Contains research summary and reports on three research projects.Joint Services Electronics Program (Contract DAALO3-86-K-0002

Birgeneau, Robert J.

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X-Ray Diffuse Scattering9.0 X-Ray Diffuse ScatteringAcademic and Research StaffProf. R.J. Birgeneau, Dr. J.W. Chung, Dr. C.J. PetersGraduate StudentsK. Evans-Lutterodt, J. Hill, H. Hong, B. Keimer, A. Mak, D.Y. Noh, E. Specht, T.ThurstonJoint Services Electronics Program (Contract DAALO3-86-K-0002)Robert J. BirgeneauIn this research program, modern x-ray scattering techniques are used to studystructures and phase transitions in thin films and on surfaces. We have two principalexperimental facilities. At MIT we have four high-resolution computer-controlled x-rayspectrometers using high intensity rotating anode x-ray generators. The angular resol-ution can be made as fine as 1.8 seconds of arc; this enables one to probe the devel-opment of order from distances of the order of the x-ray wavelength, ~ 1A, up to30,000A. The sample temperature may be varied between 2K and 500K with a relativeaccuracy of 2X10-3 K. We also have, in collaboration with IBM, a two spectrometersystem at the National Synchrotron Light Source at Brookhaven National Laboratory.A third beam line designed to operate at short wave lengths is currently being imple-mented. These make possible high resolution scattering experiments with a flux morethan three orders of magnitude larger than that from a rotating anode x-ray generator;this, in turn, has opened up a new generation of experiments.As part of this JSEP program we have built an x-ray compatible high vacuum singlecrystal apparatus. This enables us to use synchrotron radiation to study the structureand transitions occurring at a single surface; the first generation of such experimentshas now been performed. Our current experiments in this program are concentrated intwo areas: 1) the phases and phase transitions of metal and semiconductor surfaces andsurface overlayers; and 2) the structure and phase transitions of rare gas multilayers onsimple substrates.9.1 Metal Surface StudiesFor our first detailed studies of metal surface reconstruction we have chosen tungsten,both pure and coated with hydrogen or oxygen. The tungsten system has been exhaus-tively studied with all of the standard surface techniques including LEED, STM, atombeam scattering, and Rutherford back scattering. In spite of this effort, there are still majorcontroversies concerning the nature of the structures and surface transitions. Our mostsuccessful experiments were for W (100) with sub-monolayer coverages of H.The system of atomic hydrogen chemisorbed onto the surface of tungsten has longbeen considered a model system for the study of chemisorption. Extensive experimentalX-Ray Diffuse Scatteringstudies have shown that the W(100) surface goes through an elaborate sequence ofphases and phase transitions as a function of increasing hydrogen coverage; however,the microscopic nature of these successive states remains problematic. We carried outa synchrotron x-ray glancing-angle diffraction study of W(1 00) at room temperature fora range of hydrogen coverages. The experiments illustrate clearly the utility of x-raysin obtaining quantitative information about the structures and the nature of the disorderin such chemisorbed surface systems.Our data suggest a simple picture in which at very low coverages there is a tradeoffwith increasing coverage, analogous to that in a two-phase coexistence region, be-tween h drogen-poor and hydrogen-rich regions until a uniform but still disordered( 2 x 2 )R45' -H phase is attained. This disagrees with the currently accepted modelwhich assumes that the hydrogen atoms are always uniformly distributed on thetungsten surface. With further increase in coverage the surface layer exhibits acommensurate-incommensurate transition (CIT) which, rather than being a solid-solidtransition as previously assumed, is actually a lattice-gas melting transition into adomain-wall fluid phase. Such surface domain-wall fluid phases should occur com-monly in chemisorbed systems.We find that the line-shape in the (,2-x,2-)R450 -H phase is well-described as aLorentzian raised to the 3/2 power. This line-shape is indicative of random field effectsin two dimensions. Specifically, in surface lattice-gas systems, impurities and defectswhich fix the sublattice locally act as random fields which round the freezing transitionand prevent the achievement of true long range order. Such random field effects havenot been seen in previous surface diffraction experiments; only x-rays provide the re-solution and simple kinematical interaction to allow such quantitative line-shape anal-ysis.9.2 Rare Gases on GraphiteIn the last year, we completed two major experiments on monolayer rare gas films.First we carried out an encyclopaedic high resolution x-ray diffraction study of thestructures and phase transitions of monolayer krypton, adsorbed on both powder andsingle crystal graphite substrates. A comprehensive series of powder diffraction profileswas used to construct the two-dimensional phase diagram. The melting of thecommensurate solid was shown to be strongly first order throughout the region wheretricritical behavior was previously thought to occur; fluid-solid coexistence extends upto the termination of the commensurate phase at 130 K. A disordered weaklyincommensurate phase with lattice constant smaller than that of the 3 x 3 phasewas shown to be reentrant fluid, a system which may be described as a disorderednetwork of domain walls and which evolves continuously into a more conventional 2Dfluid. This evolution is marked by the disappearance of satellite peaks which are causedby the modulation of the overlayer by the substrate. The freezing of the reentrant fluidinto the commensurate phase was shown to be consistent with a chiral Potts transition,its freezing into the incommensurate solid consistent with a dislocation binding transi-tion. Single crystal experiments revealed the orientation of the weakly incommensuratephase. The reentrant fluid was found to have no visible orientational fluctuations,manifesting isotropic diffraction peaks. This is attributed to the strong epitaxy of do-main walls. The incommensurate solid was shown to undergo an aligned-rotatedtransition which is well described by zero-temperature calculations.76 RLE Progress Report Number 130X-Ray Diffuse ScatteringSecond, we carried out an extensive synchrotron x-ray study of monolayer xenonon single crystal graphite. A monolayer of xenon freezes at -~135K into anincommensurate solid. The incommensurate solid undergoes the sequence of phaseswith decreasing temperatures aligned -+ rotated -+ reentrant aligned, before transform-ing at -70K into a commensurate -x,3-R45 0 solid. The current theory oforientational epitaxy correctly predicts only the point at which the rotated -+ reentrantaligned transition occurs. None of the current theories can explain the entire sequence.The reentrant aligned phase at intermediate temperatures is an incommensurate solidwith a network of superlight domain walls. The observed phase transition into thecommensurate phase is first order consistent with domain wall theories. Theincommensurability decreases asST - To 1/3until the first order C-IC transition point is encountered. This 1/3 power law is appar-ently a universal behavior shared with Kr on graphite.We have estimated the potential corrugation of the surface adsorbtion from the in-tensity ratios between the main and satellite peaks in the domain wall incommensuratephsae. The corrugation of the potential estimated is close to that predicted by Steele.Along with this estimation from the onset of the rotated phase, we are able to set thelimits for the dimensionless wall width, 36 < 10 < 42. Also we estimated the wallcrossing energy which turned out to be +60K. However, the striped phase, predictedby Halpin-Healy using a similar value was not observed.9.3 Surface Roughening of AgDirect evidence of the thermal roughening of Ag (110) surface has been observedin a high resolution x-ray scattering experiment at the National Synchrotron LightSource of the Brookhaven National Laboratory. Information about the surfaceroughness, as manifested in the height-height correlation function, is obtained from thescattering intensity profile of the bulk forbidden (110) surface peak. The results indi-cate that surface roughening occurs at T, = 450 + 25 0 C. Below Tr , the Ag (110) sur-face is smooth, as characterized by delta function Bragg scattering, whereas above T,,the scattering intensity obeys a power law lineshape, indicating a rough surface withlogarithmically divergent height fluctuations. Since the transition is reversible withtemperature, it is indeed an equilibrium phase transition. Furthermore, the fact that theintegrated intensity of the bulk Ag (111) peak undergoes little change during the tran-sition, apart from the Debye-Waller factor, demonstrates that the observed changes inbulk forbidden (110) peak are due exclusively to changes in the surface morphology.In contrast to atom scattering and LEED experiments which have significant multiplescattering and inelastic scattering, x-ray experiments allow relatively simple data anal-ysis. Coupled with the high resolution of our experiment, we have demonstrated a verypowerful technique for studying surface roughening.X-Ray Diffuse ScatteringPublicationsMochrie, S.G.J., A.R. Kortan, P.M. Horn, and R.J. Birgeneau, "Novel Melting Transi-tion in a Two-Dimensional Stripe-Domain System," Phys. Rev. Lett. 58:690 (1987).Chung, J.W., K. Evans-Lutterodt, E.D. Specht, R.J. Birgeneau, P.J. Estrup, and A.R.Kortan, "Grazing-Incidence X-Ray Study of the Structures and Phase Transitions ofHydrogen on Tungsten (100)," Phys. Rev. Lett. 59:2192 (1987).Specht, E.D., A. Mak, C. Peters, M. Sutton, R.J. Birgeneau, K.L. D'Amico, D.E.Moncton, S.E. Nagler, and P.M. Horn, "Phase Diagram and Phase Transitions ofKrypton on Graphite in the Extended Monolayer Regime," Z. Phys. B 69:347 (1987).Held, G.A., J.L. Jordan-Sweet, P.M. Horn, A. Mak, and R.J. Birgeneau. "X-Ray Scat-tering Study of the Thermal Roughening of Ag(110)," Phys. Rev. Lett. 59:2075(1987).78 RLE Progress Report Number 130

X-Ray Diffuse Scattering

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X-Ray Diffuse Scattering

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