1,303 research outputs found
Two regimes for effects of surface disorder on the zero-bias conductance peak of tunnel junctions involving d-wave superconductors
Impurity-induced quasiparticle bound states on a pair-breaking surface of a
d-wave superconductor are theoretically described, taking into account
hybridization of impurity- and surface-induced Andreev states. Further a theory
for effects of surface disorder (of thin impurity surface layer) on the
low-bias conductance of tunnel junctions is developed. We find a threshold
for surface impurity concentration , which separates the two regimes
for surface impurity effects on the zero-bias conductance peak (ZBCP). Below
the threshold, surface impurities do not broaden the ZBCP, but effectively
reduce its weight and generate impurity bands. For low impurity bands can
be, in principle, resolved experimentally, being centered at energies of bound
states induced by an isolated impurity on the surface. For larger
impurity bands are distorted, move to lower energies and, beginning with the
threshold concentration , become centered at zero energy. With
increasing above the threshold, the ZBCP is quickly destroyed in the case
of strong scatterers, while it is gradually suppressed and broaden in the
presence of weak impurity potentials. More realistic cases, taking into account
additional broadening, not related to the surface disorder, are also
considered.Comment: 9 pages, 7 figure
Transport in two dimensional periodic magnetic fields
Ballistic transport properties in a two dimensional electron gas are studied
numerically, where magnetic fields are perpendicular to the plane of two
dimensional electron systemsand periodically modulated both in and
directions. We show that there are three types of trajectories of classical
electron motions in this system; chaotic, pinned and runaway trajectories. It
is found that the runaway trajectories can explain the peaks of
magnetoresistance as a function of external magnetic fields, which is believed
to be related to the commensurability effect between the classical cyclotron
diameter and the period of magnetic modulation. The similarity with and
difference from the results in the antidot lattice are discussed.Comment: 4 pages, 7 figures, to appear in J. Phys. Soc. Jpn., vol. 67 (1998)
Novembe
Quasiclassical negative magnetoresistance of a 2D electron gas: interplay of strong scatterers and smooth disorder
We study the quasiclassical magnetotransport of non-interacting fermions in
two dimensions moving in a random array of strong scatterers (antidots,
impurities or defects) on the background of a smooth random potential. We
demonstrate that the combination of the two types of disorder induces a novel
mechanism leading to a strong negative magnetoresistance, followed by the
saturation of the magnetoresistivity at a value determined
solely by the smooth disorder. Experimental relevance to the transport in
semiconductor heterostructures is discussed.Comment: 4 pages, 2 figure
Giant negative magnetoresistance in high-mobility 2D electron systems
We report on a giant negative magnetoresistance in very high mobility
GaAs/AlGaAs heterostructures and quantum wells. The effect is the strongest at
kG, where the magnetoresistivity develops a minimum emerging at K. Unlike the zero-field resistivity which saturates at K, the resistivity at this minimum continues to drop at an accelerated rate
to much lower temperatures and becomes several times smaller than the
zero-field resistivity. Unexpectedly, we also find that the effect is destroyed
not only by increasing temperature but also by modest in-plane magnetic fields.
The analysis shows that giant negative magnetoresistance cannot be explained by
existing theories considering interaction-induced or disorder-induced
corrections
Quasiclassical magnetotransport in a random array of antidots
We study theoretically the magnetoresistance of a
two-dimensional electron gas scattered by a random ensemble of impenetrable
discs in the presence of a long-range correlated random potential. We believe
that this model describes a high-mobility semiconductor heterostructure with a
random array of antidots. We show that the interplay of scattering by the two
types of disorder generates new behavior of which is absent for
only one kind of disorder. We demonstrate that even a weak long-range disorder
becomes important with increasing . In particular, although
vanishes in the limit of large when only one type of disorder is present,
we show that it keeps growing with increasing in the antidot array in the
presence of smooth disorder. The reversal of the behavior of is
due to a mutual destruction of the quasiclassical localization induced by a
strong magnetic field: specifically, the adiabatic localization in the
long-range Gaussian disorder is washed out by the scattering on hard discs,
whereas the adiabatic drift and related percolation of cyclotron orbits
destroys the localization in the dilute system of hard discs. For intermediate
magnetic fields in a dilute antidot array, we show the existence of a strong
negative magnetoresistance, which leads to a nonmonotonic dependence of
.Comment: 21 pages, 13 figure
Short--range impurity in the vicinity of a saddle point and the levitation of the 2D delocalized states in a magnetic field
The effect of a short--range impurity on the transmission through a
saddle--point potential for an electron, moving in a strong magnetic field, is
studied. It is demonstrated that for a random position of an impurity and
random sign of its potential the impurity--induced mixing of the Landau levels
diminishes {\em on average} the transmission coefficient. This results in an
upward shift (levitation) of the energy position of the delocalized state in a
smooth potential. The magnitude of the shift is estimated. It increases with
decreasing magnetic field as .Comment: LaTeX, 20 page
Free particle scattering off two oscillating disks
We investigate the two-dimensional classical dynamics of the scattering of
point particles by two periodically oscillating disks. The dynamics exhibits
regular and chaotic scattering properties, as a function of the initial
conditions and parameter values of the system. The energy is not conserved
since the particles can gain and loose energy from the collisions with the
disks. We find that for incident particles whose velocity is on the order of
the oscillating disk velocity, the energy of the exiting particles displays
non-monotonic gaps of allowed energies, and the distribution of exiting
particle velocities shows significant fluctuations in the low energy regime. We
also considered the case when the initial velocity distribution is Gaussian,
and found that for high energies the exit velocity distribution is Gaussian
with the same mean and variance. When the initial particle velocities are in
the irregular regime the exit velocity distribution is Gaussian but with a
smaller mean and variance. The latter result can be understood as an example of
stochastic cooling. In the intermediate regime the exit velocity distribution
differs significantly from Gaussian. A comparison of the results presented in
this paper to previous chaotic static scattering problems is also discussed.Comment: 9 doble sided pages 13 Postscript figures, REVTEX style. To appear in
Phys. Rev.
Ratchet effects induced by terahertz radiation in heterostructures with a lateral periodic potential
We report on the observation of terahertz radiation induced photogalvanic
currents in semiconductor heterostructures with one-dimensional lateral
periodic potential. The potential is produced by etching a grating into the
sample surface. The electric current response is well described by
phenomenological theory including both the circular and linear photogalvanic
effects. Experimental data demonstrate that the inversion asymmetry of the
periodic lateral pattern can be varied by means of electron beam lithography to
produce classical lateral ratchets. A novel microscopical mechanism for the
polarization-dependent photogalvanic effects has been proposed to interpret the
experimental findings. The photocurrent generation is based on the combined
action of the lateral periodic potential and the modulated in-plane pumping.
The latter modulation stems from near-field effects of the radiation
propagating through the grating.Comment: 8 pages, 6 figure
Monotonic growth of interlayer magnetoresistance in strong magnetic field in very anisotropic layered metals
It is shown, that the monotonic part of interlayer electronic conductivity
strongly decreases in high magnetic field perpendicular to the conducting
layers. We consider only the coherent interlayer tunnelling, and the obtained
result strongly contradicts the standard theory. This effect appears in very
anisotropic layered quasi-two-dimensional metals, when the interlayer transfer
integral is less than the Landau level separation.Comment: 4 pages, no figure
Mechanical land clearing to promote establishment of coastal sandplain grassland and shrubland communities
Author Posting. © The Author(s), 2005. This is the author's version of the work. It is posted here by permission of Blackwell Publishing for personal use, not for redistribution. The definitive version was published in Restoration Ecology 14 (2006): 220-232, doi:10.1111/j.1526-100X.2006.00124.x.The decline in grasslands and other species-rich early-successional habitats on the coastal sandplains of the northeastern U.S. has spurred management to increase the area of these declining plant communities. We mechanically removed overstory oak and applied seed from a nearby sandplain grassland on the island of Martha’s Vineyard, Massachusetts to evaluate this technique for creating an open oak community able to support sandplain herbaceous species. We compared vegetation structure and composition before and after clearing in an area of total tree removal (clearcutting), an area where 85% of tree basal area was removed (savanna cutting) and in adjacent coastal oak forest. Plant responses to clearcutting and savanna cutting were similar. Sandplain herbs colonized at high frequencies after seeding and increasing herbaceous cover from <7% before clearing to 22-38% three growing seasons later. Carex pensylvanica (Pennsylvania sedge) increased in cover ~ 6-fold, accounting for 84-90% of the increased herbaceous cover. Other native ruderals, and exotic herbs reached 6%, 2%, and 1%, cover respectively, after three years. Species richness across cleared treatments increased from 30 to 79 species. All forest species were retained. Forest shrubs and trees initially declined from their dominant cover, but rebounded after three years. Tree clearing plus seeding appeared to be a viable management practice for increasing cover of herbaceous sandplain species while causing minimal increases in exotic herbaceous cover. The long-term persistence of sandplain herbs may require periodic disturbances that limit woody regrowth.This work was funded by grants from the A. W. Mellon Foundation and the Massachusetts Environmental Trust to MBL and from the Kohlberg Foundation to TNC
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