The coexistence of ferroelectricity and metallicity seems paradoxical,
since the itinerant electrons in metals should screen the long-range
dipole interactions necessary for dipole ordering. The recent discovery
of the polar metal LiOsO
3
was therefore surprising [as
discussed earlier in Y. Shi et al.,
Nat. Mater
.
2013
,
12
, 1024]. It is thought that the coordination
preferences of the Li play a key role in stabilizing the LiOsO
3
polar metal phase, but an investigation from the combined
viewpoints of core-state specificity and symmetry has yet to be done.
Here, we apply the novel technique of extreme ultraviolet second harmonic
generation (XUV-SHG) and find a sensitivity to the broken inversion
symmetry in the polar metal phase of LiOsO
3
with an enhanced
feature above the Li K-edge that reflects the degree of Li atom displacement
as corroborated by density functional theory calculations. These results
pave the way for time-resolved probing of symmetry-breaking structural
phase transitions on femtosecond time scales with element specificity.
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