20 research outputs found
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
Electrostatic potential in a superconductor
The electrostatic potential in a superconductor is studied. To this end
Bardeen's extension of the Ginzburg-Landau theory to low temperatures is used
to derive three Ginzburg-Landau equations - the Maxwell equation for the vector
potential, the Schroedinger equation for the wave function and the Poisson
equation for the electrostatic potential. The electrostatic and the
thermodynamic potential compensate each other to a great extent resulting into
an effective potential acting on the superconducting condensate. For the
Abrikosov vortex lattice in Niobium, numerical solutions are presented and the
different contributions to the electrostatic potential and the related charge
distribution are discussed.Comment: 19 pages, 11 figure
Interaction between ionic lattices and superconducting condensates
The interaction of the ionic lattice with the superconducting condensate is
treated in terms of the electrostatic force in superconductors. It is shown
that this force is similar but not identical to the force suggested by the
volume difference of the normal and superconducting states. The BCS theory
shows larger deviations than the two-fluid model.Comment: 6 pages no figure
The concept of correlated density and its application
The correlated density appears in many physical systems ranging from dense
interacting gases up to Fermi liquids which develop a coherent state at low
temperatures, the superconductivity. One consequence of the correlated density
is the Bernoulli potential in superconductors which compensates forces from
dielectric currents. This Bernoulli potential allows to access material
parameters. Though within the surface potential these contributions are largely
canceled, the bulk measurements with NMR can access this potential. Recent
experiments are explained and new ones suggested. The underlying quantum
statistical theory in nonequilibrium is the nonlocal kinetic theory developed
earlier.Comment: 14 pages, CMT30 proceeding
Surface deformation caused by the Abrikosov vortex lattice
In superconductors penetrated by Abrikosov vortices the magnetic pressure and
the inhomogeneous condensate density induce a deformation of the ionic lattice.
We calculate how this deformation corrugates the surface of a semi-infinite
sample. The effect of the surface dipole is included
Charge Induced Vortex Lattice Instability
It has been predicted that superconducting vortices should be electrically
charged and that this effect is particularly enhanced for, high temperature
superconductors.\cite{kho95,bla96} Hall effect\cite{hag91} and nuclear magnetic
resonance (NMR) experiments\cite{kum01} suggest the existence of vortex
charging, but the effects are small and the interpretation controversial. Here
we show that the Abrikosov vortex lattice, characteristic of the mixed state of
superconductors, will become unstable at sufficiently high magnetic field if
there is charge trapped on the vortex core. Our NMR measurements of the
magnetic fields generated by vortices in BiSrCaCuO
single crystals\cite{che07} provide evidence for an electrostatically driven
vortex lattice reconstruction with the magnitude of charge on each vortex
pancake of x, depending on doping, in line
with theoretical estimates.\cite{kho95,kna05}Comment: to appear in Nature Physics; 6 pages, 7 figure
Contrasting behavior of covalent and molecular carbon allotropes exposed to extreme ultraviolet and soft x-ray free-electron laser radiation
All carbon materials, e.g., amorphous carbon (a-C) coatings and C60 fullerene
thin films, play an important role in short-wavelength free-electron laser
(FEL) research motivated by FEL optics development and prospective
nanotechnology applications. Responses of a-C and C60 layers to the extreme
ultraviolet (SPring-8 Compact SASE Source in Japan) and soft x-ray
(free-electron laser in Hamburg) free-electron laser radiation are investigated
by Raman spectroscopy, differential interference contrast, and atomic force
microscopy. A remarkable difference in the behavior of covalent (a-C) and
molecular (C60) carbonaceous solids is demonstrated under these irradiation
conditions. Low thresholds for ablation of a fullerene crystal (estimated to be
around 0.15 eV/atom for C60 vs 0.9 eV/atom for a-C in terms of the absorbed
dose) are caused by a low cohesive energy of fullerene crystals. An efficient
mechanism of the removal of intact C60 molecules from the irradiated crystal
due to Coulomb repulsion of fullerene-cage cation radicals formed by the
ionizing radiation is revealed by a detailed modeling