Thermodynamic Properties of the First-Generation Hybrid Dendrimer with “Carbosilane Core/Phenylene Shell” Structure

The molar heat capacity of the first-generation hybrid dendrimer with a “carbosilane core/phenylene shell” structure was measured for the first time in the temperature range T = 6–600 K using a precise adiabatic vacuum calorimeter and DSC. In the above temperature interval, the glass transition of the studied compound was observed, and its thermodynamic characteristics were determined. The standard thermodynamic functions (the enthalpy, the entropy, and the Gibbs energy) of the hybrid dendrimer were calculated over the range from T = 0 to 600 K using the experimentally determined heat capacity. The standard entropy of formation of the investigated dendrimer was evaluated at T = 298.15 K. The obtained thermodynamic properties of the studied hybrid dendrimer were compared and discussed with the literature data for some of the first-generation organosilicon and pyridylphenylene dendrimers.     

Magnetic Phase Transition Driven by Frustrated Interactions in a Longitudinal‐Field Ising Chain

The ground‐state magnetic phase transitions in a classical spin chain with long‐range interactions in a system of trapped ions are investigated. The tunable competing interactions, mediated by both longitudinal and transverse photon modes, lead to competition among different spins, due to which magnetic frustration occurs. Various ground‐state spin configurations are separated by multiple phase transitions when the strength and sign of these interactions are tuned continuously. The spin chains are highly degenerated because of the frustration, so there is some melting of several magnetic phases.     

Topological Phase Transition and Eigenstates Localization in a Generalized Non‐Hermitian Su–Schrieffer–Heeger Model

The topological properties of a generalized non‐Hermitian Su–Schrieffer–Heeger model are investigated and it is demonstrated that the non‐Hermitian phase transition and the non‐Hermitian skin effect can be induced by intra‐cell asymmetric coupling under open boundary conditions. Through investigating and calculating the non‐Hermitian winding number with generalized Brillouin zone theory, it is found that the present non‐Hermitian system has an exact bulk‐boundary correspondence relationship. Meanwhile, the non‐Hermitian winding number is used to characterize the non‐Hermitian phase transition and determine the phase transition boundary, and it is found that the non‐Hermitian phase transition is not completely induced by the asymmetric coupling strength. By means of the mean inverse participation ratio, the factors that affect the eigenstates localization are shown and it is revealed that large system size or large asymmetric coupling strength can leave the system in the localized state. Additionally, it is found that for the asymmetric coupling strength and the system size, the eigenstates localization is much more sensitive to the asymmetric coupling strength.     

Kinetics of the Ordered Phase Growth across the Isotropic-Nematic Phase Transition in Liquid Crystal Azomethine Polymers during Cooling

Main-chain azomethine liquid crystal (LC) polymers varying in spacer length were studied using differential scanning calorimetry and polarizing optical microscopy. The nematic droplets appearing across the isotropic-nematic phase transition during cooling the polymer melt were treated statistically and their size distributions were described with equation based on the principles of irreversible thermodynamics. Kinetics of the ordered phase growth was described analytically with the universal law for cluster growth. Analysis of the mean LC droplet diameter as a function of time allowed recognition of two regimes of the ordered phase growth: i) nucleation and fast LC droplet growth and ii) consequent cluster coarsening. The length of the spacer in the polymer chain was shown to have an influence on the duration of the LC droplet growth regimes.     

Investigation of strain‐induced phase transformation in ferroelectric transistor using metal‐nitride gate electrode

In this work, we report a ferroelectric memory with strained‐gate engineering. The memory window of the high strain case was improved by ～71% at the same ferroelectric thickness. The orthorhombic phase transition (from ferroelectric to anti‐ferroelectric transition) plays a key role in realizing negative capacitance effect at high gate electric field. Based on a reliable first principles calculation, we clarify that the gate strain accelerates the phase transformation from metastable monoclinic to orthorhombic and thus largely enhances the ferroelectric polarization without increasing dielectric thickness. This ferroelectric strain technology shows the potential for emerging device application.

     

Laser‐induced phase changes in olivine FePO4: a warning on characterizing LiFePO4‐based cathodes with Raman spectroscopy

Raman spectroscopy is an excellent technique for probing lithium intercalation reactions of many diverse lithium ion battery electrode materials. The technique is especially useful for probing LiFePO₄‐based cathodes because the intramolecular vibrational modes of the PO₄³⁻ anions yield intense bands in the Raman spectrum, which are sensitive to the presence of Li⁺ ions. However, the high power lasers typically used in Raman spectroscopy can induce phase transitions in solid‐state materials. These phase transitions may appear as changes in the spectroscopic data and could lead to erroneous conclusions concerning the delithiation mechanism of LiFePO₄. Therefore, we examine the effect of exposing olivine FePO₄ to a range of power settings of a 532‐nm laser. Laser power settings higher than 1.3 W/mm² are sufficient to destroy the FePO₄ crystal structure and result in the formation of disordered FePO₄. After the laser is turned off, the amorphous FePO₄ compound crystallizes in the electrochemically inactive α‐FePO₄ phase. The present experimental results strongly suggest that the power setting of the excitation laser should be carefully controlled when using Raman spectroscopy to characterize fundamental lithium ion intercalation processes of olivine materials. In addition, Raman spectra of the amorphous intermediate might provide insight into the α‐FePO₄ to olivine FePO₄ phase transition that is known to occur at temperatures higher than 450 °C. Copyright © 2008 John Wiley & Sons, Ltd.     

Computer Simulation of Nonequilibrium Growth of Crystals in a Two-Dimensional Medium with a Phase-Separating Impurity

Two-dimensional nonequilibrium growth of crystals (quasistable faceted and dendritic) in the presence of a phase separating impurity is studied by computer simulation. It is shown that there is a gradual modification in this system from quasistable faceted growth to the formation of dendrites when the impurity concentration increases. If there is dendritic growth in the presence of a phase-separating impurity, the cyclic changes in the morphology, expressed through the periodic occurrence of tertiary branches of a dendrite, are observed when the phase-separating impurity concentration is raised. This behavior of the morphology is considered as a reentrant nonequilibrium phase transition.     

Variable‐temperature Raman spectro‐microscopy for a comprehensive analysis of the conformational order in PEGylated lipids

The investigation of phase transitions and associated changes in the conformational order of lipids is of importance in various research areas dealing with phenomena such as the formation and fusion of vesicles, transmembrane diffusion and membrane interactions with drugs and proteins. In this article, we have focused on the study of thermotropic phase behaviors and associated changes in the conformational order of two newly developed synthetic PEGylated lipids trademarked as QuSomes. In contrast to conventional phospholipids, this new kind of lipid forms liposomes spontaneously upon hydration, without the supply of external activation energy. Variable‐temperature Raman spectro‐microscopy has been employed in order to plot the transition temperature profiles showing the phase behavior of these new lipids composed of 1,2‐dimyristoyl‐rac‐glycerol‐3‐dodecaethylene glycol (GDM‐12) and 1,2‐distearoyl‐rac‐glycerol‐3‐triicosaethylene glycol (GDS‐23). Furthermore, several spectral indicators were calculated and correlated which allowed for the deduction of various aspects of molecular structure as well as intramolecular motion and intermolecular interactions. To confirm the observations, differential scanning calorimetry (DSC) was applied and revealed a good agreement with the Raman spectroscopy results. Finally, this information may find application in various studies including the development of lipid‐based novel substances and drug delivery systems. Copyright © 2008 John Wiley & Sons, Ltd.     

Modeling of Nonthermal Solid‐to‐Solid Phase Transition in Diamond Irradiated with Femtosecond x‐ray FEL Pulse

In this contribution we review in detail our recently developed hybrid model able to trace simultaneously nonequilibrium electron kinetics, evolution of an electronic structure, and eventually nonthermal phase transition in solids irradiated with femtosecond free‐electron laser pulses. Diamond irradiated with an ultrashort intense x‐ray pulse serves as an example to show how an irradiated material undergoes an ultrafast phase transition on sub‐picosecond timescales. The transition of diamond into graphite is induced by an excitation of electrons from the valence band into the conduction band, which, in turn, induces a rapid change of the interatomic potential. Our theoretical model incorporates: a Monte‐Carlo method for tracing high‐energy electrons and K‐shell holes in diamond; a temperature equation for the valence‐band and low‐energy conduction‐band electrons; a tight binding method for calculation of the evolving electronic structure of the material and potential energy surfaces; and molecular dynamics propagating atomic trajectories. This unified approach predicts the damage threshold of diamond in a good agreement with experimentally measured values. It reveals a multi‐step nature of nonthermal phase transition being an interplay between electronic excitation, changes of the band structure, and atomic reordering. An effect of pulse parameters, such as photon energy and temporal pulse shape, on the phase transition is discussed in detail. (© 2015 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Statistical Physics Approach to Liquid Crystals: Dynamics of Mobile Potts Model Leading to Smectic Phase,Phase Transition by Wang–Landau Method

We study the nature of the smectic–isotropic phase transition using a mobile 6-state Potts model. Each Potts state represents a molecular orientation. We show that with the choice of an appropriate microscopic Hamiltonian describing the interaction between individual molecules modeled by a mobile 6-state Potts spins, we observe the smectic phase dynamically formed when we cool the molecules from the isotropic phase to low temperatures (T). In order to elucidate the order of the transition and the low-T properties, we use the high-performance Wang–Landau flat energy-histogram technique. We show that the smectic phase goes to the liquid (isotropic) phase by melting/evaporating layer by layer starting from the film surface with increasing T. At a higher T, the whole remaining layers become orientationally disordered. The melting of each layer is characterized by a peak of the specific heat. Such a succession of partial transitions cannot be seen by the Metropolis algorithm. The successive layer meltings/evaporations at low T are found to have a first-order character by examining the energy histogram. These results are in agreement with experiments performed on some smectic liquid crystals.     

Micro‐Raman study of orbiton–phonon coupling in YbVO3

First‐order and multiphonon Raman active excitations are studied in YbVO₃ as a function of temperature in the orthorhombic and monoclinic phases. Below T ≃ 170 K, a G‐type orbital ordering with a concomitant monoclinic transition occurs. They enhance the phonon polarizabilities, allowing the resolution of room‐temperature bands, and activate new excitations around 700 cm⁻¹. Below T ∼ 65 K, the 700 cm⁻¹ excitations disappear, indicating a C‐type orbital ordering and a return to the orthorhombic structure. The observed phonon combinations around 1400 cm⁻¹ with a dominant Jahn‐Teller vibration at ∼690 cm⁻¹ reflect a possible orbiton‐phonon coupling. Copyright © 2011 John Wiley & Sons, Ltd.     

Ink‐Jet Printed Nanoparticle Alignment Layers: Easy Design and Fabrication of Patterned Alignment Layers for Nematic Liquid Crystals

Ink‐jet printing of monolayer‐capped gold nanoparticles is introduced as a versatile and highly efficient means to pattern the alignment of nematic liquid crystals. Any homeotropic alignment patterns can be created quickly ranging in size from 30 μm (850 dpi) to several square inches, with high accuracy that does not deteriorate with time. Depending on the alignment underlayer, intermediate configurations between homeotropic and homogeneous are also feasible. Nematic liquid crystals with both positive and negative dielectric anisotropy can be switched by applying a DC or AC electric field in the printed vertical domains with the substrate configuration determining the electro‐optic response.     

掺杂有机给体与受体分子的向列相液晶的光折变研究

制备了一种掺杂有机给体和受体分子的向列相液晶（nematic liquid crystals）材料。参数的给体和受体分子使液晶材料的光折变性能大为提高。用此材料做成的夹心器件进行了能量二波耦合，简单并四波混频等实验。在外加低直流电场（0．5KV/cm)条件下，观察到了光折变增益Γ为441cm^-1，二波能量耦合实验中观察到的衍射光束达到6级，简并四波混频中最大衍射效率达到58％，写入混合材料的信号光栅已可保存数小时而无明显衰减。     

Orientation in Nematic Liquid Crystals Doped with Orange Dyes and Effect of Carbon Nanoparticles

研究了掺杂两种分散染料橙的向列型液晶E7的性质以及碳纳米粒子(单壁碳纳米管或富勒烯C₆₀)的影响. 两种分散染料橙11和13具有较高的溶解度和有序参数,被作为掺杂剂同时使用. 与掺杂单染料相比,同时加入两种染料橙使液晶的有序参数明显提高. 与纯液晶相比,掺杂可引起向列相向各向同性相转变温度的升高.     

Electron excitation cross section of atomic transitions in the two‐state approximation

The cross section of the 3s → 3p transition of sodium produced by electron impact has been calculated by performing a numerical integration of the coupled differential equations. The potential functions have been calculated exactly using the hydrogen‐like wave functions for the valence electron of the sodium with an effective charge adjusted to fit the experimental 3s → 3p line strength. The results compare very well with experimental data and with those obtained using more elaborate and sophisticated models.