Temperature behaviour of optical properties of Si+-implanted SiO2

Silicon nanocrystals were prepared by Si⁺-ion implantation and subsequent annealing of SiO₂ films thermally grown on a c-Si wafer. Different implantation energies (20-150 keV) and doses - cm ^-2 ) were used in order to achieve flat implantation profiles (through the thickness of about 100 nm) with a peak concentration of Si atoms of 5, 7, 10 and 15 atomic%. The presence of Si nanocrystals was verified by transmission electron microscopy. The samples exhibit strong visible/IR photoluminescence (PL) with decay time of the order of tens of μs at room temperature. The changes of PL in the range 70-300 K can be well explained by the exciton singlet-triplet splitting model. We show that all PL characteristics (efficiency, dynamics, temperature dependence, excitation spectra) of our Si⁺-implanted SiO₂ films bear close resemblance to those of a light-emitting porous Si and therefore we suppose similar PL origin in both materials. Received 1st September 1998 and Received in final form 7 September 1999     

Local field dynamics in a resonant quantum tunneling system of magnetic molecules

We observed non-exponential relaxation for a quantum tunneling molecular magnetic system at very low temperatures and argue that it results from evolving intermolecular dipole fields. At the very beginning of the relaxation, the magnetization follows a square-root time dependence. A simple model is developed for the intermediate time range that is in good agreement with the data over 4 decades in time. Detailed numerical calculations as well as measurements are presented which indicate unusual correlation effects in these systems. Received: 15 May 1998 / Revised: 10 July 1998 / Accepted: 11 July 1998     

Low frequency elastic properties of neutron-irradiated quartz. Comparison with glasses

Neutron-irradiated quartz is a promising model system to learn more about the low-energy excitations (tunneling states) in vitreous silica and in similar glasses. We present the first systematic study of the elastic properties of neutron-irradiated quartz at low frequencies and very low temperatures. Using the vibrating reed technique at frequencies of several kHz we have measured the sound velocity and internal friction of six quartz crystals irradiated with different neutron doses over a wide range of temperatures (7 mK-300 K). The results are analyzed using the tunneling model and several extensions of this theory. Comparisons are made with recent low-frequency measurements on normal and compacted vitreous silica and with ultrasound experiments on neutron-irradiated quartz. Received 23 October 1998     

N-doping and coalescence of carbon nanotubes: synthesis and electronic properties

Self-assembly pyrolytic routes to large arrays (<2.5 cm²) of aligned CN_x nanotubes (15–80 nm OD and <100 μm in length) are presented. The method involves the thermolysis of ferrocene/melamine mixtures (5:95) at 900–1000 °C in the presence of Ar. Electron energy loss spectroscopy (EELS) reveals that the N content varies from 2–10%, and can be bonded to C in two different fashions (double-bonded and triple-bonded nitrogen). The electronic densities of states (DOS) of these CN_x nanotubes, using scanning tunneling spectroscopy (STS), are presented. The doped nanotubes exhibit strong features in the conduction band close to the Fermi level (0.18 eV). Using tight-binding and ab initio calculations, we confirm that pyridine-like (double-bonded) N is responsible for introducing donor states close to the Fermi Level. These electron-rich structures are the first example of n-type nanotubes. Finally, it will be shown that moderate electron irradiation at 700–800 °C is capable of coalescing single-walled nanotubes (SWNTs). The process has also been studied using tight-binding molecular dynamics (TBMD). Vacancies induce the coalescence via a zipper-like mechanism, which has also been observed experimentally. These vacancies trigger the organization of atoms on the tube lattices within adjacent tubes. These results pave the way to the fabrication of nanotube heterojunctions, robust composites, contacts, nanocircuits and strong 3D composites using N-doped tubes as well as SWNTs. Received: 10 October 2001 / Accepted: 3 December 2001 / Published online: 4 March 2002     

Cavitation in superfluid helium-4 at low temperature

We have studied the nucleation of bubbles in pure superfluid helium-4 at temperatures down to 65 mK. We have found that the nucleation is a stochastic process, and that at temperatures below 600 mK the nucleation rate is independent of temperature. These results are consistent with the assumption that the nucleation takes place via quantum tunneling. Received: 15 November 1997 / Received in final form: 19 December 1997 / Accepted: 22 January 1998     

Magnetic properties of small Yttrium clusters

The magnetic properties of small Y_N clusters are studied by using a tight-binding Hubbard Hamiltonian in the unrestricted Hartree-Fock approximation. Several types of cluster geometries are considered in order to see the effects of the size and symmetry of the structures on the magnetic properties. The average magnetic moments are found to be constant over large domains of variations in the interatomic distance, a fact that can be explained by the existing closed shell electronic configurations at least for one spin direction in all our magnetic solutions. Small energy gains upon the onset of magnetization are obtained, which reveals the low stability of the magnetic solutions. Our results contradict the prediction of a magnetic-nonmagnetic transition at a large cluster size (about 90 atoms) for these kinds of systems. Received: 27 April 1998 / Received in final form: 23 June 1998 / Accepted: 17 July 1998     

Electrical transport measurements on single-walled carbon nanotubes

We review transport measurements on single-walled carbon nanotubes contacted by metal electrodes. At room temperature some devices show transistor action similar to that of p-channel field effect transistors, while others behave as gate-voltage independent wires. At low temperatures transport is usually dominated by Coulomb blockade. In this regime the quantum eigenstates of the finite-length tubes can be studied. At higher temperatures power law behaviour is observed for the temperature and bias dependence of the conductance. This is consistent with tunneling into a one-dimensional Luttinger liquid in a nanotube. We also discuss recent developments in contacting nanotubes which should soon allow study of their intrinsic transport properties. Received: 17 May 1999 / Accepted 18 May 1999 / Published online: 4 August 1999     

Coulomb blockade and coherent single-cooper-pair tunneling in single Josephson junctions

We have measured the current-voltage characteristics of small-capacitance single Josephson junctions at low temperatures ( T< or =0.04 K), where the strength of the coupling between the single junction and the electromagnetic environment was controlled with one-dimensional arrays of dc SQUIDs. We have clearly observed Coulomb blockade of Cooper-pair tunneling and even a region of negative differential resistance, when the zero-bias resistance of the SQUID arrays is much higher than the quantum resistance h/e(2) approximately 26 kOmega. The negative differential resistance is evidence of coherent single-Cooper-pair tunneling in the single Josephson junction.     

Local environment and electron correlation effects on the magnetic properties of clusters

The electronic and magnetic properties of clusters are investigated in the framework of the Hubbard model by treating electron correlations effects in a saddle-point slave-boson approximation. The size dependent single-particle spectrum is calculated using a third moment real-space expansion of the local density of states. Results for the magnetic moments, magnetic order, average number of double occupations and hopping renormalizations are given as a function of the local coordination number z, for different representative values of the Coulomb interaction strength U/t and band filling n. Several transitions between paramagnetic, ferromagnetic and antiferromagnetic behaviors are obtained as a function of z. The environment dependence of the magnetic behavior and of the degree of electron delocalization is analyzed. Advantages and limitations of the present approach are discussed. Received: 8 January 1998 / Revised: 22 June 1998 / Accepted: 6 August 1998     

Possible reaction coordinates in the metastable states of sodiumnitroprusside Na2[Fe(CN)5NO]2H2O: a discussion based on neutron diffraction- and spectroscopic measurements

The metastable states of sodiumnitroprusside are extremely stable at temperatures below 200 K. It is possible to allocate structural changes measured by neutron diffraction to measured spectroscopic parameters, but the amount of the structural change is relatively small for a reaction co-ordinate as the metastable states have an extremely long lifetime. New hypotheses for related systems try to explain such a phenomena in two ways: The first way is a bending of the NO-bond in the metastable state, the second one an exchange of the oxygen and nitrogen atoms in the NO-bond (which can be regarded as an bending). As such changes would be possible also from our density functional calculations, we re-investigated our neutron diffraction data using the new models. However, our results are not compatible with one of these models. On the contrary, the neutron diffraction data show partially opposite tendencies. We compare both models with EXAFS measurements, with vibrational spectroscopic results and the data found by M?ssbauer spectroscopy. We propose a potential scheme for all three states (GS, and ) extracted from absorption and thermodynamic data to explain the electronic and energetic rearrangement, and the population dynamics. Received: 23 June 1997 / Accepted: 13 October 1997     

Thermodynamic properties of small amorphous and crystalline Silica particles at low temperatures

We have measured the low-temperature (K) specific heat and heat release of small amorphous and crystalline SiO₂ particles embedded in Teflon and of Vycor. The temperature and time dependence of these properties have been interpreted in terms of the tunneling model. We found that the particle size influences the density of states of tunneling systems of the composite. The smaller the size of the particles the larger is the density of states of tunneling systems P₀. Quartz grains with dimensions in the micrometer range show similar glass-like properties as vitreous silica. In comparison with bulk vitreous silica, Vycor shows a much larger P₀ in agreement with the behavior we found for small SiO₂ particles. We discuss the implication of our results on the origin of the universal low-temperature properties of glasses. Received 9 April 1998     

Structure and magnetic properties of Co nanoclusters fabricated by ion beam synthesis in SiO2 films

Co nanoparticles fabricated by ion beam synthesis in SiO₂ films were investigated with transmission electron microscopy and superconducting quantum interference device technique. Variation of the thermal treatment enables the formation of Co nanoclusters of different sizes ranging from 2 to 40 nm. Small nanoclusters of about 2–3 nm are amorphous, whereas clusters above 7 nm show the configuration of cubic Co nanocrystals. Measurements of magnetisation at temperatures between 2 K and 360 K reveal superparamagnetic behaviour for the small nanoclusters up to 3 nm and ferromagnetism for clusters above 7 nm. Received: 12 February 2001 / Accepted: 3 May 2001 / Published online: 27 June 2001     

Study of the symmetry of single-wall nanotubes by electron diffraction

Determining precisely the atomic structure of single-wall carbon nanotubes is essential since it tailors electronic properties of this new carbon material. Here, we present a quantitative electron diffraction study of electric-arc produced single-wall carbon nanotube bundles, combined with simulations based on the kinematic theory and with real-space images. We stress the importance of the twist of the bundle in the interpretation of our data and we analyze both packing lattice parameters and chirality distribution. We show that, within a given bundle, no chirality is favoured whereas SWNT diameters are almost uniform. Received 5 February 1999     

Clusters on surfaces: high-resolution spectroscopy at low temperatures

With controlled growth in nanometer-sized pits we produced silver and gold clusters on a graphite surface. We give a summary of the preparation method and discuss the scanning tunneling imaging and the crystalline orientation of the clusters. The electronic structure of the clusters was studied by an in-situ combination of ultraviolet photoelectron spectroscopy (UPS) and scanning tunneling spectroscopy (STS). For both techniques we obtained an energy resolution in the range of 10 meV employing low sample temperatures. Dynamic final-state effects together with averaging over a cluster-size distribution result in characteristic spectral shapes in UPS, which can be understood referring to STS data taken on individual clusters. Finally, directions for future experiments are pointed out. Received: 13 April 2000 / Accepted: 6 November 2000 / Published online: 9 February 2001     

Electronic correlation effects and the Coulomb gap at finite temperature

We have investigated the effect of the long-range Coulomb interaction on the one-particle excitation spectrum of n-type germanium, using tunneling spectroscopy on mechanically controllable break junctions. At low temperatures, the tunnel conductance shows a minimum at zero bias voltage due to the Coulomb gap. Above 1 K, the gap is filled by thermal excitations. This behavior is reflected in the variable-range hopping resistivity measured on the same samples: up to a few degrees Kelvin the Efros-Shklovskii lnR infinity T(-1/2) law is obeyed, whereas at higher temperatures deviations from this law occur. The type of crossover differs from that considered previously in the literature.     

Orbital degrees of freedom and electron correlation in optical absorption of metallic LaSrMnO

The optical absorption in ferromagnetic metal La_1-xSr_xMnO₃ is anomalous; it has a wide-range absorption up to about 1 eV even at zero temperature. Since 3d electrons in La_1-xSr_xMnO₃ partially fill doubly degenerate e_g orbitals, the orbital degrees of freedom are crucial to understand this metallic system. We argue that the interband transition within e_g orbitals is important in the optical absorption. The optical spectrum is modified also by the inter-orbital Coulomb interaction. We have examined perturbatively the effect of the Coulomb interaction on the spectrum. Available experiments are discussed by comparing with the present results. Received: 13 February 1998 / Accepted: 17 March 1998     

Magnetic tunneling told to ignorants by two ignorants

Magnetic tunneling of a large spin subject to the Hamiltonian is investigated by elementary methods for weak fields H. In zero field (H=0) the tunnel frequency in the ground state is found to be equal to [0pt] multiplied by a quantity whose variation with s is slower than exponential. This result coincides with that of path integral methods [#!schi!#]. For the values of the longitudinal field which allow tunneling, the tunnel frequency is shown to vanish when H _y =0 for certain “diabolic" values of [0pt] , in qualitative agreement with experiments by Wernsdorfer and Sessoli. The particular case H _z =0 was already obtained by Garg by means of path integrals. The diabolic values of are in agreement with numerical results but, as already noticed by Wernsdorfer and Sessoli, they disagree with the experimental ones. This may be an effect of higher order anisotropy terms, which is briefly addressed in the conclusion. Received 17 May 2000     

Electronic properties of Gd@C<Subscript>82</Subscript> metallofullerene peapods: (Gd@C<Subscript>82</Subscript>)<Subscript>n</Subscript>@SWNTs

Electronic properties of Gd@C₈₂ metallofullerene peapods, (Gd@C₈₂)_n@SWNTs, were investigated by electron energy-loss spectroscopy (EELS), scanning tunneling microscopy and spectroscopy (STM/STS), and field-effect transistor (FET) transport measurements. The results indicate that the electronic structure of Gd@C₈₂ metallofullerene peapods is completely different from that of intact single-walled nanotubes (SWNTs). For example, Gd@C₈₂-peapod-FETs show ambipolar behavior which is not observed in the empty SWNT-FETs under our experimental conditions. Furthermore, in semiconducting nanotubes the band gap can be varied from ∼0.5 to ∼0.1 eV using inserted Gd@C₈₂ endohedral metallofullerenes with a spatial periodicity of 1.1 to 8.0 nm, depending on the density of the fullerenes. The present findings suggest that metallofullerene peapods may point the way toward novel electronic devices. Received: 6 September 2002 / Accepted: 25 October 2002 / Published online: 10 March 2003 RID="*" ID="*"Corresponding author. Fax: +81-52/789-1169, E-mail: noris@cc.nagoya-u.ac.jp     

Tight-binding studies of surface effects on electronic structure of CdSe nanocrystals: the role of organic ligands, surface reconstruction, and inorganic capping shells

We utilize a tight-binding model to study the effects of surface structure on electronic properties of CdSe clusters. The model takes into account experimental information about structure and shape of the nanocrystals, as well as the nature and distribution of capping ligands. The effects of both organic capping ligands and inorganic capping shells on the densities of states (DOS) and on the single-particle absorption spectra of the clusters are calculated for various cluster shapes and sizes, and are compared to results for clusters with truncated surfaces. For organic capping ligands, the effect of ligand hybridization is investigated and a simple model of surface reconstruction is developed. Both ligand hybridization and surface reconstruction are seen to have a major influence on the band edge electronic and optical properties. Inorganic capping shells give rise to differential localization of valence and conduction band edge states, with the hole primarily confined to the core region and the electron more evenly distributed over both core and shell. Received 21 September 1998 / Received in final form: 15 December 1998     

Dipolar interaction and incoherent quantum tunneling: a Monte Carlo study of magnetic relaxation

We study the magnetic relaxation of a system of localized spins interacting through weak dipole interactions, at a temperature large with respect to the ordering temperature but low with respect to the crystal field level splitting. The relaxation results from quantum spin tunneling but is only allowed on sites where the dipole field is very small. At low times, the magnetization decrease is proportional to as predicted by Prokofiev and Stamp, and at long times the relaxation can be described as an extension of a relaxed zone. The results can be directly compared with very recent experimental data on Fe₈ molecular clusters. Received 9 February 1999