489 research outputs found
Collective charge density fluctuations in superconducting layered systems with bilayer unit cells
Collective modes of bilayered superconducting superlattices (e.g., YBCO) are
investigated within the conserving gauge-invariant ladder diagram approximation
including both the nearest interlayer single electron tunneling and the
Josephson-type Cooper pair tunneling. By calculating the density-density
response function including Coulomb and pairing interactions, we examine the
two collective mode branches corresponding to the in-phase and out-of-phase
charge fluctuations between the two layers in the unit cell. The out-of-phase
collective mode develops a long wavelength plasmon gap whose magnitude depends
on the tunneling strength with the mode dispersions being insensitive to the
specific tunneling mechanism (i.e., single electron or Josephson). We also show
that in the presence of tunneling the oscillator strength of the out-of-phase
mode overwhelms that of the in-phase-mode at and finite ,
where and are respectively the mode wave vectors perpendicular
and along the layer. We discuss the possible experimental observability of the
phase fluctuation modes in the context of our theoretical results for the mode
dispersion and spectral weight.Comment: 9 pages, 3 figure
Order parameter suppression in double layer quantum Hall ferromagnets
Double-layer quantum Hall systems at Landau level filling factor have
a broken symmetry ground state with spontaneous interlayer phase coherence and
a gap between symmetric and antisymmetric subbands in the absence of interlayer
tunneling. We examine the influence of quantum fluctuations on the spectral
function of the symmetric Green's function, probed in optical absorption
experiments (cond-mat/9809373). We find that as the maximum layer separation at
which the quantum Hall effect occurs is approached, absorption in the
lowest Landau level grows in strength. Detailed line shapes for this absorption
are evaluated and related to features in the system's collective excitation
spectrum.Comment: 7 pages, 4 figures, To appear in Proceedings of 13th International
Conference on Electronic Properties of Two-Dimensional Systems (EP2DS-13
Tunneling current characteristics in bilayer quantum Hall systems
Weakly disordered bilayer quantum Hall systems at filling factor show
spontaneous interlayer phase coherence if the layers are sufficiently close
together. We study the collective modes in the system, the current-voltage
characteristics and their evolution with an in-plane magnetic field in the
phase-coherent regime.Comment: 4 pages, 2 figures, grammatical changes, To appear in SCES 2001
proceeding
Stripes in Quantum Hall Double Layer Systems
We present results of a study of double layer quantum Hall systems in which
each layer has a high-index Landau level that is half-filled. Hartree-Fock
calculations indicate that, above a critical layer separation, the system
becomes unstable to the formation of a unidirectional coherent charge density
wave (UCCDW), which is related to stripe states in single layer systems. The
UCCDW state supports a quantized Hall effect when there is tunneling between
layers, and is {\it always} stable against formation of an isotropic Wigner
crystal for Landau indices . The state does become unstable to the
formation of modulations within the stripes at large enough layer separation.
The UCCDW state supports low-energy modes associated with interlayer coherence.
The coherence allows the formation of charged soliton excitations, which become
gapless in the limit of vanishing tunneling. We argue that this may result in a
novel {\it ``critical Hall state''}, characterized by a power law in
tunneling experiments.Comment: 10 pages, 8 figures include
Spin Bottlenecks in the Quantum Hall Regim
We present a theory of time-dependent tunneling between a metal and a
partially spin-polarized two-dimensional electron system (2DES). We find that
the leakage current which flows to screen an electric field between the metal
and the 2DES is the sum of two exponential contributions whose relative weights
depend on spin-dependent tunneling conductances, on quantum corrections to the
electrostatic capacitance of the tunnel junction, and on the rate at which the
2DES spin-polarization approaches equilibrium. For high-mobility and
homogeneous 2DES's at Landau level filling factor , we predict a ratio
of the fast and slow leakage rates equal to where is the number
of reversed spins in the skyrmionic elementary charged excitations.Comment: 4 pages, 3 figures, submitted to Phys. Rev. Let
Effect of the Equivalence Between Topological and Electric Charge on the Magnetization of the Hall Ferromagnet
The dependence on temperature of the spin magnetization of a two-dimensional
electron gas at filling factor unity is studied. Using classical Monte Carlo
simulations we analyze the effect that the equivalence between topological and
electrical charge has on the the behavior of the magnetization. We find that at
intermediate temperatures the spin polarization increases in a thirty per cent
due to the Hartree interaction between charge fluctuations.Comment: 4 pages. Submitted to Phys.Rev.
Microwave resonance of the reentrant insulating quantum Hall phases in the 1st excited Landau Level
We present measurements of the real diagonal microwave conductivity of the
reentrant insulating quantum Hall phases in the first excited Landau level at
temperatures below 50 mK. A resonance is detected around filling factor
and weaker frequency dependence is seen at and 2.28.
These measurements are consistent with the formation of a bubble phase crystal
centered around these at very low temperatures
AC Magnetotransport in Reentrant Insulating Phases of Two-dimensional Electrons near 1/5 and 1/3 Landau fillings
We have measured high frequency magnetotransport of a high quality
two-dimensional electron system (2DES) near the reentrant insulating phase
(RIP) at Landau fillings () between 1/5 and 2/9. The
magneto\textit{conductivity} in the RIP has resonant behavior around 150 MHz,
showing a \textit{peak} at 0.21. Our data support the interpretation
of the RIP as due to some pinned electron solid. We have also investigated a
narrowly confined 2DES recently found to have a RIP at 1/31/2 and we
have revealed features, not seen in DC transport, that suggest some intriguing
interplay between the 1/3 FQHE and RIP.Comment: 4 pages and 1 figure (amsart format), 16th International Conference
on High Magnetic Fields in Semiconductor Physics (SemiMag16), August 2-6,
2004, Tallahasse
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