Temperature Variation Of Resistivity Research Articles

Concomitant antiferromagnetic transition and disorder-induced weak localization in an interacting electron system

In this Rapid Communication we report a phenomenon in a disordered interacting electron system. The measurements of structural, magnetic, and transport properties of $\mathrm{FeA}{\mathrm{l}}_{2\ensuremath{-}x}\mathrm{G}{\mathrm{a}}_{x}$ ($0\ensuremath{\le}x\ensuremath{\le}0.5$) show that antiferromagnetic transition in these intermetallic compounds occurs concomitantly with a disorder-induced weak localization of electrons; the temperatures ${T}_{N}$ and ${T}_{m}$, at which antiferromagnetic transition and the weak localization respectively occur, closely track each other as the Ga concentration is varied. The antiferromagnetic transition is confirmed from the magnetic and specific heat measurements, and the occurrence of weak localization is confirmed from the temperature variation of resistivity and magnetoresistance measurements. With increasing Ga concentration, substitutional disorder in the system increases, and the consequent disorder-enhanced magnetic exchange interaction and disorder-induced fluctuations simultaneously drive antiferromagnetic transition and weak localization, respectively, to higher temperatures.

Estimation of Joule heating and its role in nonlinear electrical response of Tb 0.5 Sr 0.5 MnO 3 single crystal

Abstract Highly non-linear I–V characteristics and apparent colossal electro-resistance were observed in non-charge ordered manganite Tb 0.5 Sr 0.5 MnO 3 single crystal in low temperature transport measurements. Significant changes were noticed in top surface temperature of the sample as compared to its base while passing current at low temperature. By analyzing these variations, we realize that the change in surface temperature ( Δ T sur ) is too small to have caused by the strong negative differential resistance. A more accurate estimation of change in the sample temperature was made by back-calculating the sample temperature from the temperature variation of resistance (R–T) data ( Δ T cal ) , which was found to be higher than Δ T sur . This result indicates that there are large thermal gradients across the sample. The experimentally derived Δ T cal is validated with the help of a simple theoretical model and estimation of Joule heating. Pulse measurements realize substantial reduction in Joule heating. With decrease in sample thickness, Joule heating effect is found to be reduced. Our studies reveal that Joule heating plays a major role in the nonlinear electrical response of Tb0.5Sr0.5MnO3. By careful management of the duty cycle and pulse current I–V measurements, Joule heating can be mitigated to a large extent.

Anomalous giant positive magnetoresistance and heavy fermion like behaviour in Mn11Ge8

Mn11Ge8 undergoes long range ferromagnetic ordering at 274 K followed by non-collinear antiferromagnetic structure below 150 K. Our investigations indicate the existence of large positive magnetoresistance which changes sign depending upon the applied field and temperature. In the temperature variation of resistivity and heat capacity, the sample shows very large coefficients of quadratic and linear terms, respectively. This indicates the existence of giant spin fluctuation despite the fact that the sample is in a magnetically ordered state. The Mn-moments possibly have both localized and itinerant character and the anomaly appears to be associated with the local moment spin fluctuations mediated via itinerant electrons. The large positive magnetoresistance is believed to be originated from the possible development of short range antiferromagnetic clusters on application of magnetic field.

Kinetic properties of the two‐dimensional conducting system formed by CrSi2 nanocrystallites in plane (111) of silicon

The behaviors of resistance, magnetoresistance (up to 5 T), and Hall electromotive force (EMF) with varying temperature (10–300 K) and measuring current (A–10 mA) are studied for the Si sample with CrSi2 nanocrystallites (NC) in the plane (111). The conduction in such heterostructure proceeds in the plane with the NC and is the conduction of a two‐dimensional system of charge carriers that shows some unusual effects. The temperature variation of resistivity may be treated as the result of the effect of thermal activation but in this case it is characterized by a low activation energy different in value in different temperature ranges. This suggests that the mechanism of conduction is more complex. It is found that the conduction is determined by the effect of temperature variation not only on carrier concentration but also on its mobility. Magnetoresistivity is also of different shape in different temperature ranges. All the above features are treated in terms of the proposed model of electron hopping through the conduction band (or hole hopping through the valence band). A peculiar effect of giant reduction in resistivity with increasing the measuring current has been revealed. Discussed are some possible factors responsible for this effect.

Pressure Effects on Rattling and Superconductivity in the Einstein Solids

Pressure effects on the rattling and superconductivity in the Einstein solids; AxV2Al20 (Ax = Ga0.2, Al0.3, and Y) are investigated by means of electrical resistivity measurements under high pressure. We observed bumps in resistivity at low temperatures only for Ga0.2 and Al0.3, which can be explained by scattering of conduction electrons by local phonons with Einstein temperatures of 8.5 and 22 K, respectively. The bumps gradually fade out with increasing pressure, and the temperature variations of resistivity for Ga0.2 and Al0.3 approach those of Y. This indicates that the low-energy local phonons in the Ga and Al compounds disappear at high pressures. We also observed \(T^{2}\) behavior with large \(A\) coefficients in the low-temperature limit for Ga and Al compounds, but not for Y. With increasing pressure, the \(A\) coefficient is largely suppressed, while \(H_{\text{c2}}' \equiv |\mathrm{d}H_{\text{c2}}/\mathrm{d}T|_{T_{\text{c}}}\) that also reflects the density of states at Fermi energy decreases...

Effect of multiwalled carbon nanotubes on electrical conductivity and magnetoconductivity of polyaniline

An in situ chemical polymerization method was applied in order to prepare polyaniline-multiwalled carbon nanotube (PANI-MWCNT) composites with different concentrations of MWCNT. X-ray diffraction, Fourier transform infrared spectroscopy, scanning electron microscopy, optical absorption and photoluminescence analyses of the composites were performed to investigate the structural, morphological and optical properties of the composites. Electrical transport properties of different PANI-MWCNT composites were investigated in the temperature range 77 K ⩽ T ⩽ 300 K in the presence and also in the absence of a magnetic field up to 1 T. The dc resistivity of the composites follows Mott's variable range hopping theory. Two different slopes have been observed in temperature variation of resistivity, which occurs due to the presence of MWCNT in the polymer matrix. The magnetoconductivity of the samples at different temperatures is negative, which can be explained by the wavefunction shrinkage effect.

How Are Heavy and Itinerant Electrons Born in a Dilute Kondo Alloy?

We report a continuous evolution of Fermi surface properties with temperature in a dilute Kondo alloy of Ce 0.02 La 0.98 Ru 2 Si 2 via measurements of the de Haas–van Alphen (dHvA) effect. The temperature variation of resistivity in this alloy exhibits a typical impurity Kondo behavior, i.e., the resistivity increases with decreasing temperature proportionally to -log T and becomes nearly constant below about 0.3 K. The Kondo temperature of this alloy is estimated to be about 1.3 K. The frequency of the dHvA oscillation from the main hole surface decreases with decreasing temperature implying that the hole surface shrinks with decreasing temperature. The temperature dependence of the frequency change is found to be similar to that of resistivity. The temperature dependence of this dHvA oscillation amplitude deviates largely from that expected from the Lifshitz–Kosevich formula conventionally employed to determine the effective mass from the temperature dependence. By assuming that the scattering of the co...

Conducting boron-doped single-crystal diamond films for high pressure research

Epitaxial boron-doped diamond films were grown by microwave plasma chemical vapor deposition for application as heating elements in high pressure diamond anvil cell devices. To a mixture of hydrogen, methane and oxygen, diborane concentrations of 240–1200 parts per million were added to prepare five diamond thin-film samples. Surface morphology has been observed to change depending on the amount of diborane added to the feed gas mixture. Single-crystal diamond film with a lowest room temperature resistivity of 18 mΩ cm was fabricated and temperature variation of resistivity was studied to a low temperature of 12 K. The observed minima in resistivity values with temperature for these samples have been attributed to a change in conduction mechanism from band conduction to hopping conduction. We also present a novel fabrication methodology for monocrystalline electrically conducting channels in diamond and present preliminary heating data with a boron-doped designer diamond anvil to 620 K at ambient pressure.

Electrical and switching properties of NiAl xFe 2− xO 4 ferrites synthesized by chemical method

Nickel-aluminum ferrite system NiAl x Fe 2− x O 4 has been synthesized by wet chemical co-precipitation method. The samples were studied by means of X-ray diffraction, d.c. electrical resistivity, a.c. electrical resistivity, a.c. conductivity and switching properties. The XRD patterns confirm the cubic spinel structure for all the synthesized samples. The crystallite size calculated from XRD data which confirm the nano-size dimension of the prepared samples. Electrical properties such as a.c. and d.c. resistivities as function of temperature were studied for various Al substitution in nickel ferrite. The dielectric constant and dielectric loss tangent were also studied as a function of frequency. The dielectric constant follows the Maxwell–Wagner interfacial polarization. A.C. conductivity increases with increase in applied frequency. The d.c. resistivity decreases as temperature increases, which indicate that the sample have semi-conducting nature. Verwey hoping mechanism explains the observed variation in resistivity. The activation energy is derived from the temperature variation of resistivity. Electrical switching properties were studied as I– V measurements. The current controlled negative resistance type switching is observed in all the samples. The Al substitution in nickel ferrite decreases the required switching field.

Performance Improvement of Induction Motor Speed Sensor-Less Vector Control System by the Phase Current Non-Zero Control in Low Speed Range

A lot of researches about speed sensor-less vector control methods of the IM have been published and applied to practical systems. However, because of various errors caused by dead time, temperature variation of resistance and so on, the speed estimation error is inevitable in the practical applications. The inverter voltage control error is very sensitive to phase current near zero cross. Therefore, the speed estimation becomes unstable when the phase current zero cross and the zero frequency condition occur simultaneously. In order to solve these problems, this paper proposes a novel method to reduce the inverter voltage control error. The phase current references are forbidden to stay near zero, where the voltage control error is very sensitive to the phase current. The phase current non-zero control causes the harmonics which influences the speed estimation. The harmonics is compensated appropriately by a novel method, which cancels the harmonic component in the input signals to the speed estimator. The experimental results with phase current non-zero control showed good performance in low speed range.

Electrical properties of In3+ and Cr3+ substituted magnesium–manganese ferrites

Two series of magnesium–manganese ferrites, viz. Mg0.9Mn0.1InxFe2 − xO4 and Mg0.9Mn0.1CryFe2−yO4 have been prepared by the conventional ceramic process. The effects of In3+ and Cr3+ ions on the dc resistivity, dielectric constant and dielectric loss factor are presented in this paper. The resistivity increases with increasing concentrations of In3+ and Cr3+ ions. The observed variations in resistivity have been explained by Verwey's hopping mechanism. The activation energy, deduced from the temperature variation of resistivity, was found to increase with increasing concentrations of In3+ and Cr3+ ions. The room temperature dielectric constant at 100 kHz decreases with successive addition of trivalent ions in both the series. The observed variation in dielectric constant has been explained on the basis of space charge polarization. The dielectric loss tangent (tanδ) values measured at 100 kHz and 13 MHz are found to be very low for the samples with a higher concentration. The low values of the loss factor even at a high frequency indicate that the prepared materials may have great potential for use in microwave devices.

Synthesis of γ-Fe 2O 3/polypyrrole nanocomposite materials

We report a chemical approach for the in situ synthesis of conducting polymer-magnetic inorganic nanomaterials with the integration of a high temperature organometallic method and a new microemulsion system containing organic, surfactant and aqueous phases that were established to enable the formation of our host nanocomposite materials. Polypyrrole-based γ-Fe 2O 3 nanocomposites were prepared through this colloidal system. These organic–inorganic nanocomposites were characterized using X-ray diffraction, transmission electron microscopy, thermogravimetric analysis, electrical resistivity and magnetization measurements. It was observed that the blocking temperature increases with increasing the γ-Fe 2O 3 nanocomponent in these samples. The resistivity and the temperature variation of resistivity have also been investigated.

Synthesis and physicochemical properties of La-Ce-Ni-O and Sr-Ce-Ni-O oxides

La2 – xCexNiOy and Sr2 – xCexNiOy materials were prepared, and their properties were studied. Nearly single-phase Sr2 – xCexNiOy samples (tetragonal K2NiF4 structure) could be obtained at x= 0.25 and 0.3. The lattice parameters, weight change, relative length change, and electrical resistivity of Sr1.7Ce0.3NiOy were measured from 20 to 1000°C. The oxygen content of this material, determined by hydrogen reduction and iodometric titration, was found to vary widely, depending on heat-treatment conditions. The room-temperature resistivity of Sr1.7Ce0.3NiOy is (2–5) × 10–2 Ω cm. In the range 20–450°C, this material exhibits n-type conductivity. Its thermoelectric power varies from –12 μV/K at 20°C to –34 μV/K at 450°C. The temperature variation of resistivity for Sr1.7Ce0.3NiO4 in the first heating–cooling cycle below 450°C is shown to depend on the thermal history of the sample. The resistivity reaches a maximum between 500 and 800°C. The structural and transport properties of the mixed oxide are shown to be correlated with its oxygen content.

Transport Properties of Ga-Doped PbTe Thin Films on Si Substrates

Thin PbTe films on Si substrates were doped with Ga via annealing in the vapor produced by heating a Ga(l) + GaTe(s) mixture (GaTe(s) + L1+ Vequilibrium). Electrical measurements showed that the vapor-phase doping reduced the hole concentration in the films by more than two orders of magnitude. IR irradiation was found to reduce the resistivity of the PbTe films by a factor of 40–100. The films annealed in the vapor over GaTe(s) + L1for the longest time exhibited an anomalous temperature variation of resistance, which was interpreted as due to the limited Ga solubility in PbTe.

Pressure sensors based on AlGaAs doped with Te and Sn

Resistivity and Hall concentration in AlGaAs layers have been measured as a function of pressure up to 25 kbar and of temperature from 77 K to 400 K. The layers were grown by LPE on GaAs and doped with Te and Sn. We first studied the properties of AlxGal1-x As with Al composition x from 0.1 to 0.3. Then we measured the graded-gap layers with x varying e.g., from 10% to 40%. Such graded-gap layers can be considered as the parallel connection of layers with uniform composition. They reveal strong pressure sensitivity and flat temperature variation of resistance. Tin-doped layers are good candidates for “laboratory” pressure sensors operating up to 30 kbar in the 77-300 K temperature range. Tellurium-doped layers reveal metastable properties below 120K but they can be useful for pressure sensing in the high-temperature range. The properties of graded-gap AlGaAs layers are superior to those of manganin and InSb sensors.

In-situ growth of superconducting YBa 2Cu 3O 7 - δ thin films at low oxygen partial pressures

In-situ superconducting c-axis oriented YBa 2Cu 3O 7 - δ (YBCO) thin films have been grown at T sub = 775 °C on SrTiO 3(100) and MgO(100) by r.f.-magnetron sputtering at an oxygen partial pressure, which not only lies below the stability line but is more than an order of magnitude lower than its empirically suggested limit. The effect of variation in partial pressure of oxygen ( P o 2 ) on the film characteristics is analyzed by employing a.c.-magnetic susceptibility, resistance-temperature variation, Rutherford backscattering, atomic force microscopy (AFM), scanning electron microscopy and X-ray diffraction techniques. The oxygen deficiency, δ, is calculated employing a number of techniques. The higher c-axis lattice parameters of the films are supportive of antisite disorder in the cations. The AFM investigations have established that Stranski-Krastanov-type growth mechanism is operative in the present case. The variation in critical current density ( J mag c( T)) with temperature is reported for the first time in the low pressure sputtered films exhibiting a J mag c(43 K) of ≈4.6 × 10 6 A cm -2.

Direct observation of resistive barriers in a BaTiO3 based thermistor

The remote electron beam induced current technique has been used to form resistive contrast images of the resistive barriers developed in 80 °C thermistor materials. Linescans extracted from the images have been used to calculate the temperature variation of resistivity of selected interfaces within the material. It is seen that the major resistive changes are associated with only a few of the grain boundaries in this material.

High pressure studies of cvt grown cuins2single crystals

Abstract Single crystals of CuInS2 having n-and p-type character were grown by chemical vapour transport technique and were characterised by XRD and temperature variation of resistivity. The bandgap Eg was determined from the optical absorption studies. The thermoelectric power (S) and a.c. resistivity (Rω) of both the n and p-type crystals were measured upto 8 GPa. From a.c. conductivity measurements, we deduce that the conduction in CuInS2 is dominated by the hopping mechanism.

多孔質炭素材料・ウッドセラミックスの開発 第１報 低温領域における電気的特性

Woodceramics is a carbon material sintered from wood or woodymaterial to be used as an industrial material. All the woodceramics has the following characteristics: porous structure, light weight, hard property, heat resistance, anticorrosion and low cost in production. Today, the woodceramics is being studied for practical applications in various fields.In the present study, the electric conduction of woodceramics was investigated to use it as a porous conductor in low temperature. The temperature dependence of electric resistance of the woodceramics was measured in low temperature and the carrier transport process was discussed. The temperature variation of resistivity was the same as that of semiconductor; it decreases with increasing temperature. This semiconductive behavior may be attributed to the intra-layer interaction of π-electron in graphite microcrystals.

Studies on some α-Fe 2O 3 photoelectrodes

p- and n-type α-Fe 2O 3 samples were prepared and the doping limits of Mg 2+, Cu 2+ and Ca 2+ ions ascertained from X-ray diffraction studies and electrical resistivity measurements. The doping limits are 0.2 and 0.1 at % for Mg 2+ and Cu 2+, respectively. α-Fe 2O 3 is not doped by Ca 2+ and forms a solid-solution to give CaFe 2O 4. Temperature variation of resistivity and thermoelectric power between 300 and 700 K showed that d-level conduction in α-Fe 2O 3 remains unchanged by Mg 2+ doping, but is changed by Cu 2+ doping. Optical constants (n, k) were determined by a Krämers-Kronig analysis from the reflectance spectra recorded in the 450–800 nm wavelength region following a procedure involving a straight line approximation of the reflection attenuation. Pure α-Fe 2O 3 shows an indirect band gap of 2.135 eV and Mg 2+ doped samples show an indirect band gap of 2̃.5 eV and a direct band gap of 1̃.6 eV. The calculated theoretical quantum efficiencies of the samples indicate Cu 2+-doped α-Fe 2O 3 to be better photoanodes in a photoelectrochemical cell for water decomposition than pure α-Fe 2O 3 or Mg 2+-doped α-Fe 2O 3. Experimentally, however, none of these samples show good photoresponse which is attributed to the indirect nature of the band gaps in these samples and their d-level conduction.