Dynamic phase transitions in the one-dimensional spin-phonon coupling model

Abstract

An interesting model for spin crossover compounds (SCC) is the so-called spin-phonon coupling model. In this model, the SC units are linked by springs, with elastic constants dependent on their spin states. In the present work, we investigate by means of Monte Carlo Metropolis method, the one-dimensional version of the model at T > 0 K. The relevance of our numerical analysis stands in the possibility of studying this model in the general case, i.e., outside the analytical limits of the mean-field approximation (MFA). First, we investigate a couple of model properties, such as pressure-induced-like effects on thermal hysteresis loops, system size and degeneracy influences on the dynamic phase diagrams. We find a high sensitivity of the hysteresis loops on the system size for small sizes only. Second, we qualitatively compare our results with those analytically obtained in the MFA and determine the limits of the latter. Third, we study the kinetic effects on the dynamic phase diagrams and the critical temperature. We find a slow asymptotic decrease of the critical temperature T c to zero as the kinetics becomes infinitely long. This might be important in the experimental studies of pressure-induced phase transitions for clusters or nanosize SC compounds at low temperatures, as this means that, in practice, there will always be a pseudophase transition, as the measurement time is finite. The transition temperatures are smaller than 0.1 /k B , meaning that is not a question of thermal activation, but it is really the mechanism of spin-phonon coupling that helps the transition from the low-spin to the high-spin level state.

Elastic Frustration in 1D Spin-Crossover Chains: Evidence of Multi-Step Transitions and Self-Organizations of the Spin States

“…This thermal hysteresis has a kinetic origin and can be attributed to finite size effects. Indeed, as we have demonstrated in a previous work devoted to the study of dynamic phase transitions in 1D spin-phonon coupling model, which constitutes a variant of the present Hamiltonian, the kinetic hysteresis widths increases almost linearly with the inverse of the chain length. That being said, it should also be highlighted that because the two simulations are performed using the same temperature scan rate, the presence of the kinetic hysteresis in Figure b is clearly a strengthened by the increase of the elastic cooperativity inside the system.…”

“…Here, we modelled this additional interaction through a direct next-nearest neighbour elastic coupling between the SCO atoms that generates elastic frustration with the interaction between immediate surrounding atoms. We first demonstrate, using Monte Carlo (MC) simulations, that an isolated elastic chain of interacting SCO atoms leads easily to kinetic hysteretic first-order transitions, in good agreement with previous MC simulations performed on a different type of elastic model 33 . Second, the introduction of an elastic frustration leads to a rich variety of thermal behaviour of the HS fraction, among which the famous multi-step 11,[21][22][23][24][34][35][36][37][38] and incomplete transitions which lead to residual HS fractions at low-temperature, as well as re-entrant phase transitions 39 based on size effect changes.…”

“…We first demonstrate, using Monte Carlo (MC) simulations, that an isolated elastic chain of interacting SCO atoms leads easily to kinetic hysteretic first-order transitions, in good agreement with previous MC simulations performed on a different type of elastic model. 33 Second, the introduction of an elastic frustration leads to a rich variety of thermal behavior of the HS fraction, among which the famous multistep 11,21−24,34−38 and incomplete transitions that lead to residual HS fractions at low-temperature, as well as re-entrant phase transitions 39 based on size effect changes. In addition, we observed the presence of clear spatial self-organizations modes of the HS and LS states in the plateau regions of multistep transitions that are monitored by the strength of the elastic frustration.…”

Spin-Crossover Molecular Solids Beyond Rigid Crystal Approximation

“…( 2 ). This translates into a linear increase of the experimental switching temperatures T ↑ and T ↓ with applied hydrostatic pressure and reduction of the hysteresis width down to zero [ 36 , 37 ]. Usual T ↑ and T ↓ are the transition temperatures on heating and cooling at n H =0.5.…”

Effect of compression in molecular spin-crossover chains

“…The one-dimensional (1D) Ising-like model plays an important role in statistical physics, being one of the models which have been solved exactly. Compressible Ising model also has long history of study [23][24][25] , and new results were obtained recently by numeric techniques [26][27][28][29][30][31] . Real quasi-1D spina) Electronic mail: yugudyma@gmail.com crossover materials almost perfectly correspond to the onedimensional Ising model causing particular interest for theoretical studies.…”