Phase transitions sequence in pyrochlore Cd2Nb2O7 studied by IR reflectivity

The infrared reflectivity of Cd₂Nb₂O₇ single crystal was studied in the temperature interval of 10-540 K, together with complementary dielectric measurements. A ferroelectric soft mode was revealed above the ferroelectric phase transition at T _c = 196 K coupled with a central-mode type dispersion in the near-millimetre range. This proves the mixed displacive and order-disorder nature of the transition. Below T _c many new modes were detected due to lowering of the symmetry, especially below the previously suggested incommensurate transition at 85 K. Discussion of the possible phase transitions based on symmetry considerations is presented with the conclusion that the ferroelectric transition is proper with the F_1u symmetry of the order parameter, whereas the intermediate ferroelastic transition is improper and triggered by the coupling with the ferroelectric order parameter. Received 17 July 2000     

Carbon nanotube bundles and thin layers probed by micro-Raman spectroscopy

Raman spectra of single-wall carbon nanotubes (CNTs) either in the form of micrometer sized bundles or thin layers prepared by dilution and sonication of powders have been compared. We have been able to collect the Raman spectrum of nanotube bundles that are not in touch with the substrate, and therefore not affected by interactions with the substrate surface. This spectrum resulted to be similar to that of the precursor nanotube powders, whereas relevant changes in the Raman spectrum are detected when the diluted powders form very thin layers on either metallic or insulating surfaces, as probed by confocal microraman imaging on well defined areas of the CNTs layers. In the case of thin layers, the intensity of the Raman D band, detected between 1 320 and 1 340 cm^-1 and ascribed to disorder effects, is strongly enhanced. This enhancement occurs independently on the kind of substrate. Received 2 September 2002 Published online 4 February 2003 RID="a" ID="a"e-mail: sangalet@dmf.bs.unicatt.it     

The origin of disorder in CH 3 HgX (X = Cl, Br and I) crystals investigated by temperature dependent Raman spectroscopy

Methyl-mercury(II) halides CH₃HgX (X = Cl, Br and I) were studied by means of temperature dependent Raman spectroscopy from 10 K to 410 K. In addition to the previously reported soft phonons, new changes in the low frequency spectra were observed at T ≈ 70 K in CH₃HgBr and at T ≈ 100 K in CH₃HgI. The bandwidths of the two internal modes in CH₃HgBr, the CH₃ symmetric stretching band and the C-Hg-Br bending band, rise towards a local maximum at T ≈ 50 K as the temperature is raised from 10 K to 300 K. On the other hand the bandwidths of the two corresponding modes in CH₃HgI crystals monotonously increase with temperature, obeying an Arrhenius law. Besides the three phonon modes present in the Raman spectra of CH₃HgCl at room temperature, the fourth phonon band that has been observed at temperatures below 245 K might correspond to the freezing of methyl librations. The huge bandwidth of the C-Hg-Br bending mode could suggest the presence of additional weak bonding of a mercury atom with bromine atoms from other molecules, thus inducing positional disorder. Received 19 November 1999 and Received in final form 10 November 2000     

Optical investigation of the quasi two-dimensional monophosphate tungsten bronzes K x P 4 W 8 O 32 (x = 0 - 1.57)

The potassium doped monophosphate tungsten bronzes K_xP₄W₈O₃₂ are two-dimensional metals which show a metal-to-metal transition at a critical temperature which depends on the doping level. The metal-to-metal transition is accompanied by the formation of a commensurate charge density wave with wave vector (π/b,0) which is independent of the doping level. Undoped P₄W₈O₃₂, on the other hand, has two metal-to-metal transitions which are connected to the formation of incommensurate charge density waves. We measured the infrared reflectivity of the series K_xP₄W₈O₃₂ (x = 0 - 1.57) in the spectral range from 100 to 10 000 cm^-1 for room temperature and well below the critical temperature. Polarization-dependent infrared spectra find a two-dimensional behavior in the normal and the charge density wave state and show signatures of hybridization between one- and two-dimensional conduction bands. In undoped P₄W₈O₃₂ the essentials of the charge density wave state can be understood from the nesting vectors of the calculated Fermi surface and two gaps are observed in the infrared spectra. The gap sizes are a factor of about 2.5 bigger than the predictions from mean-field theory in the weak-coupling limit which suggests medium- or strong electron-phonon coupling. For potassium doped K_xP₄W₈O₃₂ one gap is observed in the charge density wave state. The energetics of the charge density formation may be dominated by the energy required for the lattice modulation. Received 27 April 2001 and Received in final form 21 September 2001     

Importance of lattice contraction in surface plasmon resonance shift for free and embedded silver particles

The size evolution of the surface plasmon resonance was investigated for free and embedded silver particles between about 2 to 10 nm in size. The crystal lattice of such particles as analyzed by high resolution electron microscopy show linear contraction with reciprocal particle size. Based on this, a model was presented by combining the lattice contraction of particles and the free path effect of electrons to predict the size evolution of the resonance. The results reveal a contribution of the lattice contraction to the resonance shift according to a roughly linear relation that changes slightly with particle radius (> 1.0 nm) and surrounding media. This surface plasmon resonance shift proceeds linearly with reciprocal size for Ag particles in vacuum and argon, but for Ag particles embedded in glass it appears to be independent of the radius down to nearly 1 nm. All predictions are quantitatively compared to previously reported experimental data and a good agreement is obtained. An unusual red-shift observed for Ag particles in glass may be attributed to a thermal expansion mismatch induced lattice dilatation. Received 26 July 2000 and Received in final form 14 September 2000     

Raman cross section of spin ladders

We demonstrate that a two-triplet resonance strongly renormalizes the Raman spectrum of two-leg spin-ladders and moreover suggest this to be the origin of the asymmetry of the magnetic Raman continuum observed in CaV₂O₅. Received 25 April 2001 and Received in final form 9 May 2001     

A detailed Raman study on thin single-wall carbon nanotubes prepared by the HiPCO process

The Raman spectrum of single wall carbon nanotubes (SWNTs) prepared by high pressure CO decomposition (HiPCO process) has been recorded at nine excitation laser energies ranging from 1.83 eV to 2.71 eV. The characteristic nanotubes features: G band, D band and radial breathing mode (RBM) have been analyzed and compared to those of an arc discharge SWNT material of similar diameter. A strong Breit-Wigner-Fano type (metallic) contribution to the G band was found in the spectra measured with green lasers, while spectra measured with red lasers indicate resonances of semiconducting SWNTs. Analysis of the energy dependence of the position of the D band revealed sinusoid oscillations superimposed on a linear trend. The validity of full DOS calculations for HiPCO materials has been confirmed by a match found between the estimated spectral contribution of metallic SWNTs as calculated from the components of the measured G band and as predicted by the (n, m) indexes of the major scatterers of DOS simulations. The RBM region of the HiPCO spectrum is more complex than usually observed for SWNTs. The analysis performed with a Gaussian distribution and improved fitting parameters leads to a mean diameter and variance of 1.05 nm and 0.15 nm, respectively. A bimodal Gaussian distribution had little influence on the error sum but reduced the standard error slightly. The major spectral features of the RBM could be interpreted using available resonance Raman theory. Received 5 February 2002 / Received in final form 3 April 2002 Published online 19 July 2002     

Critical exponents in a percolation picture of the fluorescence quenching during the sol-gel transition

Herewith we report on the measurements of the time evolution of the fluorescence yield and the Rayleigh scattering, performed during the gelation process in the following solutions: gelatin in water, gelatin in a heavy water and agarose in water. Our results provide the experimental evidence of the universal power law, resulting from the percolation theory, which expresses the dependence of the fluorescence yield on the number of intermolecular bonds created during the sol-gel transition. The values of universal critical exponents present in this law are found to have the same values for all investigated materials. Received 4 May 2000     

Dichroism in angular resolved VUV-photoemission from the (0001) surfaces of thin Gd and Nd films epitaxially grown on W(110)

We present investigations of the electronic and magnetic structure of the Rare Earth valence states. In particular, we have examined ultra thin films of the rare earth metals gadolinium and neodymium epitaxially grown on tungsten (110). Various experiments on dichroism in angular resolved photoemission have been performed using circularly as well as linearly polarised light in the VUV-range with photon energies below 40 eV. A special emphasis was placed on the investigation of the surface state, which was observed for both Gd and Nd. A very small magnetic splitting of about 25 meV was observed for the surface state of ferromagnetic Gd. A magnetic ordering of a Nd-monolayer on a remanently magnetised Fe-film is observed. Large dichroism effects are found for the surface state as well as the valence bands of paramagnetic Nd. In the latter case, these are used to determine the dispersion of the valence bands. Different numerical approaches are presented, one based on atomic photoionisation theory, another is based on a one-step model of solid state photoemission. Atomic photoionisation theory is used together with three-step calculations to explain the non-magnetic circular dichroism observed in the Gd 4f emission. The capability of dichroism experiments for resolving details of the electronic structure and for sensitive tests of photoemission calculations is demonstrated. Received 21 September 1998     

Low temperature dependence of the penetration depth in YBCO thin films revisited by mm wave transmission and surface impedance measurements

We report results obtained with two different experimental set-ups in state-of-the-art YBCO thin films as similar as possible, prepared by pulsed laser deposition on LaAlO₃ substrates: a surface impedance measurement on 4000 ? thick films using a parallel plate resonator (10 GHz), and a far infrared transmission (100-400 GHz) measurement which requires thinner (1000 ?) samples. The former measurement yields the temperature variation of the penetration depth λ(T) and the real part of the conductivity, provided the absolute value of λ(T) is known. The latter yields the imaginary part of the conductivity, hence the absolute value of the penetration depth, as well as its temperature dependence at the measuring frequency. Combining these two experiments, we establish a quasi-linear temperature variation of λ(T), with a 2 ? K^-1 low temperature slope, and a fairly large zero temperature value λ(T = 0)=(1800±200) ? . The scattering rate of the quasi-particles calculated from a two-fluids model shows that the films compare to good quality single crystals, where twice a larger slope has been found. This surprising behavior is described in detail, including an in-depth structural analysis of the samples in order to evaluate their similarities. We find that the 10 GHz data obtained in the thickest films can be fitted to the dirty d-wave mode in the unitarity limit, with an extrapolated slope of 3 ? K^-1, but yield a scattering rate that is difficult to reconcile with the high T _c (92 K) of the films. Received 7 May 2001 and Received in final form 18 October 2001     

Effect of interfacial alloying on the surface plasmon resonance of nanocrystalline Au-Ag multilayer thin films

Nanocrystalline Au and Ag in multilayer thin film form with Au/Ag/Au structure were prepared by high pressure (∼40 Pa) d.c. sputtering techniques. The Ag concentrations in Ag_xAu_1-x films were changed from x = 0 to 1. These multilayer films with varying Ag concentration showed significant changes in microstructures obtained from TEM and XRD analyses. The optical absorption spectra of these multilayer films showed a single plasmon band confirming the formation of Au-Ag alloy. We ascribe this alloying to the interfacial reactions in nanophase limited at the Au-Ag interface. The red-shift and broadening of the plasmon bands with the increase in silver concentration could be associated to the increase in size of the nanoparticles and its distribution. The observed red shift in the plasmon band may be associated with the change in electronic structure at the Au-Ag interface due to configuration mixing of the atomic energy levels of Au and Ag. Received 17 October 2002 / Received in final form 26 February 2003 Published online 23 May 2003 RID="a" ID="a"e-mail: msakp@mahendra.iacs.res.in     

Studies on CdS nanoparticles dispersed in silica matrix prepared by sol-gel technique

SiO₂/CdS-nanoparticle composite films (SiO₂:CdS=85:15, 80:20, 75:25 and 70:30) were prepared by the sol-gel route. The films were characterized by studying microstructural (XRD and TEM) and optical (transmittance and photoluminescence) properties. Band gaps of these films annealed at different temperatures (373-473 K) for different times (10-120 min) indicated that the signature of nanocrystallinity is retained throughout the range of our experimental conditions. A thermal diffusion process controlled growth in the crystallite size with increasing annealing time and temperature. The average radii of the nanoparticles varied as the cube root of the annealing time but showed exponential dependence on the inverse of annealing temperature. Photoluminescence (PL) studies of the composite films indicated excitonic transitions. Theoretical analysis of the line shapes of the PL peaks recorded at 300 K and 80 K could be accounted for by the combined effects of size distribution and phonon broadening. It was observed that the deformation potential (E _d) effectively controlled the line shapes of the PL measurements. Received 24 May 2002 Published online 27 January 2003 RID="a" ID="a"e-mail: msakp@mahendra.iacs.res.in     

Structural phase transition in the 2D spin dimer compound SrCu 2 ( BO 3 ) 2

A displacive, 2nd order structural phase transition at T _s = 395 K from space group I 2 m below T _s to I 4/m c m above T _s has been discovered in the two-dimensional spin dimer compound SrCu₂(BO₃)₂. The temperature evolution of the structure in both phases has been studied by X-ray diffraction and Raman scattering, supplemented by differential scanning calorimetry and SQUID magnetometry. The implications of this transition and of the observed phonon anomalies in Raman scattering for spin-phonon and interlayer coupling in this quantum spin system will be discussed. Received 24 July 2000 and Received in final form 2 November 2000     

Photoinduced IR absorption in WO 3 : determination of the polaron binding energy

Photoinduced IR absorption measurements are reported on WO₃. A photoinduced midinfrared small polaron peak centered at 4800 cm^-1 (0.59 eV) was observed. The data were analyzed in the framework of the photon-assisted small-polaron hopping theory and briefly compared to previously published infrared absorption measurements in WO₃ and photoinduced IR absorption measurements in high T _c cuprates. Received 20 April 2001 and Received in final form 13 July 2001     

Determination of SWCNT diameters from the Raman response of the radial breathing mode

We report on the evaluation of the distribution of diameters for nanotube samples with a wide variation of mean diameters. Such results were obtained from a detailed analysis of the radial breathing mode Raman response and compared to results obtained from an evaluation of optical spectra and X-ray diffraction pattern. The evaluation of the Raman data needs a well refined analysis as the experimental analysis exhibits a rather complicated and oscillating relation between response and exciting laser. Both, an exact calculation where the density of states was considered explicitly and an approximate calculation were applied. Both models used for the analysis are able to explain several unexpected results from the experiment such as the oscillating behavior of the spectral moments, unusual discontinuities in the first moments of the Raman response for excitation in the IR, a fine structure for the response in optics and Raman, and an up shift of the RBM frequency as compared to qualified ab initio calculations. In detail the first moment and the variance of the spectra were used for the evaluation of the diameter distribution. To obtain good results between experimental and theoretical oscillation pattern the transition energy between the first two van Hove singularities had to be scaled up which is considered as a result from coulomb interaction of the electrons in the tubular material. On the other hand the analysis does not only allow to determine the mean value and the width of the diameter distribution but yields also a value for the average bundle diameters or, alternatively, the strength of the tube-tube interaction. The model used for the analysis of the Raman data is also appropriate to analyze the optical response, at least for the spectral range from 0.5 eV to 3.5 eV. The fine structure in the response for the transitions between the three lowest van Hove singularities is well reproduced and the mean tube diameters and their distribution is obtained in very good agreement with the results from the Raman analysis. From the X-ray analysis the same mean values and comparable distributions for the tube diameters were received whereas the bundle diameters could not be retained with high precision in this case. Received 18 February 2001 and Received in final form 3 April 2001     

Integrated quantum key distribution system using single sideband detection

We report a new quantum cryptographic system involving single sideband detection and allowing an implementation of the BB84 protocol. The transmitted bits are reliably coded by the phase of a high frequency modulating signal. The principle of operation is described in terms of both classical and quantum optics. The method has been demonstrated experimentally at 1 550 nm using compact and conventional device technology. Single photon interference has been obtained with a fringe visibility greater than 98%, indicating that the system can be used in view of quantum key distribution potentially beyond 50-km-long standard single-mode fiber. Received 13 July 2001 and Received in final form 30 November 2001     

Melting of the cooperative Jahn-Teller distortion in LaMnO 3 single crystal studied by Raman spectroscopy

We have studied the behavior of the Raman phonons of a stoichiometric LaMnO₃ single crystal as a function of temperature in the range between 77 K and 900 K. We focus on the three main phonon peaks of the Pbnm structure, related to the tilt, antisymmetric stretching (Jahn-Teller mode) and stretching modes of Mn-O octahedra. The phonon frequencies show a strong softening that can be fit taking into account their renormalization because of three phonon anharmonic effects in the pseudoharmonic approximation. Thermal expansion effects, in particular the variation of Mn-O bond lengths with temperature, are not relevant above 300 K. On the contrary, phonon width behavior deviates from the three phonon scattering processes well bellow T _c . The correlation between the magnitude of the cooperative Jahn-Teller distortion, that disappears at 800 K, and the amplitude of the Raman phonons in the orthorhombic phase is shown. Nevertheless, Pbnm phonons are still observable above this temperature. Phonon width and intensity behavior around T _c can be explained by local melting of the orbital order that begins quite below T _c and by fluctuations of the regular Mn-O octahedra that correspond to dynamic Jahn-Teller distortions. Received 25 January 2001 and Received in final form 14 March 2001     

Ferroelectricity and structure of BaTiO3 grown on YBa 2Cu3O_{7 - \delta } thin films

We have investigated the crystal structure and the ferroelectric properties of BaTiO₃ thin films with YBa₂Cu₃O as the bottom and Au as the top electrode. Epitaxial heterostructures of YBa₂Cu₃O and BaTiO₃ were prepared by dc and rf sputtering, respectively. The crystal structure of the films was characterised by X-ray diffraction. The ferroelectric behaviour of the BaTiO₃ films was confirmed by hysteresis loop measurements using a Sawyer Tower circuit. We obtain a coercive field of 30 kV/cm and a remanent polarisation of 1.25 μC/cm². At sub-switching fields the capacitance of the films obeys a relation analogous to the Rayleigh law. This behaviour indicates an interaction of domain walls with randomly distributed pinning centres. At a field of 5 MV/m we calculate a 3% contribution of the irreversible domain wall motion to the total dielectric constant. Received 24 June 1999 and Received in final form 27 August 1999     

Optical and electrical properties of hydrogenated yttrium nanoparticles

We studied the effect of hydrogen in yttrium nanoparticles on a quartz substrate, using optical spectroscopy and electrical resistance measurements. Pulsed laser deposition is used to obtain the Y clusters in an UHV environment. We show, that these clusters are highly sensitive to monoatomic H₁ produced from ambient hydrogen gas pressures, ranging from 10^-5 to 50 mbar with our experimental arrangement. The changes of optical and electrical properties due to the chemical reaction within the particles are sufficient to consider this material as a possible sensor for low concentrations of hydrogen. Received 29 November 2000     

Pure and Sb-doped SnO 2 nanoparticles studied by photoelectron spectroscopy

We describe photoemission results from pure and Sb-doped SnO₂ nanoparticles deposited on gold substrates. Photoelectron spectra with synchrotron radiation were recorded for Sn 3d, Sb 3d and O 1s core levels and valence bands in the 500-1200 eV energy range. For pure SnO₂ nanoparticles the surface is terminated by an oxygen rich layer with no obvious surface environment for Sn. When doped n-type with 9.1% or 16.7% Sb, dopant atoms are concentrated near the surface of the nanoparticles. The valence state of the dopant atoms is predominantly Sb^V. Plasmon satellite features are also observed in core level photoemission spectra and their intensity relative to the main peak increases with increasing photon energy. Received 30 November 2000