3,003 research outputs found
Universality of the Kondo Effect in a Quantum Dot out of Equilibrium
We study the Kondo effect in a quantum dot driven out of equilibrium by an
external ac field. The Kondo effect can be probed by measuring the dc current
induced by an auxiliary dc bias applied across the dot. In the absence
of ac perturbation, the corresponding differential conductance is
known to exhibit a sharp peak at , which is the manifestation of the
Kondo effect. In the equilibrium, there exists only one energy scale, the Kondo
temperature , which controls all the low-energy physics of the system;
is some universal function of . We demonstrate that the dot out of
equilibrium is also characterized by a universal behavior: conductance
depends on the ac field only through two dimensionless parameters, which are
the frequency and the amplitude of the ac perturbation, both divided
by . We find analytically the large- and small-frequency asymptotes of the
universal dependence of on these parameters. The obtained results allow us
to predict the behavior of the conductance in the crossover regime
.Comment: 18 pages, 5 figure
Kinetic simulations of ladder climbing by electron plasma waves
The energy of plasma waves can be moved up and down the spectrum using
chirped modulations of plasma parameters, which can be driven by external
fields. Depending on whether the wave spectrum is discrete (bounded plasma) or
continuous (boundless plasma), this phenomenon is called ladder climbing (LC)
or autoresonant acceleration of plasmons. It was first proposed by Barth
\textit{et al.} [PRL \textbf{115}, 075001 (2015)] based on a linear fluid
model. In this paper, LC of electron plasma waves is investigated using fully
nonlinear Vlasov-Poisson simulations of collisionless bounded plasma. It is
shown that, in agreement with the basic theory, plasmons survive substantial
transformations of the spectrum and are destroyed only when their wave numbers
become large enough to trigger Landau damping. Since nonlinear effects decrease
the damping rate, LC is even more efficient when practiced on structures like
quasiperiodic Bernstein-Greene-Kruskal (BGK) waves rather than on Langmuir
waves \textit{per~se}
Ferromagnetic and random spin ordering in diluted magnetic semiconductors
In a diluted magnetic semiconductor system, the exchange interaction between
magnetic impurities has two independent components: a direct antiferromagnetic
interaction and a ferromagnetic interaction mediated by charge carriers.
Depending on the system parameters, the ground state of the system may be
ordered either ferromagnetically or randomly. In this paper we use percolation
theory to find the ferromagnetic transition temperature and the location of the
quantum critical point separating the ferromagnetic phase and a valence bond
glass phase.Comment: 9 pages, 2 figures, a reference adde
Griffiths phase in diluted magnetic semiconductors
We study the effects of disorder in the vicinity of the ferromagnetic
transition in a diluted magnetic semiconductor in the strongly localized
regime. We derive an effective polaron Hamiltonian, which leads to the
Griffiths phase above the ferromagnetic transition point. The Griffiths-McCoy
effects yield non-perturbative contributions to the dynamic susceptibility. We
explicitly derive the long-time susceptibility, which has a pseudo-scaling
form, with the dynamic critical exponent being expressed through the
percolation indices.Comment: 4 pages, final version as publishe
Unified radio and network control across heterogeneous hardware platforms
Experimentation is an important step in the investigation of techniques for handling spectrum scarcity or the development of new waveforms in future wireless networks. However, it is impractical and not cost effective to construct custom platforms for each future network scenario to be investigated. This problem is addressed by defining Unified Programming Interfaces that allow common access to several platforms for experimentation-based prototyping, research, and development purposes. The design of these interfaces is driven by a diverse set of scenarios that capture the functionality relevant to future network implementations while trying to keep them as generic as possible. Herein, the definition of this set of scenarios is presented as well as the architecture for supporting experimentation-based wireless research over multiple hardware platforms. The proposed architecture for experimentation incorporates both local and global unified interfaces to control any aspect of a wireless system while being completely agnostic to the actual technology incorporated. Control is feasible from the low-level features of individual radios to the entire network stack, including hierarchical control combinations. A testbed to enable the use of the above architecture is utilized that uses a backbone network in order to be able to extract measurements and observe the overall behaviour of the system under test without imposing further communication overhead to the actual experiment. Based on the aforementioned architecture, a system is proposed that is able to support the advancement of intelligent techniques for future networks through experimentation while decoupling promising algorithms and techniques from the capabilities of a specific hardware platform
Scalar form factors of light mesons
The scalar radius of the pion plays an important role in CHPT, because it is
related to one of the basic effective coupling constants, viz. the one which
controls the quark mass dependence of F_pi at one loop. In a recent paper,
Yndurain derives a {\it robust lower bound} for this radius, which disagrees
with earlier determinations. We show that such a bound does not exist: the
"derivation" relies on an incorrect claim. Moreover, we discuss the physics of
the form factors associated with the operators \ubar u, \dbar d and \sbar s and
show that their structure in the vicinity of the K \Kbar threshold is quite
different. Finally, we draw attention to the fact that the new data on the
slope of the scalar K_l3 form factor confirm a recent, remarkably sharp
theoretical prediction.Comment: 13 pages, 2 figures. v2: minor changes - version to appear on Phys.
Lett. B. v3: Published version. Values of the measured K-pi radii correcte
Sum Rules from an Extra Dimension
Using the gravity side of the AdS/CFT correspondence, we investigate the
analytic properties of thermal retarded Green's functions for scalars,
conserved currents, the stress tensor, and massless fermions. We provide some
results concerning their large and small frequency behavior and their pole
structure. From these results, it is straightforward to prove the validity of
various sum rules on the field theory side of the duality. We introduce a novel
contraction mapping we use to study the large frequency behavior of the Green's
functions.Comment: v2: 23 pages (plus appendix), revised presentation, discussion of
branch cuts moved to appendix, and some minor changes; v1: 24 pages (plus
appendix
A unified radio control architecture for prototyping adaptive wireless protocols
Experimental optimization of wireless protocols and validation of novel solutions is often problematic, due to limited configuration space present in commercial wireless interfaces as well as complexity of monolithic driver implementation on SDR-based experimentation platforms. To overcome these limitations a novel software architecture is proposed, called WiSHFUL, devised to allow: i) maximal exploitation of radio functionalities available in current radio chips, and ii) clean separation between the logic for optimizing the radio protocols (i.e. radio control) and the definition of these protocols
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