Abstract
Although the frustrated (zigzag) spin chain is the Drosophila of frustrated magnetism, our understanding of a pair of coupled zigzag chains (frustrated spin ladder) in a magnetic field is still lacking. We address this problem through nuclear magnetic resonance (NMR) experiments on BiCu_2PO_6 in magnetic fields up to 45 T, revealing a field-induced spiral magnetic structure. Conjointly, we present advanced numerical calculations showing that even a moderate rung coupling dramatically simplifies the phase diagram below half-saturation magnetization by stabilizing a field-induced chiral phase. Surprisingly for a one-dimensional model, this phase and its response to Dzyaloshinskii-Moriya (DM) interactions adhere to classical expectations. While explaining the behavior at the highest accessible magnetic fields, our results imply a different origin for the solitonic phases occurring at lower fields in BiCu_2PO_6. An exciting possibility is that the known, DM-mediated coupling between chirality and crystal lattice may give rise to a new kind of spin-Peierls instability.
Highlights
The frustrated spin chain is the Drosophila of frustrated magnetism, our understanding of a pair of coupled zigzag chains in a magnetic field is still lacking
Is fully lifted, e.g.,43 by an external magnetic field and DM interactions]. This situation is similar to the aforementioned appearance of long-range AFM order following a field-induced Bose-Einstein condensation (BEC) of triplons in the unfrustrated spin ladder
We report the results of comprehensive numerical calculations for the frustrated ladder model (Fig. 2c), which clarify that (i) a field-induced chiral phase generally appears for sufficiently strong frustration and rung coupling, and (ii) no additional field-induced phases occur at lower
Summary
The frustrated (zigzag) spin chain is the Drosophila of frustrated magnetism, our understanding of a pair of coupled zigzag chains (frustrated spin ladder) in a magnetic field is still lacking. This is plausible, given that such spiral structures form the ground states of the classical zigzag chain ( S → ∞ ) and are, expected to arise as a field-induced phase of the quantum ( S = 1/2 ) zigzag chain, if residual interchain couplings permit conventional long-range magnetic o rder[20] Such spiral order corresponds to a secondary breaking of the U(1) symmetry emerging[24] from the combination of lattice-translation invariance and incommensurate spin correlations [in infinitely-extended systems this quasicontinuous U(1)-symmetry persists even if the continuous SU(2) symmetry of the magnetic moments themselves. This situation is similar to the aforementioned appearance of long-range AFM order following a field-induced BEC of triplons in the unfrustrated spin ladder
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