A study of the ground-state and thermodynamic properties of the Ising spin system was carried out including an external magnetic field where two exchange energies are arranged according to a Fibonacci sequence. For this model, the decimation renormalization transformation can be performed exactly as shown recently by Achiam, Lubensky, and Marshall. It is found that there is a new fixed plane of two-cycle limit points. Using the recursion relation of the free energy, we calculate numerically the physical quantities when the two exchange energies have opposite signs. It is shown that the chain becomes magnetized stepwise by the external field and that the magnetic susceptibility and the specific heat oscillate as a function of the temperature. These characteristic features can be understood, based on the hierarchical cluster structure of the ground state.
A study of the ground-state and thermodynamic properties of the Ising spin system was carried out including an external magnetic field where two exchange energies are arranged according to a Fibonacci sequence. For this model, the decimation renormalization transformation can be performed exactly as shown recently by Achiam, Lubensky, and Marshall. It is found that there is a new fixed plane of two-cycle limit points. Using the recursion relation of the free energy, we calculate numerically the physical quantities when the two exchange energies have opposite signs. It is shown that the chain becomes magnetized stepwise by the external field and that the magnetic susceptibility and the specific heat oscillate as a function of the temperature. These characteristic features can be understood, based on the hierarchical cluster structure of the ground state.
We investigated the critical behavior of the Ising model in a triangular lattice with ferro and anti-ferromagnetic interactions modulated by the Fibonacci sequence, by using finite-size numerical simulations. Specifically, we used a replica exchange Monte Carlo method, known as Parallel Tempering, to calculate the thermodynamic quantities of the system. We have obtained the staggered magnetization q, the associated magnetic susceptibility (χ) and the specific heat c, to characterize the universality class of the system. At the low-temperature limit, we have obtained a continuous phase transition with a critical temperature around Tc≈1.4116 for a particular modulation of the lattice according to the Fibonacci letter sequence. In addition, we have used finite-size scaling relations with logarithmic corrections to estimate the critical exponents β, γ and ν, and the correction exponents β^, γ^, α^ and λ^. Our results show that the system obeys the Ising model universality class and that the critical behavior has logarithmic corrections.
Using the Kramers–Wannier transfer matrix method, we studied several decorated Ising chains. The exact expressions for thermodynamic characteristics, including the ground state characteristics, were obtained. We considered several modeling chains with different signs and absolute values of exchange constants for the nearest- and the next-nearest neighbors. For these models, we calculated the magnetization curves. The critical values of magnetic fields and corresponding magnetization plateau parameters were obtained. Analytic expressions for the ground state entropy were obtained for the chains with different interaction constants. The dependencies of the number of states with minimum energy (the degeneration of the ground state) as the function of the number of particles were found. It was shown that these dependencies are expressed in terms of well-known numerical sequences, namely Lucas numbers and Pell numbers, which, in the limit of a large number of particles, are proportional to the powers of the golden and silver sections. Therefore, the ground state entropy (per particle) of the systems under consideration can be described in terms of these sections and, therefore, is nonzero.
Using the Kramers-Wannier transfer matrix method we studied several decorated Ising chains. The exact expressions for thermodynamic characteristics, including the ground state characteristics, were obtained. We considered a number of modeling chains with different signs and absolute values of exchange constants for the nearest- and the next-nearest neighbors. For these models we calculated the magnetization curves. The critical values of magnetic fields and corresponding magnetization plateau parameters were obtained. Analytic expressions for the ground state entropy were obtained for the chains with different interaction constants. The dependencies of the number of states with minimum energy (the degeneration of the ground state) as the function of the number of particles were found. It was shown that these dependencies are expressed in terms of well-known numerical sequences - Lucas numbers and Pell numbers, which, in the limit of a large number of particles, are proportional to the powers of the golden and silver sections. Therefore, the ground state entropy (per particle) of the systems under consideration can be described in terms of these sections and, therefore, is nonzero.
The ground state and the thermodynamics of a spin-1/2 asymmetric diamond Ising--Heisenberg chain are considered. For the XYZ anisotropic Heisenberg interaction, the exact calculations of the free energy, entropy, heat capacity, magnetization, and magnetic susceptibility are performed using the method of decoration-iteration transformation. In the case of antiferromagnetic interactions (Ising and XXZ anisotropic Heisenberg ones), the ground state, magnetization process, temperature dependence of the magnetization, magnetic susceptibility, and heat capacity are investigated. The influence of geometric frustration and quantum fluctuations on these characteristics is studied.
The work is devoted to the theoretical study of the energy spectrum and magnetic properties of the modified antiferromagnetic spin (1/2, s) diamond chain. This is a frustrated mixed spin system with the unit cells formed by two spin ½ and one spin s>1/2. On the base of extended Lieb theorem we proved the possibility of the appearance of quantum phase transitions mediated by ratio of coupling parameters at arbitrary nonzero value of the spin s for the above model. The results of our exact diagonalization study for some finite chain clusters with s=1 supports this conclusion. We also studied analytically and numerically magnetic properties of Heisenberg –Ising diamond mixed spin chain. The exact energy spectrum of this model is found in analytical form at arbitrary values of model parameters. On the base of this spectrum we studied the field dependence of two-particle correlators for neighbor Ising spins. It was found that at special relation between coupling parameters there is a critical value of external magnetic field for which the above correlator takes zero value (the absence of the correlation between Ising spins). For infinite spin chain model we studied field dependence of specific magnetization by means of classical transfer- matrix method and found intermediate plateau in the low-temperature magnetization profile. According to our calculations, the size of this plateau depends strongly on the relations between coupling parameters of the model. We hope this feature of our model gives new possibilities for the design of new magnetic chemo-sensors.
We obtain the exact physical characteristics of the triple-chain Ising model on a torus with all possible multispin interactions invariant with respect to rotation by the angle 2π/3. The exact value of the partition function in a finite cyclically closed strip of length L, as well as the free energy, internal energy, specific heat, magnetization, susceptibility, and entropy in the thermodynamic limit at L→∞ are found by the transfer-matrix method for the model. The spectrum of the transfer-matrix and the structure of its eigenvectors are found. For two special cases - for the model with multispin interactions of even number of spins and for the model with some interactions of two, three, four and six spins, simplified expressions of the mentioned physical characteristics are obtained; in the thermodynamic limit they are expressed through the logarithm of the root of the quadratic equation. For the model with multispin interactions of an even number of spins, a kind of pair correlations in the thermodynamic limit is found, and it is shown that the magnetization at zero magnetic field is equal to zero; the structure of the ground states of the system is found and examples of their projections of seven-dimensional space onto three-dimensional space and examples of configurations corresponding to these ground states are given. The correlation length is shown and its graphs are given. As special cases, we consider the planar triangular model with all possible interactions, including, perhaps, different triple interactions inside neighboring triangles, and the planar model with nearest-neighbor, next nearest-neighbor, and plaquette interactions. For them the main exact physical characteristics have been found. This allowed us to obtain them for the planar gonihedric model as well.