Davide Innocenti

Inhomogeneity of charge-density-wave order and quenched disorder in a high-Tc superconductor.

It has recently been established that the high-transition-temperature (high-Tc) superconducting state coexists with short-range charge-density-wave order and quenched disorder arising from dopants and strain. This complex, multiscale phase separation invites the development of theories of high-temperature superconductivity that include complexity. The nature of the spatial interplay between charge and dopant order that provides a basis for nanoscale phase separation remains a key open question, because experiments have yet to probe the unknown spatial distribution at both the nanoscale and mesoscale (between atomic and macroscopic scale). Here we report micro X-ray diffraction imaging of the spatial distribution of both short-range charge-density-wave ‘puddles’ (domains with only a few wavelengths) and quenched disorder in HgBa2CuO4 + y, the single-layer cuprate with the highest Tc, 95 kelvin (refs 26, 27, 28). We found that the charge-density-wave puddles, like the steam bubbles in boiling water, have a fat-tailed size distribution that is typical of self-organization near a critical point. However, the quenched disorder, which arises from oxygen interstitials, has a distribution that is contrary to the usually assumed random, uncorrelated distribution. The interstitial-oxygen-rich domains are spatially anticorrelated with the charge-density-wave domains, because higher doping does not favour the stripy charge-density-wave puddles, leading to a complex emergent geometry of the spatial landscape for superconductivity.

Resonant and crossover phenomena in a multiband superconductor: Tuning the chemical potential near a band edge

Resonances in the superconducting properties, in a regime of crossover from BCS to mixed Bose-Fermi superconductivity, are investigated in a two-band superconductor where the chemical potential is tuned near the band edge of the second miniband generated by quantum confinement effects. The shape resonances at T=0 in the superconducting gaps belonging to the class of Feshbach-like resonances is manifested by interference effects in the superconducting gap at the first large Fermi surface when the chemical potential is in the proximity of the band edge of the second miniband. The case of a superlattice of quantum wells is considered and the amplification of the superconducting gaps at the Lifshitz transition of the type neck-collapsing of Fermi surface topology is clearly shown. The results are found to be in good agreement with available experimental data on a superlattice of honeycomb boron layers intercalated by Al and Mg spacer layers.

Shape resonance for the anisotropic superconducting gaps near a Lifshitz transition: the effect of electron hopping between layers

Multigap superconductivity modulated by quantum confinement effects in a superlattice of quantum wells is presented. Our theoretical BCS approach captures the low-energy physics of a shape resonance in the superconducting gaps when the chemical potential is tuned near a Lifshitz transition. We focus on the case of weak Cooper pairing coupling channels and strong pair exchange interaction driven by repulsive Coulomb interaction that allows us to use the BCS theory in the weak-coupling regime neglecting retardation effects, like in quantum condensates of ultracold gases. The calculated matrix element effects in the pairing interaction are shown to yield a complex physics near the particular quantum critical points due to Lifshitz transitions in multigap superconductivity. Strong deviations of the ratio 2Δ/Tc from the standard BCS value as a function of the position of the chemical potential relative to the Lifshitz transition point measured by the Lifshitz parameter are found. The response of the condensate phase to the tuning of the Lifshitz parameter is compared with the response of ultracold gases in the BCS–BEC crossover tuned by an external magnetic field. The results provide the description of the condensates in this regime where matrix element effects play a key role.

Multiscale distribution of oxygen puddles in 1/8 doped YBa2Cu3O6.67.

Despite intensive research a physical explanation of high Tc superconductors remains elusive. One reason for this is that these materials have generally a very complex structure making useless theoretical models for a homogeneous system. Little is known on the control of the critical temperature by the space disposition of defects because of lack of suitable experimental probes. X-ray diffraction and neutron scattering experiments used to investigate y oxygen dopants in YBa2Cu3O6+y lack of spatial resolution. Here we report the spatial imaging of dopants distribution inhomogeneity in YBa2Cu3O6.67 using scanning nano X-ray diffraction. By changing the X-ray beam size from 1 micron to 300 nm of diameter, the lattice inhomogeneity increases. The ordered oxygen puddles size distribution vary between 6–8 nm using 1 × 1 μm2 beam, while it is between 5–12 nm with a fat tail using the 300 × 300 nm2 beam. The increased inhomogeneity at the nanoscale points toward a network of superconducting puddles made of ordered oxygen interstitials.

Anomalous isotope effect near a 2.5 Lifshitz transition in a multi-band multi-condensate superconductor made of a superlattice of stripes

The doping dependent isotope effect on the critical temperature (Tc) 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.

A Possible Mechanism for Evading Temperature Quantum Decoherence in Living Matter by Feshbach Resonance

A new possible scenario for the origin of the molecular collective behaviour associated with the emergence of living matter is presented. We propose that the transition from a non-living to a living cell could be mapped to a quantum transition to a coherent entanglement of condensates, like in a multigap BCS superconductor. Here the decoherence-evading qualities at high temperature are based on the Feshbach resonance that has been recently proposed as the driving mechanism for high Tc superconductors. Finally we discuss how the proximity to a particular critical point is relevant to the emergence of coherence in the living cell.

Shape Resonance at a Lifshitz Transition for High Temperature Superconductivity in Multiband Superconductors

We discuss the shape resonance in the superconducting gaps of a two-band superconductor by tuning the chemical potential at a Lifshitz transition for Fermi surface neck collapsing and for spot appearing. The high temperature superconducting scenario for complex matter shows the coexistence of a first BCS condensate made of Cooper pairs in the first band and a second boson-like condensate made of bosons like bipolarons, in the second band where the chemical potential is tuned near a Lifshitz transition. The interband coupling controls the shape resonance in the pair exchange between the two condensates. We discuss the particular BCS–Bose crossover that occurs at the shape resonance tuning the Lifshitz parameter (the energy difference between the chemical potential and the Lifshitz topological transition) like tuning the external magnetic field for the Feshbach resonances in ultracold gases. This superconducting phase provides a particular case of topological superconductivity with multiple condensates of different winding numbers.

Shape Resonances in superconducting gaps in a 2DEG at oxide- oxide interface

In multiband superconductivity, the case where the single electron hopping between different Fermi surface spots of different symmetry is forbidden by selection rules is recently attracting a large interest. The focus is addressed to superconductivity made of multiple condensates with different symmetry where the chemical potential crosses a 2.5 Lifshitz transition. This can now be investigated experimentally by fine-tuning of the chemical potential in the range of tens meV around a band edge using gate voltage control. We discuss here the case of a superconducting two-dimensional electron gas (2DEG), at the interface between two insulating oxides confined within a slab of 5 nanometers thickness, where the electronic structure is made of subbands generated by quantum size effects. We obtain shape resonances in the superconducting gaps, characterisc gaps to Tc ratios and the BCS-BEC crossover in the upper subband for different pairing strength in the shallow Fermi surface, pointing toward the best configurations for enhanced superconductivity in 2DEG.

VUV Pump and Probe of Phase Separation and Oxygen Interstitials in La2NiO4+y Using Spectromicroscopy

While it is known that strongly correlated transition metal oxides described by a multi-band Hubbard model show microscopic multiscale phase separation, little is known about the possibility to manipulate them with vacuum ultraviolet (VUV), 27 eV lighting. We have investigated the photo-induced effects of VUV light illumination of a super-oxygenated La2NiO4+y single crystal by means of scanning photoelectron microscopy. VUV light exposure induces the increase of the density of states (DOS) in the binding energy range around Eb = 1.4 eV below EF. The photo-induced states in this energy region have been predicted due to clustering of oxygen interstitials by band structure calculations for large supercell of La2CuO4.125. We finally show that it is possible to generate and manipulate oxygen rich domains by VUV illumination as it was reported for X-ray illumination of La2CuO4+y. This phenomenology is assigned to oxygen-interstitials ordering and clustering by photo-illumination forming segregated domains in the La2NiO4+y surface.

Superstripes and Superconductivity in Complex Granular Matter

The role of phase separation and superstripes in high temperature superconductors and related materials has been discussed at the meeting in Erice, Italy, July 11–17, 2012. The focus was on the point where we are after 25 years of research on the key role of complex structural, electronic, and magnetic matter in the mechanism of high temperature superconductivity.