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Bianconi A.,Rome International Center for Materials Science Superstripes
Journal of Superconductivity and Novel Magnetism | Year: 2014

Lattice and electronic nanoscale phase separation in strongly correlated multiband systems confined in heterostructure at atomic limit called superstripes has been an object of the scientific debate at the international conference Superstripes 2013 focusing on "Quantum in Complex Matter: Superconductivity, Magnetism and Ferroelectricity" held in Ischia, Italy (May 27-June 1, 2013). The focus was on lattice granularity due to defects self-organization, lattice modulations at a critical misfit strain, and electronic phase separation in multiband Hubbard models near a 2.5 Lifshitz transition. The emerging superstripes scenario is a particular case of percolation superconductivity in networks of superconducting multicondensates superconducting puddles and their competition with phase-separated networks of nanoscale-striped magnetic puddles. This new emerging paradigm for high-Tc superconductor-layered oxides opens new perspectives for quantum electronics by controlling the complexity in functional oxides. © 2014 Springer Science+Business Media New York.

Perali A.,University of Camerino | Perali A.,Mediterranean Institute of Fundamental Physics | Innocenti D.,University of Rome Tor Vergata | Valletta A.,CNR Institute for Microelectronics and Microsystems | And 2 more authors.
Superconductor Science and Technology | Year: 2012

The doping dependent isotope effect on the critical temperature (T c) is calculated for multi-band multi-condensate superconductivity near a 2.5 Lifshitz transition. We consider a superlattice of quantum stripes with finite hopping between stripes near a 2.5 Lifshitz transition for the appearance of a new sub-band making a circular electron-like Fermi surface pocket. We describe a particular type of BEC (Bose-Einstein Condensate) to BCS (Bardeen-Cooper-Schrieffer condensate) crossover in multi-band/multi-condensate superconductivity at a metal-to-metal transition that is quite different from the standard BEC-BCS crossover at an insulator-to-metal transition. The results show that the isotope coefficient strongly deviates from the standard BCS value 0.5, when the chemical potential is tuned at the 2.5 Lifshitz transition for the metal-to-metal transition. The critical temperature Tc shows a minimum due to the Fano antiresonance in the superconducting gaps and the isotope coefficient diverges at the point where a BEC coexists with a BCS condensate. In contrast Tc reaches its maximum and the isotope coefficient vanishes at the crossover from a polaronic condensate to a BCS condensate in the newly appearing sub-band. © 2012 IOP Publishing Ltd.

Bianconi A.,Rome International Center for Materials Science Superstripes
Journal of Physics: Conference Series | Year: 2013

A characteristic feature of a superconductor made of multiple condensates is the possibility of the shape resonances in superconducting gaps. Shape resonances belong to class of Fano resonances in configuration interaction between open and closed scattering channels. The Shape resonances arise because of the exchange interaction, a Josephson-like term, for transfer of pairs between different condensates in different Fermi surface spots in the special cases where at least one Fermi surface is near a 2.5 Lifshitz topological transition. We show that tuning the shape resonances show first, the gap suppression (like a Fano anti-resonance) driven by configuration interaction between a BCS condensate and a BEC-like condensate, and second, the gap amplification (like a Fano resonance) driven by configuration interaction between BCS condensates in large and small Fermi surfaces. Shape resonances usually occur in granular nanoscale complex matter (called superstripes) because of the lattice instability near a 2.5 Lifshitz transition in presence of interactions. Using a new imaging method, scanning nano-X-ray diffraction, we have shown the generic formation in high temperature superconductors of a granular superconducting networks made of striped puddles formed by ordered oxygen interstitials or ordered local lattice distortions (like static short range charge density waves). In the superconducting puddles the chemical potential is tuned to a shape resonance in superconducting gaps and the maximum Tc occurs where the puddles form scale free superconducting networks. © Published under licence by IOP Publishing Ltd.

Jarlborg T.,University of Geneva | Bianconi A.,Rome International Center for Materials Science Superstripes | Bianconi A.,National Research Council Italy | Bianconi A.,University of Rome La Sapienza
Physical Review B - Condensed Matter and Materials Physics | Year: 2013

Novel imaging methods show that the mobile dopants in optimum doped La 2CuO4+y (LCO) get self-organized, instead of randomly distributed, to form an inhomogeneous network of nanoscale metallic puddles with ordered oxygen interstitials interspersed with oxygen-depleted regions. These puddles are expected to be metallic, being far from half filling because of high dopant density, and to sustain superconductivity having a size in the range 5-20 nm. However, the electronic structure of these heavily doped metallic puddles is not known. In fact the rigid-band model fails because of ordering of dopants and supercell calculations are required to obtain the Fermi surface reconstruction. We have performed advanced band calculations for a large supercell La16Cu8O32+N where N=1 or 2 oxygen interstitials form rows in the spacer La16O16+N layers intercalated between the CuO2 layers as determined by scanning nano x-ray diffraction. The additional occupied states made by interstitial oxygen orbitals sit well below the Fermi level (EF) and lead to hole doping as expected. The unexpected results show that in the heavily doped puddles the altered Cu(3d)-O(2p) band hybridization at EF induces a multiband electronic structure with the formation of multiple Fermi surface spots: (a) Small gaps appear in the folded Fermi surface, (b) three minibands cross E F with reduced Fermi energies of 60, 150, and 240 meV, respectively, (c) the density of states and band mass at EF show substantial increases, and (d) spin-polarized calculations show a moderate increase of antiferromagnetic spin fluctuations. All calculated features are favorable to enhance superconductivity; however, the comparison with experimental methods probing the average electronic structure of cuprates will require the description of the electronics of a network of multigap superconducting puddles. © 2013 American Physical Society.

Campi G.,CNR Institute of Neuroscience | Campi G.,Rome International Center for Materials Science Superstripes | Innocenti D.,Ecole Polytechnique Federale de Lausanne | Bianconi A.,CNR Institute of Neuroscience | Bianconi A.,Rome International Center for Materials Science Superstripes
Journal of Superconductivity and Novel Magnetism | Year: 2015

New advances in X-ray diffraction, extended X-ray absorption fine structure (EXAFS), and X-ray absorption near edge structure (XANES) using synchrotron radiation have now provided compelling evidence for a short-range charge density wave phase (CDW) called striped phase in the CuO2 plane of all cuprate high-temperature superconductors. The CDW is associated with a bond order wave (BOW) and an orbital density wave (ODW) forming nanoscale puddles which coexist with superconducting puddles below Tc. The electronic CDW crystalline phase occurs around the hole doping 0.125 between the Mott charge transfer insulator and the 2D metal. The Van der Waals (VdW) theoretical model for a liquid of anisotropic extended objects proposed for supercooled water is used to describe the following: (a) the underdoped regime as a first spinodal regime of a “slightly doped charge transfer Mott insulator puddles coexisting with the striped polaronic CDW puddles;nd (b) the optimum doping regime as a second spinodal regime where striped polaronic CDW puddles coexist with the normal 2D metal puddles. This complex phase separation with three competing phases depends on the strength of the anisotropic electron-phonon interaction that favours the formation striped polaronic CDW phase. © 2015, Springer Science+Business Media New York.

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