Modelling the Resistive State in a Transition Edge Sensor

We have developed a model for the resistive transition in a transition edge sensor (TES) based on the model of a resistively shunted junction, taking into account phase-slips of a superconducting system across the barriers of the tilted washing board potential. We obtained analytical expressions for the resistance of the TES, R(T,I), and its partial logarithmic derivatives α _I and β _I as functions of temperature and current. We have shown that all the major parameters describing the resistive state of the TES are determined by the dependence on temperature of the Josephson critical current, rather than by intrinsic properties of the S-N transition. The complex impedance of a pristine TES exhibits two-pole behaviour due to its own intrinsic reactance.     

Experimental evaluation of the nonlinear surface resistance of YBa2Cu3O7−x step-edge grain boundary Josephson junctions

The temperature, microwave current, and dc magnetic field dependencies of the effective surface resistanceR _s of YBa₂Cu₃O_7?x grain boundary Josephson junctions have been investigated at 6 GHz. The junctions were prepared on stepped LaAlO₃ substrates and incorporated into tapered linear half-wavelength microstrip resonators. The characteristic parameters of the heterogeneous resonator were analyzed theoretically considering the junction as a lumped element of complex impedance. The microwave properties of the junction could then be extracted and related to their characteristic dc transport properties. The critical currentI _c of the junction was found to limit the linear power dissipation in the superconducting strip. At microwave currents aboveI _c , highly nonlinear microwave losses occurred, which displayed a characteristic magnetic field modulation. At even higher currents,R _s saturated at a level corresponding to the normal resistanceR _n of the junction.     

Surface Superconducting State of Heavy-Fermion Superconductor CeIrIn5 Probed by CeIrIn5-Ag Junctions

The CeIrIn₅-Ag junctions of about 2×10^–9 cm^–2 area have been made using microfabrication techniques, and the surface superconducting state of CeIrIn₅, which has two characteristic temperatures T ₀ and T _c, has been investigated, where T ₀ and T _c are the transition temperature to zero-resistivity state and the bulk, thermodynamic transition temperature, respectively. The temperature, below which superconducting anomalies are observed, varies from junction to junction, and yet it is always well above T _c=0.4 K. This result, together with no indication of transition at T _c, suggests that at least the surface of CeIrIn₅ is in the superconducting state above T _c. The data on the critical current I _c in superconducting anomalies point to the possibility to define a local transition temperature for each junction.     

Study of Defect Modes in 1d Photonic Crystal Structure Containing High and Low T c Superconductor as a Defect Layer

The present paper describes the study of defect modes in a one-dimensional photonic crystal (1d-PC) containing a high and low temperature superconductor as a defect layer at different temperatures below the superconducting transition temperature (T _c ). Since the refractive index of the superconducting material is dependent on the penetration depth, which depends on the temperature of the superconducting material, hence by changing the temperature of the superconducting material its refractive index can also be changed. Analysis of the transmission spectra of defect modes in the reflection band of 1d-PC structure shows a shift in the wavelength peak of the defect mode. The shift in peak is different for different superconducting materials and it increases with the increase in temperature whether the defect layer is high T _c or low T _c superconductors. We also study the presence two defect layers in a 1d-PC structure, one with high T _c and other with a low T _c superconductor. Further, the effect of variation in the thickness of the defect layer on the defect modes of the PC structure has also been studied In order to obtain the transmission (reflection) spectra of a 1d-PC structure with a defect, we employ the transfer matrix method (TMM). This property of the defective PC structure can be exploited in designing the temperature sensor and narrow optical filters. Further, this tunable feature of superconductor photonic crystal has technical use in the superconducting electronics and photonics.     

Noise and the superconducting transition in self-heated tunnel junctions

Distinctive structure in the tunneling characteristics of junctions having a superconducting electrode arises at a biasV ₀ where junction dissipation is sufficient to drive the electrode normal. While details of this structure are dependent upon junction-bath thermal linkage, in all cases a marked increase of junction conductance σ(V)=dI/dV occurs atV ₀ and this increase reflects an excess current at the superconducting-normal phase transition. In the case of Pb electrode junctions the transition in both normal and superfluid He occurs when junction dissipation triggersfilm boiling in the liquid. We discuss phase transition-induced tunneling structure, junction noise, and excess current for Pb electrode junctions in different thermal environments and consider the implications for tunneling spectroscopy and Josephson device applications.     

High temperature superconducting composite wires

High temperature superconducting composite wires are fast emerging as a new advanced materials technology. Rapid progress has been made towards demonstrating practical levels of superconducting performance across long lengths of high temperature superconducting composite wire fabricated using a manufacturable process. This progress has allowed a first generation of composite wire products that is driving commercialization of high temperature superconductivity. Recent developments have focused on basic materials science and processing advances to significantly broaden the market potential of high temperature superconductivity. These developments have focused on two basic processes, a powder-in-tube approach utilizing Bi₂Sr₂Ca_nCu_n+1O_y (n=1,2) and a coated-conductor approach utilizing Y₁Ba₂Cu₃O_x.     

Phase Fluctuations of s-Wave Superconductors on a Lattice

Based on an attractive U Hubbard model on a lattice with up to second neighbor hopping we derive an effective Hamiltonian for phase fluctuations. The superconducting gap is assumed to have s-wave symmetry. The effective Hamiltonian we finally arrive at is of the extended XY type. While it correctly reduces to a simple XY in the continuum limit, in the general case, it contains higher neighbor interaction in spin space. An important feature of our Hamiltonian is that it gives a much larger fluctuation region between the Berezinskii–Kosterlitz–Thouless transition temperature identified with T _c for superconducting and the mean field transition temperature identified with the pseudogap temperature.     

Superconductivity in Itinerant Ferromagnetic Systems

We studied the possible superconducting state in an electronic itinerant ferromagnetic system characterized by a density of states that presents a moderately strong peak that is controlled by a specific parameter a and is positioned near the band edge. Specifically, we investigated the superconducting critical temperature, T _c , and the zero-temperature superconducting gap, ??₀. The analysis is done in a self-consistent way, the BCS mean-field equation being solved together with the electron density equation to trace possible changes in the system??s chemical potential due to the strong correlations between the component electrons. We discussed the density dependence of the superconducting critical temperature and zero-temperature superconducting gap for various values of the control parameter a and of the electron?Celectron attractive interaction. In the zero temperature limit we derive the system??s phase diagram and discuss the possible fermionic and bosonic regimes of the diagram as function of the strength of the attractive interaction.     

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.     

Identifying the Pairing Mechanism in Fe�CAs Based Superconductors: Gaps and Isotope Effects

The temperature dependencies of the coupled superconducting gaps, observed in Fe?CAs based superconducting compounds is calculated and a universal temperature scaling observed which is only present if the coupled order parameters both have s-wave symmetry. Predictions for possible isotope effects on the superconducting transition temperature T _c are made if phonons are involved in the pairing or polaronic effects are of importance. Comparison to experimental data is given where these are available.     

Evaluation of superconducting ceramic thick films produced from a large bulk specimen of YBa2Cu3Ox

This experimental study concerns the production and evaluation of YBa₂Cu₃O _x superconducting thick films produced by slicing these films from a large bulk specimen. The resistivity/temperature behavior of these films was found to be equivalent to, or better than, the resistivity/temperature behavior of identically prepared bulk specimens made from the same material. Practical applications involving the use of this technique are probably most likely in hybrid microelectronic devices. Applications involving larger-scale electronic circuits and other superconducting devices are also possible.     

Peculiarities of the current-voltage characteristics of a Josephson medium in a YBCO high-temperature superconductor

The influence of a weak magnetic field (H < 150 Oe) on the current-voltage (I-U) characteristic of a YBa₂Cu₃O_{7 ? x} (YBCO) high-temperature superconductor (HTSC) near the superconducting transition temperature has been studied. It is established that there exist narrow (<0.2 K) temperature regions where the I-U curve exhibits sharp bending for H < 30 Oe and the ohmic behavior changes to a quadratic dependence of the voltage on current in a region of several milliamperes. At higher temperatures, the I-U curve bending exhibits smearing. This behavior is observed at a temperature below that corresponding to a zero critical current. Above a certain current, the temperature and magnetic field exhibit equivalent effects on the I-U curve of YBCO. Experimental results are explained by a sharp decrease in the critical currents of intergranular Josephson junctions under the action of magnetic field and by the current-induced formation of uncoupled (with respect to the order parameter) superconducting grains. Characteristic currents for the transition of the intergranular Josephson medium into an incoherent state are determined and the first critical fields in YBCO are evaluated.     

Superconductivity in a new quaternary phase with HgSm0.5In0.5Pb2 composition

This work reports a systematic study on the structural and superconducting properties in the Hg-Sm-In-Pb system. It is investigated how In and Sm dopings influence the superconducting properties of the HgPb₂ phase which was recently reported. Results for the HgSm_1 − xPb₂ system show that Sm atoms substitute Hg vacancies in the tetragonal phase which do not affect the onset critical temperature of the HgPb₂ compound. Simultaneous Sm and In doping effects on the properties of the HgPb₂ phase are also studied. Experimental and simulations of X-ray powder diffractograms suggest that the HgSm_1 − xIn_xPb₂ system (0.2 ≤ x ≤ 0.7) has a pseudo-cubic structure with lattice parameters near 4.88 Å. Superconducting properties measurements show that this system has an optimum doping at x ∼ 0.5 with the onset critical temperature close to 6.9 K. This is the highest superconducting critical temperature for the AuCu prototype so far reported.     

Resistive superconducting transition of κ-type BEDT-TTF organic superconductors in a magnetic field

The superconducting transition of the organic compoundsκ-(BEDT-TTF)₂ X is studied by resistive measurement in a magnetic field up to 10 T applied normal to the conducting plane. For the salts withX=Cu[N(CN)₂]Br andX=CuCN[N(CN)₂] the transition shows fanshaped broadening caused by superconductivity fluctuation. For theX=Cu(NCS)₂ salt the resistivity shows a peak in the transition region in a magnetic field below 4 T.This phenomenon is suppressed in defect-reduced samples for intralayer conduction. We discuss this peak in relation to the thermal fluctuation on the Josephson junction structures in this salt.     

Long-Range Spin-Triplet Superconductivity Induced by Magnetic Field in d Wave Superconductor/Ferromagnet Hybrid System

We theoretically study the long-range spin- triplet superconductivity in d wave superconductor/ ferromagnet/ferromagnet (S/F₁/F₂) trilayer junction, in which the magnetization of F₁ layer could be rotated in the y–z plane by an external magnetic field. The four-component Eilenberger equations were constructed to calculate the superconducting order parameters and density of states (DOS). Near the clean limit, the p wave equal-spin triplet component could be induced when the magnetization directions of F₁/F₂ layers are non-collinear, and the DOS exhibits a split zero-bias conductance peak. The various parameters such as ferromagnetic exchange energy, thickness of ferromagnetic layers, and angles between F₁/F₂ magnetization directions are studied for the effect on inducing triplet superconductivity. By magnetic field controlling the emergence of equal-spin triplet pairings or not, such a tunable S/F₁/F₂ trilayer junction based on long-range spin-triplet superconductivity could be used as a superconducting switch device, which would open up a new view of spintronics.     

Thermal treatment of a Y-123 precursor prepared using the hydroxide co-precipitation method

A Y-123 precursor has been obtained using the hydroxide co-precipitation method. The influences of the calcination time and temperature (with and without sintering and annealing) on the formation of the superconducting phase, YBa₂Cu₃O_7−δ, are examined using XRD and T_c measurements. A comparison between the co-precipitated hydroxide precursor and the conventional dry precursor, obtained by mixing oxides and carbonates, is made. It shows that thermal treatment of the hydroxide precursor at 750°C already gives a superconductor, while the oxide–carbonate precursor is only superconducting after thermal treatment at 850°C.     

Macroscopic Quantum Phenomena in High Critical Temperature Superconducting Josephson Junctions

YBa₂Cu₃O_7?δ grain boundary bi-epitaxial Josepshon junctions (JJs) allow a very clear demonstration of Josephson current variation with the misorientation angle, consistent with the d-wave symmetry of the superconducting order parameter in cuprate, high temperature superconductors. Our bi-epitaxial junctions show a strong suppression of the first harmonic, I ₁ sin ø, of the current phase relation when tunneling from a lobe into a node of the superconducting gap function. In these configurations, the contribution of the second harmonic, I ₂ sin 2ø, becomes of the same magnitude as the first one, giving rise to a characteristic two-well Josephson potential as a function of phase ø instead of the usual single well. This characteristic intrinsic property has suggested proposals of a new class of qu-bit named “quiet” because of the existence a spontaneously degenerate fundamental state without the need of applying an external field. Our experiments probe the macroscopic quantum properties in a d-wave Josephson junction by measuring macroscopic quantum tunneling and energy level quantization. The switching current out of the zero voltage state is measured as a function of temperature down to 20 mK. The temperature variation of the width of an ensemble of switching events goes over from one, which is characteristic of a thermal activation of phase fluctuations to a temperature independent width which is a token of quantum tunneling of the phase. The transition regime is affected by the two-well potential in a 45^° misorientation junction as the second harmonic term gives rise to additional thermal transitions. The difference between quantized energy levels in the harmonic potential was determined by microwave spectroscopy. From the broadening of energy levels, it was possible to extract a Q-value of about 40 for the phase oscillations. The relatively high Q indicates quantum coherence over a sizeable time in d-wave junctions and gives hopes for a realization of a “quiet” high-T _c qu-bit. The contributions of V. L. Ginzburg to several different fields of physics are impressive and long standing. In superconductivity the Ginzburg–Landau theory, for instance, still represents a very powerful approach to model a huge number of different physical systems. High Temperature Superconductors (HTS) have strongly influenced research of the last 20 years and their d-wave order parameter symmetry represents one of the most intriguing features from both the fundamental point of view and some types of innovative long-term applications.     

Observation of X-band SQUID signals in a High-T c superconducting film device

A microwave superconducting magnetometer is described in which a microstrip resonator is coupled to a two-hole high-T _c thin-film SQUID device. Both the microstrip circuit and the thin-film SQUID were fabricated by photolithography techniques. The YBCO thin film was deposited on single-crystal substrate of yttria-stabilized zirconia [YSZ(100)] by an ion beam sputtering technique producing a superconducting transition measured at a critical temperature ofT _c =92 K to within ΔT ～ 3 K. Non linear oscillatory behavior was observed in the microstrip resonator when inductively coupled to the SQUID. This nonlinear behavior yielded a microwave device in which the reflected microwave power varied with applied DC magnetic flux.     

Low-temperature magnetic and microwave studies of Gd0.95Ba2Cu3.05O7−δ: Agx composites

Gd_0.95Ba₂Cu_3.05O_7?δ: Ag_x (x=0.0, 0.02, 0.04, 0.06) ceramic superconducting samples were prepared by a co-precipation technique using organic carbonates in the presence of stable polymers. The room-temperature normal-state resistivity decreases with Ag addition. However,T _cand crystallographic parameters were uneffected. Low-temperature (4.2 K) magnetic and electrical transport studies in high magnetic fields (5 T) reveal that the addition of Ag into the Gd_0.95Ba₂Cu_3.05O_7?δ (GBCO) enhances the magnetic critical current density (J _mc), volume pinning force (F _p), flux explusion, and transport critical current density (J _tc). Microwave-induced dc voltage studies show the reduction in the total number of weak links between superconducting GBCO grains with increase in Ag concentration. The flux creep rate was also increased with increase in Ag concentration in GBCO resulting in stronger pinning potential. The increase inJ _mc, J_tc, and, hence,F _pwith Ag addition in the GBCO suggests the creation of an SNS-type proximity junction at the intergrannular region and stronger Josephson current paths between the superconducting intergrains, which may be due to the physical densification and reduction of the total number of weak links by Ag addition into the GBCO superconducting system.