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At least 55 records · Page 3

Role of quasiparticle structure in α decay of superheavy nuclei

Here, we use the superfluid tunneling model (STM) to calculate the half-lives of α decays of odd-A and odd-odd superheavy nuclei (SHN) with Z ≥ 100 and A ≥ 250. The experimental data are reproduced to accuracies comparable to other contemporary models of the α decay of the SHN. We then apply the STM to examine the influence of the quasiparticle structure on the properties of the chains of α decays arising from odd- A SHN. Using representative calculations of the one-quasi-particle structure of odd-Z and odd-N SHN, we illustrate the important role played by high-Ω orbitals in defining the observed characteristics of the α-decay chains. We point out sources of possible ambiguity that may arise due to the quasiparticle structure when assigning α-decay chains to specific SHN.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗

α decay of high-spin isomers in N = 84 isotones

The superfluid tunneling model is applied to the calculation of half-lives of the observed α decays in N = 84 isotones. Here the results of our calculations are compared to experimental data on the ground-state α decays along the isotonic chain from 144 Nd to 159 Re. Good agreement is found. The α decays of the known high-spin isomers in 155 Lu, 156 Hf, 157 Ta, and 158 W are also well reproduced, once a reduction in the pairing strength is taken into account. This includes reproduction of the main features of the recently observed fine structure from 155 Lu(25/2 - ) and 156 Hf(8 + ). Predictions for the α-decay fine structure of 157 Ta(25/2 - ) and 158 W(8 + ) high-spin isomers are presented.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗

Nuclear level densities in {sup 208}Bi and {sup 209}Po from the neutron spectra in the (p, n) reactions on {sup 208}Pb and {sup 209}Bi nuclei

The spectra of neutrons from the (p, n) reactions on the {sup 208}Pb and {sup 209}Bi nuclei were measured in the proton-energy range 8-11 MeV. These measurements were performed by using a time-of-flight spectrometer of fast neutrons on the basis of the pulsed tandem accelerator EGP-15 of the Institute of Physics and Power Engineering (Obninsk, Russian Federation). A high resolution and stability of the time-of-flight spectrometermade it possible to identify reliably low-lying discrete levels alongwith the continuum section of the neutron spectra. The measured data were analyzed on the basis of the statistical equilibrium and preequilibrium models of nuclear reactions. The respective calculations were performed by using the precise formalism of Hauser-Feshbach statistical theory together with the generalized model of a superfluid nucleus and the back-shifted Fermi gas model for the nuclear-level density. The nuclear-level densities in {sup 208}Bi and {sup 209}Po were determined along with their energy dependencies and model parameters. Our results are discussed together with available experimental data and recommendations of model systematics.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗

Mean-field theory for the Mott-insulator-paired-superfluid phase transition in the two-species Bose-Hubbard model

The standard mean-field theory for the Mott-insulator-superfluid phase transition is not sufficient to describe the Mott-insulator-paired-superfluid phase transition. Therefore, by restricting the two-species Bose-Hubbard Hamiltonian to the subspace of paired particles, and using perturbation theory, here we derive an analytic mean-field expression for the Mott-insulator-paired-superfluid transition boundary.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗

Dynamical critical exponent of the Jaynes-Cummings-Hubbard model

An array of high-Q electromagnetic resonators coupled to qubits gives rise to the Jaynes-Cummings-Hubbard model describing a superfluid to Mott-insulator transition of lattice polaritons. From mean-field and strong-coupling expansions, the critical properties of the model are expected to be identical to the scalar Bose-Hubbard model. A recent Monte Carlo study of the superfluid density on the square lattice suggested that this does not hold for the fixed-density transition through the Mott lobe tip. Instead, mean-field behavior with a dynamical critical exponent z=2 was found. We perform large-scale quantum Monte Carlo simulations to investigate the critical behavior of the superfluid density and the compressibility. We find z=1 at the tip of the insulating lobe. Hence the transition falls in the three-dimensional XY universality class, analogous to the Bose-Hubbard model.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗

Superfluid-insulator transitions in attractive Bose-Hubbard model with three-body constraint

By means of the method of the effective potential, the phase transitions from the Mott insulating state to either the atomic or the dimer superfluid state in the three-body constrained attractive Bose lattice gas are analyzed. Because of the appearance of the Feshbach resonance coupling between the two kinds of order parameters in the derived Ginzburg-Landau function, it is found that the continuous Mott-insulator-superfluid transitions can be preempted by first-order ones. Since the employed approach can provide accurate predictions of phase boundaries in the strong coupling limit, where the dimer superfluid phase can emerge, our work sheds light on the search of this phase in real ultracold Bose gases in optical lattices.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗

Decoupling of superfluid and normal modes in pulsating neutron stars

We show that equations governing pulsations of superfluid neutron stars can be split into two sets of weakly coupled equations, one describing the superfluid modes and another one, the normal modes. The coupling parameter s is small, |s|{approx}0.01-0.05, for realistic equations of state. Already an approximation s=0 is sufficient to calculate the pulsation spectrum within the accuracy of a few percent. Our results indicate, in particular, that emission of gravitational waves from superfluid pulsation modes is suppressed in comparison to that from normal modes. The proposed approach allows to drastically simplify modeling of pulsations of superfluid neutron stars.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗

Viscosity of liquid {sup 4}He and quantum of circulation: Are they related?

In the vicinity of the superfluid transition in liquid {sup 4}He, we explore the relation between two apparently unrelated physical quantities—the kinematic viscosity, ν, in the normal state and the quantum of circulation, κ, in the superfluid state. The model developed here leads to the simple relationship ν ≈ κ/6, and links the classical and quantum flow properties of liquid {sup 4}He. We critically examine available data relevant to this relation and find that the prediction holds well at the saturated vapor pressure. Additionally, we predict the kinematic viscosity for liquid {sup 4}He along the λ-line at negative pressures.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗

Superfluid-to-Mott-insulator transition in the one-dimensional Bose-Hubbard model for arbitrary integer filling factors

We study the quantum phase transition between the superfluid and the Mott insulator in the one-dimensional (1D) Bose-Hubbard model. Using the time-evolving block-decimation method, we numerically calculate the tunneling splitting of two macroscopically distinct states with different winding numbers. From the scaling of the tunneling splitting with respect to the system size, we determine the critical point of the superfluid-to-Mott-insulator transition for arbitrary integer filling factors. We find that the critical values versus the filling factor in 1D, 2D, and 3D are well approximated by a simple analytical function. We also discuss the condition for determining the transition point from a perspective of the instanton method.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗

Quark-gluon plasma (Selected Topics)

Introductory lectures to the theory of (strongly interacting) quark-gluon plasma given at the Winter School of Physics of ITEP (Moscow, February 2010). We emphasize theoretical issues highlighted by the discovery of the low viscosity of the plasma. The topics include relativistic hydrodynamics, manifestations of chiral anomaly in hydrodynamics, superfluidity, relativistic superfluid hydrodynamics, effective stringy scalars, holographic models of Yang-Mills theories.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗

Temperature effects in pulsating superfluid neutron stars

We study the effects of finite stellar temperatures on the oscillations of superfluid neutron stars. The importance of these effects is illustrated with a simple example of a radially pulsating general relativistic star. Two main effects are taken into account: (i) temperature dependence of the entrainment matrix and (ii) the variation of the size of superfluid region with temperature. Four models are considered, which include either one or both of these two effects. Pulsation spectra are calculated for these models, and asymptotes for eigenfrequencies at temperatures close to critical temperature of neutron superfluidity are derived. It is demonstrated that models that allow for the temperature effect (ii) but disregard the effect (i), yield unrealistic results. Eigenfunctions for the normal- and superfluid-type pulsations are analyzed. It is shown that superfluid pulsation modes practically do not appear at the neutron-star surface and, therefore, can hardly be observed by measuring the modulation of the electromagnetic radiation from the star. The e-folding times for damping of pulsations due to the shear viscosity and nonequilibrium modified Urca processes are calculated and their asymptotes at temperatures close to the neutron critical temperature, are obtained. It is demonstrated that superfluid pulsation modes are damped by 1-3 orders of magnitude faster than normal modes.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗

B phase with polar distortion in superfluid {sup 3}He in “ordered” aerogel

The properties of the low-temperature superfluid phase of {sup 3}He in “nematically ordered” aerogel in which strands are almost parallel to one another are investigated by nuclear magnetic resonance methods. Such a strong anisotropy of the aerogel affects the phase diagram of {sup 3}He and the structure of superfluid phases. A theoretical model of the B phase with polar distortion is developed. It is shown that this model successfully describes the observed properties of the low-temperature phase.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗

Plaquette valence bond state in the spin-$\frac{1}{2}$ J 1 –J 2 XY model on the square lattice

Here, we studied the ground-state phase diagram of spin-1/2 J 1 –J 2 XY model on the square lattice with first-neighbor J 1 and second-neighbor J 2 antiferromagnetic interactions using both infinite density matrix renormalization group (iDMRG) and DMRG approaches. We show that a plaquette valence bond phase is realized in an intermediate region 0.50 ≤ J 2 /J 1 < 0.54 between a Néel magnetic ordered phase at J 2 /J 1 < 0.50 and a stripy magnetic ordered phase at J 2 /J 1 ≥ 0.54 . The plaquette valence bond phase is characterized by finite dimer orders in both the horizontal and vertical directions. Contrary to the spin-1/2 J 1 –J 2 Heisenberg model, we do not find numerical evidence for a quantum spin liquid phase in the J 1 –J 2 XY model.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗

ON THE EXISTENCE OF SHOCKS IN IRRADIATED EXOPLANETARY ATMOSPHERES

Supersonic flows are expected to exist in the atmospheres of irradiated exoplanets, but the question of whether shocks develop lingers. Specifically, it reduces to whether continuous flow in a closed loop may become supersonic and if some portions of the supersonic flow steepen into shocks. We first demonstrate that continuous, supersonic flow may exist in two flavors: isentropic and non-isentropic, with shocks being included in the latter class of solutions. Supersonic flow is a necessary but insufficient condition for shocks to develop. The development of a shock requires the characteristics of neighboring points in a flow to intersect. We demonstrate that the intersection of characteristics may be quantified via the knowledge of the Mach number. Finally, we examine three-dimensional simulations of hot Jovian atmospheres and demonstrate that shock formation is expected to occur mostly on the dayside hemisphere, upstream of the substellar point, because the enhanced temperatures near the substellar point provide a natural pressure barrier for the returning flow. Understanding the role of shocks in irradiated exoplanetary atmospheres is relevant to correctly modeling observables such as the peak offsets of infrared phase curves.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗

Generalized equation of state for cold superfluid neutron stars

Mature neutron stars are expected to contain various kinds of superfluids in their interiors. Modeling such stars requires the knowledge of the mutual entrainment couplings between the different condensates. We present a unified equation of state describing the different regions of a neutron star with superfluid neutrons and superconducting protons in its core.

79 ASTRONOMY AND ASTROPHYSICS↗

Modeling and experimental characterization of stepped and v-shaped (311) defects in silicon

We propose an atomistic model to describe extended (311) defects in silicon. It is based on the combination of interstitial and bond defect chains. The model is able to accurately reproduce not only planar (311) defects but also defect structures that show steps, bends, or both. We use molecular dynamics techniques to show that these interstitial and bond defect chains spontaneously transform into extended (311) defects. Simulations are validated by comparing with precise experimental measurements on actual (311) defects. The excellent agreement between the simulated and experimentally derived structures, regarding individual atomic positions and shape of the distinct structural (311) defect units, provides strong evidence for the robustness of the proposed model.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗