Dependence Of Conductivity Research Articles

Topological transport properties of highly oriented Bi2Te3 thin film deposited by sputtering

Topological insulators (TIs) are the promising materials for next-generation technology due to their exotic features such as spin momentum locking, conducting surface states, etc. However, the high-quality growth of TIs by sputtering technique, which is one of the foremost industrial requirements, is extremely challenging. Also, the demonstration of simple investigation protocols to characterize topological properties of TIs using electron-transport methods is highly desirable. Here, we report the quantitative investigation of non-trivial parameters employing magnetotransport measurements on a prototypical highly textured Bi2Te3 TI thin film prepared by sputtering. Through the systematic analyses of the temperature and magnetic field dependent resistivity, all topological parameters associated with TIs, such as coherency factor , Berry phase (), mass term (m), the dephasing parameter (p), slope of temperature dependent conductivity correction () and the surface state penetration depth (λ) are estimated by using the modified ‘Hikami–Larkin–Nagaoka’, ‘Lu–Shen’ and ‘Altshuler–Aronov’ models. The obtained values of topological parameters are well comparable to those reported on molecular beam epitaxy grown TIs. The epitaxial growth of Bi2Te3 film using sputtering, and investigation of the non-trivial topological states from its electron-transport behavior are important for their fundamental understanding and technological applications.

Unravelling the crossover amongst band and various hopping conduction mechanisms in Mo doped ZnO thin films owing to carrier localization at defects

Understanding the carrier transport mechanism of transparent conducting oxides is crucial for its application in optoelectronic devices. Although, electrical transport mechanisms of various trivalent cation doped ZnO films have been reported adequately, a substantial study is needed to understand the transport mechanism in ZnO doped with higher valence (>+3) dopants. Unlike Al, In and Ga doped ZnO where metal-insulator transition is commonly observed, in this study, the crossover amongst band and various hopping mechanisms has been observed in the temperature range of 300–10 K in Mo doped ZnO films deposited at various substrate temperatures. All the films show electrons as the majority charge carriers and notably, the film deposited at 573 K shows the highest conductivity value of 167.22 (Ω.cm)−1. In-depth analyses of the conduction mechanism reveal grain boundary scattering dominated band conduction at and not far below the room temperature, which is shifted towards nearest neighbor hopping (NNH), and then to Mott variable range hopping (VRH) type of conductions as the temperature is decreased further. Unfamiliarly, a modified Efros-Shklovskii (ES) VRH type of conduction mechanism is shown to prevail in the lowest measured temperature range (∼30–10 K). Unprecedentedly, it is demonstrated here that the activation energies for various conduction mechanisms are higher at lower vacuum condition (∼10−1 mbar) as compared to those measured at higher vacuum condition (∼10−3 mbar) implying an added role of the surface traps in the charge transport process. The temperature dependent conductivity and X-ray photoelectron spectroscopy (XPS) studies proclaim that the film deposited at 573 K possess fewer defects related disorder for carrier localization as compared to the films deposited at 523 K and 698 K.

Interplay of Thermal and Electronic Effects in the Mott Transition of Nanosized VO2 Phase Change Memory Devices

Volatile memory devices relying on the Mott-type insulator-to-metal transition of vanadium oxide (VO2) are widely utilized in the field of neuromorphic computing. Such devices, however, are realized in a nanoscale geometry, where the switching relies on the self-heating of an ultrasmall spot as well as the presence of extremely high electric fields in the active region. In this paper, we investigate the interplay of such nanoscale thermal and nonlinear electronic phenomena by investigating the temperature and voltage dependent conduction properties of our custom-designed VO2 devices, where a V-shaped electrode focuses the switching to an ultrasmall single-spot active region. This simplified spatial structure of the active volume facilitates the device modeling and the identification of physical mechanisms behind the phase transition. We find that purely thermal or electronic effects fail to describe the device operation, however, according to our finite element simulations, a combined electronic and thermal model provides a precise description of the device characteristics. These results facilitate the understanding as well as the thermal and electronic design of novel VO2-based neuronal devices.

Scattering of phonons by edge dislocation and thermal conductivity of nanocrystalline silicon

The temperature dependent thermal conductivity of nanocrystalline silicon with the grain size of 500 nm, 144 nm, and 76 nm, was calculated based on the intrinsic phonon properties using a first-principles-based method and recent analytical theory of phonon-dislocation scattering. The results demonstrate, in the nanocrystalline silicon, that the low temperature (T) limit of thermal conductivity is directly proportional to T2 instead of the conventional Casimir limit of T3. Calculated frequency-dependent scattering rates also indicate regimes of dominant scattering mechanisms: frequency-independent boundary scattering dominates in the very low phonon frequency regime of less than 0.6 THz; frequency-dependent boundary scattering dominates in the intermediate phonon frequency regime between 0.6 and 6 THz; and Umklapp scattering dominates in the high phonon frequency regime beyond 6 THz. The thermal conductivity is linear proportional to nanograin size in the region of d<100nm and it shows independence at d>1μm. This work proves the importance of dislocation-phonon scattering to thermal conductivity when the size of nanograins is small or at low temperature.

Small polaron hopping transport mechanism, dielectric relaxation and electrical conduction in NiAl2O4 electro-ceramic spinel oxide

In this study, the frequency and temperature dependent dielectric relaxation and electrical conduction mechanisms in NiAl2O4 spinel oxide ceramic have been explored in a frequency range of over a measured temperature from 163–283 K. The polycrystalline NiAl2O4 was synthesized via solid state reaction route and sintered at 1000 °C. The room temperature x-ray powder diffraction pattern confirmed the formation of NiAl2O4 spinel phase with Fd3m space group. The surface morphology of the sample was investigated by scanning electron microscopy and chemical properties by Fourier transform infrared spectroscopic analysis. Complex impedance studies revealed the presence of relaxation time distribution and charge carriers were found to be thermally activated. The Nyquist plots exhibited depressed semicircles and their fitting by an equivalent circuit model with configuration (RGCG)(RGBCGB) resolved the contributions of grains and grain-boundaries to the electrical transport properties of the material. Electrical conductivity analysis followed the Jonscher’s power law behavior and the frequency exponent suggested small polaron hopping as the governing transport mechanism in NiAl2O4 spinel oxide. Non-Debye type nature of dielectric relaxation was confirmed by the complex modulus analysis whereas dielectric constant and tangent loss analysis verified that the hopping mechanism was thermally activated.

Transport and recombination properties of doped SiC nanoribbons with different atoms substituted by group-V elements

Based on semiconductor theory and optimization data derived from first-principles, we simulate the conductive and recombination properties of group-V doped zigzag SiCNRs (ZSiCNRs), through in-depth analysis of different substitutional doping models, to unveiling the mechanism of carrier transport in nanodevices. It’s found that though there are two impurity energy levels whether C or Si is substituted in doped ZSiCNRs, the conductivity is significantly larger for Si atom substituted than that of C atom replacement. Besides, different temperature dependent conductivity characters are exhibited in the weak and strong ionization regions, indicating the differences in dominant factor for the transport processes. The temperature-dependent holes trapping rate of carrier recombination increased sharply for C or Si atom substitution, indicating a negative temperature-dependence of lifetime of non-equilibrium minority carrier for doped ZSiCNRs. Further, the non-equilibrium minority carrier lifetime is longer with small injection due to the presence of donor impurities in group-V doped ZSiCNRs.

Field and temperature assisted DC/AC conduction measurements of PS/ZnO nanocomposite films

Electrical, optical and morphological properties of polymer nanocomposite materials are quite interesting to investigate due to their innovative characteristics and applications. Temperature dependent dc conductivity using different charge transport mechanisms like ionic hopping, Schottky and Poole-Frankel emissions has been investigated for pristine PS and PS/ZnO (at different wt% of ZnO) nanocomposite films in this research report. Non-ohmic conduction behaviour of these films developed an interest to carry out bulk and electrode limited conduction mechanisms. Ionic jump distance ‘a’ and conduction coefficient ‘βexp’ values for these films were calculated at various temperatures from the slopes of log I vs E and log I vs E1/2 plots respectively. The increase in dc conductivity of an order of 108 was observed by increasing the doping concentration up to 20 wt% of ZnO. The change in their ac conductivity was observed from 10−9 S/m to 10−6 S/m by varying the frequency from 1 to 300 kHz.

Moisture and frequency dependent conductivity as an obstacle to determining electrical percolation thresholds of cementitious nanocomposites made with carbon nanotubes

Impedance spectroscopy was applied to Portland cement and its carbon nanotubes (CNT) composites to measure and describe the electrical conductance phenomena and their dependency on the moisture. Two series of composites were prepared, one with multi-walled, and the other with single-walled CNTs. The percolation concentration was reached only with the single-walled CNTs between 0.10 and 0.25 wt%; it was therefore possible to compare a percolative and a non-percolative system. The kinetic of the drying process was measured in the range of 24 h and described by a decay model with a stretched exponential to be correlated with the composite composition. The polarization phenomena occurring in the materials before and after the moisture removal were modelled with logistic sigmoid and explained by the morphology. In particular, the three found sigmoid were correlated to the polarization phenomena occurring at well-defined structural levels of the specimens. Their mathematical definition was shown to be fundamental for a correct interpretation of the Cole-plots of the real conductivity. Such phenomena presented a peak of intensity at a well define frequency but their effects spread across a broad range of Hertz. Moreover, over the AC frequency of 10 Hz, the conductive effect of the moisture overlapped the conductivity increase caused by the percolative network of the CNT. A dry sample is therefore necessary for accurately evaluating the source of the conductivity, a distinction which is crucially important for sensing applications.

Anomalous magnetohydrodynamics with temperature-dependent electric conductivity and application to the global polarization

We have derived the solutions of the relativistic anomalous magnetohydrodynamics with longitudinal Bjorken boost invariance and transverse electromagnetic fields in the presence of temperature or energy density dependent electric conductivity. We consider the equations of states in a high temperature limit or in a high chiral chemical potential limit. We obtain both perturbative analytic solutions up to the order of \hbar and numerical solutions in our configurations of initial electromagnetic fields and Bjorken flow velocity. Our results show that the temperature or energy density dependent electric conductivity plays an important role to the decaying of the energy density and electromagnetic fields. We also implement our results to the splitting of global polarization for \Lambda and \bar{\Lambda} hyperons induced by the magnetic fields. Our results for the splitting of global polarization disagree with the experimental data in low energy collisions, which implies that the contribution from gradient of chemical potential may dominate in the low energy collisions.

Temperature dependent volumetric, viscometric and conductance studies of barium chloride in aqueous solution of citric acid: an insight into molecular interactions

Abstract Alkaline earth metal salts and citric acid both play a vital role in our daily life in terms of biological importance, so in this regard, in the present study, we calculated density, speed of sound, conductance and viscosity of barium chloride (BaCl2) in concentration range (0.01–0.12 mol kg−1) in a binary solution of 0.01 mol kg−1 citric acid in water (CA + H2O) at temperatures between 303.15 and 318.15 K with gap of 5 K. The various parameters like apparent molar volume ϕ v , limiting apparent molar volume ϕ v o , transfer volume Δ tr ϕ v o , and partial molar volume expansibilities Φ E o ${{\Phi }}_{E}^{o}$ were calculated using density data. Viscosity data has been applied to calculate relative viscosity and Jones Dole coefficient A and B. Other parameters like Molar conductance (Λ m ), Λ o m and Λ o m η o have also been calculated using conductance data. All these parameters show strong evidences of various molecular interactions in terms of ion–solvent and solute–solute interactions. The ion–solvent interactions were supported by isobaric thermal expansion coefficient α o . Structure maker/breaker behaviour is explained by Hepler’s constant ((d 2 ϕ v o /dT 2 ) p ), temperature coefficient of B ( d B / d T $dB/dT$ ) and Walden product’s temperature coefficient (d(Λ o m η o )/dT). Further these molecular interactions support structure breaking potential of present studied system.

Formation of silver particles in [PVA:CS:PEG]-AgNO3 based biopolymer electrolyte membranes: Structural and electric transport properties study

[Chitosan (CS)-poly(vinyl) alcohol (PVA)-polyethylene glycol (PEG-200)]+xwt% silver nitrate (AgNO3) (x = 0, 10, 20, 30, 40) based biopolymer blend electrolytes (BPBEs) were prepared using solution casting method. FTIR results show the interaction of Ag+/NO3− with the constituents of polymers CS/PVA. XRD results reveal that the amorphousness increases with increasing AgNO3 concentration. Optical morphological (OM) images show that the silver, Ag(o) particles are agglomerated on the surface of the membrane. Temperature dependent dc conductivity (σdc) and dielectric relaxation frequency (fr) for BPBEs follow the Arrhenius type behavior and the sample containing 30 wt% salt has optimum dc conductivity, σdc∼1.7×10−4 S/cm at 30oC with lowest activation energy∼ 0.16 eV. The ac conductivity spectra follow the Jonscher's power law (JPL) with power law exponent, s > 1. The scaling of AC conductivity and loss tangent (tanδ) spectra were performed with respect to temperatures/concentrations of salt. The dielectric permittivity study reveals that the electrode polarization effect predominates at low frequencies. The Ionic Transference Number (ITN) measurement was used to determine the nature species of charge carrier, either ionic or electronic. The linear sweep voltammetry (LSV), result shows that 30% AgNO3containing sample has the electrochemical stability window (ESW) ∼+1.5 to -1.5 V.

Unveiling the underlying mechanism of unusual thermal conductivity behavior in multicomponent high-entropy (La0.2Gd0.2Y0.2Yb0.2Er0.2)2(Zr1-xCex)2O7 ceramics

Previously, the research on thermal conductivity of ceramic thermal barrier coatings mainly focused on phonon and photon thermal conductivity (thermal radiation effect). However, electrical conductivity is remarkable in some systems. Hence, the contribution of phonon, photon and electronic heat conduction to thermal conductivity of high-entropy systems was evaluated in this study. The (La0.2Gd0.2Y0.2Yb0.2Er0.2)2(Zr1-xCex)2O7 (x = 0–0.5) high-entropy ceramics with single defective fluorite structure were successfully prepared via a solid reaction method. Below 600 °C, the thermal conductivities decrease with increasing temperature for x = 0.1–0.5 components, then reveal a drastic temperature dependent increase. Moreover, the composition dependent thermal conductivities are also unusual based on the conventional phonon thermal conduction mechanism. The increased electronic thermal conductivity, improved photon thermal conductivity (at high temperatures) and reduced phonon-grain boundary scattering should be responsible for the unusual thermal conductivity behavior. This can be verified by the significantly increased electrical conductivity, optical transmittance and grain size, as well as reduced emissivity for (La0.2Gd0.2Y0.2Yb0.2Er0.2)2(Zr1-xCex)2O7 high-entropy ceramics. The present study also broadens the way to investigate the thermal conductivity of ceramic thermal barrier coatings, and is helpful to design thermal barrier coatings with low thermal conductivity.

Study of dielectric, impedance and transport properties of co-doped Ba0.5Sr0.5TiO3 ceramics

Dielectric properties and Electrical response of BST ceramics having composition Ba0.5Sr0.5Ti0.98Sn0.01Zr0.01O3 synthesized by solid state reaction route are studied in this piece of work. The structural part analysis is done using XRD technique and scanning electron microscopy and is confirmed to be of cubic structure. Electrical characterization through impedance and modulus spectroscopy is made over a frequency region of 1 kHz to 1 MHz from ambient temperature to 400 °C. Near constant value of Ɛr and significantly low and flat loss tangent at room temperature are achieved in the material which suggest possible application in electronic circuits. Temperature dependent broadened loss peaks observed beyond 250 °C indicated thermally activated conductivity/ relaxation process in the material. Impedance spectroscopy results were examined in alternate formalism for the role of grain boundary and grain in various processes. The computed activation energy for charge carriers provides information of the transport phenomena. The estimated values suggest that oxygen vacancies play important role in conduction process. The activation energy is estimated by fitting temperature dependent relaxation time with Arrhenius equation and the type of charge carriers involved in the relaxation process are suggested. Frequency dependent ac conductivity plots at various temperatures obey Johnscher’s law. Variation of exponent in Johnsher’s power law with temperature well agrees with correlated barrier hopping (CBH) model for ac conduction mechanism.

The investigation of frequency dependent dielectric properties and ac conductivity by impedance spectroscopy in the Al/(Cu-doped Diamond Like Carbon)/Au structures

The investigation of frequency dependent dielectric properties and ac conductivity by impedance spectroscopy in the Al/(Cu-doped Diamond Like Carbon)/Au structures

Structural study and temperature dependent relaxation and conduction mechanism of orthorhombic tungsten bronze Ba4La28/3(Zr0.05Ti0.95)18O54 ceramic

In this study, the Ba4La28/3(Zr0.05Ti0.95)18O54 ceramic was prepared using the solid-state technique. The X-ray diffraction technique was used in this study to investigate the Ba/La order in the solid solution Ba4La28/3(Zr0.05Ti0.95)18O54. We have developed a structural model, to represent the order within the structure. The formula of model [Ba4−a□a]A2[La8+2b3□2−2b3]A1,A1′(Zr0.05Ti0.95)18O54 is described as perovskite layers with a thickness of 3 octahedral parallel to the xOz plane. Scanning electron microscopy (SEM) showed that Ba4La28/3(Zr0.05Ti0.95)18O54 ceramic exhibits a typical columnar grain with a heterogeneous distribution over the entire surface. Complex impedance spectroscopy was used to analyze the dielectric and electrical properties of Ba4La28/3(Zr0.05Ti0.95)18O54 materials over a frequency range of 10 Hz to 1 MHz and a temperature range of 350°C to 400°C. An appropriate equivalent electrical circuit was also used to investigate the contributions of grains and grain boundaries. The AC-conductivity data was analyzed using Jonscher's universal power law. The thermal behavior of the exponent parameter "s" indicates that the dominant conduction mechanism in our compound is the non-overlapping small polaron tunneling (NSPT) model. The activation energies calculated from the conduction and relaxation mechanisms are nearly identical for the Ba4La28/3(Zr0.05Ti0.95)18O54 ceramic.

Spin-valley switch and conductance oscillations in antiferromagnetic transition metal dichalcogenides

Antiferromagnetic materials are regarded as the outstanding candidates for the next generation of spintronics applications thanks to the numerous interesting features. We theoretically study the spin and valley transport in transition metal dichalcogenides in the present of antiferromagnetic exchange field. It is found that the spin and valley dependent band gap can be controlled by the exchange field. The system could become a spin-valley half metal, where a certain spin-valley electron is metallic state and other electrons are insulating states. The normal/antiferromagnetic/normal junction could work as an effective spin-valley switch controlled by the gate voltage. In the normal/antiferromagnetic/normal/ferromagnetic/normal junction, quantum beats occur in the oscillations of total conductance. The beat phenomenon results from the interference of two different spin-valley dependent conductances with similar frequencies. In addition, the junction can also work as a magnetoresistance device.

Anomalous and non-monotonic strain dependent thermal conductivity of typical negative thermal expansion material ScF3

It is well known that the lattice thermal conductivity of the vast majority of bulk materials increases under compressive strain and decreases under tensile strain. In this paper, taking the typical negative thermal expansion material ScF3 as an example, we systematically investigate the effect of strain on the phonon transport properties by means of first-principles calculation combined with the Boltzmann transport equation. The results show that the thermal conductivity of ScF3 exhibits an anomalous strain dependence, where the thermal conductivity decreases under compressive strain while increasing as the tensile strain is smaller than −4.2 GPa. When the tensile strain increases further (larger than −4.2 GPa), however, the thermal conductivity of ScF3 decreases with tensile strain, presenting a non-monotonic behavior. Through analyzing the phonon mode information, we demonstrate that such anomalous and non-monotonic strain dependent thermal transport in ScF3 mainly originated from the intrinsic character of the negative Grüneisen parameter (negative thermal expansion). Moreover, the root mean square displacement is also utilized to further demonstrate the variation of phonon anharmonicity and thermal conductivity of ScF3 with strain. The findings shed light on the underlying association between thermal expansion and thermal conductivity and provide a feasible strategy for searching for materials with anomalous strain dependent thermal conductivity.

Numerical investigation of a non-linear moving boundary problem describing solidification of a phase change material with temperature dependent thermal conductivity and convection

Problems related to the change in state of a PCM offer a great contribution in the field of heat transfer, describing melting, freezing, and sublimation. While most articles already exist on the mathematical aspects of this type of problem that account for constant thermal conductivity, there is still insufficient modeling and solutions involving convection driven by heated fluid and temperature-dependent thermal conductivity, which are presently being considered. In connection with this, the moving boundary problems become highly non-linear and their exact treatments are restricted. To deal with non-linearity, the numerical solution of the problem is obtained via the LWC technique. Convergence analysis of the non-linear model is extensively presented. Numerical codes are validated against analytical results and found to be in strong agreement. Numerical data of the solidification of the alloy is presented for the validation of the current model. It is found that the rate of freezing increases as the value of the dimensionless parameter b increases. The impact of other parameters on freezing is discussed in detail. Furthermore, in the Stefan problem with convection and temperature-dependent thermal conductivity, the rate of freezing is faster than the standard problem, and in the Stefan problem with convection and fixed thermal conductivity.

Role of Crystal Structure on the Ionic Conduction and Electrical Properties of Germanate Compounds A2Cu3Ge4O12 (A = Na, K)

Crystal structure and electrical properties of A2Cu3Ge4O12 (where A = Na, K) have been investigated by neutron diffraction and electrical impedance spectroscopy (EIS). The real and imaginary parts of the impedance, dielectric constant, and electrical modulus have been studied as a function of frequency and temperature. The crystal structures of the compounds K2Cu3Ge4O12 (KCGO) and Na2Cu3Ge4O12 (NCGO) are made of perfect 2D and quasi-2D alkali metal ion layers, respectively. The dc conductivity occurs due to small polaron hopping for both compounds with single activation energy over the entire experimentally available temperature range. The values of activation energies are 0.90(3) and 1.22(6) eV for KCGO and NCGO, respectively, as determined from dc conductivity and relaxation time analysis. The frequency dependent ac conductivity curves for both compounds follow a universal Jonscher’s power law. Soft bond valence sum analysis of neutron diffraction data reveals that the conduction pathways are confined within a plane in KCGO, whereas they form a one-dimensional channel in NCGO. The bottlenecks for ionic conduction and the role of crystal structure on them are discussed. The scaling behavior of electrical modulus indicates that the relaxation process does not change with temperature; however, a sharp decrease in the time constant has been observed. The present comprehensive study facilitates the understanding of the role of crystal structure on the microscopic mechanism of ionic conduction in both battery materials, which would be useful in designing sodium and potassium based ionic conductors. Further, the constant value of activation energy over the kHz frequency range is suitable for potential applications of the studied materials in avalanche beacons and amateur and geophysical sensors.

Structure dependent changes in pore water pressure due to stress application and consequences on the effective stress

Divergent processes in soils affect the pore systems and alter hydraulic, gaseous and thermal fluxes as soon as the internal soil strength (= the precompression stress) is exceeded. Especially the intense changes of the hydraulic flux properties during soil deformation result in retarded soil settlement due to their dependency on time and stress dependent changes in pore water pressure as well as the water saturation degree. These processes depend besides on texture and bulk density especially on aggregation level, but the linkage between changes in effective stress and soil structure in various soil horizons defined by the precompression stress is still not analyzed and is therefore subject of this investigation. In this study, stress strain and pore water pressure data from almost 900 (predrained to −60 hPa) samples from arable top- and subsoil horizons subjected to oedometer compaction tests are evaluated. Among others, hydraulic stress parameters like the effective stress and χ-factor are calculated and linked to the precompression stress values. Results are presented for 6 differently aggregated soil horizons. Ap horizons with a very weak soil structure document no defined soil rigidity due to a reduced functionality of the pore systems and a stress dependent weakening, while especially the soil deformation behavior of sandy Bw horizons is less controlled by the precompression stress and the calculated effective stresses. The E and Bg horizons indicate anthropogenic impacts on structure properties and the related stress-strain behavior. While it could be documented that the enhanced strain coincides with an increase in the calculated χ-factor, it does not coincide in correspondingly enhanced effective stresses due to the non- equilibrated pore water pressure even after 45 h of increasing stress application. We conclude that further investigations are needed to quantify the highly dynamic interactions between the stress dependent hydraulic conductivity / pore water pressure ratio and the impact on the stress affected pore water pressure reequilibration. Furthermore, the contribution of the sorptive potential in soils with more negative initial matric potential on these changes would elucidate also the physico chemical impacts on soil strength more clearly especially under drier soil conditions.