Photoinduced Enhancement of the Charge Density Wave Amplitude

Author(s): Singer, A; Patel, SKK; Kukreja, R; Uhliř, V; Wingert, J; Festersen, S; Zhu, D; Glownia, JM; Lemke, HT; Nelson, S; Kozina, M; Rossnagel, K; Bauer, M; Murphy, BM; Magnussen, OM; Fullerton, EE; Shpyrko, OG | Abstract: © 2016 American Physical Society. Symmetry breaking and the emergence of order is one of the most fascinating phenomena in condensed matter physics. It leads to a plethora of intriguing ground states found in antiferromagnets, Mott insulators, superconductors, and density-wave systems. Exploiting states of matter far from equilibrium can provide even more striking routes to symmetry-lowered, ordered states. Here, we demonstrate for the case of elemental chromium that moderate ultrafast photoexcitation can transiently enhance the charge-density-wave (CDW) amplitude by up to 30% above its equilibrium value, while strong excitations lead to an oscillating, large-amplitude CDW state that persists above the equilibrium transition temperature. Both effects result from dynamic electron-phonon interactions, providing an efficient mechanism to selectively transform a broad excitation of the electronic order into a well-defined, long-lived coherent lattice vibration. This mechanism may be exploited to transiently enhance order parameters in other systems with coupled degrees of freedom.