Signatures of field-induced Berezinskii-Kosterlitz-Thouless correlations in the three-dimensional manganite Bi0.5Sr0.5Mn0.9Cr0.1O3

Abstract

Despite experimental evidence for the two-dimensional topological Berezinskii-Kosterlitz-Thouless (BKT) transition in superconducting films and $^{4}\mathrm{He}$ superfluid films, the observation of a BKT transition in condensed matter systems has proven to be difficult. Potential signatures of BKT transitions were reported in two-dimensional magnets, however, the observation of a classical BKT transition in nominally three-dimensional systems has naturally not been investigated as the transition requires an underlying $XY$ model with spins confined to a plane. Here we report that the temperature dependence of the electron paramagnetic resonance (EPR) linewidth observed in ${\mathrm{Bi}}_{0.5}{\mathrm{Sr}}_{0.5}{\mathrm{Mn}}_{1\ensuremath{-}x}{\mathrm{Cr}}_{x}{\mathrm{O}}_{3}$ $(x=0.1 \mathrm{and} 0.04)$ as well as in certain other three-dimensional (3D) manganites undergoing antiferromagnetic transitions is described satisfactorily by the BKT model. We explain this unexpected observation of signatures of a two-dimensional topological phase transition in 3D systems in terms of an effective two-dimensional $XY$ easy plane anisotropy induced by the magnetic field applied in the EPR experiment that allows for the mediation of long-range vortex-like correlations between spin clusters formed due to phase segregation. This conclusion is supported by a re-analysis of EPR results reported earlier in La-doped ${\mathrm{CaMnO}}_{3}$ and a nonmanganite compound ${\mathrm{BaNi}}_{2}{\mathrm{V}}_{2}{\mathrm{O}}_{8}$. We infer that field-induced BKT correlations in a 3D system provide a step towards the observation of a BKT transition in a suitably chosen condensed matter system by the application of an appropriate magnetic field.