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The results of a high-resolution, synchrotron x-ray powder diffraction study of lattice constants in the P& (" rippled" ) phase of lecithin-water multilamellar mixtures are given. The variation with water volume fraction P and hydrocarbon chain length N, of the modulation wave vector Q"suggests that membrane curvature and hydration interactions between membranes play a significant role in the modulation. The dependence of the membrane thickness and area per head group on N, indicates that the conformation of hydrocarbon chains is predominantly solidlike. The appearance of significant intensity in higher harmonics of the modulation wave vector rules out simple (e.g. , sinusoidal or triangular) membrane density modulations. The results are consistent with a Lifshitz phenomenological model for lamellar phases of interacting membranes proposed by Goldstein and Liebler [Phys. Rev. Lett. 61, 2213 {1988)]. The phase behavior predicted by the model includes a multicritical point called the Lifshitz point where the wavelength of the modulation diverges. The experimental results indicate that this multicritical point lies in the vicinity of N, =9 and P =0.18.Both the chain melting (or "main") transition T and the

Thermal conduction in periodic multilayer composites can be strongly influenced by nonequilibrium electron-phonon scattering for periods shorter than the relevant free paths. Here we argue that two additional mechanisms-quasiballistic phonon transport normal to the metal film and inelastic electron-interface scattering-can also impact conduction in metal/dielectric multilayers with a period below 10 nm. Measurements use the 3ω method with six different bridge widths down to 50 nm to extract the in- and cross-plane effective conductivities of Mo/Si (2.8 nm/4.1 nm) multilayers, yielding 15.4 and 1.2 W/mK, respectively. The cross-plane thermal resistance is lower than can be predicted considering volume and interface scattering but is consistent with a new model built around a film-normal length scale for phonon-electron energy conversion in the metal. We introduce a criterion for the transition from electron to phonon dominated heat conduction in metal films bounded by dielectrics.

A diffraction study of lattice constants in the Pp ("rippled") phase of hydrated lecithins suggests that the lamellar modulation arises from a delicate interplay between the energies associated with membrane curvature and with hydration, as modified by interactions between membranes. Our results imply the existence of a Lifshitz point in the global phase diagram of the lecithin-water system. PACS numbers: 64.70. Md, 61.30.Eb, We report here the results of a systematic highresolution x-ray diffraction study of the modulated ("rippled," or Pp>) membrane phase of hydrated multilamellar diacylphosphatidylcholines (lecithin). 1 Our data for the dependence of the mean membrane thickness di and ripple wave vector Q r on the hydrocarbon chain length N c and the degree of hydration combine with existing thermodynamic data to yield a simple physical mechanism for modulated-membrane-phase stability. The data support a phenomenological model of structural phase transitions in interacting membranes recently proposed by Goldstein and Leibler. 2 Hydration interactions, which are supposed to dominate only at small intermembrane spacings, 3,4 play a central role throughout the Pp' phase. The systematic trends imply the existence of a Lifshitz multicritical point 5 within the global lecithin phase diagram, which appears to be accessible by variation of N c and water composition. Figure 1 displays the schematic phase boundaries of lecithin in the vicinity of the Pp' phase. At full hydration the Pp phase appears intermediate to the hightemperature, fluid phase L a and the low-temperature, solidlike phase Lp>. Latent heats AH m at the upper or "main" transition T m are proportional to chain length N c , while those at the lower or "pretransition" T p are independent of N c . 6,1 This trend suggests that the pretransition is dominated by membrane surface energetics. As the water content is decreased, the phase terminates at a third triple point T3, which is found at water volume fraction w -0.18. 8 Previous x-ray diffraction studies 1,9 of the Pp' phase have indicated that membrane stacking and rippling periodicities show the symmetry of a 2D monoclinic lattice, with constants insensitive to temperature but strongly dependent on

Tantalum Nitride (TaN) films carry high heat fluxes in a variety of applications including diffusion barriers in magnetoresistive random access memory and buffer/absorbers in extreme ultraviolet masks. The thicknesses of these films are usually of the same order as the thermal energy carrier mean free path, which complicates the study of heat conduction. This paper presents thermal (cross-plane) and electrical (in-plane) conductivity measurements on TaN films with thicknesses of 50, 75, and 100 nm. Picosecond thermoreflectance is used to extract the thermal boundary resistance between TaN and Al and the intrinsic thermal conductivity of TaN for temperatures of 300-700 K. The data and the relative importance of boundary resistances, electron-boundary scattering, and electron-defect scattering are interpreted using the electrical and thermal transport data. These data facilitate comparison of the phonon and electron contributions to thermal conduction in TaN. V