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In this talk, we report an unconventional Néel-ordered phase in the frustrated S=1/2 two-leg ladder antiferromagnet C₉H₁₈N₂CuBr₄ (DLCB for short). The temperature dependence of the gapped transverse excitations in the ordered phase at ambient pressure cannot be described by the conventional S=1 magnons, associated with explicit symmetry breaking. Contrary to conventional understanding, the ground state is best described as a disordered singlet with a spin gap. Accordingly, the origin of the spin gap in DLCB is not owing to the spin anisotropy and the three-dimensional magnetic order ought to emerge in an unconventional way. Haldane’s conjecture on spin-1 chains [1] is proposed to explain the opening of the spin gap in DLCB and the analysis of the free energy of the S=1/2 two-leg ladder sublattices supports that the magnetic order of DLCB can arise exclusively from thermal fluctuations by the mechanism of order-by-disorder [2, 3]. Our work indicates the presence of a symmetry-protected topological phase at ambient pressure in DLCB [4], which cannot be described by Landau’s symmetry-breaking theory.

References:

[1] F. D. M. Haldane, Phys. Lett. 93A, 464 (1983); Phys. Rev. Lett. 50, 1153 (1983).

[2] J. Villain et al., J. Physique 41, 1263-1272 (1980).

[3] C. L. Henley, Phys. Rev. Lett. 62, 2056 (1989).

[4] T. Hong et al., arXiv: 2306.06021.