D. Colson’s research while affiliated with University of Paris-Saclay and other places

Pressure is a key parameter for tuning or revealing superconductivity in materials and compounds. Many measurements of superconducting phase transition temperatures have been conducted using diamond anvil cells (DACs), which provide a wide pressure range and enable concomitant microscopic structural characterization of the sample. However, the inherently small sample volumes in DACs complicate the unambiguous detection of the Meissner effect, the hallmark of superconductivity. Recently, the Meissner effect in superconductors within a DAC was successfully demonstrated using diamond nitrogen-vacancy (N-V) widefield magnetometry, a non-invasive optical technique. In this work, we show that N-V magnetometry can also map superconductivity with micrometer resolution. We apply this technique to a microcrystal of HgBa$_2$Ca$_2$Cu$_3$O$_{8+\delta}$ (Hg-1223) mercury-based cuprate superconductor under 4 GPa of pressure. The method is capable to detect the magnetic field expulsion and heterogeneities in the sample, visible in a set of characteristic parameters as the local critical temperature $T_{c}$. Flux pinning zones are identified through flux trapping maps. This approach could enable detailed investigations of superconductivity of a broad range of materials under high-pressure conditions.

A magnetic Weyl semimetal presents the intriguing possibility of controlling topological properties through magnetic order. The kagome compound \CoSnS~has emerged as one of the most thoroughly characterized magnetic Weyl semimetals, yet the potential coexistence of a ferromagnetic state below $T_c$ = 172~K with a non-collinear antiferromagnetic phase or a glassy state remains unresolved. We employ $^{59}$Co NMR to gain a local perspective on the magnetic order. The magnetic and electric field gradient tensors at room temperature are determined by fitting the NMR spectra using evolutionary algorithms. Zero-field NMR measurements reveal that all Co sites are equivalent in the magnetic phase at low temperatures and up to 90~K. The local magnetic field follows in intensity the macroscopic magnetization as a function of temperature and is tilted from the c-axis by a few degrees toward the nearest triangle center. Above 90~K, a shoulder appears on the low-field side, which we attribute to a preferential tilting of the local field in one direction, breaking the equivalence between the three Co sites of the kagome structure. We rule out any coexistence with an in-plane antiferromagnetic phase and suggest instead that in-plane ferromagnetic-like moments appear above 90~K and play an increasing role in the magnetic order up to the magnetic transition.

The magnetic order in BaFe2Se3, an iron-based spin ladder that is superconducting under pressure, is intensely studied due to the intimate relation between magnetism and superconductivity. In the present paper, we have performed a comprehensive study of structural, magnetic, and electronic properties on Ba(Fe1−xNix)2Se3 (0≤x≤0.2) and BaFe2(Se1−yTey)3 (0≤y≤0.15). Neutron powder diffraction measurements were performed on the Ni-doped sample up to x=0.1 and BaFe2(Se0.85Te0.15)3. Our results show that the block magnetic order remains as the ground state for x≤0.05 in Ba(Fe1−xNix)2Se3. Additionally, for BaFe2(Se0.85Te0.15)3, the block magnetic structure is even more robust. As for the resistivity, it decreases with increasing Ni content while it barely changes with Te doping. The observed negligible change of the magnetic propagation wave vector as a function of Ni content seems to contradict the orbital selective Mott Phase proposed previously.

More than twenty years ago, multiferroic compounds combining in particular magnetism and ferroelectricity were rediscovered. Since then, BiFeO3 has emerged as the most outstanding multiferroic by combining at room temperature almost all the fundamental or applicative properties that may be desired: electroactive spin wave excitations called electromagnons, conductive domain walls, or a low band gap of interest for magnonic devices. All these properties have so far only been discontinuously strain engineered in thin films according to the lattice parameter imposed by the substrate. Here we explore the ferroelectricity and the dynamic magnetic response of BiFeO3 bulk under continuously tunable uniaxial strain. Using elasto-Raman spectroscopy, we show that the ferroelectric soft mode is strongly enhanced under tensile strain and driven by the volume preserving deformation at low strain. The magnonic response is entirely modified with low energy magnon modes being suppressed for tensile strain above pointing out a transition from a cycloid to an homogeneous magnetic state. Effective Hamiltonian calculations show that the ferroelectric and the antiferrodistortive modes compete in the tensile regime. In addition, the homogeneous antiferromagnetic state becomes more stable compared to the cycloidal state above a +2% tensile strain close to the experimental value. Finally, we reveal the ferroelectric and magnetic orders of BiFeO3 under uniaxial strain and how the tensile strain allows us to unlock and to modify in a differentiated way the polarization and the magnetic structure.

Due to a computational error, the values of various quantities obtained in our work are incorrect by a factor 2π. This error does not change the conclusions of our work. This requires small changes in the text to replace the incorrect values, and modified figures and tables.

The spin dynamics in the block magnetic phase of the iron-based ladder compound BaFe2Se3 has been studied by means of single-crystal inelastic neutron scattering. Using linear spin-wave theory and Monte Carlo simulations, our analysis points to a magnetic Heisenberg model with effective frustrated antiferromagnetic couplings only, able to describe both the exotic block order and its dynamics. This new and purely antiferromagnetic picture offers a fruitful perspective to describe multiferroic properties, but also understand the origin of the stripelike magnetic instability observed under pressure as well as in other parent compounds with similar crystalline structure.

The spin dynamics in the block magnetic phase of the iron-based ladder compound \bfs\ has been studied by means of single crystal inelastic neutron scattering. Using linear spin wave theory and Monte-Carlo simulations, our analysis points to a magnetic Heisenberg model with effective frustrated antiferromagnetic couplings only, able to describe both the exotic block order and its dynamics. This new and purely antiferromagnetic picture offers a fruitful perspective to describe multiferroic properties but also understand the origin of the stripe-like magnetic instability observed under pressure as well as in other parent compounds with similar crystalline structure.

Co3Sn2S2 has been established as a prototype of a magnetic Weyl semimetal, exhibiting a giant anomalous Hall effect in its ferromagnetic phase. An attractive feature of this material is that Weyl points lie close to the Fermi level, so one can expect a high reactivity of the topological properties to hole or electron doping. We present here a direct observation with angle-resolved photoemission spectroscopy of the evolution of the electronic structure under different types of substitutions: In for Sn (hole doping outside the kagome Co plane), Fe for Co (hole doping inside the kagome Co plane), and Ni for Co (electron doping inside the kagome Co plane). We observe clear shifts of selected bands, which are due both to doping and to the reduction of the magnetic splitting by doping. We discriminate between the two by studying the temperature evolution from a ferromagnetic to paramagnetic state. We discuss these shifts with the help of density-functional theory calculations using the virtual crystal approximation. We find that these calculations reproduce rather well the evolution with In, but largely fail to capture the effect of Fe and Ni, where local behavior at the impurity site plays an important role.