Anisotropic Charge Transport Research Articles

Magneto-chiral anisotropy in charge transport through single-walled carbon nanotubes

Carbon nanotubes are chiral molecular objects and therefore exist in two forms that are each other’s mirror image. Many aspects of these fascinating new materials have recently been explored but their chirality has hardly been addressed. We have investigated the charge transport through individual single-walled carbon nanotubes in the presence of a magnetic field parallel to the tube axis. For most of the investigated tubes, a dependence of the resistance that is odd in both the magnetic field and in the current through the tube is observed. This effect is ascribed to the chirality of the nanotube and is called electrical magneto-chiral anisotropy.

Anisotropic spin and charge transport in presence of spin-orbit interaction

We explore spin and charge transport phenomena in a two-dimensional electron gas (2DEG) in presence of spin-orbit coupling connected to two ideal Ferromagnetic leads with parallel magnetization. It is shown that the spin polarization transported through the 2DEG depends on the absolute direction of magnetization in a coordinate system defined by plane of 2DEG and normal to it. Conductance is also shown to be anisotropic.

Charge transport anisotropy in SmB6

The study of DC- and low-frequency AC-conductivity in combination with the Hall and Seebeck coefficients measurements has been carried out in the temperature interval 1.6–300K along different crystallographic directions in classic mixed-valence compound–narrow-gap semiconductor SmB6. The experiments on high-quality single crystals of samarium hexaboride have revealed a strong anisotropy of the low-temperature transport parameters.

Transport anisotropy in microcrystalline silicon studied by measurement of ambipolar diffusion length

We have studied charge transport anisotropy in microcrystalline silicon (μc-Si:H) by comparing diffusion length measured parallel to the substrate by steady stage photocarrier grating and perpendicular to the substrate by surface photovoltage method (SPV). We have developed a SPV evaluation procedure which allowed us to exclude the effect of light scattering at the naturally rough surface of the μc-Si:H. The procedure allows us to deduce not only the diffusion length, but also the depth of the depletion layer at the surface and recombination coefficients at both top and bottom interfaces of the film. With growing μc-Si:H film thickness the size of the crystallites increases, leading to higher roughness and thus also light scattering. At the same time density of grain boundaries decreases, resulting in an increase of the diffusion length and of the surface depletion layer depth. For all samples the diffusion length perpendicular to the substrate was several times higher than the diffusion length parallel to it, clearly confirming previous indication of the transport anisotropy resulting from the measurements of coplanar and sandwich conductivity.

The anisotropy of charge transport in pulse-irradiated polymers: from polyethylene to polydeoxyribonucleic acid

The results presented illustrate the applicability of the PR-TRMC technique for the study of the anisotropy of charge transport in aligned polymeric samples. The anisotropies found show that charge migration along the backbone direction of oriented samples of polyethylene, polysilylenes, and polydiacetylenes is favoured by approximately an order of magnitude or more. In contrast no anisotropy is found for the radiation-induced conductivity in oriented-fibre samples of DNA which leads to the conclusion that charge transport along the base pair stack in this material is not the favoured conductive pathway.

Charge transport in oligothiophene single crystals

The charge transport in α-quaterthiophene and α-hexathiophene single crystals was studied by space charge limited current spectroscopy. Record high mobilities of up to 10 cm 2/V s were observed at low temperatures. The strongly anisotropic charge transport in these materials is explained by the molecular packing giving rise to a strong π–π * overlap in the molecular planes due to short sulphur–carbon contacts, but almost no molecular overlap along the ends of the molecules. Therefore, the charge carrier can be described as a two-dimensionally extended polaron. The transport mechanism is governed by two-dimensional band-like motion within the planes and a phonon-assisted, thermally activated hopping process perpendicular to them. To determine the influence of molecular packing on the charge transport two different single crystalline modifications have been investigated.

Laser-induced off-diagonal thermoelectric voltage in La 1− xCa xMnO 3 thin films

Laser induced off-diagonal thermoelectric voltages were observed in La 0.67Ca 0.33MnO 3 thin films deposited on vicinal cut SrTiO 3 substrates, as well as in films of other doping levels. The dependence of this voltage on photon fluence, illuminated length of the film, film thickness and orientation are in remarkable analogy to the results reported for YBa 2Cu 3O 7− x thin films. That suggests an identical physical mechanism in both materials which based on the existence of off-diagonal elements in the Seebeck tensors. It demonstrates that an intrinsically anisotropic charge transport exists in these series of materials, in spite of the quasi-cubic crystal structure of the base material LaMnO 3. From the results obtained on films with a systematic variation of the doping level and materials, the anisotropy of the Seebeck tensor is suggested to be a contribution of the long-range cooperative Jahn–Teller distortions.

Pressure-Enhanced Interplane Tunneling Magnetoresistance in a Layered Manganite Crystal

The pressure-induced variations of the anisotropic charge-transport and magnetic properties have been investigated for a bilayered manganite crystal, ${\mathrm{La}}_{2\ensuremath{-}2x}{\mathrm{Sr}}_{1+2x}{\mathrm{Mn}}_{2}{\mathrm{O}}_{7}$ ( $x\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}0.3$). The magnitude of the magnetic coupling between the adjacent bilayers can be weakened by application of pressure, which makes the charge dynamics more two dimensional (2D) but highly incoherent. As a result, the restoration of the interlayer hopping of the spin-polarized carriers by application of magnetic field, namely the field-induced 2D-3D crossover, gives rise to an extremely high ratio of interplane tunneling magnetoresistance, say, up to $\ensuremath{\Delta}{\ensuremath{\rho}}_{c}/{\ensuremath{\rho}}_{c}\ensuremath{\sim}4000%$ at 4.2 K under $\ensuremath{\sim}10\mathrm{kbar}$.

Tunneling and anisotropic charge transport properties of superconducting (110)-oriented YBa2Cu3O7 thin films

We have measured the tunneling conductance of YBa2Cu3O7/Pb planar tunnel junctions fabricated on superconducting (110)-oriented YBa2Cu3O7 thin films exhibiting zero-resistance superconducting transition temperatures (Tc’s) between 88 and 89.5 K. The conductance is the same as we and other groups have measured on (100)- and (103)-oriented YBa2Cu3O7/Pb junctions and quantitatively different from c-axis tunneling. The ab-plane transport is comparable to that measured in (001)-oriented YBa2Cu3O7 thin films, and the transport anisotropy varies between 65 and 80 from 300 K to Tc, respectively. This anisotropy is higher than that previously measured in both (110)-oriented and c-axis aligned (100)-oriented films, but it is comparable to that measured in single crystals.

Photorefractive centers in LiNbO3, studied by optical-, Mössbauer- and EPR-methods

The photorefractive properties of LiNbO3∶Fe and LiNbO3∶Cu have been studied in combination with optical absorption-, Mossbauer- and EPR-measurements. The charge states of Fe in successively reduced LiNbO3∶Fe have been investigated with respect to the influence on the photorefractive sensitivity and saturation value of the refractive index change. The results of this experiment demonstrate clearly the close correlation between the concentration of Fe2+ impurities and the optical absorption band around 2.6 eV in LiNbO3∶Fe, which is known to give rise to an anisotropic charge transport upon optical excitation. The resulting photocurrents determine the photorefractive sensitivity mainly in the initial state of halographic exposure. With increasing conversion from Fe3+ to Fe2+ the photorefractive sensitivity saturates and the saturation value of the refractive index change decreases remarkably.