[Paper Review] Microscopic Route to Nematicity in Sr3Ru2O7

Christoph M. Puetter, Jeffrey G. Rau+1 more|arXiv (Cornell University)|Sep 24, 2009

Advanced Condensed Matter Physics3 citations

TL;DR

This paper proposes a microscopic mechanism for nematicity in Sr3Ru2O7 based on a tight-binding model including t2g orbitals, spin-orbit coupling, and RuO6 octahedral rotation, which generates van Hove singularities near the Fermi level. The nematic phase arises from a competition between ferromagnetic and nematic order, where nearest-neighbor interactions preempt ferromagnetism by inducing anisotropic Fermi surface distortion primarily in the dxy-dominated γ2 band near (±π,0) and (0,±π).

ABSTRACT

An anisotropic metallic phase dubbed electronic nematic phase bounded by two consecutive metamagnetic transitions has been reported in the bilayer ruthenate Sr3Ru2O7. It has also been shown that the nematic and the accompanying metamagnetic transitions are driven by an effective momentum-dependent quadrupole-type interaction. Here, we study the microscopic origin of such an effective interaction. To elucidate the mechanism behind the spontaneous Fermi surface distortion associated with the nematic, we identify a simple tight binding model based on t2g orbitals, spin-orbit coupling and the rotation of RuO6 octahedra as starting point, consistent with the Fermi surface obtained from recent angle-resolved photoemission data. Within an extended Hubbard model the nematic state, characterized by an anisotropy between the bands near $(\pm π,0)$ and $(0,\pm π)$, then strongly competes with ferromagnetic order but pre-empts it via a finite nearest neighbor interaction. We discuss experimental means to confirm our proposal.

Motivation & Objective

• To identify the microscopic origin of electronic nematicity in Sr3Ru2O7, which is observed between two metamagnetic transitions.
• To reconcile the effective momentum-dependent quadrupole interaction driving nematicity with a realistic electronic band structure based on t2g orbitals and spin-orbit coupling.
• To explain why nematic order emerges before ferromagnetic order despite strong on-site repulsion favoring ferromagnetism.
• To propose experimental tests—such as scanning tunneling microscopy and quantum oscillations—to distinguish this mechanism from alternative orbital-ordering scenarios.

Proposed method

• A single-layer tight-binding model is constructed using t2g orbitals (dxy, dxz, dyz) with spin-orbit coupling and unit-cell doubling due to RuO6 octahedral rotation.
• The model includes nearest-neighbor hopping, on-site Coulomb interactions (U), and inter-orbital interactions (V), with parameters tuned to reproduce ARPES-derived Fermi surface topology.
• The nematic state is characterized by anisotropy in the quasiparticle density between (±π,0) and (0,±π) points, captured via the order parameter ∑k[cos(kx)−cos(ky)]n_k.
• A multi-orbital Hubbard model is used to study competing instabilities, with mean-field theory applied to analyze nematic, ferromagnetic, and spin-nematic channels.
• The role of van Hove singularities (vHS) near the reduced Brillouin zone corners is analyzed as a key factor in enhancing nematic susceptibility.
• Bilayer coupling is considered but found to have minor influence on nematic ordering, with the focus on the single-layer effective model.

Experimental results

Research questions

• RQ1 What microscopic mechanism generates the nematic state in Sr3Ru2O7, given its proximity to metamagnetic transitions and anisotropic resistivity?
• RQ2 How does the interplay between spin-orbit coupling, octahedral rotation, and t2g orbital hybridization lead to the formation of van Hove singularities that favor nematic order?
• RQ3 Why does nematic order emerge before ferromagnetic order despite the on-site repulsion strongly favoring the latter?
• RQ4 What is the role of nearest-neighbor interactions in enabling nematic order to preempt ferromagnetism in the presence of competing instabilities?
• RQ5 How can this mechanism be experimentally distinguished from alternative proposals based on quasi-1D orbital ordering?

Key findings

• The nematic phase is driven by anisotropic Fermi surface distortion primarily in the γ2 band, which is dominated by the dxy orbital near (±π,0) and (0,±π) points.
• On-site intra-orbital repulsion strongly favors ferromagnetic order, while on-site inter-orbital interactions slightly favor nematic order over both ferromagnetic and spin-nematic channels.
• Nearest-neighbor interactions significantly enhance the charge and spin-nematic susceptibilities, enabling the nematic transition to occur before ferromagnetic order sets in.
• The combination of spin-orbit coupling and octahedral rotation generates flat bands and van Hove singularities near the Fermi level at the zone corners, making the system highly susceptible to nematic ordering.
• The model reproduces the observed Fermi surface topology consistent with recent angle-resolved photoemission spectroscopy (ARPES) data.
• Experimental probes such as scanning tunneling microscopy and quantum oscillations are proposed to distinguish this dxy-driven nematic mechanism from alternative orbital-ordering scenarios.