[en] Over the course of three decades of intense study, apart from the exceptionally high critical temperature, many unusual properties of cuprates have been discovered, notably including resistivity linear in temperature, electronic Raman continuum and optical absorption extending throughout the infrared region, pseudogap, hour-glass spin excitation spectrum, etc. However, each of these features have been also observed in other materials, including some that are not even superconducting at all. Here, we describe an extensive experiment in which over 2,000 films of the La_2-xSr_xCuO₄ have been synthesized and studied in detail over the course of the last twelve years. We argue here that, uniquely, in the cuprates an unusual superconducting state, that defies the standard BCS description, develops from an unusual metallic state, in which the rotational symmetry of the electron fluid is spontaneously broken.

[en] A conformal pinning array can be created by conformally transforming a uniform triangular pinning lattice to produce a new structure in which the six-fold ordering of the original lattice is conserved but where there is a spatial gradient in the density of pinning sites. Here we examine several aspects of vortices interacting with conformal pinning arrays and how they can be used to create a flux flow diode effect for driving vortices in different directions across the arrays. Under the application of an ac drive, a pronounced vortex ratchet effect occurs where the vortices flow in the easy direction of the array asymmetry. When the ac drive is applied perpendicular to the asymmetry direction of the array, it is possible to realize a transverse vortex ratchet effect where there is a generation of a dc flow of vortices perpendicular to the ac drive due to the creation of a noise correlation ratchet by the plastic motion of the vortices. We also examine vortex transport in experiments and compare the pinning effectiveness of conformal arrays to uniform triangular pinning arrays. In conclusion, we find that a triangular array generally pins the vortices more effectively at the first matching field and below, while the conformal array is more effective at higher fields where interstitial vortex flow occurs.

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[en] Accurate analysis of over voltage in the superconducting solenoid during a quench is one of the bases for quench protection system design. Classical quench simulation methods can only give rough estimation of the over voltage within a magnet coil. In this paper, for multi-sectioned superconducting solenoid, based on the classical assumption of ellipsoidal normal zone, three-dimension al temperature results are mapped to the one-dimension of the wire, the temperature distribution along the wire and the resistances of each turn are obtained. The coil is treated as circuit comprised of turn resistances, turn self and mutual inductances. The turn resistive voltage, turn inductive voltage, and turn resultant voltage along the wire are calculated. As a result, maximum internal voltages, the layer-to-layer voltages and the turn-to-turn voltages are better estimated. Utilizing this method, the over voltage of a small solenoid and a large solenoid during quenching have been studied. The result shows that this method can well improve the over voltage estimate, especially when the coil is larger.

[en] X-ray diffraction (XRD) measurements were made on silver-sheathed (Bi,Pb)₂Sr₂Ca₂Cu₃O_x (Ag/Bi-2223) composite superconductor using 25 keV X-rays produced by an insertion device beamline at a third generation synchrotron. These measurements, performed on fully sheathed multifilament-type Ag/Bi-2223 wire specimens, provided high resolution diffraction patterns that revealed (1) the phases present in the cores of the superconducting filaments and (2) the nature and quality of the Bi-2223 grain colony texture. In addition to the Bi-2223 phase, we were able to detect and to monitor specimen-to-specimen variations in the relative amounts of Bi-2212, Bi-2201, and the '3221' phase. The scattering geometry used (X-ray beam perpendicular to the Ag/Bi-2223 conductor rolling direction) resulted in the detection of several remarkably intense diffraction lines of Bi-2223, most notably the (2 0 0)/(0 2 0) pair, that were highly useful for recording pole figure maps evidencing the contiguous fiber texture microstructure of the Bi-2223 grain colonies. The nondestructive nature of the 25 keV transmission XRD method offers many advantages for the comprehensive study of phase evolution and microstructure development in Ag/Bi-2223 composite conductors.

[en] Here, we describe an extensive experimental study of La_2-xSr_xCuO₄ films synthesized by molecular beam epitaxy and investigated by angle-resolved measurements of transverse resistivity (without applied magnetic field). The data show that an unusual metallic state, in which the rotational symmetry of the electron fluid is spontaneously broken, occurs in a large temperature and doping region. The superconducting state always emerges out of this nematic metal state.

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[en] Epitaxial films of LiTi₂O₄ on single crystalline substrates of MgAl₂O₄, MgO, and SrTiO₃ provide model systems to systematically explore the effects of lattice strain and microstructural disorder on the superconducting state. Lattice strain that affects bandwidth gives rise to variations in the superconducting and normal state properties. Microstructural disorder, such as antiphase boundaries that give rise to Ti network disorder, reduces the critical temperature, and Ti network disorder combined with Mg interdiffusion lead to a much more dramatic effect on the superconducting state. Surface sensitive X-ray absorption spectroscopy has identified Ti to retain site symmetry and average valence of the bulk material regardless of film thickness

[en] We derive the current-voltage (I-V) characteristics of far from equilibrium superconducting tunneling arrays and find that the energy relaxation ensuring the charge transfer occurs in two stages: (i) the energy exchange between charge carriers and the intermediate bosonic agent, environment, and (ii) relaxing the energy further to the (phonon) thermostat, the bath, provided the rate of the environmental modes-phonon interactions is slower than their energy exchange rate with the tunneling junction. For a single junction we find I ∝ (V/R)ln(Λ/V), where R is the bare tunnel resistance of the junction and Λ is the high energy cut-off of the electron-environment interaction. In large tunneling arrays comprised of macroscopic number of junctions, low-temperature transport is governed by the cotunneling processes losing energy to the electron-hole environment. Below some critical temperature, T*, the Coulomb interactions open a finite gap in the environment excitations spectrum blocking simultaneously Cooper pair and normal excitations currents through the array; this is the microscopic mechanism of the insulator-to-superinsulator transition.