Volume 60, Number 56, 2015
Proceedings of second edition of "Advanced manybody and statistical methods in mesoscopic systems", September 15, 2014, Brasov, Romania
Preface Organizing Committee Romanian Journal of Physics 60,
646647 (2015)
Condensed Matter
Scattering Structure Factor from Fat Fractals E. M. Anitas Romanian Journal of Physics 60,
647652 (2015)
Recent results have revealed that "concave" smallangle scattering (SAS) data, (i.e. a succession of powerlaw decays with decreasing values of the scattering exponents), can be successfully modeled in the framework of fat fractals. In this paper, the SAS structure factor for deterministic fat fractals is studied in momentum space. We show that fat fractals are exact selfsimilar in the range of iterations having the same values of the scaling factor, and therefore in each of these ranges all the properties of regular fractals can be inferred to fat fractals. In order to illustrate the above findings we introduce deterministic "fattened" versions of two wellknown 3D structural models: Cantor and Vicsek regular deterministic fractals. We calculate the mono and polydisperse structure factor and study its scattering properties.
Structural Properties of Composite Elastomeric Membranes Using SmallAngle Neutron Scattering E. M. Anitas, I. Bica, R. V. Erhan, M. Bunoiu, A. I. Kuklin Romanian Journal of Physics 60,
653657 (2015)
The morphology of elastomeric membranes based on silicone rubber, at various concentrations of the catalyst (C) is determined by smallangle neutron scattering (SANS) technique. Nearly all membranes display a hierarchical organization of crystallites in which ramified mesoscale mass fractals are composed of either mass or surfacelike nano fractals. We use the Beaucae model in order to extract information about the radius of gyration of the clusters and about their fractal dimensions. We show that the fractal dimension of the mesoscale fractal is fixed at 2.68 and is independent with the addition of C. However, the nanoscale fractals are characterized by a transition from mass to surfacelike fractals at high values of C concentrations.
The Structure of Deterministic Mass, Surface and MultiPhase Fractals from SmallAngle Scattering Data A. Yu. Cherny, E. M. Anitas, V. A. Osipov, A. I. Kuklin Romanian Journal of Physics 60,
658663 (2015)
We present general results on smallangle scattering (SAS) from deterministic (i.e., exactly selfsimilar) mass, surface and multiphase fractals. We suggest a method of obtaining additional information from SAS data of deterministic mass fractals, such as the fractal iteration number, the scaling factor, and the number of structural units composing the mass and surface fractal. For a multiphase system, we show that the qualitative question can be answered from SAS data whether one fractal ‘absorbs’ another one or they are both immersed in a surrounding homogeneous medium. Surface fractals are shown to be constructed as combinations of mass fractals.
New Hints from Theory for Pumping Spin Currents in Quantum Circuits Michele Cini Romanian Journal of Physics 60,
664675 (2015)
In the ballistic regime, polygonal quantum rings connected with an external circuit can be used to inject into it charge and/or spin, with no need for an external bias. This is a ‘oneparameter’ form of quantum pumping; it requires a timedependent magnetic field and a proper selection of the geometry. With the field in the plane of a ring having one electron per site and an even number of sites a pure spin current, driven by the spinorbit interaction, is excited in both wires. One can prove that the charge current vanishes identically if the whole system is a bipartite lattice. The pure spin current can be called a magnetic current since magnetization propagates while the system behaves like a gapless insulator. One can use storage units to concentrate and save this magnetization, in much the same way as capacitors store the electric charge. The stored magnetization is long lived and can be used later. By connecting up and downpolarized reservoirs one produces spin currents in another circuit. These currents show a dynamics. They oscillate while the storage units exchange their polarizations. The intensity and amplitude of the oscillations can controlled by tuning the conductance of the wire used to connect the units.
Design of a Lambda Configuration in Artificial Coherent Nanostructures P.G. di Stefano, E. Paladino, A. D'Arrigo, B. Spagnolo, G. Falci Romanian Journal of Physics 60,
676685 (2015)
The implementation of a threelevel Lambda System in artificial atoms would allow to perform advanced control tasks typical of quantum optics in the solid state realm, with photons in the µm/mm range. However hardware constraints put an obstacle since protection from decoherence is often conflicting with efficient coupling to external fields. We address the problem of performing conventional STImulated Raman Adiabatic Passage (STIRAP) in the presence of lowfrequency noise. We propose two strategies to defeat decoherence, based on “optimal symmetry breaking” and dynamical decoupling. We suggest how to apply to the different implementations of superconducting artificial atoms, stressing the key role of nonMarkovianity.
Intraband Relaxation of PShell Excitons in DiskShaped Quantum Dots R. Dragomir, V. Moldoveanu, I. V. Dinu Romanian Journal of Physics 60,
686690 (2015)
We study the generation of pshell excitons in optically active diskshaped quantum dots subjected to ultrafast optical pulses. The singleparticle spectral properties are obtained from the fourband kp theory, whereas the Coulomb interaction is taken into account within the configurationinteraction approach, particular attention being payed to configuration mixing due to electronhole correlations. The effect of intraband relaxation processes is included in the nonunitary dynamics derived from the vonNeumann Lindblad equation. We find that the fast hole relaxation processes drive the pshell excitons to intermediate states which eventually evolve to sshell excitons via slower electron relaxation.
Glassy Behavior of Disordered Fractional Exclusion Statistics Systems G. A. Nemnes, D. V. Anghel Romanian Journal of Physics 60,
691695 (2015)
We employ a general method based on fractional exclusion statistics and Monte Carlo simulations to study the low temperature dynamics of systems with quenched disorder. Depending on the degree of disorder and type of the interacting potentials, different dynamical behaviors are observed at low temperatures. In particular, the systems with a random mixture of repulsive and atractive interactions exhibit a slow typical glassy dynamics. Analyzing the spatial and autocorrelation functions we point out aging effects which are enhanced in the presence of interactions.
Ab Initio Investigation of Optical Properties in Triangular Graphene  Boron Nitride CoreShell Nanostructures A. A. Nila, G. A. Nemnes, A. Manolescu Romanian Journal of Physics 60,
696700 (2015)
We calculate the optical properties of atomicsized coreshell graphene  boron nitride nanoflakes with triangular shaped crosssection using the density functional theory. The optical properties can be tuned by using different sizes and proportions of the coreshell materials. Anisotropic effects manifested in the absorption of unpolarized light with different orientations of the optical vector are pointed out.
NoiseInduced Detrapping in Small Quantum Networks T. Pope, A. D'Arrigo, E. Paladino, G. Falci Romanian Journal of Physics 60,
701710 (2015)
Optimizing excitation transport in quantum networks is an important precursor to the development of highlyefficient lightharvesting devices. We investigate the phenomenon of dephasingassisted leakage from trapped states, which may ensure efficient transport of excitations across a network. We consider three small networks with known trapped states and study the effect of Markovian quantum noise and classical noise sources with different correlation times, from very low frequency nonMarkovian noise to white noise. We show that the excitation  otherwise stalled in trapped states is able to diffuse rapidly through the networks from a source to a sink, with efficiency being generally maximized for intermediate correlation times.
The Rabi Hamiltonian in the Dispersive Regime Titus Sandu Romanian Journal of Physics 60,
711715 (2015)
The Rabi Hamiltonian is studied in the dispersive regime and ultrastrong coupling. We employ a recent unitary transformation to obtain not only the approximate Hamiltonian and its energy levels but also its eigenfunctions. The relationship of the approximation with other regimes and their approximations are also discussed.
Thermoelectric Effects in Nanostructured Quantum Wires in the NonLinear Temperature Regime Anda Elena Stanciu, G. A. Nemnes, A. Manolescu Romanian Journal of Physics 60,
716721 (2015)
The thermoelectric voltage of a quantum dot connected to leads is calculated using the scattering Rmatrix method. Our approach takes into account a temperature gradient between the contacts beyond the linear regime. We obtain sign changes of the thermopower when varying the temperature or the chemical potential around the resonances. The influence of the coupling strength of the contacts and of the thermoelectric field on the thermoelectric voltage is discussed.
Nuclear Physics
Pairing Coherence Length in Nuclei V.V. Baran, D.S. Delion Romanian Journal of Physics 60,
722726 (2015)
We study pairing correlations by analyzing the coherence length in a HF+BCS formalism with various types of pairing potentials. We compare the density dependent delta (DDD) pairing interaction to effective Gaussian interactions with different width parameters. We then generalize the discussion to quartet correlations, and finally we evaluate the importance of protonneutron correlations.
Collective Dynamics and Fragmentation in Nuclear Systems V. Baran, M. Marciu, D.I. Palade, M. Colonna, M. di Toro, A.I. Nicolin, R. Zus Romanian Journal of Physics 60,
727737 (2015)
In this work we discuss different features of the collective dynamics in neutron rich nuclei which can contribute to constrain the dependence with density of the symmetry energy below saturation. As two components systems, they manifest both isoscalar and isovector modes whose stability is changing with density in a different manner. Within a Fermi liquid theory it is predicted that the isovector modes remain stable at densities under saturation while the isoscalar modes become unstable below a certain value of density. In connection to the former modes the electric dipole response of exotic nuclei shows the emergence of a low energy component when the number of neutrons in excess increases, which is associated to the dynamics of the low density neutron skin against a more stable core. Within schematic models which generalize the BrownBolsterli approach as well as in meanfield descriptions based on the LandauVlasov equation, we analyze the role of the symmetry energy on the energy centroid position and on the sum rules for this mode. Then, by considering the same parameterizations with density of the symmetry energy we investigate the isospin dynamics in the conditions when the growth of the unstable isoscalar fluctuations determine the nuclear fragmentation at Fermi energies.
Emergence of Rotational Collectivity in Ab Initio noCore Configuration Interaction Calculations M. A. Caprio, P. Maris, J. P. Vary, R. Smith Romanian Journal of Physics 60,
738747 (2015)
Rotational bands have been observed to emerge in ab initio nocore configuration interaction (NCCI) calculations for pshell nuclei, as evidenced by rotational patterns for excitation energies, electromagnetic moments, and electromagnetic transitions. We investigate the ab initio emergence of nuclear rotation in the Be isotopes, focusing on ^{9}Be for illustration, and make use of basis extrapolation methods to obtain ab initio predictions of rotational band parameters for comparison with experiment. We find robust signatures for rotational motion, which reproduce both qualitative and quantitative features of the experimentally observed bands.
Pygmy Dipole Resonance in a Schematic Model A. Croitoru, V. Baran, T. Isdraila, M. Colonna, M. di Toro, M. Marciu Romanian Journal of Physics 60,
748752 (2015)
We present a model which extends the approach introduced by D. Brink to evidence the collective nature of Giant Dipole Resonance. For neutronrich nuclei the emergence of an additional low energy mode that can be associated to the Pygmy Dipole Resonance (PDR) is predicted. We explore the role of a separable dipoledipole interaction where the condition to have a unique coupling constant was relaxed in order to account for the density dependence of the symmetry energy. The values of the coupling constants are not affecting too much the position of energy centroid of the pygmy state but are strongly influencing the Energy Weighted Sum Rule (EWSR) exhausted by it.
Integrable RichardsonGaudin Models in Mesoscopic Physics Jorge Dukelsky Romanian Journal of Physics 60,
753766 (2015)
The exact solution of the SU(2) pairing Hamiltonian with nondegenerate single particle orbits was introduced by Richardson in the early sixties. The exact solution passed almost unnoticed till was recovered in the last decade in an effort to describe the disappearance of superconductivity in ultrasmall superconducting grains. Since then it has been extended to several families integrable models, called the RichardsonGaudin (RG) models. In particular, the rational family of integrable RG models has been widely applied to mesoscopic systems like small grains, quantum dots and nuclear systems where finite size effects play an important role. We will first introduce these families of integrable models and then we will describe the first applications of the hyperbolic family to spinless cold fermionic atoms in two dimensional lattices and to heavy nuclei.
Coherent State Description of the αEmission Spectrum A. Dumitrescu, D. S. Delion Romanian Journal of Physics 60,
767771 (2015)
We review the main aspects concerning the calculation of αdecay intensities to excited states in eveneven nuclei using a Coherent State Model (CSM) for the description of the daughter nucleus. The analysis of the αemission process is based on an αdaughter interaction having a monopole component and a quadrupolequadrupole (QQ) interaction. The decaying states are calculated through the coupled channels method. The decay intensities to 2^{+} states are reproduced by means of the QQ strength. This interaction strength can be fitted with a linear dependence on the deformation parameter, as predicted by the CSM. Predicted decayintensities to higher excited states are in reasonable agreement with available experimental data.
Random Matrices, PointGroup Symmetries, and ManyBody Systems Calvin W. Johnson Romanian Journal of Physics 60,
772781 (2015)
Whenever a system has a symmetry, Noether’s theorem tells us there is an associated conserved quantity. For quantum systems this means the Hamiltonian takes on a blockdiagonal form, labeled by “good” quantum numbers. While Noether’s theorem tells us this structure exists, it doesn’t tell us the ordering of the states, or which quantum numbers tend to be associated with the ground state. Yet in physical systems we know the ground state tends to belong to the most symmetric irrep. Could it be otherwise? To explore this question, I construct random matrices with pointgroup symmetries and find an order above and beyond Noether’s theorem. The pattern persists in detailed simulations of manybody systems, even when all physics besides symmetry is taken out. For atomic nuclei, for example, this means the ground states of eveneven nuclides have J=0 not due to pairing as we are taught, but due to underlying mathematical structures.
Seniority and Truncation Schemes for the Nuclear Configuration Interaction Approach Chong Qi Romanian Journal of Physics 60,
782791 (2015)
In this contribution I would like to review a few issues on the recent developments concerning the truncation schemes for the nuclear configuration interaction shell model approach. The seniority scheme is a way to solve the pairing Hamiltonian exactly and a good starting point for shell model calculations. Physically meaningful states may also be selected based on importance truncations from a perturbation perspective.
HalfLives of Thirteen Double βDecay Candidates with Two Neutrinos Yuejiao Ren, Zhongzhou Ren Romanian Journal of Physics 60,
792798 (2015)
Double β^{−}decay with two neutrinos is one of the important decay modes for an unstable nucleus which can spontaneously change into a stable nucleus. It is also a very complex decay process where three kinds of interactions, the strong interaction, the Coulomb interaction, and the weak interaction, are involved. Recently we have made a systematic analysis on the experimental data of nuclear double β^{−}decay and proposed that there is a law between the logarithm of double β^{−}decay halflives and the reciprocal of the decay energy. This is a simple and accurate law for halflives of the double β^{−}decay. We use this law to predict the halflives of the double β^{−}decay candidates where the mass number of parent nuclei ranges from A=70 (^{70}Zn) to A = 204 (^{204}Hg). Numerical results of the halflives show that ^{142}Ce is a very interesting candidate for future experiments to search new emitters of the double β^{−}decay. We also compare the halflives of double β^{−}decay and αdecay for ^{142}Ce where its decay energies of two modes are close and find that double β^{−}decay dominates in this case although double β^{−}decay is a second order process of the weak interaction. The branching ratios between double β^{−}decay and αdecay are listed for the candidates of double β^{−}decay.
ProtonNeutron Pairing in SelfConjugate Nuclei in a Formalism of Quartets M. Sambataro, N. Sandulescu, C. W. Johnson Romanian Journal of Physics 60,
799804 (2015)
The protonneutron pairing in selfconjugate nuclei is discussed in a formalism of quartets. Quartets are fourbody correlated structures built from two neutrons and two protons coupled to total isospin T = 0 and total angular momentum J = 0. The pairing ground state is described as a product of quartets. We review both the case in which the Hamiltonian has a pure isovector character and the case in which, in addition, it contains isoscalar components. The approach is tested by making comparisons with exact shell model calculations for N=Z nuclei with valence nucleons outside the ^{16}O, ^{40}Ca and ^{100}Sn cores. The quartet formalism is seen to reproduce with great accuracy the exact ground states energies.
Pairing, Quartet Condensation and Wigner Energy in Nuclei N. Sandulescu, D. Negrea Romanian Journal of Physics 60,
805810 (2015)
In this paper we summarize the study we have recently performed on the effect of isovector pairing correlations upon the symmetry and Wigner energies. First, we review the basic assumptions of the quartet condensation model (QCM) and show how this model can be used in HartreeFock (HF) mean field calculations in order to take into account the isovector pairing correlations. Then, within the HF+QCM approach we discuss the influence of protonneutron pairing on symmetry and Wigner energies for the isobaric chains of eveneven nuclei with 24 < A < 100.
Theory for Quartet Condensation in Fermi Systems with Applications to Nuclear Matter P. Schuck Romanian Journal of Physics 60,
811818 (2015)
α clustering and α condensation in lighter nuclei is presently strongly and increasingly discussed in the literature both from the experimental side as from the theoretical one. In protoneutron stars a macroscopic condensate of α particles may occur. A discussion of the present status of the theory for quartet condensation in general and for α particle condensation in nuclear matter in particular will be presented.
Impact of Mesoscopy on Nuclear Structure Phenomena V. Werner, N. Cooper, P.M. Goddard, P. Humby, R.S. Ilieva Romanian Journal of Physics 60,
819828 (2015)
Results from photonscattering experiments on ^{76}Se and ^{76}Ge are given and compared to statistical approaches, based on standard Lorentzian parametrizations.
Molecular Structures in Light Atomic Nuclei Furong Xu, Simin Wang, Junchen Pei, Wenjun Chen, Zhenxiang Xu Romanian Journal of Physics 60,
829835 (2015)
We have established the antisymmetrized molecular dynamics (AMD) calculation including a computational code. A phenomenological Gognytype nuclear force including spinorbit coupling has been used. A zerorange threebody force described by δ function is adopted. In order to reduce the computing time, in the present work we mainly performed variationbeforeprojection calculations. Again due to the limitation of computer capability, the calculations are limited to light nuclei. In the present work, we have investigated the light nuclei from the lightest ^{3}He to heaviest ^{16}C nuclei. From calculated density distributions by the AMD model, we can see clustering or molecular structures in the light nuclei. The AMD model can also give reasonable binding energies compared with data. We have calculated the spectra of the investigated nuclei, obtaining quite good results compared with experimental data. As a example, we performed a variationafterprojection calculation for ^{10}Be, showing the significant improvement in the calculation of binding energy. The calculations give that the 0^{+} and 1^{−} states have different molecular structures.
Nuclear Structure and NeutrinoNucleus Reactions at Supernova Energies E. Ydrefors, W. Almosly, J. Suhonen Romanian Journal of Physics 60,
836846 (2015)
Supernova(anti)neutrino–nucleus scattering is discussed with reference to neutralcurrent (NC) and chargedcurrent (CC) processes in heavy stable nuclei. The DonnellyWalecka method with the associated multipole expansion of the nucleonic current has been adopted as the basic framework in deriving the neutrinonucleus scattering cross sections. The needed nuclear wave functions are computed by using the quasiparticle randomphase approximation (QRPA) for the eveneven target nuclei in the NC processes and the protonneutron QRPA (pnQRPA) has been used to compute the CC processes for the mentioned nuclei. The wave functions of the stable oddmass target nuclei have been obtained by the use of the QRPAbased microscopic quasiparticlephonon model (MQPM), applied to both NC and CC processes. The obtained cross sections are folded by the energy distributions of the various (anti)neutrino flavors in order to access detection rates in Earthbased neutrino telescopes. The stable Molybdenum (Mo) nuclei serve as examples of application of the formalism, with a subsequent analysis of the results.
Quantum Mechanics
Extraordinary Subgroups Needed for the Construction of Mutually Unbiased Bases for the Dimension $d = 8$ Iulia Ghiu, Cristian Ghiu Romanian Journal of Physics 60,
847852 (2015)
The mutually unbiased bases are widely used in many applications of quantum information theory. Recently it was proved that there is a correspondence between mutually unbiased bases and mutually orthogonal extraordinary supersquares. An extraordinary supersquare is obtained starting from its generating extraordinary subgroup, therefore knowing the general expression of these kinds of subgroups is extremely important. We are focus here on finding the extraordinary subgroups for $\mathbb{F}8 \times \mathbb{F}8$. Further we construct the set of mutually orthogonal extraordinary supersquare of order 8 with the help of nine extraordinary subgroups, whose unique intersection element is the element zero. Then, we show how the complete set of mutually unbiased bases for d = 8 can be obtained. The case d = 8 corresponds to systems of three particles of spin1/2.
Suppression of Entanglement in TwoMode Gaussian Open Systems Tatiana Mihaescu, Aurelian Isar Romanian Journal of Physics 60,
853858 (2015)
We study the evolution of the entanglement of two independent bosonic modes embedded in a thermal environment, in the framework of the theory of open quantum systems. As a measure of entanglement we use the logarithmic negativity. For a nonzero temperature of the thermal reservoir the entangled initial Gaussian states become always separable in a finite time. For initial squeezed thermal states we calculate the survival time of entanglement and analyze its dependence on temperature, squeezing parameter and mean thermal photon numbers. For a zero temperature of the thermal bath an entangled initial state remains entangled for all finite times, but in the limit of asymptotically large times it becomes separable.
Gaussian Geometric Discord of TwoMode Systems in a Thermal Environment Serban Suciu, Aurelian Isar Romanian Journal of Physics 60,
859864 (2015)
In the framework of the theory of open systems based on completely positive quantum dynamical semigroups, we give a description of the Gaussian geometric discord in terms of HilbertSchmidt distance and the rescaled form. The system consists of two noninteracting nonresonant bosonic modes embedded in a thermal environment. We take as initial state of the system a twomode squeezed thermal state and describe the time evolution of the Gaussian geometric discord under the influence of the thermal bath. By tracing the distance between the state of the considered subsystem and the closest classicalquantum Gaussian state we evaluate the Gaussian geometric discord for all times and temperatures. The geometric discord has finite values between 0 and 1 and decreases asymptotically to zero at large times and temperatures, with oscillations on the time axis.
