Investigations of sol–gel-derived highly (100)-oriented Ba0.5Sr0.5TiO3 : MgO composite thin films for phase-shifter applications

Sol–gel deposition of highly oriented Ba_0.5Sr_0.5TiO₃:MgO composite thin films has shown desirable dielectric constant reduction and higher figure of merit for phase-shifter applications. In this multilayer configuration, MgO distributed homogeneously through the Ba_0.5Sr_0.5TiO₃ (BST50) matrix, and it helped in tailoring the dielectric constant and reducing the loss tangent significantly. In the present study, the high-frequency dielectric behavior of the films has been evaluated by fabricating an eight-element coupled microstrip phase shifter and measuring the degree of phase shift and insertion loss as a function of applied voltage at room temperature. An increase in phase-shifter figure of merit (degree of phase shift per dB insertion loss) from 28°/dB for pure BST50 to 71°/dB for a BST50:MgO film (at 14 GHz and 333 kV/cm) has been observed. PACS 77.55.+f; 81.20.Fw; 73.40.-c; 85.50.-n     

Enhanced superconductivity by correlations between two Kondo impurities

We study the interplay between magnetic correlations of two Kondo impurities and superconducting singlet pairing. Performing a Schrieffer-Wolff transformation in the zero-bandwidth limit of the two-impurity Anderson model we obtain the Hamiltonian of two magnetic impurities and we add a superconducting term to the conduction electrons. The model allows us to study the effect of the magnetic correlation between the impurities on the superconducting ground state. At zero temperature, different superconducting ground states can be obtained depending on the magnitude of magnetic coupling between S₁ and S₂. For increasing coupling, the superconducting region is enlarged showing an interesting result: in the strong coupling limit, where the impurities are in a very strong ferromagnetic correlation state, half of the conduction electrons are decoupled from the local moments of the impurities and take advantage of the superconducting pairing lowering the ground state energy. On the contrary, when the coupling between S₁and S₂ decreases, the scenario of the two independent Kondo impurities in presence of superconductivity emerges and all the conduction electrons are involved in the pair breaking physics. At finite temperature, we obtain the phase diagram and we observe a region of parameters where the re-entrance phenomenon occurs.     

Effective decrease in the exchange energy in <Emphasis Type="Italic">S</Emphasis>-(<Emphasis Type="Italic">FN</Emphasis>)-<Emphasis Type="Italic">S</Emphasis> Josephson structures

The critical current I _c of S-(FN)-S Josephson structures has been calculated as a function of the distance L between superconducting (S) electrodes using the Usadel quasiclassical equations for the case of specifying the supercurrent in the direction parallel to the interface between the ferromagnetic (F) and normal (N) films of the composite weak-link region. It has been shown that, owing to the interaction between F and N films, both the typical decrease scale I _c(L) and the period of the critical current oscillations can be much larger than the respective quantities for the SFS junctions. The conditions have been determined under which these lengths are on the order of the effective depth ζ_N of superconductivity penetration to a normal metal.     

Solid state monolithic variable phase shifter with operation into the millimeter wave wavelength regime

Information is provided on the theory and design, fabrication, and experimental results for a phase shifter designed to operate in the millimeter wavelength region. The device was fabricated in a manner that makes it compatible with present GaAs monolithic microwave circuit technology. Continuously variable phase shift is obtained by varying the bias voltage from –5.0 to +0.65 V on a Schottky microstrip line. Experimental phase shift and loss data are provided for two different width (w) Schottky lines, w=1.5 and 7.3 m, for frequencies 2–18 GHz.     

Superconductivity and metal-insulator transition in Cu(Ir<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>Rh<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>)<Subscript>2</Subscript>S<Subscript>4</Subscript>

A new thiospinel CuIr₂S₄ exhibits a metal-insulator (M-I) transition at 226 K, while CuRh₂S₄ shows a superconducting transition at 4.70 K. We present a systematic study of electrical and magnetic properties of Cu(Ir_1?x Rh _x )₂S₄. TheM-I transition of CuIr₂S₄ is accompanied by a structural phase transition from tetragonal symmetry in insulating phase to cubic symmetry in high temperature metallic phase. With increasing Rh contentx, the sharpM-I transition shifts to lower temperature forx≦0.10. The samples show semiconductive behavior for 0.10≦0.30 between 4.2 and 300 K, and recover the metallic state forx≧0.50. The superconducting transition may occur for very close tox=1.00. Magnetic susceptibility shows the jump at theM-I transition temperature and the variation ofx leads to a systematic change of the magnetic susceptibilities, which is consistent with the electrical characteristic feature.     

Raman scattering investigation and symmetry analysis of ferroelectric/ferroelastic Sb5O7I polytype 2MA

The polytype 2MA (-Sb₅O₇I) has the simplest acentric structure of the antimony oxideiodide family. It undergoes an antiferrodistortive phase transition at 438K and is both ferroelectric and ferroelastic below that temperature. The complete polarized Raman spectra in the ferroic phase have been measured and compared with those of the ferroelastic, centric polytype 2MC (-Sb₅O₇I). Several lines could be attributed to Sb—0 and Sb—I vibrations. A factor group analysis has been performed and compatibility relations have been established connecting phonon species in the low and high temperature phase. As a function of temperature the spectra revealed a strongly temperature dependent central line and several phonon lines whose intensities vanish aboveT _c . Using these phonon line intensities the temperature variation of the order parameter could be determined. The experimental results indicate that the phase transition is of first order.     

Irreversibility line and flux pinning properties in iron-based high-Tc superconductor SmFeAsO0.85

The irreversibility line and flux pinning properties of high-T_c superconductor SmFeAsO_0.85 were studied using DC magnetization data. Polycrystalline SmFeAsO_0.85 was prepared in a high pressure synthesis apparatus under the pressure of 6 GPa. The results of DC susceptibility showed the superconducting transition at about 55 K. A critical current density J_c(B) was calculated using Bean’s critical state model. At low temperatures (20 K), J_c(B) showed a relatively high value with weak dependence on an applied magnetic field. At higher temperatures, a stronger dependence of the magnetic field was observed, which resulted from decrease in a critical current density probably due to the flux creep effect. The irreversibility line (IL) agreed well with the flux creep theory of Matsushita et al. A comparison of normalized pinning force density with the theoretical models showed that the irreversible behavior in SmFeAsO_0.85 is dominated mainly by normal point pinning (δT_c) and surface pinning mechanisms.     

Direct determination of X-Ray reflection phases for noncentrosymmetric crystals

An experimental method is described for the phase determination of x-ray reflections from noncentrosymmetric crystals. This method results from considerations on the phase dependence of line profiles in three-beam reflection-type diffractions for wavelength above and below a critical absorption edge _E of the heaviest constituent atom in the crystals. A relationS _p =S _L ·S _R for phase determination is derived theoretically and verified experimentally for< _E ,S _p being the sign of the sine of invariant phase,S _L the sign defined from the line profile, andS _R determined by the rotation of the crystal lattice.     

Muon spin relaxation in heavy fermion systems

Heavy fermion systems have received a great deal of study by a wide variety of techniques, includingSR. In a number of systems, coexisting superconducting and magnetic states have been reported, leading to speculation of an intimate connection between magnetism and superconductivity in these compounds. We observe a spontaneous magnetic field in the superconducting phase of UPt₃. In addition, the broadening of the transverse field muon precession signal only onsets approximately 60 mK below the superconductingT _c. Our results provide evidence that the lower superconducting phase in theH-T phase diagram of UPt₃ is characterised by broken time-reversal symmetry. Measurements of URu₂Si₂ and CeCu_2.2Si₂ indicate that the magnetically ordered volume fraction is temperature dependent in both systems.     

Screening properties of multiply connected ferromagnet–superconductor hybrid structures

Superconducting phase transition temperature T _c of a ferromagnet/superconductor (SF) hybrid structure consisting of a hollow superconducting (S) cylinder (shell) with the central part (core) filled with a ferromagnetic (F) metal has been analyzed on the basis of linearized Usadel equations. It has been shown that the proximity effect between the S and F metals, as well as the exchange interaction, may induce an inhomogeneous superconducting state with Δ ~ exp(iLθ + ipz), which is characterized by nonzero circulation of phase L and wavenumber p describing the Larkin–Ovchinnikov–Fulde–Ferrell (LOFF) instability along the cylinder axis. The transitions between the states with different values of L and p, which are accompanied by a nonmonotonic dependence of superconducting transition temperature T _c and effective magnetic field penetration depth Λ into the SF structure on the characteristic size of the ferromagnetic region, have been investigated.     

Threshold perturbations in current-carrying superconducting bridges with a finite length near the critical temperature

Near the critical temperature of a superconducting transition, the energy of the threshold perturbation δF_thr that transfers a superconducting bridge to a resistive state at a current below the critical current I_c has been determined. It has been shown that δF_thr increases with a decrease in the length of a bridge for short bridges with lengths L < ξ (where ξ is the coherence length) and is saturated for long bridges with L ? ξ. At certain geometrical parameters of banks and bridge, the function δF_thr(L) at the current I → 0 has a minimum at L ~ (2–3)ξ. These results indicate that the effect of fluctuations on Josephson junctions made in the form of short superconducting bridges is reduced and that the effect of fluctuations on bridges with lengths ~(2–3)ξ is enhanced.     

Technology of Production of Integrated Devices for Millimeter Wave Electronics Based on High-Temperature Superconducting Films

In the work, the results on the analysis of phase transitions in amorphous films Y-Ba-Cu-O under their annealing are presented for the purpose to obtain high-temperature superconducting film samples intended for microwave applications. The analysis and choice of material for substrates have been carried out with the purpose to create integrated microwave circuits, using the high-temperature superconducting films. The method of producing the epitaxial Y-Ba-Cu-O films for microwave electronics has been justified. With the purpose of creation of the hybrid integrated circuits with application of high-temperature superconductor films, the data on producing the ohmic contacts with low transitional resistance in a system metal - high temperature superconducting film are presented. The analysis of critical parameters and microwave properties of YBa₂Cu₃O₇ films under their annealing in vacuum has been carried out. The justification of temperature regimes of contact photolithography concerning the YBa₂Cu₃O_7– films is given. The influence of oxygen implantation on structural properties and critical parameters of YBa₂Cu₃O_7– is demonstrated.     

Phase transitions in hybrid SFS structures with thin superconducting layers

Calculations of critical temperature T _c of the phase transition to superconducting state of a superconductor/ ferromagnet/superconductor (SFS) hybrid structure with proximity effect is performed on the base of linearized Usadel equations. It is shown that the proximity effect between S and F metals and the exchange interaction can induce an inhomogeneous superconducting state with longitudinal to layers Δ _∝ exp(ipz) modulation of the superconductivity order parameter, which is characterized by nonzero value of the wave number p, describing the Larkin–Ovchinnikov–Fulde–Ferrell instability. Influence of this instability on transitions between 0- and π-states of the SFS structure is studied. It is shown that the 0–π transition is accompanied by a nonmonotonic dependence of both the critical temperature T _c and the effective penetration depth Λ of the magnetic field into the hybrid structure on the characteristic size of the ferromagnetic region.     

Electron spectroscopy analysis on NbN to grow and characterize NbN/AlN/NbN Josephson junction

Three layers, NbN based Josephson junction, has been growth by RF and by DC sputtering within the constrain required by the photolithography technology. An interesting superconducting film with critical temperature of , well above the temperature of the commercial cryocooler, has been obtained reducing sputtering power and finding a proper N₂ concentration in the gas mixture. The search of the new sputtering parameters has been obtained with the help of electron spectroscopy and X-ray diffraction analysis.     

Multifaceted asymmetric radiation from the edge along improved confinement mode in LHCD plasmas with graphite limiters on HT-7 tokamak

Multifaceted asymmetric radiation from the edge (MARFE) phenomena during lower hybrid current drive (LHCD) Experiments on the HT-7 superconducting tokamak are summarized in this paper. The best correlation has been found between the total input (ohmic + LHCD) power and the product of the edge line average density and Z _eff. Studies show that the critical density of MARFE onset is observed in the region of Z _eff ^1/2f_GW = 0.6-0.9, where , (here is the maximum line average electron density and n_GW is the Greenwald density). These MARFEs generally appear to have the same characteristics as high f_GW MARFEs and are positionally stable throughout the LHCD pulse. Improved confinement mode induced by a MARFE is observed, and it is maintained for about 65 ms. MARFE cools the plasma edge, and the electron density profile is observed to become more narrow and peaked.     

Evaluation of the relative permittivity of Ba<Subscript>x</Subscript>Sr<Subscript>1-x</Subscript>TiO<Subscript>3</Subscript> ceramics at microwave frequencies using microstrip ring resonators

The microstrip-ring-resonance technique has been applied to determine the dielectric properties of high-permittivity barium strontium titanate (Ba_xSr_1-xTiO₃) ceramics at microwave frequencies. A microstrip ring resonator of diameter 5 mm has been fabricated on the ceramics by the standard photolithography process. The transmission coefficient S₂₁ spectra for the resonators have been measured using a vector network analyzer and simulated using a commercial electromagnetic simulation package, IE3D. By fitting the observed spectra to the simulated spectra, and tan of the Ba_xSr_1-xTiO₃ ceramics have been determined. The results obtained by this technique are in agreement with those determined by other conventional methods. This technique is relatively simple, especially for high-permittivity materials, since there are less rigorous requirements on the sample and the test conditions. PACS 77.22.-d; 77.84.Dy; 84.40.Az     

Change in the polarizability of MBBA molecules upon the nematic-isotropic liquid transition and physical consequences

The Lorentz tensor components L _j and the polarizability tensor components γ _j of 4-methoxyben-zylidene-4′-butylaniline (MBBA) liquid crystal molecules in nematic and isotropic phases have been determined experimentally. The quadratic dependences of the mean value $\bar \gamma $ (S) and anisotropy Δγ(S) of the polarizability in the nematic phase on the orientational order parameter S of molecules have been established. Within the phenomenological approach, the dependences $\bar \gamma $ (S) and Δγ(S) have been derived, and their features for MBBA and other known objects have been interpreted. The relation of the dependences $\bar \gamma $ (S) and Δγ(S) to the features of the nematic-isotropic liquid phase transition has been found.     

Topological Scenario for High-Temperature Superconductivity in Cuprates

The structure of the joint phase diagram of high-temperature superconducting cuprates has been studied within the theory of fermion condensation. Prerequisites of the topological rearrangement of the Landau state with the formation of a flat band adjacent to the nominal Fermi surface have been established. The related non-Fermi-liquid behavior of cuprates in the normal phase has been studied with focus on the non-Fermi-liquid behavior of the resistivity ρ(T), including the observed crossover from the linear temperature behavior ρ(T, x) = A₁(x)T at doping levels x below the critical value x _c ^h corresponding to the boundary of the superconducting region to the quadratic temperature behavior at x > x _c ^h , which is incompatible with predictions of the conventional quantum-critical-point scenario. It has been demonstrated that the slope of the coefficient A₁(x) is universal and is the same on both boundaries of the joint phase diagram of cuprates in agreement with available experimental data. It has also been shown that the fermion condensate is responsible for pairing in the D-wave state in cuprates. The effective Coulomb repulsion in the Cooper channel, which prevents the existence of superconductivity in normal metals in the S channel, leads to high-temperature superconductivity in the D channel.     

Dependence of the maximal superconducting current on the resonance frequency in a shunted Josephson junction

We have analyzed the phase dynamics and current–voltage characteristics of a Josephson junction shunted by an LC circuit. When the Josephson frequency ω _J becomes equal to the natural frequency ω_rc of the formed resonance circuit, the I–V curve acquires additional branches. We have studied the features of the rc branch and the superconducting circuit for different values of the resonance frequency. It is shown that the maximal superconducting current through the Josephson junction on the rc-branch depends on the resonance frequency and is determined by the closeness of the end point of the rc branch to the critical current. We have determined the dependence of the maximal superconducting current on the resonance frequency for different values of the dissipation parameters. The limiting value of the maximal superconducting current is independent (to within 1%) of the parameters of the system.     

Superimposed normal and superconducting films

Tunneling characteristicsdI/dV have been taken on both sides of “dirty”In - Bi/Zn superimposed films, the temperatureT being such thatIn - Bi was superconducting and Zn normal. Most of the attention has been devoted to the regionT～T _c where the experimental curves can be compared with a reasonably accurate microscopic theory. We find reasonable agreement for the voltage scale and, more important, for theamplitude of the effects in the superconducting phase.