Semiquantitative theory for high-field low-temperature properties of a distorted diamond spin chain

We consider the antiferromagnetic Heisenberg model on a distorted diamond chain and use the localized-magnon picture adapted to a distorted geometry to discuss some of its high-field low-temperature properties. More specifically, in our study we assume that the partition function for a slightly distorted geometry has the same form as for ideal geometry, though with slightly dispersive one-magnon energies. We also discuss the relevance of such a description to azurite.Comment: 10 pages, 4 figures; Presented at the 4-th Conference on Statistical Physics: Modern Trends and Applications (July 3-6, 2012 Lviv, Ukraine

Finite temperature second harmonic generation in Kitaev magnets

We study electric field induced second harmonic generation (2HG) in the Kitaev model. This frustrated magnet hosts a quantum spin-liquid, featuring fractionalization in terms of mobile Majorana fermion and static $\mathbb{Z}_{2}$ flux-vison elementary excitations. We show that finite temperature 2HG allows to probe characteristic features of both fractional quasiparticle types. In the homogeneous flux state at low-temperatures, the 2HG susceptibility displays an oscillatory spectrum, which is set by only the fermionic excitations and is subject to temperature induced Fermi-blocking, generic to all higher harmonic generation (HHG). In the intermediate to high temperature range, intrinsic randomness, which emerges from thermally excited visons leads to drastic changes of the 2HG susceptibility, resulting from resonance decoupling over a wide range of energies. At the flux proliferation crossover, we suggest an interpolation between these two temperature regimes. Our results satisfy previously established symmetries for electric field induced 2HG in Kitaev magnets.Comment: 11 pages, 9 figure

Effective low-energy description of almost Ising-Heisenberg diamond chain

We consider a geometrically frustrated spin-1/2 Ising-Heisenberg diamond chain, which is an exactly solvable model when assuming part of the exchange interactions as Heisenberg ones and another part as Ising ones. A small $XY$ part is afterwards perturbatively added to the Ising couplings, which enabled us to derive an effective Hamiltonian describing the low-energy behavior of the modified but full quantum version of the initial model. The effective model is much simpler and free of frustration. It is shown that the $XY$ part added to the originally Ising interaction gives rise to the spin-liquid phase with continuously varying magnetization, which emerges in between the magnetization plateaus and is totally absent in the initial hybrid diamond-chain model. The elaborated approach can also be applied to other hybrid Ising-Heisenberg spin systems.Comment: 6 pages, 4 figure

Investigation of entanglement measures across the magnetization process of a highly frustrated spin-1/2 Heisenberg octahedral chain as a new paradigm of the localized-magnon approach

The bipartite entanglement across the magnetization process of a highly frustrated spin-1/2 Heisenberg octahedral chain is examined within the concept of localized magnons, which enables a simple calculation of the concurrence measuring a strength of the pairwise entanglement between nearest-neighbor and next-nearest-neighbor spins from square plaquettes. A full exact diagonalization of the finite-size Heisenberg octahedral chain with up to 4 unit cells (20 spins) evidences an extraordinary high precision of the localized-magnon theory in predicting measures of the bipartite entanglement at sufficiently low temperatures. While the monomer-tetramer phase emergent at low enough magnetic fields exhibits presence (absence) of the bipartite entanglement between the nearest-neighbor (next-nearest-neighbor) spins, the magnon-crystal phase emergent below the saturation field contrarily displays identical bipartite entanglement between the nearest-neighbor and next-nearest-neighbor spins. The presented results verify a new paradigm of the localized-magnon approach concerned with a simple calculation of entanglement measures.Comment: 6 pages, 3 figure

The square-kagome quantum Heisenberg antiferromagnet at high magnetic fields: The localized-magnon paradigm and beyond

We consider the spin-1/2 antiferromagnetic Heisenberg model on the two-dimensional square-kagome lattice with almost dispersionless lowest magnon band. For a general exchange coupling geometry we elaborate low-energy effective Hamiltonians which emerge at high magnetic fields. The effective model to describe the low-energy degrees of freedom of the initial frustrated quantum spin model is the (unfrustrated) square-lattice spin-1/2 $XXZ$ model in a $z$-aligned magnetic field. For the effective model we perform quantum Monte Carlo simulations to discuss the low-temperature properties of the square-kagome quantum Heisenberg antiferromagnet at high magnetic fields. We pay special attention to a magnetic-field driven Berezinskii-Kosterlitz-Thouless phase transition which occurs at low temperatures.Comment: 6 figure