877 research outputs found
Current-induced nuclear-spin activation in a two-dimensional electron gas
Electrically detected nuclear magnetic resonance was studied in detail in a
two-dimensional electron gas as a function of current bias and temperature. We
show that applying a relatively modest dc-current bias, I_dc ~ 0.5 microAmps,
can induce a re-entrant and even enhanced nuclear spin signal compared with the
signal obtained under similar thermal equilibrium conditions at zero current
bias. Our observations suggest that dynamic nuclear spin polarization by small
current flow is possible in a two-dimensional electron gas, allowing for easy
manipulation of the nuclear spin by simple switching of a dc current.Comment: 5 pages, 3 fig
Intrinsic Gap of the nu=5/2 Fractional Quantum Hall State
The fractional quantum Hall effect is observed at low field, in a regime
where the cyclotron energy is smaller than the Coulomb interaction. The nu=5/2
excitation gap is measured to be 262+/-15 mK at ~2.6 T, in good agreement with
previous measurements performed on samples with similar mobility, but with
electronic density larger by a factor of two. The role of disorder on the
nu=5/2 gap is examined. Comparison between experiment and theory indicates that
a large discrepancy remains for the intrinsic gap extrapolated from the
infinite mobility (zero disorder) limit. In contrast, no such large discrepancy
is found for the nu=1/3 Laughlin state. The observation of the nu=5/2 state in
the low-field regime implies that inclusion of non-perturbative Landau level
mixing may be necessary to better understand the energetics of half-filled
fractional quantum hall liquids.Comment: 5 pages, 4 figures; typo corrected, comment expande
Contrasting Behavior of the 5/2 and 7/3 Fractional Quantum Hall Effect in a Tilted Field
Using a tilted field geometry, the effect of an in-plane magnetic field on
the even denominator nu = 5/2 fractional quantum Hall state is studied. The
energy gap of the nu = 5/2 state is found to collapse linearly with the
in-plane magnetic field above ~0.5 T. In contrast, a strong enhancement of the
gap is observed for the nu = 7/3 state. The radically distinct tilted-field
behaviour between the two states is discussed in terms of Zeeman and
magneto-orbital coupling within the context of the proposed Moore-Read pfaffian
wavefunction for the 5/2 fractional quantum Hall effect
Colossal magnetoresistance in an ultra-clean weakly interacting 2D Fermi liquid
We report the observation of a new phenomenon of colossal magnetoresistance
in a 40 nm wide GaAs quantum well in the presence of an external magnetic field
applied parallel to the high-mobility 2D electron layer. In a strong magnetic
field, the magnetoresistance is observed to increase by a factor of ~300 from 0
to 45T without the system undergoing any metal-insulator transition. We discuss
how this colossal magnetoresistance effect cannot be attributed to the spin
degree-of-freedom or localization physics, but most likely emanates from strong
magneto-orbital coupling between the two-dimensional electron gas and the
magnetic field. Our observation is consistent with a field-induced 2D-to-3D
transition in the confined electronic system
Single-shot, transverse self-wakefield reconstruction from screen images
A single-shot method to reconstruct the transverse self-wakefields acting on
a beam, based only on screen images, is introduced. By employing numerical
optimization with certain approximations, a relatively high-dimensional
parameter space is efficiently explored to determine the multipole components
of the transverse-wakefield topology up to desired order. The reconstruction
technique complements simulations, which are able to directly describe the
wakefield composition based on experimental conditions. The technique is
applied to representative simulation results as a benchmark, and also to
experimental data on wakefield observations driven in dielectric-lined
structures.Comment: 10 pages, 8 figure
Generation of angular-momentum-dominated electron beams from a photoinjector
Various projects under study require an angular-momentum-dominated electron
beam generated by a photoinjector. Some of the proposals directly use the
angular-momentum-dominated beams (e.g. electron cooling of heavy ions), while
others require the beam to be transformed into a flat beam (e.g. possible
electron injectors for light sources and linear colliders). In this paper, we
report our experimental study of an angular-momentum-dominated beam produced in
a photoinjector, addressing the dependencies of angular momentum on initial
conditions. We also briefly discuss the removal of angular momentum. The
results of the experiment, carried out at the Fermilab/NICADD Photoinjector
Laboratory, are found to be in good agreement with theoretical and numerical
models.Comment: 8 pages, 7 figures, submitted to Phys. Rev. ST Accel. Beam
Magneto-optics of massive Dirac fermions in bulk Bi2Se3
We report on magneto-optical studies of Bi2Se3, a representative member of
the 3D topological insulator family. Its electronic states in bulk are shown to
be well described by a simple Dirac-type Hamiltonian for massive particles with
only two parameters: the fundamental bandgap and the band velocity. In a
magnetic field, this model implies a unique property - spin splitting equal to
twice the cyclotron energy: Es = 2Ec. This explains the extensive
magneto-transport studies concluding a fortuitous degeneracy of the spin and
orbital split Landau levels in this material. The Es = 2Ec match differentiates
the massive Dirac electrons in bulk Bi2Se3 from those in quantum
electrodynamics, for which Es = Ec always holds.Comment: 5 pages, 3 figures and Supplementary materials, to be published in
Physical Review Letter
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