Papers by Keyword: Superconductivity

Abstract: Superconducting wires have been applied for the fabrication of superconducting magnets in nuclear magneto-resonance (NMR), Magneto-resonance imaging (MRI) and so on. MgB2 has the highest critical temperature of superconducting transition (TC39K) among intermetallic compound superconductive materials. This means that MgB2 Superconductive wire doesn’t need expensive liquid He for cooling. We used the original method of the three-dimensional penetration casting (3DPC) in this laboratory to fabricate the MgB2/Al composite. Our 3DPC method for fabricating composite materials can disperse particles in the matrix homogenously without any aggregation and control volume fractions of composites within the range of 4 – 40%, even when particle size is less than 1 m. Thus, these composite materials can be processed by machining, extrusion and rolling. In the composite material we made, MgB2 particles dispersed to the Al matrix uniformly. The TC was determined by electrical resistivity and magnetization to be about 37 – 39K. We succeeded in extruding MgB2/Al composite billet to 1mm wire. Microstructures of these samples have been confirmed by SEM method. MgB2/Al composite billet and extruded wire were showed there no cracks inside the materials.

Abstract: Multilayered approach to YBCO-based Josephson junction manufacture is introduced. Properties of monolayer YBa2Cu3O7 (YBCO) and multilayer YBCO/NdBCO/YBCO junctions of similar thickness grown by pulsed-laser deposition on MgO (100) substrates has been analysed and compared. The influence of the multilayered structure on junction behaviour and properties are presented and discussed.

Abstract: High temperature superconducting YBa2Cu3O7 films have been prepared by pulsed laser deposition on top of single crystal substrates and metallic templates. Films have been investigated in order to determine the influence of microstructural peculiarities on the critical current density. It has been found that there is an inherent mechanism limiting the superconductive current flow in YBa2Cu3O7 film deposited on metallic template. This mechanism is associated with YBa2Cu3O7 film architecture, fabrication procedure and following low temperature usage.

Abstract: We report on the interplay between ferromagnetism and superconductivity in trilayers La0.7Ca0.3MnO3/YBa2Cu3O7/La0.7Ca0.3MnO3 made of half metallic manganite and high temperature superconductor cuprate. Samples with a fully oxygenated cuprate show a magnetic field interval where the magnetizations of the manganite are aligned antiparallel. A considerable magnetoresistance accompanies the switching between magnetization configurations (parallel vs. antiparallel) of the manganite moments. Suppression of the free carrier density of the cuprate which occurs upon oxygen depletion, results in deep modifications in the shape of the normal state hysteresis loops indicating that there may be a magnetic coupling mediated by free carrier density of the cuprate. This result outlines the importance of quasiparticle transmission in the interplay between ferromagnetism and superconductivity in this kind of samples.

Abstract: We research under what condition the mean-field approximation can be applied to study ordered phases of quasi-one-dimensional metal. It is shown that the mean-field treatment is indeed permissible provided that it is applied not to the microscopic Hamiltonian (subject to severe one-dimensional high-energy fluctuations), but rather to effective Hamiltonian derived at the dimensional crossover scale. The resultant mean-field phase diagram has three ordered phases: spin density wave, charge density wave, and superconductivity. The density wave orders win if the Fermi surface nests well. Outcome of competition between the intra-chain and inter-chain electron repulsion determines the type (spin vs. charge) of the density wave. The ground state becomes superconducting (with unconventional order parameter) when the nesting is poor. The superconducting mechanism relies crucially on the one-dimensional fluctuations.

Abstract: On the base of new boundary-value problem for the Eilenberger function we investigate the superconducting and magnetic states in ferromagnet/superconductor (FM/S) nanostructures, where superconductivity is a superposition of the BCS pairing with zero total momentum in the S layers and the Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) pairing with nonzero 3D coherent momentum k in the FM layers. We originally study the interplay between the BCS and FFLO states in the pure thin FM/S/FM trilayers and two novel -phase superconducting states with electron-electron repulsion in the FM layers are predicted. The modulated FFLO states are possible in such trilayers only in presence of external magnetic field at suitable parameters of the FM and S layers. In the FM/S superlattices there are also two -phase magnetic states (0 and ) with compensation of the exchange field paramagnetic effect. This fact allows us to explain a surprisingly high Tc ~ 5K in the short period Gd/La superlattice and to predict the sign and value of the electron-electron interaction in the ferromagnetic Gd metal.

Abstract: We investigate the properties of a thin-walled superconducting shell placed in electrical contact with a ferromagnetic cylinder embedded in the superconductor. The case of a superconductor surrounded by the ferromagnet is also considered. Associated with the orbital effect, spontaneous vortex states may appear, characterized by different winding numbers. Special attention is paid to the switching between states of different vorticities and to the behaviour of the critical temperature caused by an external field.

Abstract: Definite regularity in the distribution of ferromagnetic, antiferromagnetic and superconducting elements is observed in the periodic table starting with the 4-th period. These trends were explained by distinction of degree of division of the d (f)- or р-orbitals of neighboring atoms in the crystal. We calculated also the radii of the external d (f)- and р-orbitals and the nearest to them orbitals with the Slater’s method. It is demonstrated that in the superconducting crystals the d-shells approach the nucleus of neighboring atoms are much closely those for ferromagnetic or antiferromagnetic crystals.

Abstract: The proximity effect of the ferromagnetic metal/superconductor (FM/S) bilayers in а external longitudinal magnetic field is considered in the dirty limit. The critical temperature and the superconducting current distribution versus applied magnetic field’s magnitude, film’s thicknesses and a transparency of the contact is calculated, with taking into account аn umklapp processes possibility on the interface of the FM/S contact. It is shown that superconducting current is strictly inhomogeneous and asymmetrical. It is shown also that the current inhomogeneity depends heavily from the FM/S interface transparency.

Abstract: We investigate the interplay between the BCS and 2D Larkin-Ovchinnikov-Fulde-Ferrell (LOFF) states in the dirty thin ferromagnetic metal/superconductor (FM/S) nanostructures. For the FM/S bilayers we have derived the dependencies of critical temperature on the FM layer exchange field, electronic correlations and thickness. Moreover, in the corresponding FM/S/FM trilayers we predict two new π phase superconducting states with electron-electron repulsion in the FM layers. The 2D modulated LOFF states are possible in such trilayers only in presence of a weak magnetic field and at suitable parameters of the FM and S layers. On this base we originally propose the method of proximity effect probe the magnitude and sign of the electronic correlations, the order parameter symmetry and exchange fields in various FM layers.