Real-space observation of nanoscale inhomogeneities and fluctuations in a phase transition of a surface quasi-one-dimensional system: In/Si(111)

We report direct visualizations of the fluctuation and condensation phenomena in a phase transition of a one-dimensional (1D) In/Si(111) system using scanning tunneling microscopy. The high-temperature (HT) and low-temperature (LT) phases are found to coexist on the nanometer scale near Tc. Above Tc, 1D LT-phase stripes fluctuate in the HT phase and coalesce into 2D islands with decreasing temperature. They condense to make the LT phase below Tc. Small areas of the HT phase also exist below Tc. The observed temperature-dependent evolution of the nanoscale inhomogeneities is consistent with the theoretical predictions for a second-order phase transition.     

Wetting of a plane with a narrow solvophobic stripe

ABSTRACT

We present a numerical study of a simple density functional theory model of fluid adsorption occurring on a planar wall decorated with a narrow deep stripe of a weaker adsorbing (relatively solvophobic) material, where wall-fluid and fluid-fluid intermolecular forces are considered to be dispersive. Both the stripe and outer substrate exhibit first-order wetting transitions with the wetting temperature of the stripe lying above that of the outer material. This geometry leads to a rich phase diagram due to the interplay between the pre-wetting transition of the outer substrate and an unbending transition corresponding to the local evaporation of liquid near the stripe. Depending on the width of the stripe, the line of unbending transitions merges with the pre-wetting line inducing a two-dimensional wetting transition occurring across the substrate. In turn, this leads to the continuous pre-drying of the thick pre-wetting film as the pre-wetting line is approached from above. Interestingly we find that the merging of the unbending and pre-wetting lines occurs even for the widest stripes considered. This contrasts markedly with the scenario where the outer material has the higher wetting temperature, for which the merging of the unbending and pre-wetting lines only occurs for very narrow stripes.     

Melting of bosonic stripes

We use quantum Monte Carlo simulations to determine the finite temperature phase diagram and to investigate the thermal and quantum melting of stripe phases in a two-dimensional hard-core boson model. At half filling and low temperatures the stripes melt at a first order transition. In the doped system, the melting transitions of the smectic phase at high temperatures and the superfluid smectic (supersolid) phase at low temperatures are either very weakly first order, or of second order with no clear indications for an intermediate nematic phase.     

Freezing of a stripe liquid

The existence of a stripe-liquid phase in a layered nickelate, La(1.725)Sr(0.275)NiO(4), is demonstrated through neutron scattering measurements. We show that incommensurate magnetic fluctuations evolve continuously through the charge-ordering temperature, although an abrupt decrease in the effective damping energy is observed on cooling through the transition. The energy and momentum dependence of the magnetic scattering are parametrized with a damped-harmonic-oscillator model describing overdamped spin waves in the antiferromagnetic domains defined instantaneously by charge stripes.     

Length scale dependence of chiral symmetry breaking in free-standing films of achiral smectic C

The textures of a smectic C phase in free-standing films of the Schiff base 5O.6 are studied as a function of the number of layers and temperature. Periodic stripes with alternating contrast are observed in SmA boundary layers. On further cooling a transition to the splay stripes occurs in the smectic A temperature interval. In the smectic C phase all the periodic textures disappear, whereas in films with one free boundary a periodic arrangement of splay stripes is found. Mechanisms of symmetry breaking are discussed. Pis'ma Zh. éksp. Teor. Fiz. 64, No. 1, 29–32 (10 July 1996) Published in English in the original Russian journal. Edited by Steve Torstveit.     

Pressure-induced phase transition(s) in KMnF3 and the importance of the excess volume for phase transitions in perovskite structures

We report a pressure-dependent investigation of KMnF(3) by x-ray diffraction up to 30 GPa. The results are discussed in the framework of Landau theory and in relation to the isostructural phase transition in SrTiO(3). The phase transition temperature near 186 K in KMnF(3) shifts to room temperature at a critical pressure of P(c) = 3.4 GPa; the pressure dependence of the transition point follows ΔP(c)/ΔT(c) = 0.0315 GPa K(-1). The transition becomes second order under high pressure, close to the tricritical point. The phase transition is determined by the rotation of MnF(6) octahedra with their simultaneous expansion along the rotation axis. The rotation angle was found to increase to 10.5° at 24 GPa. An additional anomaly was observed at higher pressure around 25 GPa, suggesting a further phase transition.     

Helical nuclear spin order in a strip of stripes in the quantum Hall regime

We investigate nuclear spin effects in a two-dimensional electron gas in the quantum Hall regime modeled by a weakly coupled array of interacting quantum wires. We show that the presence of hyperfine interaction between electron and nuclear spins in such wires can induce a phase transition, ordering electrons and nuclear spins into a helix in each wire. Electron-electron interaction effects, pronounced within the one-dimensional stripes, boost the transition temperature up to tens to hundreds of millikelvins in GaAs. We predict specific experimental signatures of the existence of nuclear spin order, for instance for the resistivity of the system at transitions between different quantum Hall plateaus.     

Magnetic-field-induced Mott transition in a quasi-two-dimensional organic conductor

We investigated the effect of magnetic field on the highly correlated metal near the Mott transition in the quasi-two-dimensional layered organic conductor, kappa-(BEDT-TTF)(2)Cu[N(CN)(2)]Cl, by the resistance measurements under control of temperature, pressure, and magnetic field. It was demonstrated that the marginal metallic phase near the Mott transition is susceptible to the field-induced localization transition of the first order, as was predicted theoretically. The thermodynamic consideration of the present results gives a conceptual pressure-field phase diagram of the Mott transition at low temperatures.     

Stress induced stripe formation in Pd/W(110)

A stress-induced stripe phase of submonolayer Pd on W(110) is observed by low-energy electron microscopy. The temperature dependence of the pattern is explained by the change both in the boundary free energy and elastic relaxation energy due to the increasing boundary width. The stripes are shown to disorder when the correlation length of the condensed phase becomes comparable to its period, while the condensate to lattice-gas transition takes place at a higher temperature, as revealed by low-energy electron diffraction.     

Finite-temperature density instability at high landau level occupancy

We study here the onset of charge density wave instabilities in quantum Hall systems at finite temperature for Landau level filling nu>4. Specific emphasis is placed on the role of disorder as well as on an in-plane magnetic field. Beyond some critical value, disorder is observed to suppress the charge density wave melting temperature to zero. In addition, we find that a transition from perpendicular to parallel stripes (relative to the in-plane magnetic field) exists when the electron gas thickness exceeds approximately 60 A. The perpendicular alignment of the stripes is in agreement with the experimental finding that the easy conduction direction is perpendicular to the in-plane field.     

Monte Carlo phase diagram for diblock copolymer melts

The phase diagram for diblock copolymer melts is evaluated from lattice-based Monte Carlo simulations using parallel tempering, improving upon earlier simulations that used sequential temperature scans. This new approach locates the order-disorder transition (ODT) far more accurately by the occurrence of a sharp spike in the heat capacity. The present study also performs a more thorough investigation of finite-size effects, which reveals that the gyroid (G) morphology spontaneously forms in place of the perforated-lamellar (PL) phase identified in the earlier study. Nevertheless, there still remains a small region where the PL phase appears to be stable. Interestingly, the lamellar (L) phase next to this region exhibits a small population of transient perforations, which may explain previous scattering experiments suggesting a modulated-lamellar (ML) phase.     

Luttinger stripes in antiferromagnets

We propose a model for the physics of stripes in antiferromagnets in which the stripes are described by Luttinger liquids hybridized with antiferromagnetic domains. Using bosonization techniques we study the model in the limit where the magnetic correlation length is larger than the inter-stripe distance and propose an explanation for the commensurate-incommensurate phase transition seen in neutron scattering in the underdoped regime of La₂-_χx Sr_χx Cu O₄. The explanation is based on a phase to anti-phase domain transition in the spin configuration which is associated with the transverse motion of the stripes. Using a non-linear σ model to describe the antiferromagnetic regions we conjecture the crystalization of the stripes in the magnetically ordered phase.     

Critical fluctuations and quenched disordered two-dimensional charge stripes in La(5/3)Sr(1/3)NiO4

Using high-resolution x-ray scattering, we have demonstrated the existence of quenched disordered charge stripes in a single crystal of La (5/3)Sr (1/3)NiO (4) at low temperatures. Above the second-order transition critical scattering was observed due to fluctuations into the charge stripe phase. The charge stripes are shown to be two dimensional in nature both by measurements of their correlation lengths (xi(a) approximately 185 A, xi(b) = 400 A, and xi(c) approximately 25 A) and by the critical exponents of the charge strip transition. The charge stripe ordering did not develop long-range order even at low temperatures, indicating that the charge stripes are disordered and that the length scale of the disorder is quenched.     

The thermoelectric power of MEM(TCNQ)2

MEM(TCNQ)₂ undergoes a first order semiconductor to metal transition at 340.8 K. We have measured the thermoelectric power (TEP) of MEM(TCNQ)₂ in the temperature range above 335 K. Above the transition the TEP is ?65 μV/°K, in the low temperature phase it is strongly temperature dependent and approaches zero near the transition. The indicated loss of spin entropy at the transition is discussed.     

Observation of stripe mobility in a dipolar frustrated ferromagnet

We have discovered two novel aspects of the stripe-domain to paramagnetic transition in perpendicularly magnetized Fe films on Cu(100). First, the width of the stripes carrying oppositely oriented spins decreases, close to the transition temperature, with a power law. Second, in a small temperature interval close to the transition temperature, the stripes--which form stationary patterns at low temperatures--become mobile. Various theoretical works have predicted stripe mobility in similar frustrated systems but no direct proof of this phenomenon has been reported so far.     

Supersolid phase of hard-core bosons on a triangular lattice

We study properties of the supersolid phase observed for hard-core bosons on the triangular lattice near half-integer filling factor, and the phase diagram of the system at finite temperature. We find that the solid order is always of the (2m, -m, -m) with m changing discontinuously from positive to negative values at half filling, in contrast with phases observed for Ising spins in a transverse magnetic field. At finite temperature we find two intersecting second-order transition lines: one in the 3-state Potts universality class and the other of the Kosterlitz-Thouless type.     

Behavior of the layer compression elastic modulus near,above, and below a smectic C–hexatic I critical point in binary mixtures

We present the first study of the layer compression modulus B carried out near, above and below the Smectic C–Hexatic I critical point in racemic mixtures of methylbutyl phenyl octylbiphenyl-carboxylate (8SI) and the octyloxy biphenyl analog (8OSI), at frequencies ranging from 0.2 Hz to 2 ×103 Hz. The behavior of B as a function of temperature shows a progressive evolution from a first order transition in 8SI to a continuous supercritical behavior in 8OSI. The latter is characterized by an increase in B, which appears above the transition, and which is followed by a leveling off when the temperature is decreased towards the transition. It is proposed that this behavior stems from the relaxation of the hexatic domains which are frozen in the frequency range studied. For the supercritical and near-critical compounds, B exhibits a small dip near the transition temperature, which is visible in the low frequency range only, indicating that the dynamics associated with the critical point is very slow. We also report measurements in the Crystal-J phase of the pure compounds, and show that 8SI behaves mechanically as a hexatic phase and 8OSI as a soft crystal phase.     

高压下LiKSO4介电和弹性特性的反常及p—T相图

本文通过对不同温度T(150—350K)和压力p(0—10kbar)范围内LiKSO₄晶体介电和弹性性质的研究,发现在室温8.2kbar(对应于升压过程),和4kbar(对应于降压过程)附近介电和弹性系数有突变。我们把它归结为压力诱导的LiKSO₄相转变。这个相变过程伴随着很大的压力滞后现象。本文还给出了LiKSO₄在上述温度和压力范围内的p-T相图。并发现在3.8kbar和281K处存在一个三相点。 关键词：     

Observation of finite-size effects on a structural phase transition of 2D nanoislands

We study a reversible, temperature-driven structural surface phase transition of Pb/Si(111) nanoislands with a variable-temperature scanning tunneling microscope. Our quantitative measurements indicate that the transition temperature decreases with decreasing island and domain size. The boundaries of the nanoislands also influence the transition. Careful examination of the change in the interior structure of nanoislands near the transition temperature allows us to image the effects of the thermal fluctuations.     

Quantum depinning transition of quantum Hall stripes

We examine the effect of disorder on the electromagnetic response of quantum Hall stripes using an effective elastic theory to describe their low-energy dynamics, and replicas and the Gaussian variational method to handle disorder effects. Within our model we demonstrate the existence of a depinning transition at a critical partial Landau level filling factor Deltanu(c). For DeltanuDeltanu(c). For Deltanu> or =Deltanu(c), we find a partial RSB solution in which there is free sliding only along the stripe direction. The transition is analogous to the Kosterlitz-Thouless phase transition.