The interplay between the surface band structure and possible surface reconstructions of Mo(112)

The experimental band structure of Mo(112) and the effects by temperature and adsorbate are presented. A surface resonance, identified as crossing the Fermi level at about 1/3 from to of surface Brillouin zone, was observed to be very sensitive to both contamination and temperature. We find evidence of adsorbate and temperature induced reconstruction of the Mo(112) surface. Examination of low-energy electron diffraction (LEED) and scanning tunneling microscopy (STM) data provides evidence for an adsorbate induced reconstruction of the Mo(112) surface with periodicities consistent with the Fermi level crossing of the surface resonance. The reconstruction is found to occur at coverages as low as 0.03 Langmuirs of oxygen or carbon. The reconstruction and/or adsorbate affects the density of states and bands near the Fermi level of a ₁ symmetry. Received 3 March 1999 and Received in final form 1 October 1999     

Ordering phenomena in C<Subscript>60</Subscript>-tetraphenylphosphonium bromide

The fulleride salt C₆₀-tetraphenylphosphonium bromide is investigated as a function of temperature by single crystal X-ray diffuse scattering and diffraction. At room temperature, the C₆₀ orientational disorder is found to be more complex than previously expected. Moreover, a structural phase transition, due to the C₆₀ orientational ordering, is evidenced around 120 K. Its relation with the stabilization of a static Jahn-Teller effect is discussed. Received 3 November 1999     

Elastic anomalies and orientational glass transition in Na(CN) Cl mixed crystals. A Brillouin spectroscopic study

High performance Brillouin spectroscopy (BS) has been used to study the elastic properties (static and dynamic) of the orientational glassy state of Na(CN)_xCl_1-x samples ). The temperature behaviour of the elastic properties reveals a more complex scenario for the orientational glass transition than generally believed. The shear elastic constant shows the well-known c ₄₄ ( T ) anomaly, indicated by a minimum, found in other cyanide mixed crystals. The results obtained for the hypersonic attenuation are in clear contradiction with the dynamic character of the c ₄₄ ( T )-minimum. The temperature behaviour of the longitudinal elastic constant c₁₁ of very dilute Na(CN)_xCl_1-x samples shows two striking features: i) Similar to the anomalous temperature behaviour of c ₄₄ ( T ), lowering the temperature c ₁₁ ( T ) first decreases, goes through a minimum and then rises again. The minimum takes place at a temperature above the temperature, , where c ₄₄ ( T ) reaches its minimum value. ii) A kink-like anomaly of c ₁₁ ( T ) is observed at lower temperatures. This second anomaly is similar to the classical one observed in canonical glasses at their glass transition temperature . Received 8 April 1999 and Received in final form 3 June 1999     

Hypersonic wave anomalies in Cs3H(SeO4)2 at the transitions above room temperature

The elastic properties of Cs3H ( SeO4 ) ₂ are investigated by Brillouin spectroscopy in the temperature range 20-220 covering the two transitions III II and II I occurring at and , respectively. Phase I is known to be a protonic conductive one. Discontinuities of elastic constants are generally observed at both transitions, implying first orderness. In phases II and I, a slight broadening of the Brillouin lines is detected. The results are discussed in comparison with compounds of the families XHSeO4 (X = NH4, Rb and Cs) and CsH2BO4 (B = As and P) which also undergo a transition to a superionic phase. In the conductive phase, it appears that the lattice anharmonicity is weaker in Cs3H ( SeO4 ) ₂ than in these other compounds. Received 16 October 1998     

Elasticity of BaFCl single crystal under hydrostatic pressure

The sound velocities for longitudinal and transverse waves have been measured in single crystalline BaFCl at room temperature using ultrasonic pulse echo and Brillouin scattering techniques. The complete set of elastic constants is deduced and lead to the bulk moduli values of BaFCl at ambiant conditions (, , ) which are compared with those obtained by a shell model. Moreover, using the ultrasonic technique under pressure, the pressure derivatives of the second order elastic constants at 298 K have been determined up to 0.3 GPa. All moduli increase linearly with pressure in this pressure range, allowing to determine directly and separately the first derivative of the bulk modulus B'₀ = 5.8. These data are used to calculate a Murnaghan equation of state. A detailed comparison is given between our results with those recently obtained by X-ray diffraction on powder or calculated using the local density approximation method. Finally, the anisotropy of BaFCl under pressure is discussed. Received: 19 March 1998 / Revised: 15 May 1998 / Accepted: 19 May 1998     

Heterogeneous dynamics at the glass transition in van der Waals liquids, in the bulk and in thin films

It has been shown over the last few years that the dynamics close to the glass transition is strongly heterogeneous, both by measuring the diffusion coefficient of tagged particles or by NMR studies. Recent experiments have also demonstrated that the glass transition temperature of thin polymer films can be shifted as compared to the same polymer in the bulk. We propose here first a thermodynamical model for van der Waals liquids, which accounts for experimental results regarding the bulk modulus of polymer melts and the evolution of the density with temperature. This model allows us to describe the density fluctuations in such van der Waals liquids. Then, by considering the thermally induced density fluctuations in the bulk, we propose that the 3D glass transition is controlled by the percolation of small domains of slow dynamics, which allows to explain the heterogeneous dynamics close to T _g. We show then that these domains percolate at a lower temperature in the quasi-2D case of thin suspended polymer films and we calculate the corresponding glass transition temperature reduction, in quantitative agreement with experimental results of Jones and co-workers. In the case of strongly adsorbed films, we show that the strong adsorption amounts to enhance the slow domains percolation. This effect leads to 1) a broadening of the glass transition and 2) an increase of T _g in quantitative agreement with experimental results. For both strongly and weakly adsorbed films, the shift in T _g is given by a power law, the exponent being the inverse of that of the correlation length of 3D percolation. Received 21 March 2000 and Received in final form 4 December 2000     

Pressure and temperature variation of the depolarized light scattering spectra of the fragile glass-former liquid salol

The pressure and temperature dependences of the depolarized light scattering spectra of salol have been measured at isobaric condition P = 1 bar for T between 328 and 393 K, and at isothermal condition T = 343 K for P between 1 and 625 bar, i.e. outside the metastable liquid region. The experimental results for both the α- and β-relaxations can be well described by the Mode Coupling Theory. The independence of the power-law parameters a, and consequently b and γ, from the thermodynamic variables T and P is demonstrated.The critical temperature T _c at 1 bar and the critical pressure P _c at T = 343 K have been determined from the normal liquid state. Received 2 May 2000     

Photoluminescence characteristics of neodymium oxide nanocrystal/titania/ormosil composite sol-gel thin films

Neodymium (III) oxide nanocrystal/titania/organically-modified silane (ormosil) composite thin films have been prepared using a chemical approach consisting of a combination of inverse microemulsion and sol-gel techniques at low temperature. Transmission electron microscopy shows that the neodymium (III) oxide nanoparticles have a needle-like nanocrystal structure. A strong room temperature emission at 1064 nm, corresponding to the ⁴ F _3/2?⁴ I _11/2 transition, has been observed as a function of the heat treatment temperature used for the production of the composite thin films. In addition to this emission, two other main emissions at 890 nm and at 1336 nm have also been observed. In particular, there was a clear shoulder peak at 1145 nm, probably be due to the host matrix, which was observed in all the measured samples and this shoulder peak gave a maximum intensity after heat treatment at 300 °C. Received: 6 September 2000 / Accepted: 15 November 2000 / Published online: 20 June 2001     

Fluid and plastic flow dynamics of the critical state for a strongly pinned 2D superconductor

The frequency of the Raman active A_1g radial breathing mode has been widely used as a tool to estimate the distribution of diameters of single wall carbon nanotubes (SWNT). However, the relation between frequency and diameter is not straightforward and results are model-dependent. Because most of the experiments are performed on bundles and not on isolated tubes, the model should especially take into account the van der Waals intertube interactions. Here, we use a pair-potential approach to account for such interactions and we derive a nonlinear relation between the SWNT diameter and the frequency of the A_1g radial breathing modes. We demonstrate a good agreement between calculations and the diameters derived from diffraction experiments on the same samples. Received 22 December 1999 and Received in final form 17 July 2000     

Preparation of Cu 2 O nanoparticles dispersed in NH 2 -terminated polyethyleneoxide matrix

Small Copper (I) oxide, Cu₂O, nanoparticles dispersed in diamine-terminated polyethyleneoxide (PEO-NH₂) matrix have been successfully prepared by vacuum evaporation of copper onto the molten PEO-NH₂. The obtained composite were characterized by TEM, electron diffraction, TG-DTA and FT-IR spectroscopy. The stable composite, in which the Cu₂O nanoparticles are stabilized through interaction between NH₂ chain end groups of PEO molecules and Cu₂O nanoparticles was obtained when the samples were heat-treated at 110 ^° C. The mean size of the Cu₂O nanoparticles increased from 2.5 to 3.5 nm in diameter upon increasing the amount of initial Cu deposition. The obtained composite material having a waxy texture was soluble in many solvents without aggregation and can be handled as a simple chemical compound for starting material in various applications. Received 29 November 2000     

Nanosize patterns as reference structures for macroscopic transport properties and vortex phases in YBCO films

This paper reports on the striking correlation between nanosize mosaic domain walls in YBCO films and 1D rows of parallel Josephson junctions, determining the J _c vs.B curves. From X-ray data analysis, it results that the average “hidden" domain wall, faceted at a nanometric scale, is almost mimicking the Josephson Junction (JJ) 1D array. The assumption that the JJs and the domain-wall arrays are coincident, enables to find out the particular scaling field, making the J _c vs.B curves independent of temperature. This scaling field can be interpreted in terms of the Josephson nature of the transport current across these particular patterns in the intermediate temperature range. By means of our model it is also possible to calculate two asymptotic behaviors of the pinning force as a function of field, for low and high fields, respectively. These behaviors are punctually repeated by the experimental results in the same asymptotic limit, so that two corresponding vortex regimes are clearly pointed out. All results can be interpreted by concluding that in the intermediate temperature range, the strong pinning observed in high quality YBCO films is due to the Josephson Junctions average patterns. These patterns are the counterpart related to the transport mechanisms of “hidden" structural nano-domains. Received 16 October 2000 and Received in final form 28 November 2000     

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     

Antiferromagnetic resonance and high magnetic field properties of NaNiO2

Magnetisation measurements up to 23 T and submillimeter wave ESR in the frequency region 48-380 GHz have been performed on NaNiO₂ powders at low temperature. Also typical spectra above the Néel temperature are shown. At 4 K the magnetisation shows a spin-flop transition at 1.8 T and saturation at 10 T. /Ni confirms the low spin state of the Ni³⁺ ions. The susceptibility exhibits a maximum at K with and K. NaNiO₂ is an A-type antiferromagnet: we derive K and K for the interactions between Ni ions within and between adjacent layers, respectively. The AFMR spectra yield an energy gap of 52.5 GHz, in agreement with the spin-flop value derived from the magnetisation. The anisotropy of the g factor observed at 100 K with can be attributed to the Jahn-Teller effect for Ni³⁺ ions in the low spin state, which stabilises the occupation. We finally comment on the isomorphic controversial Li_1-xNi_1+xO₂ compound. Received 9 March 2000 and Received in final form 13 July 2000.     

Unusually flat hole dispersion relation in the two-dimensional Hubbard model and restoration of coherence by addition of pair-hopping processes

The dispersion relation of a doped hole in the half-filled 2D Hubbard model is shown to follow a law around the and points in the Brillouin zone. Upon addition of pair-hopping processes this dispersion relation is unstable towards a law. The above follows from T=0 Quantum Monte-Carlo calculations of the single particle spectral function on lattices. We discuss finite dopings and argue that the added term restores coherence to charge dynamics and drives the system towards a d _{x2 - y2} superconductor. Received 22 March 1999     

Heat transport through a quantum dot with one-dimensional interacting leads under Coulomb blockade regime

The peculiarities of a low temperature heat transfer through a ballistic quantum dot (a double potential barrier) with interacting leads due to a long-range Coulomb interaction (in the geometrical capacitance approach) are considered. It is found that the thermal conductance K shows periodic peaks as a function of the electrostatic potential of a dot at low temperatures. At the peak maximum it is whereas near the minimum it is . Near the peak maximum the dependence K(T) is essentially nonmonotonic at the temperatures correspondent to the level spacing in the quantum dot. Received 20 October 1999 and Received in final form 20 January 2000     

Evidence for the strain critical point in high Tc superconductors

We report experimental evidence for the phase diagram of doped cuprate superconductors as a function of the micro-strain of the planar Cu-O bond length, measured by Cu K-edge EXAFS, and hole doping . The local lattice distortions are measured by EXAFS and the charge ordering is measured by synchrotron radiation diffuse X-ray diffraction. This phase diagram shows a QCP at P() where for charge-orbital-spin stripes and free carriers co-exist. The superconducting phase occurs in the region of critical fluctuations around this QCP. The function of two variables shows its maximum at the strain QCP. The critical fluctuations near this strain QCP give the self-organization of a metallic superlattice of quantum wires “superstripes" that favors the amplification of the critical temperature. Received 25 September 2000     

Non-partial-wave Coulomb-Born theory for the excitation of many-electron atomic ions II: Numerical description and application

A non-partial-wave Coulomb-Born theory is recently formulated to treat the excitation of many-electron atomic ions for impact by an arbitrary charged particle [Y.B. Duan et al., Phys. Rev. A 56, 2431 (1997)]. The multiple expansion of the transition matrix element is decomposed into the target form factor and the projectile form factor. These are the matrix elements of the tensor operators between quantum states so that any complicated wave function for the target ion can be employed. In this formal theory, an infinitesimally small positive quantity is introduced artificially to guarantee the convergence of integrals. As a supplementary part of the theory, we discuss how to choose the value of . It is found that the should be taken as functions of the momentum transfer and multipolarity . Illustrations are carried out by calculating the cross-sections for some typical transitions n _a l _a -n _b l _b of hydrogen-like ions for impact by electron, positron, and proton, respectively. The resulting cross-sections are in good agreement with ones produced by using a method available for ion targets with Slater-type orbitals [N.C. Deb, N.C. Sil, Phys. Rev. A 28, 2806 (1993)]. Comparisons demonstrate that the Coulomb-Born theory with non-partial wave analysis provides a powerful method to treat the excitation of many-electron atomic ions impact by an arbitrary charged particle. Received 6 April 1999     

Metal clusters in metal/C thin film nanosystems

Structure of metal clusters and of the C₆₀ matrix in Au/C₆₀ and Cu/C₆₀ nanosystems was investigated by X-ray diffraction. Results support a charge-transfer-type interaction at the metal-C₆₀ interface, which affects the size distribution of metal clusters, favouring interstitial location of metal ions in the fullerite lattice. Received 5 February 1999 and Received in final form 7 July 1999     

Doping-dependent electronic structure of cuprates studied using angle-scanned photoemission

Full k -maps of the electronic structure near the Fermi level of differently doped cuprates measured with angle-scanned photoelectron spectroscopy are presented. The valence band maximum of the antiferromagnetic insulator Sr₂CuO₂Cl₂, which is taken as a representative of an undoped cuprate, and the Fermi surfaces of overdoped, optimally doped and underdoped Bi₂Sr₂CaCu₂O_8+δ high-temperature superconductors are mapped in the normal state. The results confirm the existence of large Luttinger Fermi surfaces at high doping with a Fermi surface volume proportional to (1+x), where x is the hole concentration. At very low doping, however, we find that this assumption based on Luttinger's theorem is not fulfilled. This implies a change in the topology of the Fermi surface. Furthermore the intensity of the shadow bands observed on the Fermi surface of Bi₂Sr₂CaCu₂O_8+δ as a function of the doping is discussed. Received 12 October 1999 and Received in final form 12 April 2000