Speaker
Description
Most of us are familiar with ferromagnetic and antiferromagnetic materials.
Although in some cases quantum fluctuations can be strong in such systems we
would usually say that the ground state is ordered and described by a non-zero
local order parameter. In such systems, the interaction between the quantum
spins do not depend on the bond direction. Today, there is a growing class of
magnetic materials where it is believed that the interactions indeed are
bond-dependent in a way first imagined by Alexei Kitaev thereby opening a way
for realizing so called topological phases. Bond-dependent interactions are
strongly frustrating for the system and hinders conventional ordering.
However, in these Kitaev materials other interactions are also often present,
among them the well known Heisenberg coupling and also off-diagonal Gamma (Γ)
terms giving rise to an unusally rich phase diagram. Even for the simplest
models of Kitaev materials it is extremely difficult to arrive at a precise
understanding of this complex phase-diagram. Hence, in order to obtain
accurate results it is often useful to restrict the analysis to
low-dimensions and here we mainly discuss chains and two-leg ladders. Using
advanced numerical techniques, it is possible for such models to determine
the phase-diagram with very high precision, including the effects of an
applied magnetic field. An astonishing abundance of phases arises from the
combination of frustration and applied field. In this talk I will focus on
some of these phases that appear disordered, without any conventional local
magnetic ordering, but where a hidden string-order can be identified.
Surprisingly, such string-order was first suggested in the context of surface
roughening.
