Abstract
AbstractPolaron transport, in which electron motion is strongly coupled to the underlying lattice deformation or phonons, is crucial for understanding electrical and optical conductivities in many solids. However, little is known experimentally about the dynamics of individual phonon modes during polaron motion. It remains elusive whether polarons have a key role in materials with strong electronic correlations. Here we report the use of a new experimental technique, ultrafast MeV-electron diffraction, to quantify the dynamics of both electronic and atomic motions in the correlated LaSr2Mn2O7. Using photoexcitation to set the electronic system in motion, we find that Jahn-Teller-like O, Mn4+ and La/Sr displacements dominate the lattice response and exhibit a dichotomy in behaviour—overshoot-and-recovery for one sublattice versus normal behaviour for the other. This dichotomy, attributed to slow electronic relaxation, proves that polaron transport is a key process in doped manganites. Our technique promises to be applicable for specifying the nature of electron–phonon coupling in complex materials.
Highlights
The temporal correlation between electron motion and the atomic lattice distortion[1,2,3,4,5,6,7] is considered essential to electronic transport, in behaviour ranging from resistance-free flow[2] to selftrapping,[3] obtaining direct experimental information about the dynamics of electron–lattice coupling on the picosecond timescale is a daunting problem
By using the manganite LaSr2Mn2O7 as a test bed, we have demonstrated here the capability of MeV-UED to quantify very short timescale correlations of the atomic and electronic systems in a complex material
This is direct evidence for polaron formation, and shows that the motion of electrons with a 0.6° bending angle along the [040] direction) was determined by matching the intensities of ~ 40 Bragg and OO spots in the (001) zone before time zero using the atomic positions based on neutron diffraction[14] and refined by electron diffraction experiments
Summary
The temporal correlation between electron motion and the atomic lattice distortion[1,2,3,4,5,6,7] is considered essential to electronic transport, in behaviour ranging from resistance-free flow[2] to selftrapping,[3] obtaining direct experimental information about the dynamics of electron–lattice coupling on the picosecond timescale is a daunting problem. The use of ultrafast pump–probe techniques to drive electronic systems out of equilibrium has emerged as a powerful means for altering electronic states, e.g., to yield photoinduced transient superconductivity in cuprates[4,9,10] and an antiferromagnetic to ferromagnetic transition in manganites.[11]. This opens the possibility of observing the correlation of the electronic and atomic motions that occurs on entry into a nonequilibrium state.[10] Here we pursue this idea through the quantitative ultrafast and ultrahigh-energy electron diffraction (MeV-UED) characterisation of the prototypical half-doped bi-layer manganite, LaSr2Mn2O7. We are able to quantitatively follow, on the picosecond timescale, the changes in the crystal structure and electronic system of this material in response to photoexcitation, and from those observations find an unanticipated correlation of the two systems that illustrates the power of this new technique in understanding the electronic properties of condensed matter systems
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