Ac Transport Properties Research Articles

Linear ac transport in square-shaped graphene nanoconstriction devices

The linear ac transport of square-shaped graphene nanoconstriction devices are investigated, employing Buttiker’s ac transport theory. Based on Green’s function, the dynamic conductances taking account of both dc and ac contributions are presented. The square-shaped graphene nanoconstriction device displays semiconducting and resonance features in the transporting channels. The device responds capacitively or inductively to the external ac perturbations depending on the dynamic processes of the internal charges at the central nanoconstriction region. We show that as a result of the coupling and the band-mixing between the leads and the central constriction, the ac transport properties are sensitive to the geometry of the graphene systems.

Ac Conductivity and Transport Properties of [N(C3H7)4]2Zn2Cl6 Compound

The electrical properties and ac conductivity of the bis tetra-propylammonium hexachloro-dizincate have been investigated by means of impedance spectroscopy measurements in the 209 Hz–5 MHz frequency range and 303–403 K temperature interval. The Nyquist plots are fitted to an equivalent circuit modeled by a combination of a parallel bulk resistance and constant phase elements. The experimental results indicate that σac(ω) is proportional to (ωs). It was found that s increases by increasing temperature below the phase transition at 327 ± (5) K (region I) and decrease with increasing temperature above 327 ± (5) (region II). An agreement between the experimental and theoretical results are discussed and suggests that the ac conductivity can be explained by the non overlapping small polaron tunneling model and correlated barrier hopping in region I and II, respectively. Furthermore, the modulus plots can be characterized by full width at half height or in terms of a non-experiential decay function $$\varPhi \left( {\text{t}} \right) = \exp \left( {\frac{{ - {\text{t}}}}{\tau }} \right)^{\beta }$$ .

Dynamic conductance in L-shaped graphene nanosystems

Dynamic conductance of nanocircuit, which demonstrates dc and ac transport properties, is regarded as vital indicator for device feature. With the help of nonequilibrium Green's function technology and Buttiker's ac transport theory, we present dynamic conductance in L-shaped graphene nanosystems (LGNSs). It is found that electronic transport is highly sensitive to the geometric feature as well as the size of LGNSs. The armchair edge lead determines whether LGNS shows ac response or not around Dirac point. The increase of width of zigzag edge lead suppresses dc conductance and induces capacitive responses at the anti-resonance states. This is due to large dwell time originated from edge state in zigzag edge lead. In the energy region far away from Dirac point, LGNS responds inductively with the transportation channel opens. Behaviors of dynamic conductance at Dirac point and anti-resonance states are discussed by interesting spacial-resolved local density of states.

Alternating current response of carbon nanotubes with randomly distributed impurities

The increasing need for nanodevices has necessitated a better understanding of the electronic transport behavior of nanomaterials. We therefore theoretically examine the AC transport properties of metallic carbon nanotubes with randomly distributed impurities. We find that the long-range impurity scattering increases the emittance, but does not affect the DC conductance. The estimated dwell time of electrons increases with the potential amplitudes. That is, multiple scattering by the impurities increases the kinetic inductance in proportion to the dwell time, which eventually increases the emittance. We believe that our findings can contribute significantly to nanodevice development.

DC and AC transport properties on La0.8Sr0.2MnO3

Magnetoresistive sensor can be widely used in modern transportation field, such as the vehicle positioning and navigation system, vehicle detection system, and intelligent transportation system. In order to improve the efficiency of magnetoresistive sensor, we synthesized La0.8Sr0.2MnO3 polycrystalline bulks at different sintering temperatures and investigated their DC and AC transport properties in this work. As a result, all samples showed insulator–metal (I–M) phase transition, and the transition temperature (TI–M) shifted to higher temperature with the increase of sintering temperature. The TI–M measured at different AC frequencies was smaller than that measured at DC condition, which implied that the I–M phase transition was suppressed at AC frequencies. The resistivity measured at high AC frequencies was larger than that measured at low AC frequencies, which could be attributed to the change of the magnetic penetration depth (δ). However, the room-temperature AC-magnetoresistance (MR) at low frequencies was much larger than that at high frequencies and room-temperature DC-MR. These findings demonstrate that reducing the AC frequency is an effective way for enhancing the room-temperature MR, which can be used to promote the efficiency of magnetoresistive sensor.

ELECTRON TRANSPORT IN MULTI-TERMINAL GRAPHENE NANODEVICE WITH INCLINED CROSS STRUCTURES

The DC and AC transport properties are investigated in multi-terminal graphene nanoribbon (GNR) devices. The devices are composed of three or four graphene ribbons connected with different angles. It is found that DC and AC conductances depend on the structural configurations and ribbon properties. In the vicinity of Dirac point, the intersection of graphene ribbons forms band mixing and results in resonant or anti-resonant states. The edge and width, as well as, the angles of the graphene ribbons influence the DC and AC transport properties drastically. These properties can be used to build future graphene-based nanoelectronics.

From capacitive to tunnelling conduction through annealing in metal-insulating granular films: the role of ultra-small particles

In situ transmission electron microscopy (TEM) during annealing of metal-insulating granular thin films reveals significant microstructural changes: nucleation of ultra-small particles and growth of those already existing. Radio frequency impedance spectroscopy shows that by subsequent annealing, tunnelling conductance becomes more important at the expense of the decrease of capacitive contributions, giving place to an almost frequency-independent conduction, although being very far from physical percolation. These results are simulated assuming a random resistor–capacitor network, which represents the competing conduction channels between particles through thermally assisted tunnelling and capacitive conductance. The physical parameters derived from this simple model agree well with the microstructural changes observed by TEM through annealing, and evidence the increase of tunnelling conduction paths mediated by segregated ultra-small glue particles in between bigger ones. Those glue particles grow with further annealing and account for the correlation of the microstructural changes to the ac transport properties.

Dynamic response of silicon nanostructures at finite frequency: An orbital-free density functional theory and non-equilibrium Green's function study

Orbital-free density functional theory (OFDFT) replaces the wavefunction in the kinetic energy by an explicit energy functional and thereby speeds up significantly the calculation of ground state properties of the solid state systems. So far, the application of OFDFT has been centered on closed systems and less attention is paid on the transport properties in open systems. In this paper, we use OFDFT and combine it with non-equilibrium Green's function to simulate equilibrium electronic transport properties in silicon nanostructures from first principles. In particular, we study ac transport properties of a silicon atomic junction consisting of a silicon atomic chain and two monoatomic leads. We have calculated the dynamic conductance of this atomic junction as a function of ac frequency with one to four silicon atoms in the central scattering region. Although the system is transmissive with dc conductance around 4 to 5 e2/h, capacitive-like behavior was found in the finite frequency regime. Our analysis shows that, up to 0.1 THz, this behavior can be characterized by a classic RC circuit consisting of two resistors and a capacitor. One resistor gives rise to dc resistance and the other one accounts for the charge relaxation resistance with magnitude around 0.2 h/e2 when the silicon chain contains two atoms. It was found that the capacitance is around 5 aF for the same system.

Semiconducting selenium nanoparticles: Structural, electrical characterization, and formation of a back-to-back Schottky diode device

Well crystalline selenium nanoparticles having an optical band gap of 2.95 eV have been synthesized using oxalic acid. Microstructural parameters such as crystallite size, lattice strain, cell parameters, and unit cell volume are estimated from X-ray diffraction line profile analysis by Rietveld refinement technique. dc and ac transport properties of the nanoparticles in the temperature range 300 K ≤ T ≤ 390 K and frequency range 20 Hz ≤ f ≤ 2 MHz have also been studied. The values of dc activation energies in the low and high temperature regions are found to be 0.083 eV and 0.382 eV, respectively. The charge transport mechanism of the sample follows correlated barrier hopping (CBH) model and the calculated value of barrier height and relaxation time is 0.786 eV and 2.023 × 10−11 s, respectively, while grain boundary contribution being greater than the grain contribution. Considering metal electrode-semiconductor contact as a back-to-back Schottky diode device, analysis of the current-voltage and capacitance-voltage characteristics is done to extract the Schottky barrier heights, ideality parameters, built in voltage, and charge density. With ±40 V sweep the capacitance versus voltage characteristics of the sample shows hysteresis behavior which may be attributed to the presence of deep traps.

Structure and Transport Properties of Sr0.75-xCaxY0.25Co0.25Mn0.75O3-δ (0 ≤ x ≤ 0.6)

AbstractNew complex oxide Sr0.75-xCaxY0.25Co0.25Mn0.75O3-δ with a perovskite-like structure has been synthesized and characterized. DC and AC transport properties of ceramic samples have been studied in the temperature range 4.2K – 1173K. DC conductivity data at T > 300K are discussed in terms of the small polaron hopping model. Dependence of the polaron activation energy on the calcium content is found to correlate with the change of structural distortion parameters. Both the temperature dependence of DC conductivity and the frequency dependence of the real part of the admittance reveal hopping transport at low temperatures.

AC transport properties of single and bilayer graphene

We have performed a theoretical study of electronic transport in single and bilayer graphene based on the standard linear-response (Kubo) formalism and continuum-model descriptions of the graphene band structure. We are focusing especially on the interband contribution to the optical conductivity σ ( ω ) . Analytical results are obtained for a variety of situations, which allow clear identification of features in σ ( ω ) that are associated with relevant electronic energy scales. Our work extends previous numerical studies and elucidates ways to infer electronic properties of graphene samples from optical-conductivity measurements.

Electrochromic, magnetotransport and AC transport properties of vapor phase polymerized PEDOT (VPP PEDOT)

Poly(3,4-ethylenedioxy)thiophene (PEDOT) doped with tosylate ion (PEDOT–tosylate or VPP PEDOT) was synthesized by vapor phase polymerization (VPP) technique on glass as well as on glass/ITO and the electrochromic properties were investigated. Compared with that of PEDOT–PSS spin-coated on glass/ITO, the studies showed that VPP PEDOT has a lower work function and better electrochromic properties. The magneto and AC transport properties studies were done on VPP PEDOT coated on glass substrate. The system shows 2-dimensional variable range hopping and wave function shrinkage of charge carriers.

Dielectric and transport properties of carbon nanotube-CdS nanostructures embedded in polyvinyl alcohol matrix

Multiwalled carbon nanotube-CdS/polyvinyl alcohol (MWCNT-CdS/PVA) composites have been grown by a simple chemical process on one-dimensional templates. The plane-view transmission electron micrographs clearly indicate the formation of nanocrystalline CdS on the nanotube surfaces. The superior dielectric behavior of the MWCNT-CdS nanostructures over MWCNT and PVA host matrices has been demonstrated. The dc and ac transport properties of CdS carbon nanotube-insulating polymer nanocomposites have been studied using impedance spectroscopy. An enhancement in optical band gap of nanocomposites over the bulk CdS has been observed due to the quantum confinement effect in CdS nanocrystals.

Current conserving nonequilibrium ac transport theory

We develop a microscopic theory for ac transport using nonequilibrium Green's-function theory. By including the self-consistent Coulomb interaction, the current conserving and gauge-invariant conditions are satisfied. On the theoretical side, our theory can be used to calculate nonlinear ac transport properties order by order at finite frequency when the system is driven far from equilibrium. In addition, the nonlinear ac charge response such as ac electrochemical capacitance can also be calculated. On the application side, our theory can be coupled to the density-functional theory to numerically predict the ac transport properties of nanodevices.

Polaron relaxation and hopping conductivity inLaMn1−xFexO3

dc and ac transport properties as well as electric modulus spectra have been investigated for the samples ${\text{LaMn}}_{1\ensuremath{-}x}{\text{Fe}}_{x}{\text{O}}_{3}$ with compositions $0\ensuremath{\le}x\ensuremath{\le}1.0$. The bulk dc resistivity shows a temperature variation consistent with the variable range hopping mechanism at low temperature and Arrhenius mechanism at high temperatures. The ac conductivity has been found to follow a power-law behavior at a limited temperature and frequency region where Anderson localization plays a significant role in the transport mechanism for all the compositions. At low temperatures large dc resistivity and dielectric relaxation behavior for all the compositions are consistent with the polaronic nature of the charge carriers. Scaling of the modulus spectra shows that the charge transport dynamics is independent of temperature for a particular composition but depends strongly on different compositions possibly due to different charge carrier concentrations and structural properties.

Ferromagnetic ordering in pulsed laser deposited Zn 1 − xNi xO/ZnO bilayer thin films

Zn 1 − x Ni x O (0.01 ≤ x ≤ 0.163)/ZnO bilayer thin films have been grown on [0001] oriented Al 2O 3 substrates using a pulsed Nd:YAG laser. While the ZnO buffer layer is deposited at a substrate temperature of 700 °C, Zn 1 − x Ni x O films are grown at 400 °C under oxygen partial pressure of 0.01 Pa. X-ray diffraction analysis reveals that all the films correspond to wurtzite hexagonal structure similar to ZnO with preferred orientation ( c-axis normal to film plane) and c-parameter decreasing with increase in the nickel content (‘ x’). Evidence has been advanced to suggest i) occupancy of Ni 2+ ions on Zn 2+ sites (i.e. in the centre of O 2− tetrahedra) instead of phase segregation of nickel locally, and ii) strong sp– d exchange interaction between the conduction and localized ‘d’ electrons of Ni 2+ ions. As ‘ x’ increases from 0.01 to 0.163, the optical band gap absorption edge undergoes gradual red-shift from 3.27 to 3.05 eV. The dc and ac transport properties suggest the dominance of hopping conduction in the temperature range of 5–12 K while magnetic characteristics provide evidence of ferromagnetic ordering in films above room temperature.

Characterization of embedded MgO/ferromagnet contacts for spin injection in silicon

In this work we present the structural and electrical characterization of sputter-deposited CoFe(B)/MgO/Si metal-insulator-semiconductor tunneling junctions for injection and detection of spin polarized current in silicon. The multilayers have been deposited in 700 nm deep trenches, patterned in thick SiO2 dielectric, on n- and p-doped wafers. The films inside the trenches are continuous with a correlated and low roughness. The MgO barrier grows amorphous without indication of pinholes. The dc and ac transport properties of the junctions were studied as a function of temperature and frequency. A relatively high interface trap density at the MgO/Si-interface is extracted from admittance spectra measurements. Transport is dominated by majority carriers in the case of n-doped and by minority carriers for the p-doped wafers. This leads to distinct rectification characteristics for the two wafer types, which would significantly influence the spin injection efficiency of the tunneling junctions.

Effect of bulk Ca-substitution in YBaCuO thin films for microwave applications

Calcium diffusion into defects in the high-temperature superconductor (HTS) compound YBa2Cu3O7−x (YBCO) has been previously investigated [D. Seron, D.E. Oates, G. Hammerl, J. Mannhart, P.J. Hirst, R.G. Humphreys, A.C. Anderson, M.A. Hein, J. Derov, Phys. Rev. B 72 (2005) 104511] as a possible way to improve the dc, ac and rf transport properties and to clarify nucleation, growth, and doping of YBCO. In this paper, the microwave properties of homogeneously deposited Ca-substituted YBCO, Y1−yCayBa2Cu3O7−x (YCBCO) films are presented and compared with the previously published Ca-free YBCO films with a Y0.7Ca0.3Ba2Cu3O7−x capping layer. By using different substrates and growth process detailed in the paper, particular defects are created in the YBCO or YCBCO films. In every situation, Ca-substitution is probed and its effect is compared with very high quality epitaxial YBCO films close to the intrinsic limit of microwave nonlinearity for YBCO. Information on the percolation, doping effect, and the role of the defects obtained from the dc properties in the normal state are used to help interpret the nonlinear microwave behavior.

AC Conductivity, XRD and transport properties of melt quenched PbI2–Ag2O–Cr2O3 system

Composite materials used for electrode and electrolyte materials have been intensely studied in view of their advantages such as higher conductivity and better operational performance compared to their single-phase counterparts. The present work aims at studying the electrical and structural characteristics of a new composite electrolyte namely, (PbI2) x − (Ag2O–Cr2O3)100−x where x = 5, 10, 15, 20, and 25 mol%, respectively, prepared by the melt quenching technique. The room temperature X-ray diffraction spectra revealed certain crystalline phases in the samples. AC conductivity analysis for all the prepared samples was carried out over the frequency range 1 MHz–20 Hz and in the temperature window 297–468 K. The room temperature conductivity values were calculated to be in the order of 10−5–10−3 Scm−1. An Arrhenius dependence of temperature with conductivity was observed, and the activation energies calculated were found to be in the range 0.27–0.31 eV. Furthermore, the total ionic transport number (t i) values obtained for all these indicated the ionic nature of this system.

Phenomenological Model for Frequency-Related Dissipation in the Quantized Hall Resistance

Recently, ac measurements of the quantized Hall resistance have shown a linear relationship between the deviation of the Hall resistance from the perfectly quantized value DeltaR <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">H</sub> and the dissipation in the system represented by the longitudinal resistivity rho <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">xx</sub> <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">ac</sup> . In this paper, we present a phenomenological model based on electrodynamic arguments that model this relation. The dissipation is due to the displacement current flowing in the system. All the microscopic features are included in a complex tensorial dielectric susceptibility which remains to be investigated if more physical insight in the ac transport properties of the 2-D electron gas is desired