A composite electron-lattice order: electronic nematicity of 2DEG and polarization density waves at a near-ferroelectric interface

Abstract

We consider a two-dimensional electron gas (2DEG) formed at a near-ferroelectric interface and strongly coupled to polar phonons. Through a self-consistent microscopic many-body calculation, we show that the coupled system stabilizes a composite electron-lattice ordered state in which the lattice polarization spontaneously forms a polarization density wave (PDW), accompanied by an electronic stripe order in the 2DEG. This intertwined order partially reconstructs the electronic spectrum and generates a twofold quasiparticle anisotropy, giving rise to electronic nematicity at the single-particle level. However, under strong external electric fields, the nematic response becomes dominated by the collective sliding dynamics of the composite order: the sliding motion overwhelms the quasiparticle anisotropy and produces a strongly enhanced nematic signal with higher-order angular harmonics. The theory offers a natural explanation for several anomalous transport and anisotropic responses recently observed at the KTaO$_3$ (111) interface. We also estimate the mean-field transition temperature of this emergent ordered state, obtaining good agreement with experiments, and analyze its evolution with several tuning parameters. The proposed composite order, along with the field-induced crossover from quasiparticle-driven to sliding-dominated nematicity, provides a distinct mechanism of nematicity arising from many-body effects and collective dynamics in critical electron-boson systems, with applicability beyond ferroelectric platforms.

Figures (3)

• Figure 1: Formation of ( a) an electronic density-wave modulation with stripe order and ( b) a PDW. ( c) Corresponding quasiparticle spectra and ( d) the associated Fermi surface. The $x$ axis is chosen parallel to ${\bf Q}$, whose spontaneous selection signals rotational symmetry breaking. The calculated value of $|\Delta(T=0)|$ is $0.35$ meV ($\ll E_F$); it is rescaled by a factor of 100 for schematic purposes in ( c) and ( d).
• Figure 2: Angular dependence of ( a) the quasiparticle-dominated conductivity and ( c) the sliding-motion-dominated conductivity, at different temperatures. Temperature dependence of the transport nematicity $\delta N$ in ( b) the quasiparticle-dominated regime and ( d) the sliding-dominated regime. Inset of ( b): temperature dependence of the order parameter. Inset of ( d): transverse conductivity at different temperatures. Model parameters are given in Sec. SVI.
• Figure 3: ( a) Transition temperature $T_c$ as a function of electron-phonon coupling constant ${\lambda}$. Inset: $\ln T_c$ as a function of $1/\lambda$, showing a clear deviation from the linear dependence expected in conventional BCS scaling. Here $\lambda_0=0.26$. ( b) Schematic illustration of $T_c$ and the TO-mode gap $\Delta_{\rm sp}=\sqrt{a}$ versus the tuning parameter $x/x_{\rm cp}$. A phenomenological linear softening, $a(x)=a(0)(1-x/x_{\rm cp})$, is assumed to illustrate the enhancement of $T_c$ toward the ferroelectric critical point. The dashed curve denotes the dielectric response $\varepsilon(x)$, which grows and formally diverges at criticality (orange region).