arXiv subjects

Alexander Volya

Publications and source records attributed to Alexander Volya.

At least 37 records · Page 2Linked to original sources

arXiv2008

Nuclear Schiff moment and soft vibrational modes

The atomic electric dipole moment (EDM) currently searched by a number of experimental groups requires that both parity and time-reversal invariance be violated. According to current theoretical understanding, the EDM is induced by the nuclear Schiff moment. The enhancement of the Schiff moment by the combination of static quadrupole and octupole deformation was predicted earlier. Here we study a further idea of the possible enhancement in the absence of static deformation but in a nuclear system with soft collective vibrations of two types. Both analytical approximation and numerical solution of the simplified problem confirm the presence of the enhancement. We discuss related aspects of nuclear structure which should be studied beyond mean-field and random phase approximations.

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arXiv2007

Emergence of symmetry from random n-body interactions

An ensemble with random n-body interactions is investigated in the presence of symmetries. A striking emergence of regularities in spectra, ground state spins and isospins is discovered in both odd and even-particle systems. Various types of correlations from pairing to spectral sequences and correlations across different masses are explored. A search for interpretation is presented.

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arXiv2007

Thermodynamics of pairing in mesoscopic systems

Using numerical and analytical methods implemented for different models we conduct a systematic study of thermodynamic properties of pairing correlation in mesoscopic nuclear systems. Various quantities are calculated and analyzed using the exact solution of pairing. An in-depth comparison of canonical, grand canonical, and microcanonical ensemble is conducted. The nature of the pairing phase transition in a small system is of a particular interest. We discuss the onset of discontinuity in the thermodynamic variables, fluctuations, and evolution of zeros of the canonical and grand canonical partition functions in the complex plane. The behavior of the Invariant Correlational Entropy is also studied in the transitional region of interest. The change in the character of the phase transition due to the presence of magnetic field is discussed along with studies of superconducting thermodynamics.

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arXiv2006

Applications of Continuum Shell Model

The nuclear many-body problem at the limits of stability is considered in the framework of the Continuum Shell Model that allows a unified description of intrinsic structure and reactions. Technical details behind the method are highlighted and practical applications combining the reaction and structure pictures are presented.

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arXiv2005

Continuum Shell Model

The Continuum Shell Model is an old but recently revived method that traverses the boundary between nuclear many-body structure and nuclear reactions. The method is based on the non-Hermitian energy-dependent effective Hamiltonian. The formalism, interpretation of solutions and practical implementation of calculations are discussed in detail. The results of the traditional shell model are fully reproduced for bound states; resonance parameters and cross section calculations are presented for decaying states. Particular attention is given to one- and two-nucleon reaction channels including sequential and direct two-body decay modes. New calculations of reaction cross sections and comparisons with experimental data for helium and oxygen isotope chains are presented.

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arXiv2004

The nuclear pairing problem: new perspectives

The overview of the Exact Pairing technique based on the quasispin symmetry is presented. Extensions of this method are discussed in relation to mean field, quadrupole collectivity, electromagnetic transitions, and many-body level density. Realistic calculations compared with experimental data are used to support the methods as well as to emphasize the manifestations of pairing correlations in nuclear many-body systems.

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arXiv2004

Nuclear pairing and Coriolis effects in proton emitters

We introduce a Hartree-Fock-Bogoliubov mean-field approach to treat the problem of proton emission from a deformed nucleus. By substituting a rigid rotor in a particle-rotor-model with a mean-field we obtain a better description of experimental data in $^{141}$Ho. The approach also elucidates the softening of kinematic coupling between particle and collective rotation, the Coriolis attenuation problem.

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arXiv2004

Many-Body Physics on the Border of Nuclear Stability

A brief overview is given of the Continuum Shell Model, a novel approach that extends the traditional nuclear shell model into the domain of unstable nuclei and nuclear reactions. While some of the theoretical aspects, such as role and treatment of one- and two-nucleon continuum states, are discussed more in detail, a special emphasis is made on relation to observed nuclear properties, including definitions of the decay widths and their relation to the cross sections, especially in the cases of non-exponential decay. For the chain of He isotopes we demonstrate the agreement of theoretical results with recent experimental data. We show how the interplay of internal collectivity and coherent coupling to continuum gives rise to the universal mechanism of creating pigmy giant resonances.

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arXiv2004

Discrete and continuum spectra in the unified shell model approach

A new version of the nuclear shell model unifies the consideration of the discrete spectrum, where the results agree with the standard shell model, and continuum. The ingredients of the method are the non-Hermitian effective Hamiltonian, energy-dependent one-body and two-body decay amplitudes, and self-consistent treatment of thresholds. The results for helium and oxygen isotope chains well reproduce the data.

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arXiv2004

Super-Radiance: From Nuclear Physics to Pentaquarks

The phenomenon of super-radiance in quantum optics predicted by Dicke 50 years ago and observed experimentally has its counterparts in many-body systems on the borderline between discrete spectrum and continuum. The interaction of overlapping resonances through the continuum leads to the redistribution of widths and creation of broad super-radiant states and long-lived compound states. We explain the physics of super-radiance and discuss applications to weakly bound nuclei, giant resonances and widths of exotic baryons.

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arXiv2003

Nuclear Structure, Random Interactions and Mesoscopic Physics

Standard concepts of nuclear physics explaining the systematics of ground state spins in nuclei by the presence of specific coherent terms in the nucleon-nucleon interaction were put in doubt by the observation that these systematics can be reproduced with high probability by randomly chosen rotationally invariant interactions. We review the recent development in this area, along with new original results of the authors. The self-organizing role of geometry in a finite mesoscopic system explains the main observed features in terms of the created mean field and correlations that are considered in analogy to the random phase approximation.

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arXiv2003

Invariant correlational entropy as a signature of quantum phase transitions in nuclei

We study phase transformations in finite nuclei as a function of interaction parameters. The signature of a transition is given by invariant correlational entropy that reflects the sensitivity of an individual many-body state to changes of external parameters; peaks in this quantity indicate the critical regions. This approach is able to reveal the pairing phase transition, identify the isovector and isoscalar pairing regions and determine the role of other interactions. We show the examples of the phase diagram in the parameter space.

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arXiv2003

Exploring quantum dynamics in an open many-body system: Transition to superradiance

We study the dynamics of a complex open quantum many-body system. The coupling to external degrees of freedom can be viewed as a coupling to a radiation field, to continuum states or to a measuring apparatus. This perturbation is treated in terms of an effective non-Hermitian Hamiltonian. The influence of such coupling on the properties of the many-body dynamics is discussed, with emphasis on new effects related to dynamical segregation of fast and slow decays and the phase transition to Dicke superradiance. Relations to quantum optics, continuum shell model, theory of measurement, quantum chaos, percolation theory, and to quantum reactions are stressed.

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arXiv2003

Nuclear pairing: new perspectives

Nuclear pairing correlations are known to play an important role in various single-particle and collective aspects of nuclear structure. After the first idea by A. Bohr, B. Mottelson and D. Pines on similarity of nuclear pairing to electron superconductivity, S.T. Belyaev gave a thorough analysis of the manifestations of pairing in complex nuclei. The current revival of interest in nuclear pairing is connected to the shift of modern nuclear physics towards nuclei far from stability; many loosely bound nuclei are particle-stable only due to the pairing. The theoretical methods borrowed from macroscopic superconductivity turn out to be insufficient for finite systems as nuclei, in particular for the cases of weak pairing and proximity of continuum states. We suggest a simple numerical procedure of exact solution of the nuclear pairing problem and discuss the physical features of this complete solution. We show also how the continuum states can be naturally included in the consideration bridging the gap between the structure and reactions. The path from coherent pairing to chaos and thermalization and perspectives of new theoretical approaches based on the full solution of pairing are discussed.

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arXiv2003

The role of E1-E2 interplay in multiphonon Coulomb excitation

In this work we study the problem of a charged particle, bound in a harmonic-oscillator potential, being excited by the Coulomb field from a fast charged projectile. Based on a classical solution to the problem and using the squeezed-state formalism we are able to treat exactly both dipole and quadrupole Coulomb field components. Addressing various transition amplitudes and processes of multiphonon excitation we study different aspects resulting from the interplay between E1 and E2 fields, ranging from classical dynamic polarization effects to questions of quantum interference. We compare exact calculations with approximate methods. Results of this work and the formalism we present can be useful in studies of nuclear reaction physics and in atomic stopping theory.

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arXiv2003

Order generated by random many-body dynamics

New features related to collective properties generated in the systems driven by random dynamics are observed and their implications for further understanding of interplay between coherence and chaos are discussed.

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arXiv2002

Effective non-Hermitian Hamiltonian and continuum shell model

The intrinsic dynamics of a system with open decay channels is described by an effective non-Hermitian Hamiltonian which at the same time allows one to find the external dynamics, - reaction cross sections. We discuss ways of incorporating this approach into the shell model context. Several examples of increasing complexity, from schematic models to realistic nuclear calculations (chain of oxygen isotopes), are presented. The approach is capable of describing a multitude of phenomena in a unified way combining physics of structure and reactions. Self-consistency of calculations and threshold energy dependence of the coupling to the continuum are crucial for the description of loosely bound states.

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arXiv2001

A soft mode in a finite Fermi-system: anharmonic effects near the instability point

We consider a finite Fermi-system where the residual interactions create a soft mode of the excitation spectrum. Because of the large vibrational amplitude, the standard random phase approximation does not work in this situation. We develop a regular method for constructing the anharmonic potential and illustrate the application of the formalism by a simple model.

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