Random-field-driven phase transitions in the ground state of the spin chain

The ground state of the XXZ spin chain under the influence of the random magnetic field is studied by means of the exact diagonalization method. The counterpart has been investigated extensively so far. The easy-plane area, including the Haldane and the XY phases, is considered. The area suffers significantly from the magnitude of the constituent spin. Destruction of the Haldane state is observed at a critical strength of the random field, which is comparable to the magnitude of the Haldane gap. This transition is characterized by the disappearance of the string order. The XY region continues until at a critical randomness, at which a transition of the KT-universality class occurs. These features are contrasted with those of the counterpart.