10,345 research outputs found
Evidence for a Fractional Quantum Hall Nematic State in Parallel Magnetic Fields
We report magneto-transport measurements for the fractional quantum Hall
state at filling factor 5/2 as a function of applied parallel magnetic
field (). As is increased, the 5/2 state becomes increasingly
anisotropic, with the in-plane resistance along the direction of
becoming more than 30 times larger than in the perpendicular direction.
Remarkably, the resistance anisotropy ratio remains constant over a relatively
large temperature range, yielding an energy gap which is the same for both
directions. Our data are qualitatively consistent with a fractional quantum
Hall \textit{nematic} phase
Observation of An Anisotropic Wigner Crystal
We report a new correlated phase of two-dimensional charged carriers in high
magnetic fields, manifested by an anisotropic insulating behavior at low
temperatures. It appears near Landau level filling factor in hole
systems confined to wide GaAs quantum wells when the sample is tilted in
magnetic field to an intermediate angle. The parallel field component
() leads to a crossing of the lowest two Landau levels, and an
elongated hole wavefunction in the direction of . Under these
conditions, the in-plane resistance exhibits an insulating behavior, with the
resistance along more than 10 times smaller than the resistance
perpendicular to . We interpret this anisotropic insulating phase as a
two-component, striped Wigner crystal
Multicomponent fractional quantum Hall states with subband and spin degrees of freedom
In wide GaAs quantum wells where two electric subbands are occupied we apply
a parallel magnetic field or increase the electron density to cause a crossing
of the two Landau levels of these subbands and with opposite spins. Near
the crossing, the fractional quantum Hall states in the filling factor range
exhibit a remarkable sequence of pseudospin polarization transitions
resulting from the interplay between the spin and subband degrees of freedom.
The field positions of the transitions yield a new and quantitative measure of
the composite Fermions' discrete energy level separations. Surprisingly, the
separations are smaller when the electrons have higher spin-polarization
Magnetotransport in a two-dimensional electron system in dc electric fields
We report on nonequilibrium transport measurements in a high-mobility
two-dimensional electron system subject to weak magnetic field and dc
excitation. Detailed study of dc-induced magneto-oscillations, first observed
by Yang {\em et al}., reveals a resonant condition that is qualitatively
different from that reported earlier. In addition, we observe dramatic
reduction of resistance induced by a weak dc field in the regime of separated
Landau levels. These results demonstrate similarity of transport phenomena in
dc-driven and microwave-driven systems and have important implications for
ongoing experimental search for predicted quenching of microwave-induced
zero-resistance states by a dc current.Comment: Revised version, to appear in Phys. Rev.
Impact of disorder on the 5/2 fractional quantum Hall state
We compare the energy gap of the \nu=5/2 fractional quantum Hall effect state
obtained in conventional high mobility modulation doped quantum well samples
with those obtained in high quality GaAs transistors (heterojunction insulated
gate field-effect transistors). We are able to identify the different roles
that long range and short range disorders play in the 5/2 state and observe
that the long range potential fluctuations are more detrimental to the strength
of the 5/2 state than short-range potential disorder.Comment: PRL 106, 206806 (2011
Ultraslow Electron Spin Dynamics in GaAs Quantum Wells Probed by Optically Pumped NMR
Optically pumped nuclear magnetic resonance (OPNMR) measurements were
performed in two different electron-doped multiple quantum well samples near
the fractional quantum Hall effect ground state nu=1/3. Below 0.5K, the spectra
provide evidence that spin-reversed charged excitations of the nu=1/3 ground
state are localized over the NMR time scale of ~40 microseconds. Furthermore,
by varying NMR pulse parameters, the electron spin temperature (as measured by
the Knight shift) could be driven above the lattice temperature, which shows
that the value of the electron spin-lattice relaxation time lies between 100
microseconds and 500 milliseconds at nu=1/3.Comment: 6 pages (REVTEX), 6 eps figures embedded in text; published version;
minor changes to match published versio
- …