Characterization of electronic properties of novel materials is of great importance for exploratory materials development and also for the discovery of new correlated phases. As several novel compounds are available in powder form only, contactless methods, which also work on air sensitive samples, are higly desired. We present that the microwave cavity perturbation technique is a versatile tool to study conductivity in such systems. The examples include studies on semiconducting-metallic crossover in carbon nanotubes upon alkali doping, study of vortex motion in the K$_3$C$_{60}$ superconductor, and the characterization of various alkali atom doped phases of black phosphorus.

Enhanced microwave absorption, larger than that in the normal state, is observed in fine grains of type-II superconductors (MgB2 and K3C60) for magnetic fields as small as a few % of the upper critical field. The effect is predicted by the theory of vortex motion in type-II superconductors, however its direct observation has been elusive due to skin-depth limitations; conventional microwave absorption studies employ larger samples where the microwave magnetic field exclusion significantly lowers the absorption. We show that the enhancement is observable in grains smaller than the penetration depth. A quantitative analysis on K3C60 in the framework of the Coffey-Clem (CC) theory explains well the temperature dependence of the microwave absorption and also allows to determine the vortex pinning force constant.

The reduction of polycyclic aromatic hydrocarbons with alkali metals results in solids having intriguing magnetic properties the understanding of which has been hitherto severely hampered by the lack of single-phase samples. Here, we report on the successful reduction of triphenylene with stoichiometric amount of potassium in 1,2-dimethoxyethane (DME) solution. Comprehensive diffraction measurements of the obtained K2(C18H12)2(DME) solid demonstrate the importance of cation-π interactions as responsible for the characteristic stacking of the triphenylide molecular ions. Electron paramagnetic resonance and magnetization measurements reveal K2(C18H12)2(DME) is a Mott insulator with strikingly strong nearest neighbor antiferromagnetic interactions between S = 1/2 spins of (C18H12)•– radical anions. Low dimensionality hinders long-range magnetic ordering and establishes a spin state that resembles gapped quantum spin liquid state.

Alkali metal intercalation into polyaromatic hydrocarbons (PAHs) has been studied intensely after reports of superconductivity in a number of potassium- and rubidium-intercalated materials. There are, however, no reported crystal structures to inform our understanding of the chemistry and physics because of the complex reactivity of PAHs with strong reducing agents at high temperature. Here we present the synthesis of crystalline K2Pentacene and K2Picene by a solid–solid insertion protocol that uses potassium hydride as a redox-controlled reducing agent to access the PAH dianions, and so enables the determination of their crystal structures. In both cases, the inserted cations expand the parent herringbone packings by reorienting the molecular anions to create multiple potassium sites within initially dense molecular layers, and thus interact with the PAH anion π systems. The synthetic and crystal chemistry of alkali metal intercalation into PAHs differs from that into fullerenes and graphite, in which the cation sites are pre-defined by the host structure. Reports of superconductivity in KxPicene spurred interest in alkali-intercalated polyaromatic hydrocarbon (PAH) compounds, but their compositions and structures have remained unclear. Now crystalline K2Pentacene and K2Picene — neither of which are superconducting — have been prepared by mild synthesis. Structural analysis shows that the cation sites arise within the molecular layers from reorientation of the PAHs within a herringbone packing.

Molecular solids with cooperative electronic properties based purely on π electrons from carbon atoms offer a fertile ground in the search for exotic states of matter, including unconventional superconductivity and quantum magnetism. The field was ignited by reports of high-temperature superconductivity in materials obtained by the reaction of alkali metals with polyaromatic hydrocarbons, such as phenanthrene and picene, but the composition and structure of any compound in this family remained unknown. Here we isolate the binary caesium salts of phenanthrene, Cs(C 14 H 10 ) and Cs 2 (C 14 H 10 ), to show that they are multiorbital strongly correlated Mott insulators. Whereas Cs 2 (C 14 H 10 ) is diamagnetic because of orbital polarization, Cs(C 14 H 10 ) is a Heisenberg antiferromagnet with a gapped spin-liquid state that emerges from the coupled highly frustrated Δ-chain magnetic topology of the alternating-exchange spiral tubes of S = ½ (C 14 H 10 ) •− radical anions. The absence of long-range magnetic order down to 1.8 K ( T / J ≈ 0.02; J is the dominant exchange constant) renders the compound an excellent candidate for a spin-½ quantum-spin liquid (QSL) that arises purely from carbon π electrons.

The 10 GHz microwave conductivity, sigma(T) and high field, 222 GHz electron spin resonance (HF-ESR) of Li4C60 fulleride is measured in a wide temperature range. We suggest that the majority of ESR active sites and at least some of the charge carriers for sigma(T) are electrons bound to a small concentration of surplus or vacancy ions in the polymer phase. Both sigma(T) and the ESR line shape depend on ionic motion. A change of the activation energy of sigma(T) at 125 K coincides with the onset of the ionic DC conductivity. The ESR line shape is determined mainly by Li ionic motion within octahedral voids below 150 K. At higher temperatures, fluctuations due to ionic diffusion change the environment of defects from axial to effectively isotropic on the ESR time scale. sigma(T) data up to 700 K through the depolymerization transition confirm that the monomeric phase of Li4C60 is a metal.

Understanding the relationship between the superconducting, the neighboring insulating, and the normal metallic state above T-c is a major challenge for all unconventional superconductors. The molecular A(3)C(60) fulleride superconductors have a parent antiferromagnetic insulator in common with the atom-based cuprates, but here, the C(60)(3-)electronic structure controls the geometry and spin state of the structural building unit via the on-molecule JahnTeller effect. We identify the Jahn-Teller metal as a fluctuating microscopically heterogeneous coexistence of both localized Jahn-Teller-active and itinerant electrons that connects the insulating and superconducting states of fullerides. The balance between these molecular and extended lattice features of the electrons at the Fermi level gives a dome-shaped variation of T-c with interfulleride separation, demonstrating molecular electronic structure control of superconductivity.

By exposing flat and curved carbon surfaces to coronene, a variety of van der Waals hybrid heterostructures are prepared, including coronene encapsulated in carbon nanotubes, and coronene and dicoronylene adsorbed on nanotubes or graphite via π-π interactions. The structure of the final product is determined by the temperature of the experiment and the curvature of the carbon surface. While at temperatures below and close to the sublimation point of coronene, nanotubes with suitable diameters are filled with single coronene molecules, at higher temperatures additional dimerization and oligomerization of coronene occurs on the surface of carbon nanotubes. The fact that dicoronylene and possible higher oligomers are formed at lower temperatures than expected for vapor-phase polymerization indicates the active role of the carbon surface used primarily as template. Removal of adsorbed species from the nanotube surface is of utmost importance for reliable characterization of encapsulated molecules: it is demonstrated that the green fluorescence attributed previously to encapsulated coronene is instead caused by dicoronylene adsorbed on the surface which can be solubilized and removed using surfactants. After removing most of the adsorbed layer, a combination of Raman spectroscopy and transmission electron microscopy was employed to follow the transformation dynamics of coronene molecules inside nanotubes.

Metallic salts formed from fullerenes became popular because of their superconducting properties with a relatively high transition temperature, and were initially regarded as conventional metals and superconductors. Recently, owing to improved synthetic methods and a renewed interest in the study of their physical properties, many of them were found to exhibit exotic metallic and superconducting phases. In this paper, we summarize earlier results on unconventional metallic fulleride phases as well as the newly discovered expanded fulleride superconductors. The proximity of the Mott transition, a typical solid-state effect, results in molecular crystals, where molecular spectroscopic methods prove very successful. We concentrate on infrared and optical spectroscopy which is very well suited to follow metallicity and phase transitions in this class of substances.

Haromfajta szen nanoszerkezettel foglalkoztunk: fullerenek es fulleridsok, szen nanocsovek, valamint szen nanocsovek es szerves molekulak alkotta hibrid rendszerek. Az első csoportba tartozo anyagokat hőmerseklet- es nyomasfuggő rezgesi spektroszkopiaval vizsgaltuk, es kimutattuk a molekularis jellemzők es a kristalyok makroszkopikus tulajdonsagai kozti osszefuggeseket. A mikroszkopikus effektusok a molekulak kozti polimerizacios reakcio kovetese (C70), molekularis Jahn-Teller torzulas (tetrafenil-foszfonium-C60), valamint Mott-lokalizacio altal okozott elektronkorrelacio es Jahn-Teller effektus kombinacioja (Cs3C60). Szen nanocsovek szeles savu optikai spektrumabol az elektronszerkezetre, nagy erzekenyseggel mert infravoros spektrumukbol a csovek rezgesi modusaira kovetkeztettunk. Mivel kulonboző atmerőjű csovekből allo mintasorozatokat vizsgaltunk, meg tudtuk allapitani ezeknek a mennyisegeknek a csovek geometriajatol valo fuggeset. Szen nanocső-szerves molekula hibridek kotesi allapotat infravoros es optikai spektroszkopia segitsegevel hataroztuk meg. Eredmenyeinket nemzetkozi folyoiratokban publikaltuk, ezek kozul legfontosabbak ket Physical Review B, ket Journal of Physical Chemistry C, egy Journal of Applied Physics, egy Journal of Physical Chemistry Letters, egy Nature Communications folyoiratcikk, tovabba het meghivott előadas nemzetkozi konferenciakon. | We investigated three types of carbon nanostructures: fullerenes and fulleride salts, carbon nanotubes and hybrid systems composed of carbon nanotubes and organic molecules. Materials belonging to the first group were studied by temperature- and pressure-dependent vibrational spectroscopy, and determined the relationship between molecular features and macroscopic crystal properties. Such microscopic effects are molecular polimerization reactions (C70), molecular Jahn-Teller distortion (tetraphenylphosphonium-C60), and the combination of electron correlations due to Mott localization and the Jahn-Teller effect (Cs3C60). From the wide-range optical spectra of carbon nanotubes we could determine the electronic structure, and from measuring their infrared spectra with high sensitivity, their vibrational modes. Having studied series of samples with different tube diameter, the dependence of these quantities on the geometry of the tubes could be established. The bonding configuration in carbon nanotube-organic molecule hybrids was determined by infrared and optical spectroscopy as well. Our results were published in international journals, of which the most important are four papers in Physical Review B, two in Journal of Physical Chemistry, one in Science and one in Nature Materials, in addition to seven invited lectures at international conferences.

Cs3C60 is a correlated superconductor under pressure, but an insulator under ambient conditions. The mechanism causing this insulating behavior is the combination of Mott localization and the dynamic Jahn-Teller effect. We show evidence from infrared spectroscopy for the dynamic Jahn-Teller distortion. The continuous change with temperature of the splitting of infrared lines is typical Jahn-Teller behavior, reflecting the change in population of solid-state conformers. We conclude that the electronic and magnetic solid-state properties of the insulating state are controlled by molecular phenomena. We estimate the time scale of the dynamic JahnTeller effect to be above 10(-11) s and the energy difference between the conformers less than 20 cm(-1).

Coronene was encapsulated in single-walled carbon nanotubes (SWNT) by vapor-phase filling at high (450 degrees C) and low (385 degrees C) temperature and by nanoextraction from supercritical carbon dioxide. The presence of coronene inside the tubes was demonstrated indirectly via the formation of double-walled nanotubes (DWNT). To this end several subsequent annealing steps were applied and monitored by Raman spectroscopy. Our results show that the encapsulation is successful with all three methods. However, high-temperature vapor filling produces adsorbed dicoronylene, the dimerized form of coronene, as a side reaction. In order to avoid dicoronylene contamination, we suggest to use low-temperature methods for the production of coronene-filled carbon nanotubes. Coronene (top) and dicoronylene (bottom) molecules.

The 'expanded fulleride' Cs(3)C(60) is an antiferromagnetic insulator in its normal state and becomes a molecular superconductor with T(c) as high as 38 K under pressure. There is mounting evidence that superconductivity is not of the conventional BCS type and electron-electron interactions are essential for its explanation. Here we present evidence for the dynamic Jahn-Teller effect as the source of the dramatic change in electronic structure occurring during the transition from the metallic to the localized state. We apply infrared spectroscopy, which can detect subtle changes in the shape of the C(60)3- ion due to the Jahn-Teller distortion. The temperature dependence of the spectra in the insulating phase can be explained by the gradual transformation from two temperature-dependent solid-state conformers to a single one, typical and unique for Jahn-Teller systems. These results unequivocally establish the relevance of the dynamic Jahn-Teller effect to overcoming Hund's rule and forming a low-spin state, leading to a magnetic Mott-Jahn-Teller insulator.

High-pressure infrared transmission measurements up to 9.5?GPa were carried out on the rotorstator molecular cocrystal C60 center dot C8H8. Helium served as pressure transmitting medium, which intercalates into the C60 center dot C8H8 lattice. Thus, we investigated the pressure effects and effect of intercalation of helium into the C60 center dot C8H8 lattice. The pressure-induced shift of the vibrational modes of C60 center dot C8H8 shows an anomaly around 3?GPa. This anomaly can be interpreted in terms of the orientational ordering transition of fullerene molecules accompanied by a change in the crystal symmetry, which causes the splitting of the vibrational modes. We compare the value of the critical pressure to that obtained earlier [Thirunavukkuarasu et al., J. Phys. Chem. C 112, 17525 (2008); Phys. Status Solidi B 244, 3857 (2007)]. (C) 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

We revisit the infrared (IR) spectra of C-70 on pure samples as a function of temperature (20-370 K) and pressure (0-10 GPa). Although the rotation of the molecule is not completely free at ambient conditions, the measured spectra are in perfect agreement with previous density functional theory (DFT) calculations on isolated molecules, including the intensities, for 22 out of the possible 31 IR-active modes. We assign the anomalies in the infrared spectra both on decreasing temperature (similar to 280 K) and increasing pressure (similar to 0.8 GPa) to the freezing out of uniaxial rotation; the boundary between uniaxial and quasi-free rotation has a much weaker effect. At similar to 7.5 GPa an irreversible change of the spectrum occurs, which we attribute to room-temperature dimerization. The pressure at which this dimerization occurs adds a new point to the phase diagram presented by Sundqvist.

C-60 and C-70 form with cubane (C8H8) molecules rotor-stator phases in which the fullerenes are rotating and the cubanes are static. Heating the rotor-stator phase up to 470 K a fulleride copolymer, poly (C60C8H8), is obtained. Here, we study using microwave conductivity measurements, electron-spin resonance, and superconducting quantum interference device magnetometry the effect of potassium intercalation on the electric and magnetic properties of poly (C60C8H8). The resulting K2C60C8H8 copolymer is metallic for T > 200 K in contrast to the insulating pristine poly (C60C8H8) copolymer. At lower temperatures, a disorder-driven Anderson localization of the electrons takes place and the ground state is a magnetic insulator.

C-60 salts represent perfect model systems for the Jahn-Teller effect, in particular for the interplay between the molecular dynamics and the distorting crystal field. In this paper, after a brief introduction to the theoretical background, we review experimental results on salts with fulleride anions containing different charge states in the solid state. Mid-infrared (MIR) and near infrared (NIR) spectroscopic measurements and their conclusions are reported in detail, while the results obtained by nuclear magnetic resonance (NMR), electron spin resonance (ESR) and X-ray diffraction are briefly summarized. The following questions are addressed: Are fulleride ions distorted in various solids? Is the distortion dominated by the molecular Jahn-Teller effect or by the potential field of the environment? What is the shape of the distortion? Is the distortion static or dynamic, is there a pseudorotation, are there transitions between static and dynamic JT states? How do these effects manifest themselves in vibrational and electronic excitations? The experimental difficulties one has to face when studying Jahn-Teller distortions in solids are also discussed. These limitations originate not only in the performance of the spectroscopic methods used, but also in the chemistry of some of the compounds, which can lead to segregation and polymerization.

A palyazat kereten belul kifejlesztettuk a molekulakristalyok ujszerű csaladjat, amelyet szokatlan dinamikai sajatossagai alapjan rotor-sztator fazisoknak neveztunk. A nagy szimmetriaju kristalyok forgo fulleren es allo kuban molekulak alternalo halozatabol epulnek fel. Szerkezetuket a komplementer molekularis feluletek tokeletes illeszkedese stabilizalja. A fullereneket molekularis csapagy modjara korulvevő kubanok szokatlanul alacsony hőmersekletekig konnyed forgast biztositanak. Magas hőmersekleten a kristalyok egyfazisu topokemiai reakcio reven nagy hőstabilitasu kopolimerekke alakulnak. Az atalakulas utan az eredeti kristalyok kulső jegyei megmaradnak. Kulonfele cikloaddicios oligomereket izolaltunk C60 fotopolimerekből. Topokemiai modell segitsegevel ertelmeztuk a polimerizacio folyamatat es a fotopolimer szerkezetet. Tanulmanyoztuk fullerid sok es polimerek fizikai tulajdonsagait. Uj nanocső szarmazekokat allitottunk elő reduktiv funkcionalizalassal. | In the framework of the project we have developed a novel family of molecular crystals that we called rotor-stator phases, on the basis of their unusual dynamics. The high-symmetry crystals consist of alternating arrays of rotating fullerene and static cubane molecules. The structures are stabilized by the perfect match of the complementary molecular surfaces. The cubanes, surrounding the fullerenes like a molecular bearing, make possible a smooth rotation down to unusually low temperatures. At elevated temperatures the crystals transform to high-thermal-stability copolymers via single-phase topochemical reactions. The transformed materials preserve the macroscopic crystalline characteristics of the parent phases. We have isolated various cycloadduct oligomers from the photopolymers of C60. We explained the polymerization and the structure of the photopolymer in terms of a topochemical model. We have studied the physical properties of fulleride salts and polymers. We have prepared new derivatives of nanotubes via reductive functionalization.

Tobb modszerrel vizsgaltuk az alkali fulleridsok Jahn-Teller torzulasat es kimutattuk, hogy az A4C60 alkali fulleridekben es a Na2C60 magas hőmersekletu fazisaban a torzulas szimmetriaja D3d/D5d, ez folyamatos pszeudorotacio soran, dinamikusan valosul meg. Hasonloan bizonyitottuk be, hogy a Na2C60 szobahőmersekleten szegregalodik nanomeretű femes Na3C60 es szigetelő C60 tartomanyokra. Fullerenekből es kubanbol allo rotor-sztator rendszerekben spektroszkopiai modszerekkel kimutattuk a fullerenmolekulak forgasat szobahőmersekleten, es a rezgesi spektrumok szimmetria-analiziseből kovetkeztettunk a C60-C8H8 polimer szerkezetere. Szen nanocsovekből allo atlatszo, onhordo vekonyretegek szeles savu optikai spektroszkopiai vizsgalataval meghataroztuk azok optikai allandoit es ateresztesi tartomanyat. Fotovezetes es optikai abszorpcio egyuttes merese alapjan arra kovetkeztettunk, hogy a fotovalasz bolometrikus. Magneses terben orientalt nanocsőmintak optikai reflexiojaban jelentős anizotropiat talaltunk. Az oldalfalon funkcionalizalt nanocsovek spektruma alapjan megallapitottuk, hogy a kisebb atmerőjű csovek reaktivitasa nagyobb. Eredmenyeinket nemzetkozi folyoiratokban publikaltuk, ezek kozul legfontosabbak negy Physical Review B, ket Journal of Physical Chemistry, egy Science es egy Nature Materials folyoiratcikk, tovabba ot meghivott előadas nemzetkozi konferenciakon. | We investigated the Jahn-Teller distortion in alkali fulleride salts by several methods and showed that the symmetry of the distortion is D3d/D5d in both the alkali fullerides A4C60 (A=K,Rb,Cs) and in the high-temperature phase of Na2C60, and it is realized during continous pseudorotation in a dynamic way. Likewise, we proved that Na2C60 is segregated at room temperature into nanometer size regions of metallic Na3C60 and insulating C60. In rotor-stator systems of fullerenes and cubane, we found spectroscopic proof of the rotation of the fullerene molecules at room temperature, and deducted the structure of the C60-C8H8 polymer from the symmetry analysis of the vibrational spectra. We determined the wide-range optical spectrum of transparent self-supporting nanotube films and calculated the optical constants and transmission windows of several types of nanotubes. From the measurement of photoconductivity and optical absorption on the same nanotube film we concluded that the photoresponse of the tubes is bolometric. We found optical anisotropy in magnetically oriented nanotubes. From the spectra of sidewall-functionalized nanotubes we found the smaller diameter ones to be more reactive. Our results were published in international journals, of which the most important are four papers in Physical Review B, two in Journal of Physical Chemistry, one in Science and one in Nature Materials, in addition to five invited lectures at international conferences.