JVJulien Varignon

Julien Varignon

Normandie Université
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Julien Varignon has a total of 95 co-authors. Here are the top co-authors by number of shared publications:

Latest Publications

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    Importance of the crystal structure for superconductivity in infinite-layered nickel oxides

    by Álvaro Adrián Carrasco Álvarez, Wilfrid Prellier, Manuel Bibes, Julien Varignon

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    Orbital-order as the driving mechanism for superconductivity in ruthenates

    by Álvaro Adrián Carrasco Álvarez, Sébastien Petit, Wilfrid Prellier, Manuel Bibes, Julien Varignon

    Several materials transition from an insulating to a superconducting state by reducing the strength of the electron-phonon coupling associated with charge and bond orderings provided that the coupling remains strong enough to produce Cooper pairs. While the Jahn-Teller effect is at the core of a strong electron-phonon coupling producing insulating states and orbital and bond orderings, its implication in superconductivity remains unobserved. Here, with parameter-free first-principles calculations, we reveal that superconductivity in A2RuO4 (A = Sr, Ca) emerges due to an electron-phonon mechanism associated with the proximity of an orbital and bond-ordered phase. The model predicts critical temperatures Tc of 0.5–1.65 K in bulk Sr2RuO4 and 63–73 K in pressured Ca2RuO4, in agreement with experiments. Our results suggest that phonons strongly coupled to electrons, such as those involved in charge disproportionation or Jahn-Teller effects and inducing band gaps in various oxides, could also serve as mediators of Cooper pairs in metallic phases.

    Cited by 8
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    Electron-phonon mediated superconductivity in La6Ni5O12 nickel oxides

    by Álvaro Adrián Carrasco Álvarez, Sébastien Petit, Lucia Iglesias, Manuel Bibes, Wilfrid Prellier, Julien Varignon

    Cited by 9
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    Decisive role of hybridized electronic states on the appearance of superconductivity in Ba1−xKxMO3 (M=Bi,Sb)

    by Julien Varignon

    The observation of superconductivity (SC) in hole-doped antimonates Ba1−xKxSbO3 (BKSO), isoelectronic compounds to bismuthate Ba1−xKxBiO3 (BKBO) SCs, enables a pathway to clarify the role of hybridizations between electronic states of metals and ligands on the emergence of SC. Herein, using first-principles simulations, the localized electronic structure of antimonates is demonstrated to produce various charge and bond orderings (CBOs) up to x=0.6, hence hindering the appearance of a SC state. This is in contrast with bismuthates where the formation of charge orderings is suppressed by doping effects and their native hybridized electronic structure. Although quenched by doping effects, the CBOs mediate the Cooper pair formation, and the resulting electron-phonon coupling reproduces the experimental facts such as a larger Tc in BKSO than in BKBO for x>0.65 and a maximal Tc of ∼30K observed at x=0.4−0.5 for BKBO. The latter point is ascribed to a soft phonon mode at x=0.5, while the former observation is ascribed to an intrinsically stronger coupling between lattice distortions and electrons in BaSbO3. Although materials with a localized electronic structure could warrant larger T′cs, the main drawback is the resilience of such compounds to form a metallic state, and hence SC emerges far away of soft phonons possessing a strong coupling with the electronic structure.

    Cited by 2
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    Charge ordering as the driving mechanism for superconductivity in rare-earth nickel oxides

    by Álvaro Adrián Carrasco Álvarez, Lucia Iglesias, Sébastien Petit, Wilfrid Prellier, Manuel Bibes, Julien Varignon

    Superconductivity is one of the most intriguing properties of matter, described by an attractive interaction that binds electrons into Cooper pairs. To date, the highest critical temperature at ambient conditions is achieved in copper oxides. While layered nickel oxides were long proposed to be analogous to cuprates, superconductivity was only demonstrated in 2019 albeit without clarifying the pairing mechanism. Here we use density functional theory to show that superconductivity in nickelates is driven by an electron-phonon coupling originating from a charge ordering. Due to an intrinsic electronic instability in half-doped compounds, Ni1.5+ cations dismutate into more stable Ni+ and Ni2+ cations, which is accompanied by a bond disproportionation of NiO4 complexes producing an insulating charge-ordered state. Once doping suppresses the instability, the bond disproportionation vibration is sufficient to reproduce the key characteristic of nickelates observed experimentally, notably the dome of Tc as a function of doping content. These phenomena are identified if relevant degrees of freedom as well as an exchange-correlation functional that sufficiently amends self-interaction errors are involved in the simulations. Finally, despite the presence of correlation effects inherent to 3d elements that favor the formation of local Ni spins, the mechanism behind the formation of Cooper pairs in nickelate superconductors appears similar to that of nonmagnetic bismuth oxide superconductors.

    Cited by 19
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    Unexpected Competition between Ferroelectricity and Rashba Effects in Epitaxially Strained SrTiO3

    by Julien Varignon

    The Rashba parameter αR is usually assumed to scale linearly with the amplitude of polar displacements by construction of the spin-orbit interaction. On the basis of first-principles simulations, ferroelectric phases of SrTiO3 reached under epitaxial compressive strain are characterized by (i) large Rashba effects at the bottom of the conduction band near the paraelectric-ferroelectric boundary and (ii) an unexpected suppression of the phenomena when the amplitude of polar displacements increases. This peculiar behavior is ascribed to the inverse dependence of the Rashba parameter with the crystal field ΔCF induced by the polar displacements that alleviates the degeneracy of Ti t2g states and annihilates the Rashba effects. Although αR has intrinsically a linear dependance on polar displacements, the latter becomes antagonist to Rashba phenomena at large polar mode amplitude. Thus, the Rashba coefficient may be bound to an upper value.

    Cited by 8
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    Scanning Precession Electron Tomography (SPET) for Structural Analysis of Thin Films along Their Thickness

    by Sara Passuti, Julien Varignon, Adrian David, Philippe Boullay

    Accurate structure analysis of epitaxial perovskite thin films is a fundamental step towards the ability to tune their physical properties as desired. Precession-assisted electron diffraction tomography (PEDT) has proven to be an effective technique for performing ab initio structure solutions and refinements for this class of materials. As the film thickness or the region of interest (ROI) decrease in size, the capacity to collect PEDT data with smaller electron beams is a key parameter and ROI tracking becomes a major issue. To circumvent this problem, we considered here an alternative approach to acquiring data by combining PEDT with a scan over an area, extracting the intensities collected at different positions and using them to perform accurate structure refinements. As a proof of concept, a Scanning Precession Electron Tomography (SPET) experiment is performed on a 35 nm thick perovskite PrVO3(PVO) film deposited on a SrTiO3 (STO) substrate. This way, it was possible to detect small changes in the PVO structure along the film thickness, from the variation in unit cell parameters to atomic positions. We believe that SPET has the potential to become the standard procedure for the accurate structure analysis of ROIs as small as 10 nm.

    Cited by 15
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    Coexistence and coupling of ferroelectricity and magnetism in an oxide two-dimensional electron gas

    by Julien Bréhin, Yu Chen, Maria D’aNtuono, Sara Varotto, Daniela Stornaiuolo, Cinthia Piamonteze, Julien Varignon, Marco Salluzzo, Manuel Bibes

    Cited by 47
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    Strongly constrained and appropriately normed density functional theory exchange-correlation functional applied to rare earth oxides

    by Alvaro Adrian Carrasco Alvarez, Manuel Bibes, Wilfrid Prellier, Julien Varignon

    Density functional theory (DFT) is one of the main tools for studying the electronic structure of solids and molecules. Nevertheless, one of the main drawbacks of the implementation of DFT is the so-called self-interaction error (SIE) that can yield undesired delocalization errors and ultimately results in the prediction of metals instead of experimentally observed insulators. These SIEs can be amended by using more evolved exchange-correlation functionals than standard local density approximation such as the recent meta-generalized gradient approximation strongly constrained and appropriately normalized (SCAN) functional that is successful in describing electronic properties of 3d transition metal oxides. Nevertheless, the ability of such a functional to describe electronic properties of materials involving more localized states such as 4f orbitals is rather elusive. Here, we show that, even though SCAN can sometimes predict the insulating character of some compounds, it often fails in predicting the correct band edge orbital character of insulators. By comparing our SCAN results with benchmark simulations obtained with more accurate hybrid DFT calculations, we ascribe this failure to insufficiently amended SIEs by SCAN that results in an underestimation of Hund's splitting associated with 4f states. Thus, although appropriate for 3d transition metal elements, the SCAN functional is not yet a sufficient platform for studying electronic properties of materials involving rare-earth elements where 4f states play a key role in the properties.

    Cited by 15
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    Origin of superconductivity in hole doped SrBiO3 bismuth oxide perovskite from parameter-free first-principles simulations

    by Julien Varignon

    The recent discovery of nickel oxide superconductors have highlighted the importance of first-principles simulations for understanding the formation of the bound electrons at the core of superconductivity. Nevertheless, superconductivity in oxides is often ascribed to strong electronic correlation effects that density functional theory (DFT) cannot properly take into account, thereby disqualifying this technique. Being isostructural to nickel oxides, Sr1-xKxBiO3 superconductors form an ideal testbed for unveiling the lowest theory level needed to model complex superconductors and the underlying pairing mechanism yielding superconductivity. Here I show that parameter-free DFT simulations capture all the experimental features and related quantities of Sr1-xKxBiO3 superconductors, encompassing the prediction of an insulating to metal phase transition upon increasing the K doping content and of an electron-phonon coupling constant of 1.22 in sharp agreement with the experimental value of 1.3 ± 0.2. The proximity of a disproportionated phase is further demonstrated to be a prerequisite for superconductivity in bismuthates.

    Cited by 21
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