Characterization of the Fermi surface of the organic superconductor - by measurements of Shubnikov-de Haas and angle-dependent magnetoresistance oscillations and by electronic band-structure calculations

The electronic structure of the quasi two-dimensional (2D) organic superconductor -(ET)₂SF₅CH₂CF₂SO₃ was examined by measuring Shubnikov-de Haas (SdH) and angle-dependent magnetoresistance (AMRO) oscillations and by comparing with electronic band-structure calculations. The SdH oscillation frequencies follow the angular dependence expected for a 2D Fermi surface (FS), and the observed fundamental frequency shows that the 2D FS is 5% of the first Brillouin zone in size. The AMRO data indicate that the shape of the 2D FS is significantly non-circular. The calculated electronic structure has a 2D FS in general agreement with experiment. From the temperature and angular dependence of the SdH amplitude, the cyclotron and band effective masses were estimated to be and ,where g is the conduction electron g factor and the free electron mass. The band effective mass is estimated to be from the calculated electronic band structure. Received: 3 March 1997 / Revised: 5 May 1997 / Received in final form: 5 November 1997 / Accepted: 10 November 1997     

Observation of magnetic splitting in XPS MnL-spectra of Co2MnSn and Pd2MnSn Heusler alloys

The XPS MnL-spectra of Co₂MnSn, a nearly half-metallic ferromagnet (HMF) and Pd₂MnSn were investigated. The most drastical feature of the spectra observed is the well-defined magnetic splitting of the Mn 2p _3/2 , 2p _1/2 lines. This gives direct evidence of the existence of well-defined local magnetic moments in Heusler alloys in comparison with other itinerant-electron ferromagnets. The calculations of Mn2p XPS spectra of these materials were carried out using a fully relativistic generalization of the one-step model of photoemission and show excellent agreement with experiment. Received: 31 October 1997 / Received in final form: 28 November 1997 / Accepted: 18 December 1997     

Laser ablation time-of-flight mass spectrometric probing of the surface states of SiO2-based porous materials

2 -based materials of different porosity and surface states have been studied by time-of-flight mass spectrometry (TOF-MS) under 308 nm excimer laser ablation. Sequences of SiO₂-based negative ion clusters [(SiO₂)_nX]^- are observed and variations of their abundance distributions with sample properties are studied. The unique roles of surface states in cluster formation are discussed. Received: 10 January 1997/Accepted: 23 June 1997     

Generation of subpicosecond infrared pulses tunable between 5.2 μm and 18 μm at a repetition rate of 76 MHz

2 (2 mm thick) and GaSe (1 mm thick) crystals are applied as nonlinear media. GaSe has a larger tuning range and is more efficient in the whole spectral range than AgGaS₂. The average IR power reaches up to 2.0 mW at 8.5 μm when the GaSe crystal is used, and up to 1.3 mW at the same wavelength when AgGaS₂ is applied for the frequency conversion. Received: 17 February 1997/Revised version: 14 July 1997     

Optical transient bleaching/absorption of surface-oxidized CuInS2 nanocrystals

2 nanocrystals in a polymer film have been studied with the picosecond pump-probe technique. The oxidized CuInS₂ nanocrystals have an additional absorption band with a peak at 1.03 eV, which is bleached under picosecond excitation. Rapid (∼50 ps) trapping into midgap surface states results in long-lived (≫300 ps) bleaching and induced absorption features. A schematical energy-level diagram for oxidized CuInS₂ nanocrystals is given based on the experimental results. Received: 7 January 1997/Revised version: 16 April 1997     

Infrared properties of etched mercuric iodide surfaces

2 samples and on solution-grown KHgI₃,H₂O and NH₄HgI₃,H₂O crystals. A detailed study of the infrared absorption lines confirms that, taking into account small variations in the HgI₂ substrates, the residues on the surface are actually the same complexes. Received: 29 September 1997/Accepted: 31 October 1997     

Impurity scattering in d-wave superconductivity. Unitarity limit versus Born limit

As is well known, Zn-substitution of Cu in the Cu-O₂ plane in the hole-doped high T_c cuprates provides a semi-quantitative test of underlying d-wave superconductivity. Here we complement this with a parallel study of Ni-substitution, which gives rise to weak scattering described with the Born approximation. Received: 7 November 1997 / Revised: 14 November 1997 / Accepted: 24 November 1997     

Preparation of thin films of the ternary heavy fermion system CeNi2Ge2

2 Ge₂ layers on W(110). In order to produce well-ordered and atomically clean surfaces of the Ce-based intermetallic system the growth was performed under UHV conditions (p<2×10^-11 mbar). Both the polycrystalline CeNi₂Ge₂ compound and the individual elements Ce, Ni, and Ge were used as evaporants. The characterisation of the layers was made with LEED, SEM, and XPS. We find a significant influence of the substrate temperature and the evaporation power on the growth characteristics. The compound material CeNi₂Ge₂ exhibits complicated behaviour when evaporated. Under carefully selected growth conditions we obtain well-ordered films with a stoichiometry of Ce:Ni:Ge=1:2:2 and a (001) oriented surface of the body-centered tetragonal ThCr₂Si₂-type structure. The k_∥ dispersion and binding energies of the valence bands of these layers were determined with ARUPS. Received: 26 October 1997/Accepted: 27 October 1997     

Ferroelectric SrxBa1-xNb2O6 optical waveguiding thin films on SiO2-coated Si(100) substrates

x Ba_1-xNb₂O₆ (x=0.5) films (abbreviated as SBN:0.5) on SiO₂-coated Si substrates are potential components for the application of integrated electro-optics devices. SBN:0.5 optical waveguiding thin films on SiO₂-coated Si substrates with a very thin MgO diffusion buffer have been successfully prepared by pulsed laser deposition. The as-grown films have a refractive index of 2.28, which is close to that of bulk SBN. X-ray analysis showed that the as-grown films have a single-phase tetragonal tungsten bronze structure. The SBN:0.5 thin films prepared by PLD exhibit favorable ferroelectric and optical waveguiding properties. The composition and the morphology of the films were also examined by XPS and by SEM, respectively. Ferroelectric SBN:0.5 optical waveguiding thin films on SiO₂-coated Si substrates are expected to be used in integrated electro-optic devices. Received: 27 February 1997/Accepted: 17 October 1997     

Comparison of scanning probe microscopies with RBS and SEM/EDX for the analysis of RuO2, TiO2 composites

2 , TiO₂) mixed oxide thin films. A digital processing of the images leads to a bipolar distribution of values of current that reflects the difference in conductivity of metallic RuO₂ and semiconducting TiO₂. The analytical results are compared with those from Rutherford backscattering spectrometry and energy dispersive X-ray imaging on these samples. STM, with digital processing of the current mapping image, shows promises for developing into a chemically sensitive probe of composite materials with nanometer spatial resolution. Received: 29 April 1997/Accepted: 4 July 1997     

Gas contaminant effect in a discharge-excited ArF excimer laser

4 , O₂ and N₂ in a discharge-excited ArF-excimer laser. Measured characteristics include laser pulse energy, small-signal gain, and laser spectrum. Measurement results indicate that laser pulse energy degradation for such impurities (<100 ppm) is mainly due to optical absorption. It has also been found that for O₂ contaminants laser pulse energy degradation is strongly dependent on operational repetition rates; at higher repetition rates, an increased concentration of O₂ impurities results in a reduced small-signal gain and a consequent decline in laser pulse energy. Received: 12 June 1997/Revised version: 3 December 1997     

Micelle-smectic phase coexistence: Origin of the maximum swelling of a mixed lamellar phase

By use of a mean-field approach the spin-wave dispersion of the Cu degrees of freedom in the undoped high-T _C material Nd₂CuO₄is investigated. The experimentally observed sharp decrease of the Cu spin-wave gap with increasing temperature in the range is explained by a paramagnetic-like susceptibility of the Nd spins which couple to the Cu subsystem. The degeneracy of the “in-plane” and “out-of-plane” polarized Cu spin-wave branches is shown to be lifted by the uniaxial anisotropy of the Cu-Cu nearest-neighbor interaction. Received: 13 August 1997 / Revised: 4 December 1997 / Accepted: 11 December 1997     

An all-solid-state tunable 214.5-nm continuous-wave light source by using two-stage frequency doubling of a diode laser

3 and β-BaB₂O₄ crystals. The power of the generated 214.5-nm light amounts to more than 100 μW. This light source will be used for laser cooling of Cd⁺ ions. Received: 4 August 1997/Revised version: 28 October 1997     

Anomalies of attenuation and phase velocity of surface acoustic waves in YBa2Cu3O7‒δ thin films in the vicinity of Tc

We present measurements of the attenuation and phase velocity of surface acoustic waves in thin YBa₂Cu₃O films as a function of temperature, in magnetic fields up to 3.6 T applied parallel to the c-axis of the films. We have observed anomalies in both, the attenuation and the phase velocity in the vicinity of the superconducting critical temperature which do not depend on the magnetic field. Possible origins of these anomalies, observed, to our knowledge, for the first time in YBa₂Cu₃O thin films, are discussed and compared to bulk acoustic wave experiments. We present a kind of feedback technique for surface acoustic waves which improves the sensitivity of this type of measurement. The actual sensitivity limits are mentioned. Received: 7 August 1997 / Revised: 7 November 1997 / Accepted: 17 November 1997     

Phase diagram for diamond growth in atmospheric oxyacetylene flames

2 /C₂H₂ ratio for diamond synthesis in oxyacetylene flames can be theoretically predicted. When the substrate temperature is between 1000 K and 1250 K, the corresponding O₂/C₂H₂ ratio range is about between 0.8 and 1.1. The suitable range of substrate temperature is widest for O₂/C₂H₂ ratio close to unity and is narrowed rapidly when the flow ratio deviates from unity. Received: 2 March 1997/Accepted: 17 July 1997     

Structural manipulation of ultrathin organic films on metal surfaces: the case of decane thiol/Au(111)

3 (CH₂)₉]SH) have been adsorbed on Au(111) single crystals both via vacuum deposition and from an ethanolic solution. The epitaxial structure of the ultrathin organic films has been identified at room temperature via low-energy electron diffraction to be c(4×2)R30° for the solution grown film and rectangular c(23×) for the vacuum deposited film. These structures correspond to molecules adsorbed on the surface with their carbon chains flat on the surface (vacuum deposited) and nearly perpendicular (solution grown). It is demonstrated that this orientation can be changed reversibly in vacuum via either annealing the films or exposing them to additional gas. Received: 7 February 1997/Accepted: 27 May 1997     

Pulsed ESR investigations of anisotropic interactions in M@C82 (M=Sc,Y,La)

82 , Y@C₈₂, and La@C₈₂ in frozen solutions. We were able to determine the g tensors of these molecules by analysing magnetic field spectra at X-band (9.5 GHz) and W-band (94 GHz) frequencies. Moreover, in Y@C₈₂ we have investigated the hyperfine interaction of the ⁸⁹Y nuclear spin (I=1/2) with the electron spin on the C₈₂ cage. The principal values of the hyperfine tensor A and the relative orientation of g and A tensors were determined by applying three- and four-pulse electron spin echo envelope modulation techniques (ESEEM). Received: 3 September 1997/Accepted: 10 November 1997     

Positron lifetime study of vacancy-type defects in amorphous and polycrystalline nanometer-sized alumina

2 O₃ and nanocrystalline Al₂O₃ specimens. The short-lifetime (170±20 ps), intermediate-lifetime (410±20 ps) and long-lifetime components correspond to three different kinds of defects: monovacancy-like free volumes, microvoids, and larger voids. The appearance of lifetimes in the range 1–5 ns indicates the formation of positronium. The influence of thermal annealing from 873 K to 1373 K on positron lifetime parameters was also analyzed. The components with lifetimes τ₁=170 ps and τ₂=410 ps persisted even after the grains had grown to 100 nm in size, while the long-lifetime component declined significantly when grain sizes exceeded 10 nm. The interface characteristics of polycrystalline nano-Al₂O₃ prepared by the two methods were compared by analyzing the variations of the positron-lifetime parameters with grain growth. Received: 1 April 1997/Accepted: 13 August 1997     

Synthesis of CdS quantum dots by mechanochemical reaction

2 + Na₂S → CdS + 2NaCl induced by mechanical milling resulted in the formation of CdS particles with an average diameter of < 8 nm. The average particle size was controlled within the range of 4 to 8 nm by varying the size of the grinding media. The onset energy of optical absorption showed a blue shift with decreasing particle size. Received: 29 August 1997/Accepted: 25 September 1997     

High-energy, fourth-harmonic generation using CO2 lasers

2 laser to 2.38 μm has been studied experimentally by using tandem AgGaSe₂ and ZnGeP₂ crystals. Up to 24 mJ per pulse of 2.38-μm radiation at a 10% overall conversion efficiency has been achieved with a mode-locked pulse format. A computer model was also developed and its predictions are in reasonable agreement with our experimental data. Received: 6 July 1997/Revised version: 8 December 1997