54 research outputs found
Structural and electrical characterization of hybrid metal-polypyrrole nanowires
We present here the synthesis and structural characterization of hybrid
Au-polypyrrole-Au and Pt- polypyrrole-Au nanowires together with a study of
their electrical properties from room-temperature down to very low temperature.
A careful characterization of the metal-polymer interfaces by trans- mission
electron microscopy revealed that the structure and mechanical strength of
bottom and upper interfaces are very different. Variable temperature electrical
transport measurements were performed on both multiple nanowires - contained
within the polycarbonate template - and single nanowires. Our data show that
the three-dimensional Mott variable-range-hopping model provides a complete
framework for the understanding of transport in PPy nanowires, including
non-linear current-voltage characteristics and magnetotransport at low
temperatures.Comment: Phys. Rev. B Vol. 76 Issue 11 (2007
Upper Cretaceous Gosau deposits of the Apuseni Mountains (Romania) - similarities and differences to the Eastern Alps
The Apuseni Mountains were formed during Late Cretaceous convergence between the Tisia and the Dacia microplates as part of the Alpine orogen. The mountain range comprises a sedimentary succession similar to the Gosau Group of the Eastern Alps. This work focuses on the sedimentological and geodynamic evolution of the Gosau basin of the Apuseni Mts. and attempts a direct comparison to the relatively well studied Gosau Group deposits of the Eastern Alps. By analyzing the Upper Cretaceous Gosau sediments and the surrounding geological units, we were able to add critical evidence for reconstructing the Late Mesozoic to Paleogene geodynamic evolution of the Apuseni Mountains. Nannoplankton investigations show that Gosau sedimentation started diachronously after Late Turonian times. The burial history indicates low subsidence rates during deposition of the terrestrial and shallow marine Lower Gosau Subgroup and increased subsidence rates during the period of deep marine Upper Gosau Subgroup sedimentation.The Gosau Group of the Apuseni Mountains was deposited in a forearc basin supplied with sedimentary material from an obducted forearc region and the crystalline hinterland, as reflected by heavy mineral and paleocurrent analysis. Zircon fission track age populations show no fluctuation of exhumation rates in the surrounding geological units, which served as source areas for the detrital material, whereas increased exhumation at the K/Pg boundary can be proven by thermal modeling on apatite fission track data. Synchronously to the Gosau sedimentation, deep marine turbidites were deposited in the deep-sea trench basin formed by the subduction of the Transylvanian Ocean. The similarities to the Gosau occurrences of the Eastern Alps lead to direct correlation with the Alpine paleogeographic evolution and to the assumption that a continuous ocean basin (South Penninic - Transylvanian Ocean Basin) was consumed until Late Cretaceous times
Observation of a new phase transition between fully and partially polarized quantum Hall states with charge and spin gaps at
The average electron spin-polarization of two-dimensional electron
gas confined in multiple quantum-wells was measured by
nuclear magnetic resonance (NMR) near the fractional quantum Hall state with
filling factor . Above this filling factor (), a strong depolarization is observed corresponding to two spin flips per
additional flux quantum. The most remarkable behavior of the polarization is
observed at , where a quantum phase transition from a partially
polarized () to a fully polarized ()
state can be driven by increasing the ratio between the Zeeman and the Coulomb
energy above a critical value .Comment: 4 pages including 4 figure
Thermopower of Interacting GaAs Bilayer Hole Systems in the Reentrant Insulating Phase near
We report thermopower measurements of interacting GaAs bilayer hole systems.
When the carrier densities in the two layers are equal, these systems exhibit a
reentrant insulating phase near the quantum Hall state at total filling factor
. Our data show that as the temperature is decreased, the thermopower
diverges in the insulating phase. This behavior indicates the opening of an
energy gap at low temperature, consistent with the formation of a pinned Wigner
solid. We extract an energy gap and a Wigner solid melting phase diagram.Comment: to be published in Phys. Rev. Let
NMR Determination of 2D Electron Spin Polarization at
Using a `standard' NMR spin-echo technique we determined the spin
polarization of two-dimensional electrons, confined to GaAs quantum wells, from
the hyperfine shift of Ga nuclei in the wells. Concentrating on the temperature
and magnetic field dependencies of spin polarization at Landau level filling
factor , we find that the results are described well by a simple
model of non-interacting composite fermions, although some inconsistencies
remain when the two-dimensional electron system is tilted in the magnetic
field.Comment: 4 pages (REVTEX) AND 4 figures (PS
Layer charge instability in unbalanced bilayer systems in the quantum Hall regime
Measurements in GaAs hole bilayers with unequal layer densities reveal a
pronounced magneto-resistance hysteresis at the magnetic field positions where
either the majority or minority layer is at Landau level filling factor one. At
a fixed field in the hysteretic regions, the resistance exhibits an unusual
time dependence, consisting of random, bidirectional jumps followed by slow
relaxations. These anomalies are apparently caused by instabilities in the
charge distribution of the two layers.Comment: 4 pages, 4 figure
Heat Capacity Evidence for the Suppression of Skyrmions at Large Zeeman Energy
Measurements on a multilayer two-dimensional electron system (2DES) near
Landau level filling =1 reveal the disappearance of the nuclear spin
contribution to the heat capacity as the ratio between the Zeeman
and Coulomb energies exceeds a critical value 0.04. This
disappearance suggests the vanishing of the Skyrmion-mediated coupling between
the lattice and the nuclear spins as the spin excitations of the 2DES make a
transition from Skyrmions to single spin-flips above . Our
experimental is smaller than the calculated =0.054
for an ideal 2DES; we discuss possible origins of this discrepancy.Comment: Experimental paper, 6 figure
Masses of composite fermions carrying two and four flux quanta: Differences and similarities
This study provides a theoretical rationalization for the intriguing
experimental observation regarding the equality of the normalized masses of
composite fermions carrying two and four flux quanta, and also demonstrates
that the mass of the latter type of composite fermion has a substantial filling
factor dependence in the filling factor range , in agreement
with experiment, originating from the relatively strong inter-composite fermion
interactions here.Comment: 5 pages, 2 figure
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