95 research outputs found
Phased Array Systems in Silicon
Phased array systems, a special case of MIMO systems, take advantage of spatial directivity and array gain to increase spectral efficiency. Implementing a phased array system at high frequency in a commercial silicon process technology presents several challenges. This article focuses on the architectural and circuit-level trade-offs involved in the design of the first silicon-based fully integrated phased array system operating at 24 GHz. The details of some of the important circuit building blocks are also discussed. The measured results demonstrate the feasibility of using integrated phased arrays for wireless communication and vehicular radar applications at 24 GHz
Evidence for localization and 0.7 anomaly in hole quantum point contacts
Quantum point contacts implemented in p-type GaAs/AlGaAs heterostructures are
investigated by low-temperature electrical conductance spectroscopy
measurements. Besides one-dimensional conductance quantization in units of
a pronounced extra plateau is found at about which
possesses the characteristic properties of the so-called "0.7 anomaly" known
from experiments with n-type samples. The evolution of the 0.7 plateau in high
perpendicular magnetic field reveals the existence of a quasi-localized state
and supports the explanation of the 0.7 anomaly based on self-consistent charge
localization. These observations are robust when lateral electrical fields are
applied which shift the relative position of the electron wavefunction in the
quantum point contact, testifying to the intrinsic nature of the underlying
physics.Comment: 4.2 pages, 3 figure
Nonlinear eigenvalue problems
This paper presents an asymptotic study of the differential equation y'(x) = cos [πxy(x)] subject to the initial condition y(0) = a. While this differential equation is nonlinear, the solutions to the initial-value problem bear a striking resemblance to solutions to the linear time-independent Schrödinger eigenvalue problem. As x increases from 0, y(x) oscillates and thus resembles a quantum wave function in a classically allowed region. At a critical value x = xcrit, where xcrit depends on a, the solution y(x) undergoes a transition; the oscillations abruptly cease and y(x) decays to 0 monotonically as x → ∞. This transition resembles the transition in a wave function at a turning point as one enters the classically forbidden region. Furthermore, the initial condition a falls into discrete classes; in the nth class of initial conditions an − 1 < a < an (n = 1, 2, 3, ...), y(x) exhibits exactly n maxima in the oscillatory region. The boundaries an of these classes are the analogues of quantum-mechanical eigenvalues. An asymptotic calculation of an for large n is analogous to a high-energy semiclassical (WKB) calculation of eigenvalues in quantum mechanics. The principal result of this paper is that as n → ∞, , where A = 25/6. Numerical analysis reveals that the first Painlevé transcendent has an eigenvalue structure that is quite similar to that of the equation y'(x) = cos [πxy(x)] and that the nth eigenvalue grows with n like a constant times n3/5 as n → ∞. Finally, it is noted that the constant A is numerically very close to the lower bound on the power-series constant P in the theory of complex variables, which is associated with the asymptotic behavior of zeros of partial sums of Taylor series
Origins of conductance anomalies in a p-type GaAs quantum point contact
Low temperature transport measurements on a p-GaAs quantum point contact are
presented which reveal the presence of a conductance anomaly that is markedly
different from the conventional `0.7 anomaly'. A lateral shift by asymmetric
gating of the conducting channel is utilized to identify and separate different
conductance anomalies of local and generic origins experimentally. While the
more generic 0.7 anomaly is not directly affected by changing the gate
configuration, a model is proposed which attributes the additional conductance
features to a gate-dependent coupling of the propagating states to localized
states emerging due to a nearby potential imperfection. Finite bias
conductivity measurements reveal the interplay between the two anomalies
consistently with a two-impurity Kondo model
Finite-volume effects and the electromagnetic contributions to kaon and pion masses
We report on the MILC Collaboration calculation of electromagnetic effects on
light pseudoscalar mesons. The simulations employ asqtad staggered dynamical
quarks in QCD plus quenched photons, with lattice spacings varying from 0.12 to
0.06 fm. Finite volume corrections for the MILC realization of lattice
electrodynamics have been calculated in chiral perturbation theory and applied
to the lattice data. These corrections differ from those calculated by Hayakawa
and Uno because our treatment of zero modes differs from theirs. Updated
results for the corrections to "Dashen's theorem" are presented.Comment: 7 pages, 2 figures. Presented at Lattice 2014, Columbia University,
June 23-28, 201
Observation of excited states in a p-type GaAs quantum dot
A quantum dot fabricated by scanning probe oxidation lithography on a p-type,
C-doped GaAs/AlGaAs heterostructure is investigated by low temperature
electrical conductance measurements. Clear Coulomb blockade oscillations are
observed and analyzed in terms of sequential tunneling through the
single-particle levels of the dot at T_hole = 185 mK. The charging energies as
large as 2 meV evaluated from Coulomb diamond measurements together with the
well resolved single-hole excited state lines in the charge stability diagram
indicate that the dot is operated with a small number of confined particles
close to the ultimate single-hole regime.Comment: 5 pages, 5 figure
- …