Current drag in two leg quantum ladders

A two-leg ladder of either interacting bosons or tightly bound cooper pairs is investigated when a supercurrent is forced in one of the legs of the ladder. The two legs of the ladder are connected by a tunneling term. Using a bosonization representation of such an interacting ladder we show that up to a certain critical current the current in the first wire induces an identical supercurrent in the second wire. When this threshold is exceeded vortices are formed in the system and the current in the second wire reduces even if the driving current increases. Potential applications to condensed matter or cold atomic systems are discussed.     

Floquet Majorana fermions in superconducting quantum dots

We consider different configurations of ac driven quantum dots coupled to superconductor leads where Majorana fermions can exist as collective quasiparticles. The main goal is to tune the existence, localization and properties of these zero energy quasiparticles by means of periodically driven external gates. In particular, we analyze the relevance of the system and driving symmetry. We predict the existence of different sweet spots with Floquet Majorana fermions in configurations where they are not present in the undriven system.     

Soft x-ray absorption and high-resolution powder x-ray diffraction study of superconducting CaxLa1−xBa1.75−xLa0.25+xCu3Oy system

We have studied the electronic structure of unoccupied states measured by O K-edge and Cu L-edge x-ray absorption spectroscopy (XAS), combined with crystal structure studied by high resolution powder x-ray diffraction (HRPXRD), of charge-compensated layered superconducting Ca_xLa_1−xBa_1.75−xLa_0.25+xCu₃O_y (0≤x≤0.4 and 6.4≤y≤7.3) cuprate. A detailed analysis shows that, apart from hole doping, chemical pressure on the electronically active CuO₂ plane due to the lattice mismatch with the spacer layers greatly influences the superconducting properties of this system. The results suggest chemical pressure to be the most plausible parameter to control the maximum critical temperatures (T_c^max) in different cuprate families at optimum hole density.     

Investigation of the structural distortion in FeSex crystals by X-ray absorption near edge structures (XANES) spectroscopy

Crystals of the new superconductor FeSe were investigated by X-ray absorption near edge structures spectroscopy (XANES) in the temperature range 300-77 K. Crystals with Se content varying from x=1-0.8 were used. The Fe K-edge spectra show a structure distortion and increase in hole content as x decreases. The Fe K-edge spectra also show changes when the crystals are cooled to 100 K which may be related to the orthorhombic distortion reported in these crystals. The data is examined in conjunction with XRD measurements on the crystals to seek a correlation.     

What drives pseudogap physics in high-Tc cuprates? A view from the (resistance) bridge

Taking into account the evolution of the in-plane resistivity with temperature and doping, a candidate proposal for the (hole-doped) cuprate phase diagram is constructed. Many features of the phase diagram are viewed as a consequence of an anisotropic interaction that intensifies with decreasing doping from the heavily overdoped side. At a critical doping p_c, that coincides with the development of the normal state pseudogap, the interaction becomes sufficiently strong that all electronic states near the zone boundary are effectively incoherent even at T = 0.     

Optical properties of the iron-chalcogenide superconductor FeTe0.55Se0.45

The complex optical properties of the iron-chalcogenide superconductor FeTe_0.55Se_0.45 with T_c=14 K have been examined over a wide frequency range for light polarized in the Fe-Te(Se) planes above and below T_c. At room temperature the optical response may be described by a weakly interacting Fermi liquid; however, just above T_c this picture breaks down and the scattering rate takes on a linear frequency dependence. Below T_c there is evidence for two gap features in the optical conductivity at and . Less than 20% of the free carriers collapse into the condensate for T?T_c, and this material is observed to fall on the universal scaling line for a BCS dirty-limit superconductor in the weak-coupling limit.     

As-NQR study of superconductivity in LaRAsO1−xFx (R=Fe and Ni)

We report ⁷⁵As nuclear quadrupole resonance (NQR) studies on oxypnictide superconductors LaFeAsO_1−xF_x (x=0.08, 0.15) and LaNiAsO_1−xF_x (x=0, 0.06, 0.10, 0.12). In LaFeAsO_0.92F_0.08 (T_c=23 K), nuclear spin-lattice relaxation rate 1/T₁ shows no coherence peak just below T_c and decreases with decreasing temperature accompanied by a hump structure at T∼0.4T_c, which is a characteristic of the multigap superconductivity. In the normal state, the quantity 1/T₁T increases with decreasing temperature to T_c, indicating that the existence of antiferromagnetic correlation originating from its multiple electronic band structure. On the other hand, LaNiAsO_1−xF_x shows a clear Hebel-Slichter (coherence) peak just below T_c, evidencing that the LaNiAsO_1−xF_x is a BCS superconductor. In the normal state, 1/T₁T is constant in the temperature range for all LaNiAsO_1−xF_x, which indicates electron correlations are weak. We suggest that the contrasting behavior of both superconductivity and electron correlations in LaFeAsO_0.92F_0.08 and LaNiAsO_1−xF_x between them relate to the difference of electronic band structure configuration. We also provide a possible interpretation for the pseudogap-like behavior in the normal state observed in both compounds.     

Crystal growth of superconductive PrBa2Cu3O7−y

Superconductive PrBa₂Cu₃O_7−y (Pr123) crystals have been grown successfully by the travelling-solvent floating-zone (TSFZ) method using 3.7–5.8% PrO, 27–37% BaO, and 60–69% CuO composition solution with the 0.1–0.4% oxygen mixed argon gas atmosphere. The grown crystals have been identified to be Pr123 by a precession camera, X-ray powder diffractions and scanning electron microscopy–X-ray energy dispersion spectroscopy (SEM-EDS). Some portion of the grown TSFZ crystal boules show bulk superconductivity below about 80 K after annealing in oxygen.     

Influence of the Ca:Pb ratio on the superconducting properties of Bi-based ceramics of type 2223

From a study of the influence of the Ca: Pb ratio on the superconducting properties of Bibased ceramics of 2223 type it follows that as a second parameter the employed gas atmosphere must be taken into consideration. In case of air preparation good intragrain and intergrain properties are obtained for Ca rich samples (e.g. Bi_1.8Pb_0.2Sr_1.9Ca_2.1 Cu₃O_z); whereas for Pb rich materials (e.g. Bi_1.7Pb_0.5Sr_1.9Ca_1.9 Cu₃O_z) improved conditions are encountered by employment of an Ar/O₂ (8%) atmosphere during synthesis. Independently, the transition temperature is situated near 108 K.