994 research outputs found
Nonlinear optical response in doped conjugated polymers
Exciton effects on conjugated polymers are investigated in soliton lattice
states. We use the Su-Schrieffer-Heeger model with long-range Coulomb
interactions. The Hartree-Fock (HF) approximation and the single-excitation
configuration- interaction (single-CI) method are used to obtain optical
absorption spectra. The third-harmonic generation (THG) at off-resonant
frequencies is calculated as functions of the soliton concentration and the
chain length of the polymer. The magnitude of the THG at the 10 percent doping
increases by the factor about 10^2 from that of the neutral system. This is
owing to the accumulation of the oscillator strengths at the lowest exciton
with increasing the soliton concentration. The increase by the order two is
common for several choices of Coulomb interaction strengths.Comment: Accepted for publication in J. Phys.: Condens. Matte
Stability of cooperation in societies of emotional and moody agents
It is well documented that cooperation may not be achieved in societies where self-interested agents are engaging in Prisoner’s Dil
Hole Doping Effects on Spin-gapped Na2Cu2TeO6 via Topochemical Na Deficiency
We report the magnetic susceptibility and NMR studies of a spin-gapped
layered compound
Na2Cu2TeO6 (the spin gap 250 K), the hole doping effect on the
Cu2TeO6 plane via a topochemical Na deficiency by soft chemical treatment, and
the static spin vacancy effect by nonmagnetic impurity Zn substitution for Cu.
A finite Knight shift at the Te site was observed for pure
Na2Cu2TeO6.
The negative hyperfine coupling constant is an evidence for
the existence of a superexchange pathway of the Cu-O-Te-O-Cu bond. It turned
out that both the Na deficiency and Zn impurities induce a Curie-type magnetism
in the uniform spin susceptibility in an external magnetic field of 1 T, but
only the Zn impurities enhance the low-temperature Na nuclear
spin-lattice relaxation rate whereas the Na deficiency suppresses it. A spin
glass behavior was observed for the Na-deficient samples but not for the
Zn-substituted samples. The dynamics of the unpaired moments of the doped holes
are different from that of the spin vacancy in the spin-gapped Cu2TeO6 planes.Comment: 4 pages, 7 figures, to be published in J. Phys. Soc. Jpn. Vol. 75,
No. 8 (2006
Ultrafast Electronic Disordering During Femtosecond Laser Melting of GaAs
We have observed an ultrarapid electronic phase transformation to a centrosymmetric electronic state during laser excitation of GaAs with intense femtosecond pulses. Reflection second-harmonic intensity from the upper 90 atomic layers vanishes within 100 fs; reflectivity rises within 0.5 ps to a steady value characteristic of a metallic molten phase, long before phonon emission can heat the lattice to the melting temperature
Lenient multi-agent deep reinforcement learning
Much of the success of single agent deep reinforcement learning (DRL) in recent years can be attributed to the use of experience replay memories (ERM), which allow Deep Q-Networks (DQNs) to be trained efficiently through sampling stored state transitions. However, care is required when using ERMs for multi-agent deep reinforcement learning (MA-DRL), as stored transitions can become outdated because agents update their policies in parallel [11]. In this work we apply leniency [23] to MA-DRL. Lenient agents map state-action pairs to decaying temperature values that control the amount of leniency applied towards negative policy updates that are sampled from the ERM. This introduces optimism in the value-function update, and has been shown to facilitate cooperation in tabular fully-cooperative multi-agent reinforcement learning problems. We evaluate our Lenient-DQN (LDQN) empirically against the related Hysteretic-DQN (HDQN) algorithm [22] as well as a modified version we call scheduled-HDQN, that uses average reward learning near terminal states. Evaluations take place in extended variations of the Coordinated Multi-Agent Object Transportation Problem (CMOTP) [8] which include fully-cooperative sub-tasks and stochastic rewards. We find that LDQN agents are more likely to converge to the optimal policy in a stochastic reward CMOTP compared to standard and scheduled-HDQN agents
Equatorial and related non-equilibrium states in magnetization dynamics of ferromagnets: Generalization of Suhl's spin-wave instabilities
We investigate the nonlinear dynamics underlying the evolution of a 2-D
nanoscale ferromagnetic film with uniaxial anisotropy in the presence of
perpendicular pumping. Considering the associated Landau-Lifshitz spin
evolution equation with Gilbert damping together with Maxwell equation for the
demagnetization field, we study the dynamics in terms of the stereographic
variable. We identify several new fixed points for suitable choice of external
field in a rotating frame of reference. In particular, we identify explicit
equatorial and related fixed points of the spin vector in the plane transverse
to the anisotropy axis when the pumping frequency coincides with the amplitude
of the static parallel field. We then study the linear stability of these novel
fixed points under homogeneous and spin wave perturbations and obtain a
generalized Suhl's instability criterion, giving the condition for exponential
growth of P-modes under spin wave perturbations. Two parameter phase diagrams
(in terms of amplitudes of static parallel and oscillatory perpendicular
magnetic fields) for stability are obtained, which differ qualitatively from
those for the conventional ferromagnetic resonance near thermal equilibrium and
are amenable to experimental tests.Comment: 23 pages, 5 figures, To appear in Physica
Ultralow-power local laser control of the dimer density in alkali-metal vapors through photodesorption
Ultralow-power diode-laser radiation is employed to induce photodesorption of
cesium from a partially transparent thin-film cesium adsorbate on a solid
surface. Using resonant Raman spectroscopy, we demonstrate that this
photodesorption process enables an accurate local optical control of the
density of dimer molecules in alkali-metal vapors.Comment: 4 pages, 4 figure
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