We discuss in this section the obtained results using the Monte Carlo simulations under the Metropolis algorithm. Indeed, we start by the investigation of the ground state phase diagrams for a zero absolute temperature in the sub-section 3.1. Besides, for a non-zero temperature, we examine the behavior of the hysteresis electric cycles by varying temperature, exchange coupling interactions, crystal field and biquadratic exchange interaction in the sub-section 3.2.
3.1. Ground state phase diagrams
In the present study, we report the ground-state phase diagrams of a Carbon-like nanotube in the framework of a Blume-Emery-Griffiths model with mixed spins S -1 (in shell) and σ- 3/2 (in core). We use the Hamiltonian of Eq. (1) For simulating the energy of the (2S + 1) ⋅(2σ + 1) = 3 ⋅ 4 = 12 possible configurations for the ground state study.
Figure 2a is obtained in the (D/JC, EZ/JC) plane, in the absence of biquadratic exchange interaction for fixed values of the reduced exchange coupling JS/JC=1 and JCS/JC =-1. This figure exhibits only 10 stable configurations. In this phase diagram, there is a perfect symmetry of the configurations regarding the reduced external longitudinal electric field axis EZ/JC= 0. These stable configurations are: (+ 1/2, 0); (-1/2, 0); (+ 1/2, + 1); (-1/2, -1); (+ 1/2, -1); (+ 1/2, 0); (+ 3/2, + 1); (-3/2, -1); (+ 3/2, -1); (-3/2, + 1).
3.2. Hysteresis electric cycle
In this section, we use Monte Carlo simulations (MCS) under the Metropolis algorithm in order to detect the effect for several values of the reduced parameters on the hysteresis electric cycles of the mixed Carbon-like nanotube.
We present in Fig. 3 the partial (PS and Pσ) and total (Ptot) polarizations versus the reduced external longitudinal electric field EZ/JC, by varying temperature parameter T/JC = 0.1, 1, 2, 3, 4 and 7. This figure is obtained in the absence of the reduced crystal field (D/JC=0) and the biquadratic exchange interaction (K/JC=0) and for fixed reduced parameters: JS/JC =1 and JCS/JC = -1. Moreover, it is found that the surface loops decrease when increasing the temperature values and completely disappear for T/JC=7 reaching the paraelectric phase (Ptot=0). This result can be interpreted by the competition between the temperature and the external longitudinal electric field which tends to align the spins while the effect of the temperature is to disorganize them.
In Fig. 4, we investigate the effect of the exchange coupling parameter (JS/JC) on the hysteresis cycles for different values of JS/JC = 1, 2, 3, 4 and 5. Such a figure is illustrated for fixed parameters: T/JC =0.1, JCS/JC = -1, D/JC=0 and K/JC=0. As expected, increasing the exchange coupling parameter increases the coercive field and the surface loops.
Otherwise, we present in Fig. 5 the hysteresis electric cycles of the mixed Carbon-like nanotube. It is plotted for several values of the ferrielectric parameter JCS/JC = − 1,-2, -3, -4 and − 5, for fixed parameters: T/JC =0.1, JS/JC = 1, D/JC=0 and K/JC=0. This figure depicts that the increase of the parameter | JCS/JC | increases the electric coercive field resulting in an increase of the surface loops. Whereas, the electrical remanent remains invariable, as a result of the different dielectric properties of both core and shell. The system approaches the ferroelectric phase as it becomes easier to align the core and shell spins in the same direction along the applied field.
To complete this study, we plot in Fig. 6 the effect of the crystal field on the partial (PS and Pσ) and total (Ptot) hysteresis electric cycles for: T/JC =0.1, JS/JC = 1, JSC /JC= -1 and K/JC=0. From this figure, it is obvious that the increase in the absolute value of the crystal field | D/JC | decreases the surface of the loops leading to the apparition of four polarization plateaus and five electric cycle loops.
Finally, we investigate in Fig. 7 the effect of the biquadratic exchange interaction (K/JC) on the partial (PS and Pσ) and total (Ptot) hysteresis electric cycles for: K/JC = 0, -1, -2 and − 3 in the absence of reduced crystal field (D/JC=0) and for fixed parameters: T/JC =0.1, JS/JC = 1, JSC /JC= -1. This figure exhibits the central cycle at strong | K/JC = -3 | and reports that the core/shell behavior in the Carbon-like nanotube structure approaches the ferrielecrtic behavior. Due to the different ferrielectric properties in the core and shell, there are two steps that appear in the central loop. Besides, it can be observed for a low value of the intermediate reduced coupling, that it is more difficult to align the core and shell spins in the same direction along the applied external electric field, and the behavior of the core and shell in the Carbon-like nanotube approaches the antiferrielectric behavior. Whereas, it is found that the increase in the absolute value of the biquadratic exchange interaction | K/JC | decreases the surface loops leading to the apparition of one polarization plateaus and three electric cycle loops.