Anisotropic surface melting in lyotropic cubic crystals

From experiments with ice or metal crystals, in the vicinity of their crystal/liquid/vapor triple points, it is known that melting of crystals starts on their surfaces and is anisotropic. It is shown here by direct observations under an optical microscope that this anisotropic surface melting phenomenon occurs also in lyotropic systems. In the case of C12EO2/water mixture, it takes place in the vicinity of the peritectic Pn3m/L3/L1 triple point. Above the peritectic triple point, where the Pn3m and L1 phases coexist in the bulk, the surface of a Pn3m-in-L1 crystal is composed of (111)-type facets surrounded by rough surfaces. The angular junction suggests that rough surfaces are wet by a L3-like layer while facets stay “dry”. This is analogous to the pre-melting at rough surfaces in solid crystals. Upon cooling below the peritectic triple point, where L3 and L1 phases coexist in the bulk, a thick layer of the L3 phase grows from the pre-melted, rough Pn3m/L1 interface. Simultaneously, facets stay dry and their radius decreases. In this tri-phasic configuration, stable in a narrow temperature range, the L3/L1 and L3/Pn3m interfaces have shapes of constant mean curvature surfaces having common borders: edges of facets.     

Ratchet effect in faceting

The paper deals with a new phenomenon, named ratchet effect, envisioned theoretically as a likely consequence of metastability of crystal facets and expected to occur upon a temperature cycling. In experiments, Pn3m lyotropic crystals surrounded by the isotropic L1 phase in the mixture C₁₂EO₂/water are used. At equilibrium, the Pn3m/L1 interface contains small (111)-type facets in coexistence with rough surfaces. In agreement with theoretical expectations, it is shown that upon a saw-tooth-shaped temperature cycling, facets are growing until the rough surfaces are completely eliminated. A model of the ratchet effect is proposed.Received: 8 January 2004, Published online: 25 March 2004PACS: 64.70.Md Phase transitions in liquid crystals - 68.35.Md Surface thermodynamics, surface energies - 61.30.-v Liquid crystals     

Reverse roughening transition in carbon dioxide

We report the observation of a roughening transition in carbon dioxide along the melting line of phase I, which we call reverse as faceting appears with increasing temperature. The characteristics of the transition are discussed in light of modern theories of roughening and the causes of its reverse behavior investigated. We propose that high temperature faceting is related to a pressure-induced increase of the surface stiffness.     

On the nature of the γ-α transition in cerium

In 1964 Davis and Adams established that the large increase of the thermal expansion and compressibility in the critical region of the γ-to α-Ce phase transition occurs predominantly in the α phase. This provides strong evidence that a tricritical point is realized in Ce. This also means that the aforementioned transition is not isomorphic and that α-Ce should have a distorted fcc structure. A careful examination of Jayaraman’s data (1965) shows that a second-order transition line continues beyond the tricritical point to the vicinity of a triple point on the melting curve. The phase boundary with the tricritical point and the minimum of the melting curve are reconstructed within the framework of Landau theory. Pis’ma Zh. éksp. Teor. Fiz. 67, No. 2, 111–117 (25 January 1998) Published in English in the original Russian journal. Edited by Steve Torstveit.     

Surface energy anisotropy of iridium

Field-ion microscopy was used to study the faceting behavior and surface energy anisotropy of iridium in vacuum and in hydrogen. In vacuum below approximately 1300 K the order of faceting and the activation energy for growth of {111} facets agreed with previously published results of FIM studies. The unexpected faceting behavior of {210} planes was examined in terms of the geometry of field evaporated specimens. The observed anisotropy at temperatures above 1300 K was in qualitative agreement with Morse and Mie potential calculations and in nearly quantitative agreement with the pairwise bonding model using σ₂ = 0.4, σ₃ = 0.2. The observed maximum anisotropy of 8.8% for iridium at 1360 K fell within the range of extrapolated values for other metals at one-half the absolute melting temperature. Hydrogen appeared to lower the surface energy of each plane by only about 0.1%. An anisotropic effect of hydrogen on the faceting behavior, however, was observed and suggested that surface diffusion rates in {110} and {311} regions were preferentially increased in the presence of hydrogen.     

Steady state shapes of periodic profiles on (110), (100), and (111) surfaces of nickel

Periodic surface profiles with amplitudes of ≦0.4 μm and periodicities of 4–20 μm were prepared on Ni(110), (100), and (111) single crystal surfaces. These crystals were annealed in ultra-high vacuum (UHV) at 1073–1327 K after they had been cleaned by Ar ion bombardment and investigated by Auger electron spectroscopy. The geometry of the profiles was studied in UHV by laser diffraction and outside the vacuum by interference microscopy. The profiles have sinusoidal shapes on Ni(110) but trapezoidal shapes on both the (100) and (111) surfaces. This type of faceting can be understood on the basis of the anisotropic surface energy of Ni, with cusps at the (100) and (111) orientations. Model calculations show in the case of anisotropic surface energy that periodic profiles develop facets which correspond to the low surface energy orientations (close-packed surfaces).     

Surface glass transition in bimodal polystyrene mixtures

Using the cluster-embedding method of V. Zaporojchenko et al. (Macromolecules 34, 1125 (2000)), we measured the glass transition temperature T _g at the polystyrene/vacuum interface of bimodal mixtures of monodisperse polystyrenes of 3.5k and 1000k. Embedding of ≈ 1 nm Au clusters was monitored in situ by X-ray photoelectron spectroscopy (XPS). The clusters were formed by evaporation of Au onto the polymer surface. Only one glass transition was observed in the mixtures. The surface glass transition temperatures are correlated to but are below the bulk values of the mixtures and obey the Gordon-Taylor equation. The results suggest that the earlier reported molecular-weight dependence of the surface glass transition is not due to segregation of short chains to the surface.     

Use of optical anisotropy for study of ultrafast phase transformations at solid surfaces

A new method of crystalline-order detection in highly absorbing anisotropic crystals is worked out and is demonstrated experimentally on a monocrystal Zn. The method is based on partial transformation of incident p-polarized electromagnetic wave into s-polarizedreflected wave due to optical anisotropy. The method is applicable to anisotropic metals (for example, Zn, Ti, Cd) and makes it possible to follow changes of crystalline structure in thin (10–100 nm) surface layers. It must be emphasized, that the method permits the detection of changes of the long-range order, whereas most of the conventional methods provide information on changes of the short-range order, which need not be changed on melting and amorphization for certain crystals. Using picosecond laser pump pulse (time duration ≈1 ps) and streak camera “Agat”, surface melting and evaporation of Zn are studied. By means of measurement of time dependencies of s- and p-components of a reflected probe pulse (time duration ≈500 ps) the dynamics of melting and evaporation of a surface layer was studied at various flows of energy laser pump pulse. The characteristic time of disappearance of the long-range order is <3 ps. The crystal structure is restored through 100–300 ps after action of a pump pulse. The theoretical analysis of experimental results was performed. Estimations, based on the proposed model, are in satisfactory agreement with the experimental results. Pump-probe experiments with time resolution higher than 3 ps are in progress. Received: 27 November 1998 / Accepted: 21 April 1999 / Published online: 27 October 1999     

Interaction of Ag with faceted Pb1−x SnxTe single crystals

The results of an investigation of the interaction of silver atoms with Pb_1−x Sn_xTe single crystals having natural faceting are presented. A model is proposed for the mass transport of lowvolatility dopants through the vapor phase in the form of tellurides. Zh. Tekh. Fiz. 69, 128–129 (November 1999)     

Statistical mechanics of melting mediated by two types of defects

We propose a defect-mediated melting theory based on the statistics of two types of lattice defects, the point defects and dislocation pairs. The model predicts a first-order phase transition. Based on the model, phase transition temperature, latent heat and other thermodynamic functions are derived. Melting occurs due to discontinuous growth of point defects into dislocation pairs. The calculated phase transition temperature for five alkali metallic crystals are in fair agreement with measured melting temperatures, and the Richards' rule is derived by the model also.     

Thermodynamics of the surface of <Superscript>4</Superscript>He crystals

The conditions for applicability of the mean field theory for describing the thermodynamics of the surface of ⁴He crystals are investigated based on analysis of experimental data. It is found that although the faceting phase transition itself is a Berezinsky–Kosterlitz–Thouless phase transition, the thermodynamic potential outside a narrow neighborhood of the transition temperature can be expanded into a series in terms of the Landau theory of second-order phase transitions.     

Investigations on the soft-mode mechanism of the ferroelectric phase transition in

Dielectric and Raman spectroscopic measurements have been performed to investigate the ferroelectric phase transition in . Single crystals were grown by the zone melting method. The frequency dependence of the dielectric permittivity from 1 MHz to 1 GHz has been studied in a temperature range between 265 and 285 K. A Debye like dielectric dispersion was found, showing a critical slowing down around K. Polarized Raman spectra have been taken between 220 and 310 K. Two softening modes have been found, one of A- and another one of B / B g-symmetry. The phase transition mechanism in can be classified as partially order-disorder and partially displacive, confirming former structural results. It resembles strongly that of monoclinic . Received: 7 April 1998 / Revised: 5 June 1998 / Accepted: 16 June 1998     

Relativistic and many-body effects in K, L, and M shell ionization energy for elements with and the determination of the 1s Lamb shift for heavy elements

We report on a calculation of K, L and M inner-shell ionization energy in atoms with atomic numbers in the range . Many-body effects are evaluated for all n =1, 2, and 3 hole states. Those include correlation and effects due to the auto-ionizing nature of the hole states (Auger shift). For high Z we add recent corrected nuclear polarization, and several second-order vacuum polarization corrections. K and L ionization energies are compared with experimental X-ray absorption edges measurements. Excellent agreement with rare gazes and metal vapor measurements is found. We also compare our calculations with X-ray transition energies for all K and L lines that involve K, L and M holes. Finally we use K X-ray lines to deduce an hydrogenlike 1 s Lamb shift for several heavy elements, with far better accuracy than has been obtained by direct measurements of hydrogenlike ions. Received: 25 February 1998 / Accepted: 31 March 1998     

Superfluidity of grain boundaries in solid 4He

By large-scale quantum Monte Carlo simulations we show that grain boundaries in 4He crystals are generically superfluid at low temperature, with a transition temperature of the order of approximately 0.5 K at the melting pressure; nonsuperfluid grain boundaries are found only for special orientations of the grains. We also find that close vicinity to the melting line is not a necessary condition for superfluid grain boundaries, and a grain boundary in direct contact with the superfluid liquid at the melting curve is found to be mechanically stable and the grain-boundary superfluidity observed by Sasaki et al. [Science 313, 1098 (2006)10.1126/science.1130879] is not just a crack filled with superfluid.     

Formation of dissipative structures in crystals during heat and electron transport

The surface restructuring (faceting) of solids subjected to longitudinal electric-field and temperature gradients has been studied experimentally and theoretically. Tungsten crystals and wires preheated with a direct current in vacuum or a hydrogen atmosphere to a temperature higher than half the melting temperature are studied by electron microscopy and metallography. The processes of formation of bulk defects and of a regular surface structure are found to correlate. For the first time, these processes are analyzed in terms of synergetics.     

Dynamics of 2D anisotropic solids,smectics and nematics

In this research we study the critical dynamics of anisotropic layers in the various transition and crossover regimes associated with a defect unbinding picture of the melting process. We derive dynamic equations of motion for anisotropic solids, smectics and nematics, in the presence of defects and predict the critical behavior of various transport coefficients. The theory is applicable to two-dimensional layers of freely suspended liquid crystals, on which dynamic light-scattering experiments can be performed to test the predictions of the theory.     

Anisotropy-induced quantum critical behavior of magnetite nanoparticles at low temperatures

The classical, thermally driven transition from ferrimagnets to superparamagnets in Fe₃O₄ nanoparticles can be converted into another quantum phase by a transverse microwave magnetic field or by a strong internal anisotropic field. These fields, perpendicular to the Ising axis, can destroy the magnetic long-range order to quantum paramagnets as the fields exceed some critical values. We have exploited the spin resonance spectrometer to determine the dynamic spin susceptibility and the critical exponent γ, which is a power-law dependent spanning of the quantum critical point. Quantum phase transition observed at low temperatures for small magnetite nanoparticles induced by strong surface anisotropic field illustrates the fascinating interplay between thermal and quantum fluctuations in the vicinity of a quantum critical point.     

Anisotropic defect-mediated melting of two-dimensional colloidal crystals

The melting transition of anisotropic two-dimensional (2D) crystals is studied in a model system of superparamagnetic colloids. The anisotropy of the induced dipole-dipole interaction is varied by tilting the external magnetic field off the normal to the particle plane. By analyzing the time-dependent Lindemann parameter as well as translational and orientational order we observe a 2D smecticlike phase. The Kosterlitz-Thouless-Halperin-Nelson-Young scenario of isotropic melting is modified: dislocation pairs and dislocations appear with different probabilities depending on their orientation with respect to the in-plane field.     

Oxygen induced reconstruction of (h11) and (100) faces of copper

A LEED and AES study on oxygen adsorption on Cu(100) and (h11) faces with 5 h 15 has been performed under various adsorption conditions (220 K T 670 K and 1 × 10⁻⁸ P 6 × 10⁻⁵ Torr of oxygen). The dependence of adsorption temp on the oxygen surface superstructures is pointed out. At least, three oxygen surface states exist on a Cu(100) face. For low temperature exposures to oxygen, under conditions of slow surface diffusion, on the (100) face, two oxygen surface phases exist: a “four spots” and a c(2 × 2) superstructure, both observed even at saturation coverage; on all the stepped faces, a c(2 × 2) appears and no faceting is observed. For high temperature exposures, on the (100) face, two oxygen superstructures are observed, a “four spots” followed by a (2√2 × √2)R45° at higher coverages; on all the stepped faces, surface diffusion is activated and oxygen induced faceting occurs. The appearance of faceting is associated with the onset of the formation of the (2√2 × √2)R45° structure on the (100) face. The oxygen induced faceting and the oxygen surface meshes are reversible with coverages. At saturation coverage, a non-reversible surface transition between the c(2 × 2) and (2√2 × √2)R45° superstructures is observed at 420 ± 20 K. The importance of impurity traces on the surface meshes is emphasized. Oxygen coverage at saturation is independent of the studied faces and adsorption temperature. Faceting occurs at a critical coverage value, whatever the stepped faces and adsorption temperature are. Models of the oxygen structure on the (h10) stepped faces are discussed.     

Effects of milling-induced disorder on the lattice parameters and magnetic properties of hematite

A ball-milling treatment in air for 30 min is enough to reduce anhydrous bulk hematite (-Fe₂O₃) grains to nanometric sizes. For milling times, t_m, of 15, 30 min, 1 and 2 h, the crystals suffer an anisotropic lattice dilation, which is more pronounced for the smaller average grain sizes attained. Mössbauer and susceptibility results show that the process alters the effective Morin temperatures, T_M. The transition occurs less sharply than for the non-milled material and spreads over a maximum extent of 50 K for t_m=30 min. The susceptibility data indicate that the T_M for the fraction of material undergoes the transition shifts toward values not lower than 235 K. For t_m=10 h the transition is suppressed down to 12 K. Our results do not allow distinguishing bulk from surface regions of the grains. In addition, the disorder brought about by the milling reduces the magnetic response in the weak ferromagnetic state of -Fe₂O₃. The modification of T_M seems to depend mainly on the anisotropic dilation of the unit cell that affects the whole grain and it is related to the nanometric grain sizes achieved.