154 research outputs found
Novel critical field in magneto-resistance oscillation of 2DEG in asymmetric GaAs/AlGaAs double wells measured as a function of the in-plane magnetic field
We have investigated the magnetoresistance of strongly asymmetric double-well
structures formed by a thin AlGaAs barrier grown far from the interface in the
GaAs buffer of standard heterostructures. In magnetic fields oriented parallel
to the electron layers, the magnetoresistance exhibits an oscillation
associated with the depopulation of the higher occupied subband and with the
field-induced transition into a decoupled bilayer. In addition, the increasing
field transfers electrons from the triangular to rectangular well and, at high
enough field value, the triangular well is emptied. Consequently, the
electronic system becomes a single layer which leads to a sharp step in the
density of electron states and to an additional minimum in the
magnetoresistance curve.Comment: 3 pages, 3 figure
In-plane Magnetic Field Dependent Magnetoresistance of Gated Asymmetric Double Quantum Wells
We have investigated experimentally the magnetoresistance of strongly
asymmetric double-wells. The structures were prepared by inserting a thin
AlGaAs barrier into the GaAs buffer layer of a standard
modulation-doped GaAs/AlGaAs heterostructure. The resulting
double-well system consists of a nearly rectangular well and of a triangular
well coupled by tunneling through the thin barrier. With a proper choice of the
barrier parameters one can control the occupancy of the two wells and of the
two lowest (bonding and antibonding) subbands. The electron properties can be
further influenced by applying front- or back-gate voltage.Comment: 4 pages, 5 figures, elsart/PHYEAUTH macros; to be presented on the
EP2DS-15 Conference in Nara, Japan. Revised version. To appear in Physica
Temperature-Dependence of the Resistivity of a Dilute 2D Electron System in High Parallel Magnetic Field
We report measurements of the resistance of silicon MOSFETs as a function of
temperature in high parallel magnetic fields where the 2D system of electrons
has been shown to be fully spin-polarized. A magnetic field suppresses the
metallic behavior observed in the absence of a magnetic field. In a field of
10.8 T, insulating behavior is found for densities up to n_s approximately 1.35
x 10^{11} cm^{-2} or 1.5 n_c; above this density the resistance is a very weak
function of temperature, varying less than 10% between 0.25 K and 1.90 K. At
low densities the resistance goes to infinity more rapidly as the temperature
is reduced than in zero field and the magnetoresistance diverges as T goes to
0.Comment: 4 pages, including 4 figures. References adde
Identification of harmful cyanobacteria in the Sacramento-San Joaquin Delta and Clear Lake, California by DNA barcoding.
Accurate identification of cyanobacteria using traditional morphological taxonomy is challenging due to the magnitude of phenotypic plasticity among natural algal assemblages. In this study, molecular approach was utilized to facilitate the accurate identification of cyanobacteria in the Sacramento-San Joaquin Delta and in Clear Lake in Northern California where recurring blooms have been observed over the past decades. Algal samples were collected from both water bodies in 2011 and the samples containing diverse cyanobacteria as identified by morphological taxonomy were chosen for the molecular analysis. The 16S ribosomal RNA genes (16S rDNA) and the adjacent internal transcribed spacer (ITS) regions were amplified by PCR from the mixed algal samples using cyanobacteria generic primers. The obtained sequences were analyzed by similarity search (BLASTN) and phylogenetic analysis (16S rDNA) to differentiate species sharing significantly similar sequences. A total of 185 plasmid clones were obtained of which 77 were successfully identified to the species level: Aphanizomenon flos-aquae, Dolichospermum lemmermannii (taxonomic synonym: Anabaena lemmermannii), Limnoraphis robusta (taxonomic synonym: Lyngbya hieronymusii f. robusta) and Microcystis aeruginosa. To date, Dolichospermum and Limnoraphis found in Clear Lake have only been identified to the genus lavel by microscopy. During the course of this study, morphological identification and DNA barcoding confirmed A. flos-aquae as the predominant cyanobacterium in the Sacramento-San Joaquin Delta indicating a shift from M. aeruginosa that have dominated the blooms in the past decade. Lastly, the species-specific identification of Limnoraphis robusta in Clear Lake is another significant finding as this cyanobacterium has, thus far, only been reported in Lake Atitlan blooms in Guatemala
Influence of parallel magnetic fields on a single-layer two-dimensional electron system with a hopping mechanism of conductivity
Large positive (P) magnetoresistance (MR) has been observed in parallel
magnetic fields in a single 2D layer in a delta-doped GaAs/AlGaAs
heterostructure with a variable-range-hopping (VRH) mechanism of conductivity.
Effect of large PMR is accompanied in strong magnetic fields by a substantial
change in the character of the temperature dependence of the conductivity. This
implies that spins play an important role in 2D VRH conductivity because the
processes of orbital origin are not relevant to the observed effect. A possible
explanation involves hopping via double occupied states in the upper Hubbard
band, where the intra-state correlation of spins is important.Comment: 10 pages, 4 jpeg figure
Parallel Magnetic Field Induced Transition in Transport in the Dilute Two-Dimensional Hole System in GaAs
A magnetic field applied parallel to the two-dimensional hole system in the
GaAs/AlGaAs heterostructure, which is metallic in the absence of an external
magnetic field, can drive the system into insulating at a finite field through
a well defined transition. The value of resistivity at the transition is found
to depend strongly on density
Magnetoresistance and electronic structure of asymmetric GaAs/AlGaAs double quantum wells in the in-plane/tilted magnetic field
Bilayer two-dimensional electron systems formed by a thin barrier in the GaAs
buffer of a standard heterostructure were investigated by magnetotransport
measurements. In magnetic fields oriented parallel to the electron layers, the
magnetoresistance exhibits an oscillation associated with the depopulation of
the higher occupied subband and the field-induced transition into a decoupled
bilayer. Shubnikov-de Haas oscillations in slightly tilted magnetic fields
allow to reconstruct the evolution of the electron concentration in the
individual subbands as a function of the in-plane magnetic field. The
characteristics of the system derived experimentally are in quantitative
agreement with numerical self-consistent-field calculations of the electronic
structure.Comment: 6 pages, 5 figure
Tenfold Magnetoconductance in a Non-Magnetic Metal Film
We present magnetoconductance (MC) measurements of homogeneously disordered
Be films whose zero field sheet conductance (G) is described by the
Efros-Shklovskii hopping law . The low field
MC of the films is negative with G decreasing 200% below 1 T. In contrast the
MC above 1 T is strongly positive. At 8 T, G increases 1000% in perpendicular
field and 500% in parallel field. In the simpler parallel case, we observe {\em
field enhanced} variable range hopping characterized by an attenuation of
via the Zeeman interaction.Comment: 9 pages including 5 figure
Two-species percolation and Scaling theory of the metal-insulator transition in two dimensions
Recently, a simple non-interacting-electron model, combining local quantum
tunneling via quantum point contacts and global classical percolation, has been
introduced in order to describe the observed ``metal-insulator transition'' in
two dimensions [1]. Here, based upon that model, a two-species-percolation
scaling theory is introduced and compared to the experimental data. The two
species in this model are, on one hand, the ``metallic'' point contacts, whose
critical energy lies below the Fermi energy, and on the other hand, the
insulating quantum point contacts. It is shown that many features of the
experiments, such as the exponential dependence of the resistance on
temperature on the metallic side, the linear dependence of the exponent on
density, the scale of the critical resistance, the quenching of the
metallic phase by a parallel magnetic field and the non-monotonic dependence of
the critical density on a perpendicular magnetic field, can be naturally
explained by the model.
Moreover, details such as the nonmonotonic dependence of the resistance on
temperature or the inflection point of the resistance vs. parallel magnetic are
also a natural consequence of the theory. The calculated parallel field
dependence of the critical density agrees excellently with experiments, and is
used to deduce an experimental value of the confining energy in the vertical
direction. It is also shown that the resistance on the ``metallic'' side can
decrease with decreasing temperature by an arbitrary factor in the degenerate
regime ().Comment: 8 pages, 8 figure
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