Speaker
Description
We study random ( S = 2 ) antiferromagnetic Heisenberg chains with alternating bond strengths using the tensor network renormalization group method. In the clean limit,\cite{Damle2002} dimerization induces two quantum critical points separating three valence bond solid (VBS) phases: ( (\sigma, 4 - \sigma) = (2,2), (3,1), (4,0) ), characterized by ( \sigma ) valence bonds on even links and ( 4 - \sigma ) on odd links. Introducing bond randomness, we compute disorder-averaged twist order parameters and spin correlations to classify the resulting random VBS phases. The twist order parameter changes sign with ( \sigma )'s parity, distinguishing between even and odd VBS phases. Our results reveal a multicritical point at intermediate disorder and finite dimerization, where the three VBS phases converge. This point lies at the junction of three phase boundaries in the ( R\text{-}D ) plane. In the undimerized limit (( D = 0 )), the multicritical point separates a gapless Haldane phase from an infinite-randomness critical line. We further identify the (3,1)-(4,0) boundary as an infinite-randomness line even at weak disorder, and observe similar behavior near the (2,2)-(3,1) boundary close to the multicritical point.
