55 research outputs found
Particle-hole Asymmetry of Fractional Quantum Hall States in the Second Landau Level of a Two-dimensional Hole System
We report the first unambiguous observation of a fractional quantum Hall
state in the Landau level of a two-dimensional hole sample at the filling
factor . We identified this state by a quantized Hall resistance and
an activated temperature dependence of the longitudinal resistance and found an
energy gap of 40 mK. To our surprise the particle-hole conjugate state at
filling factor in our sample does not develop down to 6.9 mK. This
observation is contrary to that in electron samples in which the 7/3 state is
typically more stable than the 8/3 state. We present evidence that the
asymmetry between the 7/3 and 8/3 states in our hole sample is due to Landau
level mixing
Integrated Electronic Transport and Thermometry at milliKelvin Temperatures and in Strong Magnetic Fields
We fabricated a He-3 immersion cell for transport measurements of
semiconductor nanostructures at ultra low temperatures and in strong magnetic
fields. We have a new scheme of field-independent thermometry based on quartz
tuning fork Helium-3 viscometry which monitors the local temperature of the
sample's environment in real time. The operation and measurement circuitry of
the quartz viscometer is described in detail. We provide evidence that the
temperature of two-dimensional electron gas confined to a GaAs quantum well
follows the temperature of the quartz viscometer down to 4mK
Growth and electrical characterization of Al0.24Ga0.76As/AlxGa1-xAs/Al0.24Ga0.76As modulation-doped quantum wells with extremely low x
We report on the growth and electrical characterization of modulation-doped Al0.24Ga0.76As/AlxGa1-xAs/Al0.24Ga0.76As quantum wells with mole fractions as low as x = 0.00057. Such structures will permit detailed studies of the impact of alloy disorder in the fractional quantum Hall regime. At zero magnetic field, we extract an alloy scattering rate of 24 ns(-1) per% Al. Additionally, we find that for x as low as 0.00057 in the quantum well, alloy scattering becomes the dominant mobility-limiting scattering mechanism in ultra-high purity two-dimensional electron gases typically used to study the fragile nu = 5/2 and nu = 12/5 fractional quantum Hall states. (C) 2013 AIP Publishing LLC
Effect of strain on stripe phases in the Quantum Hall regime
Spontaneous breaking of rotational symmetry and preferential orientation of
stripe phases in the quantum Hall regime has attracted considerable
experimental and theoretical effort over the last decade. We demonstrate
experimentally and theoretically that the direction of high and low resistance
of the two-dimensional (2D) hole gas in the quantum Hall regime can be
controlled by an external strain. Depending on the sign of the in-plane shear
strain, the Hartree-Fock energy of holes or electrons is minimized when the
charge density wave (CDW) is oriented along [110] or [1-10] directions. We
suggest that shear strains due to internal electric fields in the growth
direction are responsible for the observed orientation of CDW in pristine
electron and hole samples.Comment: 10 pages, 3 figure
The OPERA experiment Target Tracker
The main task of the Target Tracker detector of the long baseline neutrino
oscillation OPERA experiment is to locate in which of the target elementary
constituents, the lead/emulsion bricks, the neutrino interactions have occurred
and also to give calorimetric information about each event. The technology used
consists in walls of two planes of plastic scintillator strips, one per
transverse direction. Wavelength shifting fibres collect the light signal
emitted by the scintillator strips and guide it to both ends where it is read
by multi-anode photomultiplier tubes. All the elements used in the construction
of this detector and its main characteristics are described.Comment: 25 pages, submitted to Nuclear Instrument and Method
Evidence for a fractional quantum Hall state with anisotropic longitudinal transport
At high magnetic fields, where the Fermi level lies in the N=0 lowest Landau
level (LL), a clean two-dimensional electron system (2DES) exhibits numerous
incompressible liquid phases which display the fractional quantized Hall effect
(FQHE) (Das Sarma and Pinczuk, 1997). These liquid phases do not break
rotational symmetry, exhibiting resistivities which are isotropic in the plane.
In contrast, at lower fields, when the Fermi level lies in the third
and several higher LLs, the 2DES displays a distinctly different class of
collective states. In particular, near half filling of these high LLs the 2DES
exhibits a strongly anisotropic longitudinal resistance at low temperatures
(Lilly et al., 1999; Du et al., 1999). These "stripe" phases, which do not
exhibit the quantized Hall effect, resemble nematic liquid crystals, possessing
broken rotational symmetry and orientational order (Koulakov et al., 1996;
Fogler et al., 1996; Moessner and Chalker, 1996; Fradkin and Kivelson, 1999;
Fradkin et al, 2010). Here we report a surprising new observation: An
electronic configuration in the N=1 second LL whose resistivity tensor
simultaneously displays a robust fractionally quantized Hall plateau and a
strongly anisotropic longitudinal resistance resembling that of the stripe
phases.Comment: Nature Physics, (2011
Double Beta Decay: Historical Review of 75 Years of Research
Main achievements during 75 years of research on double beta decay have been
reviewed. The existing experimental data have been presented and the
capabilities of the next-generation detectors have been demonstrated.Comment: 25 pages, typos adde
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