Disentangling Lattice and Electronic Instabilities in the Excitonic Insulator Candidate Ta_{2}NiSe_{5} by Nonequilibrium Spectroscopy.

Ta_{2}NiSe_{5} is an excitonic insulator candidate showing the semiconductor or semimetal-to-insulator (SI) transition below T_{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 spectroscopies.

Nematic-Fluctuation-Mediated Superconductivity Revealed by Anisotropic Strain in Ba(Fe_{1-x}Co_{x})_{2}As_{2}.

Anisotropic strain is an external field capable of selectively addressing the role of nematic fluctuations in promoting superconductivity. We demonstrate this using polarization-resolved elasto-Raman scattering by probing the evolution of nematic fluctuations under strain in the normal and superconducting state of the paradigmatic iron-based superconductor Ba(Fe_{1-x}Co_{x})_{2}As_{2}. In the parent compound BaFe_{2}As_{2} we observe a strain-induced suppression of the nematic susceptibility which follows the expected behavior of an Ising order parameter under a symmetry breaking field.

Pressure-Induced Collapse of the Charge Density Wave and Higgs Mode Visibility in 2H-TaS_{2}.

The pressure evolution of the Raman active electronic excitations of the transition metal dichalcogenides 2H-TaS_{2} is followed through the pressure phase diagram embedding incommensurate charge-density-wave and superconducting states. At high pressure, the charge-density wave is found to collapse at 8.5 GPa.

Collapse of Critical Nematic Fluctuations in FeSe under Pressure.

We report the evolution of the electronic nematic susceptibility in FeSe via Raman scattering as a function of hydrostatic pressure up to 5.8 GPa where the superconducting transition temperature T_{c} reaches its maximum. The critical nematic fluctuations observed at low pressure vanish above 1.

Crystal Growth and Characterization of HgBaCaCuO Superconductors with the Highest Critical Temperature at Ambient Pressure.

We report an original procedure for the elaboration of very high-quality single crystals of superconducting HgBaCaCuO mercury cuprates. These single crystals are unique, with very high-quality surface paving the way for spectroscopic, transport, and thermodynamic probes in order to understand the hole-doped cuprate phase diagram. Annealing allows one to optimize T up to T = 133 K.

Charge-induced nematicity in FeSe.

The spontaneous appearance of nematicity, a state of matter that breaks rotation but not translation symmetry, is one of the most intriguing properties of the iron-based superconductors (Fe SC), and has relevance for the cuprates as well. Establishing the critical electronic modes behind nematicity remains a challenge, however, because their associated susceptibilities are not easily accessible by conventional probes. Here, using FeSe as a model system, and symmetry-resolved electronic Raman scattering as a probe, we unravel the presence of critical charge nematic fluctuations near the structural/nematic transition temperature, [Formula: see text] 90 K.