Small angle neutron scattering (SANS) is used to measure the density of heavy water contained in 1D cylindrical pores of mesoporous silica material MCM-41-S-15, with pores of diameter of 15 ؎ 1 Å. In these pores the homogenous nucleation process of bulk water at 235 K does not occur, and the liquid can be supercooled down to at least 160 K. The analysis of SANS data allows us to determine the absolute value of the density of D 2O as a function of temperature. We observe a density minimum at 210 ؎ 5 K with a value of 1.041 ؎ 0.003 g/cm 3 . We show that the results are consistent with the predictions of molecular dynamics simulations of supercooled bulk water. Here we present an experimental report of the existence of the density minimum in supercooled water, which has not been described previously.liquid-liquid critical point ͉ nanopores ͉ small angle neutron scattering ͉ Widom line O f the many remarkable physical properties of liquid water (1), the density maximum is probably the most well known. The density maximum of H 2 O at T max ϭ 277 K (284 K in D 2 O) is one of only a few liquid-state density maxima known (2) and the only one found to occur in the stable liquid phase above the melting temperature. Water's density maximum is a dramatic expression of the central role played by hydrogen bonding in determining the properties of this liquid: as temperature T decreases through the region of the density maximum, an increasingly organized and open four-coordinated network of hydrogen bonds expands the volume occupied by the liquid, overwhelming the normal tendency of the liquid to contract as it is cooled.The density of bulk supercooled liquid water decreases rapidly with T before the onset of homogeneous crystal nucleation (at Ϸ235 K) precludes further measurements. The density curve of ice Ih lies below that of the liquid and almost certainly sets a lower bound on the density that the supercooled liquid could attain if nucleation were avoided, because ice Ih represents the limiting case of a perfectly ordered tetrahedral network of hydrogen bonds. Significantly, the expansivity of ice Ih in this T range is positive (3); i.e., the density increases as T decreases (see Fig. 2). The low density amorphous (LDA) ice that forms from deeply supercooled liquid water at the (in this case extremely weak) glass transition approaches very closely the structure of a ''random tetrahedral network'' (RTN) and exhibits a number of ice-like properties, including a ''normal'' (i.e., positive) expansivity (4). If the structure of deeply supercooled water also approaches that of a RTN, it is therefore possible that a density minimum occurs in the supercooled liquid (5).Consistent with this possibility, a number of recent molecular dynamics (MD) computer simulation studies predict that a density minimum occurs in water (H 2 O) (5-10). These studies achieve deep supercooling without crystal nucleation due to the small system size and short observation time explored, compared with experiment. In the literature, the five-site transfer...

The extended Q-range small-angle neutron scattering diffractometer (EQ-SANS) at the Spallation Neutron Source (SNS), Oak Ridge, is designed for wide neutron momentum transfer (Q) coverage, high neutron beam intensity and good wavelength resolution. In addition, the design and construction of the instrument aim to achieve a maximum signal-to-noise ratio by minimizing the background. The instrument is located on the high-power target station at the SNS. One of the key components in the primary flight path is the neutron optics, consisting of a curved multichannel beam bender and sections of straight neutron guides. They are optimized to minimize neutron transport loss, thereby maximizing the available flux on the sample. They also enable the avoidance of a direct line of sight to the neutron moderator at downstream locations. The instrument has three bandwidth-limiting choppers. They allow a novel frameskipping operation, which enables the EQ-SANS diffractometer to achieve a dynamic Q range equivalent to that of a similar machine on a 20 Hz source. The two-dimensional low-angle detector, based on 3 He tube technologies, offers very high counting rates and counting efficiency. Initial operations have shown that the instrument has achieved its design goals.

The basic building block of calcium-silicate-hydrate (C−S−H) gel, which is the major hydration product of a commercial Portland cement paste, is usually referred as "globule" in the Jennings' colloidal model-II developed for C−S−H. The detailed nanostructure of the globule is so far not given quantitatively. In this paper, we determine the structural parameters of the building block with good accuracy by small-angle neutron scattering technique probing an extended interval of the scattering vector, Q, from 0.015−1.0 Å −1 . In this Q-range an interlamellar peak at 0.65−0.80 Å −1 is present, shifting as a function of the water content present in the C−S−H gel. This additional feature enables us to confirm the presence of a lamellar structure and determine the thicknesses of both the water and the hydrated calcium silicate layers respectively proper of the C−S−H globules. Article pubs.acs.org/JPCC

We present an efficient and flexible method for solving the non-linear lasing equations of the steady-state ab initio laser theory. Our strategy is to solve the underlying system of partial differential equations directly, without the need of setting up a parametrized basis of constant flux states. We validate this approach in one-dimensional as well as in cylindrical systems, and demonstrate its scalability to full-vector three-dimensional calculations in photonic-crystal slabs. Our method paves the way for efficient and accurate simulations of microlasers which were previously inaccessible.

Molecular dynamics simulations and neutron scattering experiments have shown that many hydrated globular proteins exhibit a universal dynamic transition at TD = 220 K, below which the biological activity of a protein sharply diminishes. We studied the phononlike low-energy excitations of two structurally very different proteins, lysozyme and bovine serum albumin, using inelastic x-ray scattering above and below TD. We found that the excitation energies of the high-Q phonons show a marked softening above TD. This suggests that the large amplitude motions of wavelengths corresponding to this specific Q range are intimately correlated with the increase of biological activities of the proteins.