Contains research objectives and summary of research on one research project.Joint Services Electronics Program (Contract DAABO7-76-C-1400

Birgeneau, Robert J.

Contains reports on six research projects.Joint Services Electronics Program (Contract DAAB07-76-C-1400

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VII. X-RAY SCATTERING SPECTROSCOPYAcademic and Research StaffProf. Robert J. Birgeneau Dr. Dan DavidovProf. Jens Al-Nielsen Dr. Paul M. HornGraduate StudentsE. Maxine Hammond Martin C. KaplanPaul A. Heiney Cyrus R. SafinyaJoint Services Electronics Program (Contract DAAB07-76-C-1400)Robert J. BirgeneauDuring the past year we have constructed and implemented a versatile high-resolutionx-ray spectrometer system. This x-ray spectrometer is based on a Rigaku 12 kW ro-tating anode x-ray source, a Huber high-precision 2-circle goniometer and an on-linePDP 11/34 computer control system with Camac interfacing. The spectrometer anglesmay be scanned through 3600 with a step size of 1. 8 seconds of arc and an absolute accu-racy of about 10 seconds. We are constructing a second spectrometer of somewhat low-er resolution for surface layer studies and for experiments on structural phase transi-tions. As ancillary equipment we have constructed temperature control equipment whichenables us to study samples at temperatures from 10 K to 500 K with a relative accuracyof 1 to 3 X 10-3 K. Our initial experiments have been directed primarily toward phasetransitions in liquid crystals and in lower dimensional solids. The liquid crystal experi-ments show special promise and, indeed, they demonstrate that high-resolution x-raytechniques will have a major impact on that area of research. We have also initiated aproject on the structure and scattering phenomena in physiadsorbed layers on surfaces.We now discuss our individual projects.1. NEMATIC-SMECTIC-A TRANSITION IN CBOOAA number of experiments, especially in cyanobenzylideneoctyloxyaniline (CBOOA),suggested the N-A transition was characterized by two, rather than one, divergent cor-relation lengths. This would seem to conflict with our current ideas on scaling nearsecond-order transitions and would greatly complicate any attempt at a theoreticaldescription of the transition. The salient result of our experiments is that, by directmeasurement with x rays, the correlation lengths along and transverse to the moleculesdiverge with identical exponents; thus the transition is characterized by a single length.This is perhaps the simplest example of melting in nature and we hope that thesestudies, besides elucidating the nature of phase changes in liquid crystals, will alsogive insight into the general phenomena of melting. 1PR No. 120LL(VII. X-RAY SCATTERING SPECTROSCOPY)2. NATURE OF THE SMECTIC-A PHASEWhether, in nature, bodies can exist for which the density functions depend not onthree but only on one or two coordinates is a classic problem which has occupied theor-ists since the 1930's. A closely related problem is the establishment of a one-dimensional density wave in a three-dimensional system, as occurs in a smectic-A liquidcrystal. It is believed that in two-dimensional systems and also in the smectic-A phasethere should be a second-order transition to a state characterized by long-range corre-lations but without long-range order in the usual sense. Our experiments provide thefirst direct experimental evidence that the theoretical conjectures are correct. We foundthat the instantaneous density correlations for a one-dimensional density wave in a three-dimensional liquid exhibit not long-range order but rather an anisotropic power lawdependence on molecular separation. Thus smectic liquid crystals are materials inwhich these ideas may fruitfully be studied experimentally. Further experiments andmuch more theoretical efforts are called for. This work is described more fully else-where.23. SPIN-PEIERLS TRANSITION IN TTF-BDT (Cu)The organic charge transfer salt TTF-BDT (Cu) is believed to provide an ideal exam-ple of a spin-dimerization transition in an S = 1/2 one-dimensional antiferromagnet. Thissystem is of interest both because of its role as an archetype for magnetism in organicmaterials and also because of the analogy with the more familiar electronic Peierls tran-sition as observed, for example, in TTF-TCNQ. In a collaborative experiment withD. E. Moncton (performed at Bell Laboratories) we have succeeded in observing thisstructural transition at 12 K using x-ray diffuse scattering techniques. We have moni-tored both the magnitude and spatial variation of the critical fluctuations above Tc = 12 Ktogether with the growth of the order parameter below T c. The measured order param-eter agrees well with that calculated for a BCS superconductor. Further, the experi-ments demonstrate clearly that the transition involves a fundamentally different pairingthan that inferred by others from simple molecular stacking considerations. This inturn necessitates that the dominant.exchange paths differ from those which are sug-gested by a naive consideration of the TTF molecular orbitals. This work has beenpublished. 34. STRUCTURAL PHASE TRANSITION IN SQUARIC ACIDThe compound H2C404 is a hydrogen-bonded system with a planar structure. Neutronscattering experiments have indicated that it exhibits an unusual quasi two-dimensionalstructural phase transition. We have performed initial x-ray measurements which showPR No. 120- II II I I(VII. X-RAY SCATTERING SPECTROSCOPY)resolvable two-dimensional critical scattering with a variety of unusual features. Fur-ther x-ray experiments are under way to elucidate the nature of this most interestinghydrogen-bonded system.5. ELECTRONIC EXCITATIONS IN LIQUID HELIUMWe have now completed our inelastic x-ray scattering study of electronic excitationsin liquid helium (collaborators: P. E. Eisenberger and W. Marra of Bell Laboratories).We find that the electron density response function S(q,co) can be understood quantitativelyby using a simple hydrogenic atomic theory. No many-body or multielectron excitationeffects are observed above a significance level of approximately 10%. This is in sharpcontrast to simple metals like aluminum where many-body effects play a central roleand where theory is still able to provide at best a qualitative understanding. Thus thisstudy in liquid helium provides one physical system where theory and experiment for theelectron density fluctuations are in accord. This work has been published. 46. MEAN FIELD THEORY AND THE GINZBURG CRITERIONAs part of our studies of phase transitions we have also carried out the followingtheoretical analysis. By applying a real space version of the Ginzburg criterion it ispossible to assess the relative importance of critical fluctuations and thence the prob-able limitations of mean field theory for a wide variety of phase transitions. For exam-ple, for ferroelectrics and for certain soft acoustic phonon structural transitions theGinzburg arguments predict that mean field theory should be valid for spatial dimen-sionalities greater than d = 3 and 2, respectively. These "marginal dimensionalities"follow directly from the geometry of the critical fluctuations in phase space. We showalso that this approach provides a simple technique for assessing the self-consistencyof Landau theory in a vast range of physical systems.Publications1. J. Als-Nielsen, R. J. Birgeneau, M. Kaplan, J. D. Litster, and C. R. Safinya,"High Resolution X- ray Study of a Second-Order Nematic-Smectic-A Phase Tran-sition," Phys. Rev. Letters 39, 352 (1977).2. J. Als-Nielsen, R. J. Birgeneau, M. Kaplan, J. D. Litster, and C. R. Safinya,"Experimental Observation of Anomalous Ordering in a Landau-Peierls System,"Phys. Rev. Letters 39, 1668 (1977).3. D. E. Moncton, R. J. Birgeneau, L. V. Interrante, and F. Wudl, "X-ray Scat-tering Study of Spin-Lattice Dimerization in a Quasi One-Dimensional HeisenbergAntiferromagnet," Phys. Rev. Letters 39, 507-510 (1977).4. W. C. Marra, P. Eisenberger, and R. J. Birgeneau, "Inelastic X-ray Scatteringin Liquid Helium," Phys. Rev. B 15, 3325 (1977).5. J. Als-Nielsen, and R. J. Birgeneau, "Mean Field Theory, the Ginzburg Criterionand Marginal Dimensionality of Phase Transitions," Am. J. Phys. 45, 554 (1977).PR No. 120

X-Ray Scattering Spectroscopy

VI. X-RAY SCATTERING SPECTROSCOPYAcademic Research StaffProf. Robert J. BirgeneauProf. Jens Al-NielsenGraduate StudentsMartin C. KaplanCyrus R. Safinya1. HIGH-RESOLUTION X-RAY SCATTERING SPECTROSCOPYOF CONDENSED MATTERJoint Services Electronics Program (Contract DAABO7-76-C-1400)Robert J. BirgeneauOur main activity since the initiation of this project has been the design and con-struction of a multipurpose x-ray scattering spectrometer. The spectrometer isdesigned to carry out x-ray diffuse scattering studies, crystallography research,defect studies, and inelastic x-ray spectroscopy. The essential components of thissystem are a 12-kW rotating anode x-ray source and a high-precision Huber two-axisgoniometer combined with the PDP-11/34 computer control system. We have alsoconstructed a temperature control oven (1 millidegree accuracy) for liquid crystal andferroelectric phase transition studies. It is anticipated that the whole system will beoperational in February 1977. Concurrently with this construction program, we havealso carried on two separate x-ray scattering experiments, together with some theoret-ical work in structural phase transitions.a. Spin-Peirls Transition in TTF-BDT (Cu)The organic charge-transfer salt TTF-BDT (Cu) is believed to provide an idealexample of a spin-dimerization transition in an S = 1/2 one-dimensional antiferro-magnet. This system is of interest because of its role as an archetype for magnetismin organic materials and also because of the analogy with the more familiar electronicPeirls transition as observed, for example, in TTF-TCNQ. In a collaborative experimentwith D. E. Moncton performed at Bell Laboratories, we have succeeded in observingthis structural transition at 12 K, using x-ray diffuse scattering techniques. We havemonitored both the magnitude and spatial variation of the critical fluctuations above T c12 K, together with the growth of the order parameter below Tc. This work is still inprogress, but our initial experiments demonstrate clearly that the transition involvesa pairing that is fundamentally different than others have inferred from simplePR No. 119(VI. X-RAY SCATTERING SPECTROSCOPY)molecular stacking considerations. Consequently, the dominant exchange paths differfrom those implied by a naive consideration of the TTF molecular orbitals.These results are a great surprise to us. There is very little microscopic informa-tion available for this class of materials; hence, these new data could have a significantimpact on our understanding of the magnetism of one-dimensional organic charge-transfer salts.b. Electronic Excitations in Liquid HeliumWe have now completed an inelastic x-ray scattering study of the electronic excita-tions in liquid helium in collaboration with P. E. Eisenberger and W. Marra of BellLaboratories. We find that the electron density response function S(q, c) can be under-stood quantitatively by using a simple hydrogenic atomic theory. No many-body ormultielectron excitation effects are observed above a significance level of approximately10%. This is in sharp contrast to the situation for simple metals like aluminum wheremany-body effects play a central role and theory is still able, at best, to provide a quali-tative understanding. Thus this study in liquid helium supplies one physical systemwhere theory and experiment for the electron density fluctuations are in accord.c. Mean Field Theory and the Ginzburg CriterionBy applying a real-space version of the Ginzburg criterion, it is possible to assessthe relative importance of critical fluctuations, and hence the probable limitations ofmean field theory for a wide variety of phase transitions. For example, for ferroelec-trics and for certain soft acoustic phonon structural transitions the Ginzburg argumentspredict that mean field theory should be valid for spatial dimensionalities greater thand* = 3 and d* = 2, respectively. These "marginal dimensionalities" follow directlyfrom the geometry of the critical fluctuations in phase space. We have shown that thisapproach provides a simple technique for assessing the self-consistency of Landautheory in a vast range of physical systems.PR No. 119

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X-Ray Scattering Spectroscopy

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