Ground Band Research Articles

Electronic structure and magnetism in transition metal doped InSe monolayer: A GGA + U study

The electronic structures and magnetic properties of X-doped (X = Ni, Pd, Pt) InSe monolayers were investigated using first-principles calculations based on the GGA+U method. We show that the chemical doping is easier to take place at Se-rich than In-rich environment, and that Pt substitution of In is energetically most favorable among the three types of doping, followed by Ni and Pd. We further conduct calculation of ground state and band structures of Ni-, Pd- and Pt-doped InSe monolayers and find that the doped systems show diluted magnetic semiconducting nature. The magnetic moments of the X-doped systems are dominantly originated from the X dopants, and their extent of spin density expansion to the neighboring atoms is stronger in Pd- and Pt-doped cases than Ni-doped case. Further analysis of the impurity states near the Fermi energy reveals a strong spin-polarization feature in the three doped systems. Our results demonstrate that substitutional doping with Ni, Pd and Pt in InSe monolayer represents an effective manner to modulate electronic and magnetic properties of InSe for spintronic applications.

Novel solution of power law for γ-bands

The power law expression [Formula: see text] offers a single-term formula with just two parameters for expressing the level energies in the spectra of even-[Formula: see text] even-[Formula: see text] nuclei. Its application to ground band spectra for a wide range of nuclei has been demonstrated in our earlier works. Here, we extend its application to the rotational bands built on an excited state of [Formula: see text] [Formula: see text]-vibration band and [Formula: see text] beta band. A novel assumption of a virtual level with spin zero for [Formula: see text]-bands is made and its validity and use is illustrated. Here, the constancy of the parameters “[Formula: see text]” and “[Formula: see text]” with spin, offers a more realistic view of the dependence of the nuclear core deformation on spin, in the excited bands. Also, it enables a spinwise view, not available in the other energy fit expressions.

Pressure Induced Structural Phase Transition, Metallization and Superconductivity in RbBr

The structural phase transition, metallization and superconductivity of the simple alkali bromide, Rubidium bromide (RbBr) is investigated through their band structures. Under high pressure RbBr undergoes a structural phase transition from the NaCl structure (B1) to the CsCl structure (B2). The band structure, density of states (DOS) and total energy are computed as a function of volume for both B1 and B2 phases using the full potential linear muffin-tin orbital (FP-LMTO) method. The ground state properties and band gap values are compared with the experimental results. The pressure corresponding to structural phase transition from NaCl structure (B1) to the CsCl structure (B2) is 0.028 Mbar in RbBr. The metallization pressure P M is 1.172 Mbar. The superconducting transition temperatures (Tc) of RbBr is obtained as a function of pressure for both NaCl and CsCl structures. The highest Tc estimated is 2.151 K and the corresponding pressure is 4 Mbar in the NaCl structure and 0.819 K in the CsCl structure. It is also confirmed that the metallization, structural phase transition and onset of superconductivity do not occur simultaneously in ionic compounds. DOI: http://dx.doi.org/10.17807/orbital.v10i2.1125

E2/M1 mixing ratios in transitions from the gamma vibrational bands to the ground state rotational bands of 102, 104, 106, 108Mo, 108, 110, 112Ru, and 112, 114, 116Pd

E2/M1 mixing ratios have been measured for transitions from states in the $\gamma$ vibrational bands ( $ I^{+}_{\gamma}$ ) to states in the ground state bands ( $I^{+}$ or $ [I-1]^{+}$ ) of the neutron rich, even-even, deformed isotopes, 102, 104, 106, 108Mo, 108, 110, 112Ru, and 112, 114, 116Pd, including from states as high as $ 9^{+}_{\gamma}$ . These measurements were done using the GAMMASPHERE detector array, which, at the time of the experiment, had 101 working HPGe detectors, arranged at 64 different angles. A 62 $\mu$ Ci source of 252Cf was placed inside GAMMASPHERE yielding $5.7\times 10^{11}$ $\gamma$ - $\gamma$ - $\gamma$ and higher coincidence events. The angular correlations between the transitions from the $\gamma$ -bands to the ground bands, and the pure E2 transitions within the ground band were then measured. These angular correlations yielded the mixing ratios, demonstrating that these transitions are pure or nearly pure E2, in agreement with theory. In order to correct for possible attenuation due to the lifetime of the intermediate state in these correlations, the g-factors of the intermediate states needed to be known. Therefore, the g-factors of the 2+ states in the ground state band have been measured.

U(6) dynamical and quasi-dynamical symmetry in strongly deformed heavy nuclei

The low-lying collective states of the ground, β and γ bands in154Sm and238U are investigated within the framework of the microscopic proton-neutron symplectic model (PNSM). For this purpose, the model Hamiltonian is diagonalized in a U(6)-coupled basis, restricted to the symplectic state space spanned by the fully symmetric U(6) vectors. A good description of the energy levels of the three bands under consideration, as well as the intraband B(E2) transition strengths between the states of the ground band is obtained for the two nuclei without the use of an effective charge. The calculations show that when the collective quadrupole dynamics is covered already by the symplectic bandhead structure, as in the case of154Sm, the results show the presence of a very good U(6) dynamical symmetry. In the case of238U, when we have an observed enhancement of the intraband B(E2) transition strengths, then the results show small admixtures from the higher major shells and a highly coherent mixing of different irreps which is manifested by the presence of a good U(6) quasi-dynamical symmetry in the microscopic structure of the collective states under consideration.

Shell model results for T = 1 and T = 0 bands in 66As

The results of comprehensive shell model (SM) analyses, within the full model space, of the recently available experimental data (Ruotsalainen et al (2013) Phys. Rec. C 88 024320) with four T = 0 bands and one T = 1 band in the odd–odd N = Z nucleus 66As are presented. The calculations are performed by using the jj44b effective interaction developed recently by Brown and Lisetskiy for this model space. For the lowest two T = 0 bands and the T = 1 band, the results are in reasonable agreement with experimental data and the deformed shell model is used to identify their intrinsic structure. For the T = 1 band, a structural change at is predicted. For the third band with T = 0, the shell model values and quadrupole moments (in addition to energies) are consistent with the interpretation in terms of an aligned isoscalar np pair in a orbit coupled to the 64Ge ground band. Similarly, the level of band 4 and a close-lying level are found to be isomeric states in the analysis. Finally, the energies of the band 5 members, calculated using the shell model with both positive and negative parity, show that the observed levels are most likely negative parity levels. The shell model results with jj44b are compared with results obtained using the JUN45 interaction.

A systematic study of band structure and electromagnetic properties of neutron rich odd mass Eu isotopes in the projected shell model framework

The positive and negative parity rotational band structure of the neutron rich odd mass Eu isotopes with neutron numbers ranging from 90 to 96 are investigated up to the high angular momentum. In the theoretical analysis of energy spectra, transition energies and electromagnetic transition probabilities we employ the projected shell model. The calculations successfully describe the formation of the ground and excited band structures from the single particle and multi quasiparticle configurations. Calculated excitation energy spectra, transition energies, exact quantum mechanically calculated B(E2) and B(M1) transition probabilities are compared with experimental data wherever available and a reasonably good agreement is obtained with the observed data. The change in deformation in the ground state band with the increase in angular momentum and the increase in neutron number has also been established.

Outstanding problems in the band structures of Sm152

The recent data on $B(E2)$ values, deduced from the multi-Coulex excitation of the low spin states in the decay of $^{152}\mathrm{Sm}$, and other experimental findings in the last two decades are compared with the predictions from the microscopic dynamic pairing plus quadrupole model of Kumar and Baranger. The 1292.8 keV ${2}^{+}$ state is assigned to the ${{0}_{3}}^{+}$ band, and the $K=2$ assignment of the 1769 keV ${2}^{+}$ state is confirmed. The anomaly of the shape coexistence of the assumed spherical \ensuremath{\beta} band versus the deformed ground band is resolved. The values from the critical point symmetry X(5) support the collective character of the \ensuremath{\beta} band. The problem with the two-term interacting boson model Hamiltonian in predicting \ensuremath{\beta} and \ensuremath{\gamma} bands in $^{152}\mathrm{Sm}$ leads to interesting consequences. The collective features of the second excited ${K}^{\ensuremath{\pi}}=\phantom{\rule{0.16em}{0ex}}{{0}_{3}}^{+}$ band are preferred over the ``pairing isomer'' view. Also the multiphonon nature of the higher lying ${K}^{\ensuremath{\pi}}={{2}_{2}}^{+}\phantom{\rule{0.16em}{0ex}}\ensuremath{\beta}\ensuremath{\gamma}$ band and ${K}^{\ensuremath{\pi}}={4}^{+}$ band are illustrated vis-\`a-vis the new data and the nuclear structure theory.

Tetrahedral 4α and C12+α cluster structures in O16

We have investigated structures of the ground and excited states of $^{16}\textrm{O}$ with the method of variation after spin-parity projection in the antisymmetrized molecular dynamics model combined with the generator coordinate method of $^{12}\textrm{C}+\alpha$ cluster. The calculation reasonably reproduces the experimental energy spectra, $E2$, $E3$, $E4$, $IS1$ transitions, and $\alpha$-decay properties. The formation of 4 $\alpha$ clusters has been confirmed from nucleon degrees of freedom in the AMD model without assuming existence of any clusters. They form "tetrahedral" $4\alpha$- and $^{12}\textrm{C}+\alpha$-cluster structures. The $^{12}\textrm{C}+\alpha$ structure constructs the $K^\pi=0^+$ band consisting of the $0^+_2$, $2^+_1$, and $4^+_1$ states and the $K^\pi=0^-$ band of the $1^-_2$, $3^-_2$, and $5^-_1$ states. The $0^+_1$, $3^-_1$, and $4^+_2$ states are assigned to the ground band constructed from the tetrahedral 4$\alpha$ structure. The $0^+_1$ and $3^-_1$ are approximately interpreted as $T_d$ band members with the ideal tetrahedral configuration. The ground state $4\alpha$ correlation plays an important role in enhancement of the $E3$ transition strength to the $3^-_1$. The $4^+_2$ state is not the ideal $T_d$ member but constructed from a distorted tetrahedral $4\alpha$ structure. Moreover, significant state mixing of the tetrahedral $4\alpha$ and $^{12}\textrm{C}+\alpha$ cluster structures occurs between $4^+_1$ and $4^+_2$ states, indicating that the $T_d$ configuration of $4\alpha$ is rather fragile at $J^\pi=4^+$.

Modified particle-rotor model and low-lying rotational bands in odd-A triaxial nuclei

The low-lying rotational bands of triaxially deformed nuclei 137Pr, 137Pm and 139Eu are studied with a modified particle-rotor model following the nonadiabatic quasiparticle approach. The matrix elements of the odd-A nucleus are obtained in terms of a coupling matrix and the rotational energies of the even–even core. The spectra of the cores 136Ce, 136Nd and 138Sm indicate a strong influence of triaxial deformation and vibrational degrees of freedom. These properties are appropriately carried forward to the calculations for the odd-A nucleus. We demonstrate that the ground and side bands of the odd-A nucleus and its core can be explained with the same set of deformation parameters (, γ). We argue that this method could be useful in studying the low-lying states in exotic nuclei also.

A new perspective of ground band energy formulae

A host of alternative energy formulae for the ground bands of even Z even N nuclei are available in the literature. The usual approach is to compare the relative numerical accuracy of the predictions of the level energies by these formulae, for varying deformations of the nuclear core and for high spins. The soft rotor formula and variable moment of inertia model, the ab and pq formulae, the rotation vibration interaction and power index formulae are illustrated. Here, a new perspective is presented, with emphasis on the limitation of the region of their physical validity and on deriving useful meaning of their parameters.

Transition from vibrational to rotational character in low-lying states of hypernuclei

In order to clarify the nature of hypernuclear low-lying states, we carry out a comprehensive study for the structure of $^{144-154}_{~~~~~~~~\Lambda}$Sm-hypernuclei, which exhibit a transition from vibrational to rotational characters as the neutron number increases. To this end, we employ a microscopic particle-core coupling scheme based on a covariant density functional theory. We find that the positive-parity ground-state band in the hypernuclei shares a similar structure to that of the corresponding core nucleus. That is, regardless of whether the core nucleus is spherical or deformed, each hypernuclear state is dominated by the single configuration of the $\Lambda$ particle in the $s_{1/2}$ state ($\Lambda s_{1/2}$) coupled to one core state of the ground band. In contrast, the low-lying negative-parity states mainly consist of $\Lambda p_{1/2}$ and $\Lambda p_{3/2}$ configurations coupled to plural nuclear core states. We show that, while the mixing amplitude between these configurations is negligibly small in spherical and weakly-deformed nuclei, it strongly increases as the core nucleus undergoes a transition to a well-deformed shape, being consistent with the Nilsson wave functions. We demonstrate that the structure of these negative-parity states with spin $I$ can be well understood based on the $LS$ coupling scheme, with the total orbital angular momentum of $L=[I\otimes 1]$ and the spin angular momentum of $S=1/2$.

Reexamining the nuclear structure of Gd154 in the dynamic pairing plus quadrupole model

In a previous study of the collective multiphonon bands in $^{154}\mathrm{Gd}$, using the microscopic dynamic pairing plus quadrupole model, data for eight $K$ bands were analyzed. In the last four decades, its decay scheme is significantly revised and the nuclear theory has undergone a significant change. Special focus is on new weak intensity transitions in several bands and on the reassigned levels in its decay scheme. The present study represents a detailed revised analysis of the collective even parity bands below 2.1 MeV. Also, a discussion is given on the nature of the ${K}^{\ensuremath{\pi}}={0}^{+}$ excited bands, validity of band mixing approach, and of the assumption of shape coexistence of \ensuremath{\beta} band with ground band. Comparison is made with the $X$(5) analytical symmetry and the algebraic interacting boson model predictions. Discussion of the $2n$ transfer reactions is given. The validity of the multiphonon view of the ${K}^{\ensuremath{\pi}}={4}^{+}$ and ${2}_{2}^{+}$ bands is also studied.

Coexistence and B(E2)'s in 42Ca

New data have just appeared for Coulomb excitation of 42Ca, providing E2 transition matrix elements connecting several low-lying states. I have applied a simple two-state mixing model to the lowest two 0+, 2+, and 4+ states in order to fit the experimental matrix elements. Results indicate that the 0+ and 4+ states are nearly pure, whereas the 2+ states are strongly mixed. My excited band is found to be about 3.6 times more collective than the ground band.

Alternative description of chiral properties in 138Nd

The phenomenological generalized coherent-state model Hamiltonian is amended with a many-body term describing a set of nucleons moving in a spherical shell-model mean field and interacting among themselves with pairing, as well as with a particle–core interaction involving a harmonic quadrupole–quadrupole, an anharmonic hexdecapole–hexdecapole and a spin–spin interaction. The model Hamiltonian is treated in a restricted space consisting of the core projected states associated to the bands ground, and and two proton-aligned quasiparticles coupled with the states of the ground band to a total angular momentum. The chirally transformed particle–core states are also included. The Hamiltonian contains two terms which are not invariant to the chiral transformations relating the right-handed trihedral and the left-handed ones , , where are the angular momenta carried by fermions, proton and neutron bosons, respectively. The energies defined with the particle–core states form four chiral bands, two of them being degenerate. The electromagnetic properties of the chiral bands are investigated and the results are compared with the experimental data of 138Nd.

Finite valence nucleon number and rotation-vibration interactions

Background: A characteristic observable of nuclear collective motion is the relative $B(E2)$ values from the $\ensuremath{\gamma}$ band to the ground band in even-even deformed nuclei. The Alaga rules provide an idealized set of benchmarks for these observables. However, deviations from the Alaga rules are universally observed and have been traditionally and successfully interpreted in terms of parameterized $\ensuremath{\gamma}$-band--ground-band bandmixing. An alternate approach, partial dynamical symmetries, has no bandmixing whatsoever and is parameter free, yet mimics closely the effects of bandmixing, due solely to the effects of finite valence nucleon number.Purpose: To investigate the relation between these two seemingly contradictory approaches to understand how they can produce such similar results.Method: To derive approximate relations between the two formalisms.Results: A consistent relationship is found linking bandmixing to finite valence nucleon number effects on interband $\ensuremath{\gamma}$ to ground-band $B(E2)$ values.Conclusions: Two disparate approaches to one of the iconic characteristics of deformed nuclei are shown to be intimately related. Moreover, a systematic difference in their predictions also emerges naturally from the derivation. The qualitative linkage of valence nucleon number and the separation of vibrational and rotational degrees of freedom has long been assumed but never before explicitly demonstrated through complementary models.

Triaxial models description of the low-lying properties in 192Os

Several typical triaxial models have been parallel addressed and applied to describe the energy values and [Formula: see text] transitional rates in the ground band and the [Formula: see text] band for [Formula: see text]Os. It is shown that the different triaxial model presents different triaxial dynamics but each of them can only succeed to explain part of the spectral properties of this nucleus, which indicates that the triaxial shape of [Formula: see text]Os may be more complicated than that reflected by an ideal triaxial model. In addition, the staggering signature in experiments hints that [Formula: see text]-rigid triaxiality should be more or less involved in [Formula: see text]Os.

Gravitational-wave phasing for low-eccentricity inspiralling compact binaries to 3PN order

[abridged] Although gravitational radiation causes inspiralling compact binaries to circularize, a variety of astrophysical scenarios suggest that binaries might have small but nonnegligible orbital eccentricities when they enter the low-frequency bands of ground and space-based gravitational-wave detectors. If not accounted for, even a small orbital eccentricity can cause a potentially significant systematic error in the mass parameters of an inspiralling binary. Gravitational-wave search templates typically rely on the quasi-circular approximation, which provides relatively simple expressions for the gravitational-wave phase to 3.5 post-Newtonian (PN) order. The quasi-Keplerian formalism provides an elegant but complex description of the post-Newtonian corrections to the orbits and waveforms of inspiralling binaries with any eccentricity. Here we specialize the quasi-Keplerian formalism to binaries with low eccentricity. In this limit the non-periodic contribution to the gravitational-wave phasing can be expressed explicitly as simple functions of frequency or time, with little additional complexity beyond the well-known formulas for circular binaries. These eccentric phase corrections are computed to 3PN order and to leading order in the eccentricity for the standard PN approximants. For a variety of systems these eccentricity corrections cause significant corrections to the number of gravitational wave cycles that sweep through a detector's frequency band. This is evaluated using several measures, including a modification of the useful cycles. We also evaluate the role of periodic terms that enter the phasing and discuss how they can be incorporated into some of the PN approximants. While the eccentric extension of the PN approximants is our main objective, this work collects a variety of results that may be of interest to others modeling eccentric relativistic binaries.

LINEAR ACENES LINKED THIOPHENE, ELECTRONIC AND CHEMICAL PROPERTIES: PROSPECTS FOR MOLECULAR ORGANIC ELECTRONIC MATERIAL

We report a theoretical study of linear acene (n=1 to 7) linked thiophene properties functionality. The total ground state and band gap energies, Coulomb potential and nuclear repulsion energy are calculated by DFT, MP2 at B3LYP exchange level of the theory and 6-311G* basis set. The results are in good agreement with the experimental and theoretical values. It is found that the total ground state energy of the system and band gap energy decreases with an increasing number of electrons in the rings. The addition of thiophene molecules tends to improve the electronic and chemical properties of the linear acenes, the material exhibit potential application in the organic molecular electronic material.

Algebraic benchmark for prolate-oblate coexistence in nuclei

We present a symmetry-based approach for prolate-oblate and spherical-prolate-oblate shape coexistence, in the framework of the interacting boson model of nuclei. The proposed Hamiltonian conserves the SU(3) and $\overline{\rm SU(3)}$ symmetry for the prolate and oblate ground bands and the U(5) symmetry for selected spherical states. Analytic expressions for quadrupole moments and $E2$ rates involving these states are derived and isomeric states are identified by means of selection rules.