Optical study of RE 1 + x Ba 2 - x Cu 3 O 6 (RE = Nd, Sm) aaand YBa 2 Cu 3 O 6 in the mid infrared range

Infrared reflectance, and transmission measurements as well as Raman scattering have been used to study the RE_1+xBa_2-xCu₃O₆ (RE = Nd, Sm) and YBa₂Cu₃O₆ absorption bands in the 1100-1500 cm^-1 infrared range as a function of temperature and beam polarization. In addition to two-phonon absorption between 1100 and 1170 cm^-1, we observe excitations around 1400 cm^-1, occurring in oxygen rich enclosures within the samples, and assign them to an excitation involving two-phonons plus the 270 cm^-1 local mode related to Cu-O broken chains. Thus, the previously reported possible magnetic origin of the 1436 cm^-1 sharp absorption band in YBa₂Cu₃O₆ is contested. Received 14 February 2001 and Received in final form 12 April 2001     

Magnetic contributions to the low-temperature specific heat of the ferromagnetic insulator Pr 0.8 Ca 0.2 MnO 3

The Pr _1-x Ca_xMnO₃ system exhibits a ferromagnetic insulating state for the composition range x ? 0.25. A metallic ferromagnetic state is never realized because of the low hole concentration and the very small averaged A-site cation radius. In the present study, the nature of the magnetic excitations at low temperature has been investigated by specific heat measurements on a Pr _0.8 Ca_0.2MnO₃ single crystal. The decrease of the specific heat under magnetic field is qualitatively consistent with a suppression of ferromagnetic spin waves in a magnetic field. However, at low temperature, the qualitative agreement with the ferromagnetic spin waves picture is poor. It appears that the large reduction of the specific heat due to the spin waves is compensated by a Schottky-like contribution possibly arising from a Zeeman splitting of the ground state multiplet of the Pr³⁺ ions. Received 21 May 2001 and Received in final form 14 December 2001     

Hopping conductivity and activated transport in InxGa1-xAs quantum wells

We present measurements of the diagonal R_xx and off-diagonal R_xy magnetoresistance under quantum Hall conditions on several high electron mobility transistors (HEMT) based on In_xGa_1-xAs quantum wells. From the magnetoresistance tensor we obtain the longitudinal conductivity σ _xx . We study the transport mechanisms near the σ _xx minima at temperatures ranging between 2 K and 35 K; activated transport is the dominant mechanism for temperatures above 7 K while variable range hopping conductivity is significant for lower temperatures. We show that electron-electron correlations should be taken into account to explain the conductivity vs temperature behaviour below 5 K. Finally, we study the behaviour of the localization length as a function of Landau level filling and obtain a critical exponent γ = 3.45±0.15. Received 6 June 2001 and Received in final form 16 October 2001     

Oscillator strength of the strongly “forbidden” Pb 6 p 2 3 P 0 ?6 p 2 3 P 1 transition at 1278.9 nm

The “forbidden” Pb 6p^{2 3}P ₀ ↦6 p ^{2 3}P₁ line at 1278.9 nm is measured by diode laser absorption in a resistively heated hot-pipe filled with a lead vapour and argon as buffer gas. The measurements performed at a temperature of 1170 K and a lead number density of 2.4×10 ¹⁵ cm^-3 yield the oscillator strength f _F = (4.5±1.1)×10 ^-7 which corresponds to a radiative transition probability A _F = (6.1±1.5) s^-1. Within the error bars, the result is in agreement with theoretical data published by different authors. Received 11 October 2000 and Received in final form 5 January 2001     

Inversion analysis of K 2 coupled electronic states with the Fourier grid method

We present a new determination of the potential curves and interactions of the coupled electronic states A ¹ Σ ⁺ _u and b ³ Π _u of the potassium dimer, based on new laser spectroscopy measurements within a molecular beam, combined with data available in the literature. We used a new global deperturbation approach, involving the Fourier Grid Hamiltonian method for energy level calculation. A standard deviation of 1.2 is obtained corresponding to a variance of 7.5×10 ^-3 cm^-1, representing a significant improvement compared to the standard deviation of 4 yielded by the traditional local deperturbation approach. Received 12 June 2001 and Received in final form 3 September 2001     

Some experimental and theoretical studies on the Mn magnetic moment in Y ( Ni 1 - x Mn x ) 2 B 2 C

The magnetic moment of the Mn impurities was obtained from magnetization measurements of Y ( Ni _{1 - x} Mn _x ) ₂ B ₂ C as a function of the concentration x less than 0.15. Using the coherent potential approximation and starting from 3 d density of states, obtained from the first principles calculations, the magnetic moments are obtained within a two sublattice model. For adequately estimated values of the Coulomb interactions U, the position of the energy level of Mn and adopting values for the intersublattice hybridization term, a qualitative agreement with the observed experimental data is obtained. Received 23 April 2001 and Received in final form 29 October 2001     

Magnetic phase transition and magnetic structure of ground state in CsDyW O

Magnetic phase transition in the CsDyW₂O₈ magnet has been studied by means of low temperature specific heat C ( T ) measurements. The magnetic ordering temperature of the Dy³⁺ sublattice was established to be 1.34 K. The experimental results indicate on the antiferromagnetic character of interactions between Dy³⁺ ions. The behavior of the C ( T ) dependencies above and below T _N is discussed in frames of different theoretical models. The measurements data on temperature and field dependencies of magnetization are used to calculate the exchange and dipole-dipole interactions energy and to determine the possible magnetic structure of the ground state. Received 7 January 2002 / Received in final form 15 May 2002 Published online 7 September 2002     

Magnetic resonance on LiCuVO 4

EPR and ⁷Li NMR measurements were performed in the distorted inverse spinel V(LiCu)O₄ down to 1.5 K. Anisotropy effects on magnetic resonance spectra due to the Jahn-Teller distortion of the oxygen octahedra surrounding the copper ions are discussed. The estimation of the spin-spin interactions deduced from the EPR-relaxation rate Δ H reveals a situation comparable to the prototypical one-dimensional S = 1/2 Heisenberg antiferromagnet CuGeO₃. Approaching three-dimensional antiferromagnetic order ( T _N ≈ 2 K) from above, both magnetic relaxation rates, Δ H _EPR and ⁷ (1/ T ₁ ), respectively, exhibit nearly the same critical divergence reminding to the onset of three-dimensional order in two-dimensional layered systems. Received 22 January 2001 and Received in final form 6 May 2001     

Hole doping by Li substitution and antiferromagnetism in YBa Cu O studied by neutron powder diffraction measurements

The magnetic structure of tetragonal insulating YBa₂Cu_3-xLi_xO_y has been studied as a function of x and y. The Néel temperature and the mean ordered magnetic moment on the Cu2 sites were determined by neutron powder diffraction measurements. The decrease of these two parameters as compared to YBa₂Cu₃O₆ is much stronger for lithium than for zinc substitution. The difference is quantitatively explained by the presence of holes created in the CuO₂ planes. These holes arise from the substitution of plane Cu²⁺ by Li⁺. We suggest an explanation why such holes are not seen for the same substitution of plane Cu²⁺ by Li⁺ in orthorhombic superconducting YBa₂Cu_3-xLi_xO _{7 - δ} . Received 31 October 2001 and Received in final form 6 March 2002 Published online 25 June 2002     

( Sr / Ca ) 14 Cu 24 O 41 spin ladders studied by NMR under pressure

⁶³Cu-NMR measurements have been performed on two-leg hole-doped spin ladders Sr_14-xCa_xCu₂₄O₄₁ single crystals 0 ? x ? 12 at several pressures up to the pressure domain where the stabilization of a superconducting ground state can be achieved. The data reveal a marked decrease of the spin gap derived from Knight shift measurements upon Ca substitution and also under pressure and confirm the onset of low lying spin excitations around P _c as previously reported. The spin gap in Sr ₂ Ca ₁₂ Cu ₂₄ O ₄₁ is strongly reduced above 20 kbar. However, the data of an experiment performed at P = 36 kbar where superconductivity has been detected at 6.7 K by an inductive technique have shown that a significant amount of spin excitations remains gapped at 80 K when superconductivity sets in. The standard relaxation model with two and three-magnon modes explains fairly well the activated relaxation data in the intermediate temperature regime corresponding to gapped spin excitations using the spin gap data derived from Knight shift experiments. The data of Gaussian relaxation rates of heavily doped samples support the limitation of the coherence length at low temperature by the average distance between doped holes. We discuss the interplay between superconductivity and the spin gap and suggest that these new results support the exciting prospect of superconductivity induced by the interladder tunneling of preformed pairs as long as the pressure remains lower than the pressure corresponding to the maximum of the superconducting critical temperature. Received 8 March 2001 and Received in final form 27 July 2001     

Photophysics of C<Subscript>60</Subscript> and C<Subscript><Emphasis Type="Bold">60</Emphasis></Subscript><Superscript><Emphasis Type="Bold">-</Emphasis></Superscript> in faujasite zeolites

While the intercalation of C₆₀ and the formation of C ₆₀ ^- in the supercages of NaX and NaY are confirmed by using ¹²⁹Xe NMR and ESR, the photophysical properties of C₆₀ and C ₆₀ ^- are characterized by monitoring transient reflectance spectra and kinetics, fluorescence kinetics, and diffuse reflectance spectra. C ₆₀ ^- is considerably more abundant in NaY than in NaX. This difference is explained in terms of polarity difference between two zeolites. Both C₆₀ and C ₆₀ ^- have remarkably elongated excited-state lifetimes due to their collision-free environment in zeolitic nanocavities although C ₆₀ ^- has much shorter lifetimes than C₆₀. C ₆₀ ^- , in particular, shows intense absorption and emission due to its reduced symmetry in zeolites. Received 13 July 2001 and Received in final form 8 October 2001     

High accuracy Dirac-finite-element (FEM) calculations for H 2 + and Th 2 179 +

A two-dimensional, fully numerical approach to the four-component first-order Dirac-differential-equation utilizing the Finite-Element-Method (FEM) is employed for H ₂ ⁺ and Th ₂ ^{179 +} . Using elliptic-hyperbolic coordinates and further one-dimensional singular transformations, scaling transformations and extrapolation techniques (geometrical over iteration steps and logarithmic over grid points) we achieve for the molecules H ₂ ⁺ and Th ₂ ^{179 +} relative accuracies better than 10^-12 for 1(1/2) g energies. Received 16 February 2001 and Received in final form 12 July 2001     

Coexistence of superconductivity and spin density wave orderings in the organic superconductor (TMTSF) 2 PF 6

The phase diagram of the organic superconductor (TMTSF)₂PF₆has been revisited using transport measurements with an improved control of the applied pressure. We have found a 0.8 kbar wide pressure domain below the critical point (9.43 kbar, 1.2 K) for the stabilisation of the superconducting ground state featuring a coexistence regime between spin density wave (SDW) and superconductivity (SC). The inhomogeneous character of the said pressure domain is supported by the analysis of the resistivity between T _SDW and T _SC and the superconducting critical current. The onset temperature T _SC is practically constant ( 1.20±0.01 K) in this region where only the SC/SDW domain proportion below T _SC is increasing under pressure. An homogeneous superconducting state is recovered above the critical pressure with T _SC falling at increasing pressure. We propose a model comparing the free energy of a phase exhibiting a segregation between SDW and SC domains and the free energy of homogeneous phases which explains fairly well our experimental findings. Received 3 September 2001 and Received in final form 9 November 2001     

Widely usable interpolation formulae for hyperfine splittings in the 127I2 spectrum

Based on new systematic high precision measurements of hyperfine splittings in different rovibrational bands of ¹²⁷I₂ in the near infrared spectral range between 778 nm and 816 nm, and the data in the range from 660 nm to 514 nm available from literature, the quantum number dependence of the different hyperfine interaction parameters was reinvestigated. As detailed as possible parameters were re-fitted from the reported hyperfine splittings in literature, considering that the interaction parameters should vary smoothly with the vibrational and rotational quantum numbers, and follow appropriate physical models. This type of consistency has not been sufficiently taken into account by other authors. To our knowledge it is now possible for the first time to separate the hfs contributions of the two electronic states B ³ and X ¹ Σ ⁺ _g for optical transitions in a very large wavelength range. New interpolation formulae could be derived for both states, describing the quantum number dependences of the nuclear electric quadrupole, of the nuclear spin-rotation and also of the nuclear spin-spin interactions. Using these new interpolation formulae the hyperfine splittings for the components with the quantum number condition F - J = 0 can be calculated with an uncertainty of ≤30 kHz for transitions in the wavelength range between 514 nm and 820 nm. Received 17 July 2001 and Received in final form 17 October 2001     

Densification and nanocrystallisation of sol-gel ZrO 2 thin films studied by surface plasmon polariton-assisted Raman spectroscopy

Very thin ZrO ₂ films (few nanometers) have been prepared by sol-gel process. These films were deposited onto a stack of a thin silver layer evaporated on a glass substrate for Surface Plasmons Resonance (SPR) experiments. The first aim of this work is to study the high densification of the sol-gel films followed by the refractive index and thickness accurate measurements at each step of the annealing procedure, using an optical set-up based on SPR. Secondly, SPR excitation coupled with micro-Raman experiment has also been performed to determine the thin films structure depending on layer thickness. Finally, Conventional Transmission Electron Microscopy (CTEM) and High Resolution (HRTEM) studies have been conducted to check and complete Raman spectroscopy results. A discussion compares the optical results and the Transmission Electron Microscopy observations and shows that ultra thin layers structure is strongly depends on films thickness. Received 14 May 2001 and Received in final form 2 January 2002     

Ab initio calculation of excited state dipole polarizability - Application to the first 1, 3 Σ + g,u states of Li 2

Firstly, imaginary frequency-dependent dipole polarizabilities of Li in its ² S ground state and the first ² P excited state are calculated from a time-dependent gauge-invariant method using effective core pseudo-potentials and the multi-configuration one-electron wave function. C₆ dispersion coefficients of the 2 s + 2 s and 2 s + 2 p dissociations are deduced and also compared with all-electron values. Then, static polarizabilities of Li₂ in its ground and first excited ^{1, 3} Σ ⁺ _g,u states are obtained at interatomic distance corresponding to the energy minimum of each state. Received 10 May 2001 and Received in final form 14 September 2001     

Broad-band dielectric spectroscopy of Ba 2NaNb 5O 15 single crystal

Barium sodium niobate (BNN) single crystals are studied by IR spectroscopy, time-domain THz transmission spectroscopy, HF coaxial wave-guide technique and LF dielectric spectroscopy to cover the frequency range 10²-10¹⁴ Hz in a wide temperature interval. The dielectric response parallel and perpendicular to the polar c-axis is discussed. The ferroelectric transition at T _c = 830 K is driven by a relaxational soft mode coupled with another central-mode type relaxation which both gradually disappear on cooling in the ferroelectric phase. Below T _i the parameters of the expected IR active amplitudon were estimated. The low-temperature permittivity increase on cooling for the field direction has been explained by an incipient proper ferroelectric-ferroelastic transition driven by an IR and Raman active B₂-symmetry soft mode. Received 24 August 2002 Published online 19 December 2002 RID="a" ID="a"e-mail: buixader@fzu.cz     

Laser-induced fluorescence measurements of a 3 Π metastable carbon monoxide in a high enthalpy flow

Excitation of carbon monoxide molecules has been carried out in a cold cell and in a low-pressure plasma jet using an ArF narrow-band excimer laser. The different excitation models are discussed and the relevance of atomic carbon absorption into the laser cavities is pointed out. Excitation spectra of Cameron bands have been obtained in a room-temperature cell and compared with calculated spectra. A value of the constant σ related to the interaction strength between a ³ Π( v = 2) state and its neighbouring singlet states is derived: 0≤σ≤0.05. The fluorescence spectrum following broad band excitation of CO has been observed both in UV and visible. Similar experiments carried out in a high enthalpy flow have allowed to point out the presence of a ³ Π metastable carbon monoxide. A method for relative measurements of this species concentration is proposed. Received 9 April 2001 and Received in final form 13 June 2001     

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     

A multispectrum fitting procedure to deduce molecular line parameters: Application to the 3-0 band of 12 C 16 O

To deduce accurate infrared molecular line parameters (positions at zero pressure, pressure-shifting and pressure-broadening coefficients, collisional narrowing coefficients, and intensities) from rovibrational spectra, an automatic method based upon a multispectrum fitting procedure has been set up, able to treat simultaneously several laboratory Fourier transform spectra. A validation of this method, using absorption spectra of the 3-0 vibrational band of CO around 6 350 cm^-1, already used to measure line intensities and self-broadening coefficients, is presented, and the advantages of the method are pointed out. The self-collisional narrowing of CO was observed and determined for the first time in Fourier transform spectra: β = 0.028±0.004 cm^-1 atm^-1 at about 296 K. Received 21 September 2000 and Received in final form 15 January 2001