Weak Coupling and Continuous Limits for Repeated Quantum Interactions

We consider a quantum system in contact with a heat bath consisting in an infinite chain of identical sub-systems at thermal equilibrium at inverse temperature β. The time evolution is discrete and such that over each time step of duration τ, the reference system is coupled to one new element of the chain only, by means of an interaction of strength λ. We consider three asymptotic regimes of the parameters λ and τ for which the effective evolution of observables on the small system becomes continuous over suitable macroscopic time scales T and whose generator can be computed: the weak coupling limit regime λ → 0, τ = 1, the regime τ → 0, λ²τ → 0 and the critical case λ²τ = 1, τ → 0. The first two regimes are perturbative in nature and the effective generators they determine is such that a non-trivial invariant sub-algebra of observables naturally emerges. The third asymptotic regime goes beyond the perturbative regime and provides an effective dynamics governed by a general Lindblad generator naturally constructed from the interaction Hamiltonian. Conversely, this result shows that one can attach to any Lindblad generator a repeated quantum interactions model whose asymptotic effective evolution is generated by this Lindblad operator.     

Effects of risedronate in a rat model of osteopenia due to orchidectomy and disuse: densitometric, histomorphometric and microtomographic studies

Dual energy X-ray absorptiometry (DXA), histomorphometry and X-ray microtomography (microCT) were used to assess effects of risedronate and testosterone in a combined rat model of orchidectomy (ORX) and local paralysis induced by botulinum neurotoxin (BTX). Four groups of mature rats were studied for 1 month: SHAM operated; ORX and right hindlimb immobilization (BTX); ORX+BTX+risedronate or testosterone. Changes in bone and body composition were measured by DXA (BMC, lean and fat mass), histomorphometry (BV/TV(2D), Tb.Th and microarchitectural parameters) and microCT (BV/TV(3D), SMI and cortical parameters). ORX and BTX had additive effects on bone loss since differences were maximized on the immobilized bone. The decrease in BMC on the tibial metaphysis reached -33.6% vs. -11.3% in the non-immobilized limb. BV/TV and Tb.N decreased and Tb.Sp increased in both hindlimbs whereas Tb.Th was significantly lower only in the immobilized limb. Decrease of tibial cortical area and thickness was greater in the immobilized limb. Risedronate prevented BMC, BV/TV and architecture loss but not reduction in Tb.Th. Cortical bone was preserved only in the non-immobilized limb. Testosterone was unable to prevent trabecular and cortical bone loss, but it prevents loss of whole body lean mass. In conclusion, ORX and BTX resulted in additive effects on bone loss. Risedronate had protective effects on trabecular bone loss but was less effective on cortical bone.     

A novel femtosecond-laser formation of CdS nanocrystallites in zirconia matrices

A novel method for direct laser writing of two-dimensional cadmium sulfide (CdS) semiconductor nanoparticle microstructures is reported. A two photon or a higher-order multiphoton absorption process, originating from femtosecond laser pulses, was used to decompose CdS precursors dispersed in a zirconia thin film previously dip-coated on a glass substrate. The kinetics of nanoparticle formation as a function of laser power were monitored in situ by photoluminescence spectroscopy. Raman spectroscopy was also performed to characterize the structural changes of the zirconia matrix under irradiation and to verify the formation of CdS nanoparticles. Results show that CdS nanoparticles were formed by two-photon absorption (TPA) with or without the help of an additional carbazole photoinitiator.     

New excited superdeformed bands in heavy Pb nuclei: Clue for an octupole softness near the N = 118 gap at large deformation

We report the identification of six new superdeformed (SD) bands in ^197,198Pb observed with the EUROBALL IV spectrometer. The results are interpreted in the framework of cranked Hartree-Fock calculations with approximate projection on the particle number by means of the Lipkin-Nogami method. A mixing between quasi-particle excitations and an octupole vibration is suggested in the two SD isotopes. We have estimated the ordering of the neutron valence orbitals and confirm indirectly a N = 118 SD gap. Received: 20 December 2000 / Accepted: 24 January 2001     

The Role of Radial Electric Fields in the Tokamaks TEXTOR-94, CASTOR, and T-10

Radial electric fields (E _r) and their role in the establishment of edge transport barriers and improved confinement have been studied in the tokamaks TEXTOR-94 and CASTOR, where E _r is externally applied to the plasma in a controlled way using a biased electrode, as well as in the tokamak T-10 where an edge transport barrier (H-mode) is obtained during electron-cyclotron resonance heating (ECRH) of the plasma.The physics of radial currents was studied and the radial conductivity in the edge of TEXTOR-94 (R = 1.75 m, a = 0.46 m) was found to be dominated by recycling (ion-neutral collisions) at the last closed flux surface (LCFS) and by parallel viscosity inside the LCFS. From a performance point of view (edge engineering), such electrode biasing was shown to induce a particle transport barrier, a reduction of particle transport, and a concomitant increase in energy confinement. An H-mode-like behaviour can be induced both with positive and negative electric fields. Positive as well as negative electric fields were shown to strongly affect the exhaust of hydrogen, helium, and impurities, not only in the H-mode-like regime.The impact of sheared radial electric fields on turbulent structures and flows at the plasma edge is investigated on the CASTOR tokamak (R = 0.4 m, a = 0.085 m). A non-intrusive biasing scheme that we call "separatrix biasing" is applied whereby the electrode is located in the scrape-off layer (SOL) with its tip just touching the LCFS. There is evidence of strongly sheared radial electric field and E×B flow, resulting in the formation of a transport barrier at the separatrix. Advanced probe diagnosis of the edge region has shown that the E×B shear rate that arises during separatrix biasing is larger than for standard edge plasma biasing. The plasma flows, especially the poloidal E×B drift velocity, are strongly modified in the sheared region, reaching Mach numbers as high as half the sound speed. The corresponding shear rates ( 5×10⁶ s^-1) derived from both the flow and electric field profiles are in excellent agreement and are at least an order of magnitude higher than the growth rate of unstable turbulent modes as estimated from fluctuation measurements.During ECRH in the tokamak T-10 (R = 1.5 m, a = 0.3 m), a regime of improved confinement is obtained with features resembling those in the H-mode in other tokamaks. Using a heavy ion beam probe, a narrow potential well is observed near the limiter together with the typical features of the L-H transition. The time evolution of the plasma profiles during L-H and H-L transitions is clearly correlated with that of the density profile and the formation of a transport barrier near the limiter. The edge electric field is initially positive after the onset of ECRH. It changes its sign during the L-H transition and grows till a steady condition is reached. Similar to the biasing experiments in TEXTOR-94 and CASTOR, the experimentally observed transport barrier is a barrier for particles.     

High-spin excitations of 81, 82, 83, 85Se : Competing single-particle and collective structures around N = 50

The ^{81, 82, 83, 85}Se nuclei have been produced as fission fragments in the fusion reaction ¹⁸O + ²⁰⁸Pb at 85MeV bombarding energy and studied with the Euroball IV array. Their high-spin level schemes have been built from the triple -ray coincidence data and - angular correlations have been analyzed in order to assign spin and parity values to many observed states. The lowest-spin states of the two-neutron and three-neutron configurations are strongly mixed with two-proton excitations among the fp orbits. On the other hand, the highest-spin states of these neutron configurations are found to remain almost pure. Neutron excitation across the N = 50 gap is observed both in ⁸³Se₄₉ and in ⁸⁵Se₅₁ .     

Classical and quantum mechanical resonance fluorescence

We study resonance fluorescence from a two-level atom illuminated by coherent and incoherent light. Especially, we treat the case of an intense incoherent component which is broad band and chaotic in character.New insights into the phenomenon of resonance fluorescence are obtained by constructing certain analogies with the precession of a classical (Bloch) vector around a classical stochastic field. The analogies are based on a representation of the density operator of the two-level atoms as a diagonal mixture of directed angular momentum states.As long as the whole light field is an imposed one the weight function of the mixture mentioned above describes a random sequence of rotations of the Bloch vector and obeys a simple Fokker Planck equation. If, however, the incoherent component of the light field acts as a zero- or finite temperature heat bath, the equation of motion for the weight function is no longer a Fokker Planck equation. Nontheless, we find the exact solution and calculate the correlation functions relevant to a discussion of the spectrum and of antibunching effects.     

On the stationary measures of anharmonic systems in the presence of a small thermal noise

We consider certain small stochastic perturbations of ad-dimensional infinite system of coupled anharmonic oscillators. The evolution law is reversible in the Yaglom sense, thus Gibbs states with the given interaction and temperature are stationary measures. If d<3 then some stability properties of the interaction imply the converse statement; if d>2 then the same is proven for translation invariant measures only. The methods and results of Ref. 4, 6–8 are extended to second-order systems of stochastic differential equations.     

Mechanisms of fragment production in heavy-ion reactions at intermediate energies

Comparative analysis of three typical models of nuclear disintegration, the statistical multifragmentation model (SMM), the quantum statistical model (QSM) and a generalized evaporation model (GEM), is carried out. The thermodynamical properties of a decaying system as well as observable characteristics in heavy ion collisions predicted by the different models are discussed. It is shown that these models yield quite similar results for low charge yields at higher excitation energies (E/A>6 MeV per nucleon) and it is suggested that the coincidence measurements of the intermediate mass fragment multiplicity and the neutron and proton multiplicity (or alternatively, the total bound charge) may be very useful for deducing the decay mechanism. The GEM is shown to differ from the other models in predicting a high Z residue peak.     

Electron capture into few-electron heavy ions: Independent particle model

We apply the density matrix theory to re-investigate the radiative electron capture into heavy ions with one valence electron. Attention has been paid particularly to the magnetic sublevel population of the residual ions, as described in terms of alignment parameters. Simple method, based on an independent particle model, which takes into account the Pauli principle, is proposed for evaluating the alignment of the excited ionic states. By making use of this method, detailed calculations are performed for electron capture into (initially) hydrogen-like and lithium-like europium, gold and uranium ions, and are compared with the results of the multiconfiguration Dirac-Fock approach.As seen from the calculations and from the comparison with available experimental results, the independent particle model provides a good estimate for the alignment parameters of few-electron heavy ions. Therefore, our simple model may help to understand the basic properties of the X-ray emission from heavy, few-electron ions without the need for invoking sophisticated MCDF calculations.