Thermodynamics and Magnetic Excitations in Quantum Spin Trimers: Applications for the Understanding of Molecular Magnets

Emergent many-body composite excitations of interacting spin-1/2 trimers

Understanding exotic forms of magnetism in quantum spin systems is an emergent topic of modern condensed matter physics. Quantum dynamics can be described by particle-like carriers of information, known-as quasiparticles that appear from the collective behaviour of the underlying system. Spinon excitations, governing the excitations of quantum spin-systems, have been accurately calculated and precisely verified experimentally for the antiferromagnetic chain model. However, identification and characterization of novel quasiparticles emerging from the topological excitations of the spin system having periodic exchange interactions are yet to be obtained. Here, we report the identification of emergent composite excitations of the novel quasiparticles doublons and quartons in spin-1/2 trimer-chain antiferromagnet Na₂Cu₃Ge₄O₁₂ (having periodic intrachain exchange interactions J₁-J₁-J₂) and its topologically protected quantum 1/3 magnetization-plateau state. The characteristic energies, dispersion relations, and dynamical structure factor of neutron scattering as well as macroscopic quantum 1/3 magnetization-plateau state are in good agreement with the state-of-the-art dynamical density matrix renormalization group calculations.

It was recently predicted that, in addition to well-known spinon excitations, a 1D spin-trimer chain with periodic exchange couplings hosts novel composite excitations. Bera et al. experimentally demonstrate and characterize such excitations, termed doublons and quartons, in a spin-trimer compound Na₂Cu₃Ge₄O₁₂.

Magnonic contribution in thermal conductance of a nanostructure in the presence of magnon-phonon interaction

Using Green's function method, we study thermal transport properties of magnons transmitting through a magnetic nanowire at a certain temperature. In a small part of the nanowire in the middle, we are supposed to have two types of local and nonlocal magnon-phonon interactions. The self-energies for this part due to the other parts of the wire are analytically derived. First, we calculate the phonon mode-dependent magnon transmission coefficient in the classical canonical ensemble. Then, by taking an average of the phonon modes, we obtain the total magnon transmission coefficient and use it for computing magnon thermal conductance. We use the model to investigate four configurations of magnetic nanowires composed of ferromagnetic and/or antiferromagnetic parts. The results show that, when the scattering region has an antiferromagnetic alignment, the magnons transfer in the structure more weakly than for ferromagnetic alignment. There is a phonon-assisted mechanism for tunneling of magnons which are transmitted through the gap or between magnon quasifrequencies of the scattering region. Generally, in the same values for local and nonlocal strengths of magnon-phonon interaction, the nonlocal one has a greater effect on the magnonic thermal transport properties. We found the fitting functions in order to relate the macroscopic quantities of the magnon transmission coefficient and thermal conductivity to microscopic parameters of the strengths of the local and nonlocal magnon-phonon interaction.

Thermodynamics of General Heisenberg Spin Tetramers Composed of Coupled Quantum Dimers

Advances in quantum computing technology have been made in recent years due to the evolution of spin clusters. Recent studies have tended towards spin cluster subgeometries to understand more complex structures better. These molecular magnets provide a multitude of phenomena via exchange interactions that allow for advancements in spintronics and other magnetic system applications due to the possibility of increasing speed, data storage, memory, and stability of quantum computing systems. Using the Heisenberg spin–spin exchange Hamiltonian and exact diagonalization, we examine the evolution of quantum energy levels and thermodynamic properties for various spin configurations and exchange interactions. The XXYY quantum spin tetramer considered in this study consists of two coupled dimers with exchange interactions α1J and α1′J and a dimer–dimer exchange interaction α2J. By varying spin values and interaction strengths, we determine the exact energy eigenstates that are used to determine closed-form analytic solutions for the heat capacity and magnetic susceptibility of the system and further analyze the evolution of the properties of the system based on the parameter values chosen. Furthermore, this study shows that the Schottky anomaly shifts towards zero as the ground-state of the system approaches a quantum phase transition between spin states. Additionally, we investigate the development of phase transitions produced by the convergence of the Schottky anomaly with both variable exchange interactions and external magnetic field.

Low-Temperature Magnetocaloric Properties of V12 Polyoxovanadate Molecular Magnet: A Theoretical Study

The paper presents a computational study of the magnetocaloric properties of the V12 polyoxovanadate molecular magnet. The description is restricted to low-temperature range (below approximately 100 K), where the magnetic properties of the system in question can be sufficiently modelled by considering a tetramer that consists of four vanadium ions with spins S=1/2. The discussion is focused on the magnetocaloric effect in the cryogenic range. The exact and numerical diagonalization of the corresponding Hamiltonian is used in order to construct the thermodynamic description within a version of the canonical ensemble. The thermodynamic quantities of interest, such as magnetic entropy, specific heat, entropy change under isothermal magnetization/demagnetization, temperature change under adiabatic magnetization/demagnetization, refrigerant capacity, and magnetic Grüneisen ratio, are calculated and discussed extensively. The importance of two quantum level crossings for the described properties is emphasized. The significant ranges of direct and inverse magnetocaloric effect are predicted. In particular, the maximized inverse magnetocaloric response is found for cryogenic temperatures.

Magnetocaloric Effect in Cu5-NIPA Molecular Magnet: A Theoretical Study

We calculated the magnetocaloric properties of the molecular nanomagnet Cu5-NIPA, consisting of five spins S = 1 / 2 arranged in two corner-sharing triangles (hourglass-like structure without magnetic frustration). The thermodynamics of the system in question was described using the quantum Heisenberg model solved within the field ensemble (canonical ensemble) using exact numerical diagonalization. The dependence of the magnetic entropy and magnetic specific heat on the temperature and the external magnetic field was investigated. The isothermal entropy change for a wide range of initial and final magnetic fields was discussed. Due to plateau-like behavior of the isothermal entropy change as a function of the temperature, a high degree of tunability of magnetocaloric effect with the initial and final magnetic field was demonstrated.

Trinuclear copper(ii) bromide complex [C3H5N3Br]2n[Cu3Br8]n. Structure, magnetic properties and DFT calculations

Halidocuprates(ii) show unusual magnetic properties and a high degree of structural flexibility.