Julien Varignon

• PhD
• Professor (Associate) at École Nationale Supérieure d'Ingénieurs de Caen

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...

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 str...

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 ba...

Four new metallophosphonates with the chemical formulae M(H 2 O)PO 3-S 2 C 12 H 7 (M = Cu, Zn) and M(H 2 O) 2 (PO 2 OH-S 2 C 12 H 7) 2 (M = Mn, Co) were synthesized using a hydrothermal route from the original bent rigid thianthrene-2-ylphosphonic acid (TPA). This organic precursor crystallizes in a non-centro-symmetric space group P2 1 2 1 2 1 and...

Nickel oxide superconductors offer an alternative playground for understanding the formation of Cooper pairs in correlated materials such as the famous cuprates. By studying the Lan+1NinO2n+2 phase diagram on the basis of hybrid and spin-polarized density functional theory simulations, we reveal the existence of charge and bond ordered (CBO) insula...

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 fil...

Multiferroics are compounds in which at least two ferroic orders coexist, typically ferroelectricity and some form of magnetism. While magnetic order can arise in both insulating and metallic compounds, ferroelectricity is in principle only allowed in insulators, although ferroelectric metals have been proposed and several two-dimensional systems h...

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 experi...

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...

Superconductivity is one of the most intriguing properties of matter described by an attractive interaction that bounds 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 demo...

The recent discovery nickel oxides 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 p...

Epitaxial strain is a useful handle to engineer the physical properties of perovskite oxide materials. Here, we apply it to orthorhombic chromites that are a family of antiferromagnets showing fruitful functionalities as well as strong spin–lattice coupling via antisymmetric exchange interaction along Cr–O–Cr bonds. Using pulsed laser deposition, w...

Multiferroics are compounds in which at least two ferroic orders coexist - typically (anti)ferromagnetism and ferroelectricity. While magnetic order can arise in both insulating and conducting compounds, ferroelectricity is in principle not allowed in metals although a few two-dimensional (semi)metals were reported to behave as ferroelectrics. Yet,...

Rare-earth nickelates RNiO2 adopting an infinite-layer phase show superconductivity once La, Pr, or Nd issubstituted by a divalent cation. Either in the pristine or doped form, these materials are reported to adopt a high-symmetry, perfectly symmetric, P4/mmm tetragonal cell. Nevertheless, bulk compounds are scarce, hinderinga full understanding of...

In addition to the well-known dependence of band gaps of compounds on crystal symmetry and the orbital character of the constituent atoms, d-electron perovskites manifest an additional dependence of gaps on the magnetic spin configuration, a dependence deciding both metal-insulator transitions and magnetic application. We find via electronic struct...

The quest for a Ni-based oxide analog to cuprate Cu2+(d9) superconductors was long known to require a reduced form of Ni1+(d9) as in A3+Ni1+O2, being an extremely oxygen-poor form of the usual A3+Ni3+O3 compound. Through CaH2 chemical reduction of a parent R3+Ni3+O3 perovskite form, superconductivity was recently achieved in Sr-doped NdNiO2 on a Sr...

Rare-earth nickelates RNiO$_2$ adopting an infinite-layer phase show superconductivity once La, Pr or Nd are substituted by a divalent cation. Either in the pristine or doped form, these materials are reported to adopt a high symmetry, perfectly symmetric, P4/mmm tetragonal cell. Nevertheless, bulk compounds are scarce, hindering a full understandi...

Rare-earth nickelates RNiO$_2$ adopting an infinite-layer phase show superconductivity once La, Pr or Nd are substituted by a divalent cation. Either in the pristine or doped form, these materials are reported to adopt a high symmetry, perfectly symmetric, P4/mmm tetragonal cell. Nevertheless, bulk compounds are scarce, hindering a full understandi...

Understanding the controlling principles of band gaps trends in d electron perovskites is needed both for gauging metal-insulator transitions, as well as their application in catalysis and doping. The magnitude of this band gap is rather different for different magnetic spin configurations. We find via electronic structure theory that the factors t...

Rare-earth nickelates R3+Ni3+O3 (R=Lu−Pr, Y) show a striking metal-insulator transition in their bulk phase whose temperature can be tuned by the rare-earth radius. These compounds are also the parent phases of the newly identified infinite layer RNiO2 superconductors. Although intensive theoretical works have been devoted to understand the origin...

Perovskite rare-earth titanates are prototypical Mott insulators in which Ti3+ ions with 3d1 electronic configuration exhibit ferromagnetic or antiferromagnetic spin order, depending on the rare-earth size. This peculiar magnetic behavior has, however, been barely studied with element-specific probes, either in bulk or in thin films. The recent fin...

Rare-earth nickelates R$^{3+}$Ni$^{3+}$O$_3$ (R=Lu-Pr, Y) show a striking metal-insulator transition in their bulk phase whose temperature can be tuned by the rare-earth radius. These compounds are also the parent phases of the newly identified infinite layer RNiO2 superconductors. Although intensive theoretical works have been devoted to understan...

The Jahn-Teller distortion that can remove electronic degeneracies in partially occupied states and results in systematic atomic displacements is a common underlying feature to many of the intriguing phenomena observed in 3d perovskites, encompassing magnetism, superconductivity, orbital ordering, and colossal magnetoresistance. Although the semina...

In parallel with the revival of interest for magneto-electric multiferroic materials in the beginning of the century, first-principles simulations have grown incredibly in efficiency during the last two decades. Density functional theory calculations, in particular, have so become a must-have tool for physicists and chemists in the multiferroic com...

The existence of band gaps in Mott insulators such as perovskite oxides with partially filled 3d shells has been traditionally explained in terms of strong, dynamic interelectronic repulsion codified by the on-site repulsion energy U in the Hubbard Hamiltonian. The success of the “DFT+U approach” where an empirical on-site potential term U is added...

Compounds with the ABX3 perovskite structure (X=F, O), where B is a 3d transition metal element, have fascinated the solid-state communities for decades due to their broad range of properties and functionalities, encompassing magnetism, superconductivity or colossal magnetoresistance. A frequent phenomenon behind many of these properties is the Jah...

Transition-metal oxides with an ABO3 perovskite structure exhibit strongly entangled structural and electronic degrees of freedom and thus one expects to unveil exotic phases and properties by acting on the lattice through various external stimuli. Using the Jahn-Teller active praseodymium vanadate Pr3+V3+O3 compound as a model system, we show that...

With their broad range of properties, ABO3 transition metal perovskite oxides have long served as a platform for device applications and as a testing bed for different condensed matter theories. Their insulating character and structural distortions are often ascribed to dynamical electronic correlations within a universal, symmetry-conserving parad...

In transition-metal perovskites (ABO3) most physical properties are tunable by structural parameters such as the rotation of the BO6 octahedra. Examples include the Néel temperature of orthoferrites, the conductivity of mixed-valence manganites, or the band gap of rare-earth scandates. Since oxides often hold large internal electric dipoles and can...

Transition-metal oxides with an ABO$_3$ perovskite structure exhibit strongly entangled structural and electronic degrees of freedom and thus, one expects to unveil exotic phases and properties by acting on the lattice through various external stimuli. Using the Jahn-Teller active praseodymium vanadate Pr$^{3+}$V$^{3+}$O$_3$ compound as a model sys...

With their broad range of magnetic, electronic and structural properties, transition metal perovskite oxides ABO3 have long served as a platform for testing condensed matter theories. In particular, their insulating character - found in most compounds - is often ascribed to dynamical electronic correlations through the celebrated Mott-Hubbard mecha...

The existence of band gaps in Mott insulators such as perovskite oxides with partially filled 3d shells has been traditionally explained in terms of strong, dynamic inter-electronic repulsion codified by the on-site repulsion energy U in the Hubbard Hamiltonian. The success of the "DFT+U approach" where an empirical on-site potential term U is adde...

Gray tin, also known as α-Sn, can be turned into a three-dimensional topological insulator (3D-TI) by strain and finite-size effects. Such room-temperature 3D-TI is peculiarly interesting for spintronics due to the spin-momentum locking along the Dirac cone (linear dispersion) of the surface states. Angle-resolved photoemission spectroscopy (ARPES)...

Some ABX3 perovskites exhibit different local environments (DLE) for the same B atoms in the lattice, an effect referred to as disproportionation, distinguishing such compounds from perovskites that have single local environments (SLE). The basic phenomenology of disproportionation involves the absence of B-atom charge ordering, the creation of dif...

Some ABX3 perovskites exhibit different local environments (DLE) for the same B atoms in the lattice, an effect referred to as disproportionation, distinguishing such compounds from common perovskites that have single local environments (SLE). The basic phenomenology associated with such disproportionation involves the absence of B-atom charge orde...

In transition metal perovskites (ABO3) most physical properties are tunable by structural parameters such as the rotation of the BO6 octahedra. Examples include the N\'eel temperature of orthoferrites, the conductivity of mixed-valence manganites, or the band gap of rare-earth scandates. Since oxides often host large internal electric dipoles and c...

Gray tin, also known as $\alpha$-Sn, can be turned into a three-dimensional topological insulator (3D-TI) by strain and finite size effects. Such room temperature 3D-TI is peculiarly interesting for spintronics due to the spin-momentum locking along the Dirac cone (linear dispersion) of the surface states. Angle resolved photoemission spectroscopy...

The variety of emergent phenomena occurring at oxide interfaces has made these systems the focus of intense study in recent years. We argue that spin–orbit effects in oxide interfaces provide a versatile handle to generate, control and convert spin currents, with a view towards low-power spintronics.

The structural, electronic and magnetic properties of AMO3 perovskite oxides, where M is a 3d transition metal, are highly sensitive to the geometry of the bonds between the metal-d and oxygen-p ions (through octahedra rotations and distortions) and to their level of covalence. This is particularly true in rare-earth nickelates RNiO3 that display a...

Rare-earth titanates RTiO$_3$ are Mott insulators displaying a rich physical behavior, featuring most notably orbital and spin orders in their ground state. The origin of their ferromagnetic to antiferromagnetic transition as a function of the size of the rare-earth however remains debated. Here we show on the basis of symmetry analysis and first-p...

Rare-earth titanates RTiO$_3$ are Mott insulators displaying a rich physical behavior, featuring most notably orbital and spin orders in their ground state. The origin of their ferromagnetic to antiferromagnetic transition as a function of the size of the rare-earth however remains debated. Here we show on the basis of symmetry analysis and first-p...

The present paper theoretically studies the possibility to control the orbital ordering in manganite superlattices. Indeed, favored dz2eg-orbital occupancy is one of the proposed interpretations for the formation of a “dead” layer at the interfaces in manganite thin films and superlattices. We show here that favored dz2eg-orbital occupancy at the i...

At interfaces between conventional materials, band bending and alignment are classically controlled by differences in electrochemical potential. Applying this concept to oxides in which interfaces can be polar and cations may adopt a mixed valence has led to the discovery of novel two-dimensional states between simple band insulators such as LaAlO3...

Poster MRS

Transition metal oxides having a perovskite structure form a wide and technologically important class of compounds. In these systems, ferroelectric, ferromagnetic, ferroelastic, or even orbital and charge orderings can develop and eventually coexist. These orderings can be tuned by external electric, magnetic, or stress field, and the cross-couplin...

Using a combination of first-principles calculations based on density functional theory and Boltzmann semiclassical transport theory, we compute and study the properties of pristine layered calcium cobaltite Ca3Co4O9. We model the system with the B1WC hybrid functional. Two supercells of increasing size which approximate the incommensurate crystall...

The present work theoretically studies the possibility to hinder the formation of a "dead" layer at the interfaces in manganite superlattices. We showed that this goal can be reached by using alkaline-earth simple oxides as alternating layers in very thin superlattices. Indeed, such alternating layer promotes the contraction of manganite layers at...

The properties of AMO3 perovskite oxides, where M is a 3d transition metal, depend strongly on the level of covalency between the metal d and oxygen p orbitals. With their complex spin orders and metal-insulator transition, rare-earth nickelates verge between dominantly ionic and covalent characters. Accordingly, the nature of their ground state is...

At interfaces between conventional materials, band bending and alignment are classically controlled by differences in electrochemical potential. Applying this concept to oxides in which interfaces can be polar and cations may adopt a mixed valence has led to the discovery of novel two-dimensional states between simple band insulators such as LaAlO3...

Using a combination of first-principles calculations and experimental transport measurements, we study the electronic and magnetic structure of the unfilled skutterudite FeSb3. We employ the hybrid functional approach for exchange correlation. The ground state is determined to be antiferromagnetic with an atomic magnetic moment of 1.6μB/Fe. The Née...

The Jahn-Teller distortion, by its very nature, is often at the heart of the various electronic properties displayed by perovskites and related materials. Despite the Jahn-Teller mode being non- polar in nature, we devise and demonstrate in the present letter an electric field control of Jahn-Teller distortions in bulk perovskites. The electric fie...

In magnetic materials, the Pauli exclusion principle typically drives anti-alignment between electron spins on neighbouring species resulting in antiferromagnetic behaviour. Ferromagnetism exhibiting spontaneous spin alignment is a fairly rare behaviour, but once materialized is often associated with itinerant electrons in metals. Here we predict a...

Interest in first-principles calculations within the multiferroic community has been rapidly on the rise over the last decade. Initially considered as a powerful support to explain experimentally observed behaviours, the trend has evolved and, nowadays, density functional theory calculations has become also an essential predicting tool for identify...

Perovskite oxides are already widely used in industry and have huge potential for novel device applications thanks to the rich physical behaviour displayed in these materials. The key to the functional electronic properties exhibited by perovskites is often the so-called Jahn-Teller distortion. For applications, an electrical control of the Jahn-Te...

This paper proposes the direct calculation of the microscopic contributions to the magneto-electric coupling, using ab initio methods. The electrostrictive and the Dzyaloshinskii-Moriya contributions were evaluated individually. For this purpose a specific method was designed, combining density functional theory calculations and embedded fragment,...

We performed magnetic and ferroelectric measurements, associated with Landau theory and symmetry analysis, in order to clarify the situation of the YMnO3 system, a classical example of type I multiferroics. We found that the only magnetic group compatible with all experimental data (neutrons scattering, magnetization,polarization, dielectric consta...

Using first-principles calculations, we study theoretically the stable 2H hexagonal structure of BaMnO3. We show that from the stable high temperature P63/mmc structure, the compound should exhibit an improper ferroelectric structural phase transition to a P63cm ground state. Combined with its antiferromagnetic properties, 2H-BaMnO3 is therefore ex...

The lattice dynamics of the $\rm YMnO_3$ magneto-electric compound has been investigated using density functional calculations, both in the ferroelectric and the paraelectric phases. The coherence between the computed and experimental data is very good in the low temperature phase. Using group theory, modes continuity and our calculations we were a...

We performed magnetic and ferroelectric measurements, first principle calculations and Landau theory analysis on hexagonal YMnO3. The polarization and the AFM order parameter were found to present different temperature dependence at TN. A linear coupling between these two order parameters is thus forbidden in the Landau theory and P63cm cannot be t...

In multiferroic magnetoelectric materials, magnetic properties can be controlled by applying an electric field and conversely. Those materials are generally divided into two classes : for the type I, ferroelectric and magnetic phase transitions are uncorrelated whereas for the type II, ferroelectricity is a consequence of a particular magnetic arra...

We propose a new ab initio method designed for the accurate calculation of effective exchange integrals between atoms with numerous open shells. This method applies to ferromagnetic as well as antiferromagnetic exchange, direct or ligand-mediated exchange. Test calculations on high spin transition metal oxides such as KNiF(3), Ba(2)CoS(3) or YMnO(3...

We measured the temperature dependent infrared reflectivity spectra of MnF2 between 4 K and room temperature. We show that the phonon spectrum undergoes a strong renormalization at TN. The ab-initio calculation we performed on this compound accurately predict the magnitude and the direction of the phonon parameters changes across the antiferromagne...