arXiv subjects

Davide Innocenti

Publications and source records attributed to Davide Innocenti.

12 recordsLinked to original sources

arXiv2018

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 multi-band Hubbard model show microscopic multiscale phase separation little is known on the possibility to manipulate them with vacuum ultraviolet (VUV) 27 eV lighting. We have investigated the photo-induced effects of VUV 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 as due to clustering of oxygen interstitials by band structure calculations for large supercell of La2CuO4.125. We finally show that it 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

cond-mat.str-el↗

arXiv2017

Possible Fano resonance for high Tc multi-gap superconductivity in p-Terphenyl doped by K at the Lifshitz transition

Recent experiments have reported the emergence of high temperature superconductivity with critical temperature $T_c$ between 43K and 123K in a potassium doped aromatic hydrocarbon para-Terphenyl or p-Terphenyl. This achievement provides the record for the highest Tc in an organic superconductor overcoming the previous record of Tc=38 K in Cs3C60 fulleride. Here we propose that the driving mechanism is the quantum resonance between superconducting gaps near a Lifshitz transition which belongs to the class of Fano resonances called shape resonances. For the case of p-Terphenyl our numerical solutions of the multi gap equation shows that high Tc is driven by tuning the chemical potential by K doping and it appears only in a narrow energy range near a Lifshitz transition. At the maximum critical temperature, Tc=123K, the condensate in the appearing new small Fermi surface pocket is in the BCS-BEC crossover while the Tc drops below 0.3 K where it is in the BEC regime. Finally we predict the experimental results which can support or falsify our proposed mechanism: a) the variation of the isotope coefficient as a function of the critical temperature and b) the variation of the gaps and their ratios 2Delta/Tc as a function of Tc.

cond-mat.supr-con↗

arXiv2014

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 YBa2Cu3O 6+y lack of spatial resolution. Here we report the spatial imaging of dopants distribution in-homogeneity 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 1x1 μm2 beam, while it is between 5-12 nm with a fat tail using the 300x300 nm2 beam. The increased inhomogeneity at the nanoscale points toward an intrinsic granular complexity.

cond-mat.supr-con↗

arXiv2013

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 the 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 focus on the case of the BCS-BEC transition in the upper subband for different pairing strength in the shallow Fermi surface.

cond-mat.supr-con↗

arXiv2013

Superstripes : superconductivity in complex granular striped 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 July 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.

cond-mat.supr-con↗

arXiv2013

Interface reconstruction in superconducting CaCuO2/SrTiO3 superlattices: A hard x-ray photoelectron spectroscopy study

Here we report about the interface reconstruction in the recently discovered superconducting artificial superlattices based on insulating CaCuO2 and SrTiO3 blocks. Hard x-ray photoelectron spectroscopy shows that the valence bands alignment prevents any electronic reconstruction by direct charge transfer between the two blocks. We demonstrate that the electrostatic built-in potential is suppressed by oxygen redistribution in the alkaline earth interface planes. By using highly oxidizing growth conditions, the oxygen coordination in the reconstructed interfaces may be increased, resulting in the hole doping of the cuprate block and thus in the appearance of superconductivity.

cond-mat.str-el↗

arXiv2012

Small scale rotational disorder observed in epitaxial graphene on SiC(0001)

Interest in the use of graphene in electronic devices has motivated an explosion in the study of this remarkable material. The simple, linear Dirac cone band structure offers a unique possibility to investigate its finer details by angle-resolved photoelectron spectroscopy (ARPES). Indeed, ARPES has been performed on graphene grown on metal substrates but electronic applications require an insulating substrate. Epitaxial graphene grown by the thermal decomposition of silicon carbide (SiC) is an ideal candidate for this due to the large scale, uniform graphene layers produced. The experimental spectral function of epitaxial graphene on SiC has been extensively studied. However, until now the cause of an anisotropy in the spectral width of the Fermi surface has not been determined. In the current work we show, by comparison of the spectral function to a semi-empirical model, that the anisotropy is due to small scale rotational disorder ($\sim\pm$ 0.15$^{\circ}$) of graphene domains in graphene grown on SiC(0001) samples. In addition to the direct benefit in the understanding of graphene's electronic structure this work suggests a mechanism to explain similar variations in related ARPES data.

cond-mat.mes-hall↗

arXiv2012

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 focus on multi-band effects that arises in nano-structures and in density wave metals (like spin density wave or charge density wave) as a result of the band folding. We consider a superlattice of quantum stripes with finite hopping between stripes near a 2.5 Lifshitz transition for appearing 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 electron wave-functions are obtained by solving the Schrödinger equation for a one-dimensional modulated potential barrier. The k-dependent and energy dependent superconducting gaps are calculated using the k-dependent anisotropic Bardeen-Cooper-Schrieffer (BCS) multi-gap equations solved joint with the density equation, according with the Leggett approach currently used now in ultracold fermionic gases. 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. On the contrary Tc reaches its maximum and the isotope coefficient vanishes at the crossover from a polaronic condensate to a BCS condensate in the new appearing sub-band.

cond-mat.supr-con↗

arXiv2011

Electronic structure of graphene on single crystal copper substrates

The electronic structure of graphene on Cu(111) and Cu(100) single crystals is investigated using low energy electron microscopy, low energy electron diffraction and angle resolved photoemission spectroscopy. On both substrates the graphene is rotationally disordered and interactions between the graphene and substrate lead to a shift in the Dirac crossing of $\sim$ -0.3 eV and the opening of a $\sim$ 250 meV gap. Exposure of the samples to air resulted in intercalation of oxygen under the graphene on Cu(100), which formed a ($\sqrt{2} \times 2\sqrt{2}$)R45$^{\rm o}$ superstructure. The effect of this intercalation on the graphene $π$ bands is to increase the offset of the Dirac crossing ($\sim$ -0.6 eV) and enlarge the gap ($\sim$ 350 meV). No such effect is observed for the graphene on Cu(111) sample, with the surface state at $Γ$ not showing the gap associated with a surface superstructure. The graphene film is found to protect the surface state from air exposure, with no change in the effective mass observed.

cond-mat.mes-hall↗

arXiv2010

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 Femi 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

cond-mat.supr-con↗

arXiv2010

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

The 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 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 $2Δ/T_c$ 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.

cond-mat.supr-con↗

arXiv2010

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 mini-band 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 mini-band. The case of a superlattice of quantum wells is considered and the amplification of the superperconducting gaps at the 3D-2D Fermi surface topological transition 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.

cond-mat.supr-con↗