28 research outputs found
Scaling and the Metal-Insulator Transition in Si/SiGe Quantum Wells
The existence of a metal-insulator transition at zero magnetic field in two-
dimensional electron systems has recently been confirmed in high mobility
Si-MOSFETs. In this work, the temperature dependence of the resistivity of
gated Si/SiGe/Si quantum well structures has revealed a similar metal-
insulator transition as a function of carrier density at zero magnetic field.
We also report evidence for a Coulomb gap in the temperature dependence of
the resistivity of the dilute 2D hole gas confined in a SiGe quantum well.
In addition, the resistivity in the insulating phase scales with a single
parameter, and is sample independent. These results are consistent with the
occurrence of a metal-insulator transition at zero magnetic field in SiGe
square quantum wells driven by strong hole-hole interactions.Comment: 3 pages, 3 figures, LaTe
"Forbidden" transitions between quantum Hall and insulating phases in p-SiGe heterostructures
We show that in dilute metallic p-SiGe heterostructures, magnetic field can
cause multiple quantum Hall-insulator-quantum Hall transitions. The insulating
states are observed between quantum Hall states with filling factors \nu=1 and
2 and, for the first time, between \nu=2 and 3 and between \nu=4 and 6. The
latter are in contradiction with the original global phase diagram for the
quantum Hall effect. We suggest that the application of a (perpendicular)
magnetic field induces insulating behavior in metallic p-SiGe heterostructures
in the same way as in Si MOSFETs. This insulator is then in competition with,
and interrupted by, integer quantum Hall states leading to the multiple
re-entrant transitions. The phase diagram which accounts for these transition
is similar to that previously obtained in Si MOSFETs thus confirming its
universal character
Projetivismo dos valores em Nietzsche
Abstract: The aim of this paper is to claim Nietzsche’s place
within the philosophical tradition of projectivism. Indeed, as will
be shown, although Nietzsche is almost unanimously ignored by
scholars working on projectivism, during the whole development
of his philosophical thought, he holds a position which can be
reasonably defined as “projectivist”.
Resumo: Este artigo tem por objetivo reivindicar o lugar da
filosofia nietzschiana na tradição filosófica do projetivismo. Com
efeito, como mostrarei, mesmo se Nietzsche Ă© quase unanimemente
ignorado nas obras dos especialistas nessa tradição, ele mantém,
ao longo de seu desenvolvimento filosófico, uma posição que se
pode com razão definir como “projetivista”
Metal Insulator transition at B=0 in p-SiGe
Observations are reported of a metal-insulator transition in a 2D hole gas in
asymmetrically doped strained SiGe quantum wells. The metallic phase, which
appears at low temperatures in these high mobility samples, is characterised by
a resistivity that decreases exponentially with decreasing temperature. This
behaviour, and the duality between resistivity and conductivity on the two
sides of the transition, are very similar to that recently reported for high
mobility Si-MOSFETs.Comment: 4 pages, REVTEX with 3 ps figure
The barocaloric effect: A Spin-off of the Discovery of High-Temperature Superconductivity
Some key results obtained in joint research projects with Alex M\"uller are
summarized, concentrating on the invention of the barocaloric effect and its
application for cooling as well as on important findings in the field of
high-temperature superconductivity resulting from neutron scattering
experiments.Comment: 26 pages, 9 figure
Maximum Metallic Conductivity in Si-MOS Structures
We found that the conductivity of the two-dimensional electron system in
Si-MOS structures is limited to a maximum value, G_{max}, as either density
increases or temperature decreases. This value G_{max} is weakly disorder
dependent and ranging from 100 to 140 e^2/h for samples whose mobilities differ
by a factor of 4.Comment: 3 pages, 3 ps-figs, RevTex, new dat
Effect of Tilted Magnetic Field on the Anomalous H=0 Conducting Phase in High-Mobility Si MOSFETs
The suppression by a magnetic field of the anomalous H=0 conducting phase in
high-mobility silicon MOSFETs is independent of the angle between the field and
the plane of the 2D electron system. In the presence of a parallel field large
enough to fully quench the anomalous conducting phase, the behavior is similar
to that of disordered GaAs/AlGaAs heterostructures: the system is insulating in
zero (perpendicular) field and exhibits reentrant insulator-quantum Hall
effect-insulator transitions as a function of perpendicular field. The results
demonstrate that the suppression of the low-T phase is related only to the
electrons' spin.Comment: 4 pages, including 3 figures. We corrected several typos in the
figures and caption
Metallic behavior and related phenomena in two dimensions
For about twenty years, it has been the prevailing view that there can be no
metallic state or metal-insulator transition in two dimensions in zero magnetic
field. In the last several years, however, unusual behavior suggestive of such
a transition has been reported in a variety of dilute two-dimensional electron
and hole systems. The physics behind these observations is presently not
understood. We review and discuss the main experimental findings and suggested
theoretical models.Comment: To be published in Rev. Mod. Phy