SO(n) AKLT Chains as Symmetry Protected Topological Quantum Ground States

TL;DR

This thesis explores the properties of $SO(n)$ AKLT chains, a family of exactly solvable 1D quantum spin models with symmetry protected topological phases, revealing their ground state structure, symmetry breaking, and topological indices.

Contribution

It introduces a new class of exactly solvable $SO(n)$ AKLT models, analyzes their ground states, symmetry breaking, and computes their SPT indices within a generalized framework.

Findings

Ground states admit matrix product state description.

For even n, exhibit $O(n)$-to-$SO(n)$ symmetry breaking.

States have arbitrarily large correlation and injectivity lengths.

Abstract

This thesis studies a pair of symmetry protected topological (SPT) phases which arise when considering one-dimensional quantum spin systems possessing a natural orthogonal group symmetry. Particular attention is given to a family of exactly solvable models whose ground states admit a matrix product state description and generalize the AKLT chain. We call these models ``$SO(n)$ AKLT chains'' and the phase they occupy the ``$SO(n)$ Haldane phase''. We present new results describing their ground state structure and, when $n$ is even, their peculiar $O(n)$-to-$SO(n)$ symmetry breaking. We also prove that these states have arbitrarily large correlation and injectivity length by increasing $n$, but all have a 2-local parent Hamiltonian, in contrast to the natural expectation that the interaction range of a parent Hamiltonian should diverge as these quantities diverge. We extend Ogata's definition of an SPT index for a split state for a finite symmetry group $G$ to an SPT index for a compact Lie group $G$. We then compute this index, which takes values in the second Borel group cohomology $H^2(SO(n),U(1))$, at a single point in each of the SPT phases. The two points have different indices, confirming the two SPT phases are indeed distinct. Chapter 1 contains an introduction with a detailed overview of the contents of this thesis, which includes several chapters of background information before presenting new results in Chapter 7 and Chapter 8.