Quantum phase transitions (QPT) in the infinite-long Ising-XXZ diamond chain, characterized by the quantum correlations measured by the quantum discord (QD) and the entanglement of formation (EoF), are investigated exactly by the transfer-matrix method. QD and EoF are calculated numerically for...

The quantum phase transitions (QPTs) and magnetization plateau properties, together with magnetocaloric effect (MCE) of the tetrameric chain are investigated by means of Green’s function theory. We reveal the uniform, dimeric and gapped (gapless) tetrameric phases, which are explicitly...

The two-magnon-bound-state mass gap m2 for the two-dimensional quantum Ising model was investigated by means of the numerical diagonalization method; the low-lying spectrum is directly accessible via the numerical diagonalization method. It has been claimed that the ratio m2/m1 (m1: one-magnon...

In this work we study two different spin-boson models. Such models are generalizations of the Dicke model, it means they describe systems of N identical two-level atoms coupled to a single-mode quantized bosonic field, assuming the rotating wave approximation. In the first model, we consider the...

We have studied the anisotropic two-dimensional nearest-neighbor Ising model with competitive interactions in both uniform longitudinal field H and transverse magnetic field Ω. Using the effective-field theory (EFT) with correlation in cluster with N=1 spin we calculate the thermodynamic...

We report on the fact that microscopically enforcing fulfillment of thermodynamics’ third law on a system of fermions automatically yields the values of the external parameter (here coupling strengths in the pertinent Hamiltonian) at which quantum phase transitions take place. Our...