Breakdown of a Magnetization Plateau in Ferrimagnetic Mixed Spin-(1/2,S) Heisenberg Chains Due to a Quantum Phase Transition Towards the Luttinger Spin Liquid

Magnetization curves of the ferrimagnetic mixed spin-(1/2,S) Heisenberg chains are calculated with the help of density-matrix renormalization group method for several quantum spin numbers S=1, 3/2, 2 and 5/2. It is shown that the ferrimagnetic mixed spin-(1/2,S) Heisenberg chains exhibit irrespective of the spin value S exactly one intermediate magnetization plateau, which can be identified with the gapped Lieb-Mattis ferrimagnetic ground state. The magnetization plateau due to the Lieb-Mattis ferrimagnetism breaks down at a quantum phase transition towards the Luttinger spin liquid, which is characterized by a continuous change of the magnetization with the magnetic field until another quantum critical point is reached at the saturation field.Comment: 3 pages, 1 figure, presented at CSMAG16 conferenc

Unconventional quantum ordered and disordered states in the highly frustrated spin-1/2 Ising-Heisenberg model on triangles-in-triangles lattices

Ground state and thermodynamics of geometrically frustrated spin-1/2 Ising-Heisenberg model on two different but topologically related triangles-in-triangles lattices is investigated in particular. A rigorous mapping based on generalized star-triangle transformations enables one to derive exact analytical results for the free and internal energy, spontaneous magnetization, entropy and specific heat from the known exact results for the corresponding spin-1/2 Ising model on a triangular lattice. It is demonstrated that several unconventional quantum ordered and disordered phases may become the ground state in addition to the classical ferromagnetic phase with a perfect alignment of all constituent spins. A mutual competition between the ferromagnetic Ising and XXZ Heisenberg interactions may lead to an appearance of a peculiar quantum ferromagnetic phase, in which the magnetic moment of the Heisenberg spins undergoes a quantum reduction to one third of the saturation value in contrast with the fully saturated magnetic moment of the classical Ising spins. In the highly frustrated regime, we have found either the disordered quantum paramagnetic phase with the substantial residual entropy or a similar but strikingly spontaneously long-range ordered phase, in which a quantum reduction of the magnetic moment of the Heisenberg spins causes a small quantum reduction of the magnetic moment of otherwise purely classical Ising spins. It is evidenced that the specific heat exhibits diverse thermal dependences with or without a logarithmic divergence superimposed either on one or two round Schottky-type maxima.Comment: 16 pages, 13 figure

Magnetization curves and low-temperature thermodynamics of two spin-1/2 Heisenberg edge-shared tetrahedra

A full energy spectrum, magnetization and susceptibility of a spin-1/2 Heisenberg model on two edge-shared tetrahedra are exactly calculated by assuming two different coupling constants. It is shown that a ground state in zero field is either a singlet or a triplet state depending on a relative strength of both coupling constants. Low-temperature magnetization curves may exhibit three different sequences of intermediate plateaux at the following fractional values of the saturation magnetization: 1/3-2/3-1, 0-1/3-2/3-1 or 0-2/3-1. The inverse susceptibility displays a marked temperature dependence significantly influenced by a character of the zero-field ground state. The obtained theoretical results are confronted with recent high-field magnetization data of the mineral crystal fedotovite K2Cu3(SO4)3.Comment: 4 pages, 4 figures, presented at ICM 2018 conferenc

Phase diagrams of the spin-1/2 Ising-Heisenberg model on a triangle-hexagon lattice

Ground-state and finite-temperature phase diagrams of a geometrically frustrated spin-1/2 Ising-Heisenberg model on a triangle-hexagon lattice are investigated within the generalized star-triangle mapping transformation. It is shown that the ground state is constituted by two different spontaneously long-range ordered phases and one disordered spin-liquid phase.Comment: 5 pages, 2 figures, presented at 14th Czech and Slovak Conference on Magnetism 6.-9. July 2010, Kosice, Slovaki