Modern Physics Letters B

We consider a lattice model in which phonons scatter with pairs of electrons carrying a net spin current. All the energy eigenvalues and eigenvectors of this model can be obtained analytically. For a suitable choice of parameters the ground state consists of a Fermi sea of non-interacting electrons, with a layer of paired electrons on top of it. The binding energy of one electron pair is partly canceled by increased kinetic energy of another pair of electrons. This results in a momentum-dependent gap in the spectrum of the electrons.

In the absence of thermal fluctuations and other external forces, a single particle will be pinned at the bottom of a flashing ratchet potential under the overdamped condition. Unidirectional motion becomes possible if there exist interactions among particles. In this paper, the overdamped directed motion of a dimer with two heads in a flashing ratchet potential is investigated. We propose the mechanism to reveal how two heads of the dimer cooperate and help each other to move processively. The necessary condition of directed motion is analytically given, which is in good agreement with numerical simulation results. Furthermore, the resonant steps of the current are observed and discussed. The scenario and the results proposed in this paper may shed light on the elaborations of artificial molecular motors.

A kind of large-mode-area (LMA) erbium-doped fiber (EDF) is required to get high output power with good beam quality. This new LMA EDF has a multi-layer-core structure which makes the mode area bigger. The key process is deposition of multi-layer-core structure by using modified MCVD to make every layer of the core with different refractive index. Furthermore, the key optical specifications of the fiber are measured. The absorption peak is 10.6 dB/m at 980 nm and 20 dB/m at 1550 nm. The cutoff wavelength is about 1300 nm. The mode field diameter (MFD) is 12.6 μm and 13.2 μm at 1550 nm according to Petermann-2 and Gaussian definitions, respectively. The core diameter is about 12 μm. The MFD of conventional standard EDFs is in a limited range ~5–8 μm, so the experimental result has made a big progress for MFD. Especially, we can improve the parameters of the multi-layer-core structure to get single mode EDF with larger mode area for high power fiber laser.

We have studied fluctuations on the electrical conductivity in granular Tm_1-xPr_xBa₂Cu₃O_7-δ (0 ≤ x ≤ 0.10) superconductors. Samples were prepared by the standard solid-state reaction technique. The results are described in terms of the temperature derivative of the resistivity, dρ/dT, and of the logarithmic temperature derivative of the conductivity, dln (Δσ)/dT. The results show clearly that the resistive transition proceeds in two stages: intragranular and intergranular. Also, it was not observed in dρ/dT, near the transition, a structure consisting of two closely separated maxima, as seen in studies with other rare earth elements. Yet, the analysis of the conductivity in the normal phase has shown contributions from critical fluctuations and they were not affected by Pr doping. Below T_C, on approaching the zero resistance state, our results show a power-law behavior with critical exponent s ≅ 4.1 for x = 0.00 and s ≅ 2.8 for x = 0.05 sample, indicating a phase transition from a paracoherent to a coherent state of the granular array. This behavior was not observed for x = 0.10 sample, showing that Pr affected the coherence transition.

Based on first-principle calculation, the geometry and electronic transport properties of the boron and nitrogen co-doping single-walled carbon nanotubes are investigated by using density functional theory combined with non-equilibrium Green's functions. The results show that the BN atoms energetically tend to form covalent bond of BN along axis in the nanotubes. In contrast to solely B or N doping, the co-doping do not generate accepter or donor subbands near the Fermi level. The co-doping give rise to the reduction of band gap in semiconducting (10, 0) tube and, furthermore, introduces the band gap to the metallic (5, 5) tube.

Nonlinear optical properties of gold aqueous colloid obtained with chemical reaction method are investigated using Z-scan technique with 38 ps pulses. Experimental results indicate that the colloid exhibits large nonlinear refraction and nonlinear absorption under the excitation of ps 532 nm pulses, while it presents large two photon absorption under ps 1064 nm pulses excitation. Meanwhile, the fluorescence emission spectrum of gold aqueous colloid is recorded under the excitation of 350 nm, and two fluorescence peaks are observed at 454 nm and 704 nm. Moreover, the average fluorescence lifetime of colloid is estimated to be 1.33 ns.

A gel was formed when an aqueous solution of BaCl₂, NbF₅ and citric acid in stoichiometric ratio is heated on a water bath at 100°C. This gel on decomposition at 700°C yielded the nano phase of BaNb₂O₆ as confirmed by X-ray diffraction study. The crystal structure is orthorhombic with unit cell parameter a = 12.2136 Å, b = 10.2413 Å, c = 7.8831 Å. The estimated grain size is 88 nm. Dielectric studies reveal the dielectric relaxation mechanism. Conducting charges and free charges both contribute to the dielectric relaxation in this material. Electrical properties of the material were studied using impedance spectroscopy technique. Detailed analysis of impedance spectrum suggested that the electrical properties are strongly temperature-dependent. The relaxation is polydispersive and conduction is mainly through grains. AC conduction activation energies are estimated from Arrheneus plots and conduction mechanism is discussed.

Fe-based alloy of the composition, Fe₇₄Cu_0.8 Nb_2.7Si_15.5B₇ has been studied thoroughly in order to research the evolution of nanograins and soft magnetic properties. The composition has been significantly deviated from FINEMET type composition given by Fe_73.5Cu₁ Nb₃Si_13.5B₉. It is hard to optimize composition to obtain equally good soft magnetic properties as FINEMET significantly deviating from conventional type alloy. Fe-based alloy of the composition, Fe₇₄Cu_0.8Nb_2.7Si_15.5B₇ has been prepared by single roller melt spinning machine. X-ray diffraction studies confirmed that the ribbon is in the amorphous state. Evolution of α-Fe(Si) nanograins from amorphous matrix were carried out by isothermal annealing in the temperature range from 550°C to 650°C for 1, 3, 5, 10, 20, 30, 40, 50 and 60 minutes. Frequency spectrum of real and imaginary part of complex initial permeability has been measured for the samples at different annealing conditions. Short time annealing has been proved to be more efficient than long time annealing for the samples of this composition for most of the annealing temperatures.

The transient voltage response of the charge density wave (CDW) to current pulses in Tl_0.3MoO₃ and K_0.15Tl_0.15MoO₃ have been studied. Contrasted with the data reported previously for other blue bronze systems, a kind of superimposed (overshoot and sluggish) response waveform is observed for the first time. These results are discussed in analogy with the coexisting of "inductive" and "capacitive" effects at the onset of the CDW depinning.