The resistivity of nearly solid-density Al was measured as a function of temperature over 4 orders of magnitude above ambient by observing the self-reflection of an intense, <0.5 psec, 308-nm light pulse incident on a planar Al target. As an increasing function of electron temperature, the resistivity is observed initially to increase, reach a maximum which is relatively constant over an extended temperature range, and then decrease at the highest temperatures. The broad maximum is interpreted as "resistivity saturation," a condition in which the mean free path of the conduction electrons reaches a minimum value as a function of temperature, regardless of the extent of any further disorder in the material.PACS numbers: 72.15.Cz, 52.25.Fi, 78.47.+p We report the first experimental study of the electrical resistivity of a solid-density material, in this case a simple Drude metal, over an extended range (4 orders of magnitude) of elevated temperature with little or no change in its density. The results show three general regions of the dependence of the resistivity on the temperature: Initially, the resistivity increases with increasing temperatures, reaching a relatively constant value that extends over a wide temperature range, and then it decreases as the temperature is further increased. We argue that these regions reflect differing mechanisms controlling the mean free path of the conduction electrons in different temperature ranges. In particular, the region of maximum resistivity is a result of "resistivity saturation," a condition in which the electron mean free path reaches a minimum value, independent of the degree of material disorder.The ability to study the resistivity of a well characterized solid-density material over a great range of elevated temperatures was made possible here by the use of ultrashort (<0.5 psec), relatively high-energy (0-5 mJ) laser pulses. The self-reflection of a laser pulse, focused onto a smooth target at fixed pulsewidth and spot size, was monitored over 4 orders of magnitude in energy. Frequency shifts of the reflected light were also recorded, and as discussed in detail below, these frequency shifts are shown to arise directly from the expansion velocity of the solid-vacuum interface. From the dependence of the interface velocity upon the laser intensity, we were able to determine the electron temperature and degree of interface expansion for each recorded value of the reflectivity. This information is sufficient to determine the resistivity of solid-density Al as a function of temperature up to 10 6 K.It is important to differentiate between the type of heating-reflectivity experiment reported here and those conducted with high-energy (> 100 mJ), long-duration ( > 50 psec) pulses. In the latter, the majority of the en-ergy is absorbed not by the dense target, but rather by the material expanding away from the interface over a scale of many wavelengths. Detailed hydrodynamic calculations are usually required to analyze the data from this type of experiment, l and no simpl...

X-ray studies have been performed on a new liquid-crystal material which exhibits a noncrystalline B phase. Using free-standing liquid-crystal film techniques, we find that this B phase has short-range in-plane positional correlations but long-range, threedimensional, sixfold bond-orientational order. We interpret our results in terms of a system of interacting two-dimensional hexatic layers.

X-ray studies of thick, free-standing films of the liquid-crystal compound «-butyl 4'-nhexyloxybiphenyl-4-carboxylate (460BC) have determined the critical behavior at the threedimensional hexatic-5-to-smectic-^4 phase transition. The position and width of the scattering peaks exhibited 1 -a singularities in agreement with theory and heat-capacity results. Measurements on two-layer films of 460BC provided evidence for a continuous, twodimensional, hexatic-to-liquid transition.PACS numbers: 64.70.Ew, 68.60. + qThe layered hexatic-i? liquid-crystal phase is the only three-dimensional (3D) physical system in which "hexatic" order has been observed. 1,2 This ordering involves a long-range sixfold symmetric, orientational alignment of the "bonds" connecting neighboring in-plane molecules even though their in-plane positional correlations remain short ranged and is characterized by a local order parameter \lf(T) = e 6i0(r) where 0(7) is the angle between the "bonds" and some reference axis. 3 The loss of bond-orientational order upon heating the hexatic-i? phase results in a layered phase with liquidlike inplane order-the smectic-^ phase. Heat-capacity studies 4 of the hexatic to smectic-^ (hex-A ) transition in the liquid-crystal compound 650BC found the transition to be continuous with a large, nearly symmetric heat-capacity peak and critical exponent a ~ 0.6. This heat-capacity behavior implies large hexatic fluctuations which couple to the density fluctuations, causing sixfold symmetric x-ray scattering with a singular critical temperature dependence for both the position and width of the scattering peaks. In this Letter we present measurements of this singular behavior at the 3D hex-^4 transition and evidence for a continuous (2D) hexatic-liquid transition. The 3D measurements involved thick films (several hundred molecular layers) while the 2D measurements were on two-layer films. The films were free standing and thus substrate-free.The liquid-crystal compound studied was «-butyl 4'-«-hexyloxybiphenyl-4-carboxylate (460BC). In comparison to 650BC, this compound exhibited the hexatic and smectic-^ phases over wider temperature ranges. Furthermore, the range of stability for two-layer films was enhanced, and this material showed no crystalline surface-ordering transition as observed in 650BC. 1 Heat-capacity measurements 5 on 460BC indicated that the hex-^4 transition was first order, but only weakly so. The heat capacity, C p , could be fitted by a power-law divergence up to 30 mK of T c . Explicitly C p -A ± r a± , where t = \(T -T c )/T c \, the upper and lower signs refer to T > T c and T < T c , respectively, and the fitted parameters were A + /A ~ = 0.75 ± 0.03 and a + = a" =0.49 ±0.02. Unfortunately, 460BC films rupture when cooled from the hexatic phase into a monoclinic crystalline phase, and so the hexaticcrystal transition could not be studied.For our structural studies, the layers of the liquid-crystalline phase were aligned by drawing free-standing films across a 6x6-mm 2 hole in a glass cover slide....

The results of a synchrotron x-ray study of the crystalline~jB to smectic-A melting transition in 4-w-pentylbenzenethio-4'-n-tetradecyloxybenzoate (14S5) are presented. Samples of two, three, five, and twelve molecular layers were investigated with the freestanding thin-film technique. A single, abrupt, hysteretic transition is observed in twolayer films with temperature-independent positional correlations above the transition. All thicker films melt by two first-order phase transitions; the character of the intermediate phase depends strongly on film thickness. PACS numbers: 64.70.Ew, 61.30.Eb, 68.60.+ qThe planar melting behavior which occurs within the layers of smectic liquid crystals provides an opportunity to study the evolution of the melting transition from two to three dimensions with freestanding thin-film techniques/ The thinnest films consisting of only two layers are the only experimentally viable example of substrate-free two-dimensional (2D) melting known. The present study has been motivated by the expectations (1) that melting in two-dimensional systems might involve qualitatively new physical mechanisms and (2) that the behavior versus thickness would be relevant to our understanding of 3D melting transitions in smectics in particular and in solids in general. Additional interest in the problem has been stimulated by a detailed theory of second-order 2D melting via a dislocation mechanism,^ by controversial molecular-dynamics calculations/ and by recent experimental studies demonstrating second-order melting in both incommensurate Xe and Ar monolayers adsorbed on a pyrolytic graphite substrate."^ Liquid-crystal thin films were prepared as previously described/ However, a new material, 4-n-pentylbenzenethio-4'-n-tetradecyloxybenzoate (TiS5), was used, which has two distinct advantages over N-(4-w-butyloxybenzylidene)-4-woctylaniline (40.8) used in our previous studies of crystalline-B thin films: (1) T4S5 is thermally stable and (2) it has a wide smectic-A range (66.5 to 86.6 °C). This last point is important because thin-film transitions occur some 10 °C above the respective bulk transition temperatures,^ and films cannot be made when the bulk material is above the smectic regime. Our experiments were carried out on the focused wiggler beam line VII-2 at the Stanford Synchrotron Radiation Laboratory. Two different resolution configurations were used. A high-resolution mode using two Si(lll) crystals in the monochromator and one in the analyzer had a longitudinal resolution component AQn =7xlO"'*A"^ [full width at half maximum (FWHM)] at Q ~ 1.4 A" \ For studying broad liquid scattering, we used pyrolytic graphite (002) monochromator and analyzer crystals, which relaxed the longitudinal resolution to AQii c^O.03 A"^ (FWHM).The x-ray scattering work presented here is closely related to studies of the mechanical shear properties described in a Letter by Bishop et aL^ We begin with results for the two-layer ( AT = 2) system. The mechanical measurements^ found three distinguishable features, but x-ra...

A structural investigation of the liquid crystal phases of 4-(2'-methylbutyl)phenyl 4'-n-octylbiphenyl-4-carboxylate