The dynamical crossover phenomenon in bulk water, confined water and protein hydration water

We discuss a phenomenon regarding water that was until recently a subject of scientific controversy, i.e. the dynamical crossover from fragile-to-strong glass-forming material, for both bulk and protein hydration water. Such a crossover is characterized by a temperature T(L) at which significant dynamical changes occur, such as violation of the Stokes-Einstein relation and changes of behaviour of homologous transport parameters such as the density relaxation time and the viscosity. In this respect we will consider carefully the dynamic properties of water-protein systems. More precisely, we will study proteins and their hydration water as far as bulk and confined water. In order to clarify the controversy we will discuss in a comparative way many previous and new experimental data that have emerged using different techniques and molecular dynamic simulation (MD). We point out the reasons for the different dynamical findings from the use of different experimental techniques.     

Water confined in MCM-41: a mode coupling theory analysis

In this paper we analyze molecular dynamics simulation results on supercooled water in a MCM-41 pore in order to test the mode coupling theory. A layer analysis must be performed for water in the pore in order to exclude the contribution of water bound to the strongly hydrophilic surface. Upon supercooling a range of temperatures is reached where the liquid follows the mode coupling theory. From the power law behavior of the relaxation times extracted from the Kohlrausch-William-Watts fit to the self-intermediate scattering function, we obtain the crossover temperature T(C) and the γ exponent of the theory. The time-temperature superposition principle is also satisfied. A fit to the von Schweidler law yields a coefficient b from which all the other parameters of the theory have been calculated. In particular, we obtained the same value of γ as extracted from the power law fit to the relaxation times, in agreement with the requirements of the theory. For very low temperatures, the mode coupling theory no longer holds as hopping processes intervene and water turns its behavior to that of a strong liquid.     

Pumping of confined water in carbon nanotubes by rotation-translation coupling

Bidirectional single file water transport in a carbon nanotube is known to occur in "bursts" in short nanotubes. Here we show that in long carbon nanotubes, when the orientation of the water molecules is maintained along one direction, a net water transport along that direction can be attained due to coupling between rotational and translational motions. The rotations of the water molecules are correlated more with the translation of the neighboring water molecule with the acceptor oxygen than the neighbor with the donor hydrogen. This mechanism can be used to pump water through nanotubes.     

Source of reaction-diffusion coupling in confined systems due to temperature inhomogeneities

Diffusion is often accompanied by a reaction or sorption which can induce temperature inhomogeneities. Monte Carlo simulations of Lennard-Jones atoms in zeolite NaCaA are reported with a hot zone presumed to be created by a reaction. Our simulations show that localized hot regions can alter both the kinetic and transport properties. Further, enhancement of the diffusion constant is greater for larger barrier height, a surprising result of considerable significance to many chemical and biological processes. We find an unanticipated coupling between reaction and diffusion due to the presence of hot zone in addition to that which normally exists via concentration.     

低温下二氧化硅介孔内水的振动性质

利用拉曼谱测量了100 K-303 K温度范围内受限于二氧化硅介孔内水的振动性质. 利用水分子在亲水介孔内, 先径向后轴向的吸附生长特点, 改变孔内界面水和位于孔中心水的相对含量. 发现越接近界面, 水低温相的振动谱越偏离体相六角冰的振动谱. 当界面水层减小到小于两个水分子层厚度时, 界面水在降温过程中不具有晶化行为, 其低温相与体相非晶冰相的拉曼谱主峰位在不同的温区内随温度的变化趋势相同、 连续.     

Diffusion in a metallic melt at the critical temperature of mode coupling theory

According to mode coupling theory, liquidlike motion becomes frozen at a critical temperature T(c) well above the caloric glass transition temperature T(g). Here, for the first time, we report on radiotracer diffusion in a supercooled Pd43Cu27Ni10P20 alloy from T(g) to the equilibrium melt. Liquidlike motion is seen to set in exactly above T(c) as evidenced by a gradual drop of the effective activation energy. This strongly supports the mode coupling scenario. Isotope effect measurements, which have never been carried out near T(c) in any material, show atomic transport up to the equilibrium melt to be far away from the hydrodynamic regime of uncorrelated binary collisions.     

Excitonic condensation under spin-orbit coupling and BEC-BCS crossover

The condensation of electron-hole pairs is studied at zero temperature and in the presence of a weak spin-orbit coupling (SOC) in coupled quantum wells. Under realistic conditions, a perturbative SOC can have observable effects in the order parameter of the condensate. First, the fermion exchange symmetry is absent. As a result, the condensate spin has no definite parity. Additionally, the excitonic SOC breaks the rotational symmetry yielding a complex order parameter in an unconventional way; i.e., the phase pattern of the order parameter is a function of the condensate density. This is manifested through finite off-diagonal components of the static spin susceptibility, suggesting a new experimental method to confirm an excitonic condensate.     

Freezing of water confined at the nanoscale

Freezing of water in hydrophilic nanopores (D=1.2 nm) is probed at the microscopic scale using x-ray diffraction, Raman spectroscopy, and molecular simulation. A freezing scenario, which has not been observed previously, is reported; while the pore surface induces orientational order of water in contact with it, water does not crystallize at temperatures as low as 173?K. Crystallization at the surface is suppressed as the number of hydrogen bonds formed is insufficient (even when including hydrogen bonds with the surface), while crystallization in the pore center is hindered as the curvature prevents the formation of a network of tetrahedrally coordinated molecules. This sheds light on the concept of an ubiquitous unfreezable water layer by showing that the latter has a rigid (i.e., glassy) liquidlike structure, but can exhibit orientational order.     

Critical temperature curve in BEC-BCS crossover

The strongly correlated regime of the crossover from Bardeen-Cooper-Schrieffer pairing to Bose-Einstein condensation can be realized by diluting a system of two-component fermions with a short-range attractive interaction. We investigate this system via a novel continuous-space-time diagrammatic determinant Monte Carlo method and determine the universal curve Tc/epsilonF for the transition temperature between the normal and the superfluid states as a function of the scattering length with the maximum on the Bose-Einstein condensation side. At unitarity, we confirm that Tc/epsilonF=0.152(7).     

反演声场简正波耦合系数矩阵

研究了有内波传播时声场的耦合简正波形式,分析表明各阶简正波系数的时间信号包含多个频率成分,各成分的频率为对应的两地波数差与内波速度的乘积,各频率成分的振幅与对应简正波之间的耦合系数成正比。因此即使内波的波形不随其传播而变化,接收器处的各阶简正波系数仍然具有多频的复杂结构。由此并根据简正波耦合强度与声场简正波系数起伏强度的对应关系提出了一种反演简正波耦合系数矩阵的方法;并用实验中获得的内波数据,反演了声场;计算结果表明:该方法有效地反演了内波传播情况下的简正波耦合系数矩阵。     

Nonresonant cascaded acousto-optic mode coupling

Nonresonant cascaded acousto-optic mode coupling under an overall phase-matching condition is presented, where phase matching is not satisfied for each of the coupling stages but the sum of the two acoustic momenta matches the optical phase detuning between the source mode and the destination mode. As a special nonresonant cascaded case, conversion from LP(01) to LP(21) and LP(02) driven by a single acoustic frequency is predicted theoretically and demonstrated experimentally.     

Transport properties of supercooled confined water

This article presents an overview of recent experiments performed on transport properties of water in the deeply supercooled region, a temperature region of fundamental importance in the science of water. We report data of nuclear magnetic resonance, quasi-elastic neutron scattering, Fourier-transform infrared spectroscopy, and Raman spectroscopy, studying water confined in nanometer-scale environments. When contained within small pores, water does not crystallise, and can be supercooled well below its homogeneous nucleation temperature T_h. On this basis it is possible to carry out a careful analysis of the well known thermodynamical anomalies of water. Studying the temperature and pressure dependencies of water dynamics, we show that the liquid-liquid phase transition (LLPT) hypothesis represents a reliable model for describing liquid water. In this model, water in the liquid state is a mixture of two different local structures, characterised by different densities, namely the low density liquid (LDL) and the high-density liquid (HDL). The LLPT line should terminate at a special transition point: a low-T liquid-liquid critical point. We discuss the following experimental findings on liquid water: (i) a crossover from non-Arrhenius behaviour at high T to Arrhenius behaviour at low T in transport parameters; (ii) a breakdown of the Stokes-Einstein relation; (iii) the existence of a Widom line, which is the locus of points corresponding to maximum correlation length in the p-T phase diagram and which ends in the liquid-liquid critical point; (iv) the direct observation of the LDL phase; (v) a minimum in the density at approximately 70 K below the temperature of the density maximum. In our opinion these results represent the experimental proofs of the validity of the LLPT hypothesis.     

Perspectives on water science: transport and application of confined water

The confinements of water can be divided into two main categories,namely,the confinements on surface or interface and the confinements in bulk water.By adding ions or applying electric field,the intensity and distribution of the hydrogen bonds can be greatly affected.These are collectively known as confinement on water surface or interface,which has potential applications in life science and industries involving evaporation control.Confined bulk water could be found everywhere in nature,such as in granular and porous materials,macromolecules and gels,etc.The investigation of the physical properties and the transports of the confined bulk water will contribute to understanding certain types of life activities such as the water transport in plant and in new application of extracting the shale oil and water.     

The critical temperature in mode coupling theory and magnetic resonance data on molecular dynamics in glassy liquids

For different molecular glass-forming liquids the critical temperaturesT _c predicted by the mode coupling theory of glass transition are compared with the temperaturesT _b at which a single relaxation process seems to bifurcate into slow and faster relaxations. The latter temperatures are taken from the magnetic resonance data available in the literature. Both temperatures coincide within experimental accuracy.     

ESR investigation on the breather mode and the spinon-breather dynamical crossover in Cu benzoate

A "breather excitation" is observed directly by electron spin resonance in the quantum spin chain Cu benzoate, in which an unexpected field-induced gap has recently been found. The nonlinear field dependence of the resonance field agrees well with the formula based on a quantum sine-Gordon model. The power-law temperature dependence of the linewidth is observed in the gapless spinon regime while the width decreases exponentially for the gapped breather regime. In the intermediate range, a distinct anomaly is found, which is the manifestation of "the spinon-breather dynamical crossover."     

Electron-phonon coupling and the soft phonon mode in TiSe2

We report high-resolution inelastic x-ray measurements of the soft phonon mode in the charge-density-wave compound TiSe(2). We observe a complete softening of a transverse optic phonon at the L point, i.e., q=(0.5, 0, 0.5), at T≈T(CDW). Detailed ab initio calculations for the electronic and lattice dynamical properties of TiSe(2) are in quantitative agreement with experimental frequencies for the soft phonon mode. The observed broad range of renormalized phonon frequencies, (0.3, 0, 0.5)≤q≤(0.5, 0, 0.5), is directly related to a broad peak in the electronic susceptibility stabilizing the charge-density-wave ordered state. Our analysis demonstrates that a conventional electron-phonon coupling mechanism can explain a structural instability and the charge-density-wave order in TiSe(2) although other mechanisms might further boost the transition temperature.     

Three-dimensional mode coupling around a seamount

A recently developed three-dimensional normal-mode model is adopted to investigate mode coupling around a seamount in a deep water environment. As indicated by the theoretical analysis and verified by the numerical results, strong mode coupling occurs at the edge of a seamount under certain conditions. Therefore, mode coupling is critical for the investigation of the acoustic field in the presence of a seamount. In addition, the issue regarding the number of sectors assuring convergence is also presented. T...     

Some properties of mode coupling equations

In this paper it is proven that some classes of mode coupling equations for correlation functions have a unique solution which exhibits all the standard properties like causality and stability. This is done by demonstrating the uniform convergence of an iteration procedure, which was previously used for a numerical solution of these equations.