16,606 research outputs found
Ab-initio analysis of superstructures revealed by STM on bilayer graphene
In this work we performed density functional theory calculations for a
twisted bilayer graphene (BLG). Several conmensurable rotation angles were
analyzed and for each one a constant height mode STM image was obtained. These
STM images, calculated under the Tersoff-Hamman theory, reproduce the main
features experimentally observed, paticularly superstructures and giant
corrugations. In this way we confirm that STM characterization of twisted BLG
can produce superstructures whose tunneling current intensity maxima occur over
regions with stacking. Additionally we give new evidence in favour of an
electronic origin for the superstructures instead another physical grounds
Ladders for Wilson Loops Beyond Leading Order
We set up a general scheme to resum ladder diagrams for the quark-anti-quark
potential in N=4 super-Yang-Mills theory, and do explicit calculations at the
next-to-leading order. The results perfectly agree with string theory in
AdS(5)xS(5) when continued to strong coupling, in spite of a potential
order-of-limits problem.Comment: 18 pages, 5 figure
Negative-energy perturbations in cylindrical equilibria with a radial electric field
The impact of an equilibrium radial electric field on negative-energy
perturbations (NEPs) (which are potentially dangerous because they can lead to
either linear or nonlinear explosive instabilities) in cylindrical equilibria
of magnetically confined plasmas is investigated within the framework of
Maxwell-drift kinetic theory. It turns out that for wave vectors with a
non-vanishing component parallel to the magnetic field the conditions for the
existence of NEPs in equilibria with E=0 [G. N. Throumoulopoulos and D.
Pfirsch, Phys. Rev. E 53, 2767 (1996)] remain valid, while the condition for
the existence of perpendicular NEPs, which are found to be the most important
perturbations, is modified. For ( is the
electrostatic potential) and ( is
the total plasma pressure), a case which is of operational interest in magnetic
confinement systems, the existence of perpendicular NEPs depends on ,
where is the charge of the particle species . In this case the
electric field can reduce the NEPs activity in the edge region of tokamaklike
and stellaratorlike equilibria with identical parabolic pressure profiles, the
reduction of electron NEPs being more pronounced than that of ion NEPs.Comment: 30 pages, late
The Resonance Overlap and Hill Stability Criteria Revisited
We review the orbital stability of the planar circular restricted three-body
problem, in the case of massless particles initially located between both
massive bodies. We present new estimates of the resonance overlap criterion and
the Hill stability limit, and compare their predictions with detailed dynamical
maps constructed with N-body simulations. We show that the boundary between
(Hill) stable and unstable orbits is not smooth but characterized by a rich
structure generated by the superposition of different mean-motion resonances
which does not allow for a simple global expression for stability.
We propose that, for a given perturbing mass and initial eccentricity
, there are actually two critical values of the semimajor axis. All values
are
unstable in the Hill sense. The first limit is given by the Hill-stability
criterion and is a function of the eccentricity. The second limit is virtually
insensitive to the initial eccentricity, and closely resembles a new resonance
overlap condition (for circular orbits) developed in terms of the intersection
between first and second-order mean-motion resonances.Comment: 33 pages, 14 figures, accepte
MAMA: An Algebraic Map for the Secular Dynamics of Planetesimals in Tight Binary Systems
We present an algebraic map (MAMA) for the dynamical and collisional
evolution of a planetesimal swarm orbiting the main star of a tight binary
system (TBS). The orbital evolution of each planetesimal is dictated by the
secular perturbations of the secondary star and gas drag due to interactions
with a protoplanetary disk. The gas disk is assumed eccentric with a constant
precession rate. Gravitational interactions between the planetesimals are
ignored. All bodies are assumed coplanar. A comparison with full N-body
simulations shows that the map is of the order of 100 times faster, while
preserving all the main characteristics of the full system.
In a second part of the work, we apply MAMA to the \gamma-Cephei, searching
for friendly scenarios that may explain the formation of the giant planet
detected in this system. For low-mass protoplanetary disks, we find that a
low-eccentricity static disk aligned with the binary yields impact velocities
between planetesimals below the disruption threshold. All other scenarios
appear hostile to planetary formation
Negative-Energy Perturbations in Circularly Cylindrical Equilibria within the Framework of Maxwell-Drift Kinetic Theory
The conditions for the existence of negative-energy perturbations (which
could be nonlinearly unstable and cause anomalous transport) are investigated
in the framework of linearized collisionless Maxwell-drift kinetic theory for
the case of equilibria of magnetically confined, circularly cylindrical plasmas
and vanishing initial field perturbations. For wave vectors with a
non-vanishing component parallel to the magnetic field, the plane equilibrium
conditions (derived by Throumoulopoulos and Pfirsch [Phys Rev. E {\bf 49}, 3290
(1994)]) are shown to remain valid, while the condition for perpendicular
perturbations (which are found to be the most important modes) is modified.
Consequently, besides the tokamak equilibrium regime in which the existence of
negative-energy perturbations is related to the threshold value of 2/3 of the
quantity , a new
regime appears, not present in plane equilibria, in which negative-energy
perturbations exist for {\em any} value of . For various analytic
cold-ion tokamak equilibria a substantial fraction of thermal electrons are
associated with negative-energy perturbations (active particles). In
particular, for linearly stable equilibria of a paramagnetic plasma with flat
electron temperature profile (), the entire velocity space is
occupied by active electrons. The part of the velocity space occupied by active
particles increases from the center to the plasma edge and is larger in a
paramagnetic plasma than in a diamagnetic plasma with the same pressure
profile. It is also shown that, unlike in plane equilibria, negative-energy
perturbations exist in force-free reversed-field pinch equilibria with a
substantial fraction of active particles.Comment: 31 pages, late
Classical emulation of quantum-coherent thermal machines
The performance enhancements observed in various models of continuous quantum
thermal machines have been linked to the buildup of coherences in a preferred
basis. But, is this connection always an evidence of `quantum-thermodynamic
supremacy'? By force of example, we show that this is not the case. In
particular, we compare a power-driven three-level continuous quantum
refrigerator with a four-level combined cycle, partly driven by power and
partly by heat. We focus on the weak driving regime and find the four-level
model to be superior since it can operate in parameter regimes in which the
three-level model cannot, it may exhibit a larger cooling rate, and,
simultaneously, a better coefficient of performance. Furthermore, we find that
the improvement in the cooling rate matches the increase in the stationary
quantum coherences exactly. Crucially, though, we also show that the
thermodynamic variables for both models follow from a classical representation
based on graph theory. This implies that we can build incoherent
stochastic-thermodynamic models with the same steady-state operation or,
equivalently, that both coherent refrigerators can be emulated classically.
More generally, we prove this for any N-level weakly driven device with a
`cyclic' pattern of transitions. Therefore, even if coherence is present in a
specific quantum thermal machine, it is often not essential to replicate the
underlying energy conversion process.Comment: 13 pages, 4 figures; references updated; appendix adde
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