324 research outputs found
Localization and Equipartition of Energy in the beta-FPU Chain : Chaotic Breathers
The evolution towards equipartition in the -FPU chain is studied
considering as initial condition the highest frequency mode. Above an
analytically derived energy threshold, this zone-boundary mode is shown to be
modulationally unstable and to give rise to a striking localization process.
The spontaneously created excitations have strong similarity with moving exact
breathers solutions. But they have a finite lifetime and their dynamics is
chaotic. These chaotic breathers are able to collect very efficiently the
energy in the chain. Therefore their size grows in time and they can transport
a very large quantity of energy. These features can be explained analyzing the
dynamics of perturbed exact breathers of the FPU chain. In particular, a close
connection between the Lyapunov spectrum of the chaotic breathers and the
Floquet spectrum of the exact ones has been found. The emergence of chaotic
breathers is convincingly explained by the absorption of high frequency phonons
whereas a breather's metastability is for the first time identified. The
lifetime of the chaotic breather is related to the time necessary for the
system to reach equipartition. The equipartition time turns out to be dependent
on the system energy density only. Moreover, such time diverges as
in the limit and vanishes as
for .Comment: 20 pages, Revtex - Submitted to Physica
Phase-dependent interaction in a 4-level atomic configuration
We study a four-level atomic scheme interacting with four lasers in a
closed-loop configuration with a (diamond) geometry. We
investigate the influence of the laser phases on the steady state. We show
that, depending on the phases and the decay characteristic, the system can
exhibit a variety of behaviors, including population inversion and complete
depletion of an atomic state. We explain the phenomena in terms of multi-photon
interference. We compare our results with the phase-dependent phenomena in the
double- scheme, as studied in [Korsunsky and Kosachiov, Phys. Rev A
{\bf 60}, 4996 (1999)]. This investigation may be useful for developing
non-linear optical devices, and for the spectroscopy and laser-cooling of
alkali-earth atoms.Comment: 4 figure
Functional advantages offered by many-body coherences in biochemical systems
Quantum coherence phenomena driven by electronic-vibrational (vibronic)
interactions, are being reported in many pulse (e.g. laser) driven chemical and
biophysical systems. But what systems-level advantage(s) do such many-body
coherences offer to future technologies? We address this question for pulsed
systems of general size N, akin to the LHCII aggregates found in green plants.
We show that external pulses generate vibronic states containing particular
multipartite entanglements, and that such collective vibronic states increase
the excitonic transfer efficiency. The strength of these many-body coherences
and their robustness to decoherence, increase with aggregate size N and do not
require strong electronic-vibrational coupling. The implications for energy and
information transport are discussed.Comment: arXiv admin note: text overlap with arXiv:1706.0776
Microscopic origins of the ferromagnetic exchange coupling in oxoverdazyl-based Cu(II) complex
The exchange channels governing the experimentally reported coupling constant Jexpt=6 cm−1
value in the verdazyl-ligand based Cu II complex Cu hfac 2 imvdz are inspected using wave
function-based difference dedicated configuration interaction calculations. The interaction between
the two spin 1/2 holders is summed up in a unique coupling constant J. Nevertheless, by gradually
increasing the level of calculation, different mechanisms of interaction are turned on step by step.
In the present system, the calculated exchange interaction then appears alternatively ferromagnetic/
antiferromagnetic/ferromagnetic. Our analysis demonstrates the tremendously importance of some
specific exchange mechanisms. It is actually shown that both parts of the imvdz ligand
simultaneously influence the ferromagnetic behavior which ultimately reaches Jcalc=6.3 cm−1, in
very good agreement with the experimental value. In accordance with the alternation of J, it is
shown that the nature of the magnetic behavior results from competing channels. First, an
antiferromagnetic contribution can be essentially attributed to single excitations involving the
network localized on the verdazyl part. In contrast, the ligand-to-metal charge transfer LMCT
involving the imidazole moiety affords a ferromagnetic contribution. The distinct nature / of the
mechanisms is responsible for the net ferromagnetic behavior. The intuitively innocent part of the
verdazyl-based ligands is deeply reconsidered and opens new routes into the rational design of
magnetic object
SU(4) Skyrmions and Activation Energy Anomaly in Bilayer Quantum Hall Systems
The bilayer QH system has four energy levels in the lowest Landau level,
corresponding to the layer and spin degrees of freedom. We investigate the
system in the regime where all four levels are nearly degenerate and equally
active. The underlying group structure is SU(4). At the QH state is a
charge-transferable state between the two layers and the SU(4) isospin
coherence develops spontaneously. Quasiparticles are isospin textures to be
identified with SU(4) skyrmions. The skyrmion energy consists of the Coulomb
energy, the Zeeman energy and the pseudo-Zeeman energy. The Coulomb energy
consists of the self-energy, the capacitance energy and the exchange energy. At
the balanced point only pseudospins are excited unless the tunneling gap is too
large. Then, the SU(4) skyrmion evolves continuously from the
pseudospin-skyrmion limit into the spin-skyrmion limit as the system is
transformed from the balanced point to the monolayer point by controlling the
bias voltage. Our theoretical result explains quite well the experimental data
due to Murphy et al. and Sawada et al. on the activation energy anomaly induced
by applying parallel magnetic field.Comment: 22 pagets, 6 figures, the final version to be published in PR
Pressure-induced unconventional superconductivity in the heavy-fermion antiferromagnet CeIn3: An 115In-NQR study under pressure
We report on the pressure-induced unconventional superconductivity in the
heavy-fermion antiferromagnet CeIn3 by means of nuclear-quadrupole-resonance
(NQR) studies conducted under a high pressure. The temperature and pressure
dependences of the NQR spectra have revealed a first-order quantum-phase
transition (QPT) from an AFM to PM at a critical pressure Pc=2.46 GPa. Despite
the lack of an AFM quantum critical point in the P-T phase diagram, we
highlight the fact that the unconventional SC occurs in both phases of the AFM
and PM. The nuclear spin-lattice relaxation rate 1/T1 in the AFM phase have
provided evidence for the uniformly coexisting AFM+SC phase. In the HF-PM phase
where AFM fluctuations are not developed, 1/T1 decreases without the coherence
peak just below Tc, followed by a power-law like T dependence that indicates an
unconventional SC with a line-node gap. Remarkably, Tc has a peak around Pc in
the HF-PM phase as well as in the AFM phase. In other words, an SC dome exists
with a maximum value of Tc = 230 mK around Pc, indicating that the origin of
the pressure-induced HF SC in CeIn3 is not relevant to AFM spin fluctuations
but to the emergence of the first-order QPT in CeIn3. When the AFM critical
temperature is suppressed at the termination point of the first-order QPT, Pc =
2.46 GPa, the diverging AFM spin-density fluctuations emerge at the critical
point from the AFM to PM. The results with CeIn3 leading to a new type of
quantum criticality deserve further theoretical investigations
Landau-Zener quantum tunneling in disordered nanomagnets
We study Landau-Zener macroscopic quantum transitions in ferromagnetic metal
nanoparticles containing on the order of 100 atoms. The model that we consider
is described by an effective giant-spin Hamiltonian, with a coupling to a
random transverse magnetic field mimicking the effect of quasiparticle
excitations and structural disorder on the gap structure of the spin collective
modes. We find different types of time evolutions depending on the interplay
between the disorder in the transverse field and the initial conditions of the
system. In the absence of disorder, if the system starts from a low-energy
state, there is one main coherent quantum tunneling event where the
initial-state amplitude is completely depleted in favor of a few discrete
states, with nearby spin quantum numbers; when starting from the highest
excited state, we observe complete inversion of the magnetization through a
peculiar ``backward cascade evolution''. In the random case, the
disorder-averaged transition probability for a low-energy initial state becomes
a smooth distribution, which is nevertheless still sharply peaked around one of
the transitions present in the disorder-free case. On the other hand, the
coherent backward cascade phenomenon turns into a damped cascade with
frustrated magnetic inversion.Comment: 21 pages, 7 figures, to be published in Phys.Rev.
Monte Carlo Study of the Spin Transport in Magnetic Materials
The resistivity in magnetic materials has been theoretically shown to depend
on the spin-spin correlation function which in turn depends on the
magnetic-field, the density of conduction electron, the magnetic ordering
stability, etc. However, these theories involved a lot of approximations, so
their validity remained to be confirmed. The purpose of this work is to show by
extensive Monte Carlo (MC) simulation the resistivity of the spin current from
low- ordered phase to high- paramagnetic phase in a ferromagnetic film.
We take into account the interaction between the itinerant spins and the
localized lattice spins as well as the interaction between itinerant spins
themselves. We show that the resistivity undergoes an anomalous behavior at the
magnetic phase transition in agreement with previous theories in spite of their
numerous approximations. The origin of the resistivity peak near the phase
transition in ferromagnets is interpreted here as stemming from the existence
of magnetic domains in the critical region. This interpretation is shown to be
in consistence with previous theoretical pictures. Resistivity in a simple
cubic antiferromagnet is also shown. The absence of a peak in this case is
explained
Slippery Wave Functions V2.01
Superfluids and superconductors are ordinary matter that show a very
surprising behavior at low temperatures. As their temperature is reduced,
materials of both kinds can abruptly fall into a state in which they will
support a persistent, essentially immortal, flow of particles. Unlike anything
in classical physics, these flows engender neither friction nor resistance. A
major accomplishment of Twentieth Century physics was the development of an
understanding of this very surprising behavior via the construction of
partially microscopic and partially macroscopic quantum theories of superfluid
helium and superconducting metals. Such theories come in two parts: a theory of
the motion of particle-like excitations, called quasiparticles, and of the
persistent flows itself via a huge coherent excitation, called a condensate.
Two people, above all others, were responsible for the construction of the
quasiparticle side of the theories of these very special low-temperature
behaviors: Lev Landau and John Bardeen. Curiously enough they both partially
ignored and partially downplayed the importance of the condensate. In both
cases, this neglect of the actual superfluid or superconducting flow interfered
with their ability to understand the implications of the theory they had
created. They then had difficulty assessing the important advances that
occurred immediately after their own great work.
Some speculations are offered about the source of this unevenness in the
judgments of these two leading scientists.Comment: 30 pages, 3 figure
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