Session U45: Structural, Surface and other Phase Transitions

Effect of external strain on the order-disorder phase transition of the Si(001) surface

The Si(001) surface exhibits the phase transition from c(4$\times $2) to the (2$\times $1) structure at about 200 K[1, 2]. This is an order-disorder phase transition with respect to the buckling of the dimmer: the c(4$\times $2) structure results from an antiferromagnetic ordering of the buckled-dimmer and the (2$\times $1) structure is attributed to the time average of the flip-flop motion of the buckled-dimers. Externally applied tensile strain along the $<$110$>$ direction on the Si(001) surface is found to induce the flip-flop motion of the buckled dimmer below the critical temperature. This motion occurs cooperatively to form the disordered domain of the (2$\times $1) structure. Then the shape of the ordered domain as well as the size change with the strain. These results can be interpreted by the spontaneous shape instability originated from the strain relaxation energy. References [1] J. Ihm, D.H. Lee, J.D. Joannopoulos and J.J. Xiong, Phys. Rev. Lett. \textbf{51}, 1872(1983). [2] T. Tabata, T. Aruga and Y. Murata, Surf. Sci. \textbf{179}, L63(1987).   

Faceting and defaceting phase transitions of Pd/W(111)

We have studied the faceting and the defaceting phase transitions of the Pd/W(111) surface. Our studies show that for creating the largest facets, the best annealing temperature is right below the defaceting transition temperature, confirming the prediction by Oleksy (Surf. Sci. 549 (2004) 246). As we vary the programmed heating/cooling rate from 1/8 to 8 K/s, the paths of faceting transitions show normal retardation effect and shift to lower temperatures as the cooling rate increased. Surprisingly, the paths of defaceting transitions show negligible dependence on the heating rate. Detailed studies of this peculiarity lead us to propose that the defaceting transition is initiated at places subject to loss of too much Pd due to thermal desorption. As such loss can more readily be replenished at places near any one of the Pd 3-d islands, we propose that the rate independent path of defaceting transition is the consequence of a temperature dependent balance between the loss and the supply of Pd. Such balance should depend on the density of the Pd islands. Indeed, we find the paths of defaceting transition can be shifted to lower temperature by reducing the density of the Pd 3-d islands.   

Ultrafast lattice dynamics of FeRh

FeRh undergoes magnetic and structural phase transitions at $\sim 100^{\circ}$ C where a transition from antiferromagnetic to ferromagnetic orders occurs upon heating. Commensurate with this magnetic transition is a $\sim$1\% expansion in the lattice parameter. Recent optical measurements have shown that the magnetic transition can be quite fast, i.e., on the picosecond or sub-picosecond time scales [1,2]. We have used ultrafast x-ray diffraction techniques at the Advanced Photon Source to probe the speed of the corresponding structural transition. An epitaxial FeRh thin film on a MgO(001) substrate was driven through the phase transition by ultrafast laser excitation, and the response of the lattice was directly observed via picosecond-time-resolved x-ray diffraction. The temporal evolution of the FeRh lattice is reported as a function of laser fluence. [1] J.-U.\ Thiele \textit{et al}., Appl.\ Phys.\ Lett.\ 85, 2857 (2004). [2] G.\ Ju \textit{et al}., Phys.\ Rev.\ Lett.\ 93, 192301 (2004).   

Phase stability and Jahn-Teller distortion in doped lithiated manganese oxides: A LSDA+U study

We discuss how the rhombohedral phase of lithiated manganese oxide can be stabilized by doping with various impurities. Our study is based on LSDA+U calculations as implemented in the VASP code. We have considered rhombohedral and monoclinic phases using a supercell of 16 atoms. Our results are based on total energy calculations for 25{\%} dopant concentration and pure lithiated manganese oxide. Several dopants such as Co, Fe, Ni, Mg and Zn are considered. We find that oxidation state of the dopant plays an important role in suppressing the Jahn-Teller distortion. Divalent impurities are found to be most effective. The effect of including U in the calculation is discussed.   

Crystal Fields and the $ \gamma \rightarrow \alpha$ transition in Ce

In the $\gamma \rightarrow \alpha$ transition of Cerium, the material undergoes an isostructural change at which the volume changes by 15\% and the magnetic character changes. Recently, the transition has been described in terms of a balance between the free energy of the magnetic moments and the characteristic energy scale of the $\alpha$ phase. The field-temperature dependence of the phase diagram has been predicted, and was confirmed by experiment. Inelastic neutron scattering experiments on the $\gamma$-phase of Ce have shown indications of crystal field splittings, and similar experiments have determined the energy scale of the $\alpha$ phase. We shall examine the effects of the crystalline field splittings within the framework of NCA calculations on the single-impurity Anderson model, and examine their consequence for the phase diagram.   

Bifurcation Techniques for Structural Phase Transitions

A new technique for studying structural phase transitions in crystals has been developed which uses bifurcation theory to investigate a material's free energy landscape. In this method a material's behavior is numerically interrogated by beginning with its high temperature structure and mapping out the equilibrium branch corresponding to the distortions of the crystal structure that occur due to changes in parameters such as temperature or stress. The investigation of this equilibrium branch is continued into unstable regions of the material's free energy landscape, i.e., regions which are physically unobservable. In these unstable regions the equilibrium branch bifurcates, or splits, and leads to other stable regions corresponding to different crystal structure branches (phases), thus revealing the links and interactions between the various phases of the material. Often, unexpected stable phases are identified in this way. It is common to encounter non-generic bifurcation points, where a single equilibrium branch splits into many (instead of two) new equilibrium branches. In these complex situations, the current bifurcation method is guaranteed to systematically identify all of the new branches. To illustrate the method, an atomistic model for shape memory alloys is investigated and a commonly observed hysteretic transformation is identified between a cubic $B2$ (austenite) structure and an orthorhombic $B19$ (martensite) structure.   

Imaging the evolution of a glassy magnetic transition in a disordered ferromagnetic manganite

An intriguing glass-like transition in (La,Pr,Ca)MnO$_3$ is, for the first time, imaged using a variable-temperature magnetic force microscope. Images showing the temperature and magnetic- field evolution of the local magnetic structure illustrate the microscopic origin of the bifurcation of magnetic susceptibility, which is a ubiquitous phenomenon in heavily-disordered ferromagnets, and traditionally considered as a signature of a ``cluster glass transition.'' The observed avalanche-type behavior reveals the collective nature of the glassy transition in the manganites, where ferromagnetic and antiferromagnetic phases are intricately mixed.   

Quantum Relaxation in a Proton Glass

Rb$_{1-x}$(NH$_{4}$)$_{x}$H$_{2}$PO$_{4}$ is a dipolar structural glass with spatial frustration from the mixture of ferroelectric RDP and antiferroelectric ADP. We measure the ac dielectric susceptibility of RADP:72 and RADP:35 over 7 decades of frequency for 0.3 $<$ T $<$ 300 K. The relaxation is quantitatively similar for both concentrations at low temperatures, pointing to a local mechanism. We correlate the dielectric susceptibility with the potential energy landscape derived from neutron Compton scattering experiments and solve for the tunneling parameters of the protons, finding correlated rearrangements of the hydrogen network. By analogy with vortex tunneling in high-Tc superconductors, we relate the logarithmic decay of the polarization to the quantum mechanical action.   

Switching between one and two dimensions: Pb induced chain structures on Si(557)

The conductivity of epitaxially grown Pb-structures on Si(557) has been measured. Different characteristic transport mechanisms have been found: For coverages above the percolation limit(0.6ML) up to 3ML the electronic transport in the annealed Pb-films is activated. Furthermore, the uniaxial symmetry of the Si(557) surface is reflected directly in a higher conductance in the parallel direction compared to the direction perpendicular to the steps. For coverages higher than 3ML a metallic behavior is found for both directions, i.e. the conductance decreases with increasing temperature. In contrast, already one ML, but annealed to 640K, leads to the formation of atomic wires, as seen by STM, with an extremely high and quasi one-dimensional surface state conductance along the wire direction. At a critical temperature of T$_c$=78K, the system switches from low to high conductance anisotropy, with a metal-insulator transition in the direction perpendicular to the chain structure, while in the direction along the chains conductance with a (1/T + const.) temperature dependence was found. STM has shown further, that the 1D/2D transition is associated with an order-disorder phase transition of a 10- fold superperiodicity along the Pb chains.   

Spontaneous polarization in one-dimensional Pb(ZrTi)O$_3$ nanowires

Formation of spontaneous polarization in one-dimensional structures is the key phenomenon that reveals collective behaviors in systems of reduced dimension, but has remained unsolved for decades. Here we report {\it ab initio} studies on finite-temperature structural properties of infinite-length nanowires of Pb(Zr$_{0.5}$Ti$_{0.5}$)O$_3$ solid solution. Whereas existing studies have ruled out the possibility of phase transition in 1D chains, our atomistic simulations demonstrate an unambiguous otherwise. We show that phase transitions in 1D wires occur on a remarkable macroscopic length scale, but not necessarily on an infinite length scale as assumed in the general theories of 1D phase transition. Such phase transitions are chracterized by large longitudinal $d_{33}$, $\chi _{33}$ responses and a large $c/a$ strain. The long rang ordering in PZT nanowires is explained by use of depolarizing effects associated with finite thickness of wires. Our results suggest no fundamental constraint that limits the use of ferroelectric nanowires and nanotubes arising from the absence of spontaneous ordering.   

Computational investigation of wetting and prewetting phase behavior

Fluids in the presence of one or more surfaces exhibit a rich variety of phase transitions that are absent in bulk fluids. Even the simplest of systems display a broad range of phase behavior. In this presentation, we describe our recent efforts aimed towards obtaining a better understanding of surface phase behavior through the use of molecular simulation. The first part of the presentation will be used to provide an overview of transition-matrix based Monte Carlo algorithms that enable one to efficiently locate and characterize phase transitions. Results will then be presented that describe how the wetting behavior of a model substrate-fluid system evolves with temperature and the relative strength of the substrate-fluid interaction. Simulation results will be compared with density functional theory calculations. Finally, we will describe a series of calculations that enable us to estimate the boundary tension along the prewetting saturation line. This quantity is related to the line tension associated with the formation of liquid droplets on a solid substrate. The magnitude of this tension has been the subject of debate recently, with experimental values spanning several orders of magnitude.   

Magnetic properties of weak itinerant ferromagnetic Ni-V alloys

Magnetization measurements of a serie of Ni-V alloys at high magnetic fields and low temperatures will be presented. Characteristic exponents observed in the low field susceptibility and spontaneous magnetization can be extracted and traced upon dilution. Especially the changes around a critical V-concentration of 1/9 will be discussed probing and comparing the validity of itinerant models and significance of spin fluctuations and magnetic cluster contributions.   

Scaling Behavior of Classical Wave Transport in Mesoscopic Media at the Localization Transition

The propagation of classical wave in disordered media at the Anderson localization transition is studied. Our results show that the scaling behavior of wave transport depends on the sample's geometry. It is found that the averaged static diffusion constant $D(L)$ scales like $\ln L/L$ in cubes or slabs, and the corresponding transmission follows $\left\langle {T(L)} \right\rangle \propto \ln L/L^2$. This is in contrast to the scaling behavior of $D(L)\propto 1/L$ and $\left\langle {T(L)} \right\rangle \propto 1/L^2$ obtained previously for electrons or spherical samples. For wave dynamics, we solve the Bethe-Salpeter equation in a disordered slab with the recurrent scattering incorporated in a self-consistent manner. All of the static and dynamic transport quantities studied are found to follow the new scaling behavior of $D(L)$.   

Optical Near-Field Based Nanoscale Rapid Melting and Crystallization of Amorphous Silicon Thin Films

Nanostructuring of thin films is gaining widespread importance owing to ever-increasing applications in a variety of fields. The current study details nanosecond laser-based rapid melting and crystallization of thin amorphous silicon (a-Si) films at the nanoscale. Two different near-field processing schemes were employed. In the first scheme, local field enhancement in the near-field of a SPM probe tip irradiated with nanosecond laser pulses was utilized. As a second approach, the laser beam was spatially confined by a cantilevered near field scanning microscope tip (NSOM) fiber tip. Details of various modification regimes produced as a result of the rapid a-Si melting and crystallization transformations that critically depend on the input laser fluence are presented. At one extreme corresponding to relatively high laser fluence, ablated area surrounded by a narrow melt region was observed. At the other extreme, where the incident laser energy density is much lower, single nanostructures with a lateral dimension of $\sim$90 nm were defined. The ability to induce nucleation and produce single semiconductor nanostructures in a controlled fashion may be crucial in the field of nano-opto-electronics.   

Explosive Crystallization of Amorphous Semiconductor Films in the Presence of Melting

Explosive crystallization (EC) of thin amorphous solid films of germanium is investigated theoretically and experimentally. EC regime characterized by a propagating melting layer between the amorphous and the crystalline phases is considered. Laser-induced, linear EC fronts, uniformly propagating over large distances are achieved in films with various thicknesses deposited on quartz substrate. Depending on the front speed, the film thickness and the substrate temperature, different types of morphology of the resulting crystal phase are observed: columnar, scalloped and mixed. A theory of EC in the presence of melting is developed. The EC front propagation speed is calculated as a function of the substrate temperature and the film thickness; it is found to be in a good agreement with experiments. Linear stability analysis of a uniformly propagating planar EC front is performed. It is shown that for the parameter values where the columnar crystalline structure was observed the front is unstable with respect to a fingering instability similar to the Mullins-Sekerka instability of a solidification front in an undercooled melt. Nonlinear evolution of this instability is simulated numerically and is shown to exhibit a structure similar to the columnar one.