Abstract
Metallic LiOsO$_3$ undergoes a continuous ferroelectric-like structural phase transition below $T_c$ = 140 K to realize a polar metal. To understand the microscopic interactions that drive this transition, we study its critical behavior above $T_c$ via electromechanical coupling - distortions of the lattice induced by short-range dipole-dipole correlations arising from Li off-center displacements. By mapping the full angular distribution of second harmonic electric-quadrupole radiation from LiOsO$_3$ and performing a simplified hyper-polarizable bond model analysis, we uncover subtle symmetry-preserving lattice distortions over a broad temperature range extending from $T_c$ up to around 230 K, characterized by non-uniform changes in the short and long Li-O bond lengths. Such an extended region of critical fluctuations may explain anomalous features reported in specific heat and Raman scattering data, and suggests the presence of competing interactions that are not accounted for in existing theoretical treatments. More broadly, our results showcase how electromechanical effects serve as a probe of critical behavior near inversion symmetry breaking transitions in metals.
Highlights
Ferroelectric phase transitions typically occur in insulating materials where long-range electrostatic forces between local electric dipoles are unscreened [1]
By detecting such electrostrictive effects in LiOsO3 using high-multipole optical second harmonic generation rotational anisotropy (SHG-RA), we reveal a wide critical region extending from Tc up to T ∼ 230 K, the characteristic temperature where short-range correlations start to grow
We cannot reliably extract χiEjQkl values because the fits are underdetermined, especially considering the χiEjQkl ’s can be complex for absorbing materials, but the important observation is their atypical nonuniform temperature dependence. This indicates that a nonuniform but symmetry-preserving lattice distortion, i.e., a polymorphous representation of the R3 ̄c structure, takes place between T and Tc, revealing an extended critical region over which DiD j grows from dissimilar Li displacements that are correlated over the probe volume
Summary
Ferroelectric phase transitions typically occur in insulating materials where long-range electrostatic forces between local electric dipoles are unscreened [1]. We detect short-range dipolar correlations arising from Li cation displacements above Tc in LiOsO3 through electromechanical coupling, which is normally seen in insulators. In polar metals, this effect can be large due to microscopic dipole-strain interactions. DiD j can be tracked via subtle symmetrypreserving changes in atomic coordinates By detecting such electrostrictive effects in LiOsO3 using high-multipole optical second harmonic generation rotational anisotropy (SHG-RA), we reveal a wide critical region extending from Tc up to T ∼ 230 K, the characteristic temperature where short-range correlations start to grow. The observation of strong fluctuations suggests the presence of competing short-range interactions in the system, which is not captured by existing density functional theory or effective model calculations [6] and is consistent with specific heat data reporting a low entropy loss across Tc [2]
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