Revealing extremely low energy amplitude modes in the charge-density-wave compound LaAgSb$_2$

TL;DR

This study uses optical spectroscopy and ultrafast measurements to reveal extremely low energy amplitude modes in LaAgSb$_2$, a charge-density-wave compound, showing novel low-frequency collective excitations linked to its phase transitions.

Contribution

It reports the discovery of unusually low-frequency amplitude modes in LaAgSb$_2$, highlighting their connection to small modulation wave vectors and low energy scales of the CDW states.

Findings

Observation of low-frequency amplitude modes at 0.12 THz and 0.34 THz.

Energy gaps develop below phase transition temperatures, removing free carrier spectral weight.

Amplitude and relaxation times remain unchanged across phase transitions.

Abstract

The layered lanthanum silver antimonide LaAgSb$_2$ was known to experience two charge density (CDW) phase transitions, which were proposed recently to be closely related to the newly identified Dirac cone. We present optical spectroscopy and ultrafast pump-probe measurement on the compound. The development of energy gaps were clearly observed below the phase transition temperatures in optical conductivity, which removes most part of the free carrier spectral weight. Time resolved measurement demonstrated the emergence of strong oscillations upon entering the CDW states, which were illuminated to come from the amplitude mode of CDW collective excitations. The frequencies of them are surprisingly low: only 0.12 THz for the CDW order with higher transition temperature and 0.34 THz for the lower one, which shall be caused by their small modulation wave vectors. Furthermore, the amplitude and relaxation time of photoinduced reflectivity stayed unchanged across the two phase transitions, which might be connected to the extremely low energy scales of amplitude modes.