Current-carrying states in superconducting channels with nonhomogeneities

The stability of a current-carrying normal state in a superconducting channel with nonhomogeneities is considered at Tc. A value of the current is found below which the normal state becomes absolutely unstable with respect to nucleation and growth of a superconducting domain. Furthermore, the superconducting state appearing near a nonhomogeneity characterized by a critical temperatureT _c1 higher than the critical temperatureT _c of the main material is studied for the case ofT>T _c .     

Can the Lateral Proximity Effect Be Used to Create the Superconducting Transition of a Micron-Sized TES?

Recent measurements of micron-sized Mo/Au bilayer TESs have demonstrated that the TES can behave like an S-S′-S weak link due to the lateral proximity effect from superconducting leads. In this regime the T _c is a function of bias current, and the effective T _c shifts from the bilayer T _c towards the lead T _c . We explore the idea that a micron-sized S-N-S weak link could provide a new method to engineer the TES T _c . This method would be particularly useful when small size requirements for a bilayer TES (such as for a hot-electron microbolometer) lead to undesirable shifts in the bilayer T _c . We present measurements of a variety of micron-sized normal Au ‘TES’ devices with Nb leads. We find no evidence of a superconducting transition in the Au film of these devices, in dramatic contrast to the strong lateral proximity effect seen in micron-sized Mo/Au bilayer devices. The absence of a transition in these devices is also in disagreement with theoretical predictions for S-N-S weak links. We hypothesize that a finite contact resistance between the Nb and Au may be weakening the effect. We conclude that the use of the lateral proximity effect to create a superconducting transition will be difficult given current fabrication procedures.     

c-axis phonons and the enhancement of pairing correlations in high-temperature superconductors

We consider the two-dimensional Hubbard model including electron-phonon interaction. Strong local correlations (U→∞ limit) are taken into account within the mean-field approximation for auxiliary boson fields. Phonon-assisted transitions between intraand interlayer states are introduced as the source of coupling between two-dimensional CuO₂ layers. This type of processes effectively leads to the nonlinear (quadratic) interaction of intralayer electrons withc-axis phonons. We construct the Eliashberg equations for the resulting Hamiltonian and evaluate the superconducting transition temperatureT _c. Our model calculation demonstrates that a pronounced enhancement ofT _c in thed-wave channel is possible. The largest enhancement ofT _c tends to take place for small hole concentrations. This means that the coupling toc-axis phonons could compete with two-dimensional correlations responsible for the onset of antiferromagnetic order. It is remarkable that the two-dimensional features in the normal state are hardly affected by this specific interlayer interaction. Therefore,c-axis two-phonon-mediated interlayer coupling can cooperate with interlayer pair tunneling and substantially contribute to an increased pairing.     

Influence of Long-Range Coulomb Interaction and On-Site Hubbard Repulsion on the Formation of d-Wave Copper-Pairing in High-T c Cuprates

We develop a diagram technique for the self-consistent treatment of the long-range Coulomb interaction and on-site Hubbard repulsion in the normal and superconducting state of high-T _c cuprates. The resultant analytical expression for the “screened” matrix elements taking into account long-range and on-site repulsion has been derived. In particular, it accounts for processes with and without spin-flip due to an exchange of spin and charge density fluctuations. Furthermore, we derive the expressions for the normal and anomalous self-energy parts near the superconducting transition temperature T _c that takes into account the vertex corrections including crossing diagrams. The contribution of the crossing parts is taken within the ladder approximation (similar to Fluctuation-Exchange approximation) where the role of Hubbard on-site interaction is replaced by the Coulomb matrix element with a spin-flip averaged over the momentum. Finally, the developed scheme allows to analyze the formation of d-wave superconductivity and its stability in presence of the long-range Coulomb repulsion within a self-consistent anisotropic Eliashberg-like approach.     

Reduction of fault current peak in an inductive high-Tc superconducting fault current limiter

This paper dealt with current-limiting performances of an inductive high-T_c superconducting fault current limiter with an auxiliary coil. The fault current limiter mainly consists of the primary copper coil, secondary high-T_c superconducting rings, and auxiliary high-T_c superconducting coils, which are magnetically coupled through three-legged core. The superconducting fault current limiter as a series element in the power system is inserted to limit the fault current. The device presents fast variable-impedance features in the event of a fault condition. The fault current peak can become relatively large for certain ranges of the flux and the fault instant due to the core saturation. The auxiliary coil proposed in this paper was proven to increase the impedance of the SFCL up to more than 31% while preventing the core saturation.     

Multiband Superconductors Close to a 3D–2D Electronic Topological Transition

Within the two-band model of superconductivity, we study the dependence of the critical temperature T _c and of the isotope exponent α in the proximity to an electronic topological transition (ETT). The ETT is associated with a 3D–2D crossover of the Fermi surface of one of the two bands: the σ subband of the diborides. Our results agree with the observed dependence of T _c on Mg content in A $_{1-x}{\rm Mg}_x{\rm B}_2$ (A?=?Al or Sc), where an enhancement of T _c can be interpreted as due to the proximity to a ‘shape resonance.’ Moreover we have calculated a possible variation of the isotope effect on the superconducting critical temperature by tuning the chemical potential.     

Physical properties of a new cuprate superconductor Pr2Ba4Cu7O15−δ

We present studies of the thermal, magnetic, and electrical transport properties of reduced polycrystalline Pr₂Ba₄Cu₇O_15−δ (Pr247) showing a superconducting transition at T_c=10-16 K, and compare them with those of as-sintered non-superconducting Pr247. The electrical resistivity in the normal state exhibited T² dependence up to approximately 150 K. A clear specific heat anomaly was observed at T_c for Pr247 reduced in a vacuum for 24 h, proving the bulk nature of the superconducting state. By the reduction treatment, the magnetic ordering temperature T_N of Pr moments decreased from 16 to 11 K, and the entropy associated with the ordering increased, while the effective paramagnetic moments obtained from the DC magnetic susceptibility varied from 2.72 to 3.13μ_B. The sign of Hall coefficient changed from positive to negative with decreasing temperature in the normal state of a superconducting Pr247, while that of the as-sintered one was positive down to 5 K. The electrical resistivity under high magnetic fields was found to exhibit T^α dependence (α=0.08-0.4) at low temperatures. A possibility of superconductivity in the so-called CuO double chains is discussed.     

Superconductivity in β-Tin Germanium

The superconducting properties of the β-tin germanium under pressure have been investigated by the first-principles calculations based on density functional perturbation theory. The electron–phonon coupling strength as a function of pressure was calculated, and it deceases with the application of pressure. Using the calculated electron–phonon coupling strength, the superconducting transition temperature T _c has been estimated. Assuming a nominal Coulomb repulsion parameter of 0.1, the calculated T _c is in excellent agreement with experimental data. The T _c correlates with electron–phonon coupling strength, indicating phonon-mediated superconductivity in β-tin Ge phase. Our findings have great implications to other Group IVa elements.     

High Superconducting T c and Suppressed Isotope Effect in the Instantonic Condensate State of the Fermi-System: Analytic Solution

A superconducting pairing instability and isotope effect on the superconducting transition temperature T _c in the electronic system with instantonic condensate are derived analytically using previously proposed theory (Mukhin, J. Supercond. Nov. Magn. 22:75–80, 2009; J. Supercond. Nov. Magn. 24:1165–1171, 2011) of the instantonic nature of the “hidden order” in high-T _c cuprates. The self-organized instantonic condensate breaks Matsubara “time” invariance of the Fermi-system, and occurs due to strongly nonlinear interaction between antiferromagnetic or charge density fluctuations. The Cooper-channel susceptibility (polarization loop) is calculated using a single-particle Green’s function that we have derived recently (Mukhin and Galimzyanov, Physica B 407:1882–1884, 2012) for the Fermi-system with instantonic condensate. We found that superconducting T _c could be strongly enhanced and an isotope effect strongly suppressed in the presence of the instantonic condensate. The usual weak-coupling BCS results are recovered in the limit of vanishing the instantons. Relevance of the theoretical predictions for the high-T _c compounds is discussed.     

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.     

Model for the a.c. hall effect of high-T c superconductors

The sign of the Cooper pairs in the superconducting state of the currently found high-T _c superconductors can be best measured by utilizing the so-called a.c. Hall effect. In this experiment a d.c. magnetic field is applied normal to the surface of the superconductor and a planar electromagnetic wave perpendicularly polarized incident upon the sample surface. By measuring the reflected electric field polarized in the incident plane, one could determine the sign as well as the magnitude of the a.c. Hall coefficient. The measurement provides a direct means to determine the sign and density of the Cooper pairs in the superconducting state of the high-T _c superconducting sample.     

Nonequilibrium Carrier Dynamics Studied in Underdoped Bi2212 by Polarized Optical Pump-Probe Spectroscopy

We investigate nonequilibrium quasiparticle dynamics measured by ultrafast optical spectroscopy on underdoped Bi2212 crystals, which provide direct evidence that superconducting (SC) and pseudogap (PG) quasiparticles coexist below T _c . We verify that the ratio of signals from SC and PG quasiparticles depends on both excitation energy and polarization of the probe beam due to the anisotropy of the probe transition matrix elements and the interband transition probability. Based on this property, we successfully separate the SC or PG component and precisely evaluate the temperature dependence of them across T _c .     

Superconductivity of barium-doped Bi2Sr2CaCu2O y

Sintered ceramic samples of Bi₂Sr_2?x Ba _x CaCu₂O_y with nominal barium fraction 0≤x≤0.3 have been prepared by the solid-state reaction method. WDS studies verified that barium enters the superconducting phase. For slowly cooled samples, the midpointT _c of the superconducting transition is significantly increased by barium doping, whereas for quenched samplesT _c is little affected. The increase ofT _c with increasing barium fraction is consistent with a decrease in the hole concentration in the superconducting layers.     

On the Presence of 1D-Like Stripes in Superconducting Epitaxial La2 -x Sr x CuO4 thin Films

A large positive magnetoresistance (up to tens of percents) is observed in both underdoped (x?<?0.15) and overdoped superconducting La_{2? x}Sr_xCuO₄ (LSCO) epitaxial thin films, at temperatures far above the superconducting critical temperature T _c . In the underdoped regime, this magnetoresistance cannot be described by the Kohler rule and is due to the influence of superconducting fluctuations. On the other hand, in the overdoped regime, the Kohler rule does not seem to be violated. ?The strong magnetoresistance above T _c can be related to the preformed superconducting pairs existing well above T _c but forming a phase coherent superconducting state below T _c . The observations support the idea of a close relation between the pseudogap and the superconducting gap and provide evidence for the presence of pre-pairs above T _c . Both the observed fluctuations and the observed magnetoresistance are in accordance with the existence of 1D-like stripes. These results are further supported by recent high magnetic field measurements (up to 55 T) of the transverse magnetoconductivity σ _xy, which goes to zero for T→0 K.     

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.     

Influence of the Extra Layer on the Transport Properties of NdFeAsO1?0.14F0.14 and FeSe0.88 Superconductors from Magneto Dynamic Analysis

In this letter, the electromagnetic response of the NdFeAsO_1?0.14F_0.14 (T _c =49?K) superconductor system, characterized by FeAs and NdO alternating layers, has been compared with that of FeSe_0.88. We have studied the flux dynamics of these two systems by means of ac multi-harmonic magnetic susceptibility. The analysis shows that although characterized by larger thermal fluctuations due to its higher T _c , NdFeAsO_1?0.14F_0.14 exhibits a stronger pinning force relative to FeSe_0.88. The further Irreversibility Line (IL) analysis also points out that both superconductors have a 3D flux pinning behavior. We associate the stronger pinning force in the NdFeAsO_1?0.14F_0.14 structure to the presence of the extra NdO layer. Different pinning contributions can be associated to the structural stress associated to FeAs superconducting layers and/or to the Nd³⁺ ions magnetic moment (????3.6???_B) contribution on the flux cores. We will also show that these pinning are over imposed to a weak collective contribution due to the dopant F atoms.     

Superconductivity in ternary Heusler intermetallic compounds

We report the discovery of superconductivity in a number of stoichiometric A₂BC ordered ternary intermetallic alloys isostructural to the prototype ferromagnetic Heusler alloy, Cu₂MnAl. Ni in the superconducting Ni₂BC phases appears to be behaving as a full d-band constituent. The highest superconducting critical temperature T_c, for the Ni phases surveyed is exhibited by Ni₂NbSn (T_c = 3.4 K).     

Effect of negative pressure on superconducting transition temperature of MgB2

Atomistic simulation has been performed with interatomic potentials to investigate the effect of negative hydrostatic pressure on the superconducting transition temperature (T_c) of MgB₂. The calculation reveals that T_c can be greatly enhanced by applying negative pressure and it can reach up to 52.2 K at ?13 GPa, about 13 K higher than that at ambient pressure. The mechanism for the enhancement of T_c is attributed to that negative pressure reduces high-frequency phonon vibrations of B and thus dramatically enhances the electron–phonon coupling. Our result seems to open up a possible way for the enhancement of T_c in MgB₂.     

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.