Evidence for second-phonon nuclear wobbling

The nucleus 163Lu has been populated through the reaction 139La(29Si,5n) with a beam energy of 157 MeV. Three triaxial, strongly deformed (TSD) bands have been observed with very similar rotational properties. The first excited TSD band has earlier been assigned as a one-phonon wobbling excitation built on the lowest-lying (yrast) TSD band. The large B(E2)(out)/B(E2)(in) value obtainable for one of four observed transitions between the second and first excited TSD bands is in good agreement with particle-rotor calculations for a two-phonon wobbling excitation.     

Dynamical moments of inertia associated with wobbling motion in the triaxial superdeformed nucleus

The three moments of inertia associated with the wobbling mode built on the triaxial superdeformed states in the Lu-Hf region are investigated by means of the cranked shell model plus random-phase approximation to the configurations with aligned quasiparticle(s). The result indicates that it is crucial to take into account the direct contribution to the moments of inertia from the aligned quasiparticle(s) so as to realize in positive- shapes.Received: 17 October 2002, Published online: 2 March 2004PACS: 21.10.Re Collective levels - 21.60.Jz Hartree-Fock and random-phase approximations     

Classical nuclear motion in quantum transport

An ab initio quantum-classical mixed scheme for the time evolution of electrode-device-electrode systems is introduced to study nuclear dynamics in quantum transport. Two model systems are discussed to illustrate the method. Our results provide the first example of current-induced molecular desorption as obtained from a full time-dependent approach and suggest the use of ac biases as a way to tailor electromigration. They also show the importance of nonadiabatic effects for ultrafast phenomena in nanodevices.     

Particle motion in guide potentials and the collective nuclear motion

The motion of a particle which is constrained by a guide potential to move on a curve is studied in the framework of the Generator Coordinate Method (GCM). In the limit of narrow guide potentials a differential equation for the wave function of the constrained motion is obtained which differs from the corresponding Schrödinger equation by an additional potential. This additional potential is due to the embedding of the curve in the space and depends on the form of the guide potential and on the curvature of the curve. Nonadiabatic transitions in the constrained motion are possible for finite widths of the guide potential. The coupling terms are given explicitly and it is shown that an adiabatic limit exists. Since the GCM can equally well describe the collective motion of nuclei, some insight into the more complicated problem of collective motion is obtained from its analogies to the studied problem of constrained particle motion: The collective motion of a nucleus can be considered as the motion of a particle with variable mass along a curve in a guide potential which is given by the interaction potential between the nucleons. It is shown that Schrödinger's quantized kinetic energy is correctly used in the cranking model and that the additional potential terms mentioned above are included there by the definition of the collective potential energy. Approximations to the idealized GCM used here are discussed and the connection with the method of Born, Oppenheimer and Villars is indicated.     

Evidence for wobbling excitation in 161Lu

High-spin states in ¹⁶¹Lu were investigated using the EUROBALL spectrometer. A previously known triaxial superdeformed band has been extended to higher spins and a new band with similar characteristics has been discovered. Comparison to systematically occurring wobbling bands in Lu isotopes strongly suggests that these two bands represent the n_w = 0 and 1 wobbling excitations in ¹⁶¹Lu.     

Dissipation of nuclear collective motion

The collective transport theory provides a framework for understanding damped collective motion. The irreversibility of collective motion is traced to the fact that the nucleus is an open system. The finite lifetime of single-particle excitations causes the relaxation of the nuclear collective response. Both vibrational states and damped heavy-ion collisions can be understood quantitatively by computations without free parameters.     

Anharmonicity induced thermal modulation in stressed graphene

Thermal properties are essentially decided by atomic geometry and thus stress is the most direct way for manipulating. In this paper, we investigate stress modulation of thermal conductivity of graphene by molecular dynamics simulations and discuss the underlying microscopic mechanism. It is found that thermal conductivity of flexural-free graphene increases with compression and decreases with strain, while thermal conductivity of flexural-included graphene decreases with both compression and strain. Such difference in thermal behavior originates from the changes in the anharmonicity of the interatomic potential, where the wrinkle scattering is responsible for the thermal conductivity diminishment in flexural-included graphene under strain. By comparing the results obtained from the Tersoff and AIREBO potentials, it is revealed that the degree of the symmetry of interatomic potential determines the thermal conductivity variation of graphene. Our results indicate that the symmetry of interatomic potential should be taken into careful consideration in constructing the lattice model of graphene.     

Microscopic nuclear collective motion studied by classical equations of motion

The time-dependent variation principle is used to obtain generally non-canonical equations of motion from any class of quantum states which are parameterized by a set of continuous complex quantities. A class of states is presented whose associated classical dynamics is described by the five collective quadrupole degrees of freedom. Information about the classical dynamics of the system can be obtained from the non-canonical equations by finding physically interesting quantities which are coordinate independent and which characterize the low-energy collective motion. Approximate collective hamiltonians, of either a Bohr-Mottelson or an IBM type, can be found by insisting that the interesting physical quantities which describe the low-energy classical behavior of the many-body system are the same as those describing the classical behavior of the system given by the collective hamiltonian. The method is applied to two simple schematic models, one vibrational and one rotational, and IBM hamiltonians are obtained.     

Anharmonicity in aluminum hydride at high pressures

Aluminum hydride has been predicted to be a superconductor with a transition temperature of 24 K at 110 GPa, in disagreement with the experimental observation. In this work, it is shown that the bulk of the electron–phonon coupling can be associated with modes that are highly anharmonic according to frozen phonon calculations. This large anharmonicity could partially explain the origin of the disagreement between previous predictions and experiments.     

Chaotic motion and collective nuclear rotation

The regular and chaotic motion in the classical and quantal versions of a model hamiltonian with two degrees of freedom are investigated. This model contains a parameter which is identified with a conserved quantum number, the total spin. In particular, transitions between states differing in spin by one unit are studied. The transition is strongly collective for regular motion, and collectivity is destroyed with increasing stochasticity of the model.     

Boson description of nuclear collective motion

Nuclear system interacting via quadrupole and octupole particle-hole forces is studied by the boson expansion technique. Energy spectra of the negative parity yrast band and the ground state band are calculated and compared with experiment for¹⁰⁰Ru,¹¹²Cd,¹⁵⁰Sm and¹⁵⁰Gd. ExperimentalB(E1)/B(E2) ratios show strong hindrance for E1 transitions, and are used to deduce the static polarizability of E1 transitions.     

Boson description of nuclear collective motion

Nuclear system with octupole-octupole interaction is studied by means of the boson expansion method. Expressions of the fourth-order collective Hamiltonian and third-order octupole moment operator are derived. For¹¹²Cd and¹⁴⁸Sm, characteristics of octupole vibrational spectra are discussed in comparison with the quadrupole vibration.     

Generalized cranking model for collective nuclear motion

The Inglis cranking model is generalized to take into account effects of any velocity dependence present in the single-particle potential and the reaction of the pairing field to the collective motion. The generalized model is applied to translations, rotations and some special types of vibrations. Some of our results and our numerical calculations are obtained with a harmonic-oscillator single-particle potential. Unlike the inertia calculated with the Inglis cranking model, the inertia calculated with the generalized cranking model is independent of the effective mass and approaches the irrotational value in the limit of large pairing.     

Damping of nuclear collective and single-particle motion

The collective motion of the nuclear system is studied. In the independent-particle model, the motion is completely reversible. The neglected residual interactions couple the ph states to more complicated states. This coupling is taken into account by the optical model potential assuming independent decay of particle and hole states. Irreversibility is thereby introduced and damping of collective motion described in terms of the widths of the ph states. The validity of the assumption of independent decay is discussed. It is argued that spreading widths to low-frequency collective states are not part of the optical model, and do not contribute to damping of collective motion.     

Anharmonicity and superconductivity in Ba0.6K0.4BiO3

The temperature dependence of the x-ray absorption spectra above the L ₃ absorption edge of bismuth in the superconducting oxide Ba_0.6K_0.4BiO₃ are investigated. It is found that the local structure is different from the simple cubic structure indicated by x-ray and neutron-diffraction data. It is shown that the oxygen atoms move in an anharmonic double-well potential arising as a result of the existence of two nonequivalent types of octahedral environments of bismuth. Vibrations in such potential modulate the Bi-O bond lengths at the low frequency of the rotational (“tilting” type) mode of the oxygen octahedra and thus give rise to a strong electron-phonon interaction, which explains the quite high superconducting transition temperatures T _c ∼30 K. Pis’ma Zh. éksp. Teor. Fiz. 67, No. 12, 977–982 (25 June 1998)     

Interatomic electronic decay driven by nuclear motion

The interatomic electronic decay after inner-valence ionization of a neon atom by a single photon in a neon-helium dimer is investigated. The excited neon atom relaxes via interatomic Coulombic decay and the excess energy is transferred to the helium atom and ionizes it. We show that the decay process is only possible if the dimer's bond stretches up to 6.2 ?, i.e., to more than twice the equilibrium interatomic distance of the neutral dimer. Thus, it is demonstrated that the electronic decay, taking place at such long distances, is driven by the nuclear motion.     

Molecular motion in liquid benzene by nuclear magnetic resonance

The proton spin-lattice relaxation time, T ₁, has been measured for a series of mixtures of benzene in perdeuterobenzene for the liquid in equilibrium with its vapour over the temperature range from below the normal freezing point up to the critical temperature. The two contributions to T ₁ due to interactions within the molecule (T _{1 intra}) and between molecules (T _{1 inter}) have been separated and are found to be very different in magnitude and in variation with temperature. The variation and magnitude of T _{1 inter} correlates well with other translational motion dependent properties such as self diffusion and viscosity. The correlation of T _{1 intra} with other re-orientation dependent properties such as deuteron T ₁ and Rayleigh scattering is poor.

The observed variation in T _{1 intra} and in particular the broad maximum at higher temperatures is then interpreted as due to a combination of dipolar and spin-rotation effects. This interpretation results in good agreement between the activation energies for re-orientational molecular motion deduced from proton T ₁ and deuteron T ₁. It supports the Hubbard theory for the relation between the dipolar and spin-rotation correlation times τ_d and τ_sr. It gives a rough value, 3·8 kc/s, for the spin-rotation interaction constant for protons in benzene. Reasonable values for τ_d and τ_sr are predicted and for all temperatures τ_sr < τ_d as expected.

There is clearly a considerable difference between the re-orientational and translational motion of the molecules in liquid benzene but the exact nature of the difference cannot be elucidated.     

Anharmonicity and the mean square displacement in perovskite-type crystals

The theoretical expression is derived for the free energy (FE) of perovskite- type crystals (PTC) on the basis of the model of the crystal with several anharmoniously vibrating atoms in a unite cell. The expression for the mean square displacement (MSD) of any atom in (PTC) is also obtained as a function of potential coefficients of the Hamiltonian of a crystal.The evaluation of the numerical values of potential coefficients of the Hamiltonians of BaTiO₃ and SrTiO₃ on the basis of the available experimental data is made.