Mixed state Hall effect in the multiphase superconductors

Hall effect below Tc in multiphase superconductors has been studied on Bi-based superconductors. Samples with different relative content of 2212 and 2223 phase have been prepared. The phase content has been verified by X-ray diffraction. Results show that while resistance and ac susceptibility is almost insensitive to the content of 2212 phase, the qualitative behavior of the Hall resistance is strongly influenced by the presence of both phases. Theoretical calculation of Hall resistance has been made based on effective medium approximation and compared with experimental results.Comment: 7 pages, 7 figure

Scaling of resistivities and guided vortex motion in MgB2 thin films

Longitudinal and transverse voltages have been measured on thin films of MgB2 with different superconducting transition widths. The study has been performed in zero and non-zero external magnetic fields. The non-zero transverse voltage has been observed in close vicinity of the critical temperature in zero external magnetic field, while further away from Tc this voltage becomes zero. In magnetic field it becomes a transverse voltage which is an even function with respect to the direction of the field. The usual Hall voltage starts to appear with increasing magnetic field and in large fields the even voltage disappears and only the Hall voltage is measurable (i.e. the transverse even voltage is suppressed with increasing magnetic field and increasing transport current). New scaling between transverse and longitudinal resistivities has been observed. This correlation is valid not only in the zero magnetic field but also in nonzero magnetic field where transverse even voltage is detected. Several models trying to explain observed results are discussed. The most promising one seems to be guided motion of the vortices, though further theoretical work will be required to confirm this

Transverse voltage in high-Tc superconductors in zero magnetic fields

Longitudinal and transverse voltages have been measured in zero external magnetic fields. In close vicinity of the superconducting transition nonzero transverse voltage has been observed while far away from Tc, both above and below no such voltage has been detected. The value of the transverse resistivity depends on the value of the transport current. Several models have been discussed taking into account also the penetration of self field due to the applied transport current. It seems that observed results can be explained using the Kosterlitz-Thouless model as a result of an unpairing of vortex-antivortex pairs created below Tkt due to fluctuations. At Tkt free vortices and antivortices are created and can contribute to a dissipation of energy. Their movement should also be responsible for the observed nonzero transverse voltage.Comment: 3 pages in Latex, 3 figs.ep

Electron magnetotransport in GaAs/AlGaAs superlattices with weak and strong inter-well coupling

We report on magnetotransport measurements in two MBE-grown GaAs/AlGaAs superlattices formed by wide and narrow quantum wells and thin Si-doped barriers subject to tilted magnetic fields. It has been shown that illumination of the strongly coupled superlattice with narrow wells leads to reduction of its dimensionality from the 3D to 2D. The illumination-induced transition is revealed by remarkable change of magnetoresistance curves as compared to those measured before illumination. The experimental data along with tight-binding model calculations indicate that the illumination not only enhances the electron concentration but also suppresses the electron tunneling through the barriers.Comment: 3 pages, 3 figures, elsart/PHYEAUTH macros; presented on the LDSD 2007 Conference in the Caribbean Archipelago San Andres, Colombia. To be published as a special issue of Microelectronics Journal (Elsevier

In-plane Magnetic Field Dependent Magnetoresistance of Gated Asymmetric Double Quantum Wells

We have investigated experimentally the magnetoresistance of strongly asymmetric double-wells. The structures were prepared by inserting a thin Al$_{0.3}$Ga$_{0.7}$As barrier into the GaAs buffer layer of a standard modulation-doped GaAs/Al$_{0.3}$Ga$_{0.7}$As heterostructure. The resulting double-well system consists of a nearly rectangular well and of a triangular well coupled by tunneling through the thin barrier. With a proper choice of the barrier parameters one can control the occupancy of the two wells and of the two lowest (bonding and antibonding) subbands. The electron properties can be further influenced by applying front- or back-gate voltage.Comment: 4 pages, 5 figures, elsart/PHYEAUTH macros; to be presented on the EP2DS-15 Conference in Nara, Japan. Revised version. To appear in Physica

Systematic study of Mn-doping trends in optical properties of (Ga,Mn)As

We report on a systematic study of optical properties of (Ga,Mn)As epilayers spanning the wide range of accessible substitutional Mn_Ga dopings. The growth and post-growth annealing procedures were optimized for each nominal Mn doping in order to obtain films which are as close as possible to uniform uncompensated (Ga,Mn)As mixed crystals. We observe a broad maximum in the mid-infrared absorption spectra whose position exhibits a prevailing blue-shift for increasing Mn-doping. In the visible range, a peak in the magnetic circular dichroism blue shifts with increasing Mn-doping. These observed trends confirm that disorder-broadened valence band states provide a better one-particle representation for the electronic structure of high-doped (Ga,Mn)As with metallic conduction than an energy spectrum assuming the Fermi level pinned in a narrow impurity band.Comment: 22 pages, 14 figure

Cyclotron effective mass of 2D electron layer at GaAs/AlGaAs heterojunction subject to in-plane magnetic fields

We have found that Fermi contours of a two-dimensional electron gas at \rmGaAs/Al_xGa_{1-x}As interface deviate from a standard circular shape under the combined influence of an approximately triangular confining potential and the strong in-plane magnetic field. The distortion of a Fermi contour manifests itself through an increase of the electron effective cyclotron mass which has been measured by the cyclotron resonance in the far-infrared transmission spectra and by the thermal damping of Shubnikov-de Haas oscillations in tilted magnetic fields with an in-plane component up to 5 T. The observed increase of the cyclotron effective mass reaches almost 5 \% of its zero field value which is in good agreement with results of a self-consistent calculation.Comment: 4 pages, Revtex, figures can be obtained on request from [email protected]; to appear in Phys. Rev. B (in press). No changes, the corrupted submission replace