Abstract
Charge density waves (CDWs), i.e. the periodic spatial modulation of coupled electronic and lattice density, are ubiquitous in low-dimensional conductors and have taken on renewed relevance due their role in state-of-the-art materials, e.g. high-Tc superconductors, topological insulators and low-dimensional carbon. As CDWs are described by a complex order parameter to represent both the amplitude and phase, they are formally analogous to BCS superconductors and spin-waves, providing a prototype of collective phenomena for the further development of field theories and ab-initio calculations of complex solids. The low-energy excitations are mixed electron-phonon quanta which ideally separate into an amplitude and phase channel, and provide a sensitive probe of the ground state and non-equilibrium dynamics, including ultrafast photoinduced phase transitions. While recent studies of the amplitude modes have brought substantial progress aided by a phenomenological Ginzburg-Landau framework, we focus here on the phase modes using ultrafast terahertz spectroscopy. Experiments on K0.3MoO3 provide a more complete picture, and reveal a high sensitivity to interactions with impurities and screening effects from photogenerated carriers, both of which can be accounted for by generalizations of the model. Moreover, our considerations emphasize the need to revisit the treatment of inherent electronic damping in quantum-mechanical CDW theories.
Highlights
The low-temperature Charge density waves (CDWs) phase in organic and inorganic solids[1] serves as an important prototype for a broad range of collective phenomena involving strongly coupled degrees of freedom and spontaneously symmetry-broken ground states[2]
While the phase channel has been probed in certain CDW materials with terahertz (THz) and infrared spectroscopy[10] including non-equilibrium dynamics[14, 15], these studies did not interpret the results in terms of the QM or time-dependent Ginzburg-Landau (TDGL) models
While the extended TDGL model allows us to assign and account for all three phase-phonons here, it is important to note that this contrasts with the predictions of the nominal QM theory[8, 9], where one of the renormalized modes is driven to a near-zero-frequency “phason”
Summary
The low-temperature CDW phase in organic and inorganic solids[1] serves as an important prototype for a broad range of collective phenomena involving strongly coupled degrees of freedom and spontaneously symmetry-broken ground states[2]. The model describes the oQwro =drek 0rtpdoaiisrntaocmlruteditoeenres(ffEoeOfcttPsh,se∼∆uCc)hDwaWisthtivmliianee-cadlarespcsoeicnuadplleiennqtguptaeotrittouhnresbbaoatfiromenposhtoioofnnthofneospr(otξ thnee,nFtciioagml.s1pa)nl.edTxhhciiosgohprredorivsnipdaatetesisaalodvfeitmrhseeanteisllieeocfntrrsao1m2n, 1ei3c-, which should find more general applicability to other collective phenomena and photo-induced phase transitions In such optical-pump optical-probe (OP-OP) experiments[11,12,13] one probes the amplitude channel via coherent oscillations in the interband permittivity. Further experimental studies of the phase-channel and theory development are required to reach a complete and unified description of the CDW physics Towards this goal, in the present paper we target the quasi-1D conductor blue bronze (K0.3MoO3)[21], which forms an incommensurate CDW below Tc = 183 K, and whose low-energy spectrum has been intensely studied via neutron scattering[22, 23], Raman[24, 25], and far-infrared[10, 26] spectroscopy. We apply optical-pump THz-probe spectroscopy (OP-TP), which probes the phase-phonons via the broadband, transient complex conductivity, and reconcile both the non-equilibrium dynamics and T-dependence of the bands with a generalized TDGL model
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