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Unraveling the Pseudogap Phase in Stripe-phase Nickelates

Unraveling the Pseudogap Phase in Stripe-phase Nickelates
Pseudogap dynamics in the transient reflectivity ΔR reveals ultrafast hole localization (left side & top), inducing a rapid increase of electron-phonon coupling (Fano-assymetry – right side)

Scientific Achievement
Members of the Ultrafast Materials Program identified a “pseudogap” phase in the model stripe system La1.75Sr0.25NiO4 as the dynamic precursor of stripe formation, characterized by femtosecond charge localization which drives strong electron-phonon coupling.

Significance and Impact
Clarifying the nickelate pseudogap opens new paths to understanding this puzzling phase in other, e.g. high-Tc complex oxides.

Summary
Self-organization of charges into atomic-scale patterns is a recurring feature in correlated materials, which are materials whose electron’s behaviors cannot be described in terms of non-interacting entities. This results in, for example, fluctuating stripes of electric charges whose role in high-temperature superconductivity is under debate. In high-Tc superconductors, recent studies have observed fluctuating charge order above critical temperatures, which raises questions about the causal relation between dynamic charge stripe fluctuations and the yet unexplained ‘pseudogap’ phase, or transition phase to superconductivity.

A particularly intriguing model system to explore is the nickelate compound La(2-x)Sr(x)NiO(4). Electromagnetic response of nickelates is characterized by a mid-infrared (mid-IR) gap in optical conductivity along with temperature-dependent lattice vibrations. To better understand the relationship between stripe formation and these excitations, the Berkeley Lab Ultrafast Materials Program combined broadband ultrafast mid-IR spectroscopy (this excites extremely small timescale dynamics, thus avoiding the loss of information in time-averaged studies) with equilibrium optical and X-ray diffraction studies to explore low-energy excitations of a stripe-phase nickelate and their coupling in space and time. This led to identification of the mid-IR optical gap as a pseudogap feature, distinct from long-range stripe ordering. The pseudogap appears at a temperature far above stripe formation, acting as a precursor phase. These results suggest the strong and rapid dependence of electron-phonon coupling on local charge arrangement, and open a path to understanding the mysterious phase in a broad class of complex oxides.

G. Coslovich et al., Nature Communications 4, 2643 (2013).