Quasiparticle Configurations Research Articles

Gamma bands in doubly odd rhenium and iridium nuclei

Structure of the |K ± 2| bands in doubly-odd nuclei belonging to the transitional deformation region at A∼190 is discussed. Relation of these quasi gamma-bands with the non-axial deformation of the parent two- quasiparticle configurations is studied. Using available experimental information, new tentative |K ± 2| bands are proposed in 188 Re, and 192,194 Ir nuclei. Coexistence of two-quasiparticle states with different deformation modes is considered in the case of 188 Re and 194 Ir.

Electromagnetic Transition Strengths and New Insight into the Chirality in 106Ag

Excited states in 106Ag are populated through the heavy-ion fusion evaporation reaction 100Mo(11 B,5n)106 Ag at a beam energy of 60 MeV. Lifetimes are measured for transitions of the two negative-parity rotational bands in the nucleus 106 Ag. The reduced transition probabilities show a great difference between the two bands. The staggering of the B(M1) and B(M1)/B(E2) values with spin are not observed. The bands are identified to be built on two distinct quasiparticle configurations. These results are contrary to an earlier suggestion that the pair of bands in 106 Ag are chiral doublet bands.

Exploring the Origin of Nearly Degenerate Doublet Bands inAg106

The electromagnetic transition probabilities of the excited levels for the two nearly degenerate bands of $^{106}$Ag have been measured using the Doppler Shift Attenuation Method. A comparison with the calculated values using triaxial projected shell model approach indicates that these bands originate from two different quasi-particle configurations but constructed from the same mean-field deformation.

Quantum droplets of electrons and holes

Interacting many-body systems are characterized by stable configurations of objects--ranging from elementary particles to cosmological formations--that also act as building blocks for more complicated structures. It is often possible to incorporate interactions in theoretical treatments of crystalline solids by introducing suitable quasiparticles that have an effective mass, spin or charge which in turn affects the material's conductivity, optical response or phase transitions. Additional quasiparticle interactions may also create strongly correlated configurations yielding new macroscopic phenomena, such as the emergence of a Mott insulator, superconductivity or the pseudogap phase of high-temperature superconductors. In semiconductors, a conduction-band electron attracts a valence-band hole (electronic vacancy) to create a bound pair, known as an exciton, which is yet another quasiparticle. Two excitons may also bind together to give molecules, often referred to as biexcitons, and even polyexcitons may exist. In indirect-gap semiconductors such as germanium or silicon, a thermodynamic phase transition may produce electron-hole droplets whose diameter can approach the micrometre range. In direct-gap semiconductors such as gallium arsenide, the exciton lifetime is too short for such a thermodynamic process. Instead, different quasiparticle configurations are stabilized dominantly by many-body interactions, not by thermalization. The resulting non-equilibrium quantum kinetics is so complicated that stable aggregates containing three or more Coulomb-correlated electron-hole pairs remain mostly unexplored. Here we study such complex aggregates and identify a new stable configuration of charged particles that we call a quantum droplet. This configuration exists in a plasma and exhibits quantization owing to its small size. It is charge neutral and contains a small number of particles with a pair-correlation function that is characteristic of a liquid. We present experimental and theoretical evidence for the existence of quantum droplets in an electron-hole plasma created in a gallium arsenide quantum well by ultrashort optical pulses.

The study of energy bands in neutron-deficient 123,125Ce nuclei

Theoretical investigation of energy bands of 123,125 Ce is performed by applying the projected shell model approach. The present calculation is viewed to reproduce the known experimental data of these nuclei, qualitatively. Not only this, the configurations of diverse bands are also obtained by an analysis of the band diagrams. The low-lying states of the bands are seen to arise from one quasiparticle configuration whereas the high-spin states arise from multi-quasiparticle configurations. The signature partner band of negative parity band is predicted in 123 Ce. The ground state bands and relative energies of some bands with respect to them are predicted in 123,125 Ce that can be helpful for future experiments.

Particle-number conserving analysis for the 2-quasiparticle and high-K multi-quasiparticle states in doubly-odd 174,176Lu

Two-quasiparticle bands and low-lying excited high-K four-, six-, and eight-quasiparticle bands in the doubly-odd174,176 Lu are analyzed by using the cranked shell model (CSM) with the pairing correlations treated by a particle-number conserving (PNC) method, in which the blocking effects are taken into account exactly. The proton and neutron Nilsson level schemes for174,176, Lu are taken from the adjacent odd-A Lu and Hf isotopes, which are adopted to reproduce the experimental bandhead energies of the one-quasiproton and one-quasineutron bands of these odd-A Lu and Hf nuclei, respectively. Once the quasiparticle configurations are determined, the experimental bandhead energies and the moments of inertia of these two- and multi-quasiparticle bands are well reproduced by PNC-CSM calculations. The Coriolis mixing of the low-K (K=|Ω1−Ω2|) two-quasiparticle band of the Gallagher-Moszkowski doublet with one nucleon in the Ω=1/2 orbital is analyzed.

Small quadrupole deformation for the dipole bands in112In

High spin states in $^{112}$In were investigated using $^{100}$Mo($^{16}$O, p3n) reaction at 80 MeV. The excited level have been observed up to 5.6 MeV excitation energy and spin $\sim$ 20$\hbar$ with the level scheme showing three dipole bands. The polarization and lifetime measurements were carried out for the dipole bands. Tilted axis cranking model calculations were performed for different quasi-particle configurations of this doubly odd nucleus. Comparison of the calculations of the model with the B(M1) transition strengths of the positive and negative parity bands firmly established their configurations.

Diverse collective excitations in159Erup to high spin

A spectroscopic investigation of the gamma decays from excited states in Er-159 has been performed to study the changing structural properties exhibited as ultrahigh spins (I > 60 (h) over bar) are approached. The nucleus of Er-159 was populated by the reaction Cd-116(Ca-48, 5n gamma) at a beam energy of 215 MeV, and the resulting gamma decays were studied using the Gammasphere spectrometer. New rotational bands and extensions to existing sequences were observed, which are discussed in terms of the cranked shell model, revealing a diverse range of quasiparticle configurations. At spins around 50 (h) over bar, there is evidence for a change from dominant prolate collective motion at the yrast line to oblate non-collective structures via the mechanism of band termination. A possible strongly deformed triaxial band occurs at these high spins, which indicates collectivity beyond 50 (h) over bar. The high-spin data are interpreted within the framework of cranked Nilsson-Strutinsky calculations. (Less)

Decay Schemes of Three-Quasiparticle Isomers in 119,121Sb and 121,123I

The gamma-ray decay of isomers in near spherical Sb-119,Sb-121 and transitional I-121,I-123 have been studied using gamma-ray spectroscopy with Li-7 pulsed beams delivered from 14 UD Pelletron accelerator at the Australian National University. Isomers of J(pi) = 25/2(+), 19/2(-) and 21/2(-) with half-lives of a few nanoseconds to hundreds microseconds have been observed at 2.4 < E-x < 2.9 MeV. Decay schemes of the isomers in these nuclei are presented and their properties are interpreted in terms of three quasiparticle configurations based on the two-neutron (h(11/2))(2) 10(+) and (d(3/2)h(11/2)) 7(-) states in even-A Sn isotones coupled to an extra proton.

Structure changes inEr160from low to ultrahigh spin

A spectroscopic investigation of the $\ensuremath{\gamma}$ decays from excited states in $^{160}\mathrm{Er}$ has been performed in order to study the changing structural properties exhibited from low spin up toward ultrahigh spin ($I~60\ensuremath{\hbar}$). The nucleus $^{160}\mathrm{Er}$ was populated by the reaction $^{116}\mathrm{Cd}$($^{48}\mathrm{Ca}$,4$n$$\ensuremath{\gamma}$) at a beam energy of 215 MeV, and resulting $\ensuremath{\gamma}$ decays were studied using the Gammasphere spectrometer. New rotational structures and extensions to existing bands were observed, revealing a diverse range of quasiparticle configurations, which are discussed in terms of the cranked shell model. At spins around 50$\ensuremath{\hbar}$ there is evidence for oblate states close to the yrast line. Three rotational bands that have the characteristics of strongly deformed triaxial structures are observed, marking a return to collectivity at even higher spin. The high-spin data are interpreted within the framework of cranked Nilsson-Strutinsky calculations.

Energy levels in 141Nd from fusion evaporation study

The nucleus 141Nd has been investigated using two different heavy ion fusion evaporation reactions, namely 130Te(16O, 5n) at Elab = 80–85 MeV and 124Sn(24Mg,α3n) at Elab = 107 MeV, and the de-exciting gamma rays were detected using HPGe(GDA) and Clover detector (INGA) arrays, respectively. More than 30 new gamma transitions have been placed in the proposed level scheme, which has been extended up to 9 MeV excitation energy and spin parity values of 45/2−. Spin parity assignments have been made to most of the newly proposed levels. The structure up to approximately 6 MeV excitation energy and spin 31/2− can be explained using the shell model in a truncated basis. The levels with higher spin/energy cannot be described using even the full 50–82 shell, indicating the influence of neutron excitations across the shell gap in this mass region. One M1 band that has been observed is proposed to have a five quasiparticle configuration.

Quenching of the E1 strength in 149Nd

Lifetime measurements of excited states in 149Nd have been performed using the advanced time-delayed \( \beta\)\( \gamma\)\( \gamma\)(t) method. Half-lives of 14 excited states in 149Nd have been determined for the first time or measured with higher precision. Twelve new \( \gamma\) -lines and 5 new levels have been introduced into the decay scheme of 149Pr based on results of the \( \gamma\)\( \gamma\) coincidence measurements. Reduced transition probabilities have been determined for 40 \( \gamma\) -transitions in 149Nd . Configuration assignments for 6 rotational bands in 149Nd are proposed. Enhanced E1 transitions indicate that the ground-state band and the band built on the 332.9keV level constitute a pair of the \(\ensuremath K^{\pi} = 5/2^{\pm}\) parity doublet bands. Potential energy surfaces on the \(\ensuremath (\beta_{2},\beta_{3})\) -plane have been calculated for the lowest single quasi-particle configurations in 149Nd using the Strutinski method and the axially deformed Woods-Saxon potential. The predicted occurrence of the octupole-deformed K = 5/2 configuration is in agreement with experiment. Unexpectedly low \(\ensuremath \vert D_0\vert\) values obtained for the \(\ensuremath K^{\pi} = 5/2^{\pm}\) parity doublet bands may result from cancellation between the proton and neutron shell correction contributions to \(\ensuremath \vert D_0\vert\) .

Multi-phonon γ-vibrational bands in odd-mass nuclei studied by triaxial projected shell model approach

Inspired by the recent experimental data [J.-G. Wang, et al., Phys. Lett. B 675 (2009) 420], we extend the triaxial projected shell model approach to study the γ-band structure in odd-mass nuclei. As a first application of the new development, the γ-vibrational structure of 103Nb is investigated. It is demonstrated that the model describes the ground-state band and multi-phonon γ-vibrations quite satisfactorily, supporting the interpretation of the data as one of the few experimentally-known examples of simultaneous occurrence of one- and two-γ-phonon vibrational bands. This generalizes the well-known concept of the surface γ-oscillation in deformed nuclei built on the ground-state in even–even systems to γ-bands based on quasiparticle configurations in odd-mass systems.

Competing quasiparticle configurations inW163

Excited states in the neutron-deficient nuclide W163 were investigated using the Cd106(Ni60,2pn)W163 reaction at a beam energy of 270 MeV. The level scheme for W163 was extended significantly with the observation of five new band structures. The yrast band based on a 13/2+ isomeric state is extended up to (57/2+). Two band structures were established on the 7/2− ground state. Quasiparticle configuration assignments for the new band structures were made on the basis of cranked Woods-Saxon shell-model calculations. The results reported in this article suggest that the negative-parity ν(f7/2,h9/2) orbitals are responsible for the first rotational alignment in the yrast band.Received 30 October 2009DOI:https://doi.org/10.1103/PhysRevC.81.014307©2010 American Physical Society

Band structures in near spherical 138Ce

The high spin states of N = 80 138Ce have been populated in the fusion evaporation reaction 130Te( 12C, 4n) 138Ce at E beam = 65 MeV . The γ transitions belonging to various band structures were detected and characterized using an array of five Clover Germanium detectors. The level scheme has been established up to a maximum spin and excitation energy of 23 ℏ and 9511.3 keV, respectively, by including 53 new transitions. The negative parity Δ I = 1 band, developed on the 6536.3 keV 15 − level, has been conjectured to be a magnetic rotation band following a semiclassical analysis and comparing the systematics of similar bands in the neighboring nuclei. The said band is proposed to have a four quasiparticle configuration of [ π g 7 2 h 11 2 ] ⊗ [ ν h 11 2 ] − 2 . Other band structures are interpreted in terms of multi-quasiparticle configurations, based on Total Routhian Surface (TRS) calculations. For the low and medium spin states, a shell model calculation using a realistic two body interaction has been performed using the code OXBASH.

Shears bands in 112In

A new level scheme of 112In have been established up to 6.8 MeV in excitation energy and to a tentative spin of (21+) through the reaction 110Pd(7Li,5n)112In at a beam energy of 50 MeV. In-beam measurements involving γ-γ coincidences and directional correlation of oriented states were performed. M1 bands consisting of ΔI = 1 dipole transitions have been observed. Possible quasiparticle configurations suggest that these bands are similar to the shears bands observed in Pb nuclei.

Particle-number conserving analysis of the moments of inertia of high- K multiquasiparticle bands in 179Ta

The experimental one-, three-, and five-quasiparticle bands in 179Ta are analyzed by the particle-number conserving (PNC) method for the cranked shell model with pairing interaction, in which the particle-number is conserved and the blocking effects are considered exactly. Once the quasiparticle configurations, particularly those of the high- K three- and five-quasiparticle bands, are determined, the experimental moments of inertia and their variations with rotational frequencies can be reproduced very well by PNC calculations without any free parameter.

Multiple band structures ofCs131

Excited states in $^{131}\mathrm{Cs}$ were investigated through in-beam $\ensuremath{\gamma}$-ray spectroscopic techniques following its population in the $^{124}\mathrm{Sn}$($^{11}\mathrm{B}$, $4n$) fusion-evaporation reaction at a beam energy of 46 MeV. The previously known level scheme has been substantially extended up to $\ensuremath{\sim}$9 MeV excitation energy and $49/2\ensuremath{\hbar}$ spin with the addition of seven new band structures. The present level scheme consisting of 15 bands exhibits a variety of collective features in this nucleus at intermediate spin. The excitation energies of the observed levels in different bands and the corresponding ratios of transition strengths, i.e., $B(M1)/B(E2)$, have been compared with the results of projected deformed Hartree-Fock calculations based on various quasiparticle configurations. A strongly coupled band has been reassigned a high-$K$ three-quasiparticle $\ensuremath{\pi}{h}_{11/2}\ensuremath{\bigotimes}\ensuremath{\nu}({h}_{11/2}{d}_{3/2})$ configuration based on the properties of this band and that of its new coupled side band. The configurations of these bands are also discussed in the framework of tilted-axis cranking model calculations and the systematics of the odd-$A$ Cs isotopes. Additional three energetically closely placed coupled bands have been assigned different unpaired three-quasiparticle configurations. $\ensuremath{\gamma}$-vibrational bands coupled to the $\ensuremath{\pi}{h}_{11/2}$ and $\ensuremath{\pi}{g}_{7/2}$ single-particle configurations have been reported in this nucleus. Observation of new $E1$ transitions linking the opposite-parity $\ensuremath{\pi}{h}_{11/2}$ and $\ensuremath{\pi}{d}_{5/2}$ bands provides fingerprints of possible octupole correlations.

Electromagnetic transition rates in high-spin bands inNd136

Lifetimes have been measured for transitions in the two multiquasiparticle rotational bands in the nucleus $^{136}\mathrm{Nd}$. The extracted transition probabilities are compared with results of tilted-axis cranking and random-phase approximation calculations. The bands are identified as being built on two distinct quasiparticle configurations, with very different associated transition rates. These findings are contrary to an earlier suggestion that the bands form a chiral-band pair in this even-even nucleus.

High-K, t1/2 = 1.4(1)ms, isomeric state inLr255

An isomeric state in Lr255 with a half-life of t1/2 = 1.4(1) ms and Ex>720-keV has been observed for the first time using the GABRIELA setup at the focal plane of the VASSILISSA separator. Based on its K-forbiddeness, the configuration of the state is most probably formed by coupling the valence proton to a two quasiparticle neutron excitation. Possible three quasiparticle configurations are discussed.Received 31 December 2007DOI:https://doi.org/10.1103/PhysRevC.78.021302©2008 American Physical Society