A spatial interpretation of emerging superconductivity in lightly doped cuprates

The formation of domains comprising alternating 'hole rich' and 'hole poor' ladders recently observed by Scanning Tunneling Microscopy by Kohsaka et al., on lightly hole doped cuprates, is interpreted in terms of an attractive mechanism which favors the presence of doped holes on Cu sites located each on one side of an oxygen atom. This mechanism leads to a geometrical pattern of alternating hole-rich and hole-poor ladders with a periodicity equal to 4 times the lattice spacing in the CuO plane, as observed experimentally. To cite this article: G. Deutscher, P.-G. de Gennes, C. R. Physique 8 (2007).Comment: 4 pages, 3 figuer

Wie die Sprache unser Denken prägt

Es ist eine alte Debatte. Auf der einen Seite steht die althergebrachte Maxime, alles, was wert ist, gesagt zu werden, müsse in jeder Sprache gesagt werden können - umso mehr dann, wenn es sich um die Wissenschaft handelt, die den Anspruch erhebt, universelle Wahrheiten zu erfassen. Demgegenüber steht ein immer wieder anzutreffender Gedanke, dessen wohl inspirierteste Formulierung von Wilhelm von Humboldt stammt, nämlich, dass die Verschiedenheit der Sprachen nicht nur eine von „Schällen und Zeichen“, sondern eine der „Weltansichten selbst“ sei. Thema dieses Vortrags ist die Frage, wo die Wahrheit zwischen diesen scheinbaren Gegensätzen liegt

High Quality a-axis outgrowth on c-axis YlCa1-xBa2Cu3O7-d

The large amplitude of the high Tc (HTS) superconducting gap is attractive for improved electronic applications. However, the study of such HTS cuprates has uncovered that unlike the s-wave order parameter of the low Tc, an angle dependent dx2-y2 wave function is the dominant order parameter in such compounds. This symmetry causes low energy surface bound states, detrimental for applications, except at (100) oriented surfaces. It is therefore essential to have a smooth and well oriented surface of the crystallographic a-axis (100). In this work we present a study of an unconventional way to attain such surfaces in the form of a-axis outgrowth on a c-axis surface of sputtered Y1-xCaxBa2Cu3O7-d thin film. The grains topography was tested using X-ray, SEM and AFM together with Point Contact and Tunnel Junctions measurements.Comment: 15 pages, 5 figure

Superconducting gap and pseudogap

The discovery of the pseudogap has been a fundamental advance in uncovering the new physics of the high-Tc cuprates. Yet, its meaning is still far from being clear. In particular, its relation to the superconducting gap remains an object of controversy. While many authors consider that it is a high temperature precursor of superconductivity, which turns into the superconducting gap at low temperatures, others contend that it is a normal state property related only indirectly to superconductivity. We review a number of experiments such as single particle tunneling, Andreev–Saint–James reflections and others, and conclude that in the underdoped regime there exists considerable evidence for the existence of two distinct energy scales, the superconducting gap and the pseudogap, which appear to merge into one another in overdoped samples

Kinetic energy change with doping upon superfluid condensation in high temperature superconductors

In conventional BCS superconductors, the electronic kinetic energy increases upon superfluid condensation (the change DEkin is positive). Here we show that in the high critical temperature superconductor Bi-2212, DEkin crosses over from a fully compatible conventional BCS behavior (DEkin>0) to an unconventional behavior (DEkin<0) as the free carrier density decreases. If a single mechanism is responsible for superconductivity across the whole phase diagram of high critical temperature superconductors, this mechanism should allow for a smooth transition between such two regimes around optimal doping.Comment: 3 pages, 2 figure

Shaping a superconducting dome: Enhanced Cooper-pairing versus suppressed phase coherence in coupled aluminum nanograins

Deterministic enhancement of the superconducting (SC) critical temperature $T_c$ is a long-standing goal in material science. One strategy is engineering a material at the nanometer scale such that quantum confinement strengthens the electron pairing, thus increasing the superconducting energy gap $\Delta$, as was observed for individual nanoparticles. A true phase-coherent SC condensate, however, can exist only on larger scales and requires a finite phase stiffness $J$. In the case of coupled aluminium (Al) nanograins, $T_c$ can exceed that of bulk Al by a factor of three, but despite several proposals the relevant mechanism at play is not yet understood. Here we use optical spectroscopy on granular Al to disentangle the evolution of the fundamental SC energy scales, $\Delta$ and $J$, as a function of grain coupling. Starting from well-coupled arrays, $\Delta$ grows with progressive grain decoupling, causing the increasing of $T_c$. As the grain-coupling is further suppressed, $\Delta$ saturates while $T_c$ decreases, concomitantly with a sharp decline of $J$. This crossover to a phase-driven SC transition is accompanied by an optical gap persisting above $T_c$. These findings identify granular Al as an ideal playground to test the basic mechanisms that enhance superconductivity by nano-inhomogeneity.Comment: 6 + 6 pages (manuscript + supplementary material