Abstract. To mark the publication of the seventieth volume of Romanian Reports in Physics, printed before 1992 under the Romanian name Studii si Cercetari de Fizica, we briefly review here its history in the broader context of the evolution of Romanian Physics within the past seven decades, and outline some of the research topics that are representative both for the journal and the Romanian physics community at large. Our review takes the form of a historical resource article that references a comprehensive list of papers authored or co-authored by Romanian physicists in the period 1950-2017, selected to be indicative for the research interests of the authors and scientifically significant. Our survey covers theoretical, experimental, and computational physics articles from the main research and academic centers in Romania and sketches the scientific profiles of a series of Romanian physicists.

Abstract. Special types of exact two- and three-soliton solutions in terms of hyperbolic cosines to the Kadomtsev-Petviashvili II equation are presented, exhibiting rich intriguing interaction patterns on a finite background. The behavior of each line soliton in the far region can be characterized analytically. It is revealed that under certain conditions, there may appear an isolated bump in the interaction center, which is much higher in peak amplitude than the surrounding line solitons, and the more line solitons interact, the higher isolated bump will form. These results may provide a clue to generation of extreme high-amplitude waves, in a reservoir of small waves, based on nonlinear interactions between the involved waves.

Abstract. We describe the dynamics of the interferometric power in a system composed of two bosonic modes immersed in a thermal reservoir, in the framework of the theory of open systems based on completely positive quantum dynamical semigroups. The time evolution of the interferometric power is described in terms of the covariance matrix for Gaussian initial states. We show that, independent of the initial state, the Gaussian interferometric power is monotonically decreasing in time, and in the limit of large times it asymptotically decreases to a zero value.

Abstract. In this article we make a comparison between the behavior of entanglement and quantum degree of polarization for a special class of states of the radiation field, namely the Bell-type diagonal mixed states. For the three-photon mixed states we have plotted the concurrence and the Chernoff quantum degree of polarization in terms of the parameter, which defines the state. We find that the entanglement and the quantum degree of polarization are incomparable measures.

Abstract. We consider a model based on two coupled fourth-order oscillators that describes the intrinsic quadrupole vibrations of atomic nuclear surface in order to investigate the classical-quantum correspondence. We explore the role of the classical phase-space structure upon the statistical properties of energy levels distribution of the associated quantum system. A mixed distribution function, encoding two possible mechanisms for energy levels generation is proposed. Its features can be related to the relative weight of the regular and irregular volumes in the classical phase-space.

Abstract. The dissipated energy is calculated for different values of the tunneling velocity by solving the time-dependent pairing equations. Two models are used for the pairing interaction: a constant value of the pairing interaction and the density dependent delta interaction. The time-dependent pairing equations supply an average value of the dissipated energy at scission. This average value is compared with experimental data. An average tunneling velocity is deduced by selecting the value that gives the best agreement between experimental and theoretical dissipation energies. The tunneling velocity is strongly model dependent, giving much lower values for the formalism that involves state-dependent pairing interactions than that characterized by the constant pairing. The investigation is made for the fission of ²³²Th, along a fission trajectory that connects the ground state of the parent nucleus to the scission configuration. This fission trajectory is obtained from the least action principle. The rearrangement of the single particle level scheme is supplied by the Woods-Saxon two-center shell model.

Abstract. The deviation ΔZ = < Z > - Z_UCD, of the charge of a fission fragment with given mass number A_F, from the unchanged charge distribution is calculated by the minimization of the total macroscopic energy at scission. The scission configuration is approximated by two spherical fragments with masses and charges (A₁, Z₂) and (A₂, Z₂) separated by a distance d between their interior surfaces. An analytical formula for ΔZ is deduced and applied to the nucleus ²³⁶U at different mass divisions. A qualitative agreement with experimental data for the ²³⁵U(n_th,f) reaction is obtained for a wide range of d values (from 6 fm to 12 fm). When the generally accepted variation of the distance d with the mass asymmetry is introduced, the agreement becomes quantitative.

Abstract. Electron capture in collisions of Li³⁺ ion with hydrogen atom in ground state in Debye plasma is studied by employing the two-center atomic orbital close-coupling method. The plasma screened interaction of the electron with the two centers is represented by the Debye-Huckel potential, appropriate for a wide class of laboratory and astrophysical plasmas (Debye plasmas). The sensitivity of nl-selective capture sections to interaction screening, as well as the electron capture enhancement in Debye plasmas in the low-energy region are confirmed. The bell-shaped local maxima of the dependence of the 2l partial sections in a region of screening length D, for lower energy, is attributed to the proximity (and intersection) of the energies of the initial and final levels in that region.

Abstract. We consider a composite model in which the Standard Model fermions are bound states of elementary spin 1/2 particles, the preons, situated in the conjugate product representation of the color group. In this framework we propose and analyze two mass Ansatze, one for the leptons, the other one for the quarks, based on mass formulae of the Gell-Mann-Okubo type. We find that these mass Ansatze can give an adequate description of the known Standard Model fermion masses.

Abstract. A scientific-methodical review of the derivation of nonlinear evolution equations describing the interaction of laser pulses with a system of two-level atoms and having solutions in the form of optical solitons is presented. Regimes of propagation of resonant and quasi-resonant envelope solitons, as well as few-optical-cycle solitons with temporal durations from nano- to femtoseconds, are considered. The review is a short travel guide to selected problems of soliton propagation in media consisting of two-level atoms.

Abstract. We experimentally demonstrate a broadly tunable dissipative soliton resonance dual-amplifier figure-of-eight fiber laser emission. The peak power is tuned continuously from 8.8 to 41.4 W by the first amplifier and pulse width from 84 to 416 ns by the second one.

Abstract. We consider the propagation of three-dimensional few-cycle optical pulses in topological insulator thin films within the framework of an effective long-wave Hamiltonian in the case of low temperature. The key features of the propagation dynamics of input Gaussian, Bessel, and Airy pulse shapes are revealed.

Abstract. For a hollow-core Bragg fiber, the field is represented by a Bessel function of the first kind in the core region, a linear combination of Bessel functions of the first and second kinds in the dielectric interior layers, a linear combination of the Hankel functions in the gold region and a Hankel function of the first kind in the external infinite medium. Our analytical method is applied for different structures made from 19, 11, and 5 layers. When a high index material just before the outermost region of a hollow-core Bragg fiber is replaced by a gold layer, the optical confinement for the TE₀₁ mode in the core is increased about ten times. If the gold layer is located between the first and the penultimate layer, the loss for the same mode is increased.

Abstract. We investigate the generation of trains of short optical pulses whose repetition frequency is imposed by the free spectral range of a silicon microring resonator embedded into a fiber laser cavity. According to the microresonator selected in a silicon-on-insulator chip, the pulse trains are obtained with repetition frequencies ranging from 110 GHz to 450 GHz. Regimes where multiple pulses are generated in the microresonator are also shown, and nonlinear broadening of the laser lines is observed.

Abstract. In the first part of this article, we briefly overview the formation of dissipative structures in various out of equilibrium systems. In the second part, we address the formation of localized structures often called cavity solitons in nonlinear optics. We will focus on the interaction between cavity solitons, the polarization properties, and the effect of delay feedback control. The following systems will be discussed: passive resonators such as optical fibers, whispering gallery mode cavities, integrated ring resonators, left-handed materials, and vertical cavity surface emitting lasers with or without saturable absorption. The year 2017 marks the 50th anniversary of the scientific concept named dissipative structures that was put forward by Ilya Prigogine. This paper is dedicated to honor the memory of Ilya Prigogine on the occasion of the anniversary in 2017 of 100 years from his birthday. The aim of this article is to provide a list of selected subjects on some important properties of cavity solitons as generic examples of dissipative structures and their applicability in diverse fields of photonics and nonlinear sciences. Therefore, this paper is meant for active research physicists and engineers working in nonlinear optics and photonics and wishing to have a quick overview of recent developments in terms of the applicability of cavity solitons.

Abstract. Here we present details on how the cooling effects of an opto-mechanical system are affected beyond the secular approximation. To this end, a laser driven two-level quantum dot (QD) embedded in a phononic nano-cavity is investigated for moderately strong QD-phonon couplings regimes. For these regimes, the use of a secular approximation within the QD-phonon interaction terms is no longer justified as the rapidly oscillating terms cannot be neglected from the system dynamics. Therefore, one shows that although being small, their contribution plays an important role when quantum cooling is achieved. The main contribution of the fast oscillating terms is analytically estimated and one compares how the quantum cooling dynamics changes within or beyond the secular approximation. The behavior of the quantum cooling effect is investigated in the steady-state regime via the phonon field statistics.

Abstract. We address the properties of optical solitons that form in media with competing cubic-quintic nonlinearity and parity-time (PT)-symmetric complex-valued external potentials. The model describes the propagation of solitons in nonlinear optical waveguides with balanced gain and loss. We study the existence, stability, and robustness of fundamental, dipole, and multipole stationary solutions in this PT-symmetric system. The corresponding eigenvalue spectra diagrams for fundamental, dipole, tripole, and quadrupole solitons are presented. We show that the eigenvalue spectra diagrams for fundamental and dipole solitons merge at a coalescence point W_c1, whereas the corresponding diagrams for tripole and quadrupole solitons merge at a larger coalescence point W_c2. Beyond these two merging points, i.e., when the gain-loss strength parameter W₀ exceeds the corresponding coalescence points, the eigenvalue spectra cease to exist. The stability of the stationary solutions is investigated by performing the linear stability analysis and the robustness to propagation of these stationary solutions is checked by using direct numerical simulations.

Abstract. Recent results are reviewed on the synthesis and properties of high magnetization ferrofluids and ferrofluid based magnetorheological (MR) fluids. Structural characteristics at nanometer level, colloidal behavior in specific environments, magnetic and flow behavior of a large variety of ferrofluids and some newly developed nano-micro composite magnetizable fluids were evaluated by transmission electron microscopy (TEM/HRTEM), X-ray photoelectron spectroscopy (XPS), dynamic (DLS) and static light scattering (SLS), small angle neutron scattering (SANS), small angle X-ray scattering (SAXS), vibrating sample magnetometry (VSM), Mossbauer spectroscopy, rheo-magnetorheometry, chemi-luminiscence (CL), and differential scanning calorimetry (DSC). The paper is focused mainly on the synthesis and properties of long-term colloidal stability and high magnetization sealing fluids and of ferrofluid based magnetorheological fluids with improved kinetic stability and high MR response.

Abstract. We review properties of two-dimensional matter-wave solitons, governed by the spinor system of Gross-Pitaevskii equations with cubic nonlinearity, including spin-orbit coupling and the Zeeman splitting. In contrast to the collapse instability typical for the free space, spin-orbit coupling gives rise to stable solitary vortices. These are semi-vortices with a vortex in one spin component and a fundamental soliton in the other, and mixed modes, with topological charges 0 and ±1 present in both components. The semivortices and mixed modes realize the ground state of the system, provided that the self-attraction in the spinor components is, respectively, stronger or weaker than their cross-attraction. The modes of both types degenerate into unstable Townes solitons when their norms attain the respective critical values, while there is no lower norm threshold for the stable modes existence. With the Galilean invariance lifted by the spin-orbit coupling, moving stable solitons can exist up to a mode-dependent critical velocity with two moving solitons merging into a single one as a result of collision. Augmenting the Rashba term by the Dresselhaus coupling has a destructive effect on these states. The Zeeman splitting tends to convert the mixed modes into the semivortices, which eventually suffer delocalization. Existence domains for the soliton families are reviewed in terms of experiment-related quantities.

Abstract. In the present work, we explore the potential of spin-orbit (SO) coupled Bose-Einstein condensates to support multi-component solitonic states in the form of dark-bright (DB) solitons. In the case where Raman linear coupling between components is absent, we use a multiscale expansion method to reduce the model to the integrable Mel'nikov system. The soliton solutions of the latter allow us to reconstruct approximate traveling DB solitons for the reduced SO coupled system. For small values of the formal perturbation parameter, the resulting waveforms propagate undistorted, while for large values thereof, they shed some dispersive radiation, and subsequently distill into a robust propagating structure. After quantifying the relevant radiation effect, we also study the dynamics of DB solitons in a parabolic trap, exploring how their oscillation frequency varies as a function of the bright component mass and the Raman laser wavenumber.

Abstract. In this work, we numerically consider the initial value problem for nonlinear Schrodinger (NLS)-type models arising in the physics of ultracold bosonic gases, with generic Gaussian wavepacket initial data. The corresponding Gaussian's width and, wherever relevant, also its amplitude serve as control parameters. First, we explore the one-dimensional, standard NLS equation with general power law nonlinearity, in which large amplitude excitations reminiscent of Peregrine solitons or regular solitons appear to form, as the width of the relevant Gaussian is varied. Furthermore, the variation of the nonlinearity exponent aims at exploring the interplay between rogue waves and the emergence of collapse. The robustness of the main features to noise in the initial data is also confirmed. To better connect our study with the physics of atomic condensates, and explore the role of dimensionality effects, we also consider the non-polynomial Schrodinger equation, as well as the full three-dimensional NLS equation, and examine the degree to which relevant considerations generalize.