Fluctuations of the Order Parameter's Phase in Layered Superconductors

The influence of the quantum fluctuations of the order parameter's phase on the critical temperature T _c is studied for a Josephson coupled layered superconductor. Two characteristic critical temperatures exist for a system, namely the superconducting critical temperature T ⁽²⁾ _c for a single layer estimated by the mean-field theory and the transition temperature for the outset or the superconducting phase coherence T* _c . The true critical temperature T _c is shown to vary inside the intervals T* _c ≤ T _c ≤ T ⁽²⁾ _c . For a strong quantum phase fluctuation limit, the superconducting layers become decoupled.     

Analysis of Temperature-Dependent Optical Properties in 1D Ternary Superconducting Photonic Crystal with Mirror Symmetry

The temperature dependence of the defect mode in a 1D ternary superconducting photonic crystal (PC) with mirror symmetry was theoretically investigated by simultaneously considering thermal expansion effect and thermal-optical effect. Three different materials H (high-refractive index dielectric), L (low-refractive index dielectric), and S (superconducting material) are used to construct the ternary superconducting PC structure. For comparison, six structures are discussed. It is found that the structure (HLS) ^N (SLH) ^N has the highest temperature sensitivity. Here, N is the number of periods. The average change in the central wavelength of the defect mode is 31.18 nm/K as the thickness of superconducting layer is 50 nm. Thus, based on the structure (HLS) ^N (SLH) ^N , a low temperature sensor with higher sensitivity can be designed to realize the low temperature sensor.     

Generalized Multiple Gap Model for the Superconductivity in MgB2

The free energy of the two-band superconductor is obtained taking into account all actual electron–phonon and Coulomb interactions. The temperature dependences of the superconducting gaps, the specific heat with its jump at T = T _c (T _c is the critical temperature) are calculated minimizing the free energy. The theory is applied to the MgB₂ superconductor.     

Localized Modes in Metamaterial-Dielectric Photonic Crystals with a Dielectric-Superconductor Pair Defect

Transmission coefficient and dispersion relation are calculated for a one-dimensional photonic crystal of alternating dielectric-metamaterial slabs with a dielectric-superconductor pair defect. The presence of the superconductor slab in the pair defect layer leads to a strong depletion of the TM transmission coefficient in a frequency range close to the edges of the non-Bragg gap and around the characteristic resonance frequency of the superconductor and introduces TM electromagnetic modes inside such gap. The features of the arising modes depend on the relation between the thicknesses of the layers involved in the defect and a low-frequency mode can arise from the low-frequency continuum of the bulk metamaterial modes at a non-zero in-plane wave vector.     

Heat treatment of Nb3Sn conductors

Magnet coils made out of Nb₃Sn superconductors usually are manufactured by the wind- and react-technique. Due to the brittleness of the A15 material the superconductive layer is formed only after the winding of the magnet. This is done by a heat treatment in which Sn diffuses via a matrix into Nb filaments and the superconducting layer is formed. Depending on the exact temperature and time of the heat treatment, the physical properties of the superconductor such as critical current density J_c, upper critical field B_c2, critical temperature T_c and n-value can be varied over a wide range. This is because the diffusion process determines the grain size distribution, the thickness of the superconductive layer as well as the Sn distribution within the layer.This article will provide a review of the investigations concerning different aspects of heat treatment over recent years.     

A Possible Explanation of Critical Temperature and Isotope Effect Coefficient of High T c Cuprate Superconductors

The expressions for superconducting transition temperature (T _c) and isotope effect coefficient (α) have been derived from a generalized integral gap equation for a strongly coupled superconductor, when electrons–phonons and electrons–biexcitons are simultaneously present in high-T _c cuprate superconductors.     

High Temperature Superconductivity on the New Superconductor System Yb1.8Sm1.2Ba5Cu8O18

The synthesis of the new high temperature superconductor system Yb_1.8Sm_1.2Ba₅Cu₈O₁₈ is reported. The new material was produced using the standard solid-state reaction method. The structural characterization of the compound was performed by X-ray diffraction and Rietveld refinement using a polycrystalline sample of the material. Structural ordering corresponds to an orthorhombic structure, from the space group Pmm2. The lattice parameters of the material are reported here. The superconducting nature of the material was determined by measuring the magnetization, as a function of its temperature, in the regime of low and mean field. A critical temperature value of T _c ～88 K was determined for Yb_1.8Sm_1.2Ba₅Cu₈O₁₈. The experimental results have shown that the production of a novel group RE₃Ba₅Cu₈O₁₈ of high-T _c superconductivity is possible, by substitution of RE instead of Y in the recently reported new superconductor material Y₃Ba₅Cu₈O₁₈.     

Fluctuation phenomena below the superconducting transition

The fluctuation propagator for the superconducting order parameter belowT _c is derived for a gapless superconductor. The fluctuation of the order parameter consists of two distinct modes: the fluctuations of the phase and the modulus of the order parameter. The former fluctuation obeys a phononlike dispersion, while the latter fluctuation is described by a damping diffusion type equation. The present theory gives an account of the fluctuation-induced Josephson effect belowT _c observed recently by Carlson and Goldman.     

Enlarged Photonic Band Gap in Heterostructure of Metallic Photonic and Superconducting Photonic Crystals

We show theoretically that the frequency range of photonic band gap of a hetero-structure which is made of a metallic photonic and superconducting photonic crystal can be enlarged due to the combination of the reflection band properties of the superconductor–dielectric (PC1) and metallic–dielectric (PC2) periodic structures. The transmittance and band structure of the considered structures are calculated using simple transfer matrix method and the Bloch theorem. Beside this, we have also calculated the transmittance of the superconducting photonic structure (PC1), metallic photonic structure (PC2) and heterostructure of metallic photonic and superconductor photonic crystals (PC1/PC2) for TE and TM-mode at the different angles of incidence.     

MgB2 Thin Films on Metal Substrates for Superconducting RF Cavity Applications

Magnesium diboride is a promising material for superconducting RF (SRF) cavity applications. Compared to the currently used superconductor for SRF cavities Nb, MgB₂ has the potential to achieve lower RF loss and higher acceleration field due to its higher critical temperature and thermodynamic critical magnetic field. Since the RF field only penetrates a few penetration depths into a superconductor, a superconducting coating of several hundred nanometers on a metal cavity is sufficient for superb SRF cavity performances. In this work, we report the properties of MgB₂ thin films deposited by the hybrid physical–chemical vapor deposition (HPCVD) technique on different metal substrates including Nb, Mo, Ta, and stainless steel. All the films were polycrystalline, as indicated by X-ray diffractometry and scanning electron microscopy, and showed T _c ～39 K, determined by resistance versus temperature, magnetic susceptibility, and dielectric resonator measurements. MgB₂ films deposited on Nb substrates polished to various degrees of smoothness exhibit similar T _c . The result is a promising step in the investigation of using MgB₂ as an alternative to Nb for SRF cavities.     

Thermometry with a nearly temperature independent sensitivity using a normal-superconducting tunnel diode biased close to the superconducting gap

We analyze the temperature dependence of the tunneling current in a normal-insulator–superconductor (NIS) tunnel junction and find that such a junction can serve as a primary thermometer below the superconducting critical temperature T_c with a nearly temperature independent sensitivity, provided the junction is suitably biased close to the superconducting gap. Deviations from expected behavior when the superconducting electrode does not follow most simple s-wave BCS density of states, and possible applications are briefly discussed.     

Evidence for two superconducting components in oxygen-annealed single-phase YBaCuO

The complex susceptibility of a sintered YBaCuO superconductor is strongly dependent on a.c. field amplitude, h. Very small values of h must be used for the real part of susceptibility, χ′, to reach a value corresponding to bulk diamagnetism just below the critical temperature, T_c. The imaginary part, χ″, represents hysteresis loss in the sample. Thus, χ″ versus temperature becomes positive when h exceeds the lower critical field, H_cl of the superconductor.Annealing the material in oxygen gives rise to two distinct components, a relatively high-T_c, high-H_cl superconductor (denoted as ‘G’ or ‘good’) and a relatively low-T_c, low-H_cl superconductor (denoted as ‘B’ or ‘bad’). Curves of susceptibility versus increasing temperature reflect the dual nature of the annealed sample: χ′ has an inflection point at T_c of the B component and approaches zero at T_c of the G component, while χ″ has a peak at each T_c. Both critical temperatures decrease linearly with increasing h, though at very different rates. H_cl of the G component is considerably greater than H_cl of the B component. The lower critical fields are linearly decreasing functions of temperature.Two models might explain the susceptibility data. In the grain model, the G component consists of superconducting grains and the B component is either intergranular material, unfavourably orientated anisotropic grains, or oxygen-depleted grain boundaries. In the surface model, the G component is in the interior of the sample and the B component is at the sample's surface. This condition could arise if there was oxygen depletion at the surface subsequent to total enrichment during annealing.     

Synthesis and properties of superconducting lanthanum sulfide films

We report the structural and transport properties of La-S films prepared by reactive planar magnetron sputtering. The La-S (Th₃P₄ cubic structure) can be prepared over a range of compositions with the electronic properties varying from a low electron density superconductor (T_c≈ 7 K) to a semiconductor which shows thermally activated transport at low temperature. This method of deposition produces small grained (≈ 200 Å) polycrystalline films whose composition can be varied continuously by tuning the deposition parameters. We discuss the TEM and X-ray data as well as the resistivity, Hall effect and superconducting properties of these films. We find that the superconducting transition temperature of the films is the same as that of bulk single crystals; however the mobility is a factor of 2 to 300 lower than that found in bulk specimens.     

Lattice Effects Across the Phase Diagram of Pnictides

We have studied by Raman spectroscopy the effect of doping, temperature, and hydrostatic pressure on selected Fe pnictides of the 1111 series. Two sets of RFeAsO_1?x F _x compounds have been examined (R=Sm and Nd) with a varying amount of doping and transition temperature. The doping dependence of the Raman active modes reveals that the rare earth phonon is correlated with the transition temperature (T _c) and not with the amount of doping. As in the case of several other pnictides, the low temperature measurements indicate phonon modifications at much higher temperatures than T _c even in the superconducting compounds. The application of hydrostatic pressure indicates a nonlinear behavior of the rare earth phonon, which increases with doping and in the superconducting compounds correlates with modifications in T _c. The results are similar with those of the cuprates, where hydrostatic pressure has induced phonon and structure modifications at characteristic pressures where the T _c dependence on pressure is also modified. All results point to some role of the lattice for superconductivity in the pnictides.     

Study of Superconducting Properties in Bismuth-Based Cerium Doped High-T c Superconductors

Superconductivity in bismuth-based high-T _c superconducting materials attracts the researchers for their unique properties. Bismuth-based superconductors commonly called BSCCO have great importance among the superconducting family. These are divided into three phases among them 2223 phase is highly studied in order to investigate its superconducting properties by substitution of different elements. We have studied the substitution of cerium (Ce) on the calcium site of bismuth-based Bi(Pb)Sr(Ba)-2223 high-T _c superconductor. The nominal compositions of Bi_1.6Pb_0.4Sr_1.6Ba_0.4(Ca_1?x Ce _x )₂Cu₃O _x ceramic superconductor were prepared by the sol–gel method. X-ray diffraction (XRD) was done at room temperature for structural analysis and different parameters were calculated. Surface morphology was done by scanning electron microscopy (SEM). DC resistivity measurements for the transition temperature of synthesized superconducting samples were taken by the standard four-probe method, apparatus for which was developed in our laboratory. Current density measurements were also taken by the same apparatus. The synthesized superconducting samples were also characterized by thermogravimetric analysis (TG) and Fourier transformation infrared radiations (FTIR). It is observed that the substitution of cerium on the calcium site favors the formation of single high-T _c 2223 phase.     

Generalized Multiple Gap Model for the Superconductivity in MgB2

The free energy of the two-band superconductor is obtained taking into account all actual electron–phonon and Coulomb interactions. The temperature dependences of the superconducting gaps, the specific heat with its jump at T = T _c (T _c is the critical temperature) are calculated minimizing the free energy. The theory is applied to the MgB₂ superconductor.     

Novel superconducting proximized heterostructures for ultrafast photodetection

Weak ferromagnet/superconductor (F/S) proximity bilayers have been fabricated and characterized for basic physics studies concerning the ultrafast carrier dynamics in layered materials. The normalized reflectivity change (ΔR/R) as a function of the time delay between the pump and the probe laser beams has been measured in F/S heterostructures formed by a low critical temperature superconductor (Nb) with a NiCu overlayer, and a high T_c superconductor (YBCO) covered by Au/NiCu and manganite (LSMO) overlayers. The attention is mainly focused to the investigation of nonequilibrium excitation dynamics inside different bilayers in the low temperature region. The presence of the weak ferromagnetic overlayer produces faster optical relaxation times with respect to sole superconducting films. The results are promising in view of potential applications as ultrafast kinetic inductance superconducting photodetectors as confirmed by preliminary time-resolved photoimpedance experiments on both Nb and NiCu(21 nm)/Nb(70 nm) samples.     

YBa<Subscript>2</Subscript>Cu<Subscript>3</Subscript>O<Subscript>7−<Emphasis Type="Italic">x</Emphasis></Subscript>/BaTiO<Subscript>3</Subscript> 1D Superconducting Photonic Crystal with Tunable Broadband Response in the Visible Range

Based on the transfer matrix method, we studied theoretically the transmittance of a 1D photonic crystal (PC), consisting of alternating layers of a dielectric material (BaTiO₃) and a superconductor (YBa₂Cu₃O_7?x ). The dielectric properties of this system are described by the two fluid model. We have investigated the transmittance intensity and its bandwidth dependence on the superconductor thickness, incident angle, and temperature in the PC. It was found that the electromagnetic wave propagation can be controlled to be forbidden or allowed in certain wavelengths in the visible and ultraviolet range, and the photonic band gap (PBG) width can also be tuned varying these parameters. We showed that by increasing the thickness of the superconductor layer it is possible to control the number of PBGs in the structure. Also, we found that the frequency ranges of PBGs are sensitive to the incident angle and the polarization of the electromagnetic waves; the bandwidth of PBGs can be notably enlarged by increasing the angle in the TE polarization, but narrowed in the TM one. Additionally, we found that transmission is not markedly affected by temperature variation, but small shifts in the PBGs are presented. We hope these results can be of technical use for developing potential applications in optoelectronic devices.     

Fluctuations of the Order Parameter''s Phase in Layered Superconductors

The influence of the quantum fluctuations of the order parameter's phase on the critical temperature T _c is studied for a Josephson coupled layered superconductor. Two characteristic critical temperatures exist for a system, namely the superconducting critical temperature T ⁽²⁾ _c for a single layer estimated by the mean-field theory and the transition temperature for the outset or the superconducting phase coherence T* _c . The true critical temperature T _c is shown to vary inside the intervals T* _c T _c T ⁽²⁾ _c . For a strong quantum phase fluctuation limit, the superconducting layers become decoupled.     

Superconducting Point Defect in a Two-Dimensional Photonic Crystal

This paper presents the tunability of the high temperature superconducting (SC) point defect modes in a two-dimensional photonic crystal composed of circular cylinders on a square lattice. By some modifications on the plane wave expansion method, we have shown that the properties of the SC point defect modes strongly depend on the temperature and external magnetic fields. The superconductor type and the nearest-neighbor of the defect effects are further investigated. Comparison between SC and conventional defects show that the SC one has only a monopole profile for different values of the defect size, and has very low sensitivity on the disorders, which is an important feature from a manufacturing point of view.