Electronic spectra of azanaphthalenes. Mixed crystal spectra of 1,6-naphthyridine-d6, cinnoline-d6, and quinoline-d7

The electronic spectra of 1,6-naphthyridine-d₆, cinnoline-d₆, and quinoline-d₇ have been recorded. For each molecule only one $\text{n,π}{}^1$ singlet state is identified and the spectral analyses proposed are in close correspondence to those reported for the fully protonated species. Features of particular interest are the reduced prominence of sites and the many instances of crystal induced coupling of vibronic bands.     

Effect of pressure on superconductivity in transition metal alloys

Measurements on twelve alloys of the series Zr-Nb-Mo show a close correlation ofdT _c /dp with thed-band structure of these alloys and suggest that thed-band is virtually rigid with respect to pressure. The results are not compatible with an empirical observation of McMillan thatT _c is governed only by a phonon factor Mω ².     

From a new smell to a new flavour −BdBd mixing,CP violation and new physics

Using the preliminary data on B_dB̄_d mixing from ARGUS, and the standard model with three families, we infer a lower bound on the top quark mass of 50–70 GeV; also, B_sB̄_s mixing has to be close to maximal. We discuss how the prospects for observing CP violation in B⁰ decays are enhanced and sketch alternative scenarios for new physics.     

Critical behavior of the optical absorption edge in EuO

Measurements of the temperature dependence of the optical absorption edge, E_g(T), in EuO, at the close vicinity of its critical temperature, T_c, are reported. In agreement with theoretical expectations the analysis of the results, with the constraints of continuous E_g and dE_g/dT, yielded the same critical exponents (α = α' ≈ -0.044) and amplitude ratio (|A/Á| ≈ 1.22) as the specific heat data analysis.     

Theoretical investigation of the intrinsic piezoelectric properties for tetragonal BaTiO3 epitaxial films

The orientation dependences of the converse longitudinal piezoelectric constant d_33,f, and the in-plane converse piezoelectric constant e_31,f, are calculated for tetragonal barium titanate epitaxial films. The calculations demonstrate that both e_31,f and d_33,f have their maximum values along an axis close to the (1 1 1) direction of the pseudo-cubic system, which are similar to the orientation dependence results for a tetragonal BaTiO₃ single crystal. The calculated piezoelectric constants for a (1 1 1) oriented BaTiO₃ epitaxial film (e_31,f = −23 C/m², d_33,f = 124 pm/V) suggest that it is a good candidate material for lead-free MEMS applications.     

The influence of interatomic distances on magnetic ordering in RMnSi compounds (R=La, Y, Sm, and Gd)

Magnetic ordering in the RMnSi (R=La, Y, Sm, and Gd) compounds is investigated. It is found that the type of magnetic ordering depends on the d _Mn-Mn distance between manganese atoms inside the magnetic layers located in the planes perpendicular to the c axis. This inference is based on the results of studies performed with SmMnSi and GdMnSi compounds in which the distances between manganese atoms are close to the critical value d _Mn-Mn that corresponds to the crossover between ferromagnetic and antiferromagnetic ordering in RMnSi compounds. The introduction of lanthanum and yttrium atoms into the rare-earth sublattice leads to an increase and a decrease in the unit cell size, respectively, and brings about magnetic phase transitions in the compounds under investigation.     

Multilayer relaxation at the Pb(110) surface

A combined experimental and theoretical study of multilayer relaxation at the Pb(110) surface is presented. Ion shadowing and blocking measurements show that the distance d₁₂ between the outermost two atomic layers is smaller than the bulk interlayer distance d by as much as Δd₁₂/d=−15.8±2.5%. For the relaxations of the second and third interlayer distances we find that they satisfy the empirical relation Δd₂₃/d+0.75Δd₃₄/d=+0.5±2.5%. Thermal vibration amplitudes are strongly enhanced at the surface. Our theoretical model, based on energy minimization of the total energy of a semi-infinite simple metal crystal, predicts oscillatory multilayer relaxation with Δd₁₂/d=−15.9%, Δd₂₃/d=+7.9%, Δd₃₄/d=−6.8% and Δd₄₅/d=+0.7%, in agreement with experiment.     

Specific heat related to the Raman frequency shifts for the rotatory mode in ammonia solid I close to the melting point

This study gives an evidence for the validity of our spectroscopic modification of the Pippard relation as applied to ammonia solid I close to the melting point. We use our calculated frequencies for the rotatory lattice (librational) mode in ammonia solid I for the pressures of 0, 1.93 and 3.07 kbars. We obtain that the specific heat C_p varies linearly with the frequency shifts 1/ν(∂ν/∂T)_p for this Raman mode at those pressures studied in this crystalline system. Our values of the slope dP_m/dT that we deduced from the linear plots, are not in satisfactory agreement with the experimental values in ammonia solid I near the melting point.     

Pressure derivatives of bulk and shear modulus of rare gas solids

Expressions for the isothermal bulk and shear modulus and also their first and second order pressure derivatives for rare gas solids are derived by a simple method considering the different interactions i.e. modified variable induce dipoles, short range overlap repulsion and the vibrational contribution. The derived relations for dK_T/dP, dC_S/dP, dC′₄₄/dP, d²K_T/dP², d²C_S/dP² and d²C′₄₄/dP² are used to compute the numerical values of these constants at P=0 for Ne, Ar, Kr and Xe.     

Formation of an ensemble of nanoclusters under rapid deposition of atoms on a surface

The results of an experimental study of the formation of nanometer-size Au clusters on NaCl(100) and HOPG(0001) surfaces under pulsed laser deposition are presented. No clusters of small sizes (d ≤ 1 nm) have been found in the cluster size distribution. The distribution itself at d < 5 nm has the form of a percolation distribution. It has been established that the perimeter of clusters with sizes d < 5 nm has a fractal structure. The fractal dimension of clusters is different for NaCl(100) and HOPG(0001) surfaces with different symmetries; it decreases with increasing cluster size from D _f ≈ 1.2–1.4 at d ≈ 1.5 nm to D _f ≈ 1 at d ≈ 5 nm. A physical mechanism of nanocluster formation is suggested. Under pulsed laser deposition, the attainable densities of adatoms are close to the percolation threshold in the region of thermodynamically unstable states and many-particle correlation regions are formed in a spatially inhomogeneous adsorbate. Clusters are formed on the surface from many-particle correlation regions in several diffusion jumps. The suggested mechanism allows the fractal dimension of the clusters forming on surfaces with different symmetries, its dependence on cluster size, and the cluster size distribution functions to be calculated.     

Thermo-optical properties of uniaxial NaT(XO4)2 laser host crystals (where T = Y, La, Gd or Bi, and X = W or Mo)

Thermo-optic coefficients dn _o/dT and dn _e/dT were measured in tetragonal double tungstate and double molybdate crystals NaT(XO₄)₂ (where T = Y, La, Gd or Bi and X = W or Mo) by a laser beam deviation method in the spectral range 0.4–1.1 μm. Thermal expansion coefficients in the directions of a and c crystallographic axes were also measured. Analytical expressions for thermo-optic dispersion formulas were derived as series in 1/λ ². All dn/dT values for NaT(XO₄)₂ crystals were found to be negative. Their absolute values satisfy the relation |dn _e/dT| > |dn _o/dT| for crystals without Bi and |dn _o/dT| > |dn _e/dT| for crystals with Bi. A clear tendency for dn/dT values to decrease with the increase of the volumetric thermal expansion coefficient α _vol of the crystal was observed. This is related with dominant contribution of volumetric thermal expansion effect to the temperature dependence of the refractive index. Thermal coefficients of the optical path W = dn/dT + (n ? 1)α _T governing thermal lensing effect were calculated for different light propagation directions and polarizations as well as crystal athermal directions.     

The influence of a demagnetizing field on hysteresis losses in a dense assembly of superparamagnetic nanoparticles

The electrodynamic method is used to measure the hysteresis losses of a dense assembly of magnetite nanoparticles with an average diameter D=25 nm in the frequency range f=10–150 kHz and for magnetic field amplitudes H₀=100–300 Oe. It is found that the specific loss power is determined by a demagnetizing factor of a whole sample. It diminishes approximately 4.5 times when the sample aspect ratio decreases from L/d=11.4 to L/d≈1, where L and d are the sample length and diameter, respectively. For H₀≤300 Oe the maximal specific loss power 120 W/g is obtained for the sample with L/d=11.4 at f=120 kHz. For comparison, the assembly specific absorption rate has been determined also by means of direct measurement of the temperature difference between the inner and outer surfaces of a flat cuvette containing magnetic nanoparticles. For both methods of measurement close values for the specific absorption rate are obtained for samples with similar demagnetizing factors.     

Coherence transition in granular high temperature superconductors

We have studied experimentally the electrical conductivity and specific heat near the superconducting transition of granular samples of YBa₂Cu₃O_7−δ, YBa₂(Cu_2.98Zn_0.02)O_7−δ and GdBa₂Cu₃O_7−δ. The results show that the transition proceeds in two stages. Careful analysis of the conductivity in the regime of approach to the zero resistance state reveals the occurrence of a coherence transition, which is related to the connective nature of the granular samples. This transition occurs when the fluctuating phases of the order parameter in individual grains become long-range ordered. We obtain the exponent for fluctuation conductivity and the relevant critical temperature, T_co, which is close to the point where resistivity vanishes. The specific heat results, when analyzed as dC/dT, show a weak but reproducible cusp-like anomaly at T_co. This finding gives strong support to the interpretation of the coherence transition as a genuine critical phenomenon.     

Search for the parity-forbidden heavy-particle width Γ′dα of the Ex = 3.562 MeV state in 6Li

Alpha-particle capture by deuterium has been investigated at excitation energies in ⁶Li close to the J^π = 0⁺, T = 1, E_x, = 3.562 MeV state. A search for resonance γ-rays from this state has yielded a negative result and an upper limit for the heavy-particle width Γ′_dα ≦ 0.017 eV.     

Controlled vortex motion in multiple interpenetrating pinning arrays

We study the effect of multiple interpenetrating pinning arrays on the vortex motion in the presence of an ac driving force, f _d (t), by using extensive molecular dynamics (MD) simulations. Firstly, the response to a square ac wave f _d (t) has been explored for the vortices interacting with a periodic square pinning array which has different pinning strengths and sizes. The effect of the type of an ac drive and its amplitude on the oscillatory dynamics of vortices have been investigated in detail. For very low displacements of the vortices, we have found that the single-particle model can produce results analytically similar to the ones obtained by the MD simulations. It is shown that the collective motion of vortices can be controlled easily by varying the number of multiple interpenetrating square pinning lattices (N _SPSL). A regular sequence of peaks has been observed for N _SPSL = 3 in the time evolution of the average velocity of the vortices (i.e., V? _{x - t} curves). The number of peaks (N _peak) strongly depends on the magnitude of f _d (t), and increases with increasing the magnitude of f _d . The close relation between N _peak and f _d is considered as an indication of controlling vortex motion in a multiple periodic pinning structure. Finally, the variation of the power spectrum of noise S(ν) with N _SPSL has been investigated. For N _SPSL = 3, it has been found that the plastic motion of the row of vortices evolves at low frequencies, i.e., 1/ν behavior, whereas, at high frequencies, S(ν) shows a typical behavior of Gaussian white noise.     

Temperature derivatives at constant volume of the elastic constants of the alkali halides

Values of dC/dT)_V have been computed from experimental dC/dT)_P and dC/dP)_T for the three elastic constants (expressed as $\text{B}{}_{\mathrm{T}}\text{, C}{}_{44}\text{and}\text{(C}{}_{11}\text{?C}{}_{12}\text{)}\text{2)}$ of each of the 16 Li, Na, K, and Rb halides. The dC/dT)_V measure the explicit dependence of C on T, the effect of thermal expansion having been removed. The dC/dT)_V are all small (compared with dC/dT)_P), are all negative, and vary quite systematically and smoothly with anion, with cation and with the three elastic constants. Negative dB_T/dT)_V is accounted for both thermodynamically and by available lattice dynamical calculations. dC/dT)_V for shear constant is expressed in terms of two vibrational mode parameters, whose values can then be estimated from the observed value, and the trend of dC/dT)_V with Debye temperature.     

Effect of correlation on electronic properties of NiO: A study from dynamical mean field theory

In order to describe a typical strongly correlated insulator NiO at electronic level, we perform a first principles calculation for temperature effect on electronic properties of NiO using a many-body method merging local density approximation (LDA) with dynamical mean field theory, so called the LDA+DMFT scheme. Band gap and density of states (DOS) are in good agreement with available experimental data and theoretical calculations, and Ni d-e_g and d-t_2g components both exhibit insulating character. Calculated hybridization functions indicate that Ni d-e_g states strongly hybrid with O p states at T = 58 K, 116 K, 145 K, 232 K and 464 K. In order to compare with experimental angle-resolved photoemission spectrum (ARPES), we also calculate momentum-resolved electronic spectrum function, which is established that obvious electronic excitation mainly arises from Ni d-t_2g states at temperature T = 232 K, and the spectrum functions between −0.5 eV and 0.0 eV are almost symmetric about certain k points. Finally, we analyze the effect of temperature on electronic properties of NiO by carrying out LDA+DMFT calculations at T = 58 K, 116 K, 145 K, 232 K and 464 K, respectively. Results show that temperature mainly influences the valence states of spectrum function and hybridization function, in particular high-lying states close to Fermi level. Electronic excitation distributions and spectrum characters in electronic spectrum function are also discussed.     

Temperature dependence of the elastic constants of single-crystal rutile between 4° and 583°K

Measurements of all the six principal elastic constants of single-crystal rutile were made in the temperature range of 298–583°K. The temperature derivatives (in kb/deg) at 298°K are: dC₁₁/dT = − 0·510, dC₃₃/dT = − 0·900, dC₄₄/dT = − 0·220, dC₆₆dT = − 0·458, dC₁₂/dT = − 0·580, and dC₁₃/dT = − 0·330. Measurements of the four modes, C₁₁, C′ = (C₁₁ — C₁₂)/2, C₆₆, and C_110L = (C₁₁ + C₁₂ + 2C₆₆)/2, were extended to 4°K. Two features related to the temperature and volume dependences of the lattice vibrational frequencies are revealed: first, all the measured dC_lj/dT except dC′/dT become less negative with increasing temperature above 100°K. Second, dC′/dT is positive at all temperatures but decreases with increasing temperature at temperatures > 300°K. Indirectly shown is that (∂C′/∂P)_T having a value of − 1·32 at 298°K, decreases with decreasing temperatures. The significance of this latter fact is discussed in light of the computation of Grüneisen mode γ's from the acoustic (∂C_ij/∂P)_T values, and the results are compared with the γ (α_v) values obtained by Kirby from thermal expansion data. It is concluded that the large increase in γ(α_v) at low temperatures cannot be ascribed to a large temperature dependence of (∂C′/∂P)_T. Therefore, Kirby's explanation, that the large increase in γ(α_v) is caused by the large volume dependence of the acoustical mode frequencies, is not substantiated.     

Low temperature heat capacities of transition metal phosphidest2

Low temperature heat capacities of transition metal phosphides, T₃P, (T = transition metal) are characterized by large γ values which indicate the existence of a narrow d-band in these materials. Anomalous γ values are observed at compositions close to the onset of a long range magnetic state.     

Proximity effect in superconductor/ferromagnet layered system: Influence of superconductivity on magnetic properties of Nb/Fe epitaxial bilayers

Magnetic and superconducting properties of Fe/Nb epitaxial bilayers have been studied. Single crystal Fe/Nb (110) bilayers with a Nb thicknessd _Nb in the range from 250 to 650 Å and with a Fe thicknessd _Fe in the range from 7 to 27 Å were prepared using molecular beam epitaxy techniques. Magnetization measurements showed the existence of a magnetically “dead” Fe-interface layer with a thicknessd _NM varying strongly with minor modifications of the growth conditions. For bilayers with a small magnetic layer thicknessd _M and a smalld _NM the FMR measurements revealed an anomalous decrease of the effective magnetization 4πM _eff of the ferromagnetic Fe layer below the superconducting transition temperatureT _c. The absolute change of 4πM _eff belowT _c depends strongly on the actual values ofd _M andd _NM: upon decreasing bothd _M andd _NM the effect increases considerably. As a possible mechanism for the anomalous temperature dependence of 4πM _eff belowT _c we discuss a spatial modulation of ferromagnetic order due to a modification of the RKKY inter-action in the superconducting state.