Logarithmic Increase Of Resistivity Research Articles

Granular Effect on Electron Conduction in Discontinuous Metal Films

The seminal work by Dolan and Osheroff [Phys. Rev. Lett. 43, 721 (1979)], which reported anomalous low‐temperature conduction of high‐resistivity thin‐film metal strips is reanalyzed. It is argued that the observed logarithmic increase of resistance with decreasing temperature in their 3 nm thick Au–Pd strips can be ascribed to the granularity effect on electron conduction in discontinuous metal films. This reanalysis is further supported by the measurements on conducting Pb x (SiO2)1−x nanogranular films, where x is the volume fraction of Pb.

Unconventional Transport Properties of Reduced Tungsten Oxide WO2.9

The temperature and magnetic field dependence of resistivity in WO2.9 was investigated. The variation of resistivity with temperature displayed unusual features, such as a broad maximum around 230 K and a logarithmic increase of resistivity below 16 K. In the temperature range 16–230 K, we observed metallic-like behavior with a positive temperature coefficient. The combined analysis of resistivity and magnetoresistance (MR) data shows that these unusual transport properties of WO2.9 can be understood by considering the (bi)polaronic nature of charge carriers. In contrast to magnetization data, superconducting transition below Tc = 80 K was not detected in resistivity measurements, indicating that the superconductivity is localized in small regions that do not percolate. We found a strong increase in positive MR below 80 K. This effect is similar to that observed in underdoped cuprates, where the substantial increase of MR is attributed to superconducting fluctuations in small clusters. Therefore, the temperature dependence of MR indicates the presence of non-percolating superconducting clusters in WO2.9 below 80 K in agreement with magnetization data.

CuO Modified CaTiO3 for NTCR Thermistor Application

CuO modified CaTiO3 ceramics exhibit logarithmic increase of resistance with decreasing temperature, which makes the compound well suited for use as a negative temperature coefficients resistance thermistor material in temperature sensing device applications. The source powder production, the experimental set-up, the process parameters and the resulting properties for each are presented and compared. Ca1−XCuXTiO3 (0.1, 0.3, 0.5 and 0.7) synthesized by High Energy Ball Mill. The phase purity of the source powder and the films was assessed by X-ray diffraction. The electrical properties were analysed by measuring the resistance–temperature characteristics. Steinhart and Hart model and time response analysis used for the verification and potential towards the thermistor industry. Experimental result shows potential of all samples in thermistor industry because of exponential electrical resistance with temperature. Time response analysis revels materials response potential to temperature also variation of response time due to effect of doping concentration variation. The sensitivity (β value found to be in the range of 4482–8001 K which is comparable to other potential thermistor industry material already in use. The aim of this experiment to introduce new thermistor material with matching electrical parameters.

Effect of synthesis conditions on the electrical resistivity of TiSe2

Dilute impurities and growth conditions can drastically affect the transport properties of TiSe$_2$, especially below the charge density wave transition. In this paper, we discuss the effects of cooling rate, annealing time and annealing temperature on the transport properties of TiSe$_2$: slow cooling of polycrystalline TiSe$_2$ post-synthesis drastically increases the low temperature resistivity, which is in contrast to the metallic behavior of single-crystalline TiSe$_2$ due to charge doping from the residual iodine transport agent. A logarithmic increase of resistivity upon cooling and negative magnetoresistance with a sharp cusp around zero field are observed for the first time for the polycrystalline TiSe$_2$ samples, pointing to weak-localization effects due to low dimensionality. Annealing at low temperatures has a similar, but less drastic effect. Furthermore, rapid quenching of the polycrystalline samples from high temperatures freezes in disorder, leading to a decrease in the low temperature resistivity.

Gate tunable Kondo effect in magnetic molecule decorated graphene

The Kondo effect is one of the most widely studied many-body phenomena. Magnetic impurity decorated graphene is predicted to exhibit the Kondo effect with intriguing properties. Here we study the Kondo effect in Cobalt(II) phthalocyanine magnetic molecule decorated graphene by low temperature transport measurements. Spin-flip scattering and a logarithmic increase of resistance are observed, which confirms the Kondo effect. At a high carrier density, Kondo screening persists up to 20 K, suggesting a strong coupling between Dirac electrons and local magnetic moments of molecules. Decreasing the carrier concentration by a gate voltage results in a strong suppression of Kondo screening. Such tuning of Kondo screening can be used to turn on/off the local moment. Our results demonstrate a new Kondo system based on graphene. The significant gate tunability of the system may be used in spintronics.

Electro-oxidized Epitaxial Graphene Channel Field-Effect Transistors with Single-Walled Carbon Nanotube Thin Film Gate Electrode

We report the effect of electrochemical oxidation in nitric acid on the electronic properties of epitaxial graphene (EG) grown on silicon carbide substrates; we demonstrate the availability of an additional reaction channel in EG, which is not present in graphite but which facilitates the introduction of the reaction medium into the graphene galleries during electro-oxidation. The device performance of the chemically processed graphene was studied by patterning the EG wafers with two geometrically identical macroscopic channels; the electro-oxidized channel showed a logarithmic increase of resistance with decreasing temperature, which is ascribed to the scattering of charge carriers in a two-dimensional electronic gas, rather than the presence of an energy gap at the Fermi level. Field-effect transistors were fabricated on the electro-oxidized and pristine graphene channels using single-walled carbon nanotube thin film top gate electrodes, thereby allowing the study of the effect of oxidative chemistry on the transistor performance of EG. The electro-oxidized channel showed higher values for the on-off ratio and the mobility of the graphene field-effect transistor, which we ascribe to the availability of high-quality internal graphene layers after electro-oxidation of the more defective top layers. Thus, the present oxidative process provides a clear contrast with previously demonstrated covalent chemistry in which sp(3) hybridized carbon atoms are introduced into the graphitic transport layer of the lattice by carbon-carbon bond formation, thereby opening an energy gap.

Absence of weak electron localization in epitaxial Fe wires on GaAs(110)

We measured the low-temperature magnetoresistance of epitaxial Fe wires on GaAs(110) for wires with widths ranging from 100 to 3000 nm. All wires exhibit a logarithmic resistance increase toward lower temperatures. The resistance increase is independent of the external magnetic field applied perpendicular to the film plane. This behavior shows that the logarithmic resistance increase is due to electron-electron interaction rather than weak localization effects. By decreasing the wire width we find that the temperature dependence of the resistance increases which is due to a $2d\text{\ensuremath{-}}1d$ transition of the electron-electron interaction.

Absence of weak electron localization in (Co/Pt)n-multilayer-nanowires with perpendicular anisotropy

We report on low temperature resistance and magnetoresistance (MR) measurements for various (Co/Pt)n-multilayer-nanowires with perpendicular magnetic anisotropy and different widths, designed as model systems for the experimental observation of weak electron localization effects in ferromagnetic systems as proposed by Dugaev et al. [Phys. Rev. B 64, 144423 (2001)]. The low temperature MR is found to originate from both the anisotropic MR and domain wall scattering. The temperature dependence of the resistance shows a logarithmic resistance increase with decreasing temperature, as it is typical for quantum corrections in two dimensional systems. Upon application of a perpendicular magnetic field, however, the slope of the logarithmic resistance increase does not change in magnitude, which proofs that weak localization effects are not present, at least within the accuracy of our measurements. Instead, the observed quantum corrections of the resistance are well explained by enhanced electron-electron interaction effects in two dimensions. The results are discussed with regard to recent experimental as well as theoretical works.

Absence of weak electron localization in (Co/Pt)n‐multilayer‐nanowires with perpendicular anisotropy

AbstractWe report on low temperature resistance and magnetoresistance (MR) measurements for various (Co/Pt)n‐multilayer‐nanowires with perpendicular magnetic anisotropy and different widths, designed as model systems for the experimental observation of weak electron localization effects in ferromagnetic systems as proposed by Dugaev et al. [Phys. Rev. B 64, 144423 (2001)]. The low temperature MR is found to originate from both the anisotropic MR and domain wall scattering. The temperature dependence of the resistance shows a logarithmic resistance increase with decreasing temperature, as it is typical for quantum corrections in two dimensional systems. Upon application of a perpendicular magnetic field, however, the slope of the logarithmic resistance increase does not change in magnitude, which proofs that weak localization effects are not present, at least within the accuracy of our measurements. Instead, the observed quantum corrections of the resistance are well explained by enhanced electron‐electron interaction effects in two dimensions. The results are discussed with regard to recent experimental as well as theoretical works.

Absence of weak electron localization in carbon-covered Co-nanowires

We have investigated the low temperature resistance and magnetoresistance behavior of ferromagnetic cobalt wires in perpendicular external magnetic fields of up to B=5 T. Magnetic force microscopy investigations show that the cobalt wires are single-domain at remanence, if the wire width is smaller than about 800 nm. At low temperatures a logarithmic resistance increase is observed which is consistently explained as originating from enhanced electron-electron interaction (EEI) in two dimensions, while the magnetoresistance is dominated by the anisotropic magnetoresistance. Additional resistance corrections due to weak electron localization (WEL) are not observed within the accuracy of our measurements, which is discussed in the framework of recent experimental and theoretical works.

Production of NTCR thermistor devices based on NiMn 2O 4+ δ

NiMn 2O 4+ δ ceramics exhibit a logarithmic increase of resistance with decreasing temperature (NTCR), which makes the compound well suited for use as a thermistor material in temperature sensing applications. In bulk material there are often problems with poor stability and reproducibility due to incomplete intergranular contact. These difficulties can be minimised in even and dense polycrystalline films. Screen-printing, electron-beam evaporation and rf magnetron sputtering processes have been developed for the production of thin and thick film NTCR thermistor devices. The source powder production, the experimental set-up, the process parameters and the resulting film properties for each of the processes are presented and compared. The surface topology was examined by scanning-electron microscopy (SEM) and atomic force microscopy (AFM). The phase purity of the source powder and the films was assessed by X-ray diffractometry (XRD). The electrical properties were analysed by measuring the resistance - temperature characteristics. This study suggests that rf magnetron sputtering processes are most advantageous.

Influence of magnetic fields on the two-dimensional electron transport in weakly disordered fluorine-intercalated graphite fibers.

We report measurements of the temperature dependence of the resistivity in zero magnetic field and in magnetic fields up to 5 tesla for weakly disordered fluorine-intercalated graphite fibers (${\mathrm{C}}_{\mathit{x}}$F) with various fluorine concentrations (3.6x5.8). These results have been used to determine the importance of weak localization and carrier-carrier interaction in these two-dimensional (2D) systems. We found that both quantum phenomena contribute to the logarithmic increase of resistivity with decreasing temperature, though the interaction effect is generally dominant. Also, the magnitude of the resistivity increase is very sensitive to the fluorine concentration. The temperature dependence of the inelastic scattering rate 1/${\mathrm{\ensuremath{\tau}}}_{\mathit{i}}$, as obtained from magnetoresistance measurements, is well fitted by a polynomial expression of the form ${\mathrm{\ensuremath{\tau}}}_{\mathit{i}}^{\mathrm{\ensuremath{-}}1}$=\ensuremath{\alpha}T+\ensuremath{\beta}${\mathit{T}}^{2}$. The linear term ${\mathrm{\ensuremath{\tau}}}_{\mathit{i}}^{\mathrm{\ensuremath{-}}1}$(T) exhibits a good quantitative agreement with the theoretical prediction for the expected disorder dependence of the carrier-carrier scattering in 2D disordered metals.

Electrical and thermal properties of fluorine-intercalated graphite fibers.

We report measurements on the electrical- and thermal-transport properties of fluorine-intercalated vapor-grown carbon fibers ${\mathrm{C}}_{\mathit{x}}$F with the carbon-to-fluorine-atom ratio x between 4.1 and 5.8. The electrical resistivity is found to be very sensitive to the fluorine concentration. Indeed, while the resistivities of the ${\mathrm{C}}_{5.2}$F and the ${\mathrm{C}}_{5.8}$F compounds are well below that of the pristine graphite fibers, the tendency is reversed when x\ensuremath{\le}4.5 and large resistivity values are obtained. All ${\mathrm{C}}_{\mathit{x}}$F compounds exhibit a logarithmic resistance increase at low temperature, which can consistently be explained by weak-localization and carrier-carrier interaction effects for two-dimensional electron systems. Large negative magnetoresistance values have been measured on the ${\mathrm{C}}_{\mathit{x}}$F compound with x=4.1. At higher temperature, an anomalous resistance-versus-temperature behavior appears for each fluorine-intercalated vapor-grown carbon fiber, possibly resulting from a phase transition. Heat transport in ${\mathrm{C}}_{\mathit{x}}$F (with x=4.1) is almost entirely governed by lattice vibrations between 2.5 and 300 K while the thermoelectric power is positive, and its temperature dependence is similar to that previously observed in other acceptor graphite intercalation compounds. Our results indicate that the high resistivity of the ${\mathrm{C}}_{\mathit{x}}$F compounds (with x\ensuremath{\le}4.5) is due to strong defect scattering rather than a low free-charge-carrier density.

Weak localization and coulomb interaction in graphite acceptor intercalation compounds

A logarithmic increase of resistance with decreasing temperature and a negative magnetoresistance have been observed on low stage acceptor graphite fiber intercalation compounds (GACs) at low temperature. These results may be explained within the framework of the recent theories of weak localization and electron-electron Coulomb interaction in two-dimensional (2D) disordered systems. It is shown that the amplitude of the effect and the temperature of the minimum resistance are directly related to the residual resistivity of the compound. The values of the temperature dependent inelastic scattering time obtained from the temperature and field dependence of the magnetoresistance are in quite good agreement with those predicted by both theories of electron-electron Coulomb interaction and electron-phonon interaction in 2D disordered systems. Because of the large variety of graphite host materials that may be intercalated, GACs are ideal for the study of weak localization and Coulomb interaction in 2D systems.

Anomalous magneto-transport properties of p-type germanium inversion layers at very low temperatures

A logarithmic increase of resistivity of p-type Ge inversion layers adjacent to a grain boundary in Ge bicrystals levels off in the mK range, whereas the longitudinal magneto-resistance continues to rise. At very low temperature a negative magneto-resistance is observed which is possibly connected with the Kondo effect.

2D-Conductivity of thin Pd films condensed at low temperatures

Resistance measurements have been made on quenched condensed Pd films with thicknesses between 25 A and 350A. The films are prepared under different evaporation conditions by varying the system pressure between 10−8 and 10−5 mbar. Resistance minima with a logarithmic increase of the sheet resistance are observed for thick films (d<350A) condensed under intentionally “bad” (10−5 mbar) vacuum conditions, as well as for thin films (d<50A) condensed at pressures around 10−8 mbar. Structure investigations show that the thick films are granular. For these films the relation of granularity to 2D localization is discussed. The thin films are continuous and the logarithmic resistance increase is in agreement with predictions of 2D-theories.

Transport properties of ferromagnetic metallic glasses: Resistivity and magnetoresistance effect

Electrical resistivity and magnetoresistance measurements from 4K to 300K on Metglas 2605 and Metglas 2605 A are reported. For both materials a resistivity minimum appears at low temperatures. Below the minimum an approximately logarithmic increase of resistivity was found. Magnetoresistance measurements were used to get additional information about the electronic conduction, magnetic texture and the nature of the magnetization process in these alloys. The anisotropic magnetoresistivity was found to increase with decreasing temperature.