78 research outputs found
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
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 , 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
. In the case of coupled aluminium (Al) nanograins, 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,
and , as a function of grain coupling. Starting from well-coupled
arrays, grows with progressive grain decoupling, causing the
increasing of . As the grain-coupling is further suppressed,
saturates while decreases, concomitantly with a sharp decline of .
This crossover to a phase-driven SC transition is accompanied by an optical gap
persisting above . 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
- …