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F. M. Bukhanko and A. F. Bukhanko
«Topological Phase Transitions in States with Low and High Density of 2D Vortical Pairs Induced by Magnetic Field in Gapless Quantum Spin Liquid with Structural Disorder»
001–013 (2017)

PACS numbers: 03.65.Vf, 03.75.Lm, 75.30.Kz, 75.47.Gk, 75.47.Lx, 75.50.Ee, 75.60.Ej

In the (Sm(1-у)Gdу)0.55Sr0.45MnO3 system, it is determined continuous transition from a 3D ferromagnetic state of manganese spins in the initial sample with у = 0 to zigzag AFM spin ordering of a CE-type in ab-planes for у = 0.5, co-existing with a disordered phase of quantum Griffiths phase type in samples with у = 0.5, 0.6 and 0.7 at temperatures below TN = 48.5 K. At the further growth of gadolinium concentration, there is a melting of zigzag AFM structure of a СЕ-type that leads to occurrence of an unusual phase with characteristic signs of a gapless Z2 quantum spin liquid in Gd0.55Sr0.45MnO3 in the region of temperatures near absolute zero in a zero external magnetic field. The stepwise changes of isotherms of the magnetization measured at 4.2 K in the range +-75 kOe of fields are explained by quantum phase transitions of a Z2 spin liquid into the phase with topological order in weak magnetic fields and the polarized phase in strong fields. The essential difference of critical fields and size of jumps of magnetization in isotherms testifies to existence of the hysteretic phenomena in processes of magnetization–demagnetization of the quantum spin liquid caused by different nature of localization–delocalization of the 2D vortex pairs induced by a magnetic field in a quantum spin liquid with the disorder.

Key words: quantum spin liquid, topological phase transitions, vortices.

https://doi.org/10.15407/nnn.15.01.0001

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