We report an optical pump-terahertz (THz) probe study of the photo-induced transient carrier dynamics in the low bandgap semiconductor indium antimonide (InSb). Using an organic N-benzyl-2-methyl-nitroaniline crystal as a broadband THz source, we access the full spectral response over more than 5 THz, for varying pump-probe delay following the optical excitation. Using the Drude-Lorentz model accounting for differences between the excited length in the material and the penetration depth of the THz beam in pumped InSb, we extract the absolute carrier density as a function of the pump-probe delay and provide insights on the diffusion length at given carrier densities, for different pump fluences. The mismatch between the THz penetration depth and the actual excited sample depth after carrier diffusion is discussed, since their evolutions with time and pump fluence are not intuitive as both quantities depend on carrier density.

Two-dimensional electron gases (2DEGs) in quantum paraelectric SrTiO3 (STO) exhibit high electron mobilities at low temperature, superconductivity, and efficient spin-charge interconversion owing to their Rashba spin-orbit coupling. However, such 2DEGs have almost exclusively been generated in STO single crystals, and few attempts to replace crystals by heteroepitaxial STO films have mostly resulted in low mobilities, limiting device integration and functional tuning such as strain-induced ferroelectricity. Here, we use hybrid oxide molecular beam epitaxy to grow high-quality strain-engineered STO films that are ferroelectric up to 165 K. By sputtering thin aluminum layers at room temperature, we generate a 2DEG at their surface. Raman spectroscopy and magnetotransport measurements indicate that the ferroelectric character is retained after 2DEG formation. These results suggest that our samples behave as ferroelectric 2DEGs up to temperatures well above previous results based on Ca-STO substrates (∼30 K), opening the way toward thin-film-based ferroelectric 2DEGs operating at room temperature.

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. 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}.

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

Due to a computational error, the values of various quantities obtained in our work are incorrect by a factor $\frac{2}{\ensuremath{\pi}}$. 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.

More than twenty years ago, multiferroic compounds combining in particular magnetism and ferroelectricity were rediscovered. Since then, BiFeO_{3} 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 BiFeO_{3} 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 BiFeO_{3} 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.

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.

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 a(Fe1 - xCox )(2)As-2 . In the parent compound BaFe2As2 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. For the superconducting compound, the suppression of the nematic susceptibility correlates with the decrease of the critical temperature T-c, indicating a significant contribution of nematic fluctuations to electron pairing. Our results validate theoretical scenarios of enhanced T-c near a nematic quantum critical point.

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 to probe the evolution of nematic fluctuations under strain in the normal and superconducting states of the paradigmatic iron-based superconductor Ba(Fe_1-xCo_x)_2As_2. In the non-superconducting parent compound BaFe_2As_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. For the superconducting compound, the suppression of the nematic susceptibility correlates with the decrease of the superconducting critical temperature T_c. Our results indicate a significant contribution of nematic fluctuations to electron pairing and validate theoretical scenarios of enhanced T_c near a nematic quantum critical point.

We report a Raman scattering study of nematic degrees of freedom in the iron-based superconductor parent compound BaFe2As2 under tunable uniaxial strain. We demonstrate that the polarization resolved arsenic (As) phonon intensity can be used to monitor the nematic order parameter as a function of both temperature and strain. At low temperature in the nematic ordered phase we use it to track the continuous and reversible orientation of nematic domains under variable strain. At higher temperature, the evolution of the As phonon intensity under strain reflects an enhanced nematic susceptibility close to the nematic transition TS . Its temperature dependence under strong strain follows qualitatively the expected behavior of an Ising order parameter under a symmetry breaking field. Our elasto-Raman study illustrates the interest of combining selective anisotropic strain with a symmetry resolved probe like Raman scattering. Elasto-Raman scattering can be applied to a wide variety of quantum materials where uniaxial strain tunes electronic orders.

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. For the superconducting compound, the suppression of the nematic susceptibility correlates with the decrease of the critical temperature T_{c}, indicating a significant contribution of nematic fluctuations to electron pairing. Our results validate theoretical scenarios of enhanced T_{c} near a nematic quantum critical point.

We followed step by step the transition from an antiferromagnetic (AFM) Mott insulator to a superconducting (SC) metal in the Bi2Sr2CaCu2O8+sigma (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, delta(pg)(p), closely follows that of the SSE, delta(sse)(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 delta(max)(sc) and the SC transition temperature Tc are linked in an extended range of doping, such as delta(max)(sc)(p)( )proportional to delta(sse)(p) Tc (p). This relation suggests that the AFM correlations play a key role in the mechanism of superconductivity.

The highly mobile electrons at the interface of SrTiO3 with other oxide insulators, such as LaAlO3 or AlOx, are of great current interest. A vertical gate voltage allows controlling a metal/superconductor-to-insulator transition, as well as electrical modulation of the spin-orbit Rashba coupling for spin-charge conversion. These findings raise important questions about the origin of the confined electrons as well as the mechanisms that govern the interfacial electric field. Here we use infrared ellipsometry and confocal Raman spectroscopy to show that an anomalous polar moment is induced at the interface that is non-collinear, highly asymmetric and hysteretic with respect to the vertical gate electric field. Our data indicate that an important role is played by the electromigration of oxygen vacancies and their clustering at the antiferrodistortive domain boundaries of SrTiO3, which generates local electric and possibly also flexoelectric fields and subsequent polar moments with a large lateral component. Our results open new perspectives for the defect engineering of lateral devices with strongly enhanced and hysteretic local electric fields that can be manipulated with various other parameters, like strain, temperature, or photons.

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. For the superconducting compound, the suppression of the nematic susceptibility correlates with the decrease of the critical temperature T_{c}, indicating a significant contribution of nematic fluctuations to electron pairing. Our results validate theoretical scenarios of enhanced T_{c} near a nematic quantum critical point.

Anisotropic strain is an external field capable of selectively addressing the role of nematic fluctuations in promoting superconductivity. We demonstrate this using polarization-resolved elastoRaman scattering to probe the evolution of nematic fluctuations under strain in the normal and superconducting states of the paradigmatic iron-based superconductor Ba(Fe1−xCox)2As2. In the non-superconducting parent compound BaFe2As2 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. For the superconducting compound, the suppression of the nematic susceptibility correlates with the decrease of the superconducting critical temperature Tc. Our results indicate a significant contribution of nematic fluctuations to electron pairing and validate theoretical scenarii of enhanced Tc near a nematic quantum critical point.

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. For the superconducting compound, the suppression of the nematic susceptibility correlates with the decrease of the critical temperature T_{c}, indicating a significant contribution of nematic fluctuations to electron pairing. Our results validate theoretical scenarios of enhanced T_{c} near a nematic quantum critical point.

We measured the optical signature of the charge density waves (CDWs) in the multiband conductor TTF[Ni(dmit)2]2 by electronic Raman scattering. At low energies, a hump develops below 60 K. This hump is associated to the amplitude mode of the CDW with an energy around 9 meV. Raman symmetry-resolved measurements show that the CDW amplitude mode is anisotropic and that the CDW can be associated to the band nesting of Ni(dmit)2 chains.

We have performed elasticity measurements in the bulk multiferroic BiFeO3 (BFO) using acoustical pump and probe spectroscopy. The sound velocities of the (quasi)-longitudinal and two (quasi)-transverse acoustic waves along three independent directions of the (110) surface have been measured. Moreover, one surface wave and one longitudinal wave propagating perpendicular to the surface have been detected. Based on initial input values of the six independent Cij elastic constants determined by our density functional theory calculations and our eleven experimental velocities, the numerical resolution of the acoustic equations allows us to determine all the Cij elastic constants of BFO. The propagation direction dependence of volume and surface waves phase velocities allows the unambiguous assignment of the waves and, hence, the polarization of phonons.

We report the evolution of nematic fluctuations in FeSe 1− x S x single crystals as a function of Sulfur content x across the nematic quantum critical point (QCP) x c ~ 0.17 via Raman scattering. The Raman spectra in the B 1g nematic channel consist of two components, but only the low energy one displays clear fingerprints of critical behavior and is attributed to itinerant carriers. Curie–Weiss analysis of the associated nematic susceptibility indicates a substantial effect of nemato-elastic coupling, which shifts the location of the nematic QCP. We argue that this lattice-induced shift likely explains the absence of any enhancement of the superconducting transition temperature at the QCP. The presence of two components in the nematic fluctuations spectrum is attributed to the dual aspect of electronic degrees of freedom in Hund’s metals, with both itinerant carriers and local moments contributing to the nematic susceptibility.

κ ‐(BEDT‐TTF) 2 Cu(NCS) 2 has been investigated by Raman scattering in both bulk and nanoparticle compounds. Phonon modes from 20 to 1600 cm −1 have been assigned. Focusing on the unexplored low frequency phonons, a plateau in frequencies is observed in the bulk phonons between 50 and 100 K and assigned to the signature of the bad metal phase. Nanoparticles of κ ‐(BEDT‐TTF) 2 Cu(NCS) 2 exhibit anomalies at 50 K associated to the crossover from a bad metal to a Fermi liquid whose origins are discussed.