arXiv subjects

M. Guennou

Publications and source records attributed to M. Guennou.

12 recordsLinked to original sources

arXiv2025

Non-reciprocal circular dichroism of axial phonons coupled to ferro-rotational order

Circular dichroism (CD) in X-ray absorption, defined as the difference in absorption between opposite circular polarizations, is fundamentally enabled by the breaking of time-reversal symmetry or inversion symmetry. It is therefore sensitive to magnetism, chirality, and their interplay. In contrast, the sample symmetry alone is insufficient to determine whether CD in resonant inelastic X-ray scattering (RIXS) is allowed. Rather, RIXS-CD is governed by both the sample symmetry and the scattering geometry. Here, using RIXS, we identify circularly polarized phonons coupled to ferro-rotational order in MnTiO$_3$, which we refer to as ferro-axial phonons. Their excitations provide a direct demonstration of non-reciprocal RIXS-CD, in which the dichroic response changes upon reversing the propagation direction of the incident X-rays, although the system globally preserves both inversion and time-reversal symmetries. We propose that a condensate of these phonons, manifested as standing waves, underlies the ferro-rotational order in MnTiO$_3$. The observed non-reciprocal CD reflects the interplay among photon helicity, phonon polarization, and ferro-rotational order.

cond-mat.str-el↗

arXiv2025

Growth orientation and magnetic properties of GdVO$_3$ tailored by epitaxial strain engineering

Transition-metal oxides with an ABO$_3$ perovskite structure exhibit significant coupling between spin, orbital, and lattice degrees of freedom, highlighting the crucial role of lattice distortion in tuning the electronic and magnetic properties of these systems. In this study, we grow a series of antiferromagnetic GdVO$_3$ thin films on different substrates introducing various strain values. The results demonstrate that the strain not only affects the material's structure and magnetic transition temperature but also influences the growth direction of the orthorhombic unit cell. A correlation is observed between the orientation of the orthorhombic long axis direction and the material's N\'eel temperature. DFT calculations highlighting the link between strain, lattice distortions and magnetic characteristics confirm these conclusions. Our findings demonstrate that the development of novel features in RVO$_3$ through material design requires control of growth orientation through strain engineering.

cond-mat.mtrl-sci↗

arXiv2024

Giant Strain Tunability in Polycrystalline Ceramic Films via Helium Implantation

Strain engineering is a powerful tool routinely used to control and enhance properties such as ferroelectricity, magnetic ordering, or metal-insulator transitions. Epitaxial strain in thin films allows manipulation of in-plane lattice parameters, achieving strain values generally up to 4%, and above in some specific cases. In polycrystalline films, which are more suitable for functional applications due to their lower fabrication costs, strains above 1% often cause cracking. This poses challenges for functional property tuning by strain engineering. Helium implantation has been shown to induce negative pressure through interstitial implantation, which increases the unit cell volume and allows for continuous strain tuning with the implanted dose in epitaxial monocrystalline films. However, there have been no studies on the transferability of helium implantation as a strain-engineering technique to polycrystalline films. Here, we demonstrate the technique's applicability for strain engineering beyond epitaxial monocrystalline samples. Helium implantation can trigger an unprecedented lattice parameter expansion of up to 3.2% in polycrystalline BiFeO3 films without causing structural cracks. The film maintains stable ferroelectric properties with doses up to 1E15 He/cm2. This finding underscores the potential of helium implantation in strain engineering polycrystalline materials, enabling cost-effective and versatile applications.

cond-mat.mtrl-sci↗

arXiv2021

Patterning enhanced tetragonality in BiFeO3 thin films with effective negative pressure by helium implantation

Helium implantation in epitaxial thin films is a way to control the out-of-plane deformation independently from the in-plane strain controlled by epitaxy. In particular, implantation by means of a helium microscope allows for local implantation and patterning down to the nanometer resolution, which is of interest for device applications. We present here a study of bismuth ferrite (BiFeO3) films where strain was patterned locally by helium implantation. Our combined Raman, XRD and TEM study shows that the implantation causes an elongation of the BiFeO3 unit cell and ultimately a transition towards the so-called super-tetragonal polymorph via states with mixed phases. In addition, TEM reveals the onset of amorphization at a threshold dose that does not seem to impede the overall increase in tetragonality. The phase transition from the R-like to T-like BiFeO3 appears as first-order in character, with regions of phase coexistence and abrupt changes in lattice parameters.

cond-mat.mtrl-sci↗

arXiv2019

Crossover in the pressure evolution of elementary distortions in RFeO3 perovskites and its impact on their phase transition

This work reports on the pressure dependence of the octahedra tilts and mean Fe-O bond lengths in RFeO3 (R=Nd, Sm, Eu, Gd, Tb and Dy), determined through synchrotron X-ray diffraction and Raman scattering, and their role on the pressure induced phase transition displayed by all of these compounds. For larger rare-earth cations (Nd-Sm), both anti- and in-phase octahedra tilting decrease as pressure increases, whereas the reverse behavior is observed for smaller ones (Gd-Dy). EuFeO3 stands at the borderline, as the tilts are pressure independent. For the compounds where the tilts increase with pressure, the FeO6 octahedra are compressed at lower rates than for those ones exhibiting opposite pressure tilt dependence. The crossover between the two opposite pressure behaviors is discussed and faced with the rules grounded on the current theoretical approaches. The similarity of the pressure-induced isostructural insulator-to-metal phase transition, observed in the whole series, point out that the tilts play a minor role in its driving mechanisms. A clear relationship between octahedra compressibility and critical pressure is ascertained.

cond-mat.str-el↗

arXiv2018

Control of surface potential at polar domain walls in a nonpolar oxide

Ferroic domain walls could play an important role in microelectronics, given their nanometric size and often distinct functional properties. Until now, devices and device concepts were mostly based on mobile domain walls in ferromagnetic and ferroelectric materials. A less explored path is to make use of polar domain walls in nonpolar ferroelastic materials. Indeed, while the polar character of ferroelastic domain walls has been demonstrated, polarization control has been elusive. Here, we report evidence for the electrostatic signature of the domain-wall polarization in nonpolar calcium titanate (CaTiO3). Macroscopic mechanical resonances excited by an ac electric field are observed as a signature of a piezoelectric response caused by polar walls. On the microscopic scale, the polarization in domain walls modifies the local surface potential of the sample. Through imaging of surface potential variations, we show that the potential at the domain wall can be controlled by electron injection. This could enable devices based on nondestructive information readout of surface potential.

cond-mat.mtrl-sci↗

arXiv2016

Multiple strain-induced phase transitions in LaNiO3 thin films

Strain effects on epitaxial thin films of LaNiO3 grown on different single crystalline substrates are studied by Raman scattering and first-principles simulation. New Raman modes, not present in bulk or fully-relaxed films, appear under both compressive and tensile strains, indicating symmetry reductions. Interestingly, the Raman spectra and the underlying crystal symmetry for tensile and compressively strained films are different. Extensive mapping of LaNiO3 phase stability is addressed by simulations, showing that a variety of crystalline phases are indeed stabilized under strain which may impact the electronic orbital hierarchy. The calculated Raman frequencies reproduce the principal features of the experimental spectra, supporting the validity of the multiple strain-driven structural transitions predicted by the simulations.

cond-mat.mtrl-sci↗

arXiv2015

Spin-orbital excitation continuum and anomalous electron-phonon interaction in the Mott insulator LaTiO$_3$

Raman scattering experiments on stoichiometric, Mott-insulating LaTiO$_3$ over a wide range of excitation energies reveal a broad electronic continuum which is featureless in the paramagnetic state, but develops a gap of $\sim 800$ cm$^{-1}$ upon cooling below the Néel temperature $T_N = 146$ K. In the antiferromagnetic state, the spectral weight below the gap is transferred to well-defined spectral features due to spin and orbital excitations. Low-energy phonons exhibit pronounced Fano anomalies indicative of strong interaction with the electron system for $T > T_N$, but become sharp and symmetric for $T < T_N$. The electronic continuum and the marked renormalization of the phonon lifetime by the onset of magnetic order are highly unusual for Mott insulators and indicate liquid-like correlations between spins and orbitals.

cond-mat.str-el↗

arXiv2014

Dynamic and structural properties of orthorhombic rare-earth manganites under high pressure

We report a high-pressure study of orthorhombic rare-earth manganites AMnO3 using Raman scattering (for A = Pr, Nd, Sm, Eu, Tb and Dy) and synchrotron X-ray diffraction (for A = Pr, Sm, Eu, and Dy). In all cases, a structural and insulator-to-metal transition was evidenced, with a critical pressure that depends on the A-cation size. We analyze the compression mechanisms at work in the different manganites via the pressure dependence of the lattice parameters, the shear strain in the a-c plane, and the Raman bands associated with out-of-phase MnO6 rotations and in-plane O2 symmetric stretching modes. Our data show a crossover across the rare-earth series between two different kinds of behavior. For the smallest A-cations, the compression is nearly isotropic in the ac plane, with presumably only very slight changes of tilt angles and Jahn-Teller distortion. As the radius of the A-cation increases, the pressure-induced reduction of Jahn-Teller distortion becomes more pronounced and increasingly significant as a compression mechanism, while the pressure-induced bending of octahedra chains becomes conversely less pronounced. We finally discuss our results in the light of the notion of chemical pressure, and show that the analogy with hydrostatic pressure works quite well for manganites with small A-cations but can be misleading with large A-cations.

cond-mat.mtrl-sci↗

arXiv2013

Morphotropic interfaces in PMN-PT single crystals

The paper describes heterostructures spontaneously formed in PMN-PT single crystals cooled under bias electric field applied along [001]pc and then zero-field-heated in the vicinity of the so-called depoling temperature. In particular, formation of lamellar structures composed of tetragonal-like and rhombohedral-like layers extending over macroscopic (mm) lengths is demonstrated by optical observations and polarized Raman investigations.

cond-mat.mtrl-sci↗

arXiv2011

Structural and insulator-to-metal phase transition at 50 GPa in GdMnO3

We present a study of the effect of very high pressure on the orthorhombic perovskite GdMnO3 by Raman spectroscopy and synchrotron x-ray diffraction up to 53.2 GPa. The experimental results yield a structural and insulator-to-metal phase transition close to 50 GPa, from an orthorhombic to a metrically cubic structure. The phase transition is of first order with a pressure hysteresis of about 6 GPa. The observed behavior under very high pressure might well be a general feature in rare-earth manganites.

cond-mat.str-el↗

arXiv2005

Orbital excitations in titanates

Raman scattering is used to observe pronounced electronic excitations around 230 meV - well above the two-phonon range - in the Mott insulators LaTiO$_3$ and YTiO$_3$. Based on the temperature, polarization, and photon energy dependence, the modes are identified as orbital excitations. The observed profiles bear a striking resemblance to magnetic Raman modes in the insulating parent compounds of the superconducting cuprates, indicating an unanticipated universality of the electronic excitations in transition metal oxides.

cond-mat.str-el↗