Imaginary Part Of Conductivity Research Articles

Gauss–Bonnet holographic superconductors in lower dimensions

We disclose the effects of Gauss–Bonnet gravity on the properties of holographic s-wave and p-wave superconductors, with higher-order corrections, in lower-dimensional spacetime. We employ shooting method to solve equations of motion numerically and obtain the effect of different values of mass, nonlinear gauge field and Gauss–Bonnet parameters on the critical temperature and condensation. Based on our results, increasing each of these three parameters leads to lower temperatures and larger values of condensation. This phenomenon is rooted in the fact that conductor/superconductor phase transition faces with difficulty for higher effects of nonlinear and Gauss–Bonnet terms in the presence of a massive field. In addition, we study the electrical conductivity in holographic setup. In four dimension, real and imaginary parts of conductivity in holographic s- and p-wave models behave similarly and follow the same trend as higher dimensions by showing the delta function and divergence behavior at low-frequency regime that Kramers–Kronig relation can connect these two parts of conductivity to each other. We observe the appearance of a gap energy at $$\omega _{\mathrm{g}}\approx 8T_{\mathrm{c}}$$ which shifts toward higher frequencies by diminishing temperature and increasing the effect of nonlinear and Gauss–Bonnet terms. Conductivity in three dimensions is far different from other dimensions. Even the real and imaginary parts in s- and p-wave modes pursue various trends. For example in $$\omega \rightarrow 0$$ limit, imaginary part in holographic s-wave model tends to infinity but in p-wave model approaches to zero. However, the real parts in both models show a delta function behavior. In general, real and imaginary parts of conductivity in all cases that we study tend to a constant value in $$\omega \rightarrow \infty $$ regime.

Photoinduced Nonequilibrium Response in Underdoped YBa2Cu3O6+x Probed by Time-Resolved Terahertz Spectroscopy

Intense laser pulses have recently emerged as a tool to tune between different orders in complex quantum materials. Among different light-induced phenomena, transient superconductivity far above the equilibrium transition temperature in cuprates is particularly attractive. Key to those experiments was the resonant pumping of specific phonon modes, which was believed to induce superconducting phase coherence by suppressing the competing orders or modifying the structure slightly. Here, we present a comprehensive study of photo-induced nonequilibrium response in underdoped YBa$_2$Cu$_3$O$_{6+x}$. We find that upon photo-excitations, Josephson plasma edge in superconducting state is initially removed accompanied by quasiparticle excitations, and subsequently reappears at frequency lower than the static plasma edge within short time. In normal state, an enhancement or weaker edge-like shape is indeed induced by pump pulses in the reflectance spectrum accompanied by simultaneous rises in both real and imaginary parts of conductivity. We compare the pump-induced effects between near- and mid-infrared excitations and exclude phonon pumping as a scenario for the photo-induced effects above. We further elaborate the transient responses in normal state are unlikely to be explained by photo-induced superconductivity.

Observation of quantum corrections to conductivity up to optical frequencies

It is well known that conductivity of disordered metals is suppressed in the limit of low frequencies and temperatures by quantum corrections. Although predicted by theory to exist up to much higher energies, such corrections have so far been experimentally proven only for $\lesssim$80 meV. Here, by a combination of transport and optical studies, we demonstrate that the quantum corrections are present in strongly disordered conductor MoC up to at least $\sim$4 eV, thereby extending the experimental window where such corrections were found by a factor of 50. The knowledge of both, the real and imaginary parts of conductivity, enables us to identify the microscopic parameters of the conduction electron fluid. We find that the conduction electron density of strongly disordered MoC is surprisingly high and we argue that this should be considered a generic property of metals on the verge of disorder-induced localization transition.

Effect of evaporative drying on complex conductivity spectra of sandstones

Dehydration is a common process occurring in natural environments, rocks, and building materials. Suitable methods for monitoring the changing moisture content should be identified. We have investigated the impact of dehydration of sandstones on complex conductivity spectra. Spectral induced polarization measurements were performed on five sandstone samples during evaporative drying. The complex conductivity spectra indicate considerable changes with decreasing saturation. The relationship between conductivity and saturation can be described by a power law known as the second empirical Archie equation. Separate saturation exponents have been determined for the real and imaginary parts of conductivity. The imaginary part of conductivity indicates higher saturation exponents for the investigated sandstones compared to the real part. Obviously, the saturation exponents depend on the method used for changing water saturation. Evaporative drying, which is used in our experiments, causes an increase of pore-water salinity and results in lower saturation exponents for the real part of conductivity but higher exponents for the imaginary part in comparison with the classical imbibition/drainage technique. We evaluate a theoretical approach that considers the influence of pore water salinity on the saturation exponents of the real and imaginary parts of conductivity. The complex conductivity spectra are processed by a Debye decomposition procedure. The resulting integrating parameters such as direct current resistivity, total chargeability, normalized chargeability, and mean relaxation time indicate a power law dependence on saturation. Our experiments indicate that the imaginary part of conductivity and normalized chargeability have a high sensitivity related to the drying process of rocks and might be suitable indicators to monitor the changes of moisture content in sandstones.

An optical fiber surface plasmon resonance biosensor for wide range detection

An optical fiber surface plasmon resonance biosensor is presented that allows to numerically demonstrate, using transfer matrix method and the finite difference time domain method, the detection range is very wide. Two different structures of graphene photonic crystal multilayer (i.e. sensor I and sensor II) are constructed in the cladding region of single-mode fiber. Graphene is used as the plasma layer instead of the traditional metal. According to the analysis, the properties of graphene can be changed by adjusting the chemical potential µ c . In the spectral region of 1.667| µ c | ћω µ c |, the imaginary part of conductivity σ ″ becomes negative. Thus the weakly bounded low-less TE-SPR is supported by graphene. The results of the numerical simulation show that the relationship between refractive index and resonant wavelength is linear. The sensor I can detect the refractive index range of 1.33–1.4, and the sensitivity is 1942 nm/RIU. The sensor II can detect the refractive index range of 1.41–1.67, and the sensitivity is up to 2315.4 nm/RIU. Therefore, the detection of wide refractive index range of 1.33–1.67 or simultaneous detection of different biological medium concentration is realized by the sensor.

Holographic conductivity in the massive gravity with power-law Maxwell field

We obtain a new class of topological black hole solutions in (n+1)-dimensional massive gravity in the presence of the power-Maxwell electrodynamics. We calculate the conserved and thermodynamic quantities of the system and show that the first law of thermodynamics is satisfied on the horizon. Then, we investigate the holographic conductivity for the four and five dimensional black brane solutions. For completeness, we study the holographic conductivity for both massless (m=0) and massive (m≠0) gravities with power-Maxwell field. The massless gravity enjoys translational symmetry whereas the massive gravity violates it. For massless gravity, we observe that the real part of conductivity, Re[σ], decreases as charge q increases when frequency ω tends to zero, while the imaginary part of conductivity, Im[σ], diverges as ω→0. For the massive gravity, we find that Im[σ] is zero at ω=0 and becomes larger as q increases (temperature decreases), which is in contrast to the massless gravity. It also has a maximum value for ω≠0 which increases with increasing q (with fixed p) or increasing p (with fixed q) for (2+1)-dimensional dual system, where p is the power parameter of the power-law Maxwell field. Interestingly, we observe that in contrast to the massless case, Re[σ] has a maximum value at ω=0 (known as the Drude peak) for p=(n+1)/4 (conformally invariant electrodynamics) and this maximum increases with increasing q. In this case (m≠0) and for different values of p, the real and imaginary parts of the conductivity has a relative extremum for ω≠0. Finally, we show that for high frequencies, the real part of the holographic conductivity have the power law behavior in terms of frequency, ωa where a∝(n+1−4p). Some similar behaviors for high frequencies in possible dual CFT systems have been reported in experimental observations.

Dynamic conductivity of an InAs two-dimensional electron gas measured using terahertz time-domain spectroscopy

We utilized a coherent terahertz time-domain spectroscopy system to measure the dynamic conductivity of an InAs two-dimensional electron gas. Based on the theoretical transmission analysis, the real and imaginary parts of the complex conductivity of the sample were extracted from the transmitted terahertz time-domain waveforms. We also measured the sample at low temperatures and fitted the real and imaginary parts of the conductivity with the Drude model to extract its densities and mobilities. The dominant scattering mechanisms are discussed according to the temperature dependence of the extracted densities and mobilities.

Investigation of Precursor Superconducting State in YBa2Cu3O7−δthrough In-Plane Optical Spectroscopy

A precursor of superconductivity has been searched in the in-plane optical spectra of underdoped YBa$_2$Cu$_3$O$_y$, in which the previous $c$-axis optical spectra showed the presence of superconducting carriers at a temperature far above $T_c$ [Uykur $et al$., Phys. Rev. Lett. 112, 127003 (2014)][Dubroka $et al$., Phys. Rev. Lett. 106, 047006 (2011)]. By carefully subtracting the normal component from the imaginary part of conductivity $\sigma_2$($\omega$), we found a clear in-plane response of superconducting condensate at the temperature consistent with the $c$-axis optical data. This confirms that the precursory superconductivity developing with reducing a doping level is an intrinsic phenomenon in the cuprates.

On evaluation of electric conductivity by mean of non equilibrium thermodynamic approach with internal variables. An application to human erythrocyte suspension for metabolic characterizations

In this paper we will approach a new method to obtain electrical conductivity by means of dielectric measurements. After a remark on non-equilibrium thermodynamics with internal variables formulated by Kluitenberg, we introduce complex conductivity and the relationship with the complex dielectric constant. Taking into account the dielectric model by us introduced we determine the real and imaginary part of conductivity as function of the frequency of perturbation. Also we determine phenomenological and state coefficients that lead to the determination of the two components in which is splitted the polarization, and to the entropy production due to electrical conductivity. These results are applied to an human erythrocyte suspension for metabolic deductions.

Petrographical and petrophysical investigations of upper cretaceous sandstones of the South West Sennan field, Western Desert, Egypt

The sandstones of the Bahariya Formation represent an important Cretaceous oil and gas reservoir in Egypt. The sandstone samples of this study originate from Abu Sennan concession in the South West Sennan (SWS) oil field that is located in the Western Desert of Egypt. The mineralogical composition for studied sandstone samples is mainly composed of detrital quartz and quartz overgrowth (69 to 82 %), kaolinite (6 to 20 %), plagioclase and microcline (2 to 9 %), and Fe-rich chlorite and framboidal pyrite (2 %). Muscovite, biotite, zircon, rutile, and apatite, as well as diagenetic calcite, are present as accessory minerals. The petrophysical properties indicate a moderate degree of diagenesis and cementation for the investigated sandstones that results in favorable reservoir conditions. We provide a detailed description of the petrophysical investigations and the resulting relationships. The magnetic susceptibility has proved to be a key parameter in evaluating porosity and permeability of the investigated sandstone samples of the SWS oil field. Obviously, Fe-rich chlorite cement that has been precipitated in the pore space control porosity, specific internal surface, and permeability. Beside the formation factor, the specific internal surface, and the imaginary part of conductivity, longitudinal and transversal relaxation times determined by nuclear magnetic resonance enable a reliable permeability prediction.

Impedance Analysis of Gellan Gum - Poly(vinyl pyrrolidone) Membranes

This paper presents the impedance study of low acyl gellan gum/poly(vinyl pyrrolidone) blend as a host matrix for ion conduction. Low salt content of lithium perchlorate was added into the blend polymer host and the membranes were prepared by the means of solution casting method. The temperature-conductivity of the investigated samples follows the Vogel-Tamann-Fulcher rule. The occurrence of electrode polarization was explained by the combination of the dielectric loss, tangent loss and imaginary part of conductivity. The relaxation time for dipole orientation and ionic conductivity were calculated from tangent loss and imaginary part of modulus.

Study the optical properties of titanium oxide thin films deposited on glass substrate at different deposition angles by resistive evaporation method

Titanium oxide thin films of 33nm thickness were deposited on glass substrates by resistive evaporation method under UHV conditions, and at different deposition angles of 0°, 20° and 30° at room temperature. Their optical spectra were measured by spectrophotometer in the spectral range of 300–1100nm wavelength (UV–vis). The optical constants such as, real part of refractive index (n), imaginary part of refractive index (k), real and imaginary parts of dielectric function ɛ1 and ɛ2 respectively, absorption coefficient (α), real part of conductivity (δ1) and imaginary part of conductivity (−δ2) were obtained using the Kramers–Kronig analysis.Band-gap energy (Eg) was also estimated for these films. Fraction of voids was determined using the Aspens theory.

Erratum: “The origin of non-Drude terahertz conductivity in nanomaterials” [Appl. Phys. Lett. 100, 132102 (2012)

In a recent letter, we proposed a model for the nonDrude terahertz conductivity of nanosilicon and hexagonal Ge2Sb2Te5 (GST) based on a series sequence of two contributions: the conductivities of free carriers and tunneling carriers. The best fit of the proposed model to nanosilicon experimental data also used e1, where e1 is the background dielectric constant, as one of the parameters. As the experimental THz conductivity reported for nanosilicon has been obtained from optical-pumping THz spectroscopy (Ref. 7), the effect of e1 is subtracted from the experiment itself. Thus, we do not need to include the effect of e1 in fitting the model to experimental data. Figs. 1(a) and 1(b) show the corrected results of the real and imaginary part of conductivity in nanosilicon. Deduced physical parameters are also modified (f1⁄4 0.83, nf1⁄4 5.3 10 cm , s1⁄4 2.0 10 14 s, nt1⁄4 2.0 10 cm , and st1⁄4 2.0 10 14 s). Note that the results for GST are unaltered, because the experimental data are not obtained by optical-pumping. Finally, we should comment on the way in which we used Eq. (5) to calculate the overall conductivity. We described the tunneling conductivity as rt*(x)1⁄4 rtR(x) þ irtI(x) in Eq. (3). Since Eqs. (2) and (3) in the literature have been derived for e ixt and e, respectively, we had to change i to i in rt*(x) in the actual calculations in Eq. (5) to maintain consistency and causality. Also, Ref. 7 cited in the text in line 10, column 2 on page 2 should have been 9.

Ballistic transport, chiral anomaly, and emergence of the neutral electron-hole plasma in graphene

The process of coherent creation of particle - hole excitations by an electric field in graphene is quantitatively described using a dynamic "first quantized" approach. We calculate the evolution of current density, number of pairs and energy in ballistic regime using the tight binding model. The series in electric field strength $E$ up to third order in both DC and AC are calculated. We show how the physics far from the two Dirac points enters various physical quantities in linear response and how it is related to the chiral anomaly. The third harmonic generation and the imaginary part of conductivity are obtained. It is shown that at certain time scale $t_{nl}\propto E^{-1/2}$ the physical behaviour dramatically changes and the perturbation theory breaks down. Beyond the linear response physics is explored using an exact solution of the first quantized equations. While for small electric fields the I-V curve is linear characterized by the universal minimal resistivity $\sigma =\pi /2(e^{2}/h)$%, at $t>t_{nl}$ the conductivity grows fast. The copious pair creation (with rate $E^{3/2}$), analogous to Schwinger's electron - positron pair creation from vacuum in QED, leads to creation of the electron - hole plasma at ballistic times of order $t_{nl}$. This process is terminated by a relaxational recombination.

Influence of ambient conditions on surface traps in nanoporous anatase

Time-resolved microvawe photoconductivity was measured for sol-gel prepared nanoporous anatase layer in different ambients. We found a short decay of the photoconductivity in the range of few µs and a long lasting persistent photoconductivity with decay times between 30 and 60 minutes. The binding energies for ambient-modified surface traps were calculated using the short decay of the real and imaginary part of conductivity and the damped driven oscillator model. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Influence of electronic and ionic processes on electrical properties of TlBr crystals

The mechanism of electrical conductivity in TlBr at room temperature is investigated. The frequency dependence of the real and imaginary part of conductivity is presented at different temperatures. Persistent conductivity has been observed in the crystals and indicates the existence of inhomogeneities in such crystals. The long-time relaxation of photoconductivity at room temperature is conditioned by ionic-electronic processes, which are sensitive to the polarity of the electric field. The frequency dependence of conductivity at different temperatures and excitations was used to estimate the role of ionic conductivity. We demonstrate experimentally that Tl + ions are responsible for the ionic conduction mechanism at temperatures higher than 250 K. The main change of the current versus time dependence (500 h) corresponds to the same amount of electric charge that the evaporated Tl layer could carry as Tl + ions. A new, unexpected result was found in direct Tl + ion transport: diffusion-limited aggregation is responsible for dendrite structures, which could grow by the electrodiffusion of Tl + ions. The analysis of fractal formation by the electrodiffusion of Tl + ions seems very promising to understanding the polarisation phenomena in TlBr.

Charge coupled cyclotron motion of electrons and holes in InGaAsN epitaxial layers

ABSTRACTTime resolved cyclotron resonance measurements are used to investigate more in further detail the effective mass of electrons and holes in InGaAsN epitaxial layers. The In and N content in the alloy are adjusted to yield the latticematching of the epilayers (200 nm thick) to GaAs. A continuous increase of the effective mass of electron and an increase of the resonance associated with holes is observed. Through the evolution of the imaginary part of conductivity as a function of the elapsed time we show that this observation is a coupled cyclotron resonance that may have maxima in the real part of conductivity but should not be necessarily correlated with the “increase” of the effective mass.

Impedance characteristics of layer-by-layer electrostatic self-assembled films of evans blue

We have carried out impedance measurements of metal–insulator–metal structures based on thin films of evans blue deposited by the layer-by-layer electrostatic self-assembly (ESA) method. The results gave us a complete understanding of the electronic properties employed in layer-by-layer self-assembled thin films. It has been shown that a parallel capacitor–resistor ( C P– R P) network can be considered to be equivalent in behaviour to such structures. Using conductive-system dispersion (CSD) model, the frequency dependence of the real and imaginary parts of conductivity has been investigated and the significant role played by the electrodes has been shown. All these studies have been made for devices with different concentrations of evans blue and under a dc bias voltage varying from 0 to 10 V in the frequency range of 10 Hz–1 MHz.

Theory of the AC Conductivity and Hall Mobility in Inhomogeneous Anisotropic Solids

The theory of ac conductivity and Hall mobility for solid systems which consist of anisotropic crystals with random dielectrical inclusions is developed. In the present paper we use the effective-medium theory. The large-frequency region, where ω < τ -1 (τ is the electron mean free time) is investigated. It is shown that in the strong frequency dispersion region both real and imaginary parts of conductivity and Hall mobility follow a power-law behavior ω S with high anisotropic exponent S which may be used for the interpretation of experimental data in oxides.

Theory of the AC Conductivity and Hall Mobility in Inhomogeneous Anisotropic Solids

The theory of ac conductivity and Hall mobility for solid systems which consist of anisotropic crystals with random dielectrical inclusions is developed. In the present paper we use the effective-medium theory. The large-frequency region, where ω < τ—1 (τ is the electron mean free time) is investigated. It is shown that in the strong frequency dispersion region both real and imaginary parts of conductivity and Hall mobility follow a power-law behavior ωS with high anisotropic exponent S which may be used for the interpretation of experimental data in oxides.