No abstract

We analyze the possible types of ordering in a boson-fermion model. The Hamiltonian is inherently related to the Bose-Hubbard model for vector two-species bosons in optical lattices. We show that such model can be reduced to the Kugel-Khomskii type spin-pseudospin model, but in contrast to the usual version of the latter model, we are dealing here with the case of spin S = 1 and pseudospin 1/2. We show that the interplay of spin and pseudospin degrees of freedom leads to a rather nontrivial magnetic phase diagram including the spin-nematic configurations. Tuning the spin-channel interaction parameter Us gives rise to quantum phase transitions. We find that the ground state of the system always has the pseudospin domain structure. On the other hand, the sign change of Us switches the spin arrangement of the ground state within domains from ferro-to aniferromagnetic one. Finally, we revisit the spin (pseudospin)-1/2 Kugel-Khomskii model and see the inverse picture of phase transitions.

Motivated by the unusual temperature dependence of the specific heat in MnSi, comprising a combination of a sharp first-order feature accompanied by a broad hump, we study the extended Heisenberg model with competing exchange $J$ and anisotropic Dzyaloshinskii-Moriya $D$ interactions in a broad range of ratio $D/J$. Utilizing classical Monte Carlo simulations we find an evolution of the temperature dependence of the specific heat and magnetic susceptibility with variation of $D/J$. Combined with an analysis of the Bragg intensity patterns, we clearly demonstrate that the observed puzzling hump in the specific heat of MnSi originates from smearing out of the virtual ferromagnetic second order phase transition by helical fluctuations, which manifest themselves in the transient multiple spiral state. These fluctuations finally condense into the helical ordered phase via a first order phase transition as is indicated by the specific heat peak. Thus the model demonstrates a crossover from a second-order to a first-order transition with increasing $D/J$. Upon further increasing $D/J$ another crossover from a first-order to a second-order transition takes place in the system. Moreover, the results of the calculations clearly indicate that these competing interactions are the primary factor responsible for the appearance of first order phase transitions in helical magnets with the Dzyaloshinskii-Moriya (DM) interaction

We present new results for the system with two species of vector bosons in an optical lattice. In addition to the standard parameters characterizing such a system, we are dealing here with the 'degree of atomic nonidentity', manifesting itself in the difference of tunneling amplitudes and on-site Coulomb interactions. We obtain a cascade of quantum phase transitions occurring with the increase in the degree of atomic nonidentity. While in the system of nearly identical vector bosons only one phase transition between two phases occurs with the evolution of the interparticle interaction, atom nonidentity increases the number of possible phases to six and the resulting phase diagrams are so nontrivial that we can speculate about their evolution from the images similar to the 'JMiró-like paintings' to 'KMalewicz-like' ones.