arXiv subjects

Didier Wermeille

Publications and source records attributed to Didier Wermeille.

4 recordsLinked to original sources

arXiv2026

Antiferroquadrupolar Order in Altermagnetic CoF$_2$

Altermagnets host non-relativistic spin-split electronic states whose microscopic origin is theoretically linked to a hidden charge order, yet experimental studies of this charge order are limited. We therefore investigate charge order within CoF$_2$, a $d$-wave altermagnetic compound with a rutile crystal structure and $Γ$-point antiferromagnetism. Combining resonant elastic X-ray scattering, symmetry analysis and ab initio calculations, we directly observe charge ordering and identify it as antiferroquadrupolar in nature. Via electronic structure calculations, we show that the experimentally observed antiferroquadrupolar order gives rise to the characteristic altermagnetic spin-splitting, thereby establishing empirical evidence for the decomposition of the altermagnetic order parameter into magnetic and charge degrees of freedom. We hence demonstrate that antiferroquadrupolar order is the microscopic origin of altermagnetism in CoF$_2$, with implications to the wider family of rutile altermagnets. Furthermore, our approach is applicable to studying altermagnetism in general, having demonstrated that resonant elastic X-ray scattering can serve as a direct probe of the charge multipoles that underpin spin-split electronic states in these materials.

cond-mat.str-el↗

arXiv2021

Field Induced Modulated State in the Ferromagnet PrPtAl

The theory of quantum order-by-disorder (QOBD) explains the formation of modulated magnetic states at the boundary between ferromagnetism and paramagnetism in zero field. PrPtAl has been argued to provide an archetype for this. Here, we report the phase diagram in magnetic field, applied along both the easy $a$-axis and hard $b$-axis. For field aligned to the $b$-axis, we find that the magnetic transition temperatures are suppressed and at low temperature there is a single modulated fan state, separating an easy $a$-axis ferromagnetic state from a field polarised state. This fan state is well explained with the QOBD theory in the presence of anisotropy and field. Experimental evidence supporting the QOBD explanation is provided by the large increase in the $T^2$ coefficient of the resistivity and direct detection of enhanced magnetic fluctuations with inelastic neutron scattering, across the field range spanned by the fan state. This shows that the QOBD mechanism can explain field induced modulated states that persist to very low temperature.

cond-mat.str-el↗

arXiv2020

Precise control of $J_\mathrm{eff}=1/2$ magnetic properties in Sr$_2$IrO$_4$ epitaxial thin films by variation of strain and thin film thickness

We report on a comprehensive investigation of the effects of strain and film thickness on the structural and magnetic properties of epitaxial thin films of the prototypal $J_\mathrm{eff}=1/2$ compound Sr$_2$IrO$_4$ by advanced X-ray scattering. We find that the Sr$_2$IrO$_4$ thin films can be grown fully strained up to a thickness of 108 nm. By using X-ray resonant scattering, we show that the out-of-plane magnetic correlation length is strongly dependent on the thin film thickness, but independent of the strain state of the thin films. This can be used as a finely tuned dial to adjust the out-of-plane magnetic correlation length and transform the magnetic anisotropy from two-dimensional (2D) to three-dimensional (3D) behavior by incrementing film thickness. These results provide a clearer picture for the systematic control of the magnetic degrees of freedom in epitaxial thin films of Sr$_2$IrO$_4$ and bring to light the potential for a rich playground to explore the physics of $5d$-transition metal compounds.

cond-mat.str-el↗

arXiv2017

Investigation of the commensurate magnetic structure in heavy fermion CePt2In7 using magnetic resonant X-ray diffraction

We investigated the magnetic structure of the heavy fermion compound CePt$_2$In$_7$ below $T_N~=5.34(2)$ K using magnetic resonant X-ray diffraction at ambient pressure. The magnetic order is characterized by a commensurate propagation vector ${k}_{1/2}~=~\left( \frac{1}{2} , \frac{1}{2}, \frac{1}{2}\right)$ with spins lying in the basal plane. Our measurements did not reveal the presence of an incommensurate order propagating along the high symmetry directions in reciprocal space but cannot exclude other incommensurate modulations or weak scattering intensities. The observed commensurate order can be described equivalently by either a single-${k}$ structure or by a multi-${k}$ structure. Furthermore we explain how a commensurate-only ordering may explain the broad distribution of internal fields observed in nuclear quadrupolar resonance experiments (Sakai et al. 2011, Phys. Rev. B 83 140408) that was previously attributed to an incommensurate order. We also report powder X-ray diffraction showing that the crystallographic structure of CePt$_2$In$_7$ changes monotonically with pressure up to $P~=~7.3$ GPa at room temperature. The determined bulk modulus $B_0~=~81.1(3)$ GPa is similar to the ones of the Ce-115 family. Broad diffraction peaks confirm the presence of pronounced strain in polycrystalline samples of CePt$_2$In$_7$. We discuss how strain effects can lead to different electronic and magnetic properties between polycrystalline and single crystal samples.

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