8,128 research outputs found
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
Quantum states in a magnetic anti-dot
We study a new system in which electrons in two dimensions are confined by a
non homogeneous magnetic field. The system consists of a heterostructure with
on top of it a superconducting disk. We show that in this system electrons can
be confined into a dot region. This magnetic anti-dot has the interesting
property that the filling of the dot is a discrete function of the magnetic
field. The circulating electron current inside and outside the anti-dot can be
in opposite direction for certain bound states. And those states exhibit a
diamagnetic to paramagnetic transition with increasing magnetic field. The
absorption spectrum consists of many peaks, some of which violate Kohn's
theorem, and which is due to the coupling of the center of mass motion with the
other degrees of freedom.Comment: 6 pages, 12 ps figure
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
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
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
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
Exciton trapping in magnetic wire structures
The lateral magnetic confinement of quasi two-dimensional excitons into wire
like structures is studied. Spin effects are take into account and two
different magnetic field profiles are considered, which experimentally can be
created by the deposition of a ferromagnetic stripe on a semiconductor quantum
well with magnetization parallel or perpendicular to the grown direction of the
well. We find that it is possible to confine excitons into one-dimensional (1D)
traps. We show that the dependence of the confinement energy on the exciton
wave vector, which is related to its free direction of motion along the wire
direction, is very small. Through the application of a background magnetic
field it is possible to move the position of the trapping region towards the
edge of the ferromagnetic stripe or even underneath the stripe. The exact
position of this 1D exciton channel depends on the strength of the background
magnetic field and on the magnetic polarisation direction of the ferromagnetic
film.Comment: 10 pages, 7 figures, to be published in J. Phys: Condens. Matte
Snake states in graphene quantum dots in the presence of a p-n junction
We investigate the magnetic interface states of graphene quantum dots that
contain p-n junctions. Within a tight-binding approach, we consider rectangular
quantum dots in the presence of a perpendicular magnetic field containing p-n,
as well as p-n-p and n-p-n junctions. The results show the interplay between
the edge states associated with the zigzag terminations of the sample and the
snake states that arise at the p-n junction, due to the overlap between
electron and hole states at the potential interface. Remarkable localized
states are found at the crossing of the p-n junction with the zigzag edge
having a dumb-bell shaped electron distribution. The results are presented as
function of the junction parameters and the applied magnetic flux.Comment: 13 pages, 23 figures, to be appeared in Phys. Rev.
Scattering of Dirac electrons by circular mass barriers: valley filter and resonant scattering
The scattering of two-dimensional (2D) massless Dirac electrons is
investigated in the presence of a random array of circular mass barriers. The
inverse momentum relaxation time and the Hall factor are calculated and used to
obtain parallel and perpendicular resistivity components within linear
transport theory. We found a non zero perpendicular resistivity component which
has opposite sign for electrons in the different K and K' valleys. This
property can be used for valley filter purposes. The total cross-section for
scattering on penetrable barriers exhibit resonances due to the presence of
quasi-bound states in the barriers that show up as sharp gaps in the
cross-section while for Schr\"{o}dinger electrons they appear as peaks.Comment: 10 pages, 11 figure
Simplified model for the energy levels of quantum rings in single layer and bilayer graphene
Within a minimal model, we present analytical expressions for the eigenstates
and eigenvalues of carriers confined in quantum rings in monolayer and bilayer
graphene. The calculations were performed in the context of the continuum
model, by solving the Dirac equation for a zero width ring geometry, i.e. by
freezing out the carrier radial motion. We include the effect of an external
magnetic field and show the appearance of Aharonov-Bohm oscillations and of a
non-zero gap in the spectrum. Our minimal model gives insight in the energy
spectrum of graphene-based quantum rings and models different aspects of finite
width rings.Comment: To appear in Phys. Rev.
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