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We model anisotropic compact stars by solving Einstein field equation with a generalized form of equation of state (EoS) and a physically reasonable metric potential $$g_{rr}$$ which allows us to extract models with various type of EoSs such as linear, quadratic, polytrope, Chaplygin and Color-flavor-locked. The generated interior metrics match smoothly with the exterior Schwarzschild metric. The radius $$R = 8.301$$ km and mass $$M=1.0359M_{\odot }$$ of the strange star candidate SMC X-1 has been used as reference to check the physical validity of the generated models. The physical quantities emanating from the analysis for all generated models are regular and well behaved throughout the interior of the star and satisfy the necessary criterion of a realistic anisotropic star. The significance of EoSs in modeling a realistic star with regard to its gross physical behavior has been illustrated by graphical analysis. A complete physical analysis has been performed for the main physical observables, such as the density $$\rho $$ , the radial and tangential pressures as well as the anisotropy factor. Moreover, the stability of the star has been checked by means of the velocities of the pressure waves and the relativistic adiabatic index. The generated models satisfy the Bondi’s stability condition and Tolman–Oppenheimer–Volkoff equation for static equilibrium for all type of EoSs. The effect of anisotropy on cracking condition has been discussed.

We study static spherically symmetric space-time to describe relativistic compact objects with anisotropic matter distribution and derive two classes of exact models to the Einstein–Maxwell system with a modified Van der Waals equation of state. We motivate a Van der Waals-type equation of state to physically signify a high-density domain of quark matter, and the generated exact solutions are shown to contain several classes of exact models reported previously that correspond to various physical scenarios. Geometrical analysis shows that the physical quantities are well behaved so that these models may be used to describe anisotropic charged compact spheres.

We report a general solution in one of the gravitational potentials to the Einstein system in static spherically symmetric spacetime, modeling anisotropic strange quark matter by imposing a linear barotropic equation of state. The model generated by choosing Tolmann IV form for the gravitational potential is shown to contain the previously known anisotropic dark energy star model of Lobo (Class. Quantum Gravity 23:1525, 2006) and isotropic models of de Sitter and Einstein. Moreover, the model is shown to be physically admissible and corroborate with experimental observations on strange star candidates such as SAX J1808.4-3658 and 4U 1820-30.

We formulate a system of field equations with Van der Waals type equation of state to the Einstein field equations in spherically symmetric static spacetime to describe anisotropic compact matter and obtain a new class of solution by choosing one of the gravitational potentials. The generated model is shown to be physically admissible by satisfying the major physical conditions such as regularity of gravitational potential at the origin, positive definiteness of energy density and the radial pressure at the origin, vanishing of radial pressure at some finite radius, and monotonic decrease of the energy density and the radial pressure with increasing radius. The model also satisfy the casuality condition and match smoothly with the Schwarzschild exterior line element at the stellar boundary. We believe that the solution found through this work may assist more detailed studies of relativistic compact stellar bodies at the extremes of densities.

This research paper describes the synthesis of a series of Y-based superconducting alloys doped with Ag oxide. The Y3Ba5Cu8–xAgxO18–d superconducting alloys were synthesized by Gholipour et al. (2012), Tavana et al. (2010) and Khosroabadi et al. (2014) using sol-gel reaction technique. In the present research Y3Ba5Cu8–xAgxO18–d superconducting alloys were synthesized by solid state reaction technique. These alloys were investigated by using X-ray diffraction pattern (XRD), Scanning Electron Microscope (SEM) image and the electrical resistivity, ρ, measurements. The ρ measurements above room temperature of Y358 alloys shows metal behavior in the normal state. The YBCO alloy family was exposed having the CuO2 planes and the Cu-O chains. Increasing the number of the CuO2 planes and the position of the Cu-O chains have positive effects on the significance of the transition temperature in the Y-based alloys. The X-ray diffraction patterns were accepted by Joint Committee of Powder Diffraction Standards (JCPDD) database software, which the Ag ions substitute in symmetrical phase of the alloy. The replacement of the Ag ions in the CuO2 plane of symmetric phase has been associated with the Ag ions in the Y123 alloys, which replace in the Cu-O chain. The ρ of these alloys were measured employing four-probe point technique and the r(T) curves agreed the transition temperature develops above 100 K.

We study the Einstein system of equations in static spherically symmetric spacetimes. We obtained classes of exact solutions to the Einstein system by transforming the condition for pressure isotropy to a hypergeometric equation choosing a rational form for one of the gravitational potentials. The solutions are given in simple form that is a desirable requisite to study the behavior of relativistic compact objects in detail. A physical analysis indicate that our models satisfy all the fundamental requirements of realistic star and match smoothly with the exterior Schwarzschild metric. The derived masses and densities are consistent with the previously reported experimental and theoretical studies describing strange stars. The models satisfy the standard energy conditions required by normal matter.

Static spherically symmetric space-time is studied to describe dense compact star with quark matter within the framework of MIT Bag Model. The system of Einstein’s field equations for anisotropic matter is expressed as a new system of differential equations using transformations and it is solved for a particular general form of gravitational potential with parameters. For a particular parameter, as an example, it is shown that the model satisfies all major physical features expected in a realistic star. The generated model also smoothly matches with the Schwarzschild exterior metric at the boundary of the star. It is shown that the generated solutions are useful to model strange quark stars.

We study static spherically symmetric spacetime to describe compact objects with anisotropic matter distribution. We express the system of Einstein field equations as a new system of differential equations using a coordinate transformation, and then write the system in another form with polytropic equation of state and obtain two classes of exact models. The models satisfy all major physical features expected in a realistic star. For polytropic index n = 2, we obtain expressions for mass and density which are comparable with the reported experimental observations.

The occurrence of a maximum at a temperature T max in the resistivity of the dense Kondo compound CeNiAl4 and in (Ce1−xY x)NiAl4 alloys is investigated. T max ( ∝ T K , the Kondo temperature) first increases, goes through a maximum value and decreases with increase in x . This behaviour is qualitatively explained by considering the competing influence of volume changes and dilution (the Kondo-hole effect) on the Kondo system. Complementary magnetic susceptibility measurements corroborate these results.

Electrical resistivity ρ(T) measurements on polycrystalline samples of the Ce (Pt1−xNix)2Si2 system exhibit dense Kondo behaviour for alloys with . The temperature at which a maximum appears in ρ(T) shifts from Tmax = 65 K for x = 0 to Tmax = 290 K for x = 0.6 as the system evolves towards the intermediate-valence regime for the CeNi2Si2 compound. At low temperatures the Ce (Pt1−xNix)2Si2 alloys show a Fermi liquid T2 contribution to ρ(T) below the coherence temperature Tcoh. It is observed that , with a transition that takes place from a dense Kondo regime of TK TCF (). TK () and TCF denote the Kondo and crystalline electric field splitting, EF and ER are the Fermi and the Abrikosov–Suhl resonance energies and j denotes the total angular momentum. An electronic Gruneisen parameter value of 19 has been obtained for the alloy system which is in reasonable agreement with Ω = 28 found from pressure studies on CePt2Si2. It is argued that volume effects are mainly responsible for the hybridization in the Ce (Pt1−xNix)2Si2 system and application of the compressible Kondo lattice model to Tmax data yields |JN(EF)| = 0.309 ± 0.008, where J is the on-site exchange interaction between conduction electrons and the localized 4f spin and N(EF) is the density of states at EF.

Electrical resistivity results of Ce ( Pt 1 - x Ni x ) Ga and CePt ( Ga 1 - y Al y ) alloys show features at low temperature indicating antiferromagnetic ordering and Kondo behaviour. The variation of Neel temperature with alloy composition is interpreted in terms of a Doniach diagram and the possible existence of a quantum critical point is indicated. Magnetoresistance data are presented and analysed in terms of the Schlottmann formalism.

We report a general approach to solve an Einstein-Maxwell system to describe a static spherically symmetric anisotropic strange matter distribution with linear equation of state in terms of two generating functions. It is examined by choosing Tolmann IV type potential for one of the gravitational potentials and a physically reasonable choice for the electric field. Hence, the generated model satisfies all the required major physical properties of a realistic star. The effect of electric charge on physical properties is highlighted.

We establish a systematic algorithmic approach that generates new classes of solutions to the EinsteinMaxwell system in static spherically symmetric spacetime from well known uncharged solutions. A particular case is shown to satisfy all major physical features of a realistic charged star including the standard point-wise energy conditions of normal matter. The solution matches smoothly with the exterior Reissner-Nordstr¨om metric at the pressure free interface. The study, which is reported for a particular choice of gravitational potential, encourages similar approaches to study electrification of well known physically realistic uncharged models.

Measurements of electrical resistivity (ρ(T)), magnetoresistivity (MR), magnetic susceptibility (χ(T)) and heat capacity (C(P)(T)) are presented for the (Ce(1-x)La(x))PtIn alloy system of which the CePtIn parent is a known dense Kondo compound that does not order magnetically down to 50 mK. χ(T) for alloys 0≤x≤0.8 exhibits Curie-Weiss behaviour. ρ(T) results indicate a transition from a dense Kondo behaviour for 0≤x≤0.2 to a single-ion Kondo region (0.3≤x≤0.8). The Kondo energy scale as given by T(K) values calculated from MR studies and by the temperature T(max)(ρ(mag)) where the magnetic contribution to ρ(T) exhibits a maximum value, is compared with theoretical models. It is shown that the experimental results not only depend on a volume effect as given by the compressible Kondo lattice model of Lavagna but in addition confirm the more complex behaviour recently presented by Burdin and Fulde for a Kondo alloy system in which the magnetic (Ce) and non-magnetic (La) atoms are distributed randomly. Non-Fermi-liquid behaviour is predicted by Burdin and Fulde at certain critical concentrations of the alloy system and experimental evidence for this is presented through χ(T), ρ(T) and C(P)(T) measurements.

Electrical resistivity (ρ(T)) and magnetoresistivity (MR) measurements are presented for the (Ce1−xLax)Pt2Si2 alloy system. ρ(T) data at low temperatures for alloys in the concentrated Kondo lattice regime 0≤x≤0.2 conform to a ρ(T) = ρ0+AT2 dependence and Fermi-liquid parameters A and Tcoh are presented. The maximum that occurs in ρ(T) in the concentrated Kondo lattice regime at a temperature of Tmax is no longer observed for samples with x≥0.3 which show single-ion Kondo behaviour. MR measurements on alloys with x = 0.5, 0.7 and 0.9 yield values of the Kondo temperature TK through Schlottmann analysis. The dependences of A, Tmax and TK on x are discussed in terms of the volume and Kondo hole effects occurring upon alloying. Anomalies in ρ(T) for LaPt2Si2 and some of the alloy samples are also reported.

The antiferromagnetic Kondo compound CePtGa and the Ce(Pt1?xNix)Ga and CePt(Ga1?yAly) alloy systems have been studied by resistivity ?(T), magnetoresistivity MR and susceptibility ?(T) measurements on polycrystalline samples to temperatures as low as 1.5?K and in magnetic fields up to 9?T. A Doniach diagram is presented of the volume dependences of the N?el temperature TN and Kondo temperature TK of these alloys and the possible existence of a quantum critical point near x = 0.38 is indicated. ?(T) measurements in different applied fields indicate a marked decrease in TN with field for the parent CePtGa compound and extrapolation of the results suggests that magnetic order should disappear in a field of 4.8?T. In contrast the Kondo energy scale, identified by a maximum in ?(T) at low temperatures, increases significantly with field.