Half-ordered state in the anisotropic Haldane-gap antiferromagnet NDMAP

TL;DR

This study uncovers a unique half-ordered state in NDMAP, where only half of the spin chains order at intermediate magnetic fields, revealing complex quantum phase behavior in anisotropic Haldane-gap antiferromagnets.

Contribution

It reports the discovery of a half-ordered phase in NDMAP with partial chain ordering, a novel phenomenon in Haldane-gap systems under high magnetic fields.

Findings

Identified two field-induced phase transitions in NDMAP.

Discovered a half-ordered state where only half the chains order.

Observed the transition from a gapped Haldane state to a long-range ordered phase.

Abstract

Neutron diffraction experiments performed on the Haldane gap material NDMAP in high magnetic fields applied at an angle to the principal anisotropy axes reveal two consecutive field-induced phase transitions. The low-field phase is the gapped Haldane state, while at high fields the system exhibits 3-dimensional long-range Neel order. In a peculiar phase found at intermediate fields only half of all the spin chains participate in the long-range ordering, while the other half remains disordered and gapped.