Optical reflectivity of CdCr2Se4 single crystals in the visible and ultra-violet spectral region

The specular reflectivity of CdCr₂Se₄ single crystals has been measured within the spectral energy interval E = 0.5 ? 12.0 eV at room temperature. The spectral dependence of the real and imaginary part of the dielectric constant has been obtained from a Kramers-Kronig analysis of the experimental data. The observed optical transitions and the origin of the red-shifting band are discussed with respect to the semiempirical ionic model of the spinel structure, based on the recently reported increase of the structure parameter u of the spinel during cooling (Göbel, J. Magn. Magn. Mat. 3 (1976) 143). Contradicting results of the optical measurements in the region of the red-shifting edge may be attributed to the sensitivity of the parameter u on the internal stress.     

Frequency modulation of fluxon oscillations

The effect of the application of an ac external electromagnetic field to a long Josephson junction is studied by analyzing the spectrum of the radiation emitted by the junction. In the absence of the external field the junction, biased on a dc current singularity of the current-voltage characteristic, emits radiation at a frequency ƒ₀ in the X-band of the microwaves. The application of an external ac field having a frequency ƒ_m of the order of several megahertz gives rise to the appearance of spectral components at frequencies ƒ₀±nƒ_m, with n integer. The experimental results are described reasonably well by classical FM modulation theory.     

The Hydrodynamic Limit for Local Mean-Field Dynamics with Unbounded Spins

We consider the dynamics of a class of spin systems with unbounded spins interacting with local mean-field interactions. We prove convergence of the empirical measure to the solution of a McKean–Vlasov equation in the hydrodynamic limit and propagation of chaos. This extends earlier results of Gärtner, Comets and others for bounded spins or strict mean-field interactions.     

Fast approximate technique for the cumulative wavenumber model to modeling radiative transfer in a mixture of real gas media

In the cumulative wavenumber (CW) model, the total range of the absorption cross-section Cη is subdivided into the supplementary absorption cross-section of gray gases C_j, j=1,…,n, where n is the number of gray gases; and the wavenumber region is subdivided into intervals Δ_i=[η_i−1, η_i], i=1, 2,…,p, where p is the number of intervals. The intersection of the two spectral subdivisions is used to define the modeling of the fractional gray gas D_ij. In the CW model, we solve the radiative transfer equation (RTE) in every subinterval D_ij; then it is necessary to solve n x p times the spectral form of the RTE for complete spectral integration. In this work, the CW model is used with a numerical approximation technique based on additive properties of radiative intensity to reduce the solution of RTE to n new fractional gray gas D_j for complete spectral integration. The CW model was first coupled with the discrete ordinates method and the accuracy of the simplified technique and the algorithm was first examined for one-dimensional homogeneous media; results are compared with line-by-line calculations and it is found that the CW model with the simplified technique is exact for the homogeneous media examined. Also, the fast approach is tested in the diffuse reflecting boundaries case. The CW model is implemented in a bi-dimensional enclosure containing real gases in isothermal cases. Afterwards, this approximate technique is extended to non-isothermal and non-homogeneous cases; the results are compared with line-by-line calculations taken from literature and good agreement was found. The results obtained using the acceleration technique for the CW model agree with the results of original CW model. With this acceleration technique the CPU time decreases p times. Spectral database HITRAN and HITEMP are used to obtain the molecular absorption spectrum of the gases.     

Noise spectral power and non-equilibrium dynamics of vortices in the mixed state of Bi2Sr2CaCu2Oy

A model is proposed to explain the observed noise spectral power generated by the moving vortex lattice in Bi₂Sr₂CaCu₂O_y single crystals. The analysis is based on the observations that the noise generated is due to the vortex velocity rather than the thermodynamic phase of the material. The motion of vortices induces a gradual transition of the vortex lattice from the plastic flow to the uniform flow with increasing magnetic field. The influence of the disorder on the dynamics of vortices is such that a uniform distribution in the activation barrier width will exist. This distribution will affect the attempt frequency of the flux line hopping and consequently the spectral shape of the noise generated. The model provides physical basis to the fact that the observed 1/f-noise power corresponds to the smaller velocity of the vortex lattice, while the deviation from the 1/f-spectrum corresponds to the larger velocity region. The results of the simulation are compared to recent experimental measurements where good agreement has been achieved.     

Magnetic quantum oscillations in the photovoltaic effect of Cd0.24Hg0.76Te

The first experimental evidence of magnetic quantum oscillations in the photovoltaic effect is reported. Experiments were carried out with the p?n junction in Cd_0.24Hg_0.76Te placed in a quantizing magnetic field. The oscillations are related with the interband Γ₈ → Γ₆ magnetoabsorption transmissions. The discrepancy between experimental data and theory is explained by the effect of the p?n junction electric field on the magneto- absorption behaviour.     

Weakly Asymmetric Non-Simple Exclusion Process and the Kardar–Parisi–Zhang Equation

We analyze a class of non-simple exclusion processes and the corresponding growth models by generalizing the discrete Cole–Hopf transformation of Gärtner (Stoch Process Appl, 27:233–260, 1987). We identify the main non-linearity and eliminate it by imposing a gradient type condition. For hopping range at most 3, using the generalized transformation, we prove the convergence of the exclusion process toward the Kardar–Parisi–Zhang (kpz) equation. This is the first universality result under the weak asymmetry concerning interacting particle systems. While this class of exclusion processes are not explicitly solvable, under the weak asymmetry we obtain the exact one-point limiting distribution for the step initial condition by using the previous result of Amir et al. (Commun Pure Appl Math, 64(4): 466–537, 2011) and our convergence result.     

Magneto transport on the surface of a topological insulator spin valve

The effects of the magnetization on the transport properties of a ferromagnet/barrier/ferromagnet spin valve fabricated with a topological insulator are studied. We consider two types of junctions, (i) an F₁/normal barrier (NB)/F₂ junction and (ii) an F₁/magnetic barrier (FB)/F₂ junction. The junctions in both cases lie in the xy-plane with the magnetizations in both ferromagnetic regions, F₁ and F₂ aligned in the z-direction. The charge carriers in the topological insulator have a Dirac like energy spectrum of a massive relativistic particle with the magnetization M playing the role of the mass. The gap opening is a special magneto feature of topological insulators. In an anti parallel alignment of the two magnetizations, the mass of the carriers is negative in the region where M is in the negative direction. The negative mass leads the behaviors of the magneto transport properties and the tunneling magneto resistance of these junctions to be quite different from those of graphene-based spin values.     

Experimental limits on the decay of reactor neutrinos

We consider the possible decay of massive reactor neutrinos into a light neutrino and either photons or electron-positron pairs. In a detector placed at the power reactor in Gösgen, Switzerland, the difference of the counting rates for reactor on minus reactor off is consistent with zero. From the experimental bounds we deduce lifetime limits for dominantly coupled light neutrinos as well as restrictions on the mixing parameter |U_eH|² for heavy, subdominantly coupled neutrinos.     

A note on the mean square deviation of the voltage of a resistively shunted Josephson junction in the running state

We show that a resistively shunted Josephson junction, in steady state, satisfies the relation 〈V²〉_T = IR〈V_T〉, where V is the voltage, I is the bias current, R is the junction resistance, and 〈 〉_T is a time average.     

Effect of charge density waves on the tunnel spectra of the Bi2Sr2CaCu2O8+δ superconductor

The differential tunnel conductance G _S of the junction between a normal metal and a superconductor with a charge density wave (CDW) is calculated as a function of the voltage V across the junction. The results are averaged over the spread of superconducting and CDW energy gaps in the nanoscale-inhomogeneous superconductor. It is shown that, if both order parameters are nonzero, a dip-hump structure is formed beyond the superconducting gap of G _S (V). If the phase of the CDW order parameter is not equal to π/2, a dip-hump structure will appear solely or mainly for one sign of the bias polarity. The results agree with the experimental data for Bi₂Sr₂CaCu₂O_8+δ and other high-temperature oxides     

Crossover behavior in fluids with Coulomb interactions

According to extensive experimental findings, the Ginzburg temperature t_G for ionic fluids differs substantially from that of nonionic fluids [W. Schr?er, H. Weig?rtner, Pure Appl. Chem. 76, 19 (2004)]. A theoretical investigation of this outcome is proposed here by a mean field analysis of the interplay of short and long range interactions on the value of t_G. We consider a quite general continuous charge-asymmetric model made of charged hard spheres with additional short-range interactions (without electrostatic interactions the model belongs to the same universality class as the 3D Ising model). The effective Landau-Ginzburg Hamiltonian of the full system near its gas-liquid critical point is derived from which the Ginzburg temperature is calculated as a function of the ionicity. The results obtained in this way for t_G are in good qualitative and sufficient quantitative agreement with available experimental data.     

Enhancement of spectral intensities of mercury triplet lines in longitudinal magnetic field

The variation of intensity of the spectral lines of the triplet series of mercury namely (7³ S ₁ → 6³ P ₀₁₂) (λ5461 Å,λ 4358 Å,λ 4047 Å) in the presence of a longitudinal magnetic field between zero and 2000 gauss has been investigated. The effect of magnetic field is found to be different as regards the variation of intensity and the occurrence of maxima in the three lines. These variations can be explained by considering the reabsorption of the spectral lines. A mathematical theory has been presented and an expression for (I _ul ) _B /I _ul has been deduced where (I _ul ) _B and (I _ul ) are the intensities of the lines with and without the magnetic field. The experimental results agree fairly well with the theoretical model.     

Maximal supercurrent of a Josephson junction in a field of magnetic particles

The maximal supercurrent I_m of a short Josephson junction formed by an edge contact of two superconducting films is calculated for the case where the junction is placed in a periodic field produced by a chain of magnetic nanoparticles. The commensurability effects occurring when the magnetic flux of a homogeneous external field H₀ through an elementary cell is equal to an integral number of magnetic flux quanta Φ₀ are considered. The effects give rise to additional maxima in the I_m(H₀) dependence.     

Temperature dependence of the self-broadening coefficients of ethane

Using a diode-laser spectrometer, self-broadening coefficients have been measured at three temperatures (246.2, 226.2 and 150.2 K) for 11 spectral lines in the ν₉ fundamental band of ¹²C₂H₆. The collisional widths have been obtained by fitting each experimental absorption profile with a Rautian model. The temperature dependence exponent n was also determined for each line, and found to be constant within experimental uncertainties. The mean value is equal to n = 0.676.     

Theoretical and experimental photophysical studies of the tris(4,4,4-trifluoro-1-(1-naphthyl)-1,3-butanedionate) (2,2′-bipiridyl)-europium(III)

The complexes tris(4,4,4-Trifluoro-1-(1-naphthyl)-1,3-butanedionate) (2,2′-bipiridyl) Ln(III), Ln(tan)₃bipy, where Ln(III)=Eu³⁺ and Gd³⁺ have been synthesized, characterized and their photophysical properties (absorption, excitation and luminescence spectra and emission quantum yield) investigated down to 4.2 K. The Eu(tan)₃bipy complex has its molecular structure experimentally determined using X-ray crystallography and theoretically using the SMLC/AM1 method as well as their electronic singlet and triplet states were calculated, using the INDO/S-CI method with a point charge model to represent the Eu³⁺ ion, where two values were adopted, +3.0e and +3.5e, to investigate the imperfect shielding of the 4f shells. The so calculated +3.5e model electronic absorption spectrum and low lying triplet state energies agreed very well with the experimental ones. The emission quantum yield of the Eu³⁺ complex is quite low at room temperature, namely 7%, probably due to the too low lying triplet state, 19,050 cm^-1, and increases by a factor of three when the temperature is lowered to 4.2 K. This strong thermal effect indicates the presence of a channel deactivating the main emitting state, what can be due to a LMCT state possibly lying in the same spectral region, as usually found in Eu³⁺ compounds.     

Josephson effect between conventional and non-centrosymmetric superconductors

We analyze the Josephson effect between a conventional and a non-centrosymmetric superconductor to examine characteristic features of such junctions and the symmetry of the superconducting phases. As a concrete example, we consider the non-centrosymmetric superconductor CePt₃Si where Rashba spin-orbit coupling plays a crucial role and affects the Josephson pair tunneling. In this case, the Josephson coupling is composed of two parts, spin-singlet-like and spin-triplet-like components. The triplet-like component can lead to a Josephson coupling shifted by π relative to the singlet-like coupling. This has important implications on the interference effects and may explain some recent experimental results for the Al/CePt₃Si junction.     

Impurity spin dynamics in the classical Kondo system Cu: Fe. NMR relaxation of impurity neighbour nuclei

The correlation time τ_i of the impurity electronic spin in the Kondo system Cu:Fe has been measured over the temperature range 4.2 K?T?300 K (T_K=27.6 K) by means of NMR relaxation of impurity neighbour nuclei. τ_i being in the sub-picosecond region varies with temperatures as predicted by the model calculation of Götze and Schlottmann.     

Influence of packing on the vibrations of homogeneous bundles of C60 peapods

The non-resonant Raman spectra of homogeneous bundles of C₆₀ peapods (C₆₀ inserted in single-walled carbon nanotubes) are calculated in the framework of spectral moment method, together with a bond-polarizability model. The evolutions of the low wavenumber range of the Raman spectrum of homogeneous bundles of C₆₀ peapods as a function of the nanotube diameter and the size of bundles are discussed. The effect of the C₆₀ filling factor is investigated in detail. The results are compared to experimental Raman data measured on various samples of C₆₀ peapods.     

Possible nature of the pseudogap anomalies in HTSC

An HTSC model, in which the interaction of valence-band electrons with diatomic negative U centers is assumed to be responsible for the anomalous properties of HTSC compounds, is proposed and used to explain the nature of the pseudogap and pseudogap anomalies (including the giant Nernst effect, the anomalous diamagnetism above T _c, the “transfer” of the optical spectral weight). For YBa₂Cu₃O_{6 + δ}, the pseudogap opening temperature T* and T _c are calculated as functions of the degree of doping δ. The calculated dependences agree quantitatively with the experimental dependences without using scale fitting parameters. The good agreement between the calculated and experimental results can serve as an argument for the proposed HTSC model.