6,287 research outputs found
Hall potentiometer in the ballistic regime
We demonstrate theoretically how a two-dimensional electron gas can be used
to probe local potential profiles using the Hall effect. For small magnetic
fields, the Hall resistance is inversely proportional to the average potential
profile in the Hall cross and is independent of the shape and the position of
this profile in the junction. The bend resistance, on the other hand, is much
more sensitive on the exact details of the local potential profile in the cross
junction.Comment: 3 pages, 4 ps figure
Resistance effects due to magnetic guiding orbits
The Hall and magnetoresistance of a two dimensional electron gas subjected to
a magnetic field barrier parallel to the current direction is studied as
function of the applied perpendicular magnetic field. The recent experimental
results of Nogaret {\em et al.} [Phys. Rev. Lett. {\bf 84}, 2231 (2000)] for
the magneto- and Hall resistance are explained using a semi-classical theory
based on the Landauer-B\"{u}ttiker formula. The observed positive
magnetoresistance peak is explained as due to a competition between a decrease
of the number of conducting channels as a result of the growing magnetic field,
from the fringe field of the ferromagnetic stripe as it becomes magnetized, and
the disappearance of snake orbits and the subsequent appearance of cycloidlike
orbits.Comment: 7 pages, 7 figure
Ginzburg-Landau theory and effects of pressure on a two-band superconductor : application to MgB2
We present a model of pressure effects of a two-band superconductor based on
a Ginzburg-Landau free energy with two order parameters. The parameters of the
theory are pressure as well as temperature dependent. New pressure effects
emerge as a result of the competition between the two bands. The theory then is
applied to MgB2. We identify two possible scenaria regarding the fate of the
two subbands under pressure, depending on whether or not both subbands
are above the Fermi energy at ambient pressure. The splitting of the two
subbands is probably caused by the E2g distortion. If only one subband is above
the Fermi energy at ambient pressure (scenario I), application of pressure
diminishes the splitting and it is possible that the lower subband participates
in the superconductivity. The corresponding crossover pressure and Gruneisen
parameter are estimated. In the second scenario both bands start above the
Fermi energy and they move below it, either by pressure or via the substitution
of Mg by Al. In both scenaria, the possibility of electronical topological
transition is emphasized. Experimental signatures of both scenaria are
presented and existing experiments are discussed in the light of the different
physical pictures.Comment: 6 pages; supersedes the first part of cond-mat/0204085 due to new
experiment
Electron scattering on circular symmetric magnetic profiles in a two-dimensional electron gas
The quasi-bound and scattered states in a 2DEG subjected to a circular
symmetric steplike magnetic profile with zero average magnetic field are
studied. We calculate the effect of a random distribution of such identical
profiles on the transport properties of a 2DEG. We show that a nonzero Hall
resistance can be obtained, although , and that in some cases it
can even change sign as function of the Fermi energy or the magnetic field
strength. The Hall and magnetoresistance show pronounced resonances apart from
the Landau states of the inner core, corresponding to the so-called quasi-bound
snake orbit states.Comment: 7 pages, 8 figure
Confined magnetic guiding orbit states
We show how snake-orbit states which run along a magnetic edge can be
confined electrically. We consider a two-dimensional electron gas (2DEG)
confined into a quantum wire, subjected to a strong perpendicular and steplike
magnetic field . Close to this magnetic step new, spatially confined
bound states arise as a result of the lateral confinement and the magnetic
field step. The number of states, with energy below the first Landau level,
increases as becomes stronger or as the wire width becomes larger. These
bound states can be understood as an interference between two
counter-propagating one-dimensional snake-orbit states.Comment: 4 pages, 4 figure
Saddle point states and energy barriers for vortex entrance and exit in superconducting disks and rings
The transitions between the different vortex states of thin mesoscopic
superconducting disks and rings are studied using the non-linear
Ginzburg-Landau functional. They are saddle points of the free energy
representing the energy barrier which has to be overcome for transition between
the different vortex states. In small superconducting disks and rings the
saddle point state between two giant vortex states, and in larger systems the
saddle point state between a multivortex state and a giant vortex state and
between two multivortex states is obtained. The shape and the height of the
nucleation barrier is investigated for different disk and ring configurations.Comment: 10 pages, 18 figure
Dynamics of self-organized driven particles with competing range interaction
Non-equilibrium self-organized patterns formed by particles interacting
through competing range interaction are driven over a substrate by an external
force. We show that, with increasing driving force, the pre-existed static
patterns evolve into dynamic patterns either via disordered phase or depinned
patterns, or via the formation of non-equilibrium stripes. Strikingly, the
stripes are formed either in the direction of the driving force or in the
transverse direction, depending on the pinning strength. The revealed dynamical
patterns are summarized in a dynamical phase diagram.Comment: 8 pages, 11 figure
Landau levels and oscillator strength in a biased bilayer of graphene
We obtain analytical expressions for the eigenstates and the Landau level
spectrum of biased graphene bilayers in a magnetic field. The calculations are
performed in the context of a four-band continuum model and generalize previous
approximate results. Solutions are presented for the spectrum as a function of
interlayer coupling, the potential difference between the layers and the
magnetic field. The explicit expressions allow us to calculate the oscillator
strength and the selection rules for electric dipole transitions between the
Landau states. Some transitions are significantly shifted in energy relative to
those in an unbiased bialyer and exhibit a very different magnetic field
dependence.Comment: To appear in Phys. Rev.
- …