Phase transitions, interfacial fluctuations and hidden symmetries for fluids near structured walls

Fluids adsorbed at micro-patterned and geometrically structured substrates can exhibit novel phase transitions and interfacial fluctuation effects distinct from those characteristic of wetting at planar, homogeneous walls. We review recent theoretical progress in this area paying particular attention to filling transitions pertinent to fluid adsorption near wedges, which have highlighted a deep connection between geometrical and contact angles. We show that filling transitions are not only characterized by large scale interfacial fluctuations leading to universal critical singularities but also reveal hidden symmetries with short-ranged critical wetting transitions and properties of dimensional reduction. We propose a non-local interfacial model which fulfills all these properties and throws light on long-standing problems regarding the order of the 3D short-range critical wetting transition.     

The order of filling transitions in acute wedges

Using a square-gradient density functional model we test the prediction that the filling transition for a fluid in a wedge geometry changes from continuous to first-order as the wedge becomes more acute. Our numerical findings confirm such a change of order, but the value of the tilt angle at which it occurs, α* ≈ 45°, is considerably smaller than the original theoretical prediction. We critically reassess this work, which was based on allowing for the self-interaction of the fluid interface, and argue that the interfacial curvature and effective wavevector dependent surface tension can further lower the predicted value of α*, in keeping with our numerical findings. Interfacial fluctuation effects, occurring beyond mean-field level, are also discussed using effective Hamiltonian theory and are shown to substantially increase the value of α*.     

Nonlocality and short-range wetting phenomena

We propose a nonlocal interfacial model for 3D short-range wetting at planar and nonplanar walls. The model is characterized by a binding-potential functional depending only on the bulk Ornstein-Zernike correlation function, which arises from different classes of tubelike fluctuations that connect the interface and the substrate. The theory provides a physical explanation for the origin of the effective position-dependent stiffness and binding potential in approximate local theories and also obeys the necessary classical wedge covariance relationship between wetting and wedge filling. Renormalization group and computer simulation studies reveal the strong nonperturbative influence of nonlocality at critical wetting, throwing light on long-standing theoretical problems regarding the order of the phase transition.     

Mean-field phenomenology of wetting in nanogrooves

ABSTRACT

In this special issue article, we bring together our recent research on wetting in confinement, in particular planar walls, wedges, capillary grooves and slit pores, with emphasis on phase transitions and competition between wetting, filling and condensation, and highlight their similarities and disparities. The results presented are obtained with the classical density functional theory (DFT) for fluids, which is a mean-field statistical mechanical framework for including the spatial variations of the fluid density into the thermodynamic equation of state. For wetting in sculpted substrates, we solve numerically the DFT equations to obtain the fluid density profiles, wetting isotherms and phase diagrams. This allows us to contrast the wetting phenomenology of grooves, planar walls, slit and wedge-shaped pores. Of particular interest are the transitions associated with capillary condensation, planar pre-wetting and mean-field wedge pre-filling lines.     

Finite element simulation and experimental determination of interfacial adhesion properties by wedge indentation

This paper presents our recent study on determination of interfacial adhesion properties of soft-film-on-hard-substrate (SFHS) systems using finite element simulation (FEM) and wedge indentation experiments. The objectives of this study are: (i) to simulate the interfacial delamination processes during wedge indentation experiments; (ii) to study the effects of interfacial delamination on the characteristics of the indentation load–displacement (P–h) curves, (iii) to determine the interfacial adhesion properties; and (iv) to compare the simulation and experimental results. During the FEM simulation, a traction-separation law is used to describe the interfacial adhesion properties due to the large-scale yielding during indentations. The effects of main parameters in the traction-separation law, i.e. interfacial strength and interfacial energy, to the initiation of interfacial delamination are studied by parametric studies. An interface energy-strength contour, which can be used to determine the interfacial adhesion properties of the thin-film/substrate systems based on a wedge indentation experiment, is developed from the outcomes of the FEM simulation of the indentations using wedge tips with the inclusion angles of 90° and 120°. Using the respective interface energy-strength contours, the interfacial energy and strength of a BlackDiamond® (BD)/Si system and a methylsilsesquioxane (MSQ)/Si system are determined. The simulated results are then compared with the previous experimentally derived interfacial fracture toughness values and some further discussions are given.     

Disjoining pressure of planar adsorbed films

Frumkin-Derjaguin theory of interfacial phase transitions and in particular the concept of the disjoining pressure of a planar adsorbed film is reviewed and then discussed in terms of statistical mechanical formulations of interfacial phase transitions beyond mean-field.     

Anyon condensation and continuous topological phase transitions in non-Abelian fractional quantum Hall states

We find a series of possible continuous quantum phase transitions between fractional quantum Hall states at the same filling fraction in two-component quantum Hall systems. These can be driven by tuning the interlayer tunneling and/or interlayer repulsion. One side of the transition is the Halperin (p,p,p-3) Abelian two-component state, while the other side is the non-Abelian Z4 parafermion (Read-Rezayi) state. We predict that the transition is a continuous transition in the 3D Ising class. The critical point is described by a Z2 gauged Ginzburg-Landau theory. These results have implications for experiments on two-component systems at ν=2/3 and single-component systems at ν=8/3.     

Stability of the smectic quantum hall state: a quantitative study

We present an effective elastic theory which quantitatively describes the stripe phase of the two-dimensional electron gas in high Landau levels ( N>/=2). The dynamical matrix is obtained with remarkably high precision using the time-dependent Hartree-Fock approximation. A renormalization group analysis shows that at T = 0, as the partial filling factor Deltanu identical withnu- left floornu right floor moves away from 1/2, the anisotropic conducting state may undergo quantum phase transitions: stripes may get pinned along their conducting direction by disorder, or may lock into one another to form a crystal. The transitions should be reflected in the temperature dependence of the dissipative conductivity.     

Ni2+掺杂近化学计量比铌酸锂晶体的生长及光谱特性

以K2O为助熔剂,在较大的固液界面温度梯度条件下,应用坩埚下降法技术生长了初始Ni2 掺杂摩尔分数为0.5%的近化学计量比铌酸锂晶体。测定了晶体的吸收光谱,观测到由Ni2 离子在八面体中3A2g(F)→3T1g(P)、3A2g(F)→3T1g(F)、3A2g(F)→3T2g(F)能级的正常自旋允许跃迁所产生的381 nm,733 nm,1280 nm吸收峰和3A2g(F)→1T2g(D)和3A2g(F)→1E(D)能级的自旋禁戒跃迁产生的430 nm与840 nm吸收峰。从晶体紫外吸收边的位置初步估算其摩尔分数比x(Li )/x(Nb5 )为0.981。根据晶体分裂场理论和吸收光谱,计算了Ni2 在该铌酸锂晶体中的晶格场分裂参量Dq=781 cm-1、Racah参量B=1096 cm-1与C=4353 cm-1。研究了在不同激发波长下晶体在可见光波段的荧光特征,观察到500~630 nm的绿色与800~850 nm的红色荧光发射带,它们归结为1T2g(D)→3A2g(F)与1T2g(D)→3T2g(F)的能级跃迁所致。     

Complete wetting of nanosculptured substrates

Complete wetting of geometrically structured substrates by one-component fluids with long-ranged interactions is studied theoretically. We consider periodic arrays of rectangular or parabolic grooves and lattices of cylindrical or parabolic pits. We show that the midpoint interfacial heights within grooves and pits are related in the same way as for complete wedge and cone filling. For sufficiently deep cavities with vertical walls and small undersaturation, an effective planar scaling regime emerges. The scaling exponent is -1/3 in all cases studied, and only the amplitudes depend on the geometrical features. We find quantitative agreement with recent experimental data for such systems.     

Interface end theory and re-evaluation in interfacial strength test methods

The experimental results of fragmentation, micro-indentation, pull-out and microdebond tests often exhibit large discrepancies. Since all specimens of the four test methods all have interface ends, the singularity theory of the interface end should be used to evaluate the exactness of the test methods. The eigenvalues of the specimens for the micro-indentation test, pull-out test and microdebond test are calculated and investigated. The results show that the stress singularity of the interface end depends on the Dundurs' parameters and the wedge angles. The interfacial shear strength (IFSS) obtained from the tests loses its rationality if the stress is singular at the interface end. In further analysis, for a carbon fiber-epoxy resin composite, it is found that the microdebond test gives the most reliable IFSS results, if the wedge angle of the resin droplet is less than 40°; the results from the pull-out test are dubious, due to the stress singularity at the interface end. In the micro-indentation test, there is a critical matrix stiffness value for a given fiber, above which the stress at the interface end will be non-singular. The fragmentation test assumes the interfacial shear stress on the fiber fragment of critical length is the IFSS. If debonding does not occur at the interface end, then apparently, the interfacial shear stress on the fiber fragment of critical length is less than the true value of IFSS.     

Behavior of the antiferromagnetic phase transition near the fermion condensation quantum phase transition in YbRh2Si2

Low-temperature specific-heat measurements on YbRh₂Si₂ at the second order antiferromagnetic (AF) phase transition reveal a sharp peak at TN=72 mK. The corresponding critical exponent α turns out to be α=0.38, which differs significantly from that obtained within the framework of the fluctuation theory of second order phase transitions based on the scale invariance, where α?0.1. We show that under the application of magnetic field the curve of the second order AF phase transitions passes into a curve of the first order ones at the tricritical point leading to a violation of the critical universality of the fluctuation theory. This change of the phase transition is generated by the fermion condensation quantum phase transition. Near the tricritical point the Landau theory of second order phase transitions is applicable and gives α?1/2. We demonstrate that this value of α is in good agreement with the specific-heat measurements.     

Influence of spin-polarized current on superconductivity and the realization of large magnetoresistance

The superconducting state can be influenced by injecting spin-polarized current in a controlled manner by properly tailoring the interfacial transmittivity between a ferromagnet (F) and a superconductor (S), resulting in a large magnetoresistance of over 1100% for a F/I/S/I/F multilayer system (I insulator). Because of the competition between ferromagnetism and superconductivity, the superconducting transition temperature (T(C)) in the spin-parallel configuration is shifted below that in the spin antiparallel configuration. The T(C) shift is attributed to ferromagnet-induced nonequilibrium spin carriers in the superconductors.     

Direct observation of capillary condensation of a solid

We describe the direct condensation of a solid from vapor in an annular mica wedge. Neo-pentanol initially condenses as a liquid from 8 to 57 degrees C (the melting point T(m)), followed by nucleation of a solid from vapor for T<45 degrees C. Menthol (T(m) = 42 degrees C) gives only liquid condensates down to 12 degrees C. The adsorbed films of neo-pentanol, which unlike those of menthol show layering transitions, and the disordered crystalline phase of bulk neo-pentanol appear to facilitate condensation of the solid phase. There is evidence for a change in the nature of the solid neo-pentanol condensate with T.     

Antiferromagnetic spin fluctuations above the dome-shaped and full-gap superconducting states of LaFeAsO1-xFx revealed by (75)As-nuclear quadrupole resonance

We report a systematic study by (75)As nuclear-quadrupole resonance in LaFeAsO(1-x)F(x). The antiferromagnetic spin fluctuation found above the magnetic ordering temperature T(N) = 58 K for x = 0.03 persists in the regime 0.04 ≤ x ≤ 0.08, where superconductivity sets in. A dome-shaped x dependence of the superconducting transition temperature T(c) is found, with the highest T(c) = 27 K at x = 0.06, which is realized under significant antiferromagnetic spin fluctuation. With increasing x further, the antiferromagnetic spin fluctuation decreases, and so does T(c). These features resemble closely the cuprates La(2-x)Sr(x)CuO(4). In x = 0.06, the spin-lattice relaxation rate (1/T(1)) below T(c) decreases exponentially down to 0.13T(c), which unambiguously indicates that the energy gaps are fully opened. The temperature variation of 1/T(1) below T(c) is rendered nonexponential for other x by impurity scattering.     

Simulations of the intermediate bandwidth fluctuations in nanostructured PV

The size dispersion and distributions of quantum dot nanoparticles (sizes from 2–5 nm) embedded in the active region of the intermediate band solar cells are important to reach the high efficiencies. An optimized size and regularity can increase the efficiency due largely to avoided non-radiative transitions which can originate from the fluctuations in the bandwidth of the intermediate layer. In this work, we propose all the energy band diagrams possible in the formation of such a cell. Five equivalent band diagrams of the cells with different size dispersions and regularity of quantum dots are considered and compared with the reported experimental profiles in the literature. Furthermore, the degree of the size fluctuation is considered by proposing a fluctuation degree for the band gap and sub-band gaps of the cell. These proposed profiles and the fluctuation theory are exploited to consider the experimental data reported in literature. The optimized size dispersion will increase the photocurrent of the cell. We believe that every quantum dot solar cell will fall into one of the proposed band diagrams. This approach gives foresight to the theoretical studies of such devices and expectation from the energy band structure and band widths since it considers the fluctuation of the band widths for the intermediate band separately.     

Dynamic aspect of bacteriorhodopsin as a typical membrane protein as revealed by site-directed solid-state 13C NMR

We demonstrate here a general feature of dynamic aspect of membrane proteins as revealed by site-directed 13C NMR studies on bacteriorhodopsin (bR) as a typical membrane protein and a variety of mutants at ambient temperature. 13C NMR signals of [3-13C]Ala- or [1-13C]Val-labeled proteins were assigned regio-specifically with reference to the data of the conformation-dependent 13C chemical shifts from model polypeptides, followed by site-specific assignment based on site-directed mutants. Revealed picture of membrane protein at ambient temperature is not static in contrast to anticipation from crystalline structures but flexible enough to undergo a variety of local fluctuations with frequencies from 10(2) to 10(8)Hz, as pointed out already. This picture was further refined by taking into account of residue-specific dynamics of interfacial domains between the surface and inner part of the transmembrane helices and conformational fluctuation induced by the presence of a kinked structure. The residue-specific dynamics of the former was revealed by observation of broadened or suppressed peaks from the interfacial domains caused by acquisition of internal fluctuation motions interfered with frequencies of proton decoupling or magic angle spinning. The presence of such suppressed peaks due to molecular fluctuations in the interfacial domains was further confirmed by insensitivity of the peak-intensities from the interfacial domains in spite of the presence of accelerated relaxation rate to nearby residues from surface bound Mn2+ ion. Further, conformational change of the transmembrane alpha-helix F due to a plausible kinked structure at Pro 186 was confirmed in view of specific displacements of Ala 184 and Val 187 13C NMR peaks from chemically synthesized [3-13C]Ala(184)-, [1-13C]Val(187)-labeled wild type and P186L mutant of transmembrane fragment F(164-194) incorporated into lipid bilayer. It is emphasized that the observed displacement of [3-13C]-labeled Ala 184 peak at 17.4 ppm in the presence of kinked structure in this model peptide is consistent with that of intact protein at 17.27 ppm.     

Thermal Effects of Interfacial Dynamics

Dynamical Ginzburg-Landau theory is applied to the study of thermal effects of motion of interfaces that appear after different phase transitions. These effects stem from the existence of the surface internal energy, entropy and temperature gradients in the interfacial transition region. Evolution equations for the interfacial motion are derived. For the experimental verification of the thermal effects the expression is derived for the amplitude of temperature waves during continuous ordering.     

Observation of soft magnetorotons in bilayer quantum Hall ferromagnets

Inelastic light-scattering measurements of low-lying collective excitations of electron double layers in the quantum Hall state at total filling nu(T)=1 reveal a deep magnetoroton in the dispersion of charge-density excitations across the tunneling gap. The roton softens and sharpens markedly when the phase boundary for transitions to highly correlated compressible states is approached. The findings are interpreted with Hartree-Fock evaluations that link soft magnetorotons to enhanced excitonic Coulomb interactions and to quantum phase transitions in the ferromagnetic bilayers.     

Characterizing symmetry breaking patterns in a lattice by dual degrees of freedom

A duality transformation in quantum field theory is usually established first through partition functions. It is always important to explore the dual relations between various correlation functions in the transformation. Here, we explore such a dual relation to study quantum phases and phase transitions in an extended boson Hubbard model at 1/3 (2/3) filling on a triangular lattice. We develop systematically a simple and effective way to use the vortex degrees of freedom on dual lattices to characterize both the density wave and valence bond symmetry breaking patterns of the boson insulating states in the direct lattices. In addition to a checkerboard charge density wave (X-CDW) and a stripe CDW, we find a novel CDW-VBS phase which has both local CDW and local valence bond solid (VBS) orders. Implications for Quantum Monte Carlo simulations are addressed. The possible experimental realizations of cold atoms loaded on optical lattices are discussed.