65 research outputs found
Wave function-dependent mobility and suppression of interface roughness scattering in a strained SiGe p-channel field-effect structure
The 4 K Hall mobility has been measured in a top-gated, inverted, modulation-doped Si/Si0.8Ge0.2 structure having a Si:B doping layer beneath the alloy. From comparisons with theoretical calculations, we argue that, unlike an ordinary enhancement-mode SiGe p-channel metalâoxideâsemiconductor structure, this configuration leads to a decrease of interface roughness scattering with increasing sheet carrier density. We also speculate on the nature of the interface charge observed in these structures at low temperature
Back gating of a two-dimensional hole gas in a SiGe quantum well
A device comprising a low-resistivity, n-type, Si substrate as a back gate to a p-type (boron), remote-doped, SiGe quantum well has been fabricated and characterized. Reverse and forward voltage biasing of the gate with respect to the two-dimensional hole gas in the quantum well allows the density of holes to be varied from 8 Ă 1011 cmâ2 down to a measurement-limited value of 4 Ă 1011 cmâ2. This device is used to demonstrate the evolution with decreasing carrier density of a re-entrant insulator state between the integer quantum Hall effect states with filling factors 1 and 3
Dualâband RFID tag antenna based on the Hilbertâcurve fractal for HF and UHF applications
Strongly Correlated Quantum Fluids: Ultracold Quantum Gases, Quantum Chromodynamic Plasmas, and Holographic Duality
Strongly correlated quantum fluids are phases of matter that are
intrinsically quantum mechanical, and that do not have a simple description in
terms of weakly interacting quasi-particles. Two systems that have recently
attracted a great deal of interest are the quark-gluon plasma, a plasma of
strongly interacting quarks and gluons produced in relativistic heavy ion
collisions, and ultracold atomic Fermi gases, very dilute clouds of atomic
gases confined in optical or magnetic traps. These systems differ by more than
20 orders of magnitude in temperature, but they were shown to exhibit very
similar hydrodynamic flow. In particular, both fluids exhibit a robustly low
shear viscosity to entropy density ratio which is characteristic of quantum
fluids described by holographic duality, a mapping from strongly correlated
quantum field theories to weakly curved higher dimensional classical gravity.
This review explores the connection between these fields, and it also serves as
an introduction to the Focus Issue of New Journal of Physics on Strongly
Correlated Quantum Fluids: from Ultracold Quantum Gases to QCD Plasmas. The
presentation is made accessible to the general physics reader and includes
discussions of the latest research developments in all three areas.Comment: 138 pages, 25 figures, review associated with New Journal of Physics
special issue "Focus on Strongly Correlated Quantum Fluids: from Ultracold
Quantum Gases to QCD Plasmas"
(http://iopscience.iop.org/1367-2630/focus/Focus%20on%20Strongly%20Correlated%20Quantum%20Fluids%20-%20from%20Ultracold%20Quantum%20Gases%20to%20QCD%20Plasmas
Repulsive polarons and itinerant ferromagnetism in strongly polarized Fermi gases
We analyze the properties of a single impurity immersed in a Fermi sea. At
positive energy and scattering lengths, we show that the system possesses a
well-defined but metastable excitation, the repulsive polaron, and we calculate
its energy, quasiparticle residue and effective mass. From a thermodynamic
argument we obtain the number of particles in the dressing cloud, illustrating
the repulsive character of the polaron. Identifying the important 2- and 3-body
decay channels, we furthermore calculate the lifetime of the repulsive polaron.
The stability conditions for the formation of fully spin polarized
(ferromagnetic) domains are then examined for a binary mixture of atoms with a
general mass ratio. Our results indicate that mass imbalance lowers the
critical interaction strength for phase-separation, but that very short
quasiparticle decay times will complicate the experimental observation of
itinerant ferromagnetism. Finally, we present the spectral function of the
impurity for various coupling strengths and momenta.Comment: Substantial improvements to the section describing quasiparticle
decays (included a discussion of two-body and three-body processes), and to
the criteria for the stability of the itinerant ferromagnetic phas
Metastability and Coherence of Repulsive Polarons in a Strongly Interacting Fermi Mixture
Ultracold Fermi gases with tuneable interactions represent a unique test bed
to explore the many-body physics of strongly interacting quantum systems. In
the past decade, experiments have investigated a wealth of intriguing
phenomena, and precise measurements of ground-state properties have provided
exquisite benchmarks for the development of elaborate theoretical descriptions.
Metastable states in Fermi gases with strong repulsive interactions represent
an exciting new frontier in the field. The realization of such systems
constitutes a major challenge since a strong repulsive interaction in an atomic
quantum gas implies the existence of a weakly bound molecular state, which
makes the system intrinsically unstable against decay. Here, we exploit
radio-frequency spectroscopy to measure the complete excitation spectrum of
fermionic 40K impurities resonantly interacting with a Fermi sea of 6Li atoms.
In particular, we show that a well-defined quasiparticle exists for strongly
repulsive interactions. For this "repulsive polaron" we measure its energy and
its lifetime against decay. We also probe its coherence properties by measuring
the quasiparticle residue. The results are well described by a theoretical
approach that takes into account the finite effective range of the interaction
in our system. We find that a non-zero range of the order of the interparticle
spacing results in a substantial lifetime increase. This major benefit for the
stability of the repulsive branch opens up new perspectives for investigating
novel phenomena in metastable, repulsively interacting fermion systems.Comment: 11 pages, 9 figure
Trajectory tracking of a quadrotor with unknown parameters and its fault-tolerant control via sliding mode fault observer
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