Abstract:
We report studies on the vibrational and elastic behavior of lithium oxide, Li2O around its superionic transition temperature. Phonon frequencies calculated using the ab-initio and empirical potential model are in excellent agreement with the reported experimental data. Further, volume dependence of phonon dispersion relation has been calculated, which indicates softening of zone boundary transverse acoustic phonon mode along [110] at volume corresponding to the superionic transition in Li2O. The instability of phonon mode could be a precursor leading to the dynamical disorder of the lithium sub lattice. Empirical potential model calculations have been carried out to deduce the probable direction of lithium diffusion by constructing a super cell consisting of 12000 atoms. The barrier energy for lithium ion diffusion from one lattice site to another at ambient and elevated temperature has been computed. Barrier energy considerations along various symmetry directions indicate that [001] is the most favourable direction for lithium diffusion in the fast ion phase. This result corroborates our observation of dynamical instability in the transverse mode along (110) wave vector. Using molecular dynamics simulations we have studied the temperature variation of elastic constants, which are important to the high-temperature stability of lithium oxide.

Abstract:
First order phase transitions occur discretely from one state to another, however they often display continuous behavior. To understand this nature, it is essential to probe how the emergent phase nucleates, interacts and evolves with the initial phase across the transition at microscopic scales. Here, the prototypical first-order magneto-structural transition in FeRh is used to investigate these phenomena. We find that the temperature evolution of the final phase exhibits critical behavior. Furthermore, a difference between the structure and magnetic transition temperatures reveals a novel intermediate phase created from the interface between the initial and nucleated final states. This emergent phase, characterized by its lack of spin order due to the competition between the antiferromagnetic and ferromagnetic interactions, leads to suppression of the dynamic aspect of the transition, generating a static mixed-phase-morphology. Understanding and controlling the transition process at this spatial scale is critical to optimizing functional device capabilities.

Abstract:
The numerical approximation of non-isothermal liquid-vapor flow within the compressible regime is a difficult task because complex physical effects at the phase interfaces can govern the global flow behavior. We present a sharp interface approach which treats the interface as a shock-wave like discontinuity. Any mixing of fluid phases is avoided by using the flow solver in the bulk regions only, and a ghost-fluid approach close to the interface. The coupling states for the numerical solution in the bulk regions are determined by the solution of local multi-phase Riemann problems across the interface. The Riemann solution accounts for the relevant physics by enforcing appropriate jump conditions at the phase boundary. A wide variety of interface effects can be handled in a thermodynamically consistent way. This includes surface tension or mass/energy transfer by phase transition. Moreover, the local normal speed of the interface, which is needed to calculate the time evolution of the interface, is given by the Riemann solution. The interface tracking itself is based on a level-set method. The focus in this paper is the description of the multi-phase Riemann solver and its usage within the sharp interface approach. One-dimensional problems are selected to validate the approach. Finally, the three-dimensional simulation of a wobbling droplet and a shock droplet interaction in two dimensions are shown. In both problems phase transition and surface tension determine the global bulk behavior.

Abstract:
We use a discrete-time formulation to study the asymmetric avalanche process [Phys. Rev. Lett. vol. 87, 084301 (2001)] on a finite ring and obtain an exact expression for the average avalanche size of particles as a function of toppling probabilities depending on parameters $\mu$ and $\alpha$. By mapping the model below and above the critical line onto driven interface problems, we show how different regimes of avalanches may lead to different types of critical interface behavior characterized by either annealed or quenched disorders and obtain exactly the related critical exponents which violate a well-known scaling relation when $\alpha \ne 2$.

Abstract:
Contact effects in devices incorporating strongly-correlated electronic materials are comparatively unexplored. We have investigated the electrically-driven phase transition in magnetite (100) thin films by four-terminal methods. In the lateral configuration, the channel length is less than 2 $\mu$m, and voltage-probe wires $\sim$100 nm in width are directly patterned within the channel. Multilead measurements quantitatively separate the contributions of each electrode interface and the magnetite channel. We demonstrate that on the onset of the transition contact resistances at both source and drain electrodes and the resistance of magnetite channel decrease abruptly. Temperature dependent electrical measurements below the Verwey temperature indicate thermally activated transport over the charge gap. The behavior of the magnetite system at a transition point is consistent with a theoretically predicted transition mechanism of charge gap closure by electric field.

Abstract:
We consider a moving interface that is coupled to an elliptic equation in a heterogeneous medium. The problem is motivated by the study of displacive solid-solid phase transformations. We show that a nearly flat interface is given by the graph of the function $g$ which evolves according to the equation $g_t (x) = -(-\Delta)^{1/2}g (x) + \varphi(x, g(x)) + F$. This equation also arises in the study of dislocations and fracture. We show in the periodic setting that such interfaces exhibit a stick-slip behavior associated with pinning and depinning. Further, we present some numerical evidence that the effective velocity of the phase boundary scales as the square-root of the excess macroscopic force above the depinning transition.

Abstract:
Recently superconductivity at the interface between the insulators LaAlO3 and SrTiO3 has been tuned with the electric field effect to an unprecedented range of transition temperatures. Here we perform a detailed finite size scaling analysis to explore the compatibility of the phase transition line with Berezinskii-Kosterlitz-Thouless (BKT) behavior and a 2D-quantum phase(QP)-transition. In an intermediate regime, limited by a gate voltage dependent limiting length, we uncover remarkable consistency with a BKT-critical line ending at a metallic quantum critical point, separating a weakly localized insulator from the superconducting phase. Our estimates for the critical exponents of the 2D-QP-transition, z=1 and nu=0.66, suggest that it belongs to the 3D-xy universality class.

Abstract:
Understanding the acid-base behavior of silica surfaces is critical for many nanoscience and bio-nano interface applications. Silanol groups (SiOH) on silica surfaces exhibit two acidity constants--one as acidic as vinegar--but their structural basis remains controversial. The atomic details of the more acidic silanol site govern not just the overall surface charge density at near neutral solution pH, but also how ions and bio-molecules interacts with and bind to silica immersed in water. Using initio molecular dynamics simulations and multiple representative crystalline silica surfaces, we determine the deprotonation free energies of silanol groups with different structural motifs. We show that previously proposed motifs related to chemical connectivity or inter-silanol hydrogen bonds do not yield high acidity. Instead, a plausible candiate for pKa=4.5 silanol groups may be found in locally strained or defected regions with sparse silanol coverage. In the process, irreversible ring-opening reactions of strained silica trimer rings in contact with liquid water are observed.

Abstract:
Superconductivity at the interface between the insulators LaAlO3 and SrTiO3 has been tuned with the electric field effect. The data provide evidence for a two dimensional quantum superconductor to insulator (2D-QSI) transition. Here we explore the compatibility of this phase transition line with Berezinskii-Kosterlitz-Thouless (BKT) behavior and a 2D-QSI transition. In an intermediate regime, limited by a finite size effect, we uncover remarkable consistency with BKT- criticality, weak localization in the insulating state and non-Drude behavior in the normal state. Our estimates for the critical exponents of the 2D-QSI-transition, z =1 and nu=3, suggest that it belongs to the 3D-xy universality class.

Abstract:
Using a matrix model, we model the deconfining phase transition at nonzero temperature for a SU(N) gauge theory at large $N$. At infinite $N$ the matrix model exhibits a Gross-Witten-Wadia transition. We show that as a consequence, both the order-disorder and the order-order interface tensions vanish identically at the critical temperature $T_d$. We estimate how these quantities vanish in the matrix model as $T \rightarrow T_d$ and as $N \rightarrow \infty$. The numerical solution of the matrix model suggests possible non-monotonic behavior in $N$ for relatively small values of $N \sim 5$.