase transition with the experimental results. For the electronic properties, the band structures and density of states are calculated and analyzed in detail. The findings obtained from the calculations are in accordance with the literature.

In this work, we provide comprehensive first-principles insights into the structural, elastic, electronic, and optoelectronic characteristics of layered LiNbCh₂ (Ch = O, S, Se) chalcogenides. Elastic analysis confirms mechanical stability and uncovers a systematic softening and anisotropy trend across the series, with all compounds displaying brittle behavior. Electronic band structure calculations reveal a semiconducting nature, where LiNbO₂ hosts a direct band gap, while LiNbS₂ and LiNbSe₂ exhibit indirect gaps. Optical investigations uncover pronounced anisotropy and birefringence, with LiNbO₂ showing strong ultraviolet absorption, positioning it as a promising candidate for UV photodetection. Conversely, LiNbS₂ and LiNbSe₂ exhibit absorption in the visible range, underlining their potential for next-generation optoelectronic and photovoltaic applications.

This work details the design and simulation of a linear variable optical filter (LVOF) that has been optimized for the mid-infrared (MIR) spectrum, specifically within the range of 2500–5500 nm. The filter employs a silicon substrate and a distributed Bragg reflector (DBR) architecture composed of Si, Ge, and SiO₂ thin-film layers, incorporating a laterally tapered SiO₂ cavity to achieve continuous wavelength tuning. The implementation of high-contrast Si/SiO₂ and Ge/SiO₂ layer pairs enables broad spectral coverage with a reduced number of DBR periods while maintaining high reflectivity. Simulations demonstrate transmission exceeding 90%, cut-off depth below 0.1%, and spectral resolution between 0.6 and 5%. The proposed structure provides multichannel filtering capability using a monolithic design, enabling compact, low-cost MIR detection systems suitable for gas-sensing applications.

A Time-Dependent Density Functional Theory (TDDFT) based study has been performed for the optical properties of the previously identified most stable Ga_xP_y (x + y = 2 to 5) nanoclusters. The structures of Ga_xP_y nanoclusters are optimized using the B3LYP-DFT/6-31G(d) method, and after that TDDFT is applied to explore the optical properties of the most stable configurations. The Zero-point energy correction is also considered in determining the most stable structures. This study presents the calculated optical absorption spectra, which are similar to the electron energy loss spectra (EELS) and the corresponding oscillator strength (f) for the most stable GaP nanoclusters of each configuration. Nearly all nanoclusters exhibit strong absorption in the ultraviolet or extreme ultraviolet regions. Some nanoclusters also show appreciable absorption in the visible region. The results reveal a trend where, in most nanoclusters, absorption occurs at lower energies in Ga-rich nanoclusters and at higher energies in P-rich nanoclusters. The development of these theoretically identified most stable GaP nanoclusters could have potential applications in the fabrication of nanodevices, depending on their specific physical properties.

This work aims to construct the generalized form of holographic dark energy density which is known as the Rényi holographic dark energy within an inhomogeneous universe scenario with interacting dark sectors. This holographic model is investigated using the IR cutoff equals the Hubble horizon. The evolutionary behavior of some basic cosmological parameters, such as the Hubble parameter, the deceleration parameter, and the equation of state parameter is studied and plotted against the redshift. The stability of these models has been studied by investigating the evolution of the square of sound speed of the cosmological matter fluctuations against the redshift. In addition, the evolution of the diagnostic Om and Lm parameters is studied. The obtained results show a very good agreement between the recent values of some of the cosmological parameters with the observations. Some major differences and similarities between these models and previously studied inhomogeneous models have been shown. The features that distinguish these models from each other and the standard cosmological model have been studied by applying the state finder analysis. To investigate the viability of these models and how they are candidates to explain a realistic case of our universe, they have been tested with the violation of the second law of thermodynamics.