Electric conductance of metal nanowires at mechanically controllable break junctions under electrochemical potential control

We have developed the mechanically controllable break junction setup with an electrochemical cell (EC-MCBJ) to measure the electric conductance of metal nanowires under electrochemical potential control. The electric conductance of Au nanowires was investigated in 0.1 M Na₂SO₄ solution using EC-MCBJ. The conductance of the Au nanowires was quantized in units of G₀ (=2e²/h), showing clear features in the conductance histogram. The atomic contact with a specific conductance value was kept for >5 s, indicating the relatively high stability of the present EC-MCBJ system.     

Theoretical study of electric conductance through nanostructures in terms of the phase-shift

A simple expression of the electric conductance through a nanostructure connected to two electrodes is obtained by using the phase-shift analysis. The Green function theory applicable to the nonequilibrium system is employed to formulate the electric conductance. The 0 bias limit, i.e., the linear response approximation is taken. The relation between Green functions and phase-shifts is obtained by extending the method applied to the single impurity problem in the metal. It is shown that a channel does not contribute to the conductance if its phase-shift is an integer multiple of π. Also shown is the importance of the effect due to multiple orbitals in nanostructures. Some concrete examples leading to simpler forms are given.     

Effect of an ac field on the conductance fluctuations for mescoscopic systems

With the use of perturbation theory to perform impurity averaging, the conductance fluctuations (CF) in mesoscopic systems are evaluated at finite frequency (ω) of the applied electric field. Calculations are carried out for frequencies much smaller than the inverse elastic mean free time, ω≪τ_el⁻¹. It is shown that the CF decrease monotonically as ω increases. Also, the frequency scale over which this decrease occurs is given by τ_diff⁻¹≪τ_el⁻¹, where τ_diff is the time for an electron to diffuse across the sample. This means that the universality of the CF at zero frequency is not preserved at finite frequency. These calculations are for a rectangular prism. Six leads covering the probe faces are attached to the cube. It is also shown that at finite frequency the sample-to-sample CF have the same size as the fluctuations of a given sample as a function of frequency.     

Converting heat to electricity by a graphene stripe with heavy chiral fermions

A conversion of thermal energy into electricity is considered in the electricallypolarized graphene stripes with zigzag edges where the heavy chiral fermion (HCF) statesare formed. The stripes are characterized by a high electric conductance G _e and by a significantSeebeck coefficient S. The electric current in the stripes is induced due toa non-equilibrium thermal injection of “hot” electrons. This thermoelectric generationprocess might be utilized for building of thermoelectric generators with an exceptionallyhigh figure of merit ZδT ?1 and with an appreciable electric power densities ~1 MW/cm².     

Interacting quantum wires: A possible explanation for the 0.7 anomalous conductance

We investigate an effective one-dimensional conducting channel considering both the contact umklapp and the Coulomb electron-electron interaction. We show that, at low electronic density, the proximity to the Wigner crystal reproduces the anomaly in conductance at 0.7G₀. The crucial ingredient of our theory is the fact that the gate voltage acts as a bias controlling the intensity of the umklapp term. At large gate voltages, the umklapp vanishes and we obtain a conducting quantum wire with a perfect conductance. At low gate voltages, the Wigner crystal is pinned by the umklapp term, giving rise to an insulating behavior with vanishing conductance. This crossover pattern has a transition point which can be identified with the anomalous conductance around 0.7G₀. This picture is obtained within the framework of a renormalization group calculation. The conductance static regime is achieved by taking first the limit of finite length and then the limit of zero frequency.     

Hopping conduction in disordered carbon nanotubes

We report electrical transport measurements on individual disordered multiwalled carbon nanotubes, grown catalytically in a nanoporous anodic aluminum oxide template. In both as-grown and annealed types of nanotubes, the low-field conductance shows an exp[−(T₀/T)^1/2] dependence on temperature T, suggesting that hopping conduction is the dominant transport mechanism, albeit with different disorder-related coefficients T₀. The electric field dependence of low-temperature conductance behaves as exp[−(ξ₀/ξ)^1/2] at high electric field ξ at sufficiently low T. Finally, both annealed and unannealed nanotubes exhibit weak positive magnetoresistance at . Comparison with theory indicates that our data are best explained by Coulomb-gap variable-range hopping conduction and permits the extraction of disorder-dependent localization length and dielectric constant.     

Ferroelectric phase transitions in NH4HSeO4 crystals

Results are presented of measurements of spontaneous polarization, electric permittivity and electrical conductance of NH₄HSeO₄ crystals in three principal crystallographic directions from liquid nitrogen temperature to 320 K. NH₄HSeO₄ crystals exhibit ferroelectric properties in the a-axis direction between 106–250 K, whereas in the b-direction they are pyroelectrie over the entire temperature range. The calculated activation energy (1.42 eV) is indicative of semiconducting behavior.     

The properties of two-dimensional photonic crystals bandgap structure with rhombus lattice

The relative bandgap ω_R of photonic crystals structure with rhombus lattice for both TE and TM modes is studied by the plane wave expansion method. Based on the analysis, a bending waveguide of U type can be easily obtained. The steady-state distribution of the electric field with ω = 0.4765(a/λ) of TM mode is given by FDTD method and the transmission is obtained by FFT of the time domain field. All of the sufficient analysis shows the conceptual framework of the proposed rhombus lattice PCs and its great potential application in photonic integration circuit.     

Temperature and junction-type dependency of Andreev reflection in MgB2

We studied the voltage and temperature dependency of the dynamic conductance of normal metal-MgB₂ junctions obtained either with the point-contact technique (with Au and Pt tips) or by making Ag-paint spots on the surface of MgB₂ samples. The fit of the conductance curves with the generalized BTK model gives evidence of pure s-wave gap symmetry. The temperature dependency of the gap, measured in Ag-paint junctions (dirty limit), follows the standard BCS curve with 2Δ/k_BT_c=3.3. In out-of-plane, high-pressure point-contacts we obtained almost ideal Andreev reflection characteristics showing a single small s-wave gap Δ=2.6±0.2 meV (clean limit).     

Role of U(3(A−1)) multiquantum excited states in calculating spectroscopic quantities for 4 8 Be

A set of equations for deriving the SU(3)-irreducible density matrix for U(3(A?1)) multiquantum excited states involved is given. For the ₄ ⁸ Be nucleus, the binding energies, radii, spectra, electric quadrupole moments, and probabilities B(E2) of electric quadrupole transitions are computed in the multiquantum approximation of the unitary scheme. The values obtained for the above spectroscopic quantities are investigated versus the number of basis functions involved.     

S-matrix and unitarity bounds for three-channel systems,with application to low-energy photoproduction of pions from nucleons

A parametrization of the S-matrix for three-channel systems is given, where we state explicitly the unitarity bounds on the free parameters. The free parameters are directly related to measurable quantities. The results are applied to low-energy photoproduction of pions from nucleons, and lower bounds for the electric dipole amplitudes E₀₊ for π⁰ production are obtained. These lower bounds follow without using the Fermi-Watson final-state theorem.     

Electric field induced exciton binding energy and its non-linear optical properties in a narrow InSb/InGaxSb1−x quantum dot

The effect of electric field on the binding energy, interband emission energy and the non-linear optical properties of exciton as a function of dot radius in an InSb/InGa_xSb_1?x quantum dot are investigated. Numerical calculations are carried out using single band effective mass approximation variationally to compute the exciton binding energy and optical properties are obtained using the compact density matrix approach. The dependence of the nonlinear optical processes on the dot sizes is investigated for various electric field strength. The linear, third order non-linear optical absorption coefficients, susceptibility values and the refractive index changes of electric field induced exciton as a function of photon energy are obtained. It is found that electric field and the geometrical confinement have great influence on the optical properties of dots.     

Magnetic field induced multi-component QED3 and quantum Hall effect

Dynamics of two dimensional electrons under the strong perpendicular magnetic field is shown to be described by a multi-component fermion theory. The electric conductance has a remarkable property known as the quantum Hall effect. The Hall conductance is quantized in units ofe ²/h in the gap region and in the localized state region. The proof of exactness is presented in general cases using quantum field theory.     

Dielectric and magnetic properties of the multiferroic Tb<Subscript>(1−<Emphasis Type="Italic">x</Emphasis>)</Subscript>Bi<Subscript>x</Subscript>MnO<Subscript>3</Subscript>: Electric dipole glass and self-organization of charge carriers

Single crystals of the new multiferroic Tb_(1?x)Bi_xMnO₃ have been grown and studied. A semiconductor compound with x = 0.05 is investigated in most detail. At temperatures T ≥ 165 K, the electric dipole glass state is realized in the crystal. Localized charge carriers form conducting drops of electrons and holes, which are located predominantly in thin layers at the boundaries of polar domains. When drops escape as the temperature increases, jumps in conductance and capacitance are observed. The state of drops is controlled by low bias voltage. The long-range magnetic order arises at temperatures T ≤ 90 K. Negative magnetoresistance is observed at temperatures of the existence of localized charge carriers.     

Hysteresis phenomena in the course of polarization evolution in a sinusoidal electric field in lead magnesium niobate crystals

The processes of polarization evolution in single crystals of the PbMg_1/3Nb_2/3O₃ model ferroelectric relaxor in a sinusoidal electric field are investigated at temperatures near and above the temperature T _d 0 of destruction of the induced ferroelectric state upon heating in zero electric field. The polarization switching current loops are measured in the ac electric field applied along the 〈111〉 and 〈110〉 pseudocubic directions. The electroluminescence intensity loops are obtained under the combined action of ac and dc electric fields applied along the 〈100〉 direction. In a certain temperature range above T _d 0 and the freezing temperature T _f in lead magnesium niobate, there are electric current anomalies, that correspond to the dynamic formation and subsequent destruction of the ferroelectric macroregions throughout each half-cycle of the ac electric field. The measurements of electroluminescence hysteresis loops demonstrate that the observed depolarization delay (related to the ac electric field amplitude) increases with an increase in the dc electric field and decreases as the ac field amplitude increases. The nature of the observed phenomena is discussed.     

Surface study of fine MgFe2O4 ferrite powder prepared by chemical methods

To study surface behaviors, MgFe₂O₄ ferrite materials having different grain sizes were synthesized by two different chemical methods, i.e., a polymerization method and a reverse coprecipitation method. The single phase of the cubic MgFe₂O₄ was confirmed by the X-ray diffraction method for both the precursors decomposed at 600-1000 °C except for a very small peak of Fe₂O₃ was detected for the samples calcined at 600 and 700 °C by the polymerization method. The crystal size and particle size increased with an increase in the sintering temperature using both methods. The conductance of the MgFe₂O₄ decreased when the atmosphere was changed from ambient air to air containing 10.0 ppm NO₂. The conductance change, C = G(air)/G(10 ppm NO₂), was reduced with an increase in the operating temperature. For the polymerization method, the maximum C-value was ca. 40 at 300 °C for the samples sintered at 900 °C. However, the samples sintered at 1000 °C showed a low conductance change in the 10 ppm NO₂ gas, because the ratio of the O₂ gas adsorption sites on the particle surface is smaller than those of the samples having a high C-value. The low Mg content on the surface affects the low ratio of the gas adsorption sites. For the reverse coprecipitation method, the particle size was smaller than that of the polymerization method. Although a stable conductance was obtained for the sample sintered at 900 and 1000 °C, its conductance change was less than that of the polymerization method.     

Conductance of atom-sized contacts of transition metals at room temperature

Exploiting the mechanically controllable break junction technique, we have measured the conductance of atom-sized contacts of Fe, Co, and Ni at room temperature under ultrahigh vacuum conditions. The conductance histogram of Fe exhibits a broad peak around 2.5 G₀ (G₀ ≡ 2e²/h), whereas those of Co and Ni show no conductance peaks. However, the histograms of Co and Ni display different structures: While the Co histogram is simply flat, the Ni histogram reveals an appreciable background. Our experimental results are compared with previous results obtained at cryogenic and room temperatures, and the observed peak missing in our room-temperature histograms of Co and Ni is discussed.     

Differential thermal analysis of phase transitions in (Bi1−xLax)FeO3 solid solution

(Bi_1?xLa_x)FeO₃ solid solution, a material exhibiting simultaneously electric and magnetic long range dipole order, is studied by the method of differential thermal analysis. The results confirm the data on ferroelectric phase transitions obtained from electric permittivity and dilatometric measurements above 500°C. The endothermal effect observed about 820°C is related to the ferroelectric phase transition of BiFeO₃.