Raman Scattering as a Selective Probe of Chiral Electronic Excitations in Bilayer Graphene

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

Higgs-mode radiance and charge-density-wave order in 2H-NbSe2_2

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

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 1.6 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 reques

Pressure-Induced Collapse of the Charge Density Wave and Higgs Mode Visibility in 2H−TaS2

The pressure evolution of the Raman active electronic excitations of the transition metal dichalcogenides 2H-TaS2 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 a new observation of such Raman active Higgs mode since the long-standing unique case 2H-NbSe2

Disentangling lattice and electronic instabilities in the excitonic insulator candidate Ta2_2NiSe5_5 by nonequilibrium spectroscopy

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 figure

Imaging simulations of selected science with the Magdalena Ridge Observatory Interferometer

We present simulated observations of surface features on Red Supergiant (RSG) stars and clumpy dust structures surrounding Active Galactic Nuclei (AGN) with the Magdalena Ridge Observatory Interferometer (MROI). These represent two of the classes of astrophysical targets enumerated in the MROI Key Science Mission that are typical of the types of complex astrophysical phenomena that the MROI has been designed to image. The simulations are based on source structures derived from recent theoretical models and include both random and systematic noise on the measured Fourier data (visibility amplitudes and closure phases) consistent with our expectations for typical such targets observed with the MROI. Image reconstructions, obtained using the BSMEM imaging package, are presented for 4-, 6- and 8- telescope implementations of the array. Although a rudimentary imaging capability is demonstrated with only 4 telescopes, the detailed features of targets are only reliably determined when at least 6 telescopes are present. By the tine 8 telescope are used, the reconstructed images are sufficiently faithful to allow the discrimination between competing models, confirming the design goal of the MROI, i.e. to offer model-independent near-infrared imaging on sub-milliarcsecond scales