Higgs-mode radiance and charge-density-wave order in 2H−NbSe2

Abstract

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-NbSe2. 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 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-densitywave 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-NbSe2 involve mutual electronic degrees of freedom. These findings fill the 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.

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

“…The proposed mechanism of observability of the Higgs mode in the presence of CDW amplitudons [3,16,30] has been shown to be consistent with the pressure dependence of the electronic excitations in NbSe 2 [29]. The present data suggest this mechanism is also at play in 2H-TaS 2 .…”

“…3 d)) likely due to Coulomb screening in the A 1g channel [52][53][54]. Above P c , at 9.5 GPa, the in-gap mode disappears with the collapse of the CDW order, thus mimicking the behavior of the in-gap mode measured in 2H-NbSe 2 [27][28][29]. Both modes in these two brothers compounds certainly share the same nature.…”

“…However nature of the measured mode and the conditions of its observability are still under debate [13,17,[22][23][24][25][26]. Early on a possible observation of the Higgs mode was also reported in the transition metal dichalcogenide (TMDC) 2H-NbSe 2 using Raman spectroscopy [27][28][29] where the Higgs mode could be visible thanks to the coupling to the CDW amplitudon [3,16,30]. At present 2H-NbSe 2 remains a unique case and other observation of Higgs modes in CDW superconductors are desirable to assess how generic is the coupling between the Higgs and the amplitudon.…”

Theory of coherent-oscillations generation in terahertz pump-probe spectroscopy: From phonons to electronic collective modes

“…Within the conventional Peierls-like description of the CDW transition 56,57 , the ordering is driven by an instability of the electronic charge susceptibility at the nesting wavevector Q, which causes the softening of the coupled phonon mode at Q, whose frequency Ω 0 is renormalized by charge fluctuations and thus softens when T approaches the critical temperature T CDW . Below T CDW the phonon couples to the amplitude (Higgs) fluctuations of the CDW order parameter [58][59][60][61] , and its frequency Ω 0 progressively increases by reducing the temperature, reaching a finite value at T = 0 that is usually much smaller than the bare one ω 0 , see Fig. 8a.…”

Classification and characterization of nonequilibrium Higgs modes in unconventional superconductors

“…1a) [1][2][3][4][5] . The charge neutral Higgs mode does not couple to linear optical probes and therefore was expected to be observable only in materials with competing orders 6,7 , for which it was measured in Raman experiments [8][9][10] . However, an impulsive excitation of Higgs oscillations in nonequilibrium is possible via a nonlinear process by quenching the Mexican hat potential with an ultrafast THz light pulse.…”