Competing networks for new materials functionality

 A second defect network, anticorrelated with a previously known network of ordered excess oxygen atoms, is shown to improve the high temperature copper oxide superconductors Electronic devices such as integrated circuits work because of the defects in the underlying semiconductor material, namely silicon.

Defects also control the electrical properties of many other important materials, including the ability of oxides of copper to superconduct at high temperatures, that is to conduct electricity with no resistance; this is a very useful property when we wish to design electromagnets, for example for hospital magnetic resonance imaging devices, which are completely stable and do not heat up upon use.For the copper oxides the organization of the defects strongly affects the superconductivity, most notably the temperatures to which the superconductivity survives.
A collaboration between the superstripes group of the Rome International Center for Materials Science Superstripes (RICMASS) and the London Centre for Nanotechnology (LCN) now reports in an article, ( published in the Proceedings of the (US) National Academy of Sciences that it is the organization of two - not one - type of defect, that determines the superconductivity of a particularly simple “parent” oxide of copper where additional oxygen atoms are introduced to induce superconductivity. The first defects are the additional oxygen atoms while the second are deviations of atoms from where they would have been in the “parent” material.

Earlier work by the Rome/LCN group has shown that the oxygen defect order can be highly inhomogeneous, even in optimal superconducting samples, which raises the question of the nature of the sample regions where the order does not exist but which nonetheless form the “glue” binding the ordered regions together. The collaboration has now used X-ray microscopy, carried out at the European Synchrotron Radiation Facility in Grenoble, France, to show that the glue regions contain ordered defects of the second kind and that connectivity of the ordered defects is most pronounced for the best superconducting samples. The latter exist in droplets anticorrelated with the ordered excess oxygen atoms whose connectivity is also most pronounced for the best superconducting samples. There are therefore two – not only the one found previously - networks of ordered defects which must be tuned to achieve optimal superconductivity. For a given concentration of excess oxygen atoms, the highest transition temperature is obtained when both the ordered oxygen and lattice defects form maximally connected yet anticorrelated patterns, as opposed to appearing in isolated spots. Prof. Bianconi of the Rome International center for Materials Science Superstripes, noted that “The results further reinforce the idea that the remarkable superconductivity of the copper oxides cannot be understood in terms of the standard approach to superconductivity from the 1950’s where we think of electron pairing in a homogeneous medium as for ordinary solids”. Prof. Aeppli, Director of the London Centre for Nanotechnology, added that “The experiments also suggest that engineered heterogeneity, along the lines of what occurs with the heat treatment of the rather simple oxide of copper which we have studied, may lead to even more remarkable superconductors in the future.”

 Map of anticorrelated defects in optimized copper oxide superconductor. Red peaks correspond to the best organized distortions of the underlying atomic lattice while blue valleys represent oxygen defect order.

Figure: Map of anticorrelated defects in optimized copper oxide superconductor. Red peaks correspond to the best organized distortions of the underlying atomic lattice while blue valleys represent oxygen defect order.


 Lev Gorkov comment on the paper'kov&id=%7BE488961F-EF1E-4AAA-861E-18CBF14F71C1%7D
London Centre for Nanotechnology LCN

Our understanding of high temperature superconductivity (HTS) in cuprates is hindered by the fact that they reveal metallic properties only after the insulating antiferromagnetic ground state of the parent compound is suppressed by “doping” with elements that supposedly supply carriers into the conducting CuO2-plane. Unlike semiconductors, the dopants’ charges are not screened. To maintain the electroneutrality, the material would break into mesoscopic regions of the insulating antiferromagnetic and the metallic ground states competing for the Free energy.

The main attention is usually paid to the in-plane carriers. To the extent that one may neglect the inhomogeneity in distribution of the charged dopant ions, the phase separation realizes itself in formation of the so-called “stripes”- the modulated superstructure with the period of the order of few nanometers incommensurate with the lattice. It is commonly assumed that the lattice effects themselves play only a secondary role in superconductivity of HTS cuprates.

The recent results from the Prof. Bianconi group show that it is not true. Their experiments were motivated by the empiric fact that for HTS cuprates the transition temperature, Tc increases with the increase in the complexity of the unit cell. At doping such complicated multilayered lattice structure may be prone to local instabilities. The authors speculated that this could favor interplay between properties of the in-plane carriers and the lattice distortions.

To make the point, the authors have carried the extensive X-ray diffraction study on the LaCuO4, the single layered cuprate doped by the interstitial oxygen: LaCuO4+y. The ingenuity with the choice of the material is that all experiments can be performed on one and the same sample (at the fixed doping level). Due to the mobility of the oxygen above nitrogen temperatures the changes in the defects’ distribution can be achieved through few warming and annealing cycles.

Generally, the obtained overall pattern of defects consists of the “puddles” of ordered interstitial oxygens and the ordered octahedral distortions between them (the latter seem to be related to the abovementioned stripes). However, the two somehow arrange themselves to provide the “fractal” paths for superconductivity. It was found that there exists such distribution of strains coming from the interstitial “puddles” that determines the optimal superconductivity network. Thus, although details of the mechanism leading to HTS superconductivity in LaCuO4+y remain unknown, the result shows the important impact of local lattice distortions on the value of the bulk transition temperature. Thereby, while it is common in the literature to consider ions of the dopants only as mere defects on which carriers in the CuO2scatter, the new results provide the convincing arguments in favor of the significant role of the lattice degrees of freedom.

Multiple networks of defects promote superconductivity

 A Cuprate superconductor in the Elettra XRD1 Highlights

Optimum inhomogeneity of local lattice distortions in La2CuO4+y

Mediterranean Institute of Fundamental Physics,
Marino, Italy
Via Appia Nuova 31, 00040 Marino (Rome)

 I superconduttori incontrano i network

LE SCIENZE edizione italiana di Scientific American

Due networks di difetti in competizione 
promuovono la superconduttività.

Centro Fermi, Roma Italy

Con due 'difetti' la superconduttività migliora

Consiglio Nazionale delle Ricerche

Due difetti sono meglio di uno.

Istituto Nazionale di Fisica Nucleare - INFN:

About Rome International Center for Materials Science Superstripes:

Is a leading centre of research located in Rome and academic excellence in the field of new materials for the XXI century needed for improving health, environment, and needs for energy. 
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