The scaling behaviour of localised and extended states in one-dimensional tight-binding models with disorder

TL;DR

This paper studies the scaling behavior of localized and extended states in two one-dimensional disordered tight-binding models, revealing how disorder affects eigenmode properties and localization lengths through analytical and numerical methods.

Contribution

It provides a detailed analysis of the scaling laws and divergence behaviors of eigenmodes and localization lengths in two novel disordered models, including a crossover between regimes.

Findings

Participation ratio remains finite at zero energy in the first model.

Localization length diverges logarithmically as energy approaches zero.

Number of system-spanning eigenmodes scales with the square root of system size.

Abstract

We investigate two one-dimensional tight-binding models with disorder that have extended states at zero energy. We use exact and partial diagonalisation of the Hamiltonian to obtain the eigenmodes and the associated participation ratios, and the transfer-matrix method to determine the localisation length. The first model has no on-site disorder, but random couplings. While the participation ratio remains finite at zero energy, the localisation length diverges logarithmically as the energy goes to zero. We provide an intuitive derivation of this logarithmic divergence based on the weak coupling of the two sublattices. The second model has a conserved quantity as the row sums of the Hamiltonian are zero. This model can be represented as a harmonic chain with random couplings, or as a diffusion model on a lattice with random links. We find, in agreement with existing analytical calculations, that the number of system-spanning eigenmodes increases proportionally to the square root of the system size, and we related this power law to other power laws that characterise the scaling behaviour of the eigenmodes, the participation ratio, the localisation length, and their dependence on energy and system size. When disorder is so strong that the smallest hopping terms can be arbitrarily close to zero, all these power laws change, and we show a crossover between the two scaling regimes. All these results are explained by intuitive arguments based on scaling.