Normal-state transport properties of fullerene superconductors

The normal-state transport properties of alkali-metal-doped fullerene crystals are explored. The Hall effect has been measured in K_xC₆₀ from room temperature toT _c . The electrical resistivity for K₃C₆₀ and Rb₃C₆₀ has been measured over a wide temperature range (20–650 K), and notable differences are observed for the materials at both low and high temperatures. The electrical resistivity of Rb₃C₆₀ has been measured in hydrostatic pressures up to 9 kbar. The resistivity is highly pressure sensitive. The transport results give an insight into the normal-state conduction mechanism and thus have consequences for the superconductivity mechanism.     

Electrical transport in light rare-earth orthochromites

Electrical conductivity, , Seebeck coefficient, S, and dielectric constant, , measurements on the pressed pellets of six light rare-earth orthochromites, RCrO₃, where R = La, Pr, Nd, Sm, Eu and Gd, have been carried out in the temperature range 300 to 1000 K. These are essentially electronic conductors, exhibiting p-type extrinsic semiconducting nature in the studied temperature range. The extrinsic charge carriers (holes) originate from Cr⁴⁺ centres which are present due to native defects in these solids. Their room-temperature electrical conductivities lie in the range 10^–7 to 10^{–5 –1} cm^–1, which become of the order of 10^{–2 –1} cm^–1 near 1000 K. The conductivity is a maximum in LaCrO₃ and drops across the RCrO₃ series, with SmCrO₃ being an exception. The mechanism involved in the electrical transport is the hopping of holes from Cr⁴⁺ centres to neighbouring Cr³⁺ ions. The activation energy of transport is nearly 0.3 eV. Typical hopping mobility of the holes is of the order of 10² cm^–2 V^–1 sec^–1 at 325 K and of the order of 10 cm² V^–1 sec^–1 at 1000 K. The mobility activation energy of the holes in a typcial RCrO₃ decreases with temperature due to the smoothing of the potential barriers between Cr⁴⁺ and Cr³⁺ sites. Several discontinuities are observed in the T against T ^–1 and S against T ^–1 plots of RCrO₃. The anomalies which these discontinuities reflect here have also been indicated.     

Electrical transport in light rare-earth vanadates

This paper reports electrical transport studies of rare-earth vanadates (RVO₄ with R = Ce, Pr, Nd, Sm, Eu and Gd), prepared by solid state reaction and characterized by X-ray diffraction studies. TGA study (300 to 1200 K) shows no weight loss; possible phase transitions in the range 1075 to 1300 K have been indicated by DTA studies. All these vanadates are typical semiconducting compounds with room temperature electrical conductivity (σ) lying between 10⁻⁴ and 10⁻²Ω⁻¹m⁻¹. Measurements of σ and the Seebeck coefficient (S) are reported in the temperature interval 400 to 1200 K. Two linear regions 400 to T ₁K and T ₁ to T ₂K have been obtained from the log σ against T ⁻¹ as well as the S against T ⁻¹ plots followed by a peak around T ₃ and minima around T ₄K. T ₄>T ₃>T ₂>T ₁, are different for different vanadates. It has been concluded that in the interval 400 to T ₁K, conduction is of the extrinsic hopping type with Ce⁴⁺ in CeVO₄, Pr⁴⁺ in PrVO₄ and V⁴⁺ in Nd to Gd vanadates as dominant defect centres. In the temperature interval T ₁, to T ₂K, the conduction has been shown to be of the intrinsic band type in all vanadates with polarons of intermediate coupling strength as the dominant charge carriers. Above T ₂ all vanadates tend to become metallic, but before this is achieved the phase change makes the conductivity smaller. T ₄ is close enough to the temperature corresponding to the DTA peak to be termed the phase transition temperature.     

Electrical transport in light rare-earth molybdates

Rare-earth molybdates of the type R₂(MoO₄)₃ with R=La, Ce, Pr, Nd, Sm and Eu were prepared and characterized, and the electrical conductivity, and Seebeck coefficient, S in the temperature range 450–1200 K were measured. These molybdates are concluded to be insulating solids with a band gap which increases slowly going down the series from 2.30 eV for La molybdate to 3.20 eV for Eu molybdate. The plots of log and S versus T ^–1 show, in general, three linear regions with two break temperatures T ₁ and T ₂ occurring due to a change in the conduction mechanism. At higher temperatures the intrinsic conduction in these solids occurs via a band mechanism. The O^2– 2p and Mo⁶⁺ 4d orbitals form the valence and conduction bands, respectively. These bands are the main support of conduction in La, Sm and Eu molybdates; however, for Ce, Pr and Nd molybdates 4f ⁿ levels fall within the band gap and become very effective in electrical conduction. The main charge-carrying entities seem to be electrons in Ce, Pr and Nd molybdates and holes in La, Sm and Eu molybdates. On the basis of mobility calculations of charge carriers it is concluded that the charge carriers in these bands become polarons which are, in fact, the charge carrying entities. At lower temperatures electrical conduction is mainly extrinsic. Cerium molybdate shows a semiconductor-semimetal transition around 940 K.     

Electrical transport properties of III-nitrides

Excellent n-type GaN layers have been grown by all of the major epitaxial techniques: MBE, MOCVD, and HVPE. In this work, we analyze the band conduction in such samples by temperature-dependent Hall-effect measurement and theory, and determine quantitative information on donor and acceptor concentrations, as well as donor activation energies. In HVPE layers it is necessary to take account of a degenerate n-type layer at the GaN/sapphire interface in order to correctly analyze the bulk material. We also investigate hopping conduction, which occurs at low temperatures in conductive material, and at both low and high temperatures in semi-insulating material. Finally, we show by analysis of electron-irradiation data that both the N vacancy and the N interstitial are electrically active, demonstrating donor and acceptor character, respectively.     

In situ Raman spectroelectrochemical study of 13C-labeled fullerene peapods and carbon nanotubes

C60 fullerene peapods and double-walled carbon nanotubes (DWCNTs) containing highly 13C enriched C60 and inner tubes, respectively, are studied using Raman spectroscopy and in situ Raman spectroelectrochemistry in order to follow the influence of 13C enrichment on the vibrational pattern of these carbon nanostructures. The Raman response of 13C60 after encapsulation in fullerene peapods differs from that of isotope-natural species, (Nat)C60. The Raman A(g)(2) mode of encapsulated 13C60 is upshifted in frequency compared to that of the (Nat)C60 peapods with the same filling factor. The chemical doping of 13C60 peapods (peapod = C(60)@SWCNT) with K-vapor leads to the downshift of the A(g)(2) mode, similar to the case of (Nat)C60 peapods. The 13C60 peapods were successfully transformed into DWCNTs, which confirms high filling of single-walled (SW) CNTs with 13C60. The DWCNTs exhibited distinctly downshifted G and D Raman modes for inner tubes, which proves that only inner tubes were enriched by 13C. The in situ Raman spectroelectrochemistry of (Nat)C60 exhibits strong anodic enhancement, while for 13C60 peapods the enhancement is only weak. On the other hand, the electrochemical charging of the inner-tube-labeled DWCNTs (13C(i)-DWCNTs) followed the behavior of ordinary (Nat)C(i)-DWCNTs as indicated by in situ Raman spectroelectrochemistry. In addition, the spectroelectrochemical behavior of the G mode of inner tubes in 13C(i)-DWCNTs is followed from the start of the electrochemical doping, which was not feasible for (Nat)C(i)-DWCNTs.     

Electrical transport properties of some liquid metals

In the present article, we report the electrical transport properties, namely, the electrical resistivity ( ρ), the thermoelectric power (TEP) ( α), and the thermal conductivity (σ) of several monovalent, divalent and polyvalent liquid metals of the different groups of the periodic table on the basis of model potential formalism. The well known empty core model (EMC) potential of Ashcroft is used for the first time with seven local field correction functions proposed by Hartree (H), Hubbard-Sham (HS), Vashishta-Singwi (VS), Taylor (T), Ichimaru-Utsumi (IU), Farid et al. (F) and Sarkar et al. (S) in the present computation and found suitable for such study. In the calculation of these properties, we have used the values of the theoretical structure factors due to hard core fluid theory. It is concluded that the comparisons of present and theoretical or experimental findings, wherever they exist, are highly encouraging. Published in Russian in Teplofizika Vysokikh Temperatur, vol. 46, No. 6, 2008, pp. 870–880.     

Electrical transport properties of copper-doped tellurium films

Polycrystalline films of copper-doped tellurium were grown by co-evaporating copper and tellurium from separate boats. Measurements were made of the electrical transport properties of these films. It was observed that the addition of copper decreases the activation energies of both the conductivity and the mobility. The low temperature conduction data were interpreted on the basis of a variable range hopping mechanism while the high temperature data indicate the dominance of the grain boundary scattering mechanism.     

Electrical transport properties of thallium-doped p-type PbTe films

Hall measurements were made in the temperature range 77–700 K on thallium- doped p-type epitaxial films of PbTe having free carrier concentrations in the range 5×10¹⁷?6×10¹⁹cm^?3 at 300 K. The various band parameters, i.e. mobility, effective mass and population ratios for light and heavy holes, were estimated as functions of temperature and carrier concentration. These results were compared with reported data on bulk samples.     

Electrical transport properties of natural and synthetic graphite

Resistivity and magnetoresistance measurements were performed on a series of natural graphite crystals and highly oriented pyrolytic graphite (HOPG) samples as a function of temperature from 4.2 K to 293 K using a contactless r.f. technique. Resistance ratios of the natural graphite between 4.2 K and 293 K ranged from about 30 to 49. Carrier mobility of natural graphite was observed to obey at T^–3/2 behaviour up to about 35 K. An anomalously high power dependence of versus T was observed below 35 K. A new model describing the dispersion of mobility of electrons and holes is presented which gives exact agreement with magnetoresistance results in the low field regime.     

Electrical transport properties of hydrogen-exposed n-type InSb films

Hall coefficients and d.c. conductivities were measured for polycrystalline n-type InSb films exposed to H₂ gas at different pressures. It was found that H₂ gas molecules act as donor impurities for the films and the donor action increases with increasing pressure of the gas. The mobility, however, was found to decrease, showing the increasing contribution of ionized impurity scattering with increasing pressure of the gas.     

Electrical transport properties of La1−xSrxFeO3

La_1? x _Sr?x _FeO3 (0·1?x?0·4) samples exhibit nearly the same low resistivity value around the Neél temperatures. AboveT _N, electrons seem to become itinerant as suggested by Mōssbauer data.     

Electrical transport properties of low-stage AsF5-intercalated graphite

The in-plane resistivity of stage 1 and stage 2 AsF₅-graphite intercalation compounds was measured using a contactless r.f. eddy current technique from 1.6 to 290 K. The magnetoresistance of a stage 1 compound was similarly measured from 4.2 K to 290 K. The low temperature stage 2 resistivity data show a well-defined intermediate T ² region in addition to the usual T high temperature region, in qualitative agreement with the Kukkonen theory and indicative of a small, elongated cylindrical Fermi surface. Stage 2 resistivity data also show, for the first time in a graphite-acceptor compound, an apparent low temperature phase transition at 21 K. Magnetoresistance data were used to determine a stage 1 carrier concentration of 9×10²⁰ holes cm^–3. Resistive anomalies were observed at 200 K and 220 K for stage 1 and stage 2 compounds, respectively.     

Electrical transport properties of iron (II) molybdate

Electrical conductivity, thermoelectric power and static dielectric constant of iron (II) molybdate have been measured in the temperature range 300 to 1000 K on pressed pellets of polycrystalline sample. It has been found that FeMoO₄ is a p-type semiconductor with energy gap 4.1 eV. Different conduction mechanisms have been found below and above 700 K. Below 700 K conduction is due to a small polaron hopping mechanism and above 700 K conduction is due to large polarons as well as normal band conduction mechanism. Activation energy W, ₀ (T) and charge carrier mobility have been estimated in the two temperature ranges 300 to 700 K and 700 to 1000 K. Dielectric constant increases slowly with temperature up to 700 K and above 700 K, it increases exponentially with temperature.     

Exact transport properties of degenerate,weakly interacting,and spin-polarized fermions

Exact results for the transport coefficients of a highly degenerate spin-polarized Fermi system are presented. The case of weakly interacting particles, such as dilute³He in⁴He or D ↓, is treated. The results are compared to the variational treatment and corrections as large as 8% are found. Detailed behavior of the polarization of an ideal Fermi gas as a function of temperature and magnetic field is discussed as a model for dilute³He in⁴He. Limits of applicability of the formulas are given. We find that at appropriate fields and temperatures, the two spin species may be considered to be a mixture of a degenerate and a classical gas.     

Electrical transport properties of peptide nanotubes coated with gold nanoparticles via peptide-induced biomineralization

We present temperature dependent electrical transport measurements of peptide nanotube devices coated with monodisperse arrays of gold nanoparticles (AuNP). As the temperature is lowered, the current-voltage (I-V) characteristics become increasingly nonlinear and below 20 K conduction only occurs above a threshold voltage V(T). The current follows the scaling behavior I ∝ [(V ? V(T))/V(T)]α for V > V(T) with α ～ 2.5 signifying two-dimensional (2D) charge transport. The temperature dependence of the resistance shows thermally activated behavior with an activation energy of 18.2 meV corresponding to the sequential tunneling of charges through 6 nm monodispersed AuNP arrays grown on a peptide surface.