Boosting large-scale superconductor applications require nanostructured conductors with artificial pinning centres immobilizing quantized vortices at high temperature and magnetic fields. Here we demonstrate a highly effective mechanism of artificial pinning centres in solution-derived high-temperature superconductor nanocomposites through generation of nanostrained regions where Cooper pair formation is suppressed. The nanostrained regions identified from transmission electron microscopy devise a very high concentration of partial dislocations associated with intergrowths generated between the randomly oriented nanodots and the epitaxial YBa(2)Cu(3)O(7) matrix. Consequently, an outstanding vortex-pinning enhancement correlated to the nanostrain is demonstrated for four types of randomly oriented nanodot, and a unique evolution towards an isotropic vortex-pinning behaviour, even in the effective anisotropy, is achieved as the nanostrain turns isotropic. We suggest a new vortex-pinning mechanism based on the bond-contraction pairing model, where pair formation is quenched under tensile strain, forming new and effective core-pinning regions.

The polar Kerr effect in the high-T c superconductor YBa 2 Cu 3 O 6x was measured at zero magnetic field with high precision using a cyogenic Sagnac fiber interferometer. We observed nonzero Kerr rotations of order 1 rad appearing near the pseudogap temperature T and marking what appears to be a true phase transition. Anomalous magnetic behavior in magnetic-field training of the effect suggests that time reversal symmetry is already broken above room temperature. , large compared to the superconducting (SC) transition temperature, T c . Two major classes of theories have been introduced in an attempt to describe the pseudogap state: One in which the pseudogap temperature T represents a crossover into a state with preformed pairs with a d wave gap symmetry [6,7], and another in which T marks a true transition into a phase with broken symmetry that ends at a quantum critical point, typically inside the superconducting dome. While at low doping this phase may compete with superconductvity, it might provide fluctuations that are responsible for the enhanced transition temperature near its quantum critical point (e.g., as in Ref. [8] In this Letter, we report high resolution optical Kerreffect measurements on YBa 2 Cu 3 O 6x crystals with various hole concentrations p. (p is, in turn, a monotonic function of the oxygen concentration x, and it also depends on oxygen ordering in the chains [12].) We identify a sharp phase transition at a temperature T s p, below which there is a nonzero Kerr angle, indicating the existence of a phase with broken time reversal symmetry (TRS). Both the magnitude and hole concentration dependence of T s are in close correspondence with those of the pseudogap crossover temperature, T , which has been identified in other physical quantities. In particular, as shown in Fig. 1, T s is substantially larger than the superconducting T c in underdoped materials, but drops rapidly with increasing hole concentration, so that it is smaller than T c in a near optimally doped crystal and extrapolates to zero at a putative quantum critical point under the superconducting dome. The magnitude of the Kerr rotation in YBa 2 Cu 3 O 6x (YBCO) is smaller by 4 orders of magnitude than that observed in other itinerant ferromagnetic oxides [13,14], and the temperature dependence is ''superlinear'' near T c , FIG. 1 (color online). The onset of the Kerr-effect signal, T s (circles), and T c (red squares) for the YBa 2 Cu 3 O 6x samples reported in this Letter. Also shown are T c p (from [12]) and T N p (from [22]).

Electrical transport through a normal metal / superconductor contact at biases smaller than the energy gap can occur via the reflection of an electron as a hole of opposite wave vector. The same mechanism of electron-hole reflection gives rise to low energy states at the surface of unconventional superconductors having nodes in their order parameter. The occurrence of electron-hole reflections at normal metal / superconductor interfaces was predicted independently by Saint James and de Gennes and by Andreev, and their spectroscopic features discussed in detail by Saint James in the early sixties. They are generally called Andreev reflections but, for that reason, we call them Andreev -Saint James (ASJ) reflections. We present a historical review of ASJ reflections and spectroscopy in conventional superconductors, and review their application to the High Tc cuprates. The occurrence of ASJ reflections in all studied cuprates is well documented for a broad range of doping levels, implying that there is no large asymmetry between electrons and holes near the Fermi level in the superconducting state. In the underdoped regime, where the pseudo-gap phenomenon has been observed by other methods such as NMR, ARPES and Giaever tunneling, gap values obtained from ASJ spectroscopy are smaller than pseudo-gap values, indicating a lack of coherence in the pseudo-gap energy range. Low energy surface bound states have been observed in all studied hole doped cuprates, in agreement with a dominant d-wave symmetry order parameter. Results are mixed for electron doped cuprates. In overdoped Y Ba 2 Cu 3 O 7−δ (δ < 0.08) and La 2−x SrxCuO 4 , ASJ spectroscopy is consistent with the presence of an additional imaginary component of the order parameter. Results of ASJ spectroscopy under applied magnetic fields are also reviewed. A short section at the end is devoted to some recent results on spin effects. * Electronic address: guyde@tau.ac.il 2

The 'pseudogap' observed in the electron excitation spectrum of underdoped copper oxide superconductors has become the focus of considerable attention in the field of high-temperature superconductivity. In conventional superconductors, described by 'BCS' theory, an energy gap appears at the superconducting transition temperature (T); the pseudogap, in contrast, is observed well above T (ref. 1) and can be large compared to the conventional BCS gap. Here I compare gap energies, measured by different experimental techniques, for the copper oxide superconductors and show that these reveal the existence of two distinct energy scales: Δ and Δ. The first, determined either by angle-resolved photoemission spectroscopy or by tunnelling, is the single-particle excitation energy-the energy (per particle) required to split the paired charge-carriers that are required for superconductivity. The second energy scale is determined by Andreev reflection experiments, and I associate it with the coherence energy range of the superconducting state-the macroscopic quantum condensate of the paired charges. I find that, in the overdoped regime, Δ and Δ converge to approximately the same value, as would be the case for a BCS superconductor where pairs form and condense simultaneously. But in the underdoped regime, where the pseudogap is observed, the two values diverge and Δ is larger than Δ. Models that may provide a framework for understanding these results involve the existence of pairing above the condensation temperature, as might occur in a crossover from BCS to Bose-Einstein condensation behaviour or from the formation of striped phases.

The properties of a device made of two point contacts between normal (N) or ferromagnetic (F) tips, and a superconductor (S), are discussed as a function of the spin polarization and the distance L between the contacts. When L is smaller than the superconductor coherence length ξ, nonlocal Andreev reflections occur: for opposite spin polarizations of the contacts, “mixed” Cooper pairs made of electrons coming one from each tip can be injected into the superconductor. This leads to unusual properties, for instance, the parallel resistance of two S/F contacts goes from infinity for full and equal polarizations, to a finite Andreev value for opposite ones.

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The short coherence length of high-T, oxides is shown to induce considerable weakening of the pair potential at surfaces and interfaces. It is argued that this eAect is responsible for the existence of internal Josephson junctions at twin boundaries, which are at the origin of the superconductive glassy state, as well as for gapless tunneling characteristics. PACS numbers: 74.30. -e, 74.70.Ya Recent susceptibility and magnetization measurements' as well as microwave absorption experiments have been interpreted as resulting from a superconducting glass state. This state consists of superconducting loops with areas typically smaller than the grain size, and thus it cannot be explained by weak links at grain boundaries. The existence of Josephson junctions inside the grains had therefore to be assumed. Analysis of high-T, superconductors ' indicate that they have extremely short coherence lengths, of the order of the size of the unit cell. Bardeen quotes for the zero-temperature coherence lengths g(0) a value of 12 A in Y~Ba2Cu307, while values of 7 and 34 A along the c and (a, b) axis, respectively, have been inferred from H, 2 measurements on single crystals. These are to be compared with the values of the lattice parameters a =3.83 A, b =3.89 A, and c =11. 71 A. We show here that the existence of a very short coherence length is responsible for a lowering of the pair potential at surfaces and interfaces, this eA'ect being particularly strong near T, . It also leads to the appearance of internal Josephson junctions responsible for the glassy behavior observed in ceramic samples' as well as in single crystals, to the microwave response of point contacts, and to extrinsic (low) critical currents in single crystals that are otherwise unexplained.We believe that it is also responsible for the gaplesslike characteristics obtained by scanning tunneling spectroscopy.The boundary condition for the pair potential 6 at a boundary can be written as (4 dA/dx ) =p =b where the "extrapolation length" b ( Fig. 1) is given by superconductor-insulator boundary, the integrand in Eq.