Large transconductance oscillations in a single-well vertical Aharonov-Bohm interferometer

Aharonov-Bohm (AB) interference is reported for the first time in the conductance of a vertical nanostructure based on a single GaAs/AlGaAs quantum well (QW). The two lowest subbands of the well are spatially separated by the Hartree barrier originating from electronic repulsion in the modulation-doped QW and provide AB two-path geometry. Split-gates control the in-plane electronic momentum dispersion. In our system, we have clearly demonstrated AB interference in both electrostatic and magnetic modes. In the latter case the magnetic field was applied parallel to the QW plane, and perpendicular to the 0.02 um^2 AB loop. In the electrostatic mode of operation the single-QW scheme adopted led to large transconductance oscillations with relative amplitudes exceeding 30 %. The relevance of the present design strategy for the implementation of coherent nanoelectronic devices is underlined.Comment: Accepted for publication on Physical Review B Rapid Communication

Mechanism of thermally activated c-axis dissipation in layered High-Tc_c superconductors at high fields

We propose a simple model which explains experimental behavior of $c$-axis resistivity in layered High-T$_c$ superconductors at high fields in a limited temperature range. It is generally accepted that the in-plane dissipation at low temperatures is caused by small concentration of mobile pancake vortices whose diffusive motion is thermally activated. We demonstrate that in such situation a finite conductivity appears also in $c$-direction due to the phase slips between the planes caused by the mobile pancakes. The model gives universal relation between the components of conductivity which is in good agreement with experimental data.Comment: RevTeX, 4 pages, 2 Postscript figure

Direct evidence of electronic ferroelectricity in YbFe2O4 using neutron diffraction and nonlinear spectroscopy

We report the first observation of room temperature spontaneous electric polarization in an electronic ferroelectric material, a YbFe2O4 single crystal. The observation was based on second harmonic generation (SHG), a nonlinear optical process. Tensor analysis of the SHG signal revealed that this material has a polar charge superstructure with Cm symmetry. This result settles the long-term discussion on the uncertainty about electronic ferroelectric properties, including the charge order structure. We present a complete picture of the polar charge ordering of this material via consistent results from two different characterization methods. The SHG signal shows the same temperature dependence as the superlattice signal observed in neutron diffraction experiments. These results prove ferroelectric coupling to electron ordering in YbFe2O4, which results in electronic ferroelectricity which is enabled by the real space ordering of iron cations with different valences. The existence of electronic ferroelectricity holds promise for future electronics technologies where devices run a thousand times faster than frequency of the present CPU (a few gigahertz) embedded in smartphones, etc

Enhanced effective mass in doped SrTiO3 and related perovskites

The effective mass is one of the main factors determining the Seebeck coefficient and electronic conductivity. Nb-doping increases the effective mass because of two reasons, lattice constants increase and electronic effects. In this ab-initio study the effective mass is estimated from the curvature of electronic bands and it could be clarified that the deformation of SrTiO3 crystals has a significant influence on bandgap and effective DOS and band mass, which are both in excellent agreement to experimental data. However, the electronic effect due to the e2g- band flattening near the Gamma-point due to Nb-doping up to 0.2 at% is the main factor for the increase of effective mass. Doping of La shows a linear decrease of the effective mass; this is explained by the different surrounding of A- and B-site. Substitution of other elements like Ba on the A-site and V on the B-site in SrTiO3 were also found to increase the effective mass.Comment: 10 pages, 11 figures, 3 table