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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$_5$., Comment: 13 pages, 10 figures

The Shastry-Sutherland compound SrCu$_2$(BO$_3$)$_2$ constituting orthogonally coupled dimers harbors a $S=0$ singlet ground state. The confluence of strong interdimer interaction and frustration engenders a spectrum of low-energy excitations including localized triplons as well as singlet and triplet bound states. Their dynamics are controlled by an external magnetic field and temperature. Here, we employ high-field Raman spectroscopy to map the field and temperature evolution of such bosonic composite quasiparticles on approaching the 1/8 magnetization plateau. Our study unveils that the magnetic field and thermal fluctuations show remarkably similar effects in melting the singlet bound states, but are disparate in their effects on the fine spectral shapes. This, together with the anti-crossing of two singlet bound states in the intermediate field $B=10-16$ T, is discussed in terms of the correlated dynamics of frustrated, interacting bosons.

Time resolved pump and probe acoustics and first-principles calculations were employed to assess elastic properties of the TbMnO3 perovskite manganite having orthorhombic symmetry. Measuring sound velocities of bulk longitudinal and shear acoustic waves propagating along at least two different directions in the high symmetry planes (100), (010) and (001), provided a powerful mean to selectively determine the six diagonal elastic constants C11= 227 GPa, C22= 349 GPa, C33= 274 GPa, C44= 71 GPa, C55= 57 GPa, C66= 62 GPa. Among the three remaining off-diagonal ones, C23= 103 GPa was determined with a bissectrice direction. Density functional theory calculations with colinear spin-polarized provided complementary insights on their optical, elastic and magnetoelastic properties.

We investigate 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 non-linear 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 new superconducting collective mode activated by nematicity., Comment: 25 pages, 3 figures. Supplementary Material available upon request

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 of the 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 to determine all the Cij elastic constants of BFO. The propagation direction dependence of volume and surface waves phase velocities allows the unambiguously assignment of the waves, hence the polarization of phonons.

Two-dimensional electron gases (2DEGs) can form at the surface of oxides and semiconductors or in carefully designed quantum wells and interfaces. Depending on the shape of the confining potential, 2DEGs may experience a finite electric field, which gives rise to relativistic effects such as the Rashba spin-orbit coupling. Although the amplitude of this electric field can be modulated by an external gate voltage, which in turn tunes the 2DEG carrier density, sheet resistance and other related properties, this modulation is volatile. Here, we report the design of a ''ferroelectric'' 2DEG whose transport properties can be electrostatically switched in a non-volatile way. We generate a 2DEG by depositing a thin Al layer onto a SrTiO$_3$ single crystal in which 1 percent of Sr is substituted by Ca to make it ferroelectric. Signatures of the ferroelectric phase transition at 25 K are visible in the Raman response and in the temperature dependences of the carrier density and sheet resistance that shows a hysteretic dependence on electric field as a consequence of ferroelectricity. We suggest that this behavior may be extended to other oxide 2DEGs, leading to novel types of ferromagnet-free spintronic architectures.

We have performed Raman scattering measurements on BiFeO3 nanoparticles and studied both magnetic and lattice modes. We reveal strong anomalies between 140 K and 200 K in the frequency of magnon and E(LO1), E(TO1) and A1(LO1) phonon modes. These anomalies are related to a surface expansion and are enhanced for nanoparticle sizes approaching the spin cycloidal length. These observations point out the strong interplay between the surface, the lattice, and the magnetism for sizes of BiFeO3 nanoparticles close to the cycloid periodicity., Comment: 5 figures

Superconducting (SC) fluctuations in cuprate superconductors have been extensively studied to gain a deep insight into preformed Cooper pairs above the SC transition temperature $T_{\text{c}}$. While the various measurements, such as the terahertz (THz) optical conductivity, Nernst effect, angle-resolved photoemission spectroscopy (ARPES), and scanning tunneling microscopy (STM) measurements have provided the signature of the SC fluctuations, the onset temperature of the SC fluctuations depends on the measurement scheme. Here, we shed light on the Higgs mode to investigate the SC fluctuations, as it is the direct fingerprint of SC order parameter and can help elucidate the development of SC phase coherence. We perform THz pump-optical probe spectroscopy for underdoped and overdoped Bi$_2$Sr$_2$CaCu$_2$O$_{8+x}$ (Bi2212) thin films. The oscillatory behavior in the pump-probe signal (THz Kerr signal) observed in the SC phase has been identified as the Higgs mode in single crystals in our previous work [K. Katsumi et al., Phys. Rev. Lett. 120, 117001 (2018)], but two onset temperatures are identified above $T_{\text{c}}$. Combined with the results of the single crystals in a wide range of doping, we find that the first onset $T_1^{\text{ons}}$ is 10-30 K above $T_{\text{c}}$. $T_1^{\text{ons}}$ coincides with that of the superfluid density $N_s$ extracted from the THz optical conductivity. Hence, $T_1^{\text{ons}}$ is interpreted as the onset of macroscopic SC phase stiffness. On the other hand, the second onset $T_2^{\text{ons}}$ is identified at substantially higher than $T_{\text{c}}$, whose origin is discussed in terms of preformed Cooper pairs., Comment: 24 pages, 9 figures

The pure Kitaev honeycomb model harbors a quantum spin liquid in zero magnetic fields, while applying finite magnetic fields induces a topological spin liquid with non-Abelian anyonic excitations. This latter phase has been much sought after in Kitaev candidate materials, such as $\alpha$-RuCl$_3$. Currently, two competing scenarios exist for the intermediate field phase of this compound ($B=7-10$ T), based on experimental as well as theoretical results: (i) conventional multiparticle magnetic excitations of integer quantum number vs. (ii) Majorana fermionic excitations of possibly non-Abelian nature with a fractional quantum number. To discriminate between these scenarios a detailed investigation of excitations over a wide field-temperature phase diagram is essential. Here we present Raman spectroscopic data revealing low-energy quasiparticles emerging out of a continuum of fractionalized excitations at intermediate fields, which are contrasted by conventional spin-wave excitations. The temperature evolution of these quasiparticles suggests the formation of bound states out of fractionalized excitations.

In high energy physics, the Higgs field couples to gauge bosons and fermions and gives mass to their elementary excitations. Experimentally, such couplings can be inferred from the decay product of the Higgs boson, i.e. the scalar (amplitude) excitation of the Higgs field. In superconductors, Cooper pairs bear a close analogy to the Higgs field. Interaction between the Cooper pairs and other degrees of freedom provides dissipation channel for the amplitude mode, which may reveal important information about the microscopic pairing mechanism. To this end, we investigate the Higgs (amplitude) mode of several cuprate thin films using phase-resolved terahertz third harmonic generation (THG). In addition to the heavily damped Higgs mode itself, we observe a universal jump in the phase of the driven Higgs oscillation as well as a non-vanishing THG above Tc. These findings indicate coupling of the Higgs mode to other collective modes and potentially a nonzero pairing amplitude above Tc., Comment: 35 pages, 20 figues

The magnetoelectric coupling, i.e., cross-correlation between electric and magnetic orders, is a very desirable property to combine functionalities of materials for next-generation switchable devices. Multiferroics with spin-driven ferroelectricity presents such a mutual interaction concomitant with magneto- and electro-active excitations called electromagnons. TbMnO3 is a paradigmatic material in which two electromagnons have been observed in the cycloidal magnetic phase. However, their observation in TbMnO3 is restricted to the cycloidal spin phase and magnetic ground states that can support the electromagnon excitation are still under debate. Here, we show by performing Raman spectroscopy measurements under pressure that the lower-energy electromagnon (4 meV) disappears when the ground state enters from a cycloidal phase to an antiferromagnetic phase (E-type). On the contrary, the magnetoelectric activity of the higher-energy electromagnon (8 meV) increases in intensity by one order of magnitude. Using microscopic model calculations, we demonstrate that the lowerenergy electromagnon, observed in the cycloidal phase, originates from a higher harmonic of the magnetic cycloid, and we determine that the symmetric exchange-striction mechanism is at the origin of the higher-energy electromagnon which survives even in the E-type phase. The colossal enhancement of the electromagnon activity in TbMnO3 paves the way to use multiferroics more efficiently for generation, conversion and control of spin waves in magnonic devices.

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. In the coexisting charge-density-wave and superconducting orders, we unravel a strong in-gap superconducting mode, attributed to a Higgs mode, coexisting with the expected incoherent Cooper-pair breaking signature. The latter remains in the pure superconducting state reached above 8.5~GPa. Our report constitutes the first observation of such Raman active Higgs mode since the longstanding unique case 2H-NbSe$_2$., Comment: 6 pages, 3 figures. Supplementary Material available upon request

We report a symmetry resolved electronic Raman scattering (ERS) study of a bilayer graphene device under gate voltage. We show that the ERS continuum is dominated by interband chiral excitations of $A_{2}$ symmetry and displays a characteristic Pauli-blocking behavior similar to the monolayer case. Crucially, we show that non-chiral excitations make a vanishing contribution to the Raman cross-section due to destructive interference effects in the Raman amplitude matrix elements. This is in a marked contrast to optical absorption measurements and opens interesting venues for the use of Raman scattering as a selective probe of chiral degrees of freedom in topological matter and other 2D crystals.

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 1.6 GPa, indicating they play a marginal role in the four-fold enhancement of $T_{c}$ at higher pressures. The collapse of nematic fluctuations appears to be linked to a suppression of low energy electronic excitations which manifests itself by optical phonon anomalies at around 2 GPa, in agreement with lattice dynamical and electronic structure calculations using local density approximation combined with dynamical mean field theory. Our results reveal two different regimes of nematicity in the phase diagram of FeSe under pressure: a d-wave Pomeranchuk instability of the Fermi surface at low pressure and a magnetic driven orthorhombic distortion at higher pressure., Comment: 7 pages, 4 figures. Supplementary Material available upon request

We investigated the terahertz (THz)-pulse driven nonlinear response in the d-wave cuprate superconductor Bi2Sr2CaCu2O8+x (Bi2212) using a THz pump near-infrared probe scheme in the time domain. We have observed an oscillatory behavior of the optical reflectivity that follows the THz electric field squared and is strongly enhanced below Tc. The corresponding third-order nonlinear effect exhibits both A1g and B1g symmetry components, which are decomposed from polarization-resolved measurements. Comparison with a BCS calculation of the nonlinear susceptibility indicates that the A1g component is associated with the Higgs mode of the d-wave order parameter., Comment: 14 pages, 4 figures + supplemental material

Despite being usually considered two competing phenomena, charge-density-wave and superconductivity coexist in few systems, the most emblematic one being the transition metal dichalcogenide 2H-NbSe$_2$. This unusual condition is responsible for specific Raman signatures across the two phase transitions in this compound. While the appearance of a soft phonon mode is a well-established fingerprint of the charge-density-wave order, the nature of the sharp sub-gap mode emerging below the superconducting temperature is still under debate. In this work we use the external pressure as a knob to unveil the delicate interplay between the two orders, and consequently the nature of the superconducting mode. Thanks to an advanced extreme-conditions Raman technique we are able to follow the pressure evolution and the simultaneous collapse of the two intertwined charge density wave and superconducting modes. The comparison with microscopic calculations in a model system supports the Higgs-type nature of the superconducting mode and suggests that charge-density-wave and superconductivity in 2H-NbSe$_2$ involve mutual electronic degrees of freedom. These findings fill knowledge gap on the electronic mechanisms at play in transition metal dichalcogenides, a crucial step to fully exploit their properties in few-layers systems optimized for devices applications.

SrTiO$_{3}$, a quantum paraelectric, becomes a metal with a superconducting instability after removal of an extremely small number of oxygen atoms. It turns into a ferroelectric upon substitution of a tiny fraction of strontium atoms with calcium. The two orders may be accidental neighbors or intimately connected, as in the picture of quantum critical ferroelectricity. Here, we show that in Sr$_{1-x}$Ca$_{x}$TiO$_{3-\delta}$ ($0.002

We have performed Raman scattering measurements on URu$_2$Si$_2$ single crystals on a large energy range up to $\sim$ 1300 cm$^{-1}$ and in all the Raman active symmetries as a function of temperature down to 15 K. A large excitation, active only in the E$_{g}$ symmetry, is reported. It has been assigned to a crystal electric field excitation on the Uranium site. We discuss how this constrains the crystal electric field scheme of the Uranium ions. Furthermore, three excitations in the A$_{1g}$ symmetry are observed. They have been associated to double Raman phonon processes consistently with ab initio calculations of the phonons dispersion.

We review the recent developments in electronic Raman scattering measurements of charge nematic fluctuations in iron-based superconductors. A simple theoretical framework of a $d$-wave Pomeranchuk transition is proposed in order to capture the salient features of the spectra. We discuss the available Raman data in the normal state of 122 iron-based systems, particularly Co doped BaFe$_2$As$_2$, and we show that the low energy quasi-elastic peak, the extracted nematic susceptibility and the scattering rates are consistent with an electronic driven structural phase transition. In the superconducting state with a full gap the quasi-elastic peak transforms into a finite frequency nematic resonance, evidences for which are particularly strong in the electron doped systems. A crucial feature of the analysis is the fact that the electronic Raman signal is unaffected by the acoustic phonons. This makes Raman spectroscopy a unique probe of electronic nematicity., Comment: Invited review to a special issue in Iron-based superconductors

We establish a relation between the Raman response function in the $B_{1g}$ channel and the electronic contribution to the nematic susceptibility within the spin-driven approach to electron nematicity of the iron based superconductors. The spin-driven nematic phase, characterized by the broken $C_4$ symmetry, but unbroken $O(3$) spin-rotational symmetry, is generated by the presence of magnetic fluctuations associated with the striped phase. It occurs as a separate phase between $T_N$ and $T_s$ in systems where the structural and magnetic phase transitions are separated. Detecting the presence of nematic degrees of freedom in iron-based superconductors is a difficult task, since it involves measuring higher order spin correlation functions. We show that the nematic degrees of freedom manifest themselves in the experimentally measurable Raman response function. We calculate the Raman response function in tetragonal phase in the large $N$ limit by considering Aslamazov-Larkin type of diagrams that contain a series of inserted fermionic boxes that resemble the nematic coupling constant of the theory. These diagrams effectively account for collisions between spin fluctuations. By summing an infinite number of such higher order diagrams, we demonstrate that the electronic Raman response function shows a clear maximum at the structural phase transition in the $B_{1g}$ channel. Hence, the Raman response function can be used to probe nematic degrees of freedom., Comment: 13 pages, 5 figures, corrected typos, discussion about A1g channel added, added references

We present Raman experiments on underdoped and overdoped Bi2Sr2CaCu2O(8+d) (Bi-2212) single crystals. We reveal the pseudogap in the electronic Raman spectra in the B1g and B2g geometries. In these geometries we probe respectively, the antinodal (AN) and nodal (N) regions corresponding to the principal axes and the diagonal of the Brillouin zone. The pseudogap appears in underdoped regime and manifests itself in the B1g spectra by a strong depletion of the low energy electronic continuum as the temperature decreases. We define a temperature T* below which the depletion appears and the pseudogap energy, omegaPG the energy at which the depeletion closes. The pseudogap is also present in the B2g spectra but the depletion opens at higher energy than in the B1g spectra. We observe the creation of new electronic states inside the depletion as we enter the superconducting phase. This leads us to conclude (as proposed by S. Sakai et al.) that the pseudogap has a different structure than the superconducting gap and competes with it. We show that the nodal quasiparticle dynamic is very robust and almost insensitive to the pseudogap phase contrary to the antinodal quasiparticle dynamic. We finally reveal, in contrast to what it is usually admitted,an increase of the nodal quasiparticle spectral weight with underdoping. We interpret this result as the consequence of a possible Fermi surface disturbances in the doping range p=0.1-0.2., Comment: 12 pages, 10 figures, JPCS conferences series 2013

Even though composite fermions in the fractional quantum Hall liquid are well established, it is not yet known up to what energies they remain intact. We probe the high-energy spectrum of the 1/3 liquid directly by resonant inelastic light scattering, and report the observation of a large number of new collective modes. Supported by our theoretical calculations, we associate these with transitions across two or more composite fermions levels. The formation of quasiparticle levels up to high energies is direct evidence for the robustness of topological order in the fractional quantum Hall effect., Comment: 4 pages, 5 figures

The spin degree of freedom in quantum phases of the second Landau level is probed by resonant light scattering. The long wavelength spin wave, which monitors the degree of spin polarization, is at the Zeeman energy in the fully spin-polarized state at $\nu$=3. At lower filling factors the intensity of the Zeeman mode collapses indicating loss of polarization. A novel continuum of low-lying excitations emerges that dominates near $\nu$=8/3 and $\nu$=5/2. Resonant Rayleigh scattering reveals that quantum fluids for $\nu<3$ break up into robust domain structures. While the state at $\nu$=5/2 is considered to be fully polarized, these results reveal unprecedented roles for spin degrees of freedom., Comment: 4 pages, 5 figures

Neutral spin texture ST excitations at nu=1/3 are directly observed for the first time by resonant inelastic light scattering. They are determined to involve two simultaneous spin- flips. At low magnetic fields, the ST energy is below that of the magneto-roton minimum. With increasing in-plane magnetic field these modes energies cross at a critical ratio of the Zeeman and Coulomb energies of eta_c=0.020 +- 0.001. Surprisingly, the intensity of the ST mode grows with temperature in the range in which the magneto-roton modes collapse. The temperature dependence is interpreted in terms of a competition between coexisting phases supporting different excitations. We consider the role of the ST excitations in activated transport at nu=1/3., Comment: 5 pages, 4 color figs