Phase transitions in ferroelectric-paraelectric superlattices

Within the phenomenological Landau–Ginzburg–Devonshire theory, we discuss the paraelectric-ferrolectric transition in superstructures consisting of ferroelectric and paraelectric layers of equal thickness. The polar axis of the ferroelectric is perpendicular to the layer plane as expected in fully strained BaTiO3/SrTiO3 superstructures on SrTiO3 substrates with pseudomorphic electrodes. We concentrate on the electrostatic effects and do not take into account the boundary conditions other than the electrostatic ones. We find that when the ferroelectric phase transition in the superstructures is into a multidomain state, both its temperature and its character, i. e., the profile of the polarization appearing at the phase transition is strongly influenced by the nature of the near-electrode region. This is also the case for the layer thickness separating the single-and multidomain regimes of the transition. Such a finding makes us question the idea that these superstructures can be thought of as infinite systems, i.e., periodic superstructures similar to a crystal. The irrelevance of this idea in certain conditions is demonstrated by comparing the phase transitions in two different superstructures consisting of ferroelectric and paraelectric layers of the same thickness. In one of them, the ferroelectric layer is in immediate contact with an ideal metallic electrode, whereas at the other boundary, it is the paraelectric layer that is in contact with the electrode. In another superstructure, one paraelectric layer is split in two equal parts which are placed as the first and last layer between the electrodes and the ferroelectric layers which are closest to the electrodes. We show (with some formal reservations) that the phase transition temperature in the first superstructure can be over 100 °C more than in the second one if the material parameters of BaTiO3/SrTiO3 are used for the estimations. Moreover, the profile of the polarization arising at the phase transition is inhomogeneous along the superstructure and has the maximum amplitude in the ferroelectric layer contacting the electrode. We argue that this situation is general and results in smearing of the phase transition anomalies for the layer thicknesses corresponding to multidomain transitions. The work is mainly analyical but numerical methods have been used to support some statements that have been put forward as hypotheses

An aerogel Cherenkov detector for multi-GeV photon detection with low sensitivity to neutrons

We describe a novel photon detector which operates under an intense flux of neutrons. It is composed of lead-aerogel sandwich counter modules. Its salient features are high photon detection efficiency and blindness to neutrons. As a result of Monte Carlo (MC) simulations, the efficiency for photons with the energy larger than 1 GeV is expected to be higher than 99.5% and that for 2 GeV/$c$ neutrons less than 1%. The performance on the photon detection under such a large flux of neutrons was measured for a part of the detector. It was confirmed that the efficiency to photons with the energy $>$1 GeV was consistent with the MC expectation within 8.2% uncertainty.Comment: 16 pages, 16 figures, submitted to Prog. Theor. Exp. Phy

General Green's function formalism for transport calculations with spd-Hamiltonians and giant magnetoresistance in Co and Ni based magnetic multilayers

A novel, general Green's function technique for elastic spin-dependent transport calculations is presented, which (i) scales linearly with system size and (ii) allows straightforward application to general tight-binding Hamiltonians (spd in the present work). The method is applied to studies of conductance and giant magnetoresistance (GMR) of magnetic multilayers in CPP (current perpendicular to planes) geometry in the limit of large coherence length. The magnetic materials considered are Co and Ni, with various non-magnetic materials from the 3d, 4d, and 5d transition metal series. Realistic tight-binding models for them have been constructed with the use of density functional calculations. We have identified three qualitatively different cases which depend on whether or not the bands (densities of states) of a non-magnetic metal (i) form an almost perfect match with one of spin sub-bands of the magnetic metal (as in Cu/Co spin valves); (ii) have almost pure sp character at the Fermi level (e.g. Ag); (iii) have almost pure d character at the Fermi energy (e.g. Pd, Pt). The key parameters which give rise to a large GMR ratio turn out to be (i) a strong spin polarization of the magnetic metal, (ii) a large energy offset between the conduction band of the non-magnetic metal and one of spin sub-bands of the magnetic metal, and (iii) strong interband scattering in one of spin sub-bands of a magnetic metal. The present results show that GMR oscillates with variation of the thickness of either non-magnetic or magnetic layers, as observed experimentally.Comment: 22 pages, 9 figure

Scattering by magnetic and spin-orbit impurities and the Josephson current in superconductor-ferromagnet-superconductor junctions

We analyze the Josephson current in a junction consisting of two superconductors (S) and a ferromagnetic layer (F) for arbitrary impurity concentration. In addition to non-magnetic impurities, we consider also magnetic ones and spin-orbit scattering. In the limit of weak proximity effect we solve the linearized Eilenberger equation and derive an analytical expression for the Josephson critical current valid in a broad range of parameters. This expression enables us to obtain not only known results in the dirty and clean limits but also in a intermediate region of the impurity concentration, which may be very important for comparison with experimental data.Comment: revised versio

High frequency oscillations of first eigenmodes in axisymmetric shells as the thickness tends to zero

The lowest eigenmode of thin axisymmetric shells is investigated for two physical models (acoustics and elasticity) as the shell thickness (2$\epsilon$) tends to zero. Using a novel asymptotic expansion we determine the behavior of the eigenvalue $\lambda$($\epsilon$) and the eigenvector angular frequency k($\epsilon$) for shells with Dirichlet boundary conditions along the lateral boundary, and natural boundary conditions on the other parts. First, the scalar Laplace operator for acoustics is addressed, for which k($\epsilon$) is always zero. In contrast to it, for the Lam{\'e} system of linear elasticity several different types of shells are defined, characterized by their geometry, for which k($\epsilon$) tends to infinity as $\epsilon$ tends to zero. For two families of shells: cylinders and elliptical barrels we explicitly provide $\lambda$($\epsilon$) and k($\epsilon$) and demonstrate by numerical examples the different behavior as $\epsilon$ tends to zero