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Hybrid Photovoltaic Thermal (PV/T) systems are energy-generation systems that transform thermal irradiance into both electrical and thermal energy at the same time. Hybrid photovoltaic thermal systems consist of a photovoltaic panel connected to a thermal collector. The main objective of this paper is to find the optimal operating conditions that can be controlled to decrease the photovoltaic panel temperature in order to improve the electrical and thermal performance of PV/T systems. In this work, we proposed the 3D numerical model that is implemented within the COMSOL Multiphysics program to study the PV/T system. The experimental input data being used in this research study reflects a typical Algerian area with semi-humid climate conditions. We study the effect of water velocity, pipe length, diameter, thickness and inlet fluid temperature on the electrical and thermal performance using the design of experiments (DOE) method. Further, the analysis of variance (ANOVA) is used to identify which of these effects impact the most the photovoltaic thermal and electrical efficiencies, the response surface methodology (RSM) is employed to describe how these effects are interacting. Based on ANOVA analysis, the following factors are reported to be important: water velocity, pipe diameter, pipe length and the inlet fluid temperature. Further, there is a significant interactions between water velocity, pipe length and pipe diameter. Among the operating conditions being calculated using the RSM, the optimal one is found when water velocity, pipe length, pipe diameter, pipe thickness and inlet water temperature have the following values, 0.05 m/s, 7.27 m, 0.01 m, 0.0008 m and 10°C, respectively. The corresponding thermal, electrical and overall efficiency were found around 80.73%, 12.87% and 93.60%, respectively. The proposed simulation model provides a reliable framework to study, improve and predict PV/T systems performance whilst ensuring low computational time.

First-principles calculations were performed on the cubic rocksalt Cd0.75TM0.25O (TM = Mn, Fe, Co, and Ni) compounds to investigate the effect of 3d transition metals (TM) substitution on the structure, magnetic, electronic, and elastic characteristics by utilizing the full-potential linearized augmented plane wave route. The calculations were performed in the framework of the density functional theory within Generalized Gradient Approximation and the modified Becke–Johnson (TB-mBJ) potential approximation. The ground-state properties were determined in the rocksalt NaCl-type B1 structure. The band structure calculations reveal that CdO and Cd0.75TM0.25O compounds have an indirect bandgap (Γ–R) in the range of 1.41–2.64 eV. The magnetic moment of the Cd0.75TM0.25O compounds decreases from 5μB to 4μB to 3μB to 2μB for Mn, Fe, Co, and Ni substitution, respectively. The exchange constants N0α and N0β have been estimated using the splitting energy depending on the direction of spins. The studied compounds exhibit mechanical stability. The Cd0.75Ni0.25O is nearly isotropic and shows higher values of the elastic properties.

We report the direct observation of interband spin-orbit (SO) coupling in a two-dimensional (2D) surface electron system, in addition to the anticipated Rashba spin splitting. Using angle-resolved photoemission experiments and first-principles calculations on Bi-Ag-Au heterostructures, we show that the effect strongly modifies the dispersion as well as the orbital and spin character of the 2D electronic states, thus giving rise to considerable deviations from the Rashba model. The strength of the interband SO coupling is tuned by the thickness of the thin film structures.

We demonstrate by first-principles calculations that the Rashba spin-orbit splitting in the $6p$ states of a Bi adlayer on ${\text{BaTiO}}_{3}(001)$ is strongly affected by the substrate termination. For the ${\text{TiO}}_{2}$ termination the absolute splitting is very large (about $0.23\text{ }{\text{\AA{}}}^{\ensuremath{-}1}$); in contrast, the splitting becomes strongly reduced for the BaO termination (less than $0.07\text{ }{\text{\AA{}}}^{\ensuremath{-}1}$). Our findings are explained by the termination-dependent hybridization of the Bi surface states with electronic states in the substrate.

The spin-dependent electronic structure of graphene is investigated by first-principles calculations, using relativistic full-potential linearized augmented plane wave and Korringa-Kohn-Rostoker methods. Our systematic study addresses various effects on the electronic states at the Dirac points: in-plane and out-of-plane deformation of graphene's honeycomb lattice, external electric fields, doping and band filling due to heavy and magnetic adatoms (Au and Ni). Having revealed the underlying mechanisms, our findings open a route to manufacture graphene with sizable spin splittings.

As demonstrated conceptually by first-principles calculations, the Rashba spin splitting in the $6p$ states of a Bi adlayer on ${\text{BaTiO}}_{3}(001)$ can be manipulated by the electric polarization in the ferroelectric substrate. Although this spin-electric effect is moderate, with a relative change in the splitting of about 5%, the absolute splitting of about $0.24\text{ }{\text{\AA{}}}^{\ensuremath{-}1}$ is unmatched. Further, the occupied $6p$ surface states display an anisotropic dispersion and deviate significantly from the free-electron model of the Rashba effect. Our findings may pave a route for spin-electronic devices.

The tiny magnetocrystalline anisotropy energies (MAEs) of bulk Gd, GdN, and ${\text{GdFe}}_{2}$ have been calculated by means of the force theorem in conjunction with the full-potential linear augmented plane-wave (FLAPW) method. The generalized gradient correction including the Hubbard interaction $U$ $(\text{GGA}+U)$ produced the best possible agreement with the experimental MAE strength compared to either the generalized gradient approximation (GGA), where the $4f$ electrons are treated as valence states, or the GGA core, where they are treated as core electrons. Indeed, the magnetic anisotropy is three times that of the $\text{GGA}+U$ if the $4f$ orbitals are prevented to hybridize correctly with the other orbitals like in the GGA-core calculation and 1 order of magnitude if they hybridize too much, like in the GGA calculation. The $\text{GGA}+U$ results can be explained in terms of orbital moment anisotropy using Bruno's model showing that the MAE is due to the orbital magnetic-moment anisotropy. In addition, because the $4f$ states of Gd are half filled their orbital moment and spin-orbit coupling are zero; the Gd MAE is tuned by the spin-orbit coupling of $5d$ states rather than by that of the $4f$ states like in other rare-earth systems, such as Tb or Dy. Nevertheless, the strength of MAE is found to depend on the energy position of the $4f$ states. The MAE of Gd is therefore much similar to that of a transition metal rather than that of a typical rare-earth metal such as Tb or Dy. It is not surprising that Gd shows a calculated easy axis along the (0001) direction like hcp cobalt. All converged calculations within the GGA, the GGA core, or the $\text{GGA}+U$ methods show that the magnetization is along the $c$ axis, in disagreement with an experiment and a recent calculation which show that the easy axis makes an angle of $20\ifmmode^\circ\else\textdegree\fi{}$ with the hcp $c$ direction. Based on the present calculations, the disagreement with experiment might be due to possible presence of symmetry-breaking imperfections, such as defect states or impurities, and cannot be explained using bulk MAE calculations. As for the MAE of GdN and ${\text{GdFe}}_{2}$ compounds, crystallizing in, respectively, cubic rocksalt and Laves phase structures, despite the qualitative agreement with Bruno's model, their interpretation is much more complex. Indeed, their predicted magnetization easy axis is along one of the symmetry equivalent (100), (010), and (001) directions rather than the (111) direction of fcc nickel, and their MAEs are much smaller than that of Gd. The removal of N from the GdN structure without changing the lattice parameter re-established the easy axis along the (111) direction as expected, showing that the easy axis of GdN is a consequence of $\text{Gd}\text{ }5d$ and $\text{N}\text{ }2p$ hybridizations.

The full-potential linear augmented plane wave method has been used to study the electronic structure, the structural phase transitions, the ground-state x-ray absorption spectroscopy XAS , and the x-ray magnetic circular dichroism XMCD of GdN pnictide under hydrostatic pressure. To produce a satisfactory excited state used to calculate the XAS, it was necessary to describe the Gd 4f electron-electron interaction within the so-called LDA+U method in the mean field approximation. We have shown that the band structure of GdN pnictide is that of a half-metal for the experimental lattice constant. The 5d orbitals of the Gd sites which dominate the GdN low-lying conduction bands are investigated by means of XAS and XMCD calculations, and the results of the L2,3 XAS and XMCD spectra are found to be in good agreement with the experimental results. Total energy calculations performed for the rocksalt, the wurtzite, the zinc blende, and the CsCl structures reveal the richness of the electronic structure of GdN and predict two structural phase transitions: from rocksalt to zinc blende structure for the extended lattice parameter, and from half-metallic rocksalt to a semiconducting wurtzite structure under hydrostatic pressure.