Ladungstransport in dünnen Filmen

The electronic charge transport in thin indium films of high purity is investigated (direct current, eddy current and magnetoresistance in a magnetic field perpendicular to the surface) at low temperatures where surface scattering of the electrons becomes important. A method is described which allows the determination of the mean free path (corresponding to a Sommerfeld-model for the conduction electrons) for a single specimen by measuring both the direct current conductivity and the decay time of eddy currents. For a multivalent metal like indium, with a Fermi surface extending over different Brillouin zones, the “mean free path” evaluated in this way can no longer be regarded as the mean value of the electronic free path calculated over the whole Fermi surface. It is shown, that for indium the ?mean free path” evaluated from our size effect investigations will mainly correspond to the electrons of the second zone, and that the electrons of the third zone only neglectingly contribute to the electronic charge transport.     

Sr2RuO4超导转变温度附近c方向磁阻行为研究

测量了Sr2RuO4正常态和超导转变温度附近(T=1.5K,4.0K,6K)c方向的磁阻,通过对正常态磁阻分析获得了Sr2RuO4在温度4K和6K下的平均自由程.通过超导涨落分析获得了Sr2RuO4的超导特性参数ξab(0)和ξc(0).     

Effect of roughness of two-dimensional heterostructures on weak localization

The effect that a longitudinal magnetic field exerts on the transverse negative magnetoresistance by suppressing the interference quantum correction is studied in GaAs/In_xGa_l?xAs/GaAs structures with a single quantum well. It is shown that the variation in the shape of the transverse magnetoresistance curve caused by a longitudinal magnetic field depends strongly on the relation between the mean free path, the phase-breaking length, and the correlation length characterizing the roughness of the two-dimensional layer. It is shown that the experimental results allow one to estimate the parameters of large-scale and small-scale roughness of the two-dimensional layer in the structures under study. The results obtained are in good agreement with the data of probe microscopy.     

Saturating and linear magnetoresistive effects in sodium and potassium

The theoretically predicted and long-sought intrinsic saturation of the transverse magnetoresistance of the alkali metals has finally been observed after sufficiently reducing the masking effect of the large superimposed linear magnetoresistance which is attributed to such classical considerations as geometry, Hall angle and non-uniform current distribution. The observed saturation values were about 100 times smaller than those observed previously in complicated metals but were still much too large to be explained only by Fermi surface anisotropy. The saturation phenomena contradict Kohler's rule but can be plausibly explained by mean free path anisotropy due predominantly to electron-phonon scattering mechanisms.     

Sample-size effects in the magnetoresistance of graphite

Conduction electrons in graphite are expected to have micrometer large de Broglie wavelength as well as mean free path. A direct influence of these lengths in the electric transport properties of finite-size samples was neglected in the past. We provide a direct evidence of this effect through the size dependence of the magnetoresistance, which decreases with the sample size even for samples hundreds of micrometers large. Our findings may explain the absence of magnetoresistance in small few graphene layers samples and ask for a general revision of the experimental and theoretical work on the transport properties of this material.     

Indirect interactions between localized magnetic moments in doped semiconductors

The coupling between localized magnetic moments via conduction electrons is calculated taking into account the temperature and the mean free path of the electrons. For a fully degenerate electron gas and an infinite electronic mean free path the oscillatory RKKY interaction is obtained. On the limit of Boltzmann population and for infinite electronic mean free path the interaction can only be ferromagnetic. Taking into account the electronic mean free path the possibility of antiferromagnetism is restored. Furthermore the range of the interaction decreases. Several intermediate cases and possible applications are discussed.     

Mean-free path effects in magnetoresistance of ferromagnetic nanocontacts

We investigated the mean-free path effects on the magnetoresistance of ferromagnetic nanocontacts. For most combinations of parameters the magnetoresistance monotonously decreases with increasing the contact cross-section. However, for a certain choice of parameters the calculations show non-monotonous behavior of the magnetoresistance in the region in which the diameter of the contact becomes comparable with the mean-free path of electrons. We attribute this effect to different conduction regimes in the vicinity of the nanocontact: ballistic for electrons of one spin projection, and simultaneously diffusive for the other. Furthermore, at certain combinations of spin asymmetries of the bulk mean-free paths in a heterocontact, the magnetoresistance can be almost constant, or may even grow as the contact diameter increases. Thus, our calculations suggest a way to search for combinations of material parameters, for which high magnetoresistances can be achieved not only at the nanometric size of the contact, but also at much larger cross-sections of nanocontacts which can be easier for fabriaction with current technologies. The trial calculations of the magnetoresistance with material parameters close to those for the Mumetal-Ni heterocontacts agree satisfactorily with the available experimental data.     

Investigating the surface morphology and magnetic contrast of epitaxial ferromagnetic structures

The surface morphology of nickel thin films is investigated via atomic force microscopy. The multistage film growth mechanism is found to be dependent on substrate temperature and film thickness. It is shown that conduction electron scattering from the irregularities of the outer and inner surfaces of structures are influenced by the surface morphology and determined by an integrated contribution of the surface’s fluctuation density spectrum. The morphology influence can be decreased under certain growth conditions so that the residual mean free path of conduction electrons can reach 1000 nm, exceeding the film thickness. Epitaxial nanostructures with high electron mobility have been fabricated. Investigation of their magnetic structure has shown that their magnetic domain dimensions are less than the residual mean free path of electrons determined by the surface morphology.     

Low temperature magnetoresistance of Al-doped ZnO films

The magnetoresistances of aluminum-doped zinc oxide thin films with thickness of 463.63, 203.03, and 66.85 nm were measured at low temperatures from 2.5 to 30 K. It is found that the samples exhibit negative magnetoresistance at all measuring temperatures. However, neither the three-dimensional nor the two-dimensional weak-localization theories can reproduce the behavior of the magnetoresistance. We find that the magnetoresistance of the three films can be well described by a semiempirical expression that takes into account the third order s-d exchange Hamiltonians describing a negative part and a two-band model for the positive contribution. This strongly suggests that the negative magnetoresistance in ZnO:Al film originates from the scattering of conduction electrons due to localized magnetic moments.     

Particularities of Heat Conduction in Nanostructures

Heat conduction in nanostructures differs significantly from that in macrostructures because the characteristic length scales associated with heat carriers, i.e., the mean free path and the wavelength, are comparable to the characteristic length of nanostructures. In this communication, particularities associated with phonon heat conduction in nanostructures, the applicability of the Fourier law, and the implications of nanoscale heat transfer effects on nanotechnology are discussed.     

Influence of Se-deficiencies on the properties of In-doped CdCr2Se4 single crystals

Lattice constant, Curie temperature, and electrical conductivity of CdCr₂Se₄:In single crystals have been measured after heat treatments of the crystals in Se atmosphere and under streaming hydrogen. By these treatments, the concentration of the Se vacancies and of the charge carrier concentration is altered drastically. The lattice constant as well as the magnetic ordering temperature have been found not to be affected by these heat treatments.Since the Se vacancies act as doubly changed donors, the electrical conductivity is strongly dependent on the concentration of the Se vacancies. A resistivity anomaly and large magnetoresistance are observed only in crystals with considerable Se deficiency. From these results it is concluded that the magnetoresistance is caused by hopping conduction between donor sites partly emptied by compensating A-site vacancy acceptors. Large magnetoresistance is found in samples with considerable Se deficit because only in this case the conduction at lower temperatures is dominated by the impurity band.     

Galvanomagnetic size effects in iron whiskers

The influence of sample dimensions on the resistance and magnetoresistance (both longitudinal and transverse) was studied in iron whiskers with square cross section of dimensions 1÷10 μm. The effective electron mean free path of bulk material of corresponding purity was determined to be λ_∞ ～40 μm. The results are analyzed in terms of available theoretical predictions which are mostly based on spherical Fermi surface models; this allows to determine two additional effective parameters of the Fermi surface. The temperature dependence of transverse magnetoresistance leads to the conclusion that surface scattering has diffuse character. This is discussed with regard to earlier experimental work on metal whiskers and to recent interpretation of both diffuse and specular scattering in tungsten byPanchenko et al.     

Untersuchung des elektrischen widerstandes und des weglängeneffektes der leitungselektronen an dünnen kupferdrähten

The mean free path of the conduction electrons is determined by the size effect on thin copper wires. The value ?·λ being a temperature-independent constant according to the model of the free electron gas for different metals is evaluated for copper from the results of the measurements.     

Mean free path dependence of RKKY interaction in AuCuFe and AuCuMn spin glass alloys

The dependence of RKKY interaction on the mean free path of the conduction electrons in spin glass alloys has been investigated by varying the concentration of copper in gold in a binary host alloy. The experimental data are in agreement with theory.     

Quantum transport of slow charge carriers in quasicrystals and correlated systems

We show that the semiclassical model of conduction breaks down if the mean free path of charge carriers is smaller than a typical extension of their wave function. This situation is realized for sufficiently slow charge carriers and leads to a transition from a metalliclike to an insulatinglike regime when scattering by defects increases. This explains the unconventional conduction properties of quasicrystals and related alloys. The conduction properties of some heavy fermions or polaronic systems, where charge carriers are also slow, present a deep analogy.     

Tunnel magnetoresistance oscillations in a ferromagnet-insulator-ferromagnet system

A model of spin-dependent transport of electrons through a ferromagnet-insulator-ferromagnet structure is developed. It takes into account the image forces, tunnel barrier parameters, and effective masses of an electron tunneling in the barrier and in the ferromagnetic electrode in the free electron approximation. Calculations for an iron-aluminum oxide-iron structure show that, with an increase in the bias voltage, the tunnel magnetoresistance decreases monotonically and then breaks into damped oscillations caused by the interference of the electrons’ wave functions in the conduction region of the potential barrier. The image forces increase the tunnel magnetoresistance by two or three times.     

Theoretical analysis of cross-plane lattice thermal conduction in graphite

A theoretical analysis of the cross-plane lattice thermal conduction in graphite is performed by using first-principles calculations and in the single-mode relaxation time approximation. The out-of-plane phonon acoustic mode ZA and optical mode ZO have almost 80% and 20% of contributions to cross-plane heat transfer, respectively. However, these two branches have a small part of total specific heat above 300 K. Phonons in the central 16% of Brillouin zone contribute80% of cross-plane transport. If the group velocity angle with respect to the graphite layer normal is less than 30?, then the contribution is 50% at 300 K. The ZA phonons with long cross-plane mean free path are focused in the cross-plane direction, and the largest mean free path is on the order of several micrometers at room temperature. The average value of cross-plane mean free path at 300 K is 112 nm for ZA phonons with group velocity angle with respect to the layer normal being less than 15?. The average value is dropped to 15 nm when phonons of all branches in the whole Brillouin zone are taken into account, which happens because most phonons have small or even no contributions.     

Conductance quantization and magnetoresistance in magnetic point contacts

We theoretically study the electron transport through a magnetic point contact (PC) with special attention given to the effect of an atomic scale domain wall (DW). The spin precession of a conduction electron is forbidden in such an atomic scale DW and the sequence of quantized conductances depends on the relative orientation of magnetizations between left and right electrodes. The magnetoresistance is strongly enhanced for the narrow PC and oscillates with the conductance.     

Anisotropic behaviour of the magnetoresistance in single crystalline iron films

Magnetoresistive properties of single crystalline Fe(0 0 1) films with thickness in the range 5–100 nm are reported. The films possess low coercive fields (∼100 A/m) but a weak irreversible behaviour of the magnetization remains to fields of the order of the anisotropy field. The anisotropic behaviour of the magnetoresistance is investigated as a function of temperature and film thickness. A reversal in sign of the anisotropic magnetoresistance from negative to positive values is found at low temperatures on decreasing the film thickness from 100 to 5 nm, or by increasing the temperature from 10 to 300 K of a sufficiently thick film. The reversal in sign is associated with a crossover from Lorentz force (ordinary) to spin–orbit (extraordinary) dominated scattering processes governing the anisotropic magnetoresistance as the length scale of the electron mean free path, λ, decreases.     

Well-defined single-walled carbon nanotube fibers as quantum wires: Ballistic conduction over micrometer-length scales

We report quasi-ballistic conduction in single-walled carbon nanotube (SWNT) fibers at room temperature, with a measured mean free path of 1.0–2.6 μm. The dynamic submersion of vertical SWNT fibers into liquid mercury (Hg) electrode shows plateaus and steps indicative of quasi-ballistic electron transport. This response is described with a newly developed network model that uses surface impurities to simplify the parallel conducting channels. The quasi-ballistic SWNT fibers exhibit a resistance per unit length of 2.5–6.5 kΩ/μm and a mean free path exceeding 1 μm, a length longer than typical via dimensions in existing Si-chip technologies. These results highlight that SWNT fiber conductivity can be enhanced by increasing the metallic to semiconducting SWNT ratio and reducing the surface impurities.