4. X-RAY LABORATORY (atom.physics.helsinki.fi/~xray_www/)

4.1. PREFACE

 

Expansion into a new field of research was the main development in the X-ray laboratory in 1999 when a group working in medical physics was created. In cooperation with the European Synchrotron Radiation Facility (ESRF) and the Helsinki University Central Hospital research programs for angiography and bronchography were initiated. The Academy of Finland and ESRF are largely responsible for the funding of this research.

Two interesting projects have been carried out by the inelastic scattering group: (i) A first quantitative study of the properties of electron gas at high pressure was performed at ESRF. By measuring the inelastic scattering cross section from a sodium sample as a function of pressure it was possible to look at the changes in the basic electronic properties like electron-electron correlation, for example. (ii) In co-operation with the University of Dortmund, Germany, a theoretical and experimental study on the fundamental properties of the inelastic scattering process was performed. It was shown that there is a resolution limit due to the final state interactions, which cannot be exceeded in high-resolution inelastic scattering experiments.

In addition to the fruitfully continuing work in polymers together with the Department of Chemistry, HUT and the University of Groningen, the group working on weakly ordered materials has been involved in two new projects: (i) Structural studies of proteins in co-operation with VTT using small angle X-ray scattering (SAXS) has produced information about the atomic distance distribution functions which has been successfully explained by molecular modelling. (ii) The first systematic study on the cell wall structure of Finnish conifers has been made using SAXS and wide angle X-ray scattering. Properties like microporosity, microfibril angles and crystallinity have been characterized using both conventional X-ray sources and synchrotron radiation at HASYLAB.

The four-circle diffractometer has been used to study, with Russian co-workers (Academy of Sciences, Moscow), some non-linear optical materials and langasites. Also co-operation with HUT and the National Academy of Science, Ukraine, continued in studies of particular shape memory and smart materials. Co-operation with the Institute of Semiconductor Physics, National Academy of Science, Ukraine, has been started to study the defect properties of semiconductors. The laboratory has also been involved in the national research program EMMA (Electronic Materials and Microsystems) with projects related to polymer and semiconductor studies.

Characterization of X-ray detectors, used in space research has been continued in a project involving the Department of Astronomy and Metorex Oy.

The major investment in instrumentation was an image plate system. It is a two-dimensional position sensitive detector, which can be used in many of the experiments of the laboratory.

The use of synchrotron radiation was in a dominant role in many projects. Altogether 29 visits to ESRF, NSLS (USA), HASYLAB (Germany) and SRS (U.K.) were made covering 365 working days. In addition Veijo Honkimäki spent the whole year at ESRF.

The external funding has been significantly increasing during the last few years. In 1999 about 3.1 MFIM was obtained; the main sources were the Academy of Finland, TEKES, the Ministry of Education and ESRF.

Seppo Manninen

 

4.2. HARD CONDENSED MATTER: ELECTRONIC AND MAGNETIC PROPERTIES

 

FINAL STATE INTERACTION IN COMPTON SCATTERING FROM ELECTRON LIQUIDS

Keijo Hämäläinen, Aleksi Soininen, Seppo Manninen, C. Sternemann*, A. Kaprolat*, G. Döring*, C.-C. Kao** and W. Schülke*

High resolution Compton scattering is a fundamental method to study the ground state properties of the scattering electron system. In many cases it is the only direct way to probe the Fermi surface topology. In a series of experiments, performed on a Li sample at Hasylab (Germany) and NSLS (USA) we have shown that there is an intrinsic resolution limit due to the final state interaction between the excited electron and hole pair. Model calculations, based on the use of Green functions (GW approximation) support the experimental observations. The interaction introduces smearing to the scattered spectrum and some asymmetry. The effect is largest at low X-ray energies and should be taken into account in high resolution Compton scattering experiments.

* Institute of Physics, Univ. Dortmund, Germany

** NSLS, Brookhaven National Lab., USA

 

STUDIES OF FREE ELECTRON GAS IN HIGH PRESSURE

Simo Huotari, Keijo Hämäläinen, Jarkko Laukkanen, Aleksi Soininen, Seppo Manninen, C.-C. Kao*, T. Buslaps** and M. Mezouar**

Inelastic scattering of synchrotron radiation in the high energy and momentum transfer limit (i.e. Compton scattering) has been successfully applied in studying the properties of free electrons in metallic Na in pressures up to 4.2 GPa. By applying the pressure using both a diamond anvil cell (DAC) and a large volume cell press (LVC), we changed the free electron gas density parameter rs in situ and recorded the intensity spectra of Compton scattered synchrotron radiation. In the spectra we clearly see the effect of changing Fermi momentum and signatures of the electron-electron interaction. The results are well explained by a free electron theory where the inter-electronic correlations are included using a random phase approximation scheme.

* NSLS, Brookhaven National Laboratory, USA

** ESRF, Grenoble, France

 

COMPTON SCATTERING STUDIES OF METALLIC Be

Simo Huotari, Keijo Hämäläinen, Seppo Manninen, T.Buslaps*, Pekka Suortti and W. Caliebe**

Rapid process in understanding the Compton scattering process have been achieved over the past couple of years. Utilizing monochromatized synchrotron radiation at the ESRF beamlines ID16 (E1=10 keV, fixed analyzer crystal and tunable incident energy) and ID15B (E1=29 keV and 56 keV, fixed incident energy and scanning crystal spectrometer), we measured high resolution Compton scattering spectra from metallic Be. The signatures of the quite complicated Fermi surface are clearly seen in the profiles and their derivatives. The deviations from impulse approximation (IA) theory fall into two classes: (i) asymmetry in the Compton profiles induced by failing IA concerning the tightly bound core electrons, and (ii) apparent smearing of the Fermi surface structure when using low energy photons, due to interactions between valence electrons in final state with the valence electron gas in ground state. While the latter effect is a major problem only with incident photon energies of the order 10 keV, the first effect can still be seen with photon energies of 60 keV and even higher.

* ESRF, Grenoble, France

** Inst. für Festkörperforschung, Forschungs-zentrum Jülich, Jülich, Germany

 

HOLLOW ATOM'S X-RAY EMISSION SPECTRA

Keijo Hämäläinen, Simo Huotari, R. Diamant*, M. Deutsch* and C.-C. Kao**

We have continued our high resolution X-ray emission studies of double-ionized atoms utilizing synchrotron radiation both at NSLS and ESRF. The first pure high-resolution photoexcited Kh a1,2 hypersatellite spectrum from a hollow Cu atom is measured. Its Kh a2 - Ka1 shift and the Kh a1,2 lines' splitting, widths and intensity ratio are accurately determined. Such spectra are uniquely suited to study relativistic correlation effects, transition from LS to intermediate coupling and the Breit-Wigner interaction. The threshold energy and the spectrum's evolution with excitation energy are also measured. The roles of relativity and Breit interaction are explored by comparison with ab-initio Dirac-Fock calculations.

* Physics Department, Bar-Ilan Univ., Israel

** NSLS, Brookhaven National Lab., USA

 

STUDY OF MAGNETIC MATERIALS USING RESONANT INELASTIC X-RAY SCATTERING

Keijo Hämäläinen, Simo Huotari, Heikki Sutinen, Szabolcs Galambosi, K. Attenkofer*, W. Caliebe**, M. Silva+ and C.-C. Kao++

It has been shown recently that the high resolution resonant Raman scattering RRS (also referred as high resolution resonant inelastic scattering, RIXS) can be used to obtain more detailed information on the electronic structure than conventional absorption spectroscopy. Furthermore, by studying the dispersion of the emission spectra one can clearly separate pre-edge features generally related to the quadrupolar transitions. This feature is especially useful in the case of rare-earth elements and their compounds where the interplay of 5d and 4f electrons constitute the basic mechanism for the magnetic properties.

We have done RIXS scattering experiments on magnetically interesting samples at beamline X21 at NSLS. Several different Eu compounds have been studied in order to investigate the influence of electronic configuration on the LIII absorption spectra. In this way we can gain information also on the 5d-electrons eg/t2g splitting. Furthermore, detailed RIXS study on an antiferromagnetic DyFe4Al8 sample has been accomplished as a function of temperature. This will give us a very high sensitivity to study the different transition matrix elements and the electronic structure changes when we vary the sample temperature through the magnetic ordering temperature.

* University of Würzburg, Germany

** Inst. für Festkörperforschung, Forschungs-zentrum Jülich, Jülich, Germany

+ University of Coimbra, Portugal

++ NSLS, Brookhaven National Laboratory, USA

 

FIRST-PRINCIPLES STUDY OF NON-RESONANT INELASTIC X-RAY SCATTERING FROM LiF AND DIAMOND

Aleksi Soininen, Keijo Hämäläinen, E. L. Shirley*, W. A. Caliebe** and C.-C. Kao+

We have developed new first-principles schemes for modeling non-resonant inelastic X-ray scattering (IXS) from real materials. These schemes can be used to model IXS with small energy transfers or energy transfers close to a core binding energy. Although the method is based partly on the local-density approximation (LDA) pseudopotential calculations, many-body effects beyond LDA are included. The effects of electron-electron and electron-hole interaction are included in these schemes. This enables us to study at an 'ab initio' level the effect these interactions have on the IXS double differential cross section. These schemes have been applied to scattering from both the core and the valence electrons of LiF and diamond.

* Physics Laboratory, NIST, Gaithersburg, USA

** Inst. für Festkörperforschung, Forschungs-zentrum Jülich, Jülich, Germany

+ NSLS, Brookhaven National Laboratory, USA

 

XMCD MEASUREMENTS USING ELLIPTICALLY POLARIZING WIGGLER

Aleksi Mattila, Keijo Hämäläinen, D. Lott* and C.-C. Kao*

In recent years the study of magnetic materials with circularly polarized X-rays has been extended by advances in polarization control of synchrotron radiation utilizing novel insertion devices and phase plates into higher energies than previously possible. One of the experimental techniques benefiting from this development is the X-ray magnetic circular dichroism, which measures the relative difference in absorption near the absorption edges of magnetically interesting elements for circularly polarized photons with helicity parallel and opposite to the magnetization direction of the sample, yielding element-specific information on the electronic states of magnetic interest. At NSLS X-13B beamline a time-varying Elliptically Polarizing Wiggler (EPW) has been used for XMCD studies at energy range making the K-edges of transition metals and the magnetically interesting L2,3-edges of rare earth metals accessible, with most of the experimental effort concentrating on utilizing the polarization switching capability of the EPW in connection with XMCD-measurements.

* NSLS, Brookhaven National Laboratory, USA

 

ELECTRONIC PROPERTIES OF DECAGONAL QUASICRYSTALS

Jarkko Laukkanen, Pekka Suortti, J. Birman1, T. Buslaps2, M. A. Chernikov3, P. Chow3, J. Kulik3, S. Moss3, I. Fisher4 A. Goldman4, C.Y. Song5 and J.-M. Gillet6

Quasicrystals possess quite unique physical and electrical properties depending on the crystal direction. On a qualitative level, their electronic structure is understood adequately, but theoretical studies meet severe difficulties due to the non-periodicity. To test the validity of the so-called many-pocketed quasi-Brillouin-Fermi surface hypothesis, we have conducted an inelastic X-ray scattering study on the decagonal quasicrystal Al0.72Co0.17Ni0.11 utilizing the high-resolution spectrometer at beamline ID15B (ESRF). Our results do support the hypothesis. Additionally, direct evidence of an anisotropy in the electronic momentum density was discovered. The next phase of the project includes similar studies on other quasicrystalline systems e.g. of icosahedral symmetry. The project was awarded a co-operative grant of the National Science Foundation, USA.

1 Dept. of Physics, City College of New York, NY, USA

2 ESRF, Grenoble, France

3 Dept. of Physics, Univ. Houston, Houston, TX, USA

4 Ames Laboratory, Iowa State Univ., IA, USA

5 Iowa State University, Iowa, USA

6 Ecole Centrale Paris, Paris, France

 

HIGH RESOLUTION COMPTON SPECTROMETRY

Pekka Suortti, Veijo Honkimäki, T. Buslaps* and A. Shukla*

This has been a long-term project, where prototype spectrometers were built at the X-Ray Laboratory in 1980's, at the ESRF in early 1990's, and the spectrometer at beamline ID15B of the ESRF became operative in 1994. In subsequent years the performance of the spectrometer was improved, and an extensive report was published in 1999. The spectrometer is of scanning type, and its special advantages are that very small samples can be studied, the background is low, and scans can be focused on the interesting parts of the spectrum only. So far, the spectrometer has been used in tens of experiments. Typically, the momentum resolution is 0.1 a.u., and the count rate at the Compton peak is a few thousand counts per second, when the incident energy is 30 keV or 60 keV.

P. Suortti et al., J. Synchrotron Rad. 6 (1999) 69-80

* ESRF, Grenoble, France

 

COINCIDENCE COMPTON SPECTROMETRY

C. Metz*, Th. Tschentscher*,**, Pekka Suortti, T. Sattler**, J.R. Schneider**, F. Bell+, A.S. Kheifets++ and D.R. Lun++

The limitation of the Compton profile measurements is that the electron momentum density (EMD) is projected on the scattering vectors. Full information of the EMD is obtained when the recoil electron is observed in coincidence with the scattered photon. This technique was introduced in some synchrotron radiation laboratories more than 10 years ago, and was developed further at the ESRF. The present set-up is based on the use of 150 keV to 200 keV radiation, monochromatized by a bent Laue-type crystal, a 13-element Ge detector, and 16x16 pixel electron detector. Due to the short mean free path of the electrons the effective sample thickness is very small, order of 100 nm only. The coincidence count rate is about 20 cps, and the momentum resolution components are typically (0.2; 0.4; 0.2) a.u. The method has been applied to graphite, fullerines, Al, and Cu-Ni alloy. Comparisons with FP-LMTO calculations show that electrons are more delocalized in fullerine than in graphite. Also the difference to the EMD determined by positron annihilation is demonstrated; this arises from a non-uniform weighing of the EMD by the positron wave function.

C. Metz et al. J. Phys. Condens. Matter 11 (1999) 3933-3942

* ESRF, Grenoble, France

** Hasylab, Hamburg, Germany

+ University of Munich, Munich, Germany

++ The Australian National University, Canberra, Australia

 

COMPTON SCATTERING STUDIES OF HIGH-Tc SUPERCONDUCTORS

A. Shukla*, B. Barbiellini**, A. Erb+, A. Manuel+, T. Buslaps*, Veijo Honkimäki and Pekka Suortti

High-resolution Compton profiles were measured from PrBa2Cu3O7-d, which is an insulator in most cases, and from YBa2Cu3O7-d, which is always superconducting below the transition temperature. The anisotropy of the Compton profiles is very different in these nominally isostructural materials, when directions [100/010] and [110] are compared, while no difference is seen in positron annihilation measurements. The wave function coherence is strongly suppressed in insulating PrBa2Cu3O7-d, and this is interpreted to arise from Pr on Ba-site substitution disorder.

A. Shukla et al. Phys Rev B59 (1999) 12127-12131

* ESRF, Grenoble, France

** Northeastern University, Boston, Mass., USA

+ University of Geneve, Geneve, Switzerland

 

FERMI SURFACE STUDIES OF Al AND Al.97Li.03

Pekka Suortti, T. Buslaps*, Veijo Honkimäki, A. Shukla*, J. Kwiatkowska**, F. Maniawski**, S. Kaprzyk+ and A. Bansil++

The band structure calculations for metals and alloys, based on the KKR-CPA scheme, have reached great precision in recent years. At the same time, the accuracy of Compton profiles has attained the level of 0.1%, which makes detailed comparisons between theory and experiment possible. In the present project, the real accuracy of the theoretical results were tested with respect to anisotropy, the contributions of Umklapp scattering, effects of alloying, and electron-electron correlation. Al and Al.97Li.03 were chosen for the subjects of the study because of the relative complexity of the Fermi surface, where alloying may have profound effects. Earlier studies have suggested that the calculations based on the local density approximation (LDA) may not count adequately for the correlation effects. The present results show that the KKR calculation yields precise results for anisotropy, overestimates Umklapp scattering, does not predict correctly the effects of alloying, but at least in a case of high valence electron density includes the effects of correlation reasonably well.

Suortti et al. J Phys Chem Solids 61 (2000) 397-401

* ESRF, Grenoble, France

** Institute of Nuclear Physics, Krakow, Poland

+ Univ. of Mining and Metallurgy, Krakow, Poland

++ Northeastern University, Boston, Mass., USA

 

4.3. HARD CONDENSED MATTER: STRUCTURE

 

DEVELOPMENT OF POWDER DIFFRACTOMETER II

Szabolcs Galambosi and Merja Blomberg

Due to expansion of research on semiconductor systems the powder diffractometer has been upgraded with a new Huber 414 2-circle goniometer. The current setup enables the study of new problems which earlier were out of reach due to experimental limitations. The new goniometer provides improved control over sample orientation and this combined with the high intensity beam (pure CuKa1 with >108 cps) obtained with the Johansson-type quartz monochromator has made us possible to use the diffractometer extensively for experiments on diffuse X-ray scattering on semiconductors. In addition, other experimental possibilities include powder diffraction with several transmission/reflection geometries with Guinier type focussing, grazing incidence powder diffraction and texture determination.

 

X-RAY DIFFRACTION METHODS IN SEMICONDUCTOR STRUCTURE CHARACTERIZATION

Szabolcs Galambosi, Kai Nordlund*, Tommy Ahlgren* and Keijo Hämäläinen

Properties of semiconductors are greatly influenced by deviations from their ideal crystal structure. Lattice misfits, stress and various defects can also be induced on purpose along the fabrication process which affect electronic and optical properties of semiconductors. X-ray diffraction provides a non-destructive mean for structure characterisation. Defects in the crystal lattice give rise to tails around the Bragg peaks. Careful investigation of these diffuse parts gives detailed information about the defect configuration in the crystal. Experimental studies are primarily carried out at synchrotron sources with some preliminary studies done with conventional X-ray tubes. For the analysis of experimental data we use an atomistic simulation method, which allows for calculating the diffuse X-ray scattering for an arbitrary finite-sized defect in any material, where reliable interatomic force model exist. Currently our main field of interest lies in determination of the effects of As diffusion in germanium and radiation defects in several materials. In the area of thin film research we have demonstrated our ability to determine stresses which lead to a relative change of 10-3 of the lattice parameter with both polycrystalline and single crystal thin films with thicknesses under 100 nm.

* Accelerator laboratory, University of Helsinki

 

STRUCTURE OF NONLINEAR OPTICAL MATERIALS

Merja Blomberg, N. Sorokina*, I. A. Verin* and V. I. Simonov*

Low-temperature studies on the structure of compounds of the KTiOPO4-family have been continued. KTiOPO4 is a well-known nonlinear optical material which is extensively used for frequency doubling of laser light. Because the efficiency of the KTiOPO4-related compounds is essentially higher than that of other crystalline materials used for this aim, synthesis of new compounds of this family and studies of their crystal structure and physical properties is of great importance. Analysis of the previously measured data of Cs0.57Rb0.43TiOAsO4 was continued. Single-crystal X-ray diffraction measurements on the structure of K1-xNbxTi1-xOPO4 at 20 K were performed.

* Institute of Crystallography, Academy of Sciences of Russia, Moscow, Russia

 

PHASE TRANSITIONS IN Cs3Sb2I9

Merja Blomberg, M.S. Novikova** and V.I. Simonov**

A low-temperature X-ray study of the trigonal modification of Cs3Sb2I9 has been carried out. Three phase transitions at 86, 78 and 72 K have been observed. The transition at 86 K preserves the trigonal symmetry and is associated with a doubling of the c-parameter. At 72 K a doubling of the parameters a and b is observed. An intermediate incommensurate phase with q1=1/2c* and q2=(1/2-d)a* + 1/2 c* has been detected in the temperature range 78-72 K. The lock-in phase transition at 72 K has a first-order character. The results of the structural study were published in [1].

** Institute of Crystallography, Academy of Sciences of Russia, Moscow, Russia

1. B.Sh. Bagautdinov et al.: X-ray study of phase transitions in Cs3Sb2I9 crystal, Solid State Communications 111 (1999) 361-366

 

STRUCTURE OF LANGASITES

Merja Blomberg, V.N. Molchanov*, I.A. Verin* and V.I. Simonov*

Langasite (La3Ga5SiO14, LGS) is one of the most promising substrate materials for surface acoustic wave filters in the next-generation mobile telecommunications equipment. Langasite offers temperature stability comparable to that of quartz crystals while providing a much better piezoelectric performance. An interesting scientific problem related to crystals of the LGS-family is the origin of dielectric and elastic anomalies in their material constants. Low-temperature data from dielectric measurements indicate a possible phase transition near 40 K. Studies of the changes in the crystal structure of langasite at temperatures between 100 K and 20 K were performed. The analysis of the data is in progress.

* Institute of Crystallography, Academy of Sciences of Russia, Moscow, Russia

 

SMART MATERIALS

Veli Eteläniemi and Sakari Vahvaselkä

Smart materials as shape memory, magnetic recording, magnetostrictive and magneto-optic materials, are new technology materials that are interesting due to their ability to react to the changes in their environment. Their properties may change due to, for instance, pressure, temperature, luminosity, electric and magnetic field.

The new approach in this project is that some new samples have been prepared in our laboratory and structure as a function of temperature has been studied when the austenite-martensite transformation takes place. There is also a new sample holder that allows us to study how the diffraction pattern changes upon transformation under magnetic field.

 

CHARACTERIZATION OF THE DEFECT STRUCTURE OF SEMICONDUCTOR CRYSTALS USING ENERGY DISPERSIVE X-RAY DIFFRACTION

D. Grigoriev*, Szabolcs Galambosi, L. Datsenko*, V. Klad'ko*, V.Machulin*, Seppo Manninen and V. Molodkin**

Defect structures in semiconductor crystals have been studied be measuring the integral reflecting power of (111) and (333) Bragg reflections in the wavelength range of 0.2-1.2 Å. The method is based on the fact that the extinction length is wavelength dependent and bymeasuring the desired reflection at different wavelengths (by changing the scattering angle) the relative size of the defect compared to the extinction length changes and this can be seen in the total reflecting power. It turned out that the method is very defect sensitive at short X-ray wavelengths and the obtained results could be interpreted using model calculations, made by the Ukrainian group. The method was first tested using well-characterized Si and Ge crystals and the project will continue in 2000 by applying this techique on binary semiconductors like GaP and GaAs.

* Institute of Semiconductor Physics, National Academy of Science of Ukraine, Kiev, Ukraine}

** Institute of Metal Physics, National Academy of Science of Ukraine, Kiev, Ukraine