Low-temperature Electrical Conductivity Research Articles

On the experimental substantiation of the hybridization of electronic states on cobalt impurities in the conduction band of a crystal

The article presents the experimental results concerning the role of cobalt impurities in low-temperature electrical conduction and in the Hall concentration of electrons in crystals of mercury selenide. In the limit of small concentrations of the impurities, a slow variation of the electron concentration was found as a function of impurity content, which can correspond to a donor character of the impurity d levels of cobalt atoms located in the conduction band of the host crystal under conditions of hybridization. Characteristic features have been observed in the concentration and temperature dependences of the electrical conductivity, similar to those that manifest themselves in crystals with hybridized states of iron impurities. As a result, the set of data obtained on crystals with cobalt impurities, including the previously published data on the concentration dependence of the Curie constant and on the temperature dependence of thermopower, can be used on a qualitative level as an experimental substantiation of the previously predicted existence of two donor hybridized electron states of a cobalt impurity atom in the conduction band.

Thermal and electrical transport across a magnetic quantum critical point

A quantum critical point (QCP) arises when a continuous transition between competing phases occurs at zero temperature. Collective excitations at magnetic QCPs give rise to metallic properties that strongly deviate from the expectations of Landau's Fermi-liquid description, which is the standard theory of electron correlations in metals. Central to this theory is the notion of quasiparticles, electronic excitations that possess the quantum numbers of the non-interacting electrons. Here we report measurements of thermal and electrical transport across the field-induced magnetic QCP in the heavy-fermion compound YbRh(2)Si(2) (refs 2, 3). We show that the ratio of the thermal to electrical conductivities at the zero-temperature limit obeys the Wiedemann-Franz law for magnetic fields above the critical field at which the QCP is attained. This is also expected for magnetic fields below the critical field, where weak antiferromagnetic order and a Fermi-liquid phase form below 0.07 K (at zero field). At the critical field, however, the low-temperature electrical conductivity exceeds the thermal conductivity by about 10 per cent, suggestive of a non-Fermi-liquid ground state. This apparent violation of the Wiedemann-Franz law provides evidence for an unconventional type of QCP at which the fundamental concept of Landau quasiparticles no longer holds. These results imply that Landau quasiparticles break up, and that the origin of this disintegration is inelastic scattering associated with electronic quantum critical fluctuations--these insights could be relevant to understanding other deviations from Fermi-liquid behaviour frequently observed in various classes of correlated materials.

Low temperature charge transport and microwave absorption of carbon coated iron nanoparticles–polymer composite films

In this paper, the low temperature electrical conductivity and microwave absorption properties of carbon coated iron nanoparticles–polyvinyl chloride composite films are investigated for different filler fractions. The filler particles are prepared by the pyrolysis of ferrocene at 980°C and embedded in polyvinyl chloride matrix. The high resolution transmission electron micrographs of the filler material have shown a 5nm thin layer graphitic carbon covering over iron particles. The room temperature electrical conductivity of the composite film changes by 10 orders of magnitude with the increase of filler concentration. A percolation threshold of 2.2 and an electromagnetic interference shielding efficiency (EMI SE) of ∼18.6dB in 26.5–40GHz range are observed for 50wt% loading. The charge transport follows three dimensional variable range hopping conduction.

Tuning the electrical transport properties of double-walled carbon nanotubes by semiconductor and semi-metal filling

Manipulating the electrical properties of carbon nanotubes through semi-metal or semiconductor filling is of paramount importance in the realization of nano-electronic devices based on one dimensional composite materials. From low temperature electrical conductivity measurements of a network, of empty and filled double-walled carbon nanotubes (DWNT’s), we report a transition in electrical transport features from hopping to weakly activated conduction by HgTe filling and also semi-metallic conduction in selenium (Se) filled DWNT’s. Magneto-resistance (MR) studies of the filled DWNT’s show suppression of the hopping conduction and a signature of 3D weak localization for Se@DWNT’s at low temperatures and high magnetic fields. These results are discussed on the basis of strength of interaction between the filler material and the inner-walls of the host DWNT’s, which enhances the electronic density of states (DOS) in the material as well as the change in the property of the filler material due to constrained encapsulation.

Anisotropic weakly localized transport in nitrogen-doped ultrananocrystalline diamond films

We establish the dominant effect of anisotropic weak localization (WL) in three dimensions (3D) associated with a propagative Fermi surface on the conductivity correction in heavily nitrogen-doped ultrananocrystalline diamond (UNCD) films based on magnetoresistance studies at low temperatures. Also, low-temperature electrical conductivity can show weakly localized transport in 3D combined with the effect of electron-electron interactions in these materials, which is remarkably different from the conductivity in two-dimensional WL or strong localization regime. The corresponding dephasing time of electronic wave functions in these systems described as $\ensuremath{\sim}{T}^{\ensuremath{-}p}$ with $p<1$, follows a relatively weak temperature dependence compared to the generally expected nature for bulk dirty metals having $p\ensuremath{\ge}1$. The temperature dependence of Hall (electron) mobility together with an enhanced electron density has been used to interpret the unusual magnetotransport features and show delocalized electronic transport in these $n$-type UNCD films, which can be described as low-dimensional superlattice structures.

Lifshitz transition in two-dimensional spin-density wave models

We argue that both pocket-disappearing and neck-disrupting types of Lifshitz transitions can be realized in two-dimensional spin-density wave models for underdoped cuprates, and study both types of transitions with impurity scattering treated in the self-consistent Born approximation. We first solve for the electron self-energy from the self-consistent equation, and then study the low-temperature electrical conductivity and thermopower. Close to the Lifshitz transition, the thermopower is strongly enhanced. For the pocket-disappearing type, it has a sharp peak, while for the neck-disrupting type, it changes sign at the transition, with its absolute value peaked on both sides of the transition. We discuss possible applications to underdoped cuprates.

Low-temperature electrical conductivity and the superconductor-insulator transition induced by indium impurity states in (Pb0.5Sn0.5)1 − x In x Te solid solutions

A study is reported of the low-temperature electrophysical (including superconducting) characteristics of the (Pb0.5Sn0.5)1 − xInxTe semiconducting solid solutions with an indium content variable within x = 0.05–0.20. A decrease in the impurity content x in the material has been found to bring about a decrease in the superconducting transition temperature Tc and the onset of an “insulating” state of the material. These effects manifest themselves in an increase in the low-temperature (T = 4.2 K) resistivity of (Pb0.5Sn0.5)0.95In0.05Te by more than three orders of magnitude as compared to that of (Pb0.5Sn0.5)0.8In0.2Te. A decrease in the In content in the solid solution also gives rise to a radical change in the shape of the temperature dependence of the electrical resistivity from a metallic behavior in the material with x = 0.20 (decrease in the electrical resistivity with decreasing temperature in the range 300–4.2 K) to a semiconducting behavior in a sample with x = 0.05 (exponential increase in the resistivity at T < 25 K). This transition to the insulating state with decreasing content of the impurity should be assigned to the displacement of the impurity band of quasi-local indium states toward the top of the light-hole valence band of the material and its emergence into the band gap of the solid solution.

Investigation of low-temperature electrical conduction mechanisms in highly resistive GaN bulk layers extracted with Simple Parallel Conduction Extraction Method

The electrical conduction mechanisms in various highly resistive GaN layers of Al x Ga1−x N/AlN/GaN/AlN heterostructures are investigated in a temperature range between T=40 K and 185 K. Temperature-dependent conductivities of the bulk GaN layers are extracted from Hall measurements with implementing simple parallel conduction extraction method (SPCEM). It is observed that the resistivity (ρ) increases with decreasing carrier density in the insulating side of the metal–insulator transition for highly resistive GaN layers. Then the conduction mechanism of highly resistive GaN layers changes from an activated conduction to variable range hopping conduction (VRH). In the studied temperature range, ln (ρ) is proportional to T −1/4 for the insulating sample and proportional to T −1/2 for the more highly insulating sample, indicating that the transport mechanism is due to VRH.

Effect of substrate temperature on structural, optical and electrical properties of pulsed laser ablated nanostructured indium oxide films

Nanocrystalline indium oxide (INO) films are deposited in a back ground oxygen pressure at 0.02 mbar on quartz substrates at different substrate temperatures ( T s) ranging from 300 to 573 K using pulsed laser deposition technique. The films are characterized using GIXRD, XPS, AFM and UV–visible spectroscopy to study the effect of substrate temperature on the structural and optical properties of films. The XRD patterns suggest that the films deposited at room temperature are amorphous in nature and the crystalline nature of the films increases with increase in substrate temperature. Films prepared at T s ≥ 473 K are polycrystalline in nature (cubic phase). Crystalline grain size calculation based on Debye Scherrer formula indicates that the particle size enhances with the increase in substrate temperature. Lattice constant of the films are calculated from the XRD data. XPS studies suggest that all the INO films consist of both crystalline and amorphous phases. XPS results show an increase in oxygen content with increase in substrate temperature and reveals that the films deposited at higher substrate temperatures exhibit better stoichiometry. The thickness measurements using interferometric techniques show that the film thickness decreases with increase in substrate temperature. Analysis of the optical transmittance data of the films shows a blue shift in the values of optical band gap energy for the films compared to that of the bulk material owing to the quantum confinement effect due to the presence of quantum dots in the films. Refractive index and porosity of the films are also investigated. Room temperature DC electrical measurements shows that the INO films investigated are having relatively high electrical resistivity in the range of 0.80–1.90 Ωm. Low temperature electrical conductivity measurements in the temperature range of 50–300 K for the film deposited at 300 K give a linear Arrhenius plot suggesting thermally activated conduction. Surface morphology studies of the films using AFM reveal the formation of nanostructured indium oxide thin films.

Low-Temperature Magnetoresistance Studies on Composite Films of Conducting Polymer and Multiwalled Carbon Nanotubes†

We report on low-temperature electrical conductivity and magnetoresistance (MR) measurements on composite films of conducting polyaniline (PANI) and multiwalled carbon nanotubes (CNT). The conductivity below 150 K follows Mott’s variable-range-hopping model in three dimensions. The MR of the CNT/PANI composite films is strongly dependent on temperature, magnetic field, and CNT loading. Evident transition from negative to positive MR was observed with lowering temperature, increasing magnetic field, or decreasing CNT loading. The positive and negative MR is discussed in terms of a wave function-shrinkage effect and a quantum interference effect on hopping conduction.

Low temperature sintering ability and electrical conductivity of SOFC interconnect material La0.7Ca0.3Cr0.97O3

Relatively low temperature (1150–1300 °C) sintering characteristics and electric conductivities of La 0.7 Ca 0.3 Cr 0.97 O 3 with a slight Cr deficiency of 0.03, prepared by an auto-ignition process, have been discussed. The material could reach 94.3% relative density at 1200 °C for 5 h in air, which is about 200 °C lower than that of with no Cr deficiency. XRD results indicated that pure perovskite phase was formed after sintered at the temperature range of 1150–1300 °C. The sample sintered at 1250 °C had the maximum conductivity of 62.0 S cm −1 at 850 °C in air, which is about 2.6 times as high as that of La 0.7 Ca 0.3 CrO 3 with no Cr deficiency. In pure H 2 , it got the maximum electrical conductivity of 3.3 S cm −1 at 850 °C as sintered at 1200 °C. The TEC of the material was about 11.4 × 10 −6 K −1 , which is very compatible to that of other common used SOFC components. Therefore, it is a simple and cost-effective interconnect material for SOFCs.

The effect of grain size on the low-temperature electrical conductivity of doped CeO 2

Alumina was added to doped ceria and its effect on the grain conductivity was studied. The electrical conductivities of Gd, Sm, or Y-doped ceria (CeO 2) with or without alumina addition were measured as a function of temperature (250–700 °C) in air. Impedance measurement enabled the separation of the grain conductivity and the grain boundary conductivity. With alumina addition of 1, 2, and 5 mol%, the grain conductivity of alumina-added samples below 400 °C increased with the accompanied grain size increase and the migration enthalpy decrease. A model was proposed to explain the observation that includes the low-conductivity region in the grain of doped ceria.

Magnetotransport in the amorphous carbon films near the metal–insulator transition

The low temperature electrical conductivity behavior of the amorphous carbon films in the non-metallic regime of the metal–insulator transition is studied in magnetic fields up to 7 T. The films are prepared from the pyrolysis of succinic anhydride in the temperature range 700–980 ∘C. A positive magnetoresistance (MR) is observed in films that are in the insulating regime and follows the variable range hopping (VRH) mechanism of conduction. In the VRH regime, at low temperatures B 2 dependence on the magnetoresistance is observed. The conductivity of the films in the critical regime follows the power law behavior. The MR is positive both at low and high field limits. At high fields the magnetoconductance is proportional to B 1 / 2 and is negligible at high temperatures.

Low temperature dependence of electrical resistivity: Implications for near surface geophysical monitoring

Electrical resistivity imaging surveys are used to monitor variations in pore fluid chemistry and saturation as well as time‐lapse changes. Temperature variations in the near surface can produce larger magnitude changes in electrical conductivity than changes due to slow moving solute plumes or spatial variations in chemistry and soil moisture. Relationships between temperature and electrical conductivity based on previous studies conducted over 25–200°C do not explain 0–25°C laboratory data. A modification to the temperature dependence within a petrophysical model is proposed that may allow general application over this temperature range. An empirical linear approximation of 1.8 to 2.2 percent change in bulk electrical conductivity per degree C is consistent with low temperature electrical conductivity studies and the predictions of the petrophysical model used. This relationship can be used to account for the effect of temperature variations within individual images or time‐lapse difference images.

Magnetotransport in the amorphous carbon films prepared from succinic anhydride

In this paper, we report the low-temperature electrical conductivity of amorphous carbon films prepared from an organic precursor succinic anhydride at different pyrolysis temperatures (700–980 °C). The films prepared at low temperatures show activation behavior. The films prepared at 900 °C and above show metal-like behavior, with positive temperature coefficient and a resistivity hump below about 25 K in the R– T behavior. The metal-like behavior at low temperatures was suppressed by the application of a magnetic field. The magnetoconductance in the metal-like films at low and high field limits were analyzed in terms of weak localization and electron–electron interaction models. Negligible magnetoconductance was observed at higher temperatures.

Low temperature electrical conductivity of low-density polyethylene/carbon black composites

The study deals with the electrical characteristics of carbon black/low-density polyethylene (CB/LDPE) composites of various CB filler concentrations (10, 15, and 20 wt.%). The DC electrical conductivity was studied as a function of filler concentration in low temperature range 25–285 K. It was found that the composites exhibit negative temperature coefficient of resistivity (TCR) at low temperatures and high enhancement in the electrical conductivity with both temperature and carbon black concentration. The observed increase of conductivity with the filler concentration was interpreted through the percolation theory. The dependence of the electrical conductivity of the given composites on temperature (25–285 K) was analyzed in terms of a formula in consistence with Mott hopping mechanism.

Transport properties of germanium-filled CoSb3

We report the transport properties of dense polycrystalline Ge filled skutterudites Ge0.25Co4Sb12 and Ge0.05Co4Sb12 perepared by a high-pressure synthesis approach. Low temperature electrical resistivity, Seebeck coefficient, Hall coefficient, and thermal conductivity measurements were performed and compared with those of Co4Sb11Ge and CoSb3. The Ge atoms residing inside the interstitial voids of the skutterudite crystal structure act as electron donors. The lattice thermal conductivity of these compounds is lower than that of CoSb3 but higher than that of other filled skutterudites. The potential for thermoelectric applications is also discussed.

Self-interference of charge carriers in ferromagnetic SrRuO3

We report a systematic study of the low-temperature electrical conductivity in a series of SrRuO3 epitaxial thin films. At relatively high temperature the films display the conventional metallic behavior. However, a well-defined resistivity minimum appears at low temperature. This temperature dependence can be well described in a weak localization scenario: the resistivity minimum arising from the competition of electronic self-interference effects and the normal metallic character. By appropriate selection of the film growth conditions, we have been able to modify the mean-free path of itinerant carriers and thus to tune the relative strength of the quantum effects. We show that data can be quantitatively described by available theoretical models.

Using experimental data to constrain theories of hopping conduction in NTD germanium

The mechanism for low temperature electrical conduction in neutron transmutation doped (NTD) germanium is believed to be variable range hopping (VRH). The resistance, R, at temperature T should then follow R(T)=R 0 exp(T 0/T) p , for constant (or nearly constant) values of T 0, R 0 and p. NTD Ge is thought to have a “Coulomb gap” in the density of states; theories then generally predict p=0.5. However, some theories suggest larger values, such as p=0.55. So far, experimental results have failed to distinguish between these values of p. We show that it is nevertheless practical to make sufficiently accurate measurements to do so. We present measurements for several NTD Ge samples with different doping levels, and discuss the various possible sources of error.

Two dimensional metallic conductivity in Pr1−xSrxMnO3 antiferromagnets

A complex magnetic, structural, transport and thermal investigation has been performed using a series of well sintered ceramics in the x=0.50–0.60 range. Fine steps of the substitution of Sr2+ for Pr3+ enabled us to evidence the evolution of magnetic transtions with x and to exemplify the impact of concentration of eg electrons on charge carrier and thermal transport. The lowering of x from 0.56 down to 0.50 led to the concomitant decrease of the low temperature electrical conductivity and thermoelectric power pointing thus to impeded charge carrier transport for samples with x∼0.5. Nonetheless, the band-like character of eg carriers for these compounds was independently confirmed by low temperature specific heat measurement.