Y. Gallais’s research while affiliated with Université Paris Cité and other places

The transition metal dichalcogenide 1T‐TaS2 exhibits a Charge Density Wave (CDW) with in‐plane chirality. Due to the rich phase diagram, the Ferro‐Rotational Order (FRO) can be tuned by external stimuli. The FRO is studied by Angle‐Resolved Photoelectron Spectroscopy (ARPES), Raman spectroscopy, and Selected Area Electron Diffraction (SAED). The in‐plane chirality of the CDW is lost at the transition from Nearly‐Commensurate (NC) to In‐Commensurate (IC) phase and can be controlled by applying shear stress to the sample while cooling it through the transition from IC‐CDW to NC‐CDW. Based on these observations, a protocol is proposed to achieve reliable, non‐volatile state switching of the FRO configuration in 1T‐TaS2 bulk crystals. These results pave the way for new functional devices in which in‐plane chirality can be set on demand.

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.

La spectroscopie Raman a déjà une longue histoire dans les études vibrationnelles des structures moléculaires et cristallines. Après une brève introduction historique, nous décrivons quelques développements récents de cette technique dans l’étude des matériaux quantiques comme les supraconducteurs. Nous abordons ensuite l’extension de cette technique à l’imagerie Raman confocale qui est maintenant devenue son application principale et que nous illustrons sur l’exemple des matériaux 2D et du vivant.

Ta2NiSe5 is an excitonic insulator candidate showing the semiconductor or semimetal-to-insulator (SI) transition below Tc=326 K. However, since a structural transition accompanies the SI transition, deciphering the role of electronic and lattice degrees of freedom in driving the SI transition has remained controversial. Here, we investigate the photoexcited nonequilibrium state in Ta2NiSe5 using pump-probe Raman and photoluminescence spectroscopies. The combined nonequilibrium spectroscopic measurements of the lattice and electronic states reveal the presence of a photoexcited metastable state where the insulating gap is suppressed, but the low-temperature structural distortion is preserved. We conclude that electron correlations play a vital role in the SI transition of Ta2NiSe5.

Electromagnons (Electroactive spin wave excitations) could prove to be decisive in information technologies but they remain fragile quantum objects, mainly existing at low temperatures. Any future technological application requires overcoming these two limitations. By means of synchrotron radiation infrared spectroscopy performed in the THz energy range and under hydrostatic pressure, we tracked the electromagnon in the cupric oxide CuO, despite its very low absorption intensity. We demonstrate how a low pressure of 3.3 GPa strongly increases the strength of the electromagnon and expands its existence to a large temperature range enhanced by 40 K. Accordingly, these two combined effects make the electromagnon of CuO under pressure a more ductile quantum object. Numerical simulations based on an extended Heisenberg model were combined to the Monte-Carlo technique and spin dynamics to account for the magnetic phase diagram of CuO. They enable to simulate the absorbance response of the CuO electromagnons in the THz range.

We investigate the collective mode response of the iron-based superconductor Ba 1− x K x Fe 2 As 2 using intense terahertz (THz) light. In the superconducting state a THz Kerr signal is observed and assigned to nonlinear THz coupling to superconducting degrees of freedom. The polarization dependence of the THz Kerr signal is remarkably sensitive to the coexistence of a nematic order. In the absence of nematic order the C 4 symmetric polarization dependence of the THz Kerr signal is consistent with a coupling to the Higgs amplitude mode of the superconducting condensate. In the coexisting nematic and superconducting state the signal becomes purely nematic with a vanishing C 4 symmetric component, signaling the emergence of a superconducting collective mode activated by nematicity.

We followed step by step the transition from an antiferromagnetic (AFM) Mott insulator to a superconducting (SC) metal in the Bi2Sr2CaCu2O8+δ (Bi-2212) cuprate using electronic Raman scattering spectroscopy. This was achieved by tracking the doping dependence of the spin singlet excitation (SSE) originating from the AFM Mott insulator, the normal-state quasiparticle excitation related to the mobile charge carriers, and the Bogoliubov quasiparticles related to the SC gap. We show that the signature of the pseudogap phase which develops during this transition can be interpreted as the blocking of charge carriers by the enhancement of the AFM correlations as the temperature drops. We find that the energy scale of the pseudogap, Δpg(p), closely follows that of the SSE, Δsse(p) with doping p. The quasiparticle lifetime considerably increases with doping when the pseudogap collapses. We reveal that the maximum amplitude of the SC gap Δscmax and the SC transition temperature Tc are linked in an extended range of doping, such as Δscmax(p)∝Δsse(p)Tc(p). This relation suggests that the AFM correlations play a key role in the mechanism of superconductivity.

Ta$_2$NiSe$_5$ is an excitonic insulator candidate showing the semiconductor/semimetal-to-insulator (SI) transition below $T_{\text{c}}$ = 326 K. However, since a structural transition accompanies the SI transition, deciphering the role of electronic and lattice degrees of freedom in driving the SI transition has remained controversial. Here, we investigate the photoexcited nonequilibrium state in Ta$_2$NiSe$_5$ using pump-probe Raman and photoluminescence (PL) spectroscopies. The combined nonequilibrium spectroscopic measurements of the lattice and electronic states reveal the presence of a photoexcited metastable state where the insulating gap is suppressed, but the low-temperature structural distortion is preserved. We conclude that electron correlations play a vital role in the SI transition of Ta$_2$NiSe$_5$.