Contains research summary and reports on three research projects.Joint Services Electronics Program (Contract DAAG29-83-K-0003

Birgeneau, Robert J.

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X-Ray Diffuse Scattering9. X-Ray Diffuse ScatteringAcademic and Research StaffProf. R.J. Birgeneau, Dr. J.W. Chung, Dr. C. Peters, Dr. S.G.J. MochrieGraduate StudentsK. Evans-Lutterodt, H. Hong, A. Mak, L.J. Martinez-Miranda, D.Y. Noh,E. Specht, T. ThurstonJoint Services Electronics Program (Contract DAAG29-83-K-0003)Robert J. BirgeneauIn this research program, modern x-ray scattering techniques are used to study the structuresand phase transitions in novel states of condensed matter. We have two principal experimentalfacilities. At M.I.T. we have two high-resolution computer controlled x-ray spectrometers using ahigh intensity rotating anode x-ray generator. Two more spectrometers are currently underdevelopment. The angular resolution can be made as fine as 1.8 seconds of arc; this enables oneto probe the development of order from distances of the order of the x-ray wavelength, -1 A , upto 30,000 A . The sample temperature may be varied between 2 K and 500 K with a relativeaccuracy of 2 x 10- 3 K. We also have, in collaboration with IBM, a two spectrometer system at theNational Synchrotron Light Source at Brookhaven National Laboratory. This makes possible highresolution scattering experiments with a flux more than three orders of magnitude larger than thatfrom a rotating anode x-ray generator; this in turn has opened up a new generation ofexperiments. This facility is under continuous development to improve the optics andspectrometers. Synchrotron x-ray scattering experiments are also carried out on a wiggler beamline at the Stanford Synchrotron Radiation Laboratory.As part of the JSEP program we have built an x-ray compatible high vacuum single crystalapparatus. This enables us to use synchrotron radiation to study the structure and transitionsoccuring at single surfaces; such experiments are now being initiated. Our current experimentsin this program are concentrated in three areas: (i) the growth, structure and phase transitions ofintercalant materials, (ii) the structure and phase transitions of smectic liquid crystals, (iii) thephases and phase transitions of metal and semiconductor surfaces and surface overlayers.9.1 Intercalation Compound Structures and TransitionsIntercalation compounds represent a family of materials in which a foreign species (e.g., Ir3) isinserted between the layers of a larnellar material such as graphite. If the intercalant enters everynth layer then the resultant material is referred to as a stage-n intercalation compound. In thisRLE P.R. No. 128X-Ray Diffuse Scatteringprogram we study both the intercalation process itself and the structure and transitions of theintercalation compound as a function of temperature, concentration, and stage index. Ourcurrent work is concentrated on the system bromine-intercalated graphite ('n Br .1Bromine-intercalated graphite has turned out to be an interesting model two dimensionalsystem. Above 69.30 C the bromine has a novel structure in which it is in-registry with thegraphite in one direction and out of registry or incommensurate in the other. This is referred to asa stripe-domain structure. One of the unusual features of such a structure, which we havecharacterized thoroughly,' is that in a diffraction pattern there are no true Bragg peaks. Rather-2 + Ithe diffraction is characterized by power law singularities, (Q-Q,) n, where the exponent -qndepends on the order (n) of the harmonic. Here the qn are predicted exactly by theory. One ofthe interesting questions which had not been addressed quantitatively to date is how such a solidmelts. Qualitatively, it is expected that one could have a continuous melting transition in whichthe bromine melts while the graphite stays crystalline. Further, on universality grounds it isexpected that such a transition, if continuous, would be isomorphous to the superfluid transitionin two-dimensional helium which is well described by a theory due to Kosterlitz and Thouless - adramatic theoretical prediction. We have used high resolution x-ray synchrotron techniques tostudy this melting transition. A high quality single crystal of stage-8 bromine intercalated graphitewas grown in situ on the x-ray spectrometer. The data from this experiment are still beinganalyzed. We find strong evidence for a sharp but continuous melting transition for the bromine.However, the liquid structure-factor in the well-correlated fluid phase is anomalous. Specifically,it does not seem to fit into the Kosterlitz-Thouless picture. We are continuing our analysis in anattempt to determine the appropriate description of this novel melting phenomenon.9.2 Smectic Liquid CrystalsLiquid crystals are made up of rod-like molecules. In the nematic phase the axes of themolecules align parallel to each other but the centers of mass of the molecules are still randomlydistributed so that one has a pure fluid structure factor. In the smectic A and C phases aone-dimensional sinusoidal density wave is set up either along (A) or at an angle (C) to themolecular axis. Thus, these smectics are like solids in one direction and fluids in the other two.These systems exhibit particularly interesting phase transitions which present an importantchallenge to modern theories of critical phenomena. At lower temperatures many liquid crystalmaterials exhibit more ordered phases such as smectic II,JG, and I. These have well-developedin-plane triangular order. If the order is truly long range then the phase is a plastic crystal.Otherwise, the smectic may be a realization of a novel phase of matter labelled a "stackedhexatic" with long range order in the crystalline axes but only short range order in the positions ofthe molecules.We are continuing to study in some detail the smectic phases and phase transitions in a numberof materials. One of the important issues in this field has been the critical behavior of theRLE P.R. No. 128X-Ray Diffuse Scatteringnematic-smectic A transition. This might be viewed as the simplest melting transition in nature.In nearly all cases in studies of phase transitions one finds that the critical behavior isindependent of the details of the system. For the N-problem, however, the exponentscharacterizing the transition vary from system to system. In order to elucidate this unusualphenomenon we have carried out a systematic study of the N-A transition 2 in two series ofcompounds: alkylbenzoate (nCB) and thiosulphate (nS5). Here the aliphatic chain length/(Cll),,+) may be varied continuously by making appropriate mixtures. We find that as oneincreases n in both series the exponents evolve continuously from T = 1.5, vlj 2 0.80 and L -0.67 to T = 1.1, P11 = 0.6 and v = 0.5; the latter are attained at the point where the transitionchanges from being second order to first order; as such, this point is a tricritical point and indeedthe measured exponents are close to the predicted theoretical values for a tricritical point. Weconclude therefore that the varied exponents reported in the literature for other systems simplyreflect a crossover from critical to tricritical behavior.As noted above, one of the most novel phases of matter occuring in the liquid crystal family isthe "stacked hexatic." We have initiated synchrotron x-ray studies of free standing films of thematerial methylbutylphenyloctyloxybiphenylcarboxylate (80SI) which is believed to exhibit astacked hexatic phase in which the molecules are tilted with respect to the layer normal; this islabeled smectic I; at -78" C with increasing temperature the S, exhibits a transition into an ,SCphase. Our initial measurements are very promising. We find that by applying a small field viaSinuCo magnets one can orient the molecules and this, in turn, fixes the hexatic axes in the S,phase. Thus we have produced the first single-domain sample in a liquid crystal hexatic; thismakes quantitative studies of the relevant order parameters and densities possible. Currently weare studying the thermal evolution of the orientational and positional order throughout the C and Iphases. Once these measurements are concluded we will begin studies in very thin films - 2 to 10molecules thick.9.3 Structures and Transitions in Two Dimensional SolidsAs is now well-recognized, surface systems provide interesting examples of two dimensionalphases and phase transitions. There are many different categories including pristine surfacereconstruction, chemically absorbed surface overlayers and physically adsorbed layers. Raregases on graphite provide the best studied examples of this third category. Much of our surfacescience research program in the past several years has concentrated on the physics of monolayerkrypton and xenon on graphite. The principal advantages of the rare gas-graphite systems arethat, first, the interactions are simple and well-known so that first principle's calculations arepossible and second, a broad range of sophisticated experiments, carried out in thermalequilibrium, are feasible.3 In the past year we have used synchrotron radiation to probe thefreezing of submonolayer films of xenon on single crystal graphite.4The single crystal substrate experiment allows one to study the evolution of the fluid correlationRLE P.R. No. 128X-Ray Diffuse Scatteringlength up to distances approaching a micron. This is unprecedented in surface structural studies.Our experiments were carried out for a coverage of 0.92 xenon monolayers at freezing. We findthat the fluid correlation length evolves continuously up to at least 2000 A before the solid phaseis entered.4 We find that the angular width of the fluid diffraction spots scales linearly with theradial width. This is the behavior expected for a "hexatic" fluid as predicted by Halperin andNelson. Unfortunately, because of our lack of knowledge of the substrate orienting field wecannot make a definitive statement about the existence of a hexatic phase in the absence of asubstrate.We are continuing to develop our "chemically-active" single crystal apparatus. We expect toinitiate synchrotron radiation studies of molybednum surfaces in the spring of 1986.References1. S.G.J. Mochrie, A.R. Kortan, R.J. Birgeneau, and P.M. Horn, Z. Phyz. B 62, 79 (1985).2. B.M. Ocko, R.J. Birgeneau, and J.D. Litster, Z. Phyz. B 62, (1986).3. R.J. Birgeneau and P.M Horn, Science (in press).4. E.D. Specht, R.J. Birgeneau, K.L. D'Amico, D.E. Moncton, S.E. Nagler, and P.M. Horn,J. Physique Lett. 46, L-561 (1985).RLE P.R. No. 128

X-Ray Diffuse Scattering

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X-Ray Diffuse Scattering

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