Landau Quantization Evolution of the Spinon Pair Spectrum in Weak Mott Insulator La.sub.0.15Sm.sub.0.85MnO.sub.3 + [delta] with Spinon Fermi Surface with Increasing Temperature and Magnetic Field Strength

Measurements of the temperature dependences of La.sub.0.15Sm.sub.0.85MnO.sub.3 + [delta] magnetization in the temperature range of 4.2-100 K detected a magnetization threshold feature near the temperature T.sub.d â 50 K, as-sociated with the existence of the small pseudogap [DELTA].sub.e in the electron spectrum, characteristic of the weak Mott insulator. An increase in the external magnetic field strength H results in dielectric pseudogap [DELTA].sub.e suppression, an increase in the density of states of free charge/spin carrier at E.sub.F, and the transformation of wave fragments of the charge/spin density. In the temperature range of 4.2-12 K, the quantization of the spectrum of pairs of low-energy magnetic excitations of Z.sub.2 quantum spin liquid is found in the form of composite spinon-gauge field quasiparticles. The formation of the continuous spectrum of quantum spin liquid excitations in the mode of "weak magnetic fields" H = 100 Oe, 350 Oe, and 1 kOe is explained within the models of Landau quantization of the spectrum of composite quasiparticles with fractional filling factors ν of three overlapping Landau bands. In the mode of the "strong external magnetic field" H = 3.5 kOe, new quantum oscillations of the temperature dependences of the magnetization of incompressible spinon liquid are detected in the form of three narrow steps (plateaus) corresponding to the complete filling of non-overlapping Landau bands with spinons with integer filling factors.