17 research outputs found
Distortions of C-60(4-) studied by infrared spectroscopy
The Jahn-Teller effect plays a crucial role in the explanation of the insulating character of
A(4)C(60) (A = K, Rb, Cs). To detect possible phase transitions arising from the interplay between the
molecular Jahn-Teller distortion and the distorting potential field of the counterions, we measured
the mid-IR spectra of A(4)C(60) compounds in the temperature range 90 - 300 K and found significant
spectral changes with temperature in all three compounds. We also compare these spectra to that
of Na(4)C(60) in its room-temperature polymeric phase, where the distortion is more pronounced and
evident from the structure
Isolation and structure of fullerene photodimer, C-120
Photopolymerization of C60 was performed on a gram scale via a new monomer
transport method. After subsequent extractions of the raw polymer in organic solvents, lOOmg
dimer and lOmg mixture of soluble oligomers were yielded. The structures of the soluble
fractions and the insoluble polymer were studied by optical spectroscopy and x-ray diffraction
methods. According to IR and UV-VIS spectroscopic data, the obtained photodimer is identical
to the (2+2) cycloadduct C12o prepared previously. In contrast to previous crystallization studies,
a fast precipitation of the dimer from toluene gives rise to the formation of a solvent-free
microcrystalline solid. The crystal structure of the pure dimer is fee (a= 14.05 A) consisting of
closely packed balls with the cycloadduct bonds oriented randomly along the 12 nearest
neighbor directions. A linear relationship is found to exist between the fee lattice parameter (a)
and the average number of bonded neighbours (nB) in various photopolymerized structures. For
insoluble polymers nB«2 is estimated, supporting previous photopolymer models of small ring or
branched clusters
A fázisprobléma megoldásának új módszerei = New methods for solving the phase problem
A kristályos anyagok szerkezetĂ©nek ab initio meghatározásához megkerĂĽlhetetlen a krisztallográfiai fázisproblĂ©ma megoldása. Kutatásunk során erre a feladatra dolgoztunk ki Ăşj eljárást, amely a klasszikus direkt mĂłdszertĹ‘l teljesen kĂĽlönbözĹ‘ elven működik. A töltĂ©salternálĂł (charge flipping) mĂłdszer meglepĹ‘en merĂ©sz alapfeltevĂ©se az, hogy kellĹ‘en jĂł felbontás esetĂ©n a megoldáshoz nincs szĂĽksĂ©g sem elĹ‘zetes kĂ©miai vagy szimmetria ismeretekre, sem az atomicitás elvĂ©re vagy statisztikus fázisösszefĂĽggĂ©sekre. SzĂĽksĂ©gesek viszont a cella ĂĽressĂ©gĂ©t kihasználĂł gyenge perturbáciĂłk, amelyekkel a hiányzĂł fázisok terĂ©nek nagy dimenziĂłszámát le lehet csökkenteni. A megvalĂłsĂtott algoritmus a valĂłs Ă©s reciprok terek között gyors Fourier-transzformáciĂłval iterál, a valĂłs tĂ©rben az elektronsűrűsĂ©g elĹ‘jelĂ©t egy kis kĂĽszöbszint alatt megváltoztatja, mĂg a reciprok tĂ©rben csak a gyenge reflexiĂłk fázisait tolja el. Az iteráciĂłs folyamat nem igĂ©nyel kĂĽlsĹ‘ beavatkozást: a megoldást valamely jĂłsági tĂ©nyezĹ‘ hirtelen zuhanása jelzi. Könnyű adaptálhatĂłságának köszönhetĹ‘en a töltĂ©salternálás mĂłdszere az elsĹ‘ közlemĂ©ny megjelenĂ©se Ăłta rohamosan terjed. AlkalmazhatĂłsága mára a diffrakciĂłs szerkezetmeghatározás több terĂĽletĂ©n nyert bizonyosságot; a listán egykristály Ă©s pordiffrakciĂłs adatok, periodikus, pszeudoszimmetrikus Ă©s modulált szerkezetek, ill. kvázikristályok is vannak. Az algoritmust már három felhasználĂłi programcsomag kĂnálja alternatĂvakĂ©nt nehĂ©z esetek elemzĂ©sĂ©re. | The solution of the crystallographic phase problem is essential for the ab inito determination of crystalline structures. For this purpose we developed a new method called charge flipping. It is based on principles very different from that of classical direct methods. The primary assumption of charge flipping is the following: given sufficiently high-resolution diffraction data, we need neither preliminary information on chemical composition or symmetry, nor the principle of atomicity or statistical phase relations. Instead, we must rely on the emptiness of the unit cell and decrease the dimensionality of the missing phases through small perturbations. The new algorithm iterates between the real and reciprocal spaces by the fast Fourier transform, in real space it changes the sign of electron density below a small positive threshold, while in reciprocal space it shifts the phases of weak reflections. The whole process is deterministic and proceeds without user intervention, the solution is signified by a sharp drop of some figure of merit. Due to its deceptive simplicity and easy implementation the charge flipping method quickly propagates. Its applicability is already proven in several fields of structural science, the list contains using single crystal and powder data of periodic, pseudo-symmetric and modulated structures and also quasicrystals. The algorithm is already offered as an alternative by three user programs for the treatment of difficult cases
Jahn-Teller distortion in Cs4C60 studied by vibrational spectroscopy
We have measured the infrared spectra of Cs(4)C(60) in the temperature range 220 - 450 K.
Two anomalies in the low-frequency modes at 270 K and 400 K point to changes in molecular or
crystal structure. The most probable explanation is a rotator phase above 400 K and a fully ordered
phase below 220 K; the intermediate structure is one where molecular Jahn-Teller distortions
compete with crystal field effects
Holografikus módszerek a szerkezetkutatásban = Holographic methods in structural research
Az atomi felbontásĂş röntgen holográfia elmĂ©leti alapjait 1991-ben dolgoztuk ki Ă©s nĂ©hány Ă©vel kĂ©sĹ‘bb vĂ©geztĂĽk el az elsĹ‘ sikeres kĂsĂ©rletet. Jelen pályázat fĹ‘ cĂ©lja az atomi felbontásĂş röntgen holográfia Ă©s egy Ăşj, a holográfiával rokon mĂ©rĂ©si mĂłdszer, a szögátlagolt rugalmas szĂłrás továbbfejlesztĂ©se Ă©s alkalmazása volt. Holográfia mĂ©rĂ©sek segĂtsĂ©gĂ©vel kimutattuk, hogy a La0.7Sr0.3MnO3 kristály fázisátalakulásánál nem lĂ©p fel statikus Jahn-Teller torzulás, ahogy korábban feltĂ©teleztĂ©k. KĂsĂ©rleti adatokbĂłl megmutattuk, hogy a szögátlagolt szĂłrás eredmĂ©nyekĂ©nt kapott kĂ©p ugyanazt az informáciĂłt hordozza, mint a hagyományos egykristály diffrakciĂł során egyenkĂ©nt összegyűjtött intenzitás adatok. MegvalĂłsĂtottunk egy Ăşjfajta elektron holográfia mĂ©rĂ©st egy erre a cĂ©lra átĂ©pĂtett pásztázĂł elektron mikroszkĂłp segĂtsĂ©gĂ©vel. EredmĂ©nyeket Ă©rtĂĽnk el a holografikus Ă©s diffrakciĂłs kiĂ©rtĂ©kelĂ©si mĂłdszerek továbbfejlesztĂ©se terĂ©n. Kidolgoztunk egy eljárást a kis intenzitásĂş szĂłráskĂ©pek osztályozására a röntgen szabadelektron lĂ©zereknĂ©l egyedĂĽlállĂł molekulákon vĂ©gzendĹ‘ szĂłráskĂsĂ©rletekhez. | The theoretical foundations of atomic resolution x-ray holography were developed in 1991, and followed by the first successful experiment a few years later. The aim of the present project was the further development and application of the atomic resolution x-ray holography and a new related method, the angular integrated elastic scattering. Based on holography measurements, we have shown that in spite of earlier predictions, no static Jahn-Teller distortion occurs at the phase transition of the La0.7Sr0.3MnO3 crystal. We have shown from experimental data that the angular integrated elastic scattering pattern contains the same information as the intensity data of the conventional x-ray diffraction experiments. A novel electron holography experiment was realized using a scanning electron microscope modified for this purpose. Further development of holographic and diffraction evaluation methods was achieved. We have also developed a new method for the classification of low-intensity diffraction patterns for the single molecule imaging experiments of the x-ray free electron lasers
Rugalmas röntgenszórás a szerkezetkutatásban = Elastic x-ray scattering in structural research
Kutatásaink az atomi szerkezet röntgensugárzással valĂł vizsgálatát cĂ©lozták. Ezen belĂĽl kutatásunk három fĹ‘ irányban folyt: 1. Megvizsgáltuk, hogy a közeljövĹ‘ben Ă©pĂĽlĹ‘ lineáris gyorsĂtĂłkon alapulĂł szabad elektron lĂ©zer tĂpusĂş röntgenforrásokkal (Free Electron Laser, FEL) lesz-e lehetĹ‘sĂ©g egyedi, kis, nem-periodikus rĂ©szecskĂ©k atomi szintű szerkezet-meghatározására. MegállapĂtottuk, hogy a sikeres szerkezet-meghatározáshoz a tervezetnĂ©l rövidebb impulzushosszal rendelkezĹ‘ forrásokra lesz szĂĽksĂ©g. Megmutattuk, hogy a folytonos szĂłráskĂ©pbĹ‘l a "Fineup input-output" algoritmus egy mĂłdosĂtott változatával rekonstruálhatĂł az eredeti atomi szerkezet. RĂ©szletesen diszkutáltuk, hogy a teljes 3D szĂłráskĂ©p előállĂtásához szĂĽksĂ©ges klasszifikáciĂłs eljárás milyen feltĂ©telek között működik. 2. A holografikus mĂłdszerekkel rokon mĂ©rĂ©si eljárások elmĂ©leti Ă©s gyakorlati megvalĂłsĂtásán dolgoztunk. SzĂ©lesĂtettĂĽk a röntgen fluoreszcens holográfia alkalmazási terĂĽletĂ©t, kĂ©t Ăşj anyag vizsgálatával (ThAs Se Kondo rendszer Ă©s La1-xSrxMnO3 mágneses kolosszális mágneses ellenállást mutatĂł rendszer). Egy Ăşj mĂ©rĂ©si eljárást vezettĂĽnk be, a szögintegrált rugalmas szĂłrást, amely számos elĹ‘nyös tulajdonsággal rendelkezik a hagyományos szerkezet-meghatározĂł mĂ©rĂ©sekkel összehasonlĂtva. 3. Hagyományos röntgendiffrakciĂłs mĂłdszerekkel vizsgáltuk ismeretlen, Ăşj anyagok szerkezetĂ©t. Meghatároztuk, fullerĂ©n Ă©s kubán molekulák alkotta vegyĂĽletek szerkezetĂ©t, Ă©s tanulmányoztuk a hĹ‘mĂ©rsĂ©kletváltozás eredmĂ©nyekĂ©nt lĂ©trejövĹ‘ fázisátalakulásokat. | We worked on atomic level structure determination by x-rays. Our research was centered on three main points: 1. We examined the possibility of structure reconstruction of small, non-periodic single particles using x-ray free electron laser type sources. We have shown that the pulse length has to be significantly shortened (compared to present day planes) for successful reconstruction. Further, we illustrated that a modified version of the Fineup input-output algorithm can be used to reconstruct the original atomic structure. We also discussed in detail the conditions in which the classification process could work. 2. We worked on theoretical and practical sides of holographic and related methods. We applied x-ray fluorescent holography to a Kondo system (ThAsSe) and a colossal magnetic resistivity material (La1-xSrxMnO3), this way widening the application of holography. We introduced a new measuring method (the angular integrated elastic scattering), which has several advantages compared to classical methods. 3. Using traditional x-ray diffraction techniques we investigated the atomic structure of new compounds. We determined the structure of new fullerene-cubane compounds and studied the phase transitions as a function of the temperature
A fázisprobléma megoldásának új módszerei II. = New methods for solving the phase problem II.
A krisztallográfiai fázisproblĂ©ma megoldására általunk felfedezett charge flipping mĂłdszert több, a gyakorlatban azonnal alkalmazhatĂł irányba fejlesztettĂĽk tovább. Ăšj algoritmusváltozatokat, adatkezelĂ©st, paramĂ©terválasztást vezettĂĽnk be, amellyel egyrĂ©szt gyorsabban, másrĂ©szt jobb minĹ‘sĂ©gben oldhatĂłk meg a kristályszerkezetek. A fejlesztĂ©sek Ăşj alkalmazásokat is lehetĹ‘vĂ© tettek; az algoritmus egyik változata a negatĂv szĂłrássűrűsĂ©gű neutrondiffrakciĂłs adatok, mĂg egy másik, kĂĽlönbözĹ‘ tĂpusĂş, sĂşlyosan hiányos adatok esetĂ©n működik kiválĂłan. UtĂłbbi esetben, akár a krisztallográfiai irodalomban szokásos adatok 1/8-val is lehetsĂ©ges jĂł minĹ‘sĂ©gű szerkezetmeghatározás. A kristályos alapállapot elmĂ©letĂ©vel is rĂ©szletesen foglalkoztunk. Az atomok közötti Fourier-transzformálhatĂł kölcsönhatások Ă©s a kristályos rend kapcsolatának matematikailag egzakt leĂrását adtuk egyes speciális, a puhaanyag-fizikához köthetĹ‘ esetekben. Kristályok helyett molekulák azonos replikáin alapul az atomi szerkezet meghatározásának egy teljesen Ăşj mĂłdszere, az egyrĂ©szecske-lekĂ©pezĂ©s. A kĂsĂ©rlet csak szabadelektron-lĂ©zereknĂ©l lehetsĂ©ges, Ă©s mĂ©g számos kihĂvást jelent. Ezek közĂĽl mi az adatelĹ‘kĂ©szĂtĂ©s problĂ©máját oldottuk meg, iteráciĂłs mĂłdszert alkottunk, amely az ismeretlen orientáciĂłjĂş Ă©s rendkĂvĂĽl zajos 2D diffrakciĂłs kĂ©peket szerkezetmeghatározásra alkalmas 3D adatkĂ©szlettĂ© alakĂtja. | Our charge flipping method, that solves the crystallographic phase problem, was further developed with practical utilization in mind. We introduced new algorithm variants, data treatments and parameter choices, that helped to determine crystal structures both faster and at better quality. These developments also allowed new applications; one of the algorithm variants works well in the case of negative scattering density, and an other in various instances of seriously incomplete data. In the latter case, high quality structure determination is feasible using only 1/8-th of the data expected in the crystallographic literature. We also investigated the ground state of crystals with mathematical rigour. The relationship of Fourier-transformable atomic interactions and crystal order was described, the special cases treated are related to soft matter physics. Our third theme is single particle imaging, a new method of structure determination that is not based on crystals but on molecular replicas. The experiment is only made possible by free electron lasers, and it still poses a number of challenges. We worked on the problem of data preparation, and solved it by creating an iterative procedure that is capable of composing a single high-quality 3D dataset from the large number of noisy 2D images of unknown orientation
Energy gap in superconducting fullerides: optical and tunneling studies
Tunneling and optical transmission studies have been performed on
superconducting samples of Rb3C60. At temperatures much below the
superconducting transition temperature Tc the energy gap is 2 Delta=5.2 +-
0.2meV, corresponding to 2 Delta/kB Tc = 4.2. The low temperature density of
states, and the temperature dependence of the optical conductivity resembles
the BCS behavior, although there is an enhanced ``normal state" contribution.
The results indicate that this fulleride material is an s-wave superconductor,
but the superconductivity cannot be described in the weak coupling limit.Comment: RevTex file with four .EPS figures. Prints to four pages. Also
available at http://buckminster.physics.sunysb.edu/papers/pubrece.htm