Low-temperature Charge Transport Research Articles

Metal–semiconductor transition in ultrathin cobalt-phthalocyanine films grown on SrTiO3 single crystal substrates

We have investigated the low temperature charge transport properties of ultrathin cobalt-phthalocyanine films grown on (100) SrTiO3 single crystal substrates. The temperature dependence of resistivity shows an anomalous behavior, i.e., a transition from semiconducting to metallic behavior at around 110 K. We demonstrated that metallic behavior in these films is triggered by compressive strains of the SrTiO3 substrate due to its cubic to tetragonal structural phase transition at 110 K.

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.

Electronic Properties of Nanodiamond Decorated Graphene

The electronic properties of graphene sheets decorated with nanodiamond (ND) particles have been investigated. The chemical fusion of ND to the graphene lattice creates pockets of local defects with robust interfacial bonding. At the ND-bonded regions, the atoms of graphene lattice follow sp(3)-like bonding, and such regions play the role of conduction bottlenecks for the percolating sp(2) graphene network. The low-temperature charge transport reveals an insulating behavior for the disordered system associated with Anderson localization for the charge carriers in graphene. A large negative magnetoresistance is observed in this insulating regime, and its origin is discussed in the context of magnetic correlations of the localized charge carriers with local magnetic domains and extrinsic metal impurities associated with the ND.

Deviation from the Wiedemann-Franz law induced by nonmagnetic impurities in overdopedLa2−xSrxCuO4

To investigate the validity of the Wiedemann-Franz (WF) law in disordered but metallic cuprates, the low-temperature charge and heat transport properties are carefully studied for a series of impurity-substituted and carrier-overdoped ${\text{La}}_{1.8}{\text{Sr}}_{0.2}{\text{Cu}}_{1\ensuremath{-}z}{M}_{z}{\text{O}}_{4}$ ($M=\text{Zn}$ or Mg) single crystals. With moderate impurity substitution concentrations of $z=0.049$ and 0.082 $(M=\text{Zn})$, the resistivity shows a clear metallic behavior at low temperature and the WF law is confirmed to be valid. With increasing impurity concentration to $z=0.13$ $(M=\text{Zn})$ or 0.15 $(M=\text{Mg})$, the resistivity shows a low-$T$ upturn but its temperature dependence indicates a finite conductivity in the $T\ensuremath{\rightarrow}0$ limit. In this weakly localized metallic state, that is intentionally achieved in the overdoped regime, a negative departure from the WF law is found, which is opposite to the theoretical expectation.

Low temperature charge carrier hopping transport mechanism in vanadium oxide thin films grown using pulsed dc sputtering

Low temperature charge transport in vanadium oxide (VOx) thin films processed using pulsed dc sputtering is investigated to understand the correlation between the processing conditions and electrical properties. It is identified that the temperature dependent resistivity ρ(T) of the VOx thin films is dominated by a Efros–Shklovskii variable range hopping mechanism [Efros and Shklovskii, J. Phys. C 8, L49 (1975)]. A detailed analysis in terms of charge hopping parameters in the low temperature regime is used to correlate film properties with the pulsed dc sputtering conditions.

Low-temperature charge transport in photosensitive nanocrystalline PbTe(In) films

We have investigated conductivity of the PbTe(In) nanocrystalline films in dc and ac modes at temperatures 4.2–300 K in the frequency interval 20 Hz - 1 MHz in darkness and under illumination. Conductivity of the as-grown films is determined by charge transport along the inversion channels at the grain surface and activation (or tunneling) through the grain barriers. Persistent photoconductivity appears in the films due to spatial separation of non-equilibrium charge carriers at temperatures lower 150 K. As grown films have been annealed in oxygen at 300 °C and 350 °C. Oxidation at 300 °C leads to resistivity increase, grain barrier height growth and higher photoresponse . Annealing at 350 °C results in resistivity reduction and photoresponse decrease. The main contribution to charge transport at low temperature in this case is related to the hopping conductivity along the inversion channels.

Low-temperature charge transport in Ga-acceptor nanowires implanted by focused-ion beams

Ga-acceptor nanowires were embedded in crystalline Si using focused-ion beams. The dc current-voltage characteristics of these wires after annealing are highly nonlinear at low temperatures. A conductance threshold of less than 50mV is observed independent of Ga+ dosage and implant beam overlap. These features suggest a Coulomb blockade transport mechanism presumably caused by a network of Ga precipitates in the substrate. This granular scenario is further supported by measurements of gated nanowires. Nanowires with metallic conductance at low temperatures could be achieved by reducing the current density of the focused-ion beams.

Vibronic Coupling in Semifluorinated Alkanethiol Junctions: Implications for Selection Rules in Inelastic Electron Tunneling Spectroscopy

Determining the selection rules for the interaction of tunneling charge carriers with molecular vibrational modes is important for a complete understanding of charge transport in molecular electronic junctions. Here, we report the low-temperature charge transport characteristics for junctions formed from hexadecanethiol molecules having varying degrees of fluorination. Our results demonstrate that C-F vibrations are not observed in inelastic electron tunneling spectroscopy (IETS). Because C-F vibrations are almost purely dipole transitions, the insensitivity to fluorine substitution implies that Raman modes are preferred over infrared modes. Further, the lack of attenuation of the C-H vibrational modes with fluorine substitution suggests that either the scattering cross section is not an additive quantity or the physical position of a vibrational mode within the junction influences whether the transition is observed in IETS.

Electron spin resonance in YbRh2Si2: The role of the residual linewidth

We present electron spin resonance (ESR) investigations of the La-doped heavy fermion metal YbRh2Si2 and of undoped samples which differ in their low-temperature charge transport properties. The dilution of Yb by non-magnetic La leaves the temperature dependence of ESR parameters qualitatively unchanged. A comparison with various undoped samples shows that the residual, zero temperature ESR linewidth is a key parameter for the spin dynamics seen by the ESR active entities in YbRh2Si2.

Structural Contributions to Charge Transport across Ni-Octanedithiol Multilayer Junctions

We report the fabrication and characterization of multilayer thin films incorporating 1,8-octanedithiols and Ni atoms. Low-temperature charge transport measurements exhibit inelastic co-tunneling and resonant tunneling features that correspond energetically to vibrational excitations of the molecular multilayer. Several junctions exhibit changes in conductance features characteristic of charge defect-gating. Transport through our junctions is shown to be dominated by the intrinsic properties of the multilayer.

Two-Channel Kondo Effect in Glasslike ThAsSe

We present low-temperature heat and charge transport as well as caloric properties of a ThAsSe single crystal. An extra -AT(1/2) term in the electrical resistivity, independent of magnetic fields as high as 14 T, provides evidence for an unusual scattering of conduction electrons. Additionally, both the thermal conductivity and the specific heat show a glass-type temperature dependence which signifies the presence of tunneling states. These observations apparently point to an experimental realization of a two-channel Kondo effect derived from structural two-level systems.

Vibronic Contributions to Charge Transport Across Molecular Junctions

We report low-temperature charge transport measurements of metal−molecule−metal junctions. Studies on insulating alkyl and π-conjugated molecular wires provide experimental insight into the coupling between tunnel charge carriers and molecular vibrations in molecular electronic systems. By comparison with other vibrational spectroscopy studies and density functional theory calculations, the observed vibrational peaks have been assigned to longitudinal modes of the molecules.

Variable-range hopping conductivity in thin film of the ladder compound [Ca1+δCu2O3

Low-temperature charge transport in [Ca1+δCu2O3]4 films with δ=0 and 0.08 is investigated at T=1.4–290 K in magnetic fields of B=0–30 T. Between Tv≈20 and 40 K, depending on δ, and Tv′≈2–3 K three-dimensional (3D) Mott variable-range hopping (VRH) conductivity is observed. Deviations from 3D Mott VRH conductivity below Tv′ can be attributed to the onset of the Shklovskii–Efros VRH regime, accompanied by violation of the 3D hopping condition, Rh≪d (where Rh is the mean hopping length and d is the thickness of the film). The density of the localized states (DOS) at the Fermi level, g3(μ)≈4.4×1020 eV−1 cm−3, the values of the localization radius, α≈46 and 34 Å, the concentration of sites involved in hopping charge transfer, N≈9.9×1018 and 9.0×1018 cm−3, the width of the band of localized electron states, 2W≈22 and 20 meV, and the width of the Coulomb gap in the DOS spectrum, 2Δ≈1 and 1.6 meV are obtained for films with δ=0 and 0.08, respectively. These values characterize the films as being near the 3D metal–insulator transition given by the critical concentration NC≈1.3×1019 cm−3.

Variable-range hopping conductivity and magnetoresistance in n-CuGaSe2

The low-temperature charge transport in n-CuGaSe2 was investigated in zero and nonzero magnetic field. Both the Mott as well as the Shklovskii–Efros regimes of the variable-range hopping are observed in different temperature intervals. The complete set of the parameters describing the properties of the localized electrons (the localization radius, the dielectric permeability, the width of the Coulomb gap, and the values of density of states at the Fermi level) are obtained by analysis of the conductivity in zero field, on one hand, and the positive magnetoresistance in a small field, on the other hand. The negative magnetoresistance in low fields is observed in all specimens in both hopping regimes. Moreover, it is interpreted as a result of quantum interference between different paths of the tunneling electrons in conditions of scattering by intermediate centers.

Effect of low-temperature interphase charge transport at the Si/SiO2 interface on the photoresponse of silicon barrier structures

The effect of low-temperature electron charge redistribution at the Si/SiO2 interface between the interphase states and the conduction band of an n-Si crystal on the temperature behavior of conductance, photovoltage, and photocurrent in Si barrier structures with edge surface electron channels was studied in the temperature range of 77–300 K. The dynamics of the channel-current response to the voltage changes in the dark and under illumination can be explained qualitatively by dispersive hopping transport of holes in SiO2, which induces electron transfer to, and accumulation at, the Si surface near the barrier contact. The leveling off of the photovoltage at low temperatures and the nonmonotonic temperature dependence of the photocurrent are attributed to the nonmonotonically increasing, localized hole density at the Si/SiO2 interface and the free electron density at the Si surface with decreasing temperature, which reflects changes in the valence of oxygen complexes.

Low-Temperature Charge Transport in Tetrathiafulvalene Tetracyanoquinodimethane (TTF-TCNQ) near Commensurability

At low pressure and at a commensurability of 3 (at \ensuremath{\sim}19 kbar) the low-temperature thermopower of TTF-TCNQ is large, negative, and varying $\ensuremath{\sim}\frac{1}{T}$. At 17 kbar it is smaller, positive, and independent of temperature. At 22 kbar it is similar to that at 17 kbar but of opposite sign. For $P\ensuremath{\lesssim}19$ kbar pulling of the distortion wave vector from $2{k}_{\mathrm{F}}$ to a commensurate value ($\frac{{b}^{*}}{3}$) leaves the TCNQ valence band underfilled and TTF valence band overfilled. For $P\ensuremath{\gtrsim}19$ kbar the reverse holds. We suggest that the excess carriers are charged solitons or polarons.