Competing orders at higher-order Van Hove points

TL;DR

This paper investigates the effects of higher-order Van Hove points in materials like intercalated graphene, revealing how they influence competing electronic orders such as ferromagnetism, superconductivity, and novel spin Pomeranchuk states through a renormalization group analysis.

Contribution

It introduces an effective low-energy model for higher-order Van Hove points and analyzes the competition of electronic orders using an unbiased renormalization group approach.

Findings

Higher-order Van Hove points exhibit power-law divergent density of states.

Competing tendencies include ferromagnetism and chiral superconductivity.

A new spin Pomeranchuk order emerges with small attractive interactions.

Abstract

Van Hove points are special points in the energy dispersion, where the density of states exhibits analytic singularities. When a Van Hove point is close to the Fermi level, tendencies towards density wave orders, Pomeranchuk orders, and superconductivity can all be enhanced, often in more than one channel, leading to a competition between different orders and unconventional ground states. Here we consider the effects from higher-order Van Hove points, around which the dispersion is flatter than near a conventional Van Hove point, and the density of states has a power-law divergence. We argue that such points are present in intercalated graphene and other materials. We use an effective low-energy model for electrons near higher-order Van Hove points and analyze the competition between different ordering tendencies using an unbiased renormalization group approach. For purely repulsive interactions, we find that two key competitors are ferromagnetism and chiral superconductivity. For a small attractive exchange interaction, we find a new type of spin Pomeranchuk order, in which the spin order parameter winds around the Fermi surface. The supermetal state, predicted for a single higher-order Van Hove point, is an unstable fixed point in our case.