arXiv subjects

Azam Zoshki

Publications and source records attributed to Azam Zoshki.

6 recordsLinked to original sources

arXiv2025

Fingerprints of cluster-based Haldane and bound-magnon states in a spin-1 Heisenberg diamond chain

We investigate magnetic and thermodynamic properties of a spin-1 Heisenberg diamond chain in a magnetic field using a combination of analytical and numerical methods including the variational approach, exact diagonalization, density-matrix renormalization group, localized-magnon theory, and quantum Monte Carlo simulations. In the unfrustrated regime, the model exhibits a quantum ferrimagnetic phase that captures key magnetic features of the nickel-based polymeric compound [Ni3(OH)2(C4H2O4)(H2O)4].2H2O such as a at minimum in the temperature dependence of the susceptibility times temperature product and an intermediate one-third magnetization plateau. In the frustrated regime, we uncover a rich variety of unconventional quantum phases including uniform and cluster-based Haldane states, fragmented monomer-dimer phase, and bound-magnon crystals. Analysis of the adiabatic temperature change and magnetic Gruneisen parameter reveals an enhanced magnetocaloric effect near field-induced transitions between these exotic quantum phases. Additionally, we demonstrate that the frustrated spin-1 diamond chain can operate as an efficient working medium of a quantum Stirling engine, which approaches near-optimal efficiency when driven into these unconventional quantum states.

cond-mat.stat-mech↗

arXiv2025

Quantum Entanglement Generation in the Heterometallic Ni$^\text{2+}_4$Gd$_4^\text{3+}$ Complexes

We investigate various types of quantum entanglement in the octanuclear heterometallic $3d/4f$ complexes denoted as Ni$^{2+}_4$Gd$^{3+}_4$ under an external magnetic field, using the exact diagonalization approach. These molecular magnets, which can be effectively described by Heisenberg spin models, consist of two identical $\{\text{Ni}^{2+}_2\text{Gd}^{3+}_2\}$ cubane subunits bridged by acetate and hydroxide ligands. Our analysis reveals that their magnetization exhibits intermediate plateaus at low temperatures, indicating distinct ground states characteristic of Ni-containing compounds. Using negativity as a measure of quantum entanglement, we examine the influence of single-ion anisotropy and magnetic field on tetrapartite, bipartite, 1$-$3 tangle, and 2$-$2 tangle entanglements in two families of Ni$^{2+}_4$Gd$^{3+}_4$ complexes: $\boldsymbol{(1)}$ without anisotropy and $\boldsymbol{(2)}$ with anisotropy. Complex $\boldsymbol{(1)}$ exhibits strong bipartite entanglement between Ni ions, which persists up to $T \approx 3.0\,\text{K}$ and $B \approx 4.0\,\text{T}$, but shows significantly weaker tetrapartite entanglement and vanishing bipartite entanglement between Gd$\cdots$Gd and Ni$\cdots$Gd pairs. In contrast, complex $\boldsymbol{(2)}$ displays nonzero and sizable values for all types of entanglement considered. These findings emphasis the crucial role of single-ion anisotropy in generating and shaping the entanglement landscape of heterometallic Ni$^{2+}_4$Gd$^{3+}_4$ complexes. Notably, we find that the 1$-$3 tangle entanglement between a Ni ion and the remaining sites in a cubane unit serves as a reliable indicator of ground-state phase transitions, exhibiting distinct changes across phase boundaries irrespective of the presence of single-ion anisotropy.

cond-mat.mtrl-sci↗

arXiv2023

Magnetization plateaus and enhanced magnetocaloric effect of a spin-1/2 Ising-Heisenberg and Heisenberg double sawtooth ladder with four-spin interaction

The ground state, the entropy and the magnetic Gr\"uneisen parameter of the antiferromagnetic spin-1/2 Ising-Heisenberg model on a double sawtooth ladder are rigorously investigated using the classical transfer-matrix technique. The model includes the XXZ interaction between the interstitial Heisenberg dimers, the Ising coupling between nearest-neighbor spins of the legs and rungs, and additional cyclic four-spin Ising term in each square plaquette. For a particular value of the cyclic four-spin exchange we found in the ground-state phase diagram of the Ising Heisenberg ladder a quadruple point, at which four different ground states coexist together. During an adiabatic demagnetization process a fast cooling accompanied with an enhanced magnetocaloric effect can be detected nearby this quadruple point. The ground-state phase diagram of the Ising-Heisenberg ladder is confronted with the zero-temperature magnetization process of the purely quantum Heisenberg ladder, which is calculated by using exact diagonalization (ED) based on the Lanczos algorithm for a finite-size ladder of 24 spins and the density-matrix renormalization group (DMRG) simulations for a finite-size ladder with up to 96 spins. Some indications of existence of intermediate magnetization plateaus in the magnetization process of the full Heisenberg model for a small but non-zero four-spin Ising coupling were found. The DMRG results reveal that the quantum Heisenberg double sawtooth ladder exhibits a quantum Luttinger spin-liquid phase that is absent in the Ising-Heisenberg counterpart model. Except this difference the magnetic behavior of the full Heisenberg model is quite analogous to its simplified Ising-Heisenberg counterpart and hence, one may bring insight into the fully quantum Heisenberg model from rigorous results for the Ising-Heisenberg model.

cond-mat.stat-mech↗

arXiv2022

Robust quantum entanglement and teleportation in the tetrapartite spin-1/2 square clusters: Theoretical study on the effect of a cyclic four-spin exchange

The whole entanglement measure so-called geometric $\Pi_4$ average of tangles and bipartite entanglement of the antiferromagnetic spin-1/2 XXX Heisenberg model on a tetranuclear square cluster with cyclic four-spin interaction are rigorously examined by the help of thermal negativities. The model comprises two nearest-neighbor exchange couplings $J_1$ and $J_2$ such that $J_1\gg J_2$. When the cyclic four-spin exchange is zero, the maximum value of whole entanglement $\Pi_4$ is achieved at low enough temperatures and relatively high magnetic fields $(B\approx J_1)$. Also, maximum bipartite entanglement between pair spins with exchange coupling $J_1$ is achievable at high temperature and high magnetic field. This quantity remains alive for sufficiently high temperature and high magnetic field values comparable with the relevant exchange coupling $J_1$. A nonzero value of the cyclic four-spin exchange notably enhances the degree of the whole entanglement, while it weakens the bipartite entanglement degree. We demonstrate that the whole entanglement reaches an unconventional minimum at a special parameter region of cyclic four-spin exchange almost ten order of magnitude smaller than $J_1$, where the system is in a quantum antiferromagnetic state. The real complex $[\text{Cu}_4\text{L}_4(\text{H}_2\text{O})_4](\text{ClO}_4)_4$ as a strong antiferromagnetic tetranuclear square compound provides us an experimental representative to estimate the strength of the whole and bipartite entanglements at high enough temperature. It is demonstrated that the entanglement negativities of this complex are yet depend on the considered cyclic four-spin interaction even though its magnitude is significantly smaller than $J_1$.

cond-mat.stat-mech↗

arXiv2019

Electrocaloric effect in the two spin-1/2 XXZ Heisenberg edge-shared tetrahedra and spin-1/2 XXZ Heisenberg octahedron with Dzyaloshinskii-Moriya interaction

In the present paper, we consider two species of small spin clusters known as; two spin-1/2 Heisenberg edge-shared tetrahedra and spin-1/2 Heisenberg octahedron with the corresponding Dzyaloshinskii-Moriya terms in a longitudinal magnetic field, then we examine magnetization process and electric polarization of the models as functions of magnetic and electric fields at Low temperature using exact numerical diagonalization. Our exact results are in a good agreement with recent analysis carried out by J. Stre\v{c}ka and K. Kar{\v l}ova \cite{str18b,str17a}. It is demonstrated that the polarization behavior coincides the magnetization curves (including sequential intermediate plateaus), also it reflects the respective stepwise changes of ground-state phase transitions. We find that the polarization has significant effects on the magnetic field dependencies of the magnetization. Furthermore, we investigate other isothermal strategies such as cooling rate, magnetocaloric effect (MCE), as well as, electrocaloric effect(ECE) for both models. Since, new electrocaloric materials with high performance is of great interest and importance in condensed matter physics, here, we report on the ECE of the both aforedescribed spin-1/2 Heisenberg small clusters. We conclude that two spin-1/2 Heisenberg edge-shared tetrahedra is a material with significant reversible temperature change capability under an external electric field compared with the spin-1/2 Heisenberg octahedron, and can be used for cooling/heating process.

cond-mat.str-el↗

arXiv2018

Magnetic properties of an antiferromagnetic spin-1/2 XYZ model in the presence of different magnetic fields: finite-size effects of inhomogeneity property

Magnetic and thermodynamic properties of the anisotropic XYZ spin-1/2 finite chain under both homogeneous and inhomogeneous magnetic fields are theoretically studied at low temperature. Using exact diagonalization method (ED), we study the magnetization, magnetic susceptibility and specific heat of the model characterized in terms of the finite correlation length in the presence of three different magnetic fields including longitudinal, transverse and transverse staggered magnetic fields. The magnetization, susceptibility and the specific heat of the model are investigated under two conditions separately: I) when the model is putted in the presence of homogeneous magnetic fields; II) when finite inhomogeneities are considered for all applied magnetic fields in the Hamiltonian. We show that for the finite-size XYZ chains at low temperature, the evident magnetization plateaus gradually convert to their counterpart quasi-plateaus when the transverse magnetic field increases. Moreover, the influence of the transverse and staggered transverse magnetic fields, and their corresponding inhomogeneities on the magnetization process, magnetic susceptibility, and specific heat are reported in detail. Our exact results illustrate that by altering the inhomogeneity parameters, magnetization plateaus gradually convert to their counterpart quasi-plateaus. The specific heat manifests Schottky-type maximum, double-peak and triple-peak, as well as, transformation between them by varying considered inhomogeneity parameters in the Hamiltonian.

cond-mat.stat-mech↗