353 research outputs found
An Investigation of Orientational Symmetry-Breaking Mechanisms in High Landau Levels
The principal axes of the recently discovered anisotropic phases of 2D
electron systems at high Landau level occupancy are consistently oriented
relative to the crystal axes of the host semiconductor. The nature of the
native rotational symmetry breaking field responsible for this preferential
orientation remains unknown. Here we report on experiments designed to
investigate the origin and magnitude of this symmetry breaking field. Our
results suggest that neither micron-scale surface roughness features nor the
precise symmetry of the quantum well potential confining the 2D system are
important factors. By combining tilted field transport measurements with
detailed self-consistent calculations we estimate that the native anisotropy
energy, whatever its origin, is typically ~ 1 mK per electron.Comment: Reference added, minor notational changes; final published versio
New collective states of 2D electrons in high Landau levels
A brief summary of the emerging evidence for a new class of collective states
of two-dimensional electrons in partially occupied excited Landau levels is
presented. Among the most dramatic phenomena described are the large
anisotropies of the resistivity observed at very low temperatures near
half-filling of the third and higher Landau levels and the non-linear character
of the re-entrant integer quantized Hall states in the flanks of the same
levels. The degree to which these findings support recent theoretical
predictions of charge density wave ground states is discussed and a preliminary
comparison to recent transport theories is made.Comment: To be published in Physica E, as part of the proceedings of the 11th
International Winterschool on New Developments in Solid State Physics held in
Mauterndorf, Austria, February, 2000. 25 pages and 9 figures in a single pdf
fil
The Onset of Anisotropic Transport of Two-Dimensional Electrons in High Landau Levels: An Isotropic-to-Nematic Liquid Crystal Phase Transition?
The recently discovered anisotropy of the longitudinal resistance of
two-dimensional electrons near half filling of high Landau levels is found to
persist to much higher temperatures T when a large in-plane magnetic field B||
is applied. Under these conditions we find that the longitudinal resistivity
scales quasi-linearly with B||/T. These observations support the notion that
the onset of anisotropy at B||=0 does not reflect the spontaneous development
of charge density modulations but may instead signal an isotropic-to-nematic
liquid crystal phase transition.Comment: 5 pages, 4 figure
Signatures of Electronic Nematic Phase at Isotropic-Nematic Phase Transition
The electronic nematic phase occurs when the point-group symmetry of the
lattice structure is broken, due to electron-electron interactions. We study a
model for the nematic phase on a square lattice with emphasis on the phase
transition between isotropic and nematic phases within mean field theory. We
find the transition to be first order, with dramatic changes in the Fermi
surface topology accompanying the transition. Furthermore, we study the
conductivity tensor and Hall constant as probes of the nematic phase and its
transition. The relevance of our findings to Hall resistivity experiments in
the high- cuprates is discussed.Comment: 5 pages, 3 figure
Microscopic Theory of the Reentrant IQHE in the First and Second Excited LLs
We present a microscopic theory for the recently observed reentrant integral
quantum Hall effect in the n=1 and n=2 Landau levels. Our energy investigations
indicate an alternating sequence of M-electron-bubble and quantum-liquid ground
states in a certain range of the partial filling factor of the n-th level.
Whereas the quantum-liquid states display the fractional quantum Hall effect,
the bubble phases are insulating, and the Hall resistance is thus quantized at
integral values of the total filling factor.Comment: 4 pages, 4 figures; minor corrections include
Quantum Hall ferromagnets, cooperative transport anisotropy, and the random field Ising model
We discuss the behaviour of a quantum Hall system when two Landau levels with
opposite spin and combined filling factor near unity are brought into energetic
coincidence using an in-plane component of magnetic field. We focus on the
interpretation of recent experiments under these conditions [Zeitler et al,
Phys. Rev. Lett. 86, 866 (2001); Pan et al, Phys. Rev. B 64, 121305 (2001)], in
which a large resistance anisotropy develops at low temperatures. Modelling the
systems involved as Ising quantum Hall ferromagnets, we suggest that this
transport anisotropy reflects domain formation induced by a random field
arising from isotropic sample surface roughness.Comment: 4 pages, submitted to Physical Review
Is Integration Enough for Fast Product Development? An Empirical Investigation of the Contextual Effects of Product Vision
Raman scattering studies of spin, charge, and lattice dynamics in Ca_{2-x}Sr_{x}RuO_{4} (0 =< x < 0.2)
We use Raman scattering to study spin, charge, and lattice dynamics in
various phases of Ca_{2-x}Sr_{x}RuO_{4}. With increasing substitution of Ca by
Sr in the range 0 =< x < 0.2, we observe (1) evidence for an increase of the
electron-phonon interaction strength, (2) an increased temperature-dependence
of the two-magnon energy and linewidth in the antiferromagnetic insulating
phase, and (3) evidence for charge gap development, and hysteresis associated
with the structural phase change, both of which are indicative of a first-order
metal-insulator transition (T_{MI}) and a coexistence of metallic and
insulating components for T < T_{MI}
True Superconductivity in a 2D "Superconducting-Insulating" System
We present results on disordered amorphous films which are expected to
undergo a field-tuned Superconductor-Insulator Transition. Based on low-field
data and I-V characteristics, we find evidence of a low temperature
Metal-to-Superconductor transition. This transition is characterized by
hysteretic magnetoresistance and discontinuities in the I-V curves. The
metallic phase just above the transition is different from the "Fermi Metal"
before superconductivity sets in.Comment: 3 pages, 4 figure
Anisotropic transport in unidirectional lateral superlattice around half-filling of the second Landau level
We have observed marked transport anisotropy in short period (a=92 nm)
unidirectional lateral superlattices around filling factors nu=5/2 and 7/2:
magnetoresistance shows a sharp peak for current along the modulation grating
while a dip appears for current across the grating. By altering the ratio a/l
(with l=sqrt{hbar/eB_perp} the magnetic length) via changing the electron
density n_e, it is shown that the nu=5/2 anisotropic features appear in the
range 6.6 alt a/l alt 7.2 varying their intensities, becoming most conspicuous
at a/l simeq 6.7. The peak/dip broadens with temperature roughly preserving its
height/depth up to 250 mK. Tilt experiments reveal that the structures are
slightly enhanced by an in-plane magnetic field B_| perpendicular to the
grating but are almost completely destroyed by B_| parallel to the grating. The
observations suggest the stabilization of a unidirectional charge-density-wave
or stripe phase by weak external periodic modulation at the second Landau
level.Comment: REVTeX, 5 pages, 3 figures, Some minor revisions, Added notes and
reference
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