103 research outputs found
Ray-optical negative refraction and pseudoscopic imaging with Dove-prism arrays
A sheet consisting of an array of small, aligned Dove prisms can locally (on the scale of the width of the prisms) invert one component of the ray direction. A sandwich of two such Dove-prism sheets that inverts both transverse components of the ray direction is a ray-optical approximation to the interface between two media with refractive indices +n and –n. We demonstrate the simulated imaging properties of such a Dove-prism-sheet sandwich, including a demonstration of pseudoscopic imaging
Compressibility of a two-dimensional hole gas in tilted magnetic field
We have measured compressibility of a two-dimensional hole gas in
p-GaAs/AlGaAs heterostructure, grown on a (100) surface, in the presence of a
tilted magnetic field. It turns out that the parallel component of magnetic
field affects neither the spin splitting nor the density of states. We conclude
that: (a) g-factor in the parallel magnetic field is nearly zero in this
system; and (b) the level of the disorder potential is not sensitive to the
parallel component of the magnetic field
Thermodynamic Signature of a Two-Dimensional Metal-Insulator Transition
We present a study of the compressibility, K, of a two-dimensional hole
system which exhibits a metal-insulator phase transition at zero magnetic
field. It has been observed that dK/dp changes sign at the critical density for
the metal-insulator transition. Measurements also indicate that the insulating
phase is incompressible for all values of B. Finally, we show how the phase
transition evolves as the magnetic field is varied and construct a phase
diagram in the density-magnetic field plane for this system.Comment: 4 pages, 4 figures, submitted to Physical Review Letters; version 1
is identical to version 2 but didn't compile properl
Absence of Floating Delocalized States in a Two-Dimensional Hole Gas
By tracking the delocalized states of the two-dimensional hole gas in a
p-type GaAs/AlGaAs heterostructure as a function of magnetic field, we mapped
out a phase diagram in the density-magnetic-field plane. We found that the
energy of the delocalized state from the lowest Landau level flattens out as
the magnetic field tends toward zero. This finding is different from that for
the two-dimensional electron system in an n-type GaAs/AlGaAs heterostructure
where delocalized states diverge in energy as B goes to zero indicating the
presence of only localized states below the Fermi energy. The possible
connection of this finding to the recently observed metal-insulator transition
at B = 0 in the two-dimensional hole gas systems is discussed.Comment: 10 pages, 4 Postscript figures, To be published in Physical Review B
(Rapid Communications) 58, Sept. 15, 199
Physics of the Insulating Phase in the Dilute Two-Dimensional Electron Gas
We propose to use the radio-frequency single-electron transistor as an
extremely sensitive probe to detect the time-periodic ac signal generated by
sliding electron lattice in the insulating state of the dilute two-dimensional
electron gas. We also propose to use the optically-pumped NMR technique to
probe the electron spin structure of the insulating state. We show that the
electron effective mass and spin susceptibility are strongly enhanced by
critical fluctuations of electron lattice in the vicinity of the
metal-insulator transition, as observed in experiment.Comment: 5 pages, 2 figures, uses jetpl.cls (included). v.4: After publication
in JETP Letters, two plots comparing theory and experiment are added, and a
minor error is correcte
Phase diagram of the integer quantum Hall effect in p-type Germanium
We experimentally study the phase diagram of the integer quantized Hall
effect, as a function of density and magnetic field. We used a two dimensional
hole system confined in a Ge/SiGe quantum well, where all energy levels are
resolved, because the Zeeman splitting is comparable to the cyclotron energy.
At low fields and close to the quantum Hall liquid-to-insulator transition, we
observe the floating up of the lowest energy level, but NO FLOATING of any
higher levels, rather a merging of these levels into the insulating state. For
a given filling factor, only direct transitions between the insulating phase
and higher quantum Hall liquids are observed as a function of density. Finally,
we observe a peak in the critical resistivity around filling factor one.Comment: 4 pages, 4 figures, some changes in the tex
Biogenic weathering bridges the nutrient gap in pristine ecosystems - a global comparison
In many pristine ecosystems there seems to be negative nutrient budget existent, meaning that export exceeds the input received by aeolian deposition and physico-chemical weathering. Such ecosystems should degrade rather quickly, but are often found surprisingly stable on the long run. Our hypothesis was that this nutrient gap is an artefact caused by not considering the contribution of photoassimilatory-mediated biogenic weathering to the overall nutrient input, which might constitute an additional, energetically directed and demand driven pathway. Here, we firstly evaluated the evolution of mutualistic biogenic weathering along an Antarctic chronosequence and secondly compared the biogenic weathering rates under mycorrhized ecosystems over a global gradient of contrasting states of soil development. We found the ability to perform biogenic weathering increasing along its evolutionary development in photoautotroph-symbiont interaction and furthermore a close relation between fungal biogenic weathering and available potassium across all 16 forested sites in the study, regardless of the dominant mycorrhiza type (AM or EM), climate, and plant-species composition. Our results point towards a general alleviation of nutrient limitation at ecosystem scale via directional, energy driven and on-demand biogenic weathering
Metallic behavior and related phenomena in two dimensions
For about twenty years, it has been the prevailing view that there can be no
metallic state or metal-insulator transition in two dimensions in zero magnetic
field. In the last several years, however, unusual behavior suggestive of such
a transition has been reported in a variety of dilute two-dimensional electron
and hole systems. The physics behind these observations is presently not
understood. We review and discuss the main experimental findings and suggested
theoretical models.Comment: To be published in Rev. Mod. Phy
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