arXiv subjects

Antonio Bianconi

Publications and source records attributed to Antonio Bianconi.

At least 55 records · Page 3Linked to original sources

arXiv2017

The road map toward room temperature superconductivity: manipulating different pairing channels in systems composed of multiple electronic components

While it is known that the amplification of the superconducting critical temperature Tc is possible in a system of multiple electronic components in comparison with a single component system, many different road maps for room temperature superconductivity have been proposed for a variety of multicomponent scenarios. Here we focus on the scenario where the first electronic component is assumed to have a vanishing Fermi velocity corresponding to a case of the intermediate polaronic regime, and the second electronic component is in the weak coupling regime with standard high Fermi velocity using a mean field theory for multiband superconductivity. This roadmap is motivated by compelling experimental evidence for one component in the proximity of a Lifshitz transition in cuprates, diborides and iron based superconductors. By keeping a constant and small exchange interaction between the two electron fluids, we search for the optimum coupling strength in the electronic polaronic component which gives the largest amplification of the superconducting critical temperature in comparison with the case of a single electronic component.

cond-mat.supr-con↗

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↗

arXiv2016

Dislocations as a boundary between charge density wave and oxygen rich phases in a cuprate high temperature superconductor

Multiple functional ionic and electronic orders are observed in high temperature superconducting cuprates. The charge density wave order is one of them and it is spatially localized in spatial regions of the material. It is also known that the oxygen interstitials introduced by chemical intercalation self-organize in different oxygen rich regions corresponding with hole rich regions in the Cu$O_2$ layers left empty by the charge density wave order domains. However, what happens in between these two order is not known, and neither there is a method to control this spatial separation. Here we demonstrate by using scanning nano X-ray diffraction, that dislocations or grain boundaries in the material can act as boundary between charge density wave and oxygen rich phases in a optimally doped La$_2$CuO$_4$$_+$$_y$ high temperature superconductor. Dislocations can be used therefore to control the anti-correlation of the charge density wave order with the oxygen interstitials in specific portion of the material.

cond-mat.supr-con↗

arXiv2016

Lattice, Charge and Spin phase inhomogeneity in complex striped quantum matter

The Superstripes 2016 conference, held on June 23-29, 2016 in the island of Ischia in Italy celebrated the 20th anniversary of this series of conferences. For 20 years structural, electronic, and magnetic phase inhomogeneities in quantum matter have been the scientific focus for a growing physics community interested in complexity in quantum matter. It has been the meeting point for different scientific communities facing the challenging project to unveil the complex space and time landscapes in quantum matter. The interesting spatial inhomogeneity length scale of multiple coexisting phase ranges from atomic to mesoscopic and the time fluctuations are spread over multiple time scales. The response of these materials changes using different experimental techniques with different spatial and time resolution probing different aspects of the quantum complexity.

cond-mat.supr-con↗

arXiv2016

Fe-As bond fluctuations in a double-well potential in LaFeAsO

While for a long time iron based superconductors have been investigated looking at the average crystalline structure there is now growing interest on the divergence of local from the average structure. Here we use advanced data analysis of EXAFS, a fast and local probe, which has key advantages compared to crystallographic measurements, in unveiling the dynamics of the local structure. The results show that the Fe-As pair oscillates in the double-well potential both in undoped and in cobalt doped LaFeAsO crystals. The parameters and the characteristics of the double-well potential are determined by curve fits of the polarized EXAFS data. Temperature dependent Fe-As pair distribution and tunneling frequency are derived from selected model of two potential wells. We observe the softening of the Fe-As oscillations at the tetragonal to orthorhombic structural phase transition at 150 K in LaFeAsO. A weak broad softening is observed while decreasing the temperature from 70 to 40 K in superconducting LaFe0.89Co0.11AsO which could be correlated with the onset of short range charge density wave order.

cond-mat.supr-con↗

arXiv2016

Temperature dependent As K-edge EXAFS studies of LaFe1-xCoxAsO (x = 0.0 and 0.11) single crystals

We report the experimental results of temperature dependent polarized As K-edge extended x-ray absorption fine structure (EXAFS) of LaFe1-xCoxAsO (x=0.0 and 0.11) single-crystals. By aligning the Fe-As bond direction in the direction of the x-ray beam polarization we have been able to identify an anomaly in the Fe-As bond correlations at the tetragonal to orthorhombic transition at 150K, while previous investigations with standard unpolarized EXAFS of undoped LaFeAsO powder samples were not able to detect any such anomaly. Using our approach we have been able to identify in the superconducting doped sample, LaFe0.89Co0.11AsO, a broad anomaly around 60 K. The low temperature anomaly has good correlations with the temperature dependence of several properties like resistivity, magnetic susceptibility, linear thermal expansion, etc indicating the emergence of the dynamical oscillations of the Fe - As pairs

cond-mat.supr-con↗

arXiv2016

Shape-resonant superconductivity in nanofilms: from weak to strong coupling

Ultrathin superconductors of different materials are becoming a powerful platform to find mechanisms for enhancement of superconductivity, exploiting shape resonances in different superconducting properties. Here we evaluate the superconducting gap and its spatial profile, the multiple gap components, and the chemical potential, of generic superconducting nanofilms, considering the pairing attraction and its energy scale as tunable parameters, from weak to strong coupling, at fixed electron density. Superconducting properties are evaluated at mean field level as a function of the thickness of the nanofilm, in order to characterize the shape resonances in the superconducting gap. We find that the most pronounced shape resonances are generated for weakly coupled superconductors, while approaching the strong coupling regime the shape resonances are rounded by a mixing of the subbands due to the large energy gaps extending over large energy scales. Finally, we find that the spatial profile, transverse to the nanofilm, of the superconducting gap acquires a flat behavior in the shape resonance region, indicating that a robust and uniform multigap superconducting state can arise at resonance.

cond-mat.supr-con↗

arXiv2016

From the pion cloud of Tomonaga to the electron pairs of Schrieffer: many body wave functions from nuclear physics to condensed matter physics

It is well known that diverse pieces of models and physical ideas coming from different areas of physics converged in the BCS theory of superconductivity. On the contrary it is little known that the formalism developed in the Tomonaga quantum field theory of the pion-nucleon system was an important ingredient for the development of BCS theory. We discuss the evolution of these ideas in quantum field theory providing an unconventional historical perspective.

cond-mat.supr-con↗

arXiv2016

High temperature superconductivity in sulfur hydride under ultrahigh pressure: A complex superconducting phase beyond conventional BCS

The recent report of superconductivity under high pressure at the record transition temperature of Tc=203K in sulfur hydride has been identified as conventional in view of the observation of an isotope effect upon deuteration. Here it is demonstrated that conventional theories of superconductivity in the sense of BCS or Eliashberg formalisms can neither account for the observed values of Tc nor the pressure dependence of the isotope coefficient. The only way out of the dilemma is a multi-band approach of superconductivity where already small interband coupling suffices to achieve the high values of Tc together with the anomalous pressure dependent isotope effect. In addition, it is shown that anharmonicity of the hydrogen bonds vanishes under pressure whereas anharmonic phonon modes related to sulfur are still active

cond-mat.supr-con↗

arXiv2016

Breakdown of the Migdal approximation at Lifshitz transitions with giant zero-point motion in H3S superconductor

While 203 K high temperature superconductivity in H3S has been interpreted by BCS theory in the dirty limit here we focus on the effects of hydrogen zero-point-motion and the multiband electronic structure relevant for multigap superconductivity near Lifshitz transitions. We describe how the topology of the Fermi surfaces evolves with pressure giving different Lifshitz-transitions. A neck-disrupting Lifshitz-transition (type 2) occurs where the van Hove singularity, vHs, crosses the chemical potential at 210 GPa and new small 2D Fermi surface portions appear with slow Fermi velocity where the Migdal-approximation becomes questionable. We show that the neglected hydrogen zero-point motion ZPM, plays a key role at Lifshitz transitions. It induces an energy shift of about 600 meV of the vHs. The other Lifshitz-transition (of type 1) for the appearing of a new Fermi surface occurs at 130 GPa where new Fermi surfaces appear at the Gamma point of the Brillouin zone here the Migdal-approximation breaks down and the zero-point-motion induces large fluctuations. The maximum Tc=203K occurs at 160 GPa where Ef/w0=1 in the small Fermi surface pocket at Gamma point. A Feshbach-like resonance between a possible BEC-BCS condensate at Gamma and the BCS condensate in different k-space spots is proposed.

cond-mat.supr-con↗

arXiv2015

Electronic structure of superoxygenated La2NiO4 domains with ordered oxygen interstitials

The electronic structure of La2NiO4+d, where additional oxygen interstitials are forming stripes along (1,1,0), are presented. Spin-polarized calculations show that ferromagnetism on Ni sites is reduced near the stripes and enhanced far from the stripes. Totally the magnetic moment becomes reduced because of oxygen interstitials. It is suggested that the oxygen interstitial concentration in oxygen rich domains in nickelates suppress magnetism and give multiband metallic domains.

cond-mat.mtrl-sci↗

arXiv2015

Superconductivity above the lowest Earth temperature in pressurized sulfur hydride

A recent experiment has shown a macroscopic quantum coherent condensate at 203 K, about 19 degrees above the coldest temperature recorded on the Earth, 184 K, in pressurized sulfur hydride. This discovery is relevant not only in material science and condensed matter but also in other fields ranging from quantum computing to quantum physics of living matter. It has given the start to a gold rush looking for other macroscopic quantum coherent condensates in hydrides at the temperature range of living matter 200<Tc<400K. We present here a review of the experimental results and the theoretical works and we discuss the Fermiology of H3S focusing on Lifshitz transitions as a function of pressure. We discuss the possible role of the shape resonance near a neck disrupting Lifshitz transition, in the Bianconi-Perali Valletta (BPV) theory, for rising the critical temperature in a multigap superconductor, as the Feshbach resonance rises the critical temperature in Fermionic ultracold gases.

cond-mat.supr-con↗

arXiv2015

Lifshitz transitions and zero point lattice fluctuations in sulfur hydride showing near room temperature superconductivity

Emerets's experiments on pressurized sulfur hydride have shown that H3S metal has the highest known superconducting critical temperature Tc=203K. The Emerets data show pressure induced changes of the isotope coefficient between 0.25 and 0.5, in disagreement with Eliashberg theory which predicts a nearly constant isotope coefficient. We assign the pressure dependent isotope coefficient to Lifshitz transitions induced by pressure and zero point lattice fluctuations. It is known that pressure could induce changes of the topology of the Fermi surface, called Lifshitz transitions, but were neglected in previous papers on the H$_3$S superconductivity issue. Here we propose that H3S is a multi-gap superconductor with a first condensate in the BCS regime (in the large Fermi surface with high Fermi energy) which coexists with a second condensates in the BCS-BEC crossover regime (located on a small Fermi surface spots with small Fermi energy) near the $Γ$ and M point. We discuss the need of Bianconi-Perali-Valletta (BPV) superconductivity theory for superconductivity in H3S. It includes both the correction of the chemical potential due to pairing and the configuration interaction between different condensates, neglected by the Eliashberg theory. Here the shape resonance in superconducting gaps, similar to Feshbach resonance in ultracold gases, gives a relevant contribution to amplify the critical temperature. Therefore this work provides some key tools needed in the search for new room temperature superconductors.

cond-mat.supr-con↗

arXiv2015

Superstripes in the Low Energy Physics of Complex Quantum Matter at the Mesoscale

Quantum physics in the 20th century was proposed to understand the phenomenology of atomic world at short length scale (below one nanometer) and it was developed to study nuclear and subnuclear world at the lowest possible spatial scale and at the highest possible energy. Today in the 21st century the hot topic is to understand the mesoscale world extending in the range between 1 nanometer and 100 microns where the energy range of interactions is between 5 meV and 250 meV. From these studies a new Low Energy Physics (LEP) of many body collective quantum phenomena is emerging. The international conference Superstripes - 2014, the last of the series of Superstripes conferences has been a key event for the scientists active in this new low energy physics. The focus has been on a) the competition between CDW and multigap superconductivity, and on b) the nanoscale phase separation in all high temperature superconductors. The conferences of the superstripes series are now the yearly major international event in the field of quantum phenomena in the complex quantum matter at the mesoscale.

cond-mat.supr-con↗

arXiv2014

Intrinsic arrested nanoscale phase separation near a topological Lifshitz transition in strongly correlated two-band metals

The arrested nanoscale phase separation in a two-band Hubbard model for strongly correlated charge carriers is shown to occur in a particular range in vicinity of the topological Lifshitz transition, where the Fermi energy crosses the bottom of the narrow band and a new sheet of the Fermi surface related to the charge carriers of the second band comes into play. We determine the phase separation diagram of this two-band Hubbard model as a function of two variables, the charge carrier density and the energy shift between the chemical potential and the bottom of the second band. In this phase diagram, we first determine a line of quantum critical points for the Lifshitz transition and find criteria for the electronic phase separation resulting in an inhomogeneous charge distribution. Finally, we identify the critical point in presence of a variable long-range Coulomb interaction where the scale invariance of the coexisting phases with different charge densities appears. We argue that this point is relevant for the regime of scale invariance of the nanoscale phase separation in cuprates like it was first observed in La$_2$CuO$_{4.1}$.

cond-mat.str-el↗

arXiv2014

Changes of statistical structural fluctuations unveils an early compacted degraded stage of PNS myelin

Degradation of the myelin sheath is a common pathology underlying demyelinating neurological diseases from Multiple Sclerosis to Leukodistrophies. Although large malformations of myelin ultrastructure in the advanced stages of Wallerian degradation is known, its subtle structural variations at early stages of demyelination remains poorly characterized. This is partly due to the lack of suitable and non-invasive experimental probes possessing sufficient resolution to detect the degradation. Here we report the feasibility of the application of an innovative non-invasive local structure experimental approach for imaging the changes of statistical structural fluctuations in the first stage of myelin degeneration. Scanning micro X-ray diffraction, using advances in synchrotron x-ray beam focusing, fast data collection, paired with spatial statistical analysis, has been used to unveil temporal changes in the myelin structure of dissected nerves following extraction of the Xenopus laevis sciatic nerve. The early myelin degeneration is a specific ordered compacted phase preceding the swollen myelin phase of Wallerian degradation. Our demonstration of the feasibility of the statistical analysis of SmXRD measurements using biological tissue paves the way for further structural investigations of degradation and death of neurons and other cells and tissues in diverse pathological states where nanoscale structural changes may be uncovered.

q-bio.BM↗

arXiv2014

Percolative superconductivity in La2CuO4.06 by lattice granularity patterns with scanning micro X-ray absorption near edge structure

The simplest cuprate superconductor La2CuO4+y with mobile oxygen interstitials exhibits a clear phase separation, but only recently a bulk multiscale structural phase separation has been observed by using scanning micro X-ray diffraction. Here we get further information on the structural phase separation, using local probe X-ray absorption near edge structure. The spatial distribution of superconducting units is a key parameter controlling percolative superconductivity in complex matter with dispersed superconducting units. These oxides form super-molecular architectures made of superconducting atomic monolayers intercalated by spacers. Oxygen interstitials enter into the rocksalt La2O2+y spacer layers forming oxygen interstitials rich puddles and poor puddles. Their spatial distribution has been determined by using scanning La L3-edge micro X-ray absorption near edge structure. Percolating networks of oxygen rich puddles are observed in different micrometer size portions of the crystals. Moreover, the complex surface resistivity shows two jumps associated to the onset of intra-puddle and inter-puddles percolative superconductivity. The similarity of oxygen doped La2CuO4+y, with the well established phase separation in iron selenide superconductors is also discussed.

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↗