Phase Transitions In Monolayers Research Articles

Strain-induced ferroelectric phase transition and second-harmonic generation enhancement in NbOCl2 monolayer

Strain-induced ferroelectric phase transition and second-harmonic generation enhancement in NbOCl2 monolayer

Strain-mediated phase transition of MoTe2 monolayer

MoTe2 has emerged as a promising candidate in the field of integrated circuits, memristive devices, and catalysts, owing to its polymorphic nature across different phases. Experimentally, strain engineering has been demonstrated as an effective approach for manipulating the phase transition of MoTe2, but the mechanism remains unclear. The strain-dependent phase transition and its micro-mechanisms have been investigated based on first principle calculations. As demonstrated, critical strain and phase transition path from H → T′ phases are strongly governed by the applied strain's orientation, magnitude, and triaxiality. At the atomic level, nonzero movements of Te atoms within the phase transition domain with mechanical unloading have been clarified, together with an advanced understanding on the impact of strain on Te-vacancies migration. These insights advanced the knowledge of MoTe2 phase transition behavior and demonstrated the large space to explore potential applications through strain, defect, and phase engineering.

Tunable Metal–Insulator and Topological Phase Transitions in Piezoelectric Janus Monolayers: Theoretical Simulations with Implications for Reconfigurable Strain-Biased Electronic Devices

Tunable Metal–Insulator and Topological Phase Transitions in Piezoelectric Janus Monolayers: Theoretical Simulations with Implications for Reconfigurable Strain-Biased Electronic Devices

In-plane magnetization orientation driven topological phase transition in OsCl3 monolayer

The quantum anomalous Hall effect resulting from the in-plane magnetization in the OsCl3 monolayer is shown to exhibit different electronic topological phases determined by the crystal symmetries and magnetism. In this Chern insulator, the Os-atoms form a two dimensional planar honeycomb structure with an easy-plane ferromagnetic configuration and the required non-adiabatic paths to tune the topology of electronic structure exist for specific magnetic orientations based on mirror symmetries of the system. Using density functional theory (DFT) calculations, these tunable phases are identified by changing the orientation of the magnetic moments. We argue that in contrast to the buckled system, here the Cl-ligands bring non-trivial topology into the system by breaking the in-plane mirror symmetry. The interplay between the magnetic anisotropy and electronic band-topology changes the Chern number and hence the topological phases. Our DFT study is corroborated with comprehensive analysis of relevant symmetries as well as a detailed explanation of topological phase transitions using a generic tight binding model.

Cell deformability drives fluid-to-fluid phase transition in active cell monolayers.

Cell deformability is an essential determinant for tissue-scale mechanical nature, such as fluidity and rigidity, and is thus crucial for tissue homeostasis and stable developmental processes. However, large-scale simulations of deformable cells have been restricted to those of polygonal-shaped cells, limiting our understanding of populations of arbitrarily deformable cells, such as mesenchymal, amoeboid cells, and nonconfluent epithelial cells. Here, we present an efficient approach for simulating large populations of nonpolygonally deformable cells with considerably higher computational efficiency than existing methods. Using the method, we demonstrate that the densely packed active cell population interacting via excluded volume interactions exhibits a fluid-to-fluid transition. An experimentally measurable index of topological defects, defined using the number of neighboring cells, is also proposed to characterize this transition. This study provides a flexible approach to tissue-scale cell population and a broader perspective on the biological fluid phases.

Density Functional Theory Study of Topological Phase Transition and Valley Polarization in RuOHCl Monolayer: Implications for Valleytronics and Spintronics Devices

Density Functional Theory Study of Topological Phase Transition and Valley Polarization in RuOHCl Monolayer: Implications for Valleytronics and Spintronics Devices

Transforming friction: unveiling sliding-induced phase transitions in CVD-grown WS2 monolayers under single-asperity sliding nanocontacts.

Transition metal dichalcogenides (TMDs) exhibit diverse properties across different phases, making them promising materials for various engineering applications. In the present work, we employed a comprehensive approach, combining experimental investigations and computational simulations to elucidate the remarkable tunable frictional characteristics of chemical vapor deposition (CVD) grown WS2 monolayers through the sliding-induced transitions between the 1H and 1T' phases. Our atomic force microscopy (AFM) measurements reveal a significant contrast in friction between the two phases, with the 1H phase displaying higher friction (∼52%) than the 1T' phase. Surprisingly, under repeated scanning at constant stress, the friction of the 1H phase decreases, eventually matching the lower friction values of the 1T' phase. It was observed that the phase transformation is irreversible and is strongly dependent on contact stresses and is accelerated as the contact stress is increased by increasing the applied normal load. Molecular dynamics (MD) simulations provide further insights into the phase transition mechanism, highlighting the role of localized lateral stress and strain induced by sliding an AFM tip on the 1H phase. The simulations confirm that sliding induced localized lateral strain plays a crucial role in the phase transition, ultimately resulting in a decrease in friction. Moreover, our simulations unveil an intriguing connection between friction, potential energy surfaces, and the localized lateral strain during the phase transformation process. Our findings not only offer insights into the tribological properties of TMD materials but also open new possibilities for tailoring their performance in various applications where reducing friction and wear is crucial.

A next-generation transistor with low supply voltage operation constructed based on 2D materials' metal-semiconductor phase transition.

Power dissipation, a fundamental limitation for realizing high-performance electronic devices, may be effectively reduced by an external supply voltage. However, a small supply voltage simultaneously brings another serious challenge, that is, a remarkable device inability in transistors. To deal with this issue, we propose a new transistor design based on the metal-semiconductor phase transition in a AsGeC3 monolayer, which provides a switching mechanism of band-to-band tunneling at on- and off-states by gate-voltage modulation. Our first-principles calculations uncover that the monolayer AsGeC3 field-effect transistors (FETs) with gate lengths of 5, 4, and 3 nm may meet well the requirements for on-state current (Ion), power dissipation (PDP), and delay period (τ) as outlined by the International Technology Roadmap for Semiconductors (ITRS) in 2013 to achieve higher performance by the year 2028. Importantly, high performances are achieved only under a very low supply voltage (VDD = 0.05/0.10 V). Significantly, the AsGeC3 FETs exhibit remarkably lower values of both PDP and τ than those of nearly all the transistors reported up to date. These novel 2D metal-semiconductor phase transition-based FETs open up a new door for designing next-generation low-power electronic devices.

Phase transition in WSe2-xTex monolayers driven by charge injection and pressure: a first-principles study.

Alloying strategies permit new probes for governing structural stability and semiconductor-semimetal phase transition of transition metal dichalcogenides (TMDs). However, the possible structure and phase transition mechanism of the alloy TMDs, and the effect of an external field, have been still unclear. Here, the enrichment of the Te content in WSe2-xTex monolayers allows for coherent structural transition from the H phase to the T' phase. The crystal orbital Hamiltonian population (COHP) uncovers that the bonding state of the H phase approaches the high-energy domain near the Fermi level as the Te concentration increases, posing a source of structural instability followed by a weakened energy barrier for the phase transition. In addition, the structural phase transition driven by charge injection opens up new possibilities for the development of phase-change devices based on atomic thin films. For WSe2-xTex monolayers with the H phase as the stable phase, the critical value of electron concentration triggering the phase transition decreases with an increase in the x value. Furthermore, the energy barrier from the H phase to the T' phase can be effectively reduced by applying tensile strain, which could favor the phase switching in electronic devices. This work provides a critical reference for controllable modulation of phase-sensitive devices from alloy materials with rich phase characteristics.

Anisotropic Phonon Behavior and Phase Transition in Monolayer ReSe2 Discovered by High Pressure Raman Scattering.

Re-based transition metal dichalcogenides have attracted extensive attention owing to their anisotropic structure and excellent properties in applications such as optoelectronic devices and electrocatalysis. The present study methodically investigated the evolution of specific Raman phonon mode behaviors and phase transitions in monolayer and bulk ReSe2 under high pressure. Considering the distinctive anisotropic characteristics and the vibration vectors of Re and Se atoms exhibited by monolayer ReSe2, we perform phonon dispersion calculations and propose a methodology utilizing pressure-dependent polarized Raman measurements to explore the precise structural evolution of monolayer ReSe2 under the stress fields. Varied behaviors of the Eg-like and Ag-like modes, along with their specific vector transformations, have been identified in the pressure range 0-14.59 GPa. The present study aims to offer original perspectives on the physical evolution of Re-based transition metal dichalcogenides, elucidating their fundamental anisotropic properties and exploring potential applicability in diverse devices.

Photoexcitation induced magnetic phase transition and spin dynamics in antiferromagnetic MnPS3 monolayer

Antiferromagnetic spin dynamics is the key issue to develop spintronic devices. We adopt ab initio nonadiabatic molecular dynamics with spin–orbit-coupling (SOC) to investigate photoinduced spin dynamics in an antiferromagnetic semiconductor MnPS3 monolayer. Optical doping triggers MnPS3 from Néel antiferromagnetic to ferromagnetic phase at an experimentally achievable electron–hole pair density of 1.11 × 1014 cm−2. This phase transition can be ascribed to the light-induced mid-gap states of S-p orbitals, which lower the electron excitation energy and strengthen the SOC effect between S-p and Mn-d orbitals. The excited S-p electrons first decay to the mid-gap states due to p–p electron–phonon-coupling and then relax to the spin-down Mn-d orbitals via SOC. Such a dramatic relaxation process prolongs the photogenerated carrier lifetime up to 648 fs, providing an explanation for the unusual optoelectronic performance of MnPS3. The reversible switching of magnetic order via optical means gives an important clue for information storage and highly efficient photocatalysts utilizing antiferromagnetic semiconductors.

Low hydrogen coverage [formula omitted] topological phase transition in plumbene

Two-dimensional group IV monolayers with low-buckled honeycomb structures are reported as Z2 topological insulators. However, despite the strong spin–orbit coupling, the 2D lead monolayer (known as plumbene) exhibits a trivial insulator electronic state. Here we have studied the possible trivial to Z2 topological phase transition in plumbene monolayer with low-density H decoration using the density-functional theory calculations. The adsorption of the H atom on the plumbene monolayer is energetically favorable. We examined different configurations for the adsorption of one and two H atoms on the plumbene surface. The H decoration of the surface causes the trivial to Z2 topological phase transition in the plumbene monolayer. The topological phase transition depends on the density and order of H atoms on the plumbene surface.

Temperature-dependent structural phase transition of Janus MoSH monolayer

Structural phase transition materials have attracted extensive attention in various fields due to their tunable physical properties. Transition metal dichalcogenide (TMD) nanomaterial is a critical representation. However, the phase transition of Janus MoSH, a sub-member of TMD nanomaterial, remains poorly understood. In this study, we employed a theoretical approach to investigate the phase transition of MoSH monolayer. Our results reveal temperature-dependent structural phase transitions of MoSH from 2H to 1T′. As the temperature reaches a critical value, a spontaneous structural phase transition occurs from 2H to 1T′ phase. Remarkably, our study identifies the 1T′ phase of MoSH as the most stable phase (including energetic, mechanical, dynamic, and thermodynamic stability) at most temperature ranges, and we provide insight into the reaction pathway of MoSH from 2H to 1T′ phase. Moreover, the structural phase transition of MoSH with hydrogen vacancies is also temperature-dependent, with the critical temperature decreasing with the incremental number of vacancies. Overall, our findings not only reveal the temperature-dependent structural phase transition of MoSH but also confirm the stable structure of MoSH in 1T′ phase at room temperature, which is significant for potential applications of 2D MoSH nanomaterial.

Strain-regulated magnetic phase transition and perpendicular magnetic anisotropy in CrSBr monolayer

Strain-regulated magnetic phase transition and perpendicular magnetic anisotropy in CrSBr monolayer

Magnetic phase transition in a machine trained spin model: A study of hexagonal CrN monolayer

By employing the combination of non-collinear density functional theory, artificial neural network, and classical Monte Carlo simulation we study the ferromagnetic phase transition in CrN monolayers. The artificial neural network is successfully trained from different spin alignments to reproduce interaction energy at the DFT level in two dimensions. The training is performed solely based on spin spatial angles and second-order interactions energy without any speculation for the shape of spin Hamiltonian. The neural network model predicts the angle-dependent spin energy and mimics the magnetic anisotropy energy. The neural network is then validated for a larger supercell size against density functional theory results. Finally, the trained neural network spin model is used to study the finite temperature magnetization and phase transition in the two-dimensional CrN monolayer. The Curie temperature of the CrN monolayer is estimated equal to 600 K from the temperature-dependent magnetization and specific heat.

Orientational phase transition in monolayers of multipolar straight rigid rods: The case of 2-thiophene molecule adsorption on the Au(111) surface.

Monte Carlo simulations and finite-size scaling theory have been carried out to study the critical behavior and universality for the isotropic-nematic (IN) phase transition in a system of straight rigid pentamers adsorbed on a triangular lattice with polarized nonhomogeneous intermolecular interactions. The model was inspired by the deposition of 2-thiophene molecules over the Au(111) surface, which was previously characterized by experimental techniques and density functional theory. A nematic phase, observed experimentally by the formation of a self-assembled monolayer of parallel molecules, is separated from the isotropic state by a continuous transition occurring at a finite density. The precise determination of the critical exponents indicates that the transition belongs to the three-state Potts universality class. The finite-size scaling analysis includes the study of mutability and diversity. These two quantities are derived from information theory and they have not previously been considered as part of the conventional treatment of critical phenomena for the determination of critical exponents. The results obtained here contribute to the understanding of formation processes of self-assembled monolayers, phase transitions, and critical phenomena from novel compression algorithms for studying mutual information in sequences of data.

Non-volatile control of topological phase transition in an asymmetric ferroelectric In2Te2S monolayer.

The coupling of topological electronic states and ferroelectricity is highly desired due to their abundant physical phenomenon and potential applications in multifunctional devices. However, it is difficult to achieve such a phenomenon in a single ferroelectric (FE) monolayer because the two polarized states are topologically equivalent. Here, we demonstrate that the symmetry of polarized states can be broken by constructing a Janus structure in a FE monolayer. We illustrate such a general idea by replacing a layer of Te atoms in the In2Te3 monolayer with S atoms. Using first-principles calculations, we show that the In2Te2S monolayer has two asymmetric polarized states, which are characterized by a metal and semiconductor, respectively. Importantly, as the spin-orbit coupling is included, a band gap (50.4 meV) is created in the metallic state, resulting in a non-trivial topological phase. Thus, it proves to be a feasible method to engineer non-volatile FE control of topological order in a single-phase system. We also demonstrate the underlying physical mechanism of topological phase transition, which is unveiled to be related to the weakened intrinsic electric field resulting from charge transfer. These interesting results provide a general way to design asymmetric FE materials and shed light on their potential application in non-volatile multifunctional devices.

Room-temperature antiferromagnetic order in monolayer Fe2C

We have studied the room-temperature long-range antiferromagnetic order in the ${\mathrm{Fe}}_{2}\mathrm{C}$ monolayer using the combination of first-principles calculations and the Green's function analysis within the anisotropic Heisenberg model. The ${\mathrm{Fe}}_{2}\mathrm{C}$ monolayer is a semimetal with the out-of-plane antiferromagnetic order between two $\mathrm{Fe}$ planes. The Green's function approach within the random phase approximation is formulated to calculate the temperature-dependent antiferromagnetic magnon energy and the spin correlation function in a two-dimensional antiferromagnetic monolayer. The correlation function is used to evaluate the sublattice magnetization and study the magnetic phase transition in monolayer ${\mathrm{Fe}}_{2}\mathrm{C}$. Moreover, the spin Hamiltonian and the Green's function formalism are developed to investigate the antiferromagnetic ${\mathrm{Fe}}_{2}\mathrm{C}$ monolayer in the presence of an external magnetic field applied along the easy axis. The H-T phase diagram shows that the antiferromagnetic to spin-flop and the spin-flop to paramagnetic phase transitions occur in low temperatures. Finally, we estimate the N\'eel temperature and the critical values of the magnetic field strength for these two phase transitions.

Anharmonic lattice dynamics and structural phase transition of SnTe monolayer from first principles

In this work we investigate the role of quartic anharmonicity on the lattice- and thermo-dynamic properties of rectangular () and square (β) phases of two-dimensional (2D) SnTe monolayer (ML) by using self-consistent phonon (SCP) theory, based on the first-principles calculations. For both phases, as compared to the usual harmonic approximation (HA), the renormalized phonon frequency at the optical modes (4–10) is found to be increased upon the inclusion of quartic anharmonicity via the SCP method, where the effects of cubic anharmonicity are neglected. At the experimentally observed transition temperature (T c = 270 K), the difference in the vibrational free-energy between the square and rectangular phases of SnTe ML, calculated by using the anharmonic SCP correction is found to be much closer to the structural energy gain as compared to that obtained by using only the quasi-harmonic contribution. This validates the significance of SCP approach over the HA to explain the lattice dynamics properties and predict the T c for SnTe ML and similar 2D compounds. The calculated lattice thermal conductivity of square SnTe ML (e.g. 10.67 W m−1K−1 at 300 K) is higher than that of the rectangular SnTe ML (e.g. 6.72 W m−1K−1 at 300 K) due to the relatively higher corresponding thermodynamic parameters: specific heat capacity, group velocity, and phonon lifetime obtained for the square SnTe ML. Particularly, the low energy phonon modes are found to transport most of the heat in the system and, hence, played major role to the total lattice thermal conductivity.

Critical properties of the Ising ferromagnet VI[formula omitted] monolayer

Motivated by the experimental observation of two-dimensional van der Waals ferromagnets, we investigated the critical properties of Ising ferromagnet VI3 monolayer using the real-space renormalization group method. It is worth noting that a block-spin construction we choose contains an even number of sites. Thereupon, we present a scheme that combines the majority rule and central rule to determine the block-spin values. The critical temperature TC is calculated to be 46.8 K under the first-order cumulant-expansion approximation, which is in excellent agreement with the recent numerical result of TC = 46 K using the Metropolis Monte Carlo simulations based on the classical Heisenberg model, and is slightly lower than the experimental value TC = 50 K of the corresponding bulk VI3. Furthermore, the critical exponents β and ν are estimated, and compared with the mean-field results and the exact solutions, respectively. Our numerical result β≅0.324 suggests that the magnetic phase transition of the VI3 monolayer appears to lie between the two-dimensional Ising universality class (β=0.125) and three-dimensional Heisenberg universality class (β=0.365). The theoretical results in this paper may stimulate further experimental studies of the VI3 monolayer.