Markovian Properties of the Spin-Boson Model

We systematically compare the Hamiltonian and Markovian approaches of quantum open system theory, in the case of the spin-boson model. We first give a complete proof of the weak coupling limit and we compute the Lindblad generator of this mode. We study properties of the associated quantum master equation such as decoherence, detailed quantum balance and return to equilibrium at inverse temperature 0 < beta <= infinity. We further study the associated quantum Langevin equation, its associated interaction Hamiltonian. We finally give a quantum repeated interaction model describing the spin-boson system where the associated Markovian properties are satisfied without any assumption

Low Density Limit and the Quantum Langevin Equation for the Heat Bath

We consider a repeated quantum interaction model describing a small system H(S) in interaction with each of the identical copies of the chain circle times(N*) C(n+1), modeling a heat bath, one after another during the same short time intervals [0, h]. We suppose that the repeated quantum interaction Hamiltonian is split into two parts: a free part and an interaction part with time scale of order h. After giving the GNS representation, we establish the connection between the time scale h and the classic allow density limit. We introduce a chemical potential mu related to the time has follows: h(2) = e(beta mu). We further prove that the solution of the associated discrete evolution equation converges strongly, when h tends to 0, to the unitary solution of a quantum Langevin equation directed by the Poisson processes

Optical investigations of La0.7Ca0.3-xKxMnO3 (x = 0.00, 0.05 and 0.10) probed by spectroscopic ellipsometry

Using spectroscopic ellipsometry, we have studied the optical properties of doped manganites at the paramagnetic state in polycrystalline La0.7Ca0.3-xKxMnO3 samples for (x = 0.00, 0.05 and 0.10) in the energy range of 3.2-5.5 eV at room temperature. The surface morphology of the samples was obtained by using atomic force microscopy (AFM). Refractive indices, extinction coefficients, the transmission ellipsometric parameters Ψ and Δ are investigated at different wavelengths. The study of the optical conductivity σ reveals that optical behaviour and the activated transport in the paramagnetic state of these materials are consistent with Jahn-Teller small polaron. In addition, the spectrum of the complex dielectric constant ε reveals peaks for all samples, the results may be explained by the presence of space charges from the strongly dipole-allowed O(2p)−Mn(3d) transition

Landau mean-field analysis and estimation of the spontaneous magnetization from magnetic entropy change

We investigated the critical exponents for the compounds La1-x□xMnO3 (x = 0.1; 0.2 and 0.3) prepared by the sol–gel method. Our samples show a second- order transition inferred from the positive slope, in accordance with the Banerjee Criterion. Using the slope of different models on the TC, the relative slope (RS) was traced. From this perspective, the best models for the three samples prove to be the mean field and the tricritical mean field models. This model is characterized by critical exponents β, γ and δ which are determined by many methods such as MAP, KF method and critical isotherm analysis. The theoritical methods and experimental results were in good agreement for the three compounds. The universality class has been shown. After determining the spontaneous magnetization for x = 0.2 from (-ΔSM) vs. M2, we detected a good agreement with those obtained from the classical extrapolation of Arrott curves (µ0H/M vs. M2). Furthermore, based on the magnetocaloric effect (MCE), Landau’s theory is valid for the compound x = 0.2.publishe

Influence of oxygen deficiency on optical and dielectric properties of La0.75Ba0.10Sr0.15FeO2.875-δ compounds

Using the conventional sol-gel process, the series of non-stoichiometric oxygen lanthanum ferrites with the formula 0.750.100.152.875 ― (δ=0.00, 0.125 and 0.25) were prepared. X-ray diffraction analysis confirmed the formation of the orthorhombic structure with Pnma space group. In the present work, we confirm the potential of 0.750.100.15 2.875 ― (δ=0.00, 0.125 and 0.25) as an efficient dielectric material. All samples show high absorbance in the visible region with wavelength above 300 nm. The optical band gaps are found to increase from 3.25 to 4.1 with increasing oxygen vacancy concentration. The dielectric constant, dielectric loss and loss factor were carried out as a function of oxygen vacancy concentration and frequency (102 -106 Hz) at room temperature. The variation of dielectric constant with frequency indicates dispersive behavior and giant dielectric response (more than 104 ) at low frequencies for compounds deficient in oxygen. Such value of makes ′ these samples an interesting material to be used in applications namely the reduction of electronic components size. To summarize, incorporating vacancy oxygen enhances the dielectric properties. Thus, the interesting dielectric constant and weak loss strengthen the use for potential applications.publishe

Ultrafast Electrochemical Self-Doping of Anodic Titanium Dioxide Nanotubes for Enhanced Electroanalytical and Photocatalytic Performance

This study explores an ultrarapid electrochemical self-doping procedure applied to anodic titanium dioxide (TiO2) nanotube arrays in an alkaline solution to boost their performance for electroanalytical and photocatalytic applications. The electrochemical self-doping process (i.e., the creation of surface Ti3+ states by applying a negative potential) is recently emerging as a simpler and cleaner way to improve the electronic properties of TiO2 compared to traditional chemical and high-temperature doping strategies. Here, self-doping was carried out through varying voltages and treatment times to identify the most performing materials without compromising their structural stability. Interestingly, cyclic voltammetry characterization revealed that undoped TiO2 shows negligible activity, whereas all self-doped materials demonstrate their suitability as electrode materials: an outstandingly short 10 s self-doping treatment leads to the highest electrochemical activity. The electrochemical detection of hydrogen peroxide was assessed as well, demonstrating a good sensitivity and a linear detection range of 3–200 µM. Additionally, the self-doped TiO2 nanotubes exhibited an enhanced photocatalytic activity compared to the untreated substrate: the degradation potential of methylene blue under UV light exposure increased by 25% in comparison to undoped materials. Overall, this study highlights the potential of ultrafast electrochemical self-doping to unleash and improve TiO2 nanotubes performances for electroanalytical and photocatalytic applications

Investigating the structural, morphological, dielectric and electric properties of the multiferroic (La0.8Ca0.2)0.9Bi0.1FeO3 material

The (La0.8Ca0.2)0.9Bi0.1FeO3 (LCBFO) compound has been synthesized by the sol-gel method and characterized by X-Ray Diffraction (XRD), Scanning Electron Microscope (SEM), Raman spectroscopy and electrical impedance spectrometry. XRD results revealed that (La0.8Ca0.2)0.9Bi0.1FeO3 crystals are orthorhombic, belonging to the Pnma space group. The SEM measurements showed that the sample presents a large distribution of nano-grains connected to each other. The relaxation process and the electrical conductivity are awarded to the same type of charge carriers characterized by similar values of the activation energy determined from loss factor tangent tg δ( ), the imaginary part of the permittivity and from the Modulus spectrum. The ac-conductivity was analysed to examine the conduction mechanism, using the Jonscher’s universal power-law given by: = +σ ω σ Aω( )ac dc s. Based on the parameter s behavior, the conductivity was studied according to the NSPT model (non-overlapping small polaron tunneling).publishe

Effect of annealing temperature on structural, morphology and dielectric properties of La0.75Ba0.25FeO3 perovskite

The effect of annealing temperature on the structure, morphology and dielectric properties of La0.75Ba0.25FeO3 compound prepared by the sol-gel method was investigated. The increase of the annealing temperature from 900 to 1100 °C, promotes an increase of the average grain size value. Two dielectric relaxations are detected using the dielectric modulus formalism, attributed to grain and grain boundary relaxations. This behavior was confirmed by both Nyquist and Argand's plots of dielectric impedance and Modulus results at different measuring temperatures. The ac conductivity could be described by Jonscher's power law revealing the presence of both overlapping large polaron tunneling and non-overlapping small polaron tunneling mechanisms.publishe