Defect-mediated Phase Transition Research Articles

Defect-mediated structural phase transition in Ta2NiSe5 visualized via in situ TEM

Extensive experimental and theoretical efforts have been devoted to elucidating the origin of either structural or electronic instability in the formation of excitonic state in Ta2NiSe5, whereas the effects of lattice defects on actual structural phase transition in Ta2NiSe5 remain elusive. Here, we perform in situ transmission electron microscopy (TEM) experiments to investigate the structural phase transition of Ta2NiSe5 thin films, with a particular focus on the influence of microstructural features and lattice defects on the phase transition. In the cyclic in situ heating-cooling TEM experiments, we track the reversible orthorhombic to monoclinic (vice versa) phase transitions via twinning variant contrast in TEM images and via changes in reflection splitting in diffraction patterns. We further investigate the effects of dislocations and grain boundaries on the phase transition of Ta2NiSe5 thin films. Our findings reveal that individual dislocations can inhibit the transition from monoclinic to orthorhombic phases while simultaneously promoting the reverse transition from orthorhombic to monoclinic phases. Furthermore, we observe a phenomenon of metastable to stable phase transition within grain-boundary-confined regions of Ta2NiSe5 thin films. These in situ TEM observations offer a comprehensive microscopic perspective on defect-mediated phase transitions in Ta2NiSe5. This insight may extend to other varieties of transition-metal chalcogenides, thus implying a broader applicability.

Atomic scale observation of a defect-mediated first-order phase transition in VO2(A).

The study of first-order structural transformations has attracted extensive attention due to their significant scientific and industrial importance. However, it remains challenging to exactly determine the nucleation sites at the very beginning of the transformation. Here, we report the atomic scale real-time observation of a unique defect-mediated reversible phase transition between the low temperature phase (LTP) and the high temperature phase (HTP) of VO2(A). In situ Cs-corrected scanning transmission electron microscopy (STEM) images clearly indicate that both phase transitions (from the HTP to the LTP and from the LTP to the HTP) start at the defect sites in parent phases. Intriguingly, the structure of the defects within the LTP is demonstrated to be the HTP of VO2 (A), and the defect in the HTP of VO2(A) is determined to be the LTP structure of VO2(A). These findings are expected to broaden our current understanding of the first-order phase transition and shed light on controlling materials' structure-property phase transition by "engineering" defects in applications.

Defect-mediated phase transitions in active soft matter.

How do topological defects affect the degree of order in active matter? To answer this question we investigate an agent-based model of self-propelled particles, which accounts for polar alignment and short-ranged repulsive interactions. For strong alignment forces we find collectively moving polycrystalline states with fluctuating networks of grain boundaries. In the regime where repulsive forces dominate, the fluctuations generated by the active system give rise to quasi-long-range transitional order, but-unlike the thermal system-without creating topological defects.

Defect-mediated phase transition temperature of VO2 (M) nanoparticles with excellent thermochromic performance and low threshold voltage

This paper reports the phase transition temperature regulation of VO2 (M) nanoparticles using interfacial defects and size effect other than the traditional doping routine. The nanoparticles exhibit excellent thermochromic performance and a low threshold voltage.

A numerical study of the phase transition in a triangular-lattice model with three-spin interactions

We study a Berezinskii-Kosterlitz-Thouless-like phase transition in a triangular-lattice three-spin interaction model. The vector sine-Gordon field theory describes defects involved, and enables us to perform the renormalization-group analysis of the transition. Based on it, we discuss properties of the spin-spin correlation function, and also analyze the helicity modulus for the present model. To provide the data to support our analysis, we perform the Monte-Carlo simulations to calculate them. Further, we exhibit snapshots of some typical defect conflgurations, which are helpful to illustrate the defect-mediated phase transition.

Monte Carlo study of the anisotropic Heisenberg antiferromagnet on the triangular lattice

We report a Monte Carlo study of the classical antiferromagnetic Heisenberg model with easy axis anisotropy on the triangular lattice. Both the free energy cost for long wavelength spin waves as well as for the formation of free vortices are obtained from the spin stiffness and vorticity modulus respectively. Evidence for two distinct Kosterlitz-Thouless types of defect-mediated phase transitions at finite temperatures is presented.

Role of defects in two-dimensional phase transitions: An STM study of the Sn/Ge(111) system

The influence of Ge substitutional defects and vacancies on the $(\sqrt{3}\ifmmode\times\else\texttimes\fi{}\sqrt{3})$$\ensuremath{\rightarrow}(3\ifmmode\times\else\texttimes\fi{}3)$ charge-density wave phase transition in the $\ensuremath{\alpha}$ phase of Sn on Ge(111) has been studied using a variable-temperature scanning tunneling microscope. Above 105 K, Ge substitutional defects stabilize regions with $(3\ifmmode\times\else\texttimes\fi{}3)$ symmetry that grow with decreasing temperature and can be described by a superposition of exponentially damped waves. At low temperatures, $Tl~105\mathrm{K}$ defect-defect density-wave-mediated interactions force an alignment of the defects onto a honeycomb sublattice that supports the low-temperature $(3\ifmmode\times\else\texttimes\fi{}3)$ phase. This defect-mediated phase transition is completely reversible. The length scales involved in this defect-defect interaction dictate the domain size $(\ensuremath{\approx}{10}^{4} {\mathrm{\AA{}}\mathrm{}}^{2}).$

Mechanism of yielding in dislocation-free crystals at finite temperatures—Part I. Theory

A new mechanism of dislocation generation that can be activated suddenly above a critical temperature is proposed. Unlike dislocation generation by Frank-Read type sources, this process is a thermally driven, stress-assisted cooperative instability of many dislocation loops (dipoles in two dimensions). The dislocation loops are formed by thermal fluctuations and are sub-critical in size at low temperatures. The small plastic strain associated with the loops invokes an effective decrease in the moduli that describe the linear relation between the applied stress and the total strain. The self-energy of a loop which forms in the presence of other loops is proportional to these effective moduli and, consequently, it is lower than the energy of an isolated loop. As the temperature increases, the density of thermally generated loops increases and the concomitant lowering of the effective moduli and the self-energy provides a feedback for its further increase. Ultimately, above a critical temperature, the free energy of the loops becomes negative and a collective unstable expansion of many loops occurs. In a solid which is not loaded, this instability corresponds to the Kosterlitz-Thouless model of a defect-mediated phase transition that occurs just below the melting temperature. However, under large applied loads, the instability appears well below the melting temperature. Above the critical temperature, the density of glissile dislocations increases precipitously and extensive yielding occurs. This paper analyzes this mechanism in two dimensions where dislocation loops are replaced by dipoles.

Monte Carlo study of the Heisenberg antiferromagnet on the triangular lattice.

We report a Monte Carlo study of the classical antiferromagnetic Heisenberg model on the triangular lattice. The free-energy cost for the formation of free vortices is obtained from a vorticity modulus. Evidence of a Kosterlitz-Thouless type of defect-mediated phase transition at a finite temperature is found.

Free-vortex formation and topological phase transitions of two-dimensional spin systems.

A numerical method to study defect-mediated phase transitions is proposed, which combines a standard Monte Carlo technique with a particular type of boundary conditions. The method is applied to the two-dimensional XY (plane rotator) model as well as to the two-dimensional SO(3) Heisenberg model which models a triangular-lattice Heisenberg antiferromagnet. Our results give a direct numerical support for the occurrence of a topological transition in these systems driven by the vortex binding-unbinding.

THE STABILITY OF LOW INDEX METAL SURFACES TO TOPOLOGICAL DEFECTS

The study of defect formation at metal surfaces is a fundamental problem in surface physics. An understanding of defect formation is pertinent to growth and diffusion mechanisms. In addition, surface roughening, faceting, and surface melting are all defect mediated phase transitions involving the formation of different topological defects. While the importance of defects at surfaces is well recognized, the study of surface defects has been hampered by the lack of sufficiently accurate experimental techniques. In fact, it is only in the past 6 years that experiments on the thermal generation of defects on metal surfaces have been performed. This review attempts to outline both the theoretical and experimental work on surface defect formation on metal systems.

Rheology an structural defects in a lyotropic lamellar phase

We measured the parallel to layers shear viscosity of the lamellar phase of the (C 12E 6-H 2O) system for various temperatures. We found two effects: at low temperature, the measured viscosity decreases regularly as temperature increases and increases with the thickness of samples. We attribute this effect to previously revealed undulations of layers, for which we give a first estimation of wavelength (0.4 μm) and of amplitude (20 nm). Approaching the lamellar isotropic transition, viscosity draws up and increases. We attribute this effect to the brutal increase of the density of dislocation loops revealed previously by electron microscopy. These loops cross and connect layers and then disturb the flow. This last phenomenon supports the idea of a defect-mediated phase transition already suggested by different experiments previously reported.

Critical behavior of the diffusion constant in the melting of the layer of nitric acid intercalated in graphite

We have studied by incoherent quasi-elastic neutron scattering the behavior of the diffusion constant and the molecular movements in the second-stage of nitric acid intercalated in graphite near the melting transition. The transition is continuous and the high temperature phase is a lattice liquid. The diffusion constant follows the critical behavior predicted in a defect mediated phase transition. However, the X-ray diffraction pattern is closer to that of a lattice liquid than that of a hexatic phase.

Domain-wall-induced XY disorder in the fully frustrated XY model

The defect-mediated phase transition in the fully frustrated XY model is discussed. A Migdal-Kadanoff position-space renormalisation group analysis (1976) is employed to investigate the critical behaviour of a similar model in the same universality class as the fully frustrated XY model. The resulting phase diagram shows that XY order cannot coexist with Ising disorder. This is in agreement with the recently suggested phase transition scenario in the fully frustrated XY model.