Magnetic Ordering, Ferroelectric And Thermodynamic Properties In Low-dimensional Organic Quantum Magnets

In recent years, the physical properties of organic quantum magnets are the pioneering issues crossing over condensed matter physics and material discipline. Based on the synthesis of organic magnetic materials experimentally, we propose theoretical models to investigate some low-dimensional quantum magnetic systems, such as organic spin-Peierls (SP) systems, organic ferroelectrics, spin ladders, one-dimensional (1D) trimer chain and quasi-1D organic ferromagnetic polymer by means of Green's function theory. The magnetic ordering, ferroelectric property, phase transition and thermodynamics are analysed and discussed in detail, which can unpuzzle the fantastic physical phenomena observed experimentally and explore new physical laws. Meanwhile, it is necessary to consider the effect of magnetic and electric fields, which can provide theoretical basis for the physical controlling on the design of molecule devices.For the organic SP systems, the effective elastic constant is an intrinsic factor that determines the order of SP transition. The magnetic field makes the transition temperature TSP decrease, and drives the SP transition from the second order to the first order. Besides, we show that the two-site thermal entanglement entropy is a good indicator of SP transition, which displays a well switching behavior. Further considering weak coupling of double-chain, the theoretical values are closer to the experimental results. It is also found that a gapless SP phase lies in the gapped dimerized phase.Different from SP systems, the anion and cation in organic charge transfer compounds carry both charge and spin degrees of freedom, wherein the interplay among the charge, spin and lattice degrees of freedom plays an important role in the emergence of spontaneous polarization. The polarization (P)-field (E) hysteresis loop is obtained, which strongly demonstrates the ferroelectric (FE) ordering. In addition, the potential magnetoelectric behavior is taken into account. On the one hand, the magnetic field makes polarization P and transition temperature Tc decrease, and drives the FE transition from the second order to the first order. On the other hand, the electric field makes the magnetization M decrease, and also makes the FE transition become a crossover behavior, since the electrostatic energy predominates over the spin-lattice coupling.Up to now, only three kinds of strong-leg spin ladder model compounds have been synthesized experimentally. However, the spin liquid at low temperature limit has not been observed. Herein, we demonstrate the magnetic-field-controlled Luttinger liquid and explore the spin liquid at low temperatures. These low-lying excitations are gapless and gapped, respectively. Furthermore, the thermal insulating and conducting behaviors can be controlled by magnetic fields effectively.1D spin chain containing multi-spin interactions would cause frustration, resulting in quantum phase transition. The quantum criticality and thermodynamics of trimerized chain with three-spin interactions are investigated. It is found that the magnetic-field-driven quantum criticality is closely related to the elementary excitation spectra, in which the energy gap responsible for the 1/3 magnetization plateau can be opened up by three-spin interactions. In addition, the gapped low-lying excitations are responsible for the observed thermodynamic behaviors, wherein a structure with three peaks in the temperature dependence of specific heat is unveiled.There are a number of papers about the ground state properties of quasi-1D organic ferromagnetic polymer. Here, based on the periodic Anderson-like model, we study the field-controlled magnetic ordering and insulator-metal transitions. In the absence of magnetic field, the ground state resides in ferrimagnetic phase. At finite temperatures, the ferrimagnetic transition to paramagnetism occurs. When the magnetic field is turned on, the 1/3 magnetization plateau emerges and two critical fields indicate insulator-metal transitions, which are related to the energy bands. Meanwhile, we propose a competition mechanism between up-spin and down-spin hole excitations, which are responsible for the thermodynamic properties under different magnetic fields.