Spin Stiffness and Domain Walls in Dirac-Electron Mediated Magnets

Abstract

Spin interactions of magnetic impurities mediated by conduction electrons is one of the most interesting and potentially useful routes to ferromagnetism in condensed matter. In recent years such systems have received renewed attention due to the advent of materials in which Dirac electrons are the mediating particles, with prominent examples being graphene and topological insulator surfaces. In this paper, we demonstrate that such systems can host a remarkable variety of behaviors, in many cases controlled only by the density of electrons in the system. Uniquely characteristic of these systems is an emergent long-range form of the spin stiffnes when the Fermi energy resides at a Dirac point, becoming truly long-range as the magnetization density becomes very small. It is demonstrated that this leads to screened Coulomb-like interactions among domain walls, via a subtle mechanism in which the topology of the Dirac electrons plays a key role: the combination of attraction due to bound in-gap states that the topology necessitates, and repulsion due to scattering phase shifts, yields logarithmic interactions over a range of length scales. We present detailed results for domain walls in a particularly rich system, the (111) surface of a model topological crystalline insulator. This hosts two-fold and six- fold degenerate groundstates, with either short-range or emergent long-range interactions among the spins. In the latter case we demonstrate in detail the presence of in-gap states associated with domain walls, and argue that this stabilizes a pseudogap regime at finite temperature. Thus the topological nature of these systems, through its impact on domain wall excitations, leads to unique behaviors distinguishing them markedly from their non-topological analogs.

25 pages, 21 figures, submitted to PRX

References in corpus (21)

• Topological Crystalline Insulators
• Electronic States of Graphene Nanoribbons
• Topological confinement in bilayer graphene
• Interactions and phase transitions on graphene's honeycomb lattice
• Edge states in Graphene: from gapped flat band to gapless chiral modes
• RKKY in half-filled bipartite lattices: graphene as an example
• Diluted Graphene Antiferromagnet
• Ordering of magnetic impurities and tunable electronic properties of topological insulators
• The Physics of Kondo Impurities in Graphene
• Dirac mass generation from crystal symmetry breaking on the surfaces of topological crystalline insulators
• Surface magnetic ordering in topological insulators with bulk magnetic dopants
• Ising and Gross-Neveu model in next-to-leading order
• RKKY Interaction On Surfaces of Topological Insulators With Superconducting Proximity Effect
• Topological Crystalline Insulator Nanostructures
• RKKY Interactions in Graphene: Dependence on Disorder and Gate Voltage
• Inducing magnetism onto the surface of a topological crystalline insulator
• Finite temperature ordering of dilute graphene antiferromagnets
• Chiral tricritical point: a new universality class in Dirac systems
• Spontaneous Layer Polarization and Conducting Domain Walls in the Quantum Hall Regime of Bilayer Graphene
• Ferromagnetism and glassiness on the surface of topological insulators
• Carrier-density-controlled anisotropic spin susceptibility of two-dimensional hole systems

Cited by in corpus (3)

• RKKY Interactions in Graphene Landau Levels
• RKKY coupling in Weyl semimetal thin films
• Topological Magnetoelectric Switching on the Atomic Scale